Higher-temperature heat exchanger material improves efficiency of conversion in concentrating solar power plants
Although the majority of solar power around the world is generated by photovoltaic panels, this is not the only method for generating electricity from the sun’s energy. Concentrating solar power is an adaptation of the oldest way of harnessing our star: the same way that greenhouses work. Large arrays of mirrors and lenses focus the sun’s rays onto a reservoir of a salt with a relatively low melting point. The hot molten salt is pumped into a heat exchanger, where it forces a working fluid to expand into a turbine, which spins to generate current. Researchers at Purdue University in Indiana have been working on the current stumbling point for the technology – the heat exchanger – and claim to have invented a new material to make this technique cheaper and more efficient.
The key to affordable concentrating solar power (CSP) is to convert as much heat as possible into electricity, which means that rather than using steam as a working fluid, it would be preferable to use supercritical carbon dioxide. The current problem is that to operate with this fluid, the heat exchangers would need to work at an extremely high temperature and pressure to cope with the conditions under which carbon dioxide becomes supercritical – and stainless steel and nickel alloys start to soften under these conditions. The Purdue team, led by professor of materials engineering Kenneth Sandhage, had previously worked on ceramic-metal composites for the exhaust nozzles of solid-fuelled rockets, and applied this experience to the heat exchanger problem.
In a paper in Nature, Sandhage and colleagues describe a composite made from tungsten and ceramic zirconium carbide, from which they made plates for a high-performance heat exchanger, capable of operating with an inlet temperature of 1023 K rather than a typical steam temperature of 823 K, leading to a 20 per cent increase in heat-to-electricity conversion efficiency. Working with a team at Georgia Tech which modelled the counterflow of molten salt and supercritical CO2 to optimise heat exchange between the two fluids, they etched channels into the plates, while further team members at Oak Ridge National Laboratory carried out mechanical tests to confirm the material could withstand the necessary conditions. More colleagues at Wisconsin-Madison contributed with corrosion studies and ascertained that it would be necessary to bond a layer of copper to the ceramic composite and add a small percentage of carbon monoxide to the CO2 to protect the composite.
The Purdue and Georgia Tech teams also carried out economic analyses which showed that the same place can be produced at comparable cost to – or even cheaper than – stainless steel or nickel alloy plates. “Ultimately, with continued development, this technology would allow for large-scale penetration of renewable solar energy into the electricity grid,” Sandhage said. “This would mean dramatic reductions in man-made carbon dioxide emissions from electricity production.”