Electronic "Shock Absorbers"

Hawaii provides a perfect laboratory for demonstrating future technologies for integrating wind power with the grid. Each island must be self-sufficient in electrical energy, because there are no large regional power systems to provide backup, as on the mainland. When wind generation reaches 5–10 percent of one of these networks' total generating capacity, sudden changes in wind speed can result in a loss of power generation great enough to trigger automatic load shedding, disrupting consumers. This is the challenge that now faces Hawaiian Electric Co. (HECO), headquartered in Honolulu, as it adds enough wind-energy installations on its Big Island grid to bring the total wind generating capacity to 30 MW. The proportion of the Big Island's load demand supplied by wind energy could soon exceed 15 percent during peak demand hours and 30 percent during hours of minimum load.

The current approach to preventing such wind-triggered load-shedding is to add conventional power capacity to the system. As an alternative, HECO has launched a proof-of-concept demonstration combining power electronics and energy storage technology, which we call the Electronic Shock Absorber (ESA). HECO's first ESA demo unit, installed at the Lalamilo Wind Farm on the Big Island, has been operating since early January.

The ESA is programmed to respond in three scenarios. It absorbs power briefly when it detects a sharp increase in the instantaneous output of the Lalamilo Wind Farm, such as one caused by a strong gust of wind. Conversely, a lull in the wind lasting a few seconds will cause the ESA to inject power into the system. More gradual deviations from the average power output that exceed specified limits can also prompt the ESA to absorb or release power accordingly.

Importantly, the ESA can regulate reactive power—the product of current on a transmission line that is alternating out of phase with its voltage. Reactive power is consumed by the energetic fields in transmission lines; too much or too little reactive power can cause a line's voltage to spike or sag, respectively. By regulating reactive power, ESA can compensate for voltage changes on the grid to improve both power quality and system stability. Commercial power electronics built into wind turbines or in stand-alone wind-farm-scale units do provide such reactive-power support, but the ESA's energy storage capacity means it can do that much more.

The ESA consists of an inverter connected in shunt to the power line (that is, it branches off the line) that interfaces between the power line's alternating current and a direct-current energy storage component. For energy storage, HECO selected ultracapacitors, which store energy electrostatically by polarizing an electrolytic solution between two highly porous conductors. Thanks to the ultracapacitors' compact design, an ESA capable of matching 15–25 percent of an average wind farm's output for 15 seconds to 1 minute may be small enough to mount on a truck trailer. The ESAs should also require little or no maintenance, and they have a much more favorable cycle life than electrochemical batteries. The demonstration unit at Lalamilo has a 1-MVA continuous rating, but it can provide 330 percent more reactive power for up to 2 seconds. To serve larger wind farms, modular units resembling the ESA at Lalamilo can be ganged together, accommodating 2–32 MVA ratings at system voltage.

Since its commissioning in January, the demo ESA has performed as designed and has provided significant stabilization to the HECO grid. It has counteracted power and voltage fluctuations from the wind farm, as well as fluctuations on the grid caused by conventional generators or load interruptions. Further tuning of the device continues as we seek to optimize the ESA's response to a variety of conditions, particularly those that would be encountered on grids with greater wind contribution and larger wind farms.

Peak Shaving

Whereas power electronics with some storage capability like the ESA can handle fluctuations on a subseconds-to-minutes time frame, other solutions can help accommodate wind power's minutes-to-hourly fluctuations. Today, such fluctuation is generally accommodated in much the same way as fluctuating demand from consumers is handled: by ramping conventional power plants up and down. This, of course, means burning fuel—a solution that requires the maintenance of extra power-generating capacity. The cost of such backup, while currently negligible, will rise as the percentage of wind energy on the power grid multiplies—a situation that is already imminent in Hawaii. Energy storage offers a backup solution that is potentially less costly, as well as being truly renewable.

HECO is evaluating the potential of adding what is known as a pumped hydro system, in which energy is stored by pumping water from a lower reservoir to a higher one using excess generating capacity during off-peak hours. Then, during peak hours, the water is released and flows back down through a hydroelectric turbine, generating additional power as needed. This type of system could smooth out hour-to-hour fluctuations at a wind farm. Recognizing this potential, HECO is evaluating the installation of 20–50 MWh of pumped hydro storage capacity to accept energy from three major wind farms; the storage will compensate for variable winds by storing energy available during off-peak hours for use during periods of high demand.

Batteries present another option for backing up wind without firing up oil- or gas-fueled plants. Whereas locations with suitable terrain for pumped hydro storage are limited, batteries can be placed almost anywhere. Strategically placed battery installations could help smooth power supply from a wind farm while also easing power management concerns for the transmission grid: wind power produced outside of peak consumption hours can be delivered to battery installations over the power lines during off-peak hours, when the lines have spare capacity.

Consider an energy storage project under way at the New York Power Authority in the New York City area. This project, in which HECO is participating, uses a battery to reduce peak demand on the heavily loaded Long Island grid. Specifically, a 1.2-MW sodium sulfur battery with a storage capacity of 7.2 MWh, manufactured by Japan-based NGK Insulators, is being installed at the Long Island Bus Refueling Station. By charging this battery during off-peak hours, the customer can run energy-intensive fuel compressors during midday hours without pulling power over the congested grid, thereby avoiding peak demand charges.