There is enough heat in the upper layers of the oceans to meet humanity’s energy needs more than four times over. A most attractive way to extract that energy is through a form of renewable energy called ocean thermal energy conversion (OTEC).
OTEC has not yet seen commercial use, but a huge amount of research and development has been devoted to it. It has undergone a development characterised by alternating periods of intense activity and stagnation, mainly reflecting rising and falling oil prices. However, it is seen as increasingly attractive, as it is a clean, green renewable energy, and it is a baseload source of energy available all the time. A large number of OTEC plants are either planned or under development around the world.
OTEC harnesses the energy from the heat engine formed by the warm waters on the surface of the oceans and the cold waters at depths down to about 1000m. Since a heat engine is most efficient when the temperature differences are largest, it is near the equator that the best chances for an OTEC plant are.
Near the equator the surface temperature is typically about 25°C, and at a depth of about 1000m it is about 5°C. This difference of about 20°C is the minimum difference that makes an OTEC plant viable. Since that is a small difference, an OTEC plant has a low efficiency when compared with other forms of renewable energy, and huge amounts of water – equivalent to a fairly sized river – must be moved from the deep. Thus, large diameter pipes of about 1000m long must be used, and this is one of the costliest and technically most challenging parts of an OTEC system.
“OTEC is not only a source of energy, but the cold waters of the deep can also be used for air conditioning, in desalination systems, and as a source of water for aquaculture due to the fact that those waters are nutrient-rich and free of pathogenic bacteria.”
It still poses daunting engineering challenges, however. Huge progress has been made in meeting those challenges, but a number of problems remain, and OTEC is still viewed as an emerging, or ‘immature’, technology.
OTEC systems are mostly either closed cycle, or open cycle. Closed cycle systems have a long, closed loop of pipeline filled with a fluid with a very low boiling point, such as ammonia. The fluid never leaves the pipe. The pipe flows through a heat exchanger in the hot surface waters of the ocean. The hot water causes the ammonia to vaporize. The ammonia vapour blows through a turbine, which extracts some of its energy, driving a generator to produce electricity. The ammonia then goes through a second heat exchanger where cold water pumped from the ocean depths condenses it back to liquid to be recycled.
In an open cycle system, the seawater is itself used as the working fluid. Water from the surface is turned to steam by reducing its pressure. The steam drives a turbine and generates electricity. It is then condensed back using cold water piped up from the depths. The water that leaves the OTEC plant is pure and desalinated. Thus, open cycle OTEC plants can double as desalination plants.
OTEC plants, closed or open cycle, can be built either onshore or offshore. Both entail advantages and disadvantages.
There are no plans to build an OTEC plant in Australia, although there are favourable conditions, mainly along the Queensland coast. In Australia, exploitation on energy from the oceans has been mainly focussed on wave energy. An OTEC workshop was held in Townsville in 2005, which looked at the possibility of building an OTEC facility there, but the workshop decided that the high capital and running costs of such a facility made OTEC uncompetitive with other sources of renewable energy and it was decided not to proceed at that time.
Several plants proposed or under development around the world aim to produce 1MW or 2MW, while others aim to produce net power in the order of 10MW or even 20MW.
Among the limiting factors in the construction and operation of OTEC plants are the heat exchangers – to heat and vaporise the working fluid and to condense the working fluid afterwards – and the large diameter pipe to pump up the cold water from the depths. This poses a limit of about 100MW for today’s OTEC plants. The 10MW or 20MW plants will be a stage in the development of the larger 100MW plants.
Since OTEC was first proposed, in the late 19th century, a large number of OTEC plants have been built and operated, but today there are few fully operational OTEC test facilities, the largest ones located in Hawaii and in Okinawa.
The Hawaiian plant, owned and operated by Makai Ocean Engineering, has a maximum capacity of 105kW and is supplying electricity to the grid. It is a closed cycle system, using ammonia as working fluid. It is located at the Natural Energy Laboratory of Hawaii Authority (NELHA) at Keahole Point, in the North Kona district of Hawaii. NELHA wants to upgrade the plant to a 1MW plant in future. Founded in 1974, NELHA has been responsible for the earliest and of much of the R&D relating to OTEC systems. Several companies have been associated with NELHA, including Lockheed Martin of Bethesda in Maryland; plant designer Makai Ocean Engineering Inc of Waimanalo in Hawaii; and Ocean Thermal Energy Corporation (OTE Corp) of Lancaster in Pennsylvania.
Makai has been engaged in research on OTEC since working on the first net-power producing plant – the ‘mini-OTEC’ offshore plant at NELHA. The company has been sub- or prime contractor for many R&D contracts in OTEC. Most recently, Makai has been involved with Lockheed Martin and others pursuing the development of 100MW OTEC plants for island communities like Hawaii and Guam.
“Several of our personnel were integral in the research, design and operation of the various research OTEC systems operated at NELHA through the 1980s and 1990s with additional contribution in the current operating OTEC test facility,” OTE Corp chief science advisor Dr Steve Oney said.
The Okinawa plant has been operational since 2013, producing 100kW, and also supplying electricity to the grid. The plant resulted from a cooperation involving the city of Kumejima on Kume Island, Okinawa, and NELHA, but was designed by staff from Saga University’s Institute of Ocean Energy in Saga City on the island of Kyushu.
Scientists and engineers from Saga University have made major contributions to the science and engineering of OTEC systems. Implementation of the Okinawa plant has been contracted to a consortium of IHI Plant Construction Co, Yokogawa Solution Services Co, and Xenesys Inc.
French company Direction des Constructions Navales (DCNS) has been operating a 15kW plant in the island of Réunion in the Indian Ocean for testing, research and training.
DCNS is also working on a project in partnership with French company Akuo Energy to develop the NEMO offshore OTEC plant on the western coast of the Caribbean island of Martinique. The NEMO plant will be 16MW and will provide electricity for 35,000 homes in Martinique. The project is expected to be operational in 2020.
Recently, DCNS signed a letter of intent with the Andhra Pradesh Economic Development Board to develop a 20MW offshore OTEC plant for the Indian Navy. The plant will be located at the Andaman and Nicobar Islands. DCNS’s media liaison officer Alix Donnelly said studies of the project have yielded positive results, “but we don’t yet have the visibility regarding the next steps. The recent agreement with Andhra Pradesh state should contribute to progress on this subject”.
Another Indian OTEC plant is proposed for Kavaratti, capital of the Lakshadweep archipelago. Its power will be less than 200kW and it will generate 100m3 of fresh water, but the power should be sufficient to run both the OTEC and desalination cycles. No power will be pumped to the grid. Dr Purnima Jalihal, a scientist at Chennai-based National Institute of Ocean Technology (NIOT), said the design is nearly complete and the target date is 2019.
A 20kW OTEC pilot plant has been operating in Gosung, South Korea, with the purpose of R&D. The plant resulted from the cooperation of several organisations, including the Korea Research Institute of Ships & Ocean Engineering (KRISO), which is affiliated with the Korea Institute of Ocean Science and Technology (KIOST).
KRISO has a plan to develop an offshore floating 1MW OTEC plant near the Republic of Kiribati in the South Pacific to supply electricity to Kiribati by submarine cable. Cold water will be taken via a pipe 1.2m in diameter from a depth of 1100m.
Several OTEC projects are in the cards of OTE Corp. The company has a contract to develop an SWAC (Seawater Air Conditioning) system for the island of New Providence in the Bahamas at the Baha Mar Resort.
It plans to develop a 15MW OTEC plant for the US Virgin Islands producing both electricity and potable water. This plant will also supply deep water for local aquaculture and mariculture.
The company also has plans for OTEC or SWAC installations in American Samoa, the Philippines, Cayman Islands, Guam, Diego Garcia, and Tanzania. It has prepared a preliminary design for the US Office of Naval Research in Diego Garcia for an 8MW OTEC plant to provide power and potable water.
Netherlands company Bluerise B V is working on an OTEC plant in Sri Lanka, to be completed in about five years, which will supply 10MW of electricity. It will be onshore and will also supply cold water from a depth of 1000m for air conditioning. Another plant in Curaçao will supply 0.5MW of electricity and should be completed in about two years.
Bardot Group, of France, has signed a contract for a 2MW OTEC system for the Maldives in the Indian Ocean.
London company Bell Pirie Power Corporation is planning a 10MW OTEC plant to be sited 10km off the coast of Zambales in the Philippines, facing the South China Sea. The offshore power bloc will be housed on a ship-shaped platform. The electricity generated will be transported to Cabangan via a subsea power cable.
With all this activity on OTEC systems and related technologies, it is expected that a commercial plant will soon emerge, probably producing 10MW to 20MW of electricity, as a stepping stone for larger, 100MW plants. Since there has not yet been a commercial OTEC operation, a useful cost analysis would be difficult to produce. The capital and operating costs of an OTEC plant may not yet be competitive with other forms of renewable energy, but technological improvements and refinements may soon lower the costs of OTEC plants.