With PV Thermal panels sunlight is converted into electricity and heat simultaneously. Per unit area the total efficiency of a PVT panel is higher than the sum of the efficiencies of separate PV panels and solar thermal collectors. During the last 20 years research into PVT techniques and concepts has been widespread, but rather scattered. This reflects the number of possible PVT concepts and the accompanying research and development problems, for which it is the general goal to optimise both electrical and thermal efficiency of a device simultaneously. The aspects that can be optimised are, amongst others, the spectral characteristics of the PV cell, its solar absorption and the internal heat transfer between cells and heat-collecting system. Another important level of optimisation is for the PVT device geometry and the integration into a system. The electricity and heat demand and the temperature level of the heat determine the choice for a certain system set-up. With an optimal design, PVT systems can supply buildings with 100% renewable electricity and heat in a more cost-effective manner than separate PV and solar thermal systems and thus contribute to the long-term international targets on implementation of renewable energy in the built environment.

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This paper describes the characterization of four salt hydrates as potential thermochemical material for compact seasonal heat storage in the built environment. First, magnesium sulfate was investigated in detail using TG-DSC apparatus. The results of this study revealed that magnesium sulfate is able to store almost 10 times more energy than water of the same volume. However, the material was unable to take up water (and release heat) under practical conditions. A new theoretical study identified three salt hydrates besides magnesium sulfate as promising materials for compact seasonal heat storage: aluminum sulfate, magnesium chloride and calcium chloride. These salt hydrates (including magnesium sulfate) were tested in a newly constructed experimental setup. Based on the observed temperature lift under practical conditions, it was found that magnesium chloride was the most promising material of the four tested salt hydrates. However, both calcium chloride and magnesium chloride tend to form a gel-like material due to melting or formation of a solution. This effect is undesired since it reduces the ability of the material to take up water again. Finally, it was observed that performing the hydration at low-pressure will improve the water vapor transport in comparison to atmospheric pressure hydration.

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