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ectr,c he size, weight, shape and technical characteristics of the energy source affect almost every other vehicle attribute. Energy density is the most important consideration but choice of the energy source will depend on a host of factors including: power density, cycle life, initial cost, maintenance cost, energy efficiency, output voltage, internal impedance, charge rate, byproducts, shelf life, temperature characteristics and overall safety. However, energy density is what really matters. This is simply the total source energy divided by the source weight to give a result in watt-hours per kilo~ gram (Wh/kg). Naturally, the source weight must include any subsystems required by the source, such as cooling fans and so on. Table 1 shows the energy densities of various fuels and other energy sources and makes it clear how very low the energy densities of man-made sources are. The values listed are nominal and may vary considerably under certain conditions but the much lower energy densities of the man-made sources relative to the natural sources are obvious. It is also important to realise that energy density may vary considerably with the rate at which the energy is used. For example, an energy density of 60Wh/kg may be claimed by the manufacturer of a particular type of T By GERRY NOLAN When you consider that one kilogram of petrol stores the equivalent of 12,000 watt-hours of energy, any electric storage medium falls a long way short. The best alternative is the aluminium-air fuel cell which has an energy density of up to 360 watt-hours per kilogram, so it is obvious that the field of energy storage is wide open to innovation. 8 SILICON CHIP battery weighing 30kg, so we would expect to obtain 1800Wh of energy. What the manufacturer may neglect to tell us, however, is that if we use the energy at double the normal rate, the energy density drops to 45Wh/kg, giving us a total energy availability of only 1350Wh. The moral of this, of course, is to specify energy density at a specific power level. As anyone reading this article probably already knows, the most common energy source for electrical vehicles is the lead acid cell. The currently obtainable energy densities of these is up to 50Wh/kg. Other batteries, fuel cells and flywheels are shown in Table 2 along with their theoretically obtainable energy densities. Power density vs acceleration An energy source with a very high energy density is highly desirable but, as with conventional engines, the rate at which the energy may be used is also very important. This rate is determined by the power density which is measured in watts per kilogram. A good example of a battery with a high energy density but a low power density is the aluminium-air fuel cell developed by Alupower in Ontario, Canada. Because it takes up to half an hour to reach its full power capacity, it needs to be used with lead acid cells which supply short term demands. Pt.2: energy storage - THE GENERAL MOTOR'S IMPACT car uses high-power sealed-lead acid batteries & shatters the perception thai electric vehicles are slow-moving golfbuggies. During trials, it beat a Nissan 300ZX in 0-100km/h acceleration tests. Power densities of up to 150W/kg may be obtained from lead acid batteries , around 200W /kg for nickel zinc, 100W /kg for nickel iron, 65W / kg for zinc chloride, 160W /kg for lithium iron and up to 200W /kg for sodium sulphur. On the basis of this comparison, the ordinary lead acid battery isn 't too bad. Replacement cost How many times can you charge and discharge (cycle) the batteries before they have to be replaced? The General Motors Impact car is designed to be powered by 32 10-volt batteries. These have an expected life of about 40,000km and a replacement cost of around $3500, or a little less than nine cents per kilometre. This is a much higher cost than the energy itself, which could be taken from the power grid for as little as 0.3 cents per kilometre. Increasing cycle life will obviously reduce the replacement cost per kilo- metre and research over the past decade has greatly improved the number of cycles possible, in some cases almost doubling it. Now lead acid batteries have a cycle life approaching 800 cycles, up from 500 in 1980, while nickel iron and sodium sulphur may be cycled (ie, fully charged and fully discharged) more than a thousand times. Another factor which needs to be taken into consideration when selecting an energy source is cell voltage , which will determine how many cells need to be hooked up and the type of array to obtain the voltage necessary to run the motor and battery efficiently. The available voltage will also be affected by the internal impedance, which generally increases as charge decreases. Battery maintenance is also an important consideration from time, material costs and skills points of view - your average EV user may not feel too comfortable topping up the battery with some exotic electrolyte every second day. Flat batteries One of the things that will worry EV users , at least until there are enough recharging stations handy, is the fear of the batteries going flat without warning. In your present vehicle, a glance at the fuel gauge and a quick mental calculation will ef\able you to work out roughly how far you can go before you need to stop for fuel. However, replacing the fuel gauge Table 1 Energy Source Nominal Energy Density (Wh/kg) Petrol 12,300 9,350 Natural gas Methanol 6,200 Hydrogen 28,000 Coal 8,200 Lead-acid battery: up to 50 Sodium-sulphur battery: 150-300 an Eldorado for innovators FEBRUARY1991 9 ELECTROLYTE STORAGE TANK \ LEAD-ACID BATTERY PACK ELECTRIC DRIVE ALUMINIUM-AIR HYBRID ELECTRIC PROTOTYPE - because the aluminiumair battery takes half an hour to build up to peak power, a lead-acid battery pack is used to supply start-up and acceleration energy. Excess energy from·the Al-air battery can then be used to recharge the lead-acid pack while the vehicle is moving. with a "charge gauge" is not such a simple matter in an electric vehicle. The definitions of a fully charged battery and the way to measure its charge vary considerably with the type of battery. The no-load voltage level may give a reasonable indication in some cases but will vary with the history of the battery, temperature and so on. Measuring the electrolyte specific gravity also gives an indication but few EV users would appreciate the inconvenience. A type of "charge gauge", which integrates the current into and out of the battery to give an actual state-ofcharge, would be a great comfort. An additional selling point would be a "range-at-present-speed" readout. Charge acceptance The capability of the energy source to take a charge is also an important ALUMINIUM-AIR CELL - an aluminium-air cell usually has 20 individual cells, a condenser and a heat exchanger at the centre of which are located the pump motor and air blower. The condenser removes oxygen depleted air from the system and the heat exchanger keeps the electrolyte temperature at about 60°C. The blower circulates air through the cathodes. 10 SILICON CHIP consideration, as it should be able to absorb high rates of energy input (eg, under regenerative braking) without exceeding acceptable temperature levels. Nickel-cadmium batteries, in particular, are able to handle high charge rates and Audi is using these in a hybrid 4-wheel drive car. The front wheels of the "duo" are driven by the normal Audi 2.3 litre, 5-cylinder petrol engine and the rear wheels by a pack of 49 nicad cells, each of 1.2 volts, powering a 9.4kW electric motor which fits into the transmission tunnel. Safety considerations The sheer weight of batteries, especially when lead acid cells are used, requires special strengthening in the design and construction of EVs from scratch or when converting conventional vehicles to EVs or hybrids. 400kg of batteries suddenly coming loose during a crash stop would be a major hazard, to say the least. "Gassing" and high temperatures during charging can also lead to problems, particularly at high rates of charge, and effective ventilation must be built-in. Silver zinc batteries Twelve of the first 13 cars in the recent World Solar Challenge used silver zinc batteries, while roughly the same number of vehicles used DUAL POWER FOR AUDI - a 180kg, high performance nickel-cadmium battery pack fits into the spare wheel-well of the Audi 100 to power the 9.4kW electric motor which fits into the transmission tunnel. Because the petrol engine is retained to drive the front wheels, a 4WD vehicle with two completely independent drive systems is the result. lead acid batteries. Although they are much more expensive than lead acid batteries, with their high energy density (100Wh/kg), superior power density and lighter weight, silver zinc batteries give an electric vehicle a decided performance advantage. So far we've talked mostly about batteries and their close relatives, fuel cells, but flywheels have been around for a long time, much longer than batteries in fact, and could conceivably have even greater potential than batteries or fuel cells as energy storage systems for EVs. Flywheel research Your silky smooth BMW, Mercedes, even the Rolls, would run very roughly, if at all , without a flywheel to maintain the crankshaft rotation through to each ignition stroke. Apart from smoothing out the staccato power delivery of the piston engine, flywheels have also been used in vehicles for energy storage since the 1930s in everything from torpedoes to draglines and helicopter hoists. Archaeologists have found one in the Middle East that they believe was used as a potter's wheel in ancient Ur of Chaldea 5,500 years ago. As far back as 1973 , researchers were predicting energy densities of 870 watt-hours per kilogram using fused silica as a material for super flywheels. What some people didn't seem to Exotic Energy Storage For EVs Zinc-hydroxide, aluminium-air, vanadium and sodium-sulphur electro-chemical batteries and fuel cells, some being recharged simply by replacing the electrolyte, are all current areas of research . Energy densities of up to 200Wh/ kg are being claimed for the zinchydroxide electrochemical cells which are being researched by a team headed by Jim Evans at the Lawrence Livermore Laboratories near San Francisco. Zinc is often used as a material for electrodes - remember the zinccopper-acid cell we all experimented Table 2 Source Lead acid Nickel zinc Nickel iron Nickel cadmium Silver zinc Zinc chlorine Energy Density (Wh/kg) Now Available Theoretical 110 50 90 60 30 100 500 90 High Temperature Batteries Lithium metal sulphide: Sodium sulphur: 170 300 Fuel cells Aluminium air: Flywheel (steel): Super flywheel (fibre): realise until much more recently was that, while the energy storage capacity is directly proportional to the mass , it is proportional to the square of the rotational velocity - so triple the speed ofrotation and you get nine times the energy storage capacity. Concentrating the mass near the circumference, where the rotational velocity is highest, also increases the energy storage capacity for a given overall weight. For a time, lightweight high-speed flywheels appeared to have real potential and a great deal of research was carried out during the early 70s with at school? - but it is generally in the form of a sheet or slab. By using the zinc as particles, the slurry of anode and electrolyte can be continuously replaced from a reservoir with the used material being stored for later replacement and recycling at a "service station". The aluminium-air cells being developed by Alupower have a claimed energy density of 360Wh/kg. Alupower is an Alcan subsidiary which is combining its technology with Moli Energy's rechargeable lithium battery knowledge. Alupower's fuel cell generates electricity by an electrochemical reaction between aluminium and oxygen , using an alkaline solution or saltwater as an electrolyte. An air 350 12-30 up to 40 870 when the Middle East put up oil prices. Unfortunately, when oil prices dropped again, flywheel research lost its momentum. Now, although research is gaining speed again, the high expected energy densities haven't materialised. Nevertheless, the availability oflight, high-tensile fibres such as Kevlar, magnetic levitation bearings, high vacuum enclosures and electronic commutation and control have enabled densities of more than 40 watthours per kilogram to be obtained. Because they are so light, relatively maintenance free and could be made stream is blown through the cell stack to supply the oxygen for the electrochemical reaction, while the electrolyte is pumped through the cell stack between the aluminium anodes and air cathodes. Electricity is produced as the alu minium oxidizes, forming aluminium hydroxide as a byproduct which precipitates out and is collected in a sump. This can be collected and recycled back into aluminium, making it a clean and non-polluting renewable power source (see illustration). Some of these research paths will be blind alleys, others will lead to developments in directions quite different from that originally intended, but that is the way of research. FEBRUARY1991 11 i THE CLEAN AIR TRANSPORT LA301 - a 4-passenger or 2-seat microvan - uses lead-acid batteries & an aluminium-air fuel cell to power an 11.9kW DC motor. It also has a propane-fuelled auxiliary power unit to achieve a maximum range of 240km and a top speed of lO0km/h. for change over at service stations instead of waiting for a recharge, the possibilities for fruitful research are very high. Another advantage of the lightweight fibres is that if the flywheel disintegrates, perhaps because of overspeeding, it just becomes a pile of fluff instead of potentially lethal chunks of high grade steel. Where are we going? One of the mistakes EV designers and builders seem to be making is to compete head on with existing fam- ily cars by designing for ranges of at least 100km at speeds up to 100km/h. You may remember from last month's article that 90% of all daily one way trips are less than 35km long. So why not design for a trip range of 50km at speeds of up to 80km/hr, at least until the technology_ is more widely accepted. For a start, this would reduce battery weight to less than half. Less than half? Yes - with a lighter battery load and the consequent reductions in strengthening required - the overall vehicle weight would be much EXPERIMENTAL ELECTRIC vehicles have been produced in Australia. This one is a converted Mazda utility powered by a tokW forklift motor & a 48V battery bank. Solar cells on the roof top up the batteries during the day. less and not so much energy would be required to push it around. The combined results would be reduced manufacturing, running and maintenance costs, making EVs a much more immediately attractive alternative, thereby greatly accelerating their acceptance by the public. sc Outside chamber liquid-filled for cooling Fibre-glass shielding · Contra-rotating flywheel rotors Gimball ing spring assembly Electrical leads to motor/generator 12 SILICON CHIP THE ADVENT OF THE "ENERGY WH~EL" an energy pack of super flywheels built as a combined motor/generator with electronic commutation and magnetic levitation bearings, running in a high vacuum, would provide very efficient energy storage. Because of the enormous speed at which the flywheels rotate, the energy would be used to power an electric motor to drive the wheels, rather than using a mechanical drive train. The pack would be self-contained so that it could be quickly replaced by a fully charged one when discharged. Flywheels used for energy storage in moving vehicles would need to be contrarotating and gymballed to dampen out gyroscopic precessing forces.