Thermal solar collector with VO2 absorber coating and V1-xWxO2 thermochromic glazing – Temperature matching and triggering
Overheating is a common problem both with the use of active and passive solar energy in thermal solar energy systems and in highly glazed buildings, even in central European latitudes. In solar thermal collectors, the elevated temperatures occurring during stagnation result in reduced lifetime of the collector materials. They lead to water evaporation, glycol degradation and stresses in the collector with increasing vapor pressure. Special precautions are necessary to release this pressure; only mechanical solutions exist nowadays. The temperature of degradation of glycols is above 160–170 °C. However, it would be preferable to limit the temperature of the collector to approximately 100 °C, avoiding likewise the evaporation of the used water-glycol mixture. Additionally, the elevated temperatures lead to degradation of the materials that compose the collector, such as sealing, thermal insulation and the selective absorber coating. A new way of protecting solar thermal systems without any mechanical device (e.g. for shading or for pressure release) is proposed. A durable inorganic thermochromic material, which exhibits a change in optical properties at a transition temperature TtTt, is vanadium dioxide (VO2). At 68 °C, VO2 undergoes a reversible crystal structural phase transition accompanied by a strong variation in optical properties. Therefore, a dynamical switching of the thermal emittance ∊th∊th can be achieved by VO2. By doping the material with tungsten, it is possible to lower the transition temperature making it suitable as a glazing coating. The possibility of using the switch in emittance of the absorber coating in order to trigger the transition of a thermochromic coating on the glazing of the solar collector has been studied. An analytical approach yielded the required transition temperature of such a switching glazing. The fascinating optical properties of these switchable films elucidate the way towards novel intelligent thermal solar collector materials.