Solar power is the fastest growing form of renewable energy and currently accounts for 3.6% of global electricity generation today. This makes it the third largest source of the renewable energy market, followed by hydropower and wind. These three methods are expected to grow exponentially over the next few decades, reaching 40% by 2035 and 45% by 2050. Collectively, renewables are projected to account for 90% of the energy market by mid-century, with solar accounting for about half. However, in order for this transition to take place, several challenges and technical issues must be overcome.
The main limiting factor for solar energy is its intermittence, which means it can only harvest energy when sufficient sunlight is available. To solve this problem, scientists have devoted decades to space-based solar energy research (SBSP), in which orbiting satellites harvest energy 24 hours a day, 365 days a year, non-stop. To develop the technology, researchers at Caltech’s Space Solar Power Project (SSPP) recently successfully completed the first wireless energy transfer using the Microwave Array for Power-transfer Low-orbit Experiment (MAPLE).
MAPLE was developed by a Caltech team led by Ali Hajimiri, the Bren Professor of Electrical Engineering and Medical Engineering and co-director of the SSPP. MAPLE is one of three key technologies tested by the Space Solar Power Demonstrator (SSPD-1). This platform consists of a series of lightweight, flexible microwave transmitters controlled by custom electronic chips. The demonstrator was built using low-cost silicon technologies designed to harvest solar energy and transmit it to desired receiving stations around the world.
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The SSPP began in 2011 when Donald Bren, a life member of the Caltech Board of Trustees, approached then-Caltech President Jean-Lou Chameau to discuss creating a SBSP research project. Bren and his wife (also a Caltech trustee) agreed to donate a total of $100 million to fund the project, with Northrop Grumman Corporation providing an additional $12.5 million. SSPD-1 was launched on January 3 on a SpaceX Falcon 9 as part of a rideshare program and was deployed from a Vigoride spacecraft (provided by aerospace company Momentus).
For SBSP to be feasible, satellites need to be lightweight so they can be launched conveniently and flexible so they can fit into payload fairings (similar to the James Webb Space Telescope (JWST). Harry Atwater, Otis Booth Leadership Chair of the Division of Engineering and Applied Sciences, Howard Hughes Professor of Applied Physics and Materials Science and Director of the Liquid Sunlight Alliance, is a principal investigator on the project. As he explained in a Caltech press release:
“Demonstrating wireless energy transfer in space using lightweight structures is an important step towards space-based solar energy and broad access to it globally. Solar arrays are already being used in space to power the International Space Station, for example, but to launch and deploy arrays large enough to deliver power to Earth, SSPP must design and create solar energy transfer systems that are ultralight, cost-effective, and flexible.”
Each SSPP unit weighs approximately 50 kilograms (~110 lbs), comparable to microsatellites which typically weigh between 10 and 100 kg (22 to 220 lbs). Each unit folds into bundles of approximately 1 m3 (~35 ft3) in volume and then unfolds into a flat square approximately 50 m (164 ft) in diameter, with solar cells on one side and wireless power transmitters on the other . SPPD-1 components are not sealed, which means they are exposed to the extreme temperature variations of space. As well as demonstrating that power transmitters can survive a space launch, the experiment provided useful feedback to SSPP engineers.
“Through the experiments we have conducted so far, we have received confirmation that MAPLE can successfully transmit energy to receivers in space,” said Hajimiri. “We were also able to program the array to direct its energy towards the Earth, which we have detected here at Caltech. Of course we had tested it on Earth, but now we know it can survive space travel and operate there.”
The demonstrator has no moving parts and relies on constructive and destructive interference between the transmitting antennas to shift the focus and direction of the radiated energy. These antennas are grouped in clusters of 16, each driven by a custom-made flexible integrated circuit chip. They also rely on precise timing control elements and the consistent addition of electromagnetic waves to ensure that the radiated energy reaches its intended purpose. Two receiver arrays are positioned approximately 30 cm (1 ft) from the transmitting antennas which convert solar energy into direct current (DC).
This is used to power a pair of LED lights, demonstrating the entire wireless power transmission sequence. MAPLE successfully demonstrated this by turning on each LED individually and switching back and forth between them. MAPLE also includes a small window through which the array can radiate energy, which was detected by a receiver at Caltech’s Gordon and Betty Moore Laboratory of Engineering. This signal was received at the expected time and frequency and had the expected frequency shift based on its orbit.
“As far as we know, no one has ever demonstrated wireless energy transfer in space even with expensive rigid structures,” said Hajimiri. “We are doing this with lightweight, flexible structures and our integrated circuits. This is the first time. The team is now evaluating the performance of individual elements of the system by testing the interference patterns of smaller groups and measuring the difference between the combinations. This process could take up to six months, giving the team plenty of time to detect irregularities and develop solutions to inform the next generation of solar satellites.
In addition to MAPLE, the SSPD-1 carries two other major experiments. These are the Deployable Orbit Ultralight Composite Experiment (SWEET), a 1.8 by 1.8 meter (6 by 6 ft) structure designed to deploy small modular spacecraft, and SUNRISE, a series of 32 different types of photovoltaic cells to test which are more effective in space. The ALBA tests are in progress while DOLCE has not yet been implemented and the results of these experiments are expected in the coming months. Meanwhile, the results of the MAPLE experiment are very encouraging and demonstrate that key SBSP technologies are feasible. Hajimiri said:
“In the same way that the internet has democratized access to information, we hope wireless power transfer will democratize access to energy. No ground-based power transmission infrastructure will be required to receive this power. This means we can send energy to remote regions and areas devastated by wars or natural disasters.”
SBSP has the potential to produce eight times more energy than solar panels located on the earth’s surface. When the project is fully realized, Caltech hopes to deploy a constellation of modular spacecraft that will harvest solar energy, turn it into electricity, and convert it into microwaves that can be transmitted wirelessly anywhere in the world. In addition to assisting the transition to clean, renewable energy, it also has the potential to expand access for underserved communities. Caltech President Thomas F. Rosenbaum said:
“The transition to renewable energy, critical to the future of the world, is today limited by the challenges of energy storage and transmission. Transmitting solar energy from space is an elegant solution that has come one step closer to being realized thanks to the generosity and foresight of the Brenses. Donald Bren has presented a formidable technical challenge that promises a dramatic payoff for humanity: a world powered by uninterrupted renewable energy.”
Further reading: Caltech
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