Hot water

Heating water using solar collectors or air-to-water heat pumps can be a viable source of energy that can reduce gas or electricity consumption for heating water by around 50 to 70%, depending on the latitude as well as the behaviour of the hot water users. Using most hot water in the evening with an electric or natural gas back-up system installed to raise the temperature over night prevents the solar system from making a significant contribution compared with using the water in the morning. Most systems are installed on dwellings, but hotels, motels and small businesses can also benefit.

At the end of 2007, global installed solar thermal capacity was nearly 150 GW from over 200 Mm2 of collector area (including around 10% of unglazed systems used for swimming pools, commercial

55. www.iea-shc.org g

building ventilation, heating air and agricultural drying) (IEA, 2007a). Evacuated tube collectors tend to have a superior performance to simple glazed collectors and therefore require a smaller collector area for the same level of heat output. The potential for freezing conditions and hail in a region need to be considered when selecting a system. A gas or electric back-up system is required in most locations to ensure hot water is available at all times, though some systems rely on a wood-burning stove with water pipes passing through the hearth to also heat the water in winter. Integrating a system including the hot water storage cylinder into a new building design is usually cheaper than retrofitting an existing building, especially if the roof orientation and angle of tilt is not ideal for the location. Integrating collectors into the design of a building as a component (such as integration into a balcony structure) is gaining interest amongst architects (SHC, 2009). Codes of practice exist in some countries to ensure that:

I i nstallations are sized to give a minimum share of the normal annual hot water requirement; I the cylinder capacity matches around 1.5-2 times the daily use;

I the ratio of cylinder volume to collector area is designed to give a balance between energy savings and rate of heat recovery after hot water use; and

I tystems are tested to ensure they meet minimum standards.

Within Europe, the leading city Vienna had around 13 000 m2 of solar collectors installed in 2007, Barcelona had 4 300 m2 (Box B) and Lyon 3 500 m2 (Ambiente Italia, 2007). Combi-solar systems are also available that can heat both water and space, although the total costs and payback periods for an installation need to be carefully analysed. Devices that combine power generation through PV panels as well as hot water collectors are under development. (Full details of all these technologies can be found on the IEA Solar Heating and Cooling web site www.iea-shc.org).

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