Device ensuring effective usage of photovoltaics for water heating

Abstract

The power graph of Nectar Sun regulator shows two operational regimes of the device: with 4 PV modules (maximum power about 540 W) and 6 PV modules (maximum power about 860 W). Nectar Sun quickly finds the MPP of PV modules and keeps it stable under different lighting conditions. Under real conditions, the boiler temperature graph is proportional to the power graph. In the system with four PV modules, the maximum water temperature does not exceed 58 °C. In the system with six PV modules, the maximum boiler water temperature increases by an average of 10 °C. Thus, the energy generated by PV modules is effectively supplied to the electric heating element. Additionally, Nectar Sun provides independent water temperature control during the day; it automatically connects to the power supply under insufficient sunlight. This guarantees normal boiler operation regardless of environmental conditions throughout the year. The integration of photovoltaic devices (PV) into the network could ensure the efficient use of solar energy. Nevertheless, when using PV systems for water heating, the main problem arises: if a heating element is directly connected to a solar module without using a device that regulates the supply of electric energy to the electric heating element in the boiler, the usage of photoelectric module becomes inefficient. When solar raying is small, the module current becomes equal to the current of the short circuit and the decrease in voltage in the heating element reduces the voltage in the solar module to almost zero. Therefore, it is not advisable to connect solar modules directly to electric heating elements. This study presents Nectar Sun—a device that provides the necessary connection. Nectar Sun is a DC to DA converter, which operates on the basis of a micro-switch, uses maximum power point tracking technology and regulates the supply of electricity to the heating element by pulse-width modulation. These qualities highly improve the characteristics of PV systems. When lighting is equal to 400 W/m2, without Nectar Sun regulator, the voltage of one module will approximately be 22 V, current—3.4 A and power—75 W. When Nectar Sun controller is added, it produces maximum power point (MPP) regime with Umax ≈ 30 V, Imax ≈ 3.2 A and Pmax ≈ 96 W. The usage of PV module power in this point is 100%. The maximum power under lighting 800 W/m2 is about 190 W. The usage of PV module power in this point is equal to about 90%. Electric heating elements under 220 V are 1.5 kW, 2 kW and 3 kW PE (nominal power), with Umax ≈ 124 V (total voltage of four PV modules in the MPP) which will accordingly be 480 W, 640 W and 1120 W PE power. Each PV module gets 1/4 of this power, which is equal to 50%, 65% and 115% of its Pmax.