The Making of Solar Champions
By Bill Moore
IMAGINE A CAR that gets the equivalent of 800 miles (1287km) to a gallon of gasoline, can travel all day at 55-65 miles per hour (90-105km/hr) without having to stop and refuel, and best of all, creates not an ounce, not a gram of pollution. In fact, its only "fuel" is pure, clean, free, unadulterated sunlight.
Sound far-fetched? Perhaps a bit too science fiction? Think again, dear reader. Those cars are here today.
Of course, there are a few caveats. You can only drive it during daylight, and although they can cost as much as $125,000, you'll have to do without some of the more high-end creature comforts like air-conditioning, heated leather seats and multiple cup-holders. Think of it as the ultimate vegan Nascar.
One of the top university teams in this high-tech race with the sun is the University of Missouri at Rolla, which has either placed first or second in the last five North American solar car races. Not always as state-of-the-art as its competitors, the team has consistently demonstrated they have what it takes to be solar champions.
|And now their faculty advisor and mentor, Dr. Douglas Carroll has written a new book -- The Winning Solar Car: A Design Guide for Soar Race Car Teams -- that distills their winning ways into a single volume now available from the Society of Automotive Engineers and Amazon.com (click the simulated book cover at right to link to the latter online bookseller or patronize your local, independent bookseller).|
EV World talked with Carroll over Christmas break. You can listen to our discussion in MP3 stereo audio using the Flash-based MP3 Player at the right, or you can download the file for offline listening or for use in your portable MP3 player.
But starting with the launch of the EV World Premium Service, we're re-introducing the much-demanded text version of our interviews, available exclusively to EV World subscribers.
We began our interview by listing the teams recent triumphs: first in the 1999 Sunrayce, second in the 2000 Formula Sun Gran Prix, second in the 2001 American Solar Car Challenge, first in the 2002 Formula Sun Gran Prix and first in the 2003 American Solar Car Challenge.
What's their secret? Curiously, its not necessarily their technology, but organization that in Carroll's view makes the difference.
"We started about eight years ago treating this as if it were just an engineering project... we made our team as if it were a small company within the university. And our secret is really we develop a plan, a schedule and a budget for this project and a good management structure, and we stay on schedule."
That schedule includes completing each car before Saint Patrick's Day, March 17th of each competition year, so that the team's car can take part in the annual school pageant parade. This enables the team to easily meet the solar race qualification dates for that year, which are typically held in April and May, with the races held in the summer.
Carroll explained that the team gets a lot of practice in the car this way, including running it on the Interstate and staging mock flat tire drills.
"That's our secret really. We prepare to get the most out of our car. We don't always have the best car in the race, but we try to be the best prepared team," he stated. His team usually consists of fifty hard-core members with a few others who occasionally attend team meetings, though Carroll suspects they're more interested in fleshing out their resumes than becoming serious contributors.
Efficiency Is The Name of the Game
What makes solar cars competitive is their overall efficiency. They have to be because, as Carroll pointed out, there is relatively very little energy available from sunlight even with the most modern, triple-junction gallium arsenide solar cells, like those used on the current World Solar Car Challenge winner, Nuna II.
"Everything is geared towards efficiency, " Carroll explained. "That's what wins the race." Just how efficient these cars are is illustrated by the fact that they can maintain 55-65 mph on a mere 1,500 watts of electrical power, that's the equivalent of just two horsepower.
The latest incarnation of the University of Missouri Rolla sun-powered racer is called Solar Miner IV. It utilizes a New Generation hub motor that is 95 percent efficient. He elaborated briefly on the advantages of this particular motor and set up. Having the electric motor in the rear drive wheel of the tricycle configuration eliminates transmission and drive belt or chain losses. The pancake-style motor has an electronically adjustable air gap that enables maximum efficiency from start to cruising speed.
"We are about ninety-five percent efficient in converting electrical energy to mechanical energy."
Beside the highly-efficient hub motor -- which he believes is how electric-drive cars will ultimately be propelled -- Solar Miner IV uses double-junction gallium-arsenide photovoltaic cells and lithium-ion batteries.
Carroll is immensely impressed with lithium-ion batteries. He told EV World that its possible to get 95 percent of the energy back out of the batteries that it takes to charge them. In his view, these remarkable new batteries have transformed solar car racing.
"They are amazing! These lithium batteries are really going to change the world of electric vehicles as we know it," he said.
He's not only impressed by their energy density -- which means you can store a lot of electrical potential in a very lightweight unit compared to other battery chemistries -- but also with their ability to cycle energy into and out of the cell. He said his team has pumped high rates of energy into and out of their battery pack without it even getting warm, something no other chemistry they've tested has ever done. As any high school student who has built a simple Electronthon-class battery electric car can attest, heat is not only bad, its a telltale sign of inefficient design.
Changing from nickel metal hybrid -- the chemistry used in the batteries of today's production hybrid-electric cars -- meant in Carroll's view that competitive solar cars could go from travelling 300 miles in a day to 450 miles, a 50% improvement in range.
Virtually all competitive solar race cars utilize a similar-looking platform, one designed to minimize frontal drag -- the biggest energy robber, followed by rolling resistance which consumes one-third of the car's energy to overcome -- and maximize the solar collection area. The rules for the one-person cars require they can be no longer than 5 meters and no wider than 1.8 meters. Two passenger cars are allowed slightly longer and wider dimensions. Solar Miner IV weighs, including a 185 pound driver -- a mere 517 pounds, thanks in large part to the lightweight lithium batteries, carbon composite materials and tubular aluminum frame.
Carroll noted that his team's solar cells are about 20-21 percent efficient at converting photons into electrons. The Delft University team's Nuna II uses cells that are an amazing 28 percent efficient, but there's a steep price to be paid for that performance edge. He estimated that to equip the new Solar Miner V with these satellite-quality cells would cost about one million US dollars, versus the $40,000US-worth of cells on the current university car.
One interesting observation Carroll made was that not only are there 3,000 individual cells on Solar Miner IV, but each of these has to have its own separate diode soldered to it to prevent electrical damage to the cell in event of cell failure or shading. He also noted that the car's lithium-ion battery pack cost $10,000US.
Carroll explained that nearly all of the car's motive energy comes directly from sunlight. He said that on a sunny day, the car can run 500 miles (805 km) without stopping, though obviously the single-driver who is sitting with his or her head in a bubble that can go well above 100 degrees F, will need to take a break periodically, usually every 5-6 hours. The car will do 50-55 mph going up hills and 65 mph down the other side, and it can do this all day long.
The $10,000US lithium-ion battery pack stories 5kW hours of energy, which at the local utility rate in Rolla equals about 35 cents worth of electrical power. If the team turned off the solar cells and just ran on battery power alone, Carroll calculates the car can travel 200 miles on that 35 cents of electricity or the equivalent of 800 miles per US gallon.
Because Solar Miner IV is so light -- as have most of the team's past cars -- I asked Carroll about potential crosswind problems with the car. He responded by saying that because the car is designed to have rounded edges, it has very little problem with crosswinds. Even on the Interstate when being passed by a semi (heavy lorry) doing 70-80 mph, the driver notices only minor disturbances.
"It's pretty stable out there," he commented.
Management Discipline Necessary
While the current team manager/president is male -- and there have been female presidents, Carroll noted -- good management is critical to maintaining team discipline, which is essential to keeping the project on track.
"We don't want him to get too involved in the technical design. Management is largely discipline, to force yourself to stay on schedule and to make the right choices to build the fastest car you can with the resources you have. That's very important."
Under the team president, who services as overall project manager, there is an electrical system manager and a mechanical system manager. In both arenas, the team relies a great deal on computer analysis and simulations from tweaking the aerodynamics using fluid computation models to design of the circuit boards, which is also done in-house.
Another skill the team has developed is a software package that helps them test and assemble their battery pack, which Carroll says is one of the more difficult elements of the program.
From his nearly decade of solar car racing experience, Carroll sees two areas where the sport has made quantum improvements. The first was the introduction of the controllable air-gap, pancake motor, and the second was the debut of lithium-ion batteries. He sees the two most critical factors in a competitive solar car are a highly efficient electric drive system and lightweight vehicle design.
"That's true for any electric car. Everyone focuses on those solar cells and trying to get more power. I'll tell you (that) in 2001 we had thirty percent less solar power than any of the top ten teams and we finished second, because that was our choice. We said we can't afford everything. We can't afford double-junction cells at this point, but we can afford the lithium batteries... We made that choice and overall that made a faster car than buying the high-power solar cells and not having as good a battery system."
In years past, the University usually contributed $25,000US to support the program, with outside sponsors making up the rest of the $125,000 price tag. However, in the current economy with most states running large deficits, funding for the solar team is harder to come by, Carroll admitted. Even some of their traditional outside sponsors -- typically businesses with U of Missouri Rolla alumni -- are hurting. Still the team plans to field an improved Solar Miner IV for the 2004 solar race and has the paper work complete for Solar Miner V, its successor.
Why No Solar-powered SUV's?
One question EV World hears asked periodically by readers is why can't we power our conventional cars with solar cells. Carroll has a quick retort. "There's not enough solar energy to power the air conditioner, let alone push your car down the road."
"Our solar cars are just incredibly efficient. Lightweight, no heat, no air conditioning, no radio... bare bone basic." He added that the ride is rough and a driver is going to get beat up and bruised in the typical 5-6 hour driving leg.
What happens when its cloudy? Interestingly, even under cloudy conditions, the solar cells do continue to generate some electrical current, although that modest amount of energy has to be carefully husbanded, as in the 1999 Sun-less-rayce from Washington, D.C. to Florida when it rained every single day from the starting line on the Mall to the finish line in what is usually billed as the Sunshine State.
Carroll -- whose team actually won that race despite the conditions -- said they had to carefully pace themselves at a modest 25 or 30 mph many days. "You have to choose the speed that will get you the furthest," he said. "We'd crawl along all day long and we'd make it."
Sunlight to Burn
In contrast, the 2003 American Solar Challenge from Chicago to Los Angeles followed a meandering course along Route 66 where the cars passed through countless small towns with low speed limits. This forced many of the teams, including Solar Miner IV, to shut off their solar arrays to prevent overcharging their battery packs.
"Until we got out West, until we got out past Albuquerque, we couldn't go fast enough to use up energy. So, for the first part of that race it was really a reliability race; who could not break down."
He noted that west of Albuquerque, the top four teams including Canadian teams from Queens University and Waterloo University, as well as the University of Minnesota, could open up their cars and let them fly. Ironically, the only stretch of rain was in the high desert of Arizona around Flagstaff where "it really rained hard," Carroll chuckled, who was running the team's weather scout vehicle, which runs 50-100 miles ahead of the solar car.
When asked which was more important, performance or reliability, Carroll said that there's still enough of a performance difference between the cars, that an occasionally breakdown in a long cross-country solar race won't necessarily end a team's chances at winning, unlike Nascar, where a single breakdown means you're out of the race.
"We're not at the level," he added. "Reliability is the key, but you have to have an efficient car that can go a long ways on the energy or you have no chance."
Obviously, the University of Missouri Rolla solar car team has honed both to a fine edge.
Author's End Note:
As I write this, a solar-powered hydrogen fuel cell car developed by the students of Tamagawa University in Tokyo has just completed a nearly 5,000 km journey across the length of Australia, the first mating of solar power and a fuel cell battery in this type of application.