Solar Energy Perspectives - IEA

Chapter 7: Solar heat
Evacuated tube collectors can be classified in two main groups:
• Direct flow tubes: the fluid of the solar loop circulates through the piping of the absorber;
• Heat pipe tubes: the absorbed heat is transferred by using the heat pipe principle without
direct contact with the HTF of the solar loop (Figure 7.4). One advantage of such
a scheme is that collectors continue to work even if one or several tubes are broken.
Damaged tubes can be easily replaced.
Figure 7.4 Heat pipe tube
Heat transfer
Solar energy
absorbed by
solar tube
Vapour rises to top
Condensed liquid returns to bottom
Source: Apricus Solar Co. Ltd.
Heat absorbed
by heat pipe
Key point
Evacuated tubes are now the most popular solar collectors in China
CPC collectors
Compound parabolic collectors concentrate the solar radiation on an absorber (Figure 7.5).
Because they are not focusing (non-imaging), they accept most of the diffuse radiation and are
not restricted (like plain concentrating technologies) to direct “beam” radiation (see Chapter 2).
CPC collectors do not need to track the sun and are stationary or require only seasonal tilt
adjustments. This publication includes stand-alone CPC collectors are included with nonconcentrating
devices. If they concentrate the sun’s rays by only a small factor (less than 10
“suns”, often only 2 or even less) this is enough to allow non-evacuated collectors to achieve
working temperatures up to 100°C with an efficiency comparable to that of evacuated tubes.
Some evacuated tube collectors routinely include small CPC collectors inside the tubes. Future
designs with fewer tubes per surface area and CPC are expected to deliver efficiencies of 60% at
working temperatures of 160°C or 50% at temperatures of 200°C with fully stationary collectors.
Ovens
Solar ovens collect the energy from the sun with or without concentration. Box cookers look
like a flat-plate collector with a larger internal space, inside which one can place what needs
to be heated or cooked (Photo 7.1). With one or several reflectors, a box cooker is similar to
a CPC collector, with low concentration level.
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© OECD/IEA, 2011