Storage Is the New Black: A Review of Energy Storage System Applications to Resolve Intermittency in Renewable Energy Systems

Abstract

As the need for more sustainable methods of power generation becomes increasingly apparent due to the planet’s ever-deteriorating conditions, the quest for sustainable power generation intensifies. Among the options for sustainable power generation, the utilization of solar and wind power in large-scale applications is problematic due to the intermittent nature of their sources. Multiple solutions exist to counteract this intermittency, but energy storage systems are the most appealing. This article reviews the intermittency in renewable energy systems that rely on solar and wind, and how energy storage systems are utilized to mitigate this issue. While energy storage systems integrated into solar and wind power generation systems exhibit promising synergy and benefits, their full implementation is still hindered by a variety of challenges, which opens different fields of research to circumvent these challenges.

1. Introduction

As the planet continues to experience changes in its climate due to the exponential increase in the use of fossil fuels—support both population growth and technological advancement—the necessity to find alternative and renewable sources for energy production is further intensified. A recent report declared that the year 2023 was the warmest year on record since global temperature tracking began in 1850, with ten of the warmest years occurring in the last decade (2014–2023) [1]. Statistics from the National Centers for Environmental Information show that while the increase in temperature is not uniform across the planet, the average rate of temperature increase since 1850 has skyrocketed to 0.11 °F (0.06 °C) per decade. Predictive models suggest that if yearly emissions continue to increase exponentially, global temperatures are expected to be at least 5 °F warmer relative to the 1901–1960 average temperature by the end of the century. However, should annual emissions decline significantly by the next half-century, global temperatures are still expected to be at least 2.4 degrees more than the previous half-century [2]. According to a synthesis report by the Intergovernmental Panel on Climate Change [3], human activities (through greenhouse gas emissions) are the primary drivers of global warming, resulting in a 1.1 °C increase in temperature from 2011 to 2020 relative to the years 1850–1900.

One of the most commonly suggested methods for altering the trajectory of the world’s climate away from eventual degradation is through the implementation and use of alternative methods for energy production, including enhanced power conversion systems through waste heat recovery, efficient energy conversion devices, and renewable energy systems [4]. One popular method being investigated involves the use of renewable energy systems; this has always been the top choice because of its ability to reduce reliance on fossil fuels. Energy production through renewable resources often relies on solar, wind, biomass, and hydropower as the primary “fuel” sources. While renewable energy systems are the most appealing, two of these energy sources are intermittent: solar and wind. Intermittency is an issue that needs to be addressed should reliable large-scale energy production using solar and wind as “fuel” sources be deemed viable.

The objective of this review is to highlight the applications of energy storage systems in resolving intermittency in renewable energy systems that rely on solar and wind power, as well as discuss possible future developments. This review also discusses how solar and wind energy are harnessed to develop energy, the origins of wind and solar power, renewable energy system integration on microgrids and smart grids, how intermittency affects their widespread and large-scale application, what energy storage systems (as well as their types) are, and how these storage systems can help address intermittency.

2. Literature Review

2.1. A Brief Overview of the Origins of Solar and Wind Power

The photovoltaic effect was serendipitously discovered by Becquerel while experimenting with an electrolytic cell composed of two metal electrodes submerged in an electricity-conducting solution that generated electricity when exposed to light [5]. From this discovery, major developments were made that would improve upon Becquerel’s discovery. Toward the end of the 19th century, the photovoltaic effect in selenium was discovered by Willoughby Smith, W.G. Adams, and R.E. Day. Through C.E. Fritz’s investigation into the PV effect of selenium, he discovered “a continuous, constant, and considerable” force when exposing an amorphous sheet of selenium with transparent gold film to sunlight. However, there was skepticism toward this claim due to the absence of quantum theory at the time.

When quantum mechanics was discovered, it highlighted the importance of single-crystal semiconductors. Following the invention of a silicon single-crystal solar cell by Bell Labs in 1954, research and development of solar cells through silicon was prevalent, with the aim of increasing their efficiencies. In the late 1970s, multi-crystalline wafers were discovered as great materials for manufacturing efficient solar cells [5]. After years of research and development, in 1980, ARCO Solar produced 1 megawatt of photovoltaic modules, and in 1982, the first photovoltaic megawatt-scale power station was established in California by ARCO Solar. Eventually, the largest photovoltaic power plants in the world could be found in India, China, and the United States, e.g., Bhadla Solar Park (2245 MW in 2018), Longyangxia Dam Solar Park (2130 MW in 2015), and the Ivanpah Solar Power Facility (377 MW in 2014) [4].

The first windmill rotating on a horizontal axis can be traced back to the Duchy of Normandy in the northwestern part of Europe. From this region, windmills of this configuration began spreading across the northern and eastern regions of Europe, even reaching Finland and Russia [6]. The American windmill counterpart can be traced back to the early 19th century when windmill technology reached its apex in Europe. Large windmills were primarily used in the Americas for pumping water. The first attempts at utilizing windmills for energy generation can be found in both Europe and America.

In Europe, Poul la Cour was one of the most prominent pioneers of electricity generation using wind power technology. He was a physicist who taught physics and life sciences at a school for general adult education; in 1890, he built a small wind turbine that produced electricity. So rural areas of Denmark could benefit from this, in 1903, la Cour and his 21 partners established Danish Wind Electricity; they trained individuals to construct small local electricity plants powered by wind turbines in rural areas. This movement heavily contributed to the electrification movement in Denmark [7]. In America, a similar need to provide power to remote areas was what motivated brothers Marcellus and Joseph Jacobs to build wind turbines for loading batteries in the 1920s. After multiple attempts at two-bladed turbines, they settled for a three-blade turbine with a rotor diameter of 4 m, coupled with a direct-drive direct current generator. These turbines became popular throughout 1920–1973 [8]. Eventually, the first megawatt plant that utilized wind power was built in Rutland, Vermont, via the Smith–Putnam partnership, operating from 1941 to 1945. Operations were shut down in 1945 as a result of the loss of a rotor blade whose replacement was out of the financial capacity of the plant. Toward the modern age, wind farms were often built as a response to climate change. Two of the largest installed capacity wind farms are the Gansu Wind Farm in China (7965 MW) as shown in Figure 1; and the Atla Wind Energy Center in the United States (1550 MW) [4].

2.2. Solar and Wind Power Generation

It was previously mentioned that the photovoltaic effect is primarily responsible for the conversion of solar energy to electricity. Solar power plants that rely on this phenomenon are dubbed photovoltaic power plants. Another type of solar power plant does not rely on the photovoltaic effect but rather, on concentrated solar heat. The concentrated solar heat is then used to power a steam turbine to generate electricity [11]. Of the two types of solar power generation, the most common (and most utilized) is photovoltaic power.

Photovoltaic power plants rely on the photovoltaic effect; hence, the most essential components in these power plants are photovoltaic panels. These panels are grouped and arranged in such a way that they can optimally harvest solar energy from the sun to convert to electricity to be sent to the grid. The photovoltaic panels for these power plants can be mounted on the ground or onto a roof. Regardless of how they are mounted, it is paramount that they are on a tilted angle to maximize solar energy harvesting. Axis trackers are sometimes employed to optimize photovoltaic panel performance, allowing the panels to track the sun as it moves. Once enough thermal energy is amassed, these solar panels directly convert it into direct current electricity. An inverter is then used to convert it from a direct to an alternating current. This type of power plant is best situated in open areas to produce the most energy to fulfill peak demand. However, due to the intermittency of solar power, this type of power generation is only limited to peak demand and not baseload demand [11].

On the other hand, concentrating solar power systems use high-temperature heat collected from concentrated solar energy using solar collectors and are often used in conjunction with any conventional power cycle (or in conjunction with fossil fuel plants) [12]. Only direct radiation from the sun can be concentrated through the use of optical systems; hence, solar trackers are also employed to maximize the collectors’ exposure to solar energy. The collected concentrated sunlight is then brought to a heat exchanger, where absorbed energy is transferred to a heat transfer fluid, as depicted in Figure 2. CSPs are often used to directly power a cycle or circulate heat in a secondary cycle. However, there are currently no power cycles that are developed to match the high temperature of the concentrating solar systems, to which only conventional fossil fuel-driven power-generating systems are adapted to take advantage of.

For wind power generation, a wind turbine is designed to absorb energy from blowing winds using specially designed blades and then convert it to mechanical energy to drive a generator to produce electricity [14]. There are two commonly used types of wind power generation systems: direct-driven wind power and double-fed wind power. Their difference lies in the type of conversion used to connect them to the grid. A direct-driven wind power-generating set operates on the basic principle of converting wind energy into mechanical energy, and then into electrical energy. To transfer electricity from the turbine to the grid, several components are present. First, the stator produces AC power of variable amplitude and frequency. This is then passed to an AC/DC rectifier to convert the AC power to DC power and then convert it back to AC power through the use of an inverter. The inverter’s AC power is then transmitted to the grid. In contrast, the double-fed wind power has its stator connected to the grid directly. Its rotor can also supply power to the grid via a generator. As such, both the stator and rotor in this setup can supply electricity to the grid.

2.3. Renewable Energy Systems on Smart Grids and Microgrids

At its current state, the present conventional grid network is focused simply on the conversion of fuel to electricity for unidirectional distribution (from the power plant to wherever the electricity is needed). According to Kataray et al. [15], 8% of produced electricity is wasted during transmission while 20% of its ability to produce electricity is to meet the peak demand. Additionally, the modern electric power system still has issues to address in terms of increasing pollution borne from the energy sector, such as greenhouse gas emissions, diminishing fossil fuel resources, and changes in the planet’s climate [16]. With the increasing need to transition to renewable energy sources, a grid must be developed that (1) is capable of supporting the primary grid but is isolated from it and (2) is capable of connecting all new electricity sources and extending its coverage, while also accommodating the integration of renewable energy systems.

Microgrids meet the first criteria for a new grid. A microgrid is a small-scale electricity distribution network that can operate on its own or in conjunction with the main grid [17]. As such, these grids are capable of providing electricity to more remote areas. Microgrids are usually composed of one or more distributed energy sources such as solar and wind, making the penetration of renewable energy systems desirable to electricity networks [18]. While promising, renewable energy system integration in microgrids still poses critical issues that must be addressed to ensure effective implementation. Due to the inherently smaller sizes of microgrids compared to the main grids, they are more sensitive to power fluctuations, exacerbating the need to account for power dynamics, flexibility, and production uncertainties [19]. Its dependency on renewable energy sources also hampers its effectiveness due to the intermittency of these sources. However, when successful, microgrids can help reduce carbon emissions, enrich energy security, and promote local energy generation. To some extent, microgrids can serve as testbeds for new technologies in the energy sector [20], such as enabling smart grids.

A smart grid is formed by integrating and using advanced sensors, and modern communication technology, and overseeing and controlling power flow in a bid to increase efficiency, reliability, and sustainability of power distribution systems. Compared to present conventional electrical grids, smart grids are capable of enabling a two-way communication channel between the consumer and provider, resulting in more effective and efficient monitoring and control of energy usage [21]. Apart from this, the development of a smart grid confers other benefits, including storage solutions, optimized utilization of electricity, and promotion of renewable energy systems [22]. However, to take advantage of these benefits, smart grids must meet requirements surrounding energy conservation, cleaner technologies, renewables, carriers, and energy storage [23]. Similar to microgrids, the success of smart grids relies on the successful integration of distributed energy from solar and wind sources, which are intermittent by nature. To mitigate the uncertainties that come from intermittent renewable energy sources, it is necessary to explore options that would promote flexibility in these systems and reduce the effect of intermittency [24].

2.4. Intermittency of Solar and Wind Power Generation and Its Implications

From the main grid to potential novel forms of an electrical grid (microgrids and smart grids), the intermittency of renewable sources is a hurdle that must be overcome. To understand the intermittency in solar and power generation, one must first look into what is expected from power generation. The minimum amount of power expected from any power-generating system to meet electricity demand is called baseload power. As such, baseload power plants are expected to nearly constantly supply power, delegating them as the backbone of everything that utilizes electricity [25]. As the backbone of the industry, even with calls to shift to renewable energy sources becoming more pressing each day, baseload plants that rely on large coal and natural gas still operate because they can provide a near-constant supply of electricity to sustain human activity. Intermittent power is the very antithesis of baseload power. Intermittent power is defined as electrical energy that is not constantly available due to external forces that cannot be controlled, which affects its widescale application in the power generation industry. Wind and solar are inherently intermittent renewable sources because of the factors that affect their availability, such as the season and time of day. Due to these reasons alone, the development of reliable power generation systems that utilize wind and solar is heavily impeded.

While it is evident that solar and wind power generation systems can be used to an extent without integrating them into the main electricity grid, their actual integration into the grid (when necessary) will prove to be challenging. The introduction of intermittent renewable energy sources, such as wind and solar, may also pose challenges to the management of the electrical network [26]. Two challenges exist, namely, balancing production/consumption and the impact of intermittent renewable energy sources on electrical grid management.

To maintain the balance between the production and consumption of electricity, whenever it is produced, it must also be consumed once sent to the grid. The grid must be able to maintain a balance between the electricity it receives and the electricity it will impart at any given moment. As such, electricity production must adapt to the changes in demand to maintain a delicate balance. Should the balance be disrupted due to a loss of production in the renewable energy system (due to uncontrollable and external circumstances), certain actions must immediately be taken to avoid disastrous consequences. If the electricity frequency goes below the reference frequency, a rapid increase in electricity production or the start-up of a backup system must be performed in order to compensate for the loss. However, the start-up of a backup system is not instantaneous, and for every minute lost, an increase in an imbalance of production/consumption leads to load shedding, which could lead to a blackout if it is too slow or insufficient. The power system is dynamic and subject to variability that can be both expected and unpredictable [27]. The reliability of a system depends on its ability to accommodate and adapt to these changes while maintaining quality output. If an inherently intermittent system is to become a reliable source of electricity when integrated into the main electrical network, it must overcome the causes and consequences of its intermittency.

Electrical grid management becomes more complex once intermittent renewable energy systems are integrated [28]. The variability of wind and solar resources requires additional and complex actions to maintain system balance, accommodating for supply-side variability. The introduction of wind or solar systems into the grid also introduces a constraint: its source [29]. The power generated by wind or solar systems is not guaranteed due to the intermittent nature of their sources. Solar and wind are not always present when they are needed and sometimes present when they are not needed. This variability leads to the necessity for a conventional energy system to compensate for the intermittency in power, which increases the complexity of planning and managing the electrical network. As such, while the upsurge in renewable energy system installations is a great step toward reducing CO₂ emissions, the inability to fully load these systems leads to an incredible amount of waste electricity generated.

2.5. Photovoltaic and Wind Renewable System Penetration in Power Systems

While renewable energy systems have gained much traction in the past, as evidenced by the increase in photovoltaic [30] and wind renewable energy systems [31] in various countries, there are issues surrounding higher levels of penetration from photovoltaic and wind power generation systems, as they suffer from intermittency, which causes oscillations in the voltage and frequency of the power system [32]. High penetration levels of photovoltaic systems in the distribution system may cause voltage fluctuations and voltage regulation problems, islanding detection during grid disconnection, and decreased reliability and security of the distribution system. On the other hand, high levels of penetration in a standard distribution system may lead to unpredictable output power predictions, impacting distribution system frequency, and causing power quality issues [33].

“Hosting capacity” is a term used to describe the acceptable levels of renewable energy system penetration. The hosting capacity refers to the threshold—when exceeded, it results in any of the listed possible impacts to the distribution grid (and mitigation will be required to resolve them) [34]. By identifying the hosting capacity of a power system, the penetration amounts coming from these intermittent renewable energy sources—without any additional investments to the grid—can be determined. Multiple methodologies have been employed in order to ascertain the hosting capacity of a particular power distribution system, as this limit may be different from one grid to another. Three hosting capacity quantification methods, namely, deterministic, stochastic, and time series, may be used to quantify the hosting capacity of a particular grid. Deterministic methods assume grid simulation inputs are fixed and known, resulting in an exact hosting capacity value. When uncertainties are introduced to the method, they become stochastic and the hosting capacity value obtained is in the form of a probability distribution [35]. Time series have an edge over the two previous methods as they can consider all time dependencies and correlations. However, the downside of this method is that it requires grid parameter measurements over a longer period.

Multiple methodologies have been employed in order to increase the hosting capacity of a grid to accommodate for higher penetration levels of renewable energy systems (without adversely impacting the power distribution system). These include, but are not limited to, demand-side management methods, voltage and reactive power controls, virtual power plants, energy storage systems, and/or a combination of these methodologies. Demand-side management methods enable the ability to control end-user devices by rescheduling their operations to periods when large amounts of renewable energy are available, and ceasing their operations when demand levels increase [36]. Demand-side management methods focus on load shifting and energy efficiency plans to decrease overall energy consumption. In one case study [37], the impact of demand-side management methods on the electricity mix on Flores Island in the Azores archipelago was investigated. The results of this case study indicate that the employment of demand-side management methods significantly delayed the investment in new generation capacity from renewables while improving the operation and capacity of the current renewable system.

Optimizing voltage and reactive power control is a viable method to increase the hosting capacity to accommodate higher levels of penetration without altering the system’s limits. Active power fluctuation from renewable sources, especially photovoltaics, is considered the leading cause of voltage fluctuations in distribution networks [38]. These voltage fluctuations can cause power quality deterioration and decreased reliability of the distribution system. With the introduction of renewable energy systems, voltage regulation has become a problem. These devices were once easily controlled by on-load tap changers and reactive power compensation from capacitor banks, now they can no longer effectively react to fast changes in energy produced from renewable systems due to their low lifespans and slow response times [39]. As such, there is a need to optimize methods to account for renewable energy systems. In Reference [40], on-load tap changers, capacitor banks, a photovoltaic system, and a battery energy storage system were coordinated into two levels: the upper level (to plan the operations of the components) and the lower level (to control real-time bus voltage fluctuations). The simulation results of the study indicate that the proposed coordination of the components can control bus voltage variations in all feeders and allow the distribution network to operate at higher levels of photovoltaic power systems while maintaining the economy and lowering losses.

To account for distributed renewable energy sources, smart distribution is an effective method to increase energy quality at the lowest cost. A virtual power plant is a smart distribution that reduces the reliance on electricity from the grid by enlisting the benefits from distributed renewable energy sources. A virtual power plant is composed of distributed generators and energy storage systems that act and function similar to a conventional power plant [41]. An energy management system can be employed to schedule and control the components involved in the virtual power plant using information and communication technologies (ICTs). Through virtual power plants, renewable energy can be supplied to the public grid without requiring large-scale infrastructure, decrease instability by optimal scheduling, and improve social equity in renewable energy. The use of energy storage systems to help resolve intermittency and increase the hosting capacity of distribution systems will be discussed in the following sections.

2.6. Energy Storage Systems and Their Types

According to the available data, renewable energy systems (primarily those that rely on photovoltaics and wind) are expected to contribute to about two-thirds of renewable energy growth. Roughly 18% of electricity generation is expected from solar energy and roughly 17% is expected from wind [42]. Glaring issues on the use of intermittent renewable energy sources may discourage their widescale application, but this has not deterred any efforts in making the impossible, possible. One of the solutions being investigated to resolve the issues of intermittency in renewable energy systems and to increase hosting capacity is the integration of energy storage systems.

To account for the intermittencies in both solar and wind sources while maintaining the balance between electricity generation and consumption, energy storage systems are regarded as some of the most realistic and effective choices. By introducing electricity storage systems into wind and solar power generation systems, there is potential to optimize and resolve [43] the issues mentioned in Section 2.4. This is due to the fact that energy storage systems are designed to harness different energies from different sources and store them for eventual use somewhere else. With energy storage systems, the balance between generation and consumption is met as these systems enable the storage of energy when demand is low and release energy when demand is high [44]. Apart from this, the use of energy storage systems is beneficial in increasing the penetration of renewable energy systems into the main grid by enhancing the flexibility and stability of power systems [45].

Energy storage systems function by first converting electricity into another form of storable energy (e.g., mechanical, thermal), only to be converted back to electricity when the moment it is to be used arrives. There are many energy storage applications and techniques that cater to the fundamental types of energy: mechanical, chemical, and thermal [46]. An energy storage system application can be classified based on what type of energy it converts electricity from and stores it in that form [47]. Mechanical storage systems include pumped hydro-storage systems, compressed air storage systems, and flywheel energy storage systems. Thermal storage systems can be classified as sensible and latent heat systems. Chemical storage systems include hydrogen energy storage systems.

2.6.1. Mechanical Energy Storage Systems

Mechanical energy storage systems, as their name suggests, transform electrical energy to mechanical energy during off-peak hours when electricity demand is low, and back to electrical energy when demand is high. Energy storage systems that can be classified under mechanical energy storage systems include flywheel energy storage systems, compressed air storage systems, and gravity storage systems. Both flywheel and compressed air storage systems can be further categorized under low- to medium-power applications while gravity storage systems can be categorized under large-scale system applications [46].

Flywheel energy storage relies on the use of a large flywheel fixed on a stator via magnetic glide bearings. These magnetic glide bearings act as the major components in flywheel energy storage systems [48]. The flywheel energy storage system application is placed in a vacuum to reduce wind shear. During the charging state of the flywheel, the flywheel system rotates at very high speeds to store the kinetic energy. When there is demand for electricity, the flywheel discharges its stored energy to an electric motor to produce electricity [47]. To store energy fit for electrical power systems, high-capacity flywheels are needed. Large installations made up of forty 25 kW-25 kWh systems are capable of storing 1 MW of energy in a matter of an hour. However, long-term storage using flywheels is not viable due to friction losses [46].

Compressed energy storage utilizes compressed air to store energy, which is fitting for any excess energy generated from renewable energy systems, as illustrated in Figure 3. The most important components in this type of energy storage are compressors operated by prime movers such as motors. There are five stages in compressed energy storage: simultaneous extraction of heat during charging, storage, discharge to expanders, electricity generation via generators coupled to turbines, and delivery. This type of energy storage system is efficient for small-scale applications of energy storage but is at a disadvantage for large-scale applications. This is due to the greater heat losses as a result of compressing more air, which can be mitigated by having a system in place to collect and store waste heat to be reused in the expansion stage [49]. Small-scale applications are best suited for the continuous and uninterrupted supply of power while large-scale applications depend entirely on the availability of accessible and impermeable caverns for air storage and pressurization. Small-scale applications of compressed energy storage have capacities reaching up to 10 kW while large-scale applications have capacities reaching up to 100 MW.

A pumped hydro energy storage system relies on the gravitational potential energy of water to store energy. The amount of energy that can be stored using this application depends on the volume and height of the water. When demand is high, water is allowed to flow from a higher reservoir to a lower reservoir to activate the turbines for electricity generation [50]. Energy is stored by pumping the water uphill using off-peak electricity and then letting the water flow back down when electricity production is needed. This energy storage system application is currently commercially available and comprises 99% of the installed energy storage system capacity [51]. Pumped hydro storage is widely and primarily used for large-scale and high-power applications and is the leading energy storage system for photovoltaic and wind power generation systems.

Figure 3. Diagram of a compressed air energy storage system. Reprinted/adapted with permission from Ref. [52].

Figure 3. Diagram of a compressed air energy storage system. Reprinted/adapted with permission from Ref. [52].

2.6.2. Heat Energy Storage System

Thermal energy storage systems reserve thermal energy by heating or cooling a medium for storage to be used at a later time for power generation, especially in renewable energy systems. Thermal energy storage is considered a useful addition to concentrating solar power systems rather than solar systems that rely on photovoltaics [53]. Apart from assisting in resolving the intermittency of solar power generation systems, thermal energy storage systems have been shown to improve performance and thermal reliability in solar power generation systems. Thermal energy storage can be further divided into sensible heat storage systems and latent heat storage systems.

In sensible heat storage systems, thermal energy is stored by heating or cooling a solid or liquid medium, preferably water since it is the cheapest option and does not pose a toxic risk. Underground storage of the storage media is often reserved for large-scale applications while water tank storage [54] is considered the most cost-effective option. A sensible heat storage system exploits the heat capacity and changes in temperature of its storage medium during the charging and discharging of energy. The amount of thermal energy stored in the medium depends on the specific heat, temperature change, and amount of the material [55]. A latent heat storage system, on the other hand, relies on the medium’s property to release or absorb energy when it undergoes a phase change. The heat in this system is stored in the phase-change process of its medium; therefore, it relies on the medium’s latent heat.

Cryogenic energy storage systems exploit temperatures significantly below ambient temperature ranges. Various materials in both solid and liquid phases can be used as storage materials for cryogenic energy storage systems such as rocks and salt solutions [56]. Of the two phases used as storage material for cryogenic energy storage systems, the use of liquid phase materials seems to be more popular. In standalone cryogenic energy storage systems utilizing liquid air, air liquefaction occurs during off-peak times and power generation occurs during peak times. At peak hours, liquid air is pumped and heated by ambient heat and then superheated by stored compression heat before being expanded to produce electricity. Compared to other storage technologies, cryogenic energy storage systems have high energy density, standardized equipment, and large charging-discharging capabilities [57].

2.6.3. Chemical Energy Storage Systems

Chemical energy storage systems are best suited for the long-term storage of chemical energy. Energy is primarily stored in the bonds between a material’s atoms and molecules and is released whenever a chemical occurs. When the energy is released, old chemical bonds break down and new ones are formed [57]. Chemical fuels currently dominate the electricity generation industry; they are first transformed into mechanical energy before being converted into electricity. Chemical energy storage systems include hydrogen energy storage systems.

For hydrogen energy storage systems, hydrogen is used as the ideal energy carrier as it is readily available, carbon-free, and zero-emissions. One of the ways renewable energy can be stored is through a hydrogen energy storage system [58], which is also considered a relatively inexpensive method of storing renewable energy. A typical renewable energy system integrated with a hydrogen storage system is composed of a renewable energy source, an electrolyzer, a hydrogen storage system, and a hydrogen energy conversion unit (usually in the form of fuel cells). Hydrogen is produced from either water through electrolysis or sunlight through a photocatalytic water-splitting method [43]. The hydrogen obtained from these sources is then stored in a hydrogen storage tank while oxygen produced from the electrolysis process is released into the atmosphere during the charging phase. When demand is high and power availability is limited, electricity is produced from stored hydrogen using fuel cells and is discharged to the grid.

2.6.4. Electrical Energy Storage Systems

Electrochemical energy storage systems have supercapacitors at the forefront of their energy storage capabilities. Supercapacitors are further divided into two types, namely, the electric double-layer capacitor (EDLC) and the pseudocapacitor. The EDLC typically consists of two electrodes with a non-faradaic capacitive storage mechanism. Pseudocapacitors can follow two configurations: two electrodes with a capacitive faradaic mechanism or a combination of one NFCS electrode and a CFS electrode [59]. The EDLC’s primary storage mechanism relies on a physical reaction in the form of electrolyte ions being absorbed into the ELDC when charging [60]. In general, the ELDC supercapacitor has the highest power density, but the lowest in terms of energy density due to the charging mechanism, only relying on electrolytic adsorption. Pseudocapacitors, on the other hand, store energy using faradaic redox reactions, which have energy storage capabilities in accompaniment with EDLCs. Reduced adsorption sites and performance may result from poor control of the pseudocapacitor’s electrode film [61].

In photovoltaic and wind systems, supercapacitors have replaced batteries as the primary energy storage systems to avoid the limitations associated with the use of battery storage systems, such as lifespan, slow charge and discharge, and low power density [62]. A supercapacitor can be integrated into solar and wind power applications to help alleviate inconsistencies in power generation due to their intermittent nature. While supercapacitors can replace batteries in energy storage system applications, a hybrid energy storage system combines a supercapacitor and a battery to address the imbalances in power conversion and storage [63]. The hybrid system is more useful in areas where grid coverage may be insufficient. The hybrid system is capable of achieving this by having the supercapacitor handle power fluctuations with high frequencies over a short duration while the battery handles power fluctuations over a longer duration [64].

Another type of electrical storage system is the superconducting magnetic storage system (SMES). Superconducting magnetic energy storage systems rely on the superconductivity of certain materials. The main operating principle that this energy storage system relies on is the notion that the current will still flow in a superconductor even though the voltage across it is removed [65]. The current flows in one direction during the charging phase of the energy storage system where the power conditioning system must maintain a positive voltage across the superconductor’s coil to store the energy. During discharge, the power conditioning system is conditioned to mimic the system as a load across the coil by reversing the voltage leading to the discharge of the stored energy [66]. A typical SMES is composed of four primary components: the magnetized superconducting coil, power conditioning system, cryogenic system, and control system. The superconducting coil is the most important component in the system as it is where the energy is stored. The size of the coil is determined by the amount of energy to be stored. The power conditioning system, on the other hand, serves as the system’s interface between the superconducting coil and the alternating current power system. A cryogenic system is utilized to keep the coil of the SMES at a low enough temperature (at −269 °C) to sustain its superconducting state [67]. The control system is responsible for the connection between the power grid and the power flows flowing in/out of the energy storage system.

2.6.5. Effectiveness of Each Energy Storage System Type

Along with an overview of how distinct each type of energy storage system is in terms of what these systems are capable of doing, how they operate, and the primary components they utilize to achieve energy storage, it is also important to understand the effectiveness of each for application. The following table summarizes the limits of each energy storage type in terms of energy density, power density, discharge time, lifespan, and efficiency.

From

Table 1. Comparison of energy storage system type capabilities [59,66,68,69,70,71,72,73].

Category	Type	Power Density (W/kg)	Energy Density (Wh/kg)	Discharge Time	Lifespan (Years)	Efficiency
Mechanical	FES	400–1500 [68]	10–30 [68]	≤15 s [68]	≤25 [69]	90–95% [70]
	CAES	30–60 [68]	0.5–2.0 [68]	≤24 h [68]	20–40 [68]	60–80% [59]
	PHES	-	0.5–1.5 [68]	≤24 h [68]	30–60 [59]	65–85% [59]
Thermal	SHSS	-	70 [66]	1 h [66]	15–20 [66]	60–90% [66]
	LHSS
	CES	-	100–200 [71]	≤12 h [72]	≤40 [72]	40–50% [71]
Chemical	H2ESS	≤10,000 [68]	≥500 [68]	≤24 h [68]	5–15 [68]	30–40% [73]
Electrical	Supercapacitors	10,000 [73]	2.5–15 [68]	≤1 h [68]	≤10 [73]	95% [73]
	SMES	500–5000 [66]	0.5–5 [66]	≤1 min [66]	20–40 [66]	95–98% [66]

, the use of supercapacitors as energy storage systems is the most effective when considering the short-term lifespan of a system. However, when considering a system with the longest longevity, pumped hydro energy storage trumps the rest. In terms of efficiency, the superconducting magnetic energy storage system has the highest efficiency while cryogenic energy storage has the lowest. While some electrical energy storage systems may outperform others based on their characteristics, it is important to note that the use of an energy storage system should also consider other factors, such as the availability of resources in the location, the type of available renewable resource in the location, and the cost-effectiveness of its implementation.

3. Applications and Developments in Energy Storage Systems for Renewable Energy Systems

As the most attractive solution to resolving intermittency in renewable energy systems, various developments and applications have been conducted throughout the years. Energy storage systems covered in this section are those that were covered in Section 2 of this review. These are the flywheel energy storage system, compressed air storage system, pumped hydro energy storage system, thermal energy system, and hydrogen energy storage systems.

3.1. Flywheel Energy Storage System

Renewable energy systems have been shown to remedy the latter’s intermittent issues to some extent. Flywheel energy storage systems have shown a range of applications, from small-scale residential to large-scale power grid applications. In power generation systems relying on wind and solar as resources, typical flywheel applications have shown improved grid frequency regulation, power smoothing [74], and reduction in fuel consumption.

System reliability decreases as a result of the intermittency in renewable energy system sources in the forms of wind and solar. It has been found that among all the energy storage systems employed for power smoothing, the flywheel energy storage system is the most reliable [75] due to its rapid response to variations in output power. The integration of a squirrel cage induction flywheel storage system into a wind power system has shown that at least 30% of wind power generated [76] from a doubly-fed induction power wind motor system was absorbed and stored when overclocked, which assists in compensating for fluctuations in wind and improving grid quality.

In Reference [77], a hybrid system consisting of wind turbines and photovoltaic cells did not achieve a reduction in fuel consumption by itself. This was the result of diesel generators consuming 40% of the fuel, even when unloaded. Since solar and wind are intermittent, the diesel generator must be turned on when there is demand and turned off when there is no demand. The introduction of a flywheel energy storage system was seen as a solution to fluctuations in both wind and solar and the frequent start/shut-down cycles of the diesel generator, thereby reducing fuel consumption and power fluctuations.

The first large-scale application of a flywheel energy storage system in the world was a high-penetration photovoltaic diesel power station in Nullagine and Marble Bar, Australia [78]. The flywheel storage system is operated as an uninterrupted power supply system that maximizes solar power injection during peak solar hours and switches to diesel generators when intermittencies in solar power begin to manifest. The integration of the flywheel energy storage as an uninterrupted power supply has resulted in saving at least 405,000 L of fuel each year, reducing greenhouse emissions by 1100 metric tons. The photovoltaic system was also shown to be able to produce 60% of the average daytime electricity consumption of both towns, amounting to at least 1 GWh of energy.

3.2. Compressed Air Energy Storage System

Compressed air storage systems are widely being investigated and considered for enhancing the performance and reliability of renewable energy systems. They are also said to be the most efficient type of storage system compared to others due to their longevity, fast start-up times, volume of energy storage capabilities, increased energy production, and capital costs [79]. Most research and development on compressed air storage systems focus on their integration with renewable energy systems, where the storage of energy becomes necessary when there is a surplus in production.

Reference [80] looked at the feasibility and performance of a compressed air storage system integrated into a hybrid solar–wind-driven energy system with multi-stage desalination. Mechanically stored energies in the underwater balloons of the compressed air storage system were utilized to generate electricity via gas turbines in multiple stages. The low-pressure steam produced from the generation of electricity in the turbine was then used for the desalination process of seawater to provide potable water. Exergy and energy efficiencies of the organic Rankine cycle used for power generation from waste heat during air compressions were 48.6% and 19.36%, respectively. The entire system is capable of producing 365 GWh of energy from wind and solar alone; the authors claim it is enough to liberate Antigua and Barbuda from its reliance on the importation of heavy fuel oil-based energy supply. The system is also capable of producing 376.4 tons of drinking water from its multi-stage desalination processes.

In one case study [81], a 2 MW wind turbine was integrated with a small-scale compressed air storage system to investigate the capabilities of a compressed air energy storage system to suppress fluctuations in power generated from a turbine. The case study also provided a dynamic model to represent how the wind turbine, compressed air energy storage system, and grid may interact, while considering different factors, such as wind speed variations, energy storage dynamics, and grid interactions. The simulation of the dynamic model shows that the power output of the 2 MW wind turbine integrated with a 2 MW compressed air energy storage system was able to decrease power fluctuations to about 38% while also extending stable operating periods to about three hours.

A hydrogen-fueled compressed air energy storage system integrated into a photovoltaic power generation system was investigated [82]. The system configuration was designed so that the system enables the connection of other intermittent energy systems in renewable energy-saturated grids. The performance (on a yearly and seasonal basis) was evaluated for a scenario involving an actual electric grid saturated with renewable energy penetration. The integrated system displayed promising results as it was capable of reducing photovoltaic loss by 50% to as low as 4%, achieving 62% overall system efficiency. As such, the integrated system is effective at achieving zero emissions while also integrating new intermittent renewable sources into the grid and reinforcing already saturated ones during the summer and spring.

3.3. Pumped Hydro Energy Storage System

Pumped hydro energy storage systems are capable of improving the performance of both wind and solar renewable energy systems. For wind power plants, pumped hydro energy storage systems have been found to address the curtailment of power generation by providing large-density storage and the ability to adapt to large variations in power generation. For solar power plants, harnessing solar energy during peak hours is maximized due to a pumped hydro energy storage system’s large energy reserves, enabling power generation when solar energy is no longer available [83].

Recent studies on the integration of pumped hydro energy storage systems and wind and solar renewable energy systems focus on system reliability and feasibility. For the reliability of wind systems, recent advances and developments focus on circumventing potential energy losses due to wind curtailment. In a study conducted to minimize wind curtailment [84] and boost system reliability, an integrated system was designed to utilize the excess power generated from wind for desalination purposes and to store excess power in a pumped hydro energy storage system. The results show that the operational strategy employed for the integrated system was capable of achieving 84% renewable energy production from both wind and the pumped hydro energy storage system, and avoiding 67% of CO₂ emissions that were forecasted for the year 2020. In a study performed to determine the feasibility of wind-pumped hydro energy storage systems, two dams in Lebanon were analyzed to determine which dam would be the most suitable for the design of a hybrid wind–hydro energy station to store excess wind energy at night [85]. Both dams were evaluated based on the amount of water that can be pumped and the energy that they can produce from the excess electricity. The results indicate that one dam has an energy production range of 17 MWh to 698 MWh and another with a production range of 17 MWh to 768 MWh. The dam with a production range of 17 MWh to 698 MWh was shown to have lesser costs at a minimum of 0.044 USD/kWh, making the installation of a pumped hydro energy storage at that dam more feasible.

For an integrated solar–hydro pumped electro storage system, as illustrated in Figure 4, a novel off-grid small power generation system simulation was proposed for remote and rural areas [85]. The system—developed through a mathematical model—showed its reliability by displaying its uninterrupted power generation capabilities even when there were changes in solar irradiation. The results show that, despite intermittencies from solar irradiation, the integrated solar–hydro pumped electro storage system outputs 300 W and 230 V at both day and night times. The appeal of the proposed system lies in its ability to be installed anywhere, but preferably for rural areas where electricity is scarce.

3.4. Thermal Energy Storage Systems

Thermal energy storage systems are primarily deployed in systems reliant on the thermal energy that can be harnessed from the sun. Apart from its reliance on storing solar thermal energy, the installation of thermal energy storage systems for concentrating solar power systems proves to be cheap and easy to install, compared to other choices for energy storage [86]. According to multiple sources, the current capital cost of equipping concentrating solar plants with thermal energy storage systems is around USD 20–25 per kWh [87], while the cost of equipping concentrating solar plants with storage systems with a four-hour capacity is around USD 345/kWh [88].

Thermal energy storage systems require little space while being able to store huge quantities of energy, allowing them to compete with other energy storage systems. Two 50 MW concentrating solar plants in China employ thermal salts as their primary thermal energy storage systems. Both concentrating power plants have exhibited uninterrupted power generation for up to fifteen hours without interruption, regardless of the availability of solar irradiation [89]. For comparison, a concentrating power plant equipped with a thermal energy storage system utilizing molten sand can run a steam turbine for ten hours, utilizing up to 1100 MWh of energy stored, which is about ten times more powerful than the largest lithium-ion battery systems for renewable power.

An application of the principles of a cryogenic energy storage system can be seen in the proposal of a new solar-aided liquid air energy storage system integrated with a heat transfer oil circuit operating on an organic Rankine cycle [90]. Three cases of a liquid air energy system aided by solar were tested against a standalone base liquid air energy storage system powered by photovoltaics. Results from the comparison of the three proposed models indicate that the optimal air liquefaction pressure is at 160 bar and the expansion pressure at 120 bar. The base standalone liquid air energy system had a roundtrip efficiency of 53.26% while the roundtrip efficiency of the best integration case was 90.49%. The dual-side introduction of an organic Rankine cycle to the new system exhibited a 15.56% increase in roundtrip efficiency.

A case study was performed on the Hainan offshore wind power and liquefied natural gas receiving station to improve the efficiency of utilizing liquefied natural gas cold energy and reduce the electricity consumption of the production of liquid hydrogen by introducing liquid air energy storage as an intermediate stage [91]. The case study also combined offshore wind energy resources and the liquefied natural gas receiving stations to utilize surplus wind energy to produce hydrogen, and the latter to store the produced liquid hydrogen. A feasibility study using specific energy consumption exergy analysis was performed on the proposed LNG-LAES-LH₂ scheme after carrying out optimizations. For a proposed 100 MW, the roundtrip efficiency was found to be at 71%, specific energy consumption at 7.87 kWh/kg, and exergy efficiency at 46.44%.

3.5. Hydrogen Energy Storage System

Compared to other energy storage systems, hydrogen energy storage systems are capable of high-energy storage capacities at longer storage periods while also being flexible [92]. These energy storage systems are primarily used to deal with disequilibrium in the supply/demand balance of grids by storing excess energy or energy generation through grid injection when needed. They are also employed to resolve issues regarding seasonal differences in energy production as their storage capacities can reach from MWh to TWh, allowing renewable sources to be shifted from one season to another.

Two promising studies showcase how the use of a hydrogen energy storage system can open new possibilities for renewable energy systems. In the first study [93], a combined renewable energy system utilizing a hybrid wind turbine, water electrolyzer, and pumped-hydro-compressed air system was developed for electricity generation with hydrogen fuel as energy storage. The designed system required about 560.6 W of electricity from the 10 kW wind turbine for the electrolyzer to produce hydrogen fuel. The wind energy generated from the turbine was around 692.32 of electricity, where 131 W could be stored in the PH-CA system. Energy and exergy analyses of the system show that the energy system efficiency was about 74.93%.

In the second study [94], a novel hybrid system consisting of a wind–solar–hydrogen multi-energy system was evaluated for its performance. The system, as shown in Figure 5, consisted of a photovoltaic and wind power system, an electrolytic cell, a hydrogen storage tank, and a proton exchange membrane fuel. A case study was then performed through MATLAB and results indicate that the system is capable of producing around 931.39 kg of hydrogen per year and generating 253,187 MWh of electricity per year at an energy efficiency of 16.03% and exergy efficiency of 17.94%. CO₂ mitigation was shown to amount to 4,220,000 tons.

3.6. Electrical Energy Storage System

For electrical energy storage systems, specifically with the use of supercapacitors to aid in resolving intermittency in photovoltaic renewable systems, a study was performed [95] to investigate the effect of integrating a supercapacitor as a fast-response energy storage system for a photovoltaic system. A system was designed to have a 3.0 kWp capacity photovoltaic component combined with a 500 F-2.7 V per module supercapacitor, whose charge only came from the photovoltaic component. A simulation was carried out to investigate the effect of the number of supercapacitor modules on the self-consumption and self-sufficiency of the integrated system. It was found that the annual self-consumption of the system increased from 21.75% to 28.74% while its self-sufficiency increased from 28.09% to 40.77%.

Another study investigated the effect on energy self-consumption by introducing a supercapacitor into a photovoltaic energy storage system and a photovoltaic wind energy storage system [96]. The system analysis was performed using available weather data and load profiles. From the simulation results, charging the ultra-supercapacitor only with renewable energy greatly enhanced energy self-consumption. With a 300 F and 2.7 V per ultra-supercapacitor, the annual energy self-consumption was boosted from 37.01% to 41.69% for the photovoltaic-only system. For a combined photovoltaic wind turbine setup, the energy consumption was found to increase from 33.50% to 49.87%. The study highlights that by including a rapid-response energy storage component in a renewable energy system, the yearly average energy self-consumption increases.

In a study on the application of a superconducting magnetic energy storage system on a hybrid wind–solar photovoltaic farm [97], the paper describes the role of the SMES in improving the hybrid renewable energy system’s reliability and stability. The hybrid renewable energy system consists of a 75 MW photovoltaic array and a wind turbine whose permanent magnet synchronous generator has a capacity of 300 MW. To profile the output voltage, a model predictive controller was used. The SMES was connected via dc/dc converter using a PID-SDC. Through MATLAB/Simulink simulations, it was found that the system’s stability was improved by 3.36 s after reducing low-frequency oscillations at the multimachine side.

The African Vulture Optimization Algorithm was used to optimize the proportional-integral control parameters for a virtual synchronous generator-based SMES unit for frequency stability on a microgrid [98]. The microgrid was modeled to have a microturbine, diesel engine, fuel cell, photovoltaic system, WTG, and VSG-based SMES units. The controller used in the study was an LFC controller due to its simple construction and balanced performance–cost relationship. Simulations were carried out using MATLAB and results show that there are significant improvements in the dynamic response, settling duration, and frequency oscillations compared to the use of other algorithms.

4. Current Challenges and Future Directions

The use of energy storage systems as a way to mitigate intermittency in renewable energy sources via solar and wind has potential, but much like other alternatives, some challenges prevent the full-scale realization of their potential. In a review of energy storage system applications for renewables [99], the widescale implementation of energy storage systems for renewable energy systems faces technological, economic, and social challenges, as follows:

• In terms of technological challenges, energy storage systems for renewable energy systems face cost-capacity issues. At this stage of development, low capital investments tend to result in low-capacity energy storage systems. At higher capital investments, higher maintenance and operation risks result from higher-capacity energy storage systems. To properly implement energy storage systems, they must be low-cost and high in capacity. Another technological challenge involves the longevity of energy storage systems, as they have very limited lifetimes possibly due to the manufacturing process, the materials selected for their production, and improper energy management strategies. Environmental concerns also pose a challenge to the use of energy storage systems [100]. The installation of a pumped hydro energy storage system requires the clearing of land to make room for its storage, while compressed air energy storage systems require the installation of underground caverns; both installation methods are intrusive to environmental balance.
• Energy storage systems should be cost-effective. The most common energy storage systems are also the most expensive to install, whereas the cost of actually purchasing the device amounts to only 30% to 40% of the overall expenses. There is a variety of research on reducing the overall cost of expenses for the use of energy storage systems, but the cost-effectiveness in practical applications is still an issue. A roadblock also exists in the form of government policies. For example, Italy has imposed expensive grid-operational charges for the implementation and use of energy storage systems for grid application. In terms of social challenges, disposition toward the use of energy storage systems is primarily ambivalent and equivocal [101], where the key to acceptance by affected groups lies in addressing the advantages, disadvantages, and prospects of using energy storage systems.

Another review [102] lists material selection and environmental impacts as key challenges to the implementation of energy storage systems for grid applications. The raw materials necessary to build and operate an energy storage system dictate its reliability and longevity. The materials to be used also influence the charging/discharging, capacity, and energy characteristics of the energy storage. Thus, the material selection process, availability of the raw material, and costs must be considered and optimized. The deployment of some energy systems may also bring about environmental hazards. For example, compressed air energy storage systems sometimes rely on the combustion of fossil fuels for their compression stages, while superconducting magnetic energy storage systems emit magnetic fields.

While the challenges make it seem that the future is bleak regarding the use of energy storage systems for renewable energy sources, the field of research is broad as a result of the many aspects of power generation and operations that energy storage systems cover. Further research can be made in material science to optimize the selection process for suitable materials, public policies, and market research, focusing on investments and the proliferation of energy systems at small and large scales [103], as well as the cost-effectivity and safety performance assessments [100] of energy storage systems. Other avenues of research include feasibility studies on the hybridization of different energy storage systems, the integration of machine learning techniques for increased renewable energy penetration, network-based research, and geography studies for the identification of suitable installation sites for renewable energy systems integrated with energy storage [82]. In terms of increasing the capabilities of existing energy storage system types,

Table 2. Prospects and research areas for improving energy storage capabilities [92,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118].

Category	Type	Prospects
Mechanical	FES	Material: Steel-based flywheels for higher-speed operations [104]. Hybridization: FES systems have low storage capacities; incorporating with other appropriate systems to increase capacity can be performed [105]. Magnetic components: A bearing system utilizing superconducting magnetic bearings can effectively reduce friction loss during the flywheel rotor rotation, reduce self-discharge loss, and improve system operation stability [106].
	CAES	Liquid piston technology: Allows a CAES system to achieve near isothermal compression and expansion by allowing high L/D geometry, as well as flexible compression and expansion speeds [107].
	PHES	Storage cycle limitations: Current PHES arrangements operate on daily and weekly storage cycles; adopting a seasonal PHES cycle that can store energy daily, weekly, and monthly can still be explored [108]. Overcoming barriers: The implementation of PHES is blocked by barriers such as the lack of infrastructure, topological issues, availability of water, and land acquisition challenges [109].
Thermal	SHSS	Nanoparticle integration of SHSS media: The properties of the SHSS media can be enhanced with the use of nanoparticles, specifically aluminum oxides, as evidenced by [110]. Improving PCM thermal conductivity: Phase change materials can store energy effectively but have low thermal conductivity, which affects heat transfer efficiency. Enhancing PCMs by introducing carbon- and metal-based materials shows enhanced thermal characteristics for energy storage applications [111]. Limited optimization theories: Current optimization theories and methods are too simplified while operating on unreasonable assumptions. There is a need for (1) performance optimization models with more complex models and (2) specific methods for both SHSS and LHSS [112].
	LHSS
	CES	Material: More favorable cryogenic temperature ranges with favorable thermal properties remain unexplored. Utilization with other energy storage systems: Current applications where CES is used in conjunction with other ESSs include hydrogen fuel cell discharging processes and superconducting flywheel energy storage systems. More research can be conducted to improve CES so that it can be utilized in conjunction with more energy systems [113].
Chemical	H2ESS	Hydrogen storage options: Compared to gas and liquid hydrogen storage methods, solid-state hydrogen storage has a higher bulk storage density [114]. Seawater as an alternative: Water is a necessary element in hydrogen energy storage systems but becomes problematic as it contributes to the global consumption of water. Saltwater, as an alternative for the electrolysis process, can still be explored [92].
Electrical	Supercapacitors	Material: Graphene sheets in combination with other materials have been shown to improve conductivity, provide large surface areas, and facilitate faradaic currents through redox reactions [115].
	SMES	Cooling medium: An investigation into the use of liquid helium as a cooling medium instead of liquid nitrogen can be explored due to the better cooling properties of liquid helium [116]. Cooling structure design: Current designs for the cooling structure of SMES require a 4-month period to cool the superconductor from ambient to cryogenic temperatures. The design of a cooling structure for the SMES (to reduce the amount of time to put the material into cryogenic temperatures) can be explored [117]. Controllers: Research on the optimal selection and utilization of a controller depending on the SMES application can be conducted to improve dynamic voltage stability [118].

summarizes the promising prospects for accomplishing this endeavor.

If, ideally, these challenges to energy storage systems are completely overcome in the future, there will still be challenges to the development and implementation of microgrids and smart grids that heavily promote the integration of renewable energy systems. Challenges currently faced by the implementation of microgrids are cybersecurity issues, financial concerns, and environmental concerns [119]. Information and communication are important components in the operation and control of microgrids, hence their reliance on electronics, which would result in the efficient and reliable flow of data in their cyber system. Data corruption may jeopardize the microgrid’s efficiency, stability, and safety [120]. In terms of financial concerns, there is a preconceived bias against microgrids due to the risks associated with the implementation of such technology [121]. Environmental concerns include the degradation of local ecosystems and/or depletion of local natural resources. Similarly, smart grids are not exempt from challenges that must be addressed so that their use can be properly implemented. Establishing a smart grid network’s infrastructure has high upfront costs and investment requirements as a result of the equipment required for the utilities of the smart grid [122]. Apart from the physical challenges that the implementation of smart grids may face, the integration of renewable energy sources itself poses a challenge to smart grids; challenges include a reduction in environmental impacts, enhancing the flexibility of the smart grid, increasing the penetration of wind and solar technologies, and managing big data [123].

In another respect, Case studies play a vital role in addressing current challenges and shaping plans for using energy storage systems to balance the intermittencies of renewable energy sources (like solar and wind). A study on the impacts of energy storage on the reliability of wind and solar power in New England assessed the potential of wind and solar to consistently meet New England’s electricity demands, with an emphasis on energy storage [124]. Forty-four years of weather data were analyzed to predict the variability of wind and solar energy resources. To achieve 100% reliability, the integration of significant energy storage and the installation of wind and solar capacities that exceeded peak demand by at least threefold was necessary. The study also highlighted the importance of spatial aggregation to enhance system reliability and required a tailored strategy, including energy storage, demand management, and flexible technologies. As part of Hydro Tasmania [125], a renewable energy integration project, a remote island power system was developed on King Island in Bass Strait, Australia, to achieve high renewable energy penetration. The system facilitates wind farm expansions, the installation of a Vanadium Redox Battery, a solar photovoltaic array, and a dynamic frequency control resistor, leading to 85% instantaneous renewable energy penetration and over 35% annual contribution. At Shagaya Renewable Energy Park in Kuwait [126], understanding seasonal and daily wind patterns is necessary. Observations from meteorological towers, weather stations, and wind turbines result in an increase in turbine power production by 24%. Studies that provide real-world data offer valuable insights into how energy storage systems (ESSs) operate in practical scenarios. Coupled with technological advancements in the field, these insights can pave the way for breakthroughs, ultimately leading to more efficient, reliable, and sustainable energy systems for various applications.

5. Conclusions

With the growing necessity for power generation systems that do not rely on fossil fuels to mitigate and reduce carbon emissions, people are turning toward renewable energy systems as a solution to environmental woes. However, due to the intermittent nature of wind and solar as sources for renewable energy generation, full-scale application of these systems is not reliable just yet. Apart from the variability of resource availability, the energy management and maintenance of the supply/demand balance of these intermittent renewable energy systems further heighten the difficulties of their integration into the grid. A solution exists in the form of energy storage systems and their promising potential and capabilities. However, there are technological, economic, and social challenges. The field of energy storage systems and their potential application to renewable energy systems is rife with research potential, ranging from further reducing renewable energy resource variability and curtailment to feasibility and economic analysis studies.

On the other hand, it should be emphasized that—when evaluating or developing energy storage technologies—it is very important to acknowledge that no single solution can meet all needs. Rather than expecting a straightforward result, a nuanced outcome recognizes that the situation is complex, and different factors or perspectives need to be considered; moreover, there might be multiple potential directions for future development. For instance, significant advancements in solid-state batteries, which offer higher energy density, fast charging, and increased safety, could lead to smaller, more efficient battery storage systems; and breakthroughs in hybrid storage systems—combining high-energy density batteries and supercapacitors—could deliver a more resilient and flexible solution for renewable energy integration into the grid. Moreover, innovations in gravity storage systems that store energy by raising heavy blocks during periods of excess energy, and then releasing them to generate electricity when energy demand surges, could provide a low-cost, long-duration storage solution for wind and solar farms, especially in cases where conventional battery storage is not feasible.