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  The Economics of Energy Conservation in Turf Irrigation*
by
Kenneth H. Solomon

CATI Publication #880803
© Copyright August 1988, all rights reserved

ENERGY SAVINGS
Reducing the pressure requirements of an irrigation system will result in reducing the horse-power required to run that system. Pressure reductions may come from the use of special products designed for lower operating pressures, or from the use of products with lower friction losses. Other factors which influence the energy savings due to reduced pressure requirements are the system flow rate per unit area, and the gross yearly application. The first of these relates to how fast the water must be pumped (GPM) while the second relates to how long the system must be run (hours per year).

An important consideration is whether or not the change contemplated to reduce the pressure requirement will also affect the irrigation uniformity and efficiency of the system. If the irrigation efficiency goes down, you may need to apply more total water to supply the moisture needs of the turfgrass. The extra energy required to pump this extra water may more than offset the energy you save by reducing the operating pressure. It does no good to operate sprinklers at lower pressures than they were designed for if that destroys the distribution pattern, causing dry spots which can only be eliminated by over-watering the rest of the area. The articles (see References) by Gilley and Watts (1977) and Gilley and Mielke (1979) provide good discussions of this point. The remainder of this article will consider situations where it is reasonable to assume that pressure reductions will not cause any reductions in irrigation efficiency.

Table 1. Energy Savings from a 1 PSI Reduction in Operating Pressure
Annual Energy Savings Gross Water per PSI of Application Pressure Reduction
(inches/year) (BHP-HRS/Acre)
12 4.5
18 6.8
24 9.1
30 11.3
36 13.6
42 15.8
48 18.1

The data in the Table 1 above illustrate the savings in Brake Horse Power Hours (BHP-HRS) per acre of irrigated area that result from a reduction in operating pressure of one PSI. (An acre equals 43,560 square feet. (A football field, with-out the two end zones, is approximately 1 acre in size.) A pump efficiency of 70% has been assumed in developing this table.

Suppose a golf course with 100 acres of irrigated area was located in climate zone where 36 inches of irrigation were required annually. A one PSI reduction in the pressure requirements for that system would save 13.6 BHP-HRS per acre, or 1360 BHP-HRS each year. To see whether or not this is financially significant, you need to consider the type of fuel and the price of the fuel you use. Different fuels contain different amounts of useful energy, and of course prices differ. The amount of energy in different fuels can be summarized in a conversion factor that gives the number of BHP-HRS per unit of fuel. Commonly used conversion rates for different types of fuel are given in Table 2. (A range of values for each fuel may be found in the literature, but the values in Table 2 are indicative of common irrigation situations.)

Dividing the BHP-HRS saved by the conversion rate listed for your fuel will give the energy savings in units of fuel per year. Multiply this by the price of fuel and you'll have the annual energy savings due to reduced operating pressure in dollars per acre per year.

Consider further the example of the golf course with 100 acres of irrigated area and an annual water application of 36 inches. As noted before, a one PSI reduction in pressure requirements saved 1360 BHP-HRS per year. Suppose the golf course uses electric motor driven pumps, and that electricity costs them $0.08 per kilowatt-hour. The conversion factor for electricity is 1.1 BHP-HRS per kilowatt-hour. Dividing 1.1 into 1360 gives 1236 kilowatt-hours per year saved (1360/1.1 = 1236). At $0.08 per kilowatt-hour, the economic value of that energy saving is $98.88 per year (1236 x $0.08 = $98.88). Remember, this is the savings associated with a pressure reduction of only one PSI. A reduction of 10 PSI would save the golf course $988.80 each year.

Table 2. Energy Conversion Rates
Fuel Unit of Fuel BHP-HRS per Unit of Fuel
Electricity Kilowatt-Hour 1.1
Gasoline Gallon 11.5
Diesel Gallon 14.6
Propane Gallon 9.2
Natural Gas 100 cu. ft. 8.9
In some cases, additional savings may be possible. In addition to the charge for each kilowatt-hour, most users of electric powered irrigation pumps pay a "connected load" or "demand" charge, which depends on the maximum motor or pump horsepower that might be drawing power from the utility. If the pressure reduction is enough to allow a horsepower reduction, a lower demand charge would result. Some industry observers feel that demand charges are likely to become a larger portion of irrigators' electric bills in the future.

Some utilities are now introducing "time-of-use" rate programs, whereby the cost of electricity depends not only on the amount used, but on the time during the day when it is used. Electric rates during the peak load time of day may be substantially higher than the average cost per kilowatt-hour. Contact your utility company representative for further details regarding demand charges and time-of-use rate programs in your area.

It's not hard to find out the current energy prices in your area, but it is a little more difficult to project what the energy prices will be for next year and on into the future. A look at historical trends indicates that energy prices may change at a rate different than that of other operating costs. Over the period 1973 - 1982 for example, operating costs increased on average at about 11% per year. Over that same period, however, the cost of energy for irrigation pumping in California increased by over 20% per year (Moore, 1981). Presently, energy costs don't seem to be increasing as fast as they were during the mid to late 70's.

FINANCIAL ANALYSIS
In determining what energy savings can mean to you financially, two factors must be considered: the energy cost inflation rate, and the time value of money. If energy costs are going up, an energy conservation measure will save you more dollars next year than it will this year. On the other hand, you won't have the benefit of those savings dollars until next year either. In terms of today's dollars, those future savings must be discounted.

The dual effects of energy cost inflation and discounting of future savings can be combined into a "Present Value Factor." This represents the value in terms of today's dollars of a series of annual energy cost savings. Tables 3 and 4 below list Present Value Factors for different interest rates, energy cost inflation rates and time periods. The time period is the economic life of the equipment involved in the energy conservation measure. Chu (1980) and Pearson (1974) discuss equations for the Present Value Factor. They use slightly different assumptions regarding exactly when costs and savings occur during the year, and there-fore derive slightly different formulae. The values tabulated here were computed according to the equations of Pearson (1974).

The annual energy savings, based on today's energy price, when multiplied by the Present Value Factor will yield a value which might be called the Present Value of Projected Energy Savings. This value is significant because it rep-resents the maximum investment that you should make to actually achieve the pressure reduction being considered.

Consider again the golf course example. Recall that a one PSI pressure reduction saved 1360 BHP-HRS per year for that golf course. With electricity at $0.08 per kilowatt hour, this amounts to a $98.88 savings per year. Suppose that the product associated with this energy conservation measure has a life of 8 years, that the energy cost inflation rate is 5%, and that the interest rate is 10%. From Table 3, the Present Value Factor is 6.22. Multiplying this by the annual savings will give the value of the energy savings over the 8 year life of the conservation measure. The total value of the savings is $615.03 (6.22 x $98.88 = $615.03). If it costs you less that $615.03 to make the change that saves one PSI in pressure requirements, you should do it.

Table 3. Present Value Factors - 8 Year Economic Life
Interest ----Energy Cost Inflation Rate----
Rate 5% 10% 15% 20%
5% 7.62 9.02 10.70 12.74
10% 6.22 7.27 8.54 10.06
15% 5.17 5.99 6.96 8.11
20% 4.38 5.01 5.77 6.67

An energy conservation measure that would reduce the pressure requirement by 10 PSI on this golf course would save $988.80 per year. If that measure had an economic life of 15 years, the Present Value Factor would be 10.05 (see Table 4), and the present value of the total savings would be $9,937.44. So you see, energy conservation can pay off pretty well.

FURTHER EXAMPLES

Example 1 - Low Friction Loss Valves
Problem - An automatic sprinkler system is used to irrigate a park area. New improved valves are now available that have about 5 PSI less friction loss than the valves you are currently using. Will the energy savings from the new valves justify the cost of replacement?

Situation - About 42 inches of water are required to irrigate this park each year. Each electric valve controls about 8 acres. If you use the new valves, they should have an economic life of at least 15 years. Assume electricity costs $0.10 per kilowatt-hour, the energy cost inflation rate is 5%, and the interest rate is 10%.

Analysis - At 42 gross inches water application per year, the annual energy savings will 15.8 BHP-HRS per acre for each PSI in pressure saved. Since each valve controls 8 acres, the savings is 15.8 x 8 acres x 5 PSI = 632 BHP-HRS per valve. The conversion factor for electricity is 1.1 BHP-HRS per kilowatt-hour, so the annual savings per valve will be 632/1.1 = 574.5 kilowatt-hours. At $0.10 per kilowatt-hour, that's 574.5 x $0.10 = $57.45 per valve per year. For a 5% energy cost inflation rate, an interest rate of 10%, the Present Value Factor for a 15 year economic life is 10.05. The total savings per valve are worth $57.45 x 10.05 = $577.42. So if you can buy and install the new valves for under $577, it'll pay off.

Table 4.  Present Value Factors - 15 Year Economic Life
Interest ----Energy Cost Inflation Rate----
Rate 5% 10% 15% 20%
5% 14.29 20.19 29.14 42.74
10% 10.05 13.64 18.96 26.88
15% 7.45 9.73 13.04 17.87
20% 5.77 7.29 9.44 12.50

Example 2 - Larger Pipe
Problem -
You are designing a pipe network to supply water to an irrigation system covering 50 acres. The network involves a section of pipe that could be either 8" or 10" PVC. The larger pipe costs more, but it would save 3 PSI in friction loss. Which size pipe should be used?

Situation -
About 36 inches of water are required for irrigation each year. The pipe will have an economic life of at least 15 years. Assume the pumping plant runs on diesel, and that diesel fuel costs $0.80 per gallon. Further, assume an energy cost inflation rate of 10%, and an interest rate of 15% for new construction.

Analysis -
At 36 gross inches water application per year, the annual energy savings will 13.6 BHP-HRS per acre for each PSI pressure saved. The total savings is 13.6 x 50 acres x 3 PSI = 2040 BHP-HRS annually. The conversion factor for diesel is 14.6 BHP-HRS per gallon, so the annual savings will be 2040/14.6 = 139.7 gallons of diesel fuel per year. At $0.80 per gallon, that's 139.7 x $0.80 = $111.78 per year. For a 10% energy cost inflation rate, an interest rate of 15%, the Present Value Factor for a 15 year economic life is 9.73. The total savings per valve are worth $111.78 x 9.73 = $1087.62. So if you can buy and install the larger pipe for under $1087, you should go with the 10". Otherwise, use the 8" PVC pipe.

REFERENCES
Gilley JR and Watts DG. 1977. "Possible Energy Savings in Irrigation." J Irrig & Drain Div ASCE 103(IR4):445-457.

Gilley JR and Mielke LN. 1979. "Energy Conservation Using Reduced Pressure Center-Pivot Irrigation Systems." Proceedings, ASCE Specialty Conference on Irrigation and Drainage in the 1980's, July 17-20, 1979.

Moore CU. 1981. "Impact of Energy Costs on Pump-Irrigated Agriculture." California Agriculture 35(1&2):23-24.

Chu S. 1980. "Pumping Energy Reduction by Modified Cost Analysis." J Irrig & Drain Div ASCE 106(IR2):149-154.

Pearson GF. 1974. "Life Cycle Costing in an Energy Crisis Era." Professional Engineer 44(7):26-29, July 1974.

About the Center for Irrigation Technology...
The Center for Irrigation Technology (CIT) conducts studies related to the art and science of irrigation, in cooperation with the irrigation industry; local, state, and federal governments; the faculty, and units of the University. CIT facilities include a field demonstration area and a hydraulic research laboratory. CIT is located on the CSUF campus on the southeast corner of Chestnut and Barstow Avenues.

As part of its educational activities, the Center conducts a series of seminars on various irrigation topics. This article was developed as part of the CIT Seminar "Large Turf Irrigation Systems: Design and Management Update," held in December 1987. The complete Proceedings of that seminar are available, as well as proceedings from other CIT seminars. For further information about irrigation seminars, publications, or other aspects of the CIT program, contact the Center at: Center for Irrigation Technology, California State University, Fresno, California 93740-0018, Telephone: (559) 278-2066.