Calculations were carried out with the simulation model Mantlsim developed at the Technical University of Denmark, [10], [3], [4] and [11]. The two tested low flow systems with the data given in Table 1 are taken into calculation. Weather data of the Danish Test Reference Year [12] is used in the calculations.

The daily hot water consumption is 4.6 kWh, corresponding to 100 l water heated from 10°C to 50°C. Hot water is tapped with an energy quantity of 1.53 kWh three times each day: 7 am, 12 am and 7 pm. The required hot water draw-off temperature is 50°C.

The thermal performance of the system with the two draw-off levels was calculated with one draw-off pipe at the very top of the tank and with different positions of the second draw-off level.

2.2.1 SIMULATION RESULTS

Fig. 3 shows the calculated yearly net utilized solar energy of the system as a function of the relative position of the second draw-off level and as a function of the auxiliary set point temperature.

Fig. 3. Calculated net utilized solar energy of the SDHW system as functions of the position of the second draw-off level and of the auxiliary set point temperature.

Fig. 4. Calculated extra percentage net utilized solar energy of the system by using two draw-off levels as functions of the position of the second draw-off level and of the auxiliary set point temperature.

Fig. 4 shows the calculated yearly percentage increase of the net utilized solar energy by using a second draw-off pipe as functions of the relative position of the second draw-off level and the set point temperature for the auxiliary energy system. If the relative position of the second draw-off level is at the top of the tank, the system is identical to the standard system with only one draw-off level at the very top of the tank. The thermal advantage of using a second draw-off pipe is strongly influenced by the auxiliary set point temperature. The higher the set point temperature the larger the advantage. If the auxiliary set point temperature is only 0.5 K higher than the required draw-off hot water temperature, the extra thermal performance by using a second draw-off level is about 1%. If the auxiliary set point temperature is 15 K higher than the required hot water temperature, the extra thermal performance by using a second draw-off level is about 13%. The second draw-off level is best placed in the middle of the tank.

Figs. 5 and 6 show the calculated yearly net utilized solar energy of the system and the calculated extra percentage net utilized solar energy of the system by using two draw-off levels as a function of the position of the second draw-off level for four different daily hot water consumptions: 50, 100, 160 and 180 l. Hot water is tapped by means of three daily draw-offs with the same draw-off volume: At 7 am, 12 am and 7 pm. Hot water is tapped at 45°C at 7 am and at 7 pm, while hot water is tapped at 40°C at 12 am. During all hours the top auxiliary volume is heated to 50.5°C. The draw-off temperatures are realistic, since hot water is not used at the same temperature level in practice. Further, in practice the set point of the auxiliary energy supply system is often 5-10 K higher than the required hot water draw-off temperature. The figures show that the net utilized solar energy is increased by about 6% by using two draw-off levels. Again, the second draw-off level is best placed in the middle of the tank. It should be mentioned, that there is a need for development of an advanced control system before solar tanks in practice can supply the consumers with different draw-off temperatures.

Fig. 5. Calculated yearly net utilized solar energy of the SDHW system as a function of the position of the second draw-off level for different daily hot water consumptions.

Fig. 6. Calculated extra percentage net utilized solar energy of the SDHW system by using two draw-off levels as a function of the position of the second draw-off level.