Effect of phase change materials on indoor thermal environment under different weather conditions and over a long time

•Indicators evaluating the performance of PCMs in greenhouses are introduced.•Real equivalent specific heat capacity of PCMs is embedded in a numerical model.•Real behaviour of PCMs has been monitored over a long time.•Efficiency of PCMs walls are compared for sunny and cloudy days.•Heat storage and release amounts of PCMs walls have been calculated. To evaluate the effect of phase change materials (PCMs) on the indoor thermal environment of greenhouses under different weather conditions and over a long time in the heating season, a study was carried out using both experimental method and numerical method. The study was conducted in a typical greenhouse located in Beijing, China, and important parameters have been monitored continuously for 61days, including indoor air temperature, outdoor air temperature, solar radiation, surface temperature of greenhouse envelopes and soil temperature. Based on these parameters, a number of indicators, namely, operative temperature, daily effective accumulative temperature, irradiated surface temperature of the north wall, average temperature of PCMs, and daily heat storage and release, have been used to evaluate the performance of PCMs in greenhouses. All indicators have provided consistent results that confirm the positive effect of PCMs on improving the indoor thermal environment of greenhouses over a long time. Additionally, the paper has demonstrated that a sunny weather could help to promote the efficiency of PCMs, comparing to a cloudy weather.