The large power cycles refer to Rankine type power generation cycles of capacity 660 MW, 800 MW and 1000 MW. Present cycles are either subcritical (SubC), super critical or ultra supercritical. The super critical and ultra super critical (SC/USC) are generally of very high cycle efficiencies as compared to the sub critical cycles and hence their heat losses are also lower.
Basically, as the energy efficiency of the power cycle increases the cooling water requirement, the auxiliary power and water make up all are reduced.
The change over from sub-critical to super critical and ultra supercritical power cycles results in considerable saving in cooling water requirements as well as power for circulating the cooling water. Improving the power cycle energy efficiency automatically improves the efficiency of the cooling water cycle.
The heat losses in a thermal power plant are:
Dry and wet flue gas losses which exit the plant through the stack. These losses generally cannot be recovered though a few technologies are in vogue for reducing the losses.
Heat losses in auxiliary power utilization system. The auxiliary power forms around 6-8 % of the maximum continuous rating of the unit (MCR).
The auxiliary motor efficiency is around 92% with a thermal loss of 8%. Out of this only 1% of the loss is accounted for by heat generation in bearings and being cooled through the bearing cooling water. Hence, only 0.08% of the MCR power is being lost as bearing cooling loss. This is a negligible quantity as compared to the major loss in the turbine cycle.
Heat generated in the operation of the generator. These are the generator cooling losses and represented by the heat carried away by hydrogen and the tertiary cooling water. The generator efficiency is quite high of the order of 98.6%. The heat losses are of the order of 1.1-1.2% of the MCR capacity. Theselosses are also quite low.
Energy required to condenser the steam from saturated vapour or in the two phase region to saturated liquid. This is the largest loss of thermal energy in the Rankine type power cycles. This is almost two times in magnitude of the power generated in the plant.
The cooling technologies available for cooling of the steam being condensed in the condenser for large power generating cycles are:
Natural draft cooling tower
Forced draft cooling tower
River/canal water cooling
Sea water cooling
Normally for coastal plants sea water cooling is being adopted because of the availability of sea water. Sea water cooling has the disadvantage that the normal cupro-nickel tubing for the heat exchange between the steam and the water in the condenser needs to be changed to titanium tubing which is quite expensive and with poorer heat transfer coefficient.
Also tube leak with sea water can result in very serious corrosion issues in the boiler and turbine due to the carry over of the salinity in sea water into the high purity steam cycle feed water.
The detection systems for the Water purity has to be extremely efficient in rapid detection of leaks since it will take time for identification and plugging of leaking tube.
The forced draft cooling towers have additional power requirement for operation of the forced draft fans…
The parameters which affect the performance of these cooling technologies are:
Quantum of water hold up required at the plant
Quantum of water make up to compensate for permanent water losses due to evaporation, percolation, drift, etc.
Auxiliary power for running the system
Results of study
The typical performance of the three types of plants are given in Table 1.
The theoretical cooling water flow required for cooling the turbine cycle is given in Table 2. Table 3 presents the conversion factors for the design and operating water requirements.
The normal operating temperature difference (temperature gain in cooling water) is of the order of 10 °C.
However, the same can be reduced to 9 of 8 °C due to either poor condenser performance or higher inlet cooling water temperature [(lower terminal temperature difference (TTD)].
The theoretical water requirements for 660 MW, 800 MW and 1000 MW, sub critical, super critical and ultra supercritical plants is given in Figures 1-3.
The conversion factors for the design and operating water requirements are given in Table 3.
The sensitivity of the cooling water requirement to energy efficiency of the plant is given in Figure 4.
The comparison of the four cooling technologies in terms of auxiliary power, water hold up, cooking water flow and make up are given in Table 5.
Typical design or operating water requirements for the, sub critical, super critical and ultra supercritical plants of 660 MW are given in Table 4.
Conclusions
For inland power plants natural draft cooling towers are by far the best. The forced/induced draft cooling towers have an additional power requirement of 0.5 % of MCR for the fans.
The drift losses in forced/induced draft cooling towers are slightly on the higher side but do not significantly affect the make up.
Sea water cooling is ideal for coastal plants. However, the risk of contamination and the secondary damage leading to corrosion needs to be addressed by rapid detection and leak plugging technologies.
