EVALUATION OF PASSIVE ANTI-FOULING TECHNOLOGY APPLIED TO CO2 HEAT PUMP WATER HEATERS

摘要

Heat pump water heaters are increasing in popularity due to their increased energy efficiency and low environmental impact. This paper describes the experimental testing of a transcritical CO2 heat pump water heater at Queen's University. A modified 4.5 kW Eco-Cute unit was studied. It sourced heat from a constant temperature water supply and rejected the heat to a 273 litre hot water tank through a gas-cooler. The high temperatures that occur in the gas-cooler of this unit make it ideally suited for natural convection, (i.e., thermosyphon) circulation on the potable water side. This has the potential to reduce pumping power, simplify system operation and design, and increase thermal stratification in the hot water storage tank. This configuration, however, is susceptible to the accumulation of sediments, scale and mineral deposits (i.e., fouling) in geographic regions where high mineral deposits may be present in the water supply. To counteract fouling in these cases, a passive back-flushing system was proposed to prevent the accumulation of deposits on the heat transfer surfaces of the gas-cooler. As hot water is drawn from the system, the cold "mains" supply water is directed through the gas-cooler in the reverse direction of normal operation, scouring the heat transfer surfaces and dissolving deposits of inverse-soluble salts which are a major contributor to fouling on hot heat transfer surfaces. The gas-cooler used was a specially designed unit that, although offering high performance in a compact unit, may be susceptible to the fouling and blockage of the heat transfer passages when used at thermosyphon flow rates. Experiments were conducted to evaluate the effects of the back-flush operation on heat pump performance (i.e., COP) and operation. These were conducted under controlled laboratory conditions, at a range of draw flow rates and temperatures, and are summarized in this paper.