Page 165 - Solar Power in Building Design The Engineer's Complete Design Resource

PUMP OPERATION CHARACTERISTICS 135

Off-grid capability customers can elect to run a pump or motor 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.
The 2003 power outages in the Northeast and Midwest highlighted a critical appli-
cation of this proprietary solar technology. The systems described here are capable of
driving pumps or ac motors up to 600 hp as backup for grid power or in combination
with the grid or other power sources, such as diesel generators. The systems can also
be assembled for mobile or emergency use or used as part of a permanent power instal-
lation. In either case, they can provide invaluable power backup in emergency power
outages and can operate independent of the electric grid, relying instead on the con-
stant power of the sun.

Pump Operation 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 experi-
enced 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.
The 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-feet = 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. The pres-
sure drop in the piping to the pump consists of friction losses as the water passes
through the pipe, fittings, and valves. The fittings and valves are converted to equiva-
lent lengths of straight pipe (from a piping manual) and added to the actual run to get
the equivalent length of suction piping.