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Hybrid Storage

Ultracapacitors and batteries work together to solve power quality problems.

Fortnightly Magazine - June 2010

present grid isn’t integrated, and trying to use it as if it is integrated presents a problem in terms of power quality. The grid has evolved with no real, long-range plan. As a consequence, the current grid system is a patchwork of transmission lines—which, all things considered, works remarkably well. However, unless the problems affecting the grid are addressed in the short term, the situation will worsen, resulting in more frequent, widespread power outages, reduced power quality and the possibility of equipment damage.

Assuming the power grid will remain somewhat fragile—and not integrated—for the foreseeable future, other methods for improving power quality must be considered. Two methodologies that suggest themselves are to reduce the demands placed on the grid infrastructure and to look at alternatives that allow end users or localities to improve power quality in ways that don’t depend on the grid infrastructure.

Storage Solutions

One solution is the installation of a power conditioner. A power conditioner provides an alternating current (AC) signal that doesn’t vary in frequency. One straightforward way to do this is to store the power ahead of the conditioner as direct current (DC) using a bank of capacitors, and then use a DC-to-AC inverter to produce perfect 60-Hz AC. The cost of such a conditioner is driven mostly by the total power required. A second approach uses a ride-through solution. Sufficient power is stored in an ultracapacitor bank. In the event of an interruption, the ride-through power supply carries the load. The cost of this system is driven by the transmission line length needed to provide power to the system.

Other solutions to providing energy storage to electrical utility operators include spinning reserve, pumped-hydro, flywheels and high-pressure air. Spinning reserve is the practice of having a generating station running, but offline, until rising demand requires bringing additional generating capacity on line. Spinning reserve is expensive and inefficient, with power plants idling and burning fuel for long periods of time. The pumped-hydro method allows a utility to produce energy at a relatively constant rate and use periods of low demand to pump water into an elevated holding area; when demand increases, the water can be used to produce hydro electricity by recapturing the gravitational potential energy. The flywheel system uses power during periods of low demand to put energy into flywheels and which energy is recaptured when demand increases. Similarly, in compressed-air energy storage systems, high pressure air is accumulated when demand is low and used to drive a turbine under conditions of increased demand.

Pumped hydro and compressed-air storage can work well in specialized conditions— i.e., when a large reservoir or underground air storage site is available, and when transmission capacity is sufficient to serve the facility. Flywheel farms show some promise, but high maintenance costs have limited their development to prototype projects.

As technology has advanced, however, a new approach to intermediate storage has emerged, using a parallel combination of batteries and ultracapacitors.

Dense Energy

The parallel combination has both high energy density and power density. Ultracapacitors, like all capacitors, have a high power density— i.e.