The innovation is an important step for putting solar heat-to-electricity generation in direct cost competition with fossil fuels, which generate more than 60 percent of electricity in the US. Solar power accounts for less than 2 percent of US electricity but could make up more than that if the cost of electricity generation and energy storage for use on cloudy days and at nighttime were cheaper.
“Storing solar energy as heat can already be cheaper than storing energy via batteries, so the next step is reducing the cost of generating electricity from the sun’s heat with the added benefit of zero greenhouse gas emissions,” said Kenneth Sandhage, Purdue’s Reilly Professor of Materials Engineering.
The research, which was done at Purdue in collaboration with the Georgia Institute of Technology, the University of Wisconsin-Madison and Oak Ridge National Laboratory, looks at concentrated solar power plants that convert solar energy into electricity by using mirrors or lenses to concentrate a lot of light onto a small area, which generates heat that is transferred to a molten salt. Heat from the molten salt is then transferred to a “working” fluid, supercritical carbon dioxide, that expands and works to spin a turbine for generating electricity.
To make solar-powered electricity cheaper, the turbine engine would need to generate even more electricity for the same amount of heat, which means the engine needs to run hotter.
The problem is that heat exchangers, which transfer heat from the hot molten salt to the working fluid, are currently made of stainless steel or nickel-based alloys that get too soft at the desired higher temperatures and at the elevated pressure of supercritical carbon dioxide.
Sandhage worked with Asegun Henry, now at the Massachusetts Institute of Technology, to develop a composite for more robust heat exchangers using ceramic zirconium carbide and tungsten.
Purdue researchers created plates of the ceramic-metal composite that host customizable channels for tailoring the exchange of heat, based on simulations of the channels conducted at Georgia Tech by Devesh Ranjan’s team.
Mechanical tests by Edgar Lara-Curzio’s team at Oak Ridge National Laboratory and corrosion tests by Mark Anderson’s team at Wisconsin-Madison helped show that this new composite material could be tailored to successfully withstand the higher temperature, high-pressure supercritical carbon dioxide needed for generating electricity more efficiently than today’s heat exchangers. The ZrC/W-based plates showed failure strengths of over 350 megapascals at 1,073 kelvin, and thermal conductivity values two to three times greater than those of iron- or nickel-based alloys at this temperature. Corrosion resistance to sCO2 at 1,023 kelvin and 20 megapascals was achieved by bonding a copper layer to the composite surface and adding 50 parts per million carbon monoxide to sCO2.
“Ultimately, with continued development, this technology would allow for large-scale penetration of renewable solar energy into the electricity grid,” said Sandhage. “This would mean dramatic reductions in man-made carbon dioxide emissions from electricity production.”
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