Patent ID: 12198831

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. Unless otherwise specifically indicated in the disclosure that follows, the drawings are not necessarily drawn to scale. The present disclosure should in no way be limited to the exemplary implementations and techniques illustrated in the drawings and described below. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.”

The phase diagram shown inFIG.1, demonstrates how matter can exist in several different phases, including solid, liquid and gas—the three common phases of matter—which can exist together at the “triple point.” However, when the temperature/pressure regime of a material exceeds a critical point, the material can exist as a supercritical fluid (SCF). In the supercritical phase, the constituent molecules of a fluid form separate independent clusters that move freely with respect to each other. Supercritical fluids tend to exhibit the low viscosity and dielectric properties of a gas and the high thermal conductivity of a liquid. SCFs tend to combine the properties of high dielectric strength, low viscosity, and excellent heat transfer capability.

The inventors of the present invention have found that supercritical phase mixtures of certain fluids give rise to a high dielectric strength, low viscosity and high heat transfer characteristics. One representative example includes a supercritical fluid mixture of carbon dioxide (CO2) and ethane (C2H6) in an azeotropic mixture. In an experimental embodiment, certain mixing ratios resulted in an ideal compromise between dielectric performance and critical points, which can enable selectivity and applicability for a wide range of applications including those mentioned above. The first and second fluids, in certain embodiments, can include: sulfur hexafluoride; carbon dioxide; oxygen; hydrogen; trifluoroiodomethane; perfluoropentanone; perfluorohexanone; perfluoronitrile; hexafluoroethane; tetrafluoromethane; perfluoropropane; octafluorocyclobutane; ethane; and combinations thereof. As will be recognized by those of skill in the art, other fluids can also be employed without departing from the scope of the invention. In one experimental embodiment, the mixture included 60% carbon dioxide and 40% ethane by volume. In another experimental embodiment, the first fluid included carbon dioxide and wherein the second fluid included oxygen (for example, 80% carbon dioxide and 20% oxygen by volume). In one mixture, CO2can be used as the first fluid (the base) with trifluoroiodomethane (CF3I) used as the second fluid to raise the dielectric strength, and oxygen (O2) added to reduce carbon deposits after arcing.

The following table sets forth some of the important properties of the fluids discussed above:

DielectricstrengthMolecularcomparedPcformulaGWPto CO2Tc (K)(MPa)ToxicityFlammabilitySulfurSF622,800+++318.723.76Non-Non-hexafluoridetoxicflammableCarbonCO211304.257.38Non-Non-dioxidetoxicflammableOxygenO2—+154.64.98Non-Non-toxicflammableHydrogenH25.8?331.3Non-FlammabletoxicTrifluoroiodo-CF3I0.4+++395.154.04Low-Non-methanetoxicflammablePerfluoro-C5F10O1+++4192.14Non-Non-pentanonetoxicflammablePerfluoro-C6F12O1+++441.81.87Non-Non-hexanonetoxicflammablePerfluoro-C4F7N2,100++++3952.5Non-Non-nitriletoxicflammableHexafluoro-C2F612,200+2933.1Non-Non-ethanetoxicflammableTetrafluoro-CF46,630+2273.7Low-Non-methanetoxicflammablePerfluoro-C3F88,830++3452.6Non-Non-propanetoxicflammableOctafluoro-C4F810,300++388.22.78Non-Non-cyclobutanetoxicflammableEthaneC2H65.5−305.34.9Non-Flammabletoxic

The following should be noted: The decomposition products of sulfur hexafluoride can be extremely toxic. Carbon dioxide is typically used as a base fluid in many embodiments. Hydrogen can be added to increase thermal conductivity and increase arc stability in the mixture. Ethane can be used in an azeotropic mixture with CO2to reduce the critical temperature of the mixture.

Applicants have discovered an unexpected result in that a CO2and C2H6mixture has a lower critical temperature than either CO2or C2H6by itself. This lower temperature can be favorable in applications in which operating at room temperature, or near-room temperature, is desirable. The heat transfer coefficient of a CO2/C2H6azeotropic mixture falls between the values for pure CO2and C2H6, which indicates that such a mixture can be used effectively as an alternative fluid in heat power cycles.

In certain embodiments, oxygen can be added to prevent carbon build up due to decomposition of carbon dioxide. In certain embodiments, hydrogen can be used in arcing environments as hydrogen has high thermal conductivity and tends to extinguish arcs. In non-arcing conditions, fluorinated compounds can prevent carbon deposits in high thermal conditions.

As shown inFIG.2, in a representative embodiment of a dielectric system100, a first fluid100and a second fluid112are put into a container120to form a mixture. A regulating system122maintains the mixture inside the container120in a supercritical phase. In a basic method, as shown inFIG.3, the first fluid is added to the container210and the second fluid is added to the container212(typically, while both fluids are still in a sub-critical phase, such as a gaseous phase). The container is then sealed214with a fluid-tight seal and the mixture is heated or pressurized (or both) until it is in the supercritical phase216. The container is then maintained at the temperature and pressure to keep the mixture in the supercritical phase218so as to use the mixture as a dielectric material with excellent heat transfer properties. Applicants have found that use of such mixtures have certain parameters that are superior to those of the first and second fluids in a supercritical phase by themselves.

One embodiment, as shown inFIG.4, includes an electrical device300that includes a plurality of conductive surfaces320disposed in a container310that is filled with a mixture312of fluids, which is maintained at a supercritical phase. Such an electrical device300could include, for example, a switching device such as a disconnect switch or a circuit breaker, a Van de Graaff generator or an electrostatic motor (or generator).

One example of a fast mechanical switch400(i.e., high speed disconnect switch), as shown inFIG.5, can employ an piezoelectric actuator422to couple the conductive surfaces of two primary conductors410with the conductive surface of a secondary conductor420. A pair of insulators404isolates the conductors410from the container402, which keeps the supercritical fluid mixture310in a supercritical phase in a high pressure chamber406defined by the container402.

One example of a circuit breaker500, as shown inFIG.6, includes a container502filled with a fluid mixture310in a supercritical state. A fixed main contact510and a fixed arcing contact512are disposed in the container502. A moving arcing contact520is also disposed in the container502and is driven by a piston drive mechanism530. Dielectric nozzles524direct the dielectric fluid from a compression volume532to the contacts.

One example of an electrostatic motor600, as shown inFIG.7, includes a container602formed from two insulating stator plates606coupled to a plate spacer604and filled with a fluid mixture310in a supercritical phase. A plurality of stator elements616extend inwardly from the insulated stator plates606. A shaft610supports a rotor plant612, from which a plurality of rotor elements614extends. The rotor elements614are interleaved with the stator elements616. This type of system can also be configured as an electrostatic generator. A particle accelerator system700is shown inFIG.8and includes a high pressure vessel container702filled with a fluid mixture310in a supercritical phase. A particle accelerator, such as a Van de Graaff generator710, is disposed inside the container702. A particle accelerating tube712, from which an ion beam can exit the system, extends through the container702.

It has been found that breakdown voltage increases with the density of CO2, and scatters more in the supercritical region. A discontinuity of slope can be observed near the critical point where the substance experiences phase change. In a supercritical phase, the composition of the fluid is characterized by inhomogeneity in the molecular distribution due to the distinct clusters of molecules. Under the conditions close to the critical point, the density fluctuation FDincreases substantially due to repeated aggregation and dispersion of clusters, which influences the breakdown strength significantly. The density fluctuation FDis defined by:

FD=〈(N-〈N〉)2〉〈N〉=(ns⁢V)2na⁢v⁢e⁢V=kTkT0where N is the total number of particles in a given volume V, andNis the average of N, nsis the standard deviation of the local number density, naveis the average number density, kTis the isothermal compressibility, and kT0is the value of kTfor an ideal gas. A larger FDresults in larger density fluctuations, and FDreaches local maxima at the critical point.

Experiments were conducted at a constant temperature of 308 K, and breakdown voltages were measured for fluids from gaseous to supercritical conditions. Both pure fluids and mixtures of fluids were studied to determine the critical points of mixtures with different mixing ratios so that the thermodynamic phase inside the high pressure chamber container were confirmed. The observed critical points of mixtures were in good agreement with the critical points calculated from the predictive Soave-Redlich-Kwong (PSRK) model for supercritical fluids.

The density inhomogeneity is caused by the clustering effect, which forms a large mean free path where electrons can gain enough energy to ionize particles. Although the phenomenon of the discontinuity in breakdown versus density near the critical point in the experiment can be pronounced, a decrease in breakdown voltage between two electrical contacts in a supercritical fluid mixture near the critical point was not observed by the inventors. This was expected because the gap length between the contacts in the experimental study was relatively large, so that even the discharges happened under the condition of being close to the critical point, and clustering and density fluctuation FDdecreased due to the local increase of the temperature caused by discharges. If the gap length is smaller than 1 μm, the cluster structure can be preserved because more effective heat dissipation is enabled by the large specific surface area. Thus resulting in a reduction of breakdown voltage.

In the experimental study, the state of mixture inside the high pressure chamber was determined by observation through an optical cell and the critical points of mixtures with different mixing ratios were determined. The critical line for CO2/C2H6mixtures was determined and then compared the PSRK model. The PSRK model was able to reliably predict the thermodynamic properties of carbon dioxide and alkanes by using one pair of temperature-dependent group interaction parameters.

A comparison of the critical points observed from the optical cell and calculated from the PSRK model with respect to the mass fraction is shown inFIG.9A. An error between two methods was expected as being caused in part due to the presence of impurities.

To ensure consistency in the experimental results with the pure CO2data, the gap between two copper electrodes was set to 0.1 mm. C2H6mass percentages of 10%, 25% (azeotropic), 30%, 40%, and 50% were tested in the breakdown experiment. An oily substance between two electrodes was observed after the first breakdown when the C2H6mass percentage (w) reached beyond 60%. Experiments to determine the decomposition of C2H6under dielectric-barrier discharges found primary decomposition products, which included molecular hydrogen (H2), methane (CH4), acetylene (C2H2), and ethylene (C2H4). A similar phenomenon in that organic compounds were formed under the influence of electric discharges with C2H6was also observed. These results indicated that the product caused by the electric discharge could be a highly cross-linked polyethylene-type polymer.

An anomalous breakdown behavior near the critical point of pure CO2was also observed in the CO2—C2H6mixture at the azeotropic mixing ratio, as shown inFIG.9B, in which breakdown voltage is related to pressure, and inFIG.9C, in which breakdown voltage is related to density. The measured breakdown voltage at different mixing ratios as a function of the density is shown inFIG.9D. The breakdown voltages can also be presented as a function of the mixing ratio of C2H6to CO2, as shown inFIG.9E. All of the breakdown values shown inFIGS.9D and9Ewere measured while the mixture was in a supercritical phase.

The average breakdown voltages tend decrease with an increase of C2H6concentration. Also, the measured breakdown voltage of fluid mixtures tend to scatter more widely compared to the values of pure CO2. At the lower density region between 220 kg/m3to 300 kg/m3, the difference in the breakdown voltage of mixtures tends to be greater than in the higher density region. The data also indicate that breakdown voltages of different mixing ratios tend to saturate at higher density. The breakdown voltage of mixtures at the lower density region also shows a more pronounced reduction comparing with pure CO2. For an azeotropic mixture of CO2and C2H6(25% mass fraction of C2H6and 75% mass fraction of CO2), the breakdown voltage shows an average of 20.5% reduction compared to the dielectric strength of pure CO2in the vicinity the critical point of CO2. At the higher density region far away from the critical point, the reduction of dielectric strength of the mixture drops to about 13.5% compared to pure CO2.

The dielectric performance of supercritical CO2and C2H6mixtures including their azeotropic mixture reveals that such mixtures can exhibit an average of dielectric strength exceeding that of sulfur hexafluoride (SF6) gas by a factor of three (SF6being the most commonly used insulation gas.) Moreover, the breakdown anomaly is observed near the critical point due to the high density fluctuation caused by molecular clustering. Unique properties of SCF mixtures with respect to dielectric strength, viscosity, heat transfer capability, and tunable critical point can be useful in applications involving power and energy. SCF mixtures can also enable affordable particle accelerators for high energy physics and medical treatment.

Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description. It is understood that, although exemplary embodiments are illustrated in the figures and described below, the principles of the present disclosure may be implemented using any number of techniques, whether currently known or not. Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. The operations of the systems and apparatuses disclosed herein may be performed by more, fewer, or other components and the methods described may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set. It is intended that the claims and claim elements recited below do not invoke 35 U.S.C. § 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim. The above-described embodiments, while including the preferred embodiment and the best mode of the invention known to the inventor at the time of filing, are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is to be determined by the claims below rather than being limited to the specifically described embodiments above.