By Josh Perry, Editor
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Researchers from Purdue University (West Lafayette, Ind.), the Georgia Institute of Technology (Atlanta, Ga.), the University of Wisconsin - Madison, and Oak Ridge (Tenn.) National Laboratory developed a new manufacturing process for making plates with channels for heat exchange out of a composite material composed of zirconium carbide and tungsten.

A recent development would make electricity generation from the sun's heat more efficient, by using ceramic-metal plates for heat transfer at higher temperatures and at elevated pressures. (Purdue University illustration/Raymond Hassan)

According to a report from Purdue, this material had been previously created by Purdue researchers to withstand high heat and pressure. The new manufacturing process produces plates that can be used to build heat exchangers for concentrated solar power applications.

Concentrated solar power plants convert solar energy into electricity with mirrors that focus the light into a small area. The heat that is generated is transferred into molten salt and then into a working fluid (supercritical carbon dioxide) to power an electricity-producing turbine.

To produce cheaper electricity, the turbine would need to produce more electricity for the same amount of heat, requiring the engine 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,” the report explained.

Georgia Tech researchers ran simulations to determine the necessary thickness of each plate in the printed circuit heat exchanger stack and the spacing between channels for the maximum allowed stresses for each material.

Mechanical and corrosion testing showed that this new 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 research was recently published in Nature. The abstract stated:

“The efficiency of generating electricity from heat using concentrated solar power plants (which use mirrors or lenses to concentrate sunlight in order to drive heat engines, usually involving turbines) may be appreciably increased by operating with higher turbine inlet temperatures, but this would require improved heat exchanger materials.

“By operating turbines with inlet temperatures above 1,023 kelvin using closed-cycle high-pressure supercritical carbon dioxide (sCO₂) recompression cycles, instead of using conventional (such as subcritical steam Rankine) cycles with inlet temperatures below 823 kelvin, the relative heat-to-electricity conversion efficiency may be increased by more than 20 per cent. The resulting reduction in the cost of dispatchable electricity from concentrated solar power plants (coupled with thermal energy storage) would be an important step towards direct competition with fossil-fuel-based plants and a large reduction in greenhouse gas emissions.

“However, the inlet temperatures of closed-cycle high-pressure sCO₂ turbine systems are limited by the thermomechanical performance of the compact, metal-alloy-based, printed-circuit-type heat exchangers used to transfer heat to sCO₂. Here we present a robust composite of ceramic (zirconium carbide, ZrC) and the refractory metal tungsten (W) for use in printed-circuit-type heat exchangers at temperatures above 1,023 kelvin.

“This composite has attractive high-temperature thermal, mechanical and chemical properties and can be processed in a cost-effective manner. We fabricated ZrC/W-based heat exchanger plates with tunable channel patterns by the shape-and-size-preserving chemical conversion of porous tungsten carbide plates. The dense ZrC/W-based composites exhibited 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. Techno-economic analyses indicate that ZrC/W-based heat exchangers can strongly outperform nickel-superalloy-based printed-circuit heat exchangers at lower cost.”

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