Calculations of the yearly net utilized solar energy of a solar combi system are carried out by means of TRNSYS, [13], [14]. The solar combi system taken into calculation is schematically shown in Fig. 7.

The solar heating system, which is a marketed system by SOLVIS Solar Systeme GmbH, was the best system investigated in [5]. The system has a compact heat storage unit with the following components integrated: A water tank with an auxiliary condensing natural gas burner, a domestic hot water flat plate heat
exchanger with a pump, a solar collector loop and a solar heat exchanger. Thermal stratification is built up in the heat storage in a good way, since SOLVIS inlet stratifiers ensure that the solar heat is transferred to the "right” level in the tank and that the water returning from the heating system enters the tank in the right level.

The volume of the tank is 650 l and the auxiliary volume heated by the natural gas burner is 136 l. The natural gas burner heats up the auxiliary volume to 57°C. The solar collectors described in the previous section are also used in the calculations on the thermal performance of the solar combi system. The solar collector area is 12.55 m2.

The water to the heating system is tapped from a level just above the lower level of the auxiliary volume.

The space heating demand of the house taken into calculation is 14970 kWh/year. The heating system is a traditional radiator system which, at an indoor temperature of 20°C and an outdoor temperature of -12°C, can supply the required heating power of the house with an inlet water temperature of 60°C and an outlet water temperature of 50°C. A daily hot water consumption of 150 l heated from 10°C to 51°C is assumed. Weather data of the Danish Test Reference Year is used in the calculations. The draw-off level for the domestic hot water heat exchanger is placed at a relative position of 0.05 from the very top of the tank and the draw-off level for the space heating system is placed at a relative position of 0.23 from the very top of the tank.

Calculations of the yearly net utilized solar energy of the system are carried out for the standard system, for the system with two draw-off levels for the domestic hot water heat exchanger with different positions of the second draw-off level, for the system with two draw-off levels for the space heating system with different positions of the second draw-off level and for a system with two draw-off levels, both for the domestic hot water heat exchanger and for the space heating system.

Fig. 8 shows the calculated yearly net utilized solar energy of the system with two draw-off levels to the domestic hot water heat exchanger and the standard draw-off level to the heating system, as well as the yearly net utilized solar energy of the system with two draw­off levels to the heating system and the standard draw-off level to the domestic hot water heat exchanger as functions of the position of the second draw-off level.

Fig. 9 shows the extra percentage net utilized solar energy for the solar combi system by utilizing two draw-off levels to the domestic hot water heat exchanger as well as the extra percentage net utilized solar energy for the solar combi system by utilizing two draw-off levels to the heating system as a function of the position of the second draw-off level. It is possible to increase the yearly thermal performance of the system by about 3%, either by using two draw-off pipes for the domestic hot water heat exchanger instead of one or by using two draw-off pipes for the heating system instead of one. The best position of the second draw-off level for the domestic hot water heat exchanger is in the middle of the tank, and the best position of the second draw-off level for the heating system is just above the middle of the tank.

Further calculations showed, that by using a second draw-off level, both to the domestic hot water heat exchanger and to the heating system, the yearly net utilized solar energy of the solar combi system is increased by about 5% compared to the standard system. Also for this design, the second draw-off pipe for the domestic hot water heat exchanger is best placed in the middle of the tank, while the second draw-off pipe for the heating system is best placed just above the middle of the tank.

3 CONCLUSIONS

The investigations showed that it is possible to increase the thermal performance of both SDHW systems and solar combi systems by using two draw-off levels from the solar tanks instead of one draw-off level at a fixed position.

The best position of the second draw-off level is for all the investigated systems in the middle or just above the middle of the tank. For SDHW systems the extra thermal performance of using a second draw-off level from the hot water tank is strongly influenced by the difference between the set point temperature of the auxiliary energy supply system and the required draw-off temperature. For increasing temperature difference the thermal
advantage of the second draw-off level increases. For a realistic draw off hot water temperature of 40°C and 45°C and an auxiliary volume temperature of 50.5°C the increase of the thermal performance by the second draw-off level is about 6%.

For the investigated solar combi system the extra thermal performance by using one extra draw-off level, either for the domestic hot water heat exchanger or for the heating system, is about 3%, while an improvement of about 5% is possible by using a second draw-off level both for the domestic hot water heat exchanger and for the heating system.

[1]