Performance Analysis of a Coal-Fired External Combustion Compressed Air Energy Storage System
2. Coal-Fired CAES
2.1. Introduction of the Traditional CAES
2.2. Proposed Coal-Fired CAES
2.3. Parameter Selection
3. Simulation of the Proposed Coal-Fired CAES
3.1. Evaluation Criterion
3.2. Process Simulation
3.3. Thermodynamic Performance Analysis
4.1. Exergy Analysis
4.2. Techno-Economic Analysis
4.2.1. Investment Cost Estimation of CAES Plant
4.2.2. Sensitivity Analysis
- For most of the “Three Norths” area in China, the off-peak electricity price can be reduced to $40/MWh at most, which means more effort should be devoted to regulating the grid purchase price to further reduce the COE. The on-peak electricity price in most areas in China is about $96/MWh to $130/MWh , exceptionally; however, this price can reach up to $193/MWh in some areas (e.g., Beijing) . Hence, more power generated by the CAES plant should be delivered to these areas. Corresponding policies should be implemented for the energy storage plants. For example, more government subsidies should be provided to regulate the grid purchase price of energy storage plants. The power grid should be more mature.
- In this study, the construction cost of the coal-fired CAES plant is estimated by referring to the existing CAES plants. The investment cost of the air storage cavern is about 16% of the total construction cost. Such a cost can be reduced significantly if suitable air storage caverns are discovered. The cost of the low-temperature turbine is also relatively low because of the low requirement for high temperature resistance. These two factors are beneficial to reduce the TCC of the coal-fired CAES plant.
- Power grids are becoming more complicated to regulate when renewable power is in parallel with the power grid. Therefore, more peak load shaving units will be needed, and CAES can make a contribution. In this way, more power can be stored to then generate more electricity annually by the CAES plant to provide more effective power peak load shaving. Therefore, the base-load time is increased, thereby significantly reducing the COE of the CAES plant.
- The improvement of the equipment performance and system configuration makes the system perform well. The recuperator reduces the exhaust heat loss considerably and increases the outlet air temperature of the gas storage cavern. However, the combustion efficiency and heat exchange efficiency of the external heater are lower than those of the combustion chamber. As a result, the overall efficiency of the coal-fired CAES reaches 48.37%, higher than that of the Huntorf CAES, but lower than the improved Huntorf CAES, noting that the efficiency of the electricity of the proposed CAES is the highest.
- The exergy efficiency of the proposed coal-fired CAES is 47.22%, approximately 7% higher than that of Huntorf. This improvement is mainly attributed to the decrement of the exergy of the exhaust stream; the largest exergy destruction observed in the external combustion heater. Therefore, the exergy efficiency can be further improved through measures, such as optimizing the external combustion heater. Additionally, the cooling compression heat exergy is wasted in vain, so utilizing the cooling compression heat properly can also increase the exergy efficiency.
- In terms of techno-economic performance, the COE of the coal-fired CAES is $106.33/MWh, which is 26% lower compared to the conventional NG-fuel CAES with a similar capacity. The reason is that the price of NG is far higher compared to that of coal. Sensitivity analysis is conducted by varying factors, such as off-peak electricity price, coal price, total construction cost and annual base-load time. The proposed CAES will be more competitive if the following conditions are addressed: (i) the power grid should be improved to encourage energy storage plants with the least delay possible; (ii) the cost of a low-temperature turbine is lower; the total construction cost can also be reduced provided that a suitable cavern is found; (iii) more power can be stored to enable the CAES plant to generate more power for power peak load shaving.
Conflicts of Interest
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|Energy storage subsystem||Working hours||h||8||8|
|Air mass flow||kg/s||108||108|
|Pressure ratio of high-pressure compressor||–||3||2.15|
|Pressure ratio of low-pressure compressor||–||3||6|
|Adiabatic efficiency of high-pressure compressor||%||86||80|
|Adiabatic efficiency of low-pressure compressor||%||86||82|
|Inlet air pressure of gas storage cavern||bar||82||46–72|
|Power generation subsystem||Working hours||h||2||2|
|Air mass flow||kg/s||417||417|
|Inlet pressure of high-pressure turbine||bar||64||42|
|Inlet temperature of high-pressure turbine||°C||580||550|
|Inlet pressure of intermediate pressure turbine||bar||25.6||–|
|Inlet temperature of intermediate pressure turbine||°C||580||–|
|Inlet pressure of low-pressure turbine||bar||5.12||11|
|Inlet temperature of low-pressure turbine||°C||580||825|
|Adiabatic efficiency of high-pressure turbine||%||88||85|
|Adiabatic efficiency of intermediate pressure turbine||%||90||–|
|Adiabatic efficiency of low-pressure turbine||%||90||85|
|LHV of coal||MJ/kg||29.31||29.31|
|LHV of nature gas||MJ/kg||50.03||50.03|
|Efficiency of external combustion heater||%||80||–|
|streams||T (°C)||P (bar)||M (kg/s)||streams||T (°C)||P (bar)||M (kg/s)|
|Parameter||Coal-fired CAES||Huntorf||Improved Huntorf|
|Main parametersof system||Power consumption by compressors (MW)||56.45||57.90||53.00|
|Generation of electricity power (MW)||317.15||295.55||306.31|
|Coal input (MJ/s)||429.85||–||–|
|NG input (MJ/s)||–||476.79||335.53|
|Power consumption by compressors||451.59||33.62||463.22||31.71||424.02||37.66|
|Exergy input of coal||889.80||66.24||–||–||–||–|
|Exergy input of NG||–||–||995.54||68.15||699.99||62.17|
|Generation of electricity power||634.29||47.22||590.97||40.46||612.61||54.41|
|Exergy of exhaust air||9.22||0.69||153.29||10.49||20.21||1.8|
|Sub-system of energy storage||–||–||–||–||–||–|
|Air storage room||27.96||2.08||35.78||2.45||34.98||3.11|
Sub-system of electricity generation
|External combustion heater||494.79||36.83||–||–||–||–|
|Total exergy output||1346.81||100.26||1455.12||99.61||1123.13||99.75|
|Error of exergy input & output (%)||−0.26||0.39||0.25|
|Exergy efficiency (%)||47.22||40.46||54.41|
|Compressor||11.67||Power Consumption: 56 MW|
|Turbine||47.55||Total Installed: 317 MW|
|Cooler||11.67||Heat Transfer Area: 714,400 ft2|
|Recuperator||3.31||Heat Transfer Area: 220,770 ft2|
|Air storage carven||31.70||12 h of storage|
|Cost, aboveground equipment||million $||161.67|
|Cost, cavern development||million $||31.70|
|Total construction cost||million $||193.37|
|Off-peak electricity price||$/MWh||40.00|
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Liu, W.; Li, Q.; Liang, F.; Liu, L.; Xu, G.; Yang, Y. Performance Analysis of a Coal-Fired External Combustion Compressed Air Energy Storage System. Entropy 2014, 16, 5935-5953. https://doi.org/10.3390/e16115935
Liu W, Li Q, Liang F, Liu L, Xu G, Yang Y. Performance Analysis of a Coal-Fired External Combustion Compressed Air Energy Storage System. Entropy. 2014; 16(11):5935-5953. https://doi.org/10.3390/e16115935Chicago/Turabian Style
Liu, Wenyi, Qing Li, Feifei Liang, Linzhi Liu, Gang Xu, and Yongping Yang. 2014. "Performance Analysis of a Coal-Fired External Combustion Compressed Air Energy Storage System" Entropy 16, no. 11: 5935-5953. https://doi.org/10.3390/e16115935