Water use by power plant cooling systems has become a critical siting issue for new plants and the object of increasing pressure for modification or retrofit at existing plants. Wet cooling typically costs less and results in more efficient plant performance. Dry cooling, while costing more and imposing heat rate and capacity penalties on the plant, conserves significant amounts of water and eliminates any concerns regarding thermal discharge to or intake losses on local water bodies.

Hybrid cooling systems have the potential of combining the advantages of both systems by reducing, although not eliminating, water requirements while incurring performance penalties that are less than those from all-dry systems. The costs, while greater than those for wet cooling, can be less than those for dry.

This paper addresses parallel wet/dry systems combining direct dry cooling using a forced-draft air-cooled condenser (ACC) with closed-cycle wet cooling using a surface (shell-and-tube) steam condenser and a mechanical-draft, counterflow wet cooling tower as applied to coal-fired steam plants, gas-fired combined-cycle plants and nuclear plants.

A brief summary of criteria used to identify situations where hybrid systems should be considered is given. A methodology for specifying and selecting a hybrid system is described along with the information and data requirements for sizing and estimating the capital costs and water requirements a specified plant at a specified site.

The methodology incorporates critical plant and operating parameters into the analysis, such as plant monthly load profile, plant equipment design parameters for equipment related to the cooling system, e.g. steam turbine, condenser, wet or dry cooling system, wastewater treatment system. Site characteristics include a water budget or constraints, e.g. acre feet of water available for cooling on an annual basis as well as any monthly or seasonal “draw rate” constraints and meteorological data. The effect of economic parameters including cost of capital, power, water and chemicals for wastewater treating are reviewed. Finally some examples of selected systems at sites of varying meteorological characteristics are presented.

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