Diurnal Load Leveling

Demand for electric power has been increasing steadily, and as it increases, the gap between daytime and nighttime demand has been widening. Since generation capacity must be matched to peak demand, this situation has driven the construction of additional generation, transmission, and distribution infrastructure. Much of this infrastructure is only lightly utilized much of the time, leading to higher electricity rates. This situation is most acute in Japan, where demand for power is increasing more rapidly than in the United States. The most desirable solution, called diurnal or daily load leveling, is to have a mechanism for storing power during the night and feeding it back into the grid during the day. The basic idea is illustrated in Fig. 4.1.

In the United States, load leveling is addressed in some localities by building "pumped hydro" storage facilities. These consist of two water reservoirs or lakes at different elevations connected by a pump/generator station. During the night, water is pumped from the lower lake to the higher one using electric pumps. During the day, the water flows from the upper lake to the lower one, generating power. The overall efficiency of these systems is usually upwards of 80%, but their applicability is obviously tied to geography -- there must be a suitable pair of lakes available. This is rarely the case in Japan, and another method of power storage is needed there. Ideally, this storage is located as close to the consumer as possible in order to decrease the peak demand on the transmission system as well as on the generation system.

Japanese Flywheel Energy Storage Program

To address this problem, two programs are active in Japan, the superconducting magnetic energy storage (SMES) program discussed in Chapter 2, and a program on superconducting flywheel energy storage. This focus on diurnal load leveling contrasts with U.S. SMES programs that focus on power quality improvement.

Fig. 4.1. The concept of daily load leveling by electric power storage system (NEDO n.d.).

Based on a systems-level analysis of the energy storage need, the Japanese flywheel effort has focused on storing energy at the substation level, rather than at the generating-site or end-user level. The required energy storage level is approximately 10 MWh, and because of the very limited availability of space in Japan, compactness of the flywheel system is a high priority. In 1995, the New Energy and Industrial Technology Development Organization (NEDO), part of Japan's Ministry of International Trade and Industry (MITI), began a five-year program in superconducting flywheel energy storage that includes construction of a 10 kWh system in ~1999 and development of the component technology for the commercial 10 MWh system (Fig. 4.2).

The NEDO budget contained no funding for flywheels until FY 1995. The funding in 1995 was ¥300 million (~$3 million) and in 1996 was ¥500 million (~$5 million).

Program Content

The NEDO program addresses three areas:

  1. the flywheel itself
  2. the high temperature superconducting magnetic bearing
  3. systems integration

Flywheel energy storage is not a new technology. In fact, it is widely used in industry in applications as diverse as punch presses and high field pulsed magnets. Its stored energy density (usually expressed in Watt-hour/kg) is, however, relatively low. In addition, its practical use has been limited to areas where the energy holding time is short because of the excessive rotational loss caused by bearings and windage. The NEDO program addresses the energy density issue through work on high-strength, high-speed flywheels and addresses holding time through development of a very low friction HTS magnetic bearing.

Fig. 4.2. NEDO's R&D schedule for flywheel energy storage (NEDO n.d.).

High strength, high speed flywheels

Most commercial flywheels are made of metal and rotate at relatively low speeds to maintain the tensile stresses in the flywheel within reasonable limits. Since the stored energy is directly proportional to the mass of the flywheel but proportional to the square of the rotational speed, increases in rotational speed yield a large benefit in energy density. Today's state-of-the-art flywheels therefore employ fiber-reinforced plastics (FRP) rather than metal. These materials can be engineered to have very high strength in the radial direction, thus permitting higher operational speeds. In addition, they can be designed to fracture into many small pieces in the event of a structural failure. This reduces the size of the required containment vessel and allows the system to be mounted above ground. (Safety considerations have led to designs of metal flywheel systems where the flywheel itself is underground). While the first demonstration flywheel in the NEDO program was made of steel, all subsequent devices will employ FRP. An additional benefit of the FRP flywheel, as seen in the following section, is that the lighter flywheel simplifies the design of the magnetic bearing.

Superconducting magnetic bearings

The primary factor preventing the application of flywheels to long-term energy storage is loss in the bearings. Any mechanical bearing with contact between the stationary and rotating parts will have enough loss to render the system uneconomical (Higasa 1994). One solution to the problem is to use a non-contact active magnetic bearing that employs conventional electromagnets. The rotational loss of such a bearing is 1-10% that of a mechanical bearing under the same operating conditions. The problem, however, is that the bearing itself consumes power, which is dissipated as heat in the copper electromagnets, and the bearing and cooling system power consumption must be included in the calculation of the overall system efficiency. A reasonable magnetic bearing consumes a few watts for each kilogram of flywheel weight, depending on the structure of the bearing and the control system, and this loss is sufficient to make a system using copper electromagnets uneconomical. Superconducting magnetic bearings, on the other hand, have demonstrated losses of 10-2 to 10-3 watts per kg for a 2,000 rpm rotor. This translates to an overall one-day, "round-trip" system efficiency of 84%, which is acceptable.

Figures 4.3 and 4.4 show the basic operation of the flywheel and bearing. The flywheel is at room temperature and carries on its lower surface a permanent ring magnet that rides above the superconducting portion of the bearing, which is simply an array of pellets of YBCO. The YBCO is kept at 77 K by an external supply of liquid nitrogen. The YBCO traps the magnetic flux produced by the permanent magnet, and as long as the pinning force of the YBCO is not exceeded, the bearing generates restoring forces to counter any relative motion of the permanent magnet and the superconductor. Thus, the bearing is completely passive and does not require the complex feedback and control circuits needed for conventional magnetic bearings.

Fig. 4.3. Flywheel system and details of superconducting magnetic bearing assembly (Higasa 1994).

Fig. 4.4. Operation of the magnetic bearing (NEDO n.d.).

The development tasks for the bearing involve

  1. increasing the pinning force of the YBCO to increase the mass of the flywheel that can be supported (presently the design target is 0.5 to 1.0 kg of flywheel per cm2 of bearing surface)
  2. refining the design of the magnetic bearing to reduce perturbations in the magnetic field seen by the superconductor when the flywheel rotates -- such perturbations lead to loss in the superconductor and drag on the bearing
System integration

The flywheel program is in the early stages, and there appears to be no published work on Japanese plans for system integration. The superconducting bearing should, however, present no major challenges since the designs published show operation near 77 K and the expected heat loads are quite modest. They could be handled either by a liquid nitrogen supply or by integration of a single-stage Gifford McMahon cryocooler. The present target is an overall energy density of the flywheel of 100-200 Wh/kg. This is to be compared with a density of about 40 Wh/kg for lead acid batteries.

German Flywheel Energy Storage Program

In Germany, the Forschungszentrum Karlsruhe (FZK) directs a flywheel energy storage program that uses melt-processed YBCO for the bearings. FZK's Institut für Technische Physik (INFP) has advertised and offered for commercial sale semifinished cubes, cylinders, and rods of melt-processed YBCO. The institute has built and tested a 300 Wh flywheel model using superconducting bearings combined with permanent magnets. The flywheel system uses disks constructed from advanced carbon fibers. It was operated from 30,000 to 50,000 rpm and produced 10 kVA/300 Wh at 50,000 rpm. The electric drive system for the flywheel was developed in cooperation with the Institut für Elektrische Maschninen und Antriebe of Stuttgart University. The next step in the program is to develop a larger flywheel with a capacity of about 7 Wh/250 kVA, together with Siemens and accompanied by application studies with utilities. This flywheel will probably be a "stacked" design similar to that used in the NEDO program. A composite, rather than a steel, flywheel is planned with a diameter of approximately 80 cm. Large, above-ground installation with appropriate protection, is favored over below-ground excavation, due to the projected lower installation expense. It is expected that large energy delivery at > 20 MW would most likely experience some problems with respect to heating of the motor/generator. Flywheels appear attractive also for railway application combined with regenerative braking for charging. Siemens is also conducting studies using flywheels for spinning reserve.

Published: September 1997; WTEC Hyper-Librarian