Optimal design and thermal performance evaluation of phase change material filling position for solar air collectors

Unreasonable filling positions of phase-change materials (PCMs) significantly reduce the heat storage capacity, heat release time, output temperature difference, heat collection efficiency, and heat peak migration capacity of PCM-based solar–air collectors in building heating systems, thereby affecting the comprehensive heating requirements of building users. To solve this problem and obtain the optimal filling position of the PCM, four PCM filling schemes were designed in this experiment: Type I was a solar air collector with the PCM filled in the upper portion, Type II was filled in the middle portion, Type III was filled on both sides, and Type IV was filled in the bottom portion. Subsequently, the four models were compared. A comparative analysis of the thermal performance evaluation index showed that compared with the other three types of collectors, the heat storage layer of Type I stored 1.505 × 106J of heat in 1.42h, indicating that the Type I solar–air collector had the best heat storage speed rate. When the solar energy supply was terminated, the continuous heat release time of the Type I collector was 13.92h, and the ratio of the heat release time to the non-solar time was 96 %. The Type I collector had the lowest output temperature and the slightest temperature fluctuation, which could increase indoor thermal comfort. The average daily heat collection efficiency of the Type I collector was the lowest, at only 38.11 %, indicating that the Type I collector has the strongest “peak cutting and valley filling” ability, which allows for transferring more heat for nighttime release. Therefore, filling the upper portion is the optimal filling scheme that can provide technical benefits for the application and development of collectors.