Paint your roof white to reduce global warming

[LeonhardEuler]

The flaw in your reasoning is the statement that painting a roof black will decrease carbon emissions. We don't know that to be true to any significant degree, and I seriously doubt that you would see any measurable benefit in cold climates. As a minimum, we don't know if a black roof would yield any advantage at all or a corresponding reduction in ghg emissions.

Basically you are suggesting that a passive solar heater would work without a glass cover, while exposed to cold or freezing temperatures.

I see what you're saying, but I wasn't making that argument. I said "If there is any energy reduction...". I was avoiding the question of whether there was any such benefit, and just pointing out the consequences that would result if it did exist.

 

[vanesch]

This is food for thinking. How does this then apply to something like DESERTEC ?
Clearly, the CSP plants will have a much darker color than the original desert ?

However, there seems something to be missing in the whole balance.
The Earth receives about 176 PW from the sun, and the potential unwanted greenhouse effect comes from a drive of the order of a few watts per square meter of Earth surface, which corresponds to about 1% of the average solar influx if I'm not mistaking. Now, humanity as a whole consumes about 16 TW of technical energy (mainly fossil fuels), which is less than 1/10000 of the solar power, while the potentially problematic greenhouse effect is rather of the order of 1 % so 100 times more. In other words, the extra "heating power" due to the greenhouse effect that we think will give us trouble, is of the order of 1.7 PW or something (1% of the total solar power we receive) - we're talking orders of magnitude.

Now, suppose that all human power consumption (the 16 TW) is now produced by black solar power plants (CSP plants for instance). Their blackness corresponds then to 16 TW of absorbed solar energy. Meaning that this 16 TW is *peanuts* as compared to the 1.7 PW we would like to reflect back in the sky.

So I don't see how black solar panels on the roofs could ever be a problem (unless it means that they produce 100 times more power than humanity needs) ; but by the same token, I don't see how painting roofs white is going to change anything either.

We'd need to reflect back about 100 times more energy than we are consuming. We'd need to paint an area white that is about 100 times bigger than the solar farm that would power humanity.

 

[sylas]

This is food for thinking. How does this then apply to something like DESERTEC ?
Clearly, the CSP plants will have a much darker color than the original desert ?

Yes, changes and landcover and changes in vegetation have a substantial effect, by altering albedo.

However, there seems something to be missing in the whole balance.
The Earth receives about 176 PW from the sun, and the potential unwanted greenhouse effect comes from a drive of the order of a few watts per square meter of Earth surface, which corresponds to about 1% of the average solar influx if I'm not mistaking.

Sounds about right. I tend to think in terms of values per square meter. The total solar input is about 340 W/m². 30% or so is reflected, and so Earth absorbs roughly 240 W/m². Some of that is absorbed by cloud and the atmosphere; we get roughly 184 W/m² at the surface; of which some more is reflected to give approx 160 W/m² absorbed at the surface, on average.

(Numbers from Trenberth and Keihl, cited and illustrated in [thread=307685]msg #1[/thread] of "Estimating the impact of CO2 on global mean temperature".)

Doubling of CO2 would give an additional 3.7 W/m²; this works out to about 2% of the solar flux seen at the surface where it could be reflected by surface cover. Actual CO2 increases from human activity are less than a doubling, so far. Your estimate seems a good guide to me.

Now, humanity as a whole consumes about 16 TW of technical energy (mainly fossil fuels), which is less than 1/10000 of the solar power, while the potentially problematic greenhouse effect is rather of the order of 1 % so 100 times more. In other words, the extra "heating power" due to the greenhouse effect that we think will give us trouble, is of the order of 1.7 PW or something (1% of the total solar power we receive) - we're talking orders of magnitude.

Yes indeed. The impact of actual direct energy production is a drop in the bucket. However, reducing your energy consumption has a much stronger impact on the Earth's energy balance indirectly. Most of our energy for power is from fossil fuels; and so when you consume less energy, you reduce the greenhouse impact. That is how energy use makes an impact, if you feel so inclined.

There's another reason why heat from consumed energy is not so important. That gives you a certain energy per unit CO2. But once the CO2 is in the atmosphere, it gives a certain power per unit CO2 of continuous additional heating.

I said I would try my own numbers. And, by the way, Chu appears to have been referring to an upcoming publication by other scientists; not his own calculations directly. The paper is not yet available. When it is, we can compare our own estimates with their methods.

Here are my numbers:

Heating effect of one ton of carbon dioxide

The atmosphere weighs about 10 tons per square meter. (This is air pressure)

The atomic weight of air averages out at about 29, carbon dioxide averages out at about 44. There's about 385 ppm(v) of carbon dioxide. Carbon dioxide therefore weighs in at about 10 * 44 / 29 * 385 * 10^-6 = 5.84 kilograms, per square meter.

Surface area of the Earth is about 5.15 * 10¹⁴ square meters.

The extra energy feeding to the surface from additional carbon dioxide is a logarithmic relation; you get 3.7 W/m² per doubling. The rate at which this increases from increasing CO2 levels is given by a derivative. Let S be the surface area, and C be the total tonnage of carbon dioxide in the atmosphere. Let P be the difference in energy by adding an additional tonnage D. The derivative dP/dD, at D=0 for the present, gives the rate at which available power increases with additional CO2. We have

$$\begin{align*} P & = 3.7 S \log_2 \left( \frac{C+D}{C} \right) \\ \frac{dP}{dD} & = \frac{3.7 S}{\log_e(2) (C+D)} \\ C & = 5.84 \times 10^{-3} \times S \\ \frac{dP}{dD}_{[D=0]} & = \frac{3.7}{0.693 \times 5.84 \times 10^{-3}} \approx 914 W \end{align*}$$​

That is; one ton of CO2 in the air is about an additional 914 W of heating; almost a kiloWatt.

To compare with how much energy you actually get to use from burning fuels; numbers vary depending on the fuel and application. For example, using coal for power generation, you get a bit less than 1 kilogram of CO2 for about 1 kWhr of generated power.

Hence, 1 ton of CO2 is emitted to get enough energy to power a 914 W heater about 1000 hours, or around 42 days.

There's another wrinkle here. Not all emissions remain in the atmosphere. About half gets flushed into other reservoirs, especially the ocean. The trend of increase in atmospheric CO2 is roughly half the rate of emissions.

The effect of painting a roof

Moving a roof from an albedo of 0.2 to 0.7 seems entirely feasible. We have about 184 W/m² available to be reflected. Reflect half of that from a 100 m² roof, and you are saving power at a rate of 9200 W. That should offset about 10 tons of CO2.

These numbers are annual global averages. You would of course reflect nothing at night, and much more in the day. But the global annual average works out as calculated.

Now looking at the reference given by Ivan Seeking back in [post=2292644]msg #19[/post], I see that the study by Akbari et al, apparently quoted by Professor Chu, speaks of retrofitting a roof to give at least 0.4 extra albedo, with 100m² of roof off-setting about 10 tons of CO2. The calculations will, I suppose, be available when the paper comes out soon.

But in the meantime, their estimate fits well with the technique I have used... except that I used CO2 actually in the atmosphere, rather than CO2 emitted. 10 tons in the atmosphere results from about 20 tons emitted.

We'd need to reflect back about 100 times more energy than we are consuming. We'd need to paint an area white that is about 100 times bigger than the solar farm that would power humanity.

I don't think we really care very much about the energy being consumed itself. It doesn't make much difference to Earth's energy balance. What makes more difference is the changes in the impact of the Sun's energy as the composition of the atmosphere changes.

Note that in Professor Chu's comparison, he compares a certain area of reflective surface with cars being used for a certain period of time. That is; a reflective surface gives you a certain wattage of reflected power, continuously. Consuming fuel gives you a certain fixed amount of energy. The emitted CO2 gives you a certain wattage of absorbed heat, which continues to have its impact year upon year long after you've finished with the energy it produced.

Cheers -- sylas

 

[vanesch]

I don't think we really care very much about the energy being consumed itself. It doesn't make much difference to Earth's energy balance. What makes more difference is the changes in the impact of the Sun's energy as the composition of the atmosphere changes.

I think I agree with what you write, but I think you missed the point I was trying to make. As I didn't want to look into numbers of surfaces, changes in albedo and all that, I wanted to reduce this problem to its most elementary constituent, which is the ratio between the "greenhouse gas forcing" power, and the "solar power plant power", in orders of magnitude.

As you confirm, with CO2 doubling, we can expect a greenhouse gas forcing which is of the order of 2% of year-average solar power on the ground, or about 1% of the solar power on the top of the atmosphere. That's the kind of heating that makes for the unwanted heating, of the order of 1.7 PW.
Humanity's energy consumption is about 100 times smaller than this, namely 16 TW or so.

Now, what probably misled you in what was my argument, is that by coincidence a big part of the greenhouse gas forcing is probably caused by human CO2 emission to produce this 16TW, but this is besides the point. Let us imagine for sake of argument that the forcing has nothing to do with human energy consumption or anything ; that we just want to get rid, one way or another, of 1.7 PW of "too much" solar heating.

Now, let us consider that we equip all roofs with (dark) solar panels, be it photovoltaic, or thermal solar panels. They would have an "extra heating" effect, but if all these solar panels are providing humanity with their energy, namely 16 TW, that would mean (assuming them to be 100% efficient for sake of argument) that their surface is such that they take about 16 TW of extra solar energy. So they "heat the earth" by 16 TW extra. Peanuts in comparison to the 1.7 PW.

Now, on the other hand, suppose that we change them into white surfaces. They won't work anymore as solar panels then. Then we can expect that they *reflect* 16 TW. Also peanuts in comparison to the 1.7 PW we want to get rid of.

So that's why I said: in order for this to set off the greenhouse gas forcing, the needed surface that has to be whitened must be about 100 times larger than the surface we would need to cover with solar panels that can provide humanity with all its energy.

This is independent of the fact whether it was this energy which was the cullprit of driving the greenhouse gas emissions. I was just comparing the surface needed to provide humanity with solar power, and the surface needed to reflect back in space a 100 times bigger power.

This reasoning was triggered by the discussion of whether, as an individual, one should put solar panels on one's roof (in order to produce energy, but they are dark), or whether one should paint one's roof white (in order to reflect sun power).

The conclusion is that you should put solar panels on your roof which give you the energy you need, and then paint a 100 times larger area white. (if you are representative of the human power consumption) Because we need to reflect about 100 times more energy to fight the greenhouse gas forcing than we need energy for our own consumption.

Another way to put this, is that in order to reflect back about 2% of surface-reaching solar power, we need, well, to paint white about 2% of the Earth's surface. Earth's surface is about 510 10^6 km^2 which means we need to paint white:
about 10^7 km^2 or about 3000 km x 3000 km.
In order to power humanity with solar power, we need in principle a 100 times smaller area, about 300 km x 300 km (assuming 100% efficiency).
This is the earth-size version of the solar panel on the roof and the area that needs to be painted white.

BTW, the surface that needs to be painted white, 10 million square km, is the surface of the entire USA. The surface of the USA needs to be painted white in order to offset the forcing of a CO2 doubling.

In fact, it needs to be bigger, because we won't go from albedo 0 to albedo 1, but this might even be compensated with solar power plants not being 100% efficient, so their ratio might still remain 100.

Very very rough rule of thumb estimation of course.

 

[archis]

So you reject the claims of a Nobel-Prize winning physicist but have no calculations. Got it.

Lets start calculations with how many roofs you will have and with what reflecting properties. I don't know, you don't know and i doubt the nobel guy knows too. I am very sceptical about this, becouse their is enough areas like Arctica and Antarctica which reflects light + the sea + deserts. In all this reflection sum i think roof area is like
waterdrop in the sea. Altough to support this idea is bether then doing nothing and then we could see test results.

 

[turbo]

Lets start calculations with how many roofs you will have and with what reflecting properties. I don't know, you don't know and i doubt the nobel guy knows too. I am very sceptical about this, becouse their is enough areas like Arctica and Antarctica which reflects light + the sea + deserts. In all this reflection sum i think roof area is like
waterdrop in the sea. Altough to support this idea is bether then doing nothing and then we could see test results.

You are missing a big part of the picture. Highly reflective roofs don't just bounce more visible light back to space. They reduce the amount of energy absorbed by the building. This energy warms the building, and in our culture, we tend to use air-conditioning to shed this unwanted heat in warm weather. The electricity that we use to cool our homes and businesses comes overwhelmingly from power-plants fired by fossil fuels. Those plants load our atmosphere with pollutants. Reduce the cooling-load with reflective roofs, and you reduce the amount of electricity that needs to be generated.

We don't need to modify the albedo of huge areas of the world to achieve these efficiencies - just reduce the cooling load building-by-building. As I mentioned earlier, when I bought this place, it had a dark asphalt-shingle roof, and the place was really tough to cool, even with a large 220V AC unit. I had 1" of Styrofoam installed over that roof, capped with reflective standing-seam Galvalum roofing. Now we can keep the place cool with just a couple of small portable 120V AV units that vent through windows, and our summertime electric bills are much smaller. I live in Maine, and the summers are generally not real brutally hot, but imagine the savings in electrical generation and transmission if such roofs were installed all across the deep south. Summer brown-outs could be a thing of the past.

 

[vanesch]

This energy warms the building, and in our culture, we tend to use air-conditioning to shed this unwanted heat in warm weather. The electricity that we use to cool our homes and businesses comes overwhelmingly from power-plants fired by fossil fuels. Those plants load our atmosphere with pollutants. Reduce the cooling-load with reflective roofs, and you reduce the amount of electricity that needs to be generated.

This is correct, but has more to do with more environmentally-friendly building than with painting everything white.

From the link of the OP:

A global initiative to change the colour of roofs, roads and pavements so that they reflect more sunlight and heat could play a big part in containing global warming, he said yesterday.

(emphasis mine)

If it were to use less air conditioning, there wouldn't be any reason to paint roads white. I don't know of any culture yet which cools pavements with airconditioning

So clearly, the message of Chu is about increasing albedo to compensate for CO2 forcing.

Another indication is that he equals the change of color of roofs and so on to a *fixed quantity* of CO2 (and hence forcing): cars times 11 years. If it were to reduce consumption, such as using less AC, he would have talked in terms of emissions PER UNIT OF TIME (so many tonnes of CO2 per year).

 

[archis]

You are missing a big part of the picture. Highly reflective roofs don't just bounce more visible light back to space. They reduce the amount of energy absorbed by the building. This energy warms the building, and in our culture, we tend to use air-conditioning to shed this unwanted heat in warm weather. The electricity that we use to cool our homes and businesses comes overwhelmingly from power-plants fired by fossil fuels. Those plants load our atmosphere with pollutants. Reduce the cooling-load with reflective roofs, and you reduce the amount of electricity that needs to be generated.

Good point but just now i see some paradoxes. I say let's paint all buildings black in the north to lover the costs of the house warming in the winter. You are worried about conditioning costs, energy in summer i am worried about house heating in the winter. There where i come from we put heath isolations in walls and the roofes to fix both of these problems.

 

[Skyhunter]

Good point but just now i see some paradoxes. I say let's paint all buildings black in the north to lover the costs of the house warming in the winter. You are worried about conditioning costs, energy in summer i am worried about house heating in the winter. There where i come from we put heath isolations in walls and the roofes to fix both of these problems.

What you are forgetting is that the Sun's rays are not as direct, nor do the days last as long during NH winter when buildings need to be heated. A slightly warmer roof for 8 to 10 hours a day in winter would have far less impact on energy consumption than the cooler roof for 14 to 20 hours in the summer. Canadians use air conditioning in the summer the same as Floridians do.

I have heard Chu mention the reduced cooling costs when discussing "paint your roof white". Citing this one example is somewhat out of context IMO.

Do a quick google of "Chu white roof air conditioning" and you will find that all the discussions include reduced energy requirements for cooling.

http://physics.suite101.com/article.cfm/obamas_energy_secretary_suggests_white_roofs"


http://climateprogress.org/2009/05/27/energy-steven-chu-white-roofs-geo-engineering-adaptation-mitigation/"

It was a geo-engineering scheme that was “completely benign” and would keep buildings cooler and reduce energy use from air conditioning, as well as reflecting sunlight back away from the Earth.

 

[vanesch]

What you are forgetting is that the Sun's rays are not as direct, nor do the days last as long during NH winter when buildings need to be heated. A slightly warmer roof for 8 to 10 hours a day in winter would have far less impact on energy consumption than the cooler roof for 14 to 20 hours in the summer. Canadians use air conditioning in the summer the same as Floridians do.

I have heard Chu mention the reduced cooling costs when discussing "paint your roof white". Citing this one example is somewhat out of context IMO.

As I said before, this part of the idea is not bad, but it doesn't have necessarily anything to do with the roof. After all, the best use of the roof is not to paint it black or white, but to install solar panels on it, energy-wise. The other thing to do, is to make more environmentally-friendly buildings (thermal insulation and so on).

So painting the roof is already questionable (use panels instead of paint). But painting the roads and the pavements can only have one goal, unrelated to airconditioning or energy use: it is to change albedo.

Well, I challenge you to point out a gross error in the rough estimate I made, which shows that one needs the entire surface of the USA to be painted white in order to offset the CO2-doubling forcing, so this proposal is akin to proposing to take buckets of water out of the sea to compensate for sea-level changes. Of course "it helps a (very little tiny) bit". But "does it matter" ?

 

[skeff]

Given that this comes from Chu, I'll make a leap of faith. If nothing else, it would probably be significant to the heat island effect.

One concern that occurred to me wrt using concrete instead of asphalt is the amount of CO2 produced. I believe that concrete processing accounts for a significant portion of our CO2 output. But I don't know how much CO2 is produced by the processes related to asphalt.

Hi!
There are open, international, databases of "emission factors" for greenhouse gases for anything you can think of. My database tells me the following:

1. Asphalt, Road&Pavement = 0.14 kg CO2equiv/kg asphalt
and in unit area (metres squared): 134 kg CO2equiv/m2 asphalt

2. Concrete, Road&Pavement = 0.13 kg CO2equiv/kg concrete
and in unit area (metres squared): 188 kg CO2equiv/m2 concrete

The source of this data is from something called ICE, which I don't really know what is, so I cannot verify this. Anyway, if this is relatively correct, seems asphalt wins because of structral differences, when comparing area covered with the same function of road and pavement.

 

[mgb_phys]

seems asphalt wins because of structral differences, when comparing area covered with the same function of road and pavement.

The asphalt coating is much thinner, more of the structural load is taken by the roadbed. Concrete is poured much thicker and takes a lot of the load, needing less material in the roadbed - so concrete freeways are overall cheaper to build.
The concrete surface also lasts longer.

 

[DaveC426913]

After all, the best use of the roof is not to paint it black or white, but to install solar panels on it, energy-wise.

The energy from those panels adds to the net energy of the Earth; it will come out somewhere as waste heat. White roofs put that energy back into space.

The asphalt coating is much thinner, more of the structural load is taken by the roadbed. Concrete is poured much thicker and takes a lot of the load, needing less material in the roadbed - so concrete freeways are overall cheaper to build.
The concrete surface also lasts longer.

If we wanted to reduce global warming, finding an alternative to concrete might be a good place to start. Concrete is the largest single material produced by Mankind. It produces 5 to 10% of the world's CO2 emissions [http://en.wikipedia.org/wiki/Concrete#CO2"]*.

*(according to Wiki)

 

[mgb_phys]

If we wanted to reduce global warming, finding an alternative to concrete might be a good place to start.

Wooden freeways?
You could use the snap-together wooden flooring from Ikea, then you wouldn't have to cone off miles of freeway for months on end to lay a bit more asphalt.

 

[skeff]

...
If we wanted to reduce global warming, finding an alternative to concrete might be a good place to start. Concrete is the largest single material produced by Mankind. It produces 5 to 10% of the world's CO2 emissions [http://en.wikipedia.org/wiki/Concrete#CO2"]*.

*(according to Wiki)

I'd be really skeptical about that number, 5-10% of net CO2 emissions being caused by concrete. As mgb_phys hinted at, time is essential in the balance of CO2 emissions for a given product or system. What I'm looking at, is plants for a certain kind of processes. The concrete for construction of a plant, is vanishingly small, when set up against the energy used for driving the process during the lifetime of the plant/construction. Chemicals and energy are the main sources of CO2, and usually you are talking about energy in the production of those chemicals too, so it all boils down to how you create energy. It all boils down to fossile fuels.

Biomass incineration, hydropower, saltpower. That's the solution. Forget about moving your car or airplane around, those stationary plants we depend on can reduce their footprint enormously.

 

[mgb_phys]

I'd be really skeptical about that number, 5-10% of net CO2 emissions being caused by concrete.

10% is probably high, 5% might be reasonable. There is a LOT of concrete made, 3ton/person/year - outside the US/Europe everybody doesn't have a car but they all use concrete.
Some of the figures are skewed by confusing cement with concrete, cement (the stuff that produces CO2) is only about 10-15% of the finished concrete (depending on how much aggregate you are using).

Chemicals and energy are the main sources of CO2, and usually you are talking about energy in the production of those chemicals too, so it all boils down to how you create energy.

Cement manufacture releases about it's own weight in CO2. Half is from the chemistry and half from the fuel used. Cement kilns use a LOT of fuel, and it pretty much has to be fossil fuel - I don't know of any electrically powered kilns.

Again you have to offset this against the low cost and long life of a cement building compared to rebuilding a wooden one repeatedly.

In theory cement is carbon-neutral, eventually it will reabsorb the CO2 driven off in the kiln and return to being Calcium Carbonate. In practice it's reused as hardcore, but if you ground it up and spread it on the oceans it would be at least as green as wood.

 

[vanesch]

The energy from those panels adds to the net energy of the Earth; it will come out somewhere as waste heat. White roofs put that energy back into space.

Yes, but the point is that the "greenhouse forcing" of fossil power is about a factor 100 over a century: 1 KW of fossil power during a century generates 100 KW of greenhouse forcing.

(roughly: I take this from the fact that the forcing for CO2 doubling results in about 1.7 PW of forcing, while humanity uses about 16 TW, essentially fossil, which is supposed to give us a doubling in about a century).

So it is 100 times more efficient to take 1 KW from a solar panel (if that replaces fossil fuel, and as such, will avoid 100 KW of forcing), than to paint that same surface white and avoid 1KW of forcing by reflecting it back.

Very roughly.

Assuming 100% efficiencies everywhere.

 

[Ivan Seeking]

Assuming 100% efficiencies everywhere.

That is a pretty big assumption given that solar is only now becoming competitive. The true lifetime energy output and the cradle-to-grave energy costs of solar panels is critical to include [made in China with coal power?].

 

[vanesch]

The true lifetime energy output and the cradle-to-grave energy costs of solar panels is critical to include [made in China with coal power?].

True, but to compensate a factor of 100, that would mean that the overall efficiency would be less than 1%, so I thought I was on the safe side there...

For bad-quality PV, this might be an issue, but for a solar water heater, I don't think so.

But, it is a point you raise. Didn't think of it. Works in the other way, too. It means that solar PV, with only a few percent efficiency, isn't worth it either.

 

[sylas]

OK... I have checked up some figures with my sister-in-law, who is interested in such things.

First generic comment. There is no silver bullet. If anyone, for any reason, is particularly interested in reducing their impact on the planet, the most effective single thing you can do as an individual is anything that reduces your own consumption. The things we are speaking of, like painting a roof white or installing solar power, are other minor changes you can make as well.

Be that as it may, what turns me on in particular is not saving the world, but learning about it. That's just how I am wired, maybe.

So... assuming we are fascinated by the physics and the numbers, here we go.

Here in Newcastle, a domestic solar power system is commonly about a 1 kWatt system, which covers 6 square meters and actually gives you roughly 4.5 kWhours/day, averaged over all year. Continuous power there means about 190 W. That's if you've done it well.

The power you generate feeds into the grid, and what you don't use yourself at the time gets knocked off your bill. Economically, it does have a substantial impact on your power bills.

However, it's not "efficient" when measured against the sunlight hitting those panels. You get roughly 180 W/m² at the surface, and so with your 6 panels you are squeezing the juice out of about 1080 Watts. So you are getting about 17%. I'm actually surprised it is that high; but so be it.

A kiloWattHour (3.6 MJ) of power generates order of magnitude 1 kilogram of carbon dioxide in emissions; and so running your solar system for a year generates about 365 * 4.5 or about 1640 KWhr, and so saves about that many kilograms of CO2.

From the calculations I gave in [post=2295904]msg #38[/post], one kilogram of carbon dioxide can be taken as altering the energy balance of the Earth itself, so that an extra 0.9 W is absorbed; but remember not all emissions STAY in the air, and so you should probably reduce that impact by about half. Let's say the 1600 kilograms is worth about 800W heating on the Earth's energy budget. It will vary depending on your local power generation; it varies quite a bit between different states in Australia. But 800W less greenhouse heating for your year's worth of solar panel sounds in the ball park.

As a result of running your panels for a year, you've avoiding adding another 800W to the Earth's energy balance, just from the reduced CO2 emission.

Now you could also have painted your 6 square meters white. Had you done this, and increased albedo by 0.5 or so, you could have saved reflected some 6*180*0.5 = 540 W or so.

The thing is, your panels keep working. Not forever, but you'd better hope to get several years out of them! Bottom line -- for Earth's energy balance, a solar panel is better than a white roof of the same size.

Or you can just save power. Households vary a lot, by 8000 kWhrs in a year is credible. If you cut this by 20%, you have 1600 kWhours in a year; which is your 6 square meter solar system right there.

Or you can drive less. Cars can be from about 0.1 to 0.4 kg CO2 per kilometer, and I'm figuring about 0.45 W in the energy budget per kilogram. So 10,000 km in your car is from 1000 to 4000 kilograms CO2, or 450 to 1800 Watts added to the energy balance of the Earth. Or just get the more efficient car.

Here's the summary to help it stand out. The following seem to be about the same value in terms of their impact on Earth's energy budget, of roughly 5400 Watts in the energy balance to the Earth.

• Having 60 square meters of white roof. (But you have to KEEP it white to keep up the 5400 albedo effect.)
• Having a 6 square meter photovoltaic system in operation, working for 6 years. Even if it breaks down after that.
• Cut your energy bill at home. Cut an 8000 kWhr/year bill by 20%, for 6 years.
• Drive less. 30,000 kilometers less in your gas guzzling SUV, or 100,000 kilometers less in your subcompact.
• If you drive 10,000 kilometers a year, replace that SUV with the subcompact; and in 4 years or so you've got that impact.

Checks on my figures actively solicited.

I have not factored into this the energy consumption in actually MAKING and installing panels. Which is significant. Solar panels do help... but they are a big expensive complex system to set up, and straight impact-wise, one of your least efficient strategies. But that's okay; there's no silver bullet and you can do all kinds of things if you really want. The solar panels do help; if that's your interest.

My sister-in-law HAS installed solar panels. 9 square meters of photovoltaics, and another 6 square meters on the Granny-flat for my Mum and Dad who live on the same block. Plus she's recently built an extension (as an owner builder) and in the process managed a lot of passive heating and cooling, with the best ways to manage insulation, grey water reuse, air flow, etc, etc. And they are trying to slash energy use. They want to get to zero on their energy bill, and that seems possible. And now she's doing a course to help do formal assessments for others. She's paid by a public government supported program, and anyone can get the assessment for themselves, which is mainly about saving your own bills. In the future, a basic energy assessment will be a required part of the information when you sell a home, much like a building report. And Cathy will be able to those as well.

So there you go, folks. If you are interested, there's lots you can do. But look first at your insulation, and where you can switch off appliances (don't leave the computer or the TV on stand by; switch it off if you aren't using it and save yourself some money), and consider public transport where feasible. Look for a holiday destination in your state rather than overseas. Solar panels if you are keen; but they don't matter as much, or save you as much money, as just seeing how you can reduce your energy bill by using less of it.

Cheers -- sylas

 

[vanesch]

Be that as it may, what turns me on in particular is not saving the world, but learning about it. That's just how I am wired, maybe.

So... assuming we are fascinated by the physics and the numbers, here we go.

You're in good company

Here in Newcastle, a domestic solar power system is commonly about a 1 kWatt system, which covers 6 square meters and actually gives you roughly 4.5 kWhours/day, averaged over all year. Continuous power there means about 190 W. That's if you've done it well.

Newcastle, Scotland ?? That seems like a very good capacity factor for solar there.

However, it's not "efficient" when measured against the sunlight hitting those panels. You get roughly 180 W/m² at the surface, and so with your 6 panels you are squeezing the juice out of about 1080 Watts. So you are getting about 17%. I'm actually surprised it is that high; but so be it.

These are rather high-quality panels. Poly-silicon, I guess. With thin-film you won't get that.

A kiloWattHour (3.6 MJ) of power generates order of magnitude 1 kilogram of carbon dioxide in emissions; and so running your solar system for a year generates about 365 * 4.5 or about 1640 KWhr, and so saves about that many kilograms of CO2.

That's for a year's consumption, but you hope for the panel to stay there for a century (even if it needs replacing 2 or 3 times).

Let's say the 1600 kilograms is worth about 800W heating on the Earth's energy budget. It will vary depending on your local power generation; it varies quite a bit between different states in Australia. But 800W less greenhouse heating for your year's worth of solar panel sounds in the ball park.

If you use it ONE YEAR. Yes. If you use it two years, that's 1600W, if you use it 10 years, that's 8 KW, if you use it for a century, that's 80 KW.

Now you could also have painted your 6 square meters white. Had you done this, and increased albedo by 0.5 or so, you could have saved reflected some 6*180*0.5 = 540 W or so.

Yup, and that would remain 540 W.

The thing is, your panels keep working. Not forever, but you'd better hope to get several years out of them! Bottom line -- for Earth's energy balance, a solar panel is better than a white roof of the same size.

That was my point.

I have not factored into this the energy consumption in actually MAKING and installing panels. Which is significant.

It used to be of the order of 10 years of energy payback time (not economical payback time), but this has apparently been reduced to a few years for polysilicon, and one or two years for thin-film...

 

[sylas]

Newcastle, Scotland ?? That seems like a very good capacity factor for solar there.

Australia, a bit North of Sydney; plenty of sunny days. Good for solar.

These are rather high-quality panels. Poly-silicon, I guess. With thin-film you won't get that.

The rate does seem pretty good, even given Australian sun. But Cathy seems to think they get that order of supply from her system. I don't know the details of the panels she has.

If you use it ONE YEAR. Yes. If you use it two years, that's 1600W, if you use it 10 years, that's 8 KW, if you use it for a century, that's 80 KW.

Yes. However, by the time you look at a century time scale, you might be starting to approach times where you could consider the lifetime of CO2 in the atmosphere as well. Perhaps a couple of centuries. I stuck with 6 years as it gives comparisons of the order of a white roof ten times the size.

Cheers -- sylas

 

[vanesch]

Australia, a bit North of Sydney; plenty of sunny days. Good for solar.

Ah that explains things.

I was thinking that a solar capacity factor of 10 is already not so bad in Scotland!
(also the power per square m was a bit high for scotland...)

But Cathy seems to think they get that order of supply from her system. I don't know the details of the panels she has.

No, for poly silicon it is a good, but not exceptional efficiency, 17%. And a capacity factor of 5 or 6 is also reasonable if you're in a sunny place (a good estimate of the capacity factor is the average solar power per square meter, over 1 KW).

 

[mheslep]

... If anyone, for any reason, is particularly interested in reducing their impact on the planet, the most effective single thing you can do as an individual is anything that reduces your own consumption.

I would have said that a little differently: anything that reduces wasted consumption, or raises productivity per kWh. The point being that living on average as they do in Ghana now or here in pre Columbian times is not that effective in the larger view.

 

[mheslep]

• Having 60 square meters of white roof. (But you have to KEEP it white to keep up the 5400 albedo effect.)
• Having a 6 square meter photovoltaic system in operation, working for 6 years. Even if it breaks down after that.
• Cut your energy bill at home. Cut an 8000 kWhr/year bill by 20%, for 6 years.
• Drive less. 30,000 kilometers less in your gas guzzling SUV, or 100,000 kilometers less in your subcompact.
• If you drive 10,000 kilometers a year, replace that SUV with the subcompact; and in 4 years or so you've got that impact.

...

or

• one less airplane trip (2 tons CO2/person) per year for five years.

 

[Richard111]

Interesting discussion on solar panels. I would like to know what is required to convert the energy from the panel to 230v at 50hz to enable household appliances to be operated.
Can you store the energy? How is it synchronised with the town mains when you are providing them power? I guess this is way off topic. Is there a thread on solar panels?

Regards Richard

 

[mgb_phys]

Interesting discussion on solar panels. I would like to know what is required to convert the energy from the panel to 230v at 50hz to enable household appliances to be operated.

A simple box of electronics

Can you store the energy?

Not efficently, some people off grid have banks of batteries but it's large and expensive

How is it synchronised with the town mains when you are providing them power?

A rather more complicated and expensive box of electronics.

This might be interesting.

https://www.youtube.com/watch?v=IELITZ2VSvk

 

[Ivan Seeking]

Interesting discussion on solar panels. I would like to know what is required to convert the energy from the panel to 230v at 50hz to enable household appliances to be operated.
Can you store the energy? How is it synchronised with the town mains when you are providing them power? I guess this is way off topic. Is there a thread on solar panels?

Regards Richard

Many areas do or will offer net metering. This allows you to effectively use the grid for your energy storage. When you produce more power than you use, you supply power to the grid and your meter runs backwards. When you need energy back, the meter runs forwards again. Rather than limiting the contributions to the grid and the value of the energy that you produce, as was done in the past, you simply see the net metered value on your electric bill or credit.

Wrt the issue of white roofs vs solar panels, obviously the cost is the definitive factor. Many people simply cannot afford to spend $30K-50K on solar panels. Whitewash is cheap.

 

[mheslep]

Interesting discussion on solar panels. I would like to know what is required to convert the energy from the panel to 230v at 50hz to enable household appliances to be operated.

Google 'inverters'

...How is it synchronised with the town mains when you are providing them power?

As mgb_phys that requires a more sophisticated inverter system, a switch between you and the town mains, and it _must_ be setup by a registered electrician.

 

[mheslep]

Not efficiently, some people off grid have banks of batteries but it's large and expensive

Battery storage is efficient, the only significant loss is the DC/AC conversion (if needed) and that is small (~10%). It expensive as you say, unless used as emergency/backup power only (lead acid) where the cycle life is small.

 

[Richard111]

Thank you mgb_phys, Ivan and mheslep for your response. I had to skip the U-tube tutorial as my "broadband" is too slow. I googled "inverters" and found I do not know enough to define my requirements.

This year my electricity charges have gone up 40% and the prediction is a further 25% rise by about December. As a pensioner this is not sustainable.

During winter I heat my house with LNG and find there is no problem keeping cool in summer as I live in Milford Haven.

The household appliance that works the hardest is the kettle. I find we boil at least four kettle fulls a day. I checked and note that the kettle filled to the max mark holds 1.5 litres of water and takes 3 minutes 20 seconds to boil and draws a steady 10 amps while switched on.

By using timer switches all appliances that use to remain on standby are now completely disconnected from 11pm to 7am. This, among other measures, has reduced my weekly consumption from about 70 units to 60 units. I will need to reduce my consumption by about a further 20% just to keep my monthly payments constant.

So, painting my roof white will bring no benefit whatever, and will cost for the paint, and the payback time of any sort of solar panel system that would ensure I enjoy my tea over just the summer months will be beyond my expected lifetime.

The only cost effective system for me is to install a multifuel stove. This will provide heating and cooking facilities.

We are returning to the conditions I lived under as a child during the war. Even the bath water was heated in large pans on the cast iron stove.

I am not impressed with this brave new world.

 

[Ivan Seeking]

Battery storage is efficient, the only significant loss is the DC/AC conversion (if needed) and that is small (~10%). It expensive as you say, unless used as emergency/backup power only (lead acid) where the cycle life is small.

Lead-acid batteries are listed as being 75% - 85% efficient.
http://wiki.xtronics.com/index.php/Sealed_Lead_Acid_Battery_Applications

In practice, efficiency is governed in large part by Peukert's Law - high rates of discharge mean less efficiency.

 

[Count Iblis]

Why not cover large uninhabited areas of the Earth by aluminium foil? Aluminium foil of thickness 0.01 mm covering 1000 by 1000 km only contains 27 million tonnes of aluminium. The price of the bauxite needed to manufacture the aluminium is only about $80 billion.

 

[wookiemeister]

a quick explanation of why white paint works to cool the planet

1 around 50 percent of sunlight is visible light

2 around 50 percent of sunlight is heat (infra red - i will call it heat for the moment)

3 visible light is a frequency of light NOT absorbed by the atmosphere

4 some HEAT (infra red) from the sun IS absorbed by the atmosphere directly (greenhouse gasses which makes the atmosphere warmer).

5 HEAT from the sun also warms the surface of the planet and this HEAT is then radiated back into the atmosphere again being absorbed by greenhouse gasses.

6 visible light is poorly absorbed by the atmosphere, some of it gets reflected back by the atmosphere (white clouds).

7 the visible light that doesn't get reflected back by the atmosphere is either reflected by the surface (eg a snow field) or absorbed by the surface (an asphalt carpark/ road). this absorbed light heats the dark surface. this heat is then radiated into the atmosphere and is absorbed by greenhouses gasses making the atmosphere warmer.

8 by painting your roof white and other surface you reflect around 50 percent of the visible energy from the sun, a significant amount of this is reflected back into space WITHOUT heating the atmosphere.

9 by painting enough roofs you make buildings cooler and cool the atmosphere. air conditioning works easier to cool houses because they are cooler, perhaps you wouldn't even need air conditioning?

if you are thinking of using white paint to cool your roof you will need a special white insulative paint that both reflects the visible frequencies of light and stops infra red frequencies from heating your roof.


white roof keep buildings cooler because less energy is absorbed by the building, with insulation even less energy makes its way into the building. the white roof reflects energy that would have otherwise heated the air above the roof and would have made the neighbourhood temperature hotter.

i remember seeing an article some time ago, an area in spain covered in white roofed greenhouses recorded slightly lower air temperatures than anywhere else in spain. if you still don't think white roofs do much, think about this, imagine if you painted all surfaces black in a city, can you imagine just how hot it would be on the streets - it might not be possible to walk the streets. the poles are not just cooleer because of the sun being at a shallower angle, the white sends a massive chunk of energy away from the area making it much cooler.

 

[vanesch]

Why not cover large uninhabited areas of the Earth by aluminium foil? Aluminium foil of thickness 0.01 mm covering 1000 by 1000 km only contains 27 million tonnes of aluminium. The price of the bauxite needed to manufacture the aluminium is only about $80 billion.

Sure ! My dream. But then, by folding this Al foil into parabolic troughs and turning it into CSP plants, why not ?