<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>On Apr 7, 2011, at 4:31 AM, Eugen Leitl wrote:</div><div><br></div><div><div style="font-size: 36px; "><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; font-family: Verdana; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; font-size: medium; ">even terrestrial solar flux is about a factor of 6000 in excess our needs, and 2000 in excess of our<br>needs just considered land area.</span></blockquote><div><br></div>It's true that the total energy that humans use only accounts to one part in 6000 of the solar energy hitting the planet, but that's still a lot, about 47 terawatts (17 of that from fossil fuels) if you include solar powered green plants that humans use for fuel or food.</div><div style="font-size: 36px; "><br></div><div style="font-size: 36px; ">Axel Kleidon has calculated the thermodynamic limits of what heat engines and other mechanisms powered by the sun could theoretically produce on the Earth at:</div><div style="font-size: 36px; "> </div><div style="font-size: 36px; "><a href="http://arxiv.org/PS_cache/arxiv/pdf/1103/1103.2014v1.pdf">http://arxiv.org/PS_cache/arxiv/pdf/1103/1103.2014v1.pdf</a></div><div style="font-size: 36px; ">and</div><div style="font-size: 36px; "><a href="http://www.earth-syst-dynam.net/2/1/2011/esd-2-1-2011.html">http://www.earth-syst-dynam.net/2/1/2011/esd-2-1-2011.html</a></div><div style="font-size: 36px; "><br></div><div style="font-size: 36px; ">The maximum theoretical amount of power you could extract from wind, is in the range of 18–68 TW, and if you took it that far the environmental change caused by disruption of wind patterns would be considerable. </div><div style="font-size: 36px; "><span class="Apple-style-span" style="font-family: Helvetica; font-size: 10px; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; "><br></span></font></span></div><div style="font-size: 36px; "><span class="Apple-style-span" style="font-family: Helvetica; font-size: 10px; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; ">Next he figured out what you could realistically expect from biofuels given that the </span></font></span><span class="Apple-style-span" style="font-family: Helvetica; font-size: 10px; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; ">overall photosynthetic efficiency of plants is only</span></font></span><span class="Apple-style-span" style="font-family: Helvetica; font-size: 10px; "><span class="Apple-style-span" style="font-family: Verdana; font-size: 36px; "> 3 to 6 percent of the solar radiation they receive. The most you could get is about 215 TW.</span></span></div><div style="font-size: 36px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; "><span class="Apple-style-span" style="font-family: Verdana; font-size: 36px; "> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; "><span class="Apple-style-span" style="font-family: Verdana; font-size: 36px; ">Gravitational engines, tides caused by the sun and moon, could give you another 5TW.</span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; "><font class="Apple-style-span" face="Verdana" size="7"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; font-size: 36px; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; "><br></span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; font-size: 36px; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; ">Geothermal: Given that the heat flux from the interior to the surface is less than 0.1 W m^2</span></font><span style="font: normal normal normal 7.4px/normal Helvetica; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; "> the efficiency would be very low and only about 40 TW of free energy could be obtained.</span></font></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; font-size: 36px; "><span style="font: normal normal normal 7.4px/normal Helvetica; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; "><br></span></font></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; font-size: 36px; "><span style="font: normal normal normal 7.4px/normal Helvetica; "><font class="Apple-style-span" face="Verdana" size="7"><span class="Apple-style-span" style="font-size: 36px; ">And given the fact that the population is increasing and the number of people demanding to live a good middle class lifestyle is increasing even faster the </span></font></span><span class="Apple-style-span" style="font-family: Verdana; ">47 terawatts that humans currently use is certain to increase, and increase considerably. Unless we have a technological breakthrough long term nuclear fission is the only technology that is economically and thermodynamically feasible. </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; font-size: 36px; "><span class="Apple-style-span" style="font-family: Verdana; "><br></span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 10px/normal Helvetica; font-size: 36px; "><span class="Apple-style-span" style="font-family: Verdana; "> John K Clark </span></div></font></div></span></font></div></div></div><div><br></div><div><br></div></div></body></html>