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The increasing energy demands to support continued (and sustainable) global economic growth, thrust for renewable power because of environmental/climate concerns, has inspired researchers to look for fundamentally new energy technologies. Space-based solar power (SBSP, SSP) is the concept of collecting solar power in outer space by solar power satellites (SPS) and distributing it to Earth.
Space-based solar power (SBSP) — in which Miles-long satellites covered with solar panels capture the Sun’s radiation, convert it to electricity and then transmit it back to Earth in the form of either microwaves or lasers could form the basis of unlimited, renewable electricity.
The advantage of collecting solar energy in space is a higher collection of energy due to the lack of reflection and absorption by the atmosphere, and the possibility of placing a solar collector in an orbiting location where there is no (or very little) night, and better ability to orient to face the sun. A considerable fraction of incoming solar energy (55–60%) is lost on its way through the Earth’s atmosphere by the effects of reflection and absorption.
SBSP is considered a form of sustainable or green energy, renewable energy, and is occasionally considered among climate engineering proposals. It is attractive to those seeking large-scale solutions to anthropogenic climate change or fossil fuel depletion (such as peak oil).
The SPS would also be useful for disaster missions, a thin, portable rectenna can be unfolded and deployed to receive microwaves from space, which can be converted into electrical energy.
In addition to providing constant renewable energy to the planet, a space solar power plant could, in theory, focus its beam outward and power spacecraft, obviating the need for solar cell wings and greatly increasing power levels and control accuracy.
That power could also used in space to meet the energy demands of future space mining and resource extraction operations. NASA is examining how space solar power could support robotic mining operations on the moon or asteroids–a stepping stone toward enabling long-term human space exploration and possible colonization of the solar system beyond Earth. The energy beams could also direct power to remote areas or even dissipate destructive weather systems like typhoons.
SBSP is being actively pursued by Japan, China, Russia, India, the United Kingdom and the US. In 2008, Japan passed its Basic Space Law which established space solar power as a national goal and JAXA has a roadmap to commercial SBSP.
In 2015, the China Academy for Space Technology (CAST) showcased their roadmap at the International Space Development Conference. In February 2019, Science and Technology Daily (科技日报, Keji Ribao), the official newspaper of the Ministry of Science and Technology of the People’s Republic of China, reported that construction of a testing base had started in Chongqing’s Bishan District. Chinese scientists were reported as planning to launch several small- and medium-sized space power stations between 2021 and 2025. In December 2019, Xinhua News Agency reported that China plans to launch a 200-tonne SBSP station capable of generating megawatts (MW) of electricity to Earth by 2035.
The US Military has also become interested in this concept as it would save their billions in fuel costs as well as provide ultimate flexibility in their expeditionary missions as solar power could be redirected anywhere on the planet. Ralph Nansen from the US-based advocacy group Solar High, urges the US to act on this because he believes that whoever develops SBSP first, will have a monopoly position in the world economy, just like England did during the industrial revolution because of coal.
Technology requirements
Space-based solar power faces major challenges including economic feasibility and manufacturing costs, cheap and reliable launch services, and efficient and safe energy transmission.
Besides the cost of implementing such a system, SBSP also introduces several technological hurdles, including the problem of transmitting energy from orbit to Earth’s surface for use.
Since wires extending from Earth’s surface to an orbiting satellite are neither practical nor feasible with current technology, SBSP designs generally include the use of some manner of wireless power transmission with its concomitant conversion inefficiencies, as well as land use concerns for the necessary antenna stations to receive the energy at Earth’s surface.
The collecting satellite would convert solar energy into electrical energy on board, powering a microwave transmitter or laser emitter, and transmit this energy to a collector (or microwave rectenna) on Earth’s surface.
Various SBSP proposals have been researched since the early 1970s, but none are economically viable due to the expense of launching material into orbit with present-day space launch infrastructure.
Contrary to appearances of SBSP in popular novels and video games, most designs propose beam energy densities that are not harmful if human beings were to be inadvertently exposed, such as if a transmitting satellite’s beam were to wander off-course. But the vast size of the receiving antennas that would be necessary would still require large blocks of land near the end-users to be procured and dedicated to this purpose. The service life of space-based collectors in the face of challenges from long-term exposure to the space environment, including degradation from radiation and micrometeoroid damage, could also become a concern for SBSP.
What once seemed impossible, space policy analyst Karen Jones of Aerospace Corporation says, may now be a matter of “pulling it all together and making it work.” Today, both space and solar power technology have changed beyond recognition. The efficiency of photovoltaic (PV) solar cells has increased 25% over the past decade, Jones says, while costs have plummeted. Microwave transmitters and receivers are a well-developed technology in the telecoms industry. Robots being developed to repair and refuel satellites in orbit could be turned to building giant solar arrays.
Space-based solar power essentially consists of three elements:
- collecting solar energy in space with reflectors or inflatable mirrors onto solar cells or heaters for thermal systems
- wireless power transmission to Earth via microwave or laser
- receiving power on Earth via a rectenna, a microwave antenna
The space-based portion will not need to support itself against gravity (other than relatively weak tidal stresses). It needs no protection from terrestrial wind or weather, but will have to cope with space hazards such as micrometeors and solar flares. Two basic methods of conversion have been studied: photovoltaic (PV) and solar dynamic (SD). Most analyses of SBSP have focused on photovoltaic conversion using solar cells that directly convert sunlight into electricity. Solar dynamic uses mirrors to concentrate light on a boiler. The use of solar dynamic could reduce mass per watt. Wireless power transmission was proposed early on as a means to transfer energy from collection to the Earth’s surface, using either microwave or laser radiation at a variety of frequencies.
If a space-based power station ever does fly, the power it generates will need to get to the ground efficiently and safely. In a recent ground-based test, Jaffe’s team at NRL beamed 1.6 kilowatts over 1 kilometer, and teams in Japan, China, and South Korea have similar efforts. But current transmitters and receivers lose half their input power. For space solar, power beaming needs 75% efficiency, Vijendran says, “ideally 90%.”
The safety of beaming gigawatts through the atmosphere also needs testing. Most designs aim to produce a beam kilometers wide so that any spacecraft, plane, person, or bird that strays into it only receives a tiny—hopefully harmless—portion of the 2-gigawatt transmission. Receiving antennas are cheap to build but they “need a lot of real estate,” Jones says, although she says you could grow crops under them or site them offshore.
Orbital location
The main advantage of locating a space power station in geostationary orbit is that the antenna geometry stays constant, and so keeping the antennas lined up is simpler. Another advantage is that nearly continuous power transmission is immediately available as soon as the first space power station is placed in orbit, LEO requires several satellites before they are producing nearly continuous power.
Power beaming from geostationary orbit by microwaves carries the difficulty that the required ‘optical aperture’ sizes are very large. For example, the 1978 NASA SPS study required a 1-km diameter transmitting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz. These sizes can be somewhat decreased by using shorter wavelengths, although they have increased atmospheric absorption and even potential beam blockage by rain or water droplets. Because of the thinned array curse, it is not possible to make a narrower beam by combining the beams of several smaller satellites. The large size of the transmitting and receiving antennas means that the minimum practical power level for an SPS will necessarily be high; small SPS systems will be possible, but uneconomic.
Earth-based receiver
The Earth-based rectenna would likely consist of many short dipole antennas connected via diodes. Microwave broadcasts from the satellite would be received in the dipoles with about 85% efficiency. With a conventional microwave antenna, the reception efficiency is better, but its cost and complexity are also considerably greater. Rectennas would likely be several kilometers across.
NRL conducts first test of solar power satellite hardware in orbit
The U.S. Naval Research Laboratory (NRL) is building a “sandwich” module; the top side is a photovoltaic panel that absorbs the Sun’s rays. An electronics system in the middle converts the energy to a radio frequency, and the bottom is an antenna that transfers the power to a target on the ground. Ultimately, the idea is to assemble many of these modules in space by robots — something the NRL’s Space Robotics Groups is already working on — to form a one kilometer, very powerful satellite.
U.S. Naval Research Laboratory engineers launched PRAM, the Photovoltaic Radio-frequency Antenna Module, aboard an Air Force X-37B Orbital Test Vehicle in May 2020 as part of a comprehensive investigation into prospective terrestrial use of solar energy captured in space. “To our knowledge, this experiment is the first test in orbit of hardware designed specifically for solar power satellites, which could play a revolutionary role in our energy future,” said Paul Jaffe, PRAM principal investigator. The 12-inch square tile module will test the ability to harvest power from its solar panel and transform the energy to a radio frequency microwave.
“PRAM converts sunlight for microwave power transmission. We could’ve also converted for optical power transmission,” said Chris Depuma, PRAM program manager. “Converting to optical might make more sense for lunar applications because there’s no atmosphere on the Moon. The disadvantage of optical is you could lose a lot of energy through clouds and atmosphere. “ The use of solar energy to operate satellites began at the start of the space age with another NRL spacecraft: Vanguard I, the first satellite to have solar cells. This current experiment focuses on the energy conversion process and resulting thermal performance. The hardware will provide researchers with temperature data, along with PRAM’s efficiency in energy production. This information will drive the design of future space solar prototypes.
Depending on the results, the team aims ultimately to build a fully-functional system on a dedicated spacecraft to test the transmission of energy back to Earth. The development of a space solar capability could potentially help provide energy to remote installations like forward operating bases and disaster response areas. This flight experiment enables researchers to test the hardware in actual space conditions. Incoming sunlight travels through the Earth’s atmosphere, both filtering the spectrum and reducing its brightness. A space solar system traveling above the atmosphere would catch more energy from each of the sunlight’s color bands.
“There’s more blue in the spectrum in space, allowing you to add another layer to solar cells to take advantage of that,” Jaffe said. “This is one reason why the power per unit area of a solar panel in space is greater than on the ground.” The National Security Space Office recommended in a 2007 feasibility study to investigate solar power satellite technology. NRL’s expertise with solar-powered satellites since the late 1950s and long history as a pioneer in space, including in the development of GPS, led researchers to further explore this emergent field.
Contributing and supporting partners for this effort included the Operational Energy Capability Improvement Fund in the Office of the Under Secretary of Defense for Research and Engineering, the U.S. Naval Research Laboratory, the Department of the Air Force Rapid Capabilities Office, the Department of Defense Space Test Program, Boeing, TSC Praxis Operations, Gulfview Research, Odin Engineering, and SpaceQuest.
Heavy lift launch vehicles
A solar power satellite big enough to replace a typical nuclear or coal-powered station will need to be kilometers across, demanding hundreds of launches. “It would require a large-scale construction site in orbit,” says ESA space scientist Sanjay Vijendran.
It took dozens of launches to construct the International Space Station in low-Earth orbit, and would likely require an order of magnitude more launches to assemble a solar power satellite that weighs in at many thousands of tonnes. In the past, due to the high costs of launch, solar power satellites were not deemed to be economically competitive with terrestrial solutions.
“We need a cheap heavy-lift launch vehicle,” says Wang, who designed China’s first carrier rocket more than 40 years ago. “We also need to make very thin and light solar panels. The weight of the panel must be less than 200 grams per square meter.” He also points out that the space solar power station could become economically viable only when the efficiency of wireless power transmission, using microwave or laser radiation, reaches around 50 percent.
A single solar power satellite of the planned scale would generate around 2 gigawatts of power, equivalent to a conventional nuclear power station, able to power more than one million homes. It would take more than six million solar panels on Earth’s surface to generate the same amount.
However, falling costs of space launches – Musk’s company plans to slash the cost of launching into space to $1,100/kg ($500/lb) from currently $20,000/kg ($10,000/lb) through reusable launch vehicles, improvement of the efficiency of solar cells from 10 to 40% over the last four decades, advancements in space robotics, development of new lightweight materials – including graphene and advanced polymers have brought back the interest in the concept of SPS once again.
A SpaceX Falcon 9 rocket lofts cargo at about $2600 per kilogram—less than 5% of what it cost on the Space Shuttle—and the company promises rates of just $10 per kilogram on its gigantic Starship, due for its first launch. “It’s changing the equation,” Jones says. “Economics is everything.”
To bypass launching the heavy station from Earth, researchers are considering having a robot factory in space to build the power station in orbit instead. Its construction would also present huge logistical issues. “(An) SSP would be assembled piece-by-piece over repeated launches and dockings,” according to the JAXA. “The construction of the structure by crew members would be prohibitively expensive and unsafe. A key phase of the program will be to develop robotic systems capable of assembling all of the components of the large orbital structure autonomously.”
NASA / LaRC “SpiderFab” for automated on-orbit construction
Company called Tethers Unlimited (TUI) is currently developing architecture and a suite of technologies called “SpiderFab” for automated on-orbit construction of very large structures and multifunctional space system components, such as kilometer-scale antenna reflectors.
This process will enable space systems to be launched in a compact and durable ’embryonic’ state. Once on orbit, these systems will use techniques evolved from emerging additive manufacturing and automated assembly technologies to fabricate and integrate components such as antennas, shrouds, booms, concentrators, and optics.
Under a NASA/LaRC Phase I SBIR contract, TUI is currently implementing the first step in the SpiderFab architecture: a machine that uses 3D printing techniques and robotic assembly to fabricate long, high-performance truss structures. This “Trusselator” device will enable construction of large support structures for systems such as multi-hundred-kilowat solar arrays, large solar sails, and football-field sized antennas. The development of economically viable SPS now depends more on the availability of adequate budgets; finally the vision of a ring of satellites in orbit to provide nearly unlimited energy for the earth’s needs may become reality.
3D printing in space
Another solution to the transportation issue could be 3D printing. “Additive manufacturing is now widely available for the aeronautics industry,” says Nobuyoshi Fujimoto, a spokesman for the Japan Aerospace Exploration Agency (JAXA), the country’s equivalent of NASA. “Therefore, this new manufacturing technology will be used for SSPs as well.” The NSS believes the necessary technologies are “reasonably near-term” and the costs involved are smaller than paying the price of global warming — particularly when the long-term environmental benefits are considered.
3D printing has been developed at a fast pace in recent years, It is thought that by sending up special 3D printers into space to manufacturer the solar panels in orbit, the installation costs can be drastically reduced, compared to sending up pre-made solar panels. In 2014 an astronaut on the International Space Station used a 3D printer to make a socket wrench in space, hinting at a future when digital code will replace the need to launch specialized tools into orbit.
Air Force makes breakthrough in space-based solar power reported in Dec 2021
Air Force researchers have made a new breakthrough in the effort to harness solar energy from space, potentially setting the foundation for a science-fiction like renewable power source for military operations and beyond.
The Arachne mission, headed by the Air Force Research Laboratory and Northrop Grumman Corp., examines the possibility of harnessing solar energy through space-based panels and then “beaming” the power back to Earth through wireless technology. The program envisions a future in which military forces and eventually the public will have access to a practically endless supply of solar energy, at any time of day or night, by simply setting up an antenna.
The Air Force Research Laboratory awarded Northrop Grumman more than $100 million for the project. The program began in 2018, but the concept of space-based solar power has been around since the 1950s. Its practicality, in both technology and cost, has been debated and often doubted by both the private and public energy sectors.
But program leaders said that recent tests of a solar panel prototype may have set a proven foundation for the future. The prototype, called a “sandwich tile” by researchers, was able to harness intense solar radiation found in outer space and then convert it into radio frequency energy that could be beamed back to Earth.
“The successful conversion of sunlight into RF energy in a lightweight and scalable architecture is a significant step forward in delivering the technology building blocks to achieve the Arachne mission,” Jay Patel, vice president for the remote sensing programs business unit at Northrop Grumman, said in a press release. “We are helping to deliver a pioneering capability that can provide a strategic advantage to our forces around the globe.”
Researchers say the technology is the first to be scalable enough to be useful for a large space-based solar operation. At the moment, the prototype can be a building block for a square-meter panel of tiles.
Eventually, the Arachne mission intends to launch an experimental satellite in space to test different aspects of power collection, conversion and transmission. The launch was originally intended for 2024 but has been delayed to 2025.
China’s developments
China’s Xidian University in June 2022 completed a 75-meter-high steel structure facility which it calls the world’s first full-link and full-system ground test system for SBSP. In another possibly related development, research into construction of kilometer-scale objects in orbit received funding last year. Such work could help to address the major challenge of assembling the giant arrays needed for solar power collection and transmission arrays.
If we choose to encourage solar power satellites (and nuclear power, another central station technology) to serve the market as a matter of public policy, we must also assure the existence of a grid that will carry that power to market. Otherwise, why waste the $20 billion?, says LEONARD S. HYMAN, Managing Director, Energy Resource Capital, LLC
References and Resources also include
http://blogs.discovermagazine.com/lovesick-cyborg/2017/09/30/4111/#.WkplrN-WY2w
https://www.greenmatch.co.uk/blog/2014/10/space-based-solar-power
https://www.nrl.navy.mil/news/releases/nrl-conducts-first-test-solar-power-satellite-hardware-orbit
https://en.wikipedia.org/wiki/Space-based_solar_power
