Offshore Power Generation Making Waves

It's an axiom as consistent as the ocean tides—as technology progresses, demand for power rises. Fortunately, two companies are looking at those tides to provide that power. Specifically, they're converting the mechanical energy of waves into electricity.

The PowerBuoy from Ocean Power Technologies is at work off the coasts of Hawaii, New Jersey, and Spain. Measuring 5 m in diameter and 15 m long, it's submerged a meter below the ocean's surface. As it rises and falls with the waves, a piston-like structure inside the buoy moves and drives a generator anchored to the ocean's floor. A standard, small-diameter submarine transmission cable carries the generated power to shore (see the figure).

Power providers can assemble arrays of connected buoys to pump out as much power as they need. Wave power can generate up to 100 kW per meter of wave front. A 10-MW power station would require four acres of ocean surface. According to the company, a 1-MW array would price out at 7 to 10 cents/kWh, while a 100-MW system would be 3 to 4 cents/kWh. This includes all maintenance and operating expenses, as well as the amortized capital cost of the equipment.

Environmental assessments are prepared before installation to prevent harmful effects on the environment. The system automatically disconnects when waves get too powerful, and it reconnects when wave heights return to normal. Systems typically are located between 0.5 and 5 miles offshore, so they don't take up any valuable real estate or create an eyesore. Applications include primary power plants, secondary power systems, desalinization plants, hydrogen production, and remote ocean environment sensing.

Meanwhile, a Scottish company has developed and tested a prototype of a similar technology. Ocean Power Delivery Ltd. recently tested its Pelamis technology off the coast of Orkney. The Pelamis Wave Energy Converter is a semi-submerged, articulated structure comprising cylindrical sections linked by hinged joints. Hydraulic rams resist the wave-induced motion of these joints and pump high-pressure oil through hydraulic motors via smoothing accumulators. The hydraulic motors drive electrical generators to produce electricity. Power is fed down a single umbilical cable to a junction on the sea bed and then sent to shore.

The full-scale Pelamis is 120 m long and 3.5 m in diameter. It contains three power-conversion modules rated at 250 kW each. Ideally, it's moored in water that's 50 to 60 m deep, often 5 to 10 km from shore. The mooring system, made up of floats and weights, maintains enough restraint to keep the Pelamis in position while allowing flexibility to swing head on into oncoming waves.

Overall power-conversion efficiency ranges from about 70% at low power levels to over 80% at full capacity. According to Ocean Power Delivery, a 30-MW array of 40 machines would occupy 1 km² and provide enough energy for 20,000 households.