In order to assess the performance of the system the Greenhouse installed a Thermomax SMT400 Pump Control Unit, which records and stores temperature and energy data. The unit provides:

The graphs plot the temperature every 10 minutes to an accuracy of 2°c. The display allows the user to examine what time of day the maximum temperatures are achieved (usually mid to late afternoon). Figure 9 shows the standard display on the wall-mounted SMT400 unit.

The system has demonstrated excellent performance in its first 2 years, showing both economic and environmental benefits. These are described in sections 4 and 5. This section discusses the amount of energy harnessed to heat the water and the maximum temperatures reached in the hot water cylinder.

The period used for this report is the first full year for which data was recorded, from 1^st April 1999 to 31^st March 2000. The total energy harnessed during this year was 9098 kWh. This is equivalent to around 1160 kWh per square metre of collector tubes.

9098 kWh could in theory heat enough water for the equivalent of 9000 showers or 3000 baths if all the energy was harnessed and directly converted to hot water.

The reality is that due to the seasonal variation in solar radiation the total energy harnessed is not enough to provide hot water throughout the winter. This is clearly demonstrated in Figure 10.

Figure 10. The variation in the daily total energy harnessed by the Greenhouse solar collectors.

Figure 11 shows that most of the total energy is harnessed in the spring and summer (72%), whereas only 9% of the year’s energy is obtained in winter.

The solar water heating performed well throughout the year with the highest temperatures occurring during summer. Figure 12 shows the maximum temperature recorded in the solar heated cylinder for each month.

In December, the maximum water temperature reached each afternoon ranged from 24°c to 54°c with an average of 40°c. On most winter days, the solar system would not provide enough heat for bathing or showering. The use of a high efficiency gas condensing boiler was required during this period. However, when you live with a solar system you pay more attention to the weather and you can often leave the washing-up or the bath until the sun comes out! Even in January, 4 or 5 hours of direct sunlight will heat the system to above 50°c.

In the warmer months the system provided plenty of hot water. The maximum temperature reached 82°c in September and the month with the highest average maximum temperature was July with 70°c.

The temperature of the hot water cylinder rose to 50°c or above for 246 days of the year. This is suitable for bathing (40-45°c) or showering (45-50°c) as well as other typical domestic uses⁹.

The Greenhouse café has an obligation to maintain water at a minimum of 60°c to prevent the growth of legionella and other hazardous organisms¹⁰. The gas boiler provides the additional energy when the solar resource is insufficient. The maximum daily temperature in the secondary cylinder (solar only) was 60°c or above for 147 days of the year. The longest period during which the gas boiler was not required was the 57 days between 14^th June to 10^th August 1999.

Figure 12. Maximum temperatures reached in each month from April 1999 to March 2000.

During the summer, the system was sometimes manually switched to heat the primary cylinder once the secondary cylinder was up to temperature. The maximum temperatures recorded in the summer are therefore underestimates of the system’s actual working capacity.

In the entire year, the collectors harnessed enough solar energy to produce around 67,000 litres of hot water (at 50°c or above).

The solar resource is naturally variable on many time-scales. In July 1999, the Greenhouse system harnessed 1459 kWh whereas the same month in 2000 only harnessed 699.5 kWh. As the climate changes, it is hard to predict changes in the solar resource. A one-year study cannot tell us if these figures will be below or above the long-term average.