An innovative low-cost and water-saving direct photovoltaic water heating system using simplified near maximum power point tracking

Abstract

Photovoltaic (PV) water heating systems are one of the water heating technologies that have been attracting significant attention in recent years. Although from the thermodynamics point of view, solar thermal systems such as vacuum tubes, are more efficient in heating water for the same area exposed to the sun, but this is not the only factor influence the utilisation of solar energy. The main reason for the interest in using PV solar systems for water heating and thermal storage is their lower cost, higher reliability, reduced maintenance, water savings and minimising complexity of retrofitting. In this work, a novel PV water heating system is proposed where the near maximum power point tracking (Near-MPPT) is performed using different rated heating elements via a simple control circuit without the need for a relatively expensive DC-DC converter. The selection of the heating elements equivalent resistance is made based on the photovoltaic array size and the solar irradiation data. An optimisation process is considered in the design to reduce cost and maximise efficiency. A novel generic methodology is developed to select the values of the hating elements for optimum performance. This has been achieved by obtaining the maximum number of possible variations in resistance configurations from two selected heating elements while sustaining high efficiency and DC voltage switching capability. The control circuit is designed by utilising electromechanical and semiconductor switching devices. A control algorithm is created to track the maximum power point via the manipulation of the switching circuit using PV power measurements. The proposed system is compared to a PV system equipped with a DC-DC converter and a PV system directly connected to fixed heating elements. The system is experimentally tested to evaluate the idea including the payback period, performance and costings. Further, simulation is conducted and validated for conditions in Jordan as well as Egypt and the UK. The results have shown that proposed system provides a lower cost and reasonable efficiency when compared to the conventional system. The results of the proposed solar technology system in Jordan, for example, have shown a small reduction in the yearly produced energy of only 3.8 % when compared to a conversional system equipped with MPPT DC-DC converter system and an increase in efficiency of about 11.5 % in comparison to a fixed load system. However, the cost saving is expected to be 44.7 % when the proposed simple switching system is compared to a standard DC-DC converter; with water savings in winter of 83.7 % in comparison to a solar thermal system. The payback period of the system for Jordan, Egypt and the UK is found approximately 2.75 years, 6.5 years and 3 years respectively. In comparison to the conventional MPPT DC-DC converter, the proposed novel system Near-MPPT is expected to provide a much lower cost system (between 24.5 % and 44.7 %) to encourage the adaptation of renewable energy and also address energy poverty with more affordable technologies, particularly in developing countries. The results also show that the proposed PV heating system has a better water saving features when compared with Solar Vacuum Tubes (SVT) heating systems, an important feature for arid countries. The flexibility of the PV system allows the use of electricity when the heating system is not needed for heating of water, particularly during the summer months.