Compositions – Frost-preventing – ice-thawing – thermostatic – thermophoric,...
Reexamination Certificate
2000-08-04
2002-09-10
Green, Anthony J. (Department: 1755)
Compositions
Frost-preventing, ice-thawing, thermostatic, thermophoric,...
C252S073000, C252S074000, C252S075000, C252S076000, C252S078100, C252S078300, C016S010000, C016S104000
Reexamination Certificate
active
06447692
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of manufacturing of dielectric heat transfer fluids which can be pumped at sub-freezing temperatures and at the same time have high reversible thermal transport properties at temperatures up to 120° C.
In particular, this invention discloses low viscosity hydrocarbon or fluorinated hydrocarbon heat transfer fluids filled with under 30% (by volume) of a nanometer sized phase change material (hereinafter, nano-PCM), which improves the fluids' thermal properties. Such fluids are particularly important for the use in fuel cells. The invention discloses heat transfer fluids(and a method of making them) with increased heat capacity and high reversible heat transfer properties at temperatures up to 120° C. while maintaining low viscosity (below 10 cSt kinematic) at temperatures as low as −400° C.
2. Description of the Related Art
There are a number of commercially available cooling fluids which have acceptably low viscosities at low temperatures. However, all these materials, typically fluorinated or aliphatic hydrocarbons, have a common flaw—very poor thermal conductivity and heat capacity. These fluids have typical heat capacity of about 1-2 J/gK (1,000-2,000 J/kgK) (compared to about 3-4 J/gK (3,000-4,000 J/kgK) for water and other hydroxyl rich fluids). If the hydrocarbon structure of these fluids is modified to improve the heat capacity, the kinematic viscosity at low temperatures also rises making these fluids less desirable over the intended temperature range of application.
A number of solutions has been offered to solve these problems but none of the materials tried in the prior art was able to reach an acceptable combination of low enough viscosity at low temperatures and high reversible transport properties at elevated temperatures. At least four different methods, as described more specifically below, were tried and none brought about fully acceptable results.
(A) Use of De-Ionized Water
De-ionized water is the current state-of-the-art fluid for cooling of fuel cell systems. De-ionized water is an attractive fluid because it has superb dielectric properties as well as the highest thermal conductivity and heat capacity for any known fluid [0.59 W/mK and 4.1819 J/gK (4,181.9 J/kgK), respectively, at 200° C.]. De-ionized water also has very low kinematic viscosity of 1.798 cSt even at a temperature as low as a freezing temperature of 0° C. The use of de-ionized water was disclosed in Grevstad's U.S. Pat. No. 3,964,929, where de-ionized water with resistivity of at least about 20,000 ohm/cm was a preferred coolant. However, de-ionized water has two serious fundamental disadvantages.
First, it reacts with the metal of the bipolar plates of a fuel cell worsening the dielectric properties of de-ionized water over time. Second, it cannot be used as a heat transfer fluid below 0° C. because it freezes at that temperature. An anti-freeze additive, such as a glycol cannot be used to prevent the freezing at 0° C., because glycol solutions are very viscous at low temperatures (for example, a 40% solution of ethylene glycol in water, a very typical anti-freeze, has an unacceptably high viscosity of about 7 cSt at just 0° C.); therefore, a large amount of energy would be required to pump such coolant. Other popular anti-freeze additives, those based on salts, are unacceptable because they are corrosive and conduct electricity.
(B) Use of Low Temperature Heat Transfer Fluids and Coolants.
A number of fluids exhibiting acceptably low viscosity at sub-freezing temperatures with reasonably high flash points are available commercially. Typically, they are liquids based on straight chain perfluorinated hydrocarbons or aliphatic hydrocarbons (for instance, Fluorinert® fluids manufactured by Minnesota Mining and Manufacturing Corp. of Saint Paul, Minn.; Dynalene HF® coolant manufactured by Loikits Technologies, Inc. of Whitehall, Pa.; and Therminol D-12® fluid manufactured by Solutia, Inc. of Saint Louis, Mo.). Aromatic hydrocarbon-based fluids, such as Therminol LT® and Dynalene MV® coolants, are also available.
Despite their excellent fluidity at low temperatures, a number of serious drawbacks common to these products restrict their use. All of them have very low flash points, often offensive odors and incompatibility with sealing materials. Most importantly, they have poor thermal performance, such as low heat capacities and low thermal conductivities. The only realistic way to improve this poor thermal performance is to modify the molecular structure of these compounds by introducing some heat retaining groups, such as hydroxyls or thiols. This, however, leads to the deterioration in fluidity and, due to increased molecular interactions, the viscosities of these products grow unacceptably high at low temperatures. The modifications with the heat retaining groups also increase the reactivity of the fluid with metal causing corrosion and a decrease in dielectric performance with time. These disadvantages of low temperature heat transfer fluids and coolants are described in V. Stepina and V. Vesely, Lubricants and Special Fluids, Chapter 1, Elsevier Science Publishers, Amsterdam (1992).
(C) Use of Encapsulated Phase Change Materials
Micron sized encapsulated phase change materials (hereinafter, PCMs) have been added to a coolant where a latent heat of transformation, usually solid-to-liquid or solid phase I-to-solid phase II, was used to absorb a significant amount of heat. See, D. P. Colvin, J. C. Mulligan, and Y. G. Bryant, “Enhanced Heat Transport in Environmental Systems Using Microencapsulated Phase Change Materials,” 22
nd
International Conference on Environmental Systems, Seattle, Wash., Jul. 13-16, 1992, paper 921224; M. Kamitomo, et. al., “Latent Thermal Storage Using Pentaerythritol Slurry,” 21
st
Intersociety Energy Conversion Engineering Conference: Advancing Toward Technology Breakout in Energy Conversion, Aug. 25-29, 1986, San Diego, Calif., vol. 2, pp. 730-736.
Colvin, et. al., added 20-30% by weight of 20-40 micrometers sized encapsulated particles containing the paraffin wax to polyalphaolephin (PAO) dielectric coolant to increase its thermal performance. Kamimoto, et. al., investigated slurries containing 70-80% by weight of pentaerythritol encapsulated plastic crystals for heat storage applications.
The micron sized PCM particles have not performed well under repeated cycling. The larger particles were often crushed during pumping, and, in order to avoid such crushing, special diaphragm pumps were needed. In addition, the phase change of the PCMs was frequently incomplete due to the poor thermal conductivity of the PCM in the particle. As a result, Colvin et. al. had to increase the percentage of 20-40 micrometer sized PCM particles to 25-30% by weight, which in turn led to the increase in the fluid's viscosity. A lower cost encapsulation process, such as use of metal coating, was not viable for high voltage environments. In addition, even after the mechanical properties of the encapsulating shell had been somewhat improved, they were not good enough to ensure sufficient life of the slurries for the millions of cycles required.
(D) Prior Attempts At Encapsulation Within Nanometer Sized Capsules
The encapsulation of precipitated, nanometer sized materials, such as gold, silver salts, cadmium sulfide, and others, in micelles (oil-in-water emulsions) or reversed micelles (water-in-oil emulsions) has been previously achieved. Micelles or reversed micelles, with the typical size of droplets on the order of 10 to 1,000 Angstroms were self-formed through the use of suitable surfactants. It was shown that the fluid environment enclosed within a micelle or a reversed micelle can be used to precipitate reactants thereby forming nanometer sized particles. See, M. P. Pileni, “Reversed Micelles as Microreactors,” J. Phys. Chem., 97 (1993), 6361-6973. These nanometer sized particles may be then more permanently encapsulated by one of the following methods:
polymerizi
Momoda Leslie A.
Phelps Amanda C.
Green Anthony J.
HRL Laboratories LLC
Ladas & Parry
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