Optimization of number of PV cells connected in series for a direct-coupled PV system with lead-acid and lithium-ion batteries

PV systems can be installed in two main topologies, which are: a PV system directly coupled to a battery through a charge controller, and a PV system connected to a battery through an MPP-Tracker. A major advantage of a system with MPP-Tracker is that it can deliver maximum available PV power at any time, which can lead to higher yield of the PV system. On the other hand, the use of an MPP-Tracker increases the system investment cost making it unaffordable for low-income people. A direct-coupled PV system with a battery can be a cost effective alternative where the MPP-Tracker is not used. In this topology, PV modules are connected in parallel with batteries through a charge controller. This topology is simple and cheap to install. Normally, a PV module consists of cells, which are connected, in series to increase output voltage. In order to increase the amount of output current, the cells are connected in parallel. The number of PV cells in series determines also the U_MPP. Since the PV modules are connected in parallel with batteries, the batteries determine their operating voltages. The battery operating voltages and the number of PV cells in series affect the PV module’s output. The major drawback of this topology occurs if the battery voltage is far from the maximum power point (U_MPP) of the PV module, this lowers the output power of the PV module. Different loads connected in the system can cause a mismatch between battery and PV module. Not only that but also there will always be a great mismatch of the operating voltages if the difference between U_MPP and battery maximum voltage is high. This leads to a waste of PV output. For these systems, appropriate matching of battery and PV voltages is very important for high efficiencies. There are a number of factors, which also affect PV output when operated at both topologies. These include intensity of solar radiation, air temperature, shading, wind speed and type of the material used to manufacture PV panels. In this study, the number of PV cells connected in series is considered as the major factor affecting PV output for the direct-coupled systems. A direct-coupled PV model, which takes different numbers of PV cells in series into consideration, was developed. Other factors like temperature and solar radiation are also taken into account. An optimization tool for such a system was developed. The developed tool has a capability of selecting the optimal number of PV cells in series and parallel. It considers changes of solar radiation, temperature and load profiles in selecting the optimal results. The optimal system designs have fewer losses during their operation. They allow saving up to 20% of the investment cost. Furthermore, the selected optimal results have low levelized cost of electricity (LCOE).Sensitivity analysis was done by changing the load profiles to see how they affect the selection of the serial number of PV cells. Different PV modules with different cell voltages were also used to find how they affect the optimal results. Additionally the interest rates, battery lifetimes, battery and MPP-Tracker costs were also varied to see how they affect the optimal results.In order to have confidence in the optimization results, validation of the model was done through laboratory tests. In this study, two battery technologies were used. These include lead acid (Pb) and lithium nickel cobalt aluminium oxide (NCA) batteries. However, in the laboratory tests, due to safety reasons, only lead acid batteries were used