Solar Energy Perspectives - IEA

Chapter 7: Solar heat
incoming radiation is much shorter-wave. Glass has the property of being relatively opaque
to long-wave radiation – radiant heat – while letting the incoming light through.
Solar radiation enters the collector through the transparent cover and reaches the absorber,
where the absorbed radiation is converted to thermal energy. A good thermal conductivity
is needed to transfer the collected heat from the absorber sheet to the absorber pipes,
where the heat is transferred to a fluid. Usually a water/glycol mixture with anticorrosion
additives is used as the heat-carrying fluid. The fluid also protects the collector from frost
damage.
Standard flat-plate collectors can provide heat at temperatures up to 80°C. Loss values for
standard flat-plate collectors can be classified as optical losses, which grow with increasing
angles of the incident sunlight, and thermal losses, which increase rapidly with the working
temperature levels (Figure 7.2). As can be seen, the efficiency of flat-plate collectors is as high
as 60%. In the same range of temperatures, advanced flat-plates or evacuated tubes have
even higher efficiencies, which compare very favourably with those of photovoltaic (PV) or
concentrating solar power (CSP).
Figure 7.2 Optimal and thermal losses of a flat-plate collector
100%
Reflection: 8%
Glass Absorption: 2%
Convection: 13%
Reflection: 8%
Radiation: 6%
Absorber 60%
Heat conduction: 3%
Source: IEA-SHC, 2008.
Key point
Good design and manufacturing minimise heat losses in flat-plate collectors.
The vast majority of flat-plate collectors use water as the heat-transfer fluid (HTF), often
complemented with glycol to prevent freezing; a minority use air. Air collectors cannot freeze
or boil and have no corrosion issues, and they are lighter than liquid-based. They can be
unglazed perforated plates or “transpired air-collectors” (Figure 7.3), or glazed flat-plate
collectors. Air collectors are usually used for ventilation heating, and in agro-industries for
crop and food drying. When used in buildings, they are hard to distinguish from passive solar
designs (see Chapter 4).
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© OECD/IEA, 2011