SWAC (Sea Water Air Conditioning) is an innovative and ecological Air Conditioning technology which uses a renewable source of cold water. It is particularly fitted to Islands in the tropical belt as they represent an important cooling need (A/C electric consumption can add up to more than 30% of the total electric consumtion) and only a few kilometers away, deep in the ocean is a vast unused reservoir of cold water. The idea of SWAC is to link them together. Sea water is pumped and cools down the distribution loop with the help of a heat exchanger.

Other sources are appropriate for this use :

  • Oceans : Difference in density with temperature results in a stratified profile where the deeper you go and the clder the sea water. It is unsually below 7°C around 1000m below the surface.
  • Lakes in temperate areas : cold water naturally stays at the bottom (depth of  about 50m). The alpine Lakes are a very suited area for SWAC implementation.
  • Cold water surface currents (As in Stockolm’s downtown AC system)

Depending on the cooling need, even surface temperature water can be put to good use for other means, see paragraph “Beyond air-conditioning”.

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How it works:

Deep sea water is pumped to the water surface. Cold water has a higher density than warm water, thus a stratification naturally occurs wherever vertical currents are not too strong. Cold water is found in depth where as upper layers are heated by the sun. The depth of the intake point depends on the customer need, the local bathymetry and the sea water gradient.

Sea water then goes through a heat exchanger to cool down a secondary loop (which runs through the cooling units of the building). During this step, the water is warmed up by a few degrees. It is eventually released back in the ocean, at a depth compatible with the environmental constraints, in order not to modify the ecosystem. This ecological technology avoids and replaces normal air conditioning systems.

Beyond air-conditioning:

If the sea water is not cold enough to provide air-conditioning on its own, a surface intake can still provide interesting performances to cool down condensers of regular cooling machines. This allows for a greater efficiency with a decreased investment. Indeed, due to its massive heat capacity, sea water temperature shows less variations than air temperature. The thermal volumetric  capacity of water is 1000 times more important than of the air.

Such a cooling technology is equivalent to an enhanced floating HP system. Energy is saved by decreasing the compression factor of the operated cycle. This works for air-conditioning, but also for below-0°C cooling machines and even heating (in wintertime in temperate climates, sea water is warmer than the air and can be used as a mild source for heat pumps).

Advantages :

Energy Savings

This is a renewable energy based AC system providing low operating costs for residential and commercial coastal buildings/facilities. SWAC saves energy consumption from 80 to 90% compared to regular AC systems.

Multi-purpose

After cooling the secondary circuit, the sea water is still fresh enough to be used in the cooling process of auxiliary equipment’s, like Heat Pumps or other devices, depending on the customer’s need for cooling. Other uses are possible as well (fisheries, studies, crops…)

Employment

DPI designs SWAC systems to promote local employment. The setup of these systems requires workers and divers. Part of the investment is therefore reinjected in the local economy instead of countries owning offshore-oriented constructing equipment.

Ecological

Savings in electricity = savings in released CO2. A sea water loop provides most of the cooling energy, reducing the installed capacity in regular cooling machines therefore the quantity of refrigerant operated, in a world where the regulations are increasingly strict and the price of these material is surging. In some locations, sea water exchangers replace wet cooling  towers which are a potentiel source of Legionellosis.

Limits :

Shoreline proximity and cooling need

A building located far away from the coast is not a good fit for a SWAC system. An moderate cooling need is also required to make the investment profitable (at least a few MWth), otherwise the return on investment is not short enough to trigger projects.

Delays

These projects have an offshore component which requires a great deal of conception work and the investment is often several Millions of dollars. The turnover in the temas of investors/decision makers is adding to the difficulty, stretching the delays. On top of this, air-conditioning may not be the core business of potential customers (they are usually hostels, hospitals or airport) : other issues can be prioritised over these projects.

Cavitation and collapse

For any aspiration system, the flowrate is limited by two physical phenomenon : cavitation (Formation of air bubbles which induce great stress and fatigue on the equipment), and pipe collapse (Destruction of the pipe following a critical depression). DPI suggests solutions to push these limits further, allowing greater flowrates and smaller pipes. This allows for great savings in investment. More information is available here.

The particular case of islands

A SWAC system is especially interesting for islands given the following :

  • Islands naturally own a vast cold sea water reservoir within the ocean surrounding them.
  • Air Conditioning represents a strong part of the electrical consumption. In addition, Electricity in islands is quite expensive  given that islands don’t generate electricty but import it. Using a SWAC system will considerably reduce the electrical consumption and reduce the island’s dependence on fossil fuels.
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