The designers of the first long-distance trans-oceanic steam-powered ships realized that they could use seawater to condense most of the exhaust steam back into useable water, allowing ships to carry only a small amount of potable water. Beginning toward the end of the 19th century and continuing to the present day, many steam-based power stations that were located near the coast and used seawater to cool the condensers to re-use exhaust steam.
In more recent years, seawater has found additional uses in providing local climate control.
The heat capacity of the ocean is known to influence climate in different locations around the world. During winter, parts of Western Canada that are on the same latitude as The British Isles experience much colder winters than occurs across the U.K. During summer, the North Atlantic absorbs abundant solar thermal energy and by winter, the slow-moving Gulf Stream Ocean current has surrounded the U.K. with relatively warm seawater. A unit volume of seawater has been measured at some 3,500 times the heat capacity of the equivalent volume of air at atmospheric pressure.
Water Versus Air Cooling/Heating
A typical automotive cooling fan operates at a peak of around 30 percent efficiency drawing air through the cross-flow coils of an air-conditioner heat exchanger that typically operates at 65 percent heat transfer effectiveness. Replacing the cross-flow air-cooled heat exchanger with a counter-flow, water-based heat exchanger increases heat transfer effectiveness to 80 percent using a cooling medium that per unit-of-weight offers four times the heat capacity of air.
Small rotary water pumps that push water through the heat exchangers of air-conditioners and heat pumps operate at around 65 percent while some piston-based water pumps can operate at 90 percent efficiency.
The switch from air-cooling to water-cooling of air-conditioners and heat pumps can include a package that involves a more effective heat exchanger, a more efficient pumping technology and a cooling/heating medium with four times the heat capacity.
Cape Town Ocean Thermal Energy
A cool (15 degrees C) northbound ocean current flows by one of the world’s premium tourist destinations of Cape Town, South Africa, that in recent years has experienced hotter summers with increased humidity. An energy efficiency initiative is underway at Cape Town that could be adopted across the city’s business district.
Several skyscraper office towers are located near Cape Town’s waterfront and within close proximity to the Victoria and Alfred (V&A) docks tourist and visitor area. After 2010, V&A management initiated plans to maintain a comfortable climate inside a few visitor areas of their campus while consuming minimal energy.
The plan included the use of cool seawater (15 degrees C) instead of warm summer air (30 degrees C) to cool the coils of some of their commercial building-sized air-conditioners. During cool winter weather (15 degrees C), some modern designs of air-conditioners require a minor adjustment to function as a heat pump to provide interior heating to buildings by extracting latent from seawater. The initiative has realized substantial energy savings.
Singapore District Cooling
Singapore Power Company operates a district cooling system consisting of a distribution pipe network that carries chilled water at five degrees C to cool buildings. Singapore has a summertime seawater temperature of 28 degrees C with summer air temperatures involving humid air reaching 32 degrees C. The heat capacity of the seawater can efficiently operate steam condensers and also large-scale, seawater-cooled water chillers.
While water, propane and sulfur dioxide can be used as refrigerants, the technology requires thick piping to withstand extreme pressure levels and extreme-performance compressors. Such refrigeration technology is most feasible and even cost-competitive when used in large-scale operations such as at a centralized plant that serves the cooling (and heating) requirements of an entire district.
A power company can use its economy-of-scale to operate a cost-competitive and energy-competitive system compared to several thousand air-conditioners. Steam-based power station exhaust steam can sustain steam-vacuum refrigeration that is most viable when built in large-scale, with seawater sustaining the operation of the condensers. A power company can use steam to directly drive a super-size heat pump at high efficiency to use seawater as a heat sink and chill water for a district cooling system.
There are many coastal cities internationally where, unlike Singapore, the main electric power station is at a distant location. A heated or cooled closed-loop water-pipe connection between a coastal city and distant steam-based power station may be impractical.
However, some coastal cities such as Vancouver, Canada, receive electric power from a hydroelectric power station. At these cities, a large amount of exhaust thermal energy or exhaust steam may be unavailable, leaving seawater as the option to operate air-conditioning (district cooling) or district heating at greater efficiency that operating a multitude of air-cooled air-conditioners and air-heated heat pumps.
A multitude of intake pipes from the sea would be impractical as would a multitude of closed-loop water pipes placed on the seafloor. Seawater behind a breakwater and inside a harbor would be attractive locations to place closed-loop water pipes, provided that the harbor has sufficient depth. An extended length closed-loop pipe would serve as a heat exchanger while maintaining purity of the water flowing inside of it. It would have to connect to a centralized location that would connect to district heating of district cooling system, using the advantages of economy-of-scale to distribute heated or cooled water.
Aquifer Water Storage
Many cities internationally need to increase seasonal potable water storage capacity to provide for growing populations. Using groundwater as a thermal reservoir to operate heat pumps and air-conditioners becomes impractical as water tables drop seasonally to lower levels to provide potable for populations.
In some non-coastal regions in the southwestern United States and parts of India, lower water tables have greatly increased the cost of drilling for ground water. Large coastal cities with seaports that will come to depend on seasonal aquifer storage of potable water to feed their populations will need to use seawater for interior climate control.
The cost of installing an extended length closed-loop pipe made of stainless steel on the seafloor at or near a coastal city would cost less than drilling deep into the earth to access groundwater-based seasonal geothermal energy. A dual circuit pipeline system would connect the land-based closed-loop water pipeline distribution system to the undersea closed-loop pipeline via any of heat exchangers or large-scale heat pump air-conditioners. A battery of mega-size heat pumps could feasibly use extreme high-pressure pipes, alternative refrigerants and multiple (redundant) compressors to improve system reliability and provide for easier maintenance and repair.
Many coastal cities around the world could use the example of Singapore and introduce central cooling of skyscraper office towers located in their central business districts. Several European and American cities have district heating systems connected to the exhaust side of nearby thermal power stations.
Combination large-scale air-conditioners that also operate as heat pumps that use seawater as a heat sink or as a source of heat could provide district heating and district cooling to large buildings in the central business districts of numerous large coastal cities around the world.
The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.