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Home Forums Archive Infrastructure OTEC

This topic contains 25 replies, has 14 voices, and was last updated by Profile photo of CrosiarCM CrosiarCM 7 years ago.

Viewing 11 posts - 16 through 26 (of 26 total)
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    Power and potable water from of-shore OTEC plants (as well as other product streams) could potentially be provided via a long-term utility purchase contract with private owner operators. The owner/operators themselves would provide the capital to build the plant (no cost to the utility users). The users would essentially sign a long term utilities purchase agreement from the owner operators of the plant. This type of arrangement is being considered for Naval Support Activity Diego Garcia in the Indian Ocean. This type of acquisition strategy avoids the high capital costs of building a plant while simultaneously benefitting from utilities and energy rates that are more competitive than importing hydrocarbon fuels to generate electricity and produce freshwater. However, the utility users must be able to guarantee a long term utilities demand sufficient enough to make the return on invetsment favorable. Concurrent other Deep Ocean Water resource product streams may also make the return on ivestment even more attractive for owner/operators.

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    OTEC is an interesting idea, but almost certainly impractical. The laws of thermodynamics say that efficiency of a heat engine is a fuction of temperature difference, and the temperature difference in OTEC is tiny compared to other heat engines. The wikipedia entry for heat engines puts the efficiency of OTEC at 3 percent which sounds reasonable given the relatively small temperature differences.


    Solar, wind and wave energy are much more promising and likely more practical for a stead.

    On the other hand, if someone can make an OTEC that successfully violates basic laws of thermodynamics, er, go for it!

    Given that the heat of the upper ocean largely comes from insolation (i.e. sunlight), it would seem to make much more sense to convert the sunlight directly to electrical or thermal energy at up to 45% efficiency through photovoltaic cells or solar thermal heat engines, than to take the same energy through OTEC at 3% efficiency. In other words, OTEC starts with a roughtly 10x efficiency disadvantage compared to direct solar conversion.

    Profile photo of vincecate

    Since the fuel input is free (cold water), the efficiency alone is not what kills you. Efficiency is part of why the economics don’t work out well though. You have a capital investment to set everything up. Then you have to use high grade energy to pump the cold water up and then get low efficiency on converting that to high grade energy. Given all the costs and how long things last, so far I think you are better off spending your money on solar. I suspect it will always be true, but the economics can change as technology changes.

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    Given that the heat of the upper ocean largely comes from insolation (i.e. sunlight), it would seem to make much more sense to convert the sunlight directly to electrical or thermal energy at up to 45% efficiency through photovoltaic cells or solar thermal heat engines, than to take the same energy through OTEC at 3% efficiency.

    • The thing about OTEC though, is that it uses the insolation from a very large area (the surface of the sea around the water intake), while only needing a couple of point intakes. So there is no need for a large area structure like with PV cells etc.
    • Not that this guarantees that it will be economical in the end. I agree with Vince that the bottom line is the cost in money per kilowatt and not an efficiency percentage from somewhere within the system.
    • If I had to guess I´d say that someone will probably come up with a clever and cheap nuclear reactor before OTEC ever becomes economical. Until then it will probably be a combination of fossil fuels and direct solar energy to power seasteads.
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    Actually, a complete engineering economic analysis, integrating all the product streams and overall business plans of the offshore community is necessary to find the optimal sustainable utility system(s) for an offshore community. –potentially a separate thread but relevant here too.

    Product streams and markets are also a key consideration when considering the economics of OTEC plants in tropical locales. Assessing OTEC economics actually requires integrating plant design with the overall business plan of the community. The Deep Ocean Water (DOW) resource that serves as the heat sink for an OTEC power cycle simultaneously represents a number of potential products for local markets as well as export.

    Though the choice of power cycle itself (open cycle vice closed cycle for example) could simultaneously produce freshwater from surface water for local consumption and/or export. The desalinization of pathogen free DOW is becoming a business. You may want to check out the Natural Energy Laboratory of Hawaii Authority (NELHA) website and read about the current deep seawater bottlers marketing this type of product. http://www.nelha.org/tenants/authorizedbottlers.html
    The effluent DOW from an OTEC power cycle is an ideal product stream for local offshore ventures doing mariculture work like these other NEHLA tenants: http://www.nelha.org/tenants/commercial.html

    Displaced cooling load for Air Conditioning, cold agriculture, or other process applications is also a simultaneous byproduct of an OTEC power cycle like this application for Curacao http://www.otecnews.org/articles/curacao-swac.html

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    The thermal efficiency comparison among renewable energy resources is more of an academic comparison. Whether you are talking about 3% of an infinite resource that is free or 45% of an infinite resource that is free the product of either of those efficiencies is an infinite resource that is free. Thermal efficiency is much more relevant when you are comparing power cycles that exploit limited resources that have a real cost. What matters more in the renewable energy economic analysis revolves more around the life cycle and operating cost of the utility infrastructure and the sum total of revenue streams it can produce. A comprehensive engineering analysis that considers the locale and integrates the entire planned community’s business plan(s) is probably the best way to determine the right “portfolio” of utilities infrastructure.

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    -interesting article from http://www.marshalislandsjournal.com copied below.

    The Marshall Islands government is making headway to get both landowner and US agreement for long-term use of Kwajalein — but Foreign Minister Tony deBrum said there is no question that to get a new land use agreement (LUA) signed by the landowners, the military use and operating rights agreements (MUORA) in the Compact will have to be changed.

    DeBrum, in an exclusive interview with the Journal, said numerous proposals are being discussed by the landowners “to break the logjam,” though he acknowledged there is much to do before a new LUA is a possibility.
    The landowners are currently facing a December 18 deadline to sign a new LUA or risk losing $20 million in rental payments that have accumulated in an escrow account.

    DeBrum said he is hopeful that the LUA issue can be resolved by then or in the alternative that if substantive progress toward a resolution is demonstrated the deadline can be removed by the US.

    DeBrum, who is one of three senators representing Kwajalein Atoll, said that the “landowners are not attracted by the $20 million, they are insulted by it.

    “Al Staymen (a key staff person with the US Senate Energy Committee) put (the December 18 deadline) in the original act (law) and he can remove it more simply than we can change the MOURA,” deBrum said. “We’ve been very forthcoming with the US about working to get agreement with the landowners.”

    Kwajalein people are not against long-term use by the US, but they are not prepared to accept an agreement that perpetuates the current situation at Ebeye, deBrum said. DeBrum sees a proposal for inexpensive power as “an imaginative way to try to break the impasse (over the LUA).”

    He is pushing a plan for an Ocean Thermal Energy Conversion (OTEC) power system for Ebeye and Kwajalein.
    “This would add a $200 million asset to the landowners’ portfolio and provide virtually free power and water (for the atoll),” he said.

    Profile photo of Patri

    Everyone – please try to summarize your conclusions on the Wiki, so that you add to our body of knowledge.


    Profile photo of CrosiarCM

    I know this discussion is about energy, but even if OTEC is not efficient enough, it seems to me that the real advantage is the cold water from the deep. What if you scrap using the cold water for energy production and instead use it for energy usage reduction. For example, 40F is more than cold enough to supply your airconditioning needs, which is a huge energy hog. You could also use it to improve the efficiency of refridgerators and freezers. If you have a larger thermal source of energy, like a solar thermal plant, you could still use the cold water on the condensing side to improve efficiency. I also believe that this colder water could still be used in the production of freshwater via condensation, especially if combined with solar. Then there is the minerals in the deep water, which I understand may have good value in aquaculture/fish farming.

    The question then is how much cold water do you need for these purposes and is it cost efficient to pump that quantity to the surface when you compare the energy used for pumping versus the energy savings potential.

    As for the cost of the pipe, I would think it would be much lower due to the much smaller quantity of cold water needed for the above purposes versus OTEC.

    What kind of pipe is needed? If you put the pump at the bottom, could you use a very thin walled plastic/fabric due to the positive pressure inside? That way you could easily pull it to the surface if the water is to shallow, etc.

    I don’t know if these ideas have merit, but I would think this is possible to research. If you are going to live on the ocean, you will have to use all of the resources available, and that 40F water seems to be a very valuable resource all by itself.

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    You’re going to need heat more than cooling at most latitudes, most of the year. Ocean water is a much better conductor than air and cooler than is comfortable for humans over most of the world.

    Profile photo of CrosiarCM

    Sorry, I’m in the Philippines, so my thinking is definately more equitorial! I know there are a lot of people here that want to be in the more northern latitiute, say SF bay area, but in my mind that is not practical for many reasons. Time will tell.

    You may get what you want, but will you want what you get?

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