In fall 2018, the DOE awarded STEEL with a $2 million grant to investigate the thermophysical properties of heat transfer media (HTM) in Generation 3 Concentrating Solar Power (CSP) systems. In the previous generation systems, the HTM (molten salts, particles, solar oils) used to thermally store energy have only been heated to 600°C. The Gen 3 HTM will be operating at temperatures >700°C, which is expected to increase the system efficiency from ~40% to above 50%. However, the thermal properties of these materials are poorly understood at such high temperatures. Our Gen3 CSP proposal is to measure the thermal conductivity of high temperature HTM using the 3ω technique.
3ω is a widely used periodic measurement technique that involves running a current (1ω) through a metal sensor; this is typically called the “heater line” because it is usually a gold- or platinum- line of negligible thickness sputtered onto a substrate. The current causes Joule heating in the sample placed on top of the heater line at a 2ω frequency, which in turn produces a 2ω temperature difference between the line and the sample. This temperature difference is converted to a voltage reading, which is now at a frequency of 3ω due to the multiplication of the 1ω current and the 2ω resistance (which varies directly with temperature at 2ω). Finally, when this experiment is performed over a range of frequencies, the raw voltage-frequency data is converted to a thermal conductivity value.
For the Gen3 CSP project, STEEL is developing a unique 3ω-based immersion probe that can measure the thermal conductivity of corrosive, high temperature HTM in an argon glove box containing a well-insulated furnace. We will have the salt or particle mixture inside the furnace in a crucible that is accessed through a narrow portal at the top of the furnace. A current will be run through a semicircular heater line, which in our case is a platinum-coated silica fiber protected with a cladded coating. This method is innovative because the heater line is not attached to the substrate, allowing free movement of the probe. The completed probe will be used to determine the thermal conductivity of HTM proposed to be the working fluid in the final Gen3 CSP system.
- Cahill, D.G. and Pohl, R.O., Thermal conductivity of amorphous solids above the plateau. Physical review B, 1987. 35(8), p.4067.
- Generation 3 Concentrating Solar Power Systems (Gen3 CSP). Office of Energy Efficiency & Renewable Enegry.
- Dames, C., Measuring the thermal conductivity of thin films: 3 omega and related electrothermal methods. Annual Review of Heat Transfer, 2013. 16.