steel material costs are relatively low compared to labor costs. And stainless steel has
proven most cost-effective for hydroturbine applications.
While hydro power does provide pollution-free energy, it can be subject to the va-
garies of the weather and climatic conditions. Thus, at the time of this writing, some 30
hydroelectric stations in the northwestern part of the United States had to cut their electri-
cal output because the combination of a severe drought and prolonged cold weather
forced a reduction in water flow to the stations. Purchase of replacement power—usually
from fossil-fuel-fired plants—may be necessary when such cutbacks occur. Thus, the
choice of hydro power must be carefully considered before a final decision is made.
This procedure is based on the work of Jason Makansi, associate editor, Power maga-
zine, and reported in that publication.
USE OF SOLAR-POWERED PUMPS IN
IRRIGATION AND OTHER SERVICES
Devise a solar-powered alternative energy source for driving pumps for use in irrigation
to handle 10,000 gal/min (37.9 m^3 /min) at peak output with an input of 50 hp (37.3 kW).
Show the elements of such a system and how they might be interconnected to provide
useful output.
Calculation Procedure:- Develop a suitable cycle for this application
Figure 19 shows a typical design of a closed-cycle solar-energy powered system suitable
for driving turbine-powered pumps. In this system a suitable refrigerant is chosen to pro-
vide the maximum heat absorption possible from the sun's rays. Water is pumped under
pressure to the solar collector, where it is heated by the sun. The water then flows to a
boiler where the heat in the water turns the liquid refrigerant into a gas. This gas is used to
drive a Rankine-cycle turbine connected to an irrigation pump, Fig. 19.
The rate of gas release in such a closed system is a function of (a) the unit enthalpy of
vaporization of the refrigerant chosen, (b) the temperature of the water leaving the solar
collector, and (c) the efficiency of the boiler used to transfer heat from the water to the re-
frigerant. While there will be some heat loss in the piping and equipment in the system,
this loss is generally considered negligible in a well-designed layout. - Select, and size, the solar collector to use
The usual solar collector chosen for systems such as this is a parabolic tracking-type unit.
The preliminary required area for the collector is found by using the rule of thumb which
states: For parabolic tracking-type solar collectors the required sun-exposure area is 0.55
ft^2 per gal/min pumped (0.093 m^2 per 0.00379 m^3 /min) at peak output of the pump and
collector. Another way of stating this rule of thumb is: Required tracking parabolic solar
collector area = 110 ft^2 per hp delivered (13.7 m^2 /kW delivered).
Thus, for a solar collector designed to deliver 10,000 gal/min (37.9 m
3
/min) at peak
output, the preliminary area chosen for this parabolic tracking solar collector will be, Ap =
(10,000 gal/min)(0.55 ft
2
/gal/min) = 550 ft
2
(511 m
2
). Or, using the second rule of thumb,
Ap = (UO)(SO) = 5500 ft
2
(511 m
2
).
Final choice of the collector area will be based on data supplied by the collector man-
ufacturer, refrigerant choice, refrigerant properties, and the actual operating efficiency of
the boiler chosen.