Thermal Cycling Techniques for Solar Panels

Robert W. Francis, Charles Sve, and Timothy S. Wall

Thermal stress testing for solar arrays is a lengthy and unavoidable part of spacecraft mission design. Aerospace has developed a method that is as fast as it is reliable.

During the last 20 years, escalating launch costs have forced spacecraft engineers to design lighter and more efficient power subsystems. Constraints on solar array size, weight, and storage volume have spurred the development of efficient multijunction solar cells and lighter substrate materials. The decreased mass and size have helped reduce costs, while the higher power levels have helped increase spacecraft payload capability.

On the other hand, each new solar panel design must be tested to ensure that it can withstand the rigors of the space environment and maintain its structural integrity throughout a mission that might last 10 years in low Earth orbit. Such testing has traditionally presented a major bottleneck in the development of new solar cell arrays. Standard thermal chambers can take more than two years to complete thermal-cycle stress testing that adequately simulates mission life environments. Testing labs have sought to accelerate this testing, but have been challenged to do so in a manner that does not reduce confidence in the test results.

The ultrafast thermal cycle chamber at Aerospace. Solar cells are affixed to a flight-like substrate, which is moved up and down between two thermally insulated chambers. Cycle times are relatively fast because each chamber can return to its setpoint temperature while the other is in operation. Dwell or delay can be added to the hot or cold phases to allow for in-situ electrical characterization of the test article.

In response to customer need, Aerospace established space-simulated thermal cycling capabilities in the mid-1980s. These capabilities progressed through a number of evolutionary stages, each offering greater speed and fidelity. The latest approach, known as ultrafast thermal cycling, has provided timely evaluation and demonstration of advanced solar array designs for numerous space programs. The automated process controls temperature uniformity, optimizes thermal transfer, reduces cycle periods, and decreases overall test time.

These tests continue to furnish mission design and confidence data to a number of spacecraft programs and provide a valuable technical database for incorporating advanced, highly efficient solar cells into the latest spacecraft designs. Programs that have benefited from the decreased testing time and cost include present and new generation national security spacecraft, the Experimental Spacecraft System (XSS-11), the Defense Meteorological Satellite Program, as well as NASA's Messenger mission to Mercury.

As a result, the ultrafast thermal cycling facility at Aerospace is now recognized by the aerospace community as a unique capability for evaluating and demonstrating new solar cell and array design features, solar cell interconnect joint integrity, and potential early life failures with a turnaround time that is fast enough to permit a redesign, if necessary.

Early Test Methods

The first Aerospace test chamber, built in 1985, was a conductive thermal cycling system geared toward performing life-cycle thermal stress tests on the new generation of gallium-arsenide solar cells. Temperature changes were achieved by cooling a fairly massive aluminum plate with liquid nitrogen and then heating the plate with electric rod heaters. The test articles were held under vacuum so that cycling would occur primarily by thermal conduction. Under these conditions, typical solar cell coupons required 60–90 minutes to cycle 100 degrees centigrade. The disadvantage was that the hot and cold phases worked against each other to drive the thermally conductive base plate, thereby limiting cycle rates.

In 1990, Aerospace brought its first radiant thermal cycle chamber into service. This vacuum chamber used a quartz-halogen lamp for heat generation and a cold shroud for heat absorption. Cycle periods of 30–60 minutes were now attainable for flight-like test panels. Temperature cycle rates depended solely on radiation to and from the suspended solar cell panel. The heating lamps immediately overcame the cold shroud in the hot phase; however, the shroud was warmed significantly and could recover only during the next cold phase, even though liquid nitrogen continuously flowed through it during the hot phase.

In 1996, the cooling efficiency and rate were improved. A partial pressure of nitrogen gas was introduced into the vacuum chamber, and this allowed conduction in addition to the radiation of heat to and from the panels by way of the cold shroud. Shorter cycle periods of 22–45 minutes were obtained. As with the earlier radiant design, only the hot phase worked thermally against the cold phase, and this provided some advantage in cycle period over the original conductive thermal cycle chamber.

Ultrafast Thermal Cycle Chamber

The next innovation was the ultrafast thermal cycler, which combines the best aspects of all previous configurations. With a cycle rate of 10 minutes, the apparatus can achieve more than 1000 thermal cycles in one week of continuous operation. This capability allows state-of-the-art performance assessments of high-performance solar cell types, interconnecting schemes, and substrate designs in much less time than commercially available thermal cyclers.

The system has two compartments—a hot compartment on top and a cold compartment on the bottom. These thermally isolated compartments are contained in an insulated chamber that is slightly pressurized with ultrapure nitrogen gas. The positive pressure of the gas mitigates moisture condensation, oxidation, and corrosion and promotes conductive heat flow. A motor, pulley, and cable system raises and lowers the test fixture from one compartment to the other.

Quartz-halogen infrared lamps in the top compartment surround the panel in the hot phase to maintain a constant high temperature. The test panel is heated rapidly and uniformly by both radiation and gas conduction. For the cold phase, the panel is lowered into the bottom compartment, which is encased in a container filled with liquid nitrogen. A marked advantage of this design is that the cold and hot phases do not work against each other. One compartment can fully recover to its designated end-point temperature while the other is in use.

This thermal chamber system provides continuous unattended temperature cycling and can easily accommodate more massive solar cell composite test panels. Thermal cycle periods of 10–12 minutes on fully populated solar cell panel substrates as large as 30 X 35 centimeters and 5 centimeters thick can be achieved, resulting in a demonstrated capability of over 50,000 thermal cycles in one year of continuous operation.

When the ultrafast thermal cycler was first introduced to perform thermo-structural stress validation and verification, contractors expressed some reservations because of the relatively high temperature rates. However, these doubts were soon abandoned when Aerospace's fatigue analyses validated the stress failures that were replicated in the much slower traditional thermal cycle chambers.

Measurements and Methodology

The ultrafast testing system offers additional benefits over traditional testing methods. In typical thermal stress testing, the solar panels are removed from the chamber after a certain number of cycles to allow for functional evaluation. But this interrupts (and lengthens) the process and provides only a general indication of how many cycles a panel can endure before failure. Therefore, in developing the ultrafast test method, Aerospace sought a way to verify electrical performance and circuit continuity without having to remove the solar cell panel from the testing apparatus.

Temperature and forward-bias current profiles. The characteristic curve for the solar cell circuit under test is monitored and compared to a baseline set of curves at both hot and cold temperatures. Anomalous changes in either the solar cell series or shunt circuit resistance produce distinctive changes in the voltage-vs.-current signature acquired during these tests.

Aerospace devised a fully automated method based on in-situ measurement of electrical resistance. A microprocessor-controlled power supply sends an increasing current through each solar cell circuit, forward and then backward. The current is stepped up or down and held constant at specific intervals to produce a resistance-dependent voltage characteristic. The voltage-vs.-current signature is continuously monitored and compared to a baseline set of curves at both hot and cold temperatures.

Anomalous changes in resistance produce distinctive changes in the voltage-vs.-current curve. For example, problems with a device shunt, interconnect, or harness produce recognizable changes in the signature. These signature changes allow immediate detection of electrical degradation or failures. In the extreme case, an indication of an open-circuit failure would safely stop the cycling process and allow for immediate failure evaluation. This avoids continued cycling of nonfunctional or degraded solar cell circuits and facilitates the timely discovery of failure mechanisms.

In the ultrafast thermal cycler, solar cell circuits are loaded with forward-bias current during every thermal cycle. The current load is varied linearly and in proportion to the solar cell temperature. The initial and maximum allowable load currents are determined from the calculated operational photocurrent temperature coefficients of the specific solar cell device under test. The intent is to evaluate a simulation of the flight-like operational electrical current produced by solar cells in sunlight while deployed in space. Traditionally, thermal cycling qualification has been performed on solar cell circuits in the passive state, i.e., with no current generation. It is believed that the forward-bias current-loading method, performed simultaneously with thermal cycling, has the potential to simulate the operational power mode of an interconnected solar cell circuit under load conditions, which is more cost effective and has less test complexity than a ground test under illumination.

Protecting Solar Cells

The ultrafast testing apparatus also has mechanisms to protect the solar cells that are being tested. In an anomalously high or low circuit-impedance condition, the constant-current power supply can potentially exceed the load current that was initially set. This can produce an excessive voltage across the solar cell circuit and potentially damage the cells. This is an unrealistic condition, compared with solar cell circuits operating under natural solar illumination in space, where the number and type of solar cells in series inherently limit the cell-circuit voltage.

Solar array panel (exploded view). Strong and lightweight structural substrates as well as durable adhesives, bonding agents, and graphite strengthening members are needed to construct high-power solar panels from individual photovoltaic (PV) solar cells.

To address this situation, Aerospace implemented a software monitor and control scheme. The voltages produced while going from the maximum and minimum temperatures and throughout the current-loaded sequence are first characterized during a beginning-of-life cycle. Using these expected voltage endpoints, a reference voltage is constantly calculated by linear interpolation with respect to the instantaneous cell temperature to compare with the actual voltage being produced by the current loading process.

A deviation above or below a specified voltage will result in either an "anomaly" or "failure" response. In the case of an anomaly response, the test continues, and a voltage signature history plot is generated, characterizing the aberration. In the case of a "failure" response, the test is immediately terminated. This protects the solar cells by removing excessive voltage across the solar cell circuit that could potentially damage them.

Case Studies

The Aerospace thermal cycling facility has benefited numerous programs, both by finding flaws and validating designs. On one national security space program, failures were observed after 8000 cycles, out of a goal of 50,000 cycles. The real-time in-situ electrical characterization measurements detected changes in the solder-joint interconnect circuit resistance and continuity. This finding was verified at the contractor's facility six months later and required a modification to the solder-joint interconnections on the solar array.

An evaluation of an all-welded solar array being developed for another program revealed "infant mortality" or early failure of the weld joints. This discovery prompted a change of material. Similarly, for an ongoing national security space program, thermal cycling of an all-welded solar cell circuit has exposed a marginal interconnect joint in the solar cell bypass diode circuit.

The Aerospace facility has been used to validate and verify the thermal cycle fatigue requirements for solar panels on other programs involving soldered and welded circuits with advanced multijunction solar cells bonded onto flight-like substrates. NASA's Messenger spacecraft will travel close to the sun and eventually orbit the planet Mercury. The Applied Physics Laboratory of the Johns Hopkins University contracted Aerospace's thermal cycle facility to evaluate solar array materials, processes, and design parameters proposed to satisfy this mission's thermally stressing requirements. After three phases of evaluation—which included development, prequalification, and qualification testing—the final solar panel design successfully completed the mission's thermal cycle profile sequence with no performance degradation. The Messenger spacecraft with this solar panel was launched early this year and is on its way to Mercury.

Despite initial skepticism, the aerospace community has come to recognize the ultrafast thermal cycle facility for its ability to evaluate and demonstrate new solar cell devices and interconnect joint features on solar arrays used in space. In fact, two contractors have now developed thermal cycle chambers similar to the Aerospace design.

Conclusion

Modern communication satellites can have primary power capacity in excess of 20 kilowatts, whereas a decade ago, they typically had less than 5 kilowatts. Advances in chemical processes, solid-state technology, and materials science have enabled the creation of solar arrays capable of this performance level. While these technological advances have benefited the mission planner, they have also increased the testing burden for the validation phase.

Early verification of solar cell stability, electrical circuit continuity, joint and bond robustness, and panel substrate integrity is a critical step in minimizing mission risk and ensuring proper spacecraft design. The thermal cycle facility at Aerospace provides a unique capability for confidence-level testing, evaluation, and qualification of solar cells with a turnaround fast enough to accommodate tight launch deadlines and even permit redesign, when necessary. Thanks to innovations in chamber design and advances in electrical testing methods, the fully automated, fail-safe test facility at Aerospace has helped numerous programs save time and money and will continue to prove its benefit as newer and more efficient solar panel designs become available.