Energy conservation using variable-frequency drives for center-pivot irrigation: Standard systems

Energy use efficiency and energy consumption rates by center-pivot irrigation systems have been of utmost importance to the scientific community. Numerous studies have addressed different approaches to achieve the greatest energy use efficiency, but each encountered problems due to lack of detailed field information. This study introduces an approach to reduce energy consumption for center-pivot irrigation by varying the pump power supply speed using variable frequency drive (VFD) technology. By varying the frequency of the power supply, the minimum required pivot point pressure can be provided at any required flow rate and position in a field. The model was developed using high-resolution digital elevation model (DEM) datasets and geographic information system (GIS) to simulate the minimum pressure required by 100 standard equipped and randomly selected pivots located in ten Nebraska counties. The variable topography within an individual field played an important role in determining the required pressure and thus energy consumption. The resulting energy reduction using a variable-speed motor approach was calculated for each center-pivot irrigated field. On average, the pivots in Cedar County showed the largest potential for energy reduction (9.6%) and also contained the largest elevation difference (13.6 m) between the pivot point and the highest point traversed by a pivot tower. In contrast, the pivots in Hamilton County showed low potential for energy reduction and contained the smallest average elevation difference (3.5 m). To determine the potential economic benefit from the variable-speed motor approach, annual monetary savings were calculated for each installation and county. The pivots in Custer County would gain the greatest annual returns ($457 average) due to relatively long hours of operation, whereas the pivots in Hamilton County would achieve negligible savings due to gentle topography and few hours of operation. The county-average payback period for the capital cost of a VFD was found to exceed the economic life of the VFD in all ten counties.