High temperature phase change materials for the overheating protection of facade integrated solar thermal collectors

• The thermal comfort in rooms behind solar thermal collectors is controlled by PCMs.
• Our numerical simulations suggest high temperature PCMs close to the absorber.
• PCMs with melting temperatures as high as 85 °C are suitable in our scenario.
• The high temperature PCMs regenerate fast via the absorber in the night.
• The regeneration process is very reliable due to the large temperature gradients.

Maintaining the thermal comfort in buildings with facade integrated solar thermal collectors is a key criterion for the acceptance of this technology. The worst case scenario is a long period of stagnation where the solar absorber reaches very high temperatures and starts to heat up the interior wall. An appealing solution to this problem is to embed a layer of phase change material (PCM) into the absorber insulation which buffers the heat during the day and releases it in the night.We carried out dynamic simulations of a facade integrated collector in stagnation to determine the optimum melting temperature of a thin layer of PCM at various positions between the absorber and the interior wall. We found that PCMs with a wide range of melting temperatures can be used if the PCM is correctly positioned. Placing the PCM close to the hot absorber allows using high temperature PCMs having melting temperatures of up to 85 °C in our reference scenario. The high melting point of the PCM and the proximity to the solar absorber have important technical benefits. The regeneration process in the night, where the PCM solidifies and recharges for the next day, is very efficient due to the large temperature gradients involved. In addition the solar absorber is now available to transfer heat from the PCM to ambient. The novel recharging process we propose is not only fast, it is also very reliable: the large temperature difference makes the regeneration immune to the changes in indoor or ambient conditions. This is a major advantage over the conventional usage of low temperature PCMs close to the interior wall, whose regeneration rate will strongly be affected by small changes in indoor temperature.We complete our analysis by discussing potential materials and designs of a facade integrated solar thermal collector equipped with an overheating protection based on a high temperature PCMs.

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