Havard Rekstad

CO2-heat pump water heater : characteristics, system design and experimental results

Abstract CO 2 is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Tap water heating is one promising application for a trans-critical CO 2 process. The temperature glide at heat rejection contributes to a very good temperature adaptation when heating tap water, which inherits a large temperature glide. This, together with efficient compression and good heat transfer characteristics of CO 2 , makes it possible to design very efficient systems. A heating-COP of 4.3 is achieved for the prototype when heating tap water from 9°C to 60°C, at an evaporation temperature of 0°C. The results lead to a seasonal performance factor of about 4 for an Oslo climate, using ambient air as heat source. Thus, the primary energy consumption can be reduced with more than 75% compared with electrical or gas fired systems. Another significant advantage of this system, compared with conventional heat pump water heaters, is that hot water with temperatures up to 90°C can be produced without operational difficulties.

Development of compact heat exchangers for CO2 air-conditioning systems

Abstract Compact and lightweight heat exchangers are needed for motor vehicle air-conditioning systems and for several types of unitary equipment. The high-pressure natural refrigerant CO2 is now being evaluated for use in such applications, and efficient heat exchangers are being developed and investigated. Carbon dioxide heat exchangers are designed for high refrigerant mass flux and use small-diameter tubes or extruded flat microchannel tubes. Refrigerant-side heat transfer coefficients are higher than with fluorocarbons, and reduced internal surface areas can therefore be tolerated. Both small-diameter mechanically expanded round-tube heat exchangers and brazed microchannel-type units have been built and tested successfully. Results show that compact heat exchangers optimized for CO2 are very competitive with baseline HFC/HCFC units in terms of physical dimensions, exchanger mass and thermal performance. Smaller tube and manifold dimensions can give reduced size compared with HFC-134a equipment. The temperature approach between air inlet and refrigerant outlet is much lower in CO2 gas coolers than in baseline system condensers of equal size and capacity, and the reduced refrigerant exit temperature has a marked influence on the coefficient of performance, Microchannel heat exchangers give the best overall efficiency. Refrigerant distribution in multiport manifolds and heat transfer tubes does not seem to be a problem.