Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar photovoltaics
collectors consist of heat pipes on the back of PV modules. Covered PV-T collectors consist
of PV modules placed inside flat-plate solar heat collectors. The non-covered PV-T collectors
increase the electricity output, while the low-temperature heat collected can be used in
combination with heat pumps. Covered PV-T collectors replace PV modules and heat
collectors, slightly decreasing the electric efficiency but significantly increasing the total solar
energy yield of a roof surface compared to side-by-side installations (Dupeyrat et al., 2011).
Solar air heaters too can be combined with PV.
Photo 6.1 PV-thermal collectors manufactured in Turkey
Source: Solimpeks Solar Energy Corp.
Key point
Solar PV and thermal can be merged into PVT collectors.
Concentrating photovoltaics
Using mirrors or lenses or a combination of both, concentrating PV (CPV) focuses the solar
radiation on small, high-efficiency cells usually made of several layers (often called “tandem” or
“sandwich”) each capturing a specific wavelength of the solar light spectrum. One assumption is
that the higher cost of these cells is outweighed by their higher efficiency (up to 38% for cells,
25% for modules) and the lower cost of the reflective surfaces or the lenses. CPV and PV,
however, are not directly comparable; the primary criterion for choosing CPV is a high ratio of
direct normal irradiance (DNI) to diffuse irradiance (as shown in Chapter 2). Another significant
difference is the distinct daily profile of electrical output exhibited by sun-tracking systems
(Figure 6.4). Tracking the sun is indispensable for CPV (with high accuracy for high concentrations,
i.e. more than 10 “suns”), but optional (with low accuracy) for other PV technologies.
CPV requires effective cooling, which makes it easier to cogenerate heat and power
(Photo 6.2). The heat can be used for some industrial process or desalination (see Chapter 5)
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© OECD/IEA, 2011