Lighting the way with solar-powered cars

8 min read

The World Solar Challenge sees the latest in solar-powered vehicle technology. But are we likely to see such cars on our roads soon? Jon Excell reports

The World Solar Challenge runs 1,689 miles race from Darwin to Adelaide

Nothing much happens in the Australian outback town of Coober Pedy; a place so inhospitable that most of its population lives beneath the ground in converted opal mines.

But later this year its citizens will step blinking into the daylight as a bizarre-looking convoy of vehicles, powered purely by energy from the sun, sweeps at high speed along its dusty main road.

That’s because Coober Pedy is one of the very few towns along the route of the 2009 World Solar Challenge, a gruelling 1,689-mile race from Darwin to Adelaide along the infamous Stewart Highway — a interminably long and straight stretch of road bedevilled by sand storms, bush fires and blistering heat.

It won’t be the first time solar cars have travelled this way. The competition has been running every couple of years since 1987.

But while the conditions still provide a stern test for the engineers involved, it is fair to say that the esoteric vehicles are up to the challenge. Indeed, in the 2007 race competitors regularly struggled to stay below the 110km/h South Australian speed limit.

This year the emphasis of the race has shifted, and a new set of regulations limits the available area for solar panels to 6m2, stipulates an upright driving position, and even insists on the use of a steering wheel. It is a barely perceptible nod to the world of production vehicles. And while few involved in the competition envisage a future for solar-powered family cars, many of the advances made along the way do have serious potential in an auto industry increasingly concerned with boosting efficiency and embracing low-carbon technology.

The Cambridge University entry

The cars taking part in this summer’s competition are essentially variations on a similar theme. Free of the stylistic pressures felt elsewhere in the auto industry, the quest for efficiency and aerodynamic purity has led to fairly uniform designs. All of the vehicles are made of lightweight composite materials and boast a large surface area covered in about 6m2 of solar panels. Most have three wheels, with an electric motor mounted in the single rear-wheel hub, and enough battery storage for a few hours’ driving if the weather turns unexpectedly cloudy.

Nevertheless, within this framework, there is still plenty of scope for innovation and, depending on their budgets, participants are finding a number of intriguing ways of giving their vehicles a competitive edge.

The car to beat is currently under development by the Nuon team from Delft University in The Netherlands, winner of the previous four competitions.

Weighing in at 190kg and with a cruising speed of 130kph, the vehicle (Nuna5) has an immediate advantage thanks to its 6m2 covering of highly efficient gallium arsenide solar cells. Though more expensive than the silicon solar cells used by many of the other teams, these cells boast a conversion efficiency of 26 per cent, around six per cent higher than the best silicon cells. This means that 26 per cent of the energy falling on the panel in the form of sunlight is converted to electricity.

Nuna 4, the Dutch entry for the 2007 race

According to team spokeswoman Mariana Popescu, Nuna5 is effectively an improved version of its predecessor Nuna4, which completed the 2007 race in 33 hours. She said: ‘We are trying to optimise everything by making it lighter, improving aerodynamic design so that it has lower drag, while also trying to improve production methods.’

Optimisation is also the name of the game over at MIT, which is applying the finishing touches to its own entry, ‘Eleanor’ — again an evolution of MIT’s previous entries to the competition.

Here, according to lead engineer George Hansel, the focus has been on aerodynamics, and the team has achieved the impressive feat of maintaining the low drag characteristics of earlier cars while increasing the frontal area to allow an upright driving position. Eleanor’s aerodynamic profile also makes clever use of crosswinds, a potential hazard for such lightweight vehicles. ‘The car can cope with crosswinds up to 30° and at about 40mph,’ said Hansel. ‘The fairings of the wheels are unusually flat and large because they are designed to take advantage of the wind in a crosswind and generate lift, actually propelling the car forward.’

The MIT team is also particularly proud of its custom-built battery pack, a 6kWhr assembly of 600 lithium cells that will power the vehicle for around 220 miles. ‘Our batteries have an energy density that is 20-30 per cent greater than many of the other teams,’ added Hansel. ‘One of the things that will distinguish Eleanor is that it will do much better when there is bad weather.’

MIT's vehicle in the wind tunnel

A healthy appreciation of the elements has also played a major role in the development of Cambridge University’s ‘Bethany’ — one of three UK entries. Appearing in the competition for the first time, Cambridge’s Eco Racing team (CUER) is on a steeper learning curve than its more seasoned rivals, and has based its design on lessons learned from driving a road-legal prototype (Affinity) from Land’s End to John O’Groats during last year’s exceptionally wet British summer.

Despite its rookie status, CUER’s team leader, Anthony Law, is bullish about Bethany’s prospects. ‘We are pretty confident we can be a serious player in our first year and are aiming for a top-10 position,’ he claimed.

Law added that CUER’s budget, which is dwarfed by the lavish amounts spent by some of the other teams, has forced the team to come up with some elegant solutions that could work to its advantage. ‘The big teams, such as Nuon and University of Michigan, spend about £500,000 on solar cells that will give them about an extra 30 per cent of power over our car. Michigan is even doing things like launching weather balloons four times a day and using a little UAV to check how much cloud cover there is further down the route. We have a budget of £200,000 so we’ve tried to be clever rather than just throw money at it.’

Much of CUER’s focus has been on the development of an advanced cruise-control system that will help make the most of the vehicle’s capabilities by figuring out the best driving strategy. ‘The solar car wirelessly transmits information to a chase vehicle, which also processes route data, information on gradients and weather forecasting,’ explained Law. ‘It then sends instructions on the optimum speed back to the solar car. The speed of the car is controlled completely remotely and if the system works we will make it drive across Australia in the most efficient manner possible.’

While remote speed control might make the car efficient, Law warned that it won’t do much for the driving experience. ‘Driving Affinity around Cambridge is really good fun, but I’m not convinced driving Bethany across Australia will be any fun at all. The driver’s only job is to steer, and there is just one right turn at Alice Springs. He or she will be sitting there making minor adjustments and trying to stay awake, and the temperatures inside the solar car can get up to about 50°C.’

Law also admitted to concerns over the carbon footprint of the competition itself. ‘There is a huge issue with the sustainability of this race. It’s a great showcase for sustainable technology, but sending 40 teams from all over the world out to Australia and driving all around Australia with one person in a solar car and 10 people following in other cars is really not a very environmentally friendly thing to do. Whichever way we do it, we are going to try to get some sponsored carbon offsetting.’

Putting these worries to one side, the efficiency and performance of the vehicles taking part is pretty staggering: they travel at highway speeds for hours on end, never need refuelling, and consume miniscule amounts of power. MIT’s car, for instance, runs on just 0.3kWhr per mile. To boast similar efficiencies, a petrol engine would have to be capable of thousands of miles per gallon.

So is there any potential for solar technology in production vehicles? Although automakers have certainly flirted with the idea, the reality is a long way from the zero-emissions dream conjured up by competitions such as the World Solar Challenge.

A number of prototype vehicles — such as the Lotus Eco-ELise — have featured solar panels as an additional source of power, but so far the only proper production vehicle to feature solar technology is the latest model of the Toyota Prius, which was launched at the Geneva Motor Show in March. Here, a solar panel on the sunroof of the luxury hybrid is used to power an electric fan that is claimed to keep the car cool when it is parked.

Those that do not dismiss such applications as gimmicks suggest that they are the only viable applications of solar cells on mainstream cars.

‘It would take an astonishing breakthrough for solar-powered cars to be feasible,’ said MIT’s Hansel. ‘The problem with solar vehicles and the current goals of solar-cell development is that solar vehicles have a limited area, and so require the highest possible efficiency. But the trend in solar production is to go for the lowest possible cost per installed watt regardless of efficiency. That is a metric that does not suit applications where size is a factor very well.’

Dr Ned Ekins-Daukes, photovoltaics expert at Imperial College’s Grantham Institute for Climate Change, expanded on this analysis. ‘The energy density that we receive from the sun on a sunny day is 1,000W/m2. That’s quite a lot of energy, but if you consider that a standard car engine is about 150kW, then you begin to see that the surface area of a car is not large enough to drive the car. It wouldn’t really matter what efficiency we had, the only way you can make a fully solar-powered car is to design these things that look like coffee tables on wheels, because they are extremely light, have relatively little in the way of battery backup, and have a huge surface area.’

A rare example of solar technology entering the automotive mainstream on the sunroof of the Toyota Prius

So does this mean that the cars entering the World Solar Challenge are merely an intriguing but irrelevant sideshow? Far from it, claims CUER’s Law, who believes that solar-car development has a range of promising spin-offs, particularly in the realm of electric vehicles. ‘By putting solar panels on a car you limit yourself to around 50 times less power than a normal car, he said. ‘And by forcing yourself to use a very small amount of power, you are forced to look at using innovative ways to make a car run at a normal sort of speed. It tends to be the case that when people are forced to do something completely different, you get these innovative new solutions.’

Law believes that CUER’s research on strategic technologies has particular promise for the auto industry. Indeed, Ricardo’s Sentience vehicle (The Engineer, 23 March), which pioneers an intelligent driving system combining GPS with more detailed information, provides strong evidence that the auto industry is already taking these kind of ideas seriously.

MIT’s Hansel added that the efficient aerodynamics demanded by solar cars could also feed into the mainstream and help to make cars more aerodynamic and streamlined than they are now. He pointed to the design of the Aptera 2E — a Californian eco-car due to go on sale later this year — as evidence that this is already happening. Based on a body shape originally developed for MIT’s 1994 solar car, the Aptera 2E has one of the lowest drag shapes for any land vehicle, Hansel claimed.

Clearly, it’s going to take time for some of the solar-car industry’s more radical ideas to find their way onto production vehicles. And in this regard, Delft’s Popescu believes there are parallels with Formula One (F1). ‘We are a race team,’ she said. ‘We don’t necessarily believe we will be riding around in these cars in 10 years’ time, and you don’t ride around in F1 cars either, but the technology is being developed and all the steps that are being taken will lead to spin-offs.’

Durham University’s Dr David Sims-Williams, team leader on another of the UK’s contenders, believes the F1 analogy is a valid one. ‘We’re not going to see production versions as everyday vehicles in the same way that we are not now driving production versions of 1980s-era F1 cars. What we are certainly going to see migrating more and more into the mainstream automotive sector are electric drive systems and more energy-efficient vehicles. Those aspects of solar race cars are certainly going to transfer to production cars.’

MIT’s Hansel added that solar racing’s emphasis on efficient battery design could also bear fruit. ‘On our battery pack we have more than 30 microprocessors monitoring as many parameters as we can,’ he said. ‘That’s the sort of thing that Detroit doesn’t know how to do.’

Coupled with the growing appetite for domestic photovoltaic panels, such improvements to battery technology could stimulate a version of solar-powered motoring that is less dramatic, but far more realistic than the prospect of production vehicles festooned with solar cells.

‘The cost of solar cells is so high that it doesn’t really make sense to put them on a car,’ said CUER’s Law. ‘But I could envisage solar cells being used on roofs to charge electric vehicles.’ Imperial’s Ekins-Daukes agreed: ‘Potentially once the technology has been optimised and we have had some breakthroughs in storage, you could use your roof at home to provide the energy that you would then funnel into your car. Once you start talking about larger areas like that then you are beginning to produce something more practical.’