The changing face of solar-powered transport

Climate change, emissions, dwindling fossil fuel supplies, environmental damage: all of these factors are forcing rapid change and serious investment from the automotive industry. But research in this area is not limited to And, naturally enough, tomorrow's generation of engineers is already seeking the solutions to the big questions. It should come as no surprise to learn that Britain's leading universities are at the forefront of research in this area. Cambridge University, in fact, will soon take part in high-profile endurance competitions involving low- (or even zero-) carbon technologies that point the way to the vehicles of the future. Cambridge University Eco Racing (CUER) is a 60-strong student organisation that designs, builds and races solar powered vehicles. Its racing cars showcase cutting-edge sustainable engineering and demonstrate the incredible potential of electric vehicle technologies. This October, CUER will take part in the World Solar Challenge, a gruelling 3,000 km marathon across the heart of Australia from Darwin to Adelaide, starting in October 2013. The car – named Resolution – is designed to use the power of the sun to take it and its drivers through one of the world's harshest environments. Every element of the ultra-light vehicle has been designed with the single objective of improving its race time. Resolution's innovative design reflects the team's knowledge of automotive engineering and aerodynamics, as well as sophisticated modelling, space-grade composites and optimised solar cells. The result is a vehicle that rewrites the rulebook for solar vehicles but still meets the race parameters. One instance of this is that, while suspension in a car usually involves a spring and a dash pot, Resolution has an in-built, carbon-based suspension system, ensuring even less energy is wasted. Locating the motor in the hub of the wheel means there is no need for gears, chains or differentials – each of which would account for a 5% loss in efficiency. Perhaps Resolution's most radical aspect, however, is the way in which it uses a set of moving solar panels to maximise power input as they move to track the sun's position. This, it is hoped, will give the team a real advantage over bigger teams entering the competition. The race crosses 22o of latitude, so it has been necessary to create a modelling programme that will adjust the solar cells to maintain the optimum position at all times. Keno Mario-Ghae, team manager for Cambridge University Eco-Racing, based at the University's Department of Engineering, said: "Resolution is different because she overcomes one of the main limitations that affect most solar cars. "Traditionally, the entire structure of a solar car has been based on a trade-off between aerodynamics and solar performance. That's how they've been designed for the past 10 years, and that's why they all tend to look the same. We turned the concept on its head. Our reasoning is that solar performance needs to adapt to the movement of the sun, but the car needs a fixed shape to be at its most aerodynamic. To make the car as fast and powerful as possible, we needed to find a way to separate the two ideas rather than find a compromise between them." The solution the team eventually hit upon involved embedding the solar panels within an aft-facing tracking plate. This plate extends from the back of the headrest to the tail of the vehicle and is pre-programmed to follow the sun's trajectory. It uses linear actuators to move the panels in such a way to ensure that they are optimally positioned at all times. The team estimates that this will give the car 20% more power than it would otherwise have had. This structure is placed under a canopy that forms part of the teardrop shape of the vehicle as a whole. The design is a departure from the 'tabletop' look of most other solar cars, but is more aerodynamic. Because it encases the solar panels rather than making them part of the shape, the question of power generation does not compromise the car's aerodynamics. One factor in maximising power was the choice of Gallium-Arsenide solar cells over the cheaper silicon versions. Gallium Arsenide has the distinct advantage of much greater efficiency than silicon (particularly in high-light environments). Indeed, Gallium Arsenide is approximately twice as effective as silicon in converting incident solar radiation to light, with a theoretical conversion rate of up to 40% and has for that reason been used in solar cells in spacecraft. This choice has allowed the team to adopt a sleeker, more aerodynamic design (as opposed to the flatter, tabletop design). The World Solar Challenge has restrictions on how much solar area is permitted – depending on the type of solar cell. If silicon were used (which is 22% efficient), the team would be allowed 6m2 of cells. If space grade Gallium Arsenide cells are used, however (which are 35%-40% efficient), the allowable array area is 3m2. This means the power ratio of the car compared to its competitors simplifies to a ratio of 4:5. So, in terms of potential to draw power, Resolution can draw 20% less power than its rivals. However, once cruising, the car is simply using the energy taken in to do work against losses such as form drag (due to pressure), skin friction (due to viscosity), and rolling resistance of tyres. If the car is more efficient at steady state it will need less power to run. The tracking plate also plays a big part in this context. If the plates follow the sun, the incident rays are normal or near normal to the solar cells. To maximise power, the incident rays need to be striking the plate at exactly 90o to the surface. It is then possible to calculate incident power based on how far from this ideal the incident rays deviate by taking the cosine of the angle between the rays and the normal. Over a day, the ability to align the solar cells as close as possible to the ideal configuration can yield gains of 15-20% over an equivalent flat array. Rigorous test simulations showed that a narrow car (0.8m wide) using 3m2 Gallium Arsenide Triple Junction solar cells would theoretically win. Placing the cells under a canopy allows them to be tilted towards the sun. The potential 20% gain in power from tracking the sun more than makes up for the 5% losses from the sleek canopy design. This design decision has allowed the aerodynamics team to optimise the shape rather than worry about pointing surfaces towards the sun. Says Mario-Ghae: "We have a combination of an extremely small frontal area, which reduces our losses, and a more efficient array, which increases our input... Efficiency is where our real strength lies and this is how we plan to compete with the bigger teams." For the CUER development team to achieve the best of both worlds when it comes to efficiency, it is clearly necessary to keep the weight of the vehicle to an absolute minimum. The car is intended to weigh 120kg. And while at the time of writing it came in at a slightly corpulent 128kg, Marie-Ghae is confident this can be rectified, saying: "Opportunities to shave weight off do exist. Every change you make to the car involves finding the balance between capability and efficiency." Tom Grimble, who was technical director for the team's 2011 race, continues the theme, saying: "The most important factor from our point of view is maximising power, but the second most important is to reduce power resistance, so it's a constant balancing act." In any race, of course, speed is of the essence. For CUER, however, this is once again a question of balance to achieve optimum performance rather than simply hitting top speed and staying there. Says Mario-Ghae: "In theory, the top speed is 140km/hour [almost 87 mph] and we've done 110km/h, but the point is to find the speed where we're travelling as fast as possible while using the car's design and resources as efficiently as possible." This process is achieved by maximising efficiency at every level, which includes the driver's size and weight. Resolution measures less than 5m in length, is 0.8m wide and about 1.1m in height. Driving her across the Australian desert is likely to be a claustrophobic experience. In fact, the driver must be a maximum of 5' 3'' tall. These, however, are deliberate concessions made by the team for the sake of making the vehicle as fast and efficient as possible in the hope of winning the race. In the future, more conventional solar vehicles may well adopt similar ideas, but opt for comfort, rather than speed. Lucy Fielding is one of the four drivers who will operate in four-hour shifts and claims that, while not exactly luxurious, the car is not quite as uncomfortable as might be thought. Of course, in the Australian desert, one of the major concerns is the heat as the driver sits inside a plastic canopy in relatively cramped conditions. Clearly air conditioning was not an option given the weight and battery drain. However, says Fielding: "There are holes through which air can get in and the airflow does keep things reasonably cool." The car is fitted with on-board telemetry, an 'intelligent cruise control' that takes into account traffic, weather and driving style, and will advise the team on how to optimise the vehicle's efficiency during the race itself. However, one area where the car is lacking is in onboard toilet facilities. According to Fielding, there are moves afoot to address what she delicately describes as "a fascinating engineering challenge", but as yet, the World Solar Challenge remains an endurance event in more senses than one. On show in October The CUER team will be appearing at the Engineering Design Show at the Ricoh Arena, Coventry, on the 2nd and 3rd October this year. CUER, which is sponsored by Altium and its UK supplier Premier EDA Solutions, will be on stand K86 (showing 'Endeavour', the previous vehicle used in the Solar Challenge) in the Electronics Design Show. The team will also give a presentation in the Engineering Design Show Conference at 12pm on the 3rd October. To register to attend these events, go to www.engineeringdesignshow.co.uk.