Establishing the most cost effective reliability for off-grid solar systems in Zimbabwe

Trade-off, between energy supply reliability and levelised cost of energy

Abstract

The traditional method for designing off-grid stand-alone solar energy systems is based on a monthly-average daily energy balance approach whose only objective is to provide 100% energy supply reliability. However, such an approach tends to grossly oversize the systems thus rendering solar off-grid systems too costly for the target communities. This study has focused on designing a cost effective off-grid solar power system to ensure balancing of the trade-off between cost and reliability of power supply. Based on a time-step energy balance approach, an Excel spreadsheet-based model was developed to optimise the solar stand-alone system. Two dimensionless variables representing the size of the two main components of a solar photovoltaic off-grid system- the solar photovoltaic (PV) array and battery- were used to define the system size. For a given level of supply reliability, there is an infinite number of combinations of PV array and battery size- as the PV array size is increased, the required battery size reduces in a certain trend. However, for the given level of reliability, only one PV array-battery combination (the Optimum Design) results in the minimum Levelised Cost of Energy (LCOE), whose coordinates depend on the relative costs of the two components. The LCOE for the Optimum Design corresponding to each level of supply reliability was plotted against supply reliability. From such a plot it was observed that the LCOE increases disproportionately above a certain level of reliability. This point, which lies near the “elbow” LCOE-reliability plot, defines the most cost-effective reliability for the stand-alone solar system, and therefore the optimum combination of PV generator and battery to deploy. The results showed that sustainable cost effective off-grid systems can be operated at 98% reliability level and still satisfy the customer requirements and at the same time ensuring affordable tariffs. Increasing the PV system components beyond the optimum (98% reliability) point, in a quest to achieve 100% reliability, results in a disproportionate 22% increase in LCOE.