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328 CHAPTER 9 Design Principles of Photovoltaic Irrigation Systems

6. Calculation of technical and economic performance of the system. From the
water and energy balance results, performance indicators of the system can be
calculated. These performance indicators allow the technical and economic
evaluation of the designed system. Technical indicators include the efficiency of
water and energy use, that is, the fraction of the irrigation needs that have been
satisfied and the fraction of the produced energy that has been used. Economic
indicators, such as the net present value (NPV), payback period, profitability
index, or internal rate of return are useful to evaluate the economic profitability
of the designed system.
7. Repeating the simulation process for other PP values and other design pa-
rameters. Once the complete simulation (steps 1 to 6) of the system has been
performed for one set of parameters, the process must be repeated by changing
the set of input parameters. The main variable that must be considered to
optimize the PV irrigation system is the PP of the PV plant. To perform the
required iterations quickly, a computer simulation program must be developed.
The authors have developed a software program on an Excel spreadsheet to
automate the process.
For the sake of illustration of the procedure discussed in this section, Table 9.4
shows the results of a daily simulation of a simple PV pumping system consisting of
a fixed speed pump that lifts water to a tank. In this example, 3-min periods are
considered and the water pump is put into operation when the produced PV power
is higher than a threshold power that is required to lift the water to the tank.
Using this procedure, the authors have optimized and assessed the profitability of
different types of more complex PV irrigation systems. They applied this methodol-
ogy to an olive orchard farm [22] and to a greenhouse farm for growing vegetables in
the south of Spain [24].
In the first case, the authors proposed combining the methodology discussed in
this section together with a deficit irrigation approach. This new approach to PV irri-
gation design, termed Photovoltaic Opportunity Irrigation, proved to be very effi-
cient both from a technical and economic point of view as it improved the
efficiency of irrigation water, the indices of energy used, and the NPVof the system.
The results found in this work show that significant reductions in the PP of the PV
system could be achieved by using the proposed methodology. The economically
optimal designs provided by the model were not achieved for a complete irrigation
of the crop but for a deficit one. This implied that not only a reduction in the invest-
ment costs of the PV system but also some water savings (from 4% to 6% in the
study case), which is very important in areas with water scarcity.
In the second case, the authors concluded that stand-alone direct pumping irriga-
tion systems were a technical and economically feasible alternative to irrigate green-
house crops, provided that an appropriate design of the irrigation system was carried
out. Particularly, the number of irrigation sectors was a variable that significantly
affected the system profitability.