The first set parameters analyzed in Figure 1 relate to collector area, slope, azimuth, mass flow and on the store volume. The highest influence on fsax, ext can be seen for the collector size. This dependency is even higher, when the ratio of collector and store is kept constant.

A pure alteration of the store volume is not that significant and in the same range as collector azimuth and collector tilt angel.

Nearly no influence can be seen for the collector mass flow.

The standard variation of these values is below 0.2 which indicates a similarity of the results for all systems.

Figure 2 shows the dependencies fsav, therm on the store volume with the collector area as parameter. The decrease of fsav, therm for small collector areas with increasing store volume is due to higher increase of heat losses than increase of solar heat input to the storage.

The optimal ratio of store volume and collector size should not become bigger than 100 l per m2 of collector area. Most plants have optimal values between 40 and 80 l per m2 of collector area over the whole range of fsav, ext.

Figure 2 Dependency of fsav, therm with store volume and collector area, example of system #15 (Jaehnig, 2003)

In Figure 3 the dependency of fsav, ext on slope and azimuth is shown exemplary for system #19 (Heimrath, 2003). The optimum savings occur at an azimuth of around 10° west and 55° tilt angle for a maximum fsav, ext of 39.4%.

The azimuth can vary between -30° to 30° (for the optimum tilt) by decreasing the fractional energy savings by 5 %. With optimal azimuth the slope can vary by ± 20° with the same reduction of fractional energy savings. Taking this into account a stand of the collectors on the roof can be mostly avoided. The optimal slope depends primarily on the latitude of the location and secondary on the achieved fractional energy savings. As higher the savings, as more slope of the collector area is feasible in order to have a better irradiation angle to the collector area in the heating season.

Figure 3 Dependency of fsav, ext on collector tilt and azimuth angle. 100% = 39.4% (fsav, ext), system #19 (Heimrath, 2003).