At the beginning of this year Dr. Paul Byrne uploaded a study on Research Gate which compared Roll-Bond condenser for propane heat pump water heaters (HPWH) with two other types of condensers. The same study was published in International Journal of Refrigeration in 2017.
The authors of the study wrote that Roll-Bond heat exchangers, which are widely used in household refrigeration applications as evaporators, have the advantage of offering a greater heat exchange surface and a low internal volume. They help reduce the risk of deterioration of the tubes in contact with food and during manual defrosting. In the case of the HPWH, a Roll-Bond used as a condenser with a flat surface on one side offers a better thermal contact with the tank.
Roll-Bond heat exchanger, Source: Talum Izparilniki d.o.o.
The compactness is a geometric criterion defined as the exchange surface to internal volume ratio. As shown in the table below the roll-bond heat exchanger has the highest compactness.
According to the research, the charge calculated in the circuit is 145g for Roll-Bond. The maximum refrigerant charge authorized in refrigeration systems for household use is 150g. With these assumptions, the objective of a heat pump water heater with less than 0.15 kg of propane is achievable only by Roll-Bond. In comparison, the minimum surface area for a system implemented with a charge of 150 g is 36 m2.
The authors of the study found out that the best performance compared to other technologies is obtained by Roll-Bond heat exchanger.
Their conclusion was that Roll-Bond heat exchanger offers a good compromise between high local heat transfer coefficients, good thermal contact with the water tank and low liquid line. With a charge of 150 g, the HPWH has a COP of 2.76 and a heating time around 9 hours.
Source: Summarized from www.sciencedirect.com, published in April 2017
Follow the link to get the entire article in pdf
https://www.sciencedirect.com/science/article/pii/S0140700717300695
or
https://www.researchgate.net/publication/313883101_Refrigerant_charge_optimisation_for_propane_heat_pump_water_heaters