Professors Charles Sullivan and Jason Stauth are brightening the future of solar energy.
By Kathryn LoConte Lapierre
From the midst of grassy fields to the peaked roofs of farmhouses and flat roofs of commercial buildings, photovoltaic (PV) installations are advancing over the landscape. Although solar power accounts for less than 1 percent of the country’s energy production, photovoltaics are expanding globally, driving the price of solar power down to a level that many people never thought possible. Optimistic predictions place solar on the threshold of being competitive with other sources of energy.
Photovoltaic installations consist of multiple solar panels, each of which contains strings of solar cells—photo diodes that convert photons into electrons. But you have to do something to the energy that is gathered on your rooftop to make it useful. After all, you can’t plug your toaster into a solar cell and get breakfast. That’s where power electronics comes into play.
“Power electronics is the glue that holds together all the different parts of an energy system,” says Sullivan. “It’s what interfaces between the solar panel and the grid, between the grid and the device that uses energy.”
PROFESSOR CHARLES SULLIVAN
Sullivan works on the inverters that convert DC to AC and drive power to and from the grid. “We want to make inverters as efficient as possible so that we don’t lose energy during that conversion process,” he says. “We want to make them inexpensive, small, light, and convenient to install.”
Progress in inverters is already making solar panels easier to use. Microinverters that handle about 200 watts have become an attractive alternative to standard inverters, which range from handling about 1.5 to about 300 kilowatts. “A typical residential installation would use one standard inverter, located in the garage or basement, connected to multiple solar panels on the roof by special DC wiring,” he says. “With microinverters, you put one on each of the solar panels on the roof. There is nothing else to install, and all of the wiring is standard AC wiring that any electrician can do. Additional advantages: each one can optimize the operation of that particular panel; the system is modular so you can add more panels easily; if something goes wrong, only one panel stops working instead of the whole system; and each panel gets monitored individually, so you know if there is a problem with one of them and you know exactly what the problem is.”
But microinverters still lose too much energy.
According to Sullivan, the magnetic components of inverters—both standard and micro—are the bottleneck. “They are expensive, large, and have a high power loss. They’re the most problematic components,” he says. “Integrated circuit companies are making electronics smaller and cheaper all the time, but this doesn’t include the components that are unique to power electronics.”
One focus of Sullivan’s research group centers on developing more accurate and sophisticated models around where power loss occurs in conventional magnetic components. “These are mostly wire-wound inductors and transformers with ferrite cores,” Sullivan explains. “Ferrite is a magnetic material that works reasonably well at frequencies up to about 1 MHz. Our optimizations include changing the geometrical parameters to get the best performance/cost ratio or performance/weight ratio for a given application, choosing the particular ferrite, and looking at the way the winding is constructed. At high frequencies, the ‘skin effect’ forces current to flow only in a thick layer on the surface of a conductor, and the related ‘proximity effect’ can multiply the losses by a factor of as much as 100. It is generally good to have the conductors in the winding have a dimension that is thin compared to the skin depth—the thickness of the layer that current flows in—but the dimension depends on the field configuration. Thin conductors inherently have high resistance, which means that it becomes necessary to connect many in parallel, but then electromagnetic coupling between them usually makes the current flow unequally or even in counter-productive ‘eddy-current’ loops. So we work on ways to configure conductors to get the benefits of thin layers without these problems, and at low cost. We also develop tools that designers can use to develop better components more easily or come up with a new configuration.”
Another focus of Sullivan’s group is developing alternative magnetic components. “We’re working in Thayer’s micro-fabrication lab to deposit new magnetic materials that have lower loss at high frequencies,” Sullivan says. “The main material we’re working with is cobalt zirconium oxide. We are also exploring related materials with different magnetic metals to build a portfolio of materials with specific characteristics for different applications.”
“Our holy grail would be to approach having 100-percent efficiency, zero size, and zero cost, but obviously we’re never going to get to any of those. We’ll never be finished in that sense,” Sullivan says. However, new semiconductors, such as gallium nitride and silicon carbide, are on the horizon. “Those materials have attractive properties that could make them very good for making very efficient, very high-frequency power electronics,” he says. “If people make them with good performance, that could allow dramatic improvement in the size and efficiency and eventually the cost.”
In the nearer term, Sullivan would like to see power electronics play a greater role in the power grid by replacing conventional transformers. “These transformers are giant, heavy, very simple devices—just steel and copper. There’s no electronics in there at all right now. A lot of people would like to replace those transformers that are running at 60 hertz with a power electronics circuit running at a much higher frequency,” he says. The conversion would be costly, he admits, but would boost efficiency and control for a smarter grid. “Being able to control the grid to make sure that you have the right power flowing to the right place at the right time is going to become more important as we have more renewable energy,” he says.
PROFESSOR JASON STAUTH TH’00
“The goal is to make solar more efficient, to improve the energy capture of solar installations, and at the same time reduce the cost,” says Stauth, who earned his B.E. at Thayer in 2000, then worked in the private sector and completed a Ph.D. before joining the Thayer faculty last year.
“Each solar cell in a panel gives you 0.5 volts, which isn’t useful since your wall circuit is running at 120/240 volts. A central inverter may require 500 volts to operate efficiently. Solar cells are wired in series to achieve higher voltage. In a typical system there may be up to 1,000 cells stacked in series to reach the 500 volts for the central inverter. Each panel may have 60 cells stacked in series and there may be 10 to 20 panels connected in series,” says Stauth.
“We try to operate the solar panel at a very specific operating point where it is achieving the maximum power. But you can’t simultaneously run cells that are connected in series at their optimum operating point,” he says. Like batteries in a flashlight, if one is bad, it limits the energy production of all the rest. If one solar cell is in shade or is coated with dust and debris, it will block the flow of current. “You’re limited to the worst case because all the cells have to operate at the same current. Current mismatch reduces energy production of the entire string of modules.”
One inexpensive solution to mismatch, says Stauth, is to put bypass diodes in the junction box behind each PV panel to let current flow around underperforming strings. “The problem is that if bypass diodes turn on, they throw away all the power that could have been available in the underperforming string of cells,” he says.
Stauth, a cofounder of the company QVSense, which was acquired by Solar Semiconductor, has developed a different approach: a converter that lets all the strings of cells operate independently. “Our converters connect to strings of cells through connections that are made in the junction box,” he says. “They move charge in parallel with the strings of cells so that if one string of cells has less current available, our converters can ‘shuffle’ it around the underperforming string. If we didn’t do this, the current of the entire panel, and the entire string of panels, would have to be reduced to the worst performing string of cells.”
The parallel architecture brings advantages: converters handle only the mismatch power, can turn off with no mismatch in the system, and the devices are exposed to only a fraction of the total voltage stress. This allows for “higher efficiency, near zero insertion loss, and higher reliability,” says Stauth. “It’s a way to optimize power at the sub-module level, around the terminals that are normally connected to bypass diodes. People have built converters that let each panel operate independently. But this is a solution that lets regions of each panel operate independently so they can operate at their maximum power point, regardless of how much variation is in the system. We are developing a solution that in the future will let each cell operate independently. We hope to do this by integrating all of the electronics on a single silicon chip that can be embedded in the PV laminate. The solution will leverage Moore’s Law, scaling to achieve exponential reductions in size and cost.”
“Power electronics is one piece of the effort to get solar to grid parity, to get the cost of PV below the cost of coal-based electricity. Reducing the cost of solar panels, improving their energy efficiency, improving the installation process, reducing overhead for systems integrators, streamlining the regulatory framework, and even little factors, like where the wiring goes—all these things are important as well. Overall it’s getting there. Power electronics is more exciting now than it has been in many decades.”
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