If it is assumed that the energy produced by fusion must be recovered as heat, then the process requiring very low temperatures would be self-quenching, at least until a means is found for recovering that heat usefully , rapidly enough to maintain the required low temperatures. But, depending on materials, a majority of the energy might not be recovered as heat. If the process was possible in a superconductor, for instance, or on a thin surface bound to a superconductor, it might be possible to recover a good portion of the energy released directly as electric current. Of course, the second law of thermodynamics is unavoidable, and requires some of the energy to be released as heat which would have to be removed from the system to prevent quenching. But nothing requires all of the energy released to be recovered as heat.
Really Cold Fusion