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A Good Idea, But....


Martha Adams

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Recent news (2009 June) around a contract by PG&E to purchase 200 megawatts of "space solar power" from Solaren, looked slightly unreal to me. That brought to mind the funny little gadget seen on the cover of a recent NSS AdAstra (Spring 2008). Which looked wrong to my engineering eye. I thought, "Something this useful for our troubled future, shouldn't look odd ... What is that?" So I tried some simple gradeschool arithmetic which I thought led to a useful sense of scale.

My assumed installation is this. A power-from-space receiving antenna farm down on Terra surface feeds its full output to a small terminal shack just outside the works. You can go into that shack, hook up whatever load you may have at hand, and draw up to 200 megawatts steady state of electric power. This power is freshly down from space. It's stable, except when Terra occults Sol from the solar power station's point of view. This is the delivery end of the system. Simple.

Now, what hardware and systems bring that power to there? Going up the chain we find: wiring and systems to deliver the power to the terminal shack. The antenna farm to receive the power as it comes down from space and deliver it into the local network. A radio transmission link from the orbiting supply machinery. A very directional transmitting antenna that delivers the power into a focussed beam to the receiving antenna farm (with appropriate control systems). A transmitter assembly that converts electric power into radio waves to send down to Terra. A large solar cells array that catches light energy from Sol to provide that electric energy. ...How much, what area of solar photocells is this system going to require?

I'm going to make an assumption here: that the system input/output efficiency, intercepted solar energy to the terminal shack, is 10%. I recognize someone could come along and make a case, No, Martha, it's 1% or even less than that.... Well, this is all speculative. I'll use 10% here, it's my ballpark guess what the well developed technology might do after incremental improvements across a few decades of practical service and maintainance.

So at 10% input/output efficiency, 200 megawatts at the terminal shack wants 2000 megawatts intercepted solar energy. Out here in Terra's orbit, the solar constant above our atmosphere is about 1400 watts per square meter. Thus to catch 2000 megawatts of solar energy wants near 1.43 x 10^6 square meters of solar cells. That's a square array (in miles), about 0.75 miles on a side.

Oops! This is where my sense comes from that something on the cover of that NSS AdAstra (Spring 2008) isn't quite right. That funny little gadget in space to beam useful power down to Terra surface? Nothing in the picture offered a strong sense of scale, but I guessed overall dimensions there in the tens of meters at most, certainly not any large hundreds of meters. And I saw no machinery space in it. Scale aside, it's incomplete and it's too small. And that observation troubled me for two reasons.

First, because for public discussion around this idea of power from space, don't we want the hardware pictured to do that job, to be shown with appropriate engineering realism? So that people talking about it, are more usefully near to talking about a reality?

And second, because this idea of bringing down electric power from space is terribly serious. It is serious because down here on Terra's surface, our need escalates for nonpolluting power. It's a forceful consequence of growing population and people seeking less onerous lifestyles. (There are stories around about the U.S. military war-gaming consequences here. I'd much prefer building power satellites to doing yet more wars.)

(I think I found at ISDC 2009 where that image came from. I guess the artist saw a picture of a bench model of a set of satellites orbiting together. This model had a framework to hold its parts in place. The artist could have seen this framework as a part of the original idea. And so the model's framework appears in the cover picture as if it were part of the actual satellite. Which picture, I notice, includes no hint of the machinery an actual power satellite would require. Seems to me, a little technical fact-checking would have been appropriate here. -- mha)

I think we can't abide, as a world culture, the waste and byproducts of our nuclear power plants. (The plutonium is also a severe political and ideological challenge.) Thermonuclear power has been 25 years in the future for 50 years or more. There doesn't seem to be a reliable low-cost way to dispose of the carbon dioxide produced by burning coal and we cannot face the consequences of not disposing of it. (Some people propose to sort it out from power plant exhaust and pump it down deep into the ground. I see no future in that idea.)

But solar power: no-waste, no byproducts, available in any amount given technology advanced and big enough, offers us one way out of our approaching crunch. Given lots of power in hand, we might even find time to solve our runaway population problems. Time! That's what I think solar power is for us, so I don't want to play childrens and PR games about it.

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