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CO2 heat pump water heater due to the good temperature fit between the CO2 and the water in a counter-flow gas cooler [5, 7]. Unfortunately, CO2 trans-critical cycles operate at high heat rejection pressures. As seen in table 1, CO2 critical pressure is 73.8 bar, therefore, above the critical point, pressures are high. Pressures from 80 to 110 bar or more are common in these cycles. High pressure presents design challenges in terms of component robustness and compressor capability. Components for R744 systems have to withstand much higher pressures than their HFC and HCFC counterparts. Due to higher operating pressures, the heat exchangers for R744 require either smaller tube diameters or thicker walls. Other issues are introduced by the particularity of R744, e.g. high discharge temperatures, compatibility with lubricating oils, potential degradation of seals after decompression, etc. These issues hinder the fast development of a “component chain” and make CO2 heat pumps to be a very expensive venture [1]. Typical capital cost of CO2 heat pumps is approximately double the cost of convectional heat pumps [8]. However, today’s manufacturing capabilities allow production of components which can meet these demands. In addition, high pressure presents some benefits e.g. CO2 has a relatively high vapor density and correspondingly a high volumetric heating capacity (3 – 10 times larger than CFC, HCFC, HFC and HC refrigerants [9]), this allows a smaller volume of CO2 to be cycled to achieve the same heating demand which allows for smaller components and a more compact system [10]. 1.3 Irreversibilities The cycle in figure 1 is a theoretical cycle where there are no losses. Practically, this cycle is hard to achieve because of effects of friction, leakage and other sources of losses. For instance, the effect of leakage and pressure losses inside a compressor has influence on the energetic (or isentropic) and volumetric efficiency of the compression process. Friction and heat dissipation in the compressor also contribute to irreversibility. Still when compared to compressors for convectional units, trans-critical compressors are better in terms of irreversibility [9]. This is related to less pressure drop due to higher pressure operation. For a fixed mass velocity, the pressure drop decreases with the operating pressure because the variation in the physical properties becomes smaller as the operating pressure increases [11]. The same effect occurs in the other parts of the equipment i.e. throttle valve and the heat exchangers. These losses modify the theoretical cycle to the one shown in figure 2 where the dotted line represent the actual (practical) cycle. In order to identify locations of energy loss within the carbon dioxide cycles, an analysis with respect to the Second Law of Thermodynamics is made on each component in the cycle. The irreversibility of the individual components is identified throughout the range of operating conditions. This indicates which components to focus on improving with respect to availability utilization. 319PDF Image | TYPICAL INITIAL OUTPUT OF A CO2 HEAT PUMP
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