Many thanks John, but please run the numbers for a white roof with a high noon sprinkler: same daily evaporation as in your original case , plus larger albedo gain.
The question is whether the grass is necessary , if you've got the water. While lawns are a good thing, they weigh quite a lot, and require much the same waterproof membrane technology as white roofs, plus increased structural investment, and thermal mass. On Saturday, November 11, 2017 at 6:27:29 PM UTC-5, jharte 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] <javascript:> > > > > On Nov 11, 2017, at 2:22 PM, Russell Seitz <[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 >> >> >> 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:>. > 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. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout.
