Thermal energy storage applications in solar water heaters: An updated review

Fossil fuel usage for heating applications must be reduced considering the issues related to the environment and the restriction of their resources. In this regard, attention is devoted to renewable energy sources to supply the energy requirements of different sectors. In the building sector, solar energy is harnessed for heating and cooling. Solar energy is applicable both directly and indirectly for heating using different technologies. The intermittent nature of solar energy obliges the use of storage units to make the solar systems applicable at night hours or during periods the low solar intensity. Various thermal energy storage materials have been utilized in different kinds of solar heaters to stabilize their performance, improve their reliability, and avoid issues related to variations in solar radiation. In this article, studies on the usage of thermal energy storage units in solar water heaters are reviewed and their key results are reflected. As one of the main conclusions of the reviewed works, it can be denoted that several factors such as the operation condition and characteristics of the storage unit are effective on the function of the systems combined with the thermal storage component. Aside from an increment in the operating hours of solar heaters, usage of storage units can boost both energy and exergy efficiencies. Furthermore, the study denotes that the power saving rate is influenced by the abundance of solar energy resources. In addition, it could be denoted that the performance of the systems is improvable by employing some ideas, such as the application of nanotechnology in storage materials.