Condenser Cooling Requirements

11.108. In a steam-electric generating plant, only a fraction of the heat supplied, e. g., by burning a fossil fuel or by nuclear fission, is converted into electrical energy. The waste heat remaining in the exhaust steam from the turbine is removed by cooling water in a condenser and, at the same time, the steam is condensed to provide the feed water for the steam generator which may be a heat exchanger (in a PWR) or a boiler (in a BWR or fossil-fuel plant). A simplified representation of the turbine — condenser system of a steam-electric plant is shown in Fig. 11.8. The cooling water carrying the heat removed in the condenser represents the thermal discharge (or thermal effluent). The additional heat present in the thermal discharge must be dispersed in such a manner as to cause the least possible disturbance of the environment.

11.109, The thermal efficiency of a steam-electric plant is based on the fraction of the heat supplied by the fuel (or heat input) that is converted into electrical energy. A distinction must be made, however, between the gross and the net efficiency. The gross efficiency is based on the total generator output and includes the electricity used for operating auxiliary equipment within the plant. The net thermal efficiency, on the other hand, is based on the net (or busbar) output of the plant, i. e., the electrical energy available to a utility for sale. For LWRs of current design, the gross efficiency is about 34 percent and the net efficiency roughly 32.5 percent. Energy losses within the plant amount to only a few percent, and so most of the difference between the heat input and the heat equivalent of the total energy generated must be removed by the condenser water.

11.110. Modern coal-fired plants have gross thermal efficiencies of about 42 percent, although there are substantial variations among different units. In such plants, generally about 10 percent of the heat input escapes through the furnace stack, and some 10 percent of the gross output may be required

to operate plant auxiliary equipment. As summarized in Table 11.2, the heat removed by the condenser water in a coal-fired plant is approximately two-thirds of that in a typical LWR of the same net electrical capacity. This is why the thermal-discharge problem is of special importance for nuclear power generation. Fast breeder reactors and high-temperature gas — cooled reactors would produce steam at higher temperatures than do LWRs; the thermal discharge would then be comparable to those from modern coal-fired plants.

11.111. In most nuclear power plants, the difference between the inlet and outlet temperatures of the condenser cooling water ranges from about 8 to 20°C, with an average near 12°C. Assuming a temperature increase of 12°C, and bearing in mind that the specific heat of water is about 4.2 x 103 J/kg • K, it follows that for a typical lOOO-MW(el) LWR plant, the removal of 2030 MW (or 2030 x 106 J/s) of heat requires a condenser cooling-water flow rate of (2030 x 106)/(4.2 x 103)(12) ~ 40,000 kg/s or 40 m3/s (630,000 gal/min).

11.112. The simplest and most economical treatment of the thermal effluent is the “once-through” or open-cycle procedure. The cooling water is drawn from an adjacent natural water body, e. g., a river, lake, ocean, or estuary, passed through the condenser, and then discharged into the same water body. As a result, the temperature of the water is raised, especially in the vicinity of the discharge point. The possibility of using once-through cooling depends on the availability of an adequate water supply and the effect of the temperature increase on the ecological balance of the water body. If the disturbance of the ecosystem is not tolerable, other methods for treating the thermal discharge are necessary; these in­clude the use of cooling ponds or canals and cooling towers.