Solar inverter sets efficiency record

Marie Freebody, Contributing Editor, marie.freebody@photonics.com

After the solar generator, the inverter is the next most important component in a grid-linked photovoltaic system. And a new solar inverter boasts a boosted efficiency of 99 percent, up from 96 in others.

Improving the way we harvest sunlight and convert its energy into electricity is the Holy Grail of solar cell manufacturers. But what happens next is also important: efficiently feeding the electricity into the power grid.

This is the task of the inverter. Its job is to convert the direct current generated by solar modules into grid-compatible alternating current. However, standard inverters lose at least 4 percent of the electricity generated by solar panels.

That is, until Dr. Heribert Schmidt at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg, Germany, realized that a simple modification to the inverter design could bring this loss down to just 1 percent.

A tiny tweak

Single-phase feed inverters consist of three essential parts: the buffer capacitor at the input, which provides intermediate storage for the direct current from the solar generator; the inverter bridge with four semiconductor switches that “chop up” the direct current by rapidly switching on and off; and the inductor at the output, which converts the alternating current into a perfect sinus current.

The solar inverter devised by Dr. Heribert Schmidt is now being used in Sunways’ NT series photovoltaic inverters. Image courtesy of Sunways AG.

In standard inverters, the current shuttles between the output inductor and the input capacitor, and the losses occur during these return flows. Schmidt sidesteps the problem by breaking the connection between the input capacitor and the inductors at defined intervals, using two additional semiconductor switches, while allowing the inductor current to continue flowing down.

When Schmidt first came up with the HERIC topology, as it is named, in 2002, the losses of commercial inverters were in the range of 4 to 8 percent, which means that peak efficiencies of 92 to 96 percent were common. With the HERIC topology, the losses were cut to less than 2 percent, leading to peak efficiencies of more than 98 percent.

But with the advent of semiconductor switches made from silicon carbide, Schmidt was able to further improve peak efficiencies to more than 99 percent in 2009, setting a new record.

“Until the invention of the HERIC topology, inverters either had a very good efficiency and a bad EMC [electromagnetic compatibility], or vice versa,” Schmidt said. “With HERIC, the two important characteristics have been combined in one topology.”

As if boosting the efficiency were not enough, the reduction in losses means that less heat is dissipated. As a result, heat sinks can be reduced, which means less material is needed in production and the weight of the inverters is reduced. Not only that, but the average temperature will decrease, improving reliability and lifetime.

The HERIC topology is already being used by Sunways, a photovoltaics company based in Konstanz, Germany, and Schmidt says that further talks are under way with other solar cell makers.

“The topology had been exclusively licensed to the company Sunways in Germany until summer 2010,” he said. “Sunways developed the NT series of PV inverters, which has been and is actually sold very successfully all over Europe. Now, we are able to provide further nonexclusive licenses, and we are already negotiating with a number of companies.”