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Development of High Performance Room Air Conditioner Using Two-Stage Compression Cycle with Gas Injection

Overview of developed high performance room air conditioner.

Fig.1 Overview of developed high performance room air conditioner.

Structure of developed cycle (under cooling operation).

Fig.2 Structure of developed cycle (under cooling operation).

Pressure-enthalpy diagram (under cooling operation).

Fig.3 Pressure-enthalpy diagram (under cooling operation).

Two-stage rotary compressor.

Fig.4 Two-stage rotary compressor.

1. Abstract

Recent demands for improved efficiency of room air conditioners have arisen from concerns over global warming. Improvements in cycle performance are difficult because each component is performing at its optimal level. Therefore, we developed a two-stage compression cycle with gas injection to reduce expansion loss, which is an important loss in conventional cycles. The gas refrigerant in the cycle we developed does not absorb heat from air that bypasses an evaporator and is directly injected into a compression chamber. The main devices developed are a "two-stage rotary compressor" that enables a desired gas injection by connecting two compression chambers in series, an "injection component" that includes a separator, a "cycle control" that optimizes gas injection rate by sensing the temperature difference between the separator and the compressor, an "inverter control", "high efficiency heat exchangers", and "high efficiency fans". Using the developed system, we obtained a coefficient of performance (COP) of 6.50 for a room air conditioner with a cooling capacity of 2.8 kW. The obtained value is the highest in the market.

2. Technology

Fig.1 shows an overview of the high performance room air conditioner we developed. A two-stage compression cycle with gas injection was used in an outdoor unit to reduce expansion loss in the conventional cycles.
Fig.2 and Fig.3 show the developed cycle and a pressure-enthalpy diagram under a cooling condition. The two-stage rotary compressor consists of first- and second-stage compression chambers. The injection component consists of two expansion valves between a condenser and an evaporator, and a separator located between the two expansion valves that separated the two-phase refrigerant into gas and liquid. After separation, the liquid refrigerant is reexpanded to lower the pressure by the second expansion valve and then it flows into the evaporator. On the other hand, the gas refrigerant bypasses the evaporator and is then injected into the second-stage compression chamber at an intermediate pressure.

This mechanism reduces expansion loss by decreasing the quality of the inlet refrigerant of the evaporator. Refrigerant mass flow rate in the evaporator and in the first-stage compression chamber decreased dramatically. As a result, this mechanism reduces pressure loss in the evaporator and compression power in the two-stage rotary compressor. Furthermore, we have developed cycle control unit to optimize gas injection rate by sensing the temperature difference between the inlet of the separator and the suction port of the second-stage compression chamber. This control unit improves cycle

performance under varying operating conditions, such as cooling, normal heating, and low outdoor temperature heating.

The two-stage rotary compressor as shown in Fig.4 is the most important component in the developed cycle and consists of two compression chambers connected in series using a connecting pipe that is outside the compressor. This compressor achieves a stable gas injection because the injected part is connected to an intermediate connecting pipe where the pressure fluctuation is small. By compressing the refrigerant in two stages, the refrigerant leakage in each compression chamber is reduced thus leading to improved compression efficiency. We optimized the dimensions of compressor elements such as the volumetric ratio of each compression chamber, the aspect ratio of the compression chambers, and the discharge port diameter, by numerical and experimental analysis to achieve an effective gas injection and high compressor efficiency. In addition, we developed a novel inverter control that minimizes compressor motor input by altering input current wave against various torque fluctuation of the two-stage compressor.

We also optimized the refrigerant path of heat exchangers and the shape of fan blades based on the developed cycle.

3. Conclusion

Room air conditioners that use the developed cycle are named “double accelerator system”. This system was first launched in 2004 for the Japanese market and the total number of units being produced is increasing. A 2.8 kW room air conditioner that operated using the developed cycle achieved a COP of 6.50 and a maximum heating capacity of 8.3 kW. The obtained values are the highest values in the market.


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