The two solar heating systems have been tested under the same test conditions in the test period March 8, 2008 — July 7, 2008: The solar radiation on the two collectors is the same and the hot

water consumption and hot water consumption pattern are the same for both systems. Daily a hot water volume of 100 l, heated from 10°C to 50°C, is tapped from each tank. Hot water is drawn from the tanks at 7 am, at noon and at 7 pm in three equally sized volumes. The hot water consumption is 32.2 kWh per week.

The tapped energy, the auxiliary energy, the solar heat transferred to the heat storage, the operation time of the circulation pump, the pump energy and the heat loss from the cold and hot part of the solar collector loop are measured for each system during the whole test period. Due to problems with the monitoring system measurements were not carried out for one week during the test period.

The measured results are shown in table 2. The net utilized solar energy is the tapped energy minus the auxiliary energy transferred to the top of the tank by means of the electrical heating element. The solar fraction is the ratio between the net utilized solar energy and the tapped energy.

Table 2. Measured energy quantities in the test period March 8, 2008 — July 7, 2008.

Figure 3 shows the performance ratio, defined as the ratio between the net utilized solar energy for the solar heating system with the lifeline and the SOLAR 15-65 pump and the net utilized solar energy for the solar heating system with the normal solar collector loop and the SOLAR 15-40 pump as a function of the solar fraction for the solar heating system with the normal solar collector loop and the SOLAR 15-40 pump. Each point in the figure represents the performance ratio for one week. For instance, a point with a solar fraction of 0.85 and a performance ratio of 1.02 corresponds to a week where the thermal performance of the solar heating system with the lifeline and the SOLAR 15-65 pump is 2% higher than the thermal performance of the solar heating system with the normal solar collector loop and the SOLAR 15-40 pump and where the solar heating system with the normal solar collector loop and the SOLAR 15-40 pump covers 85% of the hot water consumption.

The extra thermal performance of the solar heating system with the lifeline and the SOLAR 15-65 pump is influenced somewhat by the solar fraction. For most weeks, the extra thermal performance is increasing for decreasing solar fraction. However, for some weeks it is the other way around. It is also seen that the pump energy for the system with the lifeline and the SOLAR 15-65 pump is higher than the pump energy for the system with the normal solar collector loop and the SOLAR 15-40 pump. The extra pump energy for the SOLAR 15-65 pump is higher than the extra net utilized solar energy for the system with the lifeline and the SOLAR 15-65 pump. Consequently, the use of the lifeline and the SOLAR 15-65 pump is not justified from a thermal performance point of view.

0.80

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Solar fraction of the system with the normal solar collector loop and the SOLAR 15-40 pump, [-]

Fig. 3. Performance ratio for the solar heating system with the lifeline and the SOLAR 15-65 pump as a
function of the solar fraction of the solar heating system with the normal solar collector loop and the SOLAR

15-40 pump.

Figure 4 shows the solar irradiance on the solar collectors on a cloudy day, March 28, 2008. Figure 5 shows the measured volume flow rates in the solar collector loops and the pump powers for the two systems March 28, 2008. The pumps are in operation in two periods, one from about 7:45 am to 11:45 am and one from about 12:15 pm to 15:45 pm. The volume flow rates in the solar collector loops are increasing during the operation periods. This is first of all caused by the increasing solar collector fluid temperatures. Further, relatively low volume flow rates are observed in the very start of the operation periods. This is caused by the fact that the solar collector fluid is heated to relatively high temperatures in the solar collectors during the stand by periods. When the pump is first activated the solar collector fluid with the high temperature is pumped into the pipe going downwards from the solar collector to the tank. At the same time relatively cold solar collector fluid from the mantle is pumped into the pipe going upwards from the tank to the solar collector. The density difference between the cold and the hot solar collector fluid in the two pipes is relatively large in the start-up phase due to the “overheated” solar collector fluid in the solar collectors. The pumps must overcome the pressure drops caused by these density differences. Therefore the volume flow rates are especially low in the start-up phase. It is also noticed that the pump powers are decreasing during the operation period. This is especially true for the SOLAR 15­65 pump.

Fig. 4. Total solar irradiance on solar collectors, March 28, 2008.

Figures 7 and 8 show the measured solar collector fluid temperatures in the two systems March 28. It is seen that the difference between the heat loss from the hot pipes for the two systems is small and that the heat loss from the cold pipe in the system with the lifeline is small compared to the heat loss from the cold pipe for the other system. The small heat loss of the cold pipe results in a relatively high inlet temperature to the solar collector and thereby in a somewhat reduced collector efficiency. Therefore the thermal advantage by the low heat loss from the cold pipe of the lifeline is limited.

Fig. 7. Measured solar collector fluid temperatures in the solar collector loop for the system with the normal
solar collector loop and the SOLAR 15-40 pump March 28, 2008.

Fig. 8. Measured solar collector fluid temperatures in the solar collector loop for the system with the lifeline

and the SOLAR 15-65 pump March 28, 2008.

10 15 20 25 30 35 40 45

Temperature of fluid passing the pump, °C

Fig. 9. Measured pump powers and volume flow rates as a function of the temperature of the solar collector
fluid leaving the mantle for the two systems March 28, 2008.