Graduate student Seung Ryul Moon demonstrates the "V-6" converter (foreground) and a fuel-cell simulator - one of just a handful of such testbeds existing in the United States. Moon is on Jason Lai's team at the Future Energy Electronics Center, which has developed a 97-percent efficient DC/DC converter that boosts fuel-cell system efficiency nearly 20 percent.
New Virginia Tech electronics technology boosts fuel-cell-system efficiency by nearly 20 percent, moving the zero-emissions energy source closer to cost competitiveness with today's automotive engines and power generation equipment.
The U.S. Department of Energy (DOE) goal is to reduce the cost of fuel-cell systems in high-power applications (3-10 kW) to $400 per kW by 2010. Today’s internal combustion auto engines and combustion-turbine power generators average $500 per kW. When the DOE Solid State Energy Conversion Alliance (SECA) Program was started in 1999, solid-oxide fuel cells ranged between $4,500 and $6,000 per kW.
While other teams work on materials development, fuel processing and reforming, modeling, controls, and manufacturing, researchers in ECE’s Future Energy Electronics Center (FEEC) are tackling issues of converting fuel-cell output into electricity usable by household and business devices. “This is a difficult problem,” said Jason Lai, FEEC director. “Fuel cell output is low power DC, unpredictable and slow, whereas today’s devices typically require consistent, high-quality, high-power AC voltage.”
The first step in converting the power involves stepping the power up from 22 V DC to 400 V DC, before converting to AC in the second step. The two-step process typically resulted in a total loss of 20 percent of the fuel-cell energy output, plus a ripple current feedback that triggered premature shut downs.
The Virginia Tech team developed a DC to DC converter that operates at 97 percent efficiency and cuts the ripple current from 30 percent to 2 percent. The team used soft-switching techniques to reduce switching loss and split a traditional single phase into three phases. The resulting three-phase/six leg converter — referred to as the V-6 — allows the use of fuel cells for high-power applications such as providing energy for an entire household.
The converter boosts net power output and helps downsize both the fuel cell stack and its supporting electronics and the reduced ripple current eliminates the need for the costly, bulky capacitors or additional converters that are customarily used. DOE SECA studies indicate that each 1 percent improvement in converter efficiency can reduce fuel cell stack costs by $5-$10 per kW.
The team’s next effort is improving the step-two DC to AC inverter to squeeze out still more fuel-cell system efficiency. For more information, please see the Future Energy Electronics Center website at www.feec.ece.vt.edu and the DOE SECA website at www.seca.doe.gov.