Comments on: Twenty megawatts in your hands http://metcaffeination.net/weblog/2008/11/13/twenty-megawatts-in-your-hands/ cities. physics. food. environment. fatherhood. Wed, 13 Apr 2011 18:00:00 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: thm http://metcaffeination.net/weblog/2008/11/13/twenty-megawatts-in-your-hands/comment-page-1/#comment-3411 thm Sat, 15 Nov 2008 13:11:46 +0000 http://metcaffeination.net/weblog/?p=177#comment-3411 Also, <a href="http://www.teslamotors.com"Tesla Motors</a rel="nofollow">, the subject of <a href="http://www.infosnack.org/2008/06/one-reason-electric-cars-wont-be-as.html" rel="nofollow">Michael's posts</a> <a href="http://www.infosnack.org/search/label/electric%20cars"mentioned above</a rel="nofollow">, make some overall <a href="http://www.teslamotors.com/efficiency/well_to_wheel.php"well-to-wheel efficiency</a rel="nofollow"> calculations that favorably compare their car to others. Also, , the subject of Michael’s posts , make some overall calculations that favorably compare their car to others.

]]> By: thm http://metcaffeination.net/weblog/2008/11/13/twenty-megawatts-in-your-hands/comment-page-1/#comment-3401 thm Sat, 15 Nov 2008 04:00:04 +0000 http://metcaffeination.net/weblog/?p=177#comment-3401 You're limited by what's known as the <a href="http://en.wikipedia.org/wiki/Carnot_efficiency#Efficiency" rel="nofollow">Carnot efficiency</a> for any case, which is the thermodynamic upper limit for the efficiency of creating mechanical energy, or electricity, from heat. It depends on how hot you can burn, and how cold you can exhaust at. According to Wikipedia, the theoretical limit for a steel engine would be <a href="http://en.wikipedia.org/wiki/Internal_combustion_engine#Energy_Efficiency" rel="nofollow">37% efficient</a>; to get higher efficiency you'd need temperatures hotter than steel can handle. But with clever engineering, modern combined cycle power plants, burning natural gas, can get <a href="http://en.wikipedia.org/wiki/Combined_cycle" rel="nofollow">efficiencies of 60%</a>. Old coal-fired plants are probably only marginally better than car engines. That said, electric motors really are much better than gasoline motors. The efficiency of gasoline motors <a href="http://en.wikipedia.org/wiki/Power_band" rel="nofollow">depends strongly on what speed</a> they're running at; this is why cars need transmissions. <a href="http://en.wikipedia.org/wiki/Electric_power_transmission"Power transmission losses</a rel="nofollow"> are estimated at around 7%. I'm not sure what role electric cars should play, but <a href="http://www.theoildrum.com/node/4301" rel="nofollow">(re-)electrification of our railroads</a> is a very good idea. You’re limited by what’s known as the Carnot efficiency for any case, which is the thermodynamic upper limit for the efficiency of creating mechanical energy, or electricity, from heat. It depends on how hot you can burn, and how cold you can exhaust at. According to Wikipedia, the theoretical limit for a steel engine would be 37% efficient; to get higher efficiency you’d need temperatures hotter than steel can handle. But with clever engineering, modern combined cycle power plants, burning natural gas, can get efficiencies of 60%. Old coal-fired plants are probably only marginally better than car engines.

That said, electric motors really are much better than gasoline motors. The efficiency of gasoline motors depends strongly on what speed they’re running at; this is why cars need transmissions. are estimated at around 7%.

I’m not sure what role electric cars should play, but (re-)electrification of our railroads is a very good idea.

]]> By: Ken Monahan http://metcaffeination.net/weblog/2008/11/13/twenty-megawatts-in-your-hands/comment-page-1/#comment-3395 Ken Monahan Fri, 14 Nov 2008 21:16:09 +0000 http://metcaffeination.net/weblog/?p=177#comment-3395 Can you give me an idea of what the scale economies would be? Presumably massive power plants create energy from fossil fuels more efficiently than do internal combustion engines and so by making all cars electric you should be transferring those scale economies to the drivers of cars or is there some ineffciency in the use of electric motors or in transmission of power that would negate the savings? Can you give me an idea of what the scale economies would be? Presumably massive power plants create energy from fossil fuels more efficiently than do internal combustion engines and so by making all cars electric you should be transferring those scale economies to the drivers of cars or is there some ineffciency in the use of electric motors or in transmission of power that would negate the savings?

]]> By: Michael Perkins http://metcaffeination.net/weblog/2008/11/13/twenty-megawatts-in-your-hands/comment-page-1/#comment-3392 Michael Perkins Fri, 14 Nov 2008 18:25:13 +0000 http://metcaffeination.net/weblog/?p=177#comment-3392 Thanks for the calculation. I did one on my site with similar results. I didn't look into the heat dissipation aspect, though, which was interesting. Thanks for the calculation. I did one on my site with similar results. I didn’t look into the heat dissipation aspect, though, which was interesting.

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