Our Green IT blog has been tracking some interesting developments in the world of recycling. Not the usual bottle-bank business, but new semiconductors that aim to turn waste heat directly into electricity. Two startups, Eneco and Power Chips, have products in the labs that they hope to bring to market in the next year or so.

In IT, waste heat is very noticeable whether its a laptop warming your knees or a blade rack making you sweat. So the new chips promise to kill two birds with a single stone: keeping processors cool while also cutting electricity bills.

A practical product is still a little way off, but it’s worth asking, how much heat might usefully be recovered? And what impact might it have on electricity bills?

Two centuries ago it was realised that getting useful energy out of a heat source (such as a coal fire under a boiler) was not a very efficient process. In the 1820s Nicolas Carnot did the maths: proving that better efficiency was not always a matter of fitting more insulation or using tighter bolts.

His theoretical Carnot cycle put a maximum on the energy that can be extracted from a heat source. The Carnot limit depends on two things: how hot the heat source is, and how cold your heat sink happens to be. To get 100 percent efficiency you require an infinitely hot fire, or a heat sink at absolute zero (although you’ll have a tough time using it as a heat sink without raising its temperature).

Today’s processors get hot, but not that hot. The maximum efficiency you can expect when you try to extract energy from this temperature rise, assuming a 20°C (293 Kelvin) ambient and a nice hot Pentium 4 at 100°C (373K) is about 21 percent. The strongest claim made for the new chips is 80 percent of the Carnot limit, so if that pans out we can expect to recover a maximum 17 percent of a maxed-out Pentium’s 300W of waste, assuming the Pentium is completely encased in energy-recapturing chips.

Using my trusty engineer’s thumb, I estimate that recovering 10 percent of the electricity might be practical. Hopefully you’ll agree that this makes the whole venture worth pursuing: a 10 percent rebate on your power bills would certainly be better than a poke in the eye. And in fact the savings will be slightly greater if you normally dump energy into air-conditioning, or squander a few watts running a CPU fan, or can tap into a colder outdoor temperature as your heat sink.

However, there is one rather ugly fly in the ointment. The very latest CPUs don’t run as hot as a Pentium. An Athlon 64 or Core 2 Duo will max out at about 60°C, cutting the indoor Carnot limit down to 12 percent, meaning an end result that will no doubt struggle to best five or six percent.

And CPU operating temperatures are likely to get cooler still. The window of opportunity for this technology – in IT at least – might just be closing even as it starts to swing open.