Ocean thermal energy conversion finally comes of age

Aerial view of the National Energy Laboratory of Hawaii Authority (NELHA) at Kona's Keahole Point, photo courtesy DBEDT

By Paul Grad, engineering writer.

A total electric power of 14TW, corresponding to 77 per cent of the current worldwide annual energy consumption, could be harnessed from the total of 55.1 x 1012kW/sec of solar power that arrives at the oceans. The means to harness that power is called Ocean Thermal Energy Conversion (OTEC) and it offers amounts of energy that are 10 to 100 times greater than other ocean energy options, such as wave and tidal power.

OTEC utilises the difference between the temperatures at the oceans’ surface and at depths of 1km of more, which is a maximum near the equator. Since a heat engine is most efficient when the temperature differences are largest, it is in the tropics – where the temperature difference between surface and deep water is greatest about 22°C to 24°C – that OTEC offers the best potential.

It is a base load electricity generation system, i.e. it can supply electricity 24 hours per day, all year long.

However, it entails several benefits apart from power generation. It can supply cold water as by product, which can be used for refrigeration and air-conditioning. The nutrient-rich deep ocean water can feed biological technologies, including aquaculture and mariculture. Another by-product is fresh water distilled from the sea.

OTEC systems are either closed cycle or open cycle. Closed cycle systems use working fluids such as ammonia or R-134a, with low boiling points, making them suitable for powering the systems’ generator. Warm surface seawater is pumped through a heat exchanger to vaporize the fluid. The expanding vapour turns the turbo-generator. Cold water, pumped through a second heat exchanger, condenses the vapour, which is then recycled through the system. Open cycle engines use vapour from the seawater itself as the working fluid. The warm seawater is first pumped into a low-pressure container, which causes it to boil.

There are two ways of using the steam: in one of them the expanding steam drives a low-pressure turbine attached to an electrical generator. The steam has become pure fresh water. It is condensed by exposure to cold deep-sea water. This method produces water suitable for drinking, irrigation or aquaculture.

In another method, the rising steam is used to lift water, which then generates power from a hydroelectric turbine.

OTEC plants require a long, large diameter intake pipe, submerged 1km or more to bring cold water to the surface. Whenever possible, it would probably be preferable to have land-based or near-shore OTEC facilities. Land-based OTEC plants offer several advantages over those located in deep water, such as simpler and cheaper construction because they do not require mooring and long power cables and due to the costlier maintenance of the facilities in the open ocean. Besides, fresh water and cold, nutrient-rich water can be more easily transported to the point of use. Floating OTEC facilities require mooring in relatively deep water, and cables and other equipment are more susceptible to damage in rough sea conditions. Maintenance in depths of 1km or more is difficult and expensive.

There are no plans to build an OTEC plant in Australia, although there are very favourable conditions, mainly along the Queensland coast. An OTEC workshop was held in Townsville, in 2005, sponsored by SEA O2 Sustainable Development and supported by the Society for Sustainability and Environmental Engineering. The workshop looked at the possibility of building an OTEC facility in Townsville, but the high capital and running costs of OTEC technologies made OTEC uncompetitive with other renewable energy sources, and it was decided not to proceed at that time.

Although the OTEC concept is very attractive, the cost of OTEC plants has delayed their deployment. However, two main factors have brought OTEC closer to commercialization: sustained high oil prices, which have made OTEC electricity increasingly competitive, especially in remote tropical island communities; and technical advances in the offshore oil industry during the past two decades, many of which are applicable to deep cold water pipe technology for OTEC.

One of the earliest RD efforts in OTEC systems has been by the Natural Energy Laboratory of Hawaii Authority (NELHA), which administers the Hawaii Ocean Science and Technology Park (HOST Park). NELHA was founded in 1974 to do research into the uses of deep ocean water in OTEC renewable energy production and in aquaculture. It is headquartered on Keahole Point in the North Kona District of Hawaii. Several companies have been associated with NELHA, including Lockheed Martin and Makai Ocean Engineering.

Today the Okinawa Ocean Thermal Energy Conversion Demonstration Facility is the only fully operational OTEC plant in the world. The Okinawa plant generates 50kW of electricity. At the site, the surface seawater temperature is 27.1°C, and the deep-sea water temperature is 9.0°C. The plant’s construction and development resulted from a cooperation involving Kume Island, Okinawa, and NELHA, but it was designed by engineers associated with Saga University’s Institute of Ocean Energy, in Saga City, on the island of Kyushu, Japan. Implementation of the project has been contracted to a consortium of IHI Plant Construction Co, Yokogawa Solution Services Co and Xenesys Inc.

However, several plants have already been built and operated for a while throughout the world, and there are now many plans by several companies to build OTEC plants in many parts of the world.

A 10MW OTEC prototype power plant is planned off the coast of Hainan Island in southern China. The project will be jointly developed by the Beijing-based resort developer Reignwood Group and the US defence and aerospace company Lockheed Martin, of Bethesda, Maryland. The two companies signed a memorandum of understanding for his project in April 2013. Construction of the plant is scheduled for completion in 2017. It will be the world’s largest OTEC facility. It will be a closed cycle system. Turbine systems will be placed above the water surface, with warm water passing through the heat exchanger and boiling ammonia to create steam. The steam will then pass through an underwater heat exchanger to be condensed into liquid ammonia. Cold water will be pumped from 800m to 1000m below the ocean surface. The electricity generated by the plant will be supplied to a resort to be built by the Reignwood Group on Hainan Island.

The two companies expect the Hainan project to pave the way for future commercial OTEC plants ranging from 10MW to 100MW capacity off the coast of southern China.

Another US company, Ocean Thermal Energy Corporation, of Lancaster, Pennsylvania, is involved in several OTEC projects. The company and the US Virgin Islands signed, in March 2014, a memorandum of understanding for a feasibility study for the world’s first US-based commercial onshore OTEC plant and sea water air conditioning system. The French, Paris-based DCNS Group (formerly the “Direction des Constructions Navales”) – a naval defence company –will be the engineering, procurement and construction contractor for the plant.

Ocean Thermal Energy Corporation also signed a memorandum of understanding to build OTEC plants in the 10MW to 20MW range in the Bahamas and in Zanzibar, East Africa.

In January 2014, a 20kW OTEC pilot plant was publicly demonstrated in South Korea. It is a down-scaled model of a 1MW OTEC plant, planned for 2015. The 20kW plant was the result of a cooperation of several organisations, including the Korea Research Institute of Ships & Ocean Engineering (KRISO), affiliated with the Korea Institute of Ocean Science and Technology (KIOST).

Makai Ocean Engineering, Inc, of Kailua, Hawaii, is developing a closed-cycle OTEC plant at its Ocean Energy Research Center in Kona, Hawaii. The company is installing a 100kW turbine-generator at the plant and plans to connect the plant to the Hawaiian electrical grid in 2015. The plant will be the largest OTEC plant in the world. Makai’s had a long association with NELHA. In 1979, it designed and oversaw the construction of the cold water pipe and mooring for a barge dubbed “Mini-OTEC”, the world’s first net-power producing OTEC plant.

It looks like, after many years of R&D and experimental plants, the day of OTEC has finally come. Many countries have developed a roadmap for OTEC development and industrialization, including the US, Japan, Korea, France, Malaysia and the Philippines.