Two small low flow solar domestic hot water systems with mantle tanks as heat storage were tested side-by-side in a laboratory test facility. The systems are identical, with exception of the mantle tanks. One of the mantle tanks has the mantle inlet port located at the top of the mantle and the other mantle tank has the mantle inlet port moved 0.175 m down from the top of the mantle. Both of the two mantle tanks make use of electric heating elements as auxiliary energy supply systems, and the electric heating elements heat up the top volume to 51°C during all hours.

The solar collector in each system is identical and of the type ST-NA marketed by Arcon Solvarme A/S with an area of 2.51 m2.

The solar collector loop in both systems is equipped with a Grundfos circulation pump (type UPS 25-40), which has been running at stage 2 to secure a flow rate of about 0.5 l/min throughout the measuring period. The circulation pump is controlled by a differential thermostat, which measures the temperature difference between the outlet from the solar collector and the bottom of the mantle. The differential thermostat has a start/stop set point at 10/2 K.

The two solar heating systems were tested with the same daily hot water consumption of 0.100 m3. An energy quantity of 1.525 kWh, corresponding to 0.033 m3 of hot water heated from 10°C to 50°C, was tapped from each system three times each day: at 7 am, 12 am and 7 pm.

The test period was from the beginning of March to the middle of November 2003 with a duration of 252 days.

The data for the two SDHW systems are given in Table 1.

The thermal performance of the two systems is compared by the net utilised solar energy and the solar fraction of the systems. The net utilised solar energy is defined as the tapped energy from the system minus the auxiliary energy supply to the tank, and the solar fraction is the ratio between the net utilised solar energy and the tapped energy from the system.

The measured energy quantities for the two systems are shown in Table 2. From Table 2, it is seen that the thermal performance of the system is not strongly influenced by the position of the mantle inlet. Both systems had a relatively high solar fraction (0.66-0.68) in the period. The thermal performance for the system with the lower mantle inlet was about 2% higher than the thermal performance of the system with the top inlet. The accuracy of the measured net utilised solar energy is within 4%.

10/11 2003.

At high solar fractions, large periods with high inlet temperatures to the mantle are expected and when the system with the lower mantle inlet has a higher thermal performance at high solar fractions, the relative improvement by moving the inlet down is expected to be higher for smaller solar fractions where lower inlet temperatures are expected.

The 252 days’ measuring period have been divided into 36 periods of 7 days. The performance ratio as a function of the solar fraction for the system with the top inlet for the 36 periods is shown in Fig. 2. The performance ratio is defined as the ratio between the net utilised solar energy of the system with the lower mantle inlet and the net utilised solar energy of the system with the top mantle inlet.

Fig. 2 shows, as expected, that the performance ratio increases for lower solar fractions. However, the performance ratio drops below 1 for two 7-day periods at solar fractions of 0.65-0.70, which can be explained by the distribution of the solar irradiance in these two 7- day periods. Each of the two 7-day periods has 4 days with a clear sky and 3 days more or less overcast, while the other 7-day periods, where the solar fraction is around 0.6-0.7 and the performance ratio is above unity, have clouds every day, which results in lower inlet temperatures to the mantle than on the days with a clear sky. Based on the tendency that the performance ratio increases for lower solar fractions and that the solar fraction was relatively high in most of the measuring periods, it can be concluded that these measurements show that the thermal performance of this SDHW system can be somewhat increased by moving the mantle inlet down.

Fig. 2. Performance ratio as a function of the solar fraction for the system with the top inlet.