Alan et al:
Last Tuesday, voters in Denver approved a “Green Roof” ballot measure -
despite opposition by many listed in this post-election interview:
http://www.westword.com/news/green-roof-initiative-leader-discusses-big-win-over-denver-developers-9679900
<http://www.westword.com/news/green-roof-initiative-leader-discusses-big-win-over-denver-developers-9679900>
.
I live in the next county over - but am sure I would voted with the
winning side - for reasons given by the initiative leader in the above
interview.
Ron
> On Nov 12, 2017, at 12:44 PM, Alan Robock <[email protected]> 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]
> 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?
>>>>>
>>>>>
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