Reply To: Global Warming

[dennisp]

How efficient does the onion have to be to give us practical net power?

If 40% is ok, then if the onion turns out slow to develop, a turbine would work. Pressurized water reactors run at only about 315 deg C:
http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/reactor.html

If water coolant doesn’t absorb X-rays well enough, maybe we could borrow from certain fast reactors and use lead, which melts at 327C and definitely isn’t transparent to X-rays. Molten salt and sodium are other options, though I have no idea how suitable they’d be.

Ideal Carnot efficiency at 600C on a warm day is around 65% according to this calculator:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html#c1

Of course you have to keep the lead above its freeze temp, which takes you down to 28% or so. But you could have a secondary loop of water coolant to take you from >327C to outside temp, with about the efficiency of PWR turbines, getting us in the neighborhood of 50% total. Now you’re approaching the complexity of the PWR power loop, but at least you don’t have a containment dome and a bunch of complicated safety systems. You can probably get in the neighborhood of coal power cost, especially given the relative lack of fuel expense, or of the regulatory and political delays that plague fission reactors.

It might not be the ultra-cheap power source we’re hoping for, but a zero-waste, zero-pollution, reasonably-cheap, perfectly safe power source is still good enough to go to production, and kick off a lot of investment in developing better options.