A fossil fuel plant will burn fuel to produce heat. Some of this heat will be used for generation, and the remainder is waste heat. A solar panel or wind turbine, on the other hand, does not produce significant amounts of heat in operation - a PV solar panel would only get somewhat warm, a wind turbine would only achieve some friction heating - to where it is not economical to harness this relatively small amount of energy (compared to waste heat from an exothermic generation process, like nuclear fission or hydrocarbon combustion)