Design and implementation of an off-grid PV drive system for small scale fresh produce cooling

[摘要] ENGLISH ABSTRACT: The implementation of a PV drive system is discussed in this thesis with the focuson fresh produce cooling for small-scale farmer. A background study is presented, which firstly addresses hunger and food insecurity through emphasis on one of the mentioned six causes of hunger, namely, food waste. Further focus is placed on cooling and, specifically,solar cooling. Therefore, the relevant factors pertaining to cooling and solar energy as a power source are presented, adding to the background study.Thereafter, the project details are outlined which states the research significance as the investigation of feasibility, in terms of cost and performance, of the design and implementation of PV drive systems, using the proposed development process. With thatbeing said, the design of the PV drive system, elevation of low-voltage solar PV energy and cost-effective implementation were evaluated.In a high-level project overview the project parts are presented for the designed and implemented PV drive system, with the focus on the solar energy source and the cooling application. Similarly, the proposed development process is discussed through which thedrive system was implemented. In effect, this process entails using off-the-shelf subsystems,as far as possible, to construct the drive system with only the necessary self-built subsystems. The drive system consists of the following self-built subsystems, namely the DC-DC converter and control module, and a commercial VSD, with integration betweenthese subsystems.As the main means of voltage elevation, from the low-voltage solar PV source, the focus is placed on DC-DC converters. Furthermore, a comparison of DC-DC converters are made, with the defined criteria for the project, for the most suitable converter. Foreasier comparison, the converters are grouped and their advantages and disadvantages are presented. Ultimately, a converter choice was made.The chosen converter, the resonant push-pull converter, was thoroughly discussed, analysed and specifically designed for the project. Furthermore, this theoretical design of the converter was firstly simulated, for confirmation, before physical construction proceeded.Thereafter, converter tests were done to verify the design through various measurements.Most importantly, the converter output voltage range and efficiency were measured and verified to a satisfactory degree.The system control module allowed for proper operation of the converter and, moreover,the integration of the PV drive subsystems. Furthermore, the microcontroller-basedcontrol module is discussed and the implemented algorithms are presented for optimalenergy transfer; these algorithms were expressed in flow diagrams. In addition, transientcontrol results are shown for start-up and load variations for verification.After verification of the main subsystems, the complete PV drive system was operatedthroughout a whole day with an appropriate load, to ensure proper conclusions aremade. Unfortunately, a cooling system could not be acquired for testing. Details for thedemonstration are presented, which include the specific equipment used and results forthe operation. Moreover, the results present a fully operational drive system from a lowvoltagesolar PV energy source able to drive the appropriate load, which demonstratessuccessful performance. However, the drive system included performance shortfalls, whichare discussed and for which recommendations were provided.With the drive system operational, conclusions were made, regarding cost and performance,in comparison with other similar systems. In addition, the research objectivesand aims were evaluated. After evaluation, although the PV drive system could be designedand implemented to a successful measure with regards to performance, it fell shortregarding cost in comparison with the compared systems. Therefore, the proposed PVdrive system, with the development process followed, is not completely feasible in implementation-to the degree described- for fresh produce cooling for small-scale farmers.

[发布机构] Stellenbosch University