Cold fusion is suddenly in the news again because of a recent 60 minutes story. For anyone who thinks that there is anything to this, I have only one word:
Excess energy is a red herring. There might be excess energy coming from somewhere, but if you're fusing hydrogen, then you produce helium. No helium, no fusion. It's as simple as that. And I'm not talking about the trace amounts that have been reported, there should be gobs and gobs of it, certainly enough to float a toy balloon and remove all doubt. There's also the problem that all known hydrogen-based fusion reactions produce energetic neutrons. Really, cold fusion researchers should be dropping dead left and right from radiation poisoning, but failing that the radiation should at least be detectable on a geiger counter. So where are the videos of the cold fusion apparatus with a geiger counter buzzing away next to it?
If there's no fusion, there could still be excess energy, but it's almost certainly coming from some prosaic source, and it's almost certainly consuming palladium somehow. Palladium might be an economically viable catalyst, but it makes a mighty expensive fuel.
UPDATE: Here's the math.
The specific heat of deuterium fusion is about 1 MeV, which is about 10^-13 joules. To produce enough helium to fill a balloon (let's say a liter at standard temperature and pressure, or about 0.05 mole) we'd need to produce ~5x10^7 joules of excess heat. That's actually quite a lot, enough to raise the temperature of a cubic meter of water by 50 degrees C. Not enough to vaporize the lab, but definitely enough to get someone's attention.
But here's the thing: some of these experiments run for months at a time. Measuring small amounts of excess heat over long periods of time is very hard. But collecting and storing even small amounts of putative excess helium is very easy. Let's say that the excess heat is only one watt, it would only take a year to accumulate a liter of helium, and the fact helium was being produced would be inarguable after only a few days. So I still say if it were there someone would have exhibited it by now.
I made a mistake in my calculations. There are actually two different deuterium fusion reactions. One produces 3He (helium 3, with two protons and one neutron) and an extra neutron, i.e. radiation. The other produces 4He, with two protons and two neutrons, and no extra neutrons. The second "clean" reaction also produces 23 times as much energy as the first one (or, equivalently, only about 4% as much helium per unit energy produced). It is also very rare. As far as I have been able to determine (which at this point means looking at a few Wikipedia articles) it does not occur in nature, and has not been achieved in more traditional "hot fusion" experiments. So I wrote off the possibility of achieving this reaction at room temperature as too good to be true. However, there are some plausible-looking reports of 4He production correlated with excess heat production in about the right amounts. So at this point I have to say that the possibility cannot be definitively ruled out. I still wouldn't bet my life savings on it, but if a policy maker were to ask me at this point I think I would recommend funding some of this research. These experiments are dirt-cheap by contemporary standards, and if it can be made to work it would be the single greatest technological breakthrough in the history of mankind. So it's worth putting some resources into it to determine definitively what, if anything, is going on.