Wednesday, July 29, 2009

Arrival of the Solar Power Satellite

I've been following space solar power satellite technology since O'Neill's Stewart Brand days (longer than half of you have been alive) and this is the first time it appears likely to happen in the near future:

PowerSat Corporation has filed 2 important patents. One uses the solar array to propel itself from low earth orbit to geostationary orbit. The other one turns a cloud of small geostationary solar power satellites into a huge phased array. The propulsion patent plausibly reduces launch costs by 67%. The satellite cloud microwave phasing patent, however, has a huge hidden benefit that I doubt even Powersat has fully taken into account: industrial learning curve of small launchers.

A similar argument has been made before by Autodesk founder, John Walker in "A Rocket A Day keeps the High Costs Away. Basically, if you are going to deploy a system with a large number of repetitions, the total (integral) cost is given by the formula: firstunit*(units^(1-rate))/(1-rate). To replace all fossil fuel baseload generation capacity in the US (250GW) would require 20,000 Falcon 9 HL launches (78Mdollars/15000kg or $5200/kg to geostationary transfer orbit) each launch orbiting three 5000kg BrightStars (PowerSat's satellite). Walk that down an industrial learning curve at 10% per doubling, the total launch cost of a 250GW cloud would be (1-.67)*5200*3*5000*(20000^(1-.1))/(1-.1) = 212G$ or less than a dollar per installed watt of baseload electric generation capacity.

So the ability to launch small, standard modules independently has an enormous impact on the primary historic barrier to solar power satellites.

But what about the satellite itself?

Assuming 10% energy loss in transmission to the ground array, each satellite would need to generate less than 1kW/kg (around 250GW/(3*5000*200*100)kg/(1-.1)) or 5MW/satellite. This is similar to the specific power estimate arrived at in "Low Mass Solar Power Satellites Built From Terrestrial or Lunar Materials", S. D. Potter, SSI Update, Volume XX, Issue 1 (1994). Briefly, at 35% solar conversion efficiency and 1kW/m^2 solar flux most of that satellite would be in a weightless mirror that would have to be about 140m in diameter at some fraction of 350g/m^2 (5000kg/(5MW/(.35*1kW/m^2))). Weightless mirrors can be very low mass and inexpensive.

To pay for the satellite itself let's more than triple the launch cost to reflect a total installed cost of $3/W. To understand how big of a deal this is: The other near-term scalable baseload electrical sources are "clean coal" and nuclear power — both of which are, optimistically, at similar capital costs per installed watt.

UPDATE 2014/05/06: Powersat no longer exists and the patent application is still pending.  Also, the Falcon 9 is on the verge of testing a reusable first stage which would dramatically lower launch costs.  Apparently this resulted in an article recently appearing in Aviation Week predicting this would enable power satellites, but the URL now 404s.