Phase Change Materials to increase the storage potential of solar thermal systems

Thermal energy storage, is a key issue for the use of solar thermal systems in buildings so that autonomy of the system is increased and therefore solar thermal systems become a more attractive solution for buildings. It has therefore been in the focus of research over the past few decades, as it is an important technology in order to solve the problem of temporal deviation between the availability of solar energy and the utilization of the heat generated. Thermal storage is crucial in order to bridge this gap, especially in regions where prolonged periods of reduced sunshine are common and as a result solar systems are inefficient. Traditionally thermal storage is achieved by using water, which has been proven to be practical and cost effective, especially when fairly small storage capacities are required. There are however limits, mainly due to space limitations and increasing losses, as soon as bigger volumina are needed to extend the storage period. The use of Phase Change Materials (PCMs), is an upcoming, promising technology, which has drawn the scientific community's attention for quite some years now. The main idea is to substitute water as a storage medium, with PCMs, which have larger specific energy storage capacity compared to other materials. In this paper, a solar thermal system for Domestic Hot Water production and space heating with either water storage or PCM storage is studied, for two different climate conditions in Greece. The main goal is to investigate whether the use of PCM storage is an energy efficient solution or not and to calculate the potential increase of the autonomy of a solar combi system when PCM storage is used.