FAQ: How does a solar car work?

Feb 19, 2017 | FAQ

Arjo van der Ham — Co-Founder

A solar car is a car that uses the sun as its main source of energy. You might know some experimental solar cars from the World Solar Challenge, a biannual 3000 km challenge through the deserts of Australia.

The first known long distance solar car is The Quiet Achiever. Built by the brothers Larry and Garry Perkins, this car crossed the Australian continent from west to east in 1982. It reached an average speed of 23 km/h. Over the years, battery and solar technology have improved. A lot. In 2013 the World Solar Challenge introduced the Cruiser class for more practical family cars. Today the technology is at an adequate level to start the development of commercial solar cars.

How do solar cars work?

At its core, a solar car is an electric car – one with an unlimited, free and wireless power source which it takes anywhere it goes. Basically, a solar car consists of a solar panel, a battery and one or more electric motors. All these parts are connected in parallel, like this:

Simple representation of a Solar Car.

Let’s get a little bit more technical.

At any given time, the amount of sunshine determines the power output of solar panels, Psun.  Also, we have the power of the battery Pbat and the power requested by the motors, Pmot. The power into or out of the battery is determined by the difference between the power from the sun and to the motors: Pbat = Psun – Pmot. Therefore, if more power is coming from the sun than is needed for the motor, the battery is charged. For example, when the car is parked or driving slowly, the excess energy will be stored in the battery and can be used at a later time. When the motor needs more power than is coming from the sun, the battery is discharged. For example when driving at high speed.

So what determines the power requested by the motor?

For the largest part, Pmot is determined by two factors, the aerodynamic drag and rolling resistance of the car. The aerodynamic drag, CdA, is again determined by two main parameters. The frontal area of the car A and the drag coefficient Cd. The frontal area is the area of the car’s frontal outline, it is the area against which the air particles will collide. The drag coefficient describes how easy it is for the air to flow around the car. A long list of drag coefficients for cars can be found on Wikipedia. Let’s, for example, take a look at the aerodynamic drag of a Tesla Model S and Volkswagen Golf:

A [m2] Cd [-] CdA [m2] approx. aerodynamic loss @ 20 km/h [W] approx. aerodynamic loss @ 120 km/h [W]
Tesla Model S1 2.34 0.24 0.56 64 13800
Volkswagen Golf2 2.22 0.31 0.69 76 16500

The drag coefficient of the Volkswagen Golf is only a factor of 1.2 higher than that of the Tesla Model S. But this does mean that at a speed of 120 km/h the Volkswagen Golf needs 2700W more power than a Tesla Model S, which is enough to run an average tumble dryer. A huge difference in energy use. The rolling resistance is determined by the rolling resistance coefficient of the tires Crr and the weight of the car m. Typical values for Crr are around 0.01. The power that is needed to overcome the rolling resistance increases linearly with the speed. Since the power needed to overcome aerodynamic drag increases cubically with the speed. At low speed, the rolling resistance is more important, while at high-speed aerodynamic drag is more important.

How to design the best solar car.

The trick to making a solar car work is to maximise the amount of power coming from the sun, using many efficient solar cells. And, to minimise the amount of power the motors need, thus giving the car a very aerodynamic shape, and making it as light as possible. Reducing the aerodynamic drag can be accomplished by reducing the frontal area. For example by making the car lower by slightly altering seating positions, or by replacing mirrors with cameras. Improving the drag coefficient can be accomplished by making the shape of the car more smooth, and removing edges and seams, for example by adding a closed bottom plate to the car. To reduce the weight of a car, lighter materials can be used. For example, carbon fibre reinforced plastics can achieve the same strength as aluminium at 10% of the weight! 3. Also, improving the aerodynamics lowers the energy consumption of the car, and thus less powerful motors and smaller batteries are needed to achieve the same performance. These weight reductions, in turn, result in lower requirements for the structural parts, which can thus become lighter. This effect, which is called secondary weight, triggers a loop as shown in the figure below 4.

Secondary weight loop

If you multiply power and time, the amount of energy that is generated or used is obtained. The energy used by a car per driven kilometre depends mainly on the characteristics of the car (aerodynamics, weight), and speed. To make fair comparisons between cars of different manufacturers, drive cycles are used that describe how fast a car typically drives. In Europe, the New European Driving Cycle is used, but it will soon be succeeded by the WLTP cycle. Take a look at the graph below to get a feeling of the typical energy consumption of cars. Notice that a Tesla model S uses half the energy that a VW Golf uses. The biggest improvement is the electrification of the powertrain. A typical internal combustion engine as used in the VW Golf reaches an efficiency of only 20%, while the electric powertrain of the Tesla Model S can reach efficiencies of 80 % 5. Stella, a solar-powered family car, uses even less energy. This is accomplished by creating a very aerodynamic shape, and by building a car that weighs only 390 kg.

Comparison of energy use for an VW Golf, a Tesla and Stella.

In the Netherlands, the average passenger car drives 11.000 kilometres per year 6. If you fit 5 square meters of solar panels on a car with an efficiency of 22 %, they would harvest about 750 kWh of energy per year. A solar car that, on average, produces just as much energy as it uses would, therefore, need to have an energy consumption of  750.000/11.000 ≈ 70 Wh/km. To realise such a car, new materials and production techniques, such as carbon fibre are necessary to minimise the weight. The shape of the car has to be redesigned to be more aerodynamic and fit enough solar cells.

Sources:

[1] Sherman, D. (2014, June). Aerodynamics Compared. Retrieved February 10, 2017, source

[2] Volkswagen Golf 1.2 TSI DSG (2011) – technical specifications. Retrieved February 10, 2017, source
[3] Engineering Materials. (n.d.). Retrieved February 10, 2017, source
[4] Alonso E, Lee TM, Bjelkengren C, Roth R, Kirchain RE. Evaluating the Potential for Secondary Mass Savings in Vehicle Lightweighting. Environ. Sci. Technol. 2012; 46: 2893-2901
[5] Assanis, D. (n.d.). On the Road to Clean and Efficient Powertrains (Presentation). source
[6] Jaarmonitor Wegvoertuigen: Aantallen. (2016, June 03). Retrieved February 10, 2017, source
* Cover picture by Bart van Overbeeke

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