Electric Automobiles

by Andrea Vezzini

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Подпись: Charging device |2 High-voltage battery |3 Inverter with DC/DC converter to 12 V |4 Electric axle drive with separate motor-generator |5 Antiblocking system (ABS) / electronic stability program (ESP) 6 7 Cooperative regenerative braking system (Actuation control module - hydraulic (6) and brake operating unit (7)
Подпись: Renewable Energy. Edited by R.Wengenmayr, Th. Buhrke. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electric cars indeed operate emission-free when the pow­er for charging their batteries comes from renewable sources (Figure 1). Even when their power is generated from fossil sources, they show a good C02 balance, since

they utilize the electrical energy very efficiently. Our mea­surements at the Bern University of Applied Sciences in Biel, Switzerland on an all-electric city vehicle, the Mitshu – bishi i MiEV, which has been on the roads in Europe since 2010, clearly demonstrate this. They showed a reduction of about 60 % in CO2 emissions compared to a standard mod­el with an internal-combustion engine, provided that the electric power was supplied entirely from a modern gas and steam combined-cycle plant.

The automobile industry has in the meantime made it clear that they see the future in terms of all-electric drive trains, following an intermediate phase of hybrid technolo­gy (internal combustion plus electric power), and they are investing heavily. But the pioneers are having difficulties: Thus far, two models of Nissan-Renault and Chevrolet have

Подпись: FIG. 2Подпись:image201"fallen well behind the planned sales figures for 2011 [1]. A typical example, which represents this whole industrial sec­tor, is the joint venture SB LiMotive, founded in 2008 by the auto-component manufacturer Bosch and the electronics firm Samsung and ended in September 2012; both compa­nies now continuing its battery businesses alone. Starting with a development budget of 500 million dollars, they proposed to install a manufacturing capacity for lithium – ion batteries for 50,000 all-electric cars to be in place by 2013 [2].

Megacities with short-haul traffic and air pollution prob­lems will be the first major markets for all-electric vehicles. The geographically small countries Israel and Denmark, for example, also want to electrify their road traffic, and they are cooperating with the Californian concern Better Place to set up an infrastructure of recharging stations and a bat­tery leasing system [3]. Nevertheless, SB LiMotive takes a conservative tack and predicts that the world market for all-electric automobiles will take off in earnest only after 2020.

The batteries of future electric cars will be based on lithium-ion technology. According to the plans of SB LiMo – tive, for example, the first all-electric cars will have a cruis­ing range of 200 km on a fully-charged battery. At an aver­age daily driving distance of 90 km in Germany (30 km in Switzerland) for commuters, this should relieve the fears of potential buyers that they will be stranded underway. Ad­ditional obstacles to buyers are a lack of trust in the relia­bility of the batteries and the dashboard indicators for charge level and remaining cruising range, and the long recharging times, which are several hours. A serious prob­lem at present is also the high cost of the large battery, which must be drastically reduced by the industry. This is emphasized for example among current electric vehicles by the Mitshubishi i MiEV: It costs two to three times more than comparable models with internal-combustion engines.

Experience with hybrid technology will pave the way in the coming years for all-electric drives. In hybrid vehicles, the power-control electronics, drive motors and an ad­vanced battery technology are being tested to a degree which was previously unknown in the automobile industry. Electric drive trains are already superior to internal-com­bustion engines, since they deliver optimal power and torque over a much wider rpm range (Figure 2) [4]. Fur­thermore, they provide maximum torque to the wheels at stall. Thus, they require neither a clutch nor a large trans­mission. That reduces frictional losses and saves weight and space.

Modern electric motors are not only more compact than internal-combustion engines, they also have a considerably higher efficiency: They convert well over 90 % of the elec­tric power into mechanical driving force, while the best diesel engines utilize only 35 % of the chemical energy stored in their fuel. To be sure, the same weight of fuel con­tains 50 to 100 times more energy than a present-day fully charged lithium-ion storage battery. Electric vehicles are

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The torque (red curve) of an electric motor with 186 kW maximum power (Tesla Roadster), compared to that of a modern four-cylinder internal-combustion engine (ca. 120 kW maximum power, black curve); blue: the power curve of the electric motor (after [4]).

correspondingly heavy, even though their more efficient drive train requires less on-board energy.

An important step towards an all-electric vehicle will be the so-called plug-in hybrids. They are planned to travel over typical commuter distances using only electric power – including expressways. This already requires an electric drive train with comparable power to an internal-combus­tion engine. The drive battery must be correspondingly di­mensioned. In contrast to self-sufficient hybrid vehicles, they will be recharged with electric energy directly from the power grid.