Can Solar Electric Vehicles (sEVs) Actually Become a Reality?
One of the prevailing issues with solar electric vehicles (sEVs) is range. What design obstacles must be overcome to make this future vehicle tech a widespread reality?
At the end of last year, San Diego-based company Aptera Motors announced the so-called "first solar electrical vehicle (sEV) that requires no charging." This vehicle, they say, can take occupants up to 1,000 miles on a full charge and is adequate for most daily use.
Prototype of Aptera Motors' "never-charge" sEV. Image used courtesy of Aptera Motors
As the cost of solar cell technology decreases and efficiency increases, the notion of solar-paneled vehicles has even gained traction among big-name car manufacturers. For instance, Hyundai offers a version of its Sonata hybrid with integrated solar panels. Toyota likewise teamed up with solar manufacturer Panasonic to outfit the "Prius Prime" with solar panels.
While thin-film solar cells and vehicle-integrated photovoltaics have significantly advanced in recent years to make these early prototypes possible, several design challenges hinder sEVs "solar range" and subsequently, their widespread adoption.
What Is VIPV?
The advancement of thin-film solar cells and module technology has given rise to the concept of vehicle-integrated photovoltaics (VIPV), where solar cells mounted on the vehicle are used to power some of the vehicle’s electronic equipment. The generated power can be used for powering electronic control units, displays, air conditioning units, etc.
As shown below, the VIPV concept can be applied to an internal combustion engine (ICE) vehicle.
VIPV powering an ICE vehicle. Image used courtesy of Ludwig Kronthaler
However, considering the range anxiety and the scarcity of EV charging stations, developers are now turning to solar modules to extend the driving range of EVs as much as possible.
The (Many) Stumbling Blocks to Solar Range
The electrical energy generated by the VIPV system, and consequently the additional solar range we can expect, depends on several different factors such as the driving pattern, the EV consumption, roof area, vehicle location, and climate conditions.
The driving pattern might differ significantly between different users. For example, an EV that is usually parked in an open area and receives negligible shading can generate much more power compared to the one that might be parked in an underground parking lot for a few hours at midday.
Limited roof area, which is the primary location for solar cells, can restrict solar range. Image used courtesy of Energy Sage
The roof area is the primary option for integrating the solar cells and determines how many solar cells can be easily integrated into the vehicle. The roof area of today’s EVs can be in the range of about 1.7 to 2.3 m2. The energy consumption (in terms of kWh/km) determines how efficiently the vehicle uses the available energy. With today’s EVs, the energy consumption is in the range 13 to 24 kWh/100km.
Some other important factors that affect the solar range are the solar modules' peak power, the inverter efficiency, and the battery charging efficiency. Besides, when the solar power is available, the battery shouldn’t be fully charged so it can store the captured solar energy.
A Theoretical Breakdown of Solar Range
By integrating a solar module with a peak power of 250 W/m2 on an EV with a roof area of 2 m2, the vehicle can garner 500 W of maximum power. With a battery capacity of 40 kWh, the sEV still needs 80 hours to fully charge the battery.
Realistically, however, these modules won't receive maximum sunlight all day. The following figure shows how the daily sunlight intensity varies in three different months of the year in Newark, New Jersey.
Sun intensity in Newark throughout January, March, and July. Image used courtesy of Dunbar P. Birnie
This chart illustrates the need for local sunlight intensity to have a realistic estimate of the extended range. There are a few studies from different parts of the world that take this local information into account and assess the performance of a VIPV system. According to these studies, the power generated by a VIPV system can account for only about 13–23% of the yearly average driving distance of a typical user.
Lightyear Gets Creative With Extending Solar Range
There are two main approaches to increase the solar range: integrating solar cells into other surfaces of the vehicle body (in addition to its roof) and developing new solar cell technologies with a higher efficiency.
For example, Lightyear One, a prototype all-electric solar car, integrates solar back-contact cell technology into both the roof and hood of the car. This provides a total area of 5 m2—an increase of more than two times compared to a conventional EV.
Key features of the Lightyear One. Image used courtesy of Lightyear
The company claims that Lightyear One is two to three times more energy-efficient (83 Wh/km) than the electric vehicles currently on the market and its large solar panels are capable of adding an hourly 12 kilometers of range so users won't need to charge on a daily basis.
Other Methods to Stretch the Power of the Sun
Although the sides of Lightyear One do not include any solar cells, it is possible to use these areas for further extending the solar range of an EV. Lightyear chose not to include solar doors because this could increase the production complexity of the car. Besides, with solar doors, cables and glasses would need to be added to the sides of the car, making it heavier.
Note that the energy yield of solar doors is relatively limited because these cells are vertical and don't have a favorable orientation for maximum sunlight exposure. Moreover, only one side of the car is exposed to the sunlight at each time.
Interestingly, there are special types of organic solar cells that can be integrated into the transparent surfaces of an EV. These cells are partially transparent but still capable of capturing power. Vehicle manufacturers must also consider that in addition to being highly efficient, the panels integrated into the body should be durable and aesthetically pleasing.
The Challenges of Curved Solar Panels
It should be noted that the solar panels integrated into the body of a vehicle are necessarily curved. With a curved panel, the cells do not experience a uniform irradiance. This partial shading leads to an electrical mismatch between the cells and degrades the efficiency of the PV array.
With a curved module, careful investigation of the electricity yield is required for optimized performance. This can further complicate the design of the body of the vehicle. Advanced controls and solar predictions might even be required in future solar-powered EVs.
If you've designed EV systems in the past, what other issues do you foresee with sEV design and adoption? Share your thoughts in the comments below.