Three large shipping containers in Boothbay’s industrial park look like they might be storing something. They are: enough electricity to run 100 homes for a day.

This is New England’s first utility-scale electricity storage system. It’s the latest component of a pilot program aimed at meeting the heightened need for electricity during the hottest summer days on the Boothbay peninsula, at a fraction of the cost of building new transmission lines.

This storage system uses hundreds of special-purpose batteries. It’s designed to quickly deliver 500 kilowatts of electricity for up to six hours to the region’s electric grid.

But the concept also could be used to greatly expand the value of renewable energy in Maine. Battery technology is evolving, paving the way for giant battery banks that help integrate solar and wind power, by feeding electricity back to the grid at the very moment that clouds obscure the sun or the wind stops blowing. This approach now is being refined elsewhere in the country.

The Boothbay project was developed through a partnership led by New York City-based Convergent Energy + Power. The pilot program is being run by GridSolar LLC of Portland for the Maine Public Utilities Commission. Last month, Convergent invited utility representatives from around the Northeast to visit Maine and examine the storage system.

“The project in Boothbay is a perfect example of how to apply these on a grid-scale basis,” said Tremor Temchin, Convergent’s project manager. “We’re hoping there are going to be applications around New England.”

ANOTHER OVERLOAD PROTECTION

Inside the three shipping containers are racks that hold 600 batteries. They are sealed and spill-proof, using absorbent glass-mat technology that holds the electrolyte rather than the flooded, lead-acid cells of a typical car battery. Hydrogen gas given off during discharge is recombined with the batteries, so there are virtually no emissions, Convergent says.

At night, when power demand is low, the battery bank is charged from the region’s electric grid. Beginning at 9 a.m. each day, it’s ready to be dispatched as needed.

The GridSolar pilot combines several alternative strategies designed to keep power lines from being overloaded on hot, humid afternoons, when the area is jammed with tourists and air conditioners are cooling hotels, restaurants and shops.

Thousands of LED light bulbs have been installed to trim overall power demand. Hundreds of solar-electric panels now produce electricity when it’s needed most. Thermal energy storage units that make ice at night are supplementing air conditioning during the day. A diesel generator can be switched on for an added shot of power. And now, the battery bank stands ready.

Taken together, these and other measures conserve and produce 1.8 megawatts, the output of a small hydro dam.

So far, this arsenal hasn’t been called on to actually relieve an overloaded transmission line. But the components have been tested several times by Central Maine Power Co., most recently April 29.

“One important point,” said Steve Hinchman, GridSolar’s general counsel, “is unlike the active, non-transmission resources, such as the battery and backup generator, the so-called passive resources, efficiency and solar, are always on. This has the effect of buffering the grid in Boothbay from ever reaching the point of an emergency. This is what we want – an efficient, reliable grid where the active resources are essentially a last resort to avoid brownout or blackout.”

New England’s electric grid only operates near peak capacity several hours in a typical year, so the Boothbay batteries may only be needed a few times each summer. But the storage system also has value for Convergent and the broader grid day-to-day, by functioning as a small power plant. By charging the batteries at night with cleaner, cheaper power, the company can participate in wholesale energy markets during the day when prices are higher and the grid needs more energy.

Temchin declined to say how much the storage system costs to build. Documents filed by GridSolar show the energy is more expensive than any of the other four alternative measures, more than three times the net price of solar and five times that of efficiency. Even so, the total cost of the GridSolar experiment to ratepayers is coming in three times lower than estimates for building a new transmission line – $6 million, compared with $18 million.

A SOLAR AND WIND POWER TOOL

The battery storage experiment is of interest to the region’s grid operator, ISO-New England.

Moment to moment, grid operators must match power supply with demand, and make sure that voltage, frequency and other stability metrics are in balance. Batteries can help. If a power plant goes off-line, sensors can instantly inject a rush of electricity.

In 2013, a storage system using lithium ion batteries was installed next to a gas- and oil-fired power plant in Moraine, Ohio. It helps provide stability on a grid serving 60 million people in the Midwest and Middle Atlantic States. On a smaller level, the Boothbay battery bank is serving that function.

“Technologies like this are fairly small, but they respond very fast,” said Steve Rourke, vice president of planning at ISO-New England.

That fast response could be critical for integrating large amounts of solar and wind power.

ISO estimates that New England had 900 megawatts of solar-electric output installed last year, and that’s expected to double by 2023. The 1,800 megawatts could account for 13 percent of energy during certain hours, the ISO says, or 2 percent of New England’s net load. At the same time, wind energy in the region, led by Maine, is expected to reach a similar capacity.

“The more solar we see, the more wind we get, it could become more important to use batteries to ride through the variability, minute to minute,” Rourke said.

This concept is being tested elsewhere. One place is Hawaii, where petroleum is expensive and a new law aims for the state to get all of its power from renewable sources by 2045.

Kauai Island Utility Cooperative built solar-electric arrays that provide much of its power on some days. They’re backed up by battery storage, for when clouds appear. An early storage system fell short, according to media reports, because the batteries couldn’t handle repeated charging and discharging. Improved batteries and software controls have since been installed.

Battery storage also is being refined in Hawaii by SunEdison. The effort is pioneered by the former Boston-based First Wind, which SunEdison acquired this year. First Wind learned a hard lesson in 2012, when a storage system installed by a now-bankrupt battery firm caught fire at a wind farm in Kahuku, on the island of Oahu. Since then, First Wind successfully integrated battery storage in a wind farm on Maui, called Kaheawa.

SunEdison is a leading wind developer in Maine and New England. John Lamontagne, a company spokesman, said he wasn’t aware of any plans to integrate battery storage in a wind project here.

But SunEdison does have overall plans to pair solar and wind with battery storage. Last March, it acquired a company that combines battery systems with solar panels, Solar Grid Storage LLC. In a news release, SunEdison said its strategy is to increase the value of its solar and wind projects by improving their availability and ability to interact with the electric grid.