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Compare costs of irrigation pumping

Crop harvest is the priority right now, but I hope this article will be helpful a little later as growers look at irrigation considerations for next year.

The irrigation pumping requirements for this season were generally less than we usually experience. However, comparing the pumping costs using different fuel and energy sources can be helpful in planning for next year.

The approach I am using considers only the seasonal operating costs associated with irrigation pumping. With increasing energy costs, the operating cost is usually at least 75 percent of the total irrigation cost over the life of an irrigation pumping plant.

Once you have a handle on the expected operating costs, you can compare the investment or fixed cost of the different options.

The information needed for the comparison is in three accompanying tables.

Table 1 shows what I feel are reasonable energy efficiency estimates for each fuel/energy source. In order to have a level playing field, the comparison uses the same horsepower load for each fuel/energy source. I am going to use “diesel” under a 50-hp load for an example and the numbers are highlighted in each of the three tables.

Using diesel in Table 2, divide the 50-hp load by an energy efficiency average of 18.5 hp-hr/gal to get the average fuel use of 2.7 gph (gallons per hour).

Table 3 is used to calculate the average cost per hour based on the fuel/energy cost per unit. In this example the diesel is priced at $0.80/gal. and the operating energy cost is $2.16/hr (2.7 gph × $0.80/gal = $2.16/hr).

The cost of routine maintenance, like oil and filter changes for power units, is calculated as a percent of the operating energy cost. Using the 15 percent average for diesel units gets an additional $0.32 ($2.16 × 0.15 = $0.32) to add to $2.16 for a total of $2.48/hr.

Table 3 also shows total cost calculations for the other fuel/energy sources at their respective cost per unit of fuel/energy. It may be difficult to nail down a unit cost for electric and natural gas since they usually have other charges to factor in. However, you can use different fuel/energy unit costs to determine how the variability affects the comparison.

In many cases comparing diesel to electric may be all that you do because that is all that is available. Electric may not be an option if the electric motor is bigger than 15 hp and three phase power is not available.

In some areas the propane company offers a summer power unit rental program that can be attractive from standpoint of less overhead and maintenance.

Vandalism problems in some locations and the distance to the field may make power units more of a problem to maintain than electric. Some growers may choose electric because it doesn't require fuel deliveries and the in-season oil and filter maintenance. However, most growers are more comfortable with trying to fix power units than they are with determining the problem with the electric.

The variable speed capability with power units can also be a significant comparison factor.

1. Energy efficiency estimates
Fuel/energy source Energy efficiency (range) Energy efficiency (average)
Diesel 17-20 hp-hr/gal 18.5 hp-hr/gal
Natural gas 9-11 hp-hr/ccf 10 hp-hr/ccf
LP (propane) 9-11 hp-hr/gal 10 hp-hr/gal
Gasoline 10-14 hp-hr/gal 12 hp-hr/gal
Electric-con 1-1.3 hp-hr/KWH 1.15 hp-hr/KWH
Electric-sub 1-1.1 hp-hr/KWH 1.05 hp-hr/KWH
hp-hr (horsepower hours); ccf (100 cubic feet), gal (gallon); KWH (kilowatt hour).
2. Example for determining fuel/energy use - 50hp load
Fuel/energy source Load Energy efficiency (average) Fuel/energy use
Diesel 50 hp 18.5 hp-hr/gal 2.7 gph
Natural gas 50 hp 10 hp-hr/ccf 5.0 ccf/hr
LP (propane) 50 hp 10 hp-hr/gal 5.0 gph
Gasoline 50 hp 12 hp-hr/gal 4.2 gph
Electric-con 50 hp 1.15 hp-hr/KWH 43.5 KWH/hr
Electric-sub 50 hp 1.05 hp-hr/KWH 47.6 KWH/hr
ghp (gallons per hour); ccf/hr (100 cubic feet per hour); KWH/hr (kilowatt hours per hour).
3. Example for calculating operating cost per hour for 50-hp load
Routine maintenance added as percent of operating energy cost
Fuel/energy source Fuel/energy use Fuel/energy unit cost Operating energy cost Range Average Total (w/avg)
Diesel 2.7 gph $0.80/gal $2.16 10-20% 15% $2.48/hr
Natural gas 5.0 ccf/hr $0.70/ccf $3.50 10-15% 12.5% $3.94/hr
LP (propane) 5.0 gph $0.90/gal $4.50 10-15% 12.5% $5.06/hr
Gasoline 4.2 gph $1.30/gal $5.46 10-20% 15% $6.28/hr
Electric-con 43.5 KWH/hr $0.08/KWH $3.48 1-5% 3% $3.58/hr
Electric-sub 47.6 KWH/hr $0.08/KWH $3.81 3-10% 6.5% $4.06/hr
Electric: Con is conventional above-ground motor and Sub is submersible.

This is not the only way to compare pumping costs, and as I mentioned, you have investment costs and some of the other things I have listed to consider in making your decision.

Also, some may want to argue about some of the values I have used, but I feel this is a fair comparison method. I hope it is presented in a way that can be helpful.

This is one of several articles on drainage and irrigation water management. If you have questions or suggestions on topics please contact me: Phil Tacker, 501-671-2267 (office), 501-671-2303 (fax), 501-944-0708 (cell), or (e-mail).

Phil Tacker is an Arkansas Extension ag engineer.

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