Solar water heating is an extremely benign and acceptable technology. The collectors are not obtrusive, especially when integrated into roof design. There are no harmful emissions in operation and manufacture involves no especially dangerous materials or techniques. Installation requires the operatives to be trained conventionally in plumbing and construction, and to have had a short course in the solar-related principles. The technology is now developed and commercial in most countries, either extensively (e.g. Greece, Cyprus, Israel and Jordan) or without widespread deployment (e.g. USA, France and the UK). It works best everywhere in summer and especially in sunny climates, e.g. the Mediterranean, and where alternatives, such as gas or electricity, are most expensive e.g. northern Australia. Moreover in every climate, solar water heaters have pre-heating value. In the UK for instance, a 4 m2 collector is sufficient for nearly 100% supply to a family of 2-4, with careful use, from mid-April to late-September, and will pre-heat in other months.
In almost all cases, using solar energy for water heating in practice replaces brown (fossil) energy that would otherwise be used for the same purpose. This gives the benefits of improved sustainability and less greenhouse gas emissions, as described in Section 1.2. For this reason, some governments partially subsidise household purchase of solar water heaters, in an attempt to offset the 'external costs' of brown energy (see Chapter 17 for a general discussion of external costs and policy tools). The fossil fuel use might be direct (e.g. gas heating) or indirect (e.g. gas- or coal-fired electricity). Especially in colder countries, the replacement is likely to be seasonal, with the 'solar-deficit' in the cooler months being supplied by electric heating, central-heating boilers or district heating. Depending on the source of these other supplies, which may be from renewable energy but is frequently from fossil fuels, this may reduce the greenhouse gas savings. Installing a solar water heating system can be undertaken by a practical householder, although most people employ a properly trained tradesperson. The collectors (and for some systems the water tank also) are usually fixed on roofs of sufficient strength. In most situations, a 'conventional' water heater is available either as a back-up or as an alternative new installation. Nevertheless, the payback time against the running cost of a conventional system is usually 5-10 years, which is substantially less than the life of the solar system, (see Examples 17.1 and 17.2).
Solar water heaters, even relatively sophisticated ones, can be manufactured almost anywhere on a small or medium scale, thus giving employment. They do not need to be imported and there is a market, especially among the middle class and members of 'green' organisations. The technology is modular and can be scaled up for commercial uses, such as laundries and hotels. Thus by far the largest national production of solar water heaters is in China, where even basic cheap units can provide domestic hot water, even if only for half the year in the winter climate and high latitude of China. Many of these units are single glazed or even unglazed, often with relatively poor thermal connection between the plate and the tubes, but their price/performance ratio is acceptable.
All these features are examples of the benefits of renewable energy systems generally, as set out in Chapter 1.
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