Variable Frequency Drive (VFD) Controlled Irrigation Pumps – Analysis of Potential Rebate

Well pumps, booster pumps, and a combination of the two are used throughout the agricultural

sector to provide water for on-farm irrigation.  However, the pump discharge pressures for the

majority of irrigation systems are excessive (Burt, 2009), waiving considerable monetary and

power consumption savings.  This report analyzes the numerous potential benefits of

integrating a variable frequency drive (VFD) to irrigation supply systems and modifying system

design philosophies.



   Bottom line – Pump discharge pressures can be reduced with appropriate design

   procedures and the integration of a VFD on well pumps.



In 2002, ITRC (Burt and Howes, 2002) surveyed five California irrigation districts regarding the

integration of VFD controllers to supply pumps.  The results were positive across the board

including substantial reductions in energy costs, reduced peak load demand, and other savings

related to less vehicular travel and manpower.  Annual paybacks were in the 2-4 year range.



For on-farm irrigation, VFDs will not provide as many secondary benefits to the owner as for

irrigation districts, because their operations are not similar.  Nevertheless, substantial benefits

can be achieved on-farm.



Table 1 summarizes the results that are discussed in this Appendix.  Key assumptions for Table

1 are:



     Location = West side of the San Joaquin Valley, Kern County



     Price of power = $0.16/ kWh



      Table 1:   Estimated annual kWh savings in western Kern County if VFDs are installed on

                             pressurized field irrigation systems of 160 acres.



                                                       Annual kWh savings with VFD



                                                                  Category 2 – Well pump plus booster

                                        Category 1 – Booster     or well pump only supplies pressure to

                   Crop Type                 pump only.              drip/sprinkler irrigation system.



             Deciduous Trees                   21,078                           64,176



             Grape Vines                       13,672                           41,667



             Tape on Produce Crops             13,672                           41,667



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                             Variable Frequency Drive (VFD) Controlled Irrigation Pumps

http://www.itrc.org/reports/vfdanalysis.htm                                             ITRC Report No. R 11-005



Variable Frequency Drive (VFD) Controlled Pumps



Pressure Reduction with VFDs



VFD controllers for irrigation pump motors may have the greatest potential for immediate

power savings.  There are numerous reasons to promote VFDs on both well pumps and booster

pumps.  The two most significant reasons are:



1.   Designers must always over-design pumps.  Farmers do not complain if they have too much

    pressure; but they definitely complain if they do not have enough.  The uncertainties with

    pump design are:

    a.   Designers always include a “ safety factor”  of at least 5 psi in a design – whether needed

         or not.

    b.   Published pump curves often do not exactly match what goes into a field.

    c.   The pressures from irrigation district pipeline turnouts vary over time, and may not

         even be known by the designer.

    d.   Well water levels vary from year-to-year, and from Spring to Fall.  These variations can

         easily be 50 feet.



2.   Irrigation systems do not require a constant pressure.  In general, irrigation systems

    incorporate sequences of multiple blocks.  These blocks have varying elevations and sizes,

    each with unique pressure requirements.



Given the two items above, VFDs allow designers to over-design the pump to meet

uncertainties and occasional extreme conditions, without having continuous power wastage

due to an over-designed pump.



The power savings that are obtained from a VFD will depend upon the specific installation. In

the case of well pumps for which the lift from the pumping water level to the ground surface is

substantial, the power savings are not properly predicted using the affinity law equation:



                 (New kW/ Old kW) = (New RPM/ Old RPM)3



The equation above assumes that the flow rate varies proportionally as the RPM changes.  In

irrigation, the RPM of the pump is changed in the case of fluctuating water levels, to maintain a

constant GPM.  In the case of irrigation block sizes that have variable elevations and GPM

requirements, the relationship is more complex.



Readily attainable pressure savings estimates for a well pump are shown in the following table.

These values are based on design experience of ITRC staff, and are therefore somewhat

subjective.



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                                 Table 2: Readily attainable pressure savings.



                          Pressure savings category         Estimate of pressure savings (ft)



                         Over-design for fluctuating        10% of the average pumping water

                         water table                                  level in the area

                         Over-design for unknown                             10

                         factors in system design

                         Variations in block sizes and

                                                                              6

                         locations

                         Total pressure savings,             Depends on the average pumping

                         unadjusted                                water level in the area



As described in Table 2, the magnitude of pressure savings will be proportional to the average

pumping water level in the area.  Figure 1 displays average pumping water levels of the pumps

sampled, weighted by kW, for California’s various groundwater basins.  This white paper

utilizes data from a reference region in western Kern County.



                   Figure 1:   Weighted pumping water depths from surveyed pumps (ft)



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                              Variable Frequency Drive (VFD) Controlled Irrigation Pumps

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Three additional benefits derived from the use of VFDs are also substantial, although they do

not in themselves reduce electricity consumption (kWh).  These benefits are:



1.  Water hammer and subsequent damage to the pump and irrigation system are reduced

     because of the slow start and slow stop capabilities of VFD-equipped pumps.

2.   Farmers are much more likely to adopt time-of-use pumping practices with well pumps.

     This is because the slow starting of well pumps, as opposed to 100% speed starting (with

     subsequent very high flow rates), can have a drastic impact on the life of wells.  Many

    farmers will not start or stop well pumps dur ing the irrigation season because they are

     afraid the starts and stops will damage their wells.

3.   The slow start minimizes large but temporary current loads on the electric utility grid.



Baseline Data



Table 3 describes data obtained from the ITRC website (ITRC, 2003) for the reference area,

specifically in California Department of Water Resources ETo Zone 16.  In this case, ET of

irrigation water is assumed to equal the applied water – making the assumption that farmers

irrigate to the average condition of their fields and have both over- and under-irrigation on

orchards and vines, but have slight over-irrigation on taped fields.



 Table 3: Estimated annual applied irrigation water for three crop categories in the southern SJV.



                                                             Applied Irrigation

                                                                   Water

                                   Crop Category              (AF/Acre/Year)



                                   Deciduous Orchards                3.7



                                  Vines                              2.4



                                   Row Crops (Tape)                  2.4



Table 4 reflects the average pumping data of the pumps sampled during the research completed

for Figure 1 in the Kern County groundwater basin.



     Table 4: Typical well pump data in the Kern County groundwater basin, from Appendix 3C.



                                              Input                 Pumping Water

                                Kwh/AF         Kw       TDH (ft)       Level (ft)



                                   555         127        321             300



Energy Savings



The next section will highlight the substantial energy and monetary savings possible tough the

points previously discussed using collected data from the southern San Joaquin Valley (SJV)

specifically in ETo Zone 16 for reference (western Kern County).  To differentiate pressure

savings from the complex variety of irrigation supply systems, the calculations are divided into

two categories.



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Base Unit:

  A well pump that supplies a booster pump for pressurized irrigation systems.



For this analysis, the “ typical well pump data”  in Table 4 are used to compute the following:



    • GPM of this pump = 1365            (assuming a 65% pumping plant efficiency)

    • Hours necessary to pump 1 AF = 4 hours

    • This size of a pump would typically irrigate about 160 acres of drip



The new total dynamic head (TDH) (a.k.a., pressure) for the well pump is computed as:



      Eq. 1:                                             - 16’



Where,



                       = Old TDH from Table 4 (321 ft)

                       = pumping water level (ft); in other words, the static water level (SWL) plus

                         drawdown, from Table 4         (300 ft)

     0.1               = Assumes 10% overdesign of TDH due to possible water table fluctuations

      16’              = Factors from Table 2, related to unknown variables that must be included

                         in a pump design



                                            – 16’    = 275’



Assuming a constant flow rate requirement, power savings can be computed as:



      Eq. 2:



Where,



          Old kW       = energy used to pump water, in kilowatts, from Table 4



                                                   = 108.8  kW



          Power Savings = Old kW – New kW



                         = 127 kW – 108.8 kW        = 18.2 kW



The annual monetary saving per Category 1 pump is then computed as:



     Eq. 3:



Where,



                       = kW savings calculated previously; (Old kW – New kW)



                       = typical hours of operation per year, which depends upon the crop and the



                         area of the field served.



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http://www.itrc.org/reports/vfdanalysis.htm                                                      ITRC Report No. R 11-005



                   Table 5:   Hours of operation for the example well pump on 160 acres.



                                                          Annual applied,

                                   Crop                       AF/Acre               Hours/year



                        Deciduous trees                          3.7                   2368



                        Vineyard                                 2.4                   1536



                        Tape on row crop produce                 2.4                   1536



                         = typical cost per kWh.  Assume $0.16/ kWh.



     For the deciduous trees,



                   = $ 6896/ yr for the well pump, only on 160 acres.



                  Table 6: Well pump only VFD savings on a per crop basis – 160 acres.



                                     VFD on Well Pump Only – Western Kern Co.



                         Crop Category                   Hours/yr                  Δ kWh/Year



                    Deciduous Orchards                     2368                       43.098



                    Vines                                  1536                       27,995



                    Tape on Produce Crops                  1536                       27,995



Booster Pump Only

 A system that receives water from a canal or irrigation district, without any pressure.



The computations assume the following savings with a VFD on a booster pump:



                               Table 7: Savings with a VFD on a booster pump.



                          Pressure Savings category         Estimate of pressure savings, ft.



                         Over-design for "safety

                                                                            11.5

                         factor"



                         Likely overdesign of pump or

                                                                              5

                         lack of trimming impellers



                         Adjustment due to kW impact

                                                                              6

                         caused by flow adjustment



                         Total pressure savings                            22.5 ft



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Assuming the same system as the previous well pump example, with a flow rate of 1365 GPM

on 160 acres, with 65% pumping plant efficiency:



     For deciduous orchards:



     Using the same hours per year as with the well pump:

         kWh savings/ yr on deciduous trees = 8.9 kW × 2368 hours/ yr = 21,078 kWh/ yr saving



                  Table 8: Booster pump VFD savings on a per crop basis – 160 acres.



                                 VFD on Booster Pump Only – Western Kern Co.



                        Crop Category                 Hours/yr                 Δ kWh/Year



                   Deciduous Orchards                   2368                     21,078



                   Vines                                1536                      13,672



                   Tape on Produce Crops                1536                      13,672



Summary of kWh Savings



There are two general categories of pumps for pressurized on-farm systems (drip and

sprinkler):

     • Category 1 – Booster only.  Water is supplied from an irrigation district turnout,

         and a booster pump is needed to provide the pressure for the drip system.  Even

         if the irrigation district supplies pressurized water, that pressure can vary over

         time so the designer must design the pump for the worst situation (lowest

         pressure from the turnout).

     • Category 2 – A well pump provides water directly to the drip system under pressure, or

         is directly linked to the drip system booster pump.            For this case, the savings of the well

         pump only, plus the booster pump, are added.



The estimated annual power savings will depend upon the crop type and acreage.  Well pump

savings will depend upon the depth to standing water level as well .  The table below has been

developed for the western side of Kern County.



      Table 9:   Estimated annual kWh savings in western Kern County if VFDs are installed on

                             pressurized field irrigation systems of 160 acres.



                                                       Annual kWh savings with VFD



                                                                  Category 2 – Well pump plus booster

                                        Category 1 – Booster     or well pump only supplies pressure to

                   Crop Type                 pump only.              drip/sprinkler irrigation system.



             Deciduous Trees                   21,078                            64,176



             Grape Vines                       13,672                            41,667



             Tape on Produce Crops             13,672                            41,667



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                              Variable Frequency Drive (VFD) Controlled Irrigation Pumps

http://www.itrc.org/reports/vfdanalysis.htm                                                ITRC Report No. R 11-005



Rebate Programs for VFD and Pump Selection



Given that VFD controllers can provide substantial energy-related benefits with agricultural

irrigation pumps, any rebate program for VFDs should contain minimum requirements for the

purchase of VFD controllers, covering the following features:



           1.   Efficiency.  Inefficient VFDs create excess heat, which requires significant air

               conditioning power to dissipate.

           2.  Temperature rating.

           3.   Power quality.

           4.   Form of the simulated sine wave.

           5.  Audible noise.

           6.   Length of power cords that can be used.  Some low quality VFD units can

               only have a cable of about 20 feet long between them and the motor.

           7.   Means of cooling the VFD.

           8.  Allowable voltage variation between legs.

           9.  Allowable average voltage variation.



A rebate program should have a different scale for well pumps versus booster pumps.  This is

because there are additional benefits to using VFDs on well pumps, which include:



-   The slow startup will enable farmers to take advantage of off-peak power programs that

    they might not otherwise utilize.



-   The slow startup introduces less dirt into the irrigation system, which requires less filtration

     and less water used for backflushing of filters.



However, a rebate program designed primarily for kWh reduction, rather than load shedding,

would not include well pumps that discharge into canals or pipelines that supply surface

irrigation (furrow or border strip) fields.  This is because the farmers easily adapt to the

changing groundwater levels by managing their irrigation systems for less or more flow (as the

groundwater levels fluctuate over time).  In other words, the irrigation system adapts to the

well flow rate.



In contrast, the flows from wells that supply drip or sprinkler systems must adapt to the

constant or changing demands of the irrigation system.  Therefore, VFD control of w ell pumps

is desirable, and will save power, on such systems.  These are the systems that must have over-

designed pumps to provide enough pressure and flow in the worst condition – meaning excess

pressure is supplied at all other times.