23 September 2011

NATURAL CIRCULATION DEFROSTING IN FIN-AND-TUBE HEAT EXCHANGERS WITH HIGHLY DISTRIBUTED REFRIGERANT MASS FLOW


The electrical energy demand for active defrosting of air-to-water heat pumps (HP)
has been calculated from former HP field test monitoring results at Fraunhofer ISE. The
share of electrical energy consumption of the compressor was determined at about 5 % per
annum related to the overall heat energy. This energy demand can be decreased by using
non-electric defrosting. One approach is natural circulation (NC) defrosting which reduces
the electrical energy demand for defrosting to a minimum by using thermal energy from the
sink. Electrical energy is thus only necessary for recovering the used heat as the driving
force.
At our HP testbench with R1270 a fin-and-tube heat exchanger is operated as the evaporator
with a cooling capacity of about 2 kW. Consequently to the configuration the pressure drop of
the refrigerant distributor counteracts to the NC flow. We present results of some design
issues regarding flow distribution and results of NC defrosting capacity applied to this
evaporator. The evaporator is capable of using two different heat sources at the same time.
Primarily this is air and subsequently a glycol-water-driven circuit.