Dossier Océan et énergie - Énergie Thermique des Mers

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Questions and Answers on OTEC in Taiwan

Hans-jurgen KROCK
University of Hawaii at Manoa
Professor of Ocean Engineering and
Director of J.K.K. Look Laboratory of Oceanographic Engineering

1. Would you please comment why even if the technology of the Ocean Thermal Energy Conversion (OTEC) has been demonstrated, a commercial OTEC Power Plant is not yet available?

Comments and Suggestions:

After a flurry of interest in OTEC and associated research activity following the oil crisis of the 1970's, the basic designs of both the open-cycle and closed-cycle OTEC processes have been established.  The high oil prices in the late 1970's and in 1980 resulted in the design (to blue-print stage) of a 40 MW closed-cycle OTEC plant to be located at Kahe on Oahu and to be financed by private investors.   Similarly, large U.S. companies produced designs for large (several hundred MW) floating OTEC plants at that time.  However, the price of oil dropped dramatically in the early 1980's and has stayed below 20 U.S. dollars per barrel since then (with the exception of the time of the Gulf War).

Given this history, the main reason that commercial OTEC power plants have not been built is unfavorable short term economics in comparison to fossil fuel.   Also to be considered is the fact that utility companies to related governmnetal agencies are very conservative in their decision making -- they are not innovative or willing to take risks.  Consequently, the first commercial OTEC plants are likely to be located where the present economics are unfavorable to fossil fuel plants (as, for example, on remote tropical oceanic islands).

When, in the future, the price of oil rises (due to an artificially induced shortage or due to a real shortage because of the drawdown of proven reserves of easily accessible oil), the short term economic situation will favor OTEC derived energy and, those companies and countries who have prepared the technology of OTEC systems, will be able to reap large economic benefits from the design and construction of such systems.

I should also note that in addition to the short-term economic situation and the conservatism of utilities the reason that commercial scale OTEC power plants are not presently operating is that there continues to be a need for reliable engineering information for scaling up from the existing small plants (a few hundred kilowatts) to the commercial sized plants of 25 MW to hundreds of MW.  As has been noted by numerous experts, the biggest question is the upscaling of the cold water pipe with secondary questions related tot he heat exchangers for the closed-cycle design and the turbine and non-condensable gases for the open-cycle design.  None of  these engineering questions, however, are expected to be unusually difficult.  They can be resolved by a concentrated study by engineers experienced in large scale marine structures, turbine design, and chemical reactor design.  Any industrial country has the expertise to resolve the remaining engineering questions for the design and construction of commercial scale OTEC plants.

2. Would you please predict the OTEC potential of the world? Will it become a primary or important energy source in the 21st century? When and where are the most possible times and places to develop OTEC power?  Based on what studies and/or reports you can estimate the possible OTEC potential along the east coast of Taiwan? (Please list the references, if possible)

Comments and Suggestions:

The size of the global ocean thermal resource with respect to extraction of energy for human use, without over-exploitation of that resource, is sufficient to supply the needs of approximately twice the present global population at the present per capita energy usage rate.  This makes the ocean thermal resource the largest renewable energy resource in the world.  Because of its size and ready accessibility via sea lanes, it is inevitable that the ocean thermal resource will eventually become a major component of the global energy supply.  The time when this will occur depends on the economics of the fossil fuel energy base.

With respect to fossil fuel, the economic picture has primarily been dominated by the costs associated with supply, precessing and distribution.  Very little economic consideration has been given to the costs associated with the use of fossil fuels.  Some of these secondary effects are well established as being directly related to the large scale use of  fossil fuels.  These include: acid rain with associated building, forest, and lake damage; oil spills and other accidents; wars related to oil supplies; human health effects from air pollution and the loss of valuable organic materials that could in the future be used for higher economic purposed.  Less established are the economic effects related to the increased carbon dioxide content of the atmosphere.  These include: global warming, rising sea levels, changes in the global climate pattern, and increases in the severity and frequency of tropical storms (hurricanes and typhoons).

Some economists have attempted to evaluate some "social costs" associated with fossil fuel usage and compared them to those costs associated with renewable energy systems.  For example: Olav Hohmeyer, Soziale Kosten Des Energie-Verbrauchs, Springer-Verlag, Berlin, 1989.

When there is more general recognition of the larger economic picture associated with energy supply and use, there is likely to be a rapid development of renewable energy resources.  I expect this to occur in the first half of the 21st century coincident with the significant reduction in the proven reserves of easily extractable oil.

The best place to develop OTEC power is in the area of the Pacific Ocean having the highest consistent temperature difference.  This is an area approximately the size of the United States located east of the Philippines and south of Japan.  The best time to start to develop the technology to use this resource is now --initially as proof-of-concept plants(such as the 5MW plant proposed for Taiwan); then as niche markets in tropical oceanic islands (such as Majuro, Saipan, American Samoa, etc.); then as floating, grazing OTEC plants with hydrogen production to store and transport the energy to industrial countries.

The estimate of the OTEC potential along the east coast of Taiwan contained in Volume I of the Master OTEC Plan for the Republic of China seems reasonable based on the existing data.  However, the existing data base is thin and a great deal of reliance is placed on extrapolation.  More long term measurements are needed to refine the reliability of these estimates.

Two observations may be relevant with respect to the ocean thermal resource near Taiwan.  One is that the warm water layer appears to be relatively thin.  The second is that the apparent temperature difference during the winter months is marginal and will significantly reduce the net power output.

3. What are the implications of CO2 emission with an OTEC power plant?  Can you estimate the quantity of COemitted from closed cycle OTEC and from open cycle OTEC?  Who has been doing what to have a better understanding or better way of control on this issue?

Comments and Suggestions:

There are no significant carbon dioxide emissions associated with OTEC power plants.  Closed-cycle OTEC plants with discharges below the surface layer have essentially no carbon dioxide emissions.  The maximum emission rate of carbon dioxide for open-cycle plants with direct contact condensers is about 4% of an equivalent oil fired plant.  These results are based on actual measuremnets made by my graduate students and myself, using a pilot scale OTEC facility running on real seawater at Keahole Point on the Island of Hawaii.

Fears of large OTEC related carbon dioxide emissions are apparently based on misconceptions related to the carbon dioxide content of the cold water steam and the rate of gas exchange.  While the total carbon dioxide content (including dissolved CO2, HCO3, and  CO3) of the deep cold water is about 1/3 higher than that of the warm surface layer, it does not quickly exchange with the atmosphere.   In the closed-cycle OTEC system, there is no free surface -- so essentially no gas exchange occurs.  In the open-cycle system, the time of exposure to the partial vacuum conditions is too short for any significant switch from the bicarbonate form to the dissolved  CO2 is exchanged.  If fresh water production is part of the open-cycle OTEC desing, then a heat exchanger will be used in the condenser and no  CO2 will be emitted by the cold water stream.  In summary,  CO2 emission is not a problem associated with OTEC.

4. What are the considerations of the environmental impacts with by OTEC power plants?  Can you start with stating all the possible impacts and reach a conclusion regarding how many OTEC power plants, or how many MWs of electric power, can be built along the east coast of Taiwan so that a  significant temperature difference at the ocean surface will not occur, and the ecosystem will not be affected.

Comments and Suggestions:

The environmental impacts associated with OTEC are primarily those associated with the vertical relocation of relatively large water volumes.  Organisms contained in the cold water stream will be subjected to large pressure changes.   Those organisms having swim bladders (most fish) will be killed.  In the case of an open-cycle OTEC plant, a large majority of the organisms (including micro-organisms) exposed to near vacuum conditions will be killed. Depending on the design of the open-cycle plant, the discharge water could be deoxygenated.

The physical effects of the vertical displacement of the warm and cold water streams of an OTEC plant or series of OTEC plants depend on the volume of water and on the depth of the discharge.  The volume of water associated with the recommendation contained in the Volume I of the Master OTEC Plan for the Republic of   China will not have any significant physical effect on the heat content of the waters along the east coast of Taiwan -- especially when the current structure is taken into account.  Most detrimental effects from the discharge from these OTEC plants are avoided by locating the discharge point near the isothermal depth of the ambient water.

For the case of very large scale use of OTEC for global energy supply, further studies are needed of the possible effects of discharging large volumes of   water into the main tropical ocean thermocline and possibly significantly thickening it.

5. Is it possible as the present time that one can state what will be the best platform and cold water pipe design for a floating commercial size OTEC power plant (50MW up)? If not, what are the possible good designs? For these designs, can you suggestion who has the best technology and/or experience to do the design and/or private sector for our reference.

Except platform for the and cold water pipe, are there still any technologically uncertain or unavailable components, for instance, the heat exchanger, the pumping system, or the power transmitting system.

Comments and Suggestions:

There is presently no "best" platform design for a floating commercial sized OTEC plant.  We at the University of Hawaii have been exploring the concept of multiple use platforms which might include (in addition to OTEC) ocean mining and mineral processing, hydrogen production and liquefaction, and aquaculture. The design of such platforms is site specific and dependent on the mix of uses.

For the more limited concept of an OTEC plant with hydrogen production and liquefaction, a concentric layout "spar buoy" having limited wave exposure is probably optimal.  Such a design would allow for dynamic station keeping using intake and discharge forces, as well as off-loading of liquid hydrogen to transport vessels (possibly of SWATH design).

The best cold water pipe design presently available for larger pipes is probably a fiberglass syntactic foam sandwich, similar to that used in experiments in Hawaii over the last 15 years.  I should emphasize, however, that the possibilities of new pipe materials and of the flexible pipe concept (with the pump at the bottom) have not yet been fully explored and will very likely result in better pipe designs.  The engineering consulting firm located in Hawaii, Makai Ocean Engineering, is probably the best resource for up to date information on OTEC pipe design.

Some technical uncertainty is stilll associated with a large scale design of aluminum heat exchangers for closed-cycle OTEC system No real problem is anutcipated.   However, a 5 MW upscaling of the presently proposed system should confirm design assumptions.

Power transmission from fixed off-shore OTEC platforms (probably tension leg design) using cable connection is readily feasible.  The decision of AC or DC transmission depends primarily on the distance to be transmitted.  For free floating platforms, however, no reasonable cable connection desings are available and the best power transmission system will probably involve hydrogen.

6. What is the comparison between land-based and offhshore floating OTEC?

Comments and Suggestions:

Land based OTEC is appropriate for those cases where the cold water resource is close enough to shore to be reasonably accessible by pipeline.  A land based site would also allow the development of various other uses of the cold water resource (air conditioning, aquaculture, and agriculture).  In some locations, the land based OTEC system could use the cooling water discharge stream from a larger fossil fuel plant or a nuclear plant to increase the net output of the OTEC power plant.  If an open-cycle plant with fresh water production is used, the land based location will allow ready access to the water distribution system. A land based OTEC system would also be more easily connected as base load to the electricity distribution system.

The most appropriate use for offshore floating OTEC plants is to graze in the deep tropical ocean and produce a transportable form of energy (probably liquid hydrogen) for use in industrial countries.  At some sites, it may be pssible to construct tension leg structures for such OTEC-hydrogen system.  Because of the great depth of   most of  the tropical ocean thermal resource area, effective large scale use of  that resouce will necessarily involve the development of floating OTEC plants.

Land based OTEC systems are appropriate for local use, while floating OTEC plants are appropriate for large scale use of the ocean thermal resource.

7. What is your overall impression on MPOP and MOPR? Do you think it is possible that either MPOP or MOPR can be promoted as an internationally cooperated project, instead of just a project promoted by the Taiwan government? and why? What will be the best strategy that Taiwan government must consider in order to enhance the achievement of the MPOP or the MOPR? Can you also recommend possible organizations that we should get in touch with?

Comments and Suggestions:

Both MPOP and MOPR are reasonably adequate documents to initiate the respective projects.  Neither, however, have enough site specific information on which to base designs.  This is a shortcoming of the existing data base rather than a fault of these studies.  It is clear, therefore, the site specific data gathering should continue and be coordinated with any design effort for OTEC systems in Taiwan or the troical ocean area within reach of Taiwan.

The development of the 5 MW OTEC plant and associated land based use of the cold water rescue described in MPOP is most appropriately under the purview of the Taiwan government.   This is because this project is relatively small; it is entirely within the territory of Taiwan; and knowledge developed during the design, construction, and use of this project can be developed into and exprotable technology that would then be of economic benefit to Taiwan.

The larger scale use of the ocean thermal energy resource described in MOPR would be more effectively realized as an international project.  Although they have expressed no interest in OTEC, the most appropriate industry to develop the large scale use of the ocean thermal energy resource are (paradoxically) international oil companies.  This industry has the experience and know-how to design and build large scale ocean structures and to run an energy transport, storage, and distribution system. It would be a natural transition for them to switch from mining off-shore oil deposits to mining the off-shore thermal resource.  It may be possible to convince them that long term economic benefits will accrue to them as a result of relatively small present investments in research and small scale commercial OTEC systems construction. It is unfortunately likely, however, that the short term economic viewpoint will continue to blind them and prevent them from taking part in the timely development of this renewable and large energy resource.