ONT — October 2011
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Challenge And Promise Of OTEC
Duke Hartman, Makai Ocean Engineering, Inc.

Challenges and Promise of Ocean Thermal Energy Conversion

Overview

Ocean Thermal Energy Conversion (OTEC) is a process that generates electricity using temperature differences that naturally occur in the ocean. The surface of the tropical ocean captures and stores solar thermal energy in its upper layers 24/7, yearround, that can be converted to electricity using a power cycle. An offshore floating OTEC plant pumps deep cold seawater and warm surface seawater through heat exchangers to boil a refrigerant that drives a turbine to generate electricity. Rising electricity costs, increased concerns for global warming, and a political commitment to energy security have made OTEC attractive in tropical regions where a high percentage of electricity production is oil based. OTEC power plants operating a few miles from the coast could furnish baseload electrical power at a predictable cost with no significant air pollution. OTEC is an environmentally sustainable option with large, stable power producing potential. OTEC has two unique advantages over most renewables:

1. OTEC is firm or baseload power, meaning power is delivered 24/7.

2. OTEC is offshore, and does not compete for land, food, or freshwater.

Challenges

Virtually all the components required to build an OTEC plant have been in use in different industrial applications for decades or more. The challenge to OTEC is not whether building an OTEC plant is possible, but whether it can be built economically. An intensive engineering effort has been underway since 2007 to address this question by reducing costs and improving plant efficiencies as much as possible to make a utility sized system economically viable.

To retire final technical risk and demonstrate overall performance, a significant size pilot plant in the 2.5MW to 10MW range will be necessary before commercial sized plants can be built. Because OTEC exhibits economies of scale, the pilot plant would not pay for itself in electricity revenue. Long-term investment with an eye toward a massive industry is needed.

Current Work

Risk and cost reductions are a priority on every level of an OTEC design. Since 2007, engineers at Makai Ocean Engineering have been involved in risk and cost reduction work in the following areas of OTEC: discharge-plume modeling, cold water pipe fabrication and handling, heat exchanger corrosion and performance design and testing, and design of a lowcost offshore pilot plant. These studies have been supported by the US Navy (ONR and NavFac), DOE, CEROS (DARPA), and Lockheed Martin.

Hydrodynamic and biological discharge-plume modeling

Because a commercial OTEC plant has not yet operated, it is prudent and responsible for developers to study its effects on the environment to ensure that they will be benign. Meeting regulatory requirements will be a critical part in the development of a commercial OTEC industry.

A numerical hydrodynamic model was recently completed to study these effects by simulating the large discharges of OTEC plants in the ocean environment. The goal of the model was to answer the following questions: Can one or more OTEC plants be operated without affecting the local environment by disturbing local temperatures and nutrients? Can an OTEC plant be operated without cooling its own thermal resource or a neighboring plant's resource? The deep water has nutrient levels nearly 40 times the surface water levels. What configuration of discharge depth, velocity, or pre-discharge mixing will protect the thermal resource and adequately dilute the nutrient and temperature concentrations? OTEC is a capital intensive process, and cost reduction is a primary goal of OTEC plant design. Every configuration change has an associated cost. How can the plant be economically configured while achieving sustainable operations? What are the effects of realistic variable currents, eddies, and seasonal changes?

The 3D hydrodynamic model is based on the EPA-approved Environmental Fluid Dynamics Code and receives realistic oceanographic currents and density data supplied by the Hawaii Regional Ocean Modeling System. Dynamically coupled finite Element, jet-plume models simulate the entrainment and turbulent mixing of large OTEC plumes. To give an idea of the scale of these plumes, a 100MW OTEC plant is projected to have flow rates on the order of 740m3/s (12 million gpm) total flow, with 320m3/s (5.1 million gpm) cold water and 420m3/s (6.7 million gpm) warm water.

Nitrate is an important nutrient in marine biological growth, thus monitoring the impact of an OTEC plant on the ocean's nitrate distribution is important. Results from this study suggest that, with proper design, the nitrate concentration perturbations will remain within the natural variations of the ocean. The model is also being adapted to simulate any increased biochemical productivity that may occur because of the nutrientrich discharges.

Cold water pipe fabrication and handling

The long cold water pipe is considered a relatively highrisk component. Because of the large flows involved in a 100 MW OTEC plant, a large 10m diameter pipe is required for the cold water intake from a depth of 1,000m (3,300-ft.). This pipe would be the first of its kind in the world, and must be able to accommodate the dynamic motions of the platform as well as currents.

Advances in high-strength composites and fabrication techniques have greatly improved the strength of cold water pipe designs. The primary innovative design being developed by Lockheed Martin is a fiberglass reinforced pipeline that would be fabricated onboard the offshore floating platform at sea. Makai has developed for Lockheed Martin under a contract with NavFac a pipe handling apparatus that lowers the pipe safely into the ocean from the platform as new sections are fabricated on deck. A 1/20th scale model of the pipe handling apparatus was built and tested, validating the concept.

Heat exchanger corrosion, performance studies

Heat exchanger cost reduction is critical to successful commercialization of OTEC power plants because they are the single most expensive OTEC component and the primary cost driver. Heat exchangers within a 100MW OTEC power plant would be comparable to the size of a 6.1m tall (20-ft), 970 m2 (10,000 square foot) building. Heat exchanger cost reduction is three-fold: reducing capital cost, extending useful life, and improving performance.

Small changes in heat exchanger performance and failure from corrosion or fouling would have significant economic consequences. For OTEC and other low √ĄT applications, the heat exchangers should maximize heat transfer per unit area while minimizing pressure losses, corrosion, biofouling, and the cost of materials. Heat exchanger units being developed and prototyped are geared toward application in ocean renewable energy, but have applications in once-through cooling for steam condensers and other maritime applications.

In terms of materials, titanium is an industry standard for heat exchangers in marine applications, but the large OTEC demand presents problems with cost and availability. Aluminum alloy heat exchangers are a possible lower-cost solution, but will require mitigation of corrosion mechanisms for long-term seawater service. Various joining methods of these alloys, as well as various surface conditions, are being studied.

Primary OTEC heat exchanger testing is conducted at a land-based OTEC demonstration plant and heat exchanger test facility built by Makai earlier this year at the Natural Energy Laboratory of Hawaii Authority (NELHA), a state-owned business technology park located on the Big Island of Hawaii. NELHA is the only industrial park in the world that is specifically designed to provide the infrastructure to immediately and economically support such a test program, with ready access to deep and surface seawater as well as the necessary drainage facilities and utilities. The deep seawater is obtained via a 620m deep, 1.02m diameter intake pipeline, or a 914m deep, 1.40m diameter intake pipeline. NELHA can furnish a total of 2.51m3/s (40,000 gpm) cold seawater, with corresponding warm water flows.

The facility houses a nearly-complete OTEC system for demonstration and component testing purposes. This facility will allow engineers to demonstrate the actual thermodynamic performance, life expectancy, and cost of a variety of these huge heat exchangers in a cost-effective manner. The next planned step is to install a turbine at the facility to complete the OTEC cycle and produce power that would be connected to the local electrical grid.

"Makai" is a Hawaiian word meaning "toward the sea." Makai Ocean Engineering, Inc., established in1973, provides ocean engineering services worldwide, specializing in submarine cable software, visualization software, OTEC, and marine pipelines. For more information, visit www.makai.com.
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