Power satellites convert sunlight (via photovoltaic or thermal cycle) to electrical power and then turn the power into microwaves beamed to the ground and converted back to electrical power.
Critical to achieving costs of $100/kg or less to get material to geosynchronous orbit is to use spaceplanes that can fly frequently and a laser boosting system. This launch complexity would not be needed if there were simpler ways to achieve the cost target of $100/kg or less. However, Keith is putting together systems that have active development and some momentum towards actually being developed within the next 10-20 years.
Power satellites are a way of harvesting dilute solar energy with several advantages over the solar PV on the ground or rooftops:
* A system of power satellites scales to human civilization’s needs (tens of TW).
* They don’t need storage since their location (the 24 hour orbit, geosynchronous or GEO) is illuminated 99% of the time. (Satellite TV antennas point to a location on that orbit.)
* No day-night cycle and no clouds or air gives power satellites an average advantage of about nine times over the same area of solar collectors on the ground.
* Power satellites use relatively little material. Being in orbit (zero gravity), and no wind they can be much lighter per kW than collecting sunlight on the ground.
* They have a very short energy payback time.
They have some disadvantages, however:
* For optical reasons, they don’t scale down to small sizes; 5 GW is about as small as you want to make one.
* At 50% loss electricity-in space to electricity-on-the-ground, the cost is doubled from one cent per kWh to two. On the other hand, that’s 40 times less cost than transmitting the same power over wires for the same distance.
* They take a large investment to get the cost of transporting parts to GEO down to where they make economic sense.
Cost Requirements to make Space Solar Power Economical – $100/kg or less to orbit
Is a space solar project worth doing? We need to run a cost/benefit analysis to find out.
For a ten-year return on capital, a kW of power sold for a penny a kWh generates $800 of revenue (~80,000 revenue-hours in ten years). Two cents per kWh is about the most power could sell for to displace coal. That means a kW of power satellite capacity can’t cost more than $1600 or $1.6 B per GW if it is to meet this goal.
If power satellites take 5 kg of parts to generate a kW on the ground, and the transport fraction is ~1/3, then the cost to lift parts to GEO can be no more than $100/kg. That’s a reduction of 200 to one ($20,000 per kg down to $100) over current cost to deliver communication satellites to GEO.
Hiroshi Yoshida, Chief Executive Officer of Excalibur KK, a Tokyo-based space and defense-policy consulting company, and William Maness, chief executive officer of Everett, Wash.-based PowerSat Corp., both think it will take this kind of transport cost reduction for power satellites to be competitive with other power sources.
Can we get to this lift cost with conventional rockets?
Unfortunately, the answer is no, for several reasons. The chemical energy in rocket fuel vs. the required energy it takes to get to orbit is not enough. Rocket technology with chemical fuels has reached the performance limit. The most promising design is the Falcon Heavy (a proposal of SpaceX), with first launch intended for 2012 at a cost of $100 M per trip. The rocket is expected to put 53 tons in low earth orbit (190 km) above the earth’s surface, or 19.5 tons in geostationary orbit at 36,000 km. That is a reduction to $4000/kg, a factor of five below current rockets, but not enough. Launching a Falcon Heavy every hour might get the price down to $1000/kg, which is still too high by a factor of ten.
Reaction Engine’s study of Skylon indicates it will put 12 tons in LEO or (with a second stage) 5 tons in GEO for an estimated cost of $1.5 M or $300/kg. The project goal is to develop an unpiloted space plane that can be re-used up to 200-500 times.
Skylon Sub Orbital plus Laser Propulsion
The new concept presented here is to use the Skylon sub orbital maximum load of 30 tons for a second stage (see Appendix: Into Orbit—Sideways below, also Wikipedia-Multistage Rocket). The second stage propulsion would be hydrogen heated to 3000 deg K by a (relatively small) 500 MW array of ground-based laser.
The system with ground-based lasers would function as follows:
* The laser beams go up to an array of tracking mirrors in geosynchronous orbit over a point 3500 km to the east of where the second stage release point.
* The ground lasers point at the bounce mirrors which track the accelerating laser powered second stage over 11 degrees. (The mirrors move 5.5 degrees in 16 minutes.)
* Hydrogen at 3000 deg K gives the second stage ~10km/s exhaust velocity. This velocity is about twice the required delta V for a mass ratio of ~1.65.
In this system,18-20 tons of 30 will arrive at GEO per flight. The capital cost for the lasers ($5 B or $500 M/year) is about $1/kg when spread over 480,000 tons of cargo per year and it drives the lift cost for parts down to $100/kg.
Conventional use of Skylon will deliver about 5 tons per flight to GEO. For a three per hour flight rate, that’s 15 tons per hour. By adding $5 B of lasers (and the GEO bounce mirrors), laser boosting a sub orbital payload will put 60 tons per hr in GEO, that is, 4 times as much. In calculating the economics the following assumptions are made:
Operating this transport system 90% of the time, it lifts 8000 h/yr x 60 t/h or 480,000 t per year. (That would support a substantial power satellite production.)
At 5000 t/GW, it would make 96 GW per year (19 five GW power satellites).
At a price of $1.6 B/GW (2 cents per kWh paid off over ten years), the revenue stream from selling power satellites would be over $150 B per year.
The capital cost for the lasers ($500 M/year expensed at 10%/year) is about $1/kg when spread over 480,000 tons of cargo per year and it drives the lift cost for parts down to under $100/kg. The cost to GEO (not LEO) would come down to under $100/kg, which is the magic number for two cent per kWh power, i.e., half the price of coal (or less).
This is by no means a fully worked out proposal. For example, how do we get sub orbital Skylons back to their runway? Can we really heat hydrogen with a laser to 3000 deg K? That is 600 deg below melting tungsten but questionable. Carbon is a solid up to 3900 deg K, but at that temperature will it become hydrocarbons?
More analysis might find the entire project to profitability as low as $40 B (half for Skylon development). If that’s the case, it’s close to the Chunnel or Three Gorges Dam in current dollars.
Background on laser launch