EXPLAINATION

This explaination is with reference to the Question asked by me during the Guest Lecture on SHP, arranged on 4^th 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)