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Cold Seawater Air Conditioning
SeaWater Air Conditioning (SWAC)
takes
advantage of available deep cold seawater instead of energy-intensive refrigeration
systems to cool the chilled water in one or
more buildings .
This web page describes the Basics,
Environmental
Aspects, and Economic Benefits of a SWAC
system. It also describes Makai's
Engineering Services for Seawater Air Conditioning.
Makai's experience in SWAC analysis
results directly from our work with deep, cold seawater
pipes.
A seawater air conditioning system is illustrated below. The buildings to the far right are identical internally to buildings
cooled with conventional A/C. Chilled fresh water moves through these buildings
with the same temperatures and flows of conventional systems. A conventional
chiller, however, does not cool the chilled water loop in this system. The low
temperatures in the chilled water loop are maintained by passing this fresh water
through a counter-flow heat exchanger with the primary fluid being deep cold
seawater. The two fluids are on either side of a titanium plate that transfers
the heat from one fluid to the other and do not mix.
The seawater intake brings in water at a temperature lower than
the temperature maintained in the chilled water loop. Once the seawater passes
through the heat exchanger(s), it is returned to the ocean through another
pipeline.
The main components of a basic seawater air conditioning system
are the seawater supply system, the heat exchanger or cooling station and the
fresh water distribution system. These basic components can be optimized for
each specific location, climate and building.
For a large building using conventional air conditioning system,
a constant flow of cold fresh "chilled water" is circulated throughout
the building for heat removal. As this chilled water moves throughout the
building and absorbs heat, its temperature rises from an incoming value of
approximately 7-8°C to an outflow value
approximately 5°C higher. This warm
chilled water then enters the chiller, a refrigeration system that cools the
recirculating fresh water. Water enters the chiller at a nominal 12-13°C
and exits at 7-8°C. The water flow through the building varies with demand and
the temperature of the water leaving the chiller is constant. The chiller
consumes electricity as it "pumps" heat from a cold source to a warmer
source.
Seawater air conditioning is not technically complex nor is
it a high technical risk. It is established technology being applied in an
innovative way. All the components necessary exist and have been operated under
the conditions required.
A SWAC system has significant environmental
benefits: These include drastic reductions in electricity consumption which
reduces air pollution and greenhouse gas production, and substitution of simple
heat exchangers for chiller machinery which often use ozone-depleting
chlorofluorocarbons (CFCs).
 The
existence of the deep water ocean heat sink results from natural climatic
processes where water is cooled at the poles, becomes dense and sinks to deeper
water. The figure at left illustrates a temperature profile in the tropics typical
for the world’s deep oceans. 7°C
or colder can be reached at 700m depth, 5°C
or colder at 1000m. The deep-water portion of this profile changes little
seasonally and therefore cold water is available on a year round basis.
The feasibility of using cold seawater to directly cool
buildings has been studied and analyzed for many years. At certain locations,
successful installation and operation has occurred. Large lakes may also provide
nearby sources of cold water for cooling.
In 1975, the US Department of Energy funded a program entitled
"Feasibility of a District Cooling System Utilizing Cold Seawater." [Hirshman
et al] Several locations were studied and the two most favorable sites were
Miami/Ft. Lauderdale and Honolulu. The study, however, noted that one of the
limiting technical factors was the inability to deploy large diameter pipelines
to depths of 1500' and more. This technical challenge has since been addressed
and demonstrated with deep-water pipelines at the Natural Energy Laboratory of
Hawaii at Keahole Point, Hawaii. Plans have recently been approved to
provide cold deep seawater air conditioning to the Keahole airport expansion
facilities.
In 1999, the Cornell Lake Source Cooling Project installed a
63” diameter pipeline into nearby Lake Cayuga. This pipeline was 10,000‘ in
length and installed to a depth of 250’. Cold water from this pipeline, at
approximately 4°C,
will provide air conditioning for the Cornell University Campus. The volume of
cooling that this system is capable of providing is in excess of 20,000 tons of
cooling and the system is scheduled to be operational in mid-2000.
Other cold lake water projects pending are:
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The Lake Water Supply Project, New York State: The New York State Energy Research and Development Authority (NYSERDA),
Monroe County Water Authority (MRWA), and Xerox Corporation have teamed to
investigate the feasibility of establishing a cold-water district as part of a
proposed lake water supply project for the town of Webster New York along the
shores of Lake Ontario. Xerox Corporation will be the major cold-water customer
during the initial development of the project and will use the cold water to
provide air conditioning to its industrial complex. The MRWA will then send the
water to their water treatment plant or return some water back to the lake. This
project would require 3- 63” HDPE pipelines, each 3miles in length. Estimated
cost of this project is $120 million.
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Deep Water Cooling Project, Toronto, Ontario, Canada: The city of Toronto, Ontario, Canada is developing a district
cooling plan that will utilize cold water from Lake Ontario to provide air
conditioning to Toronto. Construction has already begun for the water
distribution system throughout the city. The offshore portion of the project
will begin engineering design in 2000.
The energy requirements for a large building's air
conditioning system are significant. Approximately 45 percent of a large hotel's
total electric bill goes towards air conditioning and about 2/3 of that is for
operating the chillers and cooling towers. Chilling this water requires
approximately 1 ton of cooling for an average hotel room. Large buildings may
take many hundreds to many thousands of tons, requiring a peak electrical demand
for air conditioning of 1 megawatt or larger. Therefore, operating chillers to
keep the chilled water at 7°C comes at a significant power cost. The other 1/3
goes into running the fans for the air handling inside the building and is
unaffected.
The graph to the right shows an economic comparison conducted in 2000 for the
airport in Curacao, Netherland Antilles. The economic viability of seawater air
conditioning was determined by comparing the construction and operating costs of
the seawater supply system to the construction and operating costs of
conventional air conditioning (A/C) systems, including the performance under
varying daily and seasonal load conditions. The study concluded that Seawater
Air Conditioning had a life-cycle cost one-half that of conventional systems.
In 1980, the US Naval Material Command conducted a study
entitled: "Sea/Lake Water Air Conditioning at Naval Facilities."
Computer models were developed which provided reasonable estimates of the
capital cost and energy use of seawater air conditioning systems at Point Mugu,
California and Pearl Harbor, Hawaii. The study concluded that:
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The capital cost and energy use of a SWAC system was sensitive to
the pipeline length, which is dependent on the seawater temperature near the
seafloor versus the distance from shore at the site.
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At a hypothetical typical Navy facility, a SWAC system will use
80% less energy than conventional A/C, but the capital costs of SWAC systems are
60% greater. The Life Cycle Cost of SWAC at a typical Naval facility would be
25% lower than the life cycle cost of conventional A/C.
In 1986, a joint project between the Canadian government and
Purdy's Wharf Development, Ltd. demonstrated the use of ocean water as a source
for building cooling to a 350,000 square ft. office complex along the waterfront
in Halifax, Nova Scotia. Due to the geographic conditions and annual low water
temperatures, a small diameter pipeline was deployed to a depth of less than
100' ft. This was a major factor in limiting the overall expense of installing
the cooling system. Total investment for this project was $200,000. The project
was very successful and savings were identified in the following areas: a saving
of $50-60,000 per year in avoided electrical cost, fewer maintenance staff,
reduction in fresh water, savings in water treatment, and savings in cooling
tower maintenance and replacement. The financial result in terms of a simple
payback period was two years. Today, Purdy's Wharf continues to successfully
utilize an expanded seawater air conditioning system for their waterfront
properties.
In 1986, the Natural Energy Laboratory of Hawaii Authority,
Keahole Point, Hawaii began the successful utilization of SWAC in their main
laboratory building. Deep-water pipelines were already installed to provide
cold, nutrient rich, seawater for research purposes in alternate energy and
aquaculture. Since a cold water supply was already incorporated into the
infrastructure, it was decided to utilize the cold water for cooling.
Today, the use of SWAC has been expanded to a new administration building and a
second laboratory. Estimated monthly saving in electricity is $2000.
In 1990, the US Department of Energy funded a study entitled:
"Waikiki District Cooling Utility." The purpose of this brief study
was to evaluate whether it was economically and technically feasible to utilize
seawater air conditioning as a means to provide cooling to the hotels in Waikiki
and to create a Waikiki Cooling Utility. [Darrow-Sawyer]. Waikiki was targeted
because of the high density of hotels, high electrical consumption and a large
demand for air conditioning. It was estimated by Hawaiian Electric Company that
of the 107 Megawatts consumed in Waikiki, 51.4 Megawatts were used for air
conditioning. This study concluded that economically and technically, Waikiki
could be cooled by utilizing seawater air conditioning. Hindering progress on
this concept is the difficulty of installing the distribution system throughout
a high-tourist region.
In 1995, Stockholm Energy started supplying properties in
central Stockholm with cooling from its new district cooling system.
Most of the cooling is produced by using cold water from the Baltic Sea.
The temperature of the cooling water leaving the plant is 6°C
or lower and the return temperature from the distribution grid is 16°C
at high load and a few degrees lower at low load. The district cooling system is
designed for a maximum load of 60 MW.
 A SWAC system has a fairly significant capital cost, and the
peak capacity of the system must match the peak demand of the buildings that it
serves. Cold Water Storage can reduce the average system size needed to meet the
peak cooling requirement. The seawater air conditioning system would be operated
100 percent of the time and when the building demands are low, the excess
capacity is directed into a storage system of cold fresh water. When A/C demand
is at its peak, the cold water is drained from its storage to meet the demand.
Cold water storage tanks are commercially available that are constant volume;
the warm water remains at the top and the coldest water remains at the bottom.
These tanks are now used in conjunction with conventional A/C systems to take
advantage of low, off-peak electrical rates.
In some cases, it is either too costly or impossible to supply
seawater at the necessary low temperatures to maintain minimum temperatures in
the chilled water loop. The distance offshore to reach sufficiently cold water
might be prohibitive or the ocean depth may simply not be available. As shown
below, it is possible to economically use auxiliary chillers to achieve
the desired minimum chilled water temperature. With the condenser kept cool, the
auxiliary chiller can operate at a very high efficiency.
The primary factors impacting the economic success of
seawater air conditioning are the size of the air conditioning load,
accessibility to deep cold water, the percent utilization of the system and the
local cost of electricity. Knowledge
of these factors for a given locale plus the deep ocean pipeline experience of
Makai are the key elements in conducting a critical assessment of the economic
potential for SWAC in a particular area.
Makai Ocean Engineering offers a full range of engineering
services for seawater air conditioning analysis and implementation.
Makai can provide technical and economic SWAC assessments, offshore
surveys, environmental analysis, review of permitting, deep water pipeline
design, conceptual through final engineering design, cost estimates for
construction and operation, and construction management.
The following are some of the Seawater Air Conditioning
studies conducted by Makai Ocean Engineering:
West Beach,
Oahu, Hawaii:
The State of Hawaii requested an analysis of the technical
and economic feasibility of using deep cold seawater for cooling in Hawaii.
Makai's study concentrated on West Beach, Oahu, an emerging resort with good
access to deep cold water. Once completed, it is estimated that power savings
for a large seawater air conditioning system, as compared to a conventional air
conditioning system, are greater than 80% and lifetime savings can be as high as
50%.
Netherlands
Antilles
Makai evaluated the technical and economical feasibility
of using deep cold seawater for air conditioning and the cooling of evaporators
in power/desalination plants for the Advisory Committee on Technical Policy in
Curacao. Several sites were
identified as excellent locations for seawater air conditioning and a local
group is currently formulating a development and financing plan to install a
system.
A follow-on study showed dramatic energy and cost savings for air conditioning
the airport expansion at Curacao Intl. Airport.
Tumon Bay,
Guam
A preliminary investigation of Tumon Bay has identified
this area as a prime candidate for SWAC. It
has good access to deep water, high air conditioning utilization and increasing
electrical rates. |
