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Abstract-Four numerical models have been built for the simulation of the thermal yield of a combined PV-thermal collector: a 3D dynamical model and three steady state models that are 3D, 2D and 1D. The models are explained and the results are compared to experimental results. It is found that all models follow the experiments within 5% accuracy. In addition, for the calculation of the daily yield, the simple 1D steady state model performs almost as good as the much more time-consuming 3D dynamical model. On the other hand, the 2D and 3D models are more easily adapted to other configurations and provide more detailed information, as required for a further optimization of the collector. The time-dependent model is required for an accurate prediction of the collector yield if the collector temperature at the end of a measurement differs from its starting temperature.

Water vapor sorption in salt hydrates is one of the most promising means for compact, low loss, and long-term storage of solar heat in the built environment. One of the most interesting salt hydrates for compact seasonal heat storage is magnesium sulfate heptahydrate (MgSO4⋅7H2O). This paper describes the characterization of MgSO4⋅7H2O to examine its suitability for application in a seasonal heat storage system for the built environment. Both charging (dehydration) and discharging (hydration) behaviors of the material were studied using thermogravimetric differential scanning calorimetry, X-ray diffraction, particle distribution measurements, and scanning electron microscope. The experimental results show that MgSO4⋅7H2O can be dehydrated at temperatures below 150°C, which can be reached by a medium temperature (vacuum tube) collector. Additionally, the material was able to store 2.2 GJ/m3, almost nine times more energy than can be stored in water as sensible heat. On the other hand, the experimental results indicate that the release of the stored heat is more difficult. The amount of water taken up and the energy released by the material turned out to be strongly dependent on the water vapor pressure, temperature, and the total system pressure. The results of this study indicate that the application of MgSO4⋅7H2O at atmospheric pressure is problematic for a heat storage system where heat is released above 40°C using a water vapor pressure of 1.3 kPa. However, first experiments performed in a closed system at low pressure indicate that a small amount of heat can be released at 50°C and a water vapor pressure of 1.3 kPa. If a heat storage system has to operate at atmospheric pressure, then the application of MgSO4⋅7H2O for seasonal heat storage is possible for space heating operating at 25°C and a water vapor pressure of 2.1 kPa.

The reflection and transmission spectra of Pd capped Mg y Ti 1Ày thin films (y ¼ 0:7; 0:8 and 0.9) are measured in the 0.5-5.5 eV energy range, both in the as-prepared and hydrogenated states. Upon hydrogenation these films switch reversibly from a shiny metallic state into a ''black'' absorbing one. The composition and thicknesses can be tailored to achieve high solar absorptance and low thermal emittance in the hydrogenated state. The combination of these two characteristics is interesting for the application of this material as switchable absorber in solar collectors. The use of a Mg y Ti 1Ày switchable absorber in solar collectors allows to lower the stagnation temperature from 180 to 80 1C. The collector efficiency is affected only minimally.

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