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World Solar Cha/le Advances Electric Whether people think about it or not, the vehicles competing in the recent World Solar Challenge were some of the most advanced electric vehicles ever produced. They are extremely efficient and quite fast, as this report of the race indicates. ROUND 350 BC, when Alexander asked Diogenes if he lacked anything, Diogenes replied: "Yea, that I do; that you stand out of my sun a little". Perhaps he was only speaking metaphorically but, to this A "Spirit of Biel 11" - the winner of the 1990 World Solar Challenge. It covered the 3007km route from Darwin to Adelaide in just over 46 hours at an average speed of 65km/h. 6. SILICON CHIP day, we can't be sure that Diogenes didn't mean that Alexander should get out of the way of the sun shining on his solar powered chariot. Nothing much has changed. As solar vehicles become more popular, more people are bound to be asked, with varying degrees of politeness, "excuse me please old thing, would you mind stepping out of the sunlight for a moment or two?" Also unchanged since those times is the amount of energy from the Sun reaching the Earth's surface, around lkW /m 2 • It may be as much as 1.3kWI m 2 on some days because of energy reflecting back from clouds but this is not something to rely on. The total amount of available energy will of course be altered by a number of factors, such as: season, latitude, time of day, cloud cover, air pollution and so on; all of which we have little direct control over. What can be controlled, or ·at least improved upon, is the percentage of the available light energy that is converted to electrical energy to drive a vehicle and/or be stored for later use. And this is what the World Solar Challenge (WSC) is all about - it provides the impetus, through competition, for contestants to improve every aspect of their vehicle design and, most importantly, the efficiency with nge Vehicles · which the Sun's energy is converted for use. The second World Solar Challenge started in Darwin at 8:00am on 11 November, 1990 and finished 3007km and 46 hours, 7 minutes and 51 seconds running-time later, when the Spirit of Biel II crossed the line in first place with an average speed of 65.Zkm/h. By GERRY NOLAN. The elegant "Southern Cross". Its multiple-curved panels are made possible by the use of amorphoussilicon photovoltaic cells. Experience pays off During the race, it became apparent that the teams which competed in the first WSC in 1987 were at a distinct advantage, the Spirit of Biel team being a case in point. In 198 7, after losing five hours in Alice Springs to repair damage caused by what was arguably the world's first collision between a solar car and a conventional car, the first Spirit of Biel gained third place. This time they won it. This emphasises the tremendous value of the World Solar Challenge and the similar events that are proliferating around the world, as stimulus to the development of practical solar electric vehicles. Not everyone, however, learnt from their experience. Aquila, the "secondhand" Northern Territory University's 1987 Desert Rose, rebuilt by a team from Dripstone High School in Darwin, had a drama when a bolt fell out, allowing one front wheel to collapse the same bolt that had fallen out and caused the same drama in 1987! Ironically, the new Desert Rose had a similar problem when a nut worked loose and let one of its front wheels fall off just after the 1990 start. But these were the exceptions most of the experienced teams performed better the second time around, in particular Hoxan's Phoebus, which took over 153 hours the first time but, Highly efficient solar cells are wired in a 'shingle' type arrangement so that the entire surface of the panels is photoelectrically active. The shingle strings are then assembled into modules. in 1990, finished in 57 hours 21 minutes to gain fourth place. Making solar cars faster Most SILICON CHIP readers will al- ready have a fairly good idea of the factors influencing solar electric vehicle performance. What we are going to look at more closely here is the progress that has been made in these APRIL 1991 7 Integration of the solar modules into the vehicle structure. The final solar generator is over 17% efficient and, with a bright Sun, delivers 1300 watts, making it the most efficient silicon gener.ator ever built. areas since 1987. In particular, we'll look at the progress made towards more practical solar electric vehicles. The most obvious factor influencing solar vehicle design is of course the solar panels themselves - their physical characteristics and their efficiency. Other factors include, in roughly descending order of importance: battery performance, electrical power transmission efficiency, mechanical power transmission efficiency, body weight and aerodynamic efficiency, vehicle stability, rolling friction and; the one that can assume the most importance at any time, structural integrity and reliability. Solar cell efficiency Wait a second - if the energy from the Sun is free and virtually limitless, why worry about efficiency? Well, of course, it's a weight and space problem. If we could use an unlimited area of solar cells, without worrying about the weight, the efficiency with which the solar energy was converted to electrical energy wouldn't matter. Obviously, in the case of a solar powered vehicle, we are limited. 8 SILICON CHIP Solar, or photovoltaic cells (PVs), were first developed in 195 7 and NASA was the first organisation to undertake large scale research and development because it needed them to run the batteries in satellites. Now solar arrays are as much a part of satellites as are wheels on cars. Various combinations of chemicals have been used in the search for higher efficiencies but they have narrowed down to a few that either work better than the others or are not prohibitively expensive. Silicon cells have proven to be the best option so far and vary in efficiency from 8-12% for amorphous cells, 12-16% for monocrystalline and more than 18% for laser grooved cells. Hoxan Research Laboratories, which fielded Phoebus III in the event, claim 19.3% efficiency for their mass produced PV cells, with 18.5% efficiency when they are incorporated into a module. Laboratory calculations show that, although 20% has long been regarded as the practical limit for silicon solar cells, the fundamental efficiency limit was close to 28%, with 25% being a reasonable experimental target. Gallium arsenide cells are up to 18% efficient but are extremely expensive and the production processes are not at all environmentally friendly, so they haven't caught on. The GM Sunraycer used mostly gallium arsenide cells in 1987 b~t, even though they gave it enough advantage to win, only the Sunraycer clone, Solar Flair from California State Polytechnic University, Pomona, used them in 1990. However, it seems their cells were seconds and they gained little advantage, coming 11th with an average 44.4km/h, compared with the Sunraycer's average of 66.9km/h in 1987. The Spirit of Biel team used the "Green" laser grooved, silicon PV cells with an innovative overlapping or 'shingle' arrangement to take advantage of the 'buried' contacts in the top and bottom of the cells. This gave them a packing density of 97.5% for their panel array, with a resultant increased output for the allowable area (see box for race rules). Since the 1990 WSC, Dr Green's team has announced that they have achieved a further substantial increase in efficiency with their laser grooved (see Fig.1) buried contact, silicon photovoltaic cells. The laser grooves, in two directions at right angles to each other, form tiny inverted pyramids that 'trap ' the light, reflecting it internally up to 50 times. Amorphous silicon cells Tipped by its developers, Semiconductor Energy Laboratories (SEL), to be the first PV cells to enter large scale mass production, amorphous silicon cells were used on the very elegant looking Mazda Southern Cross in the 1990 WSC. Because amorphous cells are not as efficient as monocrystalline cells or laser grooved cells, the Southern Cross took 97.5 hours to make 28th. However, they are much cheaper to produce and, because of their thin film construction, they are very flexible , making them easier both economically and from a design point of view to use for solar vehicles. Sanyo has developed amorphous silicon solar cells which need an amorphous silicon layer only five microns thick to produce PV activity, about 1/60oth that for single crystal silicon. Using amorphous silicon, The sleek Hoxan "Phoebus III" looked as though it should have done better than fourth. It covered the route in 57 hours & 21 minutes. Below: "Phoebus III" spread out to catch the last few rays of sunlight just north of Tennant Creek in the Northern Territory. Sanyo has also developed a seethrough PV cloth which has already been used to power the Sun Seeker light aircraft on its attempt to fly across America in July 1990. The aircraft remained airborne for 7 hours and 35 minutes and covered a world record breaking distance of 330km. From a practical point of view, amorphous solar cells are likely to see widespread use as battery boosters on electric vehicles, electric sunroofs and so on, before the more efficient but more expensive cells. The average of the energy generation potential from the solar arrays of all the vehicles was around 1.3kW. Lead-acid or silver-zinc? Around half the cars in the 1990 WSC used lead-acid batteries and most of the other half used silverzinc, the rest using nickel-zinc or none at all. Battery capacities varied considerably, with the silver-zinc batteries having approximately twice the capacity of the lead-acid types. The first five cars home used sil- ver-zinc batteries; the sixth, Australian Energy Research Laboratory's (AERL) secondhand Ford from 1987, used lead-acid cells. Apart from being the first Australian built car to finish, it completed the journey in a full day and one hour less than it took in 1987, when it used silver-zinc cells. Perhaps even more significant is that this car, with its $25,000 racing budget, finished only three and a half hours behind the million dollar second place Honda entry, after 60 hours of racing! Despite the success of the silverzinc cells, they are not yet a practical option for urban vehicles because they can easily be damaged if discharged completely. They have a very high initial cost, typically about $12,000 for competing cars, and they can only be recharged about 15 times before they lose much of their capacity. Compare this with the relatively low replacement cost of lead-acid batteries, plus an expected 300 or more charge/discharge cycles, and it is obvious which is the more practical option. The Solar Star hasn't even had the battery ·caps off in over 5000km of running, including the WSC and the successful world speed record attempt. Besides , as can be seen from the results of both the AERL car and the Solar Star, they are not so far behind in long range performance and well ahead in outright speed. Only two cars used nickel-zinc batteries. The first of these to finish, Kyocera Corporation's Blue Eagle, was the 14th car to cross the line, after 72.4 hours running. This car also used a solar concentrator and a Stirling engine to add to its solar power but unfortunately the engine as well as some of their batteries failed, so its potential wasn't really tested. Although they both used silver-zinc batteries, one of the main factors credited for Spirit of Biel II's win over the Honda car was its superior battery capacity of 86, 25Ah cells connected to give 129V, as opposed to Honda's 68, 20Ah cells connected to give 102V. Two cars used no batteries at all. The first of these to finish was Sofix of Japan which, although the heaviest vehicle in the event at 290kg, finished in 21st position after 96.8 hours running, most of it under overcast conditions. The other 'solar only car' was enAPRIL 1991 9 Dripstone High School's "Aquila" braves yet another big truck on the dusty bitumen of the Stuart Highway. Note how its panels are tilted to capture the morning sun. tered by the Solar Research Association (Australia) and claimed 2 7th position after running 97.3 hours. Peak power trackers Eight of the first 11 vehicles in the 1990 WSC ran Australian Energy Research Laboratory (AERL) miniature, customised 'race trim' Maximizers, with 15 teams in total using more than 50 Maximizers between them. Ironically, although they had purchased Maximizers, the secondplaced Honda entry didn't use them as the team misunderstood the importance of peak power trackers (PPT). Spirit of Biel II used seven of their own design PPTs, each handling 220W and weighing only 0.4kg. They claimed an efficiency of up to 98.6% at 30°C. Just how important are they? PPTs are to solar photovoltaics (PV) what an automatic gearbox is to a car. Both link the power source to the load and permit the most efficient operation by exactly matching the power source at all times to the ever changing requirements of the load. The gearbox does this mechanically, while PPTs like the Maximizer use a DC-DC step-down converter to automatically maximize the electrical power delivered from the PV panels to the battery. Generally speaking, each section of the solar panels that can expect to have the same amount of solar radiation reaching it at any one time should have its own peak power tracker. This reduces the need for 'averaging' between cells that are in shadow and others that are in bright sunlight. AERL claim that their Maximizer PPT can easily produce 25% higher battery charging rates than would be achieved without the Maximizer under the same conditions (ie, 25% better on the day). Obviously, when such an effort is being put into increasing the efficiency of solar cells by a few percent, being able to achieve up to 25% more by using PPTs is more practical and cost effective. Pride of Maryland, which placed third in the US Sunrayce and was one of the GM sponsored cars, used 10 NASA designed PPTs. Because these relied on manual adjustments to find the maximum power point for each array, they proved unsatisfactory. Desert Rose, run by the Northern Territory University, used 26 PPTs they designed themselves. These took a 3ms sample of the open circuit voltage every two seconds and used an open loop algorithm to establish the optimum parameters through a 65kHz FET chopper. Because a relatively low battery voltage (42V) was used, three DC-DC converters were used to step the voltage up to 350V for the motor. Using five FETs on each side of a push-pull transformer, together with transformer current sensing, the converters returned an amazing 97.8% efficiency. Getting the power down p-silicon rear contact oxide Fig.1: diagram of the 23% efficient, laser grooved, silicon solar cell recently developed by the University ofNSW and used on the "Spirit of Biel II", winner of the 1990 World Solar Challenge. 10 SILICON CHIP The Spirit of Biel II was able to convert an astonishing 86% of the solar energy collected to mechanical energy at the drive wheel. Compare that with the efficiency of converting the energy contained in petroleum fuel to mechanical energy! Several of the cars used the powerful, lightweight UNIQ motors (see SILICON CHIP, January 1991). The overwhelming majority of competitors used DC motors, many of them brushless DC permanent magnet motors, including the winning Spirit ofBiel II, which used a specially made "Grundfos" clearly showing the 'table top' construction of its solar panel. The Rules "Grundfos" clearly showing what a willy willy can do to a car with 'table-top' construction of its solar panels. motor with a nominal power of 1.lkW and a maximum power of 5kW. Pride of Maryland was able to idle along at about one eighth power most of the time and take the hills in its stride with its 14.9kW UNIQ motor, unlike Konaweena High School from Hawaii, whose UNIQ motor had so much torque that it ripped itself right out of its mountings! But then, their Kalaikaka had other problems such as their new Trogan Pacer batteries being shipped to Auckland instead of Darwin so they only had old lead-acid cells for the WSC, a blown up DC-DC converter, many flat tyres and so on. But they finished 18th after a run of 96.2 hours. Transmission chains obviously play a large part in getting the 'power to the ground' and geared belts and chains and sprockets were the most popular way of transmitting the power from the motors to the wheels and so to the road - usually via small bicycle type wheels. Tests and experience have shown that the narrow, high pressure tyres reduce rolling resistance to a fraction of the wider softer tyres. In many cases, the wheels are streamlined with plastic discs or wheel spats. The successful Solar Star uses aluminium disc wheels and tyre pressures of 80kpa to achieve a rolling resistance of only 0.004. Body design As well as low rolling resistance, lightweight aerodynamically slippery bodies are obviously of paramount importance. In many cases, to achieve the light weight, strength and durability were Essentially the vehicle has to fit into a box six metres long, two metres wide and one metre high. Competitors can do virtu ally anything they like in that box, as long as it's Sun-powered. Sunlight is the only source of power to be used for the racers. More formally, the maximum vehicle dimensions are 6m long x 2m wide x 1.6m high. The solar array may not exceed 4m long x 2m wide x 1.6m high . As many as four people are allowed to share the driving, each one ballasted to 85kg . If you happen to weigh more than 85kg, plan ahead to lose weight, have an operation or drive with a weight penalty. A handbrake, friction brakes, brake lights, turn indicators, rear vision viewers and seat belt/s are required on all cars. Cars may only race from 8:00am to 5:00pm each day and must stop wherever they happen to be at 5:00pm. Charging of the batteries, using only the car's solar panels, is allowed from 6:00am to 8:00am and from 5:00pm to 7:00pm - after which, the car must be put into a lightproof container. Vehicle maintenance or repairs may only be carried out between 6:00am and 7:00pm. Defective batteries may be replaced but only with a costly time penalty. After scrutineering, each car will be accompanied by an official observer to ensure that all the rules are complied with. APRIL 1991 11 World Solar Challenge - Advancing Electric Vehicles The instrument panel of the "Solar Star" showing the aircraft type steering yoke, computer readout and neat array of switches. The keyboard port is on the bottom right of the panel and the slot for the magnetic memory card on the bottom left. During the WSC, the drivers were able to play computer games to alleviate the boredom of travelling at a relatively low speed along a fairly straight, flat road for hours at a time. sacrificed, which.meant that time was lost repairing cracks or even complete structural failures. Most vehicles used lightweight tubing , a la Sunraycer, with a Nomex Kevlar sandwich body to carry the solar panels. To keep the profile down to a minimum, and consequently the coefficient of drag (Cd), the vehicle was designed to be driven from almost the prone position. Driver comfort is also not unimportant on such a long journey with fairly high pressure tyres. A Cd of 0.12 was claimed by several cars and the Spirit of Biel II achieved 0.13 , all less than half the Cd for the average family car. was 2000km along the track when a willy willy tore the complete panel off, lifting the car, turning it over and dumping it upside down in the process. The driver was unhurt but the panel landed 50 metres away and suddenly the race was over for the Solvogn team from Denmark. Dripstone High School had to restrict the speed of their Aquila if there was any crosswind. Alarus, driven by Dimitri Lajovic, was tipped over by crosswinds several times and Detlef Schmitz was sitting on the side of the road having a cup of tea when a willy willy destroyed his 5-wheeler completely. Stability What was gained One of the most publicised aspects of the WSC is the stability test, in which each vehicle has to drive at full speed past a 58 wheel, 3-trailer road train travelling at 80km/h in the opposite direction. All of the vehicles that competed passed this test, but not all passed the willy willy test further down the track. Grundfos Pumps, one of the several "table top" models with solar cells in a flat panel mounted on struts, For the 3007km journey, Spirit of Biel II consumed the equivalent of 12 SILICON CHIP 50kWh of energy which corresponds to 0.165 litres of petrol per 100 kilometres or 4. 95 litres for the whole journey! That's the equivalent of 2727mpg! Old problems were redefined in new ways and new problems manifested themselves. Overall there was considerable improvement in almost every area of technology associated with the vehicles, their support crews and their campaigns. More cars and people participated, more was achieved for less cost, and several million more people in the world learnt about solar cars and the potential of solar energy through the tremendous amount of publicity the event gained world wide. Personally, I think that, in the long term, the Aquila team from Dripstone Junior High School in Darwin have cause to be more satisfied with their effort than any other team. They set out to demonstrate that schooling should not be restricted to the class room and that educational opportunities extend beyond the school - and they succeeded admirably. Their 28-member team took part in a world class event at the cutting edge of the most important technology today and gained a credible 19th place. This sort of participation can be an inspiration for the rest of their lives. They will never forget it. Neither will the rest of their school, their families or their peers around the world, to whom they have shown the way. SC Changing the right hand rear disc wheel on the "Solar Star". The motor unit and ridged aluminium brake drum can be seen here.