Hot Air Always Rises
Heat is all around us—whether it be from the sun, fossil fuels, or even from our bodies, heat is an essential part of our day-to-day life. Yet, many do not understand the importance of heat’s role scientifically. The second law of thermodynamics states that the entropy (randomness) in a system is always increasing. This important law is the catalyst for heat usage in energy applications: heat is always transferred spontaneously from the hot to cold direction, but never in reverse.
By using a source of energy such as concentrated sunlight or hot gas, thermophotovoltaic systems generate low-energy infrared radiation by heating a thermal material. A photovoltaic cell is designed specifically to capture and convert that radiation into electricity. However, harnessing energy in this fashion can be incredibly inefficient. Andrej Lenert’s team researching thermoelectric energy at the University of Michigan notes that photovoltaic cells with metal mirrors aimed to increase efficiency still absorb 5% of the light from the mirror itself. To achieve high efficiencies in the past, thermophotovoltaic systems would cost a pretty penny. With Lenert’s team at the University of Michigan, an electrical engineer, Stephen Forrest, was able to invent a new cell design using a new added air layer, resulting in 99% of light reflecting back to the emitter. By just simply adding this air layer, efficiency improved from 24% to 32%.
“This thermophotovoltaic device achieves record-high efficiency by incorporating a 600 nm thick air layer between an indium gallium arsenide photovoltaic cell and a reflective metallic layer underneath.” (c&en)
So what does this mean for our community? With Forrest's innovation to thermophotovoltaic systems, we are now able to harness more heat to convert to energy with greater efficiency, without costing an unreasonable amount like before. Thermal energy is much more sustainable with cheaper costs in thermophotovoltaic systems, which could account for an increase in usage in the near future through thermophotovoltaic power plants, or even at a small scale due to their cost-effectiveness. Just like the second law of thermodynamics, the sustainability of energy will continue to increase in thermophotovoltaic systems, similar to how we see the randomness and heat continuing to rise in an open system.
