Please also see https://doi.org/10.1016/j.ecoleng.2016.06.091
On Sat, Nov 11, 2017, 9:20 PM Michael MacCracken <[email protected]> wrote: > Thank you very much. The article is right to the point. > > Mike > > > On 11/11/17 8:12 PM, [email protected] wrote: > > > > Mike, > > > > Please look at the following. It may have answered your question. > > Hosseini, M. and H. Akbari. “Heating Energy Penalties of Cool Roofs: > > The Effect of Snow Accumulation on Roof.” Advances in Building Energy > > Research, 8:1, 1-13, 2014. > > http://dx.doi.org/10.1080/17512549.2014.890541. > > > > > > Hashem > > > > Quoting Michael MacCracken <[email protected]>: > > > >> Also, it can make a difference if you are in a climate zone that gets > >> cold in the winter as the darker roof can help with warming in the > >> winter. I wonder if a good question might be whether having snow on the > >> roof provides an insulating effect in winter that would make up for > >> having a while instead of a black roof? > >> > >> Mike > >> > >> > >> On 11/11/17 7:03 PM, Andrew Lockley wrote: > >>> Being a pedant, it's important to consider the thermal mass of the > >>> roof, as this makes a big difference to temperature variation. > >>> Variations, in turn, greatly increase perception of discomfort - a > >>> draft is cold, a cave is merely cool. > >>> > >>> On 11 Nov 2017 23:27, "John Harte" <[email protected] > >>> <mailto:[email protected]>> 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] <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. > >>> 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. > >>> 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. > > > > > > > > -- > 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. > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. 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