TYPICAL  PRELIMINARY DESIGN PROCEDURE FOR DAM BASED SCHEMES

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The Sathanur Dam is situated in Dharmapuri District on the River Ponnair. The average annual yield from the catchment of 10,826 square kilometers is 410 MCM which is released through five dam sluices of 1.52m width and 1.83m height. About 13 cumecs is discharged continuously from the sluices into two irrigation canals. Two of the existing five sluices would be utilized for the proposed mini hydro scheme. The original proposal by TNEB was to install 2 x 7.5MW and to produce 27.7 GWh/year.

Flow Duration Curve: The revised proposal is based on daily discharge data at Sathanur over a 15 year period. From the flow Duration curve shown, it is apparent that multiple units could be installed to utilize 10 cumecs each; 10 cumecs would be available for the first unit for upto 73% duration; an additional 10 cumecs would also be available to the second unit for about 30% duration; and so on upto 70 cumecs for less than 10% duration.
Head Duration Curve: The head was found to be 20 meters for 60-70% duration, between 20 to 25 meters for 50-60% duration, and 25 to 30 meters for 35-50% duration. A design head of 27.5m, which corresponds to the 40% duration, was selected. Hence for this study, the design head is assumed to be 27.5 meters. Hence, by adopting a design discharge of 10 cumecs per unit, units of 2500kW would be feasible.
Energy Curve: The energy curve produced based on the average flows and the design head , provides the following information on the incremental energy that could be recovered at the dam by increasing the design discharge level in steps of 10 cumecs. Assuming 80% efficiency of the turbine-generator sets, the energy output and B/C rations from a multi unit configuration are given in the table.

INCREMENTAL ENERGY

Unit

Energy 
(GWh)

B/C Ratio

1 14.0 2.06
2 8.0 1.69
3 2.0 0.42
4 1.4 0.29
5 1.0 0.21
6 0.8 0.17
7 0.2 0.04
Capacity (kW)

2 x 2000

2 x 2500

2 x 3500

2 x 5000

Energy Output (GWh) 19.2 22.5 25.0 25.6
Plant Load Factor (%) 0.6 0.5 0.4 0.3

Cost Estimates

Civil Structures 6.5 7.0 10.5 14.0
Electro-mech 7.4 8.8 12.3 15.6
Electricals 9.3 11.2 18.4 24.4
Grid-Tie 2.7 2.7 2.7 2.7
Total 25.9 29.7 43.9 56.7
Investment (Rs/kw) 6489 5940 6271 5670
Energy Productivity (kWh/p.a/100 Rs.) 74.1 75.7 54.7 45.2
The original proposal to install 2 x 7500kW requires the use of two sluices to handle the entire 70 cumecs of discharge from the dam; this would not be feasible. However, the above analysis of unit benefit-cost ratios indicated clearly that it would not be economic to use more than two units. After some iteration and analyses of benefit-cost ratios of units for alternative configurations, the relative energy productivity with the two unit configuration for four alternatives were evaluated. As indicated in the table, the results confirm that the 2 x 2500kW would have the highest energy productivity.

Analysis of Annual Energy Productivity

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