Not many years ago, power quality was largely a technical issue that interested a few highly specialized experts. Today, it has become a national concern. The reason for this change lies in the rapid rise of digital technology. As you'll read in this article, most power quality disturbances that affect digitally dependant industries are short in duration. The best option for providing seamless power lies in energy storage technology. Before I discuss the different types of batteries and new trends in energy storage, let's briefly look at the digital economy and its associated problems and needs.
Digital controls are everywhere. Production lines in high-tech manufacturing plants operate through digital switches. Transportation companies rely on digital devices for scheduling and ticketing. Telecommunication facilities require digital equipment to provide customers with the best service. Even financial institutions use binary code to carry out numerous transactions.
All these industries are vulnerable to small power quality disturbances because they rely on digital controls. A 30% sag lasting a few cycles can close down an entire plant. Moreover, recovery from outages can take many hours. As a result, PQ events become costly. Estimates for costs associated with outages range from $35 billion to $150 billion per year. In addition, reliability issues become security issues because many digital industries are nationally connected or serve as communication links for vital systems.
As you know, reliability is often expressed as the number of “nines.” The grid yields about 3 nines, or 99.9% reliability, but digital industries would like to have no more than a single 1-cycle outage per year, which corresponds to 9 nines. This terminology is, however, somewhat misleading. The number of nines relates to total electrical supply outages. However, the important figure for facility personnel is total downtime. Downtime is more closely related to the number of power quality events than to cumulative duration. Equally misleading, at least for digital industries, are the tables that give the cost per hour of an electricity outage. Recovery time is much the same for 5-cycle outages as for five-minute ones.
A more fruitful approach is to look at scatter plots that display the magnitude and duration of voltage fluctuations. Events falling outside the CBEMA curve, or one of its many offspring, are likely to cause system failures. Extensive data have been collected by EPRI, and a new study of power quality in Silicon Valley is underway with DOE funding. Fig. 1 summarizes the results of these studies using the framework of IEEE Standard 1159. The diagram shows the percentage of fatal power quality events in different regions of the chart. The salient point is that 98% of fatal power quality events are shorter than 15 sec — and most of them are voltage sags.
Energy storage represents virtually the only option for providing seamless continuity of a power supply and maintaining it for the crucial 15 sec. Even the fastest diesel generators cannot respond fast enough, and other forms of distributed generation (DG), such as microturbines and fuel cells, are even slower.
Of course, a customer with critical loads could opt to disengage from the grid and rely entirely on DG. Using an N+1 or N+2 configuration would provide continuous high-quality power, but this option could be considerably more expensive than grid power. It may also conflict with emission standard requirements in many parts of the country.
Furthermore, if loads are variable, some energy storage will be required because most DG has bad load-following characteristics. The ideal solution combines energy storage for at least 15 sec and DG for backup. The exact balance of storage and DG will be determined by the price of the equipment, maintenance and fuel costs, and the cost of downtime to the customer.
A wide array of batteries is available for energy storage. Most common are lead-acid (LA) batteries. These types of batteries are used widely in automobiles and trucks, so they are inexpensive and their operating characteristics are fairly well known.
The largest battery system using LA batteries is located in Puerto Rico. The system provides spinning reserve as well as voltage and frequency control for the island's grid. At 20MW and 14MWh, the system delivers both power and energy. Although the tropical location makes extra demands on the batteries, officials have decided to double the capacity of the system in the near future.
A transportable LA system with carefully integrated power electronics has been developed with partial funding from the DOE. The system allows high-tech customers to avoid the disastrous effects of microoutages and voltage sags with a payback of one to two years. It has been used successfully in polymer extrusion factories and semiconductor plants in sizes ranging from 2MW to 16MW. The developers of the system received an RD100 award for their efforts.
Valve-regulated lead-acid (VRLA) batteries need less maintenance than LA batteries. They are sealed, except for a small vent that controls internal pressure, and they require no topping off with water. On the Alaskan island of Metlakatla, engineers installed a 1MW/1.4MWh VRLA system to shield a town from the frequent momentary brownouts that occurred whenever a local sawmill processed a tree. An international panel of industry experts selected this system as one of the best 100 DOE-supported projects.
Gel batteries are becoming popular in Europe. They are more robust and can take more heat and charge abuse, but they are more expensive.
Nickel cadmium (NiCd) batteries are less common for large stationary applications, but a 40MW system is being built for voltage support on a long power line to Fairbanks, Alaska. Resistance to cold may have been among the deciding factors.
The family of flow batteries presents an interesting feature: they can decouple power and energy. A central cell stack provides power, but total energy is furnished by a reservoir of rechargeable electrolyte, which can be as large as one pleases and situated anywhere that's convenient.
Zinc-bromine batteries are available off the shelf and have been deployed at a number of sites. Again, integrated power electronics are essential to successful applications.
Vanadium redox batteries (VRB) were developed in Australia and Japan. Large units up to 500kW/10 hours have been deployed for load management in Japan. One of the more interesting applications features electrolyte storage in plastic bags, which can be stuffed into available crawl spaces and other residual areas. The technology also can provide voltage control. A VRB facility at a Japanese semiconductor factory provides 1.5MW for load leveling, but it can yield 3MW for 1.5 sec to eliminate sudden sags. Currently, a 250kW/8 hour unit is being launched for end-of-line voltage support in a remote area of southeastern Utah.
Sodium bromide batteries have received considerable interest recently. A 15 MW/8 hour facility is under construction in the United Kingdom. A similar system is planned for the state of Mississippi in collaboration with the Tennessee Valley Authority (TVA). The facility is expected to cost approximately $25 million.
Advanced batteries offer vastly decreased footprint and excellent maintenance characteristics. However, they tend to be expensive for large-scale applications. Lithium ion, lithium polymer, and nickel metal hydride batteries have been developed mainly for automotive use.
The market for electric cars may widen if other states join California in mandating zero-emission vehicles. Efforts are underway to test installations of used vehicular batteries for load leveling and power quality control. A wide secondary market would reduce the cost considerably.
A special case among advanced batteries is the sodium sulfur battery. Developed in Japan, this battery operates at high temperatures. Extensive tests have demonstrated safe containment under extreme conditions. Some 38 systems totaling approximately 2MW and 124MWh have been installed in Japan. The largest of these installations, located at a substation in the foothills of Mt. Fuji, provides 6MW for 8 hours. It also can supply active and reactive power to mitigate voltage sags and frequency fluctuations. Operation of a 500kW unit should begin in the U.S. in the near future. The unit will be used for load leveling or as an uninterruptible power supply (UPS).
Flywheels and More
Classical LA batteries or novel flow batteries rely on chemistry. Flywheels, supercapacitors, and superconducting magnetic energy storage (SMES) systems rely on physics; therefore, they're more robust and less subject to the ravages of entropy. Flywheels store kinetic energy, capacitors store electrostatic energy, and SMES systems store magnetic-field energy.
Flywheels are increasingly attracting interest. They are able to charge and discharge rapidly with few effects from temperature fluctuations or discharge patterns. They have good footprint, lower maintenance requirements, and a long life span, although power loss is faster than for batteries. Flywheels are particularly suitable for power quality control. A number of hybrid systems integrating flywheels with diesel generators are commercially available now.
High-temperature superconducting flywheels are under development with funding from DOE. Such systems would offer inherent stability, minimal power loss, and simplicity of operation, as well as increased energy storage capacity.
Supercapacitors store electrical energy as charge separation in porous high-surface-area electrodes. They are capable of fast charges and discharges and apparently can go through a large number of cycles without degradation. Although these claims are impressive, no large-scale system has been fielded yet.
SMES systems store energy in the magnetic field generated by a loop of endless current. Power is available almost instantaneously. There is no loss, and there are no moving parts. However, energy content is small, and the cryogenics can be annoying. Several 1MW units are used for power quality control throughout the world.
One recent and interesting development was the deployment of a string of distributed SMES units in northern Wisconsin to enhance stability of a transmission loop. The line is subject to large sudden load changes due to the operation of paper mills and has the potential for uncontrolled fluctuations and voltage collapse.
Besides stabilizing the grid, the six SMES units also provide increased power quality to customers served by connected feeders. Other units have recently been installed in the Houston area and the TVA is installing a unit for fast power control.
There are a number of discernible trends in energy storage technology. Storage units, particularly for applications requiring appreciable amounts of power, are increasingly factory assembled and tested. Customers want plug-and-play units. They have neither the time nor the expertise to tease a system into optimal performance.
In the same vein, the trend is toward fully integrated electronics with carefully designed inverters and control systems. Eventually, the controls can be expected to acquire a certain amount of intelligence to respond to weather or market signals. At the same time, such systems will be addressable through the Internet for diagnostics or inventory.
Hybrid systems combining energy storage for seamless response with backup power for reliability are becoming available. Backup usually consists of diesel generators, but microturbines and fuel cells will play a role in the future.
Finally, energy storage systems are getting bigger. Reliability for semiconductor factories and other sensitive industries requires multimegawatt power, and transmission stability applications have even higher needs. Energy management applications may eventually result in storage facilities of greater than 100MW.
This article was intended to highlight the crucial importance of energy storage in providing reliable power. The applications described are illustrative rather than comprehensive.
An excellent starting place for further exploration is the Web site of the Electricity Storage Association (www.electricitystorage.org). Links to most of the technology providers can be found there.
To obtain the latest available information, you may wish to attend the International Conference on Electrical Energy Storage Applications and Technologies (EESAT) 2002, to be held April 18 though April 21 in San Francisco. For details, visit www.sandia.gov/EESAT/.
Dr. Imre Gyuk is program manager for Energy Storage Research at the Department of Energy in Washington, D.C. You can reach him at [email protected].