Catch the sun

Pioneering photovoltaic inverter with a silicon-carbide based field-effect transistor could improve efficiency. Siobhan Wagner reports.

German researchers believe they have created a photovoltaic inverter that will make better use of the sun’s energy.

Inverters change direct current from a solar cell array to alternating current, so that energy can be fed into public utility grids.

According to the Fraunhofer Institute researchers who designed the inverter, the device has achieved a record efficiency rating. And greater efficiency enables photovoltaic systems to yield more energy.

The inverter uses a silicon carbide-based metal-oxide-semiconductor field-effect transistor (MOSFET) — the first time such a transistor has been applied to a photovoltaic application.

Bruno Burger, head of the power electronics group at Fraunhofer, said the device had an efficiency rating of 98.5 per cent — 0.5 per cent higher than the previous best.

‘Point five per cent might not look like much, but if an inverter has 98 per cent efficiency that means 2 per cent loss and we now have 1.5 per cent loss,’ he said. ‘If an inverter has a power rating of 5kW, it would normally lose 100W and we’ve now reduced that to 75W.’

Silicon carbide components switch faster and have less power loss than traditional silicon-based transistors. The Fraunhofer researchers demonstrated that their inverter dissipates 30 to 50 per cent less power.

‘As far as we know the US manufacturer, Cree, is the only company that can make silicon-carbide-based MOSFETs,’ said Burger. ‘All other companies have either junction field effect transistors (JFETs) or bipolar transistors of silicon carbide.’

Burger said MOSFETs are the best devices for power electronics because they are compatible with insulated-gate bipolar transistors (IGBTs). These are used to switch electric power from AC to DC, or DC to AC, in many modern appliances such as electric cars and air-conditioners.

Silicon carbide is mainly applied in white LEDs, which are now a popular power-saving lighting solution. While silicon carbide diodes have been in use for some time, MOSFETs, which are necessary for the power stage in inverters, have not been available until now.

Although silicon carbide is more expensive to use than traditional silicon, Burger said the advantage is that the Fraunhofer inverter uses smaller chips. ‘It could be the same price to manufacture inverters with silicon carbide because you need only a quarter of the chip area compared to traditional silicon.’

During testing, the inverter with silicon carbide components also set a performance record across a wide range of power output values, which is essential if an inverter is to produce the highest energy output over time.

The Fraunhofer team tested the efficiency of photovoltaic inverters for single-phase grids used for household electric and triple-phase grids used in industry. They set a record for efficiency with a single-phase inverter with a nominal power rating of 5kW. They also increased the efficiency rating of a three-phase inverter with a nominal power rating of 7kW from 95.1 to 97.5 per cent.

The researchers constructed their inverters by customising the components and integrating them into existing photovoltaic inverters.

Burger said the team plans to optimise inverter circuitry specifically for silicon carbide. It believes this will achieve even greater efficiency.

‘Special inverters for silicon carbide will use higher switching frequency to save weight, space and costs of the passive components, especially of the inductors, which are very heavy and expensive,’ he said.