Aerospace Team Tests Solar Technology at Edge of Space

Holding the credit-card-sized Aerospace device known as the Intelligent Solar Cell Carrier, or ISC² for short, is Colin Mann, one of four ISC² patent holders. The other three patent holders are, from left, Dr. Justin Lee, John Nocerino, and Dr. Don Walker. (Photo: Eric Hamburg)

Solar cell testing being led by the Aerospace team of Dr. Justin Lee, Colin Mann, John Nocerino, and Dr. Don Walker, all of the Energy Technology Department, Engineering and Technology Group, represents advancements in the field that are not only innovative, they are also shaping the future.

The four are holders of a solar cell measurement patent and are testing their newly developed technology under a one-year NASA program, which requested demonstration of space technology payloads. “We’re taking technology that has been lab tested and testing that same technology in the real space environment,” says Lee, who serves as principal investigator for this project.

The Aerospace project calls for the demonstration of an automated solar cell calibration platform, using a device attached to a high-altitude balloon to capture the solar spectrum and characterize the performance of the solar cells at a high altitude up to 22 miles.

The effort is being conducted under NASA’s Flight Opportunities program, which chooses promising space technologies to test through relatively low-cost ways that simulate or just reach the edge of space using such vehicles as high-altitude balloon flights.

Aerospace’s effort, “Rapid Calibration of Space Solar Cells in Suborbital Environments,” involves multi-junction solar cells, which are made up of multiple, monolithic subcells connected in series. The concept is to be able to fly a full multi-junction solar cell accompanied by separate individual sub-cells attached to a test platform that is carried by a high-altitude balloon to the edge of space, or about 120,000 feet up, and pointed to the sun where their performance is measured.

Following the measurements, the balloon flight is terminated and a parachute safely carries the test platform back to Earth’s surface. The devices are then retrieved and returned to the lab for follow-up measurement and data analysis.

The Aerospace team’s invention, which is currently going through the patent application process, is a compact, credit-card-sized device called the Intelligent Solar Cell Carrier or ISC². This device was created and developed under the Aerospace Technical Investment Program, which is managed by the iLAB Office.

Solar cells undergo testing inside a thermal-vacuum chamber while a light shines through. Chamber conditions simulate a space-like vacuum where the solar cells are cooled to a temperature of minus 60 degrees Celsius. (Photo: Justin Lee)

ISC² revolutionizes the world of solar cell performance testing in space by eliminating the need for large solar cell calibration platforms that either require a balloon the size of a football field or high-altitude planes such as the NASA Learjet or repurposed U-2 planes, as is the current practice. Large, bulky test platforms are needed to support the size and weight of the test equipment typically used to measure the performance of solar cells. By reducing both the size and weight of the test equipment, the team can use a smaller, cheaper high-altitude balloon that can be flown at any time.

“We’ve condensed a whole lot of functionality such as electric current, voltage, and temperature measurement instrumentation into one circuit board,” explains Lee. The team has also added other functionality such as the ability to integrate with data storage, sun angle sensors, altitude sensors, as well as an easily integrated connection and protocol to allow for it to communicate with other payloads, all in one package. “This miniaturization translates into reduced cost to fly and opens up flight test opportunities using a variety of vehicles,” he adds.

Lee explains that such measurements will aid in calibrating the solar simulator used to test the performance of solar cells in the lab. The simulator is not 100 percent accurate when comparing it to sunlight in space. Reference solar cells need to be flown and measured as close to space as possible and then retrieved so that they can be used to adjust the solar simulator. Currently, however, high-altitude testing involving Learjets or repurposed U-2 spy planes doesn’t allow for measurements above the 70,000-foot level.

Why is getting above this level so important? As Lee explains, “accurate measurements require us to test above the ozone layer because in space, where these solar cells will be used, there’s no ozone layer. If we want to obtain accurate performance data that is critical for designing the solar arrays powering satellite missions, we need to be testing those solar cells near or in space and be capable of performing accurate simulations of space sunlight conditions in the lab.”

Although alternative solar simulator calibration methods are currently in development, space is still where a solar cell’s true performance data can be collected and therefore, relied upon.

Balloon solar cell calibration testing was performed annually by Jet Propulsion Laboratory but is now coordinated by an international organization and occurs on an unpredictable schedule at high cost, Lee says. There was a recent effort to revive stateside balloon flights through the Near-Space Characterization of Advanced Photovoltaics Program (NSCAP) and although some instrumentation was built under the NSCAP Program, routine balloon flights did not resume.

In the past few years, Aerospace has continued to investigate methods to reduce the cost and increase the frequency of flight opportunities to support accurate measurements needed for rapid development of solar cell technology.

“We would like to show that there is a rapid and sustainable method for gathering accurate solar cell measurements needed to support future satellite missions,” he says. “Manufacturers are continuously producing solar cells using different recipes and each cell type reacts differently to lab-simulated sunlight. It is critical we have low-cost, frequent access to real space sunlight to ensure we can accurately benchmark how each solar cell performs. Frequent solar cell tests on high-altitude balloons are an ideal way to get us accurate numbers,” says Lee.

The Aerospace team’s efforts don’t stop with this NASA proposal. “Following retrievable high-altitude solar cell balloon flights and comparing the flight data to those measured in the lab, we would like to test the same solar cells in their end-use environment, like on a CubeSat in low-Earth orbit,” he explains. “We can show that the measurements obtained by each of our testing capabilities are consistent in accuracy.”

The team’s solar cell testing capabilities are relevant for a growing number of applications, whether they are designed to operate within Earth’s atmosphere or in space.

“No one is setting the standards for agile field testing and lab measurement of solar cells, but we’re looking to fill that role in the near future,” adds Lee.

—Kimberly Locke