Page 273 - Handbook of Energy Engineering Calculations

and (2) crop watering.
Maintenance considerations are paramount with any selection; each day of
downtime is lost revenue for the plant owner. For example, bulb-type units
for heads between 10 and 60 ft (3 and 18 m) have performance characteristics
similar to those of Francis and tubular units, and are often 1 to 2 percent more

efficient. Also, their compact and, in some cases, standard design makes for
smaller installations and reduced structural costs, but they suffer from poor
accessibility. Sometimes the savings arising from the unit’s compactness are

offset by increased costs for the watertight requirements. Any leakage can
cause severe damage to the machine.
To reduce the costs of hydroturbines, suppliers are using off-the-shelf
equipment. One way this is done is to use centrifugal pumps operated in
reverse and coupled to an induction motor. Although this is not a novel

concept, pump manufacturers have documented the capability of many
readily available commercial pumps to run as hydroturbines. The peak
efficiency as a turbine is at least equivalent to the peak efficiency as a pump.

These units can generate up to 1 MW of power. Pumps also benefit from a
longer history of cost reductions in manufacturing, a wider range of
commercial designs, faster delivery, and easier servicing—all of which add
up to more rapid and inexpensive installations.
Though a reversed pump may begin generating power ahead of a turbine

installation, it will not generate electricity more efficiently. Pumps operated
in reverse are nominally 5 to 10 percent less efficient than a standard turbine
for the same head and flow conditions. This is because pumps operate at

fixed flow and head conditions; otherwise efficiency falls off rapidly. Thus,
pumps do not follow the available water load as well unless multiple units are
used.
With multiple units, the objective is to provide more than one operating
point at sites with significant flow variations. Then the units can be

sequenced to provide the maximum power output for any given flow rate.
However, as the number of reverse pump units increases, equipment costs
approach those for a standard turbine. Further, the complexity of the site

increases with the number of reverse pumps units, requiring more
instrumentation and automation, especially if the site is isolated.
Energy-conversion-efficiency improvements are constantly being sought.
In low-head applications, pumps may require specially designed draft tubes