Solar cars

by Sergio Naveira Ibáñez

1. Presentation and history

A solar car is a solar vehicle used for land transport.

Solar cars only run on solar power from the sun. To keep the car running smoothly, the driver must keep an eye on these gauges to spot possible problems. Cars without gauges almost always feature wireless telemetry, which allows the driver’s team to monitor the car’s energy consumption, solar energy capture and other parameters and thereby freeing the driver to concentrate on driving. Most solar cars have been built for the purpose of solar car races .

The first solar car invented was a tiny 15-inch vehicle created by William G. Cobb of General Motors. Called the Sunmobile, Cobb showcased the first solar car at the Chicago Powerama convention on August 31, 1955. The solar car was made up 12 selenium photovoltaic cells and a small Pooley electric motor turning a pulley which in turn rotated the rear wheel shaft. The first solar car in history was obviously too small to drive.

Now, let’s jump to 1962 when the first solar car that a person could drive was demonstrated to the public. The International Rectifier Company converted a vintage model 1912 Baker electric car (pictured above) to run on photovoltaic energy in 1958, but they didn’t show it until 4 years later. Around 10,640 individual solar cells were mounted to the rooftop of the Baker to help propel it.

In 1977, Alabama University professor Ed Passereni built the Bluebird solar car, which was a prototype full scale vehicle. The Bluebird was supposed to move from power created by the photovoltaic cells only without the use of a battery. The Bluebird was exhibited in the Knoxville, TN 1982 World’s Fair.

Between 1977 and 1980 , at Tokyo Denki University, professor Masaharu Fujita first created a solar bicycle, then a 4-wheel solar car. The car was actually two solar bicycles put together.

At the engineering department at Tel Aviv University in Israel, Arye Braunstein and his colleagues created a solar car in 1980. The solar car had a solar panel on the hood and on the roof of the Citicar comprised of 432 cells creating 400 watts of peak power. The solar car used 8 batteries of 6 volts each to store the photovoltaic energy.

In 1981 Hans Tholstrup and Larry Perkins built a solar powered racecar. In 1982, the pair became the first to cross a continent in a solar car, from Perth to Sydney, Australia. Tholstrup is the creator of the World Solar Challenge in Australia.

In 1984, Greg Johanson and Joel Davidson invented the Sunrunner solar race car. The Sunrunner set the official Guinness world record in Bellflower, California of 24.7 mph. In the Mojave Desert of California and final top speed of 41 mph was officially recorded for a “Solely Solar Powered Vehicle” (did not use a battery). The 1986 Guinness Book of World Records publicized these official records.

The GM Sunraycer in 1987 completed a 1,866 mile trip with an average speed of 42 mph. Since this time there have been many solar cars invented at universities for competitions such as the Shell Eco Marathon. There is also a commercially available solar car called the Venturi Astrolab. Time will only tell how far the solar car makes it with today’s and tomorrow’s technology.

 2. Parts and operation

Parts

Solar Array and Power Trackers
We recommend a solar array created from individual solar cells as opposed to one made of prefabricated solar panels. It enhances the students’ learning and can result in a lighter solar array. Cells can be bought from either Siemens or ASE Americas. Both sell the terrestrial-grade cells that are permitted in the Winston Solar Challenge, and the cost for terrestrial-grade cells are much lower than space-grade cells, though terrestrial-grade is less efficient. Each solar cell should produce .5 volts at about 3 amps at peak sunlight. The number of cells to use depends on their size and the allowable solar area per Winston rules. Solar cells should be wired in series on a panel and should be divided into several zones. For example, if you have 750 solar cells, you might want to wire 3 sets of 250 cells, each zone producing about 125 volts. If one zone fails, two other zones are still producing power. The solar array voltage does not need to match the system voltage of the motor if you use power trackers. Power trackers convert the solar array voltage to the system voltage. They are essential in a solar car. Be sure to verify with the power tracker vendor the necessary array voltage to feed the power trackers. If the car drives underneath shade, the power trackers automatically adjusts the power to match system voltage, allowing the system to run as efficient as possible. Power trackers are available from AERL.

Batteries
The batteries store energy from the solar array and makes them available for the motor’s use. Many different types of batteries are sold. Most high school teams use lead-acid batteries because they are inexpensive, but some teams use lithium-ion or nickel-cadmium. We recommend that you stick with lead-acid batteries because they are readily available and inexpensive. Another choice teams must make is running with flooded-cell batteries or gel-cell batteries. Flooded-cell batteries are the standard automotive batteries filled with liquid sulfuric acid. They are preferred because they can be overcharged without risk of blowing up, but they weigh more than gel-cell batteries. Gel-cell are sealed and lightweight, but when charging the batteries, check the battery voltage often. The number of batteries to choose depends on the motor (system) voltage. If the system voltage is 72 volts, you will need 6 12-volt batteries. Also be sure to check the rules for weight or watthour requirements. Buy batteries with as many amphours as allowed by the rules to maximize the amount of energy you can store.

Motor & Controller
Most teams use DC brush permanent magnet motors to drive their solar cars. Inexpensive and easy to hook up, these motors are desirable for high school teams with little financial support. Expect a maximum efficiency of 80-90%. For teams with more money, brushless motors increase the efficiency of the motor to the 94-99% range. Also, some motor and controller setups allow for regenerative braking, which allows the solar car to put energy back into the batteries when going downhill. For the beginning team, DC brush motors would be sufficient to get a solar car up and running. Another variable in choosing a motor is how much power it has. We have found that there is little need to have more than 5hp continuous power output on our motors. There are two manufacturers who supply most teams with motors and controllers: Solectria and Advanced DC Motors. Many college teams buy their motors from Solectria, but Advanced DC Motors have less expensive motors. Controllers usually drive a particular motor. Once you choose the motor that suits your needs, the same vendor would most likely have a matching controller.

Instrumentation
One of the most important pieces of instrumentation is a state-of-charge meter. A state-of-charge meter gives information about system voltage, amp draw, battery energy remaining, and estimates the how much time remains until the battery is out of energy. We found that the E-Meter, manufactured by Cruising Equipment, served out purpose well. It has a digital display and accurately counts the number of amp-hours remaining in the battery. The E-Meter is the do-it-all in instrumentation. Another instrument that may be useful is a speedometer. Instead of using a regular speedometer drive, use magnetic contact speedometers, found in many sports equipment stores. This option does not add drag to your car. To ensure that your batteries are running properly, you may invest in getting a voltmeter for each of your batteries. A failed battery may show the proper voltage when the car is not running, but while the battery is under load, the voltmeter will show a lower than normal battery voltage.

Steering & Suspension
We strongly recommend front wheel steering as it tends to be more stable and safer. A solar car uses energy frugally if it is to be competitive. If there are two front wheels, it is therefore advisable to work out the geometry so that they run parallel when the car is going straight ahead, but when the car is turning, the front wheels turn at different radii. If the car is turning left, the left front tire is making a smaller circle than the right front tire. If the tires remain parallel while turning, they will cause unnecessary drag, decreasing tire life and overall performance.
The only advice we can offer with respect to suspension is that it should be soft enough to protect the car and solar array from unnecessary jolts and firm enough to provide a stable ride.

Brakes
Disc brakes are desirable as they are predominantly hydraulic. Having hydraulic lines running to the wheels can be easier than mechanical brake arrangements. The most significant problem with disc brakes is that the brake pads do not back away from the brake rotors when pressure is released, they just relieve braking pressure. Because the pads don’t normally back away from the rotors, they continue to have a small amount of drag. While this drag may not be noticeable on the family car, it is very inefficient on solar cars. Go kart shops now have brake calipers that are spring loaded to move the pads away from the rotors. We have found these very worthwhile.

Tires & Hubs
Tire selection will affect rolling resistance which affects how far the solar car will travel with the energy available. Tires with thicker rubber and wider tread tend to have higher rolling resistance (a bad thing). Thinner tires with higher pressuer have less rolling resistance, but are more susceptible to flats. The best tires we have found are the Bridgestone Ecopia tires made for solar cars. They are very thin and operate at over one hundred pounds/inch pressure. Unfortunately, they need to be mounted on specially made wheels and require custom made hubs. On the good side, these tires and wheels are very light. Some colege teams have experimented with bicycle tires but report limited success (bicycle tires, rims and spokes are not designed for the forces placed on them by non-tilting vechicles that weigh several hundred pounds). Motorcycle tires tend to have more resistance, although there may be high pressure tires with low resistance that we don’t know about yet.
Bearing resistance can be reduced by light minimal lubrication. Bearing seals can be cut away at the contact lip to leave most of the seal protection while removing most if not all seal drag. It is a good idea to get the rolling chassis operational months before your schedule gets critical. Run the chassis as many miles as possible to prove that your bearings, axles, steering and suspension can survive.

Operation

Step 1 – Understanding the Solar Principle

Like all solar-fueled devices, solar cars use energy that comes from the sun. Direct exposure to sunlight doesn’t move cars or other devices that can use this energy, so the converter of sun rays into usable power is needed. When it comes to solar cars, specially designed batteries serve as converters. Solar energy also needs to be stored since sun is not always available. Silicon-based photovoltaic cells are still most common solar collector and storage space where, due to electron movements and interactions, accumulated sunlight moves electrons around. These movements and interactions of electrons trigger electrical current or energy that eventually runs the car.

Step 2 – Solar Panel

Solar panels are actually “boards” that collect the sun’s rays, and host the whole process of transforming energy coming from the sun into electricity and power. Placing the solar panels on the car allows you to direct solar power to the motor or you can store it in a battery, depending on the car make and design. Known a century ago as home water heaters, then going through some periods of oblivion, solar panels experience today their re-birth one more time, thanks to growing popularity of solar powered electric cars. Solar panels basically wrap the whole system of solar principle, since they work as long as electrons freely flow around. They are still a little costly, but you can definitely find affordable ones, if you wish to turn your car to a solar electric car.

Step 3 – Using Photovoltaic Power

As already said, silicon-based photovoltaic cells are the most common collector of the sun’s rays. Those cells make solar panels that convert the sun’s energy into one form of usable energy. New models of solar panels are capable of transforming about 22% of accumulated sunlight into electricity that solar cars use (future promises more but we still have to see it). Newly created usable energy, or photovoltaic power now goes to batteries that keep the electricity in tact until it is needed. It’s important to know that thousands of photovoltaic cells are needed to transform sun energy into electricity needed to operate a car.

Step 4 – Car Components

Photovoltaic array is the basic component in a car running on solar energy. Generated and produced electrical energy goes to the motor when you turn the key on. When the motor is off, a battery keeps solar power as chemical energy. The majority of solar cars have small motors (nothing like conventional motors), and there is no need to shift gears since the electronic motor does it itself. Other basic components are the already mentioned battery that collects solar energy, motor balancer, power tracker and data system, which monitor and check the electricity inside the whole system. But don’t be surprised to see some other, or customized components in solar cars that aficionados like to make on their own.

Extra Tip – The Solar Car Design

Big cars and SUV’s have been gradually losing their battle with smaller and easier-to-maneuver cars, not only because of the fuel price, but also because of eco-awareness. That trend works well for solar cars that tend to be smaller and lighter from their inception. Until recently, solar powered electric cars used solar panels to operate, but today individual solar cells are more popular, since they significantly contribute to lighter car frames.

3. Solar motors

Solar Power is the third most important renewable energy source in the world today. We at Minnesota Electric Technology are proud to have provided Solar Powered Motors to customers in this industry for many years.

Solar power, in addition to being a clean energy technology, is also considered “inherently elegant”. This means that the conversion from sunlight to electricity takes place without any moving parts or any environmental emissions throughout the process. The Solar Power Motor then runs off of this electrical source.

All of the Solar Powered Motors that we manufacture are custom designed for each customer’s application. We then sell that specific design only to the original customer. Our Solar Powered DC Motors have all been manufactured in our plant in Winnebago, MN since 1969. If you have an application for a Solar DC Motor, just send us your specifications and we will be glad to send you a no obligation quote.

The output of our Solar Motor can range from fractional horsepower up to 2 HP. Our Solar Powered Motor physical dimension ranges from 3.3” in diameter up to 6.0” in diameter. The electrical source can be 12 volts or 24 volts, or any other voltage that your solar array will produce.

There are two major kinds of DC solar power systems:

• Directly powered DC

• Indirectly powered DC

For directly powered systems the solar panels start to provide the Solar Power Motor with low power as the sun rises, increasing during the day, and dropping to zero at night. The motor performance parallels these voltage and current levels. This system is the least expensive option. Our MET Solar Power Motors can work well in some water pump and fan applications here.

With indirectly powered systems you configure a battery between the solar panels and the motor. This lets the Solar Panel Motor run when the sun is not shining, but this option requires a battery charge controller. This system allows our Solar Electric Motor to run consistently all of the time.

4. Andvantages and disadvantages

Advantages

• Unlike regular cars, solar energy powered cars are able to utilize their full power at any speed.
• Solar powered cars do not require any expense for running.
• Solar cars are quite.
• Solar cars require very low maintenance.
• Solar cars produces no harmful emissions.

Disadvantages

• Solar cars don’t have speed or power that regular cars have.
• Solar powered cars can operate only for limited distances is there is no sun
• If it is dark out for many days, the car battery will not charge and you this can seem as a  problem to many problem. This is the main reason why people don’t rely on solar cars.
• Parts used in solar cars are not produced in large quantity so they are expensive.