Sunday, 13 December 2015

One-Five Thousandth

The Climate Summit in Paris started over a week ago.

And the topic of sustainable energy kicked off the summit with this:
Filmmaker David Attenborough, whose soothing voice narrated the vicarious journey of millions of TV viewers through the wonders of the natural world, called at a climate summit on Monday (Nov 30) for scientific investment in "saving the world".
"The essence of the thing is that it should be cheap," the acclaimed documentary-maker told AFP. "Goodness me ... if we could catch one five-thousandth part of the energy that the Sun sprays onto the Moon, onto this globe every day, we would supply all the energy requirements of humanity. So how inefficient are we that we can't get that much?"
I don't know why the Moon was dragged into this. So we should have solar panels on the Moon?

Anyway, is it true that we just need to convert 1/5000th of the energy the sun "sprays" onto Earth?

One pro-solar/pro-renewable energy source claims that the Sun "sprays" 23,000 terawatt of energy per year onto the earth.

As for our energy needs, it is projected that we will need about 35 terawatts of power per year by 2050.

BUT... 1/5000th of 23,000 terawatt is... 4.6 terawatt. That's not even enough for current energy needs (13.5 terawatts). Maybe that's why he dragged in the moon. Though even with the moon, 1/5000th would still be about 6 terawatts (rough estimate). Even if the moon doubled the energy, it would only be 9.2 terawatts.

Now if he meant 1/500th of the sun's energy, that would be 46 terawatts. That would be adequate for an earth with (severely) moderated energy needs. (Unmoderated would be over 100 terawatts).

So I think Attenborough meant 1/500th not 1/5000th (but that's ok. He's a filmmaker, not a mathematician). So an area 1/500 of the earth would receive 46 terawatts of solar energy per year. What is the size of this area?

Well the surface area of the Earth is 510 million square kilometres, so 1/500 of it would be 1 million sq km (and change).

One million Sq km. That's more than 1/10th of the size of the US. Or about the size of Egypt.

BUT... the best solar panels are only 35% efficient at converting the sun's energy. Most are only about 20% efficient. The theoretical limit on solar panel efficiency is 55%.

What does that mean?

Well, it means that while 1 million sq km surface area would RECEIVE 46 terawatts of solar energy, the solar panel would only be able to CONVERT 20% (current average technology) to 35% (current best technology) to 55% (theoretical upper limit of efficiency). Which means that the area needed to convert solar power to 46 terawatts of energy would be 5 million sq km (20% efficient), 3 million sq km (35% efficient), or 2 million sq km (50% efficient). Assuming that we would eventually reach close to the theoretical maximum efficiency (say 50%), we would need an area of about 2 million sq km.

Or about the size of Greenland.

That is... not an insubstantial piece of real estate.

But say we do managed to set aside 2 million sq km of surface area (floating on the seas perhaps?) now we need to build the Global Solar Energy System
to construct a global solar-energy system would consume at least 20 percent of the world's known iron resources, take a century to build and cover a half-million square miles.
[Note to the eagle-eyed: half a million sq miles is about 1.3m sq km. Which is less than the 2 m sq km figure I have been using. 1.3m sq km would be about the size of Peru, or South Africa, or smaller than Mongolia or Iran. Why the discrepancy? Well, in one case we were working backwards from 1/500, which gave us 2m sq km. But that would produce 46 terawatts. Working from the other direction we start with a need for 35 terawatts, which is about 75% of 46 terawatts. so proportionally, we would only need about 1.5 m sq km. Which is in the ballpark of 1.3m sq km,  Whatever conversion rate or figure you use, the point is the amount of land required is large. From the land area of South Africa to Greenland. Or somewhere in-between. ]

So even if we wanted to do this, it would take about 100 years, and used 20% of our iron resources. And we need this NOW. Or at least by 2050 (which is 35 years away or 1/3 of 100 years). Which leaves the question: what do we do for energy until then?

Where are we going to get the energy before then?

Hey, if we can go to the moon...
The challenge of supplying the world with carbon neutral energy has a lot of people calling for the launching of a "Manhattan Project" or "Apollo Project." What they mean is that the Federal government should dramatically boost research and development spending for novel energy technologies. Let's recall that the Apollo Project absorbed 5.3 percent of the Federal government's budget in 1965. A comparable expenditure would be $136 billion in 2006—that's almost 5 times higher than the Energy Department's 2006 budget. It is also more than the Federal government currently spends on the agriculture, commerce, energy, homeland security, interior, justice and labor departments. Let's also recall that the Apollo program turned out to be a technological dead end that managed to get just 12 astronauts to walk on the moon. Another telling example of Federal bungling in the energy field was the $20 billion wasted on President Jimmy Carter's Synfuels Corporation which was a pilot project that aimed to make oil production from coal commercially viable. It died in 1985.
And we won't just need iron. Solar panels use a lot of rare earth elements, and the growth of solar energy will depend on the availability of these rare earth elements.

In this piece, Solar Panels is not the answer - 7 reasons, the approach is similar to the above - how much land is needed for the solar panels needed, and this will always be the first problem, because sunlight is diffused energy.

Yes, there is a lot of it (23,000 terawatts!) but it is spread out over 510 million sq kms. And yes, we only need 1/500 of it, but we need a lot of land area, and solar panels are not very efficient, and even if technology improves, there is a theoretical limit to how efficient solar panels can be.

But even if we were to solve the technological issue of the efficiency limit, there is also the issue of environmental impact.

When a solar panel is 50% efficient, it means that it is able to convert 50% of the sunlight to usable energy. Does that mean that 50% of the sunlight continues to fall on whatever was there before the solar panel was installed? If there were a rose bush there, and a 50% efficient solar panel was installed over the rose bush, would the rose bush be able to continue to thrive or maybe thrive on 50% of the sunlight that the solar panel is not converting to electricity?

Actually, the panel would block almost 100% of the sunlight, and convert only 50% of the solar power into energy and the rose bush would probably die from lack of sunlight. The rest of the sunlight would be wasted as heat or reflected back into the atmosphere or space.

Of course, solar panels would likely be built in the desert or places where there aren't any rose bushes.

You know, places without any kind of native flora and fauna, and where 2 million sq km of solar panels would not impact on the native life.

Or the sea, where a few tens of thousands of sq km of floating solar panels would probably not affect marine life.

Yes. I am being sarcastic.

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