To meet its goal of adding more renewable generation to the Oahu grid, Hawaiian Electric is seeking proposals for one or more large-scale energy storage systems able to store 60 to 200MW for up to 30 minutes.

Potential bidders are asked for proposals that include engineering, procurement, construction, testing, commissioning, start-up and performance verification.

With the continuing dramatic growth on Oahu of utility-scale wind and solar projects and rooftop solar now in use by more than 11 percent of Hawaiian Electric customers, energy storage is needed to help ride through sudden changes in availability of these variable resources.

When power supplied by these resources drops suddenly, energy storage systems could help maintain reliable service and avoid customer outages as fast-starting firm generation units are brought online.

"Energy storage is one of the key missing elements in integrating high levels of renewable energy from variable sources like solar and wind," said Colton Ching, Hawaiian Electric vice president for energy delivery.

Energy storage can provide not only electricity but also so-called "auxiliary services" to operate the grid, such as sub-second frequency response (near-instantaneous changes to keep power quality at 60-hertz) and minute-to-minute load following (power output adjustments as demand for electricity fluctuates throughout the day).

Potential contractors will be evaluated on the overall cost of their proposals and non-price factors such as design concept and feasibility, implementation and operational viability and operating flexibility.

Bidders are invited to propose the best available technologies, including batteries, mechanical flywheels, capacitors, compressed gas systems, pumped hydro storage or a combination of such technologies.

Any project selected with a cost of $2.5 million or more must be reviewed and approved by the Hawaii Public Utilities Commission with input from the Consumer Advocate. The target is to complete and file energy storage agreements with the PUC by the end of 2014.

Bidders must provide a schedule with the goal of having the energy storage system in service in the first quarter of 2017.

The deadline for proposals is July 21, 2014.