# A Solar-Thermal-Assisted Adiabatic Compressed Air Energy Storage System and Its Efficiency Analysis
## 5 Conclusions  
In this paper, we proposed a solar-thermal-assisted A-CAES, i.e., ST-CAES. Exhaust air preheats the compressed air, which is further heated by stable solar thermal energy, which greatly improves the performance of ST-CAES during the discharge process. In the charge process, water at ambient temperature was used to recover the compression heat and produce hot water, which provides heat for residents and reduces the power consumption of the compressor. Moreover, a mathematical model comprising energy and exergy analyses was developed. On an implementation of ST-CAES with a four-stage compression train and a three-stage expansion train, the ESE, RTE, and exergy efficiency were found to be 70.2%, 61%, and 50%, respectively. In addition, investigations of the system performance and the effects of key parameters have indicated that the turbine inlet temperature and pressure in the SPT are the critical parameters affecting the performance of the proposed ST-CAES system. Therefore, they can act as essential components in current smart grids and cities owing to the high efficiency and ability to accommodate renewables.  
Figure 11: Effect of SPT maximum pressure on \(P_{\text{COM}}\) and operation time.  
Figure 12: Effect of SPT maximum pressure on ESE, RTE, and \(\eta_{\text{ex}}\).  
**Author Contributions:** [PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] analyzed the data; [PERSON], [PERSON] and [PERSON] revised the paper; [PERSON] wrote the paper.  
**Funding:** This research Supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51621065), the Scientific and Technological Project of Qinghai Province (2017-GX-101), and the Natural Science Foundation of Qinghai Province (2017-ZJ-932Q).  
**Conflicts of Interest:** The authors declare not conflict of interests.  
**Nomenclature**  
**Abbreviations**  
COM & air compressor stage  
CAES & compressed air energy storage  
A-CAES & adiabatic compressed air energy storage  
RES & renewable energy sources  
TES & thermal energy storage  
PHS & pumped storage system  
H-ACAES & hybrid ACAES  
TUR & air turbine stage  
CCHP & combined cooling heating and power  
HEX & heat exchanger  
STC & solar thermal collection and storage  
PTC & parabolic trough collector  
HWT & hot water tank  
HTF & heat transfer fluid  
CWT & cold water tank  
HOT & high-temperature oil tank  
LOT & low-temperature oil tank  
ESE & electricity storage efficiency  
EXE & exergy efficiency  
RTE & round-trip efficiency  
AC & air compressor  
AT & air turbine  
OP & oil pump  
SPT & steel pipeline tank  
TV & throttle valve  
HR & heat regenerator  
ST-CAES & solar-thermal CAES  
**Greek Symbols**  
\(\eta\) & efficiency (%)  
\(\beta\) & compression ratio  
\(\tau\) & time (h)  
\(\sigma\) & Stefan-Boltzmann constant  
\(\varepsilon\) & emissivity  
**Symbols**  
\(Ex\) & exergy (kW)  
\(p\) & Power (kW)  
\(s\) & specific entropy (kJ/kg\(\cdot\)K)  
\(k\) & polytropic index  
\(h\) & specific enthalpy (kJ/kg)  
\(Q\) & energy (kW)  
m & Mass flow (kg/s)  
\(p\) & Pressure (MPa)