New material paves the way to more effective energy

In the basement of a building at kth, there is a square box lying on a table. This is a converter that converts direct current to alternating current and where traditional silicon transistors have been exchanged for silicon carbide ones. The box is placed unobtrusively in the dark. Undeservedly so, you might think, considering the attention this device has received in the research community.

Silicon carbide has long been considered an effective material for high-voltage transistors. But it is difficult to produce a good substrate for transistors made from silicon carbide. If you’re not in complete control of the technical process, defects can creep into the material, making it impossible to build a transistor from it. However, several manufacturers have recently succeeded. The new transistors differ from their silicon predecessors in several ways, and it’s not as easy as simply swapping the old components for new ones.

A number of companies and research labs have worked on these issues internationally. But it was Hans-Peter Nee, Professor of Electrical Energy Conversion and his team at kth ee and the Swedish Center of Excellence in Electrical Power Engineering who discovered the answer.

“We haven’t exactly achieved a miracle. But we can get miraculous results by using the available technology in a smart way,” Nee says.

The use of silicon carbide in transistors results in significant energy savings. About three per cent of energy is lost with silicon transistors compared to only 0.3 per cent with those made of silicon carbide. Furthermore, the material can withstand higher temperatures – more than 250C compared to silicon that tolerates only 125C. This reduces cooling need considerably, which can have significant impact on automotive applications, for example, where weight and volume play a critical role. Energy is lost each time this mass of material is accelerated and decelerated. By minimizing energy loss, smaller cooling systems can be used.

This technology can also be used for larger scale converters. In the future, we can expect to use them in high-voltage direct current transmissions in new, smart super grids. In the electrical grid, energy gains are extremely important. Lower energy loss not only saves energy, but can also realize lower temperature variations and consequently improve reliability.

“What’s good about this type of super grid is that it can help cope with large power variations when we start to use more sources of energy, which are less predictable in production terms, such as sun, wind and water,” Nee explains.

Handling the transition to hydropower located several hundred kilometres away during periods when it’s less windy or when the big wind farms don’t produce enough energy requires an electrical grid that can handle variations. But expanding today’s electrical grid isn’t done in the blink of an eye. In Central Europe, much of the land area is built up, making it difficult to lay new high-voltage cables. They simply take up too much space. Even sparsely populated areas in Sweden have the same problem. A better solution would be to lay underground cables that occupy less space and build converters fitted with silicon carbide transistors.

“This enables a smart, new electrical grid with converters that are completely controllable and can direct the power in cables in real time down to the millisecond level,” Nee adds.

This future “super grid” will also of course create challenges. For example, when something is this effective, what happens when it breaks down? It is very important to be able to disconnect faulty parts using fast circuit breakers. Here, silicon carbide can become a key technology. Furthermore, the missing energy from the faulty link must be replaced by new fresh energy from energy storage units using power electronics. And here again, silicon carbide is power electronics may provide superior technology.

There is some way to go before the use of silicon carbide in transmission networks becomes a reality. So far there is greater probability of getting a reduced output from a large chip – which is demanded in a grid – than a relatively small chip, which is being developed today. This makes it problematic to build really large components. But as material and processes become better, they will also become lighter. Price-wise, silicon carbide is more expensive than silicon. However, with the reduced cooling requirements in the system, silicon carbide ultimately leads to transmissions that are both cheaper and better.

In the first stage, the new material will be used in engine controls and electric power supply. In all likelihood the next stage will involve electrical transmission.

“I’m hoping that in this area Sweden can establish an international position. Asia has become the main producer of silicon electronics but, in the area of silicon carbide, we can establish a commercial operation in Sweden where we build components with this material. Sweden is, and has been, at the forefront in this area. Hopefully we can keep our leading position in the coming years,” Nee concludes.