Large power hybrid PV pumping for irrigation

Abstract

The aim of this thesis is to develop technical solutions for the reliable and efficient performance of large-power hybrid photovoltaic (PV) irrigation systems. These solutions have been applied to the design and implementation of two real-scale large-power hybrid PV drip irrigation demonstrators – a 140 kWp hybrid PV-diesel system in Alter do Chao, Portugal, and a 120 kWp hybrid PV-grid system in Tamelalt, Morocco. Both systems have been working since 2016 and include monitoring systems. In order to do a technical and economic validation of the systems from these monitoring data, it has been necessary to develop new performance indices because, unlike PV grid-connected systems, the operation of this type of systems is affected by factors others than its quality. So, the typical performance ratio (PR) has been factorized in 4 distinct indicators: PRPV (which includes the losses strictly related with the PV system), URIP (which varies with the particular crop and the irrigation period), URPVIS (which is intrinsic to the PVIS design), and UREF (which gives an idea of the use of the system, it is influenced by the monthly irrigation scheduling and the availability of water in the source). The main technical solutions developed include first, an algorithm that allows the elimination of the problems associated with PV-power intermittences caused, for example, by a passing cloud; second, the match between PV production and irrigation needs through the use of a North-South horizontal axis tracker (N-S); and third the integration of the PV system in the pre-existing irrigation network through solutions which maximize the use of PV energy. This thesis is structured in 2 different parts. The first one presents the results of the technical and economic validation of the demonstrators. In Portugal, the PV share (PVS) during the irrigation period is 0.49 (in 2017) and 0.36 (in 2018), and the PR is 0.16 (in 2017) and 0.22 (in 2018) extremely influenced by the use of the system (UREF of 0.29 and 0.44 respectively). In Morocco, in 2017 and 2018, the PVS is 0.48 and 0.55, and the PR is 0.24 (with a UREF of 0.32) and 0.29 (UREF of 0.36) respectively. The economic results show an initial investment cost of 1.2 €/Wp, a payback period of 8.8 years in Portugal and 7 in Morocco and, finally, a Levelized Cost of Energy of 0.13 €/kWh in Portugal and 0.07 €/kWh in Morocco, which leads to savings of 61% and 66% in Portugal and Morocco respectively. In the second part of the thesis, three other novel contributions for the design of large-power PV irrigation systems are made. The first one is a new type of PV generator structure, the Delta structure, which has the objective to achieve constant in-plane irradiance profiles when the end-users do not want to install trackers. It is worth noting that the peak power needed in this structure to achieve the same water volume of the N-S tracker is lower than the one needed with the typical static structure oriented to the Equator. The second study evaluates the losses in a PV irrigation system depending on the number of PV modules in series of a PV generator. It is possible to conclude that these losses are irrelevant in most situations, casting doubts about the complex designs that are being offered by the market to avoid them. In places with very high mean temperatures, in a stand-alone PV system, these losses can be eliminated with the increase in the number of PV modules in series. On the other hand, in a hybrid PV-grid system it is impossible to eliminate the losses, but they can be minimized. Finally, a new pump selection method for PV irrigation systems working at a variable frequency is proposed. A simulation exercise carried out for three different places in the Mediterranean zone shows that the water volume pumped by a PV irrigation system with a pump selected with this new method is 7.3 to 20.5% higher than the one pumped with the pump selected with the traditional method.