DWSC System Concepts and Design Basis
Continuous and year-round cold water is available in a number of deep lakes and along certain stretches of coastal ocean waters near populated areas and significant cooling loads.
The basic requirement is that a sufficiently cold water source is available during the associated cooling load season.
The temperature used in most contemporary chilled water systems is 40 - 50°F.
To be a viable source of cold water for direct cooling, the water source must be in the temperature range of40 - 45° F.
The continuous availability of sufficient quantities of naturally cold water is central to the viability of any facility depending solely upon DWSC for meeting its annual cooling requirements.
Lake and ocean eco-systems have several unique characteristics which affect their ability to produce a sustained supply of cold water for a DWSC facility.
Lake Based Systems
Lake based DWSC systems are affected by several factors including location, geometry and ambient annual temperature characteristics.
The thermal structure of deep lakes generally undergoes two distinct periods, stratification and mixing.
Stratification generally occurs during the winter and summer months, with mixing occurring during the spring and fall months.
Stratification is caused by the variance in water density due to the naturally occurring temperature differences.
During the stratification period, the lake is also segregated into three biological layers.
The upper layer is referred to as the epilimnion.
The epilimnion (where photosynthesis occurs) is isolated from the deeper, more dense, colder water known as the hypolimnion (where respiration occurs).
The mixing area or boundary layer between the epilimnion and the hypolimnion is the metalimnimon or thermocline.
The thickness of the thermocline varies based upon the internal wave activities, ambient air temperature and wind activity.
The seasonal characteristics require substantial study to determine the applicability of a lake for reliable cooling as well as to locate the best cold water intake point.
Figure 1 diagrams a typical lake during stratification and mixing.
The thermal properties of most deep lakes show that cold water, in the 40 - 45°F (3 - 6°C) range is available at depths of 200 to 250 feet.
Short lived temperature excursions can occur during the transition period between the stratification and mixing periods as well as during surface events such as high wind and rain conditions.
Ocean Systems
Similar to deep lake structures, the ocean also experiences temperature stratification and biological zones.
Figure 2 illustrates the average ocean water temperature and biological zones as a function of depth.
As shown, water at temperatures cold enough to be utilized for air conditioning is generally available in ocean depths of 1500 to 3000 feet.
System Description
A lake or ocean based DWSC system for air conditioning purposes is illustrated in Figure 3 and is basic in concept.
Cold water is circulated from the cooling source through heat exchangers and returned to the body of water.
The water from the heat load is circulated through heat exchangers, cooled and pumped to the cooling load.
This system eliminates the use of conventional chillers.
A DWSC lake or ocean cooling based system is easily integrated into any existing chilled water system.
Chilled water from the new DWSC system is connected into the existing system supply line.
The existing chilled water return line is also connected into the new DWSC system return line as any district cooling system would be incorporated into an existing building chilled water system.
Hybrid System Technology
The ability of the lake or ocean water based DWSC system to meet the cooling loads is a function of the ayailable cold water temperature versus the chilled water temperature required.
The system lends itself to combinations with other forms of chilled water production in order to meet the full range of cooling requirements such as peaking capacity, additional standby capacity or the production of chilled water with a supply temperature lower than is economically possible with a DWSC system.
This can take the form of locating conventional chillers at the DWSC facility to take advantage of the offshore return water discharge as a cold source of refrigeration cycle condenser water.
The incorporation of a thermal energy storage system to the supplemental, chiller based system could further increase the ability of an integrated or hybrid system to meet a diverse cooling load while potentially minimizing the capital costs associated with pure DWSC systems.
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