Application of a Phase-Change Material Heat Exchanger to Improve the Efficiency of Air-conditioners at Partial Loads
Inverter-equipped air conditioning (AC) systems offer variable speed control allowing for increased energy efficiency. However, AC systems often operate at low load ratios below where inverter control is effective, which reduces their energy efficiency. We developed an AC system that increases the apparent load ratio of the compressor in cooling seasons by using a phase-change material (PCM) as a high-performance heat exchanger. PCM cooling and heating experiments were conducted with a PCM heat exchanger, which comprised aluminum plates and fins filled with 0.8 kg of paraffinic PCM with a phase change temperature of 17°C–18°C. The outlet air temperature was close to the phase change temperature during both the PCM cooling and heating experiments, indicating a high heat transfer coefficient of >70 W/(m2·K). A simplified numerical model of the PCM heat exchanger as a lumped constant system was created using the experimentally obtained coefficient. A parametric study revealed that the effective weight for heat exchange was 5.83 kg for aluminum and 0.53 kg for PCM. The calculations generally reproduced the experimental results, with root mean squared errors of 0.39 K for PCM cooling and 0.84 K for PCM heating, confirming their accuracy. Simulations were then conducted comparing the energy performance of the proposed system to that of a system without PCM in an office space for the cooling season. While low load operation accounted for 39% of the total AC time for a non-PCM system, it was reduced to 2.7% for the proposed system. The proposed system demonstrated load ratios of 50%–60% for most of the cooling season and achieved a high coefficient of performance. The PCM heat exchanger reduced energy consumption by 11% in the whole cooling season, owing to the improved efficiency at partial load ratios.