Hi All
Chameleons have got there first.
Put two sheets of polythene together and run close lines of ultrasonic
seem-welding to form an array of small diameter tubes. You can then pump
air or fluids of different colours at different times of day to suit
temperatures or your mood. District housing groups near airports can
also sell advertising space.
Stephen
Emeritus Professor of Engineering Design. School of Engineering,
University of Edinburgh, Mayfield Road, Edinburgh EH9 3DW, Scotland
[email protected], Tel +44 (0)131 650 5704, Cell 07795 203 195,
WWW.homepages.ed.ac.uk/shs, YouTube Jamie Taylor Power for Change
On 13/11/2017 00:19, Russell Seitz wrote:
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://envsci.rutgers.edu/%7Erobock>
☮ 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 11/12/2017 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://envsci.rutgers.edu/%7Erobock>
>> ? 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/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>
>>>>
<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?
>>>>
>>>>
>>>> --
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