4628 -- A Study of a Natural Convection Immersed Condenser Heat Pump Water Heater ✓ Most Recent

This study reports the finding of an energy-efficient heat pump water heater with no water pump. The condenser is directly inserted into the tank for water heating purposes. An experimental setup of a direct exchange vapor compression heat pump water heater was used to test and characterize several immersed condenser styles in a simulated domestic water heater. The condensers were of two primary designs— bayonet style with annular flow and U-tube style. A nominal 1-ton heat pump was affixed to an insulated 55-gallon water tank in such a way that the condenser, which was immersed in the water tank, could be easily changed. Tests were performed with eight types of condensers: one circuit, two circuit and three circuit bayonet, and three through seven circuit U-tube styles. The circuits were connected in parallel. Because of the double-wall requirement, outer tube with rifle lines were used, and smooth inner tubes were adopted. The rifle lines served as the vent channels in case there was a crack of the inner tubes. Each of the eight condenser configurations was tested by using the heat pump to heat 460 pounds of water from a starting temperature of ~57°F (14°C) up to a final temperature of at least 130°F (54°C). A variety of measurements were taken both within the water tank and throughout the heat pump in order to chart relative performance. The straight vertical geometry of the single-circuit bayonet condenser caused temperature stratification within the water tank. The other seven styles had non-vertical components and caused no discernable stratification because the non-vertical section of the condenser provided enough convective heat transfer to break the water thermal stratification. This non-vertical copper tube configuration should not be difficult or expensive to manufacture. Current design without a water pump is a major improvement over the current heat pump water heaters (HPWHs) because a major moving part has been eliminated. The U-tube style condensers generally outperformed the bayonet condensers as judged by the heating coefficient of performance (COP) as a function of average water temperature. As well, COP and heating capacity increased as a function of the number of circuits in the U-tube style condensers, with the seven-circuit style maintaining a COP of 2.7 and a capacity of 12,500 Btu/h (3,662 W), with an ambient air temperature of 80°F (27°C) and an average water temperature of 130°F (54°C) as in the literature for enclosed parking garages.

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