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ACTIVE SOLAR HEATING OF A SCHOOL BUILDING

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Design and construction of a low energy school building, equipped with solar heating intended to collect 24,270 kWh/y to provide space and water heating, and reduce annual energy consumption to between 60 to 150 kWh/m2 in comparison with +/- 170 kWh/m2 in conventional school buildings constructed at the same time (circa 1980).

The school has achieved its design objective by reducing energy requirements to 110 kWh/m2 in comparison with an average of 170 kWh/m2 in comparable schools built at the same time. However, most of this reduction is attributed to building design rather than to the use of solar energy. The system itself was designed to be versatile, offering alternate heat sinks to the solar panel. To achieve this it was necessary to design a highly sophisticated system. Consequently specialist maintenance contractors are required to locate and remedy system malfunctions. Major system components have operated satisfactorily: storage tanks and panels have been problem free with the exception of leaks (not serious) between panel joints, thought to be caused by thermal expansion.

Although temperatures of up to 53 deg.C can be attained in the low temperature store in June and July, the high temperature store only reaches 30 deg.C, although daily collection efficiencies were 10 - 15%. During summer terman average collection rate of 19 kWh/d is possible.

During winter operation there were few occasions when solar heat was available at sufficiency high temperatures to make a contribution to the high temperature store, most of it being directed to the low energy store: this becomes the heat source for the space heating system once it reaches 30 deg.C. During winter, panel efficiencies rise to +/- 20% because the low temperature store is normally below 30 deg.C.

Overall, the solar contribution to the heating load of the school is small and a failure in the solar system is easily compensated for by increased output of the electrical system. It is also easy for failures in the system to go undetected for a long time. Furthermore, the collection of low grade heat depends on sensitive and accurate controls in order to respond to prevailing low temperature differences.

Based on a number of assumptions, payback on the project is estimated at 49 years, an increase on the original 34.8 years. According to the final report this scheme indicates that solar heating systems are complex and require a great deal of attention to keep them operating. The complexity of the control system increases the capital cost and makes maintenance difficult.

Tidcombe Lane School is a very low energy, sheltered building using active solar energy heating. Entrances are provided with lobbies, and to maximise passive solar gain, windows are situated on the south side, and double glazed. The roof supports the solar panels: these supplement space and water heating. Space heating is by low temperature (43 deg.C) hot water via natural and forced convection heaters. Two water storage tanks are heated indirectly by the solar panels, providing the space heating heat source. One of these tanks is also heated by off-peak electricity. In addition, the solar heat can be transferred to the electrically heated domestic hot water tank.

Fabric insulation levels have average U-values of +/- 0.5 W/m2/deg.C for walls and roof. On the school roof there are 18 'Sunsense' solar panels each of 3.6 m2, connected in 3 banks of 6 panels and mounted at 33 deg.C to the horizontal, facing south. Design flowrate through the panels is 0.45 L/s: an indirect draindown circulation system is installed.

The alternative method of providing space heating is by means of 3 electric immersion 24 KW heaters in the base of one of the high temperature stores. The domestic hot water system is also provided with a 12 kW immersion heater. Circulation through the solar panels is intended to occur when panel outlet temperature increases to more than 3 deg.C above the temperature of any one of the tanks and to cease when it equals that of the store. In winter priority is given to the high temperature store, then to the low temperature and finally the domestic hot water tank. In summer, output can be diverted to domestic hot water, and if surplus, to the low temperature space heating store, then the high temperature store.

In the solar system, flow and return temperatures are sensed, with high and low temperature stores. Heat flow from the solar collector is independently measured by a heat meter which also provides signals to the data logger for monitoring. Solar circulating pumps operation is monitored as is the diverting valve position. Space and water heating auxiliary energy consumption is monitored by recording operation of the off-peak immersion heaters (high temperature store and domestic hot water cylinder). High and low temperature space heating distribution pump operation is recorded as well as flow and return temperatures in the space heating distribution system. Another heat meter measures energy and volume flows in the domestic hot water system.