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[SANET-MG] the blue revolution
ISIS Press Release 28/07/06
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
A fully referenced version of this article is posted on ISIS members’
website. Membership details here
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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