Alloys get into power

Power stations using materials that mimic those of commercial jet engines hold the key to reduced emissions and better use of natural resources, claim Cambridge University researchers.

According to Professor of Physical Metallurgy Harry Bhadeshia, use of improved alloys able to withstand higher temperatures will improve steam turbine power generation efficiency by 16 per cent, reducing carbon dioxide output by a similar amount.

Power stations use steel turbines to produce electricity, but can only achieve efficiency of just over 40 per cent. To improve this the turbines need to work at higher temperatures. However, over time the increased heat causes carbide and nitride molecules within the material to move together to make the steel coarser and weaker.

Much of the proposed turbine would be built using a nickel alloy, FT750dc, designed by Bhadeshia and his team. The material is only three times as expensive as the ferritic steel used at present, but can withstand temperatures of 750 degrees C.

‘British power stations currently operate at around 565 degrees C, and even the most technologically advanced Japanese plants only manage around 600 degrees C,’ said Bhadeshia.

Turbine materials must be very reliable as they would be expected to last for around 30 years under high stress while avoiding stopping and starting machinery to undertake repairs that would increase fatigue.

Materials used must also be able to withstand creep deformation and oxidation over time and be capable of forming a high-strength weld with neighbouring parts while remaining as cheap as possible.

‘Though they have a much shorter life, steel aeroplane engines operate at over 1,000 degrees C,’ said Bhadeshia. ‘However, they employ technologies such as special coatings that are not feasible for turbines as turbine components are huge and would be far too expensive to produce. We have therefore taken the aeroplane engine and removed all the expensive parts.’

In critical areas the alloy would be coupled with a more expensive heat-treated mechanically alloyed oxide material made from a mixture of titanium, aluminium, yttrium and iron that can operate at up to 1,000 degrees C, further increasing the turbine’s efficiency.

‘To keep the price of the turbine down it is essential to use the special material only when it is absolutely necessary,’ Bhadeshia said. ‘While the UK is using more gas as a fuel, countries such as China and India are building coal-fired stations. Reducing emissions in the developing world would make a great difference to carbon output.’

Bhadeshia is also working on materials able to withstand neutron flux for use in the International Thermonuclear Energy Reactor (ITER) fusion reactor programme.