The invention relates generally to heat transfer of HTS elements, and more particularly to enhanced heat transfer from an HTS element in a liquid cryogen bath.
There exist HTS cooling systems that use the properties of liquid nitrogen or other cryogenic liquids to achieve cryogenic cooling. An example of a cryogenic liquid used for cooling would be liquid nitrogen, used at one atmospheric pressure (˜0.1 MPa) where its saturated temperature (boiling point) is at 77 Kelvin. However, since the critical current density of HTS materials improves significantly at temperatures lower than 77 K, methods have been developed to reduce the temperature of the liquid nitrogen by manipulating its operating environment. By reducing the pressure of liquid nitrogen, its boiling point temperature can be lowered to about 63 K below which solid nitrogen would form. One example of using such properties of liquid nitrogen to achieve lower operating temperature is provided in U.S. Pat. No. 5,477,693. It describes a method of using a vacuum pump to pump the gaseous nitrogen region in a cryogen containment vessel (cryostat) that contains both the liquid and gaseous nitrogen. Pumping reduces the pressure of the liquid nitrogen bath therefore reducing its saturation temperature (boiling point) to below 77 K. The performance of the superconductor when cooled to this reduced temperature, namely its critical current level, is then significantly improved.
During the electrical transient in an FCL device associated with a fault on the electric power grid, the essentially adiabatic, rapid temperature rise of the high-temperature superconductor elements can result in a nominal element temperature rise of 200 to 300 K. With these rapidly heated elements submerged within the bath of liquid nitrogen, the large difference between the surface temperature of the element and the temperature of the surrounding bath results in an almost instantaneous initiation of film boiling of the liquid nitrogen bath at the interface. Film boiling is the formation of a stable vapor layer between the heated element and the liquid nitrogen bath. The thermal heat transfer across this vapor layer is limited by the thermal conductivity of the vapor and results in a relatively low cooling rate of the HTS element as it recovers after the fault. This recovery can be described as a re-cooling of the HTS element to below its critical temperature so that its superconducting properties are regained. This situation is complicated further if an electric current is applied to the element after the fault, resulting in added heat load to the element which must be removed during recovery. This added condition is called Recovery Under Load (RUL).
Under film boiling conditions, the heat transfer from the HTS element to the surrounding cryogen bath is known to employ the film boiling portion of a boiling heat transfer curve, line 12 for the case of liquid nitrogen, in plot 10 in
It is known to utilize a nylon wire mesh in conjunction with a perforated outer tube to ensure the free circulation of cooling fluid, liquid or gas around the surface of the conductor to facilitate heat recovery, as is disclosed in U.S. Pat. No. 5,432,666. It is also known to use coatings to modify the heat transfer characteristics of a surface in a cryogenic liquid. For example, in the publication by R F Barron, entitled, Cryogenic Heat Transfer, section 2.7 and the publication by M N Wilson, entitled Superconducting Magnets, section 6.5 the use of coatings is taught to enhance heat transfer characteristics.
It is also known to add to superconductive paste comprising Bi, Pb, Sr, Ca and Cu and an organic binder, which may be applied to the surface of the substrate material having a thickness of about 100 μm, or more, wherein the paste is heated to form a coating encapsulating the substrate material, as disclosed in U.S. Pat. No. 6,809,042. The resulting HTS element thus will have an enhanced high critical current and critical magnetic field.
It is further known to add epoxy encapsulation around the HTS element to thermally isolate the superconductor material from the cooling medium and decrease the critical current density of the superconductor material wherein the epoxy is less than 2 mm thick and has thermal expansion properties approximately equal to the thermal expansion properties of the superconducting material, as disclosed in U.S. Pat. No. 5,761,017. The purpose of such encapsulation is to dissipate heat as quickly as possible, as disclosed, for example, in column 5, lines 6-9. However, as shown in FIG. 3 of the '017 patent and as referenced in column 5, lines 9-14, the heat dissipation into the epoxy does not extend to the surface of the epoxy in contact with the cooling medium. In addition, this patent does not disclose or teach the use of an intermediate boundary layer to enhance heat transfer and to maximize heat transfer from the HTS element to a surrounding liquid cryogen cooling bath through the encapsulation by promoting the formation of a nucleate boiling regime.
It is known to use a Teflon® coating on the interior of cryogenic transfer lines to speed cooling thereof, however, it is not taught or suggested to use Teflon on a HTS element to enhance heat transfer from the HTS element to a surrounding liquid cryogen cooling bath.
It would therefore be desirable to employ a simple, reliable and effective apparatus to speed up the temperature recovery after a fault condition has occurred in an HTS element, within an FCL system.
Briefly, in accordance with one embodiment of the present invention, a fault current limiter, having a heat transfer medium, employs a high temperature superconductor based element which has a coating material encapsulating the high temperature superconductor based element to form an intermediate boundary layer between the HTS element and the heat transfer medium, wherein the coating material has a high thermal resistance. The coating material has a thickness which enables it to maintain substantially during recovery cooling a temperature gradient between the coated surface of the high temperature superconductor and the surface of the coating in contact with the cryogenic fluid so as to develop a temperature difference between the cooled surface of the coating (Twall) and the saturation temperature of the cryogen bath (Tsat), wherein substantially all heat transfer to the cryogen bath occurs at the nucleate heat transfer rate. The thickness of the coating material is selected so that the heat flux through the coating is substantially equal to the heat transfer from the coating material to the cryogen bath.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
During a fault condition on the electric grid and the resultant electrical load transient, the temperature of an HTS element in the Fault Current Limiter (FCL) structure rises rapidly, within milliseconds, to well above the critical temperature Tc of the HTS material where it transitions from a superconductor to the non-superconducting (resistive) state. In order to return to the normal operating superconducting condition, the HTS element must be re-cooled to restore its superconducting properties. The heating is essentially adiabatic during the fault transient. Additional heat load may be encountered if normal load current is reapplied after the fault to the FCL, with some or all of the current flowing in the HTS element, the remaining current being diverted into a parallel circuit. The heated HTS element is cooled by contact with the liquid cryogen coolant, which is typically liquid nitrogen, but can be other liquid cryogens depending on the operating temperature of the FCL system. Because the temperature rises so rapidly in the HTS element, the resulting difference in temperature at the HTS wall and the coolant temperature results in the initiation of film boiling heat transfer which generally has an inherently lower heat transfer rate 14 than the more ideal nucleate boiling heat transfer 16 as illustrated in plot 10, line 12 of
A fault current limiter in the present system 18 may be a FCL comprising a superconducting based element or composite 24, such as BSCCO-2223, YBCO, BSCCO2212 or others, which has at least one high temperature superconductor element 24 which may be coupled in parallel with a shunt coil (not shown). See, for example,
The mechanism for the HTS element 24 to cool during film boiling 14 in liquid nitrogen 26 is shown in prior art
In order to achieve the desired nucleate boiling regime at the interface, as is shown in
Preliminary modeling analysis has been conducted considering a BSCCO-2212 melt cast HTS element 24, which in one exemplary embodiment is 1.6 mm thick, which is assumed to have been heated essentially adiabatically to 300 K during a transition fault. The analysis provides for symmetric cooling from one face with the internal temperature of the HTS element dropping as energy is removed. No additional heating from re-applied current load is considered. For direct cooling in the liquid nitrogen bath, the HTS element 24 can be treated as a lump parameter system (Biot number, Bi<0.1) over most of the cooling range from 300K to approximately 140 K. Below 140 K, the HTS element cools slightly faster at the wall than the core. Upon final analysis, the difference in core to wall temperature at 110 K is only approximately 2 K. The cooling curves in plot 30 illustrated in
The model was then used to consider the impact a 0.38 mm thick intermediate boundary layer Kapton® polyimide coating 29 applied between the HTS element 24 and the 77 K liquid nitrogen bath 26. The HTS wall temperature was determined iteratively, such that the heat flux through the boundary layer 29 equaled the heat flux into the liquid nitrogen 26 utilizing nucleate boiling state 16 identified in
To illustrate the impact of the thickness of the intermediate coating on the heat transfer rate to the liquid cryogen bath, a model was run using a 1 inch diameter stainless steel rod (emulating the superconducting element) having an intermediate boundary layer 29 of Teflon film wherein the Teflon has a varied number of thicknesses as indicated in
The previously described embodiments of the present invention have many advantages, including higher heat transfer rates that enable this invention to have greater design flexibility to be able to handle higher fault loads, including the ability to recover under load, and enhance the speed of recovery after a fault for a given fault load. The boundary layer materials thickness and composition can be adjusted to optimize performance for a given set of operating parameters. Adding the intermediate boundary layer 29 to the HTS element 24 can improve the cooling rate of the fault current limiter superconductor elements by two fold, which provides a broader range of design options for handling the fault load. It is also understood that the HTS element and FCL described herein may be part of a broader matrix type fault current limiter, having a plurality of HTS elements within the MFCL as described, for example, in U.S. Pat. No. 6,664,875.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
The U.S. government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for in the terms of Contract No. DE-FC36-03G013033 awarded by the Department of Energy.