Performance of a Conventional Plant
The performance of a conventional plant varies seasonally depending on the seawater temperature. Figures 2 (a). (b) and (c) show monthly deviations in the electric power of a conventional plant located at Site-N, Site-C and Site-S, respectively, based upon the design electric power output. Poor performance, i.e., a deep dip in electric power output, can be found during summer in a plant at Site-N and Site-C due to the upsurge of seawater temperature in summer. The electric power output at Site-S is rather flat over all seasons due to fewer seasonal variations in seawater temperature.
Figure 2. Monthly variation in electric power using surface seawater
Effect of deep-sea water usage in an existing plant
The effect of deep-sea water usage in an existing plant was examined, assuming that deep-sea water can be delivered using three parallel pipes of 5,000 m in length and 2.27 m in diameter. The temperatures of deep-sea water are assumed to be 3℃, 5℃ and 9℃ at Site-N, Site-C and Site-S, respectively. A key parameter, which has an impact on the performance of a deep-sea water plant, is the temperature rise of deep-sea water in a condenser. Therefore, the optimal value was analyzed by computing how performance varies with the temperature rise. Figures 3 (a), (b) and (c) depict the optimal temperature rises as 12.9℃, 15.6℃ and 19.6℃ at Site-N, Site-C and Site-S, respectively, and the corresponding discharge temperatures are 15.9℃, 20.6℃ and 28.6℃. The flow rates of the deep-sea water are in almost inverse proportion to the temperature rise, being 1.13, 0.95 and 0.78 million tons per day at Site-N, Site-C and Site-S, respectively.
The reason why the electricity output has a peak as seen in Figure 3 is because the steam turbine blades are designed so that the exhaust loss is minimal for a specific steam velocity at the end of the turbine cascade. The steam velocity is obviously dependent on the back pressure of the turbine or the pressure in the condenser which is directly related to the steam temperature and, of course, to the rise in water temperature.
Figure 3. Annual electrical output with full utility ratio (base: annual
electrical output of a conventional plant using surface seawater)
The monthly deviations in electric power output of the deep-sea water plants at Site-N, Site-C and Site-S are shown in Figures 4 (a), (b) and (c), respectively, where the monthly electric power outputs of the conventional plants are used as the bases.
The use of deep-seawater provides a gain of electric power over all seasons in comparison with a conventional plant, not only because the electric power outputs of the deep-sea water plants are constant over entire seasons, but because cold deep-sea water better heat sink for the condenser. The large gain in summer is explained by the comparatively poor performance of a conventional plant in summer. The results indicate that, for an existing power plant in central Japan, annual electric power output using deep-sea water possibly increases by 1.4% in comparison with a conventional operation using surface seawater. Especially in summer, the power generation is expected to improve by 3.5% at most in northern and central Japan.
Figure 4. Monthly electrical power using deep-sea water (base: monthly
power generation from a conventional plant using surface seawater)