Ocean Thermal Energy Conversion (OTEC) is scaling up. This summer, the largest power plant to date was connected to the grid at Big Island of Hawaii, producing 100 kilowatts.

Some 120 households can run on that power throughout the year. Dutch technology and project developer Bluerise is starting construction of a 500 kilowatt plant at the isle of Curaçao in the Caribbean. The same company is investigating the possibility of offshore OTEC plants. These floating energy plants should be able to produce 10 to 30 megawatts- enough to run a village. Pioneers of this new branch in renewable energy generation say they are ready to build 10 megawatt plants within the next five years.

It is plausible that offshore OTEC plants may produce that kind of power. Some technical problems need to be overcome before large scale thermal energy conversion can be accomplished at sea. A one kilometre long tube, at a diameter of about two and a half metres, will hang down from the power plant at the surface, to reach the cold waters down below. “Using steel pipes is out of the question,” Bluerise CTO Berend Jan Kleute begins to explain. “Steel pipes used as risers in the oil and gas industry reach considerable lengths, but they typically have a small diameter of 20 to 30 centimetres. If the OTEC pipe was made of steel, the pipe would become too heavy to be connected to the floating structure. We have found wide polyethylene pipes with diameter over 1 metre that come in standard lengths of 500 metres. These pipes are usually used for sewer outfalls.” These pipes are right for land based plants, from where they follow the sea bottom. To apply this kind of pipe hanging from a platform, some additional engineering needs to be done and pipes will probably have to be purpose built for an offshore OTEC application.

One critical point is the connection between the pipe sections. For this, another Dutch tech start-up, working in the same innovation twinning building by the name of MOCS has invented a solution. Fellow alumni from the Delft Technical University created the MOCS Tubular Curing Tool: the thinner ends of tube parts are put together into a machine that heats and thereby cures the parts together. This tool can of course be built in various diameters, including the size required for ocean thermal energy.

OTEC generates electricity by creating steam that propels a turbine. A liquid that has low boiling temperature is heated with warm water from the ocean surface. Gasses rise through a closed circuit, a turbine starts spinning. Then, the gasses are cooled with cold ocean water from the deep, causing the working fluid to condense before being heated again. A quite simple closed loop, only requiring pumping power. Kleute assures that the installation will generate so much energy, that only 20 per cent of the production is needed for pumps and other controls of the power plant. That leaves 80 per cent of the energy produced for other use; renewable energy from the sea without any input of other energy sources.

At the island based plants that are in use today, large pipes follow the sea bottom from ashore towards waters deep enough to ensure a constant low temperature, as opposed to the warm surface water. Thermal energy is best harvested in places where the differences in temperature are highest. This is the case in the tropics, with surface water at around 28 degrees Celsius and deep ocean temperatures at about 4 degrees. “The difference is only 24 degrees maximum,” Kleute explains, “so we do need a lot of water to keep the closed working liquid circuit going and the turbine humming. To keep energy consumption of the pumps as low as possible, we use wide tubes through which water flows at relatively low speeds and with low pressure drop. The pipe we will install in the Curaçao plant is 1.4 metres (4 feet 7 inches) in diameter. It runs over a length of more than 7 kilometres (4.4 miles) over the sea bottom to the nearest spot where the ocean is 1000 metres (3300 feet) deep.”

OTEC offers additional benefits with the possibility to use the cold water that is pumped up for cooling. Specifically, at the Curaçao plant, the island airport terminal will use this for air conditioning while the electricity provided by the plant will be the main source of power at the airport buildings. Furthermore, the plant will provide fresh water for agriculture – providing a solution for another urgent shortage experienced on tropical islands. The plant is believed to reduce the islands' dependency of imported fossil fuels.

“OTEC is an ideal solution for islands and coastal regions at tropical latitudes,” Kleute reasons. “There are only very small fluctuations in seawater temperature here, so the plant can run at the same level of efficiency all year, 24 hours a day. Other renewables like wind and solar energy suffer uncertainties like still or cloudy days when energy generation is way below potential. Furthermore, islands have limited land surface for solar parks or windmills. The sea surrounding them may often be too deep for offshore wind parks.” The benefit of constant supply is apprised by the OTEC Foundation, putting forward this rising star in the industry of renewable energy as a 'great potential baseload energy source'. More information at www.otecnews.org

By Hans Buitelaar