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M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 129 Fig. 17. Influence of varying high-side pressure on specific refrigerating capacity ðq0Þ; specific compressor work ðwÞ and COP in a transcritical CO2 cycle. The results are based on isentropic compression, evaporating temperature ðT0 1⁄4 5 8CÞ and a refrigerant outlet temperature ðTex Þ from the gas cooler of 35 8C (left) and 50 8C (right). the theoretical maximum COP is reached at a pressure of 8.7 MPa (87 bar), while at 50 8C, the optimum is at 13.1 MPa (131 bar). In practice, the cooling capacity ðqÞ curve will also go through a maximum, as compressor volumetric capacity drops off at higher discharge pressures. In most situations there will also be a capacity- maximum, usually at a somewhat higher pressure than the COP-maximum. High-side pressure regulation can be applied to maintain the COP at its maximum and/or to regulate the cooling or heating capacity. The optimum pressure increases steadily and almost linearly as Tex is raised, and the influence from varying evaporating temperature is quite small. 3.2. Methods of high-side pressure control The high-side pressure in a CO2 system may be either subcritical or supercritical. In case of subcritical operation, the system will behave as conventional systems, with high-side pressure determined by condensing temperature. In case of supercritical operation, however, the pressure in the high side is determined by the relationship between refrigerant charge (mass), inside volume and temperature. Refrigerant properties can be described by an equation of state in the following form: † Allowing the refrigerant temperature ðTÞ to control the pressure. While the first two options give possibilities for active pressure control, the last method is actually a passive scheme where the refrigerant charge/volume conditions are adapted to give the desired change in pressure when temperature varies. Thus, in case of leakage, the tempera- ture/pressure relation will change when using a passive scheme and this may give loss of capacity and COP. Even though high-side conditions are supercritical a large part of the time, the circuit and control system must also be designed for subcritical (condensing) high-side conditions as well, since this type of operation will be encountered when heat rejection temperatures are moderate or low. 3.2.1. Systems with high-side charge control In systems where the high-side pressure is controlled by varying the high-side refrigerant charge, the circuit must include means for controlling the momentary mass of refrigerant located between the compressor outlet and the expansion valve inlet. Assuming that the total refrigerant charge in the circuit is constant, a refrigerant buffer must be provided so that the high-side charge can be varied without flooding or drying up the evaporator. Several buffer volume locations and control concepts are possible. The various solutions can be divided into low-pressure and intermediate- pressure buffer systems. Low pressure buffer systems. Systems with low-pressure buffers include circuits with low-pressure receiver on the evaporator outlet, and systems with liquid separator using gravity, pump or possibly ejector circulation. A system with low-pressure receiver on the evaporator outlet is shown in Fig. 18 [11]. High-side pressure is controlled by p1⁄4pðv;TÞ1⁄4p V;T m ð3Þ As a result, there are three fundamentally different ways of controlling pressure [30]: † Varying the refrigerant charge ðmÞ in the high side of the circuit, † Varying the inside volume ðVÞ of the high-side, andPDF Image | CO2 Vapor Compression Systems
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