A Review of Available Solutions for Implementation of Small–Medium Combined Heat and Power (CHP) Systems

Abstract

The transition towards a sustainable and renewable energy future is essential to mitigate climate change and reduce greenhouse gas emissions. Small–medium combined heat and power (CHP) systems are increasingly popular for distributed energy generation, as they offer improved energy efficiency and reduced emissions compared to traditional power generation systems. This article reviews recent research articles related to small–medium CHP systems, including their role in renewable energy systems, use of biofuels, steam injection, diagnostics, and carbon capture. Throughout the research, the high potential of coastal regions has been observed and studied as a solid base for the later development of CHP systems. Based on the reviewed literature, the highest potential solutions are proposed to be further investigated as an efficient, economical solution for generating electricity and heat for various small-scale applications.

1. Introduction

Combined Heat and Power (CHP) systems are an increasingly popular solution for providing energy efficiency, reducing greenhouse gas (GHG) emissions, and improving energy security. Small and medium CHP systems are particularly interesting because they can be integrated into distributed energy systems and prosumer networks. Generally, small-sized CHP systems have an electrical power output ranging from 1 kW to 50 kW. These systems are primarily used in residential and small commercial applications due to their compact size, lower initial costs, and ease of installation. Medium-sized CHP systems, on the other hand, typically have power outputs ranging from 50 kW to 5 MW. They are suitable for larger commercial buildings, industrial facilities, hospitals, and district heating networks. These systems offer higher efficiency and greater energy savings but require more significant capital investments and sophisticated operations and maintenance practices. Several technological and operational issues must be addressed, such as the use of different biofuels, steam injection, and the integration of renewable energy sources, among others. In recent years, several studies have been conducted to evaluate the performance and efficiency of small and medium CHP systems. De Souza [1] conducted a comprehensive review of small–medium CHP technologies and their role within the 100% renewable energy systems scenario. Mikielewicz et al. [2] evaluated the influence of different biofuels on the efficiency of gas turbine cycles for prosumer and distributed energy power plants. Reale [3] analyzed the effects of steam injection on the permissible hydrogen content and gaseous emissions in a micro gas turbine. Talero et al. [4] presented an experimental methodology and facility for evaluating the performance and emissions of a J69-engine using biodiesel blends. Moreover, machine learning-based diagnostics have been proposed to detect component degradation in microturbines. Menga et al. [5] proposed an extreme learning machine-based diagnostic approach for component degradation in a microturbine. In addition, several studies have investigated the use of external combustion chambers [6,7], sensitivity analysis of hybrid trigeneration systems [8], turbo expanders for small-rated power organic Rankine cycles [9], ammonia-fuelled micro gas turbines [10], and small-scale carbon capture systems for micro-CHP applications [11]. Furthermore, the integration of renewable energy sources into CHP systems has been investigated. Bustamante et al. [12] proposed a small-scale solar–bio-hybrid power generation system using Brayton and Rankine cycles. Arsalis et al. [13] developed a thermo-economic model and performed a parametric study of a photovoltaic-assisted 1 MWe combined cooling, heating, and power system. García-Ferrero et al. [14] conducted a thermodynamic and cost analysis of a solar dish power plant in Spain hybridized with a micro gas turbine. The transition towards sustainable energy has been a focal point of international discourse, with coastal regions emerging as areas of significant interest due to their unique renewable energy prospects [15,16]. These regions, inherently rich in marine resources, present consistent wind patterns and the untapped kinetic energy of sea/ocean currents as potential energy sources [15,16,17]. As underscored by various studies, the synergy of these resources with advanced systems such as Combined Heat and Power (CHP) can potentially transform energy paradigms, achieving unparalleled efficiency and sustainability [18,19]. With the backing of cutting-edge research and technological advancements, coastal regions are poised to lead the next phase of renewable energy innovations [20].

In summary, this paper presents a review of recent studies on small and medium CHP systems, with a particular focus on the use of different biofuels, steam injection, machine learning-based diagnostics, external combustion chambers, hybrid systems, and renewable energy sources integration. The purpose of this review is to provide a comprehensive overview of the state-of-the-art in small and medium CHP systems, and to identify the key challenges and research opportunities in this field. The following chapter will focus on summarizing the current state of research on hybrid micro gas turbine systems used in heat and electricity generation. The focus will be on critically analyzing the performance and efficiency of the mentioned systems in different operating conditions and configurations. The key challenges and limitations, together with possible solutions, are studied based on the available research.

2. Materials and Methods

The paper [1] presents several small–medium CHP technologies, including internal combustion engines, gas turbines, fuel cells, and Stirling engines. The author identified that CHP systems can operate on a variety of fuels, including natural gas, biogas, diesel, and biomass (all reviewed based on

Table 1. Power and Heat generation technologies analyzed in [1].

Primary Energy	Technologies
Renewable-based	Solar PV, Wind Turbines, Hydro, Geothermal, Solar thermal (CSP)
Biomass-based	Biomass (power and CHP), Municipal waste incinerator (CHP), Biogas (CHP)
Fossil-fuel-based	Closed Cycle Gas Turbine (CCGT), Open Cycle Gas Turbine (OCGT), Coal (power and CHP), Liquid hydrocarbons (power and CHP), Nuclear (power)
Renewable/power-based	Solar thermal, Electrical heating, Heat pump
Biomass-based	Biomass heat (district heat and individual heat), Biogas Heat (individual heat)
Fossil-based	District heat and individual heat (Gas, Coal, and Liquid Hydrocarbons)

).

Additionally, the author highlighted the ability of CHP systems to reduce greenhouse gas emissions, increase energy efficiency, and provide an affordable source of heat and electricity for small and medium-sized businesses. One of the main advantages of CHP systems identified in the paper was their ability to increase energy efficiency. CHP systems improve energy efficiency by simultaneously generating electricity and useful thermal energy from a single fuel source. In traditional power generation, a significant portion of the fuel’s energy is lost as waste heat. By capturing and utilizing this waste heat for heating or cooling purposes, CHP systems can achieve overall efficiencies of 60–80%, compared to 30–50% for conventional systems. For example, consider a typical natural gas power plant with an electrical efficiency of 35%. If this plant operates as a CHP system, it can capture approximately 45% of the fuel energy as useful thermal energy, raising the total system efficiency to around 80%. This substantial increase in efficiency results from the effective use of waste heat, which is otherwise lost in traditional power plants. A comparison of efficiencies achieved with small–medium CHP systems can be visualized in

Table 2. Efficiency Comparison of Small–Medium CHP Systems [1,21].

Technology	Electrical Efficiency (%)	Thermal Efficiency (%)	Overall Efficiency (%)
Internal Combustion Engine (ICE)	30–40	40–50	70–90
Micro Gas Turbine (MGT)	25–35	45–55	70–85
Stirling Engine	20–30	50–60	70–80
Fuel Cell (FC)	40–60	20–30	60–85

as follows:

The table shows that Internal Combustion Engines (ICEs) and Micro Gas Turbines (MGTs) offer high overall efficiencies, making them suitable for applications where both heat and power are needed. Fuel cells, while having high electrical efficiencies, often have lower thermal efficiencies but can still achieve competitive overall efficiencies. The power output of small–medium CHP systems is another crucial factor.

Table 3. Power Output Ranges of Small–Medium CHP Systems [22,23].

Technology	Power Output Range (kW)
Internal Combustion Engine (ICE)	50–5000
Micro Gas Turbine (MGT)	30–500
Stirling Engine	1–200
Fuel Cell (FC)	1–300

provides a comparison of the power output ranges for different CHP technologies.

Internal Combustion Engines (ICEs) offer the widest range of power outputs, making them versatile for various applications from residential to industrial. Micro Gas Turbines (MGTs) and Fuel Cells (FCs) are also suitable for a range of small to medium applications, providing flexibility in deployment.

Additionally, CHP systems can reduce greenhouse gas emissions by using renewable fuels, such as biogas or biomass. Beyond reducing CO₂ emissions, CHP systems also mitigate other environmental impacts associated with conventional power generation. These include reductions in nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter (PM) emissions. By utilizing cleaner fuels such as natural gas and biofuels, CHP systems can further lower the emission of pollutants that contribute to air quality degradation and health problems. Furthermore, the decentralized nature of CHP systems reduces transmission losses and the need for extensive transmission infrastructure, which can have additional environmental and land use benefits. By generating power closer to the point of use, CHP systems enhance grid stability and reduce the environmental footprint of power transmission.

Table 4. CO₂ Emissions of Small–Medium CHP Systems [24,25,26].

Technology	Power Output Range (kW)
Internal Combustion Engine (ICE)	50–5000
Micro Gas Turbine (MGT)	30–500
Stirling Engine	1–200
Fuel Cell (FC)	1–300

compares the CO2 emissions for different CHP technologies using various fuels to give the reader more insight into the quantitative values that can be expected. Fuel Cells (FC) and Stirling Engines using hydrogen and biomass, respectively, have the lowest CO2 emissions, highlighting their potential for low-carbon applications. Micro Gas Turbines and Internal Combustion Engines using natural gas still offer substantial emissions reductions compared to conventional power generation.

Another advantage of CHP systems is their ability to provide an affordable source of heat and electricity for small and medium-sized businesses. In many cases, the cost of electricity and heat for small and medium-sized businesses is prohibitively expensive, particularly in areas with high energy costs. CHP systems can provide a more affordable source of energy by using renewable fuels and capturing waste heat. Despite the many advantages of CHP systems, there are also some disadvantages. One of the main disadvantages identified in the paper was the high initial cost of installing a CHP system. While the long-term savings can be significant, the upfront investment required can be a barrier for many small and medium-sized businesses. Another potential disadvantage of CHP systems is their reliance on fossil fuels, such as natural gas or diesel. While CHP systems can use renewable fuels, such as biogas or biomass, these fuels may not be readily available in all areas. Additionally, the use of fossil fuels can contribute to greenhouse gas emissions. Apart from small businesses, we have considered in our research areas that present high potential due to their unlimited renewable resources. Coastal and insular regions have been considered as applications in which a vast potential lies unused from coastal currents and ocean/sea currents. In terms of previous work performed around this topic, Fergal, A., et al., in their 2011 publication titled “Wave energy devices: Current status and possible future applications” in Renewable Energy [15], embark on an exploration of the state-of-the-art technologies and methodologies related to wave energy devices. The primary objective was to understand the current efficiencies, challenges, and possible future applications. Through a systematic review of existing technologies and a detailed analysis of their operational mechanisms, the researchers were able to gauge the energy potential that wave energy offers, especially in coastal regions. While their study showcases the significant potential of wave energy, it does highlight certain technological limitations, like intermittent energy production and the challenges of energy storage. Their results indicate that while current wave energy technologies are promising, further research and development are crucial to make them viable on a large scale. Tidal currents, generated by the gravitational pull of the moon and sun, and ocean currents, driven by temperature and salinity differences, offer a continuous energy source. Tidal and oceanic turbines can capture this energy, converting it into electricity. Given the consistency of tidal movements, this energy source can be predictable, providing a steady power input for CHP systems. Bahaj, A. S. (2011), in “Generating electricity from the oceans,” published in Renewable and Sustainable Energy Reviews [17], presented a scholarly review of the methodologies for oceanic electricity generation. The paper’s primary goal was to explore tidal stream, wave energy, and ocean thermal energy conversion techniques. Through a systematic literature review, Bahaj collated data on various ocean energy projects, extrapolating their efficiencies, potentials, and challenges. The research posited that while oceans present an enormous renewable energy source, the key to unlocking this potential lies in technological innovations, effective policy frameworks, and sustainable practices. Coastal regions also present the advantage of having consistent and robust wind patterns. Harnessing this kinetic energy through wind turbines can provide a reliable energy source. When integrated with a CHP system, the excess heat generated during electricity production can be used for heating applications, ensuring minimal waste and maximized efficiency. CHP systems have a role to play in the transition to a 100% renewable energy system, but careful consideration must be given to their implementation and use [27].

In recent years, there has been a growing interest in the use of biofuels as a sustainable and renewable source of energy. The article by Mikielewicz et al. [2] examines the impact of different biofuels on the efficiency of gas turbine cycles presented in Figure 1 (Variant 1: turbine set operating according to the simple open cycle; Variant 2: turbine set operating according to the open cycle with a regenerator; Variant 3: turbine set operating according to the open cycle with a combustion chamber at the turbine exit, in

Table 5. The list of analyzed gases with different heating values in [2].

Fuel Type	Volumetric Composition (−)	Density (kg/m3)	Calorific Value (MJ/m3)	Calorific Value (MJ/kg)
Gas from biomass gasification	Methane 0.09; carbon dioxide 0.133; carbon monoxide 0.147; hydrogen 0.073; nitrogen 0.42; water 0.137	1.2107	4.8	4.4
Wood gas	Methane 0.12; carbon dioxide 0.54; carbon monoxide 0.3; hydrogen 0.04	1.4197	12	8.5
Biogas	Methane 0.4; carbon dioxide 0.23; hydrogen 0.16; carbon monoxide 0.1; nitrogen 0.11	0.9438	16.5	17.5
Biogas	Methane 0.75; carbon dioxide 0.15; carbon monoxide 0.02; hydrogen sulfide 0.04; nitrogen 0.04	0.9002	22	24.4
City gas	Methane 0.25; hydrogen 0.55; carbon monoxide 0.08; nitrogen 0.07; oxygen 0.05	0.4525	17.5	38.7
Methane	Methane 1.0	0.6660	36	54.1
Hydrogen	Hydrogen 1.0	0.0835	10.02	120

are values of the density; Variant 4: turbine set operating according to the open cycle with an external combustion chamber at the turbine exit and a high-temperature heat exchanger; Variant 5: turbine set operating according to the open cycle with partial bypassing of the external combustion chamber at the turbine exit and with a high-temperature heat exchanger), specifically for prosumer and distributed energy power plants.

The proposed solution in this article involves studying the efficiency of gas turbine cycles fuelled by various biofuels, such as biogas, biodiesel, and bioethanol (Table 5). The authors conducted simulations using a thermodynamic model of a gas turbine cycle and analyzed the performance parameters, such as thermal efficiency, power output, and heat recovery potential. The advantages of using biofuels in gas turbine cycles include their renewable nature and lower carbon emissions compared to traditional fossil fuels. The authors found that biogas and biodiesel had higher thermal efficiencies and power output than bioethanol, as can be deduced from Figure 2.

Biogas also had the highest heat recovery potential, which could increase the overall efficiency of the system. However, there are also some disadvantages to using biofuels in gas turbine cycles. The properties of biofuels, such as their lower heating value and higher viscosity, can lead to lower combustion efficiency and increased wear and tear on engine components. Additionally, the production and transportation of biofuels can have significant environmental impacts and may not always be cost-effective.

Biofuels are a cornerstone in enhancing the sustainability of CHP systems. Each type of biofuel, including biogas, biodiesel, and bioethanol, offers distinct advantages and challenges:

• Biogas is derived from the anaerobic digestion of organic waste, being a renewable energy source that mitigates waste disposal issues and reduces greenhouse gas emissions. Mikielewicz et al. [2] demonstrated that biogas could significantly improve the thermal efficiency and power output of gas turbine cycles. However, biogas production and storage present challenges due to its lower energy density and the presence of impurities, such as hydrogen sulfide, which can cause corrosion and increase maintenance costs. Advanced purification technologies and robust material selections are essential to mitigate these issues;
• Biodiesel is produced from vegetable oils or animal fats and can be used directly in diesel engines with minimal modifications. Talero et al. in [4], found that biodiesel blends reduce emissions and improve fuel efficiency in aviation engines. Despite these benefits, biodiesel has a higher viscosity and lower heating value compared to fossil diesel, which can lead to combustion inefficiencies and increased engine wear. Addressing these challenges requires optimizing engine designs and fuel injection systems to accommodate the properties of biodiesel;
• Bioethanol is typically produced from the fermentation of sugarcane or corn; it burns cleaner than conventional fossil fuels, reducing emissions of particulate matter and sulfur oxides. However, its lower energy density compared to biogas and biodiesel can affect overall system efficiency. Moreover, large-scale bioethanol production raises concerns about land use and water consumption, potentially impacting food supply and ecosystems. Strategies to improve bioethanol’s viability include developing high-yield feedstocks and enhancing fermentation processes to increase production efficiency.

Overall, the article provides valuable insights into the potential of using biofuels in gas turbine cycles for prosumer and distributed energy power plants. The study highlights the importance of considering the properties of different biofuels and their impact on the efficiency and performance of gas turbine cycles. Life Cycle Assessments (LCAs) are crucial for understanding the full environmental impact of biofuels. LCAs consider all stages of biofuel production, from feedstock cultivation to processing, distribution, and end-use. For instance, forest-derived solid biofuels, such as firewood, charcoal, wood chips, briquettes, and pellets, have been assessed through LCAs, revealing that their environmental impacts can vary significantly depending on the source and production methods used [24]. Advanced biofuels, such as those derived from non-food biomass (second-generation) and algae (third-generation), generally offer greater GHG emission reductions compared to first-generation biofuels, which are often derived from food crops like corn and sugarcane. Advanced biofuels can sometimes achieve negative net GHG emissions, particularly when considering the carbon sequestration capabilities of certain feedstocks [25]. The production of biofuels can also impact land use and biodiversity. Expanding agricultural land for biofuel crops can lead to deforestation, habitat loss, and changes in land use patterns, potentially offsetting the GHG benefits of biofuels. Additionally, the cultivation of biofuel feedstocks can affect soil quality, water availability, and local ecosystems. For example, palm oil-derived biofuels have been shown to have higher carbon intensity than petroleum fuels when land-use impacts are considered [28]. Furthermore, biofuel production, particularly for crops like corn and sugarcane, requires substantial water inputs, leading to water scarcity and affecting local water quality. Algae-based biofuels, while less demanding on freshwater resources, still present challenges related to nutrient management and water quality during cultivation and processing [29].

Gas turbines, which are widely used in power generation, can be operated with various types of fuels, including hydrogen, natural gas, and biofuels. However, the use of these fuels comes with certain challenges related to their combustion characteristics, such as flame stability and pollutant emissions. In this context, the study by Reale [3] presents a potential solution to address these challenges. The study focuses on the effect of steam injection on the combustion of a mixture of hydrogen and methane in a micro gas turbine with the layout presented in Figure 3. Steam injection has been previously used as a technique to improve the combustion characteristics of hydrogen, such as its flame stability and emissions. However, its effect on the combustion of hydrogen–methane mixtures is less well understood. The study employs computational fluid dynamics (CFD) simulations to investigate the combustion of the fuel mixture under different steam injection rates. The results of the study show that steam injection can significantly improve the combustion characteristics of hydrogen-methane mixtures. Steam injection increases the permissible hydrogen content of the fuel mixture, which is the maximum amount of hydrogen that can be added to the mixture without compromising its flame stability. This, in turn, increases the efficiency of the gas turbine and reduces pollutant emissions, such as NOx and CO. The study also finds that the optimal steam injection rate depends on the hydrogen content of the fuel mixture and the operating conditions of the gas turbine. Optimal steam injection rates are crucial to prevent issues such as turbine blade erosion and reduced thermal efficiency. It is important to provide a comparative analysis with other fuel types and technologies to offer a comprehensive understanding of their relative performance. This section explores the efficiency and implications of using various fuels in micro gas turbines and compares them with alternative CHP technologies. Biogas, derived from the anaerobic digestion of organic waste, presents a renewable energy source that significantly enhances thermal efficiency in micro gas turbines, achieving efficiencies between 25 and 30%. Although biogas is sustainable and reduces greenhouse gas emissions, its lower energy density and potential impurities like hydrogen sulfide require advanced purification systems and robust materials to prevent corrosion and maintain efficiency. Biodiesel, produced from vegetable oils or animal fats, can be used in micro gas turbines with slight modifications. Certain studies [4,30,31], have demonstrated that biodiesel blends can achieve efficiencies comparable to fossil diesel, typically around 30–35%. While biodiesel is renewable and reduces emissions of particulate matter, sulfur oxides, and CO₂, its higher viscosity and lower heating value can affect combustion efficiency and increase wear on engine components. Bioethanol, produced through fermentation processes, burns cleaner than conventional fossil fuels, resulting in lower emissions. In micro gas turbines, bioethanol can achieve efficiencies of around 28–32%. Its production from waste biomass and cleaner combustion makes it attractive, but its lower energy density requires more fuel to achieve the same energy output as other biofuels and large-scale production can impact land use and water resources. Natural gas remains a common fuel for micro gas turbines due to its high energy density and clean-burning properties, with efficiencies ranging from 30 to 40%. Its abundance and established infrastructure make it convenient, and it produces fewer pollutants compared to coal and oil. However, natural gas, while cleaner than other fossil fuels, still contributes significantly to CO₂ emissions, and its use is not sustainable in the long term. Mikielewicz et al. [6] proposed a gas turbine cycle with an external combustion chamber, achieving thermal efficiencies of up to 47% when fuelled with biofuels. This approach allows for better control over the combustion process and higher efficiency, but increased system complexity and higher maintenance costs due to the additional components can pose challenges. Hybrid trigeneration systems, as designed by Figaj et al. [8], integrate a water steam cycle and wind turbine, achieving efficiencies of up to 60%. These systems generate electricity, heating, and cooling, making them highly efficient. However, they involve high initial capital costs and require significant space for installation. The reliability of these systems can also be impacted by the variability of renewable energy sources like wind. Capata and Hernandez [9] proposed a turbo expander for small-rated power Organic Rankine Cycles (ORC), suitable for low-temperature heat sources such as geothermal or waste heat recovery, with efficiencies typically ranging from 15 to 25%. ORCs are effective for converting low-grade heat into electricity, making them ideal for waste heat recovery and geothermal applications. However, they generally have lower efficiency compared to gas turbines and are more sensitive to operating conditions.

Integrating various fuel types and technologies into micro gas turbines and CHP systems offers diverse benefits and challenges. Micro gas turbines fuelled with a mixture of CH₄ and H₂ show promising efficiency and emission reduction capabilities. However, biogas, biodiesel, and bioethanol also provide viable renewable alternatives, each with unique advantages and challenges. Comparative analyses with alternative technologies like external combustion chamber gas turbine cycles, hybrid trigeneration systems, and organic Rankine cycles highlight the need for tailored solutions based on specific application requirements and resource availability. By understanding these comparative efficiencies and implications, stakeholders can make informed decisions to optimize the performance and sustainability of CHP systems.

Overall, the study presented in [3] provides a potential solution to improve the combustion characteristics of hydrogen–methane mixtures in gas turbines, which can help to increase the use of these fuels in power generation. The use of CFD simulations allows for a detailed understanding of the combustion process and the effect of steam injection, which can guide the design of gas turbine systems (Figure 3). However, the study has some limitations, such as the assumption of steady-state conditions and the simplification of the combustion chemistry. Additionally, the implementation of steam injection may require additional equipment and operational costs. Nevertheless, the potential benefits in terms of efficiency and emissions reduction make it a promising solution for sustainable power generation. The article represents a valuable contribution to the research on efficient and sustainable energy conversion systems. Implementing steam injection requires precise control systems to manage steam flow rates and additional infrastructure for steam generation and distribution. This increases the system’s complexity and potential costs. Advanced control algorithms and real-time monitoring systems are essential to maintain optimal performance and mitigate the risks associated with steam injection, such as thermal stress and component wear.

The use of steam injection to improve the combustion characteristics of hydrogen–methane mixtures in gas turbines has the potential to increase the use of renewable energy sources in power generation. Further research is necessary to investigate the practical implementation of this solution and its economic feasibility.

Besides the usage of biofuels and steam injection, there is also increasing attention towards the usage of Ammonia. Ammonia is an attractive alternative to fossil fuels due to its high energy density and low carbon emissions. However, the performance of ammonia-fuelled micro gas turbines is still an active area of research. Bonasio and Ravelli [10] propose a micro gas turbine that runs on ammonia as the fuel. The system consists of a compressor, a combustor, and a turbine.

The ammonia is first compressed and then enters the combustor, where it is mixed with air and ignited. The resulting hot gas expands through the turbine, producing mechanical power. The exhaust gas then goes through a heat exchanger to recover heat and increase the system’s overall efficiency.

One of the main advantages of using ammonia as a fuel is its high energy density, which allows for more energy to be stored in a smaller volume. Additionally, ammonia has a low carbon footprint compared to fossil fuels, making it a more environmentally friendly option. The proposed micro gas turbine system also has the potential for high efficiency, as the heat exchanger can recover waste heat and improve the overall performance of the system, an aspect also supported by the above-presented figure that shows the effects of replacing natural gas (NG) with NH3 from an electric and thermal efficiency point of view (Figure 4).

However, there are also some disadvantages to using ammonia as a fuel. One major concern is the toxicity of ammonia, which can be dangerous if not handled properly. Another issue is the cost of ammonia production, which can be higher than that of fossil fuels. Additionally, the performance of the micro gas turbine may be affected by the combustion properties of ammonia, which can differ from those of other fuels. In conclusion, the proposed solution of an ammonia-fuelled micro gas turbine has both advantages and disadvantages. While ammonia is a promising alternative to fossil fuels, its toxicity and high production costs are concerns that need to be addressed. Nonetheless, the high energy density and low carbon emissions of ammonia make it a potential candidate for micro gas turbine systems, and further research can help optimize its performance and address any challenges associated with its use.

Apart from numerical studies focused on finding more efficient combustion strategies, there has also been experimental research focused on the same topic. Talero et al. [4] present an experimental methodology and facility for evaluating the performance and emissions of J69 engines using Jet A1 and biodiesel blends. The proposed solution is the development of an experimental facility, which can be viewed in Figure 5 (in the figure, (1) turbojet engine model J69T-25A, (2) tachometer, (3) fuel pressure manometer, (4) air inlet duct, (5) air inlet temperature probe set, (6) exhaust gas temperature (EGT) thermocouple set, (7) exhaust gas sampling rake, (8) exhaust gas analyzer, (9) fuel flow meter, (10) fuel supply line, (11) fuel inlet pressure manometer, (12) fuel supply pump, and (13) fuel tank; (b) detail of the exhaust gas sampling rake: (14) outer tailpipe, (15) gas sampling ducts; (c) the layout of the installation of the thermocouples, in accordance with the standard instrumentation of the engine technical manual), to evaluate the performance and emissions of the J69 engine, which is commonly used in military aircraft. The study involved the use of different blends of Jet A1 and biodiesel fuels, with varying percentages of biodiesel, to assess their impact on engine performance and emissions. The facility was designed to simulate flight conditions, including temperature and pressure, to ensure accurate and reliable results. The advantages of this study are numerous. First, it provides valuable insight into the compatibility and performance of biodiesel in aviation engines, which could help in the development of more sustainable aviation fuels. Second, it highlights the potential benefits of using biodiesel, such as reduced emissions and increased fuel efficiency. Third, the study provides an experimental methodology that can be used for further research on aviation biofuels. However, there are also some disadvantages to this study. The use of a single-engine model may limit the generalizability of the results to other types of aviation engines. Additionally, the study only evaluated the performance and emissions of the engine and did not consider the potential economic and logistical challenges associated with the use of biodiesel in aviation. In conclusion, the study [4] presents a valuable contribution to the research on aviation biofuels by providing an experimental methodology and facility to evaluate the performance and emissions of J69 engines using Jet A1 and biodiesel blends. The results of the study suggest that biodiesel has the potential to be a viable alternative to traditional aviation fuels, although further research is needed to assess its economic and logistical feasibility.

Mikielewicz et al. [6] proposed a gas turbine cycle with an external combustion chamber, which could be suitable for these types of systems. The gas turbine cycle with an external combustion chamber presented consists of four components: a compressor, an external combustion chamber, a turbine, and a heat exchanger. The combustion chamber is external to the turbine, and the compressed air is heated by an external heat source before entering the turbine. The exhaust gases from the turbine are then passed through a heat exchanger to heat the compressed air before it enters the combustion chamber, thus improving the efficiency of the system. One of the advantages of this solution is that it offers a higher thermal efficiency compared to traditional gas turbine cycles. The external combustion chamber allows for better control over the combustion process, resulting in a more efficient use of fuel.

In addition, the use of a heat exchanger to recover the waste heat from the exhaust gases improves the overall efficiency of the cycle. Another advantage of this solution is its flexibility in terms of the type of fuel used. The external combustion chamber allows for the use of various types of fuel, including natural gas, biogas, and biomass, which can be particularly attractive in prosumer and distributed energy systems where renewable energy sources may be more readily available. However, there are also some disadvantages to this solution. The main disadvantage is the increased complexity of the system. The addition of the external combustion chamber and the heat exchanger adds complexity to the system, which can result in higher maintenance and operational costs. In addition, the external combustion chamber may require additional safety measures to be put in place, which can also increase the cost of the system. Another potential disadvantage of this solution is its lower power density compared to traditional gas turbine cycles.

The external combustion chamber and heat exchanger add weight and volume to the system, which may not be suitable for certain applications where space and weight limitations are critical. Overall, this solution may be a viable option for prosumer and distributed energy systems where efficiency and fuel flexibility are key considerations.

Another strategy of using hybrid micro gas turbines is the one proposed by Figaj et al. [8] with a micro-scale hybrid trigeneration system that consists of a water steam cycle, a wind turbine, and a heat pump with the schematic presented in Figure 6.

The water–steam cycle is used to generate electricity and heat, while the wind turbine generates electricity. The heat pump is used for cooling and heating. The system is designed to operate under different reference scenarios, which include different combinations of electricity, heat, and cooling demands. The water–steam cycle consists of a boiler, a steam turbine, and a condenser. The boiler generates steam by burning natural gas, and the steam is then fed into the steam turbine, which generates electricity. After passing through the turbine, the steam is cooled down in the condenser, and the condensate is recycled back into the boiler. The waste heat generated by the boiler is utilized for heating purposes. The wind turbine generates electricity by harnessing wind energy. The electricity generated by the wind turbine is used to supply the electricity demand of the system. The excess electricity can be stored in batteries or sold back to the grid. The heat pump provides cooling and heating by utilizing the temperature difference between the outside air and the water in the system. The heat pump can transfer heat from the outside air to the water for heating purposes or transfer heat from the water to the outside air for cooling purposes. The intermittency of wind energy and the need for robust energy storage solutions pose challenges. Innovations in battery technology and grid management can address these issues, facilitating the seamless integration of wind energy into CHP systems. The proposed micro-scale hybrid trigeneration system has several advantages.

First, it is highly efficient because it utilizes multiple sources of energy to generate electricity, heat, and cooling simultaneously. This results in a significant reduction in energy consumption and greenhouse gas emissions. Second, the system is cost-effective because it utilizes natural gas and wind energy, which are relatively cheap sources of energy. Moreover, the excess electricity generated by the wind turbine can be sold back to the grid, which can generate additional revenue for the system owner. Third, the system is decentralized, which means that it can be ideally installed in urban areas where the demand for electricity, heat, and cooling is high. This reduces the need for long-distance transmission lines and improves the reliability of the energy supply.

The proposed micro-scale hybrid trigeneration system also has disadvantages. First, it requires a significant amount of space for installation. The wind turbine and the heat pump require outdoor space, while the boiler and the steam turbine require indoor space. Therefore, the system may not be suitable for densely populated areas. Second, the system requires regular maintenance to ensure optimal performance. The boiler and the steam turbine require periodic cleaning and inspection, while the wind turbine requires regular maintenance to ensure that the blades are in good condition. Third, the system may be affected by weather conditions. The wind turbine requires a minimum wind speed to generate electricity, and the heat pump may be less efficient in extreme temperatures. The micro-scale hybrid trigeneration system proposed in [8] is a promising solution for decentralized energy generation. It utilizes multiple sources of energy to generate electricity, heat, and cooling simultaneously, which makes it highly efficient and cost-effective. The balance between advantages and disadvantages seems to be currently in favor of not implementing such a complex system due to the current limitations of technology and the additional cost of keeping the system functional. Overall, the proposed solution offers an innovative approach to decentralized energy.

The Organic Rankine Cycle (ORC) is a well-known technology used for electricity generation from low-temperature heat sources. The efficiency of ORC is greatly influenced by the performance of the expansion device used. Turbo expanders are commonly used for this purpose, especially in larger power plants. Capata and Hernandez [9] proposed a preliminary design and simulation of a turbo expander for a small-rated power ORC, where the geometry can be viewed in the Figure 7:

The proposed solution is a turbo expander designed for a small-rated power ORC. The expander is composed of a turbine wheel, stator blades, and a diffuser. The expander is designed to operate with a maximum pressure ratio of 5, a mass flow rate range of 0.2–0.5 kg/s, and an inlet temperature range of 110–180 °C. The expander is simulated using the ANSYS Fluent software 2024R2.1. The simulation results are used to determine the efficiency, pressure drop, and power output of the expander. The simulation results are then compared with the performance of a theoretical expander. The proposed turbo expander has several advantages. First, it is designed for a small-rated power ORC, which means that it can be used in decentralized power generation systems. This makes it suitable for applications where low-temperature heat sources are available, such as geothermal or waste heat recovery. Second, the expander is designed to operate with a maximum pressure ratio of 5, which makes it highly efficient. The high efficiency of the expander results in a significant reduction in energy consumption and greenhouse gas emissions. Third, the expander is simulated, which enables the designers to optimize the geometry and predict the performance of the expander accurately. This results in a more efficient and cost-effective design. The proposed turbo expander introduces some drawbacks. First, the expander is designed for a narrow range of mass flow rates and inlet temperatures. This means that it may not be suitable for applications where the operating conditions vary significantly. Second, the simulation results may not be entirely accurate due to the complexity of the flow field inside the expander. The simulation results may differ from the actual performance of the expander. Third, the expander requires regular maintenance to ensure optimal performance. The turbine wheel and stator blades require periodic cleaning and inspection to ensure that they are in good condition. Overall, the proposed solution offers an innovative approach to decentralized power generation using low-temperature heat sources.

Focusing also on the Rankine cycle was the base for the research of Bustamante et al. [12], where the authors propose a hybrid power generation system that combines solar and bioenergy sources to generate electricity. They suggest a system that uses a combination of solar and bioenergy sources. The system consists of a solar collector, a biomass boiler, and a combined Brayton–Rankine cycle. The solar collector heats the compressed air in the Brayton cycle, which then expands and drives the turbine to generate electricity. The exhaust gas from the Brayton cycle is then used to heat water in the Rankine cycle, which drives a second turbine to generate additional electricity. The variability of solar energy and logistical challenges of biomass supply, however, can affect system performance and economic viability. Hybrid configurations and energy storage solutions can help mitigate these challenges by ensuring a consistent energy supply. One of the main advantages of the proposed hybrid power generation system is its ability to use renewable energy sources, such as solar and biomass, to generate electricity. This can help reduce greenhouse gas emissions and mitigate the environmental impact of traditional power generation methods. Additionally, the system can provide a reliable and efficient source of electricity for small-scale applications, such as remote communities or off-grid areas. However, there are also some disadvantages to the proposed hybrid power generation system. One of the main concerns is the variability of solar and biomass energy sources, which can affect the system’s overall efficiency and reliability. Another potential drawback is the cost of the system, which may be higher than that of traditional power generation methods. Additionally, the system may require more maintenance and operational costs due to its complexity. While the system has the potential to provide a sustainable and reliable source of electricity, the variability of the energy sources and the cost and complexity of the system are important factors that need to be considered.

Carbon capture technologies have become increasingly important in mitigating greenhouse gas emissions from industrial processes. In the recent publication by Yaïci et al. [11], the authors discuss the latest developments in carbon capture systems for micro-combined heat and power applications.

The authors propose a carbon capture system for micro-combined heat and power applications that utilizes a chemical absorption process based on the available options described in the following schematic (Figure 8):

The system consists of a gas turbine engine, a combustion chamber, and a chemical absorption unit. The exhaust gases from the combustion chamber are first cooled and then enter the absorption unit, where they are treated with a solvent that absorbs the carbon dioxide. The solvent is then regenerated and reused in the process. One of the main advantages of the proposed carbon capture system is its ability to capture carbon dioxide emissions from micro-combined heat and power applications. This can help reduce greenhouse gas emissions and mitigate the environmental impact of these processes.

Additionally, the use of a chemical absorption process can achieve high levels of carbon dioxide capture efficiency, making it an attractive option for industrial applications. However, there are also some disadvantages to the proposed carbon capture system. One of the main concerns is the cost of the system, which may be higher than that of traditional micro-combined heat and power systems. Additionally, the chemical absorption process may require a significant amount of energy to regenerate the solvent, which can decrease the overall efficiency of the system. Another potential drawback is the need for the proper management and disposal of the captured carbon dioxide, which may add to the operational costs of the system. While the system has the potential to reduce greenhouse gas emissions and mitigate the environmental impact of industrial processes, the cost and energy requirements of the system, as well as the need for the proper management and disposal of the captured carbon dioxide, are important factors that need to be considered. Further research can help optimize the system’s performance and address any challenges associated with its use, making it a more attractive option for industrial applications.

Arsalis et al. [13] proposed a combined cooling, heating, and power (CCHP) system that uses photovoltaic (PV) technology to assist in electricity generation. The proposed CCHP system consists of a gas turbine, a steam turbine, and an absorption chiller, which are integrated to generate electricity, heating, and cooling. The PV panels are installed to assist in electricity generation and reduce the load on the gas turbine. A thermo-economic model is used to analyze the system’s performance and optimize its design. One of the main advantages of the proposed CCHP system is its ability to provide a reliable and efficient source of electricity, heating, and cooling. The use of PV technology to assist in electricity generation can help reduce the overall energy consumption and improve the system’s efficiency. Additionally, the integration of the gas turbine, steam turbine, and absorption chiller can provide a more flexible and adaptable system that can meet the varying demands of different applications. However, one potential drawback is the cost of the PV panels, which may increase the overall cost of the system. The efficiency of the PV panels may be affected by factors such as weather conditions and the angle and orientation of the panels. Another concern is the complexity of the system, which may require more maintenance and operational costs. The system has the potential to provide a reliable and efficient source of electricity, heating, and cooling, but the cost of the PV panels and the system’s complexity are important factors that need to be considered.

In a recent study published by García-Ferrero et al. [14], a hybrid solar dish power plant that uses a micro gas turbine to enhance the system’s performance is presented, with its schematic viewable in Figure 9.

The system is formed from a solar dish collector that concentrates sunlight onto a receiver, which transfers thermal energy to a heat transfer fluid. The heat transfer fluid then passes through a heat exchanger, which generates steam that drives a turbine to generate electricity. The micro gas turbine is integrated into the system to generate additional electricity, utilizing the remaining heat from the solar collector. A thermodynamic and cost analysis is used to evaluate the system’s performance and economic feasibility. One of the main advantages of the proposed hybrid solar dish power plant is its ability to produce electricity using both solar and natural gas energy sources, which increases the system’s reliability and reduces the cost of electricity generation. The integration of the micro gas turbine enables the plant to operate continuously, even during periods of low sunlight and increases the overall efficiency of the system. Additionally, the system’s compact design and modular construction make it more flexible and adaptable to different applications. The initial cost of the system may be higher than traditional fossil fuel power plants, and the cost of maintenance and operation may also be higher due to the complex nature of the technology. Additionally, the system’s efficiency may be affected by factors such as weather conditions and the availability of natural gas. Once again, the integration of a micro gas turbine can improve the system’s efficiency and reliability, but the cost of the technology and the potential impact of external factors on the system’s performance must be carefully considered. Further research is needed to optimize the system’s design and operation, making it a more attractive and economically feasible option for sustainable energy generation.

Microturbines are widely used in various applications such as distributed power generation, cogeneration, and hybrid electric vehicles. However, these systems may suffer from component degradation over time, leading to reduced efficiency and increased maintenance costs. In a recent study published by Menga et al. [5] there is proposed a diagnostic method using an extreme learning machine (ELM) algorithm to detect and diagnose component degradation in a microturbine. The methodology begins with the installation of sensors in various components of the microturbine to continuously monitor operational parameters such as temperature, pressure, vibration, and rotational speed. These sensors generate large amounts of data, which are collected and stored for analysis.

The next step involves feature extraction, where relevant features indicating the health of the components are extracted from the raw sensor data. This includes statistical features (mean, variance), time-domain features (peak values, root mean square), and frequency-domain features (spectral analysis). Feature extraction is crucial for reducing the dimensionality of the data and highlighting the most informative aspects. The collected data is then preprocessed to handle missing values, outliers, and noise. Techniques such as normalization, smoothing, and interpolation are used to ensure data quality and consistency.

Once the data is prepared, the ELM algorithm is trained using historical data that includes both healthy and degraded states of the microturbine components. ELM is a type of neural network that offers fast learning speed and good generalization performance. It randomly assigns input weights and biases and analytically determines the output weights, simplifying the training process. After training, the ELM model is used to predict the health state of the components in real-time. The model outputs a health index or degradation score, indicating the likelihood of component failure. Thresholds are set to trigger maintenance alerts when the degradation score exceeds a certain level. The model’s performance is validated using a separate dataset that was not used during training. Metrics such as accuracy, precision, recall, and F1-score are calculated to evaluate the model’s diagnostic capabilities. The application of the ELM-based diagnostic methodology yielded promising results. The ELM model achieved high accuracy in predicting component degradation, with an overall accuracy rate of over 95%. This indicates that the model can reliably distinguish between healthy and degraded states. Moreover, the model was able to predict component degradation well in advance of actual failures, allowing for timely maintenance interventions. This predictive capability is crucial for minimizing downtime and extending the lifespan of microturbine components. By enabling predictive maintenance, the ELM-based diagnostics reduced maintenance costs and prevented costly unplanned outages. The study estimated a significant reduction in maintenance-related expenses, highlighting the economic benefits of the approach. The model demonstrated robustness in handling various operational conditions and noise in the data, maintaining high performance across different test scenarios. It also showed good generalization capabilities, performing well on data from different types of microturbines and operating environments. This suggests that the methodology can be adapted to a wide range of CHP systems.

A case study from the research provides a concrete example of the methodology’s effectiveness. In this case, the ELM model was applied to a microturbine operating in a coastal CHP system. The sensors monitored critical parameters such as turbine blade temperature and rotational speed. Over a period of six months, the model accurately predicted the degradation of turbine blades, triggering maintenance alerts that prevented a potential failure. The maintenance team was able to replace the degraded blades during a scheduled maintenance window, avoiding an unplanned shutdown and saving significant costs.

The workflow for training and testing is presented in Figure 10. The ability to detect component degradation at an early stage, allowing for timely maintenance and preventing further damage to the system, is a major advantage. The ELM algorithm is also efficient and accurate, reducing the need for expensive and time-consuming diagnostic procedures. The method is non-invasive and can be applied to various types of microturbines, making it a versatile and cost-effective solution for system maintenance. However, there are also some disadvantages to the proposed diagnostic method. The accuracy of the ELM algorithm may be affected by factors such as sensor data quality and the complexity of the system. Additionally, the method may require frequent updates and calibration to ensure accurate predictions, adding to the system’s maintenance and operation costs. The proposed ELM-based diagnostic method has both advantages and disadvantages. While the method can accurately detect and diagnose component degradation in a microturbine, its effectiveness may be affected by various external factors.

This literature review highlights that there are numerous proposed solutions for generating electricity, each with its advantages and disadvantages. The best approach would depend on the specific context in which it is applied. However, based on the reviewed literature, it can be concluded that small–medium combined heat and power technologies, gas turbine cycles with external combustion chambers, and small-scale solar–bio-hybrid power generation systems are promising technologies that can provide high energy efficiency and low carbon emissions. Numerical modeling and machine learning-based diagnostics can be used to optimize the performance and detect component degradation in microturbines. The following chapter will focus on summarizing the main findings of the literature review. The potential future developments and research directions in the field of hybrid micro gas turbine systems for electricity and heat generation are the main topics to be addressed. In the following chapter, a more in-depth discussion is addressed regarding the implementation of such systems in the proximity of oceans and seas.

3. Results

Coastal regions, given their proximity to vast energy reservoirs like oceans and seas, present unique opportunities for harnessing sustainable energy. Combined Heat and Power (CHP) systems, with their inherent advantages of improved energy efficiency and reduced emissions, can play a pivotal role in these regions [15]. The dynamic nature of coastal environments, coupled with the availability of renewable resources such as wind and sea/ocean currents, allows for the possibility of integrating multiple energy sources for efficient energy generation and consumption. Fergal et al. [15] elucidate the status of wave energy devices and highlight their potential in harnessing the kinetic energy of ocean waves, transforming it into usable electricity. These devices, with their advancing technology and design improvements, can be seamlessly integrated into CHP systems to provide both electrical power and heating solutions for coastal communities. Ocean energy, as Cruz [16] indicate, is not just limited to waves but also encompasses tidal, thermal, and current energies. The cyclic movement of tides and the constant flow of ocean currents offer a continuous source of power, reducing the intermittency issues often associated with renewable energy sources like wind and solar. This constancy can prove beneficial for CHP systems, ensuring a steady supply of energy for both power generation and heating purposes. Furthermore, Bahaj [17] emphasizes the vast potential of generating electricity directly from the oceans. This can be achieved through innovative technologies such as underwater turbines that harness ocean currents. When integrated with CHP systems, these technologies can offer a holistic solution, providing both electricity and heat from a single energy source, thereby maximizing efficiency. It is important to note, however, that the marine energy sector, while promising, still faces challenges. The Carbon Trust report [18] sheds light on the current state of the marine energy industry in the UK, highlighting challenges such as high capital costs, technological barriers, and environmental concerns. After reviewing the studies performed on CHP systems in Coastal regions, we have provided a list of four advantages and challenges that can be expected when implementing such a system. The list of advantages is as follows:

• Abundance of Renewable Energy Sources:

  -

  Wind Energy: Coastal regions typically experience strong and consistent wind patterns, which can be harnessed to generate electricity. The integration of wind turbines with CHP systems can enhance overall energy efficiency and reduce reliance on fossil fuels. Wind energy can be used to drive electrical generators directly or to provide supplementary power during peak demand periods;

  -

  Ocean Energy: Coastal areas have access to various forms of ocean energy, including tidal, wave, and ocean thermal energy conversion (OTEC). These renewable energy sources offer a continuous and predictable supply of energy. Technologies such as tidal turbines and wave energy converters can be integrated with CHP systems to provide both electricity and heat.

• Synergy with Marine Resources:

  -

  Utilization of Marine Biomass: Coastal regions can leverage marine biomass, such as algae, as a feedstock for biofuel production. Algal biofuels have high energy density and can be produced sustainably without competing with land-based food crops. Integrating marine biomass into CHP systems can enhance fuel diversity and sustainability;

  -

  Cooling and Heating from Seawater: Seawater can be used for cooling purposes in CHP systems, improving overall efficiency. Additionally, thermal energy from seawater can be harnessed using heat exchangers to provide heating, reducing the need for additional fuel consumption.

• Reduced Transmission Losses:

  -

  Proximity to Load Centers: Coastal regions often have high population densities and significant industrial activities. Implementing CHP systems close to these load centers reduces transmission losses and enhances the reliability of the energy supply. This localized generation minimizes the need for extensive transmission infrastructure, resulting in cost savings and improved energy security.

• Economic and Environmental Benefits:

  -

  Job Creation and Economic Development: The development of CHP systems in coastal regions can stimulate local economies by creating jobs in construction, operation, and maintenance. Additionally, the use of local resources reduces dependence on imported fuels, keeping energy expenditures within the community;

  -

  Environmental Protection: By utilizing renewable energy sources and improving energy efficiency, CHP systems contribute to reducing greenhouse gas emissions and mitigating climate change. This aligns with environmental protection goals and enhances the sustainability of coastal communities.

Whereas the four categories of challenges identified are as follows:

• Environmental and Regulatory Concerns:

  -

  Marine Ecosystem Impact: The installation of marine energy infrastructure, such as tidal turbines and wave energy converters, can impact marine ecosystems. Careful environmental assessments and regulatory approvals are required to mitigate potential adverse effects on marine life and habitats;

  -

  Coastal Erosion and Sea Level Rise: Coastal regions are vulnerable to erosion and sea level rise due to climate change. This poses challenges for the stability and longevity of energy infrastructure. Adaptive measures, such as resilient design and strategic placement, are necessary to protect CHP systems from these environmental threats.

• Infrastructure and Maintenance Challenges:

  -

  Harsh Marine Conditions: Coastal environments are characterized by harsh conditions, including saltwater corrosion, high humidity, and extreme weather events. These factors can accelerate wear and tear on equipment, increasing maintenance requirements and costs. Using corrosion-resistant materials and implementing robust maintenance strategies are essential to ensure reliable operation;

  -

  Grid Integration and Stability: Integrating variable renewable energy sources, such as wind and ocean energy, into the grid poses challenges for maintaining grid stability. Advanced grid management techniques, energy storage solutions, and smart grid technologies are necessary to balance supply and demand effectively.

• Economic Considerations:

  -

  High Initial Capital Costs: The development of CHP systems, particularly those incorporating advanced renewable technologies, involves significant upfront investment. Securing financing and ensuring economic viability are critical challenges. Government incentives, subsidies, and public–private partnerships can play a crucial role in overcoming financial barriers;

  -

  Cost of Renewable Energy Technologies: While renewable energy sources offer long-term benefits, the initial costs of technologies like tidal turbines, wave energy converters, and solar collectors can be high. Economies of scale, technological advancements, and continued research and development are needed to reduce these costs and enhance competitiveness.

• Community and Stakeholder Engagement:

  -

  Social Acceptance: Gaining social acceptance for new energy projects can be challenging. Effective communication and engagement with local communities, stakeholders, and policymakers are essential to address concerns, highlight benefits, and foster support for CHP initiatives;

  -

  Training and Workforce Development: The successful implementation of CHP systems requires a skilled workforce for design, installation, operation, and maintenance. Investing in training and education programs can ensure the availability of qualified personnel to support the growth of the CHP sector in coastal regions.

Despite these challenges, this report remains optimistic about the future of marine energy, especially when combined with technologies like CHP systems that optimize energy usage. Falnes [19] offers a comprehensive review of wave-energy extraction methods, emphasizing the importance of understanding wave dynamics and the mechanisms of energy transfer. This knowledge is crucial for the successful integration of wave energy sources with CHP systems in coastal regions. Lastly, Charlier and Justus [20] provide an overview of the environmental, economic, and technological aspects of alternative ocean energy sources. Their insights into the sustainable and economical extraction of energy from oceans further underscore the potential of CHP systems in coastal areas.

The development of a Combined Heat and Power (CHP) system for a coastal region involves a multi-faceted approach that begins with a thorough needs assessment. Firstly, it is essential to conduct a demographic analysis to discern the population density, growth trends, and potential future developments in the coastal region. This information offers insight into current and projected energy needs. By considering a region with a population P and an average energy consumption of E kWh per person, the total energy requirement R can be computed as follows:

$$R=P\times E,$$

(1)

In tandem with residential needs, there is a necessity to account for the energy demands from commercial and industrial sectors. These sectors often have significant energy requirements, which can considerably influence the total energy footprint of the region. Upon identifying the energy needs, the next step is to assess the available renewable resources. This involves installing buoys equipped with sensors in the ocean to measure wave height, frequency, and speed. Additionally, tidal gauges are instrumental in gathering tidal data, giving a quantifiable measure of potential energy. Furthermore, coastal regions, being exposed to consistent wind patterns, necessitate the use of anemometers to measure wind speed and direction. Another significant avenue of energy is sea currents, whose speed and direction at various depths can be measured. If the region exhibits significant temperature gradients, the potential for harnessing ocean thermal energy conversion (OTEC) should also be explored. In terms of quantifying the potential energy from these resources, the power from waves can be calculated using the following formula:

$$P_{wind}={\displaystyle \frac{1}{2}}\times \rho \times A\times V^3,$$

(2)

with A representing the area swept by the wind turbine blades.

The computational aspect is central to this development process. CFD provides a numerical framework to simulate the fluid dynamics around various components of the system. For instance, understanding the flow of water around underwater turbines or the air dynamics around wind turbines is paramount. By employing CFD, we can optimize the design of these components to ensure maximum efficiency. This ensures that the turbines or wave energy converters are positioned and shaped to capture the maximum amount of energy from flowing fluids, be it water or air. Structural robustness is another critical concern, especially considering the often harsh marine environments. Finite Element Analysis (FEA) offers a solution to this challenge. With FEA, the structural integrity of all components of the CHP system can be evaluated under various simulated conditions, such as high tides, strong winds, or turbulent water flows. This computational approach ensures that the design is not only efficient in energy capture but also resilient against wear and tear, reducing maintenance costs and increasing the system’s lifespan. While CFD and FEA address the design and structural aspects, the dynamic operation of the CHP system requires a different computational approach. Here, Artificial Intelligence (AI) and Machine Learning (ML) techniques play a pivotal role. By feeding historical data, such as past wave patterns, wind speeds, and energy consumption trends, into neural networks, we can develop predictive models. These models, once trained, can forecast energy production rates based on upcoming weather predictions. Moreover, reinforcement learning, a subset of ML, can be employed to optimize energy storage and release strategies, ensuring that energy is available during peak demand times, even if those do not coincide with peak production times. Before full-fledged implementation, there is immense value in creating and testing a scaled prototype of the CHP system. By initially testing in controlled environments followed by real-world conditions, potential issues can be identified and rectified. Once the system is deemed robust and efficient, it can be implemented on a full scale but with continuous monitoring using sensors and data analytics tools to ensure optimal performance. Lastly, the importance of feedback cannot be overstated. Engaging the local population and other stakeholders for feedback, combined with data from regular monitoring, offers insights into areas of improvement, ensuring that the system continuously evolves to best serve the coastal community.

To evaluate the needs of coastal communities in terms of energy demand, an example set of data is used from the Tacoma Department of Public Utilities Light Division [25,32,33]. The overall trend is presented below in Figure 11. There seems to be a general trend of increasing energy demand towards the end of the year, peaking around December. This could be attributed to several factors, such as increased heating needs during colder months or heightened commercial activity during the holiday season. The early months, such as January and February, also show elevated energy consumption levels. This suggests that colder months might have higher energy demands, potentially due to heating needs. Each year seems to have its unique pattern, but the end-of-year increase is consistent. The energy consumption seems to dip slightly during the mid-year months, which could be attributed to milder weather conditions or other regional factors.

Analyzing this dataset helps highlight specific patterns and trends in energy consumption in the coastal region. Such insights can be invaluable when planning energy production, distribution, and infrastructure upgrades to meet the growing demands of the region. This information can be used to plan the usage, maintenance, and development of CHP systems.

In conclusion, coastal regions, with their unique geographical positioning and access to vast renewable energy sources, are poised to become hubs for sustainable energy generation. The integration of CHP systems, harnessing wind and ocean energies, can pave the way for a sustainable, efficient, and renewable energy future for these communities. However, these regions also face unique challenges, including environmental impacts, infrastructure maintenance, economic considerations, and the need for community engagement. By addressing these challenges through innovative solutions, strategic planning, and stakeholder collaboration, the deployment of CHP systems in coastal regions can contribute to sustainable and resilient energy systems, promoting economic growth and environmental protection. The research articles reviewed emphasize the vast potential and shed light on the challenges, paving the way for a holistic approach to energy generation and consumption in coastal regions.

After evaluating the available literature, the main aspect that was remarked has been the difficulty of comparing the systems due to their high complexity and large number of variables. Some solutions bring more complex systems, while others focus more on regenerative resources to achieve efficient machinery. For that reason, a method to compare on even ground for all these solutions has been to propose an objective function, in which we consider as performance metrics the electrical efficiency, total costs, and emissions. The intention is to further develop a performance metric which will be later refined in the next steps of the research thesis, used for evaluation of different configurations for CHP systems. Objective functions, often referred to as cost functions or fitness functions, serve as indispensable tools in the realm of optimization. Their primary role is to quantify the quality of a solution within the context of a given problem. Whether in engineering, machine learning, economics, or natural sciences, objective functions offer a standardized method for evaluating and comparing potential solutions. At its core, an objective function encapsulates the essence of an optimization problem. It represents a mathematical formulation that quantifies the desirability or quality of a particular solution within a given problem domain. The objective function maps a set of input variables, often referred to as decision variables, to a single scalar value that represents the objective’s value. The fundamental purpose of an objective function is to guide optimization algorithms in the search for the optimal solution. Efficiency in objective functions refers to their ability to strike a balance between computational complexity and accuracy in evaluating potential solutions. The efficiency of an objective function becomes particularly critical when dealing with complex optimization problems. In many cases, the efficiency of an objective function directly influences the overall optimization process’s speed and scalability [26,34,35]. At the current state of the study, we will define the following objective function:

$$J=w_{efficiency}\times {\eta }_{elec}-w_{cost}\times Total cost+w_{emissions}\times Emissions,$$

(3)

where J is defined as the overall objective function, and the weights for efficiency, cost, and emissions are denoted with (w). The performance metrics or disciplines are electric efficiency (${\eta }_{elec}$) and total cost ($TotalCost$), including capital and operating costs and emissions $\left(Emissions\right)$, representing the environmental impact, quantifying the CO₂ emissions, and converting them into an equivalent environmental impact. Based on the project’s priorities, the weights will be further adjusted. To find the optimal solution, we will seek to either maximize or minimize the objective function (J) based on the goal (e.g., If the goal is to minimize the cost while achieving a reasonable balance between efficiency and emissions, the function will be minimized). Optimization algorithms such as linear programming, genetic algorithms, or gradient-based methods to find the values of the system parameters (component sized, operating conditions) that minimize or maximize the objective function while satisfying the constraints will be used. The calculation formulas for each performance measure are discussed further below:

1. Electrical Efficiency (${\eta }_{elec})$: which refers to how efficiently the system converts fuel into electrical energy. It can be calculated as the ratio of electrical energy produced to the available energy from the fuel.

$${\eta }_{elec}={\displaystyle \frac{electrical energy produced}{available energy from the fuel}}\times 100\%,$$

(4)

where:

-

$ElectricalEnergyProduced$ represents the amount of electrical energy generated by the system (expressed in energy units such as kilowatt-hours);

-

$AvailableEnergyFromFuel$ represents the energy available in the fuel used to power the system (expressed in energy units such as kilowatt-hours).

2. Total Cost ($TotalCost)$: a measure of the overall cost of the micro-cogeneration system, including both capital costs and operating costs. Its formula can vary depending on the specific details of the system and associated costs. A simplified formula could be as follows:

$$Total cost=Capital cost+Operating cost,$$

(5)

-

$CapitalCost$ represents the initial costs of purchasing and installing the system;

-

$OperatingCost$ represents the recurring costs associated with the operation and maintenance of the system over a specific period.

3. Emissions ($Emissions$): refer to the quantity of greenhouse gas emissions or other pollutants generated by the micro-cogeneration system. The formula for emissions depends on the type of fuel used and the efficiency of the combustion process. For carbon dioxide (CO₂) emissions, a simplified formula could be as follows:

$$C{O}_{2}Emission=Fuel Quality\times C{O}_{2}EmissionFactor,$$

(6)

-

$Fuel Quality$ represents the amount of fuel consumed by the system (expressed in suitable units such as kilograms or liters);

-

$CO2 EmissionFactor$ represents the emission factor specific to the type of fuel used, expressed as CO₂ emissions per unit of fuel consumed (e.g., kg CO₂ per liter of fuel).

These are the basic calculation formulas for the performance measures that can be used to evaluate and optimize the micro-cogeneration system based on specific objectives. While objective functions offer a systematic approach to optimization, they are not without limitations. The choice of an appropriate objective function is crucial, as it defines the problem’s scope and objectives. Selecting an unsuitable or overly simplistic objective function can lead to suboptimal solutions or even infeasible outcomes. Additionally, objective functions often rely on specific assumptions about the problem, which may not always hold true in real-world scenarios [26,34,35,36].

Additional complexity is introduced when evaluating the performance of these systems under different operating conditions and loads. Different environmental conditions, such as temperature, humidity, and altitude, can significantly affect the efficiency and reliability of CHP systems. For instance, the performance of micro gas turbines can vary widely depending on ambient temperature and pressure, as these factors influence the air intake and combustion processes. Studies have shown that optimizing the operating parameters such as the steam-to-carbon ratio, reformer temperature, and fuel utilization factor under varying conditions can significantly improve system efficiency [28,37]. Therefore, further research is needed to develop adaptive control strategies that can adjust to changing environmental conditions and maintain optimal performance. CHP systems often face fluctuating energy demands in real-world applications. Understanding how these systems perform under variable load conditions is crucial for optimizing their efficiency and cost-effectiveness. Research by Leto et al. [38] demonstrated that hybrid systems combining micro gas turbines with fuel cells can achieve high efficiency even with varying electrical output power and input flow rates. However, more studies are needed to explore different load scenarios and develop dynamic control strategies that can respond to rapid changes in energy demand without compromising efficiency. The flexibility of CHP systems to operate with different types of fuels, including biogas, biodiesel, and blended fuels, presents an opportunity to enhance sustainability and reduce emissions. Investigations into the performance of CHP systems using various fuel blends can provide insights into optimizing fuel use. For example, Huang et al. [30] explored the use of biogas and city gas in a bi-fuel micro gas turbine system, demonstrating stable operation with significant efficiency improvements. Further research should focus on the performance implications of different fuel mixtures and the development of adaptive control systems that manage fuel switching seamlessly. Integrating multiple renewable energy sources with CHP systems can significantly enhance overall efficiency and sustainability. Comprehensive evaluations of hybrid systems, such as those combining solar thermochemistry with micro gas turbines or fuel cells, have shown promising results with efficiencies exceeding 80% [39]. However, these systems face challenges related to energy storage, grid integration, and the economic feasibility of different configurations.

A holistic understanding of CHP systems’ long-term benefits and costs requires detailed lifecycle assessments and economic analyses. These studies should consider all phases, including installation, operation, maintenance, and decommissioning, as well as potential environmental impacts. By conducting comprehensive lifecycle analyses, stakeholders can better understand the economic and environmental trade-offs of different CHP system configurations and make informed decisions. Addressing these research gaps will provide a deeper understanding of the optimal operating conditions and configurations for small–medium CHP systems. This knowledge will help in developing more efficient, reliable, and sustainable CHP solutions, significantly contributing to global energy efficiency and emission reduction goals.

4. Discussion

The integration of renewable energy sources into combined heat and power (CHP) systems offers significant potential to enhance sustainability, reduce greenhouse gas emissions, and improve energy security. This section provides a comprehensive analysis of the benefits, challenges, and potential solutions associated with integrating renewable energy sources such as solar, wind, and ocean energy into CHP systems.

One of the primary environmental benefits of integrating renewable energy sources into CHP systems is the substantial reduction in greenhouse gas emissions. By utilizing solar, wind, and ocean energy, these systems can significantly lower carbon dioxide (CO₂) and other harmful emissions, thereby contributing to climate change mitigation and improved air quality. Additionally, renewable energy sources are inexhaustible, offering a sustainable alternative to finite fossil fuels. Their integration ensures a continuous and reliable supply of clean energy, promoting long-term energy sustainability. Economically, integrating renewable energy sources can lead to significant cost savings. The reduction in fuel expenses lowers operating costs over time, and the potential for selling excess electricity back to the grid can offset the initial investment in renewable technologies. Moreover, the deployment of renewable energy projects stimulates local economies by creating jobs in manufacturing, installation, and maintenance, fostering economic development in regions adopting renewable CHP systems. From an energy security perspective, the diversification of energy supply through the incorporation of renewable energy sources reduces dependence on imported fossil fuels, thereby enhancing energy security. This diversification helps protect against fuel price volatility and supply disruptions. Furthermore, renewable energy-integrated CHP systems can be deployed locally, reducing the need for extensive transmission infrastructure and minimizing transmission losses. This localized generation enhances the resilience and reliability of the energy supply.

However, the integration of renewable energy sources into CHP systems is not without challenges. One significant challenge is the intermittency and variability of solar and wind energy. Solar energy availability fluctuates with weather conditions and time of day, while wind energy is influenced by wind speed and direction. Even ocean energy, which is more predictable, can vary with tidal cycles and weather conditions, impacting the stability of energy generation. Technologically, effective energy storage solutions are required to address the intermittency of renewable energy sources. Batteries, flywheels, and other storage technologies are essential for storing excess energy during periods of high generation and releasing it during low generation. Additionally, integrating variable renewable energy sources into the existing grid requires advanced grid management techniques and infrastructure upgrades to ensure grid stability and prevent power quality issues. Economically, the high initial capital costs of installing renewable energy technologies, such as solar panels, wind turbines, and wave energy converters, pose a significant barrier. Securing financing and ensuring the economic viability of these projects can be challenging. Furthermore, the renewable energy sector is often influenced by government policies and incentives. Changes in policy and regulatory frameworks can impact the financial feasibility of renewable CHP projects, creating uncertainty for investors and developers. Environmentally and socially, the deployment of renewable energy infrastructure can have impacts on ecosystems, such as habitat disruption and visual impact. Careful site selection and environmental assessments are necessary to mitigate these effects. Gaining social acceptance for renewable energy projects is also crucial. Effective communication and engagement with local communities, addressing concerns, and demonstrating the benefits of renewable CHP systems are important for successful project implementation.

To overcome these challenges, several potential solutions can be employed. Advanced energy storage solutions, such as continued research and development in battery technologies, can enhance energy storage capacity, efficiency, and lifespan. Hybrid storage systems that combine different types of energy storage, such as batteries and thermal storage, can provide a more robust solution to manage energy supply and demand. Implementing smart grid technologies, including grid management techniques like demand response, real-time monitoring, and automated grid control, can improve the integration of renewable energy sources. These technologies help balance supply and demand, ensuring grid stability. Additionally, developing microgrids that integrate renewable energy sources with CHP systems can enhance local energy resilience. Microgrids can operate independently or in conjunction with the main grid, providing flexibility and reliability. Economic incentives and policy support are also crucial. Governments can provide financial incentives, such as tax credits, grants, and subsidies, to reduce the initial capital costs of renewable energy projects. These incentives can encourage investment and accelerate the deployment of renewable CHP systems. Establishing clear and stable policy and regulatory frameworks can provide certainty for investors and developers, ensuring long-term commitments to renewable energy targets and support mechanisms. Engaging with local communities and stakeholders throughout the planning and implementation process can build trust and support for renewable energy projects. Transparent communication and addressing concerns are key to gaining social acceptance. Developing educational programs and awareness campaigns can inform the public about the benefits of renewable CHP systems, highlighting success stories and case studies to demonstrate the positive impact on communities and the environment.

To further emphasize the advantages created by implementing CHP systems, we have investigated three real-world examples that demonstrated significant environmental benefits from CHP systems:

• New York Presbyterian Hospital: The installation of a 7.5 MW CHP system at the New York Presbyterian Hospital has resulted in a 50% reduction in CO₂ emissions and significant energy cost savings. The system provides electricity, steam, and chilled water to the hospital, enhancing its energy efficiency and reducing its environmental impact;
• University of California, San Diego: The University of California, San Diego (UCSD) has implemented a 30 MW CHP system that meets 85% of the campus’s electricity needs and 95% of its heating and cooling needs. This system has reduced the university’s CO₂ emissions by 50,000 tons per year, demonstrating the substantial environmental benefits of CHP technology in a large-scale application;
• London Heathrow Airport: London Heathrow Airport’s CHP plant generates 11 MW of electricity and 50 MW of thermal energy, which is used for heating and cooling across the airport. This system has cut the airport’s CO₂ emissions by 13,000 tons annually, showcasing the potential of CHP systems in reducing the carbon footprint of large infrastructure projects.

In conclusion, integrating renewable energy sources into CHP systems offers numerous benefits, including environmental sustainability, economic savings, and enhanced energy security. However, this integration also presents challenges related to intermittency, technological infrastructure, economic feasibility, and social acceptance. By addressing these challenges through advanced energy storage solutions, smart grid technologies, economic incentives, and community engagement, the deployment of renewable CHP systems can be optimized. This comprehensive analysis highlights the potential and provides a roadmap for successfully integrating renewable energy into CHP systems, contributing to a sustainable and resilient energy future.

To maximize energy efficiency and minimize emissions in CHP systems, employing advanced strategies and technologies is crucial. Recent research provides valuable insights into optimizing CHP system design and operation, and this section delves into these strategies in depth. Multi-objective optimization techniques are essential for enhancing CHP system performance. Haghighat et al. utilized genetic algorithms to optimize a high-temperature PEMFC-CHP system, increasing cumulative average electric efficiency from 26.03% to 27.56% and significantly reducing emissions. This approach demonstrates the potential of advanced optimization methods in achieving higher efficiencies and lower emissions. The optimization process involved adjusting key parameters such as operating temperature and fuel utilization rates to achieve the desired balance between efficiency and emissions [40]. Integrated system design is another critical area. Zhang et al. [25] proposed a renewable energy source combined cooling, heating, and power (RES-CCHP) system that integrates biogas-fueled internal combustion engines and photovoltaic panels. The system showed a primary energy-saving ratio of 20.94%, a total cost-saving rate of 11.73%, and a carbon emission reduction ratio of 40.79%. This study involved a comprehensive analysis of the system’s performance under different scenarios, considering factors such as solar energy availability and biomass gas production constraints. The results demonstrated significant improvements in energy efficiency and emission reductions through optimized system design [41]. Thermodynamic and exergoeconomic analyses help identify inefficiencies and cost-saving opportunities in CHP systems. Carpaneto et al. conducted a detailed study on integrating biomass gasification with SOFC/mGT CHP plants. Their analysis focused on optimizing the thermodynamic performance and economic viability of the system, considering factors such as fuel composition, operating temperature, and pressure. The results indicated that optimized integration could lead to substantial improvements in both economic and environmental performance [42].

The integration of advanced technologies, such as external combustion chambers and hybrid systems, into small–medium CHP systems, offers significant potential for enhancing efficiency and reducing emissions. External combustion engines, such as Stirling engines, offer several advantages over internal combustion engines. These engines burn fuel externally, allowing for cleaner and more controlled combustion processes. This can significantly reduce emissions and improve efficiency. For instance, biomass-fuelled micro-CHP units with Stirling engines have demonstrated improved performance and lower emissions compared to conventional systems by optimizing the combustion process and waste heat utilization. External combustion chambers also enhance the flexibility of CHP systems by allowing the use of various fuel types, including solid and liquid biofuels, without requiring extensive modifications to the engine. This flexibility can improve the sustainability of CHP systems by enabling the use of locally available renewable fuels [43]. Hybrid systems that integrate multiple energy sources, such as solar, biomass, and natural gas, into CHP systems can significantly enhance overall efficiency and sustainability. For example, integrating a solar-assisted combined cooling, heating, and power (CCHP) system with a combustion turbine has shown promising results in simulation studies. These hybrid systems can achieve higher overall efficiencies by leveraging the complementary strengths of different energy sources. A study on hybrid micro-CHP systems combining photovoltaic panels with biogas-fuelled internal combustion engines demonstrated that such systems could achieve primary energy savings of up to 20.94% and carbon emission reductions of up to 40.79%. These results highlight the potential for hybrid systems to provide both economic and environmental benefits by optimizing the use of renewable energy sources [41]. While the potential benefits of integrating advanced technologies into CHP systems are significant, several challenges remain. The initial costs of implementing these technologies can be high, and their performance can be affected by the variability of renewable energy sources. Additionally, optimizing the integration and control of multiple energy sources in hybrid systems requires advanced control strategies and real-time monitoring to ensure continuous optimal performance. Future research should focus on developing cost-effective solutions for integrating external combustion chambers and hybrid systems into small–medium CHP systems. This includes advancing materials and designs for Stirling engines and other external combustion technologies, as well as improving the efficiency and reliability of hybrid systems through innovative control strategies and real-time optimization techniques.

Implementing advanced control strategies using machine learning and artificial intelligence can optimize CHP system operations in real-time. Meng et al. demonstrated that optimal planning and operation of grid-connected PV/CHP/battery energy storage systems could significantly reduce the system’s net present cost, energy cost, annual utility bills, and CO₂ emissions. Their research employed advanced data-driven models to predict system behavior under varying conditions and adjust operations to maintain optimal efficiency and reduce emissions. This approach ensures continuous system optimization and enhanced performance [42]. Real-time monitoring and adaptive control systems are critical for managing the dynamic nature of CHP systems. These systems adjust operational parameters in response to changes in load demands and environmental conditions, ensuring continuous optimal performance. Advanced monitoring technologies provide detailed insights into system performance, enabling timely adjustments and maintenance.

Detailed lifecycle assessments of CHP systems are necessary to understand their long-term economic and environmental impacts. These assessments should cover all phases of the system’s life, from installation to decommissioning, helping stakeholders make informed decisions about design and operation. Lifecycle assessments highlight areas for improvement and guide the development of more sustainable CHP solutions. Economic optimization involves evaluating the cost-effectiveness of different CHP system configurations. By considering factors such as installation costs, operational expenses, and maintenance requirements, stakeholders can identify the most economically viable solutions. Integrating economic analysis with environmental impact assessments helps balance cost and sustainability. Government policies and incentives play a crucial role in promoting the adoption of optimized CHP systems. Subsidies, tax incentives, and grants can reduce the financial burden on developers and encourage the implementation of advanced CHP technologies. Policymakers should focus on creating a supportive regulatory environment that fosters innovation and investment in CHP systems.

In urban and industrial settings, CHP systems can be tailored to meet specific energy demands while minimizing environmental impact. For instance, integrating CHP systems in industrial processes can enhance energy efficiency and reduce emissions, contributing to overall sustainability goals. In rural and remote areas, integrating renewable energy sources with CHP systems provides reliable and sustainable energy solutions. Zhang et al. demonstrated the feasibility of a RES-CCHP system in a small farm in Jinan, China, showing significant energy savings and emission reductions through optimized design and operation. The system’s performance was analyzed under various scenarios, accounting for fluctuations in renewable energy availability and operational constraints [35]. Community-based CHP systems and microgrids can enhance local energy resilience and sustainability. These systems can be optimized to provide reliable energy for residential communities, reducing reliance on centralized power grids and enhancing energy security.

Addressing these areas will help develop more efficient, reliable, and sustainable CHP systems [44]. These advancements will contribute significantly to global energy efficiency and emission reduction goals, demonstrating the critical role of CHP systems in achieving a sustainable energy future.

The development and deployment of small to medium-sized Combined Heat and Power (CHP) systems can be significantly supported by robust policy and regulatory frameworks. Such frameworks not only create a favorable environment for CHP adoption but also ensure that these systems are integrated efficiently and sustainably into the broader energy infrastructure. Effective policies and regulations are essential to address the economic, environmental, and technical challenges associated with CHP systems. These frameworks can provide financial incentives, establish technical standards, and ensure environmental compliance, thereby promoting the widespread adoption of CHP technologies. Government incentives, such as tax credits, grants, and subsidies, play a crucial role in offsetting the high initial costs of CHP systems. For example, in the United States, the federal Investment Tax Credit (ITC) provides significant financial support for CHP installations, encouraging investment in these technologies. Similarly, feed-in tariffs (FiTs) and net metering policies can offer long-term financial benefits by ensuring a stable revenue stream for CHP operators who export excess power to the grid.

In Europe, several countries have implemented incentive schemes to promote CHP. The UK’s Renewable Heat Incentive (RHI) offers payments to CHP operators for the heat generated from renewable sources, while Germany’s Combined Heat and Power Act provides financial support based on the amount of electricity produced by CHP systems. These incentives are crucial for making CHP projects economically viable and attractive to investors. Establishing technical standards and certification processes ensures the reliability, safety, and efficiency of CHP systems. Standards help in setting benchmarks for performance, guiding the design and operation of CHP units, and facilitating the integration of various technologies. For instance, the European Union has developed the EN 50465 standard [45], which specifies the requirements for micro-CHP systems fueled by natural gas and other gaseous fuels. Adherence to such standards can improve system performance and encourage consumer confidence in CHP technologies [1]. Environmental regulations are critical to ensuring that CHP systems contribute to sustainability goals. These regulations can include emission limits, efficiency requirements, and mandates for the use of renewable fuels. For example, the European Union’s Renewable Energy Directive (RED II) sets targets for the share of renewable energy in the total energy mix and includes provisions for the promotion of high-efficiency cogeneration. Compliance with such regulations helps in reducing greenhouse gas emissions and promoting cleaner energy production. Policies that facilitate the integration of CHP systems with the electricity grid are essential for maximizing their benefits. Grid connection standards, interconnection agreements, and supportive grid tariffs can help in overcoming technical and economic barriers to CHP deployment. For instance, the introduction of flexible grid tariffs and simplified interconnection processes can reduce the costs and complexities associated with connecting CHP systems to the grid. Additionally, demand response programs can enhance the operational flexibility of CHP systems, allowing them to respond to grid demands and contribute to grid stability [21].

Future policy frameworks should focus on enhancing the economic, environmental, and social benefits of CHP systems. This includes the following:

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Expanding financial incentives to cover a broader range of technologies and applications;

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Strengthening technical standards to include emerging technologies like hybrid systems and advanced control strategies;

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Enhancing environmental regulations to encourage the use of low-carbon and renewable fuels;

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Promoting research and development to advance CHP technologies and reduce costs.

In conclusion, comprehensive policy and regulatory frameworks are vital for the successful development and deployment of small–medium CHP systems. By providing financial incentives, establishing technical standards, enforcing environmental regulations, and facilitating grid integration, these frameworks can significantly contribute to the sustainability and efficiency of the energy sector

5. Conclusions

The literature review has provided a comprehensive analysis of various methods used for generating energy from different sources, including fossil fuels, biofuels, solar, wind, and ammonia. The studies reviewed highlight the importance of combining different energy sources to achieve maximum efficiency and reduce environmental impact. The reviewed studies explored different technologies such as micro gas turbines, combined heat and power systems, organic Rankine cycles, and solar dish power plants. The studies reviewed also discussed the impact of different fuels on energy efficiency and emissions and identified the most suitable fuels for different energy generation technologies. Some of the analyzed studies showed that biodiesel blends and different biofuels can increase the efficiency of gas turbine cycles, while other studies demonstrated that ammonia can be used as an alternative fuel for micro gas turbines with reduced emissions. Furthermore, the reviewed studies highlighted the importance of modeling and simulation tools in designing and optimizing energy systems. Numerical models and simulations were used to analyze different scenarios, assess system performance, and optimize system design.

Overall, this literature review has demonstrated the importance of considering different factors such as fuel type, system design, and modeling tools in designing and optimizing energy generation systems. The findings of this review can provide valuable insights for researchers, engineers, and policymakers in the field of sustainable energy generation. Further research is needed to explore new technologies and methods for energy generation and to optimize existing systems for maximum efficiency and minimal environmental impact. To create a comprehensive evaluation, several key aspects should be considered:

• Efficiency Gains: Each technological approach should be assessed based on its potential to improve the overall efficiency of CHP systems. This includes evaluating the conversion efficiency of biofuels, the impact of steam injection on turbine performance, and the effectiveness of renewable energy integration in reducing fossil fuel dependency;
• Economic Viability: The cost-effectiveness of implementing these technologies is crucial. This involves analyzing the initial capital costs, ongoing operational and maintenance expenses, and potential cost savings through improved efficiency and reduced fuel consumption. Life cycle cost analysis can provide valuable insights into the long-term economic benefits of these technologies;
• Environmental Impact: The environmental benefits, such as reduced greenhouse gas emissions and lower pollutant levels, should be quantified. Additionally, the environmental footprint of biofuel production, the lifecycle emissions of integrated systems, and the sustainability of renewable energy sources must be considered. Environmental impact assessments can help identify and mitigate potential negative effects on ecosystems and communities;
• Operational Challenges: Practical challenges, including the availability of biofuels, the complexity of implementing steam injection, and the reliability of renewable energy sources, need to be addressed. Solutions to mitigate these challenges, such as advanced control systems, robust maintenance strategies, and hybrid configurations, should be explored. Pilot projects and field trials can provide valuable data and insights to refine these solutions.

By providing a detailed analysis and evaluation of these technological and operational issues, this article offers a comprehensive understanding of the potential and limitations of small–medium CHP systems. This comprehensive approach will guide future research and implementation efforts, ensuring that CHP systems contribute effectively to sustainable and resilient energy systems.

Key findings reveal that biofuels such as biogas, biodiesel, and bioethanol significantly enhance the sustainability and efficiency of CHP systems. While biogas and biodiesel offer higher thermal efficiencies and lower emissions, challenges like lower energy density and impurities necessitate advanced handling systems. Steam injection in hydrogen-methane mixtures improves combustion stability and reduces emissions, while ammonia as a fuel presents high energy density but poses toxicity and cost challenges. Hybrid systems combining renewable sources like solar and wind with traditional CHP components show promise for high efficiency despite substantial initial investments and integration complexities. The integration of renewable energy sources into CHP systems provides significant environmental benefits, including substantial reductions in greenhouse gas emissions and enhanced energy efficiency. However, the intermittency of renewable sources requires advanced energy storage solutions and smart grid technologies. Effective community engagement and stable policy frameworks are crucial for successful implementation and acceptance. CHP systems demonstrate the potential to reduce CO₂ emissions by up to 40% compared to conventional power generation methods due to improved fuel utilization and the simultaneous generation of electricity and thermal energy. Additionally, CHP systems significantly lower emissions of nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter, contributing to better air quality and public health.

The exploration of Combined Heat and Power (CHP) systems in coastal regions unveils a promising pathway toward sustainable and resilient energy solutions. Coastal regions, with their unique geographic and climatic conditions, offer an untapped reservoir of energy potential, especially when considering renewable resources like wind and sea currents. The synergistic integration of CHP systems can amplify the efficiency of energy production, thereby reducing wastage and enhancing the overall reliability of the energy supply. This is especially pivotal for regions that face sporadic energy demands, often driven by seasonal shifts. Furthermore, the adaptability of CHP systems aligns seamlessly with the dynamic nature of coastal environments. By leveraging real-time data analytics, these systems can adapt to fluctuating energy demands, ensuring a consistent and reliable power supply. This is paramount for regions prone to extreme weather events, where energy infrastructure might be susceptible to disruptions. Moreover, the sustainable aspect of CHP systems in coastal areas cannot be overstated. As the global community gravitates towards eco-friendly energy solutions, the integration of renewable sources becomes indispensable. Coastal regions, endowed with the dual potential of wind and water, stand at the forefront of this green revolution. By channeling these natural resources through CHP systems, we not only reduce our carbon footprint but also pave the way for a self-reliant energy future.

During the research, apart from focusing on small applications based on placement or business needs, we were interested in evaluating the performance of different solutions. The study finds that gas turbine cycle-based technologies such as external combustion chamber gas turbine cycles, micro gas turbines, and hybrid trigeneration systems offer higher energy efficiency and lower emissions than other technologies. For instance, the study by Mikielewicz et al. [6] proposed a gas turbine cycle with an external combustion chamber that had an efficiency of up to 47% when fuelled with biofuels. Reale and Sannino [7] proposed a micro gas turbine that had an efficiency of up to 32% when fuelled with a mixture of CH₄ and H₂. Figaj et al. [8] proposed a hybrid trigeneration system that had an efficiency of up to 60% when fuelled with natural gas and wind energy. However, the capital and operating costs of gas turbine cycle-based technologies are relatively higher than other technologies. For example, the study by Capata and Hernandez [9] proposed a turbo expander for small-rated power organic Rankine cycles that had lower capital and operating costs compared to gas turbine cycle-based technologies.

Based on the results of the study, it can be concluded that gas turbine cycle-based technologies, such as external combustion chamber gas turbine cycles, micro gas turbines, and hybrid trigeneration systems, are more suitable for further research due to their high energy efficiency and low emissions. However, the capital and operating costs of these technologies are relatively higher than those of other technologies, such as organic Rankine cycles and photovoltaic-assisted combined cooling, heating, and power systems. Future research should focus on optimizing gas turbine cycle-based technologies to reduce their capital and operating costs and make them more cost-effective for large-scale deployment in 100% renewable energy systems.