A techno-economic evaluation of a PV system providing simultaneous water heating and emergency power

Abstract

With an increase in both loadshedding and the price of electricity, homeowners are looking to reduce their dependence on the national electricity grid, both for reliability and financial benefit. While water heating with an electric resistance heater accounts for approximately 40% of the total power consumption in a normal household, appliances like lightbulbs and a refrigerator consume relatively little power. Therefore, a PV system that can save money with water heating while providing reliable emergency power during grid loadshedding will benefit any homeowner. In this study, a combined PV system which can provide simultaneous water heating and emergency power is under techno-economic review. A simulation model was developed to evaluate the proposed PV system. A single-diode model (SDM), which uses five parameters from a solar PV cell circuit, was used to model the power generation of a solar PV array. The model uses hourly solar radiation data from a climatic design year as input. The generated PV power is then used as input for the water heating and emergency power simulation. The water heating model uses thermodynamic conservation laws to determine the change in temperature within the geyser, taking into account a standard household water consumption profile, standing loss, and energy input from the PV system. The national grid is only used as an auxiliary power source to heat the water, should the water not be up to temperature after PV heating. Simultaneously, as the water is heated, the battery is also charged to be used as emergency power during loadshedding. Again, the grid is only used to charge the battery when the PV system is not sufficient. The model was verified with external literature and validated through experimental testing and hand calculations. The study focused on a side-by-side comparison of the different PV systems, defined in the study. Therefore, the model was not the main focus of the study, but rather means to compare the different systems. Three different PV systems were simulated, each having the same water heating capacity and PV array, but with different battery capacities. The smallest system included a 12V, 20Ah battery, only used for lightbulbs and a Wi-Fi router during a 4-hour loadshedding period. The second system used a 12V, 80Ah battery to also power a refrigerator. Lastly, the third system used a 12V, 120Ah battery, which is capable of powering all the previously mentioned appliances as well as a microwave and hairdryer, albeit not at the same time. The system inverters were sized according to the load requirements of the systems. The simulation was performed for all three systems in four different locations over South-Africa, including two coastal cities and two in-land cities. The study found that the PV systems generated around 2000 kWh for water heating in in-land cities, while only producing 1400 kWh for water-heating power in coastal cities. The savings, with regard to emergency power, for each system increased as the system size increased. The PV system saved, on average, 195 kWh/year for emergency power with system C (largest system), while system A (smallest system) only saved 37 kWh/year. The cost savings for the system were calculated by comparing the grid electricity bill without the PV system to the grid electricity bill after the system was implemented. The smallest system performed the best financially with a payback period of 6 years for inland cities and levelised cost of energy of 1.73 ZAR, while the largest system, with its large battery capacity, saved the most kWh and provided more backup hours and capacity during loadshedding. The study concluded that: Firstly, the proposed combined system worked, providing reliable emergency power and financial benefit through savings in water heating. Secondly, the factors which the homeowner values more, convenience through emergency power or financial benefit through water heating, determine which system is better suited to the household.