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Renewable Cooling

Factsheet 9 Mar. 2009 Seawater cooling

Seawater Air Conditioning (SWAC) systems use the cold water from the deep ocean (and in some cases a deep lake) to cool buildings. It can largely reduce the power consumed by air conditioning (AC) systems. In various situations it has proven cost-effective. Seawater cooling is also applied for cooling industrial processes; many examples exist of power stations cooled with seawater.

SWAC components

• Cold seawater. Although the temperature of surface seawater does not deviate very much from ambient temperatures, substantial lower temperatures exist at large depths. This is due to global water circulations in the oceans: driven by density cold water from the poles flows at large depths to warmer areas. Simultaneously, the water at the ocean surface layer flows back to the poles. Roughly, the ocean water temperature at various depths is as follows:

700m below 7°C 1000m below 5°C 2000m below 3°C

• Sea water supply system: pipes for cold water in take and for returning heated water. These pipes are made out of seawater-resistant material, like polyethylene. Furthermore, the pipe design should resist the hydrodynamic impact. Adequate filters are needed to prevent accumulation of solid particles in the system.

• Heat exchanger (“cooling station”). This brings over the cold to a closed water system. Titanium heat exchangers are used for this, because titanium combines resistance to salty water with high heat conductivity.

• Chilled water distribution net (heat-isolated pipes).

• Cooling system: mostly the chilled water is cool

enough for direct cooling application.

In some situations, the cooling capacity is insufficient. Then an auxiliary chiller can be used to supplement the cooling supplied by the sea water.

Pre-requirements for SWAC

• availability of cold water in the vicinity • high cooling demand

Practical examples

SWAC have been realised in amongst others:

• Toronto:a5kmlongpipedrawscoldwater(4°C)from

Ontario Lake. This system meets up to 40% of the city’s cooling need. The cooling water is also used for drinking water purposes.

• Cornell University utilises a comparable system for cooling the campus and school (using water from the Cayuga Lake).

• Stockholm: Here, sea water is used. The system has two water inlets: one at sea level and another at a depth of 20m.

o The deep inlet is used for cooling water. 80% (280GWh) of the district cooling in Stockholm comes from seawater.

o The inlet at sea level is used for heating. In 2006 16% (1600GWh) of the heating for Stockholm is supplied by seawater.

• Hawaï: a 1m diameter pipe is used for 15MW cooling power (with intake depth at 800m) (Ryzin and Leraand, 1992).

• Other seawater cooling systems have been realised or are considered in Halifax, Tahiti, Curacao, Korea, Malta, the Cape Verde Islands, Haiti and Mauritius.

In many of these project Hawaii based Makaï Ocean Engineering is involved.

Energetic feasibility

SWAC systems only need energy for pumping water; generally this is below 10 to 20% of the energy needed for traditional cooling.

For large building and hotels in tropical and subtropical climates, air conditioning represents the major energy demand. As a rule-of-thumb, a typical hotel room requires approximately 3.5kW of air conditioning with an energy requirement of 0.9 kW. A conventional system utilizes about 900 kw/1000 tons but a similar sized A/C system using seawater requires only pumping power in the order of 40-80 kw/1000 tons, representing a 90% electrical saving over the chiller power requirement.

The Curacao project (built in 2008):

The pipeline will reach 6 kilometres out into the sea, ... down to a depth of 850 meters, fetching up seawater at a temperature of 6°C at half cubic meter per

second. The SWAC system operates with a

Scheme for a SWAC including an auxiliary chiller unit. (Source: Bellinger, 2006).

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