The electric heating elements heat up the top volume to 53°C during all hours.

The solar irradiance on the collectors and the daily hot water consumption are the same for both systems. An energy quantity of 1.53 kWh, corresponding to 33 l of hot water heated from 10°C to 50°C or 36 l of hot water heated from 10°C to 47°C is tapped from each system three times each day: 7 am, 12 am and 7 pm.

1.1.2 MEASURED RESULTS

Measured energy quantities for 6 weeks with a draw-off temperature of 50°C are given in Table 2 and measured energy quantities for 7 weeks with a draw-off temperature of 47°C are given in Table 3. The three way valve in the system with the two draw-off pipes ensures that a hot water temperature of 50°C is achieved for all draw-offs in the first test period and that a hot water temperature of 47°C is achieved for all draw-offs in the second test period. However, in periods with very high tank temperatures, the three way valve ensures that the draw-off temperature is as low as possible. In the standard system with one draw-off pipe, a hot water temperature of 52°C is achieved for all draw-offs in both test periods. The net utilized solar energy is the tapped energy from the solar tank minus the energy supply to the electric heating elements.

For the 6 weeks test period with a hot water draw-off temperature of 50°C the net utilized solar energy for the solar heating system with two draw-off levels is 8 kWh higher than the net utilized solar energy for the standard system. For the 7 weeks test period with a hot water draw-off temperature of 47°C the net utilized solar energy for the solar heating system with two draw-off levels is 18 kWh higher than the net utilized solar energy for the standard system.

The thermal performance of the systems is small, since the tests were carried out in the winter. The thermal advantage of two draw-off levels is higher for a draw-off temperature of 47°C than for a draw-off temperature of 50°C.