An internal combustion engine similar to the one in your car could play a key role in making electrical generation far more efficient.
A University of Wisconsin-Madison team led by Mechanical Engineering Assistant Professor Sage Kokjohn is developing new technology that would enable smaller, distributed electrical generation systems that are roughly twice as efficient as conventional fossil fuel-based power plants and provide more environmentally friendly power.
Additionally, this technology could offer an economical way to support a growing number of renewable energy sources being added to the power grid.
Currently, most of the electricity that’s fed into the power grid is generated by large power plants burning fossil fuels. But the efficiency of these plants is around 36 percent, and by the time the electricity reaches a power outlet in your home, the efficiency drops to 34 percent due to energy losses over transmission lines.
Kokjohn is aiming to achieve a 70-percent fuel-to-electricity efficiency by integrating two technologies—an internal combustion engine and a solid oxide fuel cell—into a hybrid system that capitalizes on the unique characteristics of each. A $1.7 million grant from the U.S. Department of Energy’s Advanced Research Projects Agency-Energy (ARPA-e) program is supporting the research.
“There are certain characteristics of the way that fuel cells operate, such as the partially consumed fuel that exits the stack, that’s actually very beneficial for these advanced combustion engines,” Kokjohn says. “So by coupling these two devices, we can take advantage of some synergies between them and achieve very high efficiency. It’ll be very exciting to show a pathway to 70-percent electric efficiency.”
Fuel cells are devices that generate power through an electrochemical process instead of by combustion. Certain types of fuel cells can use natural gas as fuel and directly generate electricity with very high efficiency.
However, these fuel cells only consume about 75 percent of the fuel’s energy, and the remaining energy in the gases that leave the fuel cell is wasted. Kokjohn sees an opportunity to put that unused fuel to work to boost efficiency.
And that’s where the engine steps in.
Kokjohn’s hybrid system will direct the exhaust from the fuel cell into an advanced compression ignition engine, allowing the engine to generate additional power from the fuel cell’s leftovers.
But because the fuel that comes out of the fuel cell is of very low quality, a conventional combustion engine isn’t able to effectively burn it, Kokjohn says. To overcome this challenge, his team is investigating several advanced combustion strategies for enabling the engine to use that low-quality fuel.
The engine provides the system with another unique advantage: the ability to easily adjust to shifting power demands. This capability is especially desirable as more renewable, and highly variable, energy sources like wind and solar are added to the grid, Kokjohn says.
“The amount of solar energy that’s being added to the grid changes a lot based on the weather,” he says. “So you’d like to be able to shut down the non-renewable energy source when the sun is really shining, and then come back up when the sun goes under a cloud.”
Kokjohn’s hybrid system will offer just this kind of responsive flexibility, complementing intermittent renewables like wind and solar by filling in the gaps when the sun isn’t shining and the wind isn’t blowing.
“Our system would provide significant benefits not only from an energy perspective but also from an economic perspective,” says Kokjohn, noting that it’s very challenging and costly for large power plants to respond to changing power demands.
Additional benefits of the technology include a substantial reduction in greenhouse gas emissions, since a higher efficiency results in lower emissions. “By going from about 36 percent efficiency to 70 percent fuel-to-electricity efficiency, we’d be cutting greenhouse gas emissions in half compared to typical fossil fuel power plants,” Kokjohn says.