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001122 Dr aft - 9/81 the back end to maintain the pressure gradient which turns the blades of the turbine. The pressure gradient depends, in part, on the capacity of the plant's cooling system. Turbine effi- ciency increases and operating costs decrease as the pressure gradient increases. The several c901ing systems discussed below vary significantly in water requ1rements, pressure gradients, capital investment, and operation and maintenance costs. There is no single figure for cooling-water requirements, although the available literature tends to indicate that for a 1,000 MW plant, cooling ponds or spray ponds would require around 10,000 acre- feet per year, while evaporative cooling towers would consume on the order of 14,000 to 15,000 acre-feet per year. At Public Service Company of Colorado's Pawnee plant, it is est imated that the wet cooling tower system will consume 10,860 acre-feet per , -- year. Once-through cooling systems withdraw water from large sources, circulate it through a condenser where it absorbs heat, and then discharge it away from the intake point. These systems, while used extensively in the past, are not likely to be utilized in Colorado in the future because of the lack of large water sources and the thermal impacts of the discharge, which are now subject to water quality control regulations. Natural or artificial cooling ponds may be used in place of a large water source. By this method, heat is dissipated through surface evaporation. Water sprayers may be used to increase evaporation where pond cooling capacity is inadequate by itself. They may also be used in a once-through system to reduce thermal impacts to the receiving body of water. 2