Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar heat
Figure 7.3 Transpired air collectors
Heated air
Plenum
Ambient air
Perforated
absorber
Source: NREL.
Key point
Unglazed solar air collectors provide heated air to buildings or agro-industries.
To improve on standard flat-plate collectors, some of the main losses need to be reduced.
Anti-reflective coating can reduce reflections to 4% to 7%. Coating the absorber can reduce
the radiation losses. To reduce the main losses from convection through the front, hermetically
sealed collectors with inert gas fillings, double covered flat-plate collectors, or vacuum flatplate
collectors may be used.
Evacuated tube collectors
Evacuated tube collectors can produce higher temperatures in the HTF and therefore are
often more appropriate for constant high-demand water heating systems or process loads.
Evacuated tubes can show a good efficiency even for temperatures as high as 170°C. All
evacuated tube collectors have similar technical attributes:
• A collector consisting of a row of parallel glass tubes;
• A vacuum (< 10 -2 Pa) inside each tube that drastically reduces conduction losses and
eliminates convection losses;
• The form of the glass is always a tube to withstand the stress of the vacuum;
• The upper end of the tubes is connected to a header pipe; and
• A getter using absorbent material maintains the vacuum and provides visual indication
of the vacuum status.
Evacuated tubes contain a flat or curved absorber coated with a selective surface and fluid
inlet/outlet pipes. Inlet and outlet tubes can be parallel or concentric. Alternatively, two
concentric glass tubes are used, with the vacuum between them. The outside of the inner
tube is usually coated with a sputtered cylindrical selective absorber.
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© OECD/IEA, 2011