18 May 2017
Increasing internal loads and outdoor temperatures raise the need for comfort cooling, in particular in office buildings. A free-cooling method seldom considered is the heat rejection by activated components of the building envelope. An even more advantageous application is the heating and cooling operation with the same component. However, requirements for the surface properties may be different, resulting in optimisation potential depending on the prevailing load situation.
Therefore, a model has been developed and validated based on test rig measurements of absorbers with different selective coating. Subsequently, the model has been integrated in a system configuration suited for multifunctional operation with source storage, heat pump and thermally activated building systems. Heating capacities up to 600 W/m2 are reached with good solar irradiation and selective coating. Cooling capacity ranges from 70 W/m2 (selective) to 150 W/m2 (non-selective) in clear sky nights.
Seasonal performance factors of 4-5 are reached in the space heating mode for the heat pump and solar combination and about 90% of the cooling demand can be covered in free-cooling operation with Zurich Meteoschweiz normal weather data depending on absorber properties.
Selective properties of the surface are less important as source energy for heat pump heating and for cooling in moderate nights. However, non-selective properties are important for radiation losses in hot summer nights, while selective coating increases the performance by direct solar heating. A further increase of the cooling capacity can be reached by wetting the Surface.