Ocean Thermal (OTEC)

Closed Cycle OTEC | Open Cycle OTEC | OTEC in Australia | Further Information | References

Ocean Thermal Energy Conversion Technologies

The concept of OTEC dates back to 1881, when Jacque's D'Arsonval envisioned the idea. However, D'Arsonval did not live to see his idea to fruition, a task completed by his student Georges Claude in 1930. Although the theoretical efficiency of OTEC is small (~2%), there are vast quantities of seawater available for use in power generation. It has been estimated that there could be as much as 107MW of ocean thermal power available worldwide.

Closed Cycle OTEC

D'Arsonval's original concept used a working fluid with a low boiling point, such as ammonia, which is vapourised using the heat extracted from the warm surface water. The heated working fluid is used to turn a turbine to produce electricity. Cold deep-sea water is used to condense the working fluid in a second heat exchanger prior to being recirculated to the first heat exchanger. This is known as a closed cycle system (see Figure 1).

Figure 1 Closed Cycle OTEC (Image adapted from National Energy Laboratory of Hawaii Authority (NEHLA)).

Closed Cycle Demonstration Projects

In 1979, Mini-OTEC was commissioned around Hawaii, and successfully generated net power (net power = total power - power required to operate the plant) using a closed cycle OTEC system. The Mini-OTEC struck significant problems, including the selection of metals in the heat exchanger having a strong influence on the economics of power generation using closed cycle OTEC.

The National Institute of Ocean Technology (NIOT) in India is working with the Saga University in Japan on an OTEC system that uses the closed cycle principle with ammonia as the working fluid. The 1 metre diameter High Density Polyethylene (HDPE) pipework containing the ammonia was installed on a barge (Sagar-Shakthi), designed to be moored in such a manner that the pipe forms part of the mooring. In order to understand the issues associated with the power module of the OTEC plant, NIOT constructed a 1 MW demonstration OTEC plant, which consists of the barge together with a warm water source, a chiller (providing the cold water source) and plate heat exchangers for the evaporator and condenser. Unfortunately there were problems when some components were damaged and fell into the sea and could not be retrieved. Sagar-Shakthi is currently as of January 2006, moored at the port Tuticorin. Saga University is also active domestically in Japan, as well as with many joint initiatives worldwide at various stages of development, and is currently one of the most active OTEC researchers worldwide.

Figure 2 “Sagar-Shakthi” the 1MW Closed Cycle OTEC power plant joint demonstration project between Saga University (Japan) and NIOT of India. (Image courtesy of The European Centre for Information on Marine Science and Technology).

Open Cycle OTEC

The first operational OTEC trialed by Georges Claude used warm surface water as the working fluid, instead of the low boiling point fluid proposed by D'Arsonval. In open cycle OTEC systems, the seawater is evaporated under a partial vacuum, creating low pressure steam which can be used to drive a turbine. The steam is then condensed either by a second heat exchanger, as in the closed cycle, or by mixing with the deep cold water (see Figure 3).

Figure 3 Open Cycle OTEC (Image adapted from National Energy Laboratory of Hawaii Authority (NEHLA)).

Open Cycle Demonstration Projects

Initial research in this area commenced at the National Energy Laboratory of Hawaii Authority (NELHA) in 1983 and resulted in the construction and operation of a 210 kW open cycle system at NELHA from 1992 to 1998 (see Figure 4). This plant generated the largest amount of net energy from any OTEC system to date and demonstrated that fresh water could be extracted successfully from the open cycle system. Following the completion of this testing the plant was demolished in January 1999.

Figure 4 The Open Cycle OTEC at Keahole Point, Hawaii Island.
(Image courtesy of Wikipedia & The US Department of Energy).

OTEC and Australia

Australia and its territories do not have significant 'reserves' of OTEC, although the ocean temperature difference in the tropical regions of Australia is between 20 and 22^oC (see Figure 1). The high solar insolation of these regions and the profile of energy users (mainly inland, isolated communities and users) means that OTEC is unlikely to be used in Australia in the near future. However, it has potential applications for Australia's neighbours in the Western Pacific region.

On an encouraging note, Townsville held its first OTEC workshop on September 29, 2005. The workshop was sponsored by SEA O₂ Sustainable Development and supported by the Society for Sustainability and Environmental Engineering. The workshop considered the opportunities for Townsville to become an international centre for the development of OTEC technology, and expressed the potential for OTEC to solve Townsville's energy and water needs. However the significant capital and running costs of current OTEC technologies combined with the technological immaturity of the industry make this technology uncompetitive with conventional sources and the many other renewable energy sources at the present time.

Further Information

RISE Resources - Information regarding available renewable energy resources.

RISE Technologies - An extensive collection of information regarding renewable energy technologies.

RISE Applications & System Design - Renewable energy application information and system designs.

RISE System Displays - Case studies and information on installed renewable energy systems & performance data.

National Renewable Energy Laboratory OTEC research

National Energy Laboratory of Hawaii Authority (NEHLA)

References

None.