https://wikimedia.org/api/rest_v1/media/math/render/svg/3ca20f2d7f27a203b076f448dfaf5d30d7073b4f

Q(H) approaches zero if you are relying on blackbody radiation into deep space for the cold side of your heat engine.

Especially if said radiation is expected to pass through an atmosphere loaded with H2O vapour, but even if it isn't (space-based or lunar applications)