The un-utilizability design method for collector-storage walls

The monthly-average auxiliary energy requirement of a building with a collector-storage (Trombe) wall is estimated using upper and lower theoretical limits to system performance. These two limits on the building auxiliary energy requirements result from considering the building and collector-storage wall to have either zero thermal capacity or infinite thermal capacity. With zero thermal capacity, all solar gain in excess of the load, on an instantaneous basis, is not useful and must be dumped. With infinite thermal capacity, the house is able to store any gain in excess of the instantaneous load and use it at some later time. Auxiliary energy use by real systems will fall between these two theoretical bounds. An empirical correlation is presented for the fraction of the load met by the collector-storage wall, F, for systems with finite capacity. The correlation is based on the solar radiation statistic, utilizability. The correlation is compared to yearly TRNSYS simulation results for a wide variety of system types. The root-mean-square difference between F found from the un-utilizability method and from TRNSYS simulations is less than 4 per cent. One advantage of this method over other simplified design methods is that this method covers a much larger range of design parameters.