Three-fluid Heat Exchanger as Evaporator in Multi-source Heat Pumps with Viable Options for Alternative Defrosting Methods
multi-source, heat pump, defrosting, three-fluid heat exchanger
Three-fluid evaporators can be used for multi-source heat pumps to keep refrigeration circuits simple in design since it avoids the usage of a second or more evaporators. In this work a defrosting control design is presented with a lowered electrical energy demand compared to the standard procedure of reverse flow defrosting with 4-way-valves. Objective of this new defrosting concept is to develop a fully autonomous defrosting strategy. The two heat sources allow direct thermal communication of an air source and a second heat source based on a hydronic circuit with the evaporating refrigerant. The proposed defrosting strategy is based on the combination of natural circulation defrosting and a hybrid defrosting strategy which allows defrosting during operation but at the expense of significantly higher heat source inlet temperatures above 0 °C of the hydronic-based heat source. The nominal operation and heat transfer from ambient air will be stopped thus the evaporation temperature can be increased up to defrosting temperature levels. Finally the two concepts of standard defrosting and this new defrosting strategy are compared in their economic efficiency when designed for single heat pumps or for larger amount of units.