Using Deep Cold Coastal Seawater for Cooling
Several coastal cities located in warm climates have a deep seafloor within less than 50 miles of the coastline. This cold seawater offers numerous benefits.
The Toronto Precedent
Several years ago, the City of Toronto’s water department in Canada installed an insulated water source pipe to access cold potable water from near the bottom of Lake Ontario. During the northern summer and prior to arriving at the water purification plant, that cold water passes through a heat exchanger to provide district cooling to several office towers located in the business district.
A cubic unit of that water provides over 3,400 times the heat capacity of the equivalent cubic unit of air. The result has been a massive reduction in energy consumption to cool building interiors.
Cold Tropical Seawater
While tropical surface seawater temperature may exceed 25 degrees Celsius, deep level seawater at 1,000 meters (3,300 feet) depth is at five degrees Celsius. Energy researchers used that difference in temperature to develop ocean thermal energy conversion engines located off the coasts of Hawaii and India that generate electric power.
However, many coastal cities have seafloor depths below 1,000 meters within less than 100 miles of land, allowing for possible installation of submerged insulated pipelines between the city and the greater depths. At some coastal cities, seafloor depths of 2,000 meters are within this distance.
The following cities are within 60 miles of 2,000 meters seafloor depth: San Juan, Recife, Santander, Cartagena, Toulon, Nice, Algiers, Western Crete, Antalya, Muscat, Chennai, Alexandria, Lagos, Port Elizabeth, East London, Nassau, Montego Bay, Cartagena, Port Macquarie and Sydney.
The following cities have cold ocean currents and 1,000 meter depth near the shore: San Juan, Lima, Valparaiso, Antofagasta, Cape Town, Perth, Albany, San Francisco, Ft Bragg, and Wellington.
Applying the Toronto Precedent
Coastal cities in tropical climates may borrow the Toronto summertime precedent of passing cold lake water through heat exchangers to cool buildings. These cities may draw cold seawater through insulated pipes from offshore depths and pass the cold seawater through counter-flow heat exchangers to cool a secondary stream of water flowing inside a closed loop pipe.
The heated seawater would be released back into the sea while the cooled water inside the closed loop pipe would flow to a large number of buildings in the coastal city, to provide low-cost cooling while reducing summertime air-conditioner related electric consumption.
While the Toronto potable-water based system is restricted to a small section of the city, an ocean based cold-seawater based system can be built to many times the order of magnitude and encompass a much larger section of the city. As a result, the reduction in air-conditioner related electrical consumption will be many times that of Toronto.
A cold seawater based system could also sustain the summertime operation of industrial refrigeration systems. Coastal cities such as Muscat (Oman), Chennai and Cape Town could achieve many times the summertime reduction in electric energy of Toronto.
Numerous companies are offering home-based and office based water-from-air technology, essential modified dehumidifiers that include UV-radiation treatment of the water and addition of minerals. These companies estimate that the atmosphere may hold over 1 million liters of potable water per capita. Many coastal cities located at tropical and subtropical locations experience hot and humid summer weather with air temperatures exceeding 30 degrees Celsius or 86 degrees Fahrenheit and a dew point of under 10 degrees Celsius. Cold water at under 10 degrees Celsius flowing through a radiator could help extract potable water from humid air.
Deep level coastal seawater could provide the necessary cooling capacity to extract potable water from humid air, using arrays of railway locomotive size radiators located sufficiently high above sea level so as not to be in the stream of coastal ocean spray.
During humid weather, any of coastal winds, drafting fans or chimney convection currents could draw humid air across the radiators to extract several thousand liters of potable water per day. Tall solar heated chimneys could draw through circular arrays of cooled radiators and perhaps deliver several hundred thousand liters of potable water per day.
Water Inside Buildings
Several companies are now marketing modified, electrically powered air dehumidifiers that not only extract water from humid air but also sanitize the water using intense UV-light treatment. Further treatment of water may include addition of minerals. Larger versions of this technology may be used in office towers.
At coastal cities where deep sea cold water is available, tall waterfront buildings have the option of using a cold stream of piped water to cool the condensers of water-from-air machines that provide several hundred liters of water per day and sufficient for the requirements of occupants of these buildings.
Economy of Scale
Economy-of-scale would justify the installation of several miles of insulated cold water pipe on the sea floor extending to depths of 1,000 meters to 2,000 meters combined with mega-size, submerged heat exchangers near the shore to transfer hear from a shore-based, closed loop insulated piping system of cold water. The closed-loop pipe of cold water would need to connect with a large number of tall office towers and related buildings located in a central business district to replace air conditioners. It would also need to connect to several large-scale, water-from-air extraction units to provide potable water.
Each building would include multiple small water-from-air extraction units, while either the municipality or related water distributor would operate multiple mega scale, water-from-air installations to extract potable water that it would add to the local water distribution system.
The onshore ocean thermal energy conversion installation at Hawaii is rated at 100 megawatts suggesting that a large-size pipe could source sufficient deep sea coastal cold water to sustain the operation of a district cooling system at a large coastal city. Populations of suitable coastal cities are provided:
Cape Town, South Africa
Port Elizabeth, S. Africa
Low-Grade Thermal Energy
In India and Hawaii, cold deep seawater at five degrees Celsius drawn through insulated pipes serves as the heat sink for ocean thermal energy conversion, with near surface seawater at 25 degrees Celsius as the high temperature reservoir.
Several countries such as Japan and South Africa (Cape Town) operate steam-based thermal power stations located next to the coast, where seawater cools the exhaust steam condensers that then release heated seawater at 40 degrees Celsius into the ocean. The exhaust heat from coastal steam-power stations could sustain the operation of modified ocean thermal energy conversion installations and perhaps generate enough power to sustain 50,000 to 100,000 homes.
Increasing population and unpredictable weather patterns could encourage many coastal cities that face water shortages, to operate desalination plants in addition to water-from-air installations. Where space is available, brine could be deposited in specially excavated coastal brine ponds that capture solar heat and raise brine temperature to 60 to 90 degrees Celsius. The temperature difference between the brine ponds and piped in deep cold seawater could sustain the operation of Organic Rankine Cycle engines either to produce electric power after sunset, or even store enough heat overnight to generate early morning peak electric power.
Many coastal cities in warm climates have a sea floor drops that to great depth near the coast. These cities will have access via insulated pipeline located on the sea floor, to cold seawater at five degrees Celsius. Given that seawater has 3,600 times the heat capacity of the equivalent cubic unit of air, many coastal cities may use the cold deep seawater to cool the interior of buildings and to operate large-scale, water-from-air extraction units cooled by cold seawater, to supplement the supply of stored rainwater and water obtained through seawater desalination.
The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.