1. VARIABLE-SPEED DRIVES FOR PRESSURE IRRIGATION SYSTEMS Energy-Efficient Technology for the 21st Century
Thanks for the opportunity to present this technology, as a way for the NRCS to move forward as we address energy conservation in agriculture.

2. WHAT IS A VARIABLE-SPEED DRIVE?
A way to control and adjust the work performed by a motor to meet changing demands and energy requirements
It is a technology that today is found in many applications
Fans and Air Compressors
Conveyor belts
Municipal water systems
Solar pumps
For example, many solar pumps have a variable-speed drive built right into the pump assembly, allowing them to work under a wide range of power (sunlight) conditions, including the ability to run on a 110-volt AC power source such as a generator
Variable speed control is becoming quite common in industry, but is just beginning to show up in agriculture.

3. WHAT IS A VARIABLE-SPEED DRIVE?
This technology has several different names:
Variable-Speed Drive (VSD)
Adjustable-Speed Drive (ASD)
Variable-Frequency Drive (VFD)
Adjustable-Frequency Drive (AFD)
Frequency Converter
Inverter (a term used by manufacturers)
etc.
These names all represent the same technology.

4. HOW DOES THIS SAVE ENERGY?
Pumps driven by electric motors typically operate at full speed even when the loads they are serving are less than the motor capacity.
To match the output of the pump to the load needed, some sort of part-load control is necessary – ie. variable speeds.
From this point on I will be addressing pumps and irrigation systems specifically.

5. HOW DOES THIS SAVE ENERGY?
The laws of physics dictate that:
The flow will vary proportionally with the speed
The pressure, or head, will vary with the square of the speed
The energy consumed will vary with the cube of the speed
For example, reducing the speed of a motor by 10% will:
Reduce the flow by 10%
Reduce the pressure by 19% (.9 x .9)
Reduce the energy used by 27% (.9 x .9 x .9)
The real energy savings is around 25%, because no control efficiency is 100%
By the way, at the end of the presentation there is a pop quiz on this subject – lunch is the reward for passing. More seriously, the real point here is to understand how a relatively small change in the speed of something like a pump can have a very significant change in the energy used.

6. HOW DO VARIABLE-SPEED DRIVES WORK?
The most common method is to use a pressure sensor at a key point in the irrigation system
The information from the sensor is sent to the variable-speed control system, which adjusts accordingly
This can be done with direct wiring, or
By radio transmitter (more on this later)
These drives can be used to control more than one pump at a time – very important for situations where there is more than one pump supplying water, or more than one field where that water is being used

7. What Do They Look Like?
Variable-speed devices are housed in a separate control panel. I’m not going to spend any time on their schematics – I do have that information for anyone that is interested.

8. HOW DOES THIS SAVE ENERGY?
Pump Curve – the relationship between pressure (often expressed as “head”) and flow
Design Point – maximum demand
These are a stylized pump curve and system requirement curve, to show the relationship between requirement and pump performance. Is everyone familiar with the conversion between pressure in psi and head in feet? Because I am going to spend a fair bit of time examining real pump curves, and I will be working in head, expressed in units of feet.
System Curve – change in requirement

9. HOW DOES THIS SAVE ENERGY?
Un-necessary work performed by motor and device
This green shaded area is excess energy consumption.

10. POTENTIAL FOR APPLICATION
Looking at southeast Arizona alone:
The Douglas work area has 17,000 acres of irrigated cropland, all of it served by pumped groundwater
The Willcox FO has almost 70,000 acres of irrigated cropland where pumped groundwater is the water source
Virtually all of these acres are served by pressurized systems.
The Safford FO has about 48,000 acres of irrigated cropland
While much of this is still flood irrigation with open-discharge pumps, there is rapidly-accelerating interest in buried drip irrigation and other types of pressurized irrigation systems

11. Center-Pivot Sprinkler Systems
Center-pivots are very common in my work area, and also in Willcox.

12. Solid-Set Sprinklers
We also have quite a few solid-set systems.

13. Buried Drip Systems
And some buried drip systems

14. POTENTIAL FOR APPLICATION
Douglas has at least 200 irrigation wells (estimated conservatively). Each one has a pump and a motor.
Willcox has at least 600 irrigation wells, each one with pumps and motors.
Safford has a mix of irrigation wells and irrigation system booster pumps – FO staff estimates 200.
Southeast Arizona alone, covered by 3 Field Offices, has at least 1,000 pumps and motors to which a Variable-Speed Drive might usefully be applied.

15. ENERGY SAVINGS – HOW MUCH? Scenario 1
A center-pivot system, 1295 feet long, that irrigates a 120-acre circle. Diameter of the circle is 2590 feet.
Pivot is designed for a water flow of 800 gpm – this flow is necessary to achieve water management during peak periods of crop growth
Pump has to overcome about 450 feet of head
410 feet of lift from groundwater level
40 feet of head for pivot pressure requirement (12 psi=28 feet of head) and friction losses ( 12 feet)
“In a perfect world…”
Irrigated circle is absolutely level
Groundwater level does not change (no drawdown, no seasonal fluctuation)
The first part of this scenario is quite real – field size, required flow, feet to groundwater, etc.

16. “In a perfect world, cont’d…”
Pump Curve
Motor is 125 HP, running slightly below its rated capacity
Pump can produce the required flow, plus a little more
450 feet of head
822 gpm
This is a very close fit of pump capacity to system requirement – but only in a perfect world where the underlying conditions do not change. Question:
Question: Will the field actually receive the extra 22 gallons per minute?
119 HP

17. Each drop hose and nozzle on a center-pivot has a pressure regulator.
Not for center-pivots. With pressure regulators and specific nozzle sizes for that pressure, 800 gpm will be delivered to the field.

18. “In a perfect world, cont’d…”
Pump Curve
800 gpm, 460 feet of head – an extra 4 pounds or so of pressure. Motor now working at full capacity of 125HP.
Even this small change reflects 7-7.5% energy use that is not needed
What we actually get with this pump is the generation of a little extra pressure, and the motor working a little harder, to produce the required 800gpm. Even this small amount of extra work represents sig. energy – remember that energy use varies with the cube of the change.

19. “In the real world…”
An increase of as little as 25 feet of head, to 475 feet, means that this pump cannot produce the required flow without over-working the motor
This pump-motor combination should not be used if the head changes by as little as 25 feet.
25 feet was chosen because:
The circle being irrigated is almost feet wide
At a slope of 1% ( a common slope), the highest point on the field is at least 25 feet higher than the lowest point
So, if 450 feet represents the head to the low elevation on the field, 475 feet will be the elevation on the higher side.

20. “In the real world…” Scenario 1 Re-visited
To meet the changing demand of this field and provide the necessary flow at all times, the pump must be larger and the motor must be at least 135HP
When the required head is 475 feet, and the flow is held at 800 gpm, this pump produces an extra psi of un-necessary pressure
When the required head is only 450 feet, the extra pressure is 30 psi.
This pump is trying to produce 890 gpm at 475 feet of head. If it “rides up the pump curve” to 800 gpm and 520 feet of head – the extra head is expressed as extra pressure.
At all times, this pump/motor combination is working harder than it needs to – on part of the field it is working much harder than it needs to
The producer is paying for this

21. Scenario 1 Re-visited with Variable-Speed Control
When the required head is 475 feet, and the speed of the pump is reduced to produce only the required 800 gpm, the 135 HP motor is working at 122 HP
800 gpm at 475 feet of head, 58 Hz
Electric frequency in cycles/second
Note that on this graph the pump speed is expressed as frequency in cycles/second, in Hertz. I’m not going to spend time on the relationship between frequency and speed at this time. It is worth noting that at full speed the normal frequency is 60 Hertz. Little-known fact…
Little-known fact: Taking electrical appliances around the world often requires adapters. As often as not this due to a different electric frequency in another country – nominal voltage may be the same.
122 HP needed

22. Scenario 1 Re-visited with Variable-Speed Control
When the required head is 450 feet, and the speed of the pump is reduced to produce 800 gpm, the 135 HP motor is working at 115 HP
800 gpm at 450 feet of head, 57 Hz
At all times, this pump/motor combination is working only as hard as necessary to get the job done and provide the flow for design efficiency of the center-pivot
This is controlled by a pressure sensor at the far end of the pivot, and a radio transmitter
115 HP needed

23. Energy and Cost Savings – Scenario 1
Without Variable-Speed Control
135 HP at all times for an entire season (2048 hours)
237,568 Kw-hrs. used
At $0.075/Kw-hr, cost to farmer is $17,818
With Variable-Speed Control
An average of HP for an entire season
206,950 Kw-hrs. used
Cost to farmer is $15,520
30,618 Kw-hrs saved (about 13% of the energy use)
Just about $2300 saved
In this case I used corn as an example crop, and took actual inches of water applied by a farmer this summer, plus an actual pivot design (one of his) to work this example.

24. Scenario 2 – elevation difference on the field, plus well drawdown and seasonal water table fluctuations
With well drawdown and a water table that drops during irrigation season, especially the pre-monsoon period when the demand is typically highest, it is not uncommon to have another 75 feet of head to contend with, for a total of 550 feet
800 gpm at 570 feet of head
550 feet
This situation requires a160 HP motor
This is a different pump curve again, for a pump that can meet the need of this situation. Again, you have a similar situation – this pump tries to produce 875 gpm at 550 feet, - what it actually produces is 800 gpm at 570 feet, with the difference of 20 feet expressed as extra pressure.
875 gpm

25. Scenario 2 – elevation difference on the field, plus well drawdown and seasonal water table fluctuations
Without a control device, the power requirement is the same – 160 HP
At times there is the potential for as much as extra 50 psi on the system
800 gpm at 570 feet of head
450 feet
Remember, however, that at certain times on this field it only needs to overcome 450 feet of head – now it is trying to produce 1222 gpm, and now it rides way up the pump curve to the 800 gpm line. As a practical matter, apart from un-necessary energy use, this extra pressure puts a serious strain on the entire system. This too is a very real situation.
1222 gpm

26. Scenario 2 with Variable-Speed Control
800 gpm at 550 feet of head, 59 Hz
HP requirement is only 150 HP
At the peak load requirement, motor has the capacity for the job and is working at almost full capacity:
550 feet of head needed
145 HP
800 gpm delivered at the pressure needed
Note that you can use a 150-HP motor at all times with variable-speed control. Without the control it had to be 160HP – even this helps, because of less power used by the smaller motor simply to function.

27. Scenario 2 with Variable-Speed Control
At the lowest load requirement, motor is working at a much-reduced speed:
450 feet of head needed
119 HP
800 gpm delivered at the pressure needed
54 Hz
Most importantly is the difference when you only need 450 feet of head

28. Energy and Cost Savings – Scenario 2
Without Variable-Speed Control
160 HP at all times for an entire season (2048 hours)
280,576 Kw-hrs. used
At $0.075/Kw-hr, cost to farmer is $21,043
With Variable-Speed Control
An average of 132 HP for an entire season
231,424 Kw-hrs. used
Cost to farmer is $17,357
49,152 Kw-hrs saved (18% of the energy use)
Almost $3700 saved
A short note on the estimate of 18% - etc.

29. Measurement of Actual Power Savings is Being Done
In June of this year, Paul White of Whitewater farms installed the first VSD control in the area
A data logger was also installed that records pressure, speed, and kilowatts. The information is time- and date-stamped.
Preliminary information suggests that when the pivot end is at the lowest point on the field the actual power used is as low as 50% of the power used without the control
Power used during one revolution of the pivot is being reduced by 20-25%.
Ultimately, the goal of the data logger is to capture the reduction in energy use over an entire irrigation season.

30. Cost Breakdown VSD controller $14,000
Radio transmitter for pressure sensor $2,000
There is enough interest in this technology in my area that we are planning to submit a Conservation Innovation Grant for 2009, on the basis of energy savings and dissemination of new technology
Part of the proposal will be the capture of actual power savings through data loggers
This information could be very useful for an NRCS standard and specification

31. Other Benefits of Variable-Speed Drives
Allows high-efficiency irrigation systems to operate at their potential
Better system operation
Higher motor and pump efficiencies
Longer pump life
Rotational wear is the primary cause of pump failure, lower operating speeds generally translate to longer pump service before maintenance is required
Longer motor life
The enemy of electric motors is current fluctuations; transmission lines can vary 10-12%. VSD technology reduces that to less than 1%
No un-necessary pressure wearing out or potentially damaging the system

32. Variable-Speed Drives and other Irrigation Systems
Well-designed solid-set sprinkler systems and buried drip systems try to achieve equal flow requirements to each set in a field or series of fields. This is not always possible, and sometimes means design “compromises” to make systems work acceptably well.
Use of VSD technology will allow irrigation system designs to focus on the best irrigation efficiency and not have to balance this with varying flows
It may also encourage the conversion to high-efficiency irrigation systems

33. Variable-Speed Drives and other Irrigation Systems
For example, in Safford (information courtesy of Eddie Foster):
There is accelerating interest in conversion of flood irrigation to buried drip systems
One major reason that holds producers back is the wide variation in conditions such as field and irrigation set size
VSD control has been installed on a few recent buried drip systems
This has allowed producers to construct a single filtration and control system (a major cost) to monitor and irrigate multiple fields under a wide range of situations

35. How can NRCS be involved?
Identify a Resource Concern that is Applicable
Depletion of Fossil Fuel Resources - Inefficient use of fossil-originated energy sources (diesel, gasoline, propane, natural gas, coal), lubricants, and other materials.
Developed by the “Energy Team” at the West NTSC
In use as a resource concern by several states
Nebraska has it on their Conservation Practice Physical Effects (CPPE) worksheet

36. How can NRCS be involved?
Identify VSD Technology as a Specific Component of a Conservation Practice
From the Standard for practice code 533, pumping plant:
Criteria: The efficiency of units, type of power, quality of building, automation, and accessories installed shall be in keeping with the value and importance of the system, and shall accomplish the conservation and environmental objectives.
The on-line NRCS energy estimator for irrigation includes an evaluation for the pumping plant

37. How can NRCS be involved?
Develop an Interim Specification for VSD Technology, as a part of 533 – Pumping Plant
Add “Depletion of Fossil Fuel Resources” to the Arizona Resource Concern worksheet and CPPE matrix
Develop guidelines for Field Office inventory and assessment of this resource concern
Add VSD control systems to the EQIP cost list

38. Why Should NRCS be Involved?
See Title IX of the 2008 Farm Bill - the Rural Energy for America Program (REAP)
A chance to be at the forefront of agricultural energy conservation here in Arizona
An opportunity to further promote efficient irrigation water use and management
The Arizona agricultural community continues to evolve and adapt their management to resource issues, and to economic and social conditions. Technology for energy conservation is merely another step.