Virginia Tech EE students are competing with 12 other schools to develop inexpensive inverter technology for ultra-clean, off-the-grid residential electricity.
|Chris Smith, leader of Tech's Future Energy team, tests a 1/3 scale inverter system to convert low voltage, high current fuel cell power for an off-the-grid house. The team's goal is a full-scale system that is 5 percent of the cost of off-the-shelf inverters.
The team of graduate and undergraduate students is designing and building a prototype system that will convert dc power from a solid oxide fuel cell into 5 kW ac electricity to run a household that is not on a utility grid.
The target costs of the system are less than $40 per kW with 90 percent efficiency. Off-the-shelf inverters are typically $1,000/kW, according to the team's advisor, Jason Lai. "Efficiency is emphasized," he said, "because less fuel will be consumed and fewer fuel cell modules will be needed a cost savings for the entire system."
The problem is one of three options for the 2003 Future Energy Challenge, which is seeking to introduce design innovations in distributed electricity generation systems. The competition is sponsored by several IEEE societies, the U.S. Department of Energy, the U.S. Department of Defense, and several national laboratories and centers. Total prize money of at least $100,000 will be awarded for the schools with the most cost-effective designs that meet or exceed the cost targets and have fully functional prototypes.
Virginia Tech's team was a finalist in the 2001 Future Energy Challenge and won the $9,000 Performance Award. "Last year we were dealing with less power," said Chris Smith (G), team leader. "This year we need the same size, the same efficiency and twice the power as last year."
High Current, Low Voltage Input
The team faces significant technical, cost, and labor challenges. The power system must be able to provide a continual 5 kW of 120 V and 240 V household power and accommodate a 10 kW peak load. The fuel cell output is not enough for a 10 kW load, so batteries will be used as a buffer. The power input from the fuel cell is from 22 to 50 volts and around 225 amps.
"Usually when you deal with that much power, you have more voltage," Smith explained. "It's more typical to have higher voltage and lower current. The fuel cell, however, is cheaper to manufacture with high current and low voltage." This complicates the team's design goal of 90 percent efficiency, he added, because higher currents generate more loss from the wires and traces.
Parallel Design to Cut Resistance
The team's strategy is to use three or four switches in parallel, to lower the resistance and the losses. "Instead of one expensive switch, we're using several cheap devices," he said. This is expected to decrease the cost of the power semiconductors, however, it makes the control and supporting circuitry more complex.
"Our overall cost should be cheaper, but the engineering is harder," he said. "Early on, we made this decision to take the hit on engineering, since you only pay for that once, whereas you pay for the semiconductors for every system manufactured."
The high current also presents challenges with heat, Smith acknowledged. "We're determined to keep a high efficiency, so we need to keep the power semiconductors as cool as possible." Additional technical issues include packaging and control.
"Control will be a tough challenge," he said. "We have a number of independent devices that must work together." The system must react quickly to changes in the load as residents in the home turn appliances on and off. "We need our system to react dynamically to maintain the power output so that there is always enough available." Another challenge for the control system is an expected time crunch, since much of the testing and iterations cannot be done until the unit is actually built, he explained.
Recruiting for Full Scale Building
The team also faces labor challenges. About 20 students are on the team, half undergraduate and half graduate. As an extracurricular project, Smith explained, the labor is basically volunteer. "Keeping people motivated and meeting deadlines becomes an issue when you're working with free labor," Smith said.
With the initial system design work complete, the team is recruiting students at all levels for the building, subsystem design, and redesign phases. A small prototype has been built and the team is scaling up to a full power system. "Many people say that if it works for lower power, it can easily scale up," Smith said, "but our experience is otherwise. We are going to build for the full 10 kW and meet every spec of the competition."
The bottom line will be getting the whole thing to work," he said. "Last year, nobody's worked perfectly. Either a system was not full power, but it worked all the time, or it was like our system, which was full power, but failed the last test. Cost is a factor, but the one that works all the time will win the competition. So, in the end, when we might be out of time, if it's more expensive but it works, that's what we will choose."
For more information, to join the Future Energy team, or to donate funds or materials for the team, please contact Chris Smith (firstname.lastname@example.org).