# Thermodynamic Evaluation and Sensitivity Analysis of a Novel Compressed Air Energy Storage System Incorporated with a Coal-Fired Power Plant

^{*}

## Abstract

**:**

## 1. Introduction

## 2. System Description

## 3. System Simulation

#### 3.1. Parameters of Reference Coal-Fired Power Plant

#### 3.2. Model Development and Simulation

## 4. Thermodynamic Analysis

#### 4.1. Basic Hypotheses

- (a)
- The net power generated from coal is deemed as constant;
- (b)
- The air of the CAES system is regarded as an ideal gas, which consists of 75.53% N
_{2}, 21.14% O_{2}, 1.29% Ar, and 0.04% CO_{2}(mass fraction). The influence of the humidity in the air is neglected (The simulation results indicated that the outlet temperatures of COMs and EXPs vary less than 1 °C and the power consumption/generation changes less than 1% under the consideration of the humidity in the air); - (c)
- The environmental temperature and pressure are 25.0 °C and 101.325 kPa.
- (d)
- The effect of the surroundings is not considered.

#### 4.2. Parameters of Proposed System

^{3}. The isentropic efficiencies of the COM and EXP are chosen as 88%. A total of 8 h is spent to compress the air for storing energy when there is redundant electricity on the grid, and the stored energy will be used for power generation in 2 h.

#### 4.3. Energy Analysis

^{3}) has been defined as Equation (7), which indicates the ratio between the total energy output during the discharging process and the air storage vessel size.

^{3}.

^{3}, and the round-trip efficiency the CEAS system can reach 64.08%.

#### 4.4. Exergy Analysis

## 5. Sensitivity Analysis

#### 5.1. Effect of Ambient Temperature

#### 5.2. Effect of ASV Storage Pressure

#### 5.3. Effect of EXP1 Inlet Temperature

#### 5.4. Effect of Coal Power Plant Load

## 6. Further Discussion

## 7. Conclusions

^{3}. The most significant exergy loss comes from the ASV and TV, followed by the compressors and the expanders, and the exergy efficiency of the CAES system is 70.01%. A sensitivity analysis was conducted to investigate the performance of the proposed CAES system under various conditions. The increments of the ambient temperature and air storage pressure cause negative effects on the performance of the CAES system, whereas, an increase of the EXP 1 inlet temperature results in an improvement in the round-trip efficiency. Moreover, the change of the coal power plant load during the charging process has little influence on the operation of the CAES system, but the impact of the coal power plant load during the discharging process has similar trends with that of the EXP 1 inlet temperature. Via the suggested integration, the total capital cost of the CAES system can be reduced by 35.69%. Above all, the novel concept is highly suitable and favorable from the thermodynamic and economic aspects.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Nomenclature

Symbols | |||

EX | exergy (kW) | s | specific entropy (kJ/kgK) |

h | specific enthalpy (kJ/kg) | t | time (h) |

m | flow rate (kg/s) | T | temperature (K) |

P | power (kW) | V | volume (m^{3}) |

Q | energy (kWh) | W | work (kWh) |

q | caloric value (kJ/kg) | η | efficiency |

Subscripts | |||

0 | environmental state | hyb | hybrid |

c | coal | in | inlet |

ch | charge | out | outlet |

c-e | coal-to-electricity | ref | reference |

disch | discharge | x | certain stream |

ex | exergy | ||

Abbreviations | |||

ASV | air storage vessel | G | generator |

CAES | compressed air energy storage | HPT | high-pressure turbine |

COM | compressor | HX | heat exchanger |

CON | condenser | IPT | intermediate-pressure turbine |

CP | condensate pump | LPT | low-pressure turbine |

DEA | deaerator | M | motor |

ESD | energy storage density (kJ/m^{3}) | RH | regenerative heater |

EP | extra pump | RTE | round-trip efficiency (%) |

EXP | expander | TV | throttle valve |

FWP | feedwater pump |

## Appendix A. Simulation Models

**Figure A1.**Simulation models of the studied systems, (

**a**) reference coal-fired power plant, (

**b**) reference CAES system, (

**c**) proposed CAES system.

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**Figure 1.**Diagram of the proposed compressed air energy storage (CAES) system incorporated with a coal-fired power plant.

**Figure 4.**Energy flow diagrams of the reference coal power plant and proposed CAES system, (

**a**) reference coal power plant, (

**b**) charging process of the proposed CAES system, (

**c**) discharging process of the proposed CAES system.

**Figure 6.**Influence of the ambient temperature on the main energy streams of the proposed CAES system, (

**a**) heat energy recovered from and transferred to the air, (

**b**) power streams and coal power consumed by the CAES system.

**Figure 8.**Influence of the ASV storage pressure on the main energy streams of the proposed CAES system, (

**a**) heat energy recovered from and transferred to the air, (

**b**) power streams and coal power consumed by the CAES system.

**Figure 10.**Influence of the EXP1 inlet temperature on the main energy streams of the proposed CAES system, (

**a**) heat energy recovered from and transferred to the air, (

**b**) power streams and coal power consumed by the CAES system.

**Figure 11.**Influence of the expander 1 (EXP1) inlet temperature on the performance of the proposed CAES system.

**Figure 12.**Influence of the coal power plant load during the charging process on the main energy streams of the proposed CAES system, (

**a**) heat energy recovered from and transferred to the air, (

**b**) power streams and coal power consumed by the CAES system.

**Figure 13.**Influence of the coal power plant load during the charging process on the performance of the proposed CAES system.

**Figure 14.**Influence of the coal power plant load during the discharging process on the main energy streams of the CAES system, (

**a**) heat energy recovered from and transferred to the air, (

**b**) power streams and coal power consumed by the CAES system.

**Figure 15.**Influence of the coal power plant load during the discharging process on the performance of the proposed CAES system.

Item | Unit | Value | |
---|---|---|---|

Coal consumption rate | kg/s | 42.29 | |

Net caloric value of coal | kJ/kg | 18,750 | |

Main steam (into turbine) | Pressure | MPa | 24.20 |

Temperature | °C | 566.0 | |

Flow rate | kg/s | 274.80 | |

Reheated steam (into turbine) | Pressure | MPa | 3.78 |

Temperature | °C | 566.0 | |

Flow rate | kg/s | 232.33 | |

Exhaust steam (out of turbine) | Pressure | kPa | 4.90 |

Temperature | °C | 32.5 | |

Flow rate | kg/s | 174.31 | |

Gross power | MW | 349.76 | |

Net power | MW | 330.52 | |

Coal-to-electricity efficiency | % | 41.69 |

Item | Design | Simulation | Relative Error (%) | |
---|---|---|---|---|

Coal consumption rate (kg/s) | 42.29 | 42.29 | 0.00 | |

Main steam (into turbine) | Pressure (MPa) | 24.2 | 24.2 | 0.00 |

Temperature (°C) | 566.0 | 566.0 | 0.00 | |

Flow rate (kg/s) | 274.80 | 274.80 | 0.00 | |

Reheated steam (into turbine) | Pressure (MPa) | 3.78 | 3.78 | 0.00 |

Temperature (°C) | 566.0 | 566.0 | 0.00 | |

Flow rate (kg/s) | 232.33 | 232.36 | +0.01 | |

Exhaust steam (out of turbine) | Pressure (kPa) | 4.90 | 4.90 | 0.00 |

Temperature (°C) | 32.5 | 32.5 | 0.00 | |

Flow rate (kg/s) | 174.31 | 174.29 | −0.01 | |

Feedwater (into boiler) | Pressure (MPa) | 26.23 | 26.23 | 0.00 |

Temperature (°C) | 276.4 | 276.4 | 0.00 | |

Flow rate (kg/s) | 274.80 | 274.80 | 0.00 | |

Exhaust gas temperature (°C) | 130.0 | 130.0 | 0.00 | |

Boiler efficiency (%) | 94.09 | 94.09 | 0.00 | |

Gross power (MW) | 349.76 | 350.00 | +0.07 | |

Net power (MW) | 330.52 | 330.75 | +0.07 | |

Coal-to-electricity efficiency (%) | 41.69 | 41.72 | +0.07 |

**Table 3.**Model validation based on the CAES system in Ref. [37].

Item | Ref. [37] | Simulation | Relative Error (%) |
---|---|---|---|

Power consumption of COM1 (kW) | 485.57 | 486.00 | +0.09 |

Work consumption of COM2 (kW) | 514.43 | 514.63 | +0.04 |

Total power consumption during charging process (kW) | 1000 | 1000.63 | +0.06 |

Air flow rate during charging process (kg/s) | 1.58 | 1.58 | 0.00 |

Work generation of EXP1 (kW) | 509.37 | 509.65 | +0.05 |

Work generation of EXP2 (kW) | 490.63 | 491.11 | +0.10 |

Total power generation during discharging process (kW) | 1000 | 1000.76 | +0.08 |

Air flow rate during discharging process (kg/s) | 2.36 | 2.36 | 0.00 |

Charge time (h) | 6.06 | 6.06 | 0.00 |

Discharge time (h) | 4.06 | 4.06 | 0.00 |

Round-trip efficiency (%) | 66.98 | 67.01 | +0.04 |

Item | Unit | Value | |
---|---|---|---|

ASV | Volume | m^{3} | 17,940 |

Air temperature | °C | 50.0 | |

Storage pressure | MPa | 2.85 | |

Release pressure | MPa | 1.63 | |

Isentropic efficiency of COM | % | 88 | |

Isentropic efficiency of EXP | % | 88 | |

Charging time | h | 8 | |

Discharging time | h | 2 |

Item | Reference Coal Power Plant | Proposed System | Variation |
---|---|---|---|

Charging process | |||

Load of coal power plant | 100% Load | 100% Load | - |

Net power of coal power plant (MW) | 330.52 | 330.52 | 0 |

Coal consumption rate of coal power plant (kg/s) | 42.29 | 42.20 | −0.09 |

Coal power conserved by CAES system (MWh) | - | 5.41 | - |

Power consumption of CAES system’s motor (MW) | - | 3.69 | - |

Power input of CAES system during charging (MWh) | - | 29.54 | - |

Discharging process | |||

Load of coal power plant | 100% Load | 100% Load | - |

Net power of coal power plant (MW) | 330.52 | 330.52 | 0 |

Coal consumption rate of coal power plant (kg/s) | 42.29 | 42.74 | +0.45 |

Coal power consumed by CAES system (MWh) | - | 7.08 | - |

Power generation of CAES system’s generator (MW) | - | 10.00 | - |

Power output of CAES system during discharging (MWh) | - | 20.00 | - |

Performance indicators | |||

Round-trip efficiency of CAES system (%) | - | 64.08 | - |

Energy storage density of CAES system (MJ/m^{3}) | - | 4.01 | - |

Total coal consumption variation in charging and discharging process (kg) | - | 769.59 | - |

Total coal power consumed by CAES system (MWh) | - | 1.67 | - |

Overall efficiency of hybrid system (%) | - | 41.76 | - |

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**MDPI and ACS Style**

Pan, P.; Zhang, M.; Peng, W.; Chen, H.; Xu, G.; Liu, T.
Thermodynamic Evaluation and Sensitivity Analysis of a Novel Compressed Air Energy Storage System Incorporated with a Coal-Fired Power Plant. *Entropy* **2020**, *22*, 1316.
https://doi.org/10.3390/e22111316

**AMA Style**

Pan P, Zhang M, Peng W, Chen H, Xu G, Liu T.
Thermodynamic Evaluation and Sensitivity Analysis of a Novel Compressed Air Energy Storage System Incorporated with a Coal-Fired Power Plant. *Entropy*. 2020; 22(11):1316.
https://doi.org/10.3390/e22111316

**Chicago/Turabian Style**

Pan, Peiyuan, Meiyan Zhang, Weike Peng, Heng Chen, Gang Xu, and Tong Liu.
2020. "Thermodynamic Evaluation and Sensitivity Analysis of a Novel Compressed Air Energy Storage System Incorporated with a Coal-Fired Power Plant" *Entropy* 22, no. 11: 1316.
https://doi.org/10.3390/e22111316