4.1. Process Simulation
In the CAES cycle, the energy storage subsystem comprises four compressors, four coolers, and a cavern with a constant volume. The air is cooled to 50 °C in both the intercoolers and the aftercooler. Water is the cooling medium, and counter flow is adopted in the cooler. The air is compressed to approximately 60 bars on average and stored in a cavern with a volume of 300,000 m
3. The electricity-generating subsystem is built based on gas turbine units. The basic parameters are listed in
Table 1. The compressed air released from the air storage cavern is mixed with natural gas in the combustion chamber. Before flowing into the HP turbine, the mixed gas is heated to 600 °C. Similarly, a certain amount of natural gas is also mixed with the gas exhausted from the HP turbine. Then, the mixed gas is heated to 1050 °C before entering the LP turbine.
In the CC cycle, the electricity-generating subsystem is based on a three-pressure reheat HRSG and a steam turbine. The steam produced in the HRSG is divided into two parts with different pressures and then fed into the HP steam turbine and the LP steam turbine, respectively. As shown in
Figure 2, the steam from the HP steam turbine is injected into the HRSG for recycling before going into the MP steam turbine, and the steam from the MP steam turbine is directly injected into the LP steam turbine. The basic parameters of the steam turbine are listed in
Table 2.
The CAES-CC system, along with the recuperated CAES and the reference CAES systems, is simulated using the commercial software ASPEN PLUS. During the physical and chemical processes involved in the simulation, the Peng–Robinson base method is adopted for all the equations of state. The thermodynamic process in each combustion chamber and gas turbine is regarded as an adiabatic process. The natural gas is assumed to be CH
4, which is to be burned completely. The pressure, temperature, mole flow, and composition of each stream corresponding to the points indicated in
Figure 3 are listed in
Table 3.
Table 3.
Simulation of stream parameters of CAES-CC.
Table 3.
Simulation of stream parameters of CAES-CC.
No. | Temperature (°C) | Pressure (bar) | Gas Flux (kg/s) | Mole Percentage (%) |
---|
N2 | O2 | CH4 | CO2 | H2O |
---|
S1 | 10.00 | 1.01 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S2 | 237.80 | 6.08 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S3 | 50.00 | 6.08 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S4 | 147.80 | 13.05 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S5 | 50.00 | 13.05 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S6 | 147.90 | 28.00 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S7 | 50.00 | 28.00 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S8 | 148.00 | 60.11 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S9 | 50.00 | 60.12 | 108.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S10 | 50.00 | 42.00 | 417.00 | 79.00 | 21.00 | 0.00 | 0.00 | 0.00 |
S11 | 600.00 | 42.00 | 422.15 | 77.30 | 16.20 | 0.00 | 2.20 | 4.30 |
S12 | 389.40 | 11.50 | 422.15 | 77.30 | 16.20 | 0.00 | 2.20 | 4.30 |
S13 | 1050.00 | 11.00 | 428.70 | 75.00 | 9.90 | 0.00 | 5.00 | 10.10 |
S14 | 583.50 | 1.130 | 429.30 | 75.00 | 9.90 | 0.00 | 5.00 | 10.10 |
S15 | 45.90 | 500.00 | 50.14 | 0.00 | 0.00 | 0.00 | 0.00 | 100.00 |
S16 | 42.10 | 5.20 | 17.16 | 0.00 | 0.00 | 0.00 | 0.00 | 100.00 |
S17 | 540.00 | 98.00 | 50.14 | 0.00 | 0.00 | 0.00 | 0.00 | 100.00 |
S18 | 540.00 | 26.00 | 50.14 | 0.00 | 0.00 | 0.00 | 0.00 | 100.00 |
S19 | 313.50 | 5.00 | 17.16 | 0.00 | 0.00 | 0.00 | 0.00 | 100.00 |
S20 | 368.90 | 31.50 | 50.14 | 0.00 | 0.00 | 0.00 | 0.00 | 100.00 |
S21 | 313.50 | 5.00 | 50.14 | 0.00 | 0.00 | 0.00 | 0.00 | 100.00 |
S22 | 37.70 | 0.06 | 67.31 | 0.00 | 0.00 | 0.00 | 0.00 | 100.00 |
S23 | 94.80 | 1.00 | 429.30 | 75.00 | 9.90 | 0.00 | 5.00 | 10.10 |
The volume of the cavern involved in this study is constant. The air pressure inside the cavern increases along with the compressed air injected continuously into the cavern in the low-load period. This procedure requires an amplitude variation in compressor outlet air pressure to effectively store air. Considering the limitation of ASPEN PLUS, we assume that the compressed air pressure (S9) entering the cavern is approximately 60 bar on average. The discharge pressure of the cavern and the power output of the gas turbine both decrease as the compressed air is continuously discharged from the cavern in the load period. The gas turbine driven by sliding air pressure is not suitable because of the load characteristics in its supply network. Thus, the air (S10) from the cavern is throttled down to the HP gas turbine inlet pressure before it is fed into the combustion chamber.
4.2. Evaluation Criteria
The CAES plant has two energy inputs (
i.e., electric energy and fuel during the charging and discharging phases, respectively) with different qualities. The evaluation criteria for CAES plants differ from those usually applied for conventional power plants. For example, specific fuel consumption cannot identify the thermodynamic merits of a CAES plant as in a conventional power cycle. Thus, some evaluation criteria are introduced [
20,
21,
22].
- (1)
Energy rate
Energy rate (
ER) represents the ratio of pumping energy at the off-peak period to the generated energy during the peak period:
Although ER does not account for fuel consumption, it is unique to CAES plants. Therefore, ER is used as another performance parameter in addition to Wt, which is the net electric energy output during the discharge phase.
- (2)
Heat rate
Heat rate (
HR) represents the kilowatt hour of heat required to produce a kilowatt hour of energy. It is written as:
where
Qf is the total fuel energy (kWh) used in the combustors and boilers during the generation period.
- (3)
Overall efficiency
The output from a CAES plant is work. It may be compared with input energy to the plant consisting of natural gas and compressor work:
This equation may be seen as an expression of storage efficiency. However, it may be questionable. It consists of two energy types that are consumed by different parts of the process at different periods. Fuel cannot be seen as part of electricity storage but is only required because of the heating of the air to the turbine.
- (4)
Efficiency of electricity storage
Efficiency of electricity storage may be defined as:
System efficiency ηsys reflects the efficiency of electricity production in the energy system. Its value depends on which system the CAES plant uses. For a gas turbine power plant, the value is between 45% and 58%. Accordingly, the ηsys of this CAES-CC is assumed to be 50%.
4.3. Thermal Performance Analysis
In this paper, the Aspen Plus software is used to perform the process simulation and to get the performance analysis results. The simulation results for the CAES-CC and the recuperated CAES and the reference CAES are presented in
Table 4. The pump power consumption is neglected because it is too little for compressor power consumption.
Table 4.
Simulation data and performance criteria of the three systems.
Table 4.
Simulation data and performance criteria of the three systems.
Parameter | CAES-CC | Recuperated CAES | Reference CAES |
---|
Value | Unit | Value | Unit | Value | Unit |
---|
Compressor power | 57.90 | MW | 57.90 | MW | 57.90 | MW |
Gas turbine power | 349.14 | MW | 342.06 | MW | 349.14 | MW |
Steam turbine power | 83.24 | MW | | | | |
Exhaust gas temperature | 94.80 | °C | 182 | °C | 583.50 | °C |
LHV of natural gas | 50,030.04 | kJ/kg | 50,030.04 | kJ/kg | 50,030.04 | kJ/kg |
Fuel flow of combustion chamber #1 | 18,549.79 | kg/h | 4,462.12 | kg/h | 18,549.79 | kg/h |
Fuel flow of combustion chamber #2 | 25,719.33 | kg/h | 24,965.51 | kg/h | 25,719.33 | kg/h |
Fuel consumption heat | 615.22 | MW | 408.96 | MW | 615.22 | MW |
Qf | 1,230.44 | MWh | 817.93 | MWh | 1,230.44 | MWh |
Total compression power Wc | 463.22 | MWh | 463.22 | MWh | 463.22 | MWh |
Total expansion power Wt | 864.76 | MWh | 684.12 | MWh | 698.28 | MWh |
ER | 0.536 | | 0.677 | | 0.663 | |
HR | 1.423 | | 1.196 | | 1.761 | |
ηsys | 0.500 | | 0.500 | | 0.500 | |
ηee | 0.511 | | 0.534 | | 0.413 | |
ηes | 0.802 | | 0.784 | | 0.648 | |
As shown in
Table 4, the CAES-CC, along with the recuperated CAES, has a lower exhaust temperature than the reference CAES. Accurately speaking, the exhaust of CAES-CC comes from the HRSG, and most of the thermal energy is absorbed by the feed water, so the exhaust temperature decreases by 488.7 °C. However, the exhaust of the recuperated CAES comes from the recuperator, and most of the thermal energy is absorbed by the compressed air, so the exhaust temperature decreases by 401.5 °C. As far as the system performance of the three systems are concerned, both CAES-CC and the recuperated CAES are superior to the reference CAES. Although CAES-CC has recycled more exhaust thermal energy than the recuperated CAES, the energy loss of the steam cycle in CAES-CC is higher than the energy loss of the heat recovery device in the recuperated CAES, so the overall efficiency
ηee of CAES-CC is lower than that of the recuperated CAES.
As for the efficiency of electricity storage, the ηes of CAES-CC is higher than that of the recuperated CAES. From the structure and the system process of the two systems, it is found that their energy-saving methods are essentially different. The LP gas turbine exhaust is used for generating extra electricity by the steam cycle in CAES-CC, so the ER of CAES-CC is significantly decreased. However, the recuperated CAES utilizes the LP gas turbine exhaust to heat the compressed air, so the HR of the recuperated CAES is significantly decreased. Decreasing the ER or the HR can either improve the ηes of the CAES system. However, from the perspective of the quality of energy, electricity is more superior to the chemical energy of the fuel, and there is an energy loss in the energy conversion process from thermal energy to electricity, on that account, compared with decreasing the HR of CAES, decreasing the ER of CAES is more favorable to improve the efficiency of electricity storage of the CAES system.
In fact, for the CAES plant with a low-power gas turbine, such as the McIntosh plant, adding a recuperator is relatively easy to implement in the engineering application. However, the proposed system which uses a gas turbine with large capacity and high parameters has a larger scale than the McIntosh CAES plant. If a recuperated CAES operates under the high parameter condition shown in
Table 1, the heat exchange capacity will be approximately 188 MW. Considering the recuperator selected in the recuperated CAES is gas-air heat transfer mode, the estimated heat transfer area of the recuperator is over 1.0 × 10
4 m
2. Such large heat transfer area will lead to large volume and high investment of the recuperator. Furthermore, it will also cause a great pressure drop for the recuperator and the outlet pressure of LP gas turbine will increase accordingly, which results in a power output decrease of the LP gas turbine. The gas turbine power output of the mentioned recuperated CAES will decrease by 0.365 MW if the LP gas turbine outlet pressure increases by 0.5 kPa.
In conclusion, with high exhaust temperature and large exhaust flow of the gas turbine, the recuperated CAES is difficult to be implemented in engineering application. However, the steam turbine and HRSG adopted in CAES-CC are technically mature, therefore, CAES-CC is more suitable under the high parameter conditions.
4.4. System Analysis according to the Second Law of Thermodynamics
As mentioned above, the thermodynamic analysis is based on the first law of thermodynamics, that is, only considering the energy balance in the “amount” and not taken the “grade” of energy into account. To further analyze the thermodynamic performance of the system, it is necessary to study the system based on the second law of thermodynamics. The entropy generation and the exergy destruction are the measurement indicators of the thermodynamic irreversibility. Through the analysis of the entropy generation and the exergy destruction, the irreversible loss of thermal process can be measured. The relationship between the entropy generation and the exergy destruction can be described by the Gouy-Stodola equation:
In this equation,
Ir represents exergy destruction,
T0 represents the environment tempreture, and Δ
Siso represents entropy generation. It is easy to see that the exergy destruction can demonstrate the entropy generation. It is common to conduct exergy analysis based on the second law of thermodynamics [
23,
24]. Therefore, to reveal the internal phenomena of the CAES-CC system, an exergy analysis is performed for the CAES-CC and reference CAES. Assuming no chemical reaction occurs and neglecting the kinematic energy effects, the exergy of air or gas stream can be expressed as:
where
H and
S are enthalpy and entropy of the steam, respectively, and the subscript 0 indicates that the properties are taken at the environmental temperature and pressure (
T0 = 25 °C,
P0 = 101 kPa). The general exergy balance of the system components can be expressed as:
In Equation (7), ∆
EX refers to the exergy destruction,
EX(in) and
W(in) refer to the exergy input and the power input, respectively,
EX(out) and
W(out) denote the exergy output and the power output, respectively.
EX(loss) represents the exergy loss caused by energy loss. However, in this paper, there is very little heat dissipation during the energy conversion process, therefore, the simulation models of the systems are considered as adiabatic process and the “energy loss” is considered as zero. Therefore,
EX(loss) in the energy conversion process is zero, and then the general exergy balance of the system components can be expressed in the following rate form:
The exergy destruction within a component of the energy conversion system can be derived from the exergy balance equation. In this paper, the CAES systems are divided into several parts to conduct a detailed study on the exergy analysis. And the exergy analysis of all the equipments mentioned in the article is conducted according to the general exergy balance.
Table 5 lists the exergy destruction within different components of the energy conversion system.
Based on the aforementioned foundation, the exergy analyses results of CAES-CC and conventional CAES are presented in
Table 6. The exergy analysis is based on the assumption that the same quantity of electric power and natural gas is consumed in the CAES-CC and reference systems. In consideration of the non-synchronization between electricity consumption and power generation, the exergy analysis is based on a ten-hour cycle of the operation of the CAES-CC system.
As shown in
Table 6, the exergy efficiency of the CAES-CC system is 49.43%, which is 9.51% higher than that of the reference system. Comparing the exergy distributions of the CAES-CC system with those of the reference system in
Table 6, it is shown that the exergy of the power production increases and that the total exergy destruction of the CAES-CC system obviously decreases. The main reduction in exergy destruction is caused by the HRSG and steam turbine. The detailed distribution of the exergy destruction of these two units is given in
Table 6.
Table 5.
Expression of exergy destruction in the main components of the system.
The exergy of the exhaust gas of the CAES-CC system decreases remarkably by 241.46 MWh compared with that of the reference system. This result can be attributed to the addition of an HRSG to the CAES cycle to recover the exhaust heat from the gas turbine. Although introducing an HRSG into the CAES causes an exergy destruction of about 47.69 MWh, the exergy of the exhaust gas decreases by approximately 13.8%. Thus, the exergy saving on exhaust recovery is the main cause of the reduction in exergy destruction in the power generating unit.
The exergy of the power production of the CAES-CC system increases by 166.48 MWh compared with that of the reference system. This increase may be attributed to the utilization of the steam turbine in the CAES-CC system. As shown in
Table 5, the extra CAES-CC components, such as the HRSG, steam turbine, condenser, and auxiliary equipment, cause an extra exergy destruction to the power generation subsystem. However, the power output notably increases.
Table 6.
Comparison of exergy analysis for the CAES-CC system and the reference CAES system.
Table 6.
Comparison of exergy analysis for the CAES-CC system and the reference CAES system.
Name | Reference CAES | CAES-CC |
---|
Value | Proportion of Exergy Input | Value | Proportion of Exergy Input |
---|
MWh | % | MWh | % |
---|
Air | 1.98 | 0.11 | 1.98 | 0.11 |
Compressor power consumption | 463.22 | 26.48 | 463.22 | 26.48 |
Natural gas input | 1284.19 | 73.41 | 1284.19 | 73.41 |
Subtotal of exergy input | 1749.39 | 100 | 1749.39 | 100 |
Subtotal of compressor series and cooler | 161.40 | 9.23 | 161.40 | 9.23 |
Storage cavern | 34.39 | 2.04 | 34.39 | 2.04 |
Subtotal of combustion chamber | 529.63 | 30.27 | 529.63 | 30.27 |
Subtotal of gas turbine | 56.18 | 3.21 | 56.18 | 3.21 |
HRSG | | | 47.69 | 2.73 |
Subtotal of steam turbine | | | 20.42 | 1.17 |
Condenser | | | 13.01 | 0.74 |
Auxiliary equipment and other parts | | | 0.44 | 0.03 |
Power output | 698.28 | 39.92 | 864.76 | 49.43 |
Exergy of exhaust gas | 265.61 | 15.18 | 24.15 | 1.38 |
Subtotal output exergy | 1745.49 | 99.78 | 1752.07 | 100.15 |
Error of exergy input and output | 0.22% | 0.15% |
Exergy efficiency | 49.43% | 39.92% |
Therefore, we conclude that introducing a steam cycle into the CAES system can effectively reduce energy waste and increase useful power output. Consequently, the exergy efficiency and performance of the CAES-CC system both improve significantly. However, some challenges still exist. These challenges include the complexity of the system and the possible high investment requirement of the system, all of which will be addressed in our following work.