Fraunhofer Institute for Solar Energy Systems ISE
Analogous to solar cells, thermophotovoltaics (TPV) directly converts electromagnetic radiation to electrical energy. Unlike solar cells, however, the radiation does not come from the sun but rather from a heated emitter in the form of thermal radiation (long-wave photons). Specially designed thermophotovoltaic cells made of absorber materials with bandgaps in the infrared range can efficiently convert thermal radiation into electricity.
In an energy system based on fluctuating renewable energy sources, thermophotovoltaic systems can make an important contribution. Excess electrical energy can be stored in the form of high-temperature heat. Upon demand the stored heat can be converted back to electricity using thermophotovoltaic modules. The high conversion efficiency of TPV cells is a decisive factor for the efficiency of the total conversion process (electric-to-electric). Also, smart photon management is essential: Mirrors integrated into the TPV cells reflect the photons, which have energy levels that are too low to be used in the photovoltaic process, back to the heated emitter to be recycled. Thus, higher system efficiencies are achieved.
At Fraunhofer ISE we develop customized low bandgap III-V absorber materials based on GaAs, InP, GaSb, and Ge as well as high transparency tunnel diodes. By means of a metamorphic buffer layer we realize engineered substrates based on GaAs in order to replace comparatively costly InP substrates. We also develop thin film processes and highly reflective, electrically conductive back surface reflector layers for efficient photon management. Finally, we offer our customers long-standing expertise in the development of dense array modules, including the packaging technology and the thermal management.
In addition, we are working on the optical modeling, the realization and the characterization of selective emitters for TPV systems. Through specially designed coatings, it is possible to generate resonance phenomena in surface structures and/or thin film systems so that they preferentially emit photons which can be used by the thermophotovoltaic cell.