Some progressive car manufacturers are looking into using steam to create electricity in forthcoming generations of hybrid cars and lorries. Honda, for example, has just released details of a new prototype hybrid car that recharges its battery using a steam engine that exploits waste heat from the exhaust pipe.
Typically, cars only convert about a quarter of the energy produced during combustion into work, with the rest being lost as heat. Honda has managed to increase this efficiency by 4% to nearly 29% by using some of this lost heat to generate electricity. Their heat-recovery system is based on the Rankine cycle, which is also the system used in most steam-driven power plants. First, heat from the car’s catalytic converter is used to boil water. The high-temperature steam (400–500 °C) produced then turns an electric generator, before a condenser finally cools the steam back into water.
Honda researcher Kensaku Yamamoto says that under normal driving conditions, the steam system recovered three times as much electric power as the hybrid’s regenerative braking system. But Honda says that this 4% increase is not sufficient to warrant commercialization.
Other companies are also looking at steam. For instance, BMW is working on a steam-based unit that generates additional mechanical power, rather than electricity. In lab tests, their so-called Turbosteamer reduced fuel consumption by as much as 15%. It may be some time, however, before recovery of waste heat reaches the mass market, because typical car drivers would probably not make a big enough saving on fuel to justify the extra several thousand dollars that these systems would presumably add to the price of a vehicle.
This is not true for long-distance lorry drivers, however, who often spend over $100,000 per year on fuel. Several diesel-engine companies are researching ways of recovering lost heat, with interest spurred by emissions reduction and rising fuel prices. The Rankine cycle can convert up to 20% of the wasted heat into useful energy.
Christopher Nelson from the engine company Cummins Inc. says that radiators have to change considerably to cope with the 80% heat that cannot be recovered. But his company plans to have a full working prototype by mid-2009, and hopes to make the system available to customers by 2013.
