EXPLAINATION
This explaination is with reference to the Question
asked by me during the Guest Lecture on SHP, arranged on 4th of
February, 2014.Given below is the question raised, followed by the explaination
I found out appropriate.
Please read and feel free to comment on the same.
Que.:
It was told by the Resp. professor that In a Hydroelectric Power plant,
Potential Energy is converted into Kinetic Energy. On that my question was why
we consider the total head to be H? Because, H is the total height from datum
to the free surface of water. But, while calculating potential energy we
consider the height from datum to the Centre of Gravity/Centre of Mass of the
system and not it’s upper boundary. Thus, we should consider H till the CoG of
the water reservoir…. and not it’s upper surface…
Proposed Answer: The Question has nothing to do with Potential
Energy. (Recalling physicist W. Pauli’s statement- “Das ist ganz Falsch.”(that
is all wrong).)
The answer lies in Bernoullis Equation(not modified,
simple Bernoullis Equation.)
If we apply Bernoulis Equation between the two points-
1. Water level in
the reservoir(Dam) and
2.
At inlet to the turbine, we get-
where, LHS refer to the reservoir upper level
and RHS refer to turbine inlet
P : atmospheric
pressure and is thus added on both sides.
Now, multiplying both sides with rg, we get -
<=>
Here, we observe that the RHS is equivalent to Kinetic
energy per unit volume
(
= 
=
Now, for this case volume can only be expressed in
terms of discharge Q (
).
).
multiplying both sides by Q, RHS turns out to be
Kinetic Energy supplied per unit time i.e. input power and it equals LHS = 
Hence, we consider the H from datum till the free
surface of water in the reservoir, and not till the CoG/CoM.
N.B. This answer is not to disrespect/dishonour
the professor/organizers; indeed, I learned valuable new things about SHP from
him. But, this question was discussed by professor & some students after
the lecture with me. And that query needed to be answered… I personally think
that we should be good at experience and formula both. So that when we get
answer of 6 Kms, we should be able to find where we went wrong &
accordingly modify or term a new formula. We should have Practical knowledge,
as said in ‘3 idiots’. But, the question is should we blindly follow Aamir
& throw away all “formulas & definitions” or we should be Honest with
Engineering??
I choose the latter…
Kalpesh N. Kothale
(BE-Mech., 9421794652)
