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:
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)