Newly Designed Plants for Deep-sea Water Usage
In the previous section, deep-sea water usage in an existing plant was examined. In this section, we try to design a new plant suitable for the effective application of deep-sea water. Since the seawater condition has a major impact on the condenser and pump designs, we restrict our design study to these components. The key design parameters of the condenser are the temperature rise in seawater, ΔT, and the steam temperature, Ts, both of which are so related that the discharge temperature of the seawater (original intake seawater temperature plus temperature rise) is lower than that of the steam temperature for a heat transfer from the steam side to the water side. In general, a larger heat transfer area of the condenser is required to realize a low steam temperature, because the temperature difference is then further reduced between the steam side and seawater side. On the other hand, a lower steam temperature tends to increase plant efficiency due to the larger steam pressure difference between the turbine inlet and outlet. Since the temperature rise is almost in inverse proportion to the flow rate of the seawater, it is obvious that any such rise has an impact on pump specifications and on the required electric power for intake. The effects of the two parameters are examined below, keeping the design net electric power at 600 MWe as a plant condition. Other conditions such as intake pipes and deep-sea water temperatures are the same as the previous ones.
Condenser and pump designs
Figure 5 shows the heat transfer area required for a new 600 MWe plant utilizing deep-sea water at Site-S. The results indicate that the condenser of the deep-sea water plant can be much smaller than that of the conventional plant, especially if the steam temperature is designed to be high and the temperature rise to be low. For example, the heat transfer area requirement can be reduced to as little as 30% of the present design under conditions of Ts= 38.5℃ and ΔT= 7.0℃. Though the pump capacity requirement is 1.4 times larger at ΔT= 7.0℃, the results in Figure 6 indicate that the pump of the deep-sea water plant can be smaller than that of the conventional plant, if the temperature rise is designed to be larger. This is because the pump capacity required for the intake and discharge of deep-sea water is in inverse proportion to the temperature rise, being independent of the steam temperature.
The impacts of the temperature rise on the hardware requirements of the condenser and the pump are conflicting as seen in Figures 5 and 6, suggesting there is an optimal value to the temperature rise. A rough estimate indicates that the optimal temperature rise seems to fall between 10℃ and 15℃. The impacts of the steam temperature on the hardware requirement of the condenser and the plant efficiency are expected to be conflicting also, since higher steam temperature tends to deteriorate the plant thermal efficiency. Such an effect of the steam temperature on the plant efficiency is examined in the next section.
Figure 5. Required heat transfer area normalized by conventional plant value,
Figure 6. Required pump capacity normalized by conventional plant value,
Plant efficiency and economic feasibility
Since the design net electric power is kept at 600 MWe in the computation, the heat input requirement represents the reciprocal of the plant efficiency. Figure 7 shows the deviation in heat input from the conventional plant. The results suggest that a 3% saving in the heat input or fuel can be realized in the deep-sea water plant if a steam temperature of 26.0℃ and a temperature rise of 12℃ are adopted.
An evaluation of economic feasibility is impossible without a clear specification of the site conditions. In particular, the installation cost of the intake pipe for deep-sea water strongly depends on oceanographic factors such as wave and typhoon conditions. If the cost-savings of the condenser and pump compensate for the installation cost of the intake pipe for deep-sea water, the construction of a deep-sea water plant may be economically feasible. Further detailed study is needed for a concrete estimation, though that is not within the scope of the resent stud.
Figure 7. Heat input required for 600 MWe electrical output (base: HO=heat
input of a conventional plant using surface seawater)
The present study shows the beneficial effects of deep-sea water on plant performance. In northern and central Japan, the use of deep-sea water seems effective in eliminating the dip in electric power generation during summer, which is a problem the conventional plants using surface seawater currently suffer from. In southern Japan, the use of deep-sea water seems effective in raising plant efficiency with a newly designed plant suitable for deep-sea water usage, because a cold heat sink is preferable for high plant performance. The use of deep-sea water in an existing plant in southern Japan did not prove very effective in that regard, because the seawater condition does not fit the condenser design which is optimized for a surface seawater condition, i.e., a much higher seawater temperature.
Perceived concerns about deep-sea water usage include biological and chemical issues, and the possible adverse effect of cold deep-sea water discharge, which can be lower than that of the environmental seawater temperature during summer. The unusual transport phenomena of such thermal flows may be of interest to fluid engineers and scientists.
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