David Rand and John Ward from CSIRO Energy Technology report on the thrills, spills and challenges of crossing a continent on photon power.
In September 2005 the Panasonic World Solar Challenge – the eighth meeting of this premier event for solar cars – set off on a 3,000km course from Darwin to Adelaide across the ‘Red Centre’ of Australia.
What attracts private individuals and teams from major organisations, small businesses, universities and high schools across the globe? After all, there is the more-than-daunting prospect of having to negotiate some of the most inhospitable countries in the world in flimsy, cramped and stifling conveyances. Not to mention the reliance on a fuel supply that depends on the vagaries of the weather, the threat of sudden dust storms and mini-whirlwinds (‘willy-willies’), and the possibility of night-time visits to one’s swag from poisonous snakes, bird-eating spiders and sharp-teethed dingoes.
The reasons behind the continuing fascination with the World Solar Challenge are as varied as the competitors themselves. In brief, companies use the event to evaluate and showcase their latest advances in aerodynamics, lightweight materials and structures, photovoltaics, batteries, electric motors, communications systems, and information technology.
It is said that the lessons learnt from taking advantage of this ‘live laboratory’ assisted motor giants such as General Motors, Honda and Toyota in their later development of hybrid electric vehicles. Universities – more than 100 around the world are involved in solar car projects – see participation as an effective means of fostering the personal development of their students, not only in science and engineering but across a wide range of disciplines. This is essentially the same objective of high schools, with the emphasis here being placed on planning, leadership and teamwork.
Undoubtedly the participation of educational institutions is of great value. It is clear that at least some of the present generation are becoming more aware of the urgent need for society to find ways to place fewer demands on the Earth’s environment and resources, not just in transportation but in all energy sectors.
Finally, private teams tend to view the event as a sport in which, much like ocean-going yachting and long-range ballooning, human ingenuity attempts to harness the forces of nature. In essence, then, the World Solar Challenge is not a race but a test of efficiency, performance and endurance spiced with a strong element of adventure.
Over the years the World Solar Challenge has attracted a large media contingent from around the world. This was especially the case with this year’s meeting – the webpage registered up to two million hits a day! What aroused this extra interest? Quite simply, the answer lay in the rising price of oil. No one, even in their wildest dreams, could imagine that solar-powered cars would ever become practical. Nevertheless, the media questioned whether there might be spin-offs to make automobiles more fuel-efficient and thereby help redress the balance between oil supply and demand. Let us, therefore, digress for a moment and examine the present oil situation.
Determination of climate-forcing mechanisms and the true extent of global warming is indeed very important, but has it directed attention away from an even more pressing issue in the quest for energy sustainability?
Most geologists and petroleum engineers are of the opinion that the Earth’s ultimate reserves of economically-recoverable petroleum are in the region of 2,000 billion barrels, of which over 40% has already been consumed. Moreover, it is claimed that 94% of all available oil has been discovered. Some major oil-producing regions (USA, North Sea) have passed their peak production rates and are in decline. If this were not serious enough, an even more alarming fact is that much of the remaining ‘conventional’ oil (over 60%) is concentrated in just five Middle Eastern countries: Saudi Arabia, Iraq, Kuwait, the United Arab Emirates and Iran.
New oilfields will probably be discovered, for instance in the countries of the former USSR and/or off the coast of West Africa, but they are unlikely to compare in size with those of the Middle East and will not significantly change the overall picture. Canada and Venezuela have, respectively, vast reserves of ‘oil sands’ and heavy oil (bitumen) that can be mined and refined to produce petroleum, but the cost would be considerable in terms of both economics and environmental impact.
Between 1973 and 2002 the world’s consumption of oil increased by 40%, compared with 54% for energy as a whole. This was because natural gas and nuclear power took over many of the duties formerly assigned to oil. Accordingly, the lower growth rate in oil consumption has not reflected the much larger increase in demand (90%) from the transportation sector over the same period. The demand for petroleum will doubtless intensify as the developing countries aspire to Western-style mobility. For example, given the sizes of the populations of China and India, it is clear that if just these two countries were to become fully mechanised their petroleum requirements would constitute a large fraction of present oil production. Clearly, this cannot happen in a sustainable-energy future.
To bring future demand back into line with the supplies available, a five- or ten-fold increase in the price of a barrel of oil is certainly on the cards – and this may well occur within the short timeframe of 10-20 years! It is salutary to recall that at US$20, the price until quite recently, a barrel of oil cost about the same as a litre of whiskey – and the latter did not take geological ages to mature. Clearly, oil was and still is, grossly undervalued in terms of its usefulness to mankind and its finite reserves.
We need to think in terms of more immediate steps. An obvious pathway is through the progressive development and introduction of hybrid electric vehicles. These use less fuel per unit distance driven than conventional petrol engines and produce smaller quantities of gaseous emissions. Although considerable progress has indeed been achieved with hybrids, their drive-trains, energy-management systems and batteries require further refinement. And this brings us back to the value of the lessons being learnt with solar cars.
Meeting the event regulations
For the benefit of BEST readers new to the World Solar Challenge, let us take a quick look at the regulations. The principal regulation is both daring and simple: once the event starts, the solar cars must be powered by direct sunlight with a little bit of battery. It is not specified how solar energy should be used to propel cars. This simplicity gives teams the freedom to solve real technical issues in new and innovative ways. The regulations specify the allowable area of the solar panel, as well as the maximum energy that may be stored in the battery. Power sources other than batteries (e.g. supercapacitors) are acceptable, but the total energy storage of these devices together with that of the battery must not exceed a set limit. The make and number of cells/batteries are at the competitor’s discretion, though the car must travel the entire course with the same make and number as were fitted at the start. Mains electricity can be used for charging before the start of the event. After that, it must be solar energy all the way. The permitted size of the battery pack is determined by weight, namely, not more than 30kg for plastic lithium-ion, 30kg for advanced lithium-ion (-159 Wh/kg), 35 kg for standard lithium-ion (-159 Wh/kg), 40kg for silver-zinc, 70kg for nickel-metal-hydride, 75kg for nickel-zinc, 100kg for nickel-iron, and 125 kg for lead-acid.
Approval may be given to replace the pack or part of it in the event of a malfunction or accident. If this should occur a time penalty is imposed (according to a CSIRO schedule based on battery type and weight) in order to prevent competitors from gaining a strategic advantage through fitting fresh batteries, e.g. during overcast skies, headwinds, or hill climbs. Otherwise, such action would have boosted car performance through, in effect, injection of fossil fuel and not solar energy – contrary, clearly, to the aim of the event. The official start and finish times each day are 8am and 5pm respectively. In the interests of comfort and safety, however, a ten-minute leeway (with a time correction imposed before the start on the next morning) is allowed in order to find a suitable stopping place, but since the Outback is sparsely-populated open country, almost any place can serve as a campsite. Outside these hours, additional battery charging is permitted when the car is stationary.
After rigorous scrutineering to ensure compliance with the event’s regulations, the cars are required to pass stability and braking tests in order to be certified for legal driving on the highway. In addition, speed over a given distance determines the vehicle’s grid position.
Lithium’s getting better all the time
Twenty-one competitors assembled in Darwin. They came from ten countries: Australia, Belgium, Canada, France, Germany, Japan, Taiwan, The Netherlands, the UK and the USA.
As expected, the favourites had fitted their cars with triple-junction solar cells. These so-called ‘tandem’ cells can convert sunlight to electricity at over 25% efficiency by combining two or three semiconductor layers that absorb light energy in different parts of the electromagnetic spectrum. The most common design has a gallium-indium phosphide top, a gallium arsenide middle and a germanium bottom layer that, respectively, can absorb blue, green and red light. In good sunlight cells of this design provide cars with over 2kW of power.
It was also not surprising that lithium cells were overwhelmingly selected for the energy-storage system. Only three teams chose otherwise: two opted for nickel-metal-hydride, and one even used lead-acid. So, lithium technology continues to be in the ascendancy (will it soon become common in commercial hybrid vehicles?).
Lithium-ion designs first appeared in the World Solar Challenge in 1996 when one entrant came tenth with 106 Wh/kg cells. In 1999 three of the top six cars employed such cells, but the specific energy had risen to 144 Wh/kg. The 2001 event saw the arrival of a variant of standard lithium-ion technology in which the liquid organic electrolyte is immobilised in a polymer matrix – a so-called ‘gelionic’ electrolyte. These cells continue to be marketed as lithium-polymer types. This is, however, quite misleading as the gel electrolyte is not a genuine polymer. A more accurate description is ‘plastic’ lithium-ion. Two cars were equipped with plastic cells in 2001. The number grew to six in 2003 and included the teams who came second, fourth and sixth.
The designs of plastic cells could provide up to 190 Wh/kg, i.e. a performance superior to that of standard lithium-ion, but their capacity was restricted to around 3 Ah (lithium-ion was available at 100 Ah). The 2005 cars, 12 of which used plastic cells, showed that there is no longer a trade-off between size and energy. Now there are 191 Wh/kg cells with a 31 Ah capacity. Interestingly, lithium-ion appears to have gone the other way, with cells reaching 200 Wh/kg, but having only 2.4 Ah capacity.
Early alarms and excursions
The 2005 event had a rocky start. The French car went missing in transit and was only recovered at the eleventh hour. The Iranian team failed to arrive – a great disappointment as their solar car was the first to be built in the Middle East. Taiwan’s Apollo 5 blew both tyres during a braking manoeuvre and ended up in a ditch. No one was hurt.
During the speed trials, the Massachusetts Institute of Technology car lost a wheel and rolled over. This was the first such incident in the history of the World Solar Challenge. Fortunately, the driver was not injured in the accident, but about half the solar array was damaged. The team patched up the vehicle to the best of their ability in the limited time available and eventually reached Adelaide in a creditable sixth position.
And the winner was…
After qualifying fastest, Japan’s Sky Ace Tiga from Ashiya University in Osaka led the field on the long journey south. By the end of the day, the first five cars had all covered well over 700km. The Nuon Solar Team from The Netherlands in their car Nuna, winner in 2001 and 2003, was in front with Momentum from the University of Michigan some 30km behind, followed by Aurora from Australia and Tiga. It is said that good fortune favours the brave, and so it was for the Dutch. By pressing ahead with their powerful array, Nuna enjoyed near-perfect weather all the way down the track. The chasing cars were not so fortunate and encountered cloudy skies, rain and strong winds over the second half of the course.
The flying Dutch zipped into Adelaide on the fourth day for a third successive victory. Nuna crossed the continent in 29 hours and 11 minutes at an average speed of 102.75 km/h. It was the first time that a solar car had beaten the elusive 100 km/h barrier. In the first World Solar Challenge in 1987 GM’s Sunraycer took the chequered flag at 66.92 km/h.
A three-way tussle for second took place. In the final stages, Tiga ran out of battery power and Momentum ate up too much energy by travelling on a low-pressure tyre. These mishaps allowed Aurora (92.03 km/h) to cross the line ahead of Momentum (90.03 km/h) and Tiga (88.84 km/h). Eventually, fifteen teams completed the event; the slowest arrived home after completing the course at an average of 52.53 km/h.
Tiga was the CSIRO Efficiency Award winner. This is given to the car that is judged to be the most energy-efficient. The decision is based on the vehicle mass, the time taken to complete the course and the number of occupants (both single- and two-seater designs take part in the event).
Finally, mention should be made of The Spirit of the Event Award. This year it was presented to the Formosun 3 team from the National Taiwan University, who used solar panels fitted to their support trailer to charge batteries recycled from the 2003 World Solar Challenge. The team thus had free electrical power for their campsite in the evening. Is there no end to the utility of lithium batteries?