Remember when solar cars were going to take over the road, and a new commuting evolution was upon us? It still is. Though you may not have heard, read, or seen anything about the event, the American Solar Challenge (ASC) is still pushing college students to design, develop, and race solar-powered cars. This year’s race, rolled from Rochester, NY to St. Paul, MN, and it was the maiden journey for Iowa State University’s 11th car, Hyperion.
It’s All About Power
None of the vehicles in the race reflect an all-American muscle car because they are all built to fine-tune efficiency from all different angles rather than speed. “It took about a year to design, and a year to build,” Evan Stumpges, Project Director for Iowa State’s Team PrISUm says, “This car had only about 150 miles of testing before we started the race in NY.”
Hyperion had some major changes from Iowa State’s previous solar cars. The design changes, sanctioned by the ASC, reduced solar panel space from 9 to 6 m2, and reduced battery pack weight from 25 to 20 kg.
“Less storage and less power, and still our goal was to drive the same speed in the end,” says Stumpges.
In order to compensate for the reduced solar panel space, the team used the next generation in solar technology, Sunpower C60 cells. “[C60 cells] are the best terrestrial grade cell you can buy at 22.3 percent efficiency.” Hyperion holds 391 solar cells, taking up 5.9995 m2 on the top of the car.
In the past, Iowa State has had issues with battery packs overheating, and with scorching summer temperatures in the Midwest, they anticipated more of the same. “A lot of the weather we’re driving in is ambient above 100° F, so it’s hard to keep a battery system cool underneath a hot car and solar cells,” Stumpges explains. Typically the manufacturers of lithium ion cells have a thermal shut-off temp at about 113° F (45° C). Expecting these circumstances, the team decided to use a battery with a much higher thermal range, but a lower energy density.
With the ability to reach a temperature of 140° F (60° C), Hyperion was able to gain some ground on the ASC leader, University of Michigan, during the hot days.
“The second day of the race, the battery pack worked to our disadvantage – we had a lot of rain and clouds, but after that it was scorching. Some teams damaged their batteries, and some even had to slow down or stop through parts of the day that were really hot. We were able to power through the whole thing without any overheating issues,” Stumpges says.
To further their thermal advantage, Iowa State improved the thermal efficiency of their battery box by designing a unique battery module. “Our battery module has a little bit of spacing between each cell, so our fans can blow air between every cell, as opposed to other modules and pack designs that have the cells crammed in as tight as they can be,” explains Stumpges.
The Most Important Design Element
Though these solar cars average around 40 mph, aerodynamic design is extremely important. “Aerodynamics is one of the first things we looked at when designing,” says Stumpges. The competition regulations drive the aerodynamics, to some extent, but when your car weighs less than 400 lbs, slicing through the air escalates in priority.
“Before 2005, everybody made cars with four wheels, as drivers could lay down,” explains Stumpges. “When [the ASC] switched to a mandatory upright seating position, many of the teams found it to their advantage to put the rear wheel directly behind the driver. A three wheel design has pretty much been the standard since.”
Every team devises different methods to minimize the frontal area. “We worked to get a nice, sleek wing-like shape, and then fit a frame and suspension inside it that will work well.” Utilizing ANSYS Workbench 12, provided by Iowa State University, the team did a lot of computational fluid dynamics analysis of the body. “Some teams are actually able to get their cars into a real wind tunnel for testing, but it is hard to find one that big, and using smaller models there are varying degrees of accuracy,” Stumpges explains. “We found it sufficient to use a virtual wind-tunnel and run the computational analysis.”
Is This Practical?
As the idea for solar-powered vehicles has been around as long as solar panels have been in existence, these vehicles beg the question; How usable is a car powered by the sun?
There is a commercial barrier to putting solar cars in the hands of consumers. The biggest problem is obvious, these vehicles require sunlight to operate. Granted, races like the ASC go on, rain or shine. However, no sun means a reliance on a large (and heavy) battery, which can hinder speed and reliability. As Stumpges says, “You can only go so far if you don’t have sun.” This battery only gets larger as the vehicle gains weight, just like the massive batteries in hybrid cars. More car and more weight means more battery, even if it is a sunny afternoon in the desert.
Another major commercial hurdle is the consumer need for luxuries – these are single passenger vehicles with no A/C, no radio, not-so-robust wheels, and no trunk space. Even the greenest of yuppies would cringe at driving a vehicle like this everyday.
Stumpges says, “I’ve been interested in the transition to electric vehicles and the alternative fuel sources for vehicles.” He agrees that a vehicle like this has a long way to go before it can be anywhere near practical for the average consumer. “I see a lot of potential in the not-so-distant future for small lightweight electric vehicles that can be used as commuter cars, and then using solar panels mounted on a permanent structure like your garage where you can get a better value for your solar panels.”
The research for making efficient, safe, and light battery packs for these vehicles – in conjunction with a better understanding of solar cell installation efficiency – segues to creating more reliable and efficient electric cars.
Working with solar cars that demand efficiency is bound to lead to innovation in aerodynamics and frame design. “We touch on technology that is directly relevant to Ford, GM, and all of the automakers as we’re working on advanced lightweight frames using aluminum, doing a lot of analysis on different parts to reduce weight, and playing with aerodynamics and rolling resistance – something cars can save a lot of energy on,” says Stumpges.
As the ASC and various other solar car competitions continue to drive us into the future (literally and figuratively), it is apparent that the technology is improving, and quickly becoming a practical commodity. Stumpges says, “This is as much a demonstration event as anything else.” A bustling city of silent, space ship-like solar cars cruising down the road may be a nostalgic Jetsons-esc dream, but the practicality and importance of this technology is more prevalent now than ever.