Sliver Cells in Thermophotovoltaic Systems

Thermophotovoltaic systems are direct conversion heat engines that use photovoltaic cells to convert radiant energy from a heated object to electrical energy. Sources of heat for thermophotovoltaic (TPV) systems include sunlight, the combustion of fossil fuels, radioactive decay and industrial waste heat. Previous modelling indicated that thin photovoltaic cells were optimal for use in TPV systems. This thesis presents an original theoretical investigation into the application of thin silicon sliver cells in TPV systems. Sliver cells are a novel type of photovoltaic cell fabricated from single crystal semiconductor wafers. They are bifacial, narrow and very thin of the order of ten times thinner than current conventional cells. In addition, sliver cells have their metal contacts at the edges of the cell. A computational model was constructed to examine and compare the performance of sliver cells with state-of-the-art conventional back-contact cells. Silicon was modelled as the semiconductor material due to the availability of modelling data and to provide an indication of the feasibility of thin silicon cells in TPV models. The model was validated using the semiconductor device modelling program PC1D under non-TPV conditions. Excellent agreement was observed. The analysis of silicon TPV cells was extrapolated to gallium antimonide. It was found that sliver cells do not offer a clear advantage over well designed conventional cells in TPV systems. This was observed to be a result of horizontal carrier transport resistances across sliver cell widths. Minimum sliver widths in this study were limited to 300μm. Within the assumptions of the model, sliver cell maximum TPV efficiencies (32± 2)% were found to be ∼ 85% of the maximum TPV efficiency achieved by conventional cells (38 ± 2)%. Sliver cell maximum output power densities (600 ± 30)mW/cm2 were lower than those for conventional cells (3300± 100)mW/cm2 by a factor of five. Optimal cells for thermophotovoltaic systems were found to be thin back-contact cells with spectral filtering that reflected high-energy photons back to the emitter. The primary advantage for thin cells in TPV systems was observed to be a result of decreased resistive losses and not spectral considerations. It is from the original investigation of this thesis into the different resistance geometry of sliver cells in TPV systems that these effects can clearly be separated. Practical considerations such as heat-sinking and cell-module circuitry were found to favour sliver cell geometries. Silicon cells were found to perform well at emitter temperatures that are above the current range of feasibility for TPV systems. Recommendations are made for the design of future TPV systems, potential fabrication processes for the manufacturing of optimal thermophotovoltaic cells are outlined, and the properties of future whole-system thermophotovoltaic models are discussed.