Photovoltaic irrigation system study – new indexes and experimental values

QPV and the Instituto de Energía Solar, from the Universidad Politécnica de Madrid, have conducted a study on 2 years of operation of a high power battery-free photovoltaic irrigation

QPV and the Instituto de Energía Solar, from the Universidad Politécnica de Madrid, have conducted a study on 2 years of operation of a high power battery-free photovoltaic irrigation system located in Aldeanueva de Ebro, La Rioja. The design of these photovoltaic systems has not yet reached maturity and there is no experimental data available on their performance or their behavior in the face of PV power fluctuations. This fact makes experimental studies of real installations of great interest. This work has been presented at the EU PVSEC 2021 conferences, which this year are being held telematically.

The article presented incorporates the definition of new indices used to evaluate both the robustness of the system to PV power fluctuations (the “Number of abrupt stops” and the “Cloud Passage Resistance Index”) and the performance (factoring the traditional PR to determine the influence of different factors external to the system). In addition, it presents the experimental values for a 213 kWp photovoltaic irrigation system (PVIS: photovoltaic irrigation system) pumped to raft without batteries.

El PVIS estudiado está formado por un generador FV de 213 kWp conectado a dos variadores de frecuencia (FCs: frequency converters) de 110 kW, cada uno de los cuales alimenta a una bomba de 75 kW. Estas bombean agua del canal de Lodosa a una balsa situada a una altura manométrica total de 225 m. El PVIS se controla por medio de un PLC que estima la potencia FV disponible y arranca o para los variadores. Cuando se pone en marcha un variador la bomba a la que alimenta comienza a bombear.

The first relevant data used to evaluate the PV irrigation system is the number of PV abrupt stops. An abrupt shutdown is a sudden and uncontrolled shutdown of a drive that is usually caused by a rapid intermittency of PV power or an instability in the control system due to a malfunction of the control system. Abrupt shutdowns cause water hammer and surges that seriously threaten the integrity of the hydraulic and electrical part of the system.

The second index that evaluates the quality of PVIS is the cloud pass resistance index. A cloud pass is a fluctuation in PV power, usually due to the passage of a cloud over the PV generator that partially or completely shadows it, which can be resisted or cause the system to abruptly shut down. The cloud passage resistance ratio is the ratio of the number of resisted cloud passages to the total number of cloud passages supported by the system.

Finally, the performance ratio (PR) is used to analyze the overall performance of a PV system. Due to the particularities of high power PV irrigation systems, it is interesting to distinguish between the PR losses due to three different reasons: the non-irrigation period, associated with the crop water needs; the intrinsic characteristics of the PV system design; and circumstances external to the system, such as the habits of the irrigation community or the different rainfall over time, which can also affect the PR. Taking these into account, the RP can be expressed as the product of 4 different factors:

  • PRPV: is the PR considering only the losses strictly related to the PV system itself.
  • URIP: use ratio related to the irrigation period.
  • URPVIS: The PVIS design-related usage ratio: type of irrigation system, ratio between the peak PV power and the PV power required for irrigation, the type of PV generator tracking, and the accuracy of the PLC control algorithms setting.
  • UREF: use ratio related to the decisions taken by the irrigation community.

The study is carried out over two years of actual system operation. The percentage of abrupt stops and the cloud passing resistance index, after proper tuning of the system, reached values of 1.3% and 99.8%, respectively. To calculate the cloud passing resistance index, the study considers clouds associated with irradiance drops of 50% that generate a power drop of 40% in 3 or 4 seconds. This criterion was established after analyzing the clouds over three months and finding that 97.3% of them were associated with irradiance drops of 50% or less and that the highest frequency of this type of clouds was for those causing a power fluctuation of 3 or 4 seconds in duration.

The PR value during the first year was 60.0% and 46.8% during the second year. To understand the values obtained, the values of PRPV, URIP, URPVIS and UREF should be analyzed. The PRPV values are similar to the expected PR values for grid-connected PV systems (>80%). The URIP value is 100% or very close when the irrigation period extends over most of the year, but drops considerably when the irrigation period is shorter. URPVIS values, which are most affected by weather conditions and drive shutdowns, are above 70.0% and improve in the second year. UREF values are very close to 100%.

As a conclusion, it can be stated that in the case of high power PV irrigation systems, without batteries and pumping to a pond:

  • A value of 95% can be required for the cloud passage resistance index.
  • The expected PR in good quality systems is 65%. The expected values for the different factors are: PRPV > 80%, similar to the expected value in grid-connected PV systems; URIP≈100% if crop irrigation needs are spread over most of the year and/or if the sizing of the reservoir and PV generator are appropriate; URPVIS > 85% when the peak PV power is properly matched to the PV power required for irrigation and the PLC control algorithms and the tuning of the variable frequency drives is correct; and UREF≈100% if the end user makes proper use of the system.

The full article can be found in the publication of the conferees.

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