Energetic optimization of a solar cooling plant with PCM heat storage

The use of solar energy in buildings is an important contribution for the reduction of fossil fuel consumption and harmful emissions to the environment. The combined use of solar energy for heating and cooling has the potential to upgrade solar thermal energy from mainly DHW provider to a major building energy supplier. The main components of the so called “Solar Combi+” systems are: the solar thermal collector to provide the heat; another heat source for back up (typically a natural gas boiler); a storage tank that can either be installed on the warm side, on the cold side or on both; the domestic hot water tank; the sorption chiller that is fed with hot water (70-100 °C); a cooling tower (dry, wet) to reject heat at intermediate temperature (30-40 °C). A key factor for the energy optimization of the solar heating/cooling plant is the design of the heat storage, also related to the solar field size and performance. Latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal nature of the storage process. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. This study aims to evaluate the potential of integrated solar absorption cooling and heating systems with “traditional” sensible versus “innovative” PCMs heat storage tank. The TRNSYS software tool was used as a basis for assessment for a typical residential building application. A first set of simulations was used to optimize the size of the solar collectors field varying the heat storage volume and the climate (three different locations were considered: Venice, Rome and Trapani (Italy)) for a sensible heat storage tank. The same optimization was implemented for a latent heat storage tank for four different PCMs and varying different characteristics (such as shape, size, arrangement) of the PCMs. The TRNSYS models allowed to study a whole year (365 days) combining cooling, heating and DHW applications in order to compare the annual energy performances of the two systems (with sensible and latent heat storage on the warm side) for a residential building in the three resorts. Results indicate that the “PCM system” allows better annual energy performances with respect to the “sensible system”. Anyway, these results are strictly connected to the temperature level of the phase change, and so to the solar collectors field size and characteristics, the storage tank volume and obviously to the temperature of the heat loads.