The solar energy available in summer is more than twice that available in winter. Virtually the opposite applies to the energy demand for space heating. Unlike the hot-water supply, the heating load is dependent upon the outside temperature. Even on winter days, energy demand and solar radiation are partially related. This two-family house in Hitzendorf, Austria, has a collector area of 92 m2 with 0.5 m3 of DHW storage and 8.3 m3 of heating storage tank.
Auxiliary heating is from a wood-fired boiler.
AEE Figure 3 shows the solar radiation on a horizontal plane at Zurich, Switzerland. It can be seen that, at this latitude, not only are there strong seasonal variations in radiation throughout the year, but that solar radiation also changes quite widely on a daily, or even hourly basis. Solar radiation on a horizontal plane, Zurich, Switzerland Heat storage To make efficient use of the available solar energy supply, these fluctuations need to be evened out, by means of storage systems, so that hot water can be supplied continuously and a constant room temperature is guaranteed. This type of system is widely sold in France. In this system the collector array is directly connected to the underfloor heating, without any store or heat exchanger.