Energy Watch
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
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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).
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