Alan, absent a white or green roof you might benefit as well from a wet black one-- Since your solar panels are designed to soak up as much of the solar spectrum as posible, their temperature may rise 25-50 C when the sun is high.
As the temperature dependence of their internal resistivity naturally reduces their efficiency, and their conductive transparent coatings retard radiative cooling at night , it might be worthwhile to run the numbers on a water misted cooler roof On Sunday, November 12, 2017 at 2:45:12 PM UTC-5, Alan Robock wrote: > > Certainly white roofs and green roofs are not free, and the green ones > require maintenance. > > I have 100% of my roof covered with solar panels, and they require no > maintenance. > > My point was, for the same roof area, are white roofs, green roofs, or a > roof with solar panels the best economic or environmental solution, > making assumptions about the cost of electricity, the source of energy > to heat or cool the home, SRECS, time of year, and climate of the > installation? > > 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] > <javascript:> > New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock > ☮ 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 > > On 11/12/2017 12:24 PM, [email protected] <javascript:> wrote: > > > > Solar panels produce electricity at a cost. > > > > Most cool roofs save you energy and money at no cost. > > > > It is an economic comparison. > > > > Not all the roofs will be covered 100% with solar panels. > > > > Hashem > > > > Quoting Alan Robock <[email protected] <javascript:>>: > > > >> 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] > <javascript:> > >> New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock > >> ? 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 > >> > >> 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/m^2 (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/m^2 *, *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/m^2 to biomass. Over a year, this is (2.5 joules/sec-m^2 ) > >>> x (3.1 x 107 sec) = 77.5 x megajoules/m^2 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/m^2 . 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 10^6 joules (see COW Appendix) and so each year about > >>> 2.4 x 10 > >>> ^6 x 1450 = 3.5 x 10^9 joules/m^2 annually are causing transpiration > >>> rather than heating the house. Expressed in power units, this is > >>> 3.5 x 10^9 (joules/m^2 )/3.1 x 10^7 sec= *113 watts/m^2 , 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/m^2 , 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] <javascript:> <mailto:[email protected] > <javascript:>> > >>> > >>> > >>> > >>>> On Nov 11, 2017, at 2:22 PM, Russell Seitz <[email protected] > <javascript:> > >>>> <mailto:[email protected] <javascript:>>> 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] <javascript:> > >>>> <mailto:[email protected] <javascript:>>. > >>>> To post to this group, send email to > >>>> [email protected] <javascript:> > >>>> <mailto:[email protected] <javascript:>>. > >>>> Visit this group at https://groups.google.com/group/geoengineering. > >>>> For more options, visit 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] <javascript:> > >>> <mailto:[email protected] <javascript:>>. > >>> To post to this group, send email to [email protected] > <javascript:> > >>> <mailto:[email protected] <javascript:>>. > >>> Visit this group at https://groups.google.com/group/geoengineering. > >>> For more options, visit 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] <javascript:>. > >> To post to this group, send email to [email protected] > <javascript:>. > >> Visit this group at https://groups.google.com/group/geoengineering. > >> For more options, visit https://groups.google.com/d/optout. > > > > > > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. 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