The Reality of Solar-Powered Cars

If you leave physics out of the picture, some ideas can be attractive. For example, solar-powered cars seem simple enough—just add a few photovoltaic (PV) panels on top of an electric vehicle (EV), and voila! Infinite range provided by the sun. 

 

Car with solar panels on the roof. Image used courtesy of  Wikimedia Commons

 

So, why are we not all driving cars powered by the sun? Truthfully, some of us are, but not how you might think.

 

How Photovoltaics Function

PV cells, also known as solar cells, convert sunlight directly into electricity. Most commercially available cells are made of semiconductor materials, such as silicon, which have unique electronic properties. When a photon of light hits a PV cell, it can excite an electron and cause it to jump from a lower to a higher energy level, creating an electric current that can power electrical devices.

French physicist Alexandre-Edmond Becquerel first discovered the photovoltaic effect in 1839. However, it was not until the 1950s that the first practical PV cells were developed. In 1954, Daryl Chapin, Calvin Fuller, and Gerald Pearson at Bell Laboratories developed a silicon PV cell with an efficiency of 6 percent. This breakthrough led to the development of the first solar panels, which were used to power satellites in the late 1950s.

 

Early design of a PV, 1956. Image used courtesy of NREL

 

The 1970s oil crisis revived interest in solar energy. PV cell technology continued to advance, and by the 1980s, PV cells were becoming affordable enough for use in residential and commercial applications.

The efficiency of a PV cell is measured by how much sunlight it converts to electricity. The most efficient PV cells currently have efficiencies of around 25 percent. Still, researchers are working to develop even more efficient cells—the most recent perovskite cells are reaching levels of nearly 30 percent.

 

Powering Transportation With the Sun

There is already a good chance some of the electricity charging EVs comes from the sun through solar farms helping to power the grid. In the U.S., about 16 percent of the grid is powered by solar energy (in California, solar electricity generation accounts for 27 percent of the total). By comparison, China’s contribution of solar energy to its power grid is 33 percent. 

About 80 percent of all EV charging in the U.S. is done at home, and a fairly large number of EV owners also have home solar electricity systems. In California, for example, about 30 to 40 percent of people with EVs also have solar panels. Solar energy to charge an EV saves money but requires a fairly robust solar charging system. 

On average, U.S. drivers travel 13,500 miles per year or about 40 miles daily. The 2022 Hyundai Ioniq 5’s fuel economy rating is 30 kWh per 100 miles. This means 12 kWh of electricity each day to travel 40 miles. The average number of peak sun hours per day in the U.S. is between five and six hours, with a reasonable estimate of an average of five hours of usable sunlight per day. Generating 12 kWh of energy over 5 hours requires 2.4 kW or 2,400 W of solar electricity production per hour. 

Common home solar panels can produce 400 W of electricity and are around 20 square feet. Discounting power losses (about 15 percent) when converting the DC PV solar panel to AC used to charge the EV, it will take a minimum of six solar panels (more like seven panels when losses are accounted for) to power the Hyundai Ioniq 5 on its typical 40-mile daily commute. The solar energy must be stored in a battery system so the EV can be charged from the saved-up solar energy at night. 

 

Solar-powered EV charging stations. Image used courtesy of Tesla 

 

In addition to solar-powered home charging stations, it is possible to build covered parking lots whose rooftops are covered with solar panels to provide charging energy to a large number of EVs at the same time. In addition, the covered parking can reduce the need for cooling the car's interior, which is normally necessary after parking for a full day in the sun. 

 

On Vehicle Solar Power

The Hyundai Ioniq 5 is about 15 feet long and about six feet wide, giving it a plane surface area of 90 square feet. A 400-watt solar panel is approximately 20 square feet, so if every inch of the Ioniq 5 could be used as a solar panel, the total electricity generated would be 1,800 Wor about 1.8 kW. During an ideal five-hour day, if the Hyundai were parked just the right way, it could generate about 9 kWh, enough energy to travel about 30 miles. The Ioniq 5, however, has windows and sloped surfaces that point in different directions, so realistically, the range provided by our solar-powered Ioniq 5 is 10-20 miles per day. 

Nevertheless, competitions have been held for experimental solar-powered cars traveling long distances, such as across the U.S. or the Australian Outback. Typically, such vehicles are long and wide to provide a maximum amount of surface area for the solar cells and are designed for a single driver and virtually no creature comforts. 

 

Video used courtesy of CNBC

 

Researchers at the Instituto Dom Luiz at the University of Lisbon, Portugal, examined the potential of solar-powered vehicles in 100 cities worldwide.  A city’s urban canyons caused by tall buildings, along with trees and other obstacles, present additional problems for onboard solar charging of EVs. The results showed that charging within a city reduced the range provided by half to roughly 6-18 miles per day. Not surprisingly, cities nearer the Equator in Africa, the Middle East, parts of southern Europe, and Southeast Asia showed the best possibilities. In contrast, cities in China, North America, and Australia could have greater challenges but still might be acceptable.   

There are uses for solar panels directly on vehicles beyond the direct charging of traction batteries. Small solar panels on the roof or trunk surface can be used to operate low-consumption equipment (such as an air-circulating fan) or to help keep a low-voltage battery fully charged. These panels cannot power the vehicle but can be used to keep it more comfortable and ready to go. 

 

Dreaming the Solar Car Dream

Even if onboard solar power for the primary charging of an EV seems a non-starter, at least three companies have been working on bringing such vehicles to market. 

 

Lightyear 

Lightyear, a Dutch startup company, announced in 2019 that it would be building an attractive $170,000 sedan with both battery plug-in capability and solar panels arrayed across 54 square feet across the roof and hood of the car. 

 

Long-range solar electric vehicle. Image used courtesy of Lightyear

 

Designed by former Ferrari and Tesla engineers, the car is purported to add 7.5 miles per hour from its solar charging, with 45 miles of driving range added on a sunny day. Given that the 90 square feet of surface area of an Ioniq 5 would provide less than 30 miles of range after a day of charging its hypothetical solar panels, the estimates from Lightyear seem far beyond optimistic. 

A $250,000 version of the Lightyear briefly went into production at the end of 2022. However, in January 2023, Lightyear’s parent company, Atlas Technologies, was declared insolvent by Dutch Courts. A month later, Lightyear announced that it had reorganized and secured enough funding to begin work on a much more affordable Lightyear 2 model that could be launched in 2025 with a U.S. price tag of $40,000.

 

Sono Sion 

Instead of building a $170,000 luxury sedan, Sono Motors was looking at building a $30,000 small crossover utility hatchback vehicle primarily powered by the sun. Sono Motors had previous solar experience, developing power systems to operate the cooling systems of refrigerator trucks. Sono claimed that the onboard battery would provide 190 miles of range, while the solar panels covering nearly every surface would add 70 to 150 miles per week. This breaks down to between 10 and 50 miles per day, which is reasonable, at least at the lower end.

 

Sono Sion, a solar electric car. Image used courtesy of Sono Motors 

 

Finnish contract manufacturer Valmet was brought in to build the vehicle. Valmet is a serious player, as it has worked for Porsche on the Boxster and Cayman and with Mercedes-Benz on several special models. Initial production of the Sono Sion model started in November of 2022 but halted abruptly at the end of the year after just a couple thousand vehicles had been produced. By February 2023, Sono said it was abandoning the Sion and would sell the project to any interested buyers. 

 

Aptera 

Instead of building luxury sedans or mainstream SUVs, U.S.-based Aptera decided that the world needed a tiny, two-passenger, three-wheel flyweight commuter vehicle. Since the car is small, light, and highly efficient aerodynamically, it takes much less energy to move it down the road. Fifteen years ago, when Aptera started, the idea was to use a small gasoline engine. Eventually, that concept transitioned to power by lithium-ion batteries and an electric motor. Now, in its latest iteration, Aptera has decided solar electric power is the way to the future.

 

Aptera solar-electric vehicle. Image used courtesy of Aptera Motors Corp. 

 

Aptera has had a strange trajectory. The company was originally founded in 2006 and, aided by a $184 million U.S. government loan for advanced vehicles, participated with its gasoline-powered three-wheel vehicle in the 2010 automotive X-Prize. Unable to find sufficient funding to remain afloat, the company was liquidated in 2011. 

In 2019, using crowdfunding, Aptera was resurrected, but this time as a 300- mile-per-gallon-equivalent electric vehicle. It still had only three wheels and was classed as a motorcycle in most states. Where the Hyundai Ioniq 5 consumes about 300 W per mile, the diminutive and highly aerodynamic Aptera uses less than 100 W of energy per mile. It also only seats two people and has almost no creature comforts. 

Because it has such a small energy consumption, the Aptera could travel up to 1,000 miles on a 100-kWh battery pack. The company also has shown a version covered in solar panels that the company claims could add up to 40 additional driving miles per day. Given the lightweight and low energy consumption, this seems reasonable. The company plans to offer a variety of solar panel, battery, and motor combinations with ranges between 250 and 1,000 miles, from roughly $26,000 to more than $50,000.

Aptera has a production facility near San Diego, California, and has begun production tooling for the vehicle’s carbon-fiber bodywork. However, financial issues and insufficient investments have repeatedly delayed the production startup. Still, unlike the other two solar-power EV companies, Aptera is still in business and may actually, finally, make it into production.