Space-Based Solar Power is getting a revisit. Peter Glaser’s original concept (1968) – rather than the vague hand-wavium of “energy beams” that Isaac Asimov invoked in his 1941 short story “Reason” – sketched the basics of beaming energy to Earth via microwaves.
The 1970’s saw the first serious studies by the USA’s Department of Energy and NASA. A typical photovoltaic concept from the day would mass 50,000 tonnes and deliver 5 gigawatts to the ground. A specific power of 10,000 tonnes per gigawatt seemed unavoidable and so too the need for massive space-launcher infrastructure.
In the +40 year since those studies new concepts have arisen, solving many of problems which the complex rotating beaming systems tackled by brute force. The classic Solar Power Satellite needed two systems which added significant mass and complications:
(1) Power distribution to get the electric energy from the photovoltaics to the microwave beaming system, which requirement large masses of cabling hardened against space conditions
(2) A rotating connector to the microwave beamer, which by necessity always points to one spot on the Earth, even while the photovoltaic array must always point towards the Sun. Therefore a moving part as a potentially fatal point of failure.
Both John Mankins and Ian Cash have developed ultra-low mass concepts (~1,000 tonnes per gigawatt) that side-step both issues by clever design, while the Chinese are getting bullish about the prospects for launching a demonstration system into orbit by 2035.
Yet we’re seeing a rapid uptake in energy storage systems that can smooth out supply from intermittent renewable energy sources here on Earth. Can space supplied energy really compete against storage on the ground?
First, how much net energy gain can be achieved by something that has to be boosted to orbit? In raw energy terms, the potential and kinetic energy of every kilogram in Geosynchronous Orbit is a combination of the difference in potential energy, compared to being on Earth, and its orbital kinetic energy. The former is 53.041 MJ/kg while the latter is 4.619 MJ/kg – 57.660 MJ/kg total. A 2,000 tonne CASSIOPEIA power satellite produces about 2 gigawatts power. It can return the total energy invested in just 57,660 seconds – 16 hours. The terrestrial energy inputs to make the satellites and assemble the raw materials are trivial by comparison.
If the mission is launched by SpaceX’s starship, a payload of 100 tonnes delivered to GSO requires refueling in Low Earth Orbit (LEO) with the empty Starship aerobraking to return to Earth for the next payload. The empty mass of the Starship is 120 tonnes and return from GSO into a re-entry trajectory requires 1.5 km/s delta-vee, so 60 tonnes of fuel is required. Thus Payload + returning Starship masses 280 tonnes. From LEO to GSO needs 3.8 km/s delta-vee. Adding in a margin, 500 tons propellant is needed per 100 tonnes to GSO. A Starship Tanker delivers 150 tonnes propellant to LEO, thus 4 Tankers can supply 600 tonnes propellant and 20 Tanker launches therefore delivers enough to deliver 600 tonnes to GSO. Four times that gets 2,400 tonnes to GSO – 1.2 CASSIOPEIA Power Satellites. 12,000 tonnes in GSO is 6 CASSIOPEIA’s delivering 6 GW to the ground.
All up a Starship Heavy launch requires 4,600 tonnes propellant – liquid methane and liquid oxygen. As the oxidiser out-masses the fuel 3.55 to 1, the energy equivalence is the same as burning ~1,000 tons of natural gas. 12,000 tonnes in GSO needs 2,392,000 tonnes propellant. According to ENGIE, a South Australian energy wholesaler, a combined cycle Gas-Steam Natural Gas power plant can get 53% thermal to electrical efficiency, which means 2,392,000 tonnes propellant (525,714 tonnes methane) is 2.63E+16 Joules of thermal energy, or 1.4E+16 joules of electrical energy. Some 648 hours of 6 gigawatt power-supply. 27 days worth of power from those CASSIOPEIA’s. As they expected to run for 20 years, at least, then it’s very much a NET gain in energy.
While the propellant mass sounds very large, consider the fact that a 1 GW Coal-fired power plant that’s 35% thermally efficient and burning coal at the calorific value of 30 MJ/kg will burn 3,000,000 tonnes of coal per annum – if it’s 100 % pure carbon. If it’s 85% carbon and 10% silicates, then it also produces 360,000 tonnes of ash. As well as 10 million tonnes of carbon dioxide.
So a Solar Power Satellite can be low-polluting and energy effective – how does it compare to Terrestrial Storage? Part Two coming soon.