Phase change materials in residential water heaters: A comparative review of thermal energy storage in heat pump, solar, and electric systems

The need for more economical and efficient heating solutions is being driven by the growing demand for home hot water around the world. By separating heat production from demand through latent heat storage, thermal energy storage—especially with phase change materials (PCMs)—offers a viable option to lower energy usage. The objectives of this review are to: (i) identify typical PCM properties, configurations, and research methods; (ii) synthesize and categorize published work on the integration of PCMs in residential water heaters, with a focus on heat pump, solar, and electric systems; and (iii) derive useful selection criteria for PCMs and system layouts in residential hot-water applications. This work offers a comparative, heater-type-oriented synthesis, in contrast to previous reviews that examine PCMs in a generic or single-technology context. It quantitatively maps PCM selection, melting temperature, latent heat, system placement, and study approaches, as well as three key water-heater technologies. By heater type, PCM placement (e.g., storage tank, collectors), collector layout, research methodology, and engineering thematic focus, pertinent studies are methodically categorized. To determine prevailing practices and parameter ranges, reported PCM thermophysical parameters and system configurations are statistically examined. According to the analysis, solar water heaters and paraffin wax predominate among the assessed works. The majority of PCMs have latent heats of 150–200 kJ/kg and melting temperatures of 50–60 °C. The most common location for PCM is storage units, with the highest amount of contributions coming from experimental investigations. Based on these results, the review suggests useful application-specific PCM selection criteria (melting point, latent heat, and placement) and provides recommendations for appropriate water-heater types and efficient research methodologies for next studies.