Hysteresis effects on the thermal performance of building envelope PCM-walls

This work presents a numerical study of the combined effects of the hysteresis temperature difference, peak melting temperature, and thickness of a building envelope PCM-wall on its thermal performance in air-conditioning and non-air-conditioning conditions. The study was carried out considering complete melting-freezing daily cycles of the PCM in a climate exhibiting both hot and cold thermal discomfort. A time-dependent one-dimensional heat conduction code, which uses the effective specific heat method to simulate the heat transfer through the PCM was developed. Insights into the effects of the hysteresis phenomenon were obtained; it was found that hysteresis improves the thermal performance of PCM-walls. The higher the hysteresis temperature difference the better the thermal performance, but there is a limit in the improvement of the thermal performance, which is achieved when the entire phase change process takes place at temperatures outside of the thermal comfort zone. Maximum improvements from 4% to 29% for air-conditioning and from 4% to 30% for non-air-conditioning, for a BioPCM wall with thicknesses from 6 mm to 18 mm, were found. Suggested criteria to achieve the maximum possible thermal performance of PCM-walls given a thickness and use condition were obtained. This work proposes the basis of a methodology to optimize simultaneously any pair of variables of a PCM-wall for different use conditions (AC, nAC, or a combined use of AC and nAC).