Optimization of Variable-Speed Irrigation Pumping
Irrigation: Optimized Variable Frequency Drive (VFD) Pressure and Flow Control vs. Non-Optimized VFD Operation
A control method of optimizing use of existing VFDs to improve energy efficiency for irrigation systems using pressure feedback, which is especially useful in hilly terrain.
Item ID: 626
VFD optimization is thought to be applicable when there is more than a 20% variation in irrigation flow rates and/or a 10% variation in pump discharge pressure requirements. Irrigation flow rates vary due to the life stage of the crop and hotter and perhaps windy weather during the summer. Generally, water application requirements are indicated through soil moisture measurements and evapotranspiration models with the irrigation time period set to meet watering requirements. Irrigators are limited when hilly terrain is present and the pump must be selected to provide enough pressure for the sprinkler at the end of a lateral and at the highest elevation. Less pressure is needed for laterals closer to the pump and that are at the bottom, rather than at the top, of a hillside. As an irrigation line rotates to serve the downhill portions, it doesn’t need to pump as hard because head is less. Installation of pressure gauges provides a real-time feedback signal to the pump controller to slow down when rotating lines are serving lower elevation portions of their irrigation circle, dropping pump speed by roughly 75%. Energy is saved when “over-pressure” operation is limited while applying the same amount of water to the crop. Irrigation pumps are often used to supply multiple center pivots or wheel lines at the same time. Different irrigated sub-systems may be located different distances from the pump and thus have different elevations, pipe friction losses and pressure requirements. As these subsystems or zones cut in and out, variable speed pumps provide needed flows by ensuring constant pressure.
BPA has co-funded research to focus on costs and savings associated with the use of optimized irrigation pump control. Additional savings of 10% to 30% are expected due to implementation of this optimization strategy, providing a range in optimization savings of about 24 to 72 kWh/hp (Wy'East, 2013).
Baseline Description: Irrigation Pump with VFD already Installed
Baseline Energy Use: 239 kWh per year per hp
Estimates of annual energy savings due to VFD installation at sites with constant speed pump operation are in the neighborhood of 239 kWh per pump horsepower (Stroh, 2012) (from 277 projects). BPA had originally developed a provisionally deemed approach assuming 400 kWh/hp for a more optimized system. The project participants are now trying to improve the savings to something closer to the optimized estimate.
Manufacturer's Energy Savings Claims:
Manufacturers don't make energy savings claims for this technology as it involves field engineers optimizing variable speed drives operations. Additional energy savings are dependent on many variables including the quality of the original installation.
BPA states that results may vary due to change in crop rotations, weather, operator practices, water availability, system changes, analysis methodology, and multiple pumps on a single electrical meter (Stroh, 2012).
Best Estimate of Energy Savings:
"Typical" Savings: 20%
Low and High Energy Savings: 10% to 30%
Energy Savings Reliability: 2 - Concept validated
Estimating energy savings is challenging as the energy savings varies depending upon annual operating hours, the amount of water applied with the specified baseline irrigation technology (which in turn is dependent upon crop type, life stage, and soil type). Energy use is also highly dependent upon pumping conditions (including required lift from surface or deep well sources), friction losses (which depend upon fluid flow velocity, irrigation pipe diameter, distance, and surface roughness), hilliness of terrain, shape of the pump curve, and pump efficiency itself. BPA states that results may vary due to change in crop rotations, weather, operator practices, water availability, system changes, analysis methodology, and multiple pumps on a single electrical meter (Stroh, 2012).
However, BPA has developed estimates based on their experience with 277 documented projects. Their estimates of annual energy savings due to VFD installation at sites with constant speed operation are in the neighborhood of 239 kWh per pump horsepower (Stroh, 2012) (from 277 projects). They further estimate that VFD optimization will provide an additional 10% to 30% energy savings. Assuming that a pump without VFD control is equipped with a VFD and reduces its baseline energy use by 30%, an “additional” 20% energy savings due to optimization is equivalent to 6% of the original pump energy use or an 8.6% reduction in the pump energy use after the basic VFD control has been employed. The 20% “typical” savings (or 48 kWh/pump hp) is the increment of additional savings that is available for a pump that already has implemented VFD control.
BPA, the Natural Resources Conservation Service (NRCS), and the Wasco County Soil Water Conservation District contributed to an innovation grant funded project to use data loggers to demonstrate the benefits associated with optimizing the use of existing agricultural irrigation pumps that are already operating with VFD flow control. The program was launched in September of 2012 with project management supplied by the Wy’East Resource Conservation and Development Council. The projects got off to a slow start with three of the expected 15 projects installed during 2014. No reports have been published as of early 2015.
Energy Use of Emerging Technology:
191.2 kWh per hp per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
See calculations above.
Potential number of units replaced by this technology:
This technology is limited to irrigation pumps that are already equipped with VFD flow control, that experience a 10% variation in pump discharge pressure requirements, and that are already equipped with VFD flow control. These include pumps supplying water to any crop type in hilly terrain. The BPA supported research is intended to develop operating “best practices” for these types of pumps including appropriate use of pressure sensors to provide feedback to the pumps. The research will also quantify benefits for various types of pumping plants including deep well, turbine, and centrifugal pumps. Some pump curve shapes i.e. flat pump curves, may limit the effectiveness of this approach as the energy use per unit of water applied would be constant.
An inventory of irrigation pumps is not available at this time. In Chelan County of Washington State alone, there are over 1,100 irrigation pumps with over 10,000 connected hp (White, et. al., 2012). The 2008 the United States Department of Agriculture (USDA) Farm and Ranch Irrigation Survey lists 81,257 irrigation well pumps in the Northwest (WA, OR, ID, MT). An additional 47,824 pumps take suction from ponds, lakes, reservoirs, and rivers. Another 11,057 pumps are employed as booster pumps (USDA, 2015), for a total of 140,138 pumps. NEEA indicates that there are 6,881,000 irrigated acres in the Northwest, with 3,165,260 acres using center pivot technology on farms over 100 acres in size (Navigant Consulting, Inc., 2013). Flows to center pivots range from 200 gpm to well over 1,000 gpm.
For the purposes of estimating VFD optimization energy savings potential, it will be assumed that 10% of the pumps can ultimately be retrofitted with VFDs. We will assume an average pump size of 40 hp to give: 140,138 pumps x 10% x 40 hp/pump = 560,552 connected hp suitable for optimization.
Regional Technical Potential:
0.03 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: hp
First costs are dependent upon the number of pressure gauges that are installed --- which in turn is dependent upon the terrain on the farm. The pumps are operated to maintain a pressure setpoint at a limiting point in a subset of the irrigation system. Water applications remain constant through the use of pressure reduction valves. In its most simple form, the operator can simply toggle a switch to select the appropriate pressure setpoint for each section of his irrigation system. Cost data will be available when the BPA and NRCS release their initial project report.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Reference and Citations:
Northwest Agricultural Irrigation Market Characterization and Baseline Study
Northwest Energy Efficiency Alliance
BPA Emerging Technologies for Energy Efficiency: Industrial & Agricultural Technology
Bonneville Power Administration
2008 Farm and Ranch Irrigation Survey
Census of Agriculture, U.S. Department of Agriculture
Agriculture Sector Update
Bonneville Power Administration, Power in Efficiency Summit
White, et. al.,
Irrigation Pump Variable Frequency Drive (VFD) Energy Savings Calculation Methodology
Chelan County Public Utility District No. 1
More Crop per Drop with Variable Speed Pumps
National VFD Optimization Grant
Wy'East Resource Conservation and Development
Variable Speed Drive (VSD) for Irrigation Pumping
Natural Resources Conservation Service, U.S. Department of Agriculture
Variable Frequency Drive (VFD) Controlled Irrigation Pumps – Analysis of Potential Rebate
Irrigation Training and Research Center for the California Energy Commission
Energy Efficiency: Variable Frequency Drive
Bonneville Power Administration