In the second installment of this series on ultra-efficient vehicle design, I cover briefly the aerodynamic design of solar cars and the related opportunity for significant improvement of production passenger cars.

[Image Credit: energy.gov]

Air resistance is very important consideration for solar cars, or any vehicle that travels at high speeds, since the power to overcome air drag increases cubically with speed. In other words, if you double your speed, the power required to overcome air resistance increases by a factor of EIGHT.

I’m going to jump right into an equation that will help illustrate some of the main concepts in this article. The force of air pushing back on a moving car, the aerodynamic drag force (), can be approximated by this formula:

where is the density of air, is the drag coefficient, is the car’s frontal area, and is the car’s velocity (speed). This equation is a simple engineering model (see my ongoing series of articles on modeling) that helps us understand how changing vehicle design and operating conditions affects aerodynamic drag force. Looking at this equation, we can see that drag force increases quadratically with speed (that is, doubling your speed increases drag force by a factor of four), and increases proportionately with frontal area and drag coefficient. Based on this equation, what can we do to reduce drag force? Obviously the most effective thing to do is reduce speed. This is why freeway speed limits were reduced to 55 mph years ago to save fuel. Let’s assume for now that the speed we want to drive our car at is fixed. What else can we do to reduce drag force? We can’t do much about reducing air density (), but we can control frontal area () and drag coefficient (). In my last post on solar car design, I explained that solar cars must make do with very limited amounts of power (less than what a hairdrier consumes). Reducing frontal area and the drag coefficient can bring solar car designers one step closer to building a car that can travel at highway speeds with only the power from the sun.

In the past, solar car racing rules allowed drivers to pretty much lie down in the car, making it possible to design cars with a very small frontal area. This really helps reduce air resistance (cyclists understand very well the importance of keeping a small profile), but makes getting in and out of the car fairly challenging. Current solar racing rules now require a more upright driver position, resulting cars that are a little closer to what commuters might consider driving. Some teams have even built two-person solar cars. While somewhat more practical, solar cars with upright seating have increased frontal area and increased air resistance.

So what about the drag coefficient? Vehicle designers can adjust the shape of the car so that air flows around it smoothly, requiring less force to push the car through the air. Solar cars are perhaps the most streamlined road going vehicles. They have smooth surfaces that taper toward the rear, ensuring that air flows over them in a smooth, laminar way. The blunt rear edge of most cars leads to a lot more air resistance, as opposed to the trailing edge of a solar car or the the Aptera 2e. Have a look at these simulation results that show how air flows smoothly over the University of Waterloo solar car without much disturbance:

The streamlined shape slices through the air without generating a turbulent wake at its tapered rear edge, in contrast to many production vehicle with blunt rear ends. Solar cars have acheived drag coefficients as low as 0.10, while the Prius sports a much larger and less efficient drag coefficient of 0.26 (which is actually the lowest of any production car). Referring to our drag equation above, that means if a Prius and a solar car had the same frontal area, the Prius would take 2.6 times as much force to overcome air resistance as the solar car at a given speed. Most production cars have noticeably higher drag coefficients; a Civic’s is 0.36, and a Hummer H2’s is 0.57. The image below illustrates how other vehicle shapes can lead to turbulent wakes, which increases a car’s drag coefficient:

[Image Credit: flickr]

While reducing drag coefficient is a paramount consideration in solar car design, it is not the only consideration. Goro Tamai, a past MIT solar car team member, discusses in his book, The Leading Edge, that aerodynamic design must be considered as a component as the overall vehicle system.

The “best” body shape for solar cars, HPVs, or Electrathoners is not the body of absolute lowest drag. The vehicle system, including the driver, chassis, and energy/drive system must work in concert to produce the maximum output.

In addition to minimizing drag, designers must ensure vehicle stability and safety, and that aerodynamic design works in concert with powertrain and power production systems to acheive the best overall vehicle performance. In the case of a solar racer, the measure of ‘best performance’ is how fast the car can safely travel a set route, powered only by the sun. I’ll explore the importance of systems engineering in vehicle design in a later article in this series.