By Paul Grad, engineering writer
It seemed like a fairly esoteric idea when first proposed about 40 years ago – placing solar panels in satellites orbiting earth and beaming down the energy from the sun in microwaves or lasers, which would then be collected by ground receivers.
This space-based solar power (SBSP) would supply clean, unlimited and based-load energy, the ultimate form of renewable energy. But the idea does not seem so abstract anymore.
Many organisations in several countries have been developing the technologies relating to SBSP, and have carried out successful tests.
Ground-based solar power is limited by several factors: it depends a lot on the amount of sunshine reaching the Earth – a lot of the sun’s energy is blocked by the atmosphere – and there is limited energy storage capacity. There would be no such problems with solar power problems in space. The sun shines all the time in space, with no clouds and no atmosphere. There is no variation of power supply during the day, or from season to season.
An SBSP cell could supply six to eight times more power than a comparable solar cell on the Earth’s surface. The satellites could be placed in geostationary orbit – the orbit around the equator at 37,000km from Earth’s surface where the satellite would be stationary overhead the
ground receivers – although several other orbits are considered. Orbit overcrowding is an issue, but does not seem to be a main concern.
Solar energy would be converted onboard the satellites into electromagnetic carrier waves, probably at microwave frequencies of 2.5GHz or 5.8GHz for subsequent beaming back to the ground. The atmosphere is transparent to those frequencies, although a little bit of the energy would still be lost during the transmission. A column of microwave energy, 2-3km across would be beamed to an oval-shaped, ground-based rectifying antenna – or “rectenna” – and from there the energy would flow into the existing electricity grid.
The diffuse energy beam from above would pose no danger to humans or animals. Microwave radiation is nonionizng, just like visible light or radio signals. It lacks the energy needed to remove an electron from an atom or molecule, producing a charged particle that could damage DNA or other biomolecules, as x-rays and gamma rays can.
There are a number of other issues that can be a cause for concern. One of them is space junk, which could collide with an SBSP satellite. Space junk has already threatened the International Space Station. An SBSP system would have to get clearance from the UN’s International Telecommunication Union, which allocates use of the electromagnetic spectrum. Some of the SBSP’s proposed microwave frequencies are already used by wireless systems such as Bluetooth and interference could be a real problem. To avoid some of those issues some of the scientists and engineers involved with SBSP have proposed lasers rather than microwave transmission. Lasers would require far smaller satellites and transmitters, and far fewer launches. Sunlight captured in space with photovoltaics would be converted to visible or infrared lasers and beamed onto solar panel arrays on the ground. Lasers are absorbed by the atmosphere, however, and laser transmission can be disrupted by the weather. Another concern regarding lasers is high energy density on the Earth’s surface.
Yet another issue is the huge size that would be required of an SBSP in space. An SBSP system could produce 1-4GW or 5GW. About 20GW is needed to power a city like New York. Therefore, several SBSP systems would be needed to power New York. The huge size of SBSP systems is one of the major questions regarding the development and deployment of such systems. The deployment of large area, ultralight surfaces in space is indeed one of the key challenges scientists and engineers are working on. Perhaps the main stumbling block is the cost of launching equipment into space. Therefore, a main issue is to get the weight down, probably by eliminating some of the satellite’s components, including the mirrors, power module and microwave emitter. These could be “freefloating” in space, orbiting in tandem. Despite the challenges posed by SBSP, a large development effort of possible SBSP systems has been underway throughout the world. Solaren Inc, a company based in Manhattan Beach, California, created to develop SBSP, has been actively developing a SBSP system under contract for the California utility Pacific Gas & Electric Company (PG&E), to deliver 200MW of SBSP for 15 years.
Solaren vice-president electricity sales Cal Boerman said securing funding is the hardest problem faced in developing an SBSP system.
“Due to funding delays we had to extend the operational date accordingly. We now estimate a 2022 operational start date,” he said.
Northrop Grumman Corporation, an aerospace and defence technology company headquartered in Virginia, signed a research agreement with the California Institute of Technology (Caltech) in April for the development of a Space Solar Power Initiative (SSPI). Under the agreement Northrop Grumman will provide up to US$17.5 million to the initiative across three years. The SSPI will develop the technology needed to enable a SBSP system capable of generating electric power at cost parity with grid-connected fossil fuel power plants. It will focus on three areas: highefficiency ultralight photovoltaics, ultralight deployable space structures, and phased array and power transmission.
The project was conceived and will be led jointly by three Caltech professors: Harry A Atwater, Howard Hughes Professor of applied physics and materials science; Ali Hajimiri, Thomas G Myers Professor of electrical engineering and medical engineering; and Sergio Pellegrino, Joyce and Kenta Kresa Professor of aeronautics and civil engineering. Atwater’s group will design and demonstrate ultralight, high-efficiency photovoltaics optimised for space conditions and compatible with an integrated, modular power conversion/transmission system. Hajimiri’s team will develop the integrated circuits and the antenna for the system’s large-scale phased array timing control and conversion of direct current into radio frequency power.
A modular approach will allow power to be generated, converted and radiated using a distributed power solution. This will significantly reduce the system mass and thereby its costs. This highly adaptive power generator will enable on-demand power supply anywhere in the world and will enable provision of power simultaneously to multiple locations on demand.
Pellegrino said, “one of the key barriers to the realisation of cost-competitive SBSP systems is the deployment in space of large surface area structures to collect solar power at low cost”.
To circumvent this barrier, his team is developing novel architectures for multifunctional deployable space structures with an overall areal density of the order of 100g/sq m.
A major R&D effort has been underway for several years at the US Naval Research Laboratory, of Washington DC. The laboratory’s systems integration section head, Dr Paul Jaffe, predicts individual space-based solar arrays would be able to produce between 250MW and 5GW of
energy. Jaffe and his group have tested two different prototypes of a “sandwich” module. On one of the prototypes, the tile module, all electrical components are packed between two square panels. The top is a photovoltaic panel. An electronics system in the middle converts the energy to radio frequency. The bottom panel is an antenna that beams the power toward a target on the ground.
A main problem with this design is the heat, which affects the electronics and reduces efficiency. To overcome this problem, the group has developed an alternative design – a step module – where the three components open up like three pages. This allows the device to receive
more sunlight, without overheating. Dr Jaffe said the step module gave higher specific power, i.e. more power per unit area. He also said the laboratory was the first organisation to test such devices in a space environment, and obtained the best efficiency in the world. Currently he and his group are focusing on reducing the system’s mass, aiming at making it costcompetitive in terms of power per mass – i.e. kW/kg. Dr Jaffe is very optimistic about the future of SBSP, saying compared with the amount of money spent on another form of renewable energy, nuclear fusion, in the past 50 to 60 years, a very small fraction of that investment has been made in SBSP.
NASA and the US Department of Energy have jointly investigated the SBSP concept on and off across several years. They proposed the Solar Power Satellite via Arbitrarily Large Phased Array (SPS-ALPHA) – a flower-shaped solar power energy collector that beams down concentrated microwave energy. The idea was promoted by NASA veteran John Mankins. Mr Mankins and other engineers have set up Artemis Innovation Management Solutions to pursue the SBSP concept. Outside the US, several countries have also pursued the SBSP concept. In March 2015, Mitsubishi Heavy Industries Ltd has successfully transmitted 10kW of electricity via microwave to a receiver 500m away. The test also confirmed advanced control technology could direct the microwave beam to stay on target. Last year, the Japanese Aerospace Exploration Agency (JAXA) presented a 25-year development roadmap of demonstrations culminating in the 2030s with a 1GW commercial SBSP system.
Members of the Chinese Academy of Sciences and the Chinese Academy of Engineering have suggested China should begin with an experimental SBSP station by 2030, and build a commercial space power station by 2050. Many difficult hurdles remain to be overcome, but it looks like some of our energy needs will soon come from space.