A thermal transfer system includes an enclosure, optional thermal insulation inside the enclosure, a dielectric fluid inside the enclosure, and a porous medium inside the enclosure. The porous medium can be at least partially immersed in the dielectric fluid. A cooling device can have a first surface contacting an interior surface of the enclosure and a second surface contacting the dielectric fluid, the porous medium, or both. The porous medium can include a cutout or cavity configured to support an electrical or electronic device immersed in the dielectric fluid. A port can be in a wall of the enclosure and a set of electrical conductors can run through the port and the electrical or electronic device immersed in the dielectric fluid.