Is There a Future for Space-Based Solar Power?, by Emmet Cole
Space-based solar energy production systems, commonly known as 'Solar Power Satellites' or SPS's, offer the prospect of effective, environmentally friendly electrical power.
However, experts involved in designing SPS systems agree that it will take at least ten years - but more likely decades - to develop SPS's capable of feeding the grid back on earth, as launch costs, unclear economic viability, and limited research funding slow the development of this potentially ground-changing energy technology.
But could a new wave of SPS designs bring space-based solar power closer to reality?
Most SPS concepts involve launching solar cells and power storage systems into geosynchronous orbit, where the energy produced is converted into microwaves (or a laser beam) and beamed down to special stations on earth, where it is then converted into electricity.
These designs offer several challenges. For example, the cost of getting the equipment into orbit is prohibitively expensive, especially when one is dealing with monolithic SPS's weighing approximately 10,000 tonnes (107 kg).
Further, the heat generated by the systems themselves can cause inefficiencies. And there is a range of energy inefficiencies associated with beaming microwaves to earth and converting them to electricity for the grid.
One possible solution to the technological and economic costs of using extremely heavy SPS systems would be to produce a modular system composed of lightweight, mass-produced parts, an idea that has been proposed by John C. Mankins, president of Artemis Innovation and Management Solutions.
Mankins' SPS-Alpha concept uses a network of autonomous robotic solar cells to create an SPS consisting of tens of thousands of cooperating pieces, none of which is more than one or two hundred kilograms in mass.
"The heart of the SPS-Alpha concept is to hyper modularize the idea of the solar powered satellite to go with a very modular biomimetic kind of architecture, and thereby achieve affordability," Mankins told Space Quarterly.
'If you think of it metaphorically, it's the difference between an elephant and a colony of ants or a swarm of bees. You can have the same kind of functionality in a network system, but by using a very large number of mass produced system elements, you have the opportunity to achieve very low hardware cost with the same functionality."
Set-up costs have been a problem associated with SPS designs since they were first studied by NASA and the U.S. Department of Energy in the late 1970s. That research resulted in a system design that could theoretically deliver about 10 gigawatt's (GW) of power on earth, using a large (10 km by 15 km) solar array located in geosynchronous orbit.
It wasn't until NASA's 1995 "Fresh Look" SPS study that new designs were proposed, using low earth orbits, sun-synchronous orbits, and large-area Fresnel lenses to focus light onto panels of concentrator cells. Subsequent Space Solar Power (SSP) Exploratory Research and Technology (SERT) research returned NASA's thinking on the topic to geosynchronous orbit designs, but looked at the possibility of increasing energy efficiency by using laser beams instead of microwaves to transmit the power to earth.
Several factors at least partially mitigate the costs associated with SPS launch and energy conversion inefficiencies.
The intensity of sunlight in space is about 30 percent greater than on the ground, where the maximum sunlight is about 900 or 1,000 watts per square meter. By contrast, the sunlight in space has a constant intensity of about 1,400 watts per square meter. SPS's don't suffer from seasonal variations, the vagaries of atmospheric water, or the light-denying cycle of day and night.
"In the case of conventional solar powered satellite concepts, there are a number of very critical technology issues like huge power management and distribution systems and so on. In the case of SPS-Alpha, all of those go away, because the system is modular and power is generated in a small area and used in that small area," explains Mankins.
If a modular approach to SPS's is adopted, Mankins predicts that a major system level demonstration could be set up in 10-12 years.
"It's critical that there be a major system level demonstration of space solar power before significant - and by that I mean billions of dollars of commercial investment - so the question is how much will it cost and how much time will it take to get to a major system level demonstration?"
In 2011, Mankins edited a comprehensive report on the findings of a space solar power study conducted by the International Academy of Astronautics (IAA). The three-year, ten-nation study, "Space Solar Power -- The First International Assessment of Space Solar Power: Opportunities, Issues and Potential Pathways Forward," suggested that SPS technology could be ready within the decade.
However, reusable launch vehicles need to become cheaper and more efficient for SPS's to become reality, says Mankins.
"The key for viability for SPS-Alpha or for any SPS is to be able to use commercial launchers and not require a special launch infrastructure that's only for solar powered satellites, and to be able to have lots and lots and lots of launches," says Mankins.
'Solar powered satellites have sometimes been characterized as the perfect market for reusable launch vehicles because they involve launching lots of essentially identical pieces. I mean thousands and thousands of packages."
Mankins' research into the concept is being supported by the NASA Institute for Advanced Concepts (NIAC).
Elsewhere, SPACE Canada (Solar Power Alternative for Clean Energy ) Solar Power, a not-for-profit organization dedicated to the promotion of solar energy from space, advocates a traditional design that uses an SPS in geosynchronous orbit that beams power back to earth.
And in Europe, EADS Astrium has been working on early designs for an SPS that uses infrared lasers rather than microwaves to transmit the energy back to earth. The company has already demonstrated power transmission via laser in its labs, and is now working on improving the efficiencies of the end-to-end system. One possible limitation of the system is that it could require huge lasers to function. The company is expected to launch an experimental system for testing within a decade.
In collaboration with Mitsubishi Electric Corp. and industrial design company IHI Corp., the Japanese government is exploring the possibility of launching a $21 billion solar-powered generator capable of producing 1GW of energy, or enough to power 294,000 homes.
Meanwhile, at NASA's John Glenn research center in Cleveland, Ohio, scientists have been working on individual technologies for the parts that could be used in SPSs.
"The largest solar power system ever sent into space is the International Space Station and that's just much, much smaller than the power systems that would be needed for space based solar power. Our particular interest here has been the solar cells and solar arrays and power distribution systems that will help us to make larger solar arrays," NASA research scientist Geoffrey Landis told Space Quarterly.
In collaboration with the U.S. Air Force, the National Renewable Energy Laboratory, and industry partners, Glenn researchers have been working on a new type of solar cell design called an inverted metamorphic solar cell (IMM).
IMM solar cells are made of lightweight crystal-like cells and offer high flexibility, making them ideal for space applications, possibly including energy production. Glenn researchers believe these cells have the near-term potential to reach efficiencies of about 35%, says Landis, "but in the far term we're hoping for much better than that."
Landis' calculations reveal that for a capital cost of about $17.5 billion a system could produce about 5GW of power. However, he also found that to sell electricity at a competitive $0.05 per kilowatt-hour, the total cost of the system must not exceed $3.50 per watt.
This represents a challenging target, says Landis, since it includes not only the solar arrays but also the entire in-space structure as well as the ground assets.
It's difficult to say when space-based solar power systems will be capable of supplying the grid on earth and we may be better off focusing on the market that will arise beaming power to spacecraft and possible lunar settlements first, says Landis.
"Space technology development is hard to predict. I would say that it would be no earlier than 15 years from whenever they decide to go forward with the idea. It would take at least 15 years to put together all of the different technologies along with regulation and safety studies. All these elements take time," Landis says.
In the meantime, a market could be created around ground-based power systems, using large aperture antennae that can send energy beams to space-based receivers.
Electrical power in space has an effective price tag that is 10,000 times the price of power on the ground, Landis notes, and it makes more sense to beam power to the place where it is expensive from the place where it is cheap.
The 'driving challenge' for space-based solar is the cost of launch vehicles, says Landis, with one possible solution to this problem being to make SPS's out of resources that you find in space.
"So, instead of digging silicon out of the ground and making solar cells, we could go to an asteroid and use its resources to make solar arrays. That would be a nice way to do it because then you wouldn't have to ship things up from the ground. Of course, this would require a big investment in infrastructure in space," Landis says.
A possible solution to the problems caused by complicated conversion systems and the prohibitive costs associated with launching traditional, heavyweight SPS's has been proposed by Lewis Fraas, president of Boeing spin-off JX Crystals, a company that designs and manufactures both aerospace and terrestrial solar cells and power systems.
In Fraas' conceptualization, a series of eight huge, 250 meter diameter mirrors, positioned 1,000 km above the earth's surface, reflect sunlight back to terrestrial solar farms, enabling those farms to operate for two extra hours at dawn and dusk each day.
"There are three ways to make solar more cost-effective: first is to make it cheaper through cheaper hardware; second is to make the cells more efficient; and, third is to increase the number of hours of sunlight. Terrestrial solar farms could significantly increase energy production with four extra hours of sunlight each day," Fraas told Space Quarterly.
"The bottom line is, imagine if you could go from 8 hours a day to 12 hours a day on a terrestrial site. The other advantage is that the design is really very simple in concept - you just reflect the sunlight. It's pretty straightforward, but not easy."
The idea of using mirrors in space to feed terrestrial solar farms was first proposed in 1978 by German rocket scientist Krafft Ehricke in 1978. Ehricke's "Power Soletta" system proposed a constellation of satellites in a 4,200 km orbit beaming power down to a 1200 sq km location in Western Europe.
"It was a brilliant idea," says Fraas, "but it was too far up. At 4,200 km altitude, the ground station was very big (180 GW) - and there are many problems involved in distributing all that from one location," says Fraas.
In total, terrestrial solar installations provide around 65 GW of energy today now and energy production is increasing at a rate of more than 30% per year. Projecting out ten years from now, it's "not unreasonable" to consider 5 GW ground solar stations, Fraas explains.
Postulating an Earth with 40 distributed ground stations and 24/7 availability of the mirrors, Fraas calculated that even at with projected launch costs of $1,100 per kg, the mirror system would pay for itself after two years in operation.
At least in part, Fraas attributes this relatively swift economic return on investment to the lightweight materials used in the mirror's design - essentially aluminum thin film coating on a mylar substrate just a few microns thick.
"The economics are uncertain," admits Fraas. "But they look better than what is being considered with conventional SPS designs. I may make some enemies by saying that but I hope not."
Fraas, who is due to deliver a paper on his proposed SPS design and the economics involved at the IEEE's PVSC 39 in Austin, Texas in June, speculates that SPSs could be feeding the grid on earth within ten years.
"It won't be much before that, although some people I've talked to say it could be sooner. A lot depends on vision, economics, and political momentum," says Fraas.
SPSs won't be supplying power to the terrestrial grid anytime soon as researchers are still working through different designs to find the most economically viable systems to launch into orbit. To some extent, the timeline is not entirely in the hands of SPS experts. Only when low-cost, reusuable launch vehicles become reality, will the future of space-based solar power systems really start looking bright.
- Lewis Fraas' IEEE PVSC 38 Paper, "Mirrors in Space for Lower Cost Terrestrial Solar Electric Power at Night."