5. Discussion
Thesizingof standalonephotovoltaicwaterpumpingsystemsis extremelyaffected bythe availabilityof solar energyand the water requirements and typically the system is sized in order to meet the worst requirements. The main focus of this paper has been then the assessment of water demand for irrigation and solar energy inordertooptimizethesystemsize.Dynamicmodellingofthesys- tem has been then used to prove the designing phase verifying the balance between crop water requirements and water raised up by the photovoltaic water pumping system. Two issues need to have further investigations: the implementation of wind energy as renewable energy to drive water pumping systems and the usage of the system when irrigation is not needed. In this work, the pos- sibility to implement wind energy as an alternative source has not been discussed as well as the related assessment of available wind energy resources mainly because of the abundant solar resources featuring the site chosen for the feasibility study and the compet- itive current cost of PV cells. Nevertheless, further works will be conducted towards the assessment of the available wind energy resources and the possibility to technically and economically implement small scale wind turbines to drive water pumping sys- tems. Comparisons between photovoltaic systems, wind energy and hybrid renewable energy systems as sources to provide elec- tricity for water pumping systems will be addressed. In addition, the assessment will be extended to the whole China, especially to the whole Chinese grassland, and not only restricted to a single area in order to identify the best feasible areas for the implemen- tation of solar and wind energy or combination of both. Since the irrigation season usually takes few months of the year and the water demand during the irrigation season varies extremely depending on the growing phase of the crop, the system is over- sized for the most of the irrigation season and is not used for the remaining months of the year. Possible solutions of this oversizing strictly depend on the locations of the system. If the systems are installed close to farms or villages, the combination of water pumping and electricity entry in mini standalone grid or in storage system such as batteries can be a possible solution. In the case the systems are located far from possible users, the mobilization of the system,especially of the photovoltaicmodules, couldbe a dodgein order to harness them for electricity production in other sites. The possibility for the same system to combine both water pumping and feeding existing electric grid represents a notable economic advantage permitting to a standalone system to absolve multiple scopes and generate profit.
6. Conclusions
In this study a dynamic simulation tool combining the models of the water demand, the solar PV power and pumping system was developed in order to be used for quick design and design val- idation. According to the achieved results the following conclusion can be pointed out:
The sizing of photovoltaic water pumping systems for irrigation is extremely affected by the dynamic character of the water demand and collectable solar energy. In order to define the
Table 4 PV water pumping system components unit costs.
Component Unit cost PV modules 0.6 $/Wp Structure 0.1 $/Wp PV tracking system 0.9 $/Wp DC/AC inverter 0.2 $/Wp DC/DC converter 0.06 $/Wp AC pump 0.1 $/W DC pump 0.4 $/W Engineering and installation 20% (IC)
Fig. 12. Total initial investment costs for the PVWPS proposed.
644 P.E. Campana et al./Applied Energy 112 (2013) 635–645
worst condition on the basis of which the system is sized, the lowest ratio between required hydraulic energy and available solar radiation has to be considered. The assessment of the overall system efficiency gs is relevant to optimally size the PV array power peak avoiding both system failure and economic losses. gs summarizes in a steady value the dynamic efficiency of the system components and the opti- mal value can only be set through system dynamic simulations verifyingthe matchbetweenpumpedwaterand waterdemand. Preliminary economic analysis based on the initial investment costs showed that AC pump powered by fixed PV array repre- sent the most cost-effective solution for water pumping.
Acknowledgements
The authors are grateful to the Swedish International Develop- ment Cooperation Agency (Project No.: AKT-2010-040) for the financial support.
References