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The albedo effect

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Editor’s Note: Scientific American‘s George Musser will be chronicling his experiences installing solar panels in Solar at Home (formerly 60-Second Solar). Read his introduction here and see all posts here.

Someone commenting on one of my recent posts posed an interesting question:

I wonder how much the albedo change of your roof offsets gains from electricity, much of the suns short wave energy must be being converted to heat therefore enhancing greenhouse   (as well as producing some electricity) can you prove you are actually energy balance positive???

In other words, solar panels both help the climate (by reducing the need to burn fossil fuels) and hurt it (by absorbing sunlight and warming the surface). Does the benefit really outweigh the cost? Nathan Myhrvold also raised this issue in a New York Times blog last week.

Silicon solar cells convert about 1/6 of incident sunlight into electricity and dissipate most of the remaining 5/6 as heat. So in terms of their direct climate effect, they have an albedo, or reflectivity, of 1/6. This is comparable to the albedo of standard asphalt shingles, so for most people, installing solar panels doesn’t have a net heating effect. But I had just restored my 1868-vintage tin roof and painted it white, giving it an albedo of about 5/6. So my panels do warm the planet.

The increase in heat absorption is a one-off climate forcing, equivalent to adding a certain amount of carbon dioxide to the atmosphere. By comparison, fossil power generation is an ever-increasing climate forcing, since each unit of energy entails the emission of more carbon dioxide. So there is some breakeven point beyond which the solar panels help the climate.

Let’s do a quick back-of-envelope calculation. My system is rated at 3 kW of DC power, so it dissipates on the order of 104 W of heat. Its area is about 10 square meters, about one part in 1014 of the Earth’s surface, so the system amounts to a global climate forcing of about 10-10 W per square meter. To convert this to a carbon-dioxide equivalent, note that the cumulative industrial emissions of 200 billion tonnes of carbon dioxide have produced a forcing of 1.5 W per square meter, or about 10-11 W per square meter per tonne.

Therefore by installing panels, I have had the same effect as releasing 10 tonnes of carbon dioxide. Burning fossil fuel emits roughly 1 tonne of carbon per MW-hr of electricity, so we need to produce 10 MW-hr to offset the albedo decrease. This will take about three years.

This is very rough and I’d be interested in other back-of-envelope calculations — please let me know if I made a mistake. (Not that I have to implore readers of a blog to point out mistakes.)

I asked Gavin Schmidt of NASA GISS and he did the calculation a slightly different way. He imagines that it would take a photovoltaic array about 200 kilometers square to power the entire U.S. and estimates the albedo forcing to be about 0.01 W per square meter, equivalent to about 1 ppm of atmospheric carbon dioxide, which the U.S. adds to the atmosphere in about three years.

So the albedo effect is not negligible. It does need to be taken into account when calculating the payback period of solar energy. But it is hardly a deal-breaker.

Dark solar panels on George’s white roof





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  1. 1. kp81 1:22 pm 10/27/2009

    One frequently overlooked part of the calculation for greenhouse gas payback is the amount of greenhouse that is created in the production of PV panels. My understanding is that, yes you still break even over the lifetime of the solar cells, but its longer than 3 years (more like 10?).
    It’s always more effective to NOT use the electricity in the first place…it’s usually impossible to shed all electricity use…so the key is to use as little as possible and that way you dont need as big of a system to cover the gap. :)

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  2. 2. candide 2:10 pm 10/27/2009

    Does the carbon footprint for fossil fuels include ALL the secondary and tertiary items needed for their production?

    Does it include items like the plants that make the steel and concrete for refineries and drilling platforms? Is the fuel burned by support vehicles included?

    Carbon "footpronts" are, at best, a rough estimate.

    Solar, wind and others renewable sources are cleaner and will only become more clean with a better return as development continues.

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  3. 3. lamorpa 2:16 pm 10/27/2009

    What are you talking about? The manufacturing process with the most concentrated and toxic byproducts is the process of making silicon solar cells.

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  4. 4. jimdoc 5:22 pm 10/27/2009

    The article seems to assume that the heating of the solar panel is transmitted directly to the house’s interior. It is NOT. Properly installed, there will be an air gap between most of the panel and the roof, and the air heated by the panel will convect up and out with little transfer to the roofing. Better still, put a passive hot water heating panel between the PV panel and the roof, and get some of your domestic hot water heat that way. Furthermore, the roof underlying all this SHOULD be super-insulated, in which case very little of the heating at its outer surface will be transmitted to the home’s interior.

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  5. 5. egghead1619 5:44 pm 10/27/2009

    I am curious about the recent plans for ‘beaming’ power to Earth from space. It seems to me that this would add a significant amount of heat since the satellite is gathering energy that never would have made it to Earth in the first place.

    Yet, I’ve heard praises from the same communities that are very concerned with the global climate. I’ve not seen any articles digging into such energy collection technologies and would love to learn more about them.

    I am concerned that by deploying such a solar collector in space and transmitting that energy to Earth will only exacerbate any changes to the local and global climate.

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  6. 6. aim42 8:41 pm 10/27/2009

    The calculation is correct as far as it goes. Its problem is that the roof panels produce 3kW PEAK DC power. The insolation (the amount of sunlight) is ~ 1.366 kW/m2 at the top of the atmosphere. By the time you take day/night averaging and atmospheric transmission into account the AVERAGE power at the point on the ground is more like 0.3kW/m2 on a sunny day. It could be as low ..03kW/m2 in winter and overcast day. This means that in winter and under adverse weather the solar system produces less than 1/10 of its rated PEAK power. When you take these into account the calculation is too low by a factor of 10 or so on the carbon footprint. At that point the use of solar becomes less attractive.

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  7. 7. aim42 8:44 pm 10/27/2009

    The transfer of the heat to the house does not mitigate the fact that you trap additional heat on Earth. We are at delicate balance with respect to that since the only way for earth to cool is by radiation into space. It is a fallacy to consider only carbon footprint. Thermal footrpint and circulation impacts must be considered as well.

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  8. 8. longri 9:07 pm 10/27/2009

    Before bolting anything to the back of a solar panel to get hot water at the same time, check your warrantee on the panels!!! I just avoided a big mistake.

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  9. 9. Michael Hanlon 2:45 am 10/28/2009

    Here’s what I’ll call the certainty principle. If you have a number system and want to count electrons, once they are counted, you no longer have a number system. Basic Quantum 101
    I agree with Jokunen, please restate the system as installed.
    .The definition of Quality is conformance to specification, not whether it’s better than another system. So, if you signed up for a 55 mpg auto and got one that did 57, you can’t complain about the color because it wasn’t specified. Had you done so, I’m sure you could have specified a system with other albedo numbers. but don’t attempt to count them.
    .The idea of adjustable angles is a correct one for Northern Latitudes. You must blame you local code people or even the State of NJ for not requiring they (brackets) be part of any installation specification.
    .And that $0.01-0.02 amount of electricity can be directly related to fossil fuels not burned, Better to measure this one not in electrons but in gallons of crude oil not oxidized.

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  10. 10. ssm1959 6:00 pm 10/28/2009

    Please enter into your equation the industrial emissions of CO2 to make the panels in the first place. Remember 90% of the environmental damage caused by anything you buy is in its manufacturing costs not its use.

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  11. 11. gmusser 10:09 pm 10/28/2009

    True, I should have used the average rather than peak heat dissipation rate. This reduces the albedo effect on climate and therefore the payback time, making solar *more* attractive.

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  12. 12. Michael Hanlon 11:42 pm 10/28/2009

    Is the issue of heat generation on the Earth’s surface a part of the "greenhouse" effect? That little plum got tied to the arguement in the question you re-posed. It may be an effect but it is not greenhouse exactly, which I thought was atmospheric CO2 content. The issue of heat generation is another aspect, not this one, of "Global Warming" and I think you were suckered into mixing apples and windmills.
    .So, in addition to the providing of "as is" specifications, could a distinction, if any, be made of the issue you intend to battle with your solar panel installation?

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  13. 13. wayt 12:25 am 10/29/2009

    George’s post, and JimDoc’s comment raises an interesting question: why aren’t all solar PV panels also solar thermal panels–that is, why don’t we tap routinely capture the heat they produce as well as the electricity, so that we offset fossil fuel use in two ways at once? Surely this would reduce the "carbon debt" that takes ~3 years of operation to pay off.

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  14. 14. ckmapawatt 2:58 pm 10/29/2009

    To all of those

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  15. 15. ckmapawatt 3:03 pm 10/29/2009

    To all of those talking about having to include the affect of solar power manufacturing, you then have to subtract any manufacturing costs of an alternative (i.e. coal).

    What was emitted getting the coal out of the ground and to the power plant?

    And here’s the kicker. A solar cell is only manufactured once. Coal has to continued to be "manufactured" over and over again to get it to the power plant.

    So you have to calculate the life of the panel, then figure out how much power the panel will produce over its life. Then you need to figure out how much coal is required to produce the same amount and then figure out the "manufacturing" costs of getting that coal out of the ground and to the power plant.

    I guarantee you 100% that getting the coal out of the ground and to the factory causes waaaaaay more emissions than manufacturing the panels.

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  16. 16. billhunter 10:46 pm 10/29/2009

    You have neglected the heat emitted by the burning Fossil fuel ( not the CO2, but the raw heat of combustion) which should be significant in the comparison.

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  17. 17. Michael Hanlon 11:27 pm 10/29/2009

    Don’t forget to add the breathing by all the workers involved in the digging. In coal, They don’t breathe as long ’cause of black lung but that worker gets replaced by another so consider quality of life in the sum. In the PV Cell factory, the workers last longer and don’t have to respire as much cause the works is easier, cutting back emmission of CO2.
    The PV/ solar hot water combination is a poser. A pro for it would be to circulate a clear glycol solution through the panel and then extract it using a heat pump. The con though is that for efficient solar hot water ganeration, you need to focus the sun’s energy to a point using a parabolic reflector. The only way to make it work was if someone comes up with a curvable silicon junction, either fixed or flexible. That way you would convert some of the incidental rays to e**-1 (filling the Bell Lab s’ holes) and reflect the rest aimed into the glycol and circulated away to be recovered at the pump. Hmmm. run the pump with the watts from the PVs and hope the PVs can stand the greater than 212 degrees involved..

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  18. 18. billhunter 12:21 am 10/30/2009

    Solar hot water heaters do not require parabolic mirrors. You are confusing energy generation from solar hot water heating, and plain old hot solar hot water heating for residential use, which is what the OP probably intended.

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  19. 19. LiamRyan 3:35 pm 10/30/2009

    aim42 at 08:41 PM on 10/27/09

    The calculation is correct as far as it goes. Its problem is that the roof panels produce 3kW PEAK DC power. The insolation (the amount of sunlight) is ~ 1.366 kW/m2 at the top of the atmosphere. By the time you take day/night averaging and atmospheric transmission into account the AVERAGE power at the point on the ground is more like 0.3kW/m2 on a sunny day. It could be as low .03kW/m2 in winter and overcast day. This means that in winter and under adverse weather the solar system produces less than 1/10 of its rated PEAK power. When you take these into account the calculation is too low by a factor of 10 or so on the carbon footprint. At that point the use of solar becomes less attractive."

    I don’t know where you got the insolation number of 300w/m2, but as a solar installer, I have access to irradiance meters. On a sunny day, irradiance, or insolation is about 1000w/m2, and I have seen it as high as 1250w/m2. However, because of night time, a we see about 5.5 peak hours of sunlight a day here in California, you never talk about power production in system size. As far as albedo goes, there is wide variance in solar modules. Some are black, some are 30% white. On any roof, he solar panels shade the roof. The asphalt comp roofs are too hot to touch in the sun, but the ones under the array are quite cool. With a 6 in gap between the roof and the array there is a convection air flow out the top. We have also installed a prototype system that boxes in the top and sides of the array and captures the hot air underneath to war the house and to heat water via a heat exchanger. a lot of time and material went into this system, but is very efficient.
    but really, albedo schmedo. To say that solar is a bad idea because it reflects energy is a crock. If we were really concerned about that , then we should stop making black roads. Solar makes power from the sum. Does your car? No. Does your swimming pool? No. Solar, in some cases does take a while to pay itself back, but it is the only thing that ever will! Your car will never pay you back. Neither anything else. Solar PV power is in a class of its own. The class of things in this world that will provide a net energy benefit. As long as the sun still shines solar wins every time. burning fossil fuels wastes up to 40% in transmission. Wind power is not distributed every where, but solar is. All you need is an antenna to pick up that free power from the sun.

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  20. 20. nicetouch99 5:10 pm 10/30/2009

    The problem with CO2, Methane, and other greenhouse gases is that they trap the heat. So if you reduce the greenhouse gases then more heat is radiated back into space. If you increase the heat radiated back into space then heat generation itself becomes less of problem doesn’t it. This situation doesn’t seem to be addressed in this discussion.

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  21. 21. ormondotvos 10:47 pm 10/30/2009

    The point is not that the panel heats the house. It’s that the panel absorbs heat the would be re-radiated into space and thus not heat the Earth. Albedo is the techie term for whiteness, from the Latin "alba": If the earth was all white, it would be cooler. If it were all black, it would be hotter, because black things absorb heat. Sooo, when you put an essentially black panel on your roof, it only converts a sixth of the new heat to electricity. I’d suggest that you paint the rest of your roof with titanium dioxide white paint, the coolest and brightest roof coating available. Like this:

    http://www.amazon.com/gp/product/B002PJ236I

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  22. 22. Innov8or 4:35 am 10/31/2009

    Don’t worry. Solar panels in space beaming energy to earth are totally impratical. First the cost is extreemly high. Cost of putting it up are astronomcal. Cost of ground receiving stations will be almost as much as cost of ground based solar. Second the beam is dangerous, birds flying into the beam will be cooked like in a microwave oven. It will never be acceptable even though people and planes will not be allowed in the beam. What happens if there is a mal-function and the beam wanders over a city? Or if a terroist group hijacks the control and deliberately aims the beam at a city.

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  23. 23. evergreensolarinstaller 10:47 am 10/31/2009

    OK, The roof heats up due to bombardment from the sun all day,. the roof heats up and traps that heat in the attic where the temperature often reaches between 140 and 154 degr. F that heated air is heating all the toxins in the asphalt shingles and plywood glues and sap in rafter timbers as well as insulation and creating a toxic brew that is then naturally vented by soffit and ridge vents into the air, Now add a protective layer of PV panels, the PV modules do heat as does the roof, yet the 4" air flow beneath the modules allows for cooling and in a sense; discharge of a form of clean carbons with out the toxins from roofing material being added..so from my point of view the argument is dumb! and another way for people to deny the benefits of Solar energy…

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  24. 24. Michael Cook 11:25 am 10/31/2009

    The problem the albedo effect creates is that white things are so vulnerable to dust and soot. In fact, the real reason for summer Arctic ice meltback soaring up until three years ago may be increased soot and dust from Asia carried by the prevailing winds and dirtying surface ice and snow, causing vastly increased efficiency of solar melting during the days when the sun is in the sky 23 hours per day.

    So, if you paint your roof white, be prepared to burn some energy power washing it every so often.

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  25. 25. Michael Hanlon 1:54 am 11/1/2009

    Why are the Dang things on the roof in the first place? It’s inaccessible. The insurance associated with the height is also elevated. I suggest a carport type a4rrangement over your driveway. You could easily clean, adjust and perform other maintenance or up-grades at the lower level. In fact, tear up the lawn, put ‘em there and save the carbon that is emmissioned by the mower at a quantity just below autos, No more, "Honey mow the law, I don’t care if the game is on!" It’ll change to, "Darling the sun’s out and we’re generating too much juice, I;ve gotta run the TV so we don’t over charge the batteries."

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  26. 26. evergreensolarinstaller 9:07 am 11/1/2009

    TESTING<…Trouble sending post?..

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  27. 27. grizzlybowman 1:33 pm 11/3/2009

    Uh huh, and isn’t the difference here that the heat generated by your solar panels does not have the effect of molecular vibration that CO2 does: Trapping heat in the Earth’s atmosphere?

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  28. 28. labellaflora 6:42 pm 11/5/2009

    Why hasn’t there been a push for lighter colored roofs? It would show more dirt I suppose, but they wouldn’t have to be white. I’m old enough to remember when it was the fashion to put white rock on the roof. (I’m in CA.)

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  29. 29. derooney56 6:35 pm 11/6/2009

    While I agree that you can’t use the peak DC output of the panel it is also true that when the panel is not producing electricity it is also not contributing to heating of the
    atmosphere. I suspect that a detailed simulation would be the best way to get at reasonably accurate numbers.

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  30. 30. Michael Hanlon 10:59 pm 11/6/2009

    Put the panels on the ground and put mirrors on out roofs (rooves?)

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  31. 31. carterjo 4:05 am 11/8/2009

    Hi al, l and thanks for the comments re my albedo question.
    My guess is somebody needs to build a solar net energy calculator that takes all into account (possibly a PhD project); background and panel albedo, direct and diffuse beam panel efficiency, construction costs in terms of CO2, (including, install and transport costs, mounting structure, wiring and inverter) life time of panel and panel efficiency as a function of time, (local climate/ solar radiation conditions). The net effect of CO2 with time may also need to be considered.

    I suspect the results should be defined in several ways (years to payback, lifetime watts or Mj of global warming/cooling, the CO2 equivalents of energy term. Years to $ payback (with some assumption about electricity prices, and units of greenhouse savings per $ invested integrated over the panel lifetime

    Such a calculator would provide a balanced view and perhaps help buyers choose between low cost low efficiency and high cost high efficiency units. It would be nice to be calculate the real cost of the energy you are substituting in the same way.

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  32. 32. tpetscha 12:30 pm 11/9/2009

    In my mind, the probem with this equation is the fact that the albedo is converted into "CO2" equivelant. I don’t belive that the radiant heat reflected back into the atmosphere is comperable to sending heat trapping CO2 from fossil fuel combustion into the atmosphere because CO2 retains radiant heat. CO2 is not radiant heat and albedo is not a heat trapping compound comparable to CO2. This calculation is comparing radiant heat (albedo) to a heat trapping compound(CO2), which I feel is inherently flawed.

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  33. 33. Sybil 3:37 am 11/11/2009

    I am confused. Do you ealy mean we can reduce the effect of global warming by painting roofs and road white. Has anybody make some calculation about it?

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  34. 34. disdaniel 1:42 am 11/12/2009

    You are asking if burning fuel year round at the power plant (be sure to include the transmission losses…) generates less "heat" than putting dark colored panels on your average roof???

    Any there are 33 responses that dont all start of with LOL?

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  35. 35. Michael Hanlon 10:40 pm 11/12/2009

    Sorry I didn’t post this smarty bit at more than one site. I erringly thought everyone would catch up to it. I was wrong and so are all who discuss this issue of black (absorbing) photovoltaic solar generating panels. At a low incident angle, the panels do appear black. But, they are covered in glass and when viewed at the solar incident angle, appear very mirror like, reflecting all E/M rays except those being caught in the Bell Lab holes to create free electrons. White panels absorb more radiation than do mirrors. If you don’t beleive it just go to this blog site masthead and see that sun reflected in the panels!

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  36. 36. Emmeran 9:02 pm 11/13/2009

    Quote: "To convert this to a carbon-dioxide equivalent, note that the cumulative industrial emissions of 200 billion tonnes of carbon dioxide have produced a forcing of 1.5 W per square meter, or about 10-11 W per square meter per tonne."

    I guess this number is annually? That would make it look very bad, if you’d need the whole production of your cells of 3 years to equate the albedo effect of 1 year….

    What I’m trying to say is: you cannot "produce" 1.5W per square meter, since it’s power, not energy. So you have to take this into the context of the time you emit those 200 billion tons in.

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  37. 37. SaveOurSeaShore 4:48 pm 11/26/2013

    This seems to be a confusing idea for people. The goal is to keep the world from warming. Solar PV many times are black. So if you carpet over 10 square miles of desert with black PV panels….how much does that black warm up the environment versus quite reflective sand We all know that cities are very warm compared to trees and grass areas. So it is a valid question to ask how much warmth do Solar panel contribute versus their “CO2″ free electricity generation. The extended questions would be if it is just cheaper to build geothermal home systems or to just make homes ultra efficient. Of course the solar panel installers will all tell us it is solar panels with their 30% capacity factor and a 100% CO2 backup system. Of course this is a bit rich as the peak use of energy is around 8pm when solar PV contributes almost ZERO! Billions are be spent…but is it being wisely spent?

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