In order to answer these questions TRNSYS-simulations were calculated in cooperation with the company Viessmann. The simulations were carried out using the 1997 weather data from the ISFH meteorological station in Hanover in a 5-minute time step. Results show that the measured similarity of solar gains under high-flow and low-flow operation is confirmed by the simulations (AQsol < 1 % on a yearly base). Furthermore the mean collec­tor loop temperatures are nearly the same for both systems.

Starting from a specific heat transfer capability of the investigated system’s heat ex­changer (UA = 140 W/m2K with respect to the collector area) the UA-value was varied and the influence on the solar gain was investigated. It turns out that for both flow rates the system benefits in the same degree from an increased heat transfer capability of the heat exchanger. The enhancement of the heat transfer capability by 43 % improves the solar gain for both systems by about 1 %. If the heat transfer capability is decreased by 43 %

the solar gains under low-flow and high-flow operation drop by 2 %. Thus a different heat transfer capability of the internal heat exchanger leads qualitatively to the same results. One question remains: What is the reason for the lower return temperatures under low — flow operation? The simulations showed that the difference of the return temperatures un­der high-flow and low-flow operation does not depend on the size of the heat exchanger.

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In Figure 5 the temperature distributions in the stores are compared for low-flow and high — flow operation in the course of the day (5 June 1997). At 7 o’clock both stores show nearly identical temperature stratifications. A few hours later the low-flow system shows a distinct thermal stratification in the lower part of the storage, whereas the stratification of the high — flow system is only slightly developed. At 10:12 h the temperature difference between the relative store heights 10 % and 40 % amounts to 12.0 K under low-flow operation, under high-flow operation this temperature gradient amounts to 5.2 K. All in all the temperature level in the region of the heat exchanger outlet (height 5 %) is much lower under low-flow operation (up to 6 K).

Another demonstration of the better stratification under low-flow operation is given in Fig­ure 6 and 7. The diagrams show the temperature development in the course of the day for the different storage levels. At 11 o’clock the temperature spreading between the storage top and bottom amounts to 16 K under low-flow operation. The high-flow system shows only a spreading of 7 K at this time. In addition to that the temperature at storage top under low-flow operation exceeds the temperature under high-flow operation by 2 K.

So it is obvious that under low-flow operation the better thermal stratification in the store combined with the lower temperatures at the bottom of the storage are responsible for the lower return temperatures of the collector loop.