Heat transfer fluid based on nanoparticles dispersed in ionic liquids

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2014

Researchers evaluate ionic liquids as heat transfer fluids for potential use in solar-power applications

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KEY CONCEPTS
• A new type of heat transfer fluid that contains a dispersion of aluminum oxide nanoparticles in an ionic liquid is known as a nanoparticle enhanced ionic liquid (NEIL).
• The NEIL displays an increase in volumetric heat capacity of 40 percent as compared to the neat ionic liquid and an increase of 70 percent compared to a commercially available aromatic heat transfer fluid.
• Researchers have filed a patent and are looking to find partners to commercialize the technology.

THE MOVEMENT OR TRANSFER OF HEAT is an important characteristic needed to ensure that machinery operates at an optimal level. An obvious example is refrigeration where heat needs to be removed in as efficient a manner as possible from a specific location.

In a previous TLT article, the heat transfer properties of transformer insulating fluids were discussed (1). The function of these fluids is to transfer heat away from transformers that are usually operating under high temperature and operating loads for extended periods of time.

Boosting the thermal conductivity characteristics of the transformer fluid is extremely important in order to dissipate heat and provide good electrical insulation. One component that has been found to work well is the use of nanoparticles because they display high thermal conductivity values. In this research, stable dispersions of hexagonal boron nitride nanoparticles in naphthenic base oil increased the thermal conductivity of the transformer oil by 80 percent when used at a 0.1 percent treat rate.

A third application that can benefit from the use of nanoparticles is heat transfer fluids. One application that is growing in interest is the use of heat transfer fluids to facilitate the conversion of solar energy into electricity. This is accomplished by using solar collectors to focus sunlight onto a receive pipe containing the heat transfer fluid.

In contrast to the transformer oil, the objective of the heat transfer fluid is to capture and retain as much heat as possible so that it can be used to generate the steam needed to run a turbine generating electricity. The important considerations are to use a heat transfer fluid that exhibits both high volumetric heat capacity and favorable physical properties.

Currently used heat transfer fluids work well, but there is need for improvement in order to further increase their effectiveness. Dr. Elise Fox, principal engineer at the Savannah River National Laboratory in Aiken, S.C., says, “While heat transfer fluids exhibit good high-temperature properties, there is still the possibility they may generate hydrogen gas, which can lead to potential flammability problems. Costly palladium getters are required to bond with any hydrogen present in the heat transfer fluid system.”

A second limitation for heat transfer fluids is they display significant vapor pressure at the temperature range required for the operation of the system. The presence of heat transfer fluid in the vapor stage reduces their effectiveness in transferring solar heat and requires the careful control of condensates and temperature gradients.

In the search for a more suitable basestock, ionic liquids are a viable option because of their exceptional thermal stability and negligible vapor pressure. In a previous TLT article, a study showed that tetraarylphosphonium cations demonstrate exceptional thermal stability after heating for 350 C for 96 hours (2).

A logical approach to develop more effective heat transfer fluids for use in solar-power applications is to combine the positive characteristics of nanoparticles with ionic liquids. Such an approach has now been conducted.

NEILs
Fox and her associates have developed and evaluated ionic liquids containing dispersed nanoparticles as heat transfer fluids for potential use in solar power. The new heat transfer fluids are known as nanoparticle enhanced ionic liquids (NEILs).

Fox says, “We found that using aluminum oxide nanoparticles dispersed in the ionic liquid known as 1-butyl-2,3-dimethylimidazolium bis- (trifluoromethyl) sulfonyl- imide [(C₄mmim) NTf₂] achieves an increase in volumetric heat capacity of 40 percent compared to the neat ionic liquid and of 70 percent compared to a commercially available aromatic heat transfer fluid currently used in solar-power applications. The nanoparticles dispersion increases the thermal conductivity of the NEIL by 7 percent.”

Figure 1 shows an image of the NEIL poured into a beaker.


Figure 1. Nanoparticle Enhanced Ionic Liquids (NEILs) have demonstrated superior performance, which makes them ideal candidates for use as heat transfer fluids in solar-power applications. (Courtesy of the Savannah River National Laboratory)

The specific ionic liquid was chosen because it has the required characteristics needed for use as heat transfer fluids. Fox explains, “[(C₄mmim) NTf₂] is relatively hydrophobic, which means hydrogen formation will be minimized and displays high temperature stability. The anion used was chosen because ionic salts prepared with this functionality display lower viscosities.”

Viscosity is an important factor because dispersion of nanoparticles in a liquid tends to significantly increase this parameter, making it difficult for the resulting fluid to efficiently transfer heat.

Besides aluminum oxide, the researchers evaluated a wide range of nanoparticles including silica, gold, zinc oxide, cupric oxide, ferric oxide and carbon nanotubes. Aluminum oxide was selected because the resulting NEIL displayed superior thermal properties.

Two different types of aluminum oxide nanoparticles were examined. Fox says, “We evaluated spherical particles with an average diameter of 50 nanometers and whiskers that are 280 nanometers in length and 4 nanometers wide. The whiskers generated slightly better thermal properties.”

A procedure used by one of their collaborators, Jamil Kahn at the University of South Carolina, to evaluate the NEILs, is a flow loop that included a pump, heat exchanger, collection tank, pressure transducer and a test section where data was collected into an acquisition system that was linked to a computer.

Ann Visser, a chemist at the Savannah River National Laboratory, also indicated that corrosion testing was done to ensure that the NEILs are compatible with a number of metals used in heat transfer systems including stainless steel, aluminum and copper alloys. She says, “We evaluated the NEILs using electrochemical measurements, stereomicroscope examination of parts exposed to the fluids and metal coupon testing.” The results for the most part are good with most alloys.

Dale Haas, commercialization program manager for the Savannah River National Laboratory, indicates that future work involves finding a partner to commercialize this technology through the use of a Cooperative Research and Development Agreement (CRADA). Further details can be obtained by contacting Haas at dale.haas@srnl.doe.gov. Details pertaining to technical information can be obtained from Fox at elise.fox@srnl.doe.gov. A patent on this technology has been filed and further information can be found in a recent publication (3).

REFERENCES
1. Canter, N. (2012), “Nano-based transformer insulating fluid,” TLT, 68 (5), pp. 10-11.
2. Canter. N. (2013), “Thermally stable ionic liquids,” TLT, 69 (11), pp. 8-9.
3. Fox, E., Visser, A., Bridges, N. and Amoroso, J. (2013), “Thermophysical Properties of Nanoparticle-Enhanced Ionic Liquids (NEILs) Heat-Transfer Fluids,” Energy & Fuels, 27 (6), pp. 3385-3393.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.