96 CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
for θ = 2°, Kbend = 0.02
for θ = 3°, Kbend = 0.02
for θ = 12°, Kbend = 0.06
for θ = 8° , Kbend = 0.04
for θ = 3° , Kbend = 0.02
Bend losses = (0.34 + 0.11+ 0.13 + 0.18 + 0.20 + 0.02 + 0.02 + 0.06 + 0.04 + 0.02) x(3.52 / 2g) = 0.70 m
Total head loss = 12.29 m + 0.12 m + 0.70 m = 13.11m
% head loss = (13.11 / 180) x 100% = 7.28%,
Since the head loss is between 5% to 10%, in case of micro-hydropower plant, this could have been accepted. However, since this
is a mini-hydropower plant (500kW installed capacity), a 450mm diameter was adopted which gives a head loss of 7.61m (4.2%)
based on 'optimisation' which is explained below:
The power gained by increasing the pipe diameter from 405 mm to 450 mm at a conservative
60% efficiency =(12.69 -7.38) x 0.450x9.81x0.6
= 14.1 kW
Annual energy gained assuming an overall plant factor of 40%
= 14.1 x 24 x 364 x 0.4
= 49,271 kWh
Expected annual incremental income at Rs 4.0 per kWh of electricity generated
= Rs. 197,084
Note: The Jhankre power plant was initially implemented to provide partial construction power at the intake site for the then under
construction 60 MW Khimti Small hydropower plant. The alternative source of energy was diesel that had a cost of generation of
Rs 4.0 per kWh.
Now the question is, “Is this gain of Rs 197,084 worth the increase in pipe diameter?
The next step is to calculate the Net Present Value of Benefits and compare it with the incremental costs.
NPV of Benefits assuing:
- discount rate i = 10%,
- Economic live, n = 15 years
- Annual income, A = Rs. 197,084
The equation for calculating NPV is: NPV benefit = A
Or NPV benefits = Rs.1,499,336
(1+i)n-1
i(1+i)n
Now the incremental cost in increasing the pipe diameter should be considered:
Assume average pipe thickness = 4.5 mm:
Additional weight (recall pipe length = 550 m)
= P(0.450 –0.405)x4.5 x7.85x550= 2747 kg
Note: A 1 m2 with 1 mm thickness steel plate weighs 7.85 kg and thus 4.5 is multiplied by 7.85 above to calculate the weight
AT Rs 200/kg market price including fabrication transport, site welding & installation the
NPV costs = NRs. 200/kg x 2747 kg = NRs 549,400
Thus, the NPV of incremental costs in increasing the pipe diameter is 36% of NPV of benefit which justifies the decision to adopt
450 mm diameter. However, the following should be noted:
1. Increasing the diameter also somewhat increases the transport and installation costs. The anchor blocks volume would also
increase. As these factors have not been taken into account, if the NPV of costs were close to NPV of benefits it would not have
been worth increasing the pipe diameter. However, at NPV costs = 36% of NPV benefits the decision is well justified as
differences are high