Review of Coupling Methods of Compressed Air Energy Storage Systems and Renewable Energy Resources

: With the strong advancement of the global carbon reduction strategy and the rapid development of renewable energy, compressed air energy storage (CAES) technology has received more and more attention for its key role in large-scale renewable energy access. This paper summarizes the coupling systems of CAES and wind, solar, and biomass energies from the perspective of system topology, and points out the advantages and limitations of each system. It is shown that the coupling of wind energy and CAES is mainly combined in series and in parallel, and sometimes part of the wind power can be converted into thermal energy when coupled to CAES. The coupling between solar heat and CAES is an important form of coupling between solar energy and CAES. Solar-heat-coupled CAES mainly uses solar energy to heat expander inlet air. The coupling forms of solar energy and CAES are based on various CAES forms, various heat heating sequences, reheating, bottom cycle, and other factors. The combination of biomass and CAES is generally based on biomass gasiﬁcation power generation technology. In the future, a wind–solar–CAES multiple coupling system is expected to become a promising large-scale form for the utilization of renewable energy, and this integrated system has great potential as a system conﬁguration, but has some technical challenges.


Introduction
Since the beginning of the 21st century, with increasingly serious energy and environmental problems, people have paid more and more attention to the clean, low-carbon, and efficient development of energy sources. The development of renewable energy is receiving more and more attention. The BP World Energy Outlook 2022 Edition states that the share of renewable energy in global primary energy will increase to about 55% in 2050 [1]. Among them, wind energy and solar energy will play a central role in providing low-carbon electricity [2]. Figure 1 shows the development of the installed capacity of global wind power and photovoltaics/solar generation in the past decade, showing a rapid upward trend, especially in China [3].
Although the installed capacity of wind and solar power generation is growing rapidly, randomness, volatility, and discontinuity are still weaknesses [2,4,5]. In addition, wind power is sometimes negatively correlated with external loads. These inherent shortcomings cause many problems for the direct use of renewable energy, such as not being able to meet the needs of user loads and the weakening of the stability of the grid after largescale access [6]. Thus, the phenomenon of the large-scale abandonment of wind and solar energy inevitably happens [7], which hinders the further development of renewable energy utilization, and results in a huge waste of energy and equipment. Energy storage technology At present, power energy storage technologies include pumped hydro storage (PHS), compressed air energy storage (CAES), flywheel energy storage, lithium batteries, flow batteries, sodium-sulfur batteries, superconductivity, capacitors/supercapacitors, etc. [9,10]. Among the various energy storage technologies, PHS and CAES can achieve both largestorage energy capacity and large input/output power, and are currently the most suitable for large-scale power storage. PHS is a widely used technology, but due to its strict geographical condition limitations, long construction time, and ecological impact, the development of PHS has been greatly limited [11]. CAES is another large-scale energy storage technology which stores excess electrical energy in compressed air and drives turbine power generation after releasing high-pressure air from the storage chamber when needed. CAES has the advantages of being low-cost, having a long life cycle and short construction time, and being environmentally friendly [9,12]; moreover, the newly developed CAES technologies have also eliminated the geographical restrictions. Thus CAES has great advantages in the adaption to renewable energy resources [13].
To cope with the large-scale utilization of renewable energy, in addition to traditional CAES systems [14,15], a variety of new CAES systems will play a central role [16], such as the adiabatic compressed air energy storage system (A-CAES), the compressed air energy storage system with thermal storage (TS-CAES), the liquid air energy storage system (LAES), the underwater compressed air energy storage system (UWCAES), the isothermal compressed air energy storage system (I-CAES), the supercritical compressed air energy storage system (SC-CAES), and the CAES system combined with liquid piston [17][18][19][20][21][22][23][24]. The various CAES system configurations are shown in Figure 2. It can be seen that in addition to wind power and photovoltaic power that can directly drive the compressors, many types of CAES systems require heat sources, and their heat sources can come from renewable energy sources such as wind energy, solar energy, and biomass energy. Therefore, CAES systems can also be combined with biomass combustion for power generation. Moreover, wind turbines and compressor/expander rotating machinery can be mechanically coupled, so that CAES systems can thoroughly couple with renewable energy sources for both power and thermal energy, to access multiple energy sources complementarily, and efficiently utilize energy.
Moreover, wind turbines and compressor/expander rotating machinery can be mechanically coupled, so that CAES systems can thoroughly couple with renewable energy sources for both power and thermal energy, to access multiple energy sources complementarily, and efficiently utilize energy. In addition, due to the volatility of wind and solar power generation, CAES systems often face frequent changes of operating conditions [25]. Therefore, when CAES is coupled to renewable energy, it is sometimes necessary to conduct system configuration improvements and operational strategy optimization to adapt to frequently changing operating conditions. It is also necessary to coordinate the volatility of renewable energy and the offdesign characteristics of compressed air energy storage, which makes the coupling of renewable energy and CAES complex.
Aiming to solve the above problems, the various topological coupling forms of CAES and renewable energy resources in system structures will be reviewed in this paper. The renewable energies focused on include wind energy, solar energy, and biomass energy. The advantages of these coupling systems for renewable energy utilization will be pointed out, providing a reference for the thermodynamic coupling of CAES systems and renewable energy resources. Finally, an efficient coupling model of CAES and renewable energy resources will be proposed, providing a foundation for the further development of the coupling system.

CAES Systems Coupled with Wind Energy
Many scholars have studied the characteristics of the coupling system of wind power and CAES. Wind-power-connecting CAES has two topological structures of series and parallel connections. In series CAES, during energy storage periods, absorbs all the wind power [26], and then CAES outputs stable electrical energy during the energy release period. When in parallel connection CAES, under energy storage conditions, absorbs the In addition, due to the volatility of wind and solar power generation, CAES systems often face frequent changes of operating conditions [25]. Therefore, when CAES is coupled to renewable energy, it is sometimes necessary to conduct system configuration improvements and operational strategy optimization to adapt to frequently changing operating conditions. It is also necessary to coordinate the volatility of renewable energy and the off-design characteristics of compressed air energy storage, which makes the coupling of renewable energy and CAES complex.
Aiming to solve the above problems, the various topological coupling forms of CAES and renewable energy resources in system structures will be reviewed in this paper. The renewable energies focused on include wind energy, solar energy, and biomass energy. The advantages of these coupling systems for renewable energy utilization will be pointed out, providing a reference for the thermodynamic coupling of CAES systems and renewable energy resources. Finally, an efficient coupling model of CAES and renewable energy resources will be proposed, providing a foundation for the further development of the coupling system.

CAES Systems Coupled with Wind Energy
Many scholars have studied the characteristics of the coupling system of wind power and CAES. Wind-power-connecting CAES has two topological structures of series and parallel connections. In series CAES, during energy storage periods, absorbs all the wind power [26], and then CAES outputs stable electrical energy during the energy release period. When in parallel connection CAES, under energy storage conditions, absorbs the low-valley electricity from the power grid, and then outputs electricity to supplement wind power, making the overall output relatively stable [27,28]. Further applications are a combination of series and parallel connections, so that CAES and wind power are in series during energy storage (absorbing the part of the wind power that exceeds the dispatch), and so that CAES and wind power are connected in parallel when energy is released (smoothly outputting power and increasing the total amount of wind power) [29]. In addition to the power coupling between wind power and CAES, some scholars have studied thermoelectric coupling and mechanical coupling [30][31][32], but there are fewer studies of these aspects. Research progress on coupling methods is described in detail below.

Wind Power Connecting to CAES in Series
A coupling system of wind power connecting with A-CAES in series has been studied by Zhang et al. [26], as shown in Figure 3. The impacts of stable and unstable wind speed on the coupling system were analyzed and compared for energy conversion efficiency and system efficiency in Zhang's work. It was found that the strength and fluctuation of wind speed have an evident impact on the energy conversion efficiency of each part of the coupling system. The existence of a properly stable wind speed makes better use of wind energy with the high efficiency of the coupling system, which is based on both the energy conversion efficiency of the wind turbine and the A-CAES system being relative high.
wind power, making the overall output relatively stable [27,28]. Further applications are a combination of series and parallel connections, so that CAES and wind power are in series during energy storage (absorbing the part of the wind power that exceeds the dispatch), and so that CAES and wind power are connected in parallel when energy is released (smoothly outputting power and increasing the total amount of wind power) [29]. In addition to the power coupling between wind power and CAES, some scholars have studied thermoelectric coupling and mechanical coupling [30][31][32], but there are fewer studies of these aspects. Research progress on coupling methods is described in detail below.

Wind Power Connecting to CAES in Series
A coupling system of wind power connecting with A-CAES in series has been studied by Zhang et al. [26], as shown in Figure 3. The impacts of stable and unstable wind speed on the coupling system were analyzed and compared for energy conversion efficiency and system efficiency in Zhang's work. It was found that the strength and fluctuation of wind speed have an evident impact on the energy conversion efficiency of each part of the coupling system. The existence of a properly stable wind speed makes better use of wind energy with the high efficiency of the coupling system, which is based on both the energy conversion efficiency of the wind turbine and the A-CAES system being relative high.  Meng et al. [33] studied a system where wind power was connected to a conventional CAES compressor directly, and considered the off-design characteristics of the components. The results show that the CAES system at a variable shaft speed mode can utilize more excess wind energy, store more compressed air, generate more electricity, and provide a longer discharging time than that at a constant shaft speed mode. It was also found that the levelized cost of electricity (LCOE) at a variable shaft speed mode is lower than that at a constant shaft speed mode. Ghorbani et al. [34] also studied coupling systems connected in series. The work presented a recovery approach by using phase change material (PCM) thermal storage as a case study. Studies have shown that the system can be more efficient, sustainable, and reduce irreversibility after using phase change heat storage. The system energy efficiency and exergy efficiency can reach 70.83 and 80.71%. Abouzeid et al. [35] presented a cooperative control framework of a wind energy conversion system (WECS) directly connected to CAES. The proposed coordination scheme aimed to solve the contradiction between the speed regulation system and the frequency modulation signal in WECS. The results confirm that the proposed coordination can improve the frequency response of the system compared to the non-coordinated control operation. Meng et al. [33] studied a system where wind power was connected to a conventional CAES compressor directly, and considered the off-design characteristics of the components. The results show that the CAES system at a variable shaft speed mode can utilize more excess wind energy, store more compressed air, generate more electricity, and provide a longer discharging time than that at a constant shaft speed mode. It was also found that the levelized cost of electricity (LCOE) at a variable shaft speed mode is lower than that at a constant shaft speed mode. Ghorbani et al. [34] also studied coupling systems connected in series. The work presented a recovery approach by using phase change material (PCM) thermal storage as a case study. Studies have shown that the system can be more efficient, sustainable, and reduce irreversibility after using phase change heat storage. The system energy efficiency and exergy efficiency can reach 70.83 and 80.71%. Abouzeid et al. [35] presented a cooperative control framework of a wind energy conversion system (WECS) directly connected to CAES. The proposed coordination scheme aimed to solve the contradiction between the speed regulation system and the frequency modulation signal in WECS. The results confirm that the proposed coordination can improve the frequency response of the system compared to the non-coordinated control operation.

Wind Power Connecting to CAES in Parallel
The research concerning wind power connecting to CAES only in parallel is limited. Hasan et al. [27] proposed a parallel connection of the CAES system with a wind turbine to provide continuous electricity to the grid system and to reduce wind power fluctuations. The results show that a parallel CAES system and wind power are able to smooth out wind power fluctuations and to provide continuous power to the grid system with low power consumption.

Wind Power Connecting to CAES in Series and in Parallel
Succar et al. [36] studied the scheduling problem when wind power is combined with CAES in series and in parallel. The system flowchart is shown in Figure 4. Studies have shown that the comprehensive optimization of wind storage systems can significantly reduce LCOE. These changes increase the capacity factor of a wind farm. Meanwhile, the coupling method reduces the storage capacity requirements of the base load plant, and reduces the GHG emission rate of the combined system compared to the coupling of the wind farm to CAES by optimizing the wind farm alone. Sriyakul et al. [28] studied the dispatch of a similar combined system considering economic risks. The system participated in three markets, including day-ahead, intraday, and balancing markets. The downside risk constraint method (DRCA) was modeled using stochastic formulas to enable better management of financial risk. The strategy proposed by DRCA is less profitable (1.97%), but brings a guaranteed risk control strategy to the power aggregator, and the risk is reduced by 100%.
The research concerning wind power connecting to CAES only in parallel is limited. Hasan et al. [27] proposed a parallel connection of the CAES system with a wind turbine to provide continuous electricity to the grid system and to reduce wind power fluctuations. The results show that a parallel CAES system and wind power are able to smooth out wind power fluctuations and to provide continuous power to the grid system with low power consumption.

Wind Power Connecting to CAES in Series and in Parallel
Succar et al. [36] studied the scheduling problem when wind power is combined with CAES in series and in parallel. The system flowchart is shown in Figure 4. Studies have shown that the comprehensive optimization of wind storage systems can significantly reduce LCOE. These changes increase the capacity factor of a wind farm. Meanwhile, the coupling method reduces the storage capacity requirements of the base load plant, and reduces the GHG emission rate of the combined system compared to the coupling of the wind farm to CAES by optimizing the wind farm alone. Sriyakul et al. [28] studied the dispatch of a similar combined system considering economic risks. The system participated in three markets, including day-ahead, intraday, and balancing markets. The downside risk constraint method (DRCA) was modeled using stochastic formulas to enable better management of financial risk. The strategy proposed by DRCA is less profitable (1.97%), but brings a guaranteed risk control strategy to the power aggregator, and the risk is reduced by 100%.

Wind Power and Wind Heat Connecting to CAES
In addition to driving the compressor, high-frequency wind power fluctuations are also converted into resistance heat to heat the inlet of the expander and increase output power. This way, CAES can absorb more fluctuate wind power compared to a situation where wind power only drives the compressor. Yang et al. [37] studied a coupling CAES system for the hybrid use of wind power, as shown in Figure 5. Wind power not only drives the compressor, but also uses a resistance wire to heat the thermal storage device to increase power output and wind energy utilization. A theoretical thermodynamic analysis shows that the coupling CAES system has a stronger ability to absorb thermal energy than the A-CAES system, with the same compressors, thermal energy storage (TES) units, and turbines of the same size. Based on the final storage temperature of TES, the recovery efficiency of this additional wind power is about 41-47%.
Zhao et al. [38,39] also proposed a similar coupling system, in which the low-frequency part of the wind energy drives the compressor and the high-frequency part of the energy uses the resistance wire to heat the expander inlet to improve the output. Meanwhile, two sets of compressors/expanders of different powers are equipped to ensure the

Wind Power and Wind Heat Connecting to CAES
In addition to driving the compressor, high-frequency wind power fluctuations are also converted into resistance heat to heat the inlet of the expander and increase output power. This way, CAES can absorb more fluctuate wind power compared to a situation where wind power only drives the compressor. Yang et al. [37] studied a coupling CAES system for the hybrid use of wind power, as shown in Figure 5. Wind power not only drives the compressor, but also uses a resistance wire to heat the thermal storage device to increase power output and wind energy utilization. A theoretical thermodynamic analysis shows that the coupling CAES system has a stronger ability to absorb thermal energy than the A-CAES system, with the same compressors, thermal energy storage (TES) units, and turbines of the same size. Based on the final storage temperature of TES, the recovery efficiency of this additional wind power is about 41-47%.
Zhao et al. [38,39] also proposed a similar coupling system, in which the low-frequency part of the wind energy drives the compressor and the high-frequency part of the energy uses the resistance wire to heat the expander inlet to improve the output. Meanwhile, two sets of compressors/expanders of different powers are equipped to ensure the wide operating conditions of the system. The simulation results show that one of the more effective ways to eliminate fluctuations in wind power generation is the CAES system with a dual-power-level turbo machinery structure. wide operating conditions of the system. The simulation results show that one of the more effective ways to eliminate fluctuations in wind power generation is the CAES system with a dual-power-level turbo machinery structure.

Motor
Hot tank

Other Coupling Types
The coupling of wind power and CAES also includes other forms, such as mechanical coupling. Sun et al. [40] proposed a mechanical coupling system of CAES and wind turbines, as shown in Figure 6. A prototype test bench was established to enable this mechanism. Through simulation and experimental research, the energy conversion efficiency of the system under different working conditions and different modes was analyzed. The proposed system is proved to be technically feasible, with an energy efficiency around 50%. Due to the capacity of typical scroll expanders, the proposed system is only suitable for small-scale wind energy utilization systems.  Sant et al. [31] coupled CAES and wind energy into a structure, presenting a concept for integrating CAES into spar-type floating wind turbine platforms, as shown in Figure  7. The system uses a hydraulic pump and turbine to complete the power transfer. The results were compared to those derived from a conventional floating offshore wind turbine (FOWT)-spar configuration without an energy storage system. Although integrating CAES into floating beams leads to a significant increase in the weight of the floating structures, studies have shown that the integration of short-term energy storage capabilities in the order of megawatt-hours is technically feasible.

Other Coupling Types
The coupling of wind power and CAES also includes other forms, such as mechanical coupling. Sun et al. [40] proposed a mechanical coupling system of CAES and wind turbines, as shown in Figure 6. A prototype test bench was established to enable this mechanism. Through simulation and experimental research, the energy conversion efficiency of the system under different working conditions and different modes was analyzed. The proposed system is proved to be technically feasible, with an energy efficiency around 50%. Due to the capacity of typical scroll expanders, the proposed system is only suitable for small-scale wind energy utilization systems. wide operating conditions of the system. The simulation results show that one of the more effective ways to eliminate fluctuations in wind power generation is the CAES system with a dual-power-level turbo machinery structure.  Figure 5. Schematic diagram of a coupled system combining wind power, wind heat, and CAES.

Other Coupling Types
The coupling of wind power and CAES also includes other forms, such as mechanical coupling. Sun et al. [40] proposed a mechanical coupling system of CAES and wind turbines, as shown in Figure 6. A prototype test bench was established to enable this mechanism. Through simulation and experimental research, the energy conversion efficiency of the system under different working conditions and different modes was analyzed. The proposed system is proved to be technically feasible, with an energy efficiency around 50%. Due to the capacity of typical scroll expanders, the proposed system is only suitable for small-scale wind energy utilization systems.  Sant et al. [31] coupled CAES and wind energy into a structure, presenting a concept for integrating CAES into spar-type floating wind turbine platforms, as shown in Figure  7. The system uses a hydraulic pump and turbine to complete the power transfer. The results were compared to those derived from a conventional floating offshore wind turbine (FOWT)-spar configuration without an energy storage system. Although integrating CAES into floating beams leads to a significant increase in the weight of the floating structures, studies have shown that the integration of short-term energy storage capabilities in the order of megawatt-hours is technically feasible. Sant et al. [31] coupled CAES and wind energy into a structure, presenting a concept for integrating CAES into spar-type floating wind turbine platforms, as shown in Figure 7. The system uses a hydraulic pump and turbine to complete the power transfer. The results were compared to those derived from a conventional floating offshore wind turbine (FOWT)-spar configuration without an energy storage system. Although integrating CAES into floating beams leads to a significant increase in the weight of the floating structures, studies have shown that the integration of short-term energy storage capabilities in the order of megawatt-hours is technically feasible.  Saadat et al. [41] proposed a compressor air-hydraulic energy storage system, as shown in Figure 8. The system used a liquid pump/turbine for energy storage and release. A liquid piston air compressor/expander with enhanced heat transfer was developed. The enhanced heat transfer was achieved by using porous media and droplet sprays and reduced leakage. The liquid piston compressors/expanders were able to loosely maintain accumulator pressure ratios, while the down-tower hydraulic pumps/motors could accurately track the required generator power. Wind energy connecting with CAES also can be combined with other energy systems to form large hybrid systems for grid frequency regulation [35,42], increasing the revenue from wind power operation [34], and reducing pollutant emissions [43]. Some of the relative research follows.
Li et al. [44] studied the scheduling problems which result from combining wind, gas, power systems, and traditional CAES in the presence of severe uncertainty caused by high wind power penetration. In their study, information gap decision theory was employed to depict the inherent uncertainty of wind power. The numerical test results show that after considering the demand response scheme, the total cost is reduced by 3.63%. Astolfi et al. [45] studied the design problems and coupling effects of a UW-CAES system connected to wind power for grid peak shaving and dispatch ability enhancement. They evaluated the impact of a realistic power input on system performance and plant flexibility. Based on realistic wind data and considering part-load characteristics, annual simulations show that global round-trip efficiency is around 75, and a 10-15% reduction in the average unplanned energy injection into the grid system is achieved. Rahmanifard et al. Saadat et al. [41] proposed a compressor air-hydraulic energy storage system, as shown in Figure 8. The system used a liquid pump/turbine for energy storage and release. A liquid piston air compressor/expander with enhanced heat transfer was developed. The enhanced heat transfer was achieved by using porous media and droplet sprays and reduced leakage. The liquid piston compressors/expanders were able to loosely maintain accumulator pressure ratios, while the down-tower hydraulic pumps/motors could accurately track the required generator power.  Saadat et al. [41] proposed a compressor air-hydraulic energy storage system, as shown in Figure 8. The system used a liquid pump/turbine for energy storage and release. A liquid piston air compressor/expander with enhanced heat transfer was developed. The enhanced heat transfer was achieved by using porous media and droplet sprays and reduced leakage. The liquid piston compressors/expanders were able to loosely maintain accumulator pressure ratios, while the down-tower hydraulic pumps/motors could accurately track the required generator power. Wind energy connecting with CAES also can be combined with other energy systems to form large hybrid systems for grid frequency regulation [35,42], increasing the revenue from wind power operation [34], and reducing pollutant emissions [43]. Some of the relative research follows.
Li et al. [44] studied the scheduling problems which result from combining wind, gas, power systems, and traditional CAES in the presence of severe uncertainty caused by high wind power penetration. In their study, information gap decision theory was employed to depict the inherent uncertainty of wind power. The numerical test results show that after considering the demand response scheme, the total cost is reduced by 3.63%. Astolfi et al. [45] studied the design problems and coupling effects of a UW-CAES system connected to wind power for grid peak shaving and dispatch ability enhancement. They evaluated the impact of a realistic power input on system performance and plant flexibility. Based on realistic wind data and considering part-load characteristics, annual simulations show that global round-trip efficiency is around 75, and a 10-15% reduction in the average unplanned energy injection into the grid system is achieved. Rahmanifard et al. Wind energy connecting with CAES also can be combined with other energy systems to form large hybrid systems for grid frequency regulation [35,42], increasing the revenue from wind power operation [34], and reducing pollutant emissions [43]. Some of the relative research follows.
Li et al. [44] studied the scheduling problems which result from combining wind, gas, power systems, and traditional CAES in the presence of severe uncertainty caused by high wind power penetration. In their study, information gap decision theory was employed to depict the inherent uncertainty of wind power. The numerical test results show that after considering the demand response scheme, the total cost is reduced by 3.63%. Astolfi et al. [45] studied the design problems and coupling effects of a UW-CAES system connected to wind power for grid peak shaving and dispatch ability enhancement. They evaluated the impact of a realistic power input on system performance and plant flexibility. Based on realistic wind data and considering part-load characteristics, annual simulations show that global round-trip efficiency is around 75, and a 10-15% reduction in the average unplanned energy injection into the grid system is achieved. Rahmanifard et al. [43] evaluated the role of CAES in district energy systems. Using CMG STARS, they developed a model to simulate the performance of CAES-geothermal power plants with or without wind power in a typical hot and dry rock reservoir. Studies have shown that wind/CAES-geothermal schemes have the lowest LCOE (7.8-11.8¢/kWh) and an emission intensity of 88-126 g CO 2 /kWh.

CAES Systems Coupled with Solar Energy
Similar to the various coupling forms of wind power and CAES, there are also many coupling forms for solar energy combined with CAES. The coupling between solar energy and CAES can be divided into photovoltaic power generation and CAES coupling, and solar heat and CAES coupling. The first type of coupling is relatively simple: photovoltaic power drives the CAES compressor directly, just as wind power does. The second type involves the matching of multiple heat sources and power generation equipment, which is relatively complex. So only the second case will be focused on here. Many scholars have studied coupling forms and mainly the combination of solar thermal power generation and CAES. In these coupling systems, the expander inlet air of the CAES is heated by solar energy, and the expander of the original solar thermal power generation system is cancelled. Differences between the coupled systems include differences in the technologies used in the CAES, whether it is coupled with other thermal systems, and whether it only provides electrical energy. Table 1 summarizes the relevant research on the coupling of solar energy and CAES. It is found that: (1) when the CAES has a combustion chamber, the system does not use compressed heat; (2) when the system is set up with a bottom cycle (organic Rankine cycle (ORC), etc.), the system generally has no regeneration; and (3) fuel, solar energy, and compressed heat can heat the expander inlet in cascades.  [47] Y N N N Y N Kandezi et al. [48] N Y N N Y N Yang et al. [49] Y + N Y N N N N Wang et al. [50] N [57] Y N N N N N Note: N is short for no, and Y is short for yes.
Several typical coupling schemes are described in detail as follows.

Solar-Assist CAES by Combining the Heating Sources at Expander Inlet
Regenerator, solar energy, and combustion chamber combined heating Jalili et al. [46] studied the scheduling of CAES systems coupled with different renewable energies that used regenerator, solar energy, and combustion chambers to heat the expander inlet sequentially. The effects of a solar-powered CAES system on the performance and efficiency of the energy hub (EH) operation was studied. Using the proposed framework on a typical EH shows the efficiency of the coupling system can reduce op-Energies 2023, 16, 4667 9 of 22 eration costs and emissions in day-ahead energy management. Mohammadi et al. [55] proposed and analyzed an integrated micro gas turbine, CAES system, and solar dish collector system. The system does not only use compression heat, but also uses regenerator and solar energy heating and fuel to heat the expander inlet. The performance of the system was studied using energy and exergy analysis methods. The results show that when the difference between the minimum pressure and the maximum pressure of the air cavern increases, the round-trip efficiency increases.
Compression heat and solar combined heating Li et al. [47] studied a combined CAES system with solar energy with no regenerator to heat the expander inlet, as shown in Figure 9, while outputting heat to the outside. From the perspective of thermodynamics and economics, the performance under five heat distribution schemes (100, 75, 50, 25, and 0%, which represents the ratio of the amount of air used to heat the expander inlet to the amount used by the user in the compression heat) was analyzed and discussed. The results show that the exergy efficiency and net present value increase with a decrease in the heat distribution ratio. The effect of the heat exchanger's effectiveness on the system performance is different in different positions. the expander inlet sequentially. The effects of a solar-powered CAES system on the performance and efficiency of the energy hub (EH) operation was studied. Using the proposed framework on a typical EH shows the efficiency of the coupling system can reduce operation costs and emissions in day-ahead energy management. Mohammadi et al. [55] proposed and analyzed an integrated micro gas turbine, CAES system, and solar dish collector system. The system does not only use compression heat, but also uses regenerator and solar energy heating and fuel to heat the expander inlet. The performance of the system was studied using energy and exergy analysis methods. The results show that when the difference between the minimum pressure and the maximum pressure of the air cavern increases, the round-trip efficiency increases.  Compression heat and solar combined heating Li et al. [47] studied a combined CAES system with solar energy with no regenerator to heat the expander inlet, as shown in Figure 9, while outputting heat to the outside. From the perspective of thermodynamics and economics, the performance under five heat distribution schemes (100, 75, 50, 25, and 0%, which represents the ratio of the amount of air used to heat the expander inlet to the amount used by the user in the compression heat) was analyzed and discussed. The results show that the exergy efficiency and net present value increase with a decrease in the heat distribution ratio. The effect of the heat exchanger's effectiveness on the system performance is different in different positions.

Compressor1
Expander 1  Guo et al. [58] proposed a solar-assist CAES system based on multi-level thermal storage, as shown in Figure 10. The system uses different temperature levels of heat to heat the inlet air of the expander according to different external load requirements. The heat sources are a room-temperature heat source, a compression heat source, and a solar heat source. The multiple heat sources can be heated in series. In addition, the compressor inlet also has two temperature levels to achieve a wide range of operating conditions. The two temperature levels are the cold source obtained from room temperature expansion and room temperature. Due to the cascade utilization of heat, this system can improve system efficiency and enhance the absorption capacity of renewable energy under a wide range of operating conditions. Guo et al. [58] proposed a solar-assist CAES system based on multi-level thermal storage, as shown in Figure 10. The system uses different temperature levels of heat to heat the inlet air of the expander according to different external load requirements. The heat sources are a room-temperature heat source, a compression heat source, and a solar heat source. The multiple heat sources can be heated in series. In addition, the compressor inlet also has two temperature levels to achieve a wide range of operating conditions. The two temperature levels are the cold source obtained from room temperature expansion and room temperature. Due to the cascade utilization of heat, this system can improve system efficiency and enhance the absorption capacity of renewable energy under a wide range of operating conditions.  Regenerator and solar combined heating Morteza Saleh Kandezi et al. [48] studied a solar CAES system which heats the inlet of the expander using regenerative heat and solar energy sequentially, as shown in Figure  11. Meanwhile, the compressed heat is used to produce cold. This hybridization simultaneously generates power, cooling capacity, and hot water for reducing peak energy demand. This combination results in a round-trip efficiency and an exergy efficiency of 67.5 and 45.6%, respectively. Meanwhile, high exergy destruction exists in the solar receiver tower, heliostat field, and pressure regulator. Yang et al. [49] studied the coupling system of LAES and solar energy which uses regenerator, compression heat, and solar energy to heat the LAES expander inlet air sequentially. At the same time, they studied the performance of this combined system under isothermal compression (no compression heat is used), and found the performance improved. When using adiabatic compression under the design conditions, the round-trip efficiencies of the coupled systems are improved by 56.46-87.17%. When using isothermal

Regenerator and solar combined heating
Morteza Saleh Kandezi et al. [48] studied a solar CAES system which heats the inlet of the expander using regenerative heat and solar energy sequentially, as shown in Figure 11. Meanwhile, the compressed heat is used to produce cold. This hybridization simultaneously generates power, cooling capacity, and hot water for reducing peak energy demand. This combination results in a round-trip efficiency and an exergy efficiency of 67.5 and 45.6%, respectively. Meanwhile, high exergy destruction exists in the solar receiver tower, heliostat field, and pressure regulator.

 Regenerator and solar combined heating
Morteza Saleh Kandezi et al. [48] studied a solar CAES system which heats the inlet of the expander using regenerative heat and solar energy sequentially, as shown in Figure  11. Meanwhile, the compressed heat is used to produce cold. This hybridization simultaneously generates power, cooling capacity, and hot water for reducing peak energy demand. This combination results in a round-trip efficiency and an exergy efficiency of 67.5 and 45.6%, respectively. Meanwhile, high exergy destruction exists in the solar receiver tower, heliostat field, and pressure regulator. Yang et al. [49] studied the coupling system of LAES and solar energy which uses regenerator, compression heat, and solar energy to heat the LAES expander inlet air sequentially. At the same time, they studied the performance of this combined system under isothermal compression (no compression heat is used), and found the performance improved. When using adiabatic compression under the design conditions, the round-trip efficiencies of the coupled systems are improved by 56.46-87.17%. When using isothermal Yang et al. [49] studied the coupling system of LAES and solar energy which uses regenerator, compression heat, and solar energy to heat the LAES expander inlet air sequentially. At the same time, they studied the performance of this combined system under isothermal compression (no compression heat is used), and found the performance improved. When using adiabatic compression under the design conditions, the round-trip efficiencies of the coupled systems are improved by 56.46-87.17%. When using isothermal compression, the round-trip efficiencies of the systems are significantly improved to 112.73 to 120.71%, respectively.

Solar-Assist CAES with Bottom Cycle
Most of the above systems could generate excess heat. The reason is that the inlet temperature of the expander is higher with solar heating, and the inlet pressure of the expander is relatively low compared to the high temperature. In addition, the expander outlet temperature is much higher than the ambient temperature [2]. Therefore, in addition to the use of wasted heat, some scholars have proposed combined systems with a bottom cycle as follows.
Wang et al. [50] used solar energy to directly heat the inlet air of a CAES turbine, as the exhaust air from the turbine provides a heat source for organic Rankine cycle (ORC)-power generation. Meanwhile, the compression heat provides heat to users. The results show that the energy efficiency and exergy efficiency of the coupling system reach 98.30 and 68.94%, respectively. Mousavi et al. [56] proposed a configuration of a CAES integrated with an ORC which utilizes geothermal and solar energy as the thermal source. In order to observe and evaluate the performance of a design system, the energy, consumption, and a consumption-economics analysis were used as potential tools. The multi-objective optimization shows that the exergy efficiency and total cost rate of the system are 29% and 18 $/h, respectively, at the optimum conditions.

Solar-Assist CAES with CCHP or Producing Other Products
Solar energy and CAES systems also can be combined with other technologies to realize a multi-product output. Wang et al. [59] proposed a CAES combined with a gas turbine and refrigeration cycle, using regenerator and solar energy to directly heat the expander inlet. At the same time, the system provides cold, heat, and electricity (CCHP) to the outside simultaneously. It has been shown that the performance of the proposed system is mainly determined by the compressor pressure ratio, the inlet pressure and temperature of turbine, and the heat exchanger effectiveness. The results show that the optimal system exergy efficiency is about 53.10 and 45.36% in maximum heating and maximum cooling conditions, respectively. Alirahmi et al. [51] proposed a novel dualpurpose green energy storage system with the aim of providing power and potable water production. The desalination energy is provided by compression heat and turbine exhaust. Under optimal design conditions, the round-trip efficiency is 48.7% and the total cost ratio is $3056/h. Using the proposed system in a case study of San Francisco as an example, a total potable water production of 226,782 m 3 cubic meters, 27,551 MWh of electricity generation, and a payback period of 2.65 years were achieved. Wen et al. [53] proposed an energy hub consisting of a CCHP system along with a wind turbine and photovoltaic cells. The energy hub system has an ice storage conditioner system and an energy storage system. They investigated the effect of a solar-powered CAES on the performance of the energy hub. According to the modeling results, the energy storage system can exhibit successful performance under a system energy management strategy.

CAES Systems Coupled with Both Solar Energy and Wind Energy
Compared to the study of CAES systems coupled with wind energy and solar energy separately, there is less literature on the coupling of wind energy, solar energy, and CAES systems as a whole. However, due to the complementary properties of wind energy and solar energy (wind power is often abundant at night, and solar energy is abundant during the day) [60], some scholars have carried out primary studies of the coupling of wind power, solar energy, and CAES, which can be divided into two categories currently. The first category is systems in which wind power drives the compressor, and solar energy heats the air at the inlet of the expander. The second category is systems in which both wind power and photovoltaics drive the compressor to complete the energy storage process, while CAES discharges without wind and solar energy being coupled. The main focus of research on the above two situations concentrates on the performance of coupling systems and their role in promoting wind power and solar energy absorption. At present, the coupling form of wind energy, solar energy, and CAES is still based on the research discussed in Sections 2 and 3, on wind energy and solar energy alone with CAES. The relevant studies are as follows.
Xu et al. [11] proposed a new type of wind-solar-complementary energy storage integration system. Wind energy is used to drive the compressor and solar energy is used to heat the air inlet of the expander, and the efficiency of the system is 59 to 67%. Garrison et al. [60] proposed a hybrid system of wind-solar-coupling CAES in which the CAES is driven by excess wind energy at night and the power is strengthened by concentrated solar thermal storage, so that these excess energy sources can be dispatched during valley and peak periods, thus highlighting the advantages of energy storage technology. Ji et al. [61] also studied the coupling system of CAES with wind power driving the CAES compressor and the solar energy heating the expander. Meanwhile, the compression heat is used for heating outside or providing heat for ORC power generation. The power storage efficiency, round-trip efficiency, and exergy efficiency of the system are 87.7, 61.2, and 65.4%, respectively. Zhao et al. [29] studied the coupling system of photovoltaic, wind energy, and CAES connected in parallel for rural mobile base stations. The results show that the probability of power loss in the coupling system is 0.988%. Meanwhile, the monthly load and the electricity consumption of a single device, as well as the supply and demand of cold energy are well matched.
In addition to the above studies, some scholars have also studied the role of CAES in the utilization of wind and solar energy, revealing the role of CAES on local energy systems. For example, Wu et al. [62] evaluated the economic management risks of combining windsolar-tidal energy and CAES systems along the coastline. Fourteen critical criteria for management, the economy, and the environment were identified. The results indicate that the risk level of the offshore hybrid system is closer to middle-high. Based on the above results, targeted risk response strategies are proposed for decision makers. The strategies are helpful to realize the reasonable allocation of resources and appropriate risk aversion. Denholm et al. [63] pointed out the role that different energy storage systems play in the application of the utilization of wind power and solar energy. Electricity storage options include batteries, pumped hydro, CAES, and TES. Results show that a combination of load shifting and storage equal to about 12 h of average demand may keep renewable energy curtailment below 10%, when ignoring long-distance transmission options. Marano et al. [64] studied the working characteristics of a coupling system including wind power, photovoltaic, power grid, and CAES in the presence of energy management. Daily periodic energy, economic, and environmental impact analyses of the integration of wind farms and photovoltaic systems were carried out using CAES technology. With the objective of maximizing profits, an optimal management strategy based on dynamic planning was proposed. The results show that the integration of the CAES system can help increase the economic viability of renewable energy sources while strongly reducing CO 2 emissions.

CAES Systems Coupled with Biomass Energy
Different from wind and solar energy, biomass energy can provide a stable and controllable energy output in an energy system with low emission rates. Because biomass is often used in biomass gasification to generate electricity, the combination of biomass energy and CAES is often accompanied by biomass gasification power generation technology, and the technical characteristics are mainly as follows: (1) Since there is a turbine in biomass gasification power generation (BGPG), CAES often shares a turbine with biomass gasification power generation; (2) When biomass is gasified and burned, and CAES has no combustion chamber, the turbine outlet waste heat of the gasification power generation system is used for heating the CAES turbine inlet air; (3) The compression heat generated during the energy storage process can be used for purposes such as preheating the gas during biomass gasification, bottom circulation feed water, or drying the biomass; (4) In some coupling technologies, more heat is produced by CAES and biomass gasification power generation than the heat demand on CAES systems, so the coupling system always provides heat to the outside.
Here are some typical scenarios. Xue et al. [65] presented a coupling system with the above technical characteristics 1 and 3. The system is a biomass-integrated gasification combined cycle (BIGCC), as shown in Figure 12. During the charging process, the heat generated by the compression process in the CAES system is recovered by the thermal regeneration system of the BIGCC system. In the discharging process, unlike in ordinary CAES systems, the bypass flue is arranged parallel to the waste heat steam generator, and the compressed air from the air storage vessel is heated by the bypass flue and fed directly into the combustion chamber of the BIGCC system. The system reduces the power consumption of the original gasification gas turbine compressor and increases the system output power. The results show that the round-trip efficiency and exergy efficiency of the CAES system are 88.43 and 64.28%, respectively. cation power generation than the heat demand on CAES systems, so the coupling system always provides heat to the outside.
Here are some typical scenarios. Xue et al. [65] presented a coupling system with the above technical characteristics 1 and 3. The system is a biomass-integrated gasification combined cycle (BIGCC), as shown in Figure  12. During the charging process, the heat generated by the compression process in the CAES system is recovered by the thermal regeneration system of the BIGCC system. In the discharging process, unlike in ordinary CAES systems, the bypass flue is arranged parallel to the waste heat steam generator, and the compressed air from the air storage vessel is heated by the bypass flue and fed directly into the combustion chamber of the BIGCC system. The system reduces the power consumption of the original gasification gas turbine compressor and increases the system output power. The results show that the round-trip efficiency and exergy efficiency of the CAES system are 88.43 and 64.28%, respectively.
Diyoke et al. [66] presented a coupling system including the above technical features 2, 3, and 4, as shown in Figure 13. It uses partial compression heat to dry the biomass and heat the biomass gas, uses the gasification gas and fuel co-combustion to drive the diesel model to generate electricity, and the exhaust gas heats the CAES expander inlet air during the discharge process (the expander inlet is heated by compression heat and exhaust gas in sequence). The system also supplies heat to the outside. The hybrid system is designed to meet the peak-load power demand of 1.3 MW from a 1 MW A-CAES system and a 0.3 MW rated dual fuel (syngas and diesel) powered engine. The results show that the overall energy and exergy efficiency of the system is approximately 38 and 29%, respectively. The exergy efficiency of the components are 61.38, 89.17, and 86.12% for the biomass gasifier, air compressor, and air expander, respectively.  Diyoke et al. [66] presented a coupling system including the above technical features 2, 3, and 4, as shown in Figure 13. It uses partial compression heat to dry the biomass and heat the biomass gas, uses the gasification gas and fuel co-combustion to drive the diesel model to generate electricity, and the exhaust gas heats the CAES expander inlet air during the discharge process (the expander inlet is heated by compression heat and exhaust gas in sequence). The system also supplies heat to the outside. The hybrid system is designed to meet the peak-load power demand of 1.3 MW from a 1 MW A-CAES system and a 0.3 MW rated dual fuel (syngas and diesel) powered engine. The results show that the overall energy and exergy efficiency of the system is approximately 38 and 29%, respectively. The exergy efficiency of the components are 61.38, 89.17, and 86.12% for the biomass gasifier, air compressor, and air expander, respectively. Lashgari et al. [68] studied a coupling system with the technical features 2, 3, and 4 mentioned above, shown in Figure 14. This is also the CAES combined with BIGCC as described in ref. [65], but the difference is that the turbine of this CAES is independent, and its inlet temperature is the exhaust temperature of the BIGCC. The coupling system is capable of multi-generating heat and power, in addition to load shifting and peak shaving for the electricity grid. The total and electrical round-trip efficiencies of the proposed system are about 71 and 47%, respectively, with a 67 and 12% efficiency improvement, in comparison to the stand-alone biomass power plant.  Zhang et al. [69] presented a coupling system with technical features 1 and 4, as shown in Figure 15. The CAES system absorbs off-peak electricity from the grid system. The high-pressure air is utilized to combust with the bio-gas derived from the biomass gasification process. Meanwhile, the waste heat is utilized by an absorption chiller and a ground source heat pump. In addition, the system enables the energy release process and biomass gasification at the same time. The energy, exergy, and economic performances of Lashgari et al. [68] studied a coupling system with the technical features 2, 3, and 4 mentioned above, shown in Figure 14. This is also the CAES combined with BIGCC as described in ref. [65], but the difference is that the turbine of this CAES is independent, and its inlet temperature is the exhaust temperature of the BIGCC. The coupling system is capable of multi-generating heat and power, in addition to load shifting and peak shaving for the electricity grid. The total and electrical round-trip efficiencies of the proposed system are about 71 and 47%, respectively, with a 67 and 12% efficiency improvement, in comparison to the stand-alone biomass power plant.  Lashgari et al. [68] studied a coupling system with the technical features 2, 3, and 4 mentioned above, shown in Figure 14. This is also the CAES combined with BIGCC as described in ref. [65], but the difference is that the turbine of this CAES is independent, and its inlet temperature is the exhaust temperature of the BIGCC. The coupling system is capable of multi-generating heat and power, in addition to load shifting and peak shaving for the electricity grid. The total and electrical round-trip efficiencies of the proposed system are about 71 and 47%, respectively, with a 67 and 12% efficiency improvement, in comparison to the stand-alone biomass power plant.  Zhang et al. [69] presented a coupling system with technical features 1 and 4, as shown in Figure 15. The CAES system absorbs off-peak electricity from the grid system. The high-pressure air is utilized to combust with the bio-gas derived from the biomass gasification process. Meanwhile, the waste heat is utilized by an absorption chiller and a ground source heat pump. In addition, the system enables the energy release process and biomass gasification at the same time. The energy, exergy, and economic performances of Zhang et al. [69] presented a coupling system with technical features 1 and 4, as shown in Figure 15. The CAES system absorbs off-peak electricity from the grid system. The high-pressure air is utilized to combust with the bio-gas derived from the biomass gasification process. Meanwhile, the waste heat is utilized by an absorption chiller and a ground source heat pump. In addition, the system enables the energy release process and biomass gasification at the same time. The energy, exergy, and economic performances of the proposed system are analyzed. The round-trip efficiency and exergy efficiency are 90.06 and 31.52%, respectively.  Based on whether there is coupling of the key processes of BGPG and CAES, whether the two share common turbines, whether there is heat transfer between the BGPG and CAES turbines, and whether it supplies heat to the outside world, the above key references are summarized in Table 2.  [66] Y N N Y Y Razmi et al. [70] Y Y Y N Y Hai et al. [71] Y Y N N Y Note: N is short for no, and Y is short for yes.

Prospects and Challenges of the Coupling Systems
The combination of CAES and wind energy or solar energy is different from the combination of CAES and biomass, because the former is usually just a CAES system combined with one or two energy sources, while the latter is a combination of both systems (CAES and BGPG systems). Therefore, the combination system of CAES and biomass energy is more complex, and there is a certain difficulty in engineering with a high cost. Meanwhile, biomass energy is much less available than wind and solar energy resources Based on whether there is coupling of the key processes of BGPG and CAES, whether the two share common turbines, whether there is heat transfer between the BGPG and CAES turbines, and whether it supplies heat to the outside world, the above key references are summarized in Table 2.  [66] Y N N Y Y Razmi et al. [70] Y Y Y N Y Hai et al. [71] Y Y N N Y Note: N is short for no, and Y is short for yes.

Prospects and Challenges of the Coupling Systems
The combination of CAES and wind energy or solar energy is different from the combination of CAES and biomass, because the former is usually just a CAES system combined with one or two energy sources, while the latter is a combination of both systems (CAES and BGPG systems). Therefore, the combination system of CAES and biomass energy is more complex, and there is a certain difficulty in engineering with a high cost. Meanwhile, biomass energy is much less available than wind and solar energy resources [3], so it is of great significance for future application prospects to combine CAES with wind and solar for the consumption of renewable energy. After an analysis, the current advantageous directions for continued development follow.

CAES Coupled with Wind Energy, Solar Energy, and Auxiliary Energy Storage (AES)
CAES systems coupled with renewable energy scenery often involve multi-energy coupling issues. With regard to renewable energy, wind power generally shows the characteristics of having a large amount of power at night and a smaller amount during the day, which often does not match the user load curve (peak electricity consumption during the day and trough electricity consumption at night). In addition to energy storage technology which can reduce this mismatch, solar energy (solar photovoltaic power generation only during the day) can also compensate for the small amount of wind power during the day. Figure 16 shows the complementarity of wind and photovoltaic power generation in general for a region in January and July. It can be seen that the total power after complementarity is closer to the user load, which shows a good complementary effect. In addition to the intermittent (power mismatch) issues, wind power fluctuations tend to have high-frequency fluctuations. In order to improve the absorption rate of wind power, except to CAES for absorbing low-frequency fluctuating loads, it is also necessary to use a faster-response AES (auxiliary energy storage) system to absorb the high-frequency part of wind power, such as flywheels, supercapacitors, and batteries [9,72]. Preliminary research shows that CAES and AES systems can greatly improve the utilization rate and energy utilization efficiency of wind energy [73], while reducing operating costs while maintaining frequency [74].  [3], so it is of great significance for future application prospects to combine CAES with wind and solar for the consumption of renewable energy. After an analysis, the current advantageous directions for continued development follow.

CAES Coupled with Wind Energy, Solar Energy, and Auxiliary Energy Storage (AES)
CAES systems coupled with renewable energy scenery often involve multi-energy coupling issues. With regard to renewable energy, wind power generally shows the characteristics of having a large amount of power at night and a smaller amount during the day, which often does not match the user load curve (peak electricity consumption during the day and trough electricity consumption at night). In addition to energy storage technology which can reduce this mismatch, solar energy (solar photovoltaic power generation only during the day) can also compensate for the small amount of wind power during the day. Figure 16 shows the complementarity of wind and photovoltaic power generation in general for a region in January and July. It can be seen that the total power after complementarity is closer to the user load, which shows a good complementary effect. In addition to the intermittent (power mismatch) issues, wind power fluctuations tend to have high-frequency fluctuations. In order to improve the absorption rate of wind power, except to CAES for absorbing low-frequency fluctuating loads, it is also necessary to use a faster-response AES (auxiliary energy storage) system to absorb the high-frequency part of wind power, such as flywheels, supercapacitors, and batteries [9,72]. Preliminary research shows that CAES and AES systems can greatly improve the utilization rate and energy utilization efficiency of wind energy [73], while reducing operating costs while maintaining frequency [74]. In this paper, a wind-solar-storage coupling system is proposed, as shown in Figure  17. In this system, CAES and AES together absorb wind power during the trough of electricity consumption, while photovoltaics can also reduce some of the fluctuations of wind power during the trough. Solar thermal energy adopts the form of graded heat storage, the purpose of which is to widen the working conditions of the CAES release process, so that during the peak period of electricity consumption, the CAES system can better make up for the lack of wind power and photovoltaics, thereby making the overall output closer to the user load. Through the above coupling method, an excellent match between wind energy, solar energy, and energy storage can be achieved, and the deep and efficient utilization of wind power and solar energy is further realized. In this paper, a wind-solar-storage coupling system is proposed, as shown in Figure 17.
In this system, CAES and AES together absorb wind power during the trough of electricity consumption, while photovoltaics can also reduce some of the fluctuations of wind power during the trough. Solar thermal energy adopts the form of graded heat storage, the purpose of which is to widen the working conditions of the CAES release process, so that during the peak period of electricity consumption, the CAES system can better make up for the lack of wind power and photovoltaics, thereby making the overall output closer to the user load. Through the above coupling method, an excellent match between wind energy, solar energy, and energy storage can be achieved, and the deep and efficient utilization of wind power and solar energy is further realized.

Integrated System
In order to promote the deep integration and collaborative efficient operation of renewable energy and energy storage systems, the energy streams from wind, solar, and storage should be fully complementary to become an integrated system, which requires that each subsystem should coordinate and unify the operation. Figure 18 shows an integrated wind-solar-storage system. The system not only couples the energy in each part, but also requires more coordinated control technology, so that the system energy can be coordinated and unified at all times to achieve the high efficiency of the whole process. In recent years, studies have shown that the integration of renewable energy and energy storage will become an important technology tendency for the utilization of renewable energy, which will greatly improve the utilization rate of renewable energy, and the efficiency and reliability of energy systems [6]. In the future, the integrated system will not only be applied to the deep utilization of large-scale wind and solar energy, but also will be used as a stand-alone energy source, eliminating the demand for regional grids in remote places.

Integrated System
In order to promote the deep integration and collaborative efficient operation of renewable energy and energy storage systems, the energy streams from wind, solar, and storage should be fully complementary to become an integrated system, which requires that each subsystem should coordinate and unify the operation. Figure 18 shows an integrated wind-solar-storage system. The system not only couples the energy in each part, but also requires more coordinated control technology, so that the system energy can be coordinated and unified at all times to achieve the high efficiency of the whole process. In recent years, studies have shown that the integration of renewable energy and energy storage will become an important technology tendency for the utilization of renewable energy, which will greatly improve the utilization rate of renewable energy, and the efficiency and reliability of energy systems [6]. In the future, the integrated system will not only be applied to the deep utilization of large-scale wind and solar energy, but also will be used as a stand-alone energy source, eliminating the demand for regional grids in remote places.

Integrated System
In order to promote the deep integration and collaborative efficient operation of renewable energy and energy storage systems, the energy streams from wind, solar, and storage should be fully complementary to become an integrated system, which requires that each subsystem should coordinate and unify the operation. Figure 18 shows an integrated wind-solar-storage system. The system not only couples the energy in each part, but also requires more coordinated control technology, so that the system energy can be coordinated and unified at all times to achieve the high efficiency of the whole process. In recent years, studies have shown that the integration of renewable energy and energy storage will become an important technology tendency for the utilization of renewable energy, which will greatly improve the utilization rate of renewable energy, and the efficiency and reliability of energy systems [6]. In the future, the integrated system will not only be applied to the deep utilization of large-scale wind and solar energy, but also will be used as a stand-alone energy source, eliminating the demand for regional grids in remote places. Figure 18. Schematic diagram of wind-solar-storage integrated system. Figure 18. Schematic diagram of wind-solar-storage integrated system.

Challenge
In terms of system structure, the wind-solar-storage integrated system is complex. The volatility and intermittency of wind and solar, which varies with the weather, complicates the operating environment of the energy storage system. Whether CAES and AES systems can face the extreme volatility and intermittency of wind/solar energy is a challenge, which requires the CAES system to make structure upgrades for adaption, so the structure improvement and the capacity configuration of the CAES system becomes a complex issue. At the same time, it needs a strong energy management and control system to maximize the use of wind energy and solar energy. The energy management system can consider the scheduling problem in real time and the efficient distribution of each energy flow direction, while ensuring the safety and reliability of the CAES and AES systems.
In terms of the theoretical research regarding the study of the CAES system in the integrated solar-wind-storage system, due to issues such as the uncertainty of renewable energy, system structural diversity, and multi-physical process coupling, there are difficulties and challenges in the coupling system integrated design and multi-subsystem collaborative control. Previous literature has only focused on the feasibility, energy efficiency, and economy of coupling systems, and has not studied possible system structures and the dynamic operation regulations of CAES when absorbing renewable energy. Moreover, the potential of CAES technology has not been fully explored, which is crucial for the optimal design and operation of CAES working under renewable energy. The above issues will become our future work.

Conclusions
This paper summarizes the coupling systems of compressed air energy storage (CAES) systems and wind, solar, and biomass energy from the perspective of system topology, and points out the advantages and limitations of each system. The specific conclusions are as follows.
The coupling of wind power and CAES can be divided into connection in series, in parallel, and series-parallel combination, with series-parallel combination being the most common coupling method. Wind power can also convert heat to achieve coupling between wind heat and CAES, achieving the full utilization of wind energy and enhancing CAES output. In addition, wind power and CAES can also achieve mechanical coupling and coupling with other forms of energy storage.
The coupling between solar heat and CAES is an important form of coupling between solar energy and CAES. Solar heat-coupled CAES mainly uses solar energy to heat expander inlet air. Based on various CAES forms and various heat heating sequences, there are a variety of coupling forms of solar energy and CAES which reasonably utilize system heat and generate work through the expander. In order to fully utilize solar heat, solar-assist CAES with bottom cycle or multi-generation systems have also been proposed in recent years. These systems have, to some extent, improved energy utilization efficiency, but the system complexity is also increased.
The combination of biomass and CAES is generally based on biomass gasification power generation technology. We summarized that the various coupled systems of CAES and biomass energy can be classified according to: whether there is coupling of the key processes of biomass gasification power generation (BGPG) and CAES, whether the two share common turbines, whether there is heat transfer between the BGPG and CAES turbines, and whether it supplies heat to users. Typical system research was presented.
Due to the complementary nature of solar and wind energy, as well as the global richness of wind and solar resources, the wind-solar-CAES multiple coupling system is expected to become a promising large-scale form for the utilization of renewable energy in the future, and the integrated system is a very potentially important system configuration. Due to issues such as the uncertainty of renewable energy, system structural diversity, and multi-physical process coupling, there are difficulties and challenges in coupling system integrated design and multi-subsystem collaborative control.
Author Contributions: H.G., conceptualization, visualization, writing-original draft, writingreview and editing, investigation, formal analysis, and methodology; H.K., writing-original draft, writing-review and editing, formal analysis, and investigation; Y.X., writing-review and editing, methodology, and supervision; M.Z., writing-review and editing, and formal analysis; Y.Z., writingoriginal draft, and investigation. H.Z., writing-review and editing, and formal analysis; H.C., writing-review and editing, and supervision. All authors have read and agreed to the published version of the manuscript.