HighPower Solar Array
Global Aerospace Corporation (GAC) has developed the HighPower solar array, which provides a lightweight, customizable and reusable power source for balloon-borne payloads, capable of traveling on repeat missions and supporting a wide range of scientific experiments. The array is applicable to the NASA Ultra Long Duration Balloon (ULDB) Program, which has the goal of flying up to 8,000 lb science payloads above >99% of the Earth’s atmosphere for up to 100 days, a factor of 3 to 30 times longer than current balloon flights. The HighPower Solar Array has been patented by GAC (Patent No. US 6,237,241 B1).
The heart of the HighPower solar array subsystem is comprised of several Solar Array Modules (SAMs). Each SAM is a 4 m2 square panel covered with two strings of 300 solar cells each and attached at the corners to tilt/power cables, as well as to a separate set of stow/deploy lines. The panels are made of aluminum honeycomb, to which laminated solar cell modules are bonded. The solar cells are arranged in such a way as to reduce the effects of shadowing along the panel sides and corners, which balances the effects on each of the parallel SAMs. Each cell produces approximately 0.5 V, adding up to a total nominal output voltage of approximately 150 V from the array. Attachment of the SAM corners to the power-conducting tilt cables and stow cables ensures acceptable cycle life, electrical connectivity, and strength.
The tilt/power cables not only support the SAMs and move them to face the sun, but also transmit the electricity from the SAMs to a carrier, or “gondola,” which carries the scientific instruments, the batteries, the power converter and the power conditioning hardware, and the communications equipment. Since the SAMs are not attached directly to the gondola, they can be angled for maximal exposure to the sun for longer periods, independently of the position of the gondola, and more panels can be added or removed without the need to increase the size or change the design of the gondola. Thus, payload mass is saved and more power can be generated, supporting a larger number and variety of scientific instruments.
The design has considerable advantages over conventional solar-powered balloon payloads, characterized by solar panels mounted directly onto the gondola. In those designs, many solar panels need to be mounted around the gondola to ensure there is enough power as the gondola rotates or a large rotator is necessary to control its motion; that rotator, along with the power transfer slip rings, increased the payload mass. Another disadvantage of conventional solar arrays on balloon gondolas is that they are often severely damaged upon termination and landing.
In contrast, the HighPower solar array power system is protected at landing and is reusable. At the end of each flight, the SAMs are pulled together by the stow/deploy lines into a protective compartment, the Solar Array Subsystem Structure (SASS, left). At the next flight, the SAMs are released when the balloon reaches float altitude. SASS protects all the hardware of the subsystem and attaches to the gondola. Aluminum sheet metal on the outside of the SASS frame provides shear stiffness and prevents intrusions into the interior. At the bottom corners of the SASS are latches that are released by a small, motorized mechanism. The photo at left shows the SASS with its stowed SAMs but with neither the stow/deploy nor power/tilt hardware nor the outer shear panels.
The reusable and refurbishable system costs less than conventional solar array systems; the latter systems’ unprotected panels are damaged when their craft lands and the balloon drags along the ground. The Tiger payload (right) is an example of the latter: the photo shows its solar panels damaged beyond use. The HighPower solar array has a variety of potential power generation applications including stratospheric balloons, space tether systems, and commercial, industrial, or residential power systems.
System Comparisons and Performance
GAC compared various balloon solar array systems in terms of cost per watt per flight and specific power. We also compared the initial HighPower solar array operational goals with the performance of the actual prototype and with the estimated operational solar array systems.
Balloon Solar Array Comparisons
For a reusable system, the cost per Watt per flight decreases nearly inversely in proportion to the number of flights. The HighPower solar array was compared with a typical NASA/NSF long duration balloon (LDB) solar array and the ULDB Balloon Craft solar array. The LDB array, usually flown in polar summer conditions, consists of 4 sets of fixed panels of terrestrial crystalline silicon cells arranged on four sides of a gondola, all at a fixed elevation angle that is optimized for the latitude of operation. Expected array output is only about 220 W at any one time. The ULDB Balloon Craft solar array, made of thin-film amorphous silicon cells, is deployable in three panels at a fixed elevation angle and is pointed at the sun by means of a rotator system. The power required for the rotator has not been included in this analysis but will further lower the overall specific power.
GAC estimated that the reusable HighPower solar array cost per watt per flight is comparable with the LDB costs even on the first flight, and the costs are lower than the ULDB Balloon Craft system after only three flights.
Comparison of Balloon Solar Array Costs
Comparison of Balloon Solar Array Specific Power with Cost
Comparison of Solar Array Goals with Actual and Estimated Performance
The HighPower solar array system operational goals were compared with the performance of the actual prototype and an estimated operational system. Power levels for both the prototype, with 6 SAMs, and the estimated operational systems approached 2,500 W—exceeding the operational goal by nearly 500 W. In addition, the operational goal for sun-tracking cycles was exceeded in ground testing by a factor of 10. While the tracking cycle life still needs to be validated in the actual environment, the preliminary test results suggest major refurbishment of the SAM may not be necessary between flights.
While the desired voltage was between 50-100 V, the designed higher voltage of 150 V results in a more robust array, with respect to shadowing and resistance losses, and is expected to be handled easily by conventional power conditioning hardware in the gondola.
Sample solar cells performed well when exposed to ultraviolet radiation to assess degradation under simulated operational conditions. Degradation was only 2%, even after exposure equivalent to approximately three ULDB 100-day flights during day/night operations.
The total prototype system mass is considerably larger than the operational goal of 100 kg, and the current estimate of an operational system is approximately 28 kg heavier than our original operational goal. Therefore, the HighPower solar array system requires further development before test flights and operational use can occur. Even at the current mass estimate, the overall specific power of the estimated operational system is close to the goal power level.