A solar transportation system?

While Elon Musk is waxing lyrical about the Tesla Model 3, a house with solar roof tiles and batteries both in the house and in the car, an innovative group from Eindhoven in the Netherlands have combined all of this into a single offering and produced a solar powered family car that can also be used to supply energy to the grid.

The Stella Vie, claimed to be the most efficient family car ever built, recently featured in the World Solar Challenge, a biennial event which sees solar vehicles travel the 3000 km route from Darwin to Adelaide in Australia. The car competed in the Cruiser category. The 2017 event saw the third running of this class, which was introduced in 2013 when a four-seater ‘family car’ travel over 3000 km with an external energy input of just 64 kWh!  To put this into context, a modern Tesla S (P100D) family car has a 100 kWh battery with a range of around 500 km, although in a very slightly modified Tesla S, a distance of over 1000 km has been achieved.

A report by The Guardian on the event, claimed that these vehicles represented ‘the future’. All of this begs the question as to whether solar powered cars are in our future?In fact, it is possible to buy a part solar powered car today. The new Toyota Prius plug-in hybrid has the option of a solar roof panel, although according to Toyota this adds just 5 km a day to its range. But to open this discussion further requires a deeper look at the numbers.

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Jaguar solar powered concept vehicle (Source: Jaguar)

We could start by assuming that a future solar powered car doesn’t just have a single roof panel, but a surface made largely of a solar material, akin to the Tesla roof tiles on a house. It is conceivable that this extends to the windscreen glass as well. That means for a car such as the Tesla Model S, there is up to 10 m2 of solar panelling. In the USA in June, solar insolation (i.e. the energy arriving from the sun) would be around 7 kWh/m2/day. If the solar material on the vehicle had a conversion efficiency of up to 40% (a leading-edge research result today), then 28 kWh of energy might be collected, but by December this would fall to as little as 8 kWh. These amounts are far in excess of the typical daily needs of the Stella Vie for family use (say 50 km/day, or ~1 kWh), but not so for the Tesla S, with heating, air-conditioning, media, various IT systems and heavier paneling for safety. In winter (50 km in a Tesla S equates to 10 kWh) there is an energy shortfall, although potential generation is in excess of typical daily needs in summer. The balance could be exported to the grid in the summer.

However, in central London, the vehicle would generate very little of its needs in winter (<2 kWh), but should still cover its own requirements in mid-summer (~18 kWh). On an annual basis, it would likely fall short on its needs (~10 kWh/day maximum generation vs. a similar daily requirement).

The numbers indicate that in some geographies, assuming efficiency improvements in both solar PV and the vehicles themselves, a family car that requires no net electrical energy on an annual basis is plausible, although considerable advances in material engineering to integrate solar PV into various surfaces would be required. In other geographies, the balance wouldn’t be as favourable, but the contribution to the annual energy requirement of the vehicle could be substantial.

But this isn’t the end of the story. For the vehicle to collect such an amount of energy it must be parked in the sunshine all day, or be on the road. It would also have to be kept relatively clean. This might be possible in towns or some (treeless) suburbs, but in larger cities with outdoor parking limitations and taller buildings, the energy collection potential could drop substantially. Nevertheless, there would be some contribution when on the road.

Given that Toyota have already started down this track with the Prius, it can’t be too long before others follow. Innovation in solar PV, materials technology and vehicle efficiency could see solar augmentation becoming widespread by 2030, with a global vehicle fleet potentially requiring little to no net energy by the early years of the second half of this century. While this may seem a long way off, even a high deployment scenario for electric vehicles takes until the mid 2050s to completely dislodge internal combustion engine vehicles from the market.

Other scenarios for vehicle development could change this balance. For example, a much smaller fleet of autonomous ride sharing vehicles would still require the same amount of total fleet energy, but with far fewer cars deployed, energy generation would likely be much lower. Nevertheless, the World Solar Challenge, like the Shell eco-Marathon where vehicle efficiency is tested to the limits, points to an interesting future for mobility.