Page 159 - Alternative Energy Systems in Building Design

SOLAR POWER SYSTEM DEPLOYMENT 135

islanding provisions of the interconnection rules, when the power to the grid is off, the
pump or motor keeps operating from solar power alone without interruption. This
solar pumping system is the only one of its kind; other grid-tied solar systems shut
down when grid power is interrupted.
In an off-grid mode, the pump or motor can be run off-grid on solar power alone. This
may be useful if there is a time of day when, for example, running the pump or motor
would incur a large demand charge that the customer wishes to avoid. The system makes
operation still possible while avoiding peak-demand charges imposed by the utility.

Pump performance characteristics The following discussion is presented to
introduce design engineers to various issues related to pump and piping operational
characteristics that affect power-demand requirements. In general, the pumping and
piping design should be trusted to experienced and qualified mechanical engineers.
Every cooling tower requires at least one pump to deliver water. Pump selection is
based on the flow rate, total head, and ancillary issues such as type, mounting, motor
enclosure, voltage, and efficiency.
Pumping volume is dictated by the manufacturer of the equipment. The total head
(in feet) is calculated for the unique characteristics of each project as follows:

Total head = net vertical lift + pressure drop at cooling-tower exit
+ pressure drop in piping to pump + pressure drop from pump to
.
destination storage compartment + pressure drop of storage + pressure
drop through the distribution system + velocity pressure necessary
to cause the water to attain the required velocity

The total head is usually tabulated as the height of a vertical water column. Values
2
expressed in pounds per square inch (lb/in ) are converted to feet by the following
formula:
Head, ft = pounds per square inch × 2.31

The vertical lift is the distance the water must be lifted before it is let to fall. Typically,
it is the distance between the operating level and the water inlet. Pressure drop in the
piping to the pump consists of friction losses as the water passes through the pipe, fit-
tings, and valves. Fittings and valves are converted to equivalent lengths of straight
pipe (from a piping manual) and added to the actual run to get the equivalent length
of suction piping.
Then the tabulated pressure drop from the piping manual for a specific length of
pipe is compared with the length and pressure drop calculated by proportion:

Pressure drop = pressure drop for specific length of pipe
× equivalent pipe length/pipe

Typically, end-suction pumps are selected and are of the close-coupled type (where
the pump impeller fastens directly to the shaft, and the pump housing bolts directly