Published in Ocean Resources 2000
Sea Technology, September 1992
Deep, cold seawater has long been recognized
as a valuable ocean energy resource. Over the past 20 years
research and experimentation has been conducted on ocean thermal
energy conversion (OTEC) and cold water mariculture. Out of this
particular research effort, one clear economic winner emerged:
seawater air conditioning.
It is technically and economically feasible today; once installed, the energy is inexhaustible and there are no adverse environmental impacts. Since air conditioning (A/C) systems in large buildings circulate water at temperatures equivalent to those commonly found in the deep ocean, the concept of using this cold seawater for that purpose is obvious. Development, however, previously had been hindered by technical and economic uncertainties associated with the required deep water pipelines and the unknowns relative to heat exchanger fouling and corrosion.
During the last decade, research in the support of OTEC has eliminated these uncertainties.
The Natural Energy Laboratory of Hawaii has installed three different deep water pipelines and conducted years of testing on heat exchangers.
OTEC PIPES IDEAL SIZE
While modest in size for OTEC purposes, these pipelines are ideally sized for large seawater A/C systems. Long-term testing of heat exchangers has shown that fouling is not a problem with deep seawater, and corrosion can be eliminated with either titanium or aluminum.
For large buildings, resorts, hotels, and military installations in tropical and subtropical climates, air conditioning represents the major energy demand. A single hotel room, for example, requires from 0.75 to 1.0 tons of A/C. These large buildings are normally air conditioned with fresh chilled water that circulates throughout the building. Typically, the temperature of the circulation water is between 7 degrees and 14 degrees C. Large refrigeration units chillers cool this water. The electrical demand is typically 0.9 kilowatts per ton of air conditioning. A hotel complex with 1,000 rooms could have an air conditioning electrical demand of 1 megawatt or more.
Generally, water at 6 degrees C can be found between 600- to 700-meter depths and water as cold as 4 degrees C can be obtained at 800 meters. Resorts, hotels, towns, and cities pay enormous energy and monetary cost to chill A/C circulating water. For those on the coastline adjacent to deep, cold seawater, an unlimited supply of cold water is often a few kilometers offshore.
SEAWATER A/C METHODS
In one scenario we generated, we show a centralized air conditioning system that could supply multiple buildings. Typical temperatures in the circulation system are on the order of 6.4 degrees C on the seawater side and 7.2 degrees C on the fresh water loop side--after cooling by the heat exchanger. The overall system is quite simple and includes:
Because of the economy of scale, the seawater A/C system is most appropriate for supplying multiple buildings or hotels in a coastal area. For servicing multiple customers, a water distribution system needs to be installed providing cold water to all buildings. Such a system can be easily installed with minimal increases in cold water temperatures.
With foam insulation added to the plastic pipelines, cold water can be distributed over many kilometers with negligible increases in temperature. The air conditioning components inside the buildings are conventional and would not change for a seawater system. These components are not exposed to seawater. As a result, existing buildings can easily be converted to seawater A/C by simply bypassing existing chiller units.
If desired, the unused chiller units can be used for backup or auxiliary cooling. For new installations. the chiller would never be installed.
Power savings realized by seawater air conditioning can be significant. A conventional air conditioning system uses 800 to 900 kilowatts/1000 tons for the refrigeration unit. With the seawater system, this refrigeration electrical demand is replaced by seawater and chilled-water pumping power. Depending upon the length and size of the pipeline and the size of the freshwater distribution system, the pumping costs are about 75-150 kilowatts/1000 tons. This corresponds to a 80 percent savings in chiller electrical power.
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Schematic at left shows fresh water and seawater loops for a large centralized air conditioning system using cold seawater. Centralized seawater A/C system above is coupled with a conventional heat pump to operate at higher seawater intake temperatures. |
DETENTE WITH CHILLERS
In some locations the seawater available may not be sufficiently cool to completely lower the freshwater to its optimal temperature (these temperatures being driven by humidity control). In these cases, a chiller unit can be installed with the heat exchanger cooling the freshwater first, followed by the chiller unit.
The electrical savings would be proportional to the amount of heat removed from the freshwater by the heat exchanger. The technology required to install and operate a seawater air conditioning system is available today. The primary components of the seawater system are the cold- water pipeline, the seawater pump, the heat exchanger, and the effluent pipeline. All the components required have been developed and, in some cases, operated for many years.
Several deep water pipelines have been installed by the state of Hawaii and are operating at Keahole Point on the Island of Hawaii, bringing in deep, cold seawater for OTEC and aquaculture research and development. These pipelines range in diameter from 300 millimeters to a meter and have intake depths ranging from 650 to 700 meters. The 1-meter-diameter pipeline, if used exclusively for air conditioning purposes, could supply almost 5,000 tons of air conditioning and replace more than 4 megawatts of normally generated electrical power. This pipeline has been in continuous operation for nearly five years. A smaller, 300-millimeter pipeline of the same design has been servicing Keahole for more than 10 years.
All the pipelines at Keahole are made of high-molecular-weight polyethylene. This rugged,flexible material is ideal for cold water pipelines in that it is completely inert in seawater, and its flexibility allows for fast and easy installation.
Designs with expected lifetimes of 20 to 30 years can be installed. With each installation, significant improvements are evident in pipeline configuration, costs, and capabilities. Advancement in both cold water pipe design and polyethylene material is pushing feasible polyethylene pipe diameters up to 1.6 meters O.D. A pipeline of this size could provide 18.000 tons of air conditioning.
SUCCESSFUL INSTALLATION KEY
The key to the successful installation of the pipelines at Keahole was development of a controlled submergence procedure for the polyethylene pipeline by Makai Ocean Engineering. Capitalizing on polyethylene's flexibility, these pipelines can be safely deployed to depths reaching 1,000 meters and the techniques can be used for the maximum polyethylene pipe available--1.6 meters. The successful pipelines at Keahole incorporated different pipe configurations and designs applicable over a variety of sea conditions. Pipelines have been buried, bolted to the bottom, gravity weighted, pendant-supported, and floated over the bottom in long, continuous buoyant spans. Combinations of these techniques can be used to reliably deploy polyethylene pipelines over a wide variety of bottom and environmental conditions.
Some of the major research supported by the existing cold water pipelines at Keahole over the last decade has been directed at biofouling and corrosion-testing of heat exchangers. These were major concerns with OTEC in the early years but research has since shown that biofouling in the heat exchangers and the pipeline on the cold water side is non-existent. Furthermore, innovative and low cost aluminum heat exchangers that are corrosion-free have been privately developed and successfully tested by Alcan Aluminum at this facility. Corrosion-free titanium heat exchangers are also ideal for this application.
Unlike OTEC, the overall size and cost of the heat exchangers in a seawater air conditioning system are small.
A PRACTICAL DEMONSTRATION
The two main buildings at the Natural Energy Laboratory in Hawaii are air conditioned with deep cold seawater. These systems have proved to be both simple and economical.
A similar air conditioning system exists at Purdy's Wharf in Halifax. Nova Scotia. Since cold seawater exists at this location at a depth of only 75 feet, this is not an application of deep sea pipeline technology. The direct transfer titanium heat exchangers, however, have been used to successfully and economically air condition two large office buildings on the Halifax waterfront for many years.
Development of low-cost seawater pipelines and heat exchangers has been a significant R&D byproduct of the OTEC effort. Over the last decade. considerable effort has gone into research on ocean thermal energy conversion, a concept by which the temperature differential between the deep, cold seawater and the warm surface water is used in a low efficiency, Carnot cycle to produce electricity.
Air conditioning with deep, cold seawater has a significant advantage over an OTEC power plant. A seawater A/C system of comparable megawatt size can be built at a small fraction of the size and capital cost of an OTEC facility.
For example, a 5 megawatt OTEC plant would require approximately a 5-meter-diameter pipeline for each cold and warm water intake, plus heat exchangers and a power plant Conversely, a seawater A/C system that replaces 5 megawatts of electrical power is considerably smaller and much less complex. It requires only a 1-meter pipeline and needs no warm water pipe or the complexity of the power system.
LOOKING AT ECONOMIC VIABILITY
Seawater A/C is suitable for coastal developments with large air conditioning demand and reasonable access to deep, cold seawater. The main factors that influence the economic viability of a seawater air conditioning system are:
Other factors are certainly influential in the design, economics, and ultimate success of a seawater A/C system. These would include local seafloor bathymetry, wave and storm data. local climate. existing vs. new buildings, environmental requirements, and secondary uses of the seawater.
As with most alternative energy systems, the heaviest expenses for seawater air conditioning system would occur in the initial capitalization. Total capital costs include the cold water intake pipe the pumping station, the onshore heat exchangers, the onshore distribution system, and the effluent pipeline. The largest cost is in the seawater supply system (intake pipe, pumps, effluent pipe). This segment typically represents 45 to 75 percent of the total capital costs. On average, approximately half the capital costs is in the seawater supply system, 15 percent is in the heat exchanger, and the last 35 percent, in the distribution system.
Larger seawater A/C systems are more economical than small systems. The cold water pipe costs per liter of water delivered drop as the pipeline size increases. Also, the temperature rise in a small pipeline can be considerably larger than for the larger pipelines. A 1,000-ton system could have a 0.5 degrees C temperature rise in the seawater before it reaches shore, but a larger pipeline supplying 5,000 tons of A/C experiences almost negligible temperature increases. In general, a system smaller than 1.000 tons is not economical.
The baseline for the pipeline costs provided here is Keahole Point, Hawaii, since all the operational and installation experience has been at this location. With installed pipelines ranging in size from 300 millimeters to 1 meter, the Keahole projects have provided an excellent base for cold water pipeline cost estimates. Other locations may have different costs depending upon the difficulty of the pipeline installation.
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Total capital costs can be calculated here (left) for an A/C system based on a 2.2-kilometer pipe. Baseline seawater A/C system payback periods are shown (right) for various electrical rates charged by utilities vs. air conditioning demands. |
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COMPUTING PAYBACK PERIODS
The economic value of the seawater A/C system is illustrated by computing the simple payback period in years. The payback period here is defined as the total capital cost divided by the annual savings.
Savings afforded by the seawater air conditioning system are defined as the value of chiller electrical demand in a conventional air conditioning system minus the value of electrical demand for the seawater pumping in a seawater system. The savings are typically 80 percent or better, not including the freshwater circulation and fan costs inside the building that are fixed for both conventional and seawater systems. Maintenance between the two systems is assumed to be comparable.
The baseline payback period can also be defined as a function of air conditioning demand and local electrical rates. Note that as local electrical rates increase, the payback period is appropriately reduced. To estimate the actual payback period, the baseline payback period should be corrected for percent utilization and for pipeline length as follows:
As an example of these calculations, we can evaluate a 1,500-ton air conditioning system used 70 percent of the time, built in an area with $0.15/kilowatt-hour electrical rates and with the required cold water available 4 kilometers offshore: Such a system would have a baseline payback period about 2.3 years, but this is further divided by 0.70 for the utilization factor and multiplied by 1.5 for pipeline length correction. The real payback period would be approximately 5.0 years.
The payback periods illustrated are quite small. Even when coupled with the correction factors for utilization and pipeline length, some coastline communities could clearly benefit from seawater air conditioning. Areas in the tropics and subtropics with large air conditioning demands are key candidates such as islands in the Caribbean, Indian Ocean, and the Pacific--including Guam and Hawaii. Other areas of interest include specific areas along the coastlines of Central and South America, India, and Africa.
The payback values only provide a general guideline relative to the economic merits of seawater air conditioning. The construction and application benefits and challenges of the cold seawater system are very site-specific.
SITE SPECIFIC ANALYSES
Each site that has reasonable demand and access to cold seawater should be analyzed separately, As an example, Makai has conducted a more detailed analysis for a system on Curacao in the Netherlands Antilles. We studied the feasibility of seawater air conditioning for three sites on the island. with air conditioning loads ranging from 540 to 2,100 tons (corresponds to cooling of 540 to 2,100 hotel rooms). The length of the seawater intake pipelines ringed between 5,150 feet to 11,750 feet depending on the site and the seawater intake temperature.
The capital cost--including pipeline, heat exchangers, and chilled water distribution system was on the order of $2 to $5 million. The payback period for these systems ranged between 5 and 6 years for feasible sites and 9 to 17 years for the site with the smallest air conditioning loading.
This payback is based on the cost of supplying existing buildings. If new construction only is considered (without chillers), the payback is considerably better.
Makai Ocean Engineering has also completed some preliminary analyses for Guam in the Tumon Bay area where there is a high density of hotel rooms. This preliminary analysis indicates that 10,000 hotel rooms could be air conditioned with cold seawater and that the capital payback period for installing such a system would be approximately 5 to 6 years.
Utilizing the seawater air conditioning system can be very attractive for large users needing a base load system and with adequate access to deep, cold seawater. The economic payback period can be quite small. The system can be attached to existing buildings or used for new developments. For new structures. where credit can be taken for not installing conventional chillers, the payback is significantly better.
Energy savings with the seawater system can be as large as 90 percent, so there is less dependence on conventional fuels and energy-cost increases.
The technology exists today to install the deep water pipelines and the heat exchangers are off-the-shelf. Several demonstration deep water air conditioning systems are in operation today in Hawaii.
Dr. Joseph Van Ryzin is a co-founder of Makai Ocean Engineering and has been its senior engineer since 1973. He has directed the successful design and deployment of existing deep water pipelines, the precise placement of deep water cables, and alternative energy-source developments. Van Ryzin earned his Ph.D. in Ocean Engineering from the university of Rhode Island.
Tore Leraand is responsible for deep water pipeline design and seawater air conditioning developments at Makai. His work has involved the development of 10-foot diameter, cold water pipelines for OTEC, the design of a 3,000-foot-deep pipeline, and analysis on air conditioning systems. Leraand has a master's degree in ocean engineering from the University of Hawaii and a degree in civil engineering from Telemark College of Engineering in Norway.