# Experimental Investigation on the Performance of Compressors for Small-Scale Compressed Air Energy Storage in Parallel Mode

^{1}

^{2}

^{*}

*Sustainability*

**2023**,

*15*(17), 13164; https://doi.org/10.3390/su151713164 (registering DOI)

## Abstract

**:**

## 1. Introduction

## 2. Experimental Setup

## 3. Results and Discussion

#### 3.1. Influence of Air Tank Pressure on the Performance of the Compressor

_{out}is the outtake pressure, and p

_{in}is the intake pressure.

_{c}is the compressor power consumption, n is the rotational speed, and T

_{r}is the torque.

_{b}is the power output (W) of the battery, U is the voltage (V), and I is the current (A).

_{1}with the pressure of the air tank. In general, when the rotational speed is low, η

_{1}increases first, then tends to be flat, and finally decreases slightly with the increase in the air tank pressure. When the rotational speed is high, η

_{1}decreases slightly with the increase in the air tank pressure. Under the same pressure as the air tank, η

_{1}increases with the rotational speed. In terms of the overall change trend, η

_{1}of compressor 2 is slightly higher than that of compressor 1, which is caused by the performance difference of the compressor itself. The maximum value of η

_{1}is approximately 75%. DC/AC conversion efficiency is higher as the rotation speed increases. There is no obvious tendency of DC/AC conversion efficiency when air tank pressure is increasing.

_{2}is defined as the enthalpy difference between the compressor intake and outtake divided by the isentropic enthalpy value at the compressor outtake minus the enthalpy value at the compressor intake.

_{2s}is the isentropic enthalpy of the compressor at the outtake, (kg/kJ); and h

_{2}and h

_{1}are the enthalpy of the compressor at the outtake and intake, respectively.

_{2}with the pressure of the air tank. With the increase in air tank pressure, η

_{2}shows a decreasing trend. With the increase in the air tank pressure, the reduction in efficiency increases. When the pressure of the air tank is the same, η

_{2}increases with the increase in the rotational speed. When the pressure of the air tank increases from 2.2 bar to 2.6 bar and the rotational speed is 2850 r/min and 1650 r/min, η

_{2}decreases from 73.8% to 50.8% and 29.2% to 22.9%, respectively. When the pressure of the air tank is 1.4 bar and the rotational speed is 2850 r/min, η

_{2}of the compressor reaches the maximum value of approximately 95%.

#### 3.2. Influence of Torque on the Performance of the Compressor

_{1}with torque. With the increase in torque, η

_{1}first increases, then tends to be flat, and finally shows a slight decrease. In addition, η

_{1}tends to increase with the increase in rotational speed. The η

_{1}of compressor 2 is higher than that of compressor 1 under different torques and rotational speeds.

_{2}with torque. It can be seen that η

_{2}decreases with the increase of in torque value but increases with the rotational speed. Under different rotational speeds and torques, η

_{2}of compressor 2 is greater than that of compressor 1.

#### 3.3. Influence of Mass Flow Rate on the Performance of the Compressor

_{1}with mass flow rate. With the increase in mass flow rate, η

_{1}first shows a linear increasing trend and then tends to be flat. Also, η

_{1}increases with the increase in rotational speed. The η

_{1}of the compressor 2 is higher than that of compressor 1 under different mass flow rates and speeds.

_{2}with mass flow rate. η

_{2}decreases with the increase in mass flow rate but tends to increase with the increase in rotational speed. When the rotational speed is less than 2550 r/min, η

_{2}of compressor 2 is greater than that of compressor 1.

#### 3.4. Influence of Air Tank Pressure on the Performance of the Compressor in Parallel Mode

_{1}with the air tank pressure in parallel mode. In general, η

_{1}shows an increasing trend with the increase in the rotational speed and the air tank pressure. In parallel mode, the η

_{1}of compressor 2 is greater than that of compressor 1.

_{2}with the air tank pressure in parallel mode. In parallel mode, the variation trend of η

_{2}with the air tank pressure is relatively gentle. η

_{2}increases with the increase in rotational speed. In addition, the η

_{2}corresponding to compressor 2 is greater than that corresponding to compressor 1 at different rotational speeds and air tank pressures.

#### 3.5. Influence of Torque on the Performance of the Compressor in Parallel Mode

_{1}with torque in parallel mode. In general, η

_{1}increases first and then tends to be flat with the increase in torque. In parallel mode, the η

_{1}of compressor 2 is higher than that of compressor 1 at different rotational speeds and torques. Higher efficiency could be achieved in small torque and maintained in a wide torque range.

_{2}with torque in parallel mode. When the rotational speed is low, η

_{2}decreases first and then tends to be flat with the increase in torque. When the speed is high, η

_{2}first increases, then tends to be flat, and finally decreases slightly with the increase in torque. In parallel mode, η

_{2}of compressor 1 is higher than that of compressor 2 at different rotational speeds and torques. In parallel mode, a lower torque leads to higher isotropic efficiency for the two compressors. As the torque increases, isotropic efficiency is promoted at the same rotation speed. The isotropic efficiency is distributed in a wide bell curve at high rotation speeds. But in the same torque range, the fluctuation is not wide. On the contrary, a small rotation speed change is not significant.

#### 3.6. Influence of Mass Flow Rate on the Performance of the Compressor in Parallel Mode

_{1}with mass flow rate in parallel mode. When the rotational speed is low, η

_{1}increases linearly with the increase in mass flow rate. When the rotational speed is high, the variation of η

_{1}with mass flow rate is relatively gentle. In parallel mode, the η

_{1}of compressor 2 is greater than that of compressor 1. In parallel mode, lower rotation speeds have great DC/AC conversion efficiency such as at 450 r/min, 750 r/min, and 1050 r/min.

_{2}with a mass flow rate in parallel mode. With the increase in mass flow rate, η

_{2}shows a gradual variation trend first and then a downward trend. At different rotational speeds and mass flow rates, the corresponding η

_{2}of compressor 2 is greater than that of compressor 1. In parallel mode, isotropic efficiency is not a downtrend compared to single mode. This curve would reach a peak in large rotation speed, such as 1650 r/min, 1950 r/min, 2250 r/min, and 2550 r/min. It is better for the compressor to operate at high efficiency.

#### 3.7. Uncertainty Analysis

_{i}is the absolute uncertainty of the measured variable.

## 4. Summary and Conclusions

- (1)
- The torque, pressure ratio, and power consumption of the compressor increase linearly with the increase in the air tank pressure. The maximum value of power consumption is approximately 1233.1 W.
- (2)
- With the increase in mass flow rate, the pressure ratio and power consumption first present a linear increasing trend, then tends to be flat, and finally shows an increasing trend again.
- (3)
- Parallel mode could extend the current and torque working conditions almost twice as much as single mode.
- (4)
- In parallel mode, DC/AC conversion efficiency and isotropic efficiency have improved significantly. The isotropic efficiency curve has a bell shape with a wide peak.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Nomenclature

m | Mass (kg) |

n | Rotation speed (r/min) |

p_{in} | Intake pressure (bar) |

p_{out} | Exhaust back pressure (bar) |

P_{c} | Power consumption of compressor (W) |

P_{t} | Power output of battery (W) |

T_{r} | Torque (N·m) |

Greek letters | |

ε | Expansion ratio |

η | Efficiency |

Acronyms | |

CAES | Compressed air energy storage |

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**Figure 7.**Efficiency of CAES with air tank pressure. (

**a**) DC/AC conversion efficiency. (

**b**) Isotropic efficiency.

**Figure 11.**Efficiency of CAES with torque. (

**a**) DC/AC conversion efficiency. (

**b**) Isotropic efficiency.

**Figure 16.**Efficiency of CAES with mass flow rate. (

**a**) DC/AC conversion efficiency. (

**b**) Isotropic efficiency.

**Figure 20.**Consumption of CAES with air tank pressure in parallel mode. (

**a**) Compressor power. (

**b**) Battery power.

**Figure 21.**Efficiency of CAES with air tank pressure in parallel mode. (

**a**) DC/AC conversion efficiency. (

**b**) Isotropic efficiency.

**Figure 24.**Consumption of CAES with torque in parallel mode. (

**a**) Compressor power. (

**b**) Battery power.

**Figure 25.**Efficiency of CAES with torque in parallel mode. (

**a**) DC/AC conversion efficiency. (

**b**) Isotropic efficiency.

**Figure 29.**Consumption of CAES with mass flow rate in parallel mode. (

**a**) Compressor power. (

**b**) Battery power.

**Figure 30.**Efficiency of CAES with mass flow rate in parallel mode. (

**a**) DC/AC conversion efficiency. (

**b**) Isotropic efficiency.

Type | Rated Rotation Speed | Rated Power | Rated Torque |
---|---|---|---|

QMH050A | 2000 r/min | 485 W | 7.0 N·m |

80ST-M02430 | 3000 r/min | 750 W | 2.39 N·m |

Name | Measuring Range | Tolerances |
---|---|---|

Pressure sensors | 0~15 bar | ±0.2% FS |

Temperature sensor | −20~100 °C | ±0.5% FS |

Torque sensor | 0~20 N·m | ±0.5% FS |

Speed sensor | 0~6000 r/min | ±0.5% FS |

Flowmeter | 0~5000 L/min | ±0.5% FS |

Voltage sensor | 0~250 V | ±0.5% FS |

Current sensor | 0~30 A | ±0.5% FS |

Parameters | Measuring Range | Accuracy | Relative Uncertainty |
---|---|---|---|

p_{in}, p_{out} | 0–15 bar | ±0.2% FS | 0.4% |

T_{in}, T_{out} | −20–100 °C | ±0.5% FS | 2.5% |

Torque | 0–20 N·m | ±0.5% FS | 1.67% |

Rotation speed | 0–6000 r/min | ±0.5% FS | 1.5% |

Volume flow rate | 0–5000 L/min | ±0.5% FS | 1.56% |

Current | 0–30 A | ±0.5% FS | 1.67% |

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## Share and Cite

**MDPI and ACS Style**

Yang, H.; Xu, Y.; Zhang, H.; Zhang, J.; Yang, F.; Wang, Y.; Wu, Y.
Experimental Investigation on the Performance of Compressors for Small-Scale Compressed Air Energy Storage in Parallel Mode. *Sustainability* **2023**, *15*, 13164.
https://doi.org/10.3390/su151713164

**AMA Style**

Yang H, Xu Y, Zhang H, Zhang J, Yang F, Wang Y, Wu Y.
Experimental Investigation on the Performance of Compressors for Small-Scale Compressed Air Energy Storage in Parallel Mode. *Sustainability*. 2023; 15(17):13164.
https://doi.org/10.3390/su151713164

**Chicago/Turabian Style**

Yang, Hailong, Yonghong Xu, Hongguang Zhang, Jian Zhang, Fubin Yang, Yan Wang, and Yuting Wu.
2023. "Experimental Investigation on the Performance of Compressors for Small-Scale Compressed Air Energy Storage in Parallel Mode" *Sustainability* 15, no. 17: 13164.
https://doi.org/10.3390/su151713164