Watching the recent soft splashdown of Space Exploration Technology’s (SpaceX’s) Falcon-9 First Stage I felt that I was witnessing an almost historic moment. Almost. When the first SpaceX Falcon achieves a soft-landing on land, then we’ll know that the real beginning of the Space Age has arrived. Widespread use of a reusable first stage will drive the costs of launch down by 70% from SpaceX’s already low value. To get lower, the volume of traffic to space must increase, but the hinted ~$40 million price-tag to launch 53 tonnes (~$800/kg) via the Falcon Heavy booster means a long expected space-technology starts looking viable: Solar Power Satellites or Power-Sats.
The dream of beaming continual solar-power from space to the ground, providing the first direct space-resource for terrestrial use, has been around in fiction since the 1940s and in serious physics-based proposals since Peter Glaser’s seminal paper in 1968, developed more fully by the series of studies conducted by NASA and the US Department of Energy from 1978-1980. One outcome of the NASA-DoE studies was the 1980 conceptual Reference Design which was the subject of a report to Congress by the Office of Technology Assessment. The Design described a 5 gigawatt Power-Sat, with an area of 55 km2 and a mass of 50,000 tonnes, which would convert raw sunlight into microwave energy and send it to Earth at efficiency of just 6.7%. Part of the system inefficiency was the assumed 13% efficiency of the photovoltaic cells used.
Since the late 1970s the efficiency of photovoltaic cells have improved, especially in so-called multi-junction cells, which use multiple layers, each tuned to a different part of the spectrum. Combined with concentrating optics, which focus the light onto the cells, the efficiency of lab cells is presently pushing 45%. Wired up into an array and they might achieve ~37% efficiency. Newer light-to-electricity concepts seek to improve upon even that high mark. Here’s an example. A new preprint, by Manor, Martin & Rotschild, describes a way of pushing photovoltaics to 69% efficiency by using heat as well: http://arxiv.org/abs/1404.7345
The operating temperatures are high, but doable with concentrators and the right materials. That makes it very attractive for Solar Power Satellite deployment, so long as the cold end can be kept cold efficiently. The hot end is ~1200 K, while the effective temperature at Earth’s orbit is ~400 K, a ratio of 3-to-1. Illumination to achieve a given temperature scales with the four-power i.e. (3)^4. Thus ~81 times concentration of sunlight is needed. For a Power-Sat delivering 1 GW power on the ground, with 50% power-transfer efficiency, that’s 2 GW (electricity) to be produced in orbit. At ~67% efficiency conversion efficiency we need 3 GW of raw sunlight, which about 2.3 square kilometres of collector – just 4.2% the size of the NASA-DoE Reference Power-Sat.
Such an area, 2.3 million square metres, sounds like a lot, but being a concentrator system it means only low-mass reflectors are needed, focussed on the much smaller convertor systems. Thus the total collector mass could be 0.01 to 0.001 of the 1970s Reference Design. A 1 gigawatt Power-Sat, based on the 2014 design, might mass ~200 tonnes for the collector and a significant mass for the microwave antenna array to beam mass back to Earth. Perhaps 400 tonnes total. Due to the nature of microwave transmission from geosynchronous orbit at 35,786 km altitude, the antenna array is about a kilometre in diameter. Such large structures can potentially be assembled via automated systems like the SpiderFab robotic construction satellite, being developed by Tethers Unlimited for NASA. The structure would be an open wire-work rather than solid sheets, thus potentially able to average less than 0.1 kg per square metre. Wired together on-site, the structure can be more fragile than if it had to be folded up for launch.
SpaceX expect to slash the cost of Space Launch by 70% with reusable 1st Stages. Imagine the Falcon Heavy lofting 55 tonne SPS modules. Part of the array opens up to power an electric propulsion system, using 5 tonnes propellant for a GEO transfer. Once in the correct orbit it opens up fully, like a field of solar flowers. Eight components are lofted, meeting up and joining together to form a 1 GW Power-Sat, sending 2 GW of microwave power Earthwards, which is picked up by a Rectenna Farm – a field of conducting wires on poles, with crops growing underneath. After factoring in all the losses, the system supplies 1 GW of totally carbon-free power to the grid. 24 hours a day, 7 days a week – even in bad weather and at night. No gigantic banks of flow-batteries needed and no vast tanks of molten salt to store heat for night-time power supply. Sold at an average rate of $0.10/kilowatt-hour that 1gigawatt from space earns $877 million/yr. Over a 15 year lifetime it might earn ~$13 billion (constant $). Space Launch and construction costs need to be some preferably small fraction of that, to produce a significant Return-on-Investment (RoI). That’s where robotics is needed, but there will be some need for manned maintenance, perhaps via teleoperated machinery, which is advancing all the time.
Of course my ~400 tonnes per gigawatt estimate is optimistic, with large low-mass reflectors focussing sunlight onto a high-efficiency power-converter combined with a gossamer microwave antenna made by SpiderFab satellites. Consider it a challenge target, but one that is starting to look achievable. If Falcon Heavy launches can drop from the current $135 million each to about $40 million, and self-propulsion via solar-electric engines can be as effective as described, then the whole thing might be assembled using just 8 launches. With launch costs of just ~$320 million, then SPS might be on its way to costing less than coal ($1-$2 billion per gigawatt, upfront costs). The true Space Age would lead to the Solar Age, which can potentially last for the next 5 billion years of the Sun’s Main Sequence lifetime.
In a previous blog I discussed the prospect for propelling starships via vast numbers of Power-Sats near the orbit of Mercury. Before we develop such advanced laser or mass-beam systems, we have the potential to propel our first star-probes via microwave beams. The same science and engineering skills for operating such systems will also give us the means to power the Earth. Thus star-flight technology could pay for its own development.