About 18 months ago, U.S. Naval Research Laboratory (NRL) engineers launched the Photovoltaic Radio-frequency Antenna Module (PRAM) on an Air Force X-37B Orbital Test Vehicle 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. “Seeing some of the effects of key differences between testing in space and on the ground has definitely been eye-opening,” he said. “The experiment is chugging along and continues to operate and provide a bounty of flight data. Sometimes we scratch our heads but that’s the beauty of science: you keep researching to understand the results.”
Added Jaffe, “It’s been exhilarating getting to this point. While we would have liked the moment to arrive sooner, it’s great to feel that we’re making forward progress.”
The 12-inch-square tile module is testing 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 have 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.”
PRAM is testing functional components of what would be part of a power satellite network that could transmit energy from space to anywhere on Earth. Since the launch, the team has been receiving data regularly. “The analysis to this point has shown that it has performed well in orbit and even in some cases, exceeded our pre-launch laboratory testing,” DePuma said. The real “wow” moment for him was when they received the very first data package from the X-37B Orbital Test Vehicle. “It confirmed all our hard work had paid off and PRAM was working in orbit and delivering valuable data to advance space solar and power beaming research.”
The 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 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. The 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.
“We are going into our extended testing phase and working with others in the community to try out a range of operating conditions,” Jaffe said. “PRAM is successfully laying the foundation for the next iteration of experiments and demonstrations for space solar.”
Given PRAM results, a next step would be fabricating a fully functional system on a dedicated spacecraft to test the transmission of energy back to Earth that could potentially help power remote installations like forward operating bases and disaster response areas. Solar power satellites would have the ability to provide clean, constant electricity anywhere in the world, capable of redirecting energy anywhere it is needed.
“If a power grid has a designated receiving site, a potential solar power satellite could redirect energy during an energy crisis,” said DePuma. “A solar power satellite could potentially function as a national disaster resource that could deploy additional energy during a time of crisis. Solar power satellites could be used internationally to support humanitarian missions as well as support military operations,” said DePuma. “These are designed to collect solar energy in space where it is readily available and then function as a power plant in the sky that is capable of delivering energy wherever it is most needed,” he added.
There is still a lot of data to be analyzed that will be used to iterate future solar power satellite designs. “The work done has been extremely accurate in predicting exactly what the module would do on orbit, so there haven’t been a lot of surprises in the performance,” DePuma said.