To reduce capital cost, most site developers choose standard-design hydroturbines.
With essentially every high-head site developed, low-head sites become more attractive to
developers. Table 5 shows the typical performance characteristics of hydroturbines being
used today. Where there is a region of overlap in Table 5 or Fig. 18, site-specific parame-
ters dictate choice and whether to install large units or a greater number of small units.
Delivery time and ease of maintenance are other factors important in unit choice. Fur-
ther, the combination of power-generation and irrigation services in some installations
make hydroturbines more attractive from an environmental view because two objectives
are obtained: (1) "clean" power, 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 18m) have performance characteristics similar to those of Francis and tu-
bular 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 com-
pactness 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 induc-
tion 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 multi-
ple 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 in-
creases, equipment costs approach those for a standard turbine. Further, the complexity of
the site increases with the number of reverse pump 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 to minimize remaining en-
ergy after the water exists from the runner blades. Other improvements being sought for
pumps are: (1) modifying the runner-blade profiles or using a turbine runner in a pump
casing, (2) adding flow-control devices such as wicket gates to a standard pump design or
stay vanes to adjust turbine output.
Many components of hydroturbines are being improved to reduce space requirements
and civil costs, and to simplify design, operation, and maintenance. Cast parts used in old-
er turbines have largely been replaced by fabricated components. Stainless steel is com-
monly recommended for guide vanes, runners, and draft-tube inlets because of better re-
sistance to cavitation, erosion, and corrosion. In special cases, there are economic tradeoffs
between using carbon steel with a suitable coating material and using stainless steel.
Some engineers are experimenting with plastics, but much more long-term experience
is needed before most designers will feel comfortable with plastics. Further, stainless
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