How much heat can we vent into space?

In this thread, matt mentions:

So are there any feasible schemes for preventing global warming by essentially building giant radiators that try and vent heat into space? With some rough back-of-the-envelope figures, how much heat are we looking at and what sort of schemes would make it remotely possible?

One thing that comes to mind is the concept of a solar shade, which is a orbital screen that is held open via rotation which would reflect sunlight away from the earth.

Having ground based reflectors should also work.

Suspending partials in the atmosphere should also work, and a large scale nuclear war should due nicely in causing this.

Going further, the earth itself generates heat from nuclear decay. This is why the middle of the earth is very hot (if you beleive that it’s hot from the formation of the earth, it doesn’t matter for this example). If we expand the use of geothermal power (and heating) we remove heat from deep in the earth and move it to the surface where is it more likely to radiate away.

But we don’t care how hot the center of the earth is, we want to reduce global warming on the surface.

It was a joke, but I’m not one to turn down a back-of-the-envelope opportunity!

According to this thread, the Earth is about 0.85 W per square meter out of energy balance:

Assuming the Earth is a sphere of 12800 km diameter, this amounts to a surface area of 515,000,000 square km, which means the excess energy is about 438000 gigawatts.

Now, I’ll assume that Arwin’s power tower has a hot temperture of 10 deg C (the North sea is COLD) and a cold temperature of -25 deg C, from the table on this page:

That’s actually somewhat more than the 20 deg. difference cited in Arwin’s reference, but let’s run with it…
Turn those temperatures to Kelvin and plug in into the Carnot efficiency expression (T1-T2)/T1, and the Carnot efficiency of such a heat engine would be 35/283, or 12.4%.

Assume our imperfect heat engine only gets about one third of the Carnot efficiency (fairly typical) and the actual efficiency of the thing comes out about 4%.

Now, Arwin’s cite gives an output of 7000 megawatts, or 7 gigawatts. Since its only 4% efficient, 25 times as much heat energy must flow through the thing, or 175 gigawatts. That’s 0.04% of the problem dealt with! We only have to build 2500 hundred of them and we’re home free!

Apologies to anyone who is pained by my use of “energy” when I meant “power”.

It hurts me more than it hurt you.

I don’t think that reflecting the sunlight away or shading the earth from part of it is a good idea. All animal life in earth is ultimately dependent on plants for energy to live and plants need sunlight, not the heat from an electric generating station. With the numbers of people constantly increasing it will get harder and harder to have enough plant life to feed everyone as it is. Reducing the amount of sunlight will make it just that much harder.

On the other hand, maybe that’s a form of population control. A little Malthusian perhaps, but by reducing the sun’s input we could increase infant mortality by malnutrition. Sounds like a plan.