QPV and the Instituto de Energía Solar – Universidad Politécnica de Madrid have carried out a study on 2 years of operation of a high power photovoltaic irrigation system without battery located in Aldeanueva de Ebro, La Rioja, Spain. 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 fluctuations in photovoltaic energy. This fact makes experimental studies of real installations of great interest. This work has been presented at the EU PVSEC 2021 conferences that this year will be carried out online.
The presented paper incorporates the definition of new indices used to assess both the robustness of the system to PV power fluctuations (the “Number of abrupt stops” and the “Passing-cloud resistance ratio”) and the performance (by factoring the traditional PR to determine the influence of different factors external to the system). In addition, it presents experimental values for a battery-free 213 kWp PV irrigation system (PVIS), pumping to a water pool.
The PVIS is composed by a single 213 kWp PV generator connected to two 110 kW frequency converters (FCs), each feeding a 75 kW pump, and pumps water from the Lodosa channel to a water pool with a total manometric head of 225 m. The PVIS is controlled by a PLC that estimates the available PV power and starts or stops the FCs. When a FC is on, the pump it feeds starts pumping.
The first relevant information used to asses the PV irrigation system is the number of abrupt stops. An abrupt stop is a sudden and uncontrolled stop of a frequency converter, usually provoked by a quick PV power intermittence or a control instability caused by a malfunction of the control system. Abrupt stops cause water hammer and AC overvoltages that seriously threaten the integrity of the hydraulic and electric components of the PVIS.
The second indicator used to asses the quality of the PVIS is the passing-cloud resistance ratio. A passing-cloud is a PV power fluctuation, usually due to a cloud passing over the PV generator that shades it totally or partially. Passing-clouds can be resisted by the system or can cause abrupt stops. The passing-cloud resistance ratio is the ratio of resisted passing-clouds to the total number of passing-clouds.
Finally, to analyse the global performance of PV systems it is used the performance ratio (PR). Due to the specificities of high-power PVIS, it is interesting to distinguish between PR losses for three different reasons: the non-irrigation period, associated to water needs of the crop; the intrinsic characteristics of the PV system design; and external circumstances that may affect the PR like the irrigation community habits or the different rainfall over time. Taking them into consideration, the PR can be expressed as a product of 4 different factors:
- PRPV is the PR considering only losses strictly related to the PV system itself.
- URIP is the utilization ratio related to the irrigation period.
- URPVIS is the utilization ratio related to the PVIS design (type of irrigation system, the ratio PV peak power - PV power required for irrigation, the tracking geometry and the accuracy of the PLC control algorithms setup).
- UREF is the utilization ratio related to the irrigator’s decisions.
The system has been analyzed over two years of real operation. The percentage of abrupt stops and the passing cloud resistance ratio after an adequate tuning were 1.3% and 99.8% respectively. In order to obtain the passing-cloud resistance ratio, the study considers clouds associated to 50% irradiance drops causing 40% power drops in a 3 or 4 seconds interval. This criterion is established after analyzing the existing clouds over three months and discovering that 97.3% of them were associated with irradiance drops of 50% or less and that the highest frequency of clouds of this type occurs for those in wich the power fluctuation lasts 3 or 4 seconds.
The PR was 60.0% the first year and 46.8% the second one. To understand these values, PRPV, URIP, URPVIS and UREF values must be analysed. PRPV values are similar to PR values expected for grid-connected PV systems (>80%). URIP is 100% or very close if the irrigation period is extended for most of the year, but decreases considerably when the irrigation period is shorter. URPVIS values, the most affected by weather conditions and FC stops, is over 70.0% and improves the second year. UREF values are close to 100%.
As a conclusion, it can be stated that for high-power PV irrigation systems without batteries and pumping to a water pool:
- A value of 95% can be required in the Passing-cloud resistance ratio index.
- The expected PR in good quality systems is 65%. The expected values of the different factors are: PRPV > 80%, similar to what is expected in PV systems connected to the grid; URIP≈100% if the needs of the crop are extended for most of the year and/or if the relative size of the pool with respect to the PV generator is appropriate; URPVIS > 85% with an adequate adjustment of the PV peak power to the PV power required for irrigation and the accuracy of the PLC control algorithms setup and tuning methodology and UREF≈100% if the end user makes an appropriate use of the system.