Building the Future: Integrating Phase Change Materials in Network of Nanogrids (NoN)

Abstract

Buildings consume 10% of global energy and 50% of global electricity for heating and cooling. Transitioning to energy-efficient buildings is essential to address the global energy challenge and meet sustainable development goals (SDGs) to limit global temperature rise below 1.5 $°\mathrm{C}$. The shift from traditional to smart grids has led to the development of micro, milli, and nanogrids, which share energy resources symbiotically and balance heating/cooling demands dealing with acute doldrums (dunkelflaute). This scoping review explores the methods by which phase change materials (PCMs) can be used in residential buildings to form a nanogrid. This review examines the components and concepts that promote the seamless integration of PCMs in residential houses. It also discusses the key challenges (e.g., scalability, stability, and economic feasibility in high summer temperatures), proposing the community-scale network of nanogrids (NoN) and the potential of thermochromic and photochromic materials. The findings of this review highlight the importance of latent heat storage methods and ingenious grid architectures such as nanogrids to construct resilient and sustainable houses in the future and thereby offer practical insights for policymakers and industries in the energy sector.

1. Introduction

Modern societies have increasing energy demands, and the world is heading towards a wilting and warming world. The global average temperature on the planet has remained 5–10 $°\mathrm{C}$ higher than the pre-industrial level for the last 500 million years. The global average temperature plunged below the pre-industrial level about 3.5 million years ago, and it is now returning to its historic normal. Changing climate demands extensive research on novel energy storage technologies and management strategies. Climate change acts as a pendulum that causes heat waves in summer and cold waves in winter. Extreme climate events require more energy to cope with heating and cooling. Phase change materials allow us to store summer heat for winter and vice versa. The rapid growth in global energy consumption, alongside rising ${\mathrm{CO}}_2$ and greenhouse gas emissions, is propelled by population growth, swift economic expansion, and an increasing reliance on energy-intensive devices. This surge in energy consumption poses significant environmental challenges, impacting human health and ecosystems. More fossil fuels inject more greenhouse gases, which mediate positive feedback. Moreover, dwindling oil, coal, and natural gas reserves demand innovative systems for converting and storing sustainable energy. The building sector significantly impacts global energy use and environmental health, chiefly through its high greenhouse gas and ${\mathrm{CO}}_2$ emissions, which also contribute to ocean acidification. This sector’s increased energy usage is evident not only during the production of construction materials and the actual building process but also, more critically, during the operational phase of buildings. Empirical evidence indicates that maintaining indoor comfort accounts for approximately 30% of the sector’s total energy demand [1].

Phase change materials represent a viable and potent thermal energy storage solution to reduce energy consumption in the building sector. PCMs can sustain thermal comfort in indoor environments without actively consuming energy. PCMs possess high thermal energy absorption and storage capacity, facilitated by their intrinsic ability to undergo phase transitions. The storage mechanism of these materials (enthalpy-based) is required for phase changes. As the temperature of the PCMs increases, they absorb sensible heat and, upon reaching the transition temperature, store a considerable amount of latent heat of fusion to transition phases. This thermal energy remains sequestered until a temperature decline triggers the reverse phase transition, releasing the stored energy. Phase change materials (PCMs) are grouped based on their phase transition types: solid to liquid, liquid to gas, and solid to solid. Examples of solid–solid PCMs include polyurethanes and cross-linked polyethylene, which exhibit relatively low latent heat in comparison to solid–liquid PCMs, the latter being more suitable for applications in residential buildings. Organic PCMs such as paraffin/fatty acids are especially beneficial for home use. However, their effectiveness is curtailed by their inherently low thermal conductivity. Prevailing methodologies include encapsulation with thermally conductive substrates to mitigate leakage by fabricating form-stable composites incorporating expanded graphite/perlite to improve the limiting thermal performance.

Extensive research has explored integrating PCM in buildings to reduce energy consumption demands [2,3,4,5] heating/cooling demands by up to 80% [6], greenhouse gas emissions consumption [7], and indoor thermal comfort [8]. PCMs are embedded inside roofs, walls [9], and ceilings [10], windows [2], outside, inside, and middle of the building envelope [7]. Active uses include heating, ventilation, air conditioning, domestic hot water (DHW), and solar facades. PCMs are integrated into building materials as passive solutions to improve the thermal efficiency of ceilings, walls, and floors, thereby increasing the energy performance of buildings [11] and may result in a 33.02% reduction in heating energy and a 55.48% reduction in cooling energy demands. The literature highlights research gaps in building design with phase change materials (PCMs) and opportunities for future research. The widespread use of PCMs in building envelopes needs further study, especially in high summer temperatures when night-time crystallization hinders PCM efficiency. The whole charging and discharging cycles of PCMs across climate zones require further study to maximize their energy-saving potential. Developing nano- or micro-sized conductive fillers to improve heat conductivity is another priority. Limited study on inorganic and solid–solid PCMs has brought forth an opportunity for thermochromic and photochromic phase change materials.

This scoping review seeks to explore the critical and relevant findings for phase change materials (PCMs) that can be used in the design of nanogrid architecture. A multitude of research has demonstrated and proved the capabilities of PCMs in improving energy efficiency, curtailing local demands, and optimizing thermal regulation in home environments. However, a comprehensive review focusing on what and how to integrate in tandem with PCMs into residential nanogrids was limited to non-existent in the literature, making it much more challenging. This review also addresses this gap by reviewing the literature on different technologies and their possible applications in designing an energy-efficient nanogrid. Moreover, the studies were very diverse, and study designs were equally heterogeneous, making it challenging to converge to commonalities. This review seeks to provide valuable insights for future directions in residential nanogrid systems by synthesizing recent developments and identifying areas requiring further research. The findings of this review will contribute to guiding researchers, policymakers, and industry professionals towards optimizing PCM integration in nanogrid systems, ultimately promoting sustainable and energy-efficient residential solutions. This review encompasses a holistic overview of PCM integration in residential buildings, storage technologies, thermal enhancement methods, and passive design techniques to maintain thermal comfort levels, meet the requirements of a nanogrid, and meet the criteria of net zero/plus energy buildings as summarized in Figure 1.

2. Method

The paper aims to explore the integration of phase change materials (PCMs) in residential buildings to form nanogrids, enhancing energy efficiency and reducing energy demands. The objective was to investigate the holistic integration of multiple melting-point PCMs in wallboard with solar hot water systems, incorporating advanced additives (e.g., graphite, silicon carbide, multi-walled carbon nanotubes [MWCNT]), innovative coatings (e.g., photochromic cool white, thermochromic vanadium dioxide, titanium oxide, barium sulfate, sodium citrate, polymer paints), and effective heat transfer enhancements (e.g., metal foam, fins) to curtail energy demands and improve energy efficiency indexes while justifying viability in residential nanogrids. It reviews recent literature on the techno-economic feasibility and integration techniques of PCMs, proposes a novel concept of community-scale nanogrids (NoN) for collaborative energy management, highlights software tools for future modeling and evaluation, and emphasizes the importance of sustainable building practices using latent heat storage and innovative grid architectures. These contributions aim to advance PCM applications in residential energy systems and promote sustainable, energy-efficient building practices.

This scoping review was conducted through a structured and systematic approach to justify the relevance and impact of the study. The research articles were sourced from credible science databases including ScienceDirect, IEEE Xplore, and Google Scholar, particularly emphasizing phase change materials (PCMs) and their case studies in residential buildings/nanogrids. These databases were selected based on their comprehensive repositories of peer-reviewed journal articles, conference proceedings, and technical reports pertinent to PCM technology, thermal energy storage systems, and energy-efficient building frameworks. Data are synthesized in tabular form and summarized using a narrative approach that comprehensively reviews the findings of diverse studies and draws meaningful connections between various topics, areas, and disciplines. Themes were framed to identify patterns and extract meaningful insights remaining within the identified scope and boundaries. The data charting process to extract relevant information from the literature involves sorting and articulating data in tabular form over fifteen columns. Data were tabulated using a standardized form, sorting data according to the scope of the research, study design, building size, types of PCMs, integration techniques, performance metrics (energy, exergy, economic), and outcomes. This structured approach ensures that the data charting process is thorough and systematic, providing a solid foundation for analyzing the integration of PCMs in residential nanogrids.

The inclusion criteria for this scoping review encompass various types of screening based on the nature of studies, scope, outcomes, publication dates, and languages to ensure relevance and comprehensiveness. Studies published between 2012 and 2024 were included to capture the most recent advancements in PCM and nanogrid technologies. Eligible studies include experimental research investigating PCM integration in building materials and ways to improve energy efficiency through additives. Only publications from peer-reviewed journals, conference proceedings, and reputable technical reports were considered, focusing on research addressing PCMs in residential energy systems, particularly in the context of nanogrids and the techno-economic feasibility of such implementations. Studies published before 2012 were excluded unless they provided significant context to the development of nanogrid systems, and articles not available in English or focusing on unrelated applications were also excluded. A combination of keywords was used to search the databases, ensuring a comprehensive collection of scientific publications. Keywords included “PCMs”, “thermal energy storage (TESS)”, “nanogrid”, “energy-efficient buildings”, “smart grids”, “poly-generation”, “photochromic PCM”, “thermochromic PCM”, and “multi-energy systems”. Relevant articles were filtered based on their titles, abstracts, scope, and keywords. The selected articles were then thoroughly reviewed. Key information was extracted, focusing on PCM material properties and classifications, phase transition processes, building integration, advancements in nanogrid technologies, and their implementation. The review excluded studies that do not focus on PCM integration in residential buildings and lack empirical evidence or a clear methodological framework. Moreover, any study focusing on commercial buildings or industrial PCM applications was excluded unless a clear advantage was to be highlighted. These criteria ensure that the included studies are directly relevant to the research objectives of understanding and optimizing PCM integration in residential nanogrids.

To guarantee the credibility of the literature, the chosen studies were evaluated by considering the reliability of their publication sources and the number of citations, as well as the studies’ methodological rigor and relevance to this review’s research questions and objectives. The review process involved initial screening of titles, figures, headings, and abstracts to identify relevant studies. Key concepts and findings were then extracted, synthesized, analyzed, and organized into thematic sections as presented in the literature review. By adopting this structured methodology, the scoping review aimed to provide a thorough and reliable analysis of the potential integration of PCMs into nanogrid systems for residential buildings, highlighting both the advancements and challenges in this field.

3. Literature Review

3.1. Zero Energy Buildings, Plus Energy Buildings, and Nanogrids

A zero-energy building uses salient architectures and integrated solar and wind energy storage systems to regulate temperature. A nanogrid is a decentralized energy system that utilizes local renewable energy generation and storage inside buildings. Thermochromic and photochromic PCMs are integrated inside construction materials to minimize the use of heat and electric energies. PCM-integrated buildings with sun and wind-based systems offer increased energy security, resilience, and efficiency by effectively managing local energy resources. Solar systems go down after sunset, and wind systems may or may not during the day or night. There are times when it is dark doldrums, a situation called dunkelflaute in Germany, which relies more on than any other European country. Contrarily, times come in summer when power prices become negative due to excess production, as has happened several times in Australia. Solar panels increase the urban heat island effect, and wind turbines kill migratory birds. We have to turn to naturally ventilated, sunshine-lit, and geothermal energy-supported systems in addition to thermochromic and photochromic phase change materials for buildings.

Plus energy buildings are prosumer-owned buildings that produce more annual energy than annual energy demands. Enhancing building envelopes and incorporating energy-efficient technologies can reduce energy consumption by 49% [12]. Designing a plus or positive energy building encompasses active and passive design, including building components, insulation materials, advanced storage (nano-enhanced phase change materials, Li-ion batteries, thermally insulated storage tanks), and renewable energy technologies, including solar thermal collectors, solar photovoltaics, biofuels, geothermal heat pumps, air-to-water heat pumps, and photovoltaic systems. Several notable studies have focused on studying plus energy buildings [13]. Retrofit options have also been assessed for old buildings with poor insulation and higher air changes per hour (ACH) with an energy payback period of 2–10 years, depending on the level of retrofitting [14]. The widespread integration of net zero energy buildings poses significant threats to the voltage stability of distribution systems, which can be tackled through nanogrid, microgrid architectures (islanding abilities), or bulk local distributed storage. Phase change materials (PCMs) provide thermal inertia to the residential buildings, enhancing the reliability of net-zero and plus-energy buildings by effectively managing thermal energy.

Nanogrids are residential houses that utilize local renewable energy sources and storage solutions to operate in islanded mode or non-islanded connected to the main grid. Nanogrid houses are gaining traction owing to trigeneration, polygeneration, and multigeneration concepts being investigated at residential scales. There are AC, DC, and hybrid AC/DC nanogrid variations being studied. DC nanogrids are found to be more efficient [15] but are preferred to be implemented in remote communities [16] or new communities. The major challenges that exist in the widespread penetration of nanogrids are the lack of smart grid standards dealing with decentralized architectures such as nanogrids, low inertia, and the requirement for electrical (Li-ion) and thermal energy storage (PCM, insulated tanks) [17]. Other thermal storage options for nanogrids also include Carnot batteries [18] and hydrogen storage. High-energy dense (PCMs) are best suited for residential nanogrids with space limitations because of their energy density capabilities. On a wider scope, nanogrids and microgrids offer ancillary services to the wider grid. Integrating nanogrids with thermal energy storage systems such as PCM and sensible heat storage entails addressing several challenges across technical, economic, social, and regulatory domains. Technical issues include energy management protocols/solutions to optimize use and maintain reliability [19], the intermittency and stability of local intermittent generation [20], and thermal energy storage efficiency [21]. Economic challenges involve high initial investment costs [22]. Regardless of the challenges, nanogrids are now being studied, integrating diverse technologies, including phase change materials within building walls, to decrease energy demands for HVAC applications.

3.2. Phase Change Materials (PCMs)

Phase change materials (PCMs) are energy-absorbing substances that store and release large amounts of latent heat during phase transitions, offering a stable thermal storage solution for residential buildings. PCMs experience phase transitions in (solid–liquid–gas) form with significant volume changes, making them impractical for thermal storage systems despite their high heat of fusion. Solid–solid and solid–liquid phase transitions are majorly used in thermal energy storage applications. Solid–liquid phase change stores significant thermal energy with minimal volumetric expansion, typically 10%. Melting and solidification occur under constant pressure if the container allows for slight volume expansion. During melting, phase change materials (PCMs) absorb thermal energy while maintaining a constant phase change temperature. After melting, additional heat is stored as sensible heat. The energy absorbed during the phase transition, called latent heat, relates to enthalpy change and is not noticeable due to minimal temperature variation. Materials that store thermal energy in this way are known as phase change materials (PCMs) [23,24]. Figure 2 illustrates the thermodynamic characteristics of a single-stage PCMstorage mechanism.

Phase change materials (PCMs) were first investigated in the 1940s by researchers Telkes and Raymond. However, their potential was not widely recognized until the energy crisis of the 1970s, which spurred renewed interest in PCMs. By the early 1980s, extensive research had been conducted to explore their applications, with a particular focus on solar energy systems [24,25,26,27,28,29,30,31,32,33,34,35,36]. The research into PCMs as thermal energy storage (TES) has since expanded to cover a broad range of applications. Significant contributions have been made in the fields of building air conditioning [37,38,39,40,41,42,43,44], solar heating [45,46,47,48,49], electronic cooling [50,51,52,53,54], waste heat recovery [55,56,57,58], textiles [59,60,61], construction and buildings [62,63,64,65,66], food and drink [67,68,69,70], and agriculture [71,72,73,74,75,76], among others. Holistic use of PCMs integrated with solar thermal storage, hydronic heaters, and other sustainable technologies for residential applications aids in building temperature regulation and thermal energy management. PCMs offer a robust solution for managing surplus solar energy and dealing with the duck curve phenomenon by leveling the load and enhancing solar energy ride-through capability, thereby ensuring a more stable and reliable decentralized energy grid. Advances in nanocomposites, microencapsulated PCMs, and shape-stabilized PCMs are being extensively studied with potential for application in nanogrid architecture, which will be discussed in each subtopic.

PCMs have positive and negative outlooks regarding their applications in polygeneration nanogrid environments. In terms of thermal storage, PCMs can store and release bulk amounts of energy in the form of latent heat in building structures. However, low thermal conductivity results in lower heat transfer rates, response time, and efficiency. Several methods are explored to resolve this issue, as discussed in Section 3.3.5, which includes embedding conductive materials such as graphite, carbon fibers, and metal foams to improve thermal conductivity. Nanogrid houses would require a stable comfort zone temperature range, which PCM can provide, but low conductivity can impact its performance in dealing with sudden fluctuations in thermal loads. A similar approach of adding nano-additives such as carbon nanotubes and metal nanoparticles increases response speed, which is discussed further in Section 3.3.8. Organic biodegradable PCMs such as paraffin and salt hydrates have low environmental impacts but may require encapsulation to prevent leakage, as discussed in Section 3.3.6. PCMs offer passive energy savings by reducing HVAC loads but have a higher price tag. PCMs with low-cost conductive fillers are a good option for providing a reasonable payback period. PCMs must endure long phase change cycles over a period of time to extend their lifespan; this can be improved by using layered, composite, multi-PCMs as discussed in Section 3.3.3. Moreover, PCM thermal response can be improved by integrating heat exchangers and finned structures to improve heat transfer, as discussed in Section 3.2.7. The thermal stability of PCMs is affected by phase segregation and sub-cooling in low-conductivity PCMs. This can be remedied by mixing additives to prevent sub-cooling. A faster response rate limits the application of PCMs in active cooling systems, and hence, hybrid PCM systems are explored. A summary of some of the latest trends is articulated in

Table 1. Recent PCM trends and application in nanogrid.

Trends	Key Findings	Residential (Nanogrid) Applications	Challenges	References
Nanocomposites	Enhanced thermal conductivity and energy storage efficiency	Potential for improving energy efficiency in walls and insulation materials	High production costs and potential agglomeration issues	[77,78,79,80,81]
Microencapsulated PCMs	Improved stability and integration with building materials	Suitable for incorporation into drywall, paint, and cement for temperature regulation	Complexity in manufacturing and potential leakage problems	[82,83,84,85,86,87]
Shape-stabilized PCMs	Better adaptability for residential applications	Effective in floor heating systems and underfloor heating applications	Limited thermal conductivity and scalability concerns	[88,89,90,91,92,93]
Bio-based PCMs	Increased sustainability and reduced environmental impact	Eco-friendly option for insulation in green buildings and sustainable housing	Variable thermal properties and higher costs	[94,95,96,97,98]
Hybrid PCMs	Combined benefits of multiple PCMs and improved thermal performance	Versatile for use in a variety of integrated thermal storage systems	Complexity in synthesis and phase separation issues	[99,100,101]
High-temperature PCMs	Industrial applications and higher operational temperature ranges	Less relevant for typical residential use and more applicable in homes using solar thermal systems	Corrosion and material compatibility issues	[102,103,104,105,106,107]
Thermal Cycling Stability	Enhanced durability and longevity of PCMs through repeated thermal cycles	Ensures long-term reliability of PCM-integrated home heating and cooling systems	Degradation over time and consistency of properties	[108,109,110]
Additive Manufacturing	Customizable PCM structures for specific applications via 3D printing techniques	Customizable heating and cooling elements that can be designed for specific residential structures	Limited material options and mechanical strength concerns	[111,112]
Phase Transition Temperature Tuning	Tailored thermal properties for specific applications	Allows for precise temperature control in different residential zones or rooms	Complexity in designing and cost of materials	[113,114]
Photochromic and Thermochromic PCMs	Dynamic response to light and temperature changes for improved energy management	Suitable for smart windows and adaptive insulation in residential buildings	Limited availability and higher cost of materials	[115,116,117,118,119,120,121,122,123,124,125,126]

.

Latent heat storage (PCM)-based TES systems have high storage capacity. This capacity is 5 to 14 times higher than other heat storage systems [127,128,129] and compatible with air conditioning applications [130,131,132], with minimal fluctuations in temperature during the cold storage phase [128,133,134,135], energy stored/released at a constant temperature [128,134,136], reversible nature, the controllable range of melting point temperatures, minimal volumetric changes [128,134] and low vapor pressures [128]. On the contrary, PCMs pose various challenges, including poor thermal conductivity [136,137], stability issues, and container compatibility issues [133,137,138]. Furthermore, accumulation/deposition of solids on heat exchanger surfaces, density changes, segregation of phases, and high costs are also major challenges [23,133,134,137,139].

3.2.1. Phase Change Materials (Types)

The classification is based on phase transitions. PCMs (solid–solid, solid–liquid phase) retain lower heat during phase transitions but are more relevant to nanogrids because of lower prices [24]. PCMs fall into two primary types: low molecular compounds and polymers. Low molecular compounds are further classified into inorganic (salts, salt hydrates, hydroxides, alloys) and organic (paraffins, alcohols, fatty acids, esters, and others). Polymers include materials such as poly (ethylene glycol). Each PCM presents distinct advantages and limitations as discussed in the literature [25,135,139,140,141]. While solid–solid phase transitions can address leakage issues associated with solid–liquid PCMs and serve as an alternative energy storage medium, they generally have lower latent heat of phase transition [23,25,140]. Conversely, PCMs with solid–gas and liquid–gas transitions are constrained by specific limitations that hinder their application in thermal energy storage [23,142]. Thermal cycles in PCM’s are articulated in Table 2.

3.2.2. Phase Change Materials (Thermal Storage)

Phase change materials (PCMs) offer various applications as reliable thermal energy storage mediums, particularly within residential building-based nanogrids. The effective implementation and integration of PCM thermal energy storage in these systems require specific types of PCMs and techniques [143]. Solar energy, a promising source of heat, can be harnessed for multiple uses, but maintaining the indoor temperature of residential nanogrid buildings is particularly relevant during the winter months [144]. Comparative analysis demonstrated that PCM-fluidized beds outperformed sand-fluidized beds in thermal efficiency and performance [145], highlighting their potential for optimizing PCM-based storage in residential nanogrids. The variations in solar energy output pose many challenges, most significantly the duck curve problem in residential nanogrids, which requires reliable thermal storage and PCM to provide thermal energy ride-through capability during the absence of the sun. Close et al. [146] pioneered solar air heater (SAH) research, resulting in innovative designs such as flat plates, fin plate v-grooves, and Chevron plates. In residential nanogrids, PCM thermal storage enhances air heating efficiency by integrating directly with solar collectors [45], helping to smooth energy supply and address the duck curve issue. Solar dryers are categorized into four types based on their heating mechanisms: (1) direct, (2) indirect, (3) mixed, and (4) hybrid. In residential buildings, the use of solar radiation and air heaters is coupled with thermal energy storage (sensible, latent, chemical), capturing excess energy to level the duck curve issue. Enhancing the efficiency of solar collectors is vital for maximizing the performance of solar heating systems within residential nanogrids. Recent studies, such as those involving the Chevron plate collector [147], have showcased a 20% boost in thermal efficiency, achieving output temperatures that are 10 $°\mathrm{C}$ higher at specific mass flow rates.

Table 2. Thermal cycles for commonly used PCMs [138].

Type of PCM	Thermal Cycles	Melting Point ($°\mathrm{C}$)	Latent Heat of Fusion (KJ/Kg)	References
Paraffin (70 wt.%) + Polypropylene (30 wt.%)	3000	44.77	136.16	[148]
Paraffin wax 54	1500	53.32	184.48	[149]
Paraffin wax 58–60	600	58.27	129.8	[149]
Paraffin wax 60–62	600	57.78	129.7	[149]
Acetanilide (C8H9NO)	500	113	169.4	[150]
Erythritol	1000	117	339	[149]
Lauric acid (C11H23COOH)	1200	42.46	176.6	[151]
Myristic acid (C13H27COOH)	450, 1200	50.4, 52.99	189.4, 181	[151,152]
Palmitic acid (C15H31COOH)	450, 1200	57.8, 61.31	201.2, 197.9	[152]
Palmitic acid (80 wt.%) + expanded graphite (20 wt.%)	3000	60.88	148.36	[152]
Stearic acid (C17H35COOH)	450, 1500	65.2, 63	209.9, 155	[152,153]
Calcium chloride hexahydrate (CaCl2.6H2O)	1000	29.8, 28, 23.26	190.8, 86, 125.4	[154,155,156]
Magnesium chloride hexahydrate (MgCl2.6H2O)	1000	23.26	125.4	[150]
Glauber’s salt (Na2SO4.10H2O)	500	111.5	155.11	[157]
Sodium acetate trihydrate (NaCH3COO.3H2O)	500	58	230	[158]
Na2SO4.1/2NaCl.10H2O	5650	20	-	[159]
Al–34%Mg–6%Zn alloy	1000	454	314.4	[160]
Ammonium alum (NH4(SO4)2.12H2O)(15%)	1100	53	170	[161]
Capric acid (65 mol%) + lauric acid (35 mol%)	360	19.6	126.5	[161]
Capric acid (73.5 wt.%) + myristic acid (26.5 wt.%)	5000	21.4	152	[162]
Capric acid (83 wt.%) + stearic acid (17 wt.%)	5000	24.68	178.64	[163]
Lauric acid (66 wt.%) + myristic acid (34 wt.%)	1460	34.2	166.8	[164]
Lauric acid (69 wt.%) + palmitic acid (31 wt.%)	1460	35.2	166.3	[164]
Myristic acid (64 wt.%) + stearic acid (36 wt.%)	1460	44.1	182.4	[164]
Myristic acid + glycerol	1000	31.96	154.3	[165]
Palmitic acid + glycerol	1000	58.50	185.9	[165]
Stearic acid + glycerol	1000	63.45	149.4	[165]
Mg(NO3)2.6H2O (93 wt.%) + MgCl2.6H2O (7 wt.%)	110	33.8	111.6	[166]

Table 2. Thermal cycles for commonly used PCMs [138].

Type of PCM	Thermal Cycles	Melting Point ($°\mathrm{C}$)	Latent Heat of Fusion (KJ/Kg)	References
Paraffin (70 wt.%) + Polypropylene (30 wt.%)	3000	44.77	136.16	[148]
Paraffin wax 54	1500	53.32	184.48	[149]
Paraffin wax 58–60	600	58.27	129.8	[149]
Paraffin wax 60–62	600	57.78	129.7	[149]
Acetanilide (C8H9NO)	500	113	169.4	[150]
Erythritol	1000	117	339	[149]
Lauric acid (C11H23COOH)	1200	42.46	176.6	[151]
Myristic acid (C13H27COOH)	450, 1200	50.4, 52.99	189.4, 181	[151,152]
Palmitic acid (C15H31COOH)	450, 1200	57.8, 61.31	201.2, 197.9	[152]
Palmitic acid (80 wt.%) + expanded graphite (20 wt.%)	3000	60.88	148.36	[152]
Stearic acid (C17H35COOH)	450, 1500	65.2, 63	209.9, 155	[152,153]
Calcium chloride hexahydrate (CaCl2.6H2O)	1000	29.8, 28, 23.26	190.8, 86, 125.4	[154,155,156]
Magnesium chloride hexahydrate (MgCl2.6H2O)	1000	23.26	125.4	[150]
Glauber’s salt (Na2SO4.10H2O)	500	111.5	155.11	[157]
Sodium acetate trihydrate (NaCH3COO.3H2O)	500	58	230	[158]
Na2SO4.1/2NaCl.10H2O	5650	20	-	[159]
Al–34%Mg–6%Zn alloy	1000	454	314.4	[160]
Ammonium alum (NH4(SO4)2.12H2O)(15%)	1100	53	170	[161]
Capric acid (65 mol%) + lauric acid (35 mol%)	360	19.6	126.5	[161]
Capric acid (73.5 wt.%) + myristic acid (26.5 wt.%)	5000	21.4	152	[162]
Capric acid (83 wt.%) + stearic acid (17 wt.%)	5000	24.68	178.64	[163]
Lauric acid (66 wt.%) + myristic acid (34 wt.%)	1460	34.2	166.8	[164]
Lauric acid (69 wt.%) + palmitic acid (31 wt.%)	1460	35.2	166.3	[164]
Myristic acid (64 wt.%) + stearic acid (36 wt.%)	1460	44.1	182.4	[164]
Myristic acid + glycerol	1000	31.96	154.3	[165]
Palmitic acid + glycerol	1000	58.50	185.9	[165]
Stearic acid + glycerol	1000	63.45	149.4	[165]
Mg(NO3)2.6H2O (93 wt.%) + MgCl2.6H2O (7 wt.%)	110	33.8	111.6	[166]

Nanogrid buildings highly depend on the thermal performance of local energy storage systems for optimal energy management. Solar air heaters (double-pass) with sensible heat storage were studied using materials like gravel, limestone, and iron scraps, demonstrating the potential for increasing thermo-hydraulic efficiency in heating systems. Gravel-based TES proved to be 22–27% higher in efficiency, making it suitable for integrating into residential nanogrids to enhance space heating efficiency [167]. Encapsulated TES modules in domestic hot water cylinders, arranged in 57 vertical tubes, enhance storage capacity and reduce operational costs, making them ideal for improving energy efficiency and availability in residential nanogrids during off-peak periods [168]. A recent study [169] combined PCM and solar hot water systems in a heat transfer fluid loop, demonstrating a significant reduction in storage volume, which is one of the main concerns in residential nanogrids that have limited space. TES technologies integrating PCMs may prove to be very critical in residential nanogrids for managing energy storage and dispatch for heating, cooling, and even power generation [170]. Nkwetta et al. [143] reviewed PCM configurations, highlighting key factors influencing PCM selection and application. Although PCM-based TESS systems have higher costs, cascaded TES configurations offer 30% higher energy utilization and 23% improved exergy efficiency, offering a promising solution for residential nanogrids. The integration of transpired solar collectors with PCM storage in residential nanogrids improves energy efficiency, reduces system size and cost, and enhances energy collection, with further optimization and automation recommended for maximizing performance. This study proposes the integration of transpired cascaded solar integrated thermal walls in residential houses with energy-sharing networks to form a near-zero energy network of nanogrids. Investigation by Erro and team [171] encompasses the integration of thermoelectric heat pumps into nanogrids with sensible thermal energy storage systems. They tested charging a sensible thermal energy storage system using air as the heating fluid. The system achieved temperatures up to 139.2 $°\mathrm{C}$ and outperformed conventional thermal storage by 30% at a storage temperature of 120 $°\mathrm{C}$.

3.2.3. PCM Thermal Storage (Building)

The development of novel architectures for energy conservation in residential and commercial sectors is crucial, with TES systems offering potential benefits including improved energy and economic efficiency, reduced electrical consumption, and lower environmental pollution and ${\mathrm{CO}}_2$ emissions. Thermal energy storage (TES) systems in building applications can be categorized into two distinct types: active and passive. Active thermal energy storage (TES) systems, utilizing forced convection and sometimes mass transfer in heat exchangers, aim to regulate indoor conditions and reduce peak energy demand in HVAC systems. The passive TES systems use solar, wind, or natural resources and architectural changes such as insulation of roofs, walls, floors, such as in passive houses, triple-glazed windows, building orientation, thermal mass, green roofs, heat recovery devices, and shading devices to maintain comfortable indoor conditions, aiming to reduce dependence on mechanical heating or cooling. A prototype nano-PCM wallboard (Gypsum and n-heptadecane) with expanded graphite nanosheets was investigated [172] to enhance the thermal conductivity and shape stability of a building. The findings revealed that wallboards significantly reduce heat gains/losses for hot/humid climates. Yuhao Qiao and team [173] employed an artificially controlled environment to simulate and study PCM wallboard for five different climatic conditions. In one climatic condition, the study made a bold claim of reducing temperature accumulation by 88.9%. Another study investigated single- and double-layer PCMs integrated into building envelopes and reiterated the fact that PCM ceilings lower reliance on air conditioning and lower energy consumption. Moreover, a double-layer PCM building envelope can reduce maximum temperature variation by 8.5 $°\mathrm{C}$ as compared with a single layer. The study [174] claimed a 6 $°\mathrm{C}$ decrease in temperature (sub-tropical climate) when using OM35 (157 J/g) and Eicosane as PCM material.

3.2.4. PCM Thermal Storage (Solar Heating)

An experimental investigation [72] was conducted using indirect solar drying and PCMs for drying applications specifically, but the results are significant for potential applications in microgrid applications. The setup included a pair of flat plate solar air heaters, a drying chamber, a PCM storage unit, and an air blower. Each heater featured a 1 mm thick copper absorber plate covered by two 5 mm thick glass sheets with a 25 mm gap and insulation (40 mm foam). The drying chamber (galvanized iron), measuring 0.6 by 1.2 by 1.7 m³, was also insulated with 40 mm foam and contained three stainless steel trays supported by aluminum frames. PCM energy storage was provided by two plastic cylinders filled with paraffin wax, insulated with foam and glass, and equipped with 32 copper tubes for heat transfer. The use of PCM maintained a constant drying temperature for 7 h daily, improving air temperature by 3.5–6.5 $°\mathrm{C}$ after 2:00 p.m. compared with systems without storage [73].

Another study with similar implications was reported [175,176], in which PCM (calcium chloride hexahydrate) integrated solar dryers with reflecting mirrors for trapping more solar radiation. Another study with H-68 PCM [177,178] also concluded improved performance using PCMs. All the above studies have used mass flow rate and discharge period as the main parameters of interest. Hybrid solar technologies with PCM result in 10–25% savings on total energy consumption [70,179,180], which prompts its use in nanogrids. Whenever we talk about thermal energy storage, it is imperative to discuss heat transfer fluids. Blood in the human body facilitates the distribution of nutrients and oxygen. Similarly, heat transfer fluids in thermal energy systems play an equally critical role in the efficient transfer. The integration of U-pipe evacuated glass tube (EGT) solar collectors into residential nanogrids may enhance energy efficiency by optimizing thermal storage. Experimental investigations were conducted for such a solar collector model. Thermal efficiency mainly depends on the mass flow rate of HTF and not on the length of the tube. A low heat loss coefficient collector was deemed suitable for cold climates. The absorption coefficient, ranging between 0.85 and 0.94, did not significantly impact efficiency under different weather conditions [181]. Double U-tube collectors demonstrated an efficiency of 80% under solar irradiance of 900 W/m² and a thermal conductivity of 100 W/mK, outperforming single U-tube collectors by 4% [182].

An absorption refrigerator is a refrigerator that uses a heat source (e.g., steam and hot water) to provide the energy needed to drive the cooling process. However, in adsorption refrigeration, the refrigerant or adsorbate vapor molecules adsorb onto the surface of a solid instead of dissolving into a liquid like LiBr and ammonia. Both absorption and adsorption chillers are well suited for residential applications, but an absorption chiller is proposed for the nanogrid because of the integration of evacuated tube collectors and insulated thermal storage, which use hot water to drive the cooling process with a higher coefficient of performance C.O.P (0.7–1.8) and cooling capacity as compared with an adsorption chiller. Nano-enhanced PCMs (Ne-PCMs) [183] find applications in solar cookers, water heaters, air warmers, and desalination systems, emphasizing their role in enhancing the thermal performance of all nanogrid elements and, most importantly, increasing heat storage capacity and energy efficiency.

Table 3. Temperature ranges for different solar thermal technologies.

No.	Component	Melting Temperature ($°\mathrm{C}$)
1.	Air conditioning	<15 [4]
2.	Absorption refrigeration	>90 [4]
3.	Solar heating	15 to 90 [4]
4.	Residential heating/cooling	0 to 65 [68]
5.	Agricultural drying products	40 to 75 [131]
6.	Solar thermal plant	>500 [111,141]

outlines the various operating temperature ranges for different solar thermal technologies.

3.2.5. PCM Thermal Storage (Integrated Hot Water Storage Unit)

In a numerical study conducted with TRNSYS, a domestic hot water tank was coupled with a thermal energy storage (TES) module containing phase change materials (PCM). The results demonstrated that integrating PCM effectively improved the tank’s energy storage density. The optimal PCM was identified as a 10% graphite and sodium acetate trihydrate composite due to its superior storage potential. Key considerations included the thermal conductivity and thickness of the PCM module, as well as ensuring safe and durable containment. The study demonstrated that PCM integration not only enhanced energy density but also effectively met energy, load shift, and peak power demands [143]. The integration of PCM in hot water tanks within residential nanogrids enhances energy storage density, enabling effective load shifting and peak demand management. This leads to optimized thermal management and reduced reliance on instantaneous heating. Figure 3 illustrates the charging durations for a variety of PCMs.

3.2.6. PCM Thermal Storage (Feasibility of PCM-TES)

There is a disparity in demand/supply at centralized and decentralized levels in the grid with the penetration of renewable technologies, especially solar and wind. Such disparities can be best addressed by reliable electrical and thermal energy storage, which also improves the energy and exergy performance of intermittent sources. Hybrid energy storage, such as synergetic electro-thermal storage technology, effectively reduces overall energy consumption. TES systems are pivotal in achieving successful thermal systems by significantly reducing thermal energy losses [184]. Sensible heat storage (energy stored through heating/cooling) and PCM thermal energy storage (melting/ vaporization/ solidification/ liquefaction stores the energy) are all feasible options for nanogrid integration. Sensible heat storage, being a mature technology, is integrated in residential buildings due to its cost-effectiveness, despite its large volumetric requirements for small temperature variations [134]. In contrast, latent heat storage offers higher energy density (kWh/m³) and superior properties overall. Latent heat storage is self-regulating, discharging energy at a constant temperature, and can store five to ten times more energy than sensible heat storage units, making them ideal candidates for small residential nanogrids [133,140]. Based on prior research, latent heat storage systems appear to be a promising and practical solution for integration in micro and nanogrids.

3.2.7. PCM Thermal Storage (Key Challenges)

The effectiveness of PCM-integrated thermal energy storage (TES) systems in nanogrids depends largely on their thermophysical characteristics at melting temperature. Key attributes include heat of fusion, specific heat, thermal conductivity, stability, non-toxicity, inertness, and density. However, significant challenges persist, such as constrained space, elevated capital costs, and the intermittent availability of solar energy. Integrating PCM thermal storage into residential nanogrids can bridge the energy supply–demand gap, enhancing energy conservation and reliability. However, the poor thermal conductivity of paraffin wax limits efficient charging and discharging in TES systems, necessitating performance improvements in connected solar air heaters. Low thermal conductivity is a major hurdle for PCMs; research favors integrating highly conductive additives such as fins and metallic foams to improve overall thermal performance. However, techno-economic case-based studies must be conducted to reach a consensus on its use in nanogrids. To advance PCM thermal energy storage technology, operational parameters must be optimized. Desired traits include a high heat transfer rate, stable inlet and outlet temperatures, consistent phase transition temperatures, uniform heat transfer fluid (HTF) temperatures, rapid charging/discharging, minimal circulation power for HTF, controlled temperature regulation, and uniform temperature distribution. Additional considerations are the compatibility of PCM with the container, minimized heat loss, and efficient solid/liquid motion during freezing and melting processes [94]. Proper structural adjustments can thus lead to a well-designed thermal energy storage system. Proper selection of PCM based on its melting characteristics and recommended working temperature is critical. Additionally, the effectiveness of PCMs depends on the convective heat exchange between the PCM walls and the surrounding room air. The current heat transfer coefficients specified in building codes might not be suitable for PCM applications [185].

3.3. Phase Change Materials (Heat Transfer Enhancement)

Contemporary research on phase change materials (PCMs) focuses on various facets such as the materials themselves, heat transfer fluids, and their properties such as temperature, flow rate, configurations, structural designs, and additives. This exploration spans both theoretical analysis and experimental investigations to uncover their diverse applications [186]. One of the most effective ways of enhancing heat transferability is the use of PCM-packed beds, improving storage density and efficiency. Figure 4 illustrates a spherical capsule-packed bed with multiple PCM arrangements.

Nanoparticles significantly boost the thermophysical properties of phase change materials (PCMs), making them ideal for nanogrid applications. The key to these enhancements lies in the nanoparticles’ high volume-to-surface area ratio, which optimizes interfacial heat transfer, thermal conductivity, and thermal diffusivity [187]. As a result, PCMs with embedded nanoparticles exhibit better energy storage and discharge capabilities. Titanium oxide ${\mathrm{TiO}}_2$ nanoparticles are claimed to achieve up to a 37% increase in efficiency but highlighted the need for further research on long-term stability and higher nanoparticle concentrations [188]. Increasing nanoparticle volume fraction can reduce energy storage efficiency due to decreased capacity and increased viscosity, while optimized long and narrow triangle fins, especially unequal length ones, enhance heat transfer and outperform rectangle fins in improving PCM melting rates without compromising storage capacity [189]. The findings indicate that nano-enhanced PCMs exhibit 8.3% more heat charged in and 25.1% more heat discharged compared with pure PCMs under winter test conditions [190].

3.3.1. Heat Transfer Fluids and Mass Flow Rates (Thermophysical Property)

A detailed study [191] on the choice of heat transfer fluids considered air, compressed air (10 bar), supercritical ${\mathrm{CO}}_2$ (100 bar), steam (10 bar), solar salt, and liquid sodium with flat plate PCM as thermal energy storage (140 MW). The study found that liquid sodium reduced the thermal storage charging duration by 25% compared with solar salt and delivered 99.4% of its stored energy during discharge, outperforming other fluids. It concluded that the choice of heat transfer fluid had a minor impact on electricity generation, suggesting possible cost savings with less efficient fluids. The implications of the aforementioned study extend to application in residential nanogrids. Residential buildings with diurnal energy demands require rapid charging cycles to provide reliable energy ride-through capabilities. Using liquid sodium as a heat transfer fluid may enhance the responsiveness and reliability of thermal storage. Additionally, the potential cost–benefit analysis for residential applications needs to be studied.

The predicted outlet air temperature during discharge in a solar air heater (SAH) equipped with a phase change material (PCM) thermal energy storage (TES) unit was assessed across mass flow rates ranging from 0.05 kg/s to 0.19 kg/s. The optimal values were determined by evaluating the outlet air temperature and the freezing time of the PCM. A paraffin wax and aluminum powder composite (0.5% by weight) enhanced thermal conductivity, revealing an inverse relationship between mass flow rate and PCM freezing time [192]. Experimental and numerical studies showed discrepancies during PCM melting and solidification, confirming the TES unit’s applicability to other air-based systems [193]. Optimizing mass flow rates and enhancing the thermal conductivity of PCM composites can develop efficient TESS for residential nanogrids, ensuring a stable energy supply and improved sustainability.

3.3.2. Shell and Tube PCM (Arrangement)

Researchers in the study [194] explored a shell and tube thermal energy storage (TES) system for district heating, using paraffin (RT100) as the phase change material (PCM) and water as the heat transfer fluid (HTF). Their findings revealed that a paraffin/graphite composite with 15% graphite content exhibited impressive performance. The configuration included vertically arranged HTF pipes in a 15 × 15 matrix. The addition of expanded graphite improved the thermal conductivity of the storage medium, maintaining the PCM’s essential thermophysical properties. The TES system reliably met heating demands ranging from 130 kW to 400 kW, depending on the operating conditions. Figure 5 illustrates a shell-and-tube PCM thermal energy storage unit.

The inclusion of nanoparticles such as Al₂O₃ and graphene nanoplatelets (GNPs) significantly enhances the thermal performance of PCMs in horizontal triple-series shell-and-tube heat exchangers by accelerating melting times and increasing effective temperatures [195,196]. These enhancements suggest promising applications for improved energy storage in a nanogrid context.

3.3.3. Multi PCM Systems (Arrangement)

An array of multiple PCMs with multiple melting points can be fused/arranged together vertically or horizontally, as shown in Figure 6, in tanks to improve the overall heat transferability of the system. To improve the heat transfer ability of the systems, multiple phase change materials (PCMs) with varying melting points are linked in horizontal/vertical tanks in ascending/descending order. This combinational approach leads to optimized charging/discharging. Employing packed bed storage with multiple PCMs significantly enhances the heat transfer efficiency of thermal storage systems. In such a unit, the temperature difference between the heat transfer fluid (HTF) and the PCMs is minimized, leading to an improved system coefficient of performance (COP). Multi-PCMs (series/parallel combinations) have significant advantages in building applications, especially in reducing energy consumption. Multiple PCMs transform the flexibility of storage mediums to adjust to changing operational conditions. In a multi-system, each PCM has a different melting point, thus providing a wider temperature control range and faster charging/discharging [197]. In building contexts, systems with multiple PCMs can balance heating and cooling demands by storing and releasing heat at various temperatures, making them ideal for year-round use with reduced energy consumption. A multi-PCM configuration (RT-35-RT27) in a PV-PCM window has been shown to lower peak interior wall temperatures by up to 46% compared with standard glass panes [198]. The use of multiple PCMs in air-phase change material heat exchangers (H.EX.) reduced incoming fresh air temperatures before reaching air conditioning systems, lowering cooling requirements by more than 11% [199].

3.3.4. Hybrid Technologies (Arrangement + Thermophysical Properties + Structure)

Residential nanogrids may also benefit from combining thermoelectric (TE) modules with evacuated glass tube heat pipe solar collectors [200], enabling simultaneous water heating and electricity generation with validated theoretical and empirical performance models. Despite having a thermal efficiency of 1–2%, slightly lower than the 3–4% efficiency of Rankine Cycle systems, the TE/evacuated glass systems offer benefits such as simple design, no moving parts, and flexible component replacement. These systems can provide both thermal energy for domestic hot water (DHW), space thermal conditioning, and electrical power generation simultaneously, realizing combined heat and power nanogrids. It has a packed bed of spherical PCM capsules that demonstrated a consistent heat output of 200 W/m² for 11 h at night during discharge. The net daily energy and exergy efficiencies ranged from 32% to 45% and 13% to 25%, respectively. The ability of such a device to provide consistent heat output during night-time is ideal for nanogrid energy ride-through capabilities, energy sharing, and reliability. The team in [201] emphasized the use of latent heat TES for thermal storage applications. They studied the inclusion of nanoparticle additives and fins to enhance thermal conductivity. Their design is a triple-tube heat exchanger with centrally installed PCM. The team compared cases with/without arc-shaped fins, different shapes of fins (upward, downward), and different angles. Higher angles improve circulation and heat transfer within PCM. The best performance was observed at 90-degree upward fins. Moreover, a higher Reynolds number and inlet temperatures enhance the system’s performance. This study has significant implications for nanogrid buildings, which depend on energy efficiency and the ability to store excess energy to provide thermal energy ride-through capabilities. The team in [202] pointed out that the solutions to low thermal conductivity include nanoparticle distribution, high conductivity materials, and composite PCMs. The study’s findings indicate that the implementation of a framed structure led to a storage rate of 20.4 watts, demonstrating 115% improvement over the 9.5-watt storage rate observed with an unframed structure. Furthermore, the research highlighted the efficacy of zigzag surface shapes over smooth ones in boosting the thermal energy storage rate through the melting process. Additionally, the study observed that an increase in the number of pitches on the zigzag surfaces correlated with a better thermal energy storage rate. The reverse arc-shaped structure emerged as the most effective, achieving a thermal energy storage rate of approximately 34 watts and completing the melting process in just over an hour (63.15 min). This superior performance was recorded when the structure was heated by water with a Reynolds number of 1000 and an inlet temperature of 50 $°\mathrm{C}$. Lastly, the research revealed that both an increase in the Reynolds number, from 1000 to 2000, and a rise in the heat transfer fluid’s (HTF) inlet temperature, from 50 $°\mathrm{C}$ to 55 $°\mathrm{C}$, contributed to enhancements in the thermal energy storage rate by around 6 watts and 10 watts, respectively. An innovative solar air heater (SAH) [74] with an illustration of the setup shown in Figure 7.

Solar air heaters are designed based on several factors. Collector covers are classified into three types: bare plate SAH, single cover SAH, and double cover. The absorber material in SAHs can be metallic, non-metallic, or a matrix structure. The shapes include slats, porous media, or fins to enhance heat absorption efficiency. The absorber flow patterns are another essential aspect that can occur over, under, or on both sides of the surface. Flow directions within SAHs can vary, including single pass, parallel pass, double pass, and triple pass configurations. Hybrid collectors are also noteworthy, combining different mediums such as water, air, or a combination of both (water–air) [203]. Low-temperature solar air heating with PCMs is an ideal candidate for nanogrids designed in cold climates. PCM with encapsulated spherical PCMs was analyzed [42] to identify three main parameters influencing the charging of thermal storage. The parameters include the size of the heat transfer surface area reducing the size of the encapsulated balls, the temperature differential between the phase transition and HTF inlet, and finally the mass flow rate of HTF. An extensive study was conducted on a compact PCM air heat exchanger. The thermal storage system comprised compact storage modules constructed from aluminum containers filled with PCM. These modules were arranged vertically, with air forced to flow vertically from the upper to the lower part of the storage unit. The study [204,205] utilized 135 kg of paraffin (RT27) as the storage medium, taking into account the thermo-physical properties of the phase change material (PCM) and a variety of parameters to refine the model. Key factors such as the air mass flow rate, surface roughness, and the thermal conductivity of the encapsulating material were specifically evaluated in the numerical analysis. Paraffin wax-integrated PCM-TES systems ensure consistent temperature regulation and energy supply during intermittent sunlight conditions. The characteristics of paraffin wax as a phase change material (PCM), as detailed by the supplier Pure Temp, are presented in

Table 4. Thermal characteristics of paraffin PCMs.

PCM	Heat (Latent) (J/g)	Phase Transition Temperature ($°\mathrm{C}$)	Density (g/cm3)	Solid (J/g $°\mathrm{C}$)	Liquid (J/g $°\mathrm{C}$)
PT-40	198	40	0.85	1.98	2.12
PT-43	180	43	0.88	1.87	1.94
PT-48	245	48	0.82	2.10	2.27
PT-50	200	50	0.86	1.82	1.94
PT-56	237	56	0.81	2.47	2.27
PT-61	199	61	0.84	1.99	2.16
PT-68	198	68	0.87	1.84	1.91

. A study [206] investigated such a system with and without PCM storage in clear and semi-cloudy conditions. In contrast to a similar system without storage, the plate temperature increased at 2.5 m from the entrance point, remaining stable over the subsequent 7.6 m of the storage path. During periods of limited or no sunlight, the liquid phase change material (PCM) released its stored heat energy to the circulating water until it fully solidified. The literature on ground-source heat pumps (GSHPs) coupled with TES systems highlighted that the coefficient of performance (COP) for GSHP with TES systems varied from 2.0 to 6.49 for both cooling and heating applications and from 2.3 to 7.95 for standalone setups [207].

3.3.5. Nanocomposite PCMs (Structure)

A reliable nanogrid requires consistent operation over multiple charge/discharge cycles, efficiency of heat storage, rapid heating/cooling, and maximum discharge efficiency. Through the in situ miniemulsion polymerization process, the team in [208] crafted nano-encapsulated phase change materials (PCMs) featuring n-octadecane (core), encased in a resilient styrene/methyl methacrylate copolymer shell. This robust polymeric armor adeptly safeguarded the n-octadecane, showcasing exceptional thermal stability and enduring up to 360 cycles of charging and discharging. An investigation was conducted on a nanocomposite latent heat storage system, employing stearic acid as the phase change material (PCM) and incorporating multi-walled carbon nanotubes (MWCNTs) as an additive for enhanced performance. Adding MWCNTs resulted in a lower melting point during charging and an increased freezing point during discharge compared with pure stearic acid. While the inclusion of MWCNTs improved the thermal heat transfer of stearic acid, it simultaneously diminished its natural convective heat transfer. Thus, the overall thermal performance of the stearic acid/MWCNT nanocomposite depends on a balance between the benefits and the drawbacks. When catering to consumer energy demands, the discharge rate of latent heat storage is more crucial than the charge rate. Findings revealed that the charge rate only saw improvement when the volumetric concentration of MWCNTs in stearic acid was kept below 5%. To ensure efficient charging and discharging, establishing an effective heat transfer network is pivotal, as it enables rapid heating or cooling of the PCM’s inner regions [132]. López et al. [209] performed experimental investigations to scrutinize the performance attributes of an organic PCM storage tank. The analysis further delved into the design parameters for the latent heat storage tank, with a particular emphasis on evaluating paraffin’s efficacy in cold storage applications. The experiments revealed that precise control over the target supply temperature could be accomplished by incorporating a sensible heat storage unit downstream of the latent heat storage tank. Measurements of both the PCM and the heat transfer fluid (HTF) indicated pronounced vertical stratification, notably within the liquid phase of the PCM. Within 4 h, the storage tank reached 78% of its maximum capacity, with stored energy showing greater sensitivity to supply temperature variations during melting tests. Despite testing various coil designs, a full phase change was not accomplished during solidification tests at temperatures 6 K below the phase change threshold. Combining latent heat storage with sensible heat storage holds potential to improve the efficiency and dependability of energy systems for homes.

Table 5. Cost of organic PCM latent heat storage tank [209].

Element	Cost (Euro)	Cost (AUD)	% Cost
Wall (tank) + lid + drainage	1700	2788	36.5
Insulation	1500	2460	32
Copper (tubes)	400	656	8.6
Collectors	240	393.6	5.1
PCM (165 kg)	814	1334.96	17.5
Total	4654	7632.56	100

summarizes the standard pricing of the organic latent heat storage (LHS) tank.

Parameshwaran et al. [210] found that embedding hybrid nanocomposites into phase change materials (PCM) significantly enhanced thermal properties and heat storage capacity, making them suitable for cooling applications in buildings. Similarly, Li et al. [211] studied the effects of various carbon nanofibers (CNFs), carbon nanofillers—short and long multi-walled carbon nanotubes (S-MWCNTs, L-MWCNTs), and graphene nanofillers (GNPs)—on paraffin-based nanocomposite PCMs. They observed a nearly linear decrease in melting/solidification enthalpies with the addition of nanofillers, with minimal dependency on filler size and shape. The thermal conductivity improvement, particularly notable with S-MWCNTs and planar GNPs, was attributed to reduced steric impedance and low thermal interface resistance, respectively. However, the performance of graphene nanofillers was highly dependent on their size and thickness, with larger and thicker layers being more effective. Consequently, graphene nanofillers emerged as optimal candidates for enhancing thermal conductivity in nanocomposite PCMs while only moderately reducing energy storage capacity. These findings have significant implications for residential nanogrids, where optimized nanocomposite PCMs can enhance energy efficiency and reliability, crucial for managing energy demands and cooling needs.

Nanocomposite phase change materials (PCMs) incorporating paraffin infused with hexagonal boron nitride (h-BN) nanosheets have been evaluated for their potential in advancing thermal energy storage solutions. A 10 wt.% concentration of h-BN nanosheets increased thermal conductivity by 60% and decreased latent heat of fusion by 12% compared with pure paraffin wax, while also accelerating solidification and melting rates by 25%. Dimensional attributes of h-BN nanosheets significantly affected thermal conductivity, and the absence of surfactants caused instability due to nanosheet precipitation during cyclical phase changes, necessitating further stabilization efforts [212]. Additionally, Teng et al. [213] investigated modified PCMs with multi-walled carbon nanotubes (MWCNTs) and graphite added to paraffin at various concentrations (1.0, 2.0, and 3.0 wt.%). Their direct-synthesis method and differential scanning calorimetry (DSC) experiments revealed that MWCNTs were more effective than graphite in enhancing the thermal storage performance of paraffin, reducing temperature differentials between PCM and heating fluids, and improving several experimental parameters. The phase change temperature variation was minimal, with a maximum decrease of only 3.69% compared with pure paraffin, indicating the significant potential of MWCNT-enhanced paraffin for future thermal storage applications.

3.3.6. Microencapsulated PCM (Arrangement)

Microencapsulation is a process of enclosing PCM materials within protective coatings, forming microcapsules. Microencapsulated phase change materials (PCMs) range from 1 to 1000 $\mathrm{\mu }\mathrm{m}$, offering enhanced heat transfer, chemical stability, and volumetric control during phase transitions. A high-concentration (45 wt.%) PCM slurry of microencapsulated Rubitherm RT6 has been developed, achieving a heat transfer coefficient up to five times that of water [186,214]. Microencapsulated PCMs, combined with nanoparticle additives, significantly enhance thermal conductivity in storage systems. Additionally, absorption and sorption technologies have been introduced for negative temperature applications. Ma et al. [215] developed a model to study heat transfer in PCM slurries, finding that TBAB CHS showed temperature variations but incomplete melting in microencapsulated PCM. PS/n-heptadecane micro/nano encapsulated PCM, synthesized via miniemulsion polymerization, exhibited good thermal properties and reliability [216]. Encapsulated PCM with tetradecane and donkey-hide gelatin measured distinct melting/freezing points and latent heats [217], while PMMA/docosane microcapsules displayed compact surfaces and high thermal reliability [218]. Polystyrene-shelled PCMs were also explored, with PMMA/octadocosane identified as the most effective PCM [219]. In the context of residential nanogrids, the improved heat transfer rates and thermal conductivity of microencapsulated PCMs, augmented by nanoparticle additives, can substantially enhance energy efficiency and storage capabilities. The application of such advanced materials in residential nanogrids could lead to more efficient thermal management, thereby contributing to the reduction of energy consumption. The characteristics of microencapsulated PCMs are given in Table 4 [220]. Encapsulation techniques at macro, micro, and nanoscales were discussed in [221] focusing on their application in thermal energy storage systems. It highlights the potential of nano-encapsulated PCMs (nano-PCMs) to enhance thermal conductivity and efficiency. Various methods, such as pan coating, micro-fluidic techniques, and interfacial polymerization, are discussed, emphasizing their role in improving the thermal performance of PCMs for nanogrid applications.

3.3.7. Shape-Stabilized PCMs (Structure)

Shape-stabilized PCMs tend to maintain their structural integrity by being embedded in polymer matrices during phase changes, minimizing leakage. Advancements in stabilized, shape-stabilized PCMs significantly impact residential nanogrids by enhancing energy storage and efficiency. Integrating high-capacity, thermally stable composites like paraffin/HDPE and bio-based PCMs with boron nitride addresses issues such as leakage and low thermal conductivity. Additionally, materials like expanded graphite and aluminum nitride improve heat transfer, optimizing overall performance. Encapsulation is expensive; an alternative to enhance thermal conductivity is to use stabilized and shape-stabilized PCMs. Polymer matrix prevents leakage of molten paraffin, resolving issues related to encapsulation [220,222,223,224,225,226,227,228,229,230,231]. Trigui et al. [232] conducted a study on PCM composites made of low-density polyethylene and paraffin wax, identifying poor heat transfer capabilities and shape stabilization as obstacles to widespread PCM application. Another study prepared bio-based PCMs with boron nitride using vacuum impregnation, highlighting their high latent heat and thermal stability but noting issues with low thermal conductivity and leakage, which boron nitride addressed effectively [233]. Polyethylene was identified as a supportive material for shape-stabilized PCMs due to its compatibility with paraffins, and the melt mixing method achieved well-dispersed paraffin/HDPE composites [234]. Further studies showed that blending PCMs with PMMA and other polymers could support up to 80 wt.% PCM without leakage [235], and adding aluminum nitride improved heat transfer capability [236]. Other findings included the successful stabilization of paraffin/expanded graphite composites through capillary forces without leakage [237], and thermal characterizations revealed the optimal mass fractions for maximizing latent heat [238]. The long-term stability of Rubitherm 21 (RT21) and a propyl stearate–palmitate ester mixture was assessed by examining changes in their thermophysical properties under sustained heating above their melting points. RT21 exhibited irreversible changes in weight and latent heat of fusion, while the ester mixture remained stable without mass loss or alteration in thermal properties [239]. These results suggest that the stable ester-based PCM is a viable option for enhancing the efficiency and durability of TESS in residential nanogrids.

3.3.8. Nitrides, Additives and Nanofluids (Additives)

A study [240] explored how sodium nitride, potassium nitride, and their combinations with expanded graphite could act as additives to boost thermal conductivities, uncovering potential opportunities for residential nanogrids. At 25 $°\mathrm{C}$, the thermal conductivities of pure sodium nitride and potassium nitride were measured at 0.224 W/mK and 0.156 W/mK, respectively. The addition of graphite significantly improved these conductivities, with mixtures containing 10% graphite showing a 30–40% increase and those with 5% graphite showing a 10–20% increase. Additionally, graphite altered the phase change temperatures and reduced the freezing/melting points of the mixtures by 5–18% compared with pure nitrides. These findings suggest that integrating graphite-enhanced nitrides could improve the efficiency of solar energy storage in residential nanogrids. An experimental study claimed that embedding metal foam in phase change materials (PCM) can boost heat transfer performance by nearly ten times compared with pure PCM. This enhancement is attributed to the metal’s superior thermal conductivity merging with the PCM. Moreover, smaller porosity and pore size within the metal foam outperformed larger ones, further elevating heat transfer efficiency [241]. When it comes to residential nanogrids, boosting the thermal conductivity of phase change materials (PCMs) is essential for optimal energy management. A study explored the incorporation of carbon fibers to enhance the thermal conductivity of fiber/paraffin composites. Two different setups were examined: fibers dispersed randomly and a suspended carbon fiber brush. The random arrangement and fiber length showed minimal effect, but the fiber brush markedly improved thermal conductivity [242]. Similarly, paraffin/copper nanofluid composites were examined, revealing that the integration of copper foam substantially boosted both heat transfer rate and thermal conductivity [243]. Another investigation focused on paraffin/graphite composites, which exhibited a 20–30% higher thermal conductivity compared with pure paraffin [244]. Furthermore, tests on paraffin integrated with ${\mathrm{SiO}}_2$ and expanded graphite, as well as paraffin combined solely with ${\mathrm{SiO}}_2$, showcased impressive enhancements in thermal conductivity—registering improvements of 94.7% and 28.2%, respectively, compared with pure paraffin [245]. Adding silica-alumina nanoparticles into NaNO₃-KNO₃ binary salt is claimed [246] to enhance thermal storage properties. Incorporating 1.0 wt.% of these nanoparticles into the nanofluid enhances its specific heat capacity, boosting it by a remarkable 15% to 57% when in the solid phase and by 1% to 22% in the liquid phase. These improvements could significantly benefit the integration of efficient PCM-TES systems in nanogrids. In a comparative study, the charging times for copper, polyethylene, and polyvinyl chloride (PVC) capsules were found to be 7 h, 8 h, 9 h, and 20 min, respectively. The minor differences in charging times suggest that polyethylene capsules are a cost-effective option for constructing TES storage tanks [247].

3.4. Photochromic and Thermochromic (Materials)

Photochromic and thermochromic materials are gaining traction as phase change materials (PCMs), which have the ability to transform their properties in response to changes in light and temperature, respectively. Photochromic materials change their colors when exposed to light (UV). These materials undergo a reversible molecular transformation when exposed to certain wavelengths of light, with a change in their absorption properties and colors. Thermochromic materials change their color in response to temperature changes. These materials can change color at specific temperature thresholds due to physical or chemical changes in their molecular structure. An ideal photochromic or thermochromic material must switch from black color at low temperature to white color at high temperature. Thermochromic material Bis(dimethylammonium) tetrachloronickelate(II) changes color from green (25 $°\mathrm{C}$) to yellow (50 $°\mathrm{C}$) as photochromic glasses change color from transparent (25 $°\mathrm{C}$) to dark (50 $°\mathrm{C}$) in sunshine. Nature is replete with color-changing reptiles (Chameleon), insects (Chrysso), and birds (Surakav). Photochromic and thermochromic materials often change color from one (say green) to another (say yellow), like chameleon with a change in temperature instead of ideal black and white coatings as shown in Figure 8.

Photochromic materials are used for smart glasses, which turn dark in the sun and translucent in the shade. Experimental studies demonstrated that cool coatings can reduce cooling loads by 18–93% and peak cooling demand by 11–27% in air-conditioned buildings [248]. Cool white coatings like titanium oxide and barium sulfate can reflect 98% visible light, whereas sodium citrate can reflect infrared. There are some polymer paints that can reflect UV too. Titanium dioxide-based solar reflective paint cost dollars 8/L compared with dollars 4/L for others. Super therm cool roof paints are good reflectors in the visible range but not very efficient in the IR and UV ranges. Ultra-white paint technologies can replace air conditioners. Measurements show at an ambient temperature of 35 $°\mathrm{C}$ the indoor temperature of a white-painted roof building reaches 37 $°\mathrm{C}$ compared with 60 $°\mathrm{C}$ in an unpainted grey rooftop building, where others claim even less than the ambient temperature inside an ultra-white building. Civil engineers are deploying thermochromic materials for cool surface pavements [249]. Bitumen membranes can achieve higher solar reflectance when coated with white materials, while elastomeric or cementitious cool white coatings typically reflect 0.7 to 0.85 of solar radiation. For thermal energy storage using granular encapsulated PCMs, the PCM serves as the core, encapsulated by a shell made of inorganic metal oxides like zirconium dioxide (${\mathrm{ZrO}}_2$). The temperature discoloration function is used as an indicator [250,251]. Cool tiles in residential buildings offer high solar reflectance and may achieve energy savings of up to 15% in hot, dry climates [248].

3.4.1. Photochromic (Materials)

Photochromism refers to a reversible change of color upon exposure to light. This color change is mediated by photoisomerization of chromic material mixed in silicon lenses or coated on plastic lenses. This change is not long-lasting and reverses with an increase in temperature. The photochromic compounds undergo reversible photochemical reactions altering absorption bands in the visible spectrum. Exposure of silver halide to UV breaks the ionic bond, and silver atoms create a blackish shade due to absorption of light. However, the temperature rise may reverse this opaque state back to a transparent condition. We may engineer the thermochromic properties by adding copper atoms. All photochromic molecules, like tenebrescence, back-isomerize at certain rates, which are accelerated by heat. Any material like nitrospiropyran that takes ten minutes to back-isomerize at room temperature is considered photochromic material. Thermally stable photochromic compounds like diarylethenes do not back-isomerize for 90 days even after heating to 80 $°\mathrm{C}$. There is a strong relationship between photochromic and thermochromic compounds. The textile industry uses photoreactive compounds, which undergo color change after exposure to light. Various classes of photochromic materials are triarylmethanes, stilbenes, azastilbenes, nitrones, fulgides, spiropyrans, naphthopyrans, spirooxazines, quinones, etc., as shown in Figure 9.

Smart photochromic compounds and meta surface or semiconductor phase change materials respond to UV, IR, light, heat, stress, humidity, and electromagnetic fields [252,253]. Ge, Sb, and Te (GST) exhibit large change in n and change in k changes, whereas Ge2Sb2Se4Te1 has a broadband response from NIR to FIR parts of the electromagnetic spectrum [254]. Photochromic technology is used in the sunglasses, data storage disks, cosmetics, and clothes industries. Photoisomerization may be used to store solar energy in dihydroazulene (DHA)/ vinylheptafulvene (VHF) photo/thermo-switch material [255,256]. The IEA technology roadmap expediting energy transition recommends using solar thermal systems where photo and thermochromic materials can play a role [257]. Rampant rise in rooftop photovoltaic and solar thermal panels impacts the urban heat island (UHI).

3.4.2. Thermochromic (Materials)

Thermochromism refers to a change in color caused by a change in the substance’s temperature. Thermochromism is found in both organic and organic compounds. Thermochromic liquid crystals with a limited color range and leuco dyes with widespread color and temperature ranges are commonly used for precision and rough estimates. The color of liquid crystals depends upon the crystalline structure of the material, which changes from crystal (cool), nematic (warm), and isotropic (hot) phases. However, liquid crystals exhibit thermochromism in the nematic phase, wherein light undergoes Bragg diffraction. Thermochromic material colors switch from black to visible colors. Low temperature results in red–orange, and high temperature results in blue–violet colors. Low (3–5 $°\mathrm{C}$), medium (17–23 $°\mathrm{C}$), and high (37–40 $°\mathrm{C}$) temperatures may be obtained by mixing cholesteryl oleyl carbonate, Cholesteryl nonanoate, and Cholesteryl benzoate in various proportions. A mood ring or baby feeder changes colors depending on temperature. Liquid crystal phase change material is used in thermometers, refrigerators, and aquariums. Thermochromic dyes (leucos) are used with other chemicals to change from no colour to some color; litmus papers are pH indicators that correspond to various colors. Commonly used dyes include spirolactones, flurans, spiropyrans, and fulgides, which with bisphrenol a, parabens, 1,2,3-triazole, and 4-hydroxycoumarin exhibit leuco switching from no to some color states. Liquid crystals have better temperature response than leuco dyes, which are used in thermochromic papers, thermoplastics, polymers, and inks. Most inorganic materials exhibit to some extent thermochromism. Titanium oxide, zinc sulfide, and zinc oxides look white at room temperature and yellow at high temperature. Indium oxide switches color from yellow to yellow–brown on heating. This phenomenon is dramatic in phase transition materials. Cuprous mercury iodide undergoes a phase transition at 67 $°\mathrm{C}$, switching from bright red to dark brown [258]. Silver mercury iodide switches from yellow at room temperature to orange at 47–51 $°\mathrm{C}$. Bis(dimethylammonium) tetrachloronickelate switches from red at low temperature to blue at 110 $°\mathrm{C}$ but returns back to red in one week [259]. Chromium and aluminum oxides in a 1:9 ratio are red at room temperature and gray at 400 $°\mathrm{C}$ [260]. Chromio–Chromium richsare reddish purplish at low temperatures, which become green at 80 $°\mathrm{C}$. Vanadium dioxide blocks IR from the rooftop to cool the building [261]. DR is the main cause of overheating in summer, so paint windows with vanadium dioxide to keep the heat out. However, copper iodide and manganese violet are irreversible thermochromic materials [262]. Thermochromic materials are applied in the building to keep the heat out to reduce energy consumption on air conditioning by up to 17% in half a million big cities facing UHI problems [263,264]. Buildings consume 30–40% of global energy consumption. Global building energy consumption was 23.7 PWh in 2010, which will increase to 38.4 PWH by 2040 at a growth rate of 490 TWh per year. The average increase of the cooling demand was 23% from 1970 to 2010, while the average reduction of the heating was 19% in the same period, though both increased by 11% in four decades, which is attributed to global warming and the urban heat island effect [265].

4. Proposed Nanogrid Integrated Architecture

4.1. Overview of Studies

The review highlights the importance of PCM technology in improving the techno-economic indicators of nanogrid buildings. Advanced PCM systems, particularly those with enhanced thermal properties and integrated designs, can significantly contribute to energy efficiency and sustainability. The findings from experimental studies on phase change material (PCM)-based thermal energy storage (TES) systems in residential nanogrids reveal several significant insights. The reduction in charging time is notably achieved through the utilization of metallic capsules, smaller capsule sizes, lower Heat Transfer Fluid (HTF) temperatures, and higher HTF flow rates. Conversely, an extension in discharge time is observed with non-metallic capsules, larger capsule sizes, lower HTF temperatures, and lower HTF flow rates. The energy storage capacity benefits from the use of metallic capsules, larger capsule sizes, increased HTF flow rates, and lower HTF temperatures. Metallic capsules, larger capsule sizes, and lower HTF flow rates also contribute to a superior energy recovery ratio. Despite their lower energy recovery ratios, non-metallic capsules offer a more cost-effective and easily manufacturable alternative. Enhancing the solid mass fraction is achieved with metallic capsules, smaller capsule sizes, higher HTF flow rates, and lower HTF temperatures. Conversely, increasing the melted mass fraction and recovering stored energy is facilitated by metallic capsules, smaller capsule sizes, higher HTF flow rates, and higher HTF temperatures. Interestingly, the thermal conductivity of metallic capsules has a minimal effect on overall charging and discharging times, solid and melted mass fractions, and the energy recovery ratio, suggesting that other factors are more critical in enhancing system performance. When comparing capsule types, metallic capsules demonstrate superior charging characteristics and shorter charging times, whereas non-metallic capsules, being economical and easy to manufacture, offer shorter charging/discharging times with lower energy recovery ratios, making them suitable for specific thermal energy storage applications. Furthermore, innovative heat transfer enhancement techniques, including double-pipe and shell-and-tube designs, have been recognized as highly effective solutions. Integrating PCMs into medium-scale air conditioning systems is advised to lower costs and minimize the size of energy storage systems. These conclusions suggest various strategies and configurations to optimize PCM-based TES systems, balancing cost, efficiency, and application suitability.

4.2. Nanogrid (Architecture)

A proposed nanogrid architecture is introduced based on the information analyzed in this review. This architecture leverages PCM technology to enhance energy efficiency and thermal storage within residential buildings. Polygeneration and multi-energy hubs are gaining attention in the decentralization of smart grids. Concepts of combined cooling, heating, and power (CCHP), combined heating and power (CHP), and trigeneration concepts have been employed in the industries at commercial levels, but only recently has the focus shifted towards residential buildings. Passive house concepts have also been introduced as the main architecture for nanogrids. In the proposed network, each nanogrid house acts as a building block to form a holistic microgrid with energy, data, and resource sharing hubs of multiple nanogrids, as illustrated in the AI-generated community of nanogrids on the left and floor plan on the right (Figure 10).

4.3. Nanogrid (Design)

A nanogrid house design should address/include consistent operation over multiple charge/discharge cycles, reliable/efficient thermal energy storage, rapid heating/cooling through any means of hydronic/radiative/convective methods, maximum discharge efficiency, and reliable backup. Reviewing various trends in phase change materials (PCM) technologies for nanogrids, it is evident that each trend offers distinct advantages and poses specific challenges. The selection of PCM should cover heat of fusion, specific heat, thermal conductivity, stability, non-toxicity, inertness, and density. The challenges in integrating PCMs into nanogrids include limited space, higher capital costs, the intermittent nature of solar, and the thermal conductivity of PCM material. The proposed nanogrid features a north-facing (Australia) PCM wall-integrated residential building forming an airtight (0.6 to 2 air changes per hour) chassis encompassing an area of 144–256 m². PCM with inorganic composites is installed in either the attic or wall of the house with a thickness of 20 mm [266]. The nanogrid will aim to produce heating, cooling, and power [267] simultaneously, with each node/nanogrid connected to another through pipes and cables to improve the reliability of microgrid architecture. A nanogrid will be assumed to accommodate a family of 2–4 occupants with an average peak energy demand of 1800 kWh (winters) and 1500 kWh (summers). For residential nanogrids, the optimal parameters for a solar water heating system in a single-family house include a 6 m² collector area, a 0.42 m³ tank capacity, and a 1.2 m tank height. Using HFOs R-1234yz and R-1234yf, which have minimal global warming potential, enhances thermal efficiency. ${\mathrm{CO}}_2$ (0.720289) and R-410a (0.723428) showed the highest annual solar fraction readings under various weather conditions [268]. The PCM may have a height in the range of 2–4 m, a width of 4–12 m, and a thickness of 0.2–0.535 m. The wall includes one glass cover with a glazing emittance of 0.1, an extinction coefficient of 0.0026, and a refractive index of 1.526. The vent outlet area is 1 m², with vents spaced 3.5 m apart. The thermal demands of this nanogrid would be around 60%, while other power demands will be 40%. Nanogrid will be integrated with two insulated thermal energy storages with a surface area of 2 m² and a tank capacity of 0.5 m³. A multilayer envelope of 0.535 m made of wood, foam, and brick, designed for low thermal mass on the outside, insulation in the middle, and high thermal mass inside. The goal is to keep air changes per hour below 1, ideally below 0.6. Nanocomposites offer improved thermal conductivity for PCMs, but the payback is higher. Microencapsulated PCMs offer improved stability and seamless integration into nanogrid buildings, resulting in efficient temperature regulation, but do pose leakage risks. Shape-stabilized PCMs may be useful for hydronic underfloor heating systems. Bio-based PCMs are very marketable due to their sustainable nature and lower environmental impact but incur higher costs. Multi-PCMs offer enhance thermal performance for TES applications but face phase separation issues. High-temperature PCMs are best suited for solar thermal applications. Photochromic/thermochromic PCMs, suitable for adaptive insulation, offer dynamic responses to environmental changes but are constrained by limited availability and higher material costs.

Nanogrid design may be validated by comparing energy flows across system boundaries with internal energy changes as shown in Figure 11. In the nanogrid system, QGains represents total energy inputs, including QCol, energy from the evacuated tube collector, and QAux, auxiliary energy from backup systems. QPump refers to energy for pumping heat transfer fluids, while QWall accounts for heat gained through PCM walls. On the losses side, QLosses measures all energy leakage within the system. IEC (Internal Energy Changes) tracks variations in stored energy, and TLoss indicates thermal losses to the environment. PLoss denotes power losses due to resistance and inefficiencies in conversion or storage, with Qconv capturing heat lost through fluid movement. Qeq represents energy losses from operating nanogrid equipment. Together, these terms quantify both energy inputs and losses within the nanogrid. Consistent energy transfer among components indicates proper timestep and convergence tolerances. An energy imbalance suggests improper model development. An annual mean energy balance of 3% is acceptable.

$$Q_{\mathrm{gains}}=Q_{\mathrm{col}}+Q_{\mathrm{aux}}+Q_{\mathrm{pump}}+Q_{\mathrm{wall}}$$

(1)

$$Q_{\mathrm{losses}}=\mathrm{IEC}+T_{\mathrm{loss}}+P_{\mathrm{loss}}+Q_{\mathrm{conv}}+Q_{\mathrm{eq}}$$

(2)

$$\mathrm{IEC}+Q_{\mathrm{col}}+Q_{\mathrm{aux}}+Q_{\mathrm{pump}}+Q_{\mathrm{wall}}+Q_{\mathrm{load}}-T_{\mathrm{loss}}-P_{\mathrm{loss}}-Q_{\mathrm{conv}}-Q_{\mathrm{eq}}=0$$

(3)

The payback period for phase change materials typically depends on factors such as installation costs, type of materials, and the specific application in buildings. Conducting a detailed cost analysis considering these factors would be essential in determining the payback period for detailed nanogrid projects. Designing an efficient residential nanogrid requires integrating thermal energy storage (TES) systems and phase change materials (PCMs) to optimize space heating and energy efficiency. The proposed design can incorporate water/gravel-based TES, which uses materials like water, gravel, limestone, and iron scraps, making it ideal for space heating in residential nanogrids. The TES system, with aluminum containers filled with paraffin wax (RT27, RT100), provides a constant thermal output and energy ride-through capability. To enhance storage density, the system includes a packed bed of spherical PCM capsules made from cost-effective polyethylene or metallic materials.

The optimal positioning of PCMs in walls [269] is influenced by several factors, especially the weather conditions that dictate the building’s heating/cooling demands. Moreover, the goal of integrating the PCM wall should be clear, such as reducing heating or cooling loads. The melting temperature and heat of fusion of the PCMs determine their effectiveness in different temperature ranges. The quantity of PCMs used affects the total thermal energy storage capacity, which decides the energy ride-through capability of the nanogrid, while the thermal properties of wall materials influence heat transfer rates as per the air conditioning requirement. Integrating PCMs with solar hot water systems is crucial for reducing storage volume, especially in space-constrained residential nanogrids. The PCMs are stored in plastic cylinders insulated with foam and glass containing paraffin wax. Enhanced thermal performance can be achieved by incorporating graphite, silicon carbide, or multi-walled carbon nanotubes (MWCNT) into the PCM. Furthermore, nano-encapsulated PCMs with an n-octadecane core and a styrene/methacrylate copolymer matrix, or microencapsulated Rubitherm RT6, offer improved thermal reliability and heat transfer coefficients.

Various heat transfer fluids (HTFs) are considered to optimize the TES, including air, compressed air (10 bar), supercritical ${\mathrm{CO}}_2$ (100 bar), steam (10 bar), solar salt, and liquid sodium, with each fluid having specific advantages. Liquid sodium, in particular, offers enhanced responsiveness and reliability, although its economic feasibility for residential applications requires further investigation. The HTFs mass flow rates can be adjusted between 0.05 kg/s and 0.19 kg/s, depending on environmental conditions, to optimize the system’s performance. Transpired solar collectors integrated with PCM storage significantly enhance energy efficiency and reduce costs. Double U-tube solar collectors, which offer a 4% higher efficiency with additional capital investment, are suitable for high solar insolation areas, while flat plate collectors and U-pipe evacuated glass tube collectors can be selected based on specific regional solar insolation levels. The walls of the nanogrid incorporate PCM with a paraffin core (melting point: 25–30 $°\mathrm{C}$), featuring additives like graphite and silicon carbide to improve thermal conductivity and stability. Additionally, the structure includes specialized coatings, such as a photochromic cool white coating for overall thermal performance enhancement. Thermochromic vanadium dioxide blocks IR radiation, reducing energy demands and keeping the nanogrid cool. Coatings with titanium oxide and barium sulfate reflect 98% of visible light, sodium citrate reflects infrared light, and specific polymer paints reflect UV light, contributing to indoor thermal comfort. The design emphasizes high thermal conductivity, facilitated by additives like graphite and silicon carbide, which can increase thermal conductivity by up to 94.7% compared with pure paraffin. The size and configuration of heat transfer surfaces are critical in optimizing the TES’s efficiency. While carbon nanotubes offer superior performance, their high cost must be justified for residential use. Proper stratification and insulation are essential for maintaining stable inlet/outlet temperatures and efficient energy storage. Overall, this detailed nanogrid design effectively employs advanced materials and technologies to create a sustainable and energy-efficient residential system. Future studies should focus on economic analysis and practical applications to refine and validate the proposed design.

5. Future Works

The integration of phase change materials (PCMs) in nanogrids offers promising benefits, primarily by improving energy efficiency through latent heat storage. Latent heat storage is critical to managing heating and cooling demands in net-zero energy nanogrid buildings. However, challenges persist, including the economic feasibility of niche technologies like liquid sodium, carbon nanotubes, and graphene-based materials for residential applications. While advanced materials like nanocomposites do offer PCM performance, they entail higher costs and complex integration/retrofit in building applications. Current studies frequently lack long-term data, leading to uncertainties about the durability and degradation of PCMs in real-world conditions, and scaling these technologies for widespread residential use presents significant challenges. These gaps highlight the need for comprehensive economic analyses of each individual element discussed in this review, the feasibility of the range of technologies through optimization modeling, and long-term performance studies to evaluate the viability and reliability of PCM systems under varying climatic conditions and through multiple charge/discharge cycles. This review underscores a range of future research opportunities focused on developing PCM-integrated nanogrids. Future studies should utilize software tools such as TRNSYS, EnergyPlus, ASPEN, MATLAB/Simulink, ANSYS/COMSOL, HOMER Energy, Helioscope, Power Factory, and PSSE to conduct detailed analyses of nanogrids connected to main grids.

Future studies could explore the cost-effectiveness of advanced PCM technologies, such as liquid sodium and nanocomposites for residential nanogrids. By evaluating initial investments, operational costs, payback periods, and potential cost savings through tools like TRNSYS and EnergyPlus, researchers can identify economic viability. Further research should assess the long-term stability, durability, and degradation of PCMs under real-world nanogrid conditions to ensure lasting performance. These investigations should compare the economic and energy performance of PCM walls against traditional solar thermal collectors, conduct exergy analyses, and assess the techno-economic performance of various PCM materials integrated into buildings. Additionally, the impact of these integrations on overall energy demand must be evaluated. A comparative analysis of energy efficiency and economic viability between PCM-integrated walls and traditional solar thermal collectors using ANSYS/COMSOL and EnergyPlus could clarify the benefits of each approach. Exergy performance evaluations of different PCM materials for thermal energy storage, facilitated by ASPEN and MATLAB/Simulink, could offer insights into optimal material selection. Additionally, the impact of PCM integration on overall energy demand in residential nanogrids should be studied, with a focus on peak load reduction, energy savings, system efficiency, and energy balance using TRNSYS, Heliscope, and Power Factory. Conducting lifecycle assessments (LCA) to quantify the environmental impacts of PCM use, including global warming potential, embodied energy, and recyclability, using tools like SimaPro, OpenLCA, GaBi, TRNSYS, and EnergyPLAN, could clarify sustainability aspects. For scalability, researchers should investigate large-scale PCM applications in networks of nanogrids, identifying challenges and using TRNSYS, PSSE, Homer, and SketchUp for modeling. An overview of cross-disciplinary approach in PCM integrated nanogrid in summarized in Figure 12.

Further studies may compare the thermal performance of PCM-integrated wall systems (e.g., gypsum, concrete, bricks) across various climates via EnergyPlus and TRNSYS, focusing on energy savings and thermal comfort. Optimal PCM placement within walls could be examined using ANSYS/COMSOL to identify configurations that maximize thermal performance and minimize material use. Computational fluid dynamic (CFD) simulations can assess the heat transfer efficiency of hexagonal PCM structures for wall integration, and finite element analysis (FEA) could analyze other geometric PCM structures (e.g., hexagonal, honeycomb, lattice) for their heat storage and transfer capabilities. The stability and dispersion of nanofluids within PCMs should be studied using particle size analysis and thermal cycling tests, evaluating the effectiveness of nanofluid-PCM combinations. Integrating PCMs with advanced insulation materials like aerogels and vacuum insulated panels could further improve thermal management. Research into the effect of phase transition temperature tuning on PCM wall applications using ASPEN and EnergyPlus would also be valuable. Future smart wall systems integrating PCMs with sensors and control mechanisms for real-time thermal management, using IoT multi-agent systems in a nanogrid or microgrid, are worth investigating. Graphene incorporation in PCMs for enhanced heat transfer could be studied using COMSOL, and an in-depth analysis of nanogrids linked to main grids using TRNSYS, EnergyPlus, and MATLAB/Simulink could reveal performance optimization strategies.

Moreover simulations of PCMs with varied metallic and non-metallic capsules can be studies for thermmal performance and energy recovery, weighing cost and efficiency trade-offs. Studies on nanofluid types and concentrations for residential buildings could summarize the comparative performance. Additionally, hybrid PCM walls integrated with heat pumps, solar thermal collectors, and hydrogen storage may offer versatile thermal solutions. An analysis of PCM layer thickness and composition for walls using FEA in ANSYS would help determine optimal thickness for economic and energetic efficiency. Research into photochromic and thermochromic materials for PCM or building integration, focusing on color change efficiency and thermal durability, may offer energy-saving innovations. Integrating these materials in smart windows within passive houses or nanogrids, with an emphasis on exergy analysis and payback, would enhance thermal regulation. Finally, hybrid coatings with photochromic and thermochromic properties, along with long-term thermal cycling tests for PCMs in nanogrids, could reveal sustainable, efficient approaches to residential energy management. Interdisciplinary research teams, including electrical, mechanical, materials, energy, and architectural engineering experts, are crucial for addressing the complexities of PCM-integrated nanogrids. Collaborative efforts will ensure these systems are both economically viable and technically robust, advancing sustainable energy solutions for residential use.

6. Conclusions

The integration of phase change materials (PCMs) in nanogrids offers significant benefits, primarily enhancing energy efficiency through latent heat storage methods, which is vital for managing heating and cooling demands in net-zero energy buildings. Additionally, additives like carbon fibers and copper foam notably improve the thermal conductivity of PCMs, facilitating better energy management in residential nanogrids. However, challenges remain, such as the economic feasibility of advanced technologies like liquid sodium for residential applications and the stability and leakage risks associated with microencapsulated PCMs. Different viewpoints highlight the potential of advanced materials like nanocomposites to boost PCM performance, though these innovations often come with higher costs and complex manufacturing processes. Environmental benefits, such as the use of low global warming potential refrigerants in solar water heating systems, are promising, yet the overall lifecycle impact of PCM materials requires further scrutiny. Existing studies often lack long-term data, leading to uncertainties about the durability and degradation of PCMs in real-world conditions, and scaling these technologies for broad residential use presents significant challenges. Identified gaps include the need for comprehensive economic analyses and long-term performance studies to evaluate PCM systems’ viability and reliability under varying climatic conditions and through multiple charge/discharge cycles. By critically analyzing these aspects, this review provides a balanced perspective on PCM integration in nanogrids and underscores areas for future research.

The proposed concept is a holistic approach in designing a nanogrid. Each nanogrid house is designed to operate independently yet contribute to a shared microgrid, allowing water, electricity, data, and resource sharing. These nanogrids, when networked, form a “Network of Nanogrids” (NoN) with improved energy reliability. Each unit supports a family of 2–4, with tailored energy demands for seasonal variations (1800 kWh in winter, 1500 kWh in summer). Energy demands are divided into two main categories 60% thermal demands and 40% electrical demands. Each nanogrid includes a north-facing PCM wall (for optimal solar exposure in Australia), aiming for an airtight envelope with 0.6 to 2 air changes per hour and covering an area between 144–256 m². PCM is incorporated into the wall or attic (20 mm thick), enhancing the thermal envelope. A multi-layered wall structure (wood, foam and brick) aims to balance thermal mass, insulation, and low thermal mass for energy retention and efficiency. Each nanogrid is equipped with a range of technologies PV (5–10 kW), Battery (5–15 kWh), Tripple glazed windows (U = 0.4, g = 0.408, glass 2500 mm), hot water fired absorption chillers 2–5 kW with COP 0.8. TESS tanks are aimed to be in range from 200–500 L per insulated tank. The solar hot water system includes a 3–6 m² solar collector and TESS with 1.2 m height. The system uses eco-friendly refrigerants (HFOs R-1234yz and R-1234yf) with minimal global warming potential. Different heat transfer fluids (HTFs) enhance TES, including options like compressed air, supercritical ${\mathrm{CO}}_2$, and solar salt. For PCM storage, transpired and double U-tube solar collectors are recommended for efficient energy capture. The walls integrate PCM with paraffin, enriched with additives like graphite for enhanced conductivity. A variety of coatings (photochromic cool white, thermochromic vanadium dioxide) regulate heat absorption, with reflective layers to maintain thermal comfort. Coatings with titanium oxide and barium sulphate reflect visible light, while sodium citrate and polymer paints address infrared and UV light reflection, respectively.

This review paper paves the way for a multitude of future studies focused on developing phase change material (PCM) integrated nanogrids. Interdisciplinary research teams, including electrical, mechanical, materials, energy, and architectural engineering experts, are essential to address the complexities of PCM-integrated nanogrids. Collaborative efforts will ensure these systems are both economically viable and technically robust, advancing sustainable energy solutions for residential use. As we look towards “Building the future”, the focus must shift from merely innovating new PCM technologies to establishing international standards that ensure their effective and widespread adoption.