In a magnetohydrodynamic energy conversion device having an electrically conductive fluid flowing through an electrically conductive fluid conduit, first and second permanent magnets aligned for forming a magnetic field intersecting said electrically conductive fluid conduit axis for inducing an electric current in said fluid, the improvement wherein each of said first and second magnets is a permanent magnet and, for cryogenically cooling each of said magnets said magnetohydrodynamic energy conversion device comprises,
- a housing having a chamber in which one of said permanent magnets is disposed, said chamber containing a cryogenic liquid surrounding and cooling said one of said permanent magnets, a heat exchanger external to said chamber for allowing heat absorbed by said cryogenic liquid to escape to the ambient atmosphere, and a pump for circulating between said chamber and said heat exchanger said cryogenic liquid,
- a container having a vacuum and a first wall mounted adjacent said housing for insulating said chamber from heat, and
- a nitrogen cooing system for further cooling said one of said permanent magnets comprising, connected in series, a liquid nitrogen chamber in a housing mounted adjacent a second wall of said container having a vacuum opposite said first wall, a liquid nitrogen heat exchanger, and a liquid nitrogen pump for pumping liquid nitrogen through said liquid nitrogen chamber and said liquid nitrogen heat exchanger whereby heat absorbed by said liquid nitrogen can be released to the ambient atmosphere.