Near-field radiation thermophotovoltaic demonstrator – DEMO-NFR-TPV

The objective of DEMO-NFR-TPV is to make the first experimental proof in laboratory conditions of the concept of near-field radiation thermophotovoltaic (NFR-TPV) energy converters for distances between a thermal radiator and a photovoltaic cell much smaller than 500 nanometers. Similarly to solar cells, thermophotovoltaic (TPV) cells convert photon energy into electrical energy, but with radiation coming from a hot body instead of the Sun. When the space between two radiating bodies is smaller than the characteristic wavelength of thermal radiation, radiative heat transfer is enhanced up to several orders of magnitude above the classical far-field blackbody radiation exchange and the spectrum of the exchanged radiation is modified. This is due to the contribution of the evanescent waves. NFR-TPV converters combine near-field radiation effects with TPV energy conversion. They provide a clever solution to enhance the efficiency of energy harvesting from waste heat sources.
An analysis of the state-of-the-art reveals specific issues that may explain why the couple of previous attempts failed to provide convincing data of near-field TPV effect involving controlled sub-micron gaps. First, the up-to-date experimental setups demonstrating near-field thermal radiation enhancements do not meet all criteria for building a NFR-TPV demonstrator: large emitter and receiver areas, small gap distances and large temperature differences. Second, there has not been real contribution from the optoelectronics community to design and build photovoltaic cells for the specific configuration of near-field photogeneration of electrical carriers. In this frame, the project has the scientific objectives to theoretically understand the contribution of each loss mechanism associated with the thermal-radiation-to-electrical-power conversion in NFR-TPV systems, to propose and optimize dedicated NFR-TPV cell architectures which minimize the losses, and to demonstrate experimentally the near-field radiation enhancement of NFR-TPV converters by building a demonstrator in laboratory conditions.
To meet the aforementioned objectives, the project will be based on four specific primary choices: a TPV cell maintained at room temperature (or below), a temperature difference between the radiator and the cell from 200 to 550 K, the use of a very low bandgap semiconductor (InSb, Eg = 0.173 eV at 300 K), a sphere–plane configuration for building and controlling the gap distance between the radiator and the cell, that will be less than 500 nm and down to a dozen of nanometers.
The first stage of the work program will consist in a full modeling for the design of NFR-TPV devices. It will include optical, electrical and thermal loss mechanisms under high injection conditions. Front windows, a back reflector and contacts will be accounted for. Then, TPV cells will be designed, fabricated and characterized. Using the modelling tool, junction architectures addressing the recombination problems coming along with near-field radiation photogeneration of electron-hole pairs, will be specifically designed. Up-to-date molecular beam epitaxy (MBE) growth conditions and processing techniques will be used to fabricate InSb TPV cells. The structural and optoelectronic properties will be characterized. The final stage will be the building, operation and characterization of a NFR-TPV device demonstrator using a system based on atomic force microscopy (AFM) to control the distance between the TPV cell and a spherical thermal radiation emitter. After assessing the capabilities of the setup for making near-field thermal radiation measurements, electrical output power of the InSb cells will be measured as a function of distance between the radiator and the cell. It is aimed at demonstrating that the NFR-TPV concept is able to provide higher power densities – at least twice – than in the far-field configuration.