Evacuated tube solar collectors integrated with phase change materials

•Integrating phase change materials (PCM) within the evacuated solar tube collectors.•Heat pipe is immersed inside the PCM, where heat is effectively stored.•We examined the novel approach during both normal and on-demand operation.•Improved functionality during on-demand operation or insufficient solar intensity.•The feasibility is tested via large scale commercial solar water heaters. This paper presents a novel method of integrating phase change materials (PCMs) within the evacuated solar tube collectors for solar water heaters (SWHs). In this method, the heat pipe is immersed inside the phase change material, where heat is effectively accumulated and stored for an extended period of time due to thermal insulation of evacuated tubes. The benefit of this method includes improved functionality by delayed release of heat, thus providing hot water during the hours of high demand or when solar intensity is insufficient. The proposed solar collector utilizes two distinct phase change materials (dual-PCM), namely Tritriacontane and Erythritol, with melting temperatures of 72°C and 118°C respectively. The operation of solar water heater with the proposed solar collector is investigated during both normal and on-demand operation. The feasibility of this technology is also tested via large scale commercial solar water heaters. Beyond the improved functionality for solar water heater systems, the results from this study show efficiency improvement of 26% for the normal operation and 66% for the stagnation mode, compared with standard solar water heaters that lack phase change materials.