As the Navy dives headlong into the challenge of meeting its alternative energy goals within the next decade, technologists are striving to help the service harness solar power trapped in ocean waters to generate electricity for its shore-side bases.
Facilities ashore consume a quarter of the Navy’s annual energy resources. Most are powered by the U.S. electrical grid, which relies on fossil fuel generators. In addition to being tied to the turbulent prices of foreign oil, the grid infrastructure is vulnerable to hacker attacks, says R. James Woolsey, senior advisor at Vantage Point and former co-chair of the Defense Science Board’s study on energy and defense.
Naval installations are shifting to grids powered by renewable energy sources, says Rear Adm. Philip Cullom, director of the Navy’s fleet readiness division. Within the next 10 years, officials plan to generate half of the service’s shore-based installation energy requirements from alternative sources.
“This is where renewables make a huge difference,” says Cullom, who is leading the Navy’s task force on energy. Officials intend to boost the use of solar, wind, ocean and geothermal energy sources on bases and in some cases also supply power to the U.S. grid.
At China Lake Naval Station, Calif., a geothermal plant produces 270 megawatts of power. A megawatt powers about 1,000 homes.
Solar and wind power too have become sources of renewable energy. But there are limitations: The sun does not always shine and the wind does not blow constantly. Grids that are powered by these resources often have to supplement the system with electricity made by conventional fuel-burning generators.
Thermal energy from the ocean is gaining interest because seawater is readily available around the clock to provide utilities with a consistent output of power, experts say.
Ocean thermal energy is a form of solar energy that is trapped in the upper layers of the sea. In tropical areas of the world, the water temperatures can be as warm as 80 degrees Fahrenheit. Several thousand feet below the surface, the water temperature drops below 40 degrees Fahrenheit.
The warm and cold waters can be used in an energy conversion system that drives turbine generators to produce electricity.
The Navy last fall awarded an $8.1 million contract to Lockheed Martin Corp. to continue development of a 10-megawatt ocean thermal energy conversion pilot plant.
“OTEC is essentially a very large heat pump,” explains Robert Varley, program manager of the contract, which was awarded through the naval facilities engineering support center in Port Hueneme, Calif. Warm ocean water drawn up through a pipe evaporates liquid ammonia. The gaseous ammonia turns the turbine generators that produce electricity. Cold ocean water condenses the generator exhaust back into liquid form. The ammonia is pumped back to the evaporator to start the cycle anew.
“We need to pump a lot of water to extract enough energy to produce a utility-scale power plant,” says Varley. Unlike fossil fuel plants, the fuel is free and carbon dioxide is not produced as a byproduct.
The company is not a newcomer to the field. In 1979, it demonstrated a floating 50-kilowatt OTEC plant off Hawaii’s big island. The plant ran for three months and produced net power of 15 kilowatts. The Department of Energy had planned to initiate the construction of a pilot plant afterwards, but government funding for renewable energy took a hit and the industry fell dormant.
In recent years, renewed interest in clean energy has spurred a resurrection of the industry. Lockheed Martin in 2008 won a grant from the Energy Department to develop and demonstrate the tooling necessary to build a large cold water pipe to suck up ocean water from depths of several thousand feet below the surface, says Dennis Cooper, OTEC business manager for Lockheed Martin. Funding for that project concludes in September, and the team plans to demonstrate some of the key components later this year.
Cooper says the team has spent the last three years developing a conceptual design for a 10-megawatt pilot plant. In the design, the plant is roughly the size of an offshore oil platform. Hanging off the platform’s 150-foot long sides are 16 cylinders containing heat exchangers that are about the size of shipping containers — 10 feet by 10 feet by 30 feet long. To produce the electricity, approximately 10,000 gallons of warm and cold seawater would be pumped up through the system per second. Moored about seven miles offshore, the plant would send electricity to the shore via underwater cables. That power then would be transferred to the local grid.
The 10-megawatt pilot plant requires a cold water pipe that is 10 feet in diameter and 3,000 feet long. The team is developing a tooling process to build the pipe at sea on the platform and lower it into the water as sections are completed.
The environmental impact on the oceans will be minimal, officials say. Water will be discharged back into the seas at depths that correspond to its temperature. Screens on the warm water intake pipes will prevent large fish from being caught up in the current. The pipes’ low velocity intake will allow smaller marine animals to swim out. The cold water pipes may not have to contend with wildlife or bio-fouling issues. But because the water’s nutrient content is higher than that of warmer water, it must be recycled carefully to not cause algae blooming in surface waters. The team plans to discharge the water in a plume that reaches the ocean’s mesopelagic zone, where it will settle down to its proper depth.
Because of the high costs associated with water pumping technology, the OTEC concept will not be able to attract investors unless a demonstration plant is built, experts say. “The solution is to get a megawatt-class plant in the water up and running so that the industry can see, one, it’s feasible. Two, it works, and three, we understand the costs,” says Varley.
If successful, the pilot plant could scale up to one capable of producing 100 megawatts of power. The cold water pipe for that plant would be more than 33 feet in diameter — roughly the size of a metro tunnel in Washington, D.C. — and 3,000 feet long.
One of the key challenges is ensuring the cold water pipe stays attached to the platform, which is affected by wind and waves. “If you want that pipe to stay connected for any length of time, you have to design that interface to handle the stresses and strains that are imposed on the pipe by the platform,” says Varley.
The platform is based on a semi-submersible design, which means buoyancy chambers below the water hold up the top deck. The platform itself would be moored in place with similar systems that anchor offshore oil platforms. “That kind of stability is what we need to maintain the integrity of that interface” for the cold water pipe, says Varley. Instead of manufacturing the pipe from conventional steel or aluminum, developers are looking to composites.
Program managers are targeting Naval Station Pearl Harbor in Hawaii as a potential candidate for the OTEC pilot plant. If funding commences this year, the plant could be in place as early as 2013, officials say.
Other naval bases in the Pacific are ideal candidates for future 100-megawatt plants, the first of which could be constructed by 2020 if the pilot plant and subsequent commercialization proceed well. Officials say that the potential is there to build plants that generate several hundred megawatts of electricity, comparable to natural gas or coal-powered plants.
Producing three to five terawatts of power — roughly 30 percent of today’s total global energy consumption — is in the realm of possibility for OTEC, experts say. “That’s high enough to more than justify going after the market,” says Varley.
Future OTEC systems could target states in the Gulf Coast region and along the southern portion of the East Coast. An example is Florida, where the base-load power grid for the entire peninsula is about 80,000 to 90,000 megawatts. Around the world, as many as 85 countries or territories could benefit from the systems via direct cabling, officials say.
Down the road, energy carriers could transport that power to other nations in much the same way that oilers haul petroleum today, they add.
“We’re hoping that Department of Defense will lead the charge to get the technology demonstrated,” says Varley.