A Development Path to the Efficient and Cost-Effective Bulk Storage of Electrical Energy

Abstract

Efficient and cost-effective means for storing electrical energy is becoming an increasing need in our electricity-oriented society. For example, for electric utilities an emerging need is for distributed storage systems, that is, energy storage at substations, at solar or wind-power sites, or for load-leveling at the site of major consumers of their electricity. One of the important consequences of distributed storage for the utilities would be the reduction in transmission losses that would result from having a local source of load-leveling power. For applications such as these there are three criteria that must be satisfied by any new system that is developed to meet such needs. These criteria are: (1) high 'turn-around' efficiency, that is, high efficiency of both storing and recovering the stored energy in electrical form, (2) long service life (tens of years), with low maintenance requirements, and, (3) acceptably low capital cost. An additional requirement for these particular applications is that the system should have low enough standby losses to permit operation on a diurnal cycle, that is, storing the energy during a portion of a given day (say during sunlight hours) followed several hours later by its use during night-time hours. One answer to the spectrummore » of energy storage needs just outlined is the 'electromechanical battery'. The E-M battery, under development for several years at the Laboratory and elsewhere in the world, has the potential to solve the above energy storage problems in a manner superior to the electro-chemical battery in the important attributes of energy recovery efficiency, cycle lifetime, and amortized capital cost. An electromechanical battery is an energy storage module consisting of a high-speed rotor, fabricated from fiber composite, and having an integrally mounted generator/motor. The rotor operates at high speed, in vacuo, inside of a hermetically sealed enclosure, supported by a 'magnetic bearing', that is, a bearing that uses magnetic forces to support the rotor against gravity. Magnetic bearings are a virtual necessity for the E-M battery in order to achieve long service life, and to minimize frictional losses so that the battery does not lose its charge (run down) too rapidly. These considerations mitigate against the use of conventional mechanical bearings in the E-M battery for most applications. The Laboratory has pioneered the development of a new form of magnetic bearing to meet the special requirements of the E-M battery: the 'ambient-temperature passive magnetic bearing'. Simpler, and potentially much less expensive than the existing 'active' magnetic bearings (ones requiring electronic amplifiers and feedback circuits for their operation) development of the ambient-temperature passive magnetic bearing represents a technological breakthrough. Beyond its use in the E-M battery, the ambient-temperature magnetic bearing could have important applications in replacing conventional lubricated mechanical bearings in electrical machinery. Here the gains would be two-fold: reduced frictional losses, leading to higher motor efficiency, and, of equal importance, the elimination of the need for lubricants and for routine replacement of the bearings owing to mechanical wear. Thus an added benefit from a vigorous pursuit of our electromechanical battery concepts could be its impact on many other areas of industry where rotating machinery in need of improved bearings is involved. If perfected, passive magnetic bearings would seem to represent an almost ideal replacement for the mechanical bearings in many types of industrial electrical machinery. Returning to the issued of energy storage, the E-M battery itself has much to contribute in the area of improving the efficiency of stationary energy storage systems. For example, many electrical utilities utilize 'pumped hydro' energy storage systems as a means of improving the utilization of their 'base-load' power plants. That is, electrical energy is stored during off-peak hours for delivery at times of peak usage. These pumped hydro systems employ upper and lower reservoirs, between which water is shuttled to store and recover the energy. Of necessity, pumped hydro storage facilities are located in mountainous areas, usually far, both from the urban centers where power uses are concentrated, and from the sites of the power plants themselves, increasing the transmission line losses that subtract from the useful energy. More importantly, pumped hydro systems themselves only return from 65 to 70 percent of the electrical energy input required to pump the water from the lower reservoir to the upper one. Thus, including the extra transmission losses from distant siting of the facility, of order 40 percent of the input electrical energy is wasted in every cycle of use of the facility.« less