With the wide application of renewable energy, energy storage technology has become a research hotspot. In order to overcome the shortcomings of energy loss caused by compression heating in compressed air energy storage technology, a novel constant-pressure pumped hydro combined with compressed air energy storage system was proposed. To deepen the understanding of the system and make the analysis closer to reality, this paper adopted an off-design model of the compressor to calculate and analyze the effect of key parameters on system thermodynamics performance. In addition, the results of this paper were compared with previous research results, and it was found that the current efficiency considering the off-design model of compressor was generally 2% - 5% higher than the previous efficiency. With increased preset pressure or with decreased terminal pressure, both the previous efficiency and current efficiency of the system increased. The exergy destruction coefficient of the throttle valve reached 4%. System efficiency was more sensitive to changes in water pump efficiency and hydroturbine efficiency.
With the rapid development of the new energy industry, it is becoming more and more urgent to solve the inherent randomness and instability of new energy sources. Researchers believe that compressed air energy storage (CAES) technology is one of the physical technologies to solve the intermittent new energy grid-connected and the power grid peaking [
A schematic diagram of a constant-pressure PHCA is depicted in
1) Pre-compression process: before the charging process, valves 1, 2 and compressor C1 were opened. When the air pressure in the storage vessel and high-pressure vessel to a set value, valves 1 and 2 were closed and compressor C1 was stopped.
2) Charging process: by opening the valves 3 and 4, water was injected from the water tank into the storage vessel through the water pump. At the same time, valve 5 was opened, and the compressor C2 was started to pressurize the air from the storage vessel into the high-pressure vessel. In this process, by controlling the flow of the water pump and compressor C2, the pressure in the storage vessel remained constant. As the injection volume reached the set value, the water pump and compressor C2 were stopped while valves were closed.
3) Dis-charging process: valves 6 and 7 were opened, and the water in the storage vessel pushed the hydroturbine to work. Then it flowed into the water tank. Meanwhile, the valve 8 was opened, which allowed the air in the high-pressure vessel flows into the storage vessel through the throttle valve. In this process, by controlling the flow of the hydroturbine and throttle valve, the pressure in the storage vessel remained constant.
In order to simplify the analysis of the constant-pressure PHCA system, the following assumptions were made:
1) Air is an ideal gas, and its dissolution in water is ignored;
2) Water is an incompressible fluid;
3) High-pressure vessels and storage vessels are adiabatic vessels;
4) Ignore pressure changes due to fluid level changes;
5) Ignore energy loss due to resistance in the pipeline.
Yao had built the basic thermodynamic model and analyzed the thermodynamic performance of the constant-pressure PHCA system while the compressor C2 is working in design conditions [
During the charging process, the air pressure in the storage vessel remained
| Parameters | Unit | Value |
|---|---|---|
| Ambient temperature | K | 298 |
| Ambient pressure | Pa | 101,330 |
| Volume of storage vessel | m3 | 40 |
| Adiabatic index | - | 1.4 |
| Gas constant | J∙kg−1∙K−1 | 287 |
| Constant pressure specific heat of air | J∙kg−1∙K−1 | 1004.5 |
| Specific heat of water | J∙kg−1∙K−1 | 4200 |
| Density of water | kg∙m−3 | 1000 |
| Efficiency of water pump | % | 88 |
| Efficiency of hydroturbine | % | 90 |
| Design efficiency of compressor | % | 88 |
| Preset pressure | Pa | 3,000,000 |
| Terminal pressure in the high pressure vessel | Pa | 4,000,000 |
constant by adjusting the flow of the water pump and compressor C2. However, the air pressure in the high-pressure vessel was constantly increasing due to the inflow of air. Therefore, the pressure ratio of compressor C2 was constantly changing, and it worked under variable working conditions. We applied off-design calculations through compressors characteristic maps that defined relative pressure ratio ε c and relative efficiency η c [
ε c = c 1 ( G ˙ c ) 2 + c 2 G ˙ c + c 3 (1)
η c = [ 1 − c 4 ( 1 − n ˙ c ) 2 ] ( n ˙ c / G ˙ c ) ( 2 − ( n ˙ c / G ˙ c ) ) (2)
where G ˙ c and n ˙ c are the reduced flow rate and the reduced speed, respectively.
G ˙ c = ( m ˙ c T c , i n / P c , i n ) / ( m ˙ c T c , i n / P c , i n ) 0 n ˙ c = ( n c / T c , i n ) / ( n c / T c , i n ) 0 (3)
c 1 = n ˙ c / [ p ( 1 − m n ˙ c ) + n ˙ c ( n ˙ c − m ) 2 ] c 2 = ( p − 2 m n ˙ c 2 ) / [ p ( 1 − m n ˙ c ) + n ˙ c ( n ˙ c − m ) 2 ] c 3 = − ( p m n ˙ c − m 2 n ˙ c 3 ) / [ p ( 1 − m n ˙ c ) + n ˙ c ( n ˙ c − m ) 2 ] c 4 = 0.3 (4)
The definitions for pressure ratio and relative efficiency of Equation (1) and Equation (2) are the following ones:
Among them, c 1 , c 2 , c 3 , c 4 , G ˙ c and n ˙ c can be calculated using the following Equation (3) and Equation (4).
Subscript 0 in Equation (3) and Equation (4) denotes design conditions while p = 1.8 and m = 1.8 .
Considering that the temperature of the gas flowing from the throttle into the storage vessel was constantly changing, the average temperature in the storage vessel would also change. From the state equation of ideal gas, the gas pressure would also change. We calculated the temperature in the storage vessel according to Equation (5).
δ Q = d E C V + h o u t δ m o u t − h i n δ m i n (5)
where δ Q is the heat transfer between the air in the storage vessel and the environment, d E C V is the inner energy change of air in the storage vessel, δ m o u t and δ m i n are the mass of air flow out and flow in storage vessel respectively.
Because the storage vessel is adiabatic, so δ Q = 0 . And calculated the pressure in the storage using the following Equation (6).
P = m s R g T / V s (6)
System efficiency varied with the main thermodynamic parameters in the system. The analyses were performed for the constant-pressure PHCA system.
the compressor was higher than the system efficiency when the compressor always working in design operating conditions. Through analysis, we found that although the efficiency of the compressor was reduced under off-design conditions, the working mode of changing the pressure ratio makes the pressure ratio of the compressor not need to be maintained at the maximum pressure ratio all the time. This mode effectively saved the power consumption of the compressor.
The effect of preset pressure and terminal pressure on the exergy destruction of the throttle valve was also illustrated in
we could also see that the system efficiency was more sensitive to the changes in water pump efficiency and hydroturbine efficiency. Therefore, in order to improve system efficiency, we should pay more attention to improving the performance of water pumps and hydroturbines.
In this paper, the off-design condition calculation model of the compressor was applied and the system efficiency varying with main thermodynamic parameters was analyzed. The analysis of the constant-pressure PHCA system suggested that the system efficiency considering the off-design model of the compressor was generally 2% - 5% higher than the system efficiency when the compressor always works in design operating conditions. What’s more, with increased preset pressure or with decreased terminal pressure, both the previous efficiency and current efficiency of the system increased. The system efficiency increased with increased mechanical efficiency, especially water pump efficiency and hydroturbine efficiency. In this system, the exergy destruction coefficient of the throttle valve reached 4%.
In short, this paper analyzed the constant-pressure PHCA system by applying the off-design model of the compressor to make the analysis result more in line with the actual situation. This paper has guiding significance for the practical application of the constant-pressure PHCA system. As research progresses, however, and the depth of knowledge increases, it is important for researchers to analyze the thermodynamic performance of the system considering off-design models of all components of the system.
The authors declare no conflicts of interest regarding the publication of this paper.
He, X., Wang, H.R., Tao, F.Y. and Ge, G.Q. (2021) Performance Analysis of Constant-Pressure Pumped Hydro Combined with Compressed Air Energy Storage System Considering Off-Design Model of Compressor. Energy and Power Engineering, 13, 11-18. https://doi.org/10.4236/epe.2020.134B002