The Gas Co. of Hawaii said it will expand its product line to produce more hydrogen gas as part of an agreement announced Tuesday with General Motors to launch a pilot fuel cell vehicles project in the Islands.

Detroit-based General Motors, one of the largest automakers in the world, said it wants to build a network of hydrogen fueling stations to power fuel cell vehicles on Oahu by 2015.

The role of The Gas Co., which manufactures and delivers synthetic natural gas, including a small amount of hydrogen, will be to tap into its 1,000-mile-long utility pipeline system on Oahu at key locations and separate hydrogen for use in fueling stations.

The companies said a total of 20 to 25 fueling stations could be built at existing petroleum gas stations on Oahu with at least six stations in northwest Oahu and at least 15 in southeast metro areas.

“The infrastructure is very important because it does no good to put these technologies on the road if there’s no ability to fuel the vehicles,” Charlie Freese, executive director of GM Global Fuel Activities said in a teleconference Tuesday.

General Motors said it will cost approximately $300,000 to $500,000 to add hydrogen pumps to existing retail gas stations.

The automaker said it is still working with potential users of fuel cell vehicles in Hawaii, but said the pilot project likely will involve “more than five, but fewer than 50” vehicles. The companies will evaluate results of the pilot before deciding whether to ramp up the project.

The Gas Co. said it will file a tariff with the Hawaii Public Utilities Commission to set the price of hydrogen.

“We believe the cost will be comparable to gasoline on a per-mile basis, but we will not be able to control any retail markup at the pump,” Jeffrey Kissel, president and CEO of The Gas Co., said in the teleconference.

General Motors, which says it has invested more than $1.5 billion in fuel cell transportation in the last 15 years, is developing a fuel cell system that could be ready for commercialization in 2015.

The company currently has fuel cell vehicles under the Chevrolet brand as part of Project Driveway, a three-year-old demonstration of fuel cell vehicles, which includes a fleet of 119 vehicles. Some of those vehicles could be brought to Hawaii for testing, Freese said.

The Gas Co., which is based in Kapolei on Oahu, owns and operates the only synthetic natural gas plant in the United States. The plant uses a catalytic process to convert the byproducts of petroleum refining into methane and hydrogen for its customers, which includes more than 68,000 customers statewide.