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Solar and Lithium-ion

Solar and Lithium-ion polymer - a potential synergy of power for tomorrow’s vehicles
BY Shirley Georgi

(October, 02) The 2002 ‘Drive the Future Tour’ was a fist-time interprovincial effort in Canada among three internationally competitive Canadian solar car racing teams. The University students involved in this project wanted to challenge perceptions of alternative energy vehicles, raise awareness about positive climate change and draw attention of the needed environmental responsibility toward clean technologies. One of the three participating schools, the University of Toronto, had a dedicated group of more than 100 students who worked to design and build an auto using solar panels and a Lithium-ion polymer battery pack. Through teamwork and innovative application of classroom knowledge, Blue Sky, as the team members are called, demonstrated that this combined technology can and does work. They have entered a number of solar races including the 2001 American Solar Challenge (ASC), where they placed 12th, and the 2001 World Solar Challenge in Australia, where they achieved 14th place. Although they did not win the blue ribbon, they built a vehicle capable of traveling in excess of 100 km/h using the same power as needed for a household toaster.
 The three solar cars pictured on the front cover are the three vehicles which participated in the Canadian interprovincial “Drive the Future Tour” this past summer. Dr. Sankar Das Gulpta (right), CEO of Electrovaya, Inc. and David Nam (left), the Strategic Development Officer and solar car driver for the University of Toronto (U of T) Blue Sky Solar Racing show off the U of T solar car called Faust (foreground).

The basic design -The U of T’s solar vehicle has an aerobody composed of layered cloth Kevlar and carbon fiber composite material with a honeycomb nomex core and is constructed using the same methods as those used in aircraft body parts. The chassis, composed of hollow aluminum tubes, has sides slightly thicker than a pop can.

The unique design has been engineered to weigh 8.2 kg and serve as the car’s skeleton, providing all structural support.

Power system -More than 3000 of SunPower’s silicon photovoltaic cells, covering eight square meters of Faust’s top surface, convert sunlight at 160 W/m2 of electrical power, into approximately 950 W total output. The 125 Lithium-ion polymer prismatic cells from Toronto based Electrovaya, Inc. comprise the battery pack which as a whole is capable of storing five kiloWatt-hrs. of energy and weighs under 30 kg. A 95% efficiency brushless DC motor from New Generation Motors allows Faust to maintain highway speeds while consuming less energy than a hair dryer.

The solar vehicles in the background are the University of Waterloo’s Midnight Sun VI and L’ Équipe d’Éclipse de l’École de Technologie Supérieure. (The photo is courtesy of iContact and Electrovaya.)

Of course, the true proof of the pudding is in the execution, and in this case, the performance of vehicle. To get a more in-depth understanding of the University of Toronto’s vehicle, David Nam, the driver, gave BD the following interview.

David, can you tell us a little more about the solar cells and how they performed?

The solar cells were monocrystalline silicon. I don’t have the exact number, but I believe there were about 2960 individual cells that constituted the solar array, Supplied by SunPower Corporation, the cells were sorted into bins according to efficiency ranging from 17.5% to 21.1%. (figures are based on the manufacturer’s data) Each cell was 22.2 centimeters square and had a Voc of ~ 0.6V and an Isc of ~0.8A. Our tests showed that the cells from the bins that were put on the car had an average efficiency of a little over 19%. Ignoring factors like geometry or wire losses (proven to have little effect on our car), the expected output of our array under good lighting conditions was 1200W.

PARAMETER

 SYMBOL
 MINIMUM
 TYPICAL
 UNITS
 Frontside Efficiency
 h f
20
22.5
%
 Rearside Efficiency
 h r
11
11.5
 %
 Frontside Power density
Pmax, f
20
22.5
 mW/cm2
 Rearside Power density
 Pmax, r
11
11.5
 mW/cm2
 Open circuit voltage
 Voc
680
mV
 Voltage at Pmax
 Vmp
565
 mV
 Front Short Circuit Current
 Jsc,f
40.3
 mA/cm2
 Rear Short Circuit Current
  Jsc,r
20.5
 mA/cm2
 Front Current at Pmax
 Jmp,f
39.8
 mA/cm2
 Front Current at Pmax
 Jmp,r
18.5
 mA/cm2
 Fill factor
 FF
80
 %


Electrical Characteristics
The Pegasus solar cells have been designed for solar powered aircraft and low-fluence space missions as well as for solar race vehicles. The chart shows electrical characteristics for AM1.5 Sunlight (std. terrestrial spectrum), 100.0 mW/cm2, TA + 250C. (Data is courtesy of SunPower.)

However, 1200W was never achieved. During the American Solar Challenge (ASC), our array on a good day produced 850 W, which translates into an overall efficiency of ~14%. During the World Solar Challenge (WSC), we discovered that some of the cells were mismatched when joined into strings by the manufacturer. (Note that 10 cells together constitutes a string.) Some of the cells had cracked from wear and tear. Those mismatched or damaged cells were bypassed, and consequently, our array produced ~ 950 W on a good day.

What about the battery pack? Can you give us more details about the cells? The pack? Its performance?

The Electrovaya battery pack was made up of 25 modules in series, where each module contained five cells in parallel. The total storage capacity was in the neighborhood of 5 kWhr or about 50Ahr since the operating Voltage was usually around 100V. A lot of efforts were taken to characterize the pack before the 2001 racing season, so the pack indeed behaved as expected during that race. We were extremely impressed with how well the pack performed in many high-current draw situations (i.e., the long climb up mountains during the ASC) and overall.
The only problems the team experienced with the pack were mechanical in nature. During the ASC some of the aluminum tabs used to connect the terminals broke off due to the extremely rough road conditions on “HISTORIC” route 66.

Adobe Photoshop Image Cross-Section of a Pegasus Solar Cell

The Pegasus Solar cell by SunPower is designed using an interdigitated back-contact with no front-side grids. This eliminates grid shading and allows for high cell packing density in module form. The cells are fabricated using very thin monocrystalline silicon wafers. The design incorporates front surface texturing and a double-layer antireflective coating. Backside grid coverage has been minimized to allow for bifacial illumination. (Artwork is courtesy of SunPower.)

For the ASC, the pack was never completely exhausted because of the way the route was set up. There were a number of days where teams who pulled in early had an opportunity to put back a significant amount of energy into their packs. Furthermore, the final stretch of the ASC was 90 miles, and every team had at least one full day to recharge their pack before setting off.

The WSC was a completely different story. There were no extended check points. Every media stop along the route was 30 minutes. On the second to the last day of the race, our telemetry system completely pooped out. Therefore, we neither had information on the state-of-charge of the pack nor exact measurements of how much energy we were consuming. As a result, we did run our pack completely dry on the second to the last day of the race. Fortunately, this happened 10 minutes before the end of the race and about 100 km shy of the finish line for the day. The next day wecharged our pack enough to complete the race, and we maintained an average speed fast enough to gain a position.

Tell us about your experience in driving the vehicle. Were you able to complete the 900 km in “Drive the Future” as planned? How fast could you travel? Max. speed? What was the longest distance you traveled in one day?
Unfortunately, we were not able to complete the entire ‘Drive the Future’ tour due to a collision involving our vehicle. However, finishing the 900 km tour would have been no problem; the car had already completed 800 km of racing in 2001.

During the ASC, we did reach speeds in excess of 140 km/h (with the help of a slight downhill) and got penalized for it. The maximum speed of our car on the flat road is 125 km/h. On the DFT tour, on non-racing days, we were cruising at maximum posted speed limits of 100 km/h on the highways. We would pull into towns and reduce our speed and then we would go back to our highway speed. We drove just like one would in a normal vehicle. There is one interesting note, however. One of the other teams with us on the tour had requested that we slow down the pace because their battery pack was starting to heat up quite a bit, which is a testament to the robustness of our Electrovaya battery pack.

We were making a lot of stops during the tour, so we didn’t really cover a lot of distance in one day. I can tell you, however, that the longest distance traveled in a single day was during the WSE where we went some 600 km.
Adobe Photoshop Image Front Side External Quantum Efficiency and Reflection

The combination of quantum efficiency and reflection combine to define the spectrum of light which produces electricity in the cell. In the visible spectrum, quantum efficiency is almost 100% while the reflection is negligible. Above that, in the infrared region, the cell surface reflects almost 50% of the light energy. (Chart is courtesy of SunPower.)

How long did it take to build the vehicle? Was it totally built at the University by the Department of Engineering?
Although the majority of students working on the project are engineering students at the U of T, there are also many arts and science students working on the project. (I, myself, am a philosophy/political science specialist.) About 90% of the vehicle was built by students, including the chassis, aerobody, moulds, plugs and the majority of the suspension components and electrical system with computers. The design along with the parts were spec’d by the engineering students.

What additional facts (or thoughts) would you like to tell our readers that you think might be of interest?
Here is some food for thought. If the environmentally friendly world in our future is going to run on electricity, including electricity produced from fuel cells, we are going to need some ‘kick-ass’ batteries.

BD