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Answering Questions on OTEC – Part II

This is the second installment responding to reader questions and comments on ocean thermal energy conversion (OTEC) by Dr. Robert Cohen. Dr. Cohen’s previous entries are:

Ocean Thermal Energy Conversion

Potential Markets and Benefits from Ocean Thermal Energy

Answering Questions on OTEC – Part I


Robert Cohen, February 16, 2010

Environmental, Operational, and CO2 Issues

Since the operation of an ocean thermal plant requires the circulation through the plant of a veritable “river of water”, careful design consideration must be given to minimizing effects on the local and downstream temperature distribution with depth. Hence a lot will depend upon how the effluent seawater is discharged following passage of the warm and cold seawater inputs through the evaporators and condensers. Fortunately there is a disincentive for the plant operator to perturb the pre-existing local temperature distribution, since plant economics are greatly improved by maintaining the largest practical temperature difference between the warm seawater and the cold seawater at depth.

Design of the discharge process—i.e., how to discharge the cooled warm water and warmed cold water effluents—can be handled in various ways. For example, by discharging the cooled warm water at a depth corresponding to its new temperature, and by discharging the warmed cold water below the sunlight-affected (phototropic) zone, to prevent formation of algae blooms within that nutrient-rich cold seawater. One of the functions of the pilot plant is to monitor the discharge plumes, compare them to modeling predictions, and allow environmental scientists to assess how the plant interacts with its surroundings.

During the heyday of the federal ocean thermal R&D program, in the 70s and early 80s (prior to public concerns about CO2), a key environmental goal of the federal R&D program on ocean thermal energy was to avoid perturbing the thermal environment of the plant. Accordingly, contracts were awarded to groups at MIT and Cornell to conduct fluid-dynamical modeling studies of water circulation.

Those studies led to another likely way of satisfactorily discharging the seawater effluents to avoid significantly perturbing the thermal environment; namely, to mix the cooled warm water and warmed cold water effluents, then to discharge the mixture at a depth within the thermocline where the ambient temperature matches the resulting temperature of the mixture.

In those modeling studies, global warming and the fate of the CO2 dissolved in the upwelled cold water were not issues of significant concern. But nowadays, avoiding liberation of CO2 to the atmosphere must also be a goal in plant operation, hence future modeling of seawater circulation in connection with the design of ocean thermal plants and plantships will need to consider both temperature and CO2 parameters.

Another important design factor in avoiding CO2 emissions is proper design and operation of the ocean thermal power cycle. According to a study by Green and Guenther (1990), proper use of the “closed” power cycle would probably suppress CO2 emissions, but if the “open” cycle is used to co-produce fresh water, special care must be taken, in the course of degasifying the warm seawater, to avoid liberating CO2 to the atmosphere. It appears that most serious plant designs for multi-megawatt offshore plants are choosing the closed cycle, because the turbines needed for open-cycle operations are too large for those applications.

There is a conjectural possibility that ocean thermal plants and plantships could—in addition to their normal operation, and for a fee—take on the additional task, if feasible, of removing CO2 from the atmosphere and sequestering it in the deep ocean. But the incremental cost of achieving such sequestration would have to be considered and internalized into the plant economics.

Accordingly, one can safely make the qualified assertion that, when and if deep-sea sequestration of CO2 extracted from the atmosphere becomes technically and economically viable, then fleets of ocean thermal plants and plantships will be well-positioned for conducting that additional function, assuming that the incremental cost of doing so can be dealt with. If such sequestration were to become a realistic option, then ocean thermal technology may be in a position to win the Branson Virgin Earth Challenge Prize for removing CO2 from the atmosphere.

There has long been interest in using for mariculture (of plants or animals) the artificial, nutrient-laden, cold-water upwelling associated with the operation of ocean thermal plants. Such mariculture would utilize for fertilizer the nutrients (phosphates, nitrates, and CO2) dissolved in the upwelled cold water. But nowadays the potential viability of this co-product application would need to be reexamined, in view of the possibility that an open-ocean mariculture operation, as an adjunct to normal plant operation, could result in liberating some of the CO2 contained in the cold water into the atmosphere. Furthermore, although kelp plants, for example, fare well in a cold-water environment, conducting an open-ocean mariculture operation near the surface could result in reducing the temperature of the warm surface water fueling the ocean thermal plant, hence make the two activities incompatible.

Besides the above thermal and CO2 considerations, there are many other environmental aspects of operating ocean thermal plants and plantships. Numerous studies have been conducted regarding possible environmental impacts of ocean thermal power plants, such as: impingement and entrainment of marine organisms; possible discharges of CO2, biocides, corrosion products and working fluids; and artificial reef, nesting, and migration aspects. Those studies indicated that such potential impacts can be satisfactorily dealt with. For example, see a 1990 report by Green and Guenther, and a 1986 study report by Myers et al. The latter, conducted by researchers at NOAA and Argonne National Laboratory for the National Marine Fisheries Service (NMFS) of NOAA, is available at this URL.

Those studies probably need updating today, in view of growing concerns about global warming. In particular, further R&D will be desirable on how to avoid liberating CO2 from ocean thermal plants, and for modeling various environmental aspects of operating a fleet of ocean thermal power plants and plantships.

Despite the absence of updated studies in these areas, conjectures are being made, often without much basis, as to what environmental effects might occur as a consequence of large-scale implementation of ocean thermal energy extraction. For example, forecasts are being made regarding how much electrical power can ultimately be extracted from the vast available ocean thermal resource. It is my contention that—in the absence of hard data resulting from significant operational experience with commercial ocean thermal plants—it is currently premature to forecast likely environmental impacts or make valid quantitative forecasts of total recoverable power.

As deployment of this technology proceeds, it will be important for the environmental community to develop the modeling tools needed to forecast possible environmental effects. The adaptation of existing finite-element modeling tools is underway for applying them to the pilot plant. As part of the procedures for satisfying the NOAA licensing requirements for siting, building, and operating the pilot plant, there will probably be a year of preliminary environmental monitoring at the proposed site off Hawaii, followed by a second year to validate those measurements, compare them with modeling results, and exclude any anomalies.

The next step will be to model an array of commercial plants located around markets such as Hawaii, Puerto Rico, and Florida. Finally, as more plants are placed in operation, global models will be needed to assess any concerns about large global ocean currents. As a fleet of ocean thermal plants and plantships emerges, data will become available to validate models and assumptions, and the parameters needed for such forecasting will start becoming available.

Much will depend upon how these plants and plantships are developed, deployed, and operated—such as how their effluent seawater is discharged—and upon the degree of implementation of this technology. Those details will evolve with time and are presently rather unpredictable, since there are too many parameters, imponderables, and unknowns to reach valid conclusions in these areas.

The evolution of ocean thermal technology from a concept to a commercial reality will probably proceed cautiously and gradually at first, then accelerate. During the initial phases of that process—sort of a shakedown cruise—much will be learned operationally about how to properly handle the seawater intakes and effluents, among other environmental aspects. And, during that gradual commercial introduction of the technology, licensing requirements ought to cautiously ensure that these early plants be operated responsibly.

By the time fleets of plants and plantships are deployed, operational experience will inform their environmental design and operation. Public Law 96-310 assigned to NOAA the responsibility for licensing the operation of ocean thermal plants and plantships. Presumably EPA will also participate in this process. Recently there have been numerous meetings between the NOAA licensing team and the LM engineering team regarding licensing, and it was NOAA that initiated the technology-readiness workshop cited above. NOAA is planning another such workshop, focused on addressing environmental issues, for the summer of 2010.

Engineering Requirements and Challenges

There are various technical requirements for constructing and operating ocean thermal power plant systems, some of which pose significant engineering challenges that must be surmounted in order to achieve the commercial viability of ocean thermal systems. System requirements include ocean engineering of the platform and its external attachments, and power engineering internal to the platform. And the system solutions to all technical and environmental requirements must be achievable at a reasonable total capital cost for the system, so that, when amortized over the plant’s lifetime, the system will provide products whose costs can be competitive in the marketplace.

One of the reasons why the successful operation of a multi-megawatt pilot plant will be a critically important step in making the transition to a first-of-a-kind commercial power plant is that it will provide a capability to assess the impacts of seawater circulation and to validate analytical circulation-modeling studies.

Building and testing a pilot plant prior to designing and constructing a commercial plant is a standard, prudent industrial practice aimed at reducing risks when making the transition from any engineering concept to a commercial reality. In the case of ocean thermal, assembling the components and subsystems into a pilot-plant working system prior to making a major investment in a commercial plant can build confidence in the viability of the concept by demonstrating that it is practical at a multi-megawatt scale and by solving any unanticipated problems that emerge. Operational data and experience obtained from successful operation of a pilot plant will provide invaluable cost, environmental, and engineering-design Inputs for moving to a commercial-size plant.

Ocean engineering requirements and challenges that must be successfully surmounted in achieving commercially viable ocean thermal power plants include:

• Designing and deploying large-diameter, kilometer-long cold water pipes (CWPs)
• Flexible, detachable coupling of the CWP to the platform
• Tolerance of the CWP to vibrations caused by vortex shedding
• Detachable mooring (or dynamic positioning) of ocean thermal power plants for stationkeeping in depths equaling or exceeding a kilometer
• Operability in storms, and survivability in severe storms and hurricanes

Similarly, means must be developed for satisfactorily connecting submarine electrical cables to ocean thermal power plants, and those cables will need to be durable and capable of transmitting power to shore from large distances at a reasonable cost and with minimal power attenuation. For plantships grazing on the high seas, the stationkeeping requirement is relaxed compared to the stationkeeping requirement—usually mooring—for plants cabling electricity to shore.

Some CWP failures during deployment have historically occurred in the course of ocean thermal experiments, yet many CWPs have been deployed successfully in the past forty years. However, the CWP diameters required for large, multi-megawatt ocean thermal plants will considerably exceed those of similar pipes that have been successfully deployed at sub-megawatt power levels, largely as intakes for seawater-cooling or lake-water-cooling installations.

In 2008 DOE awarded LM a cost-shared, multi-year R&D coöperative agreement aimed at demonstrating technology for designing and deploying a CWP made of composite material. The LM technique is to fabricate sections of the CWP aboard the platform, then to assemble and deploy them as they are manufactured. LM is developing approaches for coupling the CWP to the ocean platform, as part of the NAVFAC contract mentioned here earlier.

To meet power-engineering requirements, design of the heat exchangers must address biofouling, the buildup of a layer of ocean organisms on surfaces exposed to seawater. Formation of such a slime film on the heat exchanger surfaces inhibits heat transfer, hence prevention or removal of biofouling deposits is required.

Similarly, the corrosion of the heat exchanger surfaces would inhibit heat transfer and must be avoided. In view of their lower costs and greater availability, aluminum alloys that can resist seawater corrosion are attractive candidate materials in comparison to titanium alloys.

Open-ocean testing of the biofouling of candidate heat exchangers rated at 1 MWe was conducted aboard OTEC-1, the test facility for ocean thermal system components. As part of those tests, biofouling was controlled (Gavin & Kuzay, 1981) primarily by chlorination; i.e., injection of chlorine into the evaporator and condenser. The rate of intermittent injection was 0.4 mg per liter during one hour out of each 24-hour period that the seawater systems were in operation.

Even with stringent environmental regulations, it is anticipated that chlorination levels in the discharge can be designed so as to comply with those regulations. Indeed, use of intermittent chlorination within EPA standards has already proved successful in controlling biofouling in the condensers used in conventional coastal power plants, hence that technique is a likely means for performing the same function in ocean thermal plants.

Fortunately, during the 28-year lapse since DOE ocean-thermal R&D funding began to be curtailed in 1981, the offshore oil industry has made some remarkable technological advancements in designing and operating ocean structures, much of which will be relevant to the above technical requirements for ocean thermal systems. Consequently, many of the perceived and actual risks of moving forward today have been considerably reduced, thanks to the innovations and experience of that industry. At the annual Offshore Technology Conference held in Houston in 2009, a panel session reviewing the status of ocean thermal technology was attended by some key people from the offshore oil industry.

These events were foreshadowed years ago by Derrington, 1979; Clare, 1981; and Wortman, 1981, who observed that much of the technology developed for the offshore petroleum industry will be transferable to the construction, deployment, and operation of ocean thermal systems.

Efficiency and Cost Considerations for Generating Electricity

There is a basic question as to whether the above engineering requirements for ocean thermal power systems can be achieved at a system capital cost that will provide baseload power that is cost-competitive with other sources of electricity. For ocean thermal power systems, it is clear that, because of the low net conversion efficiency, lots of capital equipment will be required to circulate a “river of water” past extensive expanses of heat exchanger surfaces.

Practical realization of an ocean thermal plant requires a physical configuration of heat exchangers, turbines, generators, pumps, and other hardware, such as a startup engine. The plant will probably be comprised of multiple power modules, whose condensers are served by a single cold water pipe.

Engineers familiar with conventional power systems that generate electricity by the combustion of fossil fuels are accustomed to dealing with conversion efficiencies of at least 30%. Hence they find it difficult to grasp that an ocean thermal power system can be viable at a net efficiency much smaller than that.

For ocean thermal power plants, typical operating ∆T’s project theoretical (Carnot) efficiencies of about 6 or 7%, while their achievable net efficiency will be about one-third of theoretical. There are two reasons for this reduction:

1) The gross power generated will be less than the theoretical target because there is some loss of temperature across the walls of the extensive areas of heat exchangers.

2) The net power output from the plant will be less than the gross power generated because there is a need for operational “housekeeping” power, mainly used to power the pumps that circulate these large volumes of seawater.

The considerable pumping power required will be used for 1) pushing seawater against the drag experienced when it flows through the heat exchangers, and 2) for moving seawater through the cold water pipe, the warm water pipe, and the seawater-effluent pipes. Some pumping power will be used to move the ammonia working fluid from the condensers to the evaporators.

The pumping power required to lift the water through the kilometer-long cold water pipe is lower than what one might anticipate. That is because, thanks to buoyancy, the pumping power actually required for that purpose is only that needed to accelerate the cold seawater, compensate for its density gradient, and counter the drag experienced alongside the CWP’s walls.

To minimize drag losses alongside the CWP walls, it is evidently desirable to employ a single CWP rather than to have multiple CWPs. One can also draw the conclusion that the CWP is a plant component amenable to an economy of scale (up to a point) as plant size and the diameter of the CWP are increased.

Granted that it will be important for ocean thermal power systems to operate at maximum/optimum efficiency, there are two reasons why focusing solely on efficiency can be diversionary or misleading when it comes to analyzing and comparing power-plant economics:

1) Net conversion efficiency is not the economic bottom line, which is energy cost, the cost of the plant’s output electricity per kWh.

2) The life-cycle energy cost of a power plant is the sum of three components: the plant’s fuel cost, O&M cost, and amortized capital cost.

For power generation from renewable energy sources, the fuel cost is zero, but the capital cost of those systems tends to be relatively high compared to a fuel-consuming plant. For power plants that burn oil, the fuel-cost component nowadays tends to be high, and is likely to increase, while their capital cost continues to be relatively low compared to that of power plants employing renewable energy.


Clare, R., 1981, in Proceedings, Eighth Ocean Energy Conference (ed. E.M. MacCutcheon)

Derrington, J., 1979, in Proceedings, Sixth Ocean Energy Conference (ed. G.L. Dugger)

Gavin, A. P. & T. M. Kuzay, 1981, 0TEC-1 power system test program: biofouling and corrosion monitoring on 0TEC-1. Argonne National Laboratory

Green, H.J. and P.R. Guenther, 1990, Carbon dioxide release from OTEC cycles, Solar Energy Research Institute report TP-253-3594

Myers, E.P. et al., 1986, The potential impact of ocean thermal energy conversion (0TEC) on fisheries, NOAA Technical Report NMFS 40—Available at URL http://spo.nwr.noaa.gov/tr40opt.pdf

Wortman, E.J., 1981, in Proceedings, Eighth Ocean Energy Conference (ed. E.M. MacCutcheon)

February 22, 2010 Posted by | global warming, guest post, ocean thermal energy conversion, otec, Robert Cohen | Comments Off on Answering Questions on OTEC – Part II

Answering Questions on OTEC – Part I

Dr. Robert Cohen has been involved in ocean thermal energy conversion (OTEC) since the early 1970’s. He has posted two guest essays here previously:

Ocean Thermal Energy Conversion

Potential Markets and Benefits from Ocean Thermal Energy

Following both essays, a number of questions and concerns were raised, so I asked Dr. Cohen if he would respond. He has written me a thoughtful and detailed response, and I will present it here in two parts.

Dr. Cohen also has a website with more information on OTEC. His contact information is available there. Part I is a general commentary on history, current status, and the projections for cost and a market-entry outlook. Part II will delve deeper into the engineering and environmental questions that were raised.



Robert Cohen, February 16, 2010


Numerous comments were posted on this blog in response to my two previous postings here regarding ocean thermal energy. Those comments raised various issues and concerns regarding the implementation of ocean thermal energy technology. This posting is an effort to provide some perspective on the status of ocean thermal technology, written with the intention of addressing the points people raised in their comments.

The postings by viewers tended to fall into several categories, which I shall group as follows:

  • Possible environmental impacts of plant operation on the ocean, including those on parameters such as temperature and CO2-concentrations
  • Technical and economic requirements, challenges, and hurdles for ocean thermal technology to become a commercial reality
  • System conversion efficiency and system energy costs

My Perspectives on Ocean Thermal Energy Technology

Since being assigned by NSF in 1973 to serve as the first ocean thermal program manager, charged with organizing and conducting a concerted federal R&D program on ocean thermal energy, my tentative outlook has been, and continues to be, that of a cautiously optimistic advocate of this technology. Informed by my experience since then, I have yet to encounter a demonstrable or foreseeable “show-stopper” in the technical, environmental, or economic areas that would preclude the achievement of economically/technically viable and environmentally acceptable technology for harnessing ocean thermal energy.

In the mid-1970s my outlook was first bolstered by two federally sponsored industrial studies that resulted from contracts awarded to Lockheed and TRW. After conducting an engineering evaluation, both firms independently concluded that ocean thermal technology had good prospects for achieving technical and economic viability. By “economic viability” I believe that we all mean that baseload ocean thermal power systems could become cost-competitive, at least versus oil-derived electricity.

Now, some 35 years later, both Lockheed Martin (LM) and the U.S. Navy seem to have reached similar tentative conclusions about today’s outlook for this technology.

Starting around 2007, LM began rebuilding its ocean thermal engineering team by annually investing millions of dollars of its own discretionary internal R&D funds, and is continuing to make such investments. The LM team’s effort is focused on developing the design of a multi-megawatt power plant for operation off Hawaii, successful operation of which can lead to prompt design and construction of a first-of-a-kind 100 MWe commercial plant. LM regards the latter as likely to be cost-competitive in markets like Hawaii that presently rely on oil-derived electricity. Some company perspective on LM’s effort is stated at this URL.

Recently the Naval Facilities Engineering Command (NAVFAC), which is responsible for naval-base infrastructure, competitively awarded an $8.1 M contract to the LM team. That award is for technical activities aimed at reducing overall system and developmental risks for critical subsystems and components, and at maturing a pilot-plant design. The Navy has a long-term interest in helping foster the commercialization of ocean thermal technology, achievement of which would enable it to purchase, at cost-effective rates, ocean-thermal-derived electricity and fresh water from privately developed facilities at U.S. military bases located in places like Hawaii, Guam, and Diego García.

In an effort to help call attention to what harnessing ocean thermal energy can do to help mitigate global warming, I posted some information on the Copenhagen Climate Council Web site, which can be accessed via URL here. A set of slides addressing various facets of ocean thermal energy can be downloaded there. Those slides summarize many of the technical, economic, and environmental aspects of the issues raised here.

A November 2009 workshop was convened and hosted by the NOAA people who are charged under U.S. law with licensing ocean thermal plants. That workshop was specifically aimed at exploring the technical readiness of ocean thermal energy technology, and it is my understanding that the technology received high grades there. The workshop is summarized on this Web page. [Note that the Web page is replete with hot buttons, i.e., Web links, each leading to detailed information about various technical aspects of ocean thermal that were examined at the workshop. Most, but not all, of the items that are in bold face are Web links.]

Cost and Market-entry Outlooks

The largest ocean thermal power system heretofore operated (by DOE contractors in 1980) was OTEC-1, a floating test facility designed to test candidate ocean thermal components and subsystems, such as heat exchangers, rated at 1 MWe. Lacking a turbine-generator set, that facility fell short of being a complete power system. Two complete closed-cycle ocean thermal power systems of sub-megawatt size have been successfully demonstrated. They were the 50 KWe (15 kWe net power) floating facility operated off Hawaii in 1979, which was developed by a private consortium led by Lockheed, and the 100 kWe (34 kWe net power) land-based facility operated in 1981 on the island of Nauru, which was developed by the Tokyo Electric Power Services Co.

To bridge the gap to multi-megawatt commercial plants, the LM team is designing a 5/10 MWe ocean thermal pilot plant—initially containing the first of two 5 MWe power modules—to be sited off Pearl Harbor, Hawaii. Operation of the pilot plant will provide performance, cost, and environmental data preparatory to designing and constructing a 100 MWe “commercial” plant for Hawaii’s oil-driven market.

Extrapolating pilot-plant cost estimates to what the commercial plant might cost, the LM team believes that a first-of-a-kind 100 MWe commercial plant can be built at a capital cost enabling it to compete in Hawaii’s oil-driven electricity market; i.e., to produce electricity at an avoided-cost target close to what busbar electricity is currently worth there.

Assuming that LM can achieve that energy-cost target—a busbar cost of electrical energy of roughly 20¢/kWh—then, if I work backward from that energy cost, using reasonable assumptions regarding interest rate and plant-amortization lifetime, including an additional cost of ca. 2¢/kWh for O&M, I estimate that that energy cost for a first-of-a-kind 100 MWe baseload power plant would roughly correspond to a plant capital cost target of about $1 B, or $10 per watt. If one assumes that federal tax credits are available to serve as an incentive/subsidy, then the tolerable capital cost for this first-of-a-kind commercial power plant could perhaps be about 50% higher, around $1.5 B.

In contrast, the 5/10 MWe pilot plant that LM is designing—since piloting of a technology at small scale increases the cost per unit output—will probably cost roughly several hundred million dollars, corresponding to an energy cost perhaps ranging from 40 to 60¢/kWh, making that large an investment sub-economic. Hence the pilot plant will require some subsidization, the hurdle-cost for launching this new ocean industry.

But the subsidy required sounds like peanuts nowadays. Note that during its heyday—the late 70s and early 80s—the DOE ocean thermal R&D program was being funded at about $40 M annually, equivalent to $100 M/year in today’s dollars. It may well be that Recovery Act funds or DoD will provide that subsidy, but it would be reassuring if the Obama Administration and the Congress would soon explicitly embrace ocean thermal and commit to rapidly advancing it into the marketplace, as was happening during the Nixon, Ford, and Carter Administrations.

Once the pilot plant is successfully operated, the design data and cost estimates for the first 100 MWe commercial plant will become much clearer. There are various options for funding that commercial plant. For example, about 80% of its capital investment could be federally loan-guaranteed; the remainder, roughly $200 to 300 M or so, would be venture capital, and investment tax credits would offer an additional incentive.

A comparison—albeit crude—can be made between the above $10 capital cost per watt, for baseload (continuous, 24/7) ocean thermal power capacity, versus the capital costs per watt for intermittent wind and photovoltaic power. Let’s assume wind and photovoltaic power systems that cost $4 and $7 per watt, respectively, and that they generate power about one-third of the time. Then, for purposes of making a rough comparison with the capital cost of a baseload source like ocean thermal, the intermittent wind and photovoltaic capital costs can be multiplied by three, yielding $12 and $21 per watt, respectively, compared to roughly $10 to $15/watt for a first-of-a-kind, 24/7 ocean thermal plant.


Clare, R., 1981, in Proceedings, Eighth Ocean Energy Conference (ed. E.M. MacCutcheon)

Derrington, J., 1979, in Proceedings, Sixth Ocean Energy Conference (ed. G.L. Dugger)

Gavin, A. P. & T. M. Kuzay, 1981, 0TEC-1 power system test program: biofouling and corrosion monitoring on 0TEC-1. Argonne National Laboratory

Green, H.J. and P.R. Guenther, 1990, Carbon dioxide release from OTEC cycles, Solar Energy Research Institute report TP-253-3594

Myers, E.P. et al., 1986, The potential impact of ocean thermal energy conversion (0TEC) on fisheries, NOAA Technical Report NMFS 40—Available at URL http://spo.nwr.noaa.gov/tr40opt.pdf

Wortman, E.J., 1981, in Proceedings, Eighth Ocean Energy Conference (ed. E.M. MacCutcheon)

February 21, 2010 Posted by | global warming, guest post, ocean thermal energy conversion, otec, Robert Cohen | Comments Off on Answering Questions on OTEC – Part I

Osmotic Power

Did you ever wonder why the skin on your hands sometimes shrivels when you have them in water for too long? The underlying reason is called osmosis (a simple explanation in more detail below), and the same driving force is now being utilized as a power source.

Occasionally I encounter an energy story that catches me by surprise because it is so far under the radar. This morning I got one of those from a friend who e-mailed and referred me to this story:

The world’s first osmotic power plant opened!

My immediate reaction was skepticism that you could really make osmotic power work as a viable energy source. But first a bit of background before readers’ eyes glaze over at the usage of unfamiliar terminology. Students of chemistry or biology will have encountered the concept of osmosis, and most people have heard of reverse osmosis for the production of fresh water from saline or otherwise contaminated water.

In a simplified nutshell, water that is separated from a salt solution by a semi-permeable membrane (like a cell wall) will have a potential to migrate across into the salt solution – creating a pressure difference on the two sides of the membrane. (Lots of systems can create an osmotic pressure, but for illustration let’s focus on salt water and fresh water).

Osmosis is a very important concept in biology, as it is the mechanism by which water moves in and out of cells. A blood cell, for instance, will lose water and shrink if it encounters an outside environment that is more saline (saltier) than the internal environment. Water moves through plants by this process as well.

But to illustrate what is going on in the press release above, let’s talk about reverse osmosis. In reverse osmosis, a pressure is applied to the high salt concentration side to force fresh water back across the membrane – leaving the salt behind. The applied pressure must be greater than the osmotic pressure, or the fresh water will migrate to the saline side.

Now imagine that system in reverse. There is a saline solution on one side of the membrane, and it is allowed to build pressure from the migration of the fresh water across the membrane into the salt water. The build up of pressure – in this case osmotic pressure – could in theory be utilized for energy.

Imagine the way a dam works. Water pressure forces the water through a turbine, which generates electricity if it is coupled to a generator. If the osmotic pressure is likewise allowed to relieve through a turbine, then yes, in fact it could be used to produce electricity. Such a system would indeed produce osmotic power.

However, until this morning’s e-mail I had never heard of anyone actually building a system to do this. And I am skeptical that anyone can actually produce cost-effective electricity this way, because to generate a substantial pressure is going to require a lot of membrane surface area. A little bit of digging shows that the system above has a power output of only 4 kilowatts.

To put that into perspective, there are numerous power plants with outputs greater than 1,000 megawatts – which is 250,000 times the size of this osmotic power demonstration unit. So while this is perhaps newsworthy due to the novelty, they must prove that they can economically scale-up, and that is always a big hurdle.

One thing I wondered about as I read this article is whether it might not be more cost-effective to put in pipelines of fresh water to regions that are doing reverse osmosis of salt water. The idea being instead of using the fresh water in one location for osmosis and the salt water in the other for reverse osmosis, bypass the osmosis all together (reverse osmosis is very energy intensive).

Update: A reader just sent a link that says that IBM is looking into this as well: Energy From Sea Water? Consider IBM Intrigued

Footnote: I Googled the term “osmotic power” to see if that term had ever been used in this blog. My expectation was that it hadn’t, but I see that a reader linked to a story on this a couple of weeks ago in the comments following the story on OTEC (which I should be updating soon).

December 14, 2009 Posted by | ocean thermal energy conversion, osmotic power | 16 Comments

Brainstorming the Year’s Top Energy Stories

I am working on a few things right now that should be finished up in the next week or so. First, I am compiling a list of questions/comments for Bob Cohen regarding his recent guest post on ocean thermal energy conversion (OTEC). I will post his answers. If you have a relevant question that you feel wasn’t asked following that essay, post it here and I will get it to him.

Second, I am trying to put together the year’s top energy stories. In my mind the one at or near the top has been the resurgence in oil prices since January. Oil prices have more than doubled, and in a normal year that would be big news. But I think the news has been discounted merely because the levels are so much lower than the record levels of 2008. After that, there are a few stories that I think go in the Top 10, like the enormous amount of money devoted to energy in the stimulus package, the plunge in oil demand/imports, the commissioning of various alternative energy projects, Climategate, and the passage of Markey-Waxman. What other significant stories happened this year that deserve a spot in the Top 10?

Third, I have been really overwhelmed with e-mails lately. If I didn’t answer your e-mail in a timely manner, I apologize. If I missed it completely and you really need an answer, please resend. Sometimes things inadvertently end up in my spam folder. But I have gotten a couple of e-mails recently from people wishing to share links on energy saving tips, and so here those are:

20 Under $5 Tips to Improve Your Cars Gas Mileage

One of their tips:

Too much junk in the trunk – It’s a known fact that excess weight in your car caused the engine to work even harder. Being that said, having too much junk in the trunk, of your car, that is, can significantly affect the way your car utilizes gas. Please leave the unwanted items at home before starting your driving journey. Just think of all the junk you can accumulate in your car and how all those items can start to add up.


8 Painless Ways To Cut Your Electric Bill This Winter

One of their tips:

Strip : According to the experts at Lowe’s, a 1/8″ space between a standard exterior door and its threshold is equivalent to a two square inch hole in the wall. Closing those gaps can save you up to 15% in heating costs and reduce the demand on your heating system. They also offer a guide on how to accomplish this at the beginner level. Only three tools, three materials, and a day is all it requires to weatherstrip your entire home.

Finally, thanks to all who read and contribute here. I probably don’t say that enough, but this blog continues because of you. I initially started it just as an outlet for myself, unsure of whether it would attract any readers. But I enjoy writing, and would have still probably written a dozen or so essays even if nobody ever stopped by. Based on current trends, 2010 should bring in the 1 millionth visit (page views are already at 1.2 million views) and I should publish essay number 1,000.

December 10, 2009 Posted by | climate change, Media coverage, ocean thermal energy conversion | 43 Comments

Potential Markets and Benefits from Ocean Thermal Energy

Happy Thanksgiving to those who will celebrate it tomorrow. I plan to spend the long weekend with my family, and probably won’t be on here much.

In the interim, two things. First is that it is about time to start thinking about the top energy stories of the year. As in year’s past, I would like reader’s opinions on the top energy-related stories of 2009. I will put up a post late in December with the ones that I think are most significant.

Second, I present a guest post by Dr. Robert Cohen on ocean thermal energy conversion (OTEC). Dr. Cohen has been an advocate of OTEC for many years, and has posted a guest essay here previously:

Ocean Thermal Energy Conversion

There were some useful comments following that essay that I think explain the challenges for OTEC. Dr. Cohen has a website where he addresses OTEC in more detail, and his contact information is also available there.



Robert Cohen, November 24, 2009

Once the Obama Administration and the Congress put ocean thermal R&D back on a fast track, one that leads to the maturation of this technology within a few years, ocean thermal energy can foreseeably provide baseload electricity to energize at least three major geographic markets (see the ocean thermal resource map on the next page) in the following time-frames:

1) An early market to displace the use of oil, oil that is presently being burned to generate electricity in places like Hawaii, Puerto Rico, and in many developing countries. Places that are relatively accessible to the ocean thermal resource. Years ago we estimated that that early market could utilize about 50,000 MWe, which amount would achieve an oil savings of 2 million BBL/day [calculated @ 40 BBL/day per 1 MWe of ocean thermal].

2) A near-term market for electricity generated (for example) in the Gulf of Mexico and delivered via submarine cable to the U.S. electrical grid at points on the U.S. Gulf Coast, such as Key West, Tampa, New Orleans, and Brownsville. This source of baseload electricity could substantially implement the priority strategies advocated by Al Gore and T. Boone Pickens, which are aimed at providing renewable energy for propelling vehicles.

3) A potentially vast longer-term market for products derived from electricity generated aboard a fleet of ocean thermal plantships grazing on the high seas. Those plantships would convert the baseload electricity (plus air and water) to energy-intensive products such as hydrogen or ammonia, which could be shipped as energy carriers or as final products. For example, ammonia could be used as a hydrogen-carrier, combustion fuel, or for fertilizer.

Note that achieving the third option would mean that energy and energy-products could be shipped to the USA from domestically-owned ocean thermal plantships at locations that are in many cases closer to our shores than are many of our (often hostile) foreign sources of imported oil.

A global map of the OTEC Thermal Resource to supply energy to the above markets

This map shows contours of annual average temperature differences, in degrees Celsius, available in the world’s major oceans between surface waters and the cold water at 1000 meters depth that serves as a heat sink. The most desirable regions (bounded by the areas in yellow) are where that parameter equals or exceeds 20 degrees Celsius.

Energy FROM the oceans, to replace energy from ACROSS the oceans!

November 25, 2009 Posted by | geothermal, ocean thermal energy conversion, reader submission | 31 Comments

Interview with The Reef Tank

A couple of weeks ago, I was contacted by a website called The Reef Tank about doing an interview with them on Energy and The Marine World. At first I said that this was a bit outside my expertise, but they said that was OK; I could just try to answer the questions to the best of my ability. We discussed my background and my interest in energy, this blog, climate change, and energy from the oceans.

They have asked me if I would mind linking to the interview so they might reach a scientifically-minded audience that is of a somewhat different cross-section than what they might normally reach. The interview is available at the link above. Below are excerpts:

Tell me about your interest in the potential of wave and tidal power.

I became interested in the topic several years ago. I was watching a story on the Panama Canal, and watched the level of a ship fall as the water in one of the locks was pumped down. I started thinking about the energy that could be extracted as that massive ship moved to a lower level. That led me to investigate wave power, which is similar in principle to the water rising and falling in a lock. Little did I know when I started investigating that a large body of work had already taken place in this area. Looking into waves naturally caused me to happen upon tidal power as well, and the enormous potential of the flow of the Gulf Stream. [RR: See this story that I did in early 2008: Infinite Underwater Energy.]

Tell me about your interest in ocean thermal energy conversion. How does it work?

I didn’t know too much about ocean thermal energy conversion (OTEC) until I was contacted last year by Bob Cohen, who is a friend of a friend and an advocate of OTEC. [RR: See Bob’s guest post here on OTEC: Ocean Thermal Energy Conversion.] The principle is pretty straightforward. Anywhere you have temperature differentials – such as a warm ocean surface and much colder layers deep down – you can use those differentials to extract work via a heat engine. You are essentially capturing some of the heat flow from the higher temperature reservoir to the lower.

I am sitting in Amsterdam’s Schiphol Airport right now, about to fly back to the U.S. When I get home, I am going to take my first real vacation with the family in a long time. I don’t expect to post anything or answer e-mails for the next week. Please keep the discussions civil, and if trolls show up just ignore them and I will deal with them when I can. Let me also take this opportunity to thank the readers and regular posters who make this effort worthwhile.

Aloha. 🙂

May 7, 2009 Posted by | ocean currents, ocean thermal energy conversion, tidal energy, wave power | 100 Comments

Ocean Thermal Energy Conversion

There are several potential sources of alternative energy that I think are quite promising, but don’t get nearly the press of wind or solar power. Geothermal power is one. There are already a number of geothermal power plants in operation around the world, but you wouldn’t know it by the amount of press coverage. Iceland derives 20% of their electricity and nearly 90% of their heating from geothermal power, but in terms of total megawatts the U.S. is the world’s largest producer of geothermal energy (covered in a previous post here).

Ocean thermal energy conversion (OTEC) is another potential energy source that gets little press. Think of OTEC as a big heat pump that operates off of the temperature differences between the surface temperature of the ocean and the cooler temperatures in deeper waters.

I recently received an e-mail on this topic from Bob Cohen, a friend of my friend Jerry Unruh and advocate of ocean thermal energy. He included an essay that he has written on the potential of the technology. I am reproducing it below, with Bob’s permission.

Status and potential of ocean thermal energy technology

Unlike most renewable energy options, ocean thermal energy conversion (OTEC) technology is a “baseload” (continuous) renewable energy source that potentially can provide a substantial portion of global energy needs. As such, it is worthy of ample national attention and R&D funding. Yet today, OTEC technology is largely unknown to the public and has become an “orphan technology” that is being widely overlooked and left out of the public discussion on energy.

In September 1973, during President Nixon’s “Operation Energy Independence”, I left my scientific career at NOAA/Boulder in favor of becoming the first ocean energy program manager within the budding U.S. federal solar energy R&D program, then headquartered at NSF/RANN. Ocean energy R&D was later moved to DOE. The initial federal ocean energy R&D program was mainly focused on OTEC technology, in view of that technology’s potentially large energy payoff. Indeed, assuming that OTEC-derived ammonia or other OTEC-derived energy carriers become competitively viable, OTEC technology could provide the largest payoff of any renewable energy source.

Soon after arriving at NSF, I initiated an RFP seeking an objective evaluation by industry of OTEC’s viability, to get their take on what four groups of OTEC advocates (three of which were in academia) were saying about OTEC. That RFP led to our funding two parallel, buffered studies, which were conducted by Lockheed and TRW starting in 1974. Both studies independently reached favorable conclusions as to the prospects for the technical and economic viability of commercial OTEC plants. Since 2006, there has been a resurgence of interest in OTEC at Lockheed Martin, which is currently investing significant internal funds in reexamining OTEC technology.

From 1972 to 1981, the U.S. ocean energy R&D program thrived and grew under the Nixon, Ford, and Carter Administrations. But in 1981 the Reagan Administration curtailed funding, just on the verge of DOE’s realizing a 40 MWe cost-shared OTEC pilot plant. All funding of the DOE ocean energy R&D program ceased in 1995, leaving the nation hitting on only five of the original six renewable energy R&D cylinders. Meanwhile, DOE has been advancing the other five renewable-energy technologies (wind, photovoltaics, biofuels, heating/cooling of buildings, and solar thermal) toward commercialization.

Thanks largely to an emerging constituency in the coastal states—groups who have been working on generating energy from ocean waves and tidal currents—Congress recently reactivated in FY08 the former DOE ocean energy R&D program (including hydropower R&D) under the name “Water Power Energy R&D”. Congress gave DOE considerable latitude on how to allocate the $10 M FY08 appropriation among hydropower and the various ocean energy technologies.

Subsequent to the U.S./DOE R&D program on OTEC technology during the 70s and 80s, which expended a total of about $250 million, the offshore oil industry has made remarkable progress in developing and advancing marine technology, much of which can be spun off to OTEC applications. So I conclude that elements within the oil industry are likely to become interested in diversifying into an OTEC industry, since they have the capital to do so and because they are likely to be comfortable investing in this sister technology.

There is a substantial U.S. market for OTEC-derived electricity that can be delivered via submarine electrical cable. For example, from locations about 60 to 100 miles off Tampa, New Orleans, and Brownsville. Also, OTEC plants situated much closer to shore can provide electricity and fresh water to a large island market, such as in Hawaii, Puerto Rico, Virgin Islands, and U.S. military bases (e.g., Guam, Hawaii, Kwajalein, and Diego García). Abroad, there are similar potential markets in developing nations that are located in tropical and sub-tropical latitudes. The reason that many such locations are especially attractive early markets for OTEC-derived electricity is that it can be supplied—often, along with fresh water as a valuable co-product—competitively with the oil-derived electricity presently being utilized.

Each baseload megawatt of OTEC power—cabled to locations like Hawaii and Guam that presently use oil to generate electricity—could replace 40 BBL of oil each day. Projections made in 1981 estimated an early OTEC market in such places at some 50,000 MWe, achievement of which would offset a total of about 2 MBBL of oil per day.

Renewable energy technologies are often lumped together and dismissed as serving only nîche or boutique markets. However, OTEC technology is likely to be an exception, assuming that baseload OTEC electricity—harvested aboard factory “plantships” grazing on the high seas—can be converted at sea to viable energy carriers that can economically and competitively deliver those products to markets ashore. Promising candidates for OTEC energy carriers include hydrogen or, more likely, ammonia (as a hydrogen carrier, fuel for combustion or use in fuel cells, or for end-uses like fertilizer). By employing such energy carriers or energy-intensive products as an “energy bridge” to shore, OTEC has the potential to become a major global source of renewable energy.

From the standpoint of national security and energy security, achieving the goal of importing substantial amounts of renewable ocean thermal energy—harvested in international waters by a fleet of domestically-owned OTEC plantships—would be in marked contrast to importing oil from foreign, often hostile, sources. At the same time, OTEC can become an attractive means for mitigating global warming.

I responded by telling Bob that I had a favorable impression of ocean thermal, and I followed up with a few questions about cost, as well as whether anyone has actually managed to produce ammonia in this method (a similar scheme has also been proposed for wind power, but not yet demonstrated) because I am concerned about the worldwide fertilizer situation. Bob’s response (in part):

Insofar as energy cost, one current capital-cost estimate for a proposed first commercial ocean thermal plant — a 75MWe plant projected for a location in Puerto Rico — is $8,000 per kWe. Assuming that that capital-cost estimate is realistic and in the ballpark, that first plant would generate electricity roughly costing about 15¢/kWh.

You mentioned being alarmed by the fertilizer situation. So I’ve just asked the NYTimes to send you a front-page article they published in April [RR edit: Here is the article] about the threat to farmers being posed by increasing fertilizer cost and shortages.

Re deriving ammonia from offshore wind, I’m in contact with George Hart, Jr., who is looking into that possibility conceptually. George is Chief Technical Officer for the Ocean Energy Institute, which was formed a year or so ago by oilman Matt Simmons. My impression is that there has yet to be anything demonstrated along those lines. George is talking about offshore wind @ something like $3.50 per installed watt, and he estimates a duty cycle of ca. 40%. One question I was asking Jerry [Unruh] about is the feasibility of intermittently powering an ammonia plant from a source of electricity like wind.

One nice feature of producing ammonia from renewables would be that — assuming there is ever a cap-and-trade system — those processes ought to warrant a CO2 credit. As contrasted with a CO2 debit when ammonia is produced from natural gas, oil, or coal, since I gather that when those fossil fuels are used to make ammonia, the resulting CO2 is dumped into the atmosphere.

I was recently asking Jerry about the prospect of avoiding the electrolysis of water to make ammonia, by instead combining nitrogen and water. You may have already heard about the technology being proposed to do that, which is being called “Solid State Ammonia Synthesis” (SSAS). A PDF file of a presentation on that subject is available here.

My understanding is that the primary obstacle right now is capital costs. The ocean is very harsh on equipment. But the potential is there for a renewable source of energy that does not have some of the negative land usage baggage that is associated with biofuels.

Like most of the stuff I do, Bob is a pro bono advocate because he thinks this is the right thing to do. If you have some useful expertise or criticisms to add, Bob can be contacted at r.cohen ‘at’ ieee.org.

August 23, 2008 Posted by | geothermal, ocean thermal energy conversion, reader submission | 62 Comments