Solar power is becoming more competitive. Worldwide deployment is growing rapidly. In the future, orbiting satellites might be able to collect solar power with higher efficiency and without intermittency. The power could be transmitted wirelessly to a receiver anywhere on Earth, by means of microwave or laser.
The Sun is flooding us with more renewable energy than we can use. It delivers more energy to Earth in two hours than we use in a year. We are getting better at collecting the solar power; photovoltaic manufacturing costs have dropped ten percent per year for several decades, and the improved cost-efficiency is driving deployment growth.
Better technology, new business models
In many places, the electricity cost from solar photovoltaics is competing favorably with other energy sources. Even in Sweden, a country receiving fairly few sunlight hours, solar power is expanding; the newly installed capacity has doubled for years.
New service-based business models have been a driver for solar expansion lately, for instance in California. Companies such as SolarCity don’t sell panels, but rather provide installations and lease them to homeowners, becoming their new utility provider. The clients are able to adopt solar without having to make an investment upfront.
Not only is photovoltaics becoming cheaper, but also more efficient. Today, the best commercial systems have an efficency of 22 percent. Multi-junction solar cells, composed of several layers responding to different wavelengths of light, could reach much higher efficiency. Another fast-advancing technology, Perovskite solar cells, could have very low production costs compared to traditional silicon-based cells. Swedish researchers are developing printable polymer solar cells. At Stanford, researchers have come up with a thin, patterned silica layer that overlays on solar cells. The material is transparent to visible sunlight but captures and emits thermal radiation, which protects the cell and boosts efficiency.
If solar power keeps growing at the current rate, it is set to replace most of the world’s energy production within decades. But even though its share might have increased tenfold ten years from now, it represents only 1.5 percent of global electricity production today. Solar is also weather dependent and only available during daytime; if we don’t develop grid energy storage and supergrids spanning large areas anytime soon, the rate of growth might have to stall until infrastructure is able to catch up.
SBSP – Space-based Solar Power
But there might be a way to increase efficiency substantially, and at the same time overcome the infrastructure hurdles. Space-based solar power could be 10 times as efficient as Earth-based. It would be able to operate continously, and the energy could be directed to any location on Earth.
Outside the atmosphere, the sunlight is twice as bright, and no clouds would get in the way. While the average solar panel on Earth is illuminated less than one third of the day, a solar panel in space would be illuminated 24 hours a day, seven days a week. The basic idea is to build a solar power station in geostationary orbit to gather sunlight, with a high-efficiency solar panel array converting the energy to electricity.
Launching the satellites and collecting the energy is the easy part. Then comes the real issue: how can the energy be brought down to Earth so we can use it? Wires would not be possible – but the electricity can be used to power a transmitter, directing the energy via microwaves or laser beams to receiving antennas on Earth. The receiver converts it back to electricity again and feeds it to the grid. Microwaves would likely be the preferred option; they can be transmitted under any weather conditions, while laser beams cannot penetrate clouds. The phases of conversion would cause some energy loss, of course, reducing the efficiency gain compared to Earth-based solar power somewhat.
The transmission has to be precisely guided, since a 0.01 degree angular shift would correspond to missing the target by one kilometer. The idea is for the transmitter to lock on a pilot beam from the receiver, to guarantee that energy is transferred only when they are lined up.
The transmitter satellite would need to be in a geostationary orbit above the equator to stay aligned with the receivers. The receiving stations on the ground can be located anywhere (even in remote, off-grid areas in need of energy). A microwave receiver station would have the appearance of a large field, maybe several kilometers across, covered by antennas.
Space is a hostile environment, and solar panels in orbit would degrade faster. Space debris and micrometeorites are also a hazard. Maintenance operations would be very difficult, and would likely need to be performed robotically. On the other hand, maintenance of photovoltaic installations on the ground is also becoming automatized; autonomous drones are often used to inspect the facilities and look for panels in need of replacement.
Japan is leading the way
NASA investigated the concept in the 70s and 80s. Eventually, they concluded that the costs were insurmountable and the risk was too high. But since then, technology has come a long way: the efficiency of photovoltaics, Earth-to-orbit transportation and wireless power transmission have all improved significantly, and new light-weight materials have been invented. USA, China and India are once again considering space-based solar power projects, but this time Japan is leading the way. The Japanese Aerospace Exploration Agency, JAXA, is working to start commercial use within 25 years. They are currently studying a system to fold solar panels into a rocket and have them unfold in space as one large panel; the 2 km by 2 km panel envisioned would be about 750 times the size of the International Space Station.
The private sector is involved as well. Mitsubishi Electric has come up with “the Solarbird project”, aiming to put 40 small satellites with reflecting mirrors in geostationary orbit, with the hope of generating enough electricity to replace a nuclear reactor.
On 12 March 2015 Mitsubishi demonstrated transmission of 10 kW of power to a receiver unit located 500 meters away. While the test was successful, and also confirmed the advanced control system technology used to direct the microwave beam, it is still a far cry from the actual distance to geostationary orbit.
Launch cost is key
Because of the sheer size of the structures, the launch cost to put them into orbit is a deciding factor in making the concept feasible. Reusable rockets, increased competition and technology development by companies such as SpaceX have lowered the cost of Earth-to-orbit transportation to one tenth of what it was five years ago, a development which is making the case for SBSP stronger.
Another way of getting around launch costs would be to send autonomous robots to the Moon, to build the structures there instead, using lunar material. The Japanese Shimizu Corporation wants to begin construction of a lunar solar power base as early as 2035, eventually having a tissue thin solar array span the lunar circumference.
A large receiver is required
The SSPS would be located 36,000 km above Earth. Because of the distance and the microwave frequency, both the transmitter and the receiver would have to be very large. The reason for this is the diffraction of the beam; the larger the transmitter and the higher the frequency, the less is the resulting diffraction and hence the receiver size. But if the frequency would be set too high, the microwaves would not be able to penetrate the atmosphere – and increasing the size of the transmitter would come at the cost of launching more material into orbit. A 100 GHz transmitter, 30 meters in diameter, for instance, would need a receiving area more than 3 kilometers across. The intensity of the beam would not necessarily have to be so high that it would be dangerous to pass through, though, and while an energy beam from space might sound intimidating enough, ensuring safety would hardly be a major difficulty.
Not too long ago, SBSP was entirely a sci-fi concept, but the concurrent price drop of many cross-fertilizing technologies is beginning to cause a disruption. Space is more reachable than ever, and all the required components are becoming cheaper and more efficient. Though it still remains to be seen whether space-based solar power will be the most cost-efficient option compared to other renewable energy sources, it is definitely about to become one of the candidate technologies for the energy system of the future.
The article was published in May 2016.