Fig. 1 shows a schematic diagram of the proposed heating
system, which uses a unique designed evaporator to gather
convection and radiation heat transfer. The cross-section of
the unique evaporator is shown in Fig. 2. Solar energy was
collected by the plate surface of collector/evaporator. And the
energy from ambient air was mainly transferred to spiralfinned
evaporating tubes, adhered to lower surface of a
flat-plate, through the natural convection between air and
refrigerant. When available solar energy was adequate,
evaporating temperature of heat pump water heater would be
higher than ambient temperature, and solar energy would be
a low-grade energy source with high COP. On the other hand,
if inadequate, temperature of plate surface might be lower
than ambient temperature, and the heat pump water heater