Tesla Reveals High-Profile Electric Roadster; Calculates EV is More Than 3x as Efficient (WTW) as Fuel Cell Vehicle
20 July 2006
|The Tesla Roadster.|
Tesla Motors unveiled its much-anticipated all-electric two-seater roadster. The lithium-ion battery powered sportscar features a 248hp (185 kW) electric motor that accelerates the car from 0 to 60 in four seconds.
Built by Lotus for Tesla, the Roadster has a range of about 250 miles and a top speed of 130 mph. The price for the Roadster will be around $100,000.
The custom-designed battery system (Energy Storage System) weighs in at close to 1,000 pounds and uses commodity lithium-ion cells. The system addresses thermal balancing with a liquid cooling circuit.
The 3-phase, four-pole motor uses a low resistance “squirrel cage” with large copper end rings. This allows the rotor to develop high current flows and torque, with low resistance losses. The use of a small air gap allows tight inductive coupling which, combined with low loss magnetic materials, enables the development of high torque at high rpm. Together, these factors allow the induction of large currents, even at high rpm, producing much flatter power and efficiency curves from approximately 2,000 rpm to 12,000 rpm. The motor redlines at 13,500 rpm.
(Devising a cost-effective method for the production of copper motor rotors has been under investigation for years. Siemens introduced three motors with die-cast copper rotors to the US market in April.)
|Comparing Well-to-Wheel Efficiency and GHG emissions. Click to enlarge.|
In a white paper (The 21st Century Electric Car) published on the Tesla Motors website, the company calculates the tank-to-wheel (actually, the “electrical outlet to wheel”) energy efficiency of the Roadster to be 2.18 km/MJ.
Assuming electricity supplied from a combined-cycle natural-gas-fired generator, and accounting for transmissions losses over the grid leads them to calculate the “well-to-wheel” efficiency of the Roadster to be 1.14 km/MJ—double the efficiency of the Toyota Prius.
Tesla then tackles the question of hydrogen fuel-cell vehicles (FCV) fuel cars, deriving a theoretical efficiency for an FCV fueled with hydrogen produced by steam methane reforming of 0.85 km/MJ.
Theoretical efficiency of battery-electric and fuel-cell vehicles. Click to enlarge.
This is impressive when compared to a gasoline car, though it is 32% worse than our electric car. But real fuel-cell cars do not perform nearly this well.
...The best fuel-cell demonstration car measured by the EPA is the Honda FCX, which gets about 49 miles per kilogram of hydrogen, equal to 80.5 kilometers per kilogram. We know that the energy content of hydrogen is 141.9 MJ/kg, so we can calculate the vehicle efficiency to be 80.5 km/kg / 141.9 MJ/kg = 0.57 km/MJ.
...When we calculate the well-to-wheel energy efficiency of this Honda experimental car, we get 0.57 km/MJ x 61% = 0.35 km/MJ, not even as good as the ordinary diesel Volkswagen Jetta, let alone the gasoline-powered Honda Civic VX or the Honda Insight hybrid car.
However, some proponents of hydrogen fuel cells argue that it would be better to produce hydrogen through electrolysis of water. The well-to-tank efficiency of hydrogen made through electrolysis is only about 22%, and the well-to-wheel energy efficiency of our theoretical fuel-cell car would be 2.78 km/MJ x 50% x 22% = 0.30 km/MJ, and the well-to-wheel energy efficiency of the Honda FCX would be 0.57 km/MJ x 22% = 0.12 km/MJ, even less efficient than a Porsche Turbo.
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