David Rand (CSIRO Energy Technology) reports from Australia on the biennial World Solar Challenge, which provided more ‘challenges’ than usual this year.
Regular readers of BEST will know that every couple of years, some of the world’s brightest young minds travel to Darwin to take on the Australian Outback in solar-powered cars. This event that is designed to test and promote the ultimate synergy of nature, motion and innovation. The competitors are required to build a car which, with a small initial charge of mains electricity, must be capable of crossing the vast continent from Darwin to Adelaide, via the Stuart Highway, purely on the energy of the sun.
The regulations specify maximum values for the area of the photovoltaic (solar) array and for the weight of the battery pack. Based on the original notion, back in 1987, that a 1-kW solar car would complete the journey in 50 hours, weights of different battery chemistries are subject to specific limits such that a nominal stored energy of just 5 kWh is allowed; that is, 10% of the theoretical amount required, which would be just sufficient to run a hair dryer for three hours! By maintaining such a benchmark, the performance of the cars in successive meetings provides a useful indication of energy-saving advances— not only in batteries, but in all other components of the vehicle. Batteries may not be replaced, except in case of accident or malfunction; in such a situation, a penalty time will apply.
The daily driving schedule is from 8 am to 5 pm. 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 at the end of the day in order to find a suitable place to camp for the night; all teams must be fully self-sufficient. Outside these hours, additional battery charging is permitted until sunset, and from sunrise the following morning. Overnight, the battery must be removed from the car, contained in a locked box, and placed in the custody of an official observer.
During the journey there are seven mandatory ‘control stops’ where observers are changed and teams may update themselves with the latest information on the weather and their own position in the field. During this time, only the most basic of maintenance may be performed, i.e., checking and maintenance of tyre pressures, cleaning or the removal debris from the vehicle, and water cooling of the solar cells.
A Brief Retrospective
Prior to 2011, 347 teams from 29 different countries had travelled to Darwin to participate in ten meetings of the World Solar Challenge.
The first WSC, in 1987, was won by the General Motors Sunraycer— a futuristic, low-weight, wind-resistant design. Dubbed the ‘streamlined cockroach’, by virtue of its shape, the car travelled the 3 000 km route at an average speed of 66.92 km/h to finish two days in front of Ford Australia’s Sunchaser in second place (44.48 km/h). Cloudy skies, incessant headwinds and the occasional torrential downpour made the going tough in 1990. A powerful photovoltaic array of monocrystalline silicon cells greatly assisted the Swiss Spirit of Biel/Bienne II to take the chequered flag some 350 km ahead of its nearest rival, the Honda Dream, but in a time that was some 74 minutes slower than that of Sunraycer. These two positions were reversed in 1993 with the Honda Dream II reaching Adelaide well before the Swiss at an average 84.96 km/h and thereby eclipsed the event record by 9.4 hours.
Honda completed back-to-back wins in 1996 at an impressive 89.76 km/h in a two-seater, four-wheeler vehicle— the first such design to enter the WSC and, ironically, the co-driver sat back-to-back with the driver. Notably, that year marked the introduction of lithium-ion cells. Fitted with a battery pack of 32 parallel-connected sets of series-connected 28 cells (106 Wh/kg), the car entered by the Salesian Polytechnic Tokyo came a creditable tenth out of a field of 46. In 1999, only 33 minutes separated the winner, the Australian Aurora 101, from the third placegetter— for a contest that spans a continent and takes over 40 hours, this is equivalent to a photo-finish in a two-minute horse race. Nine of the 40 entrants used lithium-ion battery packs; the cell with the highest specific energy was rated at 140 Wh/kg. Nickel-metal–hydride chemistry was used for the first time. Further interest in this technology was short-lived, however, given the comparatively lower specific energy of such cells (< 90 Wh/kg) and the ongoing rapid improvement in lithium technology.
The Alpha Centauri team, which was sponsored by the Dutch energy company Nuon and consisted mainly of students from the Delft Technical University, took the WSC to a new level in 2001. Their car, Nuna, featured dual-junction and triple-junction gallium–arsenide solar cells, which had been developed primarily for satellites. By combining two or three semiconductor layers that absorb light energy in different parts of the electromagnetic spectrum, the cells convert sunlight to electricity at much greater efficiencies than the monocrystalline silicon or gallium–arsenide alternatives that had been used previously by competitors. Nuna’s array could produce 2.1 kW of power— streets ahead of its nearest rival, Aurora, which ran at 1.5 kW. The superior performance enabled Nuna to achieve an average speed of 91.81 km/h and thereby, at its first attempt, to beat comfortably the record time set by Honda in 1996.
Now participating as the Nuon Solar Team, successive groups of Delft students exercised a stranglehold on the event with victories in 2003, 2005 and 2007. In the second of these four successes, the flying Dutch crossed Australia in 29 hours and 11 minutes at an average speed 102.75 km/h (cf., 44 hours 54 minutes at 66.92 km/h for Sunraycer). Their speed record stands to this day— principally because of changes to the 2007 WSC regulations that were introduced mainly in the interests of safety, but also to ease the cost of building a competitive solar car.
Given that cars equipped with multi-junction solar cells were travelling long distances at exceptionally high speeds, and that such costly cells were only affordable to teams with high budgets, the maximum allowable area of solar panels was lowered from 9 to 6 square metres for the 2007 WSC. The new regulations also demanded a more upright seating position for the driver and thereby increased the aerodynamic drag. Similarly, the steady advancement of lithium batteries, which had overwhelmingly become the preferred technology, required reductions to be made in the permitted weight of the pack.
Levelling the playing field
Despite the 25% reduction in array area enacted in 2007, the results that year and in 2009 demonstrated that use of highly efficient, triple-junction, solar cells remained the key determinant of success. In fact, the net effect of the decrease in cell coverage was to reduce the solar energy to the cars by only a mere 4%, thanks to technological advances. In good sunlight, a 6 square-metre array of such cells could supply up to 1.8 kW so that the average speed of the winner continued to be very healthy indeed, namely: 90.87 km/h (2007) and 100.54 km/h (2009). Even at this size, the arrays are expensive. In a further attempt to intensify competition by lessening the supremacy of affluent entrants, the technical regulations were further revised for the 201l event. Just 6 square metres are now allowed for silicon cells, which are less costly, but only 3 square metres for any other photovoltaic technology. This specification has prompted most of the elite teams to move to silicon and all but one used cells supplied by the SunPower Corporation,
Tokai University, the defending champion, was the notable exception to those who had adopted SunPower photovoltaics for the 2011 WSC. Tokai Challenger 2 was fitted with an array of Sanyo HIT (Heterojunction with Intrinsic Thin layer) solar cells. The unit cell is composed of a thin wafer of monocrystalline silicon surrounded by ultra-thin layers of amorphous silicon. This structure reduces power generation losses, especially at high temperatures. Tokai claimed a conversion efficiency of 22% with a 1.32 kW maximum output. By comparison, Nuon reported 1.20 kW from the Nuna 6 array, which indicated an efficiency of 20% from the chosen grade of SunPower cell.
Mention should also be made of the array built by Stanford University for its Xenith car. Instead of the customary application of plastics or polymers, the solar cells were encapsulated in thin, flexible glass similar to that used in iPads. The team claimed that the new prototype glass developed by Corning (one of their sponsors) offered significant advantages on account of its strength, ultraviolet stability and optical clarity. This is yet another example of how the WSC provides industrial organisations with a valuable test-bed to evaluate and showcase their latest advances. Stanford also found that the Xenith design created less aerodynamic drag than a rider on a bicycle. Nuon made similar improvements— studies in Europe’s largest wind tunnel in Marknesse showed that Nuna 6 had 10% less drag than its predecessor, Nuna 5.
The Umicar Imagine entered by the Umicore Solar Team, Belgium, and the BOGT of Hochschule Bochum, Germany, were the only two cars to use gallium–arsenide cells which, in both cases, were manufactured by Azur Space Solar Power GmbH with reported conversion efficiencies of 34% and 29.2%, respectively. In addition, the Belgian car was fitted with a system of germanium concentrator cells and parabolic mirrors to capture more energy from sunlight. Given that both the Solar Team Twente (The Netherlands) and the Michigan Solar Team (USA) had achieved little or no advantage though the use of concentrators in past events— the former with Fresnel lenses and a tilting solar panel; the latter with mirrors— the unveiling of the Umicar Imagine design elicited some surprise, but it was quickly realised that the team’s sponsor is a major supplier of multi-junction III–V solar cells, for which concentrators are the best terrestrial application.
Bochum’s aim was not to win the 2011 WSC, but to build and demonstrate a vehicle that is useful for everyday life in urban surroundings. The BOGT is the next step from the BOcruiser, which competed in the 2009 WSC, to a practical version that is solar powered. Accordingly, the new car has four wheels, two doors, two seats, a panoramic windshield, and a full lighting system. The available surface area of the design dictated the use of gallium–arsenide cell for maximum collection of solar energy. After crossing Australia, the team will travel to New Zealand on the next leg of a circumnavigation of the globe. In all, BOGT will travel more than 34 000 km throughout five continents with two equator crossings, which will make it the first ‘World-Wide Solar Ride’.
Less energy storage
Given the steady improvement in the performance of the various types of rechargeable lithium battery, the maximum weight for lithium‑ion, plastic lithium‑ion and lithium iron phosphate was reduced from the 2009 specifications of 25, 25 and 50 kg to 21, 22 and 40 kg, respectively. The progressive formulation of battery weight limits to match the above mentioned 5 kWh benchmark is challenging, especially since all regulations have to be set well in advance to give prospective competitors sufficient time to design and build their cars. This task is the responsibility of the author and to his relief the first three to finish in the 2011 WSC had battery packs with rated energies of 5.055, 4.955 and 5.089 kWh, respectively.
2011 – Mother Nature turns nasty!
Following significant backing by Veolia Environmental Services, the 2011 WSC become officially known as the ‘2011 Veolia World Solar Challenge’. The event was again conducted under the auspices of the South Australian Motor Sport Board.
A total of 37 cars from 20 countries made the starting line. 21Connect of Solar Team Twente took pole position after qualifying trials put it just 3 seconds ahead of its compatriot Nuna 6 at a speed of 85.43 km/h.
The preferred battery technology was split evenly between lithium-ion (15 cars, cell specific energy 234–245 Wh/kg) and plastic lithium-ion (15 cars, 146–236 Wh/kg). Five teams opted for lithium iron phosphate (106–137 Wh/kg) and the remaining two used lead–acid although in one case this chemistry was a last-minute substitute for a lithium iron phosphate battery that failed to clear customs.
The most popular choice of lithium-ion cell was the 3.1-Ah 18650A product manufactured by Panasonic. At present, this is the commercial lithium technology with the highest reported specific energy. The cell uses a proprietary positive electrode based on lithium nickel oxide (LiNiO2) and a negative electrode made from a silicon-based alloy instead of carbon. The favoured plastic lithium-ion cell was the 5.3 Ah model supplied by Nomura Keisuke, who was also the leader of the Ashiya University Solar Car Project. The Ashiya Sky Ace V and four other entrants employed Nomura cells.
The first few hours of 2011 WSC followed a well-worn script. The favourites— Tokai, Nuon, and Michigan— streaked away in close attendance with the experienced, but lower-budget, entries from the Aurora Vehicle Association (Australia), the Apollo Solar Car Team (Taiwan), Twente University, and Ashiya University in a tight pack some one hour behind. Surprisingly, the Umicore car had fallen well off the pace.
The remaining 29 teams were meeting problems to a lesser or greater degree. For instance, the wireless telemetry data sent from Impulse (University of California, USA) to its chase vehicle disappeared. Chopper Del Sol (Massachusetts Institute of Technology, USA) was another early casualty when, 16 km from the start, its custom-made solid polyurethane tyres suffered deformation and then breakage. These tyres were mounted on specially-machined anodized aluminium wheels and designed to boost performance through lower rolling resistance. Endeavour (Cambridge University, UK) encountered problems with its steering wheel. Strong winds blew the canopy and a solar panel from Avenir (Delhi Technological University, India) and also demolished the shell of GreenManiac (Kookmin University, Korea). This weather condition compounded the customary hazard associated with the cross-winds and wakes from Australia’s famous road trains. The solar drivers had to contend with an exceptionally high number of these massive vehicles (up to 53 m long and 130 t), several of which were carrying huge mining trucks and even complete houses that took up more than their fair share of the road.
With hindsight, the winds were the first signs that the 2011 WSC was not going to enjoy the near perfect sunshine that has usually accompanied past meetings. By the end of the first day, many experienced solar challengers were lulled into a false sense of security. Elsewhere, the remaining teams and particularly the newcomers were discovering that, in the words of Chris Selwood, The Event Director: ‘the WSC is an adventure in adversity’.
Cloud cover was steadily advancing from the north and this would therefore hit the slowest competitors hardest. In addition, Central Australia was experiencing the worst bushfire season in a decade. A smoke haze was drifting across thousands of square kilometres and, in places, was thick enough to blot out the sun. An increasing number of teams were unable to maintain a reasonable speed (and a well‑charged battery) and thus were forced to trailer their cars so as to reach each successive control stop within the scheduled time.
On the afternoon of the second day, the officials stopped the event because a serious bush fire some 300 km north of Alice Springs had required the police to close the Highway. Notably, the fire passed perilously close to the petrol forecourt of the roadhouse at Barrow Creek, a remote settlement of less than 20. At Wauchope, some 100 km south, the same three leading cars camped close by each other and took advantage of a unique mid-journey opportunity to charge fully their batteries. Meanwhile, a pack of eight others had reached the control stop at Tennant Creek and had also managed to replenish their batteries.
The Highway reopened overnight. The WSC Science Faculty therefore determined that the cars held at Wauchope would depart in order from 08:00 and maintain the separations between the times of their respective arrivals there, whilst those at Tennant Creek would observe their respective separations at the time the event was disrupted. In effect, everyone was delayed for 3 hours and 52 minutes. The bush was still smoking on either the side of the road and drivers had also to contend with dust devils, or ‘willy-willies’ as they are known in Australia. Whereas Nuon lost 20 minutes due to a technical fault and Michigan drove conservatively given that clouds and rain were forecast, Tokai pressed onwards and extended its lead despite being unable to travel above 70 km/h towards the end of the day. The chasing pack fell further behind.
News broke that the plastic lithium-ion battery of Sikat II, the Philippine entry, had overheated and caught fire in the late afternoon at Tennant Creek. There were no injuries because the car had already been parked and was recharging at the time. The team, including the driver, were a safe distance away. The local Fire Department dealt quickly with the situation so that the car suffered only minor damage to the body and the solar cell system remained in good order. After working through night to repair the damaged parts and install a replacement battery, the team had Sikat II ready to resume its journey south the following morning.
On crossing the border, the weather turned from poor to absolutely terrible. Having traversed vast expanses of red baked dirt, driven through bush fires, pushed through dust storms and survived rough buffeting from high winds, the vast majority of teams departed the Northern Territory to find nearly all of South Australia spread under a huge front of clouds, steady rain and intermittent thunderstorms with spectacular bolts of lightning. Thus many faced the dismal prospect of nearly empty battery packs and weakly performing arrays (in most cases, delivering only about 200 W). Consequently, more and more teams decided to trailer their cars in order to reach Adelaide before the official close of timing on the seventh day.
It is important to remember that the WSC is held in a single stage over 3 000 km. Thus conditions that affect some may not affect others and, given this uncertainty, the fundamental strategy of managing a very limited amount of stored energy over the whole route is critical to success. Umicore were to discover this to their cost. After an unexpected poor start, the team exhausted the battery on the fifth day and decided to replace it with a spare.
Disastrously, on forcing Umicar Imagine past Ashiya and Twente into fourth position on the sixth day, the replacement battery pack caught fire, about 200 km from home. The driver and six other colleagues had inhaled some of the smoke and were therefore taken to hospital for observation and possible treatment. All were declared healthy. Give the time penalties incurred through exchanging the battery twice, it had become impossible for the car to complete the course under its own power within the allotted time.
The four elements of earth, air, fire, water had certainly put the solar cars and their crews through their paces. So much so that the 2011 WSC may well be remembered as the ‘World Element Challenge’! Nevertheless, Mother Nature’s provision of generally bad weather had produced one of the most interesting and exciting contents in recent memory.
Oh yes – the results
The three front‑runners had largely avoided the bad weather. On the fourth day at Port Augusta, the final control point, Nuon was only 25 minutes behind Tokai. The Dutch decided to throw caution to the wind in an all-out attempt to catch up during the 300-km final stage into Adelaide. This high-risk strategy entailed rapid draining of the battery and the expectation that the sun would provide a last boost to the chequered flag. Unfortunately, however, the skies clouded over and light rain began to fall. Nuna 6 slowed down and eventually arrived home in second place some 65 minutes behind Tokai Challenger 2 which took line honours in a time of 32 hours and 45 minutes at an average 91.54 km/h.
Earlier on the same day, Michigan had also launched a counter attack. By travelling at over 100 km/h, Quantum had been just three minutes from overtaking Nuna 6 when a strong gust of wind tore the right wheel fairing from the chassis (these components smooth out airflow and hence improve vehicle efficiency). Thirty valuable minutes were lost in making the necessary repairs. Further misfortune struck when a combination of cross-winds and road train wakes again pulled the fairing from the car. Quantum finally ‘crawled’ over the finish line under weak solar power and with an empty battery to claim third place some 43 minutes behind Nuna 6.Ashiya University secured fourth place the next day ahead of Solar Team Twente by just seven minutes. The following and final day witnessed an exciting battle between competitors from Australian cities which have a long-standing rivalry. After swapping positions, Sunswift IV of the University of News South Wales from Sydney and Aurora Evolution from Melbourne secured sixth and seventh positions, and were similarly separated by a mere seven minutes.
All the remaining thirty cars failed to cover the distance under their own power. Istanbul University, Solar Energy Racers of Switzerland and the Apollo Solar Car Team made up the top ten. All had come tantalising close to completing the journey; they clocked up 2735, 2663 and 2650 solar kilometres, respectively. Despite the dramatic episode at Tennant Creek, fire-damaged Sikat II arrived home in a creditable 19th position after recording 1815 solar km— well ahead of notable teams from Canada, Italy, the USA and UK, amongst others.
Solar cars test the ultimate boundaries of energy efficiency— they demonstrate the possibilities that reside deeper into the future and, in essence, show how much more can be done with how much less. The innovations on display are at the heart of all electric vehicles, whether they be powered by hybrid engines, batteries alone, or even hydrogen fuel cells. This is why the emphasis that the WSC places on real-life operating conditions is so important. The event is run on real roads under gruelling conditions so that the competing technologies are put to the test under the conditions in which they would be used should they achieve critical mass for market acceptance. Moreover, from year to year, the technical regulations are revised to reflect those conditions more accurately. The concepts that are continually honed to perfection in the WSC undoubtedly contribute to the automotive industry’s efforts to reduce its carbon footprint through the realisation of more efficient road transportation.