As the world’s energy demand increases, there is an increasing cognition that nonrenewable resources are limited and can drastically impact the environment. On the other hand, renewable energy sources are commonly unstable and/or intermittent in nature. To mitigate the impacts of the intermittence of renewable energy sources, TES may be a viable option for addressing future electricity demand challenges and assuring a sustainable and steady power supply. Despite the wealth of literature available in the renewable energy and energy storage systems field, previous reviews typically investigated only specific topics related to renewable energy sources or energy storage systems, including thermal ones.
The wide range of studies demonstrates the growing interest in developing innovative combined RES–TES systems to address energy supply and storage challenges sustainably. Several studies that investigate solar energy combined with sensible TES employ technologies such as parabolic trough solar concentrators, concentrated solar power (CSP) systems, and ambient solar radiation collectors 
. Other studies focus on solar energy combined with latent TES, utilizing materials such as phase change materials (PCMs) and copper foam 
. Wind energy is also considered in combination with various TES systems, including sensible storage using thermal energy grid storage and variable TES systems such as compressed air energy storage (CAES), pumped hydroelectric storage, and sodium–sulfur batteries 
. Moreover, biomass energy is explored with sensible and latent TES systems, often involving bioproducts and different storage materials 
. Geothermal energy is frequently combined with sensible TES, often in the form of high-temperature aquifer thermal energy storage (HT-ATES) and mobile thermal storage systems (M-TES) 
. Ocean energy, another renewable source, is also examined in combination with both sensible and latent TES systems 
. Some studies investigate hybrid renewable energy systems incorporating multiple renewable sources, such as solar–geothermal, ocean–solar, and solar–wind–ocean systems 
. These hybrid systems typically involve sensible or latent TES to optimize the combined use of different renewable resources.
2. Combined Solar/TES System
The combined use of solar energy and thermal energy storage systems has been examined in several studies. Many researchers investigated solar energy storage in the form of sensible energy. Soni and his team 
devised a pioneering mathematical model to simulate the process of thermal energy storage (TES) drawn from solar energy, purposed explicitly for winter residential heating. This renewable system employed a parabolic trough solar concentrator with a novel design for the TES medium, utilizing compressed CO2
. This design aimed to minimize heat loss by integrating several layers of highly insulating materials, thereby maximizing solar energy storage within the gas medium. The study resulted in an innovative solar/TES system for home heating that offers a compelling solution for year-round heating at a cost that is 42.5% less than traditional methods. Promising opportunities abound for the adoption and enhancement of such technology. With the steady rise in fossil-fuel prices, renewable energy alternatives such as solar-powered TES systems could become increasingly appealing economically, despite their substantial initial costs. Furthermore, as societies become more aware and responsive to climate changes, the demand for renewable energy sources is projected to grow. This creates a more conducive environment for the implementation of TES technologies. The relentless pace of technological advancement, particularly in material science, also offers the prospect of addressing current limitations. As more efficient, resilient, and cost-effective TES solutions are developed, the potential of TES as a residential heating option is likely to grow. However, there are significant constraints. A substantial initial financial outlay is required for the installation of TES systems, including both the cost of the system and the installation. In addition, deploying solar-powered TES systems often requires large land areas to house the solar collectors. This requirement could present a significant hurdle, particularly in urban or densely populated areas with limited space. Weather vulnerability also poses a substantial challenge, as TES systems are susceptible to extreme weather events. This vulnerability is particularly pertinent in regions prone to such conditions, where damage could compromise system performance or cause complete failure. Despite these challenges, the potential benefits and opportunities presented by TES systems could serve as compelling reasons for further research and development.
Temiz and Dincer 
crafted an innovative model, merging solar and geothermal systems to create an array of valuable outputs, including hydrogen, electric power, and freshwater. This was accomplished through comprehensive thermodynamic analysis. Their approach incorporated concentrated solar power (CSP) as a renewable energy source and harnessed geothermal-based plate tectonic boundaries for thermal energy storage (TES) to generate electricity, hydrogen generation, space heating, and freshwater production within a hypothetical community near the Geysers area in California. This sophisticated system was thermodynamically scrutinized using Aspen Plus, NREL’s SAM, and HOMER Pro software packages. TES and CSP systems were modeled using tanks of 15 m height and absorber tubes of 8 cm diameter. The CueCl hydrogen production plant, when combined with desalination processes, generated approximately 160,390 tons of fresh water. Additionally, it contributed 48 GWh of electricity, 453 GWh of heat, and 297 tons of hydrogen. CSP and geothermal systems, despite their complexities, offer unique opportunities in the renewable energy realm. One significant opportunity lies in the burgeoning demand for hydrogen. The rising interest in clean energy sources has highlighted hydrogen’s potential as a green fuel in various sectors. Furthermore, the dual-purpose nature of CSP systems is an attractive prospect. These systems generate electricity and produce and store heat for future use, thereby enhancing energy use efficiency and reducing dependency on the electrical grid. Geothermal systems also present valuable opportunities. They provide a stable, reliable heat source, making them an ideal choice for district heating and industrial processes. Their heat can also be harnessed for electricity generation, presenting a sustainable and steady power source. However, several constraints demand a critical attention. The high cost associated with CSP systems is a significant limitation. These systems necessitate considerable initial investments, primarily due to the requirement of large-scale solar collector installations, energy storage systems, and associated infrastructure. The expense can be particularly daunting for smaller-scale projects. Geothermal energy, while promising, is limited by the availability of suitable geological conditions—specifically, heat emanating from the Earth’s crust. This inherent constraint restricts the scalability of geothermal energy. Moreover, geothermal systems, despite being renewable, pose potential environmental impacts. These include the release of trapped greenhouse gases, surface instability due to geothermal fluid extraction, and a risk of water contamination if the extraction process is not meticulously managed. Balancing these opportunities and constraints is critical for successfully integrating and advancing CSP and geothermal systems in the renewable energy sector.
proposed an innovative approach of amalgamating solar–thermal energy storage (TES) systems with thermochemical water-splitting cycles to produce cost-effective hydrogen. The envisaged system anticipated the generation of 0.1 GW of continuous electricity, thereby facilitating the production of approximately 2750 kg/h of hydrogen daily by the year 2030. The model utilized concentric solar power (CSP), deploying photovoltaic (PV) panels for solar energy-harvesting, and advanced ultra-supercritical CO2
as the medium for storing the produced energy. Opportunities for this initiative are abundant. The system presents a promising path toward hydrogen production from renewable energy sources, potentially reducing the dependence on fossil fuels. Moreover, the generated hydrogen could find myriad applications, including its use in fuel cells, ammonia production, and rocket fuel, thereby encouraging cleaner and more efficient energy sources. The system also offers the prospect of mitigating climate change by enabling cleaner energy production and reducing carbon emissions. Beyond environmental benefits, the broad application of the produced hydrogen could extend to various sectors, including transportation, power generation, and industrial processes, thus broadening the reach of renewable energy utilization. Furthermore, developing and implementing this system could stimulate job creation and economic growth within the clean energy sector. Despite these opportunities, certain constraints demand thoughtful consideration. One such limitation is that the study is based on a theoretical model; hence, its effectiveness and efficiency in real-world scenarios still need to be verified. Moreover, as the system is in its infancy, vital factors such as cost and performance still need to be defined, potentially discouraging immediate adoption. Additionally, the system’s effectiveness may depend on geographical location and climatic conditions, limiting its applicability. The high initial investment could also present a significant barrier to implementing such systems for potential users or applications. Moreover, the system may necessitate regular maintenance, potentially inflating the total cost of ownership over time. Lastly, the system’s susceptibility to external factors, such as dust, corrosion, or extreme weather conditions, could compromise its durability and efficiency. Balancing these opportunities and constraints will be crucial for the successful implementation and broader acceptance of this promising technology.
In solar/TES systems, concrete can be used as a TES medium to store the heat generated by solar collectors during the day and release it at night or during periods of low sunlight. However, in order for concrete to be an effective storage medium, it must be able to withstand high temperatures without significant degradation or loss of structural integrity. In a study investigating concrete as a thermal energy storage medium, John and his coworker 
undertook a novel endeavor, developing 26 unique concrete mixtures using sandstone, limestone, and syenite, combined with washed river sand in varied proportions. These mixtures underwent casting, curing, and a month-long immersion in water at room temperature. Subsequently, they were cured in saturated air at 90 °C for a couple of days. The endurance of these concrete mixtures was then tested in molten salt at 585 °C for 500 h, followed by exposure to 30 cycles in the air. A notable outcome of thermal cycling in molten salt was a substantial reduction in the compressive strength of both concrete and mortar when subjected to temperatures up to 600 °C. This offers intriguing opportunities for applying concrete in thermocline solar energy storage systems. The methodology introduced by John and his team reveals an innovative pathway toward more efficient solar energy harnessing. Utilizing concrete as a storage medium, solar thermal energy can be effectively absorbed and held for subsequent use. The inherent properties of concrete, such as its commendable heat capacity and thermal conductivity, position it as an ideal candidate for solar thermal energy storage systems. This implies that concrete can absorb, retain, and transmit heat effectively. Furthermore, the concept of a modular cement-based solid–liquid heat storage system suggests a potential for scalability and adaptability. This system offers the flexibility to be easily modified or expanded based on specific requirements. While the opportunities are substantial, a few constraints necessitate careful consideration. A concrete-based storage medium requires a heat transfer system, generally a fluid, for optimal functioning. This requirement introduces an additional component into the system, increasing its complexity and cost. Moreover, the limited scope of the study presents another constraint. The focus was primarily on comparing the heat capacity and thermal conductivity of concrete used as a filler material, leaving out other potentially influential factors such as durability, long-term performance, and environmental impact, which indicates the necessity for a more comprehensive research approach in this domain. Balancing these opportunities and constraints will be integral to effectively use concrete as a thermal energy storage medium in solar energy systems.
Paksoy and coworkers 
employed the CONFLOW simulation program to conceive a system that optimizes energy conservation by harnessing solar power and an aquifer for seasonal thermal energy storage. Explicitly envisioned for a hospital in Adana, Turkey, this system served for heating and cooling purposes. It exhibited the capacity to store 7000 MW/year at an average temperature exceeding 98 °C. During winter, the proposed system ingeniously utilized hospital ventilation air and proximal surface water to cool the aquifer, storing thermal energy. Concurrently, the system prewarmed the ventilation air with the assistance of two heat exchangers. This operation was reversed during the summer. Despite the promising aspects, some constraints exist and could be improved. One concern is the lack of generalizability, as the study was conducted in a specific hospital setting in Turkey. Thus, the applicability of its findings to hospitals located in different regions or countries could be questioned due to variations in climate, architectural design, and pre-existing energy infrastructure. A noteworthy omission in the study was the cost of implementing such a system. Considering that hospitals function within prescribed budget constraints, the financial implications of adopting this system could play a crucial role in its broader usage. The study also overlooked the environmental impact of deploying this system, a significant consideration given the escalating concern about sustainability across all sectors, including healthcare. On the flip side, there are significant opportunities to be explored. Foremost is the potential for significant energy cost savings. The savings could be substantial, considering the high-energy demands typically associated with hospital operations. The system also provides an opportunity to reduce reliance on nonrenewable energy sources, potentially aiding hospitals in reducing their carbon emissions and aligning with global climate change mitigation efforts. Furthermore, establishing a well-regulated and efficient energy system could create a more comfortable environment for patients and staff. This could improve patient satisfaction and enhance staff productivity, thereby underscoring the system’s holistic benefits. Balancing these opportunities against the constraints will be essential in determining the system’s broader applicability and effectiveness.
Many researchers investigated the storage of solar energy in the form of latent energy. In an experimental study, Wang and colleagues 
embarked on a study exploring the potential of latent thermal energy storage by leveraging n-tetradecane as a phase change material (PCM) embedded in gradient porosity copper foam. This was aimed at the efficient storage of mid-temperature solar energy. Solar energy was harnessed via a shell-and-tube heat exchanger, where a transparent silica glass shell was utilized to capture solar radiation. Simultaneously, the TES system was endowed with PCM encapsulated within the copper foam to amplify its hardness, thermal conductivity, and stability. The team devised a gradient porosity metal foam (GPMF) to circumvent the issue of low conductivity typically associated with PCM in mid-temperature solar energy storage systems. The experimental results indicated a substantial improvement in heat transfer with the employment of GPMF. The study, however, presents several constraints. A critical constraint is the scalability of the production process for gradient porosity copper foam. Given the specificity of this material, it could be challenging to mass-produce, potentially impeding the broad adoption of this technology. Additionally, the availability of gradient porosity copper foam could be restricted, making it challenging to source and potentially hindering the technology’s widespread deployment. Furthermore, the findings necessitate more exhaustive research to validate the conclusions drawn and explore potential long-term impacts across various applications. The initial study may not fully address the practical realities and challenges that emerge in real-world scenarios and applications. Conversely, the study also highlights several promising opportunities. Employing gradient porosity copper foam could drastically enhance heat transfer capacity in mid-temperature solar energy storage systems, improving their efficiency and effectiveness. Another significant benefit is the potential for better temperature uniformity achieved through this material, which could lead to the more consistent and efficient performance of solar energy storage systems. Lastly, the study indicates that using gradient porosity copper foam could reduce the melting time by 37.6%. This could lead to solar energy storage systems becoming quicker and more efficient in capturing and storing solar energy.
For a solar energy-assisted drying process, Atalay 
represents a crucial step toward understanding the energy and cost implications of the packed bed and phase change material (PCM) thermal energy storage systems. By comprehensively evaluating these media for storing sensible and latent energies, Atalay underscores the economic advantage of packed bed due to its lower initial investment cost but points to the superior thermal storage capacity of paraffin wax PCM. Atalay’s investigation opens up promising opportunities. Integrating solar dryers and thermal energy storage techniques can enhance energy efficiency, leading to more sustainable and potentially cost-effective drying processes. Moreover, adopting these energy storage systems could significantly reduce drying time, boosting productivity. An intriguing insight from the study suggests that these systems aid in preserving product quality during the drying process. This potential benefit could result in higher-quality dried goods, enhancing their marketability. Furthermore, the study underscores the utility of computational numerical modeling for optimizing drying systems, thus spotlighting an avenue for further improvement in system efficiency and performance. Despite these promising opportunities, the study also presents considerable constraints that warrant further attention. The research underlines the need for more in-depth exploration to identify optimal materials and configurations for these energy storage systems, highlighting that the full potential of these systems is yet to be realized. The challenge of implementing these systems under varying climates and conditions is also identified. This suggests that customization is necessary to suit specific environmental factors, which could amplify complexity and cost. Fei and his team 
innovatively developed a wearable solar energy management system that leverages visible solar thermal energy storage for complete solar spectrum utilization. Their invention termed the “visible solar storage fabric” (VSSF), employs a unique combination of “Azo-PCM@PS” nanocapsules and “Cs0.32
” nanoparticles. The outcome is a “thermochemical–thermophysical coupled energy” storage system with visible to near-infrared (Vis–NIR) light-harvesting capabilities. Remarkably, the designed wearable solar energy system has demonstrated the ability to release significant solar heat up to about 85 °C, which could provide notable protection against cold injuries to the human body. Fei’s team’s pioneering work presents several exciting opportunities. First, it has the potential to provide a sustainable and reliable energy source for a wide array of wearable devices, including but not limited to smartwatches, fitness trackers, and medical devices. The technology’s versatility powers a broad spectrum of wearable tech applications. Additionally, the system can be harnessed to collect environmental data such as temperature, humidity, and air quality, thus providing valuable insights for various applications. Beyond these notable points, the solar energy management system could extend its applications to power other portable devices such as smartphones and laptops, further enhancing its usefulness. This technology could be a crucial renewable energy source in off-grid situations where traditional power infrastructure is lacking. Furthermore, if these systems gain widespread acceptance, the data collected could enhance weather forecasting accuracy and refine climate modeling. Despite these promising opportunities, the study by Fei et al. also uncovers potential constraints that require careful attention. For one, the testing environment was a laboratory, and it remains to be seen how the system will perform under real-world conditions, which include weather fluctuations, outdoor wear and tear, and varied user behaviors. Secondly, being in the early stages of development, the cost implications and total system performance still need to be defined, potentially posing a challenge to immediate implementation. A few more concerns also arise. The system’s application may be restricted by certain factors, such as location and climate, which can influence the availability of sufficient solar energy. Additionally, wearability and durability questions surface; the system might be too heavy or bulky, negatively affecting user comfort, or fragile, leading to concerns about its durability with frequent use. Lastly, regular maintenance might be necessary for optimal system functioning, potentially contributing to higher total ownership costs.
Overall, the combined use of solar energy and thermal energy storage systems presents several opportunities, including the potential for cost-effective hydrogen production, significant energy cost savings, and reduced reliance on nonrenewable energy sources. It also offers the prospect of mitigating climate change and creating a more comfortable environment for patients and staff. However, several constraints demand critical attention, including the high cost associated with CSP systems, the lack of generalizability of some studies, and the need for regular maintenance. Additionally, the effectiveness of the system may depend on geographical location and climatic conditions, and the cost implications and total system performance still need to be defined.
3. Combined Wind/TES System
Several studies have explored the integration of wind energy and thermal energy storage (TES) systems to enhance the efficiency and performance of renewable energy applications. Many researchers investigated the storage of wind energy in the form of sensible energy, but no research was found on storing wind energy in the form of latent energy. Al-Mashakbeh and colleagues 
devised a compelling combination of a simulated wind farm with “thermal energy grid storage multijunction photovoltaics” (TEGS-MPV) in Jordan. Their approach harnessed the capabilities of HOMER®
software to simulate a 60 MW wind power production scenario involving wind turbines, inverters, and TEGS-MPV, resulting in an attractive energy cost of 0.04252 USD/kWh. The study opens up many opportunities, beginning with its potential to generate significant environmental benefits. It could substantially curb greenhouse gas emissions, with an estimated annual reduction of 293,764 tons of carbon emissions. Another highlight of the study is the importance placed on optimization and sensitivity analysis, achieved through the application of HOMER®
software. This process enables precise calculation of levelized cost of energy (LCOE) values, assisting in identifying optimal scenarios for the TEGS-MPV system deployment. Moreover, the research promotes an innovative strategy for integrating wind energy with thermal energy grid storage. This creative approach fosters broader acceptance and use of renewable energy sources while enhancing energy efficiency. Despite these promising opportunities, the study also brings to light several constraints that merit attention. One notable constraint is the need for practical validation of the proposed system. Given the study’s theoretical nature, it is crucial to test the feasibility of the TEGS-MPV system in real-world settings. Another constraint lies in translating this theoretical approach into practical application. There may be technical, financial, or regulatory challenges during the implementation phase of the TEGS-MPV technology that need to be surmounted. Additionally, the study is geographically specific, focusing on Mafraq, Jordan. Therefore, the transferability and effectiveness of the TEGS-MPV system in different regions or countries might be influenced by various factors, including local climatic conditions and regulations.
Caralis and his team 
delve into the evaluation of potential energy storage systems in Crete, designed to harness the fluctuations of wind energy. They put a spotlight on a “compressed air energy storage” (CAES) system, integrated with a thermal energy storage (TES), in comparison to other energy storage systems. Several storage solutions, including CAES, “pumped hydroelectric storage”, and “sodium–sulfur batteries”, were scrutinized for their efficacy in storing electrical energy derived from wind power during periods of curtailment exploitation. In terms of economics, CAES was slightly more advantageous, costing 0.21 EUR/kWh. However, despite being economically less viable, sodium–sulfur batteries emerged as strong contenders for positively balancing generated loads. Starting with opportunities, it is clear that Crete boasts significant untapped potential for exploiting wind and solar energy. With the right infrastructure and energy storage systems, this latent renewable energy could be harnessed more efficiently. Energy storage systems are key in storing excess energy wind turbines produce. Utilizing these technologies could reduce, if not eliminate, the need for energy curtailment. This would augment the wind energy system’s overall efficiency and economic viability. Furthermore, this presents a comprehensive examination of different energy storage technologies, assessing their potential to mitigate the issue of wind energy curtailment. This analysis is a valuable foundation for future research and developments to maximize the benefits of renewable energy utilization in Crete. Turning to constraints, the study points out potential technical limitations with existing thermal units. These issues may impede the effective harvesting of wind energy, necessitating appropriate measures to overcome these limitations. Another challenge lies in the inconsistency of wind turbine output, which can fluctuate due to varying wind speeds. This could lead to challenging, often curtailed energy surpluses, reducing the wind energy system’s overall efficiency and economic appeal.
Karasu and Dincer 
evaluated the effectiveness of a hybrid system combining electromagnetic induction and thermal energy storage (TES), designed to convert wind energy into heat directly. A distinctive feature of their system is the placement of the TES cycle within wind turbine nacelles, which allows the system to operate continuously. Using induction heating to heat the TES within wind turbine nacelles empowers the system to operate without interruptions, facilitating the direct conversion of wind energy to heat. Overall energy efficiencies of 7.0% and 8.6% were achieved when using electromagnetic induction and TES, respectively. The innovative wind energy/TES system is a potential substitute for traditional fossil-fuel-based facilities and nuclear power plants. Opportunities present themselves with the possibility of enhanced energy efficiency. The direct transformation of wind energy into heat sidesteps unnecessary intermediary conversion steps, which can reduce overall energy losses. This approach is in harmony with a broader global shift toward renewable energy and could be instrumental in curtailing greenhouse gas emissions. Moreover, the innovative system proposes a more effective way of harnessing renewable energy resources. As the global demand for cleaner, renewable energy surges, technologies such as this wind energy/TES system could significantly affect future energy landscape. Despite these promising prospects, there are a few constraints that need consideration. The research underscores the need for a comprehensive understanding of thermal energy storage and the cost-effectiveness of wind energy-TES technology. A thorough economic and technical feasibility assessment is crucial for its large-scale implementation. Furthermore, given the relative novelty of this approach, technical challenges and uncertainties might arise during the development, deployment, and operation of these systems. Hence, further research and development are imperative to refine this technology and ensure its reliability and efficiency.
Overall, the opportunities of combining wind energy and thermal energy storage systems include the potential for more efficient use of renewable energy resources, reduction in greenhouse gas emissions, and the possibility of replacing traditional fossil-fuel-based facilities and nuclear power plants. However, constraints include technical limitations with existing thermal units, the inconsistency of wind turbine output, and the need for a comprehensive understanding of thermal energy storage and the cost-effectiveness of wind energy/TES technology. Further research and development are necessary to refine this technology and ensure its reliability and efficiency.
4. Combined Biomass/TES System
The combined use of biomass energy and thermal energy storage systems has been examined in several studies. Some researchers investigated the storage of biomass energy in the form of sensible thermal energy. Fushimi 
offers an insightful review of biomass power generation systems, evaluating their economic worth and inherent limitations. He delves into a thorough examination of combustion technologies, the issues of lower energy conversion rates from solar power to electricity, and high fuel costs. To mitigate the mismatch between the sporadic electricity supply from various renewable energy sources and the demand for electricity, the review proposes integrating next-generation biomass energy systems with other fluctuating renewable energy sources such as solar and wind power and energy storage systems. This approach could potentially address the non-steady operations induced by an intermittent power supply. The opportunities provided by the study illuminate the potential for biomass to serve as a sustainable alternative for producing electricity, fuels, and chemicals. Substituting fossil fuels with biomass opens up promising prospects for a greener future. Moreover, the study emphasizes the potential of establishing flexible renewable-based utility plants. Such plants could generate a stable stream of renewable energy, significantly enhancing the power grid’s reliability. Moreover, it prompts a discussion on the economic and technological feasibility of power-generating systems that utilize biomass resources. This comprehensive analysis could inform policymaking decisions, advocating for the broader adoption of biomass power generation. However, several constraints need to be considered. The limitation of biomass resources emerges as a critical challenge. The availability of these resources can vary significantly across regions, demanding sustainable management strategies to prevent adverse environmental effects such as deforestation and biodiversity loss. The complexity and potential expense of the biomass-to-energy conversion processes can also pose challenges to the widespread deployment of biomass power generation. Furthermore, environmental impacts associated with improper resource management can lead to ecological degradation, highlighting the need for careful consideration of the environmental implications. Additionally, the carbon released during biomass combustion, although often deemed part of a closed carbon cycle, can contribute to short-term spikes in atmospheric carbon levels, posing another potential constraint.
Rezaei and colleagues 
undertake a thorough review of the integration of biomass-powered combined heat and power (BCHP) systems with thermal energy storage (TES) in district heating. They scrutinize many optimization models, including economic and environmental ones incorporating parametric or sensitivity analyses. Various parameters related to thermodynamics, hydraulics, chemistry, costing, and decision-making variables are considered. Since the heat value of syngas produced from biomass is lower than fossil fuels, the researchers investigate the alternatives of combining syngas with natural gas or utilizing it in a coal-fired subsystem. To deal with uncertainties in the biomass supply chain, they propose a post-optimization evaluation to estimate the probability of the optimal scenarios. Rezaei and colleagues spotlight a significant opportunity to augment energy efficiency by integrating BCHP systems with district heating and thermal energy storage. Their approach opens up the potential for substantial reductions in greenhouse gas emissions, offering a significant step forward in combating climate change. Moreover, the installation and upkeep of BCHP systems have the potential to stimulate local job creation, contributing to economic development. There is also an enticing prospect of opening up new revenue streams by exploiting biomass waste, which adds an economic incentive to adopting BCHP systems. Furthermore, BCHP integration could boost energy security by diversifying the energy portfolio and diminishing dependence on fossil fuels. Integrating BCHP systems could expedite the transition toward a more sustainable energy infrastructure by increasing the utilization of renewable resources. However, several constraints emerge in this field. One of the significant challenges lies in securing a consistent and sustainable supply of biomass, which is subject to regional and seasonal fluctuations. There is also a pressing need for the evolution and broad acceptance of efficient and environmentally friendly technologies for converting biomass to energy. Integrating BCHP systems into the existing energy infrastructure presents a significant challenge due to potential technical and regulatory barriers. Issues related to biomass combustion emissions, regulatory obstacles, and public acceptance are crucial and need to be addressed adequately.
In their study, Wang and colleagues 
implemented a simulation of a biomass-fueled boiler based on field data harvested from a wood pellet boiler equipped with radiant floor heating. Key variables such as thermal energy storage (TES) tank discharge efficiency, maximum product temperature, and boiler on/off times are considered. Using these parameters, the simulation provides insights into the optimal TES reboiler volume, capacity, and heat demand profiles. Findings suggest that a reboiler operating on a medium heat demand profile requires the smallest TES tank volume. Conversely, an intermittently used reboiler demands a considerably larger TES tank. Interestingly, a reboiler with a high heat demand profile leads to the highest TES tank discharge efficiency but also calls for a substantial storage volume. For economic efficiency, the team recommends sizing the thermal energy storage system such that the boiler’s nominal capacity is roughly 45% of the average building heat demand. The research by Wang et al. elucidates the potential of thermal energy storage tanks in the context of biomass boiler heating systems, offering an avenue for optimizing the operation and improving the overall energy efficiency of these systems. Integrating TES systems into district heating (DH) infrastructure represents a significant opportunity. These systems can provide several benefits, including peak load leveling, enhanced operational flexibility and reduced production costs; which can lead to more sustainable and economically viable heating solutions. In their comparison of different heat demand scenarios, the researchers highlight the potential of TES in minimizing costs and emissions. This is especially noteworthy in the context of sustainable industrial operations and aligns with the global push for reducing the environmental impact of energy production and consumption. Despite these promising opportunities, the study also uncovers some constraints. Technically, challenges related to the design, implementation, and integration of TES systems into existing infrastructures might pose barriers to their adoption. Economic factors also present significant constraints. The capital and operational costs of implementing and maintaining TES systems might be high. They could deter widespread adoption, especially in areas where the initial investment is not immediately apparent or outweighed by the economic benefits. The study also points to geographical and temporal constraints that can influence the effectiveness and feasibility of TES systems. Factors such as the availability and type of waste heat, heat demand patterns, and seasonal fluctuations can all impact the overall system efficiency.
Zhang and colleagues 
embarked on a trailblazing investigation into biomass’s latent energy storage potential. The researchers innovatively developed a multifunctional form-stable composite PCM that leverages Guar gum, a natural polysaccharide, to create a carbon aerogel encapsulating polyethylene glycol. The biomass-derived composite PCMs demonstrated exceptional characteristics such as robust structural stability, comprehensive energy storage performance, and leakproof quality. When these synthesized PCMs were applied to solar-thermal energy conversion and storage, they performed admirably, marking a substantial advancement in the renewable energy landscape. A key opportunity highlighted is the potential to enhance solar-thermal energy conversion and storage significantly. Developing such innovative carbon aerogels can escalate solar energy systems’ efficiency and storage capacity, rendering them more attractive for broader, large-scale energy production. This could revolutionize the renewable energy sector and expedite the global transition toward a more sustainable energy economy. Additionally, the successful utilization of biomass materials in creating aerogels showcases a promising approach to developing eco-friendly energy solutions. This novel use of biomass supports the sustainability agenda and provides a pathway to efficiently utilize biomass waste, thereby contributing to the circular economy concept. This approach also paves the way for more cost-effective energy solutions, assuming that the production cost of these carbon aerogels is economically viable. However, despite these promising opportunities, the study also reveals significant constraints. One of the primary challenges is the difficulty in achieving homogeneity in the biomass materials used to create carbon aerogels. Maintaining consistency in these materials is a critical factor that directly influences the performance and efficiency of the aerogels in their solar-thermal energy conversion and storage roles. Another significant constraint is the need to optimize the properties of carbon aerogels to boost solar-thermal energy conversion efficiency. Determining the proper parameters for this optimization is a complex task and can affect the overall efficacy of the aerogels. It highlights the need for additional research and development to fully unlock the potential of these innovative materials.
Overall, the combined use of biomass energy and thermal energy storage systems presents several opportunities, including reducing greenhouse gas emissions, stimulating local job creation, opening up new revenue streams, diversifying the energy portfolio, and increasing the utilization of renewable resources. Thermal energy storage systems can optimize the operation and improve the overall energy efficiency of biomass boiler heating systems, as well as provide peak load leveling, enhanced operational flexibility, and reduced production costs. Additionally, the use of biomass materials in creating carbon aerogels can escalate solar energy systems’ efficiency and storage capacity, rendering them more attractive for broader, large-scale energy production. However, there are several constraints, including securing a consistent and sustainable supply of biomass, developing efficient and environmentally friendly technologies for converting biomass to energy, technical and regulatory barriers to integrating BCHP systems into the existing energy infrastructure, and high capital and operational costs of implementing and maintaining thermal energy storage systems. Additionally, geographical and temporal constraints can influence the effectiveness and feasibility of thermal energy storage systems, and achieving homogeneity in the biomass materials used to create carbon aerogels is a critical factor that directly influences their performance and efficiency.
5. Combined Geothermal/TES System
The combined use of geothermal energy and thermal energy storage systems has been examined in several studies. Some researchers investigated the storage of geothermal energy in the form of latent energy. The research conducted by Matuszewska and her team 
provides an insightful analysis of using a mobile thermal energy storage (M-TES) system for delivering geothermal heat to individual recipients, specifically in Polish conditions. They present an innovative solution using a phase change material (PCM) energy storage container of 55 kWh capacity, demonstrating its potential to overcome critical logistical challenges in the application of geothermal energy. This sustainable energy resource holds significant potential in terms of environmental benefits, making it an appealing solution for energy generation. Another significant opportunity lies in the high-capacity factors achievable through geothermal resources. The study illustrates that up to 60% of capacity factors are viable, offering a promising avenue for economically and environmentally efficient use of geothermal energy. Moreover, the researchers propose a mobile thermal energy storage (M-TES) system as a novel solution to tackle the challenges associated with the long-distance transportation of geothermal heat. This M-TES system enhances the versatility and feasibility of geothermal energy usage, allowing for efficient storage and transportation of geothermal heat to individual recipients over longer distances, thereby improving the economic viability of such operations. However, alongside these opportunities, the study also acknowledges a notable constraint. Climate factors are recognized as a significant challenge that can limit the effective use of geothermal resources in certain regions. This necessitates developing and implementing additional technical measures to optimize system performance and maintain efficient energy delivery in varying climate conditions.
In the study conducted by Fleuchaus and colleagues 
, they address the risks associated with high-temperature aquifer thermal energy storage (HT-ATES) systems and present a risk assessment framework to mitigate those risks, thereby bridging the seasonal gap between the demand and supply of thermal energy. A key focus of their work lies in the exploration of potential opportunities while also acknowledging inherent constraints. The significant opportunities highlighted by this involve risk identification and mitigation strategies for HT-ATES systems. The proposed risk assessment framework paves the way for a more robust and secure approach toward designing and operating these systems. Moreover, the study also stresses the importance of promoting sustainable energy solutions. Their research encourages the integration of renewable energy sources and enhances thermal energy storage system efficiency, contributing a valuable dimension to sustainable energy. On the other hand, the study recognizes the constraints associated with these systems. A notable challenge is the limited availability of data on HT-ATES systems, which poses an obstacle to a comprehensive understanding and efficient design. Uncertainties inherent in subsurface conditions also pose a considerable challenge to the development and operation of these systems, impacting their reliability and performance. Furthermore, the potential environmental impacts associated with the heat storage and extraction processes of HT-ATES systems are also a cause for concern, necessitating careful risk assessment and mitigation strategies.
In their compelling study, Arslan and Arslan 
investigated the potential for integrating residential-scale latent heat thermal energy storage (RS-LTES) using phase change material (PCM) energy storage containers in a geothermal district heating system. Located in Simav, Turkey, the system exhibited numerous opportunities and certain constraints that are imperative to consider for potential implementation. On the positive side, Arslan and Arslan’s research illuminates several opportunities. Their investigation underscores the promise of geothermal energy as a sustainable and renewable energy source for district heating systems. This energy choice is environmentally friendly and a viable path to reduce greenhouse gas emissions substantially. They further highlight the enhanced performance and reliability of integrating thermal energy storage into the district heating system. This feature contributes to the system’s long-term sustainability and operational efficiency. Lastly, the study introduces an important tool: a multicriteria decision analysis. This framework supports informed decision making, thoroughly evaluating different design options and technologies. This approach is instrumental in fostering the development of efficient, environmentally friendly district heating systems. Despite these promising opportunities, Arslan and Arslan also shed light on the constraints accompanying the use of geothermal energy. These include the difficulty of accurately calculating the size and capacity of the generator plant, distribution system, and substations using geothermal energy as the source. Precise dimensioning of the district heating system is essential for optimal performance and economic feasibility. They underscore the challenge of selecting the appropriate thermal energy storage technologies for district heating systems. The efficiency and cost of these technologies can pose limitations, which need careful consideration during the design and implementation phases.
Overall, the combined use of geothermal energy and thermal energy storage systems offers promising opportunities such as sustainable and renewable energy sources for district heating systems, reduced greenhouse gas emissions, enhanced performance and reliability of the system, and a multicriteria decision analysis framework for informed decision making. However, there are also constraints, such as the difficulty of accurately calculating the size and capacity of the generator plant, distribution system, and substations, and the challenge of selecting appropriate thermal energy storage technologies for district heating systems due to their efficiency and cost limitations.
6. Combined Ocean/TES System
The combined use of ocean energy and thermal energy storage systems has been examined in several studies. Some researchers investigated the storage of ocean energy in the form of sensible energy. In their notable study, Li and colleagues 
developed a thermodynamic model for the ocean thermal energy conversion system (OTEC), offering a load-following control strategy. Their analysis quantitatively evaluated the impact of manipulated variables on the system’s power output and the superheating of the evaporator outlet. Despite certain constraints, this also illuminated several opportunities that could profoundly influence the effectiveness and efficiency of the OTEC systems. The study opened the door to several positive prospects. Firstly, it presented the opportunity for developing advanced control strategies for OTEC systems, enhancing the power output and mitigating temperature fluctuations in the evaporator outlet superheating degree. Such improvements could significantly improve the overall efficiency of OTEC systems. Moreover, their findings offer valuable insights into renewable energy sourcing for islands. Reducing reliance on fossil fuels could substantially decrease greenhouse gas emissions, contributing to environmental conservation efforts. Lastly, the study provided valuable insights, which could enhance the overall efficiency and effectiveness of OTEC systems, further establishing its potential as a robust and sustainable energy solution. However, the study by Li et al. also brings certain constraints to light. One major limitation lies in comparing only two controllers—the model predictive control (MPC) and the proportional integral (PI) controller. This focus on their performance leaves room for exploring other potential control strategies for OTEC systems. It indicates a need for broadening the scope to evaluate a more comprehensive range of control strategies, potentially leading to more optimized OTEC systems.
In a comprehensive heuristic review, Zhou 
scrutinized the integration of ocean energy into intelligent energy systems, focusing on diversified ocean energy systems for coastal residential communities. This identified potential limitations and illuminated remarkable opportunities that could shape the future of ocean energy applications. In terms of opportunities, Zhou’s review uncovered several promising prospects. The review explores the potential utilization of advanced ocean energy converters to increase energy extraction efficiency from ocean resources. Furthermore, the study pointed toward the benefits of diversified ocean energy systems and hybrid energy storages, which could bolster energy production and storage capacity and consequently improve the reliability and effectiveness of the energy system. The study also highlighted the integration of artificial intelligence as a significant opportunity. This integration could enhance the sustainability and efficiency of these systems through improved prediction, optimization, and control mechanisms. Lastly, Zhou proposed strategies for complementary hybrid renewable system integrations. This effective strategy could help overcome constraints and seize opportunities, leading toward a carbon-neutral transition. Nevertheless, the study has its limitations. The primary challenge identified is the fluctuating power frequency, a side-effect of vertical cascade ocean energy systems. Ensuring stable and grid-friendly operations necessitates identifying practical solutions to this challenge. This constraint underscores the need for further exploration and innovation in this realm to ensure that the efficiency and reliability of these systems are not compromised.
Similarly, in an insightful review, Wang and colleagues 
critically assessed the potential of harnessing ocean energy as latent energy for crewless underwater vehicles (UUVs) operation. The researchers explored the mechanics of ocean thermal energy formation, the development of ocean-based thermodynamic UUVs, and the current challenges faced in their application. This comprehensive review lends itself to opportunities and constraints in ocean thermal energy storage and application. Wang and colleagues identified significant opportunities for advancing ocean thermal energy technology. The potential utilization of ocean thermal energy, a vast yet largely untapped renewable energy source, was emphasized for powering UUVs. Furthermore, the review identified PCM-based thermal energy-harvesting systems as the most promising technology in the context of these vehicles. This recognition suggests room for considerable advancements and innovation in PCM-based systems, thus paving the way for more efficient and sustainable energy solutions in UUVs. However, the application of these technologies has its limitations. The review noted that the slow heat transfer rates of the phase change material (PCM) thermal-harvesting systems impede their efficiency. Moreover, ocean thermal energy conversion technologies, including PCM-based systems, are grappling with low conversion efficiency. Energy storage technologies used in ocean thermal UUVs, such as PCM thermal-harvesting systems, were highlighted as having a low energy storage density, which presents an obstacle to their operational effectiveness. Lastly, Wang and colleagues pointed out that many ocean thermal UUV concepts, such as shape memory alloys, thermoelectric generators, and thermodynamic cycles, are still in the conceptual design phase, indicating an urgent need for further research and development. Despite these constraints, the study by Wang and his team provides a valuable resource for understanding the current state and future potential of ocean thermal energy-powered UUVs. As the study explores opportunities for innovation, it also outlines the areas where substantial improvements are needed, thereby contributing significantly to the development of more efficient, sustainable, and advanced ocean thermal energy applications.
Overall, the combined use of ocean energy and thermal energy storage systems presents significant opportunities for advancing ocean thermal energy technology, particularly in powering crewless underwater vehicles (UUVs). PCM-based thermal energy-harvesting systems are identified as the most promising technology in this context. However, the slow heat transfer rates of PCM thermal-harvesting systems and the low conversion efficiency of ocean thermal energy conversion technologies present constraints to their operational effectiveness. Additionally, many ocean thermal UUV concepts are still in the conceptual design phase, indicating an urgent need for further research and development.
7. Renewable Polygeneration/TES System
Renewable resources can be hybridized to complement the individual deficiencies found in these resources. For instance, solar and geothermal energies can be used as a di-generation system in some areas to maximize the generated power. Similarly, solar, ocean, and wind energies can be used as a di- or tri-generation system to enhance electricity and heat 
In their noteworthy research, Temiz and Dincer 
, introduce an ocean and solar-based multigeneration system tailored to the challenging conditions of Arctic communities. This comprehensive system incorporates a variety of components, such as a concentrated solar plant (CSP), bifacial photovoltaic (BiPV) cells, cascaded heat pumps, a multi-effect desalination process, and a polymer electrolyte membrane (PEM) electrolyzer. Additionally, the system utilizes fuel cell systems and thermal energy storage for energy retention. Through an in-depth analysis using multiple methodologies and software, the researchers explore the potential of this system in providing essential services, including food and energy production, for Arctic communities. In terms of opportunities, Temiz and Dincer’s work highlights the potential of renewable energy technologies, such as ocean thermal energy conversion, concentrated solar plants, and bifacial photovoltaic systems, in addressing the energy needs of isolated Arctic communities. Their proposed system also pioneers the integration of hydrogen production and thermal energy storage, providing promising avenues for enhancing the efficiency and sustainability of energy solutions in these harsh climates. Moreover, the study advocates a comprehensive approach to food production by amalgamating a fish farm, a greenhouse, and a food drying facility, all powered by their innovative energy system. Through these innovative strategies, this opens the door to significant improvements in the quality of life for Arctic communities by addressing their perennial food and energy shortages. Despite the promising opportunities, certain constraints are identified in the study. The harsh and often unpredictable Arctic conditions pose substantial challenges to implementing and operating such a sophisticated and integrated system. These conditions require robust and resilient systems that withstand freezing, ice, and limited sunlight. Another hurdle lies in system optimization; achieving maximum efficiency and effectiveness necessitates extensive analysis and simulations, which can be time-consuming and complex.
Additionally, Assareh and colleagues 
proffer an intriguing model of an integrated energy system combining an ocean thermal energy convertor (OTEC), a wind turbine, and a solar flat plate panel to cater to the electricity needs of Iranian households. Using Rankine cycles for power generation and wind turbines for backup, the system offers a synergistic approach to harnessing multiple renewable energy sources. This innovative setup underwent thorough thermo-economic analysis, was gauged against an Iranian household’s annual electricity consumption, and was further honed using the NSGA-II optimization algorithm. Their system could feasibly supply electricity to 38 Iranian households annually, operating with an energy conversion efficiency of 12.53% and producing a net power output of 448 kW. Furthermore, the proposed system promises an affordable electricity supply of 57.6 USD per hour. Commencing with the opportunities, Assareh and his team’s study highlights the potential of crafting a sustainable and efficient energy system through the concerted utilization of diverse renewable energy sources such as solar, wind, and ocean energy. This integrated approach offers a promising pathway to reducing dependence on fossil fuels, thus curbing greenhouse gas emissions—an increasingly crucial goal in the face of global warming. Moreover, their proposed system enhances energy security by leveraging locally available renewable energy resources, thereby safeguarding against geopolitical uncertainties and supply chain disruptions. Conversely, several constraints come into play. Foremost among these is the geographical specificity of the study, which focuses on Bandar Abas, Iran. This localization could restrict the direct applicability of the findings to regions with different climatic conditions and resource availability. Additionally, deploying such an integrated system necessitates substantial initial investment and infrastructure development, which could pose challenges in regions with limited resources. Lastly, the inherent intermittency of renewable energy sources, particularly solar and wind, raises the need for further exploration into effective energy storage solutions to ensure the system’s reliability and stability.
Similarly, solar and geothermal energies can be used as a di-generation system in some areas to maximize the generated power. A recent study by Li, Tao, Zhang, and Fu 
illuminates a suite of opportunities inherent in their innovative solar–geothermal hybrid system. Foremost among these is the efficient utilization of renewable resources. The proposed system leverages both strengths by seamlessly integrating solar and geothermal energy sources, mitigating dependence on fossil fuels and significantly curtailing greenhouse gas emissions. In addition, implementing thermal energy storage bolsters the system’s reliability, delivering unwavering flexibility in power generation, hydrogen production, and freshwater supply, regardless of the intermittent nature of the solar input. Furthermore, pioneering a transcritical CO2
cycle heightens thermodynamic performance while concurrently diminishing environmental impact, providing a compelling alternative to conventional power cycles. The two-objective optimization process delineated by the researchers also opens up the prospect of optimizing system configurations. This method uncovers the most efficient configurations, paving the way for future renewable energy projects by gleaning invaluable insights. However, a critical appraisal of Li’s research also brings to light some constraints that could impede widespread adoption. A fundamental limitation lies in the high initial capital costs associated with the complex nature of the hybrid system. This considerable financial barrier could limit the accessibility of the technology and slow its adoption. Geographical and climatic dependency presents another hurdle. The effectiveness of the solar and geothermal elements is heavily contingent upon the geographical location and local climate, which can vary markedly and consequently influence the system’s overall performance. The efficacy of the thermal energy storage component also hinges on the availability of suitable materials and technologies. As these might still be in the nascent stages of development, this could stymie the practical deployment of the system. Lastly, technical challenges in optimizing the transcritical CO2
cycle require advanced control strategies and high-level technical acumen. Given the limited pool of expertise, this might present an obstacle to widespread implementation.
Picone and team 
meticulously explored the plethora of opportunities in integrating Aquifer Thermal Energy Storage (ATES) with solar collectors, soil remediation, and other geothermal energy systems. Doing so paves the way for enhanced energy efficiency, a crucial asset in today’s energy-demanding era. Their simulations at sites in Belgium and the Netherlands demonstrated that such an innovative combination increases efficiency and energy savings. This, in turn, enables ATES to be used even under water scarcity conditions, thereby expanding its application spectrum. Furthermore, the promising results showing reduced dechlorination of chlorinated ethenes attest to the potential for reducing environmental impact, a critical consideration in today’s climate-sensitive scenario. Picone’s research not only introduces a novel combination of energy systems but also presents a blueprint for the cost-effective integration of renewable energy sources. This provides a creative solution to ongoing challenges in sustainable energy production and management, further underlining the value of the study. However, even with the positive strides made in this direction, it is important to recognize emerging constraints. Integrating diverse energy systems efficiently is riddled with technical challenges that must be overcome to function optimally. Additionally, legal and regulatory barriers might present obstacles, potentially limiting the widespread adoption of these innovative combinations. Of equal significance is the fact that the effectiveness and feasibility of these combinations are intrinsically tied to local conditions, particularly the geothermal and hydrogeological factors. Therefore, despite the myriad benefits these integrated systems offer, addressing these hurdles to unlock their full potential remains paramount.
The research conducted by Senturk Acar and Arslan 
, along with the insights gleaned from Boretti’s 
investigation, presents a compelling case for the integration of solar and geothermal energy to power an organic Rankine cycle (ORC). By incorporating these renewable sources, there is significant potential for enhanced energy efficiency and a subsequent reduction in dependence on fossil fuels. Consequently, this transition could drastically cut greenhouse gas emissions, contributing to a more sustainable energy landscape. Moreover, using both solar and geothermal energy has been shown to optimize the performance of the ORC, paving the way for improved energy conversion rates. In particular, Boretti’s exploration of the considerable geothermal resources within Saudi Arabia’s Earth’s crust demonstrates the potential for “enhanced geothermal systems” (EGS) to increase thermal cycle efficiencies by 40% more than traditional methods. The good coupling of “concentrated solar power” (CSP) and EGS with “thermal energy storage” (TES) could boost thermal cycle efficiency by over 50%, particularly in regions abundant in CSP and EGS resources, such as Saudi Arabia. Despite the promising prospects, however, certain constraints could impede the wide-scale deployment of these hybrid systems. For one, the availability and suitability of geothermal resources are primarily confined to specific geographical regions, which could limit the overall adoption. High initial costs associated with the necessary geothermal and solar energy infrastructure can also pose a significant financial barrier, potentially deterring investments in these systems. Lastly, the inherently intermittent nature of solar energy may impact the hybrid system’s overall efficiency and reliability, adding to its implementation’s complexities. Hence, while these studies underscore the potential of integrating solar and geothermal energy sources, they also illustrate the need to address these challenges further to maximize the potential of these promising renewable energy systems.
Overall, the combined use of renewable polygeneration and thermal energy storage systems offers opportunities such as reducing dependence on fossil fuels, enhancing energy security, and mitigating greenhouse gas emissions. However, there are constraints such as geographical specificity, high initial investment and infrastructure development, intermittency of renewable energy sources, technical challenges, legal and regulatory barriers, and dependence on local conditions. Overcoming these constraints is necessary to unlock the full potential of these integrated systems.