As far as energy is concerned, efficiency is one of its least alluring facets. It has a far lower profile than renewables and is much less talked about than nuclear. But improving energy efficiency has the potential to make a huge contribution to the low-carbon economy. Much of the current infrastructure is outdated and inefficient as power is lost at critical junctions between the generating stations and the devices that use it. The handling and converting of power from gigawatts to milliwatts can be done efficiently using power electronics.
This type of electronics has the potential to transform the infrastructure through which our energy is delivered. It can reduce losses, making the energy that is produced go further, therefore improving efficiency.
The resulting savings can be considerable, says Professor Bill Drury from the University of Bristol. When used in drives for industrial electric motors, for example, power electronics can reduce the energy used by 30-40%. Such motors account for 60% of all electricity consumption, and power electronic drives could be fitted in 50% of applications.
This means that just by using this technology, which is already available, total energy consumption could be cut by 9%. Drury says: “There is probably a level of ignorance that these savings are possible.”
The government is trying to change that. It has launched a national power electronics strategy to promote greater awareness of the energy-saving opportunities. Mark Prisk, business and enterprise minister, says: “Power electronics systems tend to be highly specialised with high added-value, so there are real chances to grow the UK manufacturing base in this field.” The power electronics market is worth £135 billion and is growing at a rate of 10% every year.
The field will be critical for developing a digitally enabled “smart grid” to make our power supply stable and reliable. It will also be essential for converting the energy generated from wind turbines, photovoltaic systems and tidal generators into a fixed voltage suitable to connect to the grid.
Drury says that there are three materials suitable for power electronic devices, and each brings a little of its own benefit to the table. The first, silicon carbide, is suited to applications that demand high temperatures. The second, diamond, is suited to very high-voltage applications, and the third, gallium nitride, will bring higher efficiency.
Professor Phil Mawby works with silicon carbide in his lab at the University of Warwick and says that it is the most interesting of the materials available. It is very hard and has been used in sandpaper and grinding wheels for years. Many blue LEDs are also made of silicon carbide.
Rectifiers and semiconducting diodes made of silicon carbide have been available for 10 years. But in recent months, semiconductor switches made of silicon carbide have come to the market.
Mawby says that, compared to semiconductors made of silicon, those made of silicon carbide can withstand much higher voltages. The material has a higher field strength too, which means less of it can be used to do the same job.
Mawby explains: “It allows you to build very high-voltage systems for power transmission that you can’t build at the moment.” Using silicon carbide to build these systems will reduce the space that these devices take up. “At a power system level it could make something the size of a house the size of something that would fit in a room,” says Mawby.
Semiconducting switches made of silicon carbide can be used in niche applications, such as pulsed power for X-ray sources and within machines used to drill oil boreholes, where it gets very hot. The material retains its semiconducting properties at temperatures over 600°C, whereas silicon loses these properties at 175°C.
As a thermal conductor, silicon carbide is almost as good as copper and is three times better than silicon.
In aerospace, where every ounce counts, the material will also be useful, says Mawby. Silicon carbide is not damaged by radiation and it will reduce the weight of aircraft electronics, allowing more passengers on board. Aircraft systems controlled by electrical power rather than hydraulics and pneumatics will improve efficiency and functionality.
Potentially the biggest market for silicon carbide is in electric vehicles, says Mawby. The technology can control the efficient flow of energy from the battery to the motor and back again. As systems that include silicon carbide components can operate at higher temperatures and do not dissipate as much heat, in-built cooling systems become redundant. “You can get rid of all the radiators, pumps, and the pipework and the system becomes a lot simpler and more reliable,” explains Mawby.
“The Japanese government are investing hundreds of millions of pounds into silicon carbide research, primarily aimed at the automotive sector,” he adds.
But creating a high-quality pure crystal of silicon carbide is tricky. When reacted together, silicon and carbon form silicon carbide but further work is required to make a single crystal. This is done using a process known as sublimation growth, which starts with a small seed crystal of silicon carbide. Additional crystalline silicon carbide is then heated up to create a gas that condenses on to the seed material, and a crystal of silicon carbide grows.
But the growth conditions for this process are highly sensitive. “The problem is that there are over 200 different types of crystal that you can grow, and you can easily change from one polytype to another if you’re not careful.”
This makes it difficult to produce large wafers of the material that have few defects. At the moment only 4in wafers are available, but industry would prefer 6in wafers, explains Mawby. Research is ongoing as to how to improve this process.
Silicon carbide is also more expensive than silicon, by at least an order of magnitude. So at the moment it can only be used when there is an absolute need for it. But Mawby is confident that the price will come down in time. “People always say that a new technology is too expensive. It will become an important technology in electrical energy conversion,” he says.
He believes that we are entering the golden age of power electronics and that the UK is poised to take a leading role internationally. “In this area we are still world leaders,” he says. “It sometimes depresses me, the tales that we hear on the news about the state of UK manufacturing. There are many companies in the UK that manufacture high-volume, high-pay products – we just don’t know about them.”