Are solar cars the ultimate electric vehicles?

All-Electric Vehicles (EVs) are a hot topic these days in terms of design, manufacturing and sales for a number of reasons. While only about 2% of cars sold in the United States in 2020 were pure electric vehicles, that number is expected to increase dramatically over the next five, ten and twenty years. The magnitude of this increase is guessable, as the crystal balls of market analysts and experts who make these predictions are everywhere, with forecasts ranging from a modest gain to a significant gain.

But why just think of EVs that are recharged via the grid or an advanced home solar / hydro / geothermal system? Why not go all the way and install solar cells on the roof of the car, potentially eliminating the need to connect anything? It’s personal, it’s portable, it’s the ultimate type of energy harvesting from an endless free source, what could be better than that?

As ambitious as it sounds, a few companies are working on cars that charge through such panels (they also include an on-board charger for conventional EV charging). These vehicles are not the ones you might have seen for the famous Australian World Solar Challenge which has been operating for over 30 years with three categories of cars (Figure 1), none of which is even close to “street legal”.

Figure 1 The Australian World Solar Challenge has been held every two years since 1987; its entrants are unique in their kind and far from being legal on the streets. (Image source: Australian World Solar Challenge)

Among the companies working on viable solar cars include Lightyear (based in the Netherlands) with their Lightyear One (Figure 2). It has about 50 square feet of solar cells (4.6 m2) and four lightweight electric motors (one in each wheel) to minimize weight and extend reach, rather than a single motor and gearbox.

Figure 2 The Lightyear One looks like a conventional sedan, but it is capable of being fully recharged using solar power. (Image Source: Lightning)

The Lightyear One prototype claims a range of over 440 miles (700 km) on a full charge; a full day in the sun offers a range of just over 64 km. If you are wondering “how much does it cost?” and “when will it be available?” The answers are “$ 175,000” and “next year” for production. Before saying ‘this is crazy’, the company claims that over 160 vehicles have already been booked in Europe, most of them prepaid (US sales are scheduled for a later date) .

Another developer of these solar-powered vehicles is San Diego-based Aptera Motors Corp. with the Luna, a two-person, gull-winged three-wheeler (Figure 3). With approximately 24 square feet (2.2 m2) solar cells, they claim a range of up to 40 miles after a full day of charging in the summer sun; the 350V battery is supposed to be good for 250-1000 miles with a full charge depending on the battery capacity, which can go up to 700W-hr), whether it comes from solar power or of the network.

figure 3 The Aptera Luna is a three-wheeled solar-powered vehicle that claims 40 miles of range on a full day of optimal charge. (Image source: Aptera Motors Corp.)

As for the price, they say they’ll deliver a version with a 400-mile range to US customers next year, with prices for a base model starting at $ 29,800, significantly less than the Lightyear. One. They categorize it as a three-wheeled motorcycle rather than a car, to avoid some of the regulatory mandates such as airbags. (They say most of the United States states that these vehicles do not require a motorcycle license, only a regular driver’s license.)

In addition to the lower rolling resistance of three wheels compared to four and extremely light weight due to the use of highly advanced composite materials, Aptera claims that one of their keys to success is a coefficient of drag. (wind resistance) much lower than that of conventional vehicles (Chart 4). Note that the drag coefficient is extremely critical since the impact of drag increases with the square of the vehicle speed).

Figure 4 The Aptera Luna’s drag coefficient is about half that of standard vehicles, but double that of a typical World Solar Challenge vehicle (designed for a single competition scenario). (Image source: Aptera Motors Corp.)

All of these promises are fine, but maybe it’s time to face reality. While I have no doubts about the sincerity of these startups, I do wonder about any optimistic assumptions they may make: Basic physics numbers are hard to beat. Under the best conditions of sunshine, season and terrestrial latitude, the available surface of these photovoltaic cells simply cannot provide much energy; although their efficiency increases dramatically, the amount of solar radiation reaching Earth is modest at best.

Here’s the situation: About 99% of the solar, or shortwave, radiation on the Earth’s surface is in the region of 0.3 to 3.0 µm, between ultraviolet and near infrared. Above the Earth’s atmosphere, solar radiation has an intensity of about 1380 W / m2, called the solar constant. At 40⁰ latitude, the surface value is approximately 1000 W / m2 on a clear day at solar noon during the summer months due to atmospheric losses (see US Dept. of Energy, “Solar Radiation Basics”).

Do your own “back of envelope” rough calculation (5) using figures that you find realistic now and in the near future on the efficiency of photovoltaic cells, energy conversion and management circuits, inverters, battery and motors. You will probably find that the amount of solar energy that can be harnessed and then available is quite low even under “ideal” conditions of full summer sun (and remember that power equals around 750 watts) .

Figure 5 Using this one-of-a-kind personalized notepad encourages general estimations in numerical analysis and reminds you that accuracy is not always a good thing; it is often better to be roughly speaking rather than implying unrealistic precision in the final answer. (Image source: author)

Of course, this situation deteriorates rapidly due to clouds, shade, shorter days and many other factors. Additionally, increasing the potential range by adding more batteries to capture more when the sun is fully available and the car is not in use means additional weight and range penalties.

What do you think about the viability of these solar cars? Among the many questions are these:

  • Are they the first steps in a longer, more ambitious and perhaps exciting product development journey?
  • Or are they just expensive “toys” for those who only need to walk a few miles a day with minimal charge?
  • Do they really look more like externally charged EVs, but with the added “feature / gimmick” of those solar panels?
  • Are they a solution for only very, very few people, especially those who have another vehicle available for longer trips or cloudy days?
  • Rather than looking to park in the shade, will the owners of these vehicles look for sunny spots (and when the sun shines, the shade moves as well) and avoid parking garages?
  • Will people accept vehicles without standard safety features (many are mandatory), as well as now standard comfort features such as heating and air conditioning?

So many questions, so many points of view, it looks like we’ll have to check it out in a few years!

Bill Schweber is an EA who has written three manuals, hundreds of technical articles, opinion columns and product specifications.

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