Another tidbit from Boskone. At the Sunday afternoon panel on global warming, one of the panelists brought up the fact that the Second Law of Thermodynamics ensures that power generating systems always generate a great deal of waste heat. He wondered about how the waste heat released into the environment compares to the effects of greenhouse warming from CO2 emissions. Or, to put it another way, if you could develop some sort of magic carbon sequestration program that would capture every gram of greenhouse gas emitted by a coal-fired power plant, would you still have a problem because of the direct heat produced by burning the coal?
My immediate reaction would be to say "no," that is, that the greenhouse gases are a much bigger problem. This is partly because of the localization of the effect-- greenhouse gases spread out over the entire atmosphere, and lead to warming everywhere, while a power plant just heats its immediate neighborhood, and that's more easily contained and mitigated. But it's mostly because of the scale of the numbers involved.
An electric power plant will have an overall efficiency of something like 30%, meaning that for every megawatt of electricity produced, there will be an additional two megawatts released as waste heat. Total world electricity consumption is something like 16 trillion kilowatt-hours annually, which works out to generating about 1.8 trillion watts on a continuing basis, so let's call it three trillion watts (3 x 1012 W) of heat produced by electrical generation.
Greenhouse gases, on the other hand, are (loosely speaking) trapping some fraction of the energy dumped onto the Earth by the Sun. That works out to about 1.7 x 1017 W on average (according to Wikipedia). You don't need to be trapping a large fraction of that to match the direct heating from burning fossil fuels to generate electricity. And, of course, the greenhouse gases only compound any effects from direct heating, because they help retain all of the heat generated at the surface of the planet, whatever the source.
It turns out to be maddeningly difficult to get any more quantitative than this, though. I can easily Google up figures like the average production of CO2 by various types of power plants (roughly 8,000 tons per year for a one-megawatt plant), and find that that's roughly one one-millionth of the total annual emission of CO2, which would be around 0.25% of the total mass of CO2 in the atmosphere (3 x 1015 kg). I'm coming up blank on any kind of warming-per-ton figure, though, probably because it's a fiendishly difficult calculation requiring computer simulations of inflow and outflow and all that fun stuff.
- Log in to post comments
I'm sure I've seen figures for the total production of energy by humans, all of which ends up as heat but for a very small amount packaged in space probes and photons that leave the planet. It's tiny compared to even the slight variation in total solar insolation across a sunspot cycle on average. I'll dig for it if nobody's found it by the time I have time to revisit.
I'm always frustrated in my discussions of global warming with non-scientists because they just don't understand the scale of the discussion. Non-scientists will dismiss global warming -- "It's only one degree!" -- and I say to them, "Do you have any notion how much energy it takes to raise the temperature of the entire atmosphere of the Earth one degree!?"
Non-scientists will dismiss global warming -- "It's only one degree!" -- and I say to them, "Do you have any notion how much energy it takes to raise the temperature of the entire atmosphere of the Earth one degree!?"
Roughly 10^21 joules, or about a hundred years' worth of the world's electricity consumption.
(Assuming you mean one degree C, anyway. About half that if you mean Fahrenheit...)
This reminds me a little of the "Water vapour holds more heat than Carbon Dioxide" argument I hear from AGW denialists once in a while.
Even if you ignore all the other reasons it's wrong, it's still the waste product of burning fossil fuels, so the solution (reducing fossil fuel use) would be nearly identical.
Round numbers are as you said: couple of teraWatts for electricity, bit more for waste heat.
Insolation is about 100,000 TeraWatts (don't forget albedo!)
Carbon emission is about 8 GigaT, of which roughly half remains resident.
CO2 reservoir is 3000 GigaT
Doubling the Carbon is about 1% forcing, but emissivity goes like T^4 so you give back an order of magnitude for the 1/4 in the linear expansion of delta T and another 1/2 for geometric effects.
So... energy forcing is ~0.1% per carbon doubling in the linear approximation or 100 TeraWatts per 3000 GigaT
So current cumulative Carbon emissions are factor of several more important than ongoing power production and Carbon forcing becomes increasingly dominant.
Note that to match CO2 doubling power generation would have to be about factor of 20 greater - it is highly inadvisable to let sustained power generation exceed 100 TW on the surface of the Earth (I always though this was one of the better arguments for going to space - there are interesting things to do that require much more than 100TW...)
Note also this is close to what we would need to bring world to US consumption at current efficiency levels!
You gain a bit by two factors power stations as you note are localised, and hot spots emitt more efficiently because of the T^4
Also the higher temperature emission peaks outside the CO2 absorption bands (but might move straight into some H2O bands instead...)
It is a nasty problem. Be nice if people like the CEO of GM were numerate enough to comprehend it.
Minor quibble: almost all of the energy produced by a power station will eventually end up as waste heat, not just the portion lost as waste heat during generation...
Still, that doesn't change the fact that (a) it's orders of magnitude less than the heat trapped by increased CO2 levels, and (b) it would be radiated into space anyway, if it weren't for (a).
PV = energy, 101.326 J/liter-atm. Carbon sequestration would be an immense energy hog consuming at least 30% the power output of a fossil fuel plant. An advocate makes virtue of failure. The worse the cure the better the treatment - and the more that is required. Hunger is due to weather, famine is due to politics.
With the US consuming 70% of terrestrial resources and China consuming 70% of terrestrial resources, one anticipates an ongoing need for Federal subsidies.
One of us has slipped a digit. 16 trillion kWh is about 6e19 Joules, so that's just less than 10 years of electricity production waste heat. But raising that atmosphere's temperature isn't the right metric, since the land and water will tend to heat up as well.
It gets more complicated, because most of the waste heat is disposed of in the form of hot water vapor, which convects to an unknown altitude before either mixing with the surrounding atmosphere or radiating directly into space.
Also, wind, hydro, and solar energy effectively produce negligible waste heat.
This is off subject but sorta related. Do photovoltaics have a cooling effect by converting sunlight into electricity?