My solar panels only occupy about 20% of my total roof area. Both are important.
John Harte Professor of Ecosystem Sciences ERG/ESPM 310 Barrows Hall University of California Berkeley, CA 94720 USA [email protected] > On Nov 12, 2017, at 8:21 AM, Alan Robock <[email protected]> wrote: > > Wouldn't solar panels on your roof be preferable? Obviously they would > create energy for you. But they would also shade the roof in the summer, > preventing almost all sunlight from reaching it. One would then have to > figure out the additional downward longwave from them to the roof, estimating > the temperature of the bottom of them and their emissivity. Does anyone know > of such a calculation? In the winter, the longwave would be good, as it > would make up for the missing Sun. > > Ignoring the initial cost of the solar panels, would this be cost-effective > in terms of cooling and heating a house? And if the cost were distributed > over time, and accounting for the electricity you would generate, how long > would they take to pay for themselves? In NJ we get SRECS of about $0.20 per > kWh in addition to the electricity, but that changes with the market. And > currently the Federal tax credit pays for 1/3 of the initial cost. > Alan > > Alan Robock, Distinguished Professor > Editor, Reviews of Geophysics > Department of Environmental Sciences Phone: +1-848-932-5751 > Rutgers University Fax: +1-732-932-8644 > 14 College Farm Road E-mail: [email protected] > <mailto:[email protected]> > New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock > <http://envsci.rutgers.edu/~robock> > ☮ http://twitter.com/AlanRobock <http://twitter.com/AlanRobock> 2017 > Nobel Peace Prize to ICAN! > Watch my 18 min TEDx talk at http://www.youtube.com/watch?v=qsrEk1oZ-54 > <http://www.youtube.com/watch?v=qsrEk1oZ-54> > On 11/11/2017 6:27 PM, John Harte wrote: >> I assigned that problem as a homework assignment in a course I teach. >> >> >> 2. Consider a house in a relatively hot, sunny location such as Southern >> California. >> >> a. To keep the house cool without air conditioning, and thereby reduce >> energy demand, its inhabitants decide to do one of two things: >> >> i. They can paint the roof white, increasing its albedo from 0.1 to 0.8, >> or >> >> ii. They can grow a green roof, using a productive species of grass that >> will increase the albedo of the roof from 0.1 to 0.2 and that, if watered >> and fertilized adequately, will cool the house by transpiration. The rate >> of transpiration can be estimated from the following: for every kg of grass >> produced, 300 kg of water are transpired, and the grass grows with an >> overall photosynthetic efficiency of 1%. >> >> a. Ignoring the issue of water supply, which of these strategies (i. or ii.) >> will result in a cooler house? (20 pts.) >> >> Solution: 2. a. First, let’s examine the effect of painting the roof white. >> We’ll assume an average solar flux on the roof of 250 watts/m2 (if you >> assumed anything between 170 and 300 we will accept it.). By changing the >> albedo from 0.1 to 0.8, the home is avoiding the absorption of 0.7 (250) = >> 175 watts/m2, which is the benefit of plan i. For plan ii., we need to >> estimate NPP on the roof first. At 1% of available energy, the plants are >> converting 2.5 watts/m2 to biomass. Over a year, this is (2.5 joules/sec-m2) >> x (3.1 x 107 sec) = 77.5 x megajoules/m2 incorporated into biomass. Using >> the conversion: of 16 megajoules(dry biomass) per kg, we find that biomass >> is produced at an annual rate of 77.5/16 = 4.8 kg (dry biomass0/m2. Now >> using the 300:1 ratio of transpired water to photosynthesized biomass, we >> get 4.8 x 300 = 1450 kg(transpired water)/year. Transpiring a kilogram of >> water requires about 2.4 x 106 joules (see COW Appendix) and so each year >> about 2.4 x 106 x 1450 = 3.5 x 109 joules/m2 annually are causing >> transpiration rather than heating the house. Expressed in power units, this >> is 3.5 x 109 (joules/m2)/3.1 x 107 sec= 113 watts/m2, which is the >> transpiration benefit of plan ii. But there is also a small albedo >> benefit of grass versus dark shingle, so we get an additional benefit which >> is 1/7 of the plan i. benefit (due to an albedo increase of 0.1 rather than >> 0.7), so now we have 113 + (1/7) 175 = 138 watts/m2, which is the albedo >> benefit of plan ii. So plan i. wins by a little. >> >> >> The problem went on to evaluate the added benefit if you burn the grass on >> the roof for fuel. >> >> I actually replaced my dark shingle roof this autumn with light-colored >> composition shingle. It makes a huge difference! >> >> >> >> John Harte >> Professor of Ecosystem Sciences >> ERG/ESPM >> 310 Barrows Hall >> University of California >> Berkeley, CA 94720 USA >> [email protected] <mailto:[email protected]> >> >> >> >>> On Nov 11, 2017, at 2:22 PM, Russell Seitz <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>> How do green roofs, which cool by evapotransportation ( rooftop lawns >>> require water much as those on the ground do) compare in cooling efficiency >>> with higher albedo white roofs combined with an equal volume of water >>> spraying when the sun is high? >>> >>> On Saturday, November 11, 2017 at 12:16:10 AM UTC-5, E Durbrow wrote: >>> >>> Perhaps, tangental. Seville planners think they can cool their city despite >>> significant temperature increase with 204-700 hectares of green roofs. >>> >>> Summary: >>> >>> https://www.sciencedaily.com/releases/2017/11/171110113938.htm >>> <https://www.sciencedaily.com/releases/2017/11/171110113938.htm> >>> >>> >>> Comment: My layperson’s understanding is that it is very difficult to >>> predict and simulate city-wide changes in temperature when a modification >>> (e.g. reflective roofs, green space, etc) occurs. I though I remember that >>> reading that reflective roofs might have no effect on local temperature >>> (city’s micro-climate). Modelers, is this the case? >>> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "geoengineering" group. >>> To unsubscribe from this group and stop receiving emails from it, send an >>> email to [email protected] >>> <mailto:[email protected]>. >>> To post to this group, send email to [email protected] >>> <mailto:[email protected]>. >>> Visit this group at https://groups.google.com/group/geoengineering >>> <https://groups.google.com/group/geoengineering>. >>> For more options, visit https://groups.google.com/d/optout >>> <https://groups.google.com/d/optout>. >> >> -- >> You received this message because you are subscribed to the Google Groups >> "geoengineering" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected] >> <mailto:[email protected]>. >> To post to this group, send email to [email protected] >> <mailto:[email protected]>. >> Visit this group at https://groups.google.com/group/geoengineering >> <https://groups.google.com/group/geoengineering>. >> For more options, visit https://groups.google.com/d/optout >> <https://groups.google.com/d/optout>. > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. 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