China has recently announced that it will build the world’s first commercial-scale solar power station in Earth’s GES(geosynchronous) orbit, which is located 36,000 km 22,369 miles above the equator, by 2050. Between 2021 to 2025, China will launch small and medium-sized space solar power projects in the stratosphere and later on launch a small scale space-based solar power station in 2030.
In the backdrop of imminent catastrophic anthropogenic climate changes, the lure of infinite zero-carbon space solar power is tempting which overcomes the intermittent nature of terrestrial renewable energy sources(wind and solar)due to weather, seasons or time of day. The idea of tapping solar power in space and transmitting it through microwave beams is not new. It was popularized by Isaac Asimov in his science fiction short story “Reason” in 1941. In that story, he envisioned that space stations transport energy from the sun to other planets with microwave beams. While In his other short story, “The Last Question” Asimov had mentioned limitless space solar energy for terrestrial application.
But in 1968, Peter Glaser an American aerospace engineer brought Asimov’s vision near to reality by proposing a solar-based power system in space. Three years later, he filed for a U.S. patent for a “Method and Apparatus for Converting Solar Radiation to Electrical Power,” which was granted in 1973. He argued that his invention will not only solve fossil fuel and nuclear power-related environment and waste disposable problems but also overcome many of the practical limitations of generating electricity from solar power on the Earth, such as atmospheric absorption, cloud cover, and day-night cycles. Hitherto Space stations and satellites use solar panel arrays for their power needs but Peter Glaser’s stand-alone Solar Power Satellite (SPS) concept was path-breaking albeit merits economical and operational feasibility.
According to this concept, solar power satellite orbiting in GES orbit with a large(10X5 km) solar array can work as a power plant. It converts solar energy into electricity through photovoltaic and converts electricity into microwaves(RF electrical power) which can be transmitted wirelessly to large receiving antennas (rectennas) on earth. These large receiving antenna on earth converts the microwaves to DC electrical power (with80% efficiency) for distribution on the national electric power grid.
According to Space solar power mechanism, space solar satellite placed in a fixed point in GES(geostationary) orbit is tethered to the ground station. It consists of three functional units in space and one unit on earth.
Solar energy collectors: They collect solar energy in space with reflectors and inflatable mirrors onto solar PV cells to convert solar energy into DC (Direct current) electricity.
Microwave converter: It converts DC to Microwave converter
Large antenna array: They beam the Microwave/ LASER( light amplification by stimulated emission of radiation) at a variety of frequencies from space to the earth.
Rectenna: microwave antennas collect the microwave beams and convert it into electricity
Keeping in the mind the cost, technical feasibility, and safety microwaves, high-frequency microwave or laser can be explored to transmit energy via an RF antenna at the non-ionizing radiation frequency (between2–8 gigahertz ) which can pass through the Earth\’s atmosphere without interference to a rectenna on the ground from space.
Compare to the lower frequency range, the higher frequency range microwaves require much smaller transmission and reception antennae and will produce less ionospheric heating. Microwaves are more efficient than laser but need large transmitting antennae on space and also large receiving antennae on the ground. Although less efficient, The laser is more compact needs more accurate beam pointing and much smaller ground receivers.
Advantages of space solar power
AS space solar power can be delivered to any part of the world, tapping infinite uninterrupted solar power in space without the restriction of the day-night cycle or atmospheric attenuation, or cloud cover, it can solve both climate change and energy security-related issues for the entire human race. Space solar array produces ten to four times more energy compared to the terrestrial solar array located in low-isolation and high-resolution locations, respectively.
Potential terrestrial and space application of Space solar power
- Ultra-long-duration unmanned surveillance drones
- In case of disruption of power generation due to Natural disaster
- In the case of Forward-deployed combat units
- Long-duration space mission, interplanetary human exploration and space vehicle on moon or Mars
- As a peak load power complementing ground-mounted solar power as part of a larger energy portfolio
Technical barriers and safety concerns regarding space solar power project implementation Size and weight of the solar power system
The largest structure currently in LEO (Low Earth Orbit)orbit, the International Space Station has a mass of450 metric tons (MT) and 356 x240 feet size. So placing a single Space Solar Power Satellite of above 3,000MT mass,( 5×10 km) solar collector size and 1 km diameter size antenna in GEO to deliver (1 to 10GWe) electricity is a real technical challenge. It is a technical and economic challenge to place a constellation of 109 to 120 such large and heavy satellite satellites, each with a service life of 30 years.
Space launch cost
The advent of new technologies for reusable launch vehicles can make space launch economically viable with present-day space launch infrastructure but space lift, ground-tracking support and satellite command facilities needed for such large projects will remain a challenge.
Cost of Solar PV
The availability of less expensive, lighter, space radiation-tolerant ( insensitive to ionizing radiation), thin-film solar panels(with 20% efficiency) can reduce launch cost without sacrificing efficiency.
Transporting material from earth to space
As not all material needed for setting up a solar station be delivered to its eventual orbit GES immediately, SPS(space power station) material can be transported from LEO (low earth orbit)to GEO at an acceptable cost akin to ion thrusters or nuclear propulsion. In future SPS can be launched from the moon due to its lower gravity and lack of atmospheric drag.
Fabrication and assembly of large structures in space
The barrier to fabrication and assembly of large structures(50km)and more than 3000 MT weight in space may be overcome by using advanced modular manufacturing and telerobotics. These techniques may enable China to build a huge deployable structure from modules less than a ton weight and obviate their need for heavy-lift vehicles.
Similarly, China may explore to build a constellation of smaller solar satellites that can easily connect together in space to form a much larger array and harvest sunlight.
Maintenance of solar panel in space
Maintenance and repairs can be carried out by robots astronauts and scientists.
Large track of land required for rectenna
The Earth-based rectenna( consist of many short dipole antennas)spread across several kilometers (10×13 KM) and need a large buffer zone. Desert or lake or non-agriculture land can be used for such purpose.
Disposal of waste heat in space
The disposal of waste heat generated during several phases of energy conversion from photons to electrons to photons and back to electrons in space power systems can be intractable when the entire spacecraft is designed to absorb as much solar radiation as possible.
Increase temperature on earth
Importing solar power from space and transmit it into the earth will raise the earth’s temperature and may have unintended consequences for various forms of life on earth. China needs to address these concerns and ensure that the energy densities of space
solar power should not exceed what we get from the sun now.
Space hazards for solar satellite
Except for the magnetosphere where solar satellites are protected from solar hazards, solar space panels are more susceptible to radiation, solar flares, micrometeoroid, charged particles and planetary debris.
The geopolitical danger of putting the satellite on GES
Solar satellite on GES always remains in the view from the ground station and antenna geometry stays constant. But other nations may come under direct observation from GES orbit it can create geopolitical concerns from neighboring nations. Placing huge satellites in already crowded GES can exacerbate the risk of space debris and safe operations for all other satellites.
Weaponisation of space solar power
High-frequency space-based laser cannons could be concentrated to serve as directed-energy weapons.
Safety from incoming microwave
While beaming microwave or LASER from space en route to its intended ground receiver on earth, care must be taken to protect satellites, aircraft, or humans and birds. As all incoming microwaves are well within the safe exposure limit(10 mW/cm2) for humans and wildlife, the safety of the human population is not an issue.
Fencing around the rectenna can protect humans and wild animals from microwave radiation. Faraday cages intercept the microwaves and provide passengers with a protective metal shell while aircraft flying through the microwave beam. Flight control spaces enable other aircraft to avoid exposure to the microwave beam.
Prior to China, the USA\’s space agency NASA has worked on the same project but abandoned it due to practical intractability. In March 2015 Japan Space Systems succeeded in converting 1.8 kilowatts of electricity into microwaves and transmitting it to the earth. By employing the latest technology in robotic assembly, drones, warehouse robots, and modular manufacturing China may overcome basic technical challenges related to building large structures in space. To produce satellites of desired size and configuration, China may explore Solar Power Satellite via Arbitrarily Large Phased Array (SPS-ALPHA) technology in which microwave transmission apertures distributed across the solar array and synthesized into a beam.
Even if, China succeeds in demonstrating space solar power, remaining issues of power efficiency, precise beam-pointing control requirements, and beam-exposure hazards are challenging. The cost parity of space solar power with other zero-carbon energy technology such as terrestrial solar power or hydrogen fuel is still a big challenge. Only Time will tell whether space solar concept will remain in the realm of Asimov’s science fiction forever or will become reality.