A Photovoltaic Solar Water Heating System

Abstract

A novel solar water heating system was patented in 1994. This system uses photovoltaic cells to generate electrical energy that is subsequently dissipated in multiple electric resistive heating elements. A microprocessor controller continually selects the appropriate heating elements such that the resistive load causes the photovoltaic array to operate at or near maximum power. Unlike other residential photovoltaic systems, the photovoltaic solar water heating system does not require an inverter to convert the direct current supplied by the photovoltaic array to an alternating current or a battery system for storage. It uses the direct current supplied by the photovoltaic array and the inherent storage capabilities of a residential water heater. A photovoltaic solar hot water system eliminates the components most often associated with the failures of solar thermal hot water systems. Although currently more expensive than a solar thermal hot water system, the continued decline of photovoltaic cell prices is likely to make this system competitive with solar thermal hot water systems within the next decade. This paper describes the system, discusses the advantages and disadvantages relative to solar thermal water heating systems, reviews the various control strategies which have been considered, and presents experimental results for two full-scale prototype systems.