Study on the influence of expansion section parameters on the performance of compressed air energy storage system

Compressed air energy storage (CAES) gains attention because of its promising future application. In this paper, the parameters related to the expansion section, such as the heat exchanger efficiency, the high-pressure storage tank outlet air temperature and the high-pressure storage tank outlet air pressure, are selected to study their effects on the performance and setting of the CAES system. The results show that with the change in heat exchanger efficiency, air temperature and pressure at the tank outlet, the compression and expansion stage numbers are significantly affected. Because of the influence of the change of compression and expansion stage numbers, the hot water residual temperature, the efficiency of the first cycle, the efficiency after multiple cycles, and the area of the heat exchanger have changed. It can be seen that the change of expansion section parameters mainly affects the system performance by affecting the setting of compressor and expander stages.


Introduction
With the increasingly severe energy situation, China has proposed a strategy of "achieving carbon peak and carbon neutralization".To achieve efficient use of energy, researchers began to pay attention to high-efficiency energy using technology.
Currently known energy storage technologies [1][2][3][4][5] include water-pumped, compressed air, flywheel, electrochemical, etc. Considering the duration of energy storage, the environmental friendliness, and the device service life, compressed air energy storage is one of the technologies with obvious advantages.However, water storage and energy storage have a high demand on the terrain, and compressed air energy storage can avoid the dependence of the device on the terrain by using highpressure air tanks instead of underground caves.Considering various factors, compressed air energy storage technology gets researchers' attention.
A CAES system is mainly composed of a gas path and a thermal cycle loop.The gas path includes a compression section, a high-pressure gas storage tank, an expansion section, etc., while the thermal cycle is composed of a heat exchanger and thermal storage device.The parameter settings of each part may affect the performance of the energy storage system.
A series of studies focused on parameter settings of various parts of the CAES system have been published, but did not discuss in detail the influence of parameter settings of the expansion section on system performance.Therefore, this paper selects the heat exchanger efficiency, air pressure and temperature at the inlet of the expansion section (i.e., the outlet of the high-pressure air storage tank) and other parameters involved in the expansion section to carry out research, so as to clarify their impact on the system performance and provide theoretical guidance for the parameter setting of the CAES system.Figure 1 shows the multi-stage compression and cooling CAES system.COMPi is the compressor for stage i compression, EXPAi is the expander for stage i expansion, HEC1, HEC2 and HEC3 are the heat exchangers for interstage cooling, HEC4 is the heat exchanger for aftercooling, HEE1 is the heat exchanger for preheating, HEE2, HEE3 and HEE4 are the heat exchangers for interstage heating, VALVE1 is the throttle valve, CAS (compressed air storage) is the gas storage device, and TES (thermal energy storage) is a heat storage device.In this study, pressurized water is chosen as the heat storage cycle medium.

Effects of heat exchanger efficiency
As shown in Figure 2(a), under the condition of constant maximum heat storage temperature, the expansion stage decreases at 0.5 heat exchanger efficiency and 0.6 heat exchanger efficiency, which has an impact on the work capacity of the subsequent expansion section.As shown in Figures 2(b)-(c), the effect of heat exchanger efficiency on the energy storage efficiency of the system after the first cycle and multiple cycles under the condition of fixed maximum heat storage temperature.It can be seen from the figure that under the given maximum temperature, the influence of the improvement of heat exchanger efficiency on the system energy storage efficiency is the same as that under the constant water flow.As shown in Figure 2(d), the influence of the improvement of heat exchanger efficiency on the residual temperature of hot water at the outlet of the intermediate heat exchanger under the condition of constant maximum heat storage temperature is the same as that under the condition of constant water flow.In Figure 2(e), the total UA of the expansion section heat exchanger also shows the same change trend as that under constant water flow.

Effects of expansion section inlet air temperature
As shown in Figure 3(a), the change of total flow of heat exchange water versus air temperature at the expansion section inlet.Considering the long-term stable operation of the energy storage system, the total hot water flow exchange in the compression section is set to be the same as that in the expansion section.In the figure, it is obvious that with the increase of the expansion section inlet temperature, the total hot water flow rate for exchange gradually decreases.This is because the increase of the air temperature at the expansion section inlet means that the total amount of heat energy transferred from the air in the compression section to the heat exchange water is gradually reduced, and the required water flow is gradually reduced when the heat absorbed by the unit mass water is unchanged.On the other hand, the increase in the gas storage temperature means that the initial work capacity of the air in the expansion section is enhanced, thus affecting the subsequent changes in the energy storage efficiency.Figures 3(b)-(c) show the influence of the expansion section inlet temperature change on the system energy storage efficiency of the first cycle and multiple cycles respectively.With the increase of the expansion section inlet temperature, the energy storage efficiency of the first cycle gradually decreases, and the energy storage efficiency after multiple cycles is basically unchanged.In the first cycle, the compression section needs to do work to increase the pressure in the high-pressure air tank from atmospheric pressure to the maximum gas storage pressure.When the air is released, when the pressure of the gas tank is lower than the set outlet pressure of the throttle valve, it is impossible to continue to vent outwards.From the previous analysis, increasing the inlet air temperature of the expansion section can improve the initial work capacity of the air.Similarly, the internal energy of the air remaining in the CAS will also increase.
With the increase of the expansion section inlet air temperature, the hot water residual temperature at the intermediate heater outlet shows a rising trend, as shown in Figure 3(d).The hot water residual temperature of the intercooler is related to the heat transferred between water and air.As mentioned above, when the expansion section inlet air temperature increases, the air's ability to do work increases, and the amount of heat energy needed to obtain from the hot water decreases.
Figure 3(e) shows the influence of the change of the expansion section inlet air temperature on the heat exchanger UAs in the compression and expansion section.It can be seen from the figure that the total heat exchanger UA in the expansion section increases with the expansion section inlet air temperature.This is because with the increase of the inlet air temperature, the temperature gradient between air and water decreases, and the heat transfer capacity decreases.Therefore, a larger heat transfer area is required for heat transfer.The total UA of the heat exchanger in the compression section increases first and then decreases with the expansion section inlet air temperature.The coupling effect between two opposite influence trends caused by air temperature change dominates this rule.

Effects of expansion section inlet air pressure
Figure 4(a) shows the influence of the change in the inlet pressure of the expansion section on the compression and expansion stages.With the increase of inlet pressure, the number of compression stages and expansion stages increases gradually.With the increase of the maximum gas storage pressure, the single-stage pressure ratio in the compression section increases, and the single-stage exhaust temperature increases.When the limit is exceeded, the compression stages need to be increased to reduce the single-stage exhaust temperature.With the increase of the maximum gas storage pressure, each stage expansion ratio increases, and the outlet air temperature of expanders decreases.The total flow of hot water exchange is also affected by the inlet pressure of the expansion section, as shown in Figure 4(b).Since the total hot water flow rate exchange in the compression and expansion section are the same, this section discusses the total hot water flow rate exchange in the compression section.Influenced by the increase of the inlet pressure of the expansion section, the total flow of the heat exchange water gradually increases.As shown in Figure 4(c), the influence on the hot water residual temperature in the expansion section is obvious.Under the influence of the increasing inlet pressure, hot water residual temperature shows a coupling law of significantly decreasing and slowly increasing, which is jointly affected by the maximum gas storage pressure and the change of expansion stage setting.With the increase of the expansion stage, the water flow rate in the single heating heat exchanger decreases, so the heat transferred from the unit mass of water to cold air increases, and the residual temperature of water decreases.When the maximum gas storage pressure increases, the compression heat received by unit water mass increases, so the hot water residual temperature increases.Figures 4(d)-(e) respectively show the influence of the expansion section inlet pressure on the energy storage efficiency of the first cycle and multiple cycles.When the expander inlet air pressure is relatively small, the system efficiency increases with its increase, and then remains basically unchanged.As shown in Figure 4(f), the total UA of the heat exchanger in the compression section and expansion section is affected by the inlet pressure of the expansion section.When the expansion stage number is fixed, the total expansion section heat exchanger UA decreases with the increase of the inlet pressure, and the increase of the expansion stage will lead to a significant increase in the total UA of the expansion section heat exchanger.

Summary and outlook
In this paper, the influence of expansion section parameters, such as heat exchanger efficiency, inlet air temperature and inlet air pressure on the CAES system, is studied.Seen from the influence of the change of the inlet parameters of the expansion section, it is not completely monotonous.Therefore, for the parameter selection and system setting of a specific 100 kW CAES system, it is necessary to comprehensively consider the optimal combination under the coupling of multiple parameters, and finally realize the optimization of the system energy storage efficiency.

Figure 2 .
The influence of heat exchanger efficiency on (a) expansion stage number, (b) first time cycle efficiency, (c) stable cycle efficiency, (d) hot water residual temperature, (e) UA.

Figure 3 .
Figure 3. Influence of inlet temperature of expansion section on (a) total flow rate of heat exchange water, (b) first cycle efficiency, (c) stable cycle efficiency, (d) hot water residual temperature, (e) UA.

Figure 4 .
Figure 4. Influence of inlet pressure of expansion section on (a) number of compression stage and expansion stage, (b) total flow rate of heat exchange water, (c) hot water residual temperature, (d) first cycle efficiency, (e) stable cycle efficiency, (f) UA.