Generating Electricity from the Ocean Floor
- Published on Tuesday, 01 June 2010 11:48
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Ocean Thermal Energy Conversion - often referred to as OTEC, is a process that uses the heat stored within the Earth’s oceans to generate electricity. There is a belief that electricity can be generated with minimal environmental impact along with many other benefits that outweigh those of other alternative energies! We decided to get a deeper insight into this technology and to see whether it is likely OTEC will become a widespread process in the future. |
We approached Dominic Michaelis, an architect and engineer who runs the Solar Energy Developments Consultancy, and Jan C. War, who works for Natural Energy Laboratory of Hawaii Authority! They share their expertise with us on the OTEC phenomenon …
Hi Dominic, how does Ocean Thermal Energy Conversion work?
DM- OTEC relies on the temperature difference between surface and deep seawaters. To harness this vast energy resource, OTEC, in its simplest form, pumps surface tropical or near tropical waters to a heat exchanger evaporator, where a low temperature boiling liquid is vaporised. The vapour contained first activates a turbine, which it then enters a heat exchanger condensation chamber. Here, it is cooled by water drawn up by a Cold Water Pipe, (CWP) from around 1000 metre’s depth. The vapour then returns to its liquid state and is pumped back to the evaporator, which complete its Rankine cycle. This is known as a closed cycle OTEC. See Dr Luis Vega diagram.
There is also another OTEC cycle developed by French Engineer, Georges Claude. The Claude process is now called open cycle OTEC, or OC OTEC because the original working fluid is not returned, but simply replaced by fresh surface seawater.
There are now other variants like hybrid systems, Kalina and Uehara cycles. These are generally more complicated and therefore more expensive to assemble, but that, in some cases, may prevail.
(Video description: http://www.youtube.com/watch?v=x59MptHscxY)
Dominic, What are the benefits of this technology?
DM- The potential benefits of OTEC technology are that it is capable of generating electricity on a nuclear scale, from a renewable energy resource, but unlike solar, wind or wave and tidal energies. These other processes are variable and discontinuous, whereas OTEC operates continuously all year round and is capable of constituting a base load electrical supply on its own. Additionally OC OTEC can supply around 2,4 million litres of desalinated water a day, acting like a powerful reverse osmosis plant.
Jan, Can such electric plants be launched from a technical perspective (is technology ready to make electricity)?
JW- Closed Cycle, Kalina and hybrid cycles of OTEC are thought to be feasible today with slight advances in current day technology. Due to the requirement of large diameter turbines for the open-cycle process, this version of OTEC is still considered in the “research” phase and is limited to a maximum output of 10-25 mW. OTEC technology in many respects is ready to make electricity today.
DM- The technology is relatively simple, most components being already available. OTEC plants will generally be offshore, in which case specialised vessels or platforms will need to be designed and built. Some plants may also eventually be used to create hydrogen or ammonia as fuels, which will demand further research. But for basic Closed Cycle and Open Cycle OTEC, there would be no problem in designing and building plants now.
How much does it cost to build OTEC plant?
JW- This will be unknown until someone builds the first commercial power plant and obtains some useful operating experience. Commercial scale (10-40 mW) demonstration plants will cost in the hundreds of millions (USD) and the first 100 mW commercial plant will probably cost several (2-4) billion dollars (US).
In this case, what is the situation with funding for OTEC plants? Are governments/private companies ready to invest/build OTEC plants?
DM- There are various new initiatives taken recently that are encouraging. The US Dept of Energy has launched a new Centre in Hawaii; the ‘Hawaii National Marine Renewable Energy Center’ (HNMREC) whose mission is to “accelerate development and testing of Ocean Thermal Energy Conversion technologies.” Dr Luis Vega, our consultant who led the team that designed, built and monitored the 210 kW plants from 1992 to 1996 in Hawaii, directs the centre. This is the only plant to have successfully worked and proven the concept, allowing electrical and desalinated water production output figures to be confirmed.
As well as the success in Hawaii, the US Government has given funds to Lockheed Martin to design a 10MW C OTEC plant, with the view that is can be build and monitored in association with the HNMREC.
In France the French navy shipbuilders, DCNS, are also planning to build a 10 MW CC OTEC plant off the island of Reunion.
A Filipino client of ours is negotiating with the Government for a concession to be allowed to build a 50 MW OC OTEC plant.
JW- As my chart demonstrates, OTEC funding and R&D in the United States has closely followed the price of crude oil. Consequently, most of the progression of OTEC technology has occurred between the 1975-1984 and 1992-1999.
Other than a few small scale (100-250 kW) research and development programs currently underway or planned, there are no operating OTEC power plants anywhere around the world. Although the governments of the United States, Japan, Taiwan, France, the Netherlands, India, and to some degree Korea appear willing to invest in OTEC R&D. Hawaii currently has a strong desire to be the first location in the world where a closed-cycle, commercial demonstration OTEC plant will be constructed and hopes to achieve this goal within the next 5-7 years.
Are there any issues with environment? There are some opinions that such plants can increase temperature of the ocean and be a threat to marine life.
JW- Although computer models and small-scale tests have been conducted to investigate the potential environmental impact of operating OTEC plants, this is also an area that requires further research and actual operating experience to validate. Potential environmental impacts of OTEC are: thermal interference of surrounding waters, nutrient enhancement, organism entrainment, impacts to marine mammals, hazards to navigation, hazardous material spills, etc.
There is also information that OTEC plants need specific location, so it cannot be built anywhere. How does this influence the feasibility/interest/future perspectives of this project?
DM- This map shows, in red and orange, areas where OTEC can work. The emerald blue areas show where shallow continental shelves or cold currents prevent OTEC from operating. Starting from the left, only 4 West African countries can benefit, as can India, Myanmar, Indonesia and Australia, but close to shore, only Queensland. It can also work in the Philippines, Papua New Guinea, South China, and Taiwan, many mid Pacific islands, and then Mexico, Central America down to Ecuador.
JW- OTEC’s limited development locations will certainly make it more difficult to promote; especially in the United States where only Hawaii, a few locations in the Gulf of Mexico and around Puerto Rico are thought to be ideal. Most of the promoters of OTEC will also say that OTEC can be developed both on land and at sea
What about other sustainable energy sources such as solar/wind power? Are there any competitive technologies which could be better that OTEC?
JW- At the present time, most renewable energy concepts such as wind, solar thermal, solar electric, hydroelectric, geothermal, biomass, biofuels, etc. are more competitive than OTEC because they are proven technologies that can be utilized in more locations around the planet. To use the United States as an example, it is much more cost effective (and politically rewarding) to invest in the expansion of wind and solar technologies than OTEC because these technologies offer a more immediate benefit for a much larger percentage of the American population.
DM- Considering the fact the wind or solar energy sources usually do not provide constant energy; OTEC plan can be more cost effective if we take into account other benefits of OTEC such as desalinated water.
What do you think about the future of this technology and when do you think we will be seeing commercially profitable OTEC plants?
JW- I am optimistic that a commercial sized, closed-cycle OTEC demonstration power plant will be built within the next 7-10 years. It is very important that this plant be scalable to larger sizes and develops real world operating experience for at least 3-5 years. If the first commercial demonstration plant is found to be successful and competitive with fossil fuels and proven renewable energy alternatives, then the construction industry and financial institutions will be willing to support the expansion of the concept.
From our report into OTEC and our experts’ contributions, we see there are promising developments being made through this energy converting process. Before any large-scale commercialisation of plants, here is the overall outline of the challenges faced, presented by Jan C. War:
1) Obtaining Sustained Funding for more OTEC R&D. OTEC R&D is needed in the areas of; Maximizing Power Extraction Efficiency
Large Diameter Pipeline Construction, Heat Exchanger Cost and Efficiency, Dynamic Cable to Shore / Moorings and Potential Environmental Impacts.
2) Obtaining Funding for a Demonstration Plant of Pre-Commercial Size that is scalable
3) Obtaining Valuable Operating Experience at Sea for 3-5 years or longer
4) Constructing the World’s First Commercial Power Plant
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