The need of a heat mirror

Hybrid thermoelectric - photovoltaic generators (HTEPV) need to be equipped with strategies to reduce heat losses. In particular radiative heat losses can play an important role in setting the final system efficiency. Here you can find an explanation why.

The efficiency of a hybrid thermoelectric – photovoltaic generator can be written as

η_{htepv =} η_{pv +} η_teg η_ot

where η_htepvg , η_pv, η_teg, and η_ot are respectively the hybrid, the photovoltaic, the thermoelectric, and the opto-thermal efficiency. The opto-thermal efficiency is the system efficiency in converting the input power coming from the Sun into heat flowing through the thermoelectric generator (TEG). Actually all the heat not flowing through the TEG legs is a loss in the energetic balance, and therefore contributes to decrease the system efficiency.

In HTEPV generators (but generally in all solar-to-thermal conversion systems) the prevention of heat losses is fundamental to guarantee optimal efficiencies. Heat can be exchanged by conduction, convection, and radiation.

In HTEPV generators, conduction happens exclusively in the TEG legs, and contributes to set the system output power. Convection and radiation happen instead at the top, and bottom system surfaces, and between thermoelectric legs.

Convection need a fluid (typically air) and it is normally prevented encapsulating the HTEPV system in vacuum.

Radiative heat exchange is more difficult to be contained and depends on the emittance of the surfaces exchanging heat with the environment. Normally in solar-to-thermal application the easiest strategy is the implementation of materials with low emittances. In HTEPV devices this can be done for the TEG part (typically using copper plates that exhibit very low emittances) but not for the solar cell top surface, which is given by the materials implemented within the cell.

Emittances of solar cells are reported to exhibit values that can be in general split into two groups. Silicon, GaAs, and multi-junction solar cells were shown to exhibit high emittances, ranging between 0.7 and 0.9, while CIGS and related alloys (such as CZTS and CGS) show values between 0.2 and 0.4. In both cases, it is fundamental to limit as much as possible the loss from the top of the system by heat radiation. Actually for Th ∼400 K, and emittance ranging between 0.7 and 0.9, the amount of thermal power lost by radiation is ∼700–900 W/m², which is almost all the incoming power at one sun.

However, since it is not possible to modify solar cell emittance without modifying their basic structures, and thus impairing their efficiencies, different solutions are needed. One possible approach comes from so-called heat mirrors (HM). A HM is a layer or a multi-layer of materials that exhibit high optical transmittance for the solar spectrum, showing instead high reflectance for the infrared, as shown in the figure.

The change between highly transparent and highly reflective behavior happens at the plasma frequency of the material free carriers. During the ’80s of the last century, several publications appeared suggesting Transparent Conductive Oxides (TCOs), such as In:SnO2 (ITO) and Al:ZnO (AZO) as viable candidates for HMs. The recent increasing attention raised around these materials for their application as the transparent conductive front contact in solar cells, or as spectrum splitters in hybrid PV–thermal strategies, joint with the possibility of tuning their optical properties by changing the deposition parameters, make this solution very interesting for future development of HSTEPVG devices.