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]
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] 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]>:
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]
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] <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.
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]
<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.
