The identical design of the two systems does not guarantee equal thermal performance under identical flow rates, weather and boundary conditions. Therefore comparative measurements with equal flow rates in both collector loops were carried out. Results show that the thermal performance differs by less than 1 % between the two systems.

During the following tests both systems were exposed to the same weather and boundary conditions, the specific flow rates were set to 15 l/m2h (low-flow) and 40 l/m2h (high-flow). A total of 27 test days with different draw-off profiles and a high variability of solar irradia­tion have been measured and evaluated. In order to recognize correlations between the weather conditions and the differences of the solar gains of the two systems, the results have been evaluated day by day.

Results

The analysis of the test days shows an interesting effect: The difference between the daily solar gains of the two systems depends on how often the sunshine is interrupted by clouds. On clear and cloudless days the system gain under high-flow operation exceeded the gain of the low-flow system by up to 5.7 %, while for unsettled irradiation the thermal performance of the low-flow system was up to 4.6 % better. In the sum of all test days the system gain of the high-flow system is superior by only 0.5 %.

The decisive influence of irradiance characteristics becomes obvious when regarding two representative test days. On 26 June 2003 and 16 July 2003 the daily solar irradiation amounts to 7.59 kWh/m2d, but the irradiance characteristics is quite different. On 26 June 2003 the weather was changeable, whereas the 16 July 2003 was a clear and cloudless day. The daily results are shown in Figure 2 and 3.

1000 800

03 ^

ЧГ <D

600

І. з

400 200 0

4 6 8 10 12 14 16 18 20 t [h]

Figure 3: Irradiance, flow rates and switching cycles of the collector loop pumps on 16 July 2003. The clear and cloudless weather leads to a better performance under high-flow operation.

On 26 June 2003 the solar system gain of the low-flow operation exceeds the system gain of the high-flow operation by 1.4 %, whereas on the cloudless day (16 July 2003) the high — flow system shows a better thermal performance (2.3 %). Under intermittent sunshine the low-flow system benefits from a reduced number of switching cycles and from longer op­eration intervals. Periods of low irradiance are better utilized under low-flow operation.

For a meaningful evaluation of the different flow rates it is necessary to consider the en­ergy demand of the collector loop pumps. The operation time as well as the electrical power demand of the pumps are quite different for low-flow and high-flow operation. For a comparison of the primary energy savings the electrical power consumption are weighted by the primary energy factor 3. The savings of fossil fuel by the solar DHW system is weighted by the primary energy factor 1.1. Thus the primary energy savings Esav, prim turn out to:

= 1.1 • ^ — 3 • £

The calculation shows that every investigated day brings up higher primary energy savings of the sys­tem with low-flow rate (be­tween 2.2 % and 8.7 %). Totalized over the whole measurement period, the low-flow operation saves 5.2 % more primary energy than the high-flow operation. In consideration of the par­ticularly low efficiency of the collector loop pump under low-flow operation (low-flow: power level 1, 45 W / high — flow: power level 3, 90 W), this result is very surprising.

The differences of solar gain AQsOi and primary energy savings AEsav, prim for each test day are presented in Figure 4. The most important results are listed in Table 1.

Dynamic System Test (DST)

Besides the comparative analysis of single test days, the two systems were measured in a complete dynamic system test (DST) according to ISO/DIS 9459-5. With the system pa­rameters obtained from the dynamic fitting procedure a long term prediction for the loca­tion Wurzburg was calculated. The fractional solar gain under high-flow operation amounts to 43.0 %, whereas the low-flow operation achieves a fractional solar gain of 42.8 %. With regard to the high uncertainty of the DST procedure of 5 % the solar gain is equal for both flow rates.

Analysis of the Mean Collector Loop Temperatures

Due to higher collector outlet temperatures and a minor degree of collector efficiency un­der low-flow operation, a lower solar gain of the low-flow system was expected. A careful investigation of the collector loop temperatures showed that the return temperatures are significantly lower under low-flow operation. This leads, in interaction with the higher col­lector outlet temperatures of the low-flow system, to nearly equal mean collector loop tem­peratures and consequently to similar collector efficiencies for both flow rates. Thus the result of the comparative measurements — nearly equal solar gains under high-flow and low-flow operation — is explainable.

Two questions remain unsolved:

1. What is the reason for the lower return temperatures under low-flow operation?

2. Is it possible to enhance the performance of the high-flow system compared to the low-flow system by the use of a heat exchanger with a better heat transfer capability?