Solar collectors

The two standard types of solar collectors are the flat-plate and the evacuated-tube systems. Flat-plate collectors consist of a metal heat-absorbing plate, closely bonded to copper water pipes which transport the heated water to a storage system. The maximum efficiency is achieved using a low-emissivity matt black absorbing plate, which limits the loss of energy through re-radiation from the hot surface. A low iron-content double-glazed cover, which admits the maximum quantity of short-wave energy, protects the absorbing plate and retains the entrapped heat. The underside of the pipework is insulated with fibreglass or polyisocyanurate foam to prevent heat loss to the aluminium casing and the underlying roof structure or support system.

Evacuated-tube collectors consist of a double layer glass tube, with a vacuum between the two layers. The outer glass is clear, admitting light and heat with minimal reflection. The inner tube is coated to absorb the maximum quantity of radiation. The heat from the inner tube is transferred in a sealed unit vaporising and condensing system to a heat exchanger within the main liquid flow to the heat storage system. Evacuated-tube collectors are substantially more expensive than flat-plate collectors, but are more efficient if angled correctly and will produce higher temperatures.

Flat-plate solar collectors may be located in any unshaded location, at ground level or attached to buildings. The best orientation is directly towards the midday sun, but a variation of up to 15° east or west will have little adverse effect. The optimum tilt from horizontal for solar hot water collectors for maximum all year round efficiency equals the location's latitude. However, for increased winter efficiency, when solar gains are at a premium, the tilt from horizontal should be increased by 10°, to pick up more energy at lower sun altitudes. Solar hot water systems are heavy and must be fixed securely to suitable substrates. On tiled or slated pitched roofs an air gap should allow for the clear passage of rainwater and melting snow.

Hot water from the solar collector is usually circulated through an indirect system to a solar storage tank (Fig. 14.4). This acts as a heat store of preheated water to be fed into a standard hot water cylinder system, where the temperature can then be boosted from a boiler to the required level. Circulation may either be a gravity thermosyphon system operated by hot water convection with the storage tank located above the collector, or through a pumped system, in which case the tank may be below the collector. The circulated water must contain antifreeze and a rust inhibitor. An alternative direct system feeds tap water directly into the solar collector, but scaling and corrosion of the pipework can be problematic. A 5 m2 solar collector panel will heat 250 litres of hot water per day, which is a typical four person family demand.

Solar energy district-heating plants in Europe, including Scandinavia, contribute significantly to a direct reduction in the energy requirements for small town domestic hot water systems. Water is preheated by large arrays of solar collectors before the local conventional-fuel heating system tops up the temperature to the required domestic level. Furthermore, solar heating systems in conjunction with large underground heat storage tanks can significantly reduce winter energy consumption by preheating the water supplies during periods when direct solar gain is ineffective.


Back-up boiler


Note: Expansion tanks to heating systems not illustrated. Fig. 14.4 Solar collector and domestic hot water system



Fig. 14.5 Light pipe. Illustration: Courtesy of Monodraught

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