For many reasons, all-electric vehicles (EV) are a hot topic in design, manufacturing, and sales today. Although only about 2% of vehicles sold in the United States in 2020 will be pure electric vehicles, this number is expected to increase significantly in the next five, ten and twenty years. Anyone is guessing how much this growth will be, because the crystal balls of market analysts and experts who make these predictions are ubiquitous, and the range of predictions has grown from modest to significant growth.

But why only consider electric cars that are charged via the grid or advanced household solar/hydro/geothermal systems? Why not put solar cells on the roof of the car all the way, making it possible to eliminate the need to connect anything? It's personal, it's portable, it's the ultimate type of collecting energy from never-ending free resources-what could be better than this?

Although it sounds ambitious, some companies are developing cars that are charged through such panels (they also include on-board chargers for charging traditional electric cars). These vehicles may not be the ones you saw in the famous Australian World Solar Challenge, which has been held for more than 30 years. There are three types of vehicles (Figure 1), none of which is close to "street legal".

Figure 1 The Australian World Solar Challenge has been held every two years since 1987; its entrants are unique and far less legal than on the street. (Image source: Australia World Solar Challenge)

Lightyear (headquartered in the Netherlands) and its Lightyear One (Figure 2) are among the companies committed to developing viable solar cars. It has approximately 50 square feet of solar cells (4.6 square meters) and four light electric motors (one per wheel) to reduce weight and expand range instead of a single electric motor and gearbox.

Figure 2 Lightyear One looks like a traditional car, but it can be charged by solar energy completely independently. (Image source: Lightyear)

The Prototype Lightyear 1 has a cruising range of more than 440 miles (700 kilometers) when fully charged; it provides a range of more than 40 miles (64 kilometers) in the sun throughout the day. If you want to know "How much will it cost?" and "When will it be available?" The answer is "$175,000" and "Next year" for production. Before you say "this is crazy", the company claims to have booked more than 160 cars in Europe, most of which are prepaid (planned to be sold in the US in the future).

Another developer of these solar cars is Aptera Motors Corp. in San Diego, whose Luna is a two-person, gull-wing, three-wheeled vehicle (Figure 3). They have approximately 24 square feet (2.2 square meters) of solar cells, and after charging for a full day in the summer sun, they have a range of up to 40 miles; the 350-V battery pack should be able to travel when fully charged 250 to 1,000 miles, depending on the battery pack capacity (up to 700 W-hr), whether it comes from solar or the grid.

Figure 3 Aptera Luna is a three-wheeled solar car that can travel 40 miles on the best charge in a whole day. (Image source: Aptera Motors Corp.)

As for the price, they said they will offer a 400-mile version to American customers next year. The basic model starts at $29,800-much lower than the Lightyear One. They classify it as a three-wheeled motorcycle instead of a car to avoid certain regulatory requirements, such as airbags. (They say that in most states in the United States, this kind of vehicle does not require a motorcycle license, only a normal driver's license.)

In addition to the use of highly advanced composite materials that make the rolling resistance of the three wheels lower than that of the four wheels and extremely light weight, Aptera claims that one of the keys to its success is far lower than the drag coefficient (wind resistance) of traditional vehicles ( Figure 4). Please note that the drag coefficient is very important because the effect of drag increases with the square of the vehicle speed).

Figure 4 The drag coefficient of Aptera Luna is about half that of a standard vehicle, but twice that of a typical World Solar Challenge vehicle (designed for unique competition scenarios). (Image source: Aptera Motors Corp.)

All these promises are good, but maybe it's time to check it out. Although I do not doubt the sincerity of these startups, I do doubt the optimistic assumption they may make: basic physical numbers are hard to beat. Under the best conditions of the sun, the seasons, and the latitude of the earth, the available surface area of ​​these photovoltaic cells cannot provide much power; even if their efficiency is significantly improved, the amount of solar radiation reaching the earth is moderate at best.

The situation is like this: About 99% of the solar radiation or shortwave radiation on the earth's surface lies between 0.3 and 3.0 microns, between ultraviolet and near infrared. Above the earth’s atmosphere, the intensity of solar radiation is about 1380 W/m2, which is called the solar constant. At 40⁰ latitude, due to atmospheric loss, when the sun is clear at noon in summer, the value of the earth's surface is about 1000 W/m2 (see US Department of Energy, "Solar Radiation Fundamentals").

Use the numbers you think are realistic for the efficiency of PV cells, power conversion and management circuits, inverters, batteries, and motors now and in the near future, and make a rough "back cover" calculation yourself (Figure 5). You may You will see that even under "ideal" conditions full of summer sun, the amount of solar energy that can be captured and subsequently available is quite low (remember that one horsepower is approximately 750 watts).

Figure 5 The use of this customized, unique "back of the envelope" pad encourages extensive estimates in numerical analysis and reminds you that accuracy is not always a good thing; compared to accuracy that implies that the final answer is impractical, A rough idea is usually better. (Image source: author)

 Of course, this situation will quickly deteriorate due to clouds, shadows, shorter days and many other factors. In addition, when the sun is full and the car is not in use, adding more batteries to increase the potential cruising range means increasing weight and cruising range.

How do you see the feasibility of these solar cars? Many problems include:

So many questions, so many points of view-it seems we have to come back in a few years!

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns and product features.

With tens of percent efficiency, solar photovoltaic can provide hundreds of watts of electricity per square meter. 10 square meters per hour per day will allow you to charge 2 kWh per day, which will allow you to drive 8 miles in a Tesla, or drive 2 times or 3 times in a vehicle with a smaller cross-section/lower drag coefficient. (The resistance is directly proportional to the cross section and the drag coefficient.) Therefore, the full solar figure for driving 20 miles a day is not difficult to understand, while wall charging is used for further travel. The picture of the envelope is ironic.

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