ARTIGO TÉCNICO ENERGY EFFICIENCY OF A CENTER PIVOT IRRIGATION SYSTEM

This study aimed to evaluate the energy efficiency of a center pivot irrigation system operating in a terrain of variable topography. Values of Pumping Energy Efficiency (PEE), Supply Energy Efficiency (SEE), Global Energy Efficiency (GEE) and Specific Energy (Es in kWh m -3 ) computed at 18 different angular positions of the lateral line were used as energy efficiency indicators. An ultrasonic flow meter, digital pressure transducers and a power quality analyzer were used in order to evaluate hydraulic (total system flow-Q and total dynamic head-TDH) and electrical parameters (active electrical power AEP) of the center pivot pumping unit that were required for evaluating the selected energy efficiency indictors. Topographic elevations of the water source, the pumping unit and of the center lateral line were also determined. For the center pivot lateral line, it was necessary to determine, at the 18 angular positions considered, the altitude of the track of each center pivot support tower. Results indicated that currently, even after more than 10000h of use, the center pivot system operates with satisfactory energy efficiency, as indicated by an average GEE value equal to 42.5%, that is classified as “good”. Statistical analysis indicated that the topographic disposition of the center pivot lateral line, as characterized by a uphill or downhill disposition, resulted on different PEE, SEE and GEE values, while the average Es value (0.42 kWh m -3 ) was not affected by the lateral line disposition.


INTRODUCTION
In the near future due to water and energy constraints, production costs in agricultural areas tend to increase, and with this there will be the need to explore new technologies and methodologies for the management of irrigation systems (Evans & King, 2012).
In Brazil, the use of center pivot has been widely diffused on agricultural irrigated areas due to several advantages (automation, uniformity, reduction of manpower) that this equipment provides in relation to other types of irrigation system.Nowadays, there are 1.28 million hectares irrigated by center pivot system in Brazil.This represents an increase of 43.3% in relat ion to the 2006 center pivot irrigated area in Brazil (Gu imaraes & Landau, 2016).
In this context of irrigated agriculture expansion and need of water and energy efficiency monitoring, different energy efficiency indicators were proposed (Rodríguez Díaz et al., 2011;Córcoles et al., 2010).According to Tarjuelo et al. (2015), it is important to implement an energy evaluation routine not only to determine the energy efficiency of an irrigation system, but also to assist in the decisions making process regarding improvements in the water distribution system, in order to optimize energy consumption and economic planning.
For the specific case of center pivot irrigation systems in Brazil, Cezar-de- Lima et al. (2008) and Schons et al. (2012) used a specific normalized energy consumption index that is expressed in kWh.mm −1 .ha−1 .100m−1 .Th is index represents the amount of active electrical energy (kWh) that is required in order to pump a normalized volu me o f water (mm.ha= 10m 3 ) with an standart values of total dynamic head (100m).
For the case of pressurized irrigation systems, Abadia et al. (2008) proposed a procedure for calculation of a Global Energy Efficiency (GEE) indicator.This indicator is the result of the product of two other energy efficiency ind icators: Pu mping Energy Efficiency (PEE) and Supply Energy Efficiency (SEE).The Pu mping Energy Efficiency (PEE) is related to the efficiency of the pumping unit (electric motor efficiency mu ltiplied by centrifugal pu mp efficiency).Supply Energy Efficiency (SEE) is related to the design and management of the water distribution system.
The Specific Energy (Es in kWh m -3 ) index is widely used to evaluate the energy performance of water distribution systems of different configurations (Córco les et al., 2010;Abadia et al., 2010).The ES represents the amount of active electrical energy (kWh) that is required in order to pump a normalized volume of water (1,0 m 3 ).For practical p roposes, the amount of electricity energy consumed by an irrigation system, that is measured in kWh, can be determined by multip lying the total volume of irrigated water pumped in the cropped area ( m 3 ) by the Specific Energy value (Es, in kWh m -3 ) of the pumping unit.According to Urrestarazu & Burt (2012) reduction on the amount of energy (kWh) consumed for pumping irrigation water in a irrigation system can be achieved by both methods: (i) reduction in the pumped water volu me (m 3 ); and (ii) reduction on the pumping unit Specific Energy (kWh m -3 ) value.
Cezar-de- Lima et al. (2008) pointed out that indicators of irrigation systems energy consumption mu st be in some way normalized, allo wing the co mparison of different irrigation systems, installed in areas of different topographic conditions.Abadia et al. (2010), Abadia et al. (2008) also warn that, for the particular case of irrigation systems operating in terrains with variable topographic conditions, energy efficiency indicators should be analyzed with caution, as this analysis may lead to errors of interpretation and, therefore, in non-representative GEE values.
The aim of this study was to evaluate the energy efficiency use of a center pivot irrigation system that operates in a terrain of varied topography by analyzing the behavior of different energy efficiency indicators (PEE, SEE, GEE, Es) that were adapted for this type of pressurized irrigation system.

Characterizati on of the area
The study was carried out in a center pivot irrigation equip ment located at the Fazenda Invernada in the municipality of Bo m Sucesso-MG, with UTM coordinates of 23K 50940 2.45 m E, 7662306.20 m S.

Equi pment Characteristics
The technical characteristics of the center pivot system used in this study, as described in the original design datasheet of the manufacturer's, are reproduced in Table 1.Along the center pivot lateral line, at the outlet end of ¾"flexib le drop pipes that are regularly spaced by approximately 2,3, there are 189 Senninger R Iwob type emitters.The pressure head at the inlet section of each one of these emitters is controlled by a 10 psi nominal pressure regulating valve (PRV), that according to the manufacturer specifications (Senninger, 2017) operates adequately in pressure range from 69 kPa (hPRV = 7.02 m) up to 690 kPa (70.36 m).

Topography of the terrain
The planialt imetric survey of the irrigated area and the elevations of the pumping unit and the water supply level were determined with the aid of a GPS device of Topcom brand, Hyper Lite + model (accuracy of 5 mm).
In order to generate topographic level curves on the irrigated area, the topographic data from this survey was submitted to an interpolation process provided by the QGIS software, 2.12 version.As indicated in Figure 1, the 18 different angular position of the center pivot lateral line, that were considered in this study represent different angular positions (α) assumed by the mobile lateral line along its rotation (0 0 < α ≤ 360 0 ) around the fixed p ivot point, located at the center of the irrigated area.These angular positions were determined by the different positions assumed in the field by each one the 8 mobile towers (T1 to T8) used to move the lateral line around the pivot point.Abadia et al. (2008) define the overall energy efficiency of an irrigation system (GEE) as the ratio between the required energy (Er) for irrigation and the actual energy consumed (Ec) in the irrigation process.For each one of the 18 angular position (α) assumed by the moving lateral, the GEE values were co mputed as percentage (%), based on [eq.( 1)] the product of two efficiency components: the supply energy efficiency (SEE) and the pumping energy efficiency (PEE).The energy efficiency of the pumping unit (PEE) is expressed by the ratio between the delivered hydraulic power (P H ) and the active electrical power (A EP), as expressed by [eq.( 2)] ( Abadia et al., 2008):
Total system flow values (Q (α) ) were determined using a portable FMS 175 brand ultrasonic flowmeter with built-in data logger.The flow meter was installed on the pump unit suction pipe, observing minimu m values of straight-pipe runs upstream and downstream fro m sensitive flow elements.According to the manufacturer's specifications, for flo w velocity above 0.18 m/s, the equipment used has an accuracy of 1%, with ± 0.5% linearity and ± 0.2% repeatability.Pressure values at inlet and outlet sections of the pump were measured.Absolute pressure values at the pump inlet section were measured with an Instrutherm pressurure transducer, VA-318 model, connected to a VDR-920 d igital reader.A digital Instrutherm brand, PS100-20 BAR model, pressure head transducer, with a 20 bar maximu m capacity, connected to a digital reader, Instrutherm M RV-87 model, was used for recording pressure head values at the inlet section of the pump.Both digital readers are equipped with a RS232 output that was used to store the measured pressure values in a co mputer file.
TDH values were obtained by correcting the difference between relative pressure head values taken at the pump outlet section (PL out /) and at the pumping inlet section for differences on water velocity head values and elevation values in these sections [eq.( 3)].Relative pressure head values at the inlet pumping section were obtained by subtracting from the absolute pressure head values mensured at this section (PL entrance /γ) the absolute value of the atmospheric pressure (P atm /γ) observed at the same time in which the inlet pressure measurement were taken. ) where, PL outlet (α) -pressure at the outlet of the pump in the angular position (α), kPa; PL entrance (α) -absolute pressure at the pump entrance in the angular position (α), kPa; P atm -at mospheric pressure, 91.39 kPa; V outlet (α) -averange water flow velocity at the pump outlet section, m.s -1 ; V inlet (α) -averange water flo w velocity at the pump inlet section, m.s -1 ; Z inlet (α -geometric elevation of the absolute pressure sensor at the pump in let, m; Z outlet (α) -geometric elevation of the effect ive pressure sensor at the pump outlet, m, irrigation water specific weight, kN.m -3 .
Active electrical power values (A EP) were measured with a Flu ke® power quality analyzer, 435-II model, with 0.5% accuracy, with built-in data logger.The power quality analy zer was installed in a three-phase three-wire connection at the starter protection system of the 110 kW nominal power asynchronous motor used to drive the center pivot pump.
Supply Energy Efficiency (SEE) values are directly related to the effects of topographic variations on the spatial distribution of the ratio of required pressure available p ressure at the irrigated area.In order to apply the methodology proposed by Abadia et al. (2008) to compute SEE values based on the ratio of system water head balance (ΔWH) and TDH, each one of the 18 center pivot lateral line angular positions of the was considered as a different irrigation project.Further, it was considered that which one of these 18 "irrigation pro jects" is supposed to pump irrigation water to feed 189 different irrigated fields, that were identified by an J index in the range 1 ≤ J ≤189.By attributing to each "irrigation pro ject" a total number of irrigated fields equal to the total number of emitters installed along the center pivot lateral line, it was possible to estimate to each one of "irrigated field", that is part of an irrigation project, values of field irrigated area (Aj), field average topographic elevation (Zj), required inlet pressure head (hprv), equal to the ones corresponding to the emitter with the same J index value along the center pivot lateral line.Th is adaptation to the methodology proposed by Abadia et al. (2008) to co mpute SEE is shown in [eq.( 4)]: (5) where, Z e j, i (α) -elevation of the emitter (j) in the tower (i), m; (Zt i-1 ) (α) -wheel track elevation, at angular position α of the tower demarking the inlet end of the i th span, m; rg j -distance fro m center pivot in let to the inlet point of index j emitter's drop pipe, m; rt i-1 -wheel track radius of the tower demarking the inlet end of the index i span, m; In [eq.( 5)] the constant 2.15 m represents the height of the water entrance point in the pressure regulating valves installed in each emitter.

Es calculation
At each angular position (α), simultaneous measured values of active electric power ( A EP in kW) and total flow system flow ( Q in m3s -1) were used to determine specific energy consumption values (Es in kWh m-3) as indicated in In [Eq.( 5 Abadia et al. (2008) in their study found a PEE value equal to 63.8%.These authors attributed this values to the capacity of the pumping unit to provide a flow and pressure head constant.This study, it observed that the values of Q showed a maximu m variation of 7% and a maximu m of 6% for TDH, and according to the characteristic curve of the centrifugal pu mp in studing (Figure 2), these pairs of points are close to the highest efficiency points, corroborating with the in formation described above by Abadia et al. (2010).

RES ULTS AND DISCUSS ION
The analysis of the PEE adequacy value is relatively simple and when there is no equipment installed in the pumping unit such as frequency inverters and cable efficiency values, the PEE can be expressed by the product of efficiency of the pump and the electric motor, in which "theoretical" PEE values can be calculated from pump and motor performance data taken from commercial catalogs (Moreno et al., 2010).However, Urrestarazu & Burt (2012) emphasize the importance of verifying in the field the validity of such "theoretical" values.
Thus, the analysis of the adequacy of specific energy consumption values must necessarily consider the adequacy of the observed value of active electric power (kW) required to the activation of the pumping unit operating at the desired flow rate.For a same pumped flo w value, the active electrical power required for activation of a pumping unit (kW) it's affected by both the total TDH value supplied by the pump and the PEE value.
Therefore, for the same pu mp value, any analysis of the reduction possibility in the observed value of specific energy consumption (Es in kWh m -3 ) is necessarily conditioned: (i) to the analysis of the adequacy of the observed value of TDH provided by the pump; (ii) to the analysis of the adequacy of the PEE value of the motorpump set used.Luc et al. (2006) report that the PEE may have a deviation in its values due to the interference of other factors such as TDH or motor and pump efficiency, which corroborates with the data obtained here in this study where TDH (Table 2) and the pump and motor efficiency (Figure 2) have their values changed in function of the angular position of the lateral line in the area (Table 2).This behavior is also observed in this s tudy through a quadratic polynomial regression analysis of the data described in Table 1 of TDH correlated with PEE, resulting in a R² value of 0.49.Urrestarazu & Burt, (2012) report that for operating conditions of pumping units in California the factor that most correlates with PEE is TDH.When the PEE is correlated with Q values, the obtained value of R² is only 0.25.
The PEE can be classified according to the methodology proposed by Abadia et al. (2008) adapted fro m Pelli & Hit z (2000), which varies fro m unacceptable efficiency (PEE <45%) up to excellent (PEE> 65%).

Suppl y Energy Efficiency (S EE)
The average value of SEE in all the tested positions was 57%, registering a maximu m value of 61.66%, referring to the angular position of 356° (uphill) (Figure 3), region with the highest WHD value due to greater geometric elevation in irrigated area (Figure 2).The lowest value of SEE was 50.7% for angular position of 96° (downhill) (Figure 3), position with lower WHD value due to the lower geo metric elevation.Abadia et al. (2012) report that SEE values are influenced by a pumping of water with excessive pressure head to regions downhill slopes, thus generating a waste of energy.The verification of leaks in lateral and mainline pipe should also be verified, as it influences the SEE of system.
An alternative to improve SEE and PEE is the installation of frequency inverters in pump ing unit the center pivot when this system operates in irregular areas.According to Dos Santos Lima et al. (2015) the use of frequency inverters reduces energy consumption due to the use of the resulting TDH fro m the difference in level between the center of the pivot and its end the lateral line.However, the implementation of a frequency inverter is conditioned a previous economic feasibility study that varies in each case study (Campana et al., 2000).

Gl obal Energy Efficiency (GEE)
For the GEE values, according to the data in Table 2 and Figure 3, there was a variation between maximu m and min imu m values of 7%, with the maximu m value being 45% for uphill region (356°) and minimu m of 36 % for the angular position 76° (downhill).According to the GEE qualification proposed by Abadia et al. (2008) adapted fro m Pelli & Hit z (2000) the average value of 42.52% is classified as "excellent".This behavior of the GEE (Figure 3) fo llo ws the PEE and SEE behaviors computed during lateral line rotation, since the energy excess applied in the region where is downhill causes the efficiency of the system to reduce.

Specific consumpti on (Es)
The specific consumption data (Es) of the center pivot pumping unit studied here presented an average value of 0.42 kWh m -3 (Table 2).Co mparing the positions of the lateral line in area irrigated by center pivot, we can see higher values of Es for areas of higher elevation in the last tower (maximu m value equal 0.43 kWh m -3 for the 336° position) and smaller Es values for lower altitude area (0.416 kWh m -3 referring to the 96° position).Schons et al. (2012) find an estimated value of 0.41 kWh m -3 for a center pivot in the state of Rio Grande do Sul.These authors also report that Es can be used to compare projects within the same irrigation system, and to evaluate the efficiency and depreciation of equip ment.Plappally & Lienhard, (2012) found Es values in water pump ing stations.For the specific case of irrigation, these authors present values varying from 0.32 to 1.1 kWh m -3 for the localized irrigation system, and fro m 0.6 to 1.3 kWh m -3 for sprinkler irrigation system, for the case of the center pivot system to Australia conditions operating at a pressure head of approximately 400 kPa, the approximate Es value was 0.18 kWh m -3 .However, the comparison of specific energy consumption values for referring to different pumping stations should be made with caution.

Energy indicators x Topography of the terrai n
The PEE, SEE, GEE and Es indicators were submitted to the statistical Student t test at significance level of 5%, as described in Table 3, in function of the angular position (uphill and downhill) of the lateral line.The average values of Es it was considerate statistically equal for the angular positions in uphill and downhill.This observation differs fro m that made by Schons et al. (2012) where these authors relate values of Es only in function of the topography.Thus, the Es values may be more related to other factors, and according to Luc et al. (2006) who analyzed a group of 115 pump ing units with a capacity greater than 80 m³.h -1 , and found that there is a linear relat ion with determination index (R²) equal to 0.98 of the Es values with the TDH values.

CONCLUS IONS
It was possible to estimate the energy efficiency through the indicators adapted to the center pivot irrigation system.
The periodic monitoring of the center pivot irrigation system is essential for maintaining the energy efficiency levels as appropriate.
FIGURE 1. Angular positions of the center pivot movable lateral line.
) -balance of water head in the system, m; WHD (α) -required water head in the irrigated area at the angular position (α), m; WHI (α) -water head injected into the system in the angular position (α), m; Z j -geometric elevation of each emitter (j) installed on the lateral line, m; h P RV -minimu m pressure head required by the pressure regulating valve (PRV), 7.02 m; A j -area irrigated the emitter identified by the index j, m², Z a -geo metric elevation of the water supply level of the irrigation sys tem, m. ∆WH (α) values were co mputed considering the geometric elevation of water source level (Za = 806 m) and the minimu m pressure head value (hPRV = 7.02 m) required by the pressure regulator valve model (10psi or 69kPa) used on the emitters installed along the lateral line length.For each angular position (α), emitter elevation values (Zj, α) were co mputed based on the topographic elevation of each one of the eight A -framed mobile towers (T1 to T8) and on the distance among then: )] (Viholainen et al. 2012, Urrestarazu & Burt (2012), and Schons et al. (2012)).

TABLE 1 .
Characteristics of center pivot system as described at the original design datasheet.

Table 2
shows the values of Q, TDH, AEP and the energy efficiency indicators (PEE, SEE, GEE and Es) calculated for each angular position of the lateral line.

TABLE 3 .
Values of statistical Student test applied to the energy efficiency indicators PEE, SEE, GEE and Es.It was observed the average values of PEE, SEE and GEE differ statistically for uphill and downhill lateral line positions.
* Significant at a 5% probability level.