Hybrid Storage


Ultracapacitors and batteries work together to solve power quality problems.

Fortnightly Magazine - June 2010

Power quality was not the important issue it is today when electrical appliances consisted of incandescent bulbs, induction motors and other devices of similar design. None of these items was overly sensitive to voltage sag, voltage swell, momentary interruptions or transients. Maybe a bulb would flicker or a motor would momentarily operate at reduced power; however, with the advent of microcomputers and sophisticated control circuitry, power quality has become important. Recent advances in storage devices such as improved batteries and ultracapacitors have made intermediate storage of electrical energy more practical and cost effective—which in turn helps utilities to deliver high-quality power at the local level.

Unlike most other products, electrical energy isn’t easily stored, so when an unexpected spike in demand occurs, it often leads to a reduction in power quality and manifests in voltage sag, harmonic distortion and variations in frequency. In severe cases, excessive load can cause a low frequency, forcing a generating facility to drop off line. This can put excessive load on other generating facilities, in some cases causing a domino effect and widespread power outage. The lack of efficient storage methods requires utilities to keep a vigilant eye on demand and be prepared to bring reserve facilities into action at a moment’s notice in order to prevent power interruptions.

New storage technologies are helping to make that job easier.

Power Problems

Demand changes come in two flavors: long- and short-term. The long-term changes are those that occur at a gradual rate, and are illustrated by the summertime increase in demand that accompanies a period of sultry summer weather resulting in the widespread use of air conditioners. This type of demand increase can be handled by the utility, provided it has sufficient capacity or can buy reserves from a neighboring utility via the grid network. More ubiquitous are short-term increases in demand that result from a sudden change in load that might occur when large machinery is brought online in a manufacturing facility. Indeed, the majority of fluctuations occur for a period of less than two seconds, and such momentary changes in demand represent the biggest headaches for utilities, as they lead to reduced power quality.

Power quality also is affected by aging delivery infrastructure. Transmission lines in many areas are overloaded, leading to higher line and power losses due to reactance created by an ever-increasing inductive load. With the proliferation of electronic devices and electric motors, inductive load is increasing, which in turn requires more and more reactive power to reduce the real power that’s delivered by utilities. This not only reduces profits to operators, but exacerbates the problems associated with the efficient delivery of quality power to the end user.

It would be advantageous for independent system operators (ISO) to have a fully integrated network so that power could be shifted from locations that are separated by great distances. However the 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., they can deliver a great deal of current from a small package. What differentiates ultracapacitors from their traditional counterparts, electrolytic capacitors, is their high energy density, allowing them to store a vast amount of energy in a small package.

The capacitors with which most design engineers are familiar have short time constants, which means their voltage cycles quickly, whereas ultracapacitor arrays have time constants between tens of seconds and minutes in length. The large capacitance and extremely low frequency time constants allow ultracapacitors to be used in applications that haven’t been practical or economical for other types of capacitors. Using such capacitor supplies in concert with power electronic techniques brings the design and cost of power conditioning equipment within reach of most volume users of electrical energy. Further, as the sophistication of power conditioners increases, their costs will come down, and such systems probably will become available for a much wider spectrum of power consumers.

Because ultracapacitors have a much lower internal resistance and much faster charge rate than batteries do, they can make a battery-powered system run much more efficiently. An array of ultracapacitor cells in series, coupled to a load in parallel with a storage battery, creates a hybrid power source with higher power and energy density than either device in a stand-alone configuration. By gradually taking on a load, batteries are insulated from high current drains that cause thermal, chemical, and mechanical stresses. And by reducing current spikes, the internal temperature of batteries is decreased substantially, extending the life of the batteries by as much as 400 percent, depending on the application. Additionally, there are times when a battery simply can’t deliver the current needed for an application. In this situation, an ultracapacitor can be used to augment the battery.

The primary limitation associated with ultracapacitors is their low voltage rating, which may be overcome to some extent in lower voltage applications by constructing a parallel-series array of devices; the series connections increase voltage standoff, while the parallel connections increase capacitance and reduce equivalent series resistance (ESR). The arrays can be interconnected easily and allow for capacitor banks that will function well up to intermediate voltage levels (400 to 600 V).

In most instances, it’s necessary to incorporate a controller and appropriate power electronic circuitry to meet specific needs. For example, an array of ultracapacitors can be used as a power source to compensate for power sags of short duration; however such systems require, in addition to the capacitor bank, a controller and power electronic circuitry to make them really useful. With these additions, a module can be constructed to compensate for power sag and do real-time power factor correction for loads of various sizes. The cost-to-benefit ratio of such a system is at present questionable, but as prices of ultracapacitors decline, such applications will become widespread. These systems also have the ability to be scaled up to higher voltage levels. Experimental applications at grid level still are being evaluated.

Ultracapacitors allow design engineers to separate energy and power needs. In most applications there’s a continuous power demand that’s handled by a primary energy source, and at times, there are peak power demands that require additional capacity. Engineers either cansize the batteries to handle peak demands, or use ultracapacitors to bridge the demand, which has the added benefit of being able to size the primary energy source.

Combinations of ultracapacitors and batteries in energy storage systems can reduce the size, weight and the number of batteries in a system. Such hybridized systems are more efficient and use fewer expensive materials. They also can extend the cycle life of the battery component, which makes the whole system greener.

Hybrid power sources consisting of batteries and ultracapacitors are being used in low- and intermediate-voltage applications. Because these systems are scalable, they have potential for use at high-voltage levels when augmented with the appropriate power electronic circuitry. In order for there to be a burgeoning of the ultracapacitor market, manufacturers will have to increase energy density further, which will make the devices more attractive from both a cost and size point of view. A number of companies are working on this problem, and preliminary research suggests energy densities of ultracapacitors have the potential to exceed the energy densities of many battery types. Some estimates suggest energy densities could be increased by as much as a factor of 100, which would allow for the wholesale replacement of batteries in many applications. Even a modest increase in energy density has the potential for replacing lead-acid storage batteries with ultracapacitors.

The future for ultracapacitors looks bright. If the energy densities and voltage level issues can be overcome at an economical price point, their usefulness in ensuring power quality will increase by orders of magnitude.