Stick your hand behind a car exhaust. OK, don’t: It’s really hot. The burning sensation you feel is wasted energy. Most car engines have a mechanical efficiency of around 20 percent, climbing to 40 percent or so at optimum efficiency. That means a lot of waste: 36 percent of energy is lost to the radiator, and another 38 percent goes out the tailpipe.
But with car makers like Honda and BMW exploring heat reclamation, and recent advances in thermoelectrics and nanotechnology, this kind of inefficiency may soon be a thing of the past. Honda recently announced that it’s exploring the use of a heat-reclamation technology known as a Rankine cycle co-generation unit to reclaim some of the waste.
Alas, Honda’s presentation only improves the engine’s thermal efficiency by 3.8 percent; a far cry from the 80 percent that’s wasted. So we need something better.
Well, many researchers at universities and in car maker R&D centers have actually been hard at work on this problem for some time. Back in 2005, BMW announced plans to recapture wasted heat energy, claiming a 15% improvement in fuel efficiency from cars fitted with their Turbosteamer technology.
Cars are a great place to try and reclaim energy, since the differential between the engine (hot) and the air whizzing by (relatively cool) is high. But heat reclamation hasn’t been successfully implemented on a wide scale.
There are a variety of ways to convert heat into energy. One involves heat engines — mechanical devices like the Stirling engine. Others use thermoelectric materials to turn heat differences into electricity. But while the thermoelectric approaches are convenient — they have fewer moving parts, for starters — they’re generally less efficient, turning only 7 percent of the heat they use into electricity versus 20 percent for traditional heat engines.
More recently even greater advancements have appeared on the horizon. Spurred by high oil prices and breakthroughs in nano-level engineering, inventors are hard at work rethinking heat reclamation. Last February, University of Berkeley researchers combined organic molecules and metal at the nano level to generate heat from relatively cost-effective materials at smaller temperature differentials.
Back in November we profiled MIT Institute Professor Mildred S. Dresselhaus’ work in thermoelectric materials. And this January, super-soaker inventor and nuclear physicist Lonnie Johnson claimed a new approach that might reach as high as 60 percent efficiency under the right conditions.
More efficient engines should be the first step in transforming transportation, before electric vehicles and biofuels are seriously considered. If we can combine the car makers’ push for efficiency with advances in thermoelectrics and heat engines, we may be able to dramatically improve the efficiency of modern cars and cool off those tailpipes.