AIR CONDITIONING & ENERGY FROM DEEP
WATER
Deep lake and ocean water and even ground water is being exploited for cooling buildings, providing drinking water, and generating electricity.
The cities of Toronto and Stockholm, and the Cornell University campus have been using cold deep water to cool large buildings and making big savings in energy and carbon emissions and cutting other pollution from energy generating plants.
Toronto, for example, draws cold water from the depths of Lake Ontario to Toronto Island where the water is filtered and treated with chlorine as it is delivered to taps in homes and businesses. After treatment, part of the very cold water flows to a city plant, and via heat exchanger, cools a closed 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. A total of 46 buildings signed up to the system, saving 85 GWh and reducing 79 000 tonnes CO2 emission annually.
Honolulu has been investigating the possibility of converting the energy of sun-warmed surface water to electricity (ocean thermal energy conversion, or OTEC). 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 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 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. The first OTEC was deployed in Hawaii in 1979.
Japan began pumping up deep ocean water in 1979 to support fisheries that had been depleted by over-grazing of seaweed beds that support fish and marine mammals.
Pumping deep ocean water to air condition cities, produce energy and fresh water, and to fertilize the productive surface waters, appears a promising approach to mitigating global warming by reducing the consumption of polluting oil and coal and the impact of overgrazing on marine food production.
But is large-scale pumping of deep ocean water sustainable? The deep ocean is ventilated through a giant thermohaline circulatory system that moves deep waters from north to south as salt-laden cooled water sinks into the depths in the North Atlantic and energizes a global conveyor belt that sends nutrient laden deep waters naturally to the surface in the North Pacific, north Indian Ocean, and south- east Pacific. This circulatory system is already being seriously disturbed by global warming.
There is 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 system could damage the deep sea habitat and its wild life. These applications, if practised on a large scale could contribute to warming the oceans, thereby decreasing their net primary production and impacting on all marine life.
Many big projects have remained on the drawing board also because the technology is expensive. Nevertheless, small scale air conditioning projects are definitely sustainable, and there are increasing examples, including the use of ground water to cool the tunnels of the London underground in the UK, and deep-mine flood water for air- conditioning in Springfield, Nova Scotia in Canada, and Park Hill Missouri in the US.
REEF NOT BARRAGE TO TAP THE TIDES
The Severn estuary has the third highest tidal range in the world, and a barrage across the estuary to trap the high tide could contribute 0.6 percent of UK’s primary energy use and 2 percent of its electricity. The barrage, estimated to cost of £15 billion many decades back, had triggered widespread environmental concerns as it would lead to the loss of hundreds of square kilometres of mudflats and salt marsh, home to waders and other coastal birds and a host of migratory species. The powerful surge of water over the turbines when the barrage gates open will profoundly disturb estuarine life, including fisheries and salmon runs.
A possible solution proposed by Cornish hydraulics engineer Rupert Armstrong Evans is to build a reef instead of a barrage that would generate as much electricity and far more steadily than the big barrage. This would consist of a semi-floating set of box structures housing the turbines and stretching across the estuary riding over a fixed base on the estuary floor. By using a moveable ‘crest gate’ to track the tide level and therefore to maintain a small head difference, irrespective of the stage of the tide, the turbines would operate for long periods, at least double the generation period of the proposed big barrage.
The reef would minimise environmental effects, save on construction and costs and still allow big ships to pass. The UK government announced in 2008 it believes the Severn tidal reef to have merit and would consider it. In July 2009, however, a row broke out as Evans’ idea, entered in a Department of Energy and Climate Change competition, was rejected in favour of a similar design put forward by another engineering firm.
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