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Ocean Thermal Energy Conversion: Power from the Sea
History | Innovations | Power Systems | Pipe Systems
The Tropical Ocean — the world’s largest — and most efficient solar energy collector and storage medium - represents a vast and yet untapped natural resource which can provide the globe with all of its required energy needs in perpetuity.
Each day, the solar radiation incident upon, and absorbed by, the tropical ocean is significantly more than 1000 times the current global energy consumption over the same twenty four-hour period. Thus, even anticipating the eventual industrialization of the rest of the planet, we will likely never consume more energy than the natural daily fluctuation of this enormous thermal resource, thereby ensuring an environmentally friendly and sustainable energy economy for all peoples for many generations to come.
Ocean Thermal Energy Conversion is a process where we can tap into that natural cycle and reap enrgy benefits from that natural fluctuation - with ultiple peripheral benefits.
During the latter part of the last century, the young sciences of Oceanography and Thermodynamics had each separately developed past the initial rush of discovery to attain a certain level of maturity. The science of Oceanography was being built upon the results of measurements made in several voyages of discovery, under a number of flags, towards a broad understanding of the structure and dynamics of the ocean. Thermodynamics, after the development of theoretical underpinnings relating mechanical and thermal energy and work, flowered into practical application in the form of the steam engine and further, the growing Industrial Revolution.
Jaques-Arsène d'Arsonval, a famous French scientist, noted for his numerous contributions to the physical, physiological, and medical sciences, bridged the two sciences of Thermodynamics and Oceanography with his realization that the ocean and atmosphere acted like a giant steam engine. In the natural solar energy cycle, heat flows horizontally from the sun-warmed tropics to cooler temperate zones. This same heat flow could be tapped vertically in the tropical regions of the ocean via human technological means. This concept could, in effect, be actualized to harness a potentially limitless amount of energy.
Ocean Thermal Energy Conversion uses the world’s largest renewable energy resource, the ocean. Solar energy stored in the warm tropical ocean is converted to electrical energy using a modified refrigeration technology and cold, deep ocean water. All heat engines collectively share the principle that energy will flow from an area of higher temperature to an area of lower temperature. All modern power plants use temperature differences to produce electricity. Geothermal, coal, nuclear and diesel power plants all employ a high temperature gaseous medium to spin a turbine, which turns a dynamo, which generates electricity. These power plants use a medium with a temperature difference of several hundred degrees. On the other hand, an OTEC energy generation system uses a temperature differential of far less- approximately 20oC to 24oC Ocean waters in the equatorial region of the earth have the necessary temperature gradient. Surface water is kept warm all year via the radiation of an ever-present tropical sun. Below the surface, the water remains at Arctic temperatures of approximately 4oC. The degree of difference in temperature is adequate to power a thermal engine of a relatively low efficiency. Most modern power plants have a resource to electricity conversion efficiency approaching 30 to 40 percent. An OTEC plant operates at a much lower efficiency, but it works with such a large volume of water, 24 hours a day, 365 days a year, that it produces substantially large amounts of energy, negating the apparent disadvantages of operating at a perceived lower efficiency. In reality, the net positive effect is substantial. The fact that the Ocean Energy Resource is extremely large and free makes the importance of technical efficiency less dominant.
One of d'Arsonval's students, Goerges Claude, took this idea to application, first in 1928, in what is now known as a bottom cycle application. He used warm cooling water from a steel plant in Belgium, and cool Meuse river water from near the plant as the operating medium for an Open-Cycle OTEC plant that produced electricity. It was the first of it's kind to operate at the scale. Modern designs for OTEC plants provide continuous, base-load electric power because ocean water temperatures are stable, unlike other renewable energy sources such as wind and solar which are inherently intermittent in nature. Using the sun for its heat source, the OTEC process is free of any kind of pollution or emissions.
Claude favored the use of open-cycle OTEC and he developed a system of pre-dearation (taking out dissolved gases from a liquid prior to evaporation.), to remove the non-condensable gases and thus, to improve the efficiency of the whole process. d'Arsonval had suggested the use of a closed cycle system in line with theoretical descriptions of such a cycle by Carnot and later, more realistically, by Rankine. Recently, Exergy, Inc. has developed a new cycle, the Kalina Cycle®, which considerably improves upon the efficiency of the OTEC concept.
The OTEC idea lay largely dormant during the period of artificially inexpensive and readily available oil and was resurrected after the OPEC oil crisis in the 1970's. Hawaii became the world center for exploring the potential for development and application of the OTEC concept. In the last 25 years, numerous OTEC relevant research efforts have been conducted in Hawaii. Included in these efforts were MiniOTEC and OTEC-1 that demonstrated power generation, and the development by PICHTR of an operational freshwater producing OPEN-CYCLE OTEC prototype. Other technological advancements included biofouling and corrosion solutions in heat exchanger design; problems that were inherent in previous OTEC designs.
What has occurred at the research facilities in Hawaii is the realization that not only can electric power be produced using the ocean’s natural thermal gradients, but also abundant, clean, and potable drinking water. In addition, cold, deep seawater is nutrient rich and pathogen free, ideal for aquaculture and mariculture applications such as fish and shellfish farming. The cold seawater causes natural condensation in the soil enhancing growth rates for agriculture. Further, an OTEC power system pumps a large amount of very cold seawater to the surface and therefore can provide air-conditioning which is extremely cost effective. By providing air-conditioning as a separate sub-system within an OTEC facility, electrical load requirements and maintenance costs are drastically reduced. Additionally, through the process of electrolysis, fresh water from the OTEC process can be the basis for a hydrogen production system, of which commercial scale quantities could be constantly produced. This realization has thus led to the development of an OTEC systems approach that can be implemented to produce various optimized amounts of all of these products for a particular installation. The systems approach results in extremely favorable economic conditions for a profitable governmental or commercial entity, energy corporation, division, or consortium.
An Integrated OTEC System can create harmonious, self-sustaining island communities independent of imported fossil fuels and their associated costs. Pollution and emission free energy with reduced, predictable and stable operational costs largely independent of foreign imposed pricing and politics. Island communities can now create a complete infrastructure providing food, water, and electricity which can reliably support industry, tourism, and trade - effectively bolstering their developing economies.
The specific market needs for this technology are quite extensive. Commercial OTEC facilities must be located in an environment that is stable enough for efficient system operation. The natural ocean thermal gradient necessary for OTEC operation is generally found between latitudes 20oN and 20oS. Within this tropical zone, are portions of several industrial nations (i.e. United States, Taiwan, Japan, etc.), as well as 29 territories and 66 developing nations and nearly a dozen U.S. Department of Defense operated military installations (i.e. Diego Garcia, B.I.O.T.; Guantanamo Bay, Cuba; AUTEC, Bahamas; Kwajalein, etc.). Of all these possible sites, tropical islands with growing power requirements and a dependence on expensive imported oil are the most likely areas for initial OTEC development. Most of these military installations, developing countries and territories have significant needs for reliable, sanitary potable water and/or food sources, afforded through cold water agriculture and/or aquaculture application, adding to the social and political desirability of an integrated OTEC system.
Recent Advancements
The last evaluation of OTEC was based on the technology demonstrated by the mid-1980’s. Technical advances since that time include:
These technical advances bring OTEC systems to the point of development where projects can be confidently designed, constructed, and operated. Each component exists and has been shown to work reliably. No significant technological questions remain in OTEC systems. This is in contrast to other renewable energy systems, such as nuclear fusion, which also have the potential to replace fossil fuel on a large scale. We at OCEES International, Inc. applaud the efforts of all involved in researching these various aspects of OTEC technology. This combined effort, building upon the work of d'Arosonval, Claude, and Lockheed, in conjunction with the research conducted in Hawaii, will lead to the commercialization of OTEC.
Many of these technological advances over the past decade are directly attributable to the principals of OCEES International, Inc. and/or its consortium of corporate partners intent on commercializing this extremely important technology. OCEES and its partners represent the world’s foremost experts in this field with substantial experience in design and practical application of OTEC systems and ancillary technologies. OCEES has positioned itself through research, partnerships, governmental relationships and experience to take a leading role in the imminent global commercialization of the OTEC systems technology.
The innovation to the power cycle which OCEES International, Inc. employs is provided by OCEES strategic corporate partner Exergy, Inc. in the form of their patented Kalina Cycle®. The Kalina Cycle® is a variation of the more conventional closed-cycle OTEC system incorporating aqueous ammonia (ammonia/water mixture) as the working fluid instead of the conventional ammonia or propylene working fluid employed in earlier designs of closed-cycle OTEC power systems. The Kalina Cycle® is a break-through technology for OTEC power systems providing a nearly 80% increase in efficiency over previous closed-cycle designs. Because the ammonia/water concentrations can be varied throughout the system to optimize according to system temperatures (sort of a "designer" working fluid) and by adding an extra component – the recuperator, heat losses generally experienced in other closed-cycle designs can be minimized and recovered, thereby improving the overall efficiency of the power cycle.
The Kalina Cycle® is a modified Rankine cycle with increased efficiency resulting from the altered properties of its ammonia/water working fluid, rather than the pure water or ammonia working fluid in a standard Rankine cycle. The Kalina Cycle® specifically exploits the variable boiling and condensing temperatures of a variable concentration working fluid since an ammonia/water mixture can more closely follow the straight-line temperature change of the heat source or condensing medium in a counterflow heat exchanger.
Compared to a conventional Rankine cycle, the temperature rise of the cooling water for the ammonia/water can be higher than that for condensing the more traditional anhydrous ammonia, thereby minimizing the cold deep seawater requirements saving capital costs and increasing the net power output of the plant over previous OTEC system designs! By utilizing the more efficient Kalina Cycle® in the integrated OTEC system design, OCEES International, Inc. is able to reduce costs of power generation and provide emission free energy to the tropical island communities it services.
The OTEC Kalina Cycle® uses the four typical Rankine cycle phases: evaporation, expansion, condensation and feed. The evaporator and condenser components in an integrated OTEC Kalina Cycle® system consist of numerous large plate heat exchangers with titanium plates for maximum corrosion resistance against seawater and maximum life-time efficiency. An additional piece of equipment, the recuperator, recovers heat from the warm but unvaporized liquid leaving the separator vessel. A brief simplistic process description follows.
Generally, for OTEC applications and system temperatures, a mixture of approximately 60% ammonia – 40% water (by weight) enters the counterflow evaporator where it is heated by the warm surface seawater. The warmed vapor/liquid mixture travels to the separator where high quality ammonia vapor goes to the turbine. Warm "lean" liquid from the separator drains through the recuperator and heats an incoming quantity of 60/40 mixture working fluid.
The high quality ammonia vapor from the separator enters a radial flow ammonia turbine and expands creating mechanical energy which is then converted into electrical energy via the attached generator system. Design of such ammonia turbine/generator systems is well established with system efficiencies approaching 85% or greater for OTEC applications.
The turbine exhaust vapor is recombined with the cooled post-recuperator lean mixture. Both condensation and ammonia absorption then occur inside the counterflow condenser cooled by the cold, deep seawater supplied by the Cold Water Pipeline. The 60/40 liquid mixture then flows to the condenser hotwell where feed pumps take suction from the hotwell and pump the ammonia/water mixture back into the evaporator, completing the cycle.
OCEES International, Inc. utilizes integrated OTEC systems power cycle designs which are scalable and modular for implementation into nearly every tropical island application with suitable access to cold, deep seawater. Sufficient redundancy of components is incorporated into every design to ensure periodic maintenance and repair procedures can be performed without compromising the supply of base-line power to the local community or power grid. Likewise, the modular nature of the system ensures proper "down stream" protection of discharge water streams (either to aquaculture applications or system discharge streams to the ambient ocean) from ammonia contamination, should an unlikely leak in the system occur, without compromising the plant’s performance and availability. Instrumentation and sensors to protect against any leakage is standard in OCEES International, Inc. OTEC system designs.
Ocean Thermal Energy Conversion (OTEC) requires huge quantities of deep cold seawater and warm surface water to operate. The fabrication and installation of deep-water pipelines to provide this water represents the single most expensive portion of any OTEC plant and the highest risk during construction. Because of these associated costs and risks, it is the least demonstrated major component of a large OTEC plant.
Hawaii has been the center for OTEC development in the United States over the past twenty years and OCEES and its strategic partners have been an integral part of that research and have gained very valuable experience in cold water pipe technology. The State of Hawaii is currently operating several pipelines at the Natural Energy Laboratory of Hawaii Authority (NELHA) , the largest of which is 55" OD. Pipelines as large as 8’ OD have been tested at the facility in both down-the-slope mode and suspended mode. In fact, all of the world’s large deep seawater pipelines have been deployed at NELHA. All phases of design, manufacture and deployment of each of these pipelines have been overseen by OCEES International, Inc.'s strategic partner Makai Ocean Engineering. The techniques learned and experience accumulated through the deployment of the deep water pipelines at NELHA and the research accomplished on large diameter pipelines performed in Hawaii have led to the confident conceptual design and deployment scenarios for OTEC pipelines up to three (3) meters in diameter. These segmented pipes of fiber-reinforced plastic can be deployed with the same controlled submergence techniques that have already been implemented in Hawaii and can be installed in either the gravity anchor mode, pendant mode or the long, inverted catenary mode.
Pipeline configuration design and development requires analysis of the pipeline under two very differing environments: the near shore region and the deep-water pipeline.
Near Shore Region
We define the near shore region as the shore landing through to a depth of approximately 60 — 80 feet. In this region a designer is faced with a variety of challenges including severe current and wave loads, aesthetic and environmental considerations, pump station interface and offshore pipeline interface. Because the pipeline is in shallow water, wave action on the pipeline can be quite extreme, especially in areas susceptible to hurricane activity. This is where experience in ocean related design cannot be understated - OCEES International, Inc. and it’s strategic partner’s all possess the necessary ocean training and experience to confidently design, deploy and construct a working plan in nearly any ocean environment.
Large hurricane waves and associated large design wave heights, as are common to tropical island communities, place very high lateral and lift loads on exposed pipelines and effectively eliminate consideration of a near shore traverse using gravity anchors alone. Several near shore crossing techniques are available for consideration, including:
Buried or Trenched Pipelines are well protected from the environmental loads and, for multiple pipes, are the most cost effective approach for the near shore route. However, digging and blasting the trench can be environmentally damaging to reefs and the near shore region; therefore should only be pursued if other, more environmentally attractive means of traversing the near shore region prove unrealistic.
Bolt Down Pipelines are sometimes utilized in applications where the near shore region is exposed to extreme wave conditions thereby exposing a hard seafloor or large boulders partially submerged in the sand of sufficient size to stabilize the attached pipeline under extreme wave conditions (hurricanes, etc.). Under these conditions, bolt down pipelines become extremely cost effective. Rock bolts are relatively inexpensive and divers can accomplish maintenance through periodic replacement of the sacrificial zinc anodes for corrosion protection. The disadvantage of this technique is that the seafloor is highly irregular and it is difficult to design in advance the exact location of each rock bolt and clamp on the pipeline.
Tunneling consists of two differing techniques for accomplishing the same effect — slant drilling and micro tunneling. Slant drilling uses oil drilling techniques with a drill oil rig onshore pressing a drill bit and drill pipe through the soil at a fairly shallow angle to the approximately 60 foot depth. In the micro tunneling approach, a large dry jacking pit is constructed onshore reaching well below sea level. A micro tunneling machine with a drill bit equal in size to the outer diameter of the desired tunnel is pushed through the vertical wall of this onshore pit and continues to drill by adding drill pipe sections until reaching the offshore regions of 60 — 80 foot depths. Under each of these tunneling configurations, the shoreline and seafloor regions are relatively undisturbed and prove the best protection for the near shore pipeline as well as the most environmentally favorable means of traversing the fragile shallow water region to a shore mounted OTEC facility.
Deep Water Pipeline
The deep-water portion of any pipeline sees smaller environmental loads than the near shore region. Waves have less impact upon this portion of the pipe, there are less visible environmental concerns, and currents diminish with depth. In general, the design approach and installation procedures are completely different than the near shore region.
Three different techniques for mounting deep-water pipelines have been developed in Hawaii, they are:
In many deep pipeline applications, it is necessary to incorporate a combination of the above approaches to accommodate different deep-water terrains.
Gravity Anchored Pipelines are most appropriate in regions where the seafloor is relatively smooth over a gentle slope. Gravity anchoring consists of attaching a series of cement bottom anchors at regular intervals before deployment over the length of the pipeline with sufficient clearance to prevent damage to the pipe from the seafloor. A very detailed survey route and precise deployment of the pipeline is imperative in this technique to prevent damage to the pipeline during deployment — a pipelines most vulnerable moment. Because of its simplicity in design and minimal cost, it is the preferred method of anchoring the deep-water pipelines associated with an OTEC facility. However, since ideal conditions rarely exist in reality, as mentioned previously, this method is often combined with the other techniques over the course of the pipeline.
Pendant Pipelines are utilized under buoyant pipe conditions to traverse rugged bottom environments (sharp rocks, etc.) by allowing the pipeline to float above the bottom at considerable spacing to avoid damaging the pipe during deployment and periods of very large waves which may cause some lateral motion to the pipeline. A pendant system consists of a buoyant pipe attached to anchors via a cable system allowing the pipe motion flexibility without permitting contact with the seafloor. One drawback of this approach is the cost of buoyancy, which, if utilized over large portions of the pipe length, can become cost prohibitive.
Catenary Pipelines take advantage of the deep water pipe’s natural buoyancy and flexibility to traverse very rugged and often very steep bottom terrain. When this technique is to be employed, detailed route surveys of the bottom are not necessary (only at the two ends of the catenary). Like the pendant pipeline, the catenary does not experience high inertial loads and, being mostly detached from the seafloor is not susceptible to earthquake damage.
The catenary approach cannot be implemented everywhere, it is not suitable for shallow water applications or gentle slope considerations where the peak of the catenary could approach the ocean’s surface.
Pipeline Deployment
The design of the installation process of the pipeline is the single most important aspect of the overall pipeline design. The most formidable obstacles to the installation of a deep-water pipeline are the extreme depths, currents and waves that the installer will encounter during installation. A pipeline will often experience its most extreme lifetime loads during deployment and certainly the highest risk of loss. The proper design process of a deep-water pipeline must constantly involve the deployment procedure.
In most cases in engineering the role of designer and installer are clearly separated. However, this is not the case in deep water pipelines — it is unlikely the contractor hired to deploy the pipe has installed such an instrument, therefore, it is up to the designer to make sure the pipeline can be constructed and deployed both reliably and economically. It is inherently necessary for the pipe designer to provide detailed deployment plans and specifications together with the final design in order to make sure the contractor fully understands the intended installation procedure. Because of the extreme expense associated with marine construction operations, it is also necessary that the pipelines be designed for fast and efficient deployment as well.
Most of the work on pipeline construction is performed on land or in a nearby harbor. All weights, pendants, and other attachments are attached to the pipeline with shore crews. Often the pipeline is assembled right at the shoreline in a harbor and as each section of pipe material is added to the pipeline, the other attachments are added and the pipeline is "pushed" into the harbor. The pipeline is designed to float when air filled; it supports the weights and other pipeline attachments. Therefore the whole pipeline is assembled and pushed into protected waters while using a minimal amount of marine equipment.
At the time of deployment, the pipeline is towed to the site and aligned over the design pipeline path. The shore end of the floating pipeline is anchored in about 60 feet of water and attached to a set of flooding pumps at the shore end. While the pipeline is under tension (being pulled by barges at pre-designed speeds and directions), water is being pumped into the pipeline at the shore end. The non-flooded buoyant section of the pipeline supports the heavy flooded end at any given point and the pipeline takes on an S-shape. By carefully monitoring the pull of the pipeline, the internal pressure, the distribution of weight on the pipeline, and the temperature and time of deployment, the pipeline can be safely placed on the bottom to depths up to and exceeding 3000 feet. The alignment and flooding of a mile-long pipeline can be accomplished in one day.
As one can imagine, careful deployment design and detailed planning is critical to the success and installation of a deep-water pipeline. The deploying contractor needs to pre-analyze all conceivable problems and be prepared for any contingency. Therefore, experience in the design and deployment of such a project requires the best, most experienced companies available — OCEES International, Inc.'s strategic partner Makai Ocean Engineering represents that experience and possesses the expertise necessary to confidently design and deploy the large diameter cold water pipelines necessary for the successful implementation of an integrated OTEC system.