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128 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 Fig. 15. Prandtl number of CO2. CO2 seem to be favorable in terms of heat transfer and pressure drop, compared to other typical refrigerants. 3. Transcritical vapor compression cycle Compared to conventional refrigerants, the most remark- able property of CO2 is the low critical temperature of 31.1 8C Vapor compression systems with CO2 operating at normal refrigeration, heat pump and air-conditioning temperatures will therefore work close to and even partly above the critical pressure of 7.38 MPa. Heat rejection will in most cases take place at supercritical pressure, causing the pressure levels in the system to be high, and the cycle to be ‘transcritical’, i.e. with subcritical low-side and super- critical high-side pressure (for a single-stage cycle). Some peculiarities of transcritical cycles and systems are discussed in the following text. 3.1. Fundamentals of transcritical cycle During operation at high ambient air temperatures the CO2 system will operate in a transcritical cycle most of the time Heat rejection then takes place by cooling the compressed fluid at supercritical high-side pressure. The low-side conditions remain subcritical, however, as shown in Fig. 16. At supercritical pressure, no saturation condition exists and the pressure is independent of the temperature. In conventional subcritical cycles, the specific enthalpy in point 3 is mainly a function of temperature, but at supercritical high-side conditions the pressure also has a marked influence on enthalpy. This effect may be observed as non-vertical or S-shaped isotherms in the supercritical and near-critical region. An important consequence of this is that it is necessary to control the high-side pressure, since the pressure at the throttling valve inlet will determine specific Fig. 16. Transcritical cycle in the CO2 pressure–enthalpy diagram. refrigeration capacity. As in conventional systems, the compressor work and thereby also the COP will depend on the discharge pressure. However, while the COP tends to drop with increasing pressure in conventional cycles, the behavior is quite different in a transcritical cycle, as will be shown in the following [28]. Fig. 17 shows the theoretical influence from varying high-side pressure on specific refrigerating capacity ðq0Þ; specific compressor work ðwÞ and cooling COP. The refrigerant outlet temperature from the gas cooler ðTexÞ is assumed to be constant. In practice, this temperature will be some degrees higher than the coolant inlet temperature. The curves are based on ideal cycle calculations, with evaporating temperature ðT0 1⁄4 5 8CÞ and a minimum heat rejection temperature ðTexÞ of 35 8C (left), and 50 8C (right). Note that all curves are normalized (Fig. 17). As the high-side pressure is increased, the COP reaches a maximum above which the added capacity no longer fully compensates for the additional work of compression. In Fig. 16, it may be observed that the Tex-isotherm becomes steeper as the pressure increases, thereby reducing the capacity enhancement from a given pressure increment. In contrast, the isentropic (compression) line shows a nearly linear shape. Differentiation of cooling COP 1⁄4 ðh1 2 h3 Þ=ðh2 2 h1 Þ with respect to the high-side pressure gives maximum COP for ›COP=›p 1⁄4 0 at a pressure ðpÞ defined by Inokuty [29] ›h3 ›h2 ›p 1⁄42COP ›p ð2Þ Ts That is, the ‘optimum’ pressure is reached when the marginal increase in capacity equals COP times the marginal increase in work. The enthalpy h1 is constant. Curves in Fig. 17 are normalized by the values for COP, q0 and w at the optimum high-side pressure. At Tex 1⁄4 35 8CPDF Image | CO2 Vapor Compression Systems
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