Sunday, September 1, 2013

Energy Storage Application 4: Regulation

Utility operators work very hard to predict demand changes across many intervals (i.e. within a given hour, daily, weekly, monthly, etc.) in order to match supply to demand. No matter how accurate these predictions are, the grid is characterized by short interval variations in demand. If the momentary difference between supply and demand exceeds a tolerable threshold, the grid may experience unfavorable power quality events. These unfavorable events may include changes in the power frequency, voltage sags, variation to the waveform of the AC power and presence of harmonic currents within the power supply. If these irregularities continue to a longer interval, noticeable brownouts or even blackouts can occur (EPRI, 2003). All of these out of specification power events can be very damaging to distribution equipment on the grid itself as well as to equipment consuming the power. In the case of blackouts, the financial impact can be exacerbated by lost productivity and damage or loss of product because of interrupted manufacturing processes (Lineweber & McNulty, 2001). On various timescales energy storage, with its highly variable output and short response times, can be used for power regulation. An energy storage device is appropriate for regulation when it can be quickly discharged or charged to reconcile the many momentary differences between supply and demand.
As some energy storage technologies are highly modular and distributable, they are particularly well suited for voltage support and reactive power. Reactance is a localized phenomenon experienced in AC circuits caused by equipment in the circuit that acts as a capacitor or an inductor (i.e. equipment such as asynchronous induction motors commonly used for air conditioning equipment) (Banta, 2012). This equipment causes the accumulation of electric or magnetic fields within the circuit proximal to the equipment. This is known as reactive power, which in turn produces an opposing electromotive force in the circuit causing the voltage and current to come out of phase. This out of phase condition reduces the real power, or usable power, in the circuit (Bhatia). The impact of this effect is represented by the circuit’s power factor:

If a circuit has a power factor below 1, then power is being lost due to reactance, in this case, generation equipment must provide more power to compensate in the form of reactive power (VAR; Volt-Ampere Reactive). The problem with using central generation to provide reactive power is that reactive power cannot be effectively transmitted to the reactive load center over the distances common in a traditional centralized generation power grid. Many major power outages can be attributed, in some part, to problems with transmitting reactive power to load centers. Placing an energy storage device at the load center, where the most reactance occurs, can significantly reduce the required amount of reactive power produced by central generation equipment thereby improving grid stability (Eyer & Corey, 2010).

Works Cited

Banta, R. L. (2012, November 23). Mechanical Engineer, Plant Operations. (M. R. Banta, Interviewer)

Bhatia, A. (n.d.). Inductive and Capacitive Reactance. Retrieved December 9, 2012, from CED Engineering: http://www.cedengineering.com/upload/Inductive%20and%20Capacitive%20Reactance.pdf

EPRI. (2003). EPRI-DOE Handbook of Energy Storage for Transmission & Distribution Applications. Washington DC: EPRI, Palo Alto, CA, and the U.S. Department of Energy.

Eyer, J., & Corey, G. (2010). Energy Storage for the Electricity Grid: Benefits and Market Potential Assessment Guide. Albuquerque, New Mexico: Sandia National Laboratories.

Lineweber, D., & McNulty, S. (2001). The Cost of Power Disturbances to Industrial & Digital Economy Companies. Madison, WI: EPRI.



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