A highly efficient solar-air dual-source heat pump (SAHP) for water heating applications has been designed with a blower fan that enables forced convection during partial sunshine and diffuse light conditions.

The direct expansion modified SAHP includes a rotary hermetic compressor, an expansion valve and submerged water-cooled condenser coils. The system's novel evaporator, designed to overcome the limitation of natural convection, is constructed by stacking two roll-bonded bare plates with blower fans in between.

The system has a collector area pf 1.6 m² and an evaporator area of 3.2 m². Ambient air is forced through the dual source bare plate evaporator by the blower fans with individual motor capacities of 35 W. The rotary hermetic compressor, with rated input power of 910 W and a heating capacity of 3,500 W, compresses the vapor refrigerant.

During periods of abundant solar insolation, the system operates in a solar-air source natural mode, enabling the evaporator to gather solar energy and heat from the surrounding air through natural convection. When insolation is absent during dusk or night hours, the system uses an air-source forced mode in which the evaporator absorbs only ambient thermal energy from the air via forced convection.

The capacity of the system to heat 300 l of water from an initial temperature of 31° C to a final one of 50° C was tested under summer and winter conditions in southern India. An average daily coefficient of performance of 3.24. was documented for the solar-air source natural mode; the air-source forced mode achieved a coefficient of performance of 3.05. The average heating time for these two modes was 163 minutes and 187 minutes, respectively.

The solar water heating system engineered by researchers from Bannari Amman Institute of Technology (India), Sri Krishna College of Technology (India) and Sunway University (Malaysia) is described in International Communications in Heat and Mass Transfer.