You could define free energy research in a number of ways but we will focus on just three. One meaning is that it is a device that can generate electricity with the help of some kind of source that has not been recognized scientifically. Next, free energy is referred to as a kind of perpetual motion machine. Last, it most commonly referred to as a device with the ability to collect and distribute energy for free.
The one scientific definition that seems to be the most agreeable would be the first one. If a device can generate free energy with the help of say a nuclear reactor, then it would be considered as a free energy device but only if it were being used in the 1890′s unfortunately.
In the present context, if a scientist found out some way to generate electricity out of neutrino flux or harness gravity, then that would be free energy as well. But there is a catch here.
Out of all these definitions it is the first scientific definition that is the most feasible. By going with this definition, in theory, nuclear reactors could be considered as a free energy device (in the 1890′s).
For example, Becquerel discovered that fog plates could effectively filter uranium ore and he invented a device based on this discovery. Uranium ore was considered as free energy up until scientists summed this unusual phenomenon up as radioactivity of unstable elements.
As an example we will use Becquerel who discovered that fog plates can filter uranium ore. Based on this discovery, he invented a workable device that could do just that. But, scientists could not explain the strange phenomenon by way of radioactivity of unstable elements and because of that uranium ore ceased to be classified as free energy.
But the problem with free energy is that a lot of people do not believe in it. So though there are a lot of enthusiasts among this group, it usually has to face criticism in the hands of self acclaimed specialists.
This has made research in this field difficult. But the research continues to take place and scientists continue to make successful the devices in the midst of all odds.