VARIABLE-SPEED DRIVES FOR PRESSURE IRRIGATION SYSTEMS

Energy-Efficient Technology for the 21st Century

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

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.

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.

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%

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

What Do They Look Like?

HOW DOES THIS SAVE ENERGY?

Pump Curve – the relationship between pressure (often expressed as “head”) and flow

System Curve – change in requirement

Design Point – maximum demand

HOW DOES THIS SAVE ENERGY?

Un-necessary work performed by motor and device

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

Center-Pivot Sprinkler Systems

Solid-Set Sprinklers

Buried Drip Systems

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.

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)

“In a perfect world, cont’d…”

Pump Curve

822 gpm

450 feet of head

119 HP

•Motor is 125 HP, running slightly below its rated capacity•Pump can produce the required flow, plus a little more

Question: Will the field actually receive the extra 22 gallons per minute?

Each drop hose and nozzle on a center-pivot has a pressure regulator.

“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

“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 2600 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

“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 19-20 psi of un-necessary pressure•When the required head is only 450 feet, the extra pressure is 30 psi.

•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

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

122 HP needed

Electric frequency in cycles/second

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.

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

•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

800 gpm at 450 feet of head, 57 Hz

115 HP needed

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 118.5 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

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

•This situation requires a160 HP motor

800 gpm at 570 feet of head

550 feet

875 gpm

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

1222 gpm

Scenario 2 with Variable-Speed Control

•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

800 gpm at 550 feet of head, 59 Hz

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

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

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.

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

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

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

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

How can NRCS be involved?

1. 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

How can NRCS be involved?

2. 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

How can NRCS be involved?

3. Develop an Interim Specification for VSD Technology, as a part of 533 – Pumping Plant

4. Add “Depletion of Fossil Fuel Resources” to the Arizona Resource Concern worksheet and CPPE matrix

5. Develop guidelines for Field Office inventory and assessment of this resource concern

6. Add VSD control systems to the EQIP cost list

Why Should NRCS be Involved?

1. See Title IX of the 2008 Farm Bill - the Rural Energy for America Program (REAP)

2. A chance to be at the forefront of agricultural energy conservation here in Arizona

3. 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.