In this technology, evacuated tubes enclosed within an insulated steel casing work by exploiting the vacuum around the collector. This reduces the heat loss from the system, making them particularly suited to cooler climates like that experienced in the UK. They heat water to around 60°C but sometimes significantly higher. This means that domestic hot
water systems may have no need of additional heating. To realize their full potential they should be linked to a storage facility which stores excess warmth in summer to supplement winter heating (see Fig. 2.1).
Solar panels have traditionally been associated with providing domestic hot water (DHW). Solar water heaters comprise a solar collector array, an energy transfer system and a thermal storage unit. There are two basic principles involved:
• passive or thermosyphon systems in which circulation of the working fluid is driven by thermal buoyancy
• active solar whereby a heat transfer fluid is mechanically circulated through the collector.
There is a further division into direct or 'open loop' systems, in which potable water is circulated through the collectors, and 'closed loop' or indirect systems, which use an antifreeze heat transfer circulating fluid.
In thermosyphon systems the storage tank must be above the collectors. There are three configurations which can be directly connected to a horizontal tank:
• flat bed collectors
• parabolic trough collectors with the heat pipe absorber (see below) feeding directly into the base of the storage cylinder
• evacuated tube collectors.
This combined collector/storage system is limited to providing hot water during the day (see Figs 2.2-2.5).
Active solar systems can be open loop (Fig. 2.6) or closed loop, in which the circulating fluid passes through a coil heat exchanger within the storage tank (Fig. 2.7). This technology is being promoted by the International Energy Agency (IEA) via its solar heating and cooling programme. The IEA is an agency of the Organization for Economic Co-operation and Development (OECD). The potential for solar domestic hot water is about 1 m2 per person.
Countries like those in the Middle East and China are exploiting this technology at an accelerating pace, with 10 million square metres already installed in China and yearly sales
of 3 million square metres. The market in China is five times larger than that in Europe. In 2001 China produced 20 million all-glass evacuated tube collectors. By 2003 the installed total was 50 million square metres. In its tenth five-year plan the government set a target of 100 million square metres by 2010.
Whilst this technology is one of the cheapest renewable options it is still not cost-effective weighed against fossil-based energy with a payback time of around 20 years. The market will only grow significantly at this stage if:
• there is direct or indirect government support for the technology
• following this intervention, the market expands to achieve economies of scale
• the product is of the highest quality and fitting procedures are simplified and standardized
• there is an adequate network of trained and accredited specialist installers
• complete roof modules incorporating solar collectors are made available to the construction industry for new build, making the marginal cost of the collectors relatively moderate.
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