In the late 1980’s,
the question “Does improving pumping plant efficiency save energy?”
was posed to a group of growers and utility company representatives
in Salinas. Most of the growers said no, while most of the utility
employees said yes. Before this meeting the utility company had spent
a great deal of money to improve the efficiency of irrigation pumping
plants in the Salinas area. The goal was to improve the ratio of pump
output horsepower to input.
Pump efficiency is a measure of the amount of power produced by the
pump per unit of input power. The power (rate of energy use) produced
by a pump, called the water horsepower, depends on the pump capacity
or flow rate, pressure developed by the pump, and amount of pumping
lift in the well (pumping lift is the elevation difference between
the discharge of the pump and the pumping water level in the well).
The higher the efficiency, the more power that is produced per unit
of input power.
Electricity users pay for energy based on kilowatt-hours(kwh) consumed
(plus fixed charges). The kilowatt is the power demand of the electric
motor; 1 kilowatt equals 1.34 horsepower. The hours are the operating
time of the motor. To reduce electrical energy use, the kilowatt-hours
must decrease because of fewer kilowatts or less operating time, or
both. Regardless of the claims about a proposed energy-saving measure,
if the number of kilowatts or the operating time is not reduced, no
energy savings will occur.
Improving Efficiency
Options for improving pumping plant efficiency include adjusting impellars,
repairing or replacing worn pumps, replacing mismatched pumps, and
converting to energy efficient electric motors. The goal is to improve
the ratio of pump output horsepower to input horsepower; in other
words, to make the plants more efficient by pumping a greater volume
or pressure of water while using the same or less energy.
Adjusting Impellars
Maintaining the appropriate clearance between the bottom of the vanes
of a semi-open impellar and the bowl housing is critical for efficient
pump perfomance. Wear caused by sand in the well water can increase
the clearance between the impellars and housing, and reduce pumping
plant efficiency. Efficiency can be partially restored by adjusting
the impellars. This involves slightly lowering the pump shaft and,
inturn, the impellars, by rotating the nut at the top of the shaft.
This adjustment will not work for enclosed impellars.
When we adjusted the impellars of four pumps, both pump capacity and
overall efficiency increased considerably. Total head increased slightly,
because pumping lift only contributed to total head. However, often
times, impellar adjusments increase input horsepower. Therefore, if
the pumps are operated for the same amount of time after the adjustment
(a common practice) energy use will increase. Energy use will only
decrease if the operating time is decreased by pumping the same volume
of water after the adjustment as before. The increase in pump capacity
can be insufficient to offset the increase in input horsepower because
part of the increased pump output also contributes to the increase
in total head.
Repairing Worn Pumps
Repairing a worn pump can increase capacity, total head and overall
efficiency. A summary of 63 data sets of pump performance before and
after repair shows increases of 39%, 0.5% and 33% in pump capacity,
total head and overall efficiency, respectively. The small increase
in total head occurred because pumping lift was the main contributor.
However, the repair increased the input horsepower for 58% of the
pumping plants, wth an average increase of 17%. For these pumping
plants, using the same operating time before and after the pump repair
will increase energy use by 17%. However, pumping the same volume
will decrease the average energy use by 22%. For many pumping plants,
reducing the operating time may be necessary to realize any energy
savings from pump repairs.
Operators of irrigation pumping plants commonly run repaired or adjusted
pumps for the same amount of time after repairs or adjustments as
they did before.
Replacing Mismatched Pumps
A performance charcteristic of deep-well turbine and centrifugal (booster)
pumps is that as pump capacity increases, pump efficiency increases
to a maximum and then decreases. New pumps should be selected to provide
the desired flow rate and total head near the point of maximum efficiency,
which minimizes the horsepower demand of the pump. Initially efficient
pumps can become inefficient because of changes in operating conditions,
such as different groudwater levels or alterations in discharge to
pressurized irrigation systems, even though the pump is operating
properly (no wear).
A pump operating properly but not near the point of maximum efficiency
is said to be mismatched to the operating conditions. Used pumps are
also candidates for being mismatched. To resore the pumping plant’s
efficiency, the mismatched pump must be replaced with one providing
the desired total head and capacity near maximum efficiency. This
change reduces the kilowatt demand of the pump and results in energy
savings even if the operating time is unchanged.
Using Energy-Efficient Electric Motors
Energy-efficient electric motors need less input horsepower than standard
motors. Buying an energy-efficient motor for a new irrigation pumping
plant is more economical than retrofitting an existing pumping plant.
For example, and energy-efficient, 100 horsepwer motor can cost $6,000,
compared with $5,000 for a standard motor. The input horsepower of
the energy-efficient motor will be 104 compared with 109 for the standard
motor. At a typical cost of $0.1 per kilowatt-hour operating the pump
for 2000 hours per year will save $746, with a simple payback of 1.3
years. The payback period for retrofitting, on the other hand, is
8 years.
Energy Savings
Simply imroving pumping plant efficiency does not guarantee energy
savings. In fact, adjusting or repairing worn pumps may increase energy
use unless the operating time of the pumping plant is reduced. Sometimes,
adjusting the operating time still will not save energy if part of
the increase in pump output contributes to a significant total head
increase in addition to a capacity increase. With a higher flow rate,
pumping plant operators can reduce operating time, with either less
irrigation time per set or greater acreage irrigated per set. Opportunities
for reducing the operating time will depend on site-specific conditions,
such as the irrigated method and its design and management characteristics.
If reducing the operating time is not possible, the improved efficiency
may result in more crop yield and revenue due to more water applied
to a field.
Multiple pump tests (at least three) of a pumping plant are recommended,
to help evaluate possible reasons for low efficiency. Pumping plant
operators should also obtain the manufacturer’s performance
curves to use in the evaluation process.
