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Air-cooled heat pump hot and cold water units have been widely used in China since the 1990s, with their application rapidly expanding from south to north. These units offer two main advantages: they combine cooling and heating functions in a single system, improving overall efficiency. During cooling, air-cooled condensers eliminate the need for a separate water-cooling system, while heating is achieved through a heat pump mode, which is energy-efficient and environmentally friendly by avoiding the use of boilers that cause pollution. Additionally, these units are easy to install, often placed on rooftops without requiring a dedicated space.
As the economy develops, power shortages have become more severe, and the energy consumption of air conditioners during summer has increased significantly. This has led to stricter energy-saving requirements for air conditioning systems. One trend in the development of these systems is replacing thermal expansion valves with electronic expansion valves. Electronic expansion valves offer a wide range of flow control, high precision, and adaptability to load changes, ensuring stable operation of the evaporator and improving the dynamic performance of the air conditioner during startup and load variations.
**Experimental Setup and Test Method**
Figure 1 shows the schematic diagram of the experimental test device. A prototype unit with a nominal cooling capacity of 18.40 kW (MH008 type) was tested under standard conditions for various air-conditioning applications. Temperature sensors and turbine flow meters were installed at the inlet and outlet of the water-side heat exchanger to measure thermal performance. Cooling towers and electric heaters were used to regulate water temperature in different seasons.
**Test System Operation**
During testing, four shut-off valves were used to switch between the thermal expansion valve and the electronic expansion valve. When using the thermal expansion valve, valves 5 and 13 were closed, and valves 4 and 10 were opened. For the electronic expansion valve, the opposite configuration was used. To enhance the performance of the electronic expansion valve, two capillaries were connected in parallel.
**Test Results and Analysis**
According to the standard JB/T 4329-1997, the system was tested under maximum load, standard cooling, low-temperature, maximum heating, standard heating, and defrost conditions. Figures 3–6 show the comparison of cooling capacity, energy efficiency, heating capacity, and energy efficiency between the two types of expansion valves.
From the figures, it is clear that when using an electronic expansion valve, the system’s cooling and heating capacities are higher than those with a thermal expansion valve. In heating mode, the increase can be as high as 3,000 W, and in cooling mode, over 1,000 W. Energy efficiency also improves under the same operating conditions, proving that replacing the traditional thermal expansion valve with an electronic one enhances system performance and efficiency.
To evaluate the thermodynamic cycle from an energy perspective, exergy analysis was conducted. It revealed the conversion, transmission, utilization, and loss of energy, helping identify areas for improvement. The exergy losses in each process—compression, condensation, throttling, and evaporation—were calculated and compared.
**Table 1: Exergy Loss Calculation Results**
| Process | Thermal Expansion Valve | Electronic Expansion Valve |
|--------|--------------------------|----------------------------|
| Compression (%) | 32.42 / 39.06 / 35.75 / 36.45 / 37.98 / 38.42 | 22.9 / 16.95 / 14.07 / 19.92 / 20.5 / 17.57 |
| Condensation (%) | 35.61 / 55.08 / 36.91 / 62.94 / 41.34 / 66.33 | 35.61 / 55.08 / 36.91 / 62.94 / 41.34 / 66.33 |
| Evaporation (%) | 18.86 / 15.33 / 12.15 / 14.17 / 12.94 / 13.44 | 18.86 / 15.33 / 12.15 / 14.17 / 12.94 / 13.44 |
| Total Exergy Loss (%) | 13.11 / 6.69 / 11.87 / 5.94 / 9.97 / 6.16 | 13.11 / 6.69 / 11.87 / 5.94 / 9.97 / 6.16 |
The use of an electronic expansion valve reduces throttling losses and improves system efficiency. Future improvements could involve optimizing heat transfer and reducing exergy loss in the condensation process. Using variable frequency technology to adjust compressor speed and refrigerant flow can further enhance system performance.
**Conclusion**
Replacing the thermal expansion valve with an electronic one improves system performance, increases cooling and heating capacity, and enhances energy efficiency. The electronic expansion valve offers faster response and more accurate superheat regulation. Future work should focus on optimizing the condenser and using variable frequency technology to improve system efficiency under varying conditions.
**References**
1. "Electronic Expansion Valve vs. Thermal Expansion Valve Comparison." World Shipping, October 2004.
2. Xue Ting. "Crisp Declining Caries? Excellent Head Oxybutadiene Ometoate Smart Mechanical Fatigue Ether Ether?" April 2002.
3. Lei et al. "Comparison of Electronic Expansion Valve and Thermal Expansion Valve in Air-Cooled Heat Pump Units." Refrigeration Technology, No. 4, 2001.
4. Zhang Zhi. "Refrigeration Principle and Equipment." Beijing: Machinery Industry Press.
5. Zhang Long et al. "Exergy Analysis of Subcooled Small Ice Storage System." Energy Saving Technology, April 2005.
6. Mohsen Farzad, Dennis Lo, Neal. "System Performance Characteristics of An Air Conditioner over a Range of Charging Conditions." Int. J. Ref., 1991, 14.
7. JB/T 4329-1997: Standard Performance Conditions for Volumetric Cold Water (Heat Pump) Units.
8. GB/T 10870-2001: Methods for Testing Performance of Volumetric and Centrifugal Cold Water (Heat Pump) Units.
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