Power transfer cables using high-temperature superconducting (HTS) materials are currently being developed for utility demonstration projects. The initial urban retrofit applications may employ lengths from 100 to 1,000 meters or longer. Cryogenic refrigeration systems are required to maintain these cables at their operating temperature range from about 70 to 80 kelvin (K). Thermal losses are a key factor in the successful application of HTS power cables. The phase I project, Thermal Insulation Performance of Flexible Piping for Use in HTS Power Cables, in collaboration with the Department of Energy and Oak Ridge National Laboratory, was completed at the Cryogenics Test Laboratory in 2001. A series of heat transfer tests under cryogenic vacuum conditions using flexible corrugated piping (simulated thermal insulation system for an HTS power cable) was performed. The mechanical effects created by bending and effects of insulation compression created by spacers were simulated. Over 90 tests of 12 different thermal insulation systems, including standard multilayer insulation (MLI) and the new layered composite insulation (LCI), were tested and evaluated.

Existing HTS power cable prototypes rely on the use of vacuum jacketing with MLI systems inside to reduce the ambient heat leak rates to manageable levels. MLI systems are subject to large variations in actual performance. The small space available for the thermal insulation materials makes the application even more difficult because of bending considerations, mechanical loading, and the arrangement between the inner and outer piping. Each of these mechanical variables affects the heat leak rate. For all applications, it is critical that the thermal insulation and vacuum enclosure be robust. For any MLI to function properly, the vacuum level must be maintained below 0.0001 torr cold vacuum pressure (CVP). Furthermore, a maintenance-free insulation system (high-vacuum level for 20 years or longer) is a practical requirement. Overall heat leak targets of around 1 watt per meter (W/m), depending on the diameter of the cable, are achievable, but manufacturing and maintenance can be a problem because of the high-vacuum requirement.

This experimental research study of flexible piping for HTS power cables shows three basic levels of thermal performance: ideal MLI, MLI on rigid piping, and MLI between flexible piping. The thermal performance varies widely with both the vacuum level and the materials. The performance of ideal MLI is defined as a k-value of 0.05 milliwatt per meter-kelvin (mW/m-K) for a vacuum level below 0.0001 torr and boundary temperatures of 80 and 293 K. At a high-vacuum level, the k-values of MLI on rigid piping were about 0.09 mW/m-K. Under similar conditions, the k-values of MLI between corrugated piping were 0.19 mW/m-K. The new LCI, on the smooth sleeve or between the corrugated piping, performed as well as MLI at high vacuum and much better than MLI at soft vacuum (only 3.1 mW/m-K at 1 torr). The total insulating effectiveness of an insulation system is the key factor when considering the cryogenic refrigeration requirements for an HTS power cable. The simulated spacers tests and the simulated bending tests showed significant degradation in the thermal performance of a given insulation system (typically greater than 50 percent) at high vacuum conditions. A typical k-value of 1.0 mW/m-K, based on commercial double-wall flexible piping, for thermal loss calculations appears reasonable for a well-executed MLI construction operating at the high vacuum level.

The results from the study begun at NASA Kennedy Space Center will be used to decrease the refrigeration load for HTS power cables. Soft-vacuum systems have much lower vacuum burden costs, which is key to lowering the overall cost of building, operating, and maintaining long, flexible power cables as part of a utility infrastructure. The plan for continuing this work includes the construction and testing of a long flexible cryostat to address basic heat transfer and fluid flow questions. In this approach we can leverage the ongoing insulation material development work and the existing test infrastructure of the Cryogenics Test Laboratory including our 18-meter-long Cryogenic Pipeline Test Apparatus. The target is to be able to make flexible piping with thermal performance approaching that of rigid piping to help make energy-efficient HTS power cables become an industrial reality.

Contacts: J.E. Fesmire (James.Fesmire-1@ksc.nasa.gov), YA-C2, (321) 867-7557; and K.G. Thompson, YA-C2, (321) 867-7555
Participating Organizations: Dynacs Inc. (Dr. S.D. Augustynowicz, Z.F. Nagy, and K.W. Heckle) and Oak Ridge National Laboratory (Dr. J.A. Demko)