4.3.3.7 Solar heating and cooling
Solar heating and cooling of buildings can reduce conventional fuel consumption and reduce peak electricity loads. Buildings can be designed to use efficient solar collection for passive space heating and cooling (Chapter 6), active heating of water and space using glazed and circulating fluid collectors, and active cooling using absorption chillers or desiccant regeneration (US Climate Change Technology Program, 2003). There is a risk of lower performance due to shading of windows or solar collectors by new building construction or nearby trees. Local ‘shading’ regulations can prevent such conflicts by identifying a protected ‘solar envelope’ (Duncan, 2005). A wide range of design measures, technologies and opportunities are covered by the IEA Solar Heating and Cooling implementing agreement (www.iea-shc.org).
Active systems of capturing solar energy for direct heat are used mainly in small-scale, low-temperature, domestic hot water installations; heating of building space; swimming pools; crop drying; cook stoves; industrial processes; desalination plants and solar-assisted district heating. The estimated annual global solar thermal-collector yield of domestic hot water systems alone is around 80 TWh (0.3 EJ) with the installations growing by 20% per year. Annual solar thermal energy use depends on the area of collectors in operation, the solar radiation levels available and the technologies used including both unglazed and glazed systems. Unglazed collectors, mainly used to heat swimming pools in the USA and Europe, represented about 28 million m2 in 2003.
More than 130 million m2 of glazed collector area was installed worldwide by the end of 2003 to provide around 0.5 EJ of heat from around 91 GWth capacity (Weiss et al., 2005). In 2005, around 125 million m2 (88 GWth) of active solar hot-water collectors existed, excluding swimming pool heating (Martinot et al, 2005). China is the world’s largest market for glazed domestic solar hot-water systems with 80% of annual global installations and existing capacity of 79 million m2 (55 GWth) at the end of 2005. Most new installations in China are now evacuated-tube in contrast with Europe (the second-largest market), where most collectors are flat-plate (Zhang et al., 2005). Domestic solar hot-water systems are also expanding rapidly in other developing countries. Estimated annual energy yields for glazed flat-plate collectors range between 400 kWh/m2 in Germany and 1000 kWh/m2 in Israel (IEA, 2004d). In Austria, annual solar yields were estimated to be 300 kWh/m2 for unglazed, 350 kWh/m2 for flat-plate, and 550 kWh/m2 for evacuated tube collectors (Weiss et al., 2005). The retail price for a solar water heater unit for a family home differs with location and any government support schemes. Installed costs range from around 700 US$ in Greece for a thermo-siphon system with a 2.4 m2 collector and 150 L tank, to 2300 US$ in Germany for a pumped system with antifreeze device. Systems manufactured in China are typically 200–300 US$ each.
Nearly 100 commercial solar cooling technologies exist in Europe, representing 24,000 m2 with a cooling power of 9 MWth. High potential energy savings compared with conventional electric vapour-pressure air-conditioning systems do not offset the higher costs (Philibert and Podkanski, 2005).