Re: Power budgets was [Re: Understanding nanotech]

Robert J. Bradbury (
Tue, 31 Aug 1999 21:09:59 -0700 (PDT)

On Tue, 31 Aug 1999, dan wrote:

> ... a typical household really will be able to use solar and geothermal
> energy, contributing zero net heat to the environment to achieve a
> lifestyle that is wildly luxurious by today's standards.

True. Energy resources of the future that we can use will exceed by many orders of magnitude (> 10^5) what we actually require.

> If the energy budget is computed to be constrained by global
> warming, then there is no true constraint: we can use lazers
> to beam energy into space to dump the heat. This is a violation
> of the second law of thermodynamics: The earth is not a closed system.
Yes, I understand the thermodynamics part, but I've never seen anyone propose a real "physical system" that would allow the efficient removal of the waste heat/entropy. There are some interesting suggestions regarding slow ballistic transport and fast ballistic transport in Frank & Knight (1997) [ {esp. Sect 7.1}], but I've never seen anyone discuss the nuts and bolts of such an approach.

While I agree that beaming the entropy into space is a great idea, I'm under the impression that there are significant losses when you attempt to drive energy "uphill" (say from waste heat generated by nano-assembly, into light light as laser emissions). The best energy efficiency that I know of for lasers is ~60%, and that is in diodes lasers converting from electricity into light. Unless you can propose a system that is >90% efficient at converting heat into laser energy, I don't think your argument buys us much.

It is certainly true that we could circulate the heat in a cooling fluid into the upper atmosphere, but one faces losses here due to gravity and friction.

I really do think we are limited as far as planetary heat production.