Solar Energy Perspectives - IEA

Solar Energy Perspectives: Buildings
Simple systems such as thermo-siphon not protected against freezing, with flat-plate or
evacuated tube collectors (see Chapter 7), can be installed on terraces and horizontal rooftops
in mild climates (Photo 4.1 and Photo 4.2). Building integration of pumped systems
allows storage for several days in stratified water tanks, where a back-up from another
energy source is often installed. Manufacturers have overcome early technical issues, but
installation requires trained and experienced installers. The most cost-effective systems
cover 40% to 80% of the heating loads for sanitary hot water, however, covering 100%
requires over-sized collectors and storage capacities. The additional cost is generally
unjustifiable and over-sizing increases the risk of overheating, which could damage the
collectors. Systems are usually designed to fully cover the low season for hot water
demand (summer).
Photo 4.1 Chinese thermo-siphon solar
water heater
Photo 4.2 Solar water heaters in Kunming,
China
Source: Popolon, Wikimedia.
Source: Raffaele Miraglia.
Key point
Solar water heaters represent the bulk of solar heat, and most are installed in China.
Costs vary greatly according to climate conditions and the associated levels of complexity, as
well as other factors such as labour. A SWH thermo-siphon system for one family unit
consisting of a 2.4 m 2 collector and 150 litre tank costs EUR 700 in Greece, but EUR 150 in
China (with no government support). In central Europe, a pumped system of 4 m 2 to 6 m 2
and 300-litre tank, fully protected against freeze, costs around EUR 4 500. Systems of this
size might be used only for water heating, or also contribute – marginally – to space heating
(as some do in the Netherlands), thereby increasing their value.
Solar domestic hot water systems cost in Europe from EUR 85/MWh to 190/MWh of heat,
which is competitive with retail electricity prices in some countries, if not yet with natural
gas prices. These costs are expected to decline by 2030 to EUR 50/MWh to 80/MWh for solar
hot water systems.
In China, Cyprus and Turkey, low-cost solar water heaters are already an economic alternative
for households. In Israel, they are ubiquitous and save 6% of total electricity demand. In
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© OECD/IEA, 2011