Performance study of the supplemental combustion type compressed air energy storage system

Compressed air energy storage technology is considered to be the most promising energy storage technology, but it has not been applied commercially on a large scale, partly because of the low system efficiency, with the existing efficiency being about 70%. To improve the round trip efficiency of the system, this paper proposes a supplementary combustion compressed air energy storage system based on adiabatic compressed air energy storage. The system adds supplementary combustion equipment to increase expansion machines’ inlet air temperature by burning fuels such as syngas, hydrogen, and natural gas to increase the power generation of the system. The thermodynamic performance analysis has been conducted through the thermodynamic model of the system, and the effects of parameters such as compressor discharge temperature, supplementary combustion, and air-fuel ratio on the system performance have been further investigated. The research results show that the efficiency of the system is improved by nearly 6% compared with the conventional adiabatic compressed air energy storage system. Meanwhile, the system’s round-trip efficiency can be further increased by appropriately raising the compressor discharge temperature and supplemental combustion quantity.


Introduction
Energy storage technology is an integral part of the energy revolution, which can solve the grid connection problem of renewable energy and improve the utilization rate of sustainable energy, and is an extremely critical safeguard for the maintenance of grid security as well as new energy consumption.Compressed air energy storage system (CAES) has a low environmental impact, long operation time, and long operating life, which can effectively solve the problem of using a large amount of sustainable energy.
Germany and the United States have built and put into operation two large commercial CAES power stations, both of which are traditional combustion CAES systems, but they have not been widely applied due to their low round-trip efficiency (RTE).The main reason for the low RTE of traditional combustion CAES systems is that a large amount of compression heat generated during the energy storage process is directly emitted into the atmosphere.Therefore, some scholars have proposed new CAES systems such as advanced adiabatic CAES and isothermal CAES to improve the round-trip efficiency of the systems by recycling the compressed heat.While the advanced adiabatic CAES system uses a heat storage facility to hold compression heat, the inevitable heat exchange temperature difference during the heat exchange process reduces the energy level of the compression heat, resulting in lower compressed air temperatures entering the expander and insufficient power capacity.Therefore, the improvement in the system's RTE is limited.
To deal with the lack of energy level of compression heat, the CAES system can be coupled to other systems for increasing the inlet air temperature of the expander as a way to increase the RTE of the system.Zhang [1] coupled a CAES system with a coal gasification system, using the gas produced by coal gasification to replace the fuel natural gas in the conventional CAES system; Guang et al. [2] presented a new CCHP system coupled with compression air and thermochemical energy storage, using syngas fuel produced by methanol cracking mixed with high-pressure air to drive the gas turbine for power generation; Xue et al. [3] introduced a new scheme to integrate a biomass gasification combined cycle system with a CAES system, using the biomass gasification reaction to produce syngas, and using high-pressure air in the combustion chamber to produce flue gas to drive the gas turbine for power generation.As can be seen, there has been a lot of research on CAES coupled with various energy systems and the use of syngas combustion to supplement combustion.However, advanced research is needed to explore the system integration between compression heat recovery and increasing the temperature of the compressed air inlet to the expander through supplementary combustion.
In this paper, a supplementary combustion CAES system (SC-CAES) is proposed on the basis of the adiabatic CAES system.The system can enhance the inlet gas temperature of the expander and the RTE of the system through the integration of compression heat recovery and supplementary combustion.This paper constructs a model of the SC-CAES system and further investigates the influence of parameters including compressor exhaust temperature and air-fuel ratio on the system performance, and then reveals the internal parameter interaction mechanism and optimization direction of the SC-CAES system.

Description of the system
The principle of the SC-CAES system is illustrated in Figure 1.The SC-CAES system includes compressors, heat exchangers, gas storage chambers, burners, expanders, and heat storage tanks.The system work operation is divided into two processes, one is the energy storage procedure, and the other is the energy release procedure.In the energy storage phase, compressor units can be driven by renewable energy generation or low-cost electricity from the grid.The temperature and pressure of the air are increased by the compression of the compressor unit.Thermal energy is exchanged with heat exchangers at multiple levels to transfer thermal energy to the heat storage working medium, realizing the stepwise compression and heat exchange, and finally, it was stored in a cryogenic and hyperbaric gas storage chamber; the low-temperature heat storage working medium after heat exchange stores the thermal energy in the heat storage tank, thus completing the conversion of electric energy to air internal energy.In the process of energy release, hyperbaric air is released from the gas storage chamber.First, it is preheated in the heat exchanger, and then enters the combustion chamber to burn with fuel, forming hyperpyrexia and hyperbaric flue gas that enters the hyperbaric turbine expander for work.Then, the flue gas enters the low-pressure expander through the heat exchanger for heat exchange to do work, multistage expansion, multistage heat exchange, and staged supplementary combustion, so that the turbine expansion unit can work to drive the generator to generate electricity and transmit the electric energy back to the power grid, so as to realize the conversion of the internal energy and electric energy of high-pressure air.

Model of the SC-CAES system
For the purpose of studying the property of the SC-CAES system, it is necessary to first establish a thermodynamic model of the relevant components, and then build a model of the whole system based on the energy balance and mass conservation principles to perform simulations.
In order to streamline the system model, several assumptions are made in this paper as follows [4].
1) The air is the ideal gas.
2) Air pressure losses in the piping and at the component connections are ignored.
3) The whole system is in a steady-state process.4) All the effects of kinetic energy and potential are neglected.
5) The heat storage tank is adiabatic.6) Both the compressor and the turbine expander operate as an isentropic process.

Compressor
According to the relevant adiabatic compression theory equation, we calculate the compressor outlet temperature, pressure and power consumption, and other parameters.For the SC-CAES system with n compressor stages, the temperature of the outlet air of a particular compressor stage may be represented in the following way: ( The pressure of the compressor outlet mass can be expressed as follows： Ci,out Ci Ci,in 2) The power consumed by the compressor can be described as follows: ) Therefore, the overall power of the compressor set can be derived as follows: where the subscripts i =1,2,3 represent the number of compressor stages in the system, Ci,in T is the temperature of the compressor inlet mass; is the adiabatic index; Ci  is the compressor pressure ratio; Ci  is the compressor isentropic efficiency; Ci G is the mass flow rate of the mass entering the first compressor stage; Ci,in h and Ci,out h are the specific enthalpy of the compressor inlet and outlet air, which are respectively.

Heat exchanger
The heat exchanger can recover the compression heat in the energy conservation procedure and decrease the specific work of the compressor unit.We can establish the heat exchange model of the heat exchanger and get the exchanged heat as shown in the following equation. (5)

Gas Storage chamber
According to the principle of mass conservation, the filling time of the gas storage chamber can be obtained as follows: The venting time of the energy release procedure is: ac,in ac,out ac dis air,dis ( ) where ac V denotes gas storage chamber volume; ac,in  and ac,out  denote the density of air entering and leaving the storage chamber, separately.

Combustion chamber
Combustion chamber refers to the devices that mix fuel and air for combustion in a certain way.In this paper, natural gas is selected as the fuel for the SC-CAES system.More than 95% of natural gas is methane, so the chemical reaction equation in the combustion chamber is as follows: (8) Assuming that the combustion reaction process occurring in the combustion chamber is an isobaric process, the mass conservation equation can be expressed as follows: air,in air,in CH CH cc cc cc,out cc,out where cc,out G , cc,out h and cc,out T are the high-temperature flue gas mass flow rate, specific enthalpy, and temperature, respectively; cc Q is the heat released from fuel combustion; cc  is the combustion chamber efficiency.

Expander
High-pressure air through the expansion unit to do external work, the expansion procedure can be considered as an adiabatic process.The energy release of the SC-CAES system has stages of expansion machine, and the temperature of the expander outlet mass can be expressed as follows: (1 ) Ti Ti,out Ti,in Ti The pressure of the expander outlet mass can be derived as follows: Where the subscripts i =1, 2 represents the level of the expansion unit, Ti,in T is the temperature of the inlet mass of the expansion machine; Ti  is the pressure ratio of the expansion machine; Ti  is the isentropic efficiency of the expansion machine; Ti G is the mass flow rate of the inlet mass of the expansion machine; Ti,in h and Ti,out h are the specific enthalpy of the inlet and outlet mass of the expansion machine, respectively.

Evaluation Indicator
The cyclic electrical efficiency index of the SC-CAES system refers to the ratio of the output of the generator of the expansion unit to the combination of the power consumption of the compressor set motor and the converted power of the external input heat source in a complete cycle.The RTE of the system is specifically calculated as follows： where tb E represents the power output of the generator of the expansion unit in one complete cycle; cp E represents the power consumption of the motor of the compressor unit in one complete cycle, which is equal to the power input to the energy storage system through the main transformer minus the power consumption of the auxiliary engine in the compression storage stage; g F represents the heat of gas consumed in the combustion chamber re-fueling procedure in one complete cycle; g  represents the gasto-electric efficiency of a single cycle of a combustion engine of similar capacity class or g  denotes the gas-to-electricity efficiency of a single cycle of an internal combustion engine; h F denotes the heat from heat sources other than gas introduced in a complete cycle (excluding compression heat); h  denotes the heat-to-electric efficiency of a waste heat utilization generator set of similar capacity class; cp  and tb  are the efficiency of the electric motor-driven compressor in the system energy storage procedure and the efficiency of the expander-driven generator in the energy emission procedure, respectively.

Results and discussion
This section presents the results of the system calculations and analysis.It should be noted that the thermodynamic properties of the working fluid were calculated using REFPROP software.The system in steady state operation was simulated and a program was written using EES (Engineering Equation Solver) software to calculate and analyze the thermodynamic performance of the system and to further investigate the effect of parameters such as compressor discharge temperature, supplementary combustion, etc., on the performance of the system.

Calculation results
According to the existing literature and the design parameters of domestic and foreign power plants [5][6], the hierarchical combustion CAES system in this paper adopts the scheme of a three-stage compressor arrangement and two-stage expander arrangement with a design power of 10 MW.In order to reduce the power consumption of the compressor unit and the similar exhaust temperature of the compressors at all levels, the compression ratios of the system at all levels are set to 4.9, 4.52, and 4.52, respectively, and the expansion ratios at all levels are equal expansion ratios.The pinch point temperature of the heat exchanger is 10℃.The energy release procedure will be distributed to the heat exchangers at each stage according to a set ratio for heating the air at the expander inlet.The principal operating parameters of the system under design conditions are shown in Table 1, and the calculated thermal performance of the system is shown in Table 2. % 85 From Table 1 and Table 2, it can be seen that the RTE of the system is 85% under the design working condition.When 4 CH mol is 0 mol/s, it can be regarded as a non-supplemental combustion CAES system, and the RTE of the system is 79% at this time.The fuel in the combustion chamber undergoes an oxidation-reduction reaction, which releases heat to increase the inlet temperature of the expander, and the working fluid in the expander is more capable of doing work, further improving the power generation capacity of the system.

Sensitivity Analysis
This section examines the effect of the most important parameters on system performance.These parameters include: outlet pressure of gas storage chamber, ambient pressure, ambient temperature, compressor discharge temperature, expander inlet temperature, air-fuel ratio, and the supplemental methane molar flow rate.In studying each parameter, only the parameter under study will be changed and the other parameters will remain unchanged. mol/s, as shown in Figure 2, the RTE of the SC-CAES system changes with the compressor exhaust temperature.It can be seen from Figure 2 that the RTE of the system increases with the increase of the compressor exhaust temperature.Under design conditions, as the exhaust temperature increases from 130 ℃ to 160 ℃, the RTE of the system increases from 84.4% to 86.0%, with an increase of 1.6%.The RTE of the system increases linearly with the increase of the exhaust temperature.It can be seen that the exhaust temperature of the air compressor has a great impact on the RTE of the system.Therefore, properly increasing the exhaust temperature of the compressor during the system operation can significantly improve the system's performance.
Under the design conditions that ,  mol/s, the change curves of system compressor unit power and expansion unit power with compressor discharge temperature are shown in Figure 3. From Figure 3, it can be seen that the compressor unit power and expansion unit power increase with the increase of compressor discharge temperature, but the expansion unit power increases more than the compressor unit power.As seen from the changes in Figure 2 and Figure 3, as the compressor exhaust temperature increases, the compressor unit in the CAES system needs to consume more electrical energy to compress the air from the environment, so the compressor unit in the energy storage system has an increase in power.With the high-pressure storage chamber pressure, ambient pressure temperature and heat exchanger pinch point temperature kept constant, the temperature of the heat storage mass in the heat exchanger will increase with the compressor discharge temperature.Subsequently, during the energy release process, the inlet temperature of the expander increases further, resulting in an increase in the power of the air entering the expander and therefore the power of the system expansion unit for the same amount of fuel consumed by the system.Although the increase in compressor exhaust temperature causes the system compressor unit to consume more energy, the increase in system expansion unit power capacity is greater than the increase in system compressor unit energy consumption, and therefore the system RTE increases.T  ℃ respectively, and compressor discharge temperature of 142 °C, the RTE of the SC-CAES storage system with the supplemental combustion volume is shown in Figure 4, and it can be seen from Figure 4 that the RTE of the system is 79% under the design conditions when the supplemental methane molar flow rate is 0 mol/s.At this time, the system   ℃ respectively, and the exhaust temperature is 142 ℃, the change curve of the inlet air temperature of the system expander with the air-fuel ratio is shown in Figure 5.It can be seen from Figure 5 that the inlet temperature of the expansion machine decreases with the increase in the air-fuel ratio.This is because the methane molar flow rate into the combustion chamber gradually grows and the air-fuel ratio gradually decreases as the amount of supplementary fuel increases with a constant air flow rate, but the thermal compression generated during the compression process remains unchanged, so the temperature of the inlet to the expansion machine gradually increases, thereby enhancing the working capacity of the expander and improving the RTE of the system.

Conclusion
In this paper, the SC-CAES system is proposed for the integration of recycling compressed heat and supplementary combustion, and the main findings are listed below： (1) Under the design conditions, when 4 CH mol is 7 mol/s, the RTE of the system is 85%; while when 4 CH mol is 0 mol/s, the RTE of the system is 79% at this time.After the addition of the supplemental combustion equipment, the corresponding system RTE increases to 6%.Therefore, this paper proposes an SC-CAES system that can effectively improve the efficiency of the A-CAES system.
(2) Properly raising the compressor exhaust temperature can increase the temperature of the heat storage working medium and further increase the inlet temperature of the expander.Under the condition that the system consumes the same amount of fuel, the operating capacity of the air entering the expander will increase, and the RTE of the system will increase.
(3) Properly increasing the amount of supplemental fuel and lowering the air-fuel ratio can improve fuel economy increase the power delivery of the system and further improve its RTE.

Figure 1 .
Figure 1.SC-CAES system diagram The output power of a particular stage of the expander can be indicated as: The output power of the entire expansion unit is shown below: .1088/1742-6596/2592/1/012047 5

4. 2 . 1 .
The effect of compressor discharge temperature on system performance.Under the design condition that

Figure 2 .
Figure 2. Variation of RTE with compressor exhaust temperature.

Figure 3 .
Figure 3. Power changes of the compressor unit and expansion unit with compressor exhaust temperature.

4. 2 . 2 .
The effect of compressor discharge temperature on system performance.Under the design conditions that , .1088/1742-6596/2592/1/012047 8 can be regarded as a non-combustion CAES system.With 4 CH mol changing from 0 mol/s to 10 mol/s, the RTE of the system is increased by 8%.

Figure 4 .
Figure 4. Variation of RTE with the amount of supplementary fuel.

Figure 5 .
Figure 5. Change of inlet temperature of the expander with air-fuel ratio.