Free Neutron Decay as an Energy source I suspect there would be enormous engineering and practical difficulties using free neutron decays as a energy source (more as a battery than a net energy supplier) but assuming those could be overcome what sort of energy characteristics would it have such as energy density per cubic metre? Have there been theoretical studies of this subject?   
 A: The energy released when a neutron decays is 0.782343 MeV. To put this into perspective, one mole of neutrons (about a gram) would produce about 75GJ which is about the same as the energy from burning 1.7 tonnes of diesel (although some of the energy would be carried away by the anti-neutrino and lost).
So that's a hell of a lot of energy! The trouble is that there is no way to easily generate free neutrons, or to store them once we have made them. A free neutron will decay with a half life of about ten minutes and there is no way to prevent this except by putting the neutron back into a nucleus - and then of course we can't get it out again. So as a practical way to store and use energy it's a non-starter.
As Gary says in a comment, we do sort of do this already. There are plenty of radioactive nuclei that decay by beta decay, which is basically the decay of a neutron. If you take a lump of such material it will get hot due to the energy released in the decay, and you can use the heat to produce power.
A: The problem with neutrons as an energy source is that once created, neutrons escape. Their speed is measured in km/s and you can't put them in a container because they would either react with nuclei in the container wall or go right through the wall. Remember: they are neutral and won't be stopped by electric fields--which is what prevents ordinary objects from going right through each other.
Even cold neutrons have considerable speed and at a half-life of >10min your container to catch the energy would have to be really big.
And what if we could cool them down to only a few Kelvin, where they wouldn't noticeably move any more? Then the remaining force on them would be gravity. The precious neutrons (in an evacuated chamber) would sink toward the center of the Earth, literally like a stone, and accelerating just as quickly. Once it gets in contact with regular matter, it will quickly be absorbed by some nucleus.
