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Save Our Oceans, Save Our Planet

The Blue Revolution: Air Condition and Energy from Deep Waters of Lakes and Oceans

Deep lake and ocean water is being exploited for cooling buildings, provide drinking water and generate electricity. Prof. Joe Cummins

How cities and campuses keep cool

Many great cities around the world are located near ocean shores or deep lakes. The cities of Toronto, Stockholm and Honolulu, and the Cornell University campus are showing the world what can be done using cold deep water to power the cooling of large buildings, providing a large saving in energy and cutting down on carbon emissions and pollution from energy generating plants.

Toronto initiated the cooling system in 2004 by the company Enwave District  Energy Ltd. A five-kilometre long pipe draws cold (4 C) water from the depths  (83 metres down) of Lake Ontario to Toronto Island  (just offshore of Toronto) where the water is filtered and treated with chlorine as it is delivered to taps in homes and businesses. After treatment, the very cold water flows to a city plant that employs a heat exchanger to transfer heat from the water to cool a closed cooling water loop that circulates to the distribution network, where more heat exchangers cool the water circulating through the air conditioning systems in the office towers. The system will meet up to about 40 percent of the city’s cooling needs. Toronto, like most Midwestern Cities, has very hot and humid summers, which put a huge demand on the electrical supply, so that the lake cooling system brings very welcome relief and protection against electrical ‘brown out’. Cooling is provided for office towers, sports and entertainment facilities and waterfront developments. Currently, government buildings including the Ontario legislative complex are being modified for lake-water cooling [1-3].

Cornell University draws cold water from a nearby deep lake, Lake Cayuga. The water is pumped to a heat exchanger at the shore where the campus and a school share a cooling loop, and the warm water from the buildings flows down to push cool water up to the campus. The system is both elegant and cost effective [4].

Stockholm is employing cold deep-sea water to cool buildings. In central Stockholm, the cooling plant comprises four heat pumps that obtain their energy from seawater. The plant has two seawater inlets, one at the surface and the other at a depth of 20 meters. Cooling is produced by cold water drawn through the inlet to a heat pump and then passes to heat exchangers that cool the water used to cool buildings in the central district. The heat exchangers are made of titanium to withstand the corrosive seawater. The surface inlet delivers water to the heat pump, which produces heating energy for delivery to the heating network [5].

Honolulu has been investigating alternative uses of seawater in cooling. The results were published as the proceedings of a 2003 workshop.  One system draws very cool water from the offshore depths and delivering it to heat exchangers to cool hotels and other large buildings. The other system generates electrical energy using the stored energy of sun-warmed water to energize the evaporation of ammonia to drive turbines to create electricity (see OETC below) [6]. 

In 1986, the Natural Energy Laboratory of Hawaii Authority, Keahole Point, Hawaii began the successful utilization of seawater air-conditioning 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. As a cold water supply was already incorporated into the infrastructure, they decided to use it for cooling.  Today, seawater air-conditioning has been expanded to a new administration building and a second laboratory. Installations for deep water cooling have been proposed for other locations in Hawaii including Kahoolawe, Kona Airport and the new town of Kapolei, Oahu [7]. Currently, seawater cooling systems are under construction in Tahiti, Curacao, Korea, Malta, the Cape Verde Islands, Haiti and  Mauritius [6,7].The Guam Power Authority  put together an extensive report on the project at Tumon Bay [8].

Environmental impact study

A territory-wide system for cool water air-conditioning is planned for Hong Kong, the proposed project included consideration of environmental impact [9]. China undertook a study of the impact of proposed Chinese coastal municipal air conditioning using deep ocean water. The study dealt with the issue of warming deep water on the intensity of El Nino effects, and concluded that the impact of deep water-cooling to air-condition coastal cities was negligible at a coarse-grained level, but there could be local hotspots in temperature changes [10].

Deep-water air-conditioning could be considered for other major cities located near the ocean or near deep lakes, as it has the advantages of low cost, great savings on energy and on air-conditioning chemicals. From the systems described above, deep-water air-conditioning may be suitable for both large and midsize to small communities or for universities, hospitals or hotel resorts.

Energy and water from the deep ocean

The deep ocean has also been put forward for the “blue revolution”, a sink for converting the energy of sun-warmed surface water to electricity (ocean thermal energy conversion or OTEC) and at the same time enriching the surface waters with nutrients from the depths to support the growth of phytoplankton that sustains both fish and marine mammals [11]. Electricity can be generated from surface water warmed by the sun, while the cool water from the depths is used in the cooling cycles to drive turbines generating electricity.

The first OTEC was deployed in Hawaii in 1979 [11]. OETC systems include the closed-cycle system that uses a working fluid, such as ammonia, pumped around a closed loop with three components: a pump, turbine and heat exchanger (evaporator and condenser). The warm seawater passes through the evaporator and converts the ammonia liquid into high-pressure ammonia vapour. The high-pressure vapour is then fed into an expander where it drives a turbine connected to a generator. Low-pressure ammonia vapour leaving the turbine is passed through a condenser, where the cold seawater cools the ammonia, returning the ammonia back into a liquid. The open-cycle system is generally similar to the closed-cycle system and uses the same basic components. The open-cycle system uses the warm seawater as the working fluid. The warm seawater passing through the evaporator is converted to steam, which drives the turbine/generator. After leaving the turbine, the steam is cooled by the cold seawater to form desalinated water. The desalinated water is fresh water fit for domestic and commercial use.

The hybrid system uses parts of both open-cycle and closed-cycle systems to produce electricity and desalinated water. In this arrangement, electricity is generated in the closed-cycle system and the warm and cold seawater discharges are passed through the flash evaporator and condenser of the open-cycle system (i.e, the original open-cycle system with the turbine/generator removed) to produce fresh water [12,13].

Deep ocean water has also been used to provide fresh water from warm moist ocean air [14] or from warm surface water evaporated at low pressure then condensed using cool deep water  [15]. With rapidly decreasing supplies of unpolluted fresh water, methods such as these can provide fresh water at relatively low cost without adding to global warming.

In 1979, Japan began pumping deep ocean water to support fisheries whose productivity had been reduced by over-grazing. Up welling of deep water replenishes surface water nutrients naturally, but productivity of offshore fisheries can be enhanced by pumping up deep water. Seaweed beds that support fish and marine mammals are frequently over- grazed and changed into barren sea. It has been possible to restore productivity by pumping up nutrient-rich deep water [17].

Pumping deep ocean water to air condition cities, produce energy and fresh water, and to fertilize the productive surface waters appears to be a promising approach to mitigate global warming by reducing consumption of polluting oil and coal burning and to reduce the impact of overgrazing on marine food production.

Is the large scale pumping of deep ocean water sustainable?  As indicated earlier, current evaluations suggest that even the most ambitious projects are unlikely to significantly impact ocean-related climate controls. The deep ocean is ventilated through open ocean convection and cascading down coastal waters [18]. The relatively puny human efforts seem unlikely to impact the natural processes, at least for the immediate future. The other concern has been that of eutrification, a process by which an excess of plant nutrients, mainly nitrogen and phosphorous, causes the overgrowth of microbes and reduces oxygen needed to support fish life. This often occurs where sewage is discharged into harbours, fjords, coastal waters and lakes The deep waters provide a range of needed nutrients for overgrazed waters, but are not rich enough in nitrogen and phosphorous to cause eutrification.       

Even though London, England, is not located near a useful source of cool water for air-conditioning, the underground railway has begun to use cool ground water to cool the tunnels for the comfort of the passengers. Groundwater seepage has been a growing problem causing damage to tracks and switches, so the seepage is simply bled off and used to cool the tunnels. The system promises to be both cost effective and cost-saving with regard to the maintenance of the railway [19].

The cooling systems discussed earlier are not suitable for single-family homes. For homes in hot climates it would be desirable to have cooling systems that require minimal energy expenditure. Of those systems, roof ponds seem to be the most desirable, though they must be installed with caution. The most effective system may be a roof pond upon which white cotton towels were floated on the surface using polystyrene strips; gunny bags also serve in place of towels. The towels resist heat transfer from the sun to the lower depths of the shallow pond [20]. The system is developed for tropical climates but might serve very well in areas with cold winters where the roof pond would accumulate insulating snow.

(Editor’s end note: A new UN report [21] (Oceans in distress, this issue) points to a potential threat to deep sea communities as food particles and organisms are sucked up with the cold water and hence removed from the deep water environment. Furthermore, the construction and maintenance of the pump and pipe systems could damage the surrounding habitat and its wildlife.

Another reservation is that these applications, if practised on a large enough scale, would contribute to warming the oceans, thereby decreasing net primary production (Shutting down the oceans Act III, global warming and plankton, this issue).

Article first published 28/07/06


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