For directly converting heat to electrical energy, for example in a spacecraft operating in space, at least one thermoelectric generator module is arranged in an evacuated chamber so as to receive thermal radiation from a heat source through the interior vacuum within the interior of the chamber. The thermoelectric generator module converts a portion of the received heat to electrical energy and rejects a portion of the heat by thermal conduction to a thermally conducting cooling body such as the chamber wall or directly by thermal radiation to the surrounding vacuum of space.