If your hot source is really hot, thermophotovoltaic (https://en.wikipedia.org/wiki/Thermophotovoltaic_energy_conversion https://en.wikipedia.org/wiki/Thermopho

If your hot source is really hot, thermophotovoltaic (https://en.wikipedia.org/wiki/Thermophotovoltaic_energy_conversion https://en.wikipedia.org/wiki/Thermophotovoltaic_energy_conv...) makes sense and can offer much better efficiency...