This is in contrast with the belief that slip, due to a relative rotational
velocity within the blade passages, is only present in pump
mode such that flow should be increased in order to minimise
incidence losses. However, a degree of slip is also present in turbine
operation, and an increase in incidence loss due to higher flow is
balanced in some degree by the mechanical efficiency. This
decreasing influence of mechanical losses can be seen in Fig. 6
whereby the overall efficiency approaches the hydraulic efficiency
at normalised flow Q/QBEP
> 1.
Of practical interest is the general flatness of the efficiency map,
particularly around the BEP. This is also shown in Fig. 5, presented
for three cases of constant specific energy corresponding to tank
minimum supply level, BEP condition and tank maximum supply
level. The variation in head represents a range of static head between
approximately 0.85HBEP and 1.2HBEP. The efficiency in each
case remains considerably high even with significant decreases in
flow. Indeed, in all cases a 30% flow reduction is required to see a
10% reduction in overall efficiency, and a further 20% decrease in