Energy expenditure in central pivot irrigation

Limiting the commercial availability of predetermined powers of electric motors may be the cause of excessive expenditure on electrical energy in irrigation systems, increasing expenses in this operation.

In recent years, electricity has become a fundamental factor for the agricultural sector, having a direct impact on the cost of production. It is known that rural properties that have a good supply of electricity have become more productive due to the use of technologies, such as the use of electric motors in irrigation systems, a practice that, in addition to increasing productivity, can provide of a differentiated product, of better quality and with the prospect of good prices in the market.

With the development of the agricultural sector, there was also an increase in the consumption of electrical energy, which is, for the most part, used by equipment that requires traction such as electric motors, equipment that transforms electrical energy into mechanical energy, being of great use for traction of loads in rural areas.

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The most used electric motors in rural areas, mainly in central pivot irrigation systems, are induction motors, as they present some advantages such as greater robustness, simple construction, low cost, ease of transportation, simple control, great versatility in adapting to loads of the most diverse types and are lighter and have better performance (Oliveira Filho et al.

The limitation of commercial availability of predetermined powers of electric motors is one of the reasons why many central pivot irrigation systems have oversized motors, working empty, that is, not fully using their nominal power.

When induction motors work at no load, their loading rate decreases. This index represents how much of the available power is being effectively used and the closer its value approaches 100%, the more efficient the system is, directly influencing electricity costs.

In the region of Cristalina (GO), a survey was carried out in commerce to verify the conditions most used in the sizing of central pivots. Information was collected on the service pressures of sprinklers working at low pressure, which corresponds to a service pressure of 196kPa for areas of 100, 110, 120 and 130 hectares, as well as their respective flow rates (Table 1).

To estimate the cost of electricity (see Equation 2 in the box), 3.500 hours of operation per year were considered, green hourly-seasonal tariff for subgroup A3a, supplied with a voltage of 34,5kV, with prices charged by Celg by Aneel resolution No. 02/2015.

The sizing of the pumps and electric motors, as well as the yields based on the area defined as irrigated, the consumption and expenditure on electrical energy, for the service pressure of 196kPa, can be seen in Table 1.

It is also observed that due to the fact that there are only a few predetermined powers of commercial engines, an engine of the same power (100 hp) was designed for the four predefined areas, causing oversizing and a consequent increase in electrical energy consumption by 10%, 8%, 6% and 5% per year, for areas of 100, 110, 120 and 130 hectares, respectively. The variation in the motor loading index is inversely proportional to the size of the areas analyzed, that is, the larger the area, the smaller the variation in this index, since the closer the power required on the pump shaft approaches the commercial power value ( 100hp), the higher its charging rate, reaching 97,18% in the case of the largest area (130 hectares).

It can be seen that the loading index varied from 76,73% to 97,18%, which means that the engines are not working with the efficiency provided by the manufacturer. In the smallest area (100ha), the loading rate is 23,27% below ideal, it is noted that the same engine, whose commercial power is 100hp, could irrigate 30 more hectares, reducing the variation in its loading index to 2,82%, making the system more efficient.

Because there are only a few predetermined powers of commercial engines, most of the time, irrigation systems are oversized, working empty and decreasing their efficiency, which causes a significant increase in consumption and expenditure on electrical energy, and the more the value The power requested on the pump shaft is close to the value of the power of the commercial engine, the value of its loading index is closer to 100% and, consequently, its efficiency is higher and the expenditure on electrical energy is lower.

Eloiny Guimarães Barbosa, Maria Joselma de Moraes, Marcos Eduardo Viana de Araujo, UEG

Article published in issue 169 of Cultivar Máquinas