SUPERCONDUCTING UTILITY POWER ENERGY RETRIEVAL (SUPER)©
Bob Brown

Abstract

The fact that Electric power utilities have developed, built, and studied many load leveling concepts is ample proof of their continuing need. Also, of great importance, are any improvements in power transmission lines that will enhance system efficiency.

Existing reliable superconductors and the exciting prospect of high temperature superconductors make possible some innovative concepts for storing energy. Cryogenic cost are a major factor with all superconducting energy storage concepts. The Department of Energy supports a concept called SMES (Superconducting Magnetic Energy Storage) for storing energy inductively. SMES is basically a very large air core magnet buried underground. Very large magnetic forces are contained by the surrounding bedrock. The concept presented in this paper is an air core magnet with a bore so large it also serves as a transmission line. Call this concept SUPER (Superconducting Utility Power Energy Retrieval). This concept consist of a loop of superconductors having a very large bore but with a small winding cross-section. The bore may be measured in hundreds of miles but need not be circular or even be confined to the same plane. In practice, a very elongated loop (parallel lines) may be most attractive. The shape is determined by the capacity requirements, location of power sources, location of load centers, geography, and environmental impact. Calculations indicate that extremely large amounts of DC inductive energy will be stored with this concept. Power sources or loads may tap on this loop at any point, thus using the loop as a transmission line. Although this concept is ambitious, the needed technology is well established. Significant advantages of SUPER (when compared with SMES) are that magnetic forces are greatly reduced or trivial, there are no fundamental limits to its capacity, and it also serves as a transmission line. The stray field is high and localized with SMES. The stray field with SUPER is low but encompasses a large area. The environmental impact of stray fields for both systems is important and must be considered carefully. SUPER will not replace SMES but will offer another alternative for load leveling. It also has the bonus of being a power transmission line. A proof of concept study of all aspects of SUPER is justified. The study will be a first effort to determine physical size, capacity, superconducting materials, insulation, cryogenic needs, fault detection and protection, charging and discharging characteristics, stray field impact, and cost. Finally, the proof of concept study may demonstrate justification for building a small scale prototype (approximately 20 meter bore) as a first phase development program.

Background

At the time of the energy crisis in the early seventies, many new plans for generating or saving energy by the power utilities were proposed. As conservation programs were implemented the projected power needs were scaled back. Indeed, for several years many generating plants were delayed, postponed, or canceled. Inevitably, demand is now catching up with supply. Load leveling is an attractive alternative to building more generating capacity. Not only is less generating capacity needed for peak loads, systems are more efficient when operated at a constant load level.

Existing load leveling plants first convert electrical energy to mechanical, chemical, or thermal energy. When needed, some hours later, the energy is converted back into electrical energy. Examples are flywheels, batteries, hydraulic storage reservoir, etc.. Efficiency is improved if the energy remains in the electrical state. This is accomplished with both SMES and SUPER that store energy inductively. In addition, SUPER serves as a transmission line, delivering even greater system efficiency. Inductive transmission lines are transformed into an asset instead of a liability.

Description and Justification

The SUPER concept is an energy storage DC superconducting loop. It may have multiple turns in this loop. The ends of the loop are connected to each other. Electrically there is no beginning or ending. When a current is induced in a superconducting loop, the current persist because of the complete absence of electrical resistance in the conductor. This phenomenon makes the storage of inductive energy possible.

The loop may have any irregular shape and its elevation at any point may depend upon the terrain. See Fig. 1. In essence, the loop is a very large magnet coil with relatively few turns and an extremely large bore. The inductance of a coil is proportional to the area included in its bore and the product of the number of turns and current in each turn. The inductive capacity of this system is derived from the large bore more than by having many turns or large currents. See Fig. 2. A connected system of SUPER's might be constructed as shown in Fig. 3. A connected system permits systems to be linked for matching generators and loads over large areas.

In areas where stray fields are a serious problem, a very elongated loop (parallel lines) may be most attractive. Obviously, the bore area is reduced and energy storage capacity is sacrificed as a trade-off for stray field cancelling. Since most of the energy is stored very close to the conductor bundle, the sacrifice will not be as severe as it might appear. There may be some savings in the cost of right of way land and maintenance cost.

If the conductor bundle is buried in the ground, the earth may have enough electrical resistance to absorb energy (eddy currents) while charging and discharging the loop. As in the above description, most of the energy is stored very close the conductor bundle. The amount of energy absorbed by the earth can be reduced to acceptable levels by enclosing the conductor bundle in a larger pipe or tunnel that permits air or any medium that has zero resistance. Charging and discharging the loop is slow and is accomplished over a period of hours. While the energy absorption by the earth may be significant enough to consider, it may not be large enough to be a major problem. A very important feature of SUPER is that essentially all of the magnetic forces are contained in the winding pack. These are all compressive forces completely within the cryogenic envelope. By contrast, the large winding crossection of SMES and relatively small bore create extremely large radial magnetic forces. These forces must be supported from a cryogenic environment to the surrounding bedrock at normal temperatures. This is one of the biggest technical problems of SMES. With SUPER the radial magnetic forces are distributed over such a large area that they are much reduced or trivial. Only the weight of the windings are transferred through cryogenic barriers.

Another potential feature may be attractive at some time in the future. It has been predicted that hydrogen may become a fuel of choice as other fuels become scarce or environmentally unpleasant. By that time superconductors that operate at temperatures greater than 20 K should be reliable and available in large amounts. Liquid hydrogen will be adequate to keep the superconductors cold and the containing pipe for the conductor bundle will be available to transport the liquid hydrogen. It is not proposed that this liquid hydrogen feature be included in the proof of concept study.

Development of SUPER will be a long term program. Although the potential for electric utilities is great, the payoff is far in the future for them. The first step is to initiate a proof of concept study to explore its feasibility in detail. Many organizations have suitable skills to perform the proof of concept study. East Tennessee has ORNL (Oak Ridge National Laboratory) and UT (University of Tennessee) that already have much depth in these technologies.

After the proof of concept is proven, a prototype system may be reasonable as the next step. A 20 m (66 feet) prototype would demonstrate a working system. It would test the design concept, energy handling, conductor and insulation, and cryogenic system. SUPER will draw heavily upon the work already documented by SMES. It is anticipated there will be considerable cooperation and collaboration between SMES and SUPER.

Proof of Concept Study

The study should first select an example that represents the typical requirements of a power utility. The following system appears interesting. The system would have a radius of 161 km (100 miles), a conductor bundle radius of .04 m (1.6 inches), and a peak field of .34 T at the surface of conductor bundle. A rough calculation indicates that this system, having a current density of 7 kA/cm2, would store 1,100 MWh of energy. This corresponds to one TVA (Tennessee Valley Authority) nuclear reactor like one located at their Sequoyah Nuclear Plant. The stray field is 2 gauss at 100 m (328 feet) from the conductors.

Consider a really fantastic system. The system would have the same radius of 161 km (100 miles), a conductor bundle radius of .2 m (7.9 inches), and a peak field of 8.5 T at the surface of conductor bundle. A rough calculation indicates that this system, having a current density of 7 kA/cm2, would store 27,000 MWh of energy. This is over 24 times the total capacity of the same TVA nuclear unit mentioned above. The stray field is 44 gauss at 100 m (328 feet) from the conductors.

Much of the technology that is needed for SUPER has already been studied in depth in the SMES program. This material will be used as much as possible. An important task of the study will be to size the system. Other tasks are: Investigating Grantz bridge circuits for transforming AC to DC and charging and discharging the system. A selection of the conductor, its size, and insulation. Voltage breakdown during charging and discharging. Other fault possibilities and protection. Supports for the weight of the windings and heat loss analysis. Size cryogenic systems. Estimates of cost.

Information gathered in the proof of concept study should assist in specifying a prototype or other test for subsequent technical development.

The initial study is primarily a paper project. It may make a good thesis for a graduate student. Elaborate computer models are not anticipated. A desk computer should be adequate.

This idea was conceived while under contract with the DOE (Department Of Energy). Therefore patents that might result may be the property of the DOE. Although the idea has been reviewed by a number of professionals, it has never been made public. Before any public disclosure is made, it must be cleared with the Patent Office of the DOE in Oak Ridge. There should be no problems licensing this technology to private industry.

Figure #1. For larger display click on figure.

Figure #2. For larger display click on figure.

Figure #3. For larger display click on figure.

Comment

Sooner or later someone will snap their fingers and say, "Why now combine energy storage with transmission systems?" Click on article below to enlarge.

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