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Lunacy! February 25, 2014

Posted by Maury Markowitz in solar power satellites.
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The Luna Ring

The Luna Ring, of DOOM!

I’ve written on the topic of solar power satellites on several occasions. I thought it would be obvious to anyone that the negatives are so great that it’s a dumb idea, even if there is no night time cutting off your power half the time.

So what’s an even dumber idea? Putting them on the Moon. Now you have all the problems the problems of a satellite in terms of launch costs and beaming losses, and a night time that lasts two weeks!

And yet the interwebs are all alight with this silly idea.

Good news everyone!

Imaging a solar panel here on Earth, fixed to the ground and pointed up at the sky. When the sun is directly overhead and shining flat onto the panel, we get the maximum output, what we call AM1.5. That’s about 1000 Watts/m2, which is a lot of power, way more than you need to power your house if you could capture all of it.

But for most of the day the sun isn’t right overhead, it’s somewhere else, maybe hiding. The most obvious example is at night, which is half the time on average. There’s also weather to consider, and even things like snow or dust on the panels. But there’s another issue that’s a little less obvious, but is important for what follows.

Obviously it’s colder in winter. The reason is that the sun is at a lower angle. This means you get less hours of sunlight during the day, and even when its up it’s just grazing us. That second item is the “cosine error”. If you take a compass and measure the angle between the panel and the direct line to the sun, you’ll get an angle that’s less than 90 degrees for almost all day – the sun moves. If you calculate the cosine of that angle, you’ll get the correction for how much power you’re losing. For instance when the sun is 45 degrees off flat, you’re receiving 71% of the maximum. At sunup and sunset you’re making basically nothing.

Your panel will only make full power for a short period of time, from about 10am to 2pm. And even then, because the sun moves north and south with the seasons, even at noon it’s rarely right above the panel. When you do the math and include weather and everything else too, during the summer we only get about 5.5. hours of “bright direct sunlight” here in Toronto – and we have 14 hours of daylight. In the winter the same math gets you 2.4 hours. And that’s why it’s cold.

So the big reason to put panels in space is that your satellite can easily follow the sun and eliminate the cosine error, and nighttime as well. Instead of 5.5. hours of sunlight in the best case, it gets 24 hours in every case.

The bad news

So I can get four or five times as much energy in space, this is a no-brainer, right?

Not so fast. You see, you have to get that power to market, and even our best methods will lose half of it on the way down.

Still 2.5 times as much power sounds pretty attractive, right? Sure, but then you have to remember that space is an incredibly nasty place, and solar panels in space burn out in half the time they do here.

So you lose half in transmission, and another half in lifetime, which leaves you with about a quarter of the power left by the time it gets to market. You started with a little less than five times…

Not so attractive any more, is it?

It’s worse

So if the major reason for putting panels in space is to eliminate night and cosine error, putting the panels on any body that has a day/night cycle would be pretty stupid, right?

So them why the heck would a construction company like Shimizu think it was a good idea to put them on the Moon? That eliminates the weather problems, but everything else remains, along with the transmission problems. And dust, the Moon is dirty.

But then when you look at their other proposed projects, like floating glass cities and old images of Moon colonies, the reason becomes clear. This is a fun project to keep the art department in whiskey.

Of course what passes for the news these days can’t tell the difference. But obviously I’m tilting at windmills here; if they can’t tell that a wolf walking around a  dorm in Sochi is a fake, I really can’t expect them to figure this out.

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Comments»

1. Malus Malum - February 26, 2014

The joys of dealing with regolith not withstanding, it’s still a dumb idea. http://boingboing.net/2013/12/18/moon.html


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