Pressure distribution on center-pivot lateral lines: comparative results and energy conservation with a variable speed drive

Abstract

The use of variable speed drives to control irrigation pumping running speed is one of the strategies to reduce energy consumption of center pivot systems operating on undulating topography. By varying the frequency of the power supply as the lateral line rotates in the field, pivot point pressure may be adjusted in order to decrease the excess pressure that is dissipated by pressure regulators at each outlet. However, in order to reduce energy consumption of a center pivot system operating with a variable speed drive without compromising the center pivot water application uniformity, the pumping unit behavior, the digital elevation model of the irrigated field, and the pressure distribution along the lateral line must all be fully characterized. A detailed description of pressure distribution along a center pivot lateral line and adequate knowledge of the minimum pressure point position along the center pivot lateral are required in order to define strategies to reduce energy consumption in center pivots operating with a variable pumping running speed. In this sense, this research aimed to study the spatial distribution of pressure along center pivot lateral line in plots with variable topography, and to analyze the energy conservation promoted by using variable speed drives in center pivot pumping units. As described in article 1, pressure distributions along a center pivot lateral line determined by four different analytical models were compared to pressure distribution determined by numerical solutions provided by the EPANET hydraulic simulator. Several different lateral line configurations were considered in this study: lateral line with one and two pipe diameters, application of different gross irrigation depths per revolution time, different values of the pipe wall roughness coefficients, different values of the ground slope, and lateral lines operating with and without an end gun. For all different center pivot lateral line configurations considered, two different friction head loss equations were used: the Darcy-Weisbach equation, and the Hazen-Williams equation. Regardless of friction head loss equation considered, the estimates of head loss distribution obtained with the four analytical solutions were very similar to those calculated with the EPANET hydraulic simulator. These results indicate that the detailed description of the flow distribution along the center pivot lateral line that is required by the numerical method did not improve the accuracy of the head loss distribution. In addition, the tedious process of editing input files required by EPANET may be restricted for its use. In this sense, in article 2 the VSPM (Variable Speed Pivot Model) was developed to perform hydraulic and energy analyses of center pivot systems using the EPANET hydraulics engine. Besides improving the process of editing EPANET input files, this tool can determine, using any type of digital elevation model, the elevation of each tower for each angular position of the center pivot lateral line. It is also capable of simulating, in an accurate manner, the performance of the center pivot controlled with a VSD. The possible reduction in energy consumption with the use of a VSD was simulated for a center pivot system located in Albacete (Spain), in an irrigated area presenting a 15.3 m maximum topographic elevation difference. This simulation indicated that energy consumption with a VSD is 12.2% lower than the energy consumption observed with the commonly used fixed pumping running speed. This reduction in energy consumption correspond to a reduction of 2,821.47 € on energy consumption expenses.