SiC modules promise grid efficiency boost

We d all be better off with a world economy that is less dependent on oil for power generation - even President Bush agrees, recently proposing a 22% increase in funding for clean energy research. While this includes nuclear energy, renewable sources like wind and solar power are of course central to the drive and will in time account for a much larger proportion of grid power.

This is all well and good, but saving the planet is clearly a complicated business and innovation at the lower-profile "nuts and bolts" level of the electrical grid is just as important. One difficulty with renewables such as wind and solar energy is that these sources produce a direct current, which then needs to be switched to the alternating current used by the grid. This power conversion requires inverter modules, which are currently based on silicon devices.

The problem with these inverters is that they operate at "only" 90-96% efficiency, depending on the application. While high, this can be further improved by SiC to between 95 and 98%. That might not sound like much, but across a grid it could make a huge impact.

A key proponent of SiC-based inverters is the utilities company Kansai Electric Power (KEPCO), which supplies electricity throughout much of Japan. As well as running dozens of conventional power stations and nuclear facilities, KEPCO is keen to develop renewables and has a 21% stake in the wind-power generator Eco Power. Yoshitaka Sugawara manages the SiC program at KEPCO. "Tremendous energy savings could be realized by switching from silicon to SiC inverters," said the executive researcher.

Now, following an eight-year development program with SiC chipmaker Cree, KEPCO has built a three-phase inverter module rated at 100 kVA. Until recently, the best power rating that had been achieved by the team was just 12 kVA. "Typically, we need 300-600 kVA inverters," Sugawara told Compound Semiconductor. "We can get to 300 kVA by connecting three 100 kVA inverters in parallel, but we will have to increase the output power of the inverter to reach 600 kVA."

Each inverter module uses two SiC devices - a commutated gate turn-off thyristor (SiCGT) to act as a switch, and a pin diode. Thyristors, also known as silicon-controlled rectifiers, have an anode, a cathode and a gate. Usually, the gate current extracted from a thyristor is much smaller than the current flowing between its anode and cathode, but in the commutated device the two are equivalent. According to Sugawara, this means high-speed, low-power-loss switching.

The 100 kVA performance of the latest inverter was met by connecting six of these modules together, and it operated with a pulse-width modulation frequency of 2 kHz. The next step for the collaboration will be to scale up the power capability so that KEPCO can use SiC inverters in a wider range of commercial applications. "This will be done by increasing both the SiC device chip size and the current density," explained Sugawara. Currently, the SiCGTs measure 8 × 8 mm, their large size having been enabled by a major reduction in SiC crystal defect density and improved epitaxy. Increasing the chip area will require further defect reduction and a more homogenous flow of current.

Although additional development is required, Cree s John Palmour says that the technology is moving closer to the point at which - if deployed widely - it should significantly reduce the need for new power stations and, in turn, decrease carbon dioxide emissions.

Palmour added that demand for Cree s SiC Schottky diodes for inverter designs was already being fielded by a number of European customers. "A few are DC for battery storage, but most are grid-connected," he said. The European demand stems largely from a European Union directive that pays around half a euro per kilowatt hour for solar-generated power.

Though these solar designs have a much smaller power requirement than the KEPCO inverters, larger systems can be built by connecting modules in parallel. These systems boost the variable voltage input to the grid that the solar panels produce, with the SiC Schottky diodes again yielding greater power efficiency than their silicon equivalents.

Although Palmour won t make a prediction on what the total addressable market for SiC-based inverters could amount to, he does believe that the market for power devices based on the wide-bandgap material will be "very large". Schottkys from the likes of Cree and Infineon, which is soon to release a new version of its SiC diodes, are already widely used for power factor correction in computer server switch-mode power supplies.

While wide-bandgap materials have received a lot of funding to exploit their potential in military applications, the knock-on effect promises to be altogether more environmentally friendly. And any reduction in oil dependency must be encouraged on both environmental and political grounds. Just ask George W Bush.