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Sustainable Energy

Engine Reinvention

Novel engine designs could help meet our growing demand for energy.

Today, the U.S. electrical grid delivers power from fossil-based fuels to the end user with an average efficiency of just 33 percent. We need to improve that not just for the sake of the environment but to find the additional 214 gigawatts of electrical generation capacity that the Edison Foundation says we’ll need by 2030.

Most of today’s innovation for higher-efficiency power systems focuses on new technologies such as solar, wind, and fuel cells. Work in areas such as improving the reaction rates at fuel-cell cathodes and lowering manufacturing costs for solar cells has advanced those technologies significantly, but they still need a lot of work to become cost-effective. In the meantime, the bulk of our power generation comes from more established technologies such as conventional reciprocating engines and turbines. Gas and steam turbines, such as those in power plants, typically operate at power scales greater than 10 megawatts, while generators with reciprocating engines similar to those in cars usually operate at lower power. Improving conventional reciprocating engines could combine their existing benefits such as high reliability and low cost with significantly higher efficiencies and lower emissions.

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One way to improve the efficiency of conventional engines is to modify existing designs. For instance, new technology for the piston rings that seal in high-pressure gases could lower frictional losses as the pistons move inside the engine cylinder; adjustments to valve timing could reduce the energy that escapes with an engine’s exhaust. However, engine designs are already approaching the theoretical limits of their current architectures. Calculations tell us that a naturally aspirated conventional reciprocating engine can have a maximum efficiency near 45 percent, and such engines achieve near 35 percent in practice. Reaching efficiencies of 50 percent or greater in practice requires completely new engine architectures with theoretical efficiencies greater than 60 percent.

That’s an approach we are taking, by starting with the fundamental thermodynamics at work in an engine. Models suggest areas that we can optimize to reduce energy losses, and that can point to new architectures. Engines that operate at higher compression ratios, for instance, reduce both combustion losses and the energy lost in exhaust. The resulting engines will be different from those we use today and will require significant development, but progress should be faster than it will be for less established new energy technologies.

Shannon Miller is a cofounder and the CEO of EtaGen, a startup working on high­-efficiency engine designs for power generation.

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