Issues in the Storage of Electric Power

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Issues in the Storage of Electric Power

By Ruth Howes and Sekazi Mtingwa

Most of us are now well aware that the economic security of the United States requires development of renewable energy sources, and that our aging electrical grid needs renewal. The storage of electric power is essential to both these objectives. POPA recently prepared a report on this problem that can be downloaded at http://www.aps.org/policy/reports/popa-reports/ . This article summarizes that report.

There are at least three critical properties of an electric power supply system: 1) Stable voltage at any current; 2) Stable frequency (critical for digital equipment) and 3) No interruption of service even for very short times. One study (1) estimates that power outages cost US consumers $79 billion annually with 2/3 of this due to outages lasting less than 5 minutes. Short power outages are rarely critical for domestic consumers, but they cause large computer processing operations to reboot all the computers in their systems. For the Fabs where chips are manufactured, even a brief power outage means the loss of an entire batch of melt material plus the labor of cleaning up the resulting mess. Electricity storage can function as a backup source of electric power during unavoidable power outages.

In the U.S., demand for electric power varies by about a third with time of day as well as with seasons of the year. Therefore power companies vary the energy sources used to generate the electric power they provide. For example, nuclear reactors cannot be quickly shut down and restarted while natural gas generation plants can be quickly started and stopped. Nuclear power is excellent for providing power to meet a stable base load while natural gas plants are used to respond to surges in demand or peak loads that occur when people come home from work or when hot weather causes people to run their air conditioners. Electricity storage can stockpile electric power from a background source and release it to meet peak needs. Increased reliance on nuclear power plants will require storage to meet peak demand in a timely manner. Many renewable energy sources, for example solar energy and wind energy, are intrinsically variable, and storage will be needed to allow them to supply electricity when demand is highest.

Many renewable sources of electricity are geographically distributed and must be connected to the grid both to allow back up to a local generator and to enable local generators to add power to the grid. Such systems will need storage to enable frequency and voltage matching. Power matching applications of electricity storage will become more important with the growth of new generation technologies based on renewable sources.

Finally, local electric power companies can use large scale storage to supply power to isolated subdivisions while they plan future construction and spread expenses evenly over time. No storage technology can be seriously evaluated without considering economic feasibility. Any commercially viable technology will have to have a reasonably long life time. Table 1 below describes the properties of storage systems needed to make them effective for various applications. Rated properties include the rate at which energy can be discharged (discharged power), total energy stored, efficiency required, and total lifetime. Applications include power matching ( providing more power to meet peak demand or sudden surges in demand), backup power during outages, enabling renewable technologies (by storing power generated by intermittent renewable technologies such as solar power or wind power for use when the power source is not available), and power quality (that is keeping voltage and frequency stable during demand fluctuations).

Table 1: Requirements for Different Applications of Electricity Storage
Based on data from Schoenung (reference 2)

Energy Storage Technologies

There are currently six technologies for electricity storage that are under active consideration for commercial deployment: pumped hydropower, compressed air storage (CAES), batteries, flywheels, superconducting magnetic energy storage (SMES) and “super” capacitors. They are in various stages of development and commercialization and offer differing advantages.

Pumped hydropower storage uses off-peak electric power to operate pumps that fill a water reservoir. At peak demand, the stored water is released through a hydroelectric generating plant. The technology is well understood and has been commercially deployed, for example, by the TVA at the Raccoon Mountain Plant which has a generating capacity of 1600 megawatts. Hydropower responds quickly to changes in demand and can generate high levels of power for long times. The difficulty with pumped hydropower is that it requires a large reservoir with attendant environmental problems, and systems are very expensive to construct. Projected improvements rely on variable speed pumps and turbines which can lead to at least a 3% increase in efficiency.

Compressed air storage uses off-peak power to pump compressed air into a storage container. On a commercial scale, the container will probably be a limestone cavity. Should CAES be used to support distributed generation, the container will a pressurized tank. There are two large CAES facilities built as demonstration plants although there are no commercial facilities as yet. CAES is less environmentally damaging than pumped hydro and as a distributed system is projected to work as a natural partner with wind generation. Large scale systems require a reservoir to store the compressed air, and small scale systems have safety problems with the possibility of exploding containers. Technical advances include development of small scale systems for distributed generation and better storage containers for the compressed air.

Batteries are a major technology for portable energy storage and find wide application in transportation and portable appliances. Here we consider only their application to the storage of electric power. In these applications which are primarily commercialized at facilities like the Fabs where power outages are disastrous, battery banks are located close to the facility that is being protected. Local power companies also use battery banks to supply emergency power in areas where power demand has rapidly growing peak demand. Batteries offer high energy storage densities, rapid response times, and they are portable. However, they are very expensive and have limited life times. The materials of which they are made pose environmental hazards. There are major research efforts underway to develop batteries that cost less and have longer life times. The research is creative but there is a long way to go before batteries will be an affordable option for electricity storage on a residential or industrial scale.

Flywheels store energy as rotational kinetic energy. They can store more energy if they operate at greater rotational velocities or if they are larger. They are limited by the properties of the materials of which they are made since large wheels tend to break apart at high angular velocities and by dynamical instabilities in rotation. Flywheels respond very quickly and can be connected in “farms” for large energy storage. At present, they are in a prototype phase and are very expensive. The obvious research needs are in materials science.

Superconducting magnetic energy storage uses high currents in superconducting coils to store electrical energy. SMES systems offer the possibility of very fast response with discharge of high power. For large scale energy storage, they can be networked, and they have long lifetimes because they have no moving parts. However, they require cryogenic systems which do wear out, and they are very expensive and currently experimental. However, it is possible that developments in materials such as high Tc superconductors could make this appealing technology a practical method of storing electrical energy.

Conventional capacitors store energy as a charge on electrodes separated by a dielectric material. Charge storage depends on the area of the electrodes. “Super” capacitors increase the electrode area by using porous electrodes and vary materials to increase operating voltages. They are potentially capable of rapid and high power discharges. Like SMES systems, they have no moving parts and potentially long lifetimes. At present, they are experimental, expensive and able to store little energy.

Figure 1, prepared by John Scofield, compares the capabilities of electricity storage technologies as of this writing.

A final issue in electrical energy storage is the power conversion system that releases or stores power and matches voltage and frequency to the grid. Power conversion systems need to operate with rapid response time at high currents and voltages to produce power with stable voltages and frequencies. They must be reliable and efficient. Although they are often overlooked in discussions of storage technologies, power conversion systems account for at least 20%, and as much as 70%, of the cost of an electricity storage system because they are one-of-a-kind. They require thermal backup and are limited by currently available thermal materials. Finally, computer models of systems do not include power conversion. There is a genuine need for software for modeling storage systems as a part of the grid that will allow for efficient planning.

Figure 1: Capabilities of Existing Electricity Storage Technologies

Political Issues in Implementing Electrical Storage

Even if the technical issues in storing electrical power can be resolved, there are a number of political barriers to implementing new systems, particularly on a distributed basis. The first question is, who should pay for implementing storage systems and for demonstration projects? Power companies argue that the federal and state governments should support research and development until technologies have been shown to be commercially feasible. Demonstration projects whose costs are shared between the government and the utility company seem promising for large storage project. In the case of distributed storage systems, one must ask whether the owner of the grid or the owner of the generating system owns the storage system. It is possible that a system could be developed whereby power companies charge premium prices for high quality power that is uninterruptible and has very stable voltage and frequency.

On the federal level, research and development of electrical power storage systems are spread across agencies from DOE and DOD to NASA. There is an urgent need for a central panel to coordinate efforts as well as to recommend pricing and regulatory policies to facilitate the development and deployment of electrical storage systems. It is also critical to consider the environmental impact of diverse storage systems. electric power energy

In conclusion, it is clear that increased reliance on either nuclear fission or renewable energy sources for generation of electric power will require the use of electrical power storage systems. While electricity storage systems and the power conversion systems they require are not a glamorous as large wind farms or the huge mirrors of solar thermal power, they will be an essential component of the grid of the future.

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