These would also count as "direct" power generation, I think (how should it be defined?). None of them work at scale, yet. https://en.wikipedia.org/wiki/Betavo

These would also count as "direct" power generation, I think (how should it be defined?). None of them work at scale, yet. https://en.wikipedia.org/wiki/Betavoltaic_device https://en.wikipedia.org/wiki/Betavoltaic_device https://en.wikipedia.org/wiki/Thermoelectric_generator https://en.wikipedia.org/wiki/Thermoelectric_generator https://en.wikipedia.org/wiki/Thermophotovoltaic https://en.wikipedia.org/wiki/Thermophotovoltaic https://en.wikipedia.org/wiki/Direct_energy_conversion https://en.wikipedia.org/wiki/Direct_energy_conversion