The efficiency of a TPV generator is very dependent on selective optical properties of the chain emitter-filter-TPV cell-reflective backing. If a selective reflective edge filter using multiple reflections in dielectric layers is employed, the slope of the edge depends on the incidence angle interval. A special geometry, consisting of a double square cone between the (square) emitter and the (square) cell array, which narrows this angle interval significantly, has been constructed and investigated experimentally as well as with ray tracing analysis.
A joint project between NREL, SLU and UCFB aims at building a wood powder fueled TPV generator. The progress of the project is presented.
The optical efficiency is a week link in all TPV systems. Many different approaches have been studied earlier, but not much on the possibility to use interference filters. Such filters can be designed to get alsmost any desired optical performance, as long as it is allowed to use a large number of layers in the multilayer stack. In practical applications this is very often not acceptable. Nevertheless such filters can still be quite good, at least for a very narrow range of incident angles. Therefore a double-cone geometry is proposed to be used in combination with such filters. This geometry will limit the range of incident angles onto the filter, which will then act more efficiently. The influence of the geometry is evaluated by means of model experiments, and a computer program has been constructed to be able to simulate this type of geometry using ray-tracing technique.
An apparatus for measuring TPV cell efficiencies at different radiation intensities and for different graybody emitter temperatures has been constructed. The apparatus has been used for measuring V-I characteristics, efficiencies and fill factors for several InGaAs TPV cells. Measured results are used to determine how cells may function together with edge filters, and those results are compared with theory.
The development of a thermophotovoltaic converter that uses combustion of wood powder as energy source has started with development of the combustion source. During the last few months, we have constructed and tested a feeding mechanism and a combustion chamber that seem very promising. We manage to keep a 10 kW flame steadily burning for several minutes at the time, generating a temperature exceeding 1400 K. The plans for continued development of this and other components of the converter are discussed in the paper.
Wood fuel has a high energy density, 18.3 MJ/kg dry matter, and it is combustible also in existing oil furnaces (in the range 1-15 MW) with little alteration. In principle, wood powder would be a possible fuel for small scale combustion, and we are in the process of developing such a furnace. During the last few months, we have constructed and tested a feeding mechanism and a combustion chamber that seem very promising. As of November 1994, we have reached 1481 K and been able to keep the temperature around 1400 K for any length of time at the time in one of our two prototype burners. A joint TPV R&D program, in which we cooperate with researchers at National Renewable Energy Laboratory, Golden, CO, USA, is in the planning, aiming at co-generation of electricity and heat from refined wood fuel.