Combined Solar Thermal System Research Articles

Hot Water Preparation along with Combined Solar Thermochemical Energy Storage System: Experimental Validation and Performance Simulation

Hybrid solar thermal systems are considered to be a promising solution for delivering clean thermal energy for the building sector, especially while combining them with other renewable energy sources. Usually, solar energy production does not match the thermal energy demand, and hence the energy storage must be integrated. Thermochemical energy storage is adapted particularly to be used along with solar thermal applications. In this article, the design of the combined solar thermochemical energy storage system is presented. The experimental prototype was built and tested within atmospheric conditions. The model of the thermochemical reactor was developed using Matlab® and it was validated by the experimental data. The dynamic simulations of the combined solar thermal system for the preparation of domestic hot water were carried out in the TRNSYS environment. The experimental energy storage density of the fully dehydrated material under non-equilibrium conditions was measured between 102 and 158 kWh/m³. Dynamic simulations performed in a broader scope of climate conditions showed that the energy storage density in the material under non-equilibrium conditions could vary between 71 and 247 kWh/m³.

Alternative storage options for solar thermal systems

The efficient implementation of solar systems in buildings depends on the storage of energy yielded, as it can both increase the solar system's autonomy and make it a feasible solution for zero energy buildings, and make storage vessels more compact, reducing precious space requirements. This is of particular important in places with reduced time of sunshine, where solar systems are less effective, because of the deviation between solar radiation and the demand. The traditional storage options use water, which is practical, safe and low-cost, especially when the storage requirements are small. However, when larger storage is needed, limits concerning the use of water exist, mainly due to the need for larger installation space and the increased thermal losses. The use of phase change materials (PCM) for thermal energy storage seems an upcoming technology. The main idea is the substitution of water with PCM, which feature larger specific energy storage capacity compared to other conventional materials. In the context of the specific paper, a combined solar thermal system used for the preparation of domestic hot water (DHW) and space heating (Solar Combi System) with two different types of storage is studied, for two Greek cities. The aim is to find out which is the most efficient way of storing energy with respect to the autonomy of the system, for a solar combi system. This is being achieved by determining the comparative autonomy of PCM and water storage system for various climates. It was proven that using PCM is advantageous, as it can extend the autonomy duration of the solar system for 2 to 8 hours, depending on the season and the climatic conditions. However, it was also seen that in solar combi systems used throughout the whole year, PCM are inefficient during summer period.