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Racing Solar Panel Design: Part Four

Michael Hackleman

©1993 Michael Hackleman

While publishing Alternative Transportation News, I began a series on designing and fabricating the solar panel for the Solar Eagle, a world-class solar race car. In 1990, the Solar Eagle placed 4th of 32 cars in the transcontinental Sunrayce USA, and 10th of 42 cars in the transcontinental World Solar Challenge in Australia. Although this was, therefore, a highly competitive design, there are more similarities in various solar panel designs than differences between them. This series, then, was intended to reveal design and construction techniques that would apply to a scratchbuilt solar panel for mobile or stationary applications.

In the first of my articles on Racing Solar Panel Design (ATN, Jun/Aug 91), I toured the design issues, the constraints due to panel shape and size, and the method we selected to string cells in series-parallel configurations. In the second article (ATN, Sep/Oct 91), I detailed the actual module construction: interconnect options, soldering, applying the glazing, light testing, and soldering the sub-strings. In the third article (ATN, Nov-Dec 91), I covered module design, construction issues, assembly, mass production, carriers, attachment to the substrate, and vacuum bonding. (These three back issues are available from Home Power.)

This is the fourth and final article in the Racing Solar Panel Design series: finishing, testing, and racing the Solar Eagle panel. The process picks up at the point where the modules have been assembled on the substrate and the bonding agent has dried.

Module interconnection

Once all of the modules had been bonded to the substrate, the array team began the process of connecting them together electrically. Typically, there were two terminating strips to each module to handle the end-of-row junctions. These were pinned in place overlapping the adjacent module's terminators. Individual modules were connected with others by soldering. In the wide portion of the center panel, there were many "center" terminators to join. These continued the rows of cells all the way across the width of the panel. These terminators were also bent into position and soldered.

As strings were completed, they were tested with a voltmeter under ambient light conditions to ensure that the connections had indeed been made, and that the Voc (voltage, open circuit) was approximately the correct value. Once the interconnection of modules was complete, the solar array was cleaned of any adhesive that had leaked out from under the cells.

Actually, a substantial amount of adhesive had pushed out from under the cells. Partway through the bonding process, I was shocked to discover that most of the solar cells that had been bonded to the substrate had "dished". Dishing occurs when the center of the solar cells bonds closer to the substrate than the edges of the cell. This is partially due to the soldered interconnect under each cell. I hadn't noticed this with panels built at Spectrolab. Admittedly, none of the cells had cracked, but I did have nightmares about what would happen when the panel got hot or was subjected to road shock. Nevertheless, we continued the bonding process, and completed the panel. Fortunately, no cells were lost because of this effect. Subsequently, however, I discovered that a vital ingredient was missing from our adhesive recipe — microballoons. This acts like both a filler and a thickener. It would have prevented most of the dishing and kept a larger percentage of the adhesive under the cell, too.

Bypass Diodes

Bypass diodes were added to each of the solar strings. A bypass diode is wired in parallel with a portion of a string of series cells to provide a current path around them in the event a cell becomes resistive or open. This way, a string can supply power at reduced voltage rather than no power. To ease the fabrication effort, our design put diodes across rows of cells. Diodes, then, bypassed as few as fifteen and as many as forty seven cells. A more standard procedure is to install bypass diodes across a specific number of cells throughout a string. However, since we could not find any reason not to do it, we opted for this irregular procedure. It clearly simplified our assembly process!

To simplify the testing procedure and to help with any subsequent troubleshooting effort, all of the bypass diodes were mounted on small pieces of perforated

Above: Richard Benevides connects a wire from a bypass diode to a terminator strip on the PV panel.

circuit board. These boards contained as many as ten diodes and were mounted underneath the substrate surface, facing out, for access from under the panel. Each diode was soldered to the next diode on these boards; the finished product represented a string of diodes wired in series. From each of these soldered connections, a wire of the correct length ran to a hole drilled close to a terminator (an end or center of a cell row) on the modules. The end of this #22 size wire was stripped, tinned, pushed through the hole, bent over, and soldered to the terminator. Additional wires were added to connect the diode boards together, as needed. These wires and the diode boards themselves were fixed in position with Kapton tape. The tape was transparent enough to allow us to inspect the wiring and diode polarity without ripping off the tape — a real plus.

Blocking Diodes

Larger wires (#16) were also inserted through holes drilled in the substrate near the junctions of the solar strings. Both positive and negative leads were soldered to each end of the strings and run to the frontmost section of the panel. These were identified and labeled with a string number, and terminated in separate, perforated circuit boards.

The negative leads from all strings ended in one board. While it would have been possible to tie all of the negative leads of the strings directly together and save some wire, we elected to run both the positive and negative wires from each string forward for several reasons. The major one was to preserve isolation of the strings from one another. This helped considerably with troubleshooting, as there were several clever ways that the team managed to miswire diode boards and strings. It was difficult enough to sort out the anomalies even with the isolation of the strings from one another! Also,

Above: Michael Hackleman routes wires from the diode boards on the back of the PV panel.

the individual wires helped us to isolate the inputs (pos and neg) of the three PPTs (Peak Power Trackers) from each other for fine tuning and troubleshooting.

Each positive lead from a 432-cell string was terminated in a blocking diode. A blocking diode permits parallel wiring of strings on a panel without incurring the risk of a damaged string dragging down adjacent strings. There were twenty of these 432-cell strings on the panel and, thus, twenty blocking diodes. These were mounted together on one board. We selected Unitrode JANTXV diodes (600 PIV, 1 A) to isolate the strings from one another.

At this point, the panel was taken outside, mounted on its stand, and left to sit in the sun. Without positive air movement over the panel, it quickly heated up. Both Voc and Vpp (voltage, peak power) measurements were then taken. The panel was realigned as we took readings of each facet of the three sections. A resistive load was fashioned so that we could observe the voltage and current of the strings as we varied the load resistance manually.

Our first readings were a disappointment. Despite all of our efforts, all of the strings seemed to be reading much too low, and there was a considerable variation in string output. I recall that we initially read a paltry 700 Watts of power! Low readings of a solar string when compared with others in the same section led to further checks and we quickly isolated faulty wiring, reversed diode boards, and the like. These were corrected and the readings improved. A few weeks later, the Solar Eagle was taken for a test run in the San Joaquin Valley and the readings improved even more. Even on a sunny day, the smog in the Los Angeles basin was robbing the panel of as much as 15% of its power!

The Umbilical Cable

The Solar Eagle's panel was connected with the rest of the vehicle via a connector and a long length of cable. When finished, it had the look of an umbilical cord. This cable was composed of a number of wires, most of which carried power from the three panel sections to each of their respective PPTs mounted just behind the driver in the chassis. To keep line losses to a minimum, the wire sizes were large, and each wire carried only the current from several strings. These were ganged at the terminal boards at the panel head, and at the connector pins. This way, if any wire in the cable was severed or if a pin failed, paralleled wires would safely route the power around the fault and onward to the PPTs.

Each PPT (peak power tracker) was a special low-weight version of the (Maximizer) commercial unit and was rated for 1,200 watts. In the Solar Eagle, one of the PPTs handled almost 450-500 Watts of power, while each of the other two PPTs carried only half of that amount.

The connector cable also routed some low-voltage power from the panel to various loads in the racing chassis — fans, lights, etc. These cells filled up leftover spaces on each of the substrates that was too small to accommodate enough cells to form one of the twenty 432-cell strings.

Conformal Coating

Once the panel had been tested, a conformal coating was applied. Industrial syringes were used to apply adhesive (essentially the same one used under the cells) around the cells, like grout on a tile floor. Once and for all, this sealed the panel, preventing any water from getting under a cell to short it out or corrode it. This was a laborious effort that I only observed, since it was at this point that I left the project — to start the magazine Alternative Transportation News.

How Did it All Work?

Following each of the two races, the 1990 GM Sunrayce and the 1990 World Solar Challenge, I talked with Solar Eagle team members. During this whole time, no cells broke, chipped, cracked, or opened up due to shock, heat, rocks, mishandling, or rain. If they had, a replacement technique designed and perfected by Richard Benevides (one of the array's builders) was ready to go. But — it was not required.

In retrospect, there are only two changes I would make to the way the Solar Eagle panel was done. One was to add the aforementioned microballoons to the adhesive solution that bonded the cells to the substrate. At least, this would have minimized dishing. It might have saved cleaning adhesive off all those cells, too. It also might have eliminated the task of adding a conformal coating.

Another change I'd make is to hold to a smaller tolerance at the angular edge of the panel's facets. (That's the big white V in the panel.) We worried about getting the edges of the cells in the modules too close to a curved joint along the angled edges of the array's facets. We did not custom fit the solar modules to the panel substrate. Rather, construction of the array substrate and the modules occurred simultaneously. We were running out of time. The modules had to be ready to bond to the substrate as soon as it was done.

I was way too optimistic about the amount of power our panel would produce for the number and quality of cells involved. Consequently, while we were careful, we were also somewhat conservative. Anyway, it looks like we could have squeezed another 90-180 cells onto each of the three panel surfaces. Coupled with the cells (used as fill) that were wired for 12 VDC loads, the additional cells might have resulted in a 10-14% gain in the panel's output power. Of course, that's 20/20 hindsight! No one on the team had ever built a panel before. When you've got a couple of hundred thousand dollars in a prototype, you must be somewhat conservative or risk a major goof.

Still, as ignorant as we were to begin with, I'm amazed that things turned out as well as they did. It was an enormous task. Anyone who worked on the project was virtually a slave to the process and effort. The project consumed more than a year of my life, and I know that it took me almost a year to recover. It's been said that building a solar car is once-in-a-lifetime experience. Let me translate that! I learned a lot, I'm glad that I did it, but I'm not likely to do it again!

This information aims to ease the way of others that may walk this path. Good luck!


Michael Hackleman, c/o Home Power, POB 520, Ashland, OR 97520

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Above: Michigan State's HEV at the Dearborn Proving Ground.
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