18-04-2019 | By Paul Whytock
Have you ever left your hire car parked in the Spanish sun? If so, you’ll know just how blisteringly powerful that ball of solar energy can be.
I mention this because I recently became embroiled in a pub conversation with a couple of friends who have properties in Spain, and both of them make full use of the sun’s energy by having solar panels on their roof terraces. Well, why wouldn’t you? Malaga, for example, has close to 3000 hours of sunshine a year.
Anyway, the gist of the conversation was that while my chums are basking in the sun on their roof terraces, guzzling the occasional chilled San Miguel, their electricity costs are being slashed by the energy generated by their photovoltaic (PV) solar panels.
This prompted the question of why their cars were not harvesting the same solar energy while being toasted by the Spanish sun on the villa driveway?. This makes enormous sense.
One of my mates was convinced he had hit on the groundbreaking idea of vehicle manufacturers offering solar panels on car roofs as an optional extra.
But before he rushed out of the pub on his way to the Patents Office to register his fortune-making theory, I told him that quite a few car makers were already developing models with solar panel roofs. And there is even a solar paint idea, albeit, at the experimental stage, that uses photovoltaic technology that, in the future, could be painted onto cars. More about that later.
But is it really such a good idea to have solar panels on cars? Would it really provide significant energy harvesting to make a noticeable contribution to powering electric and hybrid vehicles? And how would they look? Nothing turns off potential car buyers if the vehicle they spend bundles of hard-earned cash on is not visually appealing.
So efficiency and aesthetics are the two big questions when it comes to solar cars.
The aesthetics are easily dealt with. Take a look at this Tesla Model 3 with a solar roof. Nothing ugly about that.
No, the much harder question to answer is, would it be worth it in terms of electricity-generating efficiency?
The first pragmatic consideration has to be the size of the vehicle. Obviously, a bigger surface area is better, so those Steroidal (sorry, I mean Sports) Utility Vehicles (SUV) would be the ideal car choice rather than a Mini.
An SUV roof may accommodate a 250-300W solar panel, and if this gets says 7 hours of daily sunshine (depending on what part of the planet you’re on), then the amount of electricity you’d gain would be chicken feed. It may get you an extra 7 miles in your overweight SUV.
However, to be fair, it could be handy for some supplementary charging issues like providing a trickle charge to the starter battery or running your in-car music and entertainment systems while you’re parked.
But it’s frustrating in terms of freeing Electric Vehicles (EVs) and their owners from the tedium of finding a public plug-in charge point. And let’s face it; the speed of the politically-promised EV charging infrastructure implementation makes a Galapagos tortoise look turbo-charged!
But things could change. Painting your wagon could take on a new slant if the conceptual idea of solar paint comes to efficient fruition. Yes, your future car could be painted with electricity-generating solar absorbing paint that uses photovoltaic technology.
This paint would use colloidal quantum dots. These are semiconductor particles that are already used in solar panels, LEDs and computers. They are microscopic and have been reduced below the size of the Exciton Bohr radius, which means they are between 2- 10 nanometres in size.
They are often called artificial atoms and can be generated in various sizes, which means they can exhibit a variety of bandgaps. These bandgaps can be adjusted during the synthesis process, and this flexibility on bandgaps makes quantum dots extremely useful in solar panels.
Basically, these quantum dots have to be integrated into a paint-like liquid that could be used on buildings, glass and even your future car to generate electricity. But don’t hold your breath. Currently (sorry), the efficiency levels of about 1% are way too low to make it technically or financially viable, but scientists believe it will happen.
Meanwhile, there are other ways to integrate solar power to make driving greener. They will not be here tomorrow either because the big challenge is getting those efficiency figures up. At the moment, it would take at least 100 hours of constant sunny weather to fully charge a small electric vehicle using a solar roof panel. And the chances of that happening anywhere north of Barcelona are slim. But there are technologies that could cut that figure.
Gallium Arsenide is one, and developers believe it could boost solar panel efficiency to 45%, but it’s not cheap.
More economical are perovskite solar cells. Their efficiency levels are about 30%, and price-wise it would be economical to not only perovskite solar paint the car but your garage and house as well.
And while going about the business of electrifying our lives, we best not forget the humble solar inverter’s role in all this.
Before all your solar harvested electricity can actually be used, it has to be in a form that can be used by most of your devices and appliances. It must be converted from direct current (DC) to utility-grade alternating current (AC). This is also the case for fuelling your electric car with solar energy.
Solar power inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.
Powerpoint tracking is all about operating efficiencies and how varying levels of sunlight can impact load characteristics. The job of power point tracking is to maintain a load characteristic that provides the highest power transfer efficiencies, and this is called the maximum powerpoint.
So all the technical ideas are there but, for the moment, remain conceptual, and the idea of solar panels on cars remains a good but pricey one when compared to the amount of power that can currently be generated. For example, those living under predominantly cloudy skies would be lucky to generate 500 driving miles of electricity a year.
And as much as the idea of solar power paint is intriguing, I’m still not sure how you’d get the electricity generated by the paint into the car battery. I might ask my pals that question next time I’m in the pub.