However, based on first principles from physics, the power
equation should be
P=eo x γ x Q x hgross
where:
Other variables are same as above and
γ is the unit weight of water = 9.8 kN/m3 Note that in the
power equation based on first principles,
γ is replaced by ‘g’ and since both have the same absolute
value (9.81), the results are the same, i.e., same power output
in kW for same flow (m3/s), gross head (m) and overall system
efficiency
Net head, hn, is the pressure head at the entrance to the
turbine. That is, the gross head minus conveyance losses in
the penstock. For micro-hydropower schemes the penstock is
generally designed such that the net head is 90-95% of the
gross head measured from the forebay (refer to Section 6.4
for penstock sizing).The overall system efficiency, eo, is the
ratio of useful power output to hydraulic power input. It is the
product of separate efficiencies for several components of the
system, i.e.,
e = ep et eg etr
where,
ep is the penstock efficiency, typically 0.90 - 0.95 (hn=hgrossep)
et is the turbine efficiency, typically 0.65 - 0.80 depending on
turbine type
eg is the generator efficiency, typically 0.65 - 0.90 dependingon
size
etr is the transmission efficiency including transformers if
used, typically 0.85 - 0.90
For preliminary planning of micro-hydropower schemes in
Nepal it is common to assume an overall system efficiency of
0.5 to 0.6. However, it may be as low as 0.3 for very small
installations and as high as 0.7 for larger schemes. Therefore
at detailed design stage it is important to recalculate the
power output based on the actual design and manufacturers’
data for the proposed equipment.