Saturday, March 5, 2011

The Watchman's Rattle and Space-based Solar Power


So as stated in my last post, I'm reading "The Watchman's Rattle" by Rebecca Costa, which, while it's hitting a lot of good psychological points, seems to be simply incoherent on energy issues.

I've gotten to Ms. Costa's story concerning space-based solar power, claiming that it's a fine example of what she calls "Silo Thinking".

Silo thinking, or bunkering, as I've called it at work, is essentially when the wall you have to throw ideas over to collaborate are a bit taller than your arm is strong.  It's what happens when you have territorial management.  It's a clear point, it happens fractally as you escalate the scope of organization, and it's something that restricts the ability of an organization to handle large and complex projects.

The problem isn't with the example, which, if true, would be a prime example of the issue.  The problem is, she's over-selling space-based solar.

I won't bother quoting the book.  Her explanation is pretty bad where it hits details, and pretty light on the details in general.

Space-based solar is a simple concept: you throw a satellite into geosynchronous orbit*, unfurl a PV or heat capturing device, convert the incoming energy into electricity, and fire a microwave laser at the earth's surface where, hopefully, there's an energy capture dish.

There are obvious potential safety issues with the concept, but they're entirely soluble with good engineering.  Meanwhile, materials science helps as well with potential efficiency.

However, Costa sells this as "unlimited free energy", which it isn't, at least, no more than rooftop solar is.  She also sells space-based solar as having efficiencies "magnitudes greater than what we achieve by laying solar panels on our roofs".

Solar flux is limited to about 450 watts / sq picoradian. That equates to about 1 kW / m^2 at the equator on a sunny day, or 1.4 kW / m^2 at geostationary orbit.  To convert this flux, we use either a heat engine or photovoltaics.  Neither solution has very high efficiencies: 33% for thermal, 25% for PV.  Both ground-based and geostationary solar is only active about 1/3 of every 24 hour cycle.  Ground-based has about a 50% duty cycle even from that, due to weather conditions.  Space based solar must then transmit the power down to earth, at an efficiency of about 84%.

Now, while that means that space-based solar can be as much as 3.1 times as efficient, that's only a single order of magnitude if you're using e (which I do endorse, as it's a more "natural" growth measure), but I believe the conventional wisdom is that a 10x improvement constitutes an order of magnitude.

Then there's the cost.  Even assuming the use of lightweight, thin-film solar panels, held out using a minimum scaffolding, you're still only talking about 300 W/kg, at ~$3/W for panel capacity alone ($9/W to accommodate the 33% duty cycle), and a cost to orbit of about $1200/kg.

In power generation, we talk about dollars per watt (Power cost).  The current prevailing limit of profitability for electricity production is at about $3/W.  SBSP, including just those factors, is $13/W.  That is not free.  That has to be re-spent every time the panels get too degraded by micrometeors, an estimated 10 years.  That's $0.14/kWh just for generation, without profit added - which is currently more than I pay for all electricity-related service.

It's no secret I'm a fan of nuclear power, and moreover of LFTR.  The reason for this is that I've looked into solar, wind, and even the lame "magnetic motor perpetual motion" self-delusions that people have, and have found them to be too diffuse, too expensive, too variable, or any combination of the three.  Nuclear power, well executed**, is an effective solution to climate change.

* Or use geosync sats as relays for giant SBSPs at the leading and trailing lagrange points of the earth.

** i.e., with reprocessing where it's needed, and with a focus on building reactors that continuously reprocess, such as LFTR.

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