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134 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 throttling loss [36]. In some high-lift applications such as refrigeration or space heating where a highly effective internal heat exchanger may produce compressor discharge temperatures high enough to damage the lubricant, the internal heat exchanger may employ a parallel-flow configuration. In case of given capacity requirement, the reduced high-side pressure needed in a system with internal heat exchanger may give a compressor discharge temperature which is comparable to a system without internal heat exchanger [37]. 4.2. Expansion with work recovery Internal heat exchange is only one option for reducing expansion losses Another approach is to extract and make use of the work potentially available from the process. Owing to the high throttling loss of CO2, there is a considerable potential for COP improvement by the introduction of an expander. Several authors have therefore studied this potential. Regarding cycles and circuits with work-producing expansion, Negishi [38] devised a system based on the Plank cycle [39] where the supplementary pump/compres- sor is driven by an expander in a self-contained unit. Ikoma et al. [40] suggested another approach that expansion from supercritical state in a single-stage cycle is allowed to continue until near the saturation curve, and a throttling valve controls the remaining pressure reduction down to the evaporator pressure. Thus, the expander operates with a single-phase fluid only. From a hardware standpoint, the practical challenges are substantial because cooling systems experience a wide range of mass flow rates, requiring a robust design as detailed in Maurer and Zinn [41]. Positive-displacement devices, specifically internal and external gear pumps are theoretically more desirable because of the edge losses inherent in small turbines and even pistons. Research has focused on finding an instantaneous use for the highly variable work output, because of the inevitable losses associated with electric generators and motors. As a result, recent investigations have aimed to explore direct mechanical linkages to the high stage of a two-stage compressor [42–45]. Aside from implementation issues, the recovery of expansion work involves interesting thermodynamic tradeoffs with alternative methods of reducing expansion losses, such as internal heat exchange. A detailed parametric analysis revealed that internal heat exchange could increase cycle COP if the expander efficiency was only 30%, but would substantially decrease COP if the expander isentropic efficiency was 60% [36]. These results were based on a rather large assumed gas cooler outlet approach temperature difference (5 8C), so the impacts would be smaller if the gas cooler were more effective in reducing potential expansion losses (ineffective gas coolers lead to high evaporator inlet quality, hence more potentially recoverable expansion work). Nevertheless, the results of this parametric analysis reflect a fundamental reality: the large difference in specific heats between the suction gas and supercritical hot stream limit the second-law effectiveness of an internal heat exchanger, even as the first-law effectiveness approaches unity. An expander is subject to no such theoretical limit, only practical ones which have to date made internal heat exchange the technology of choice in prototype and production systems. Maurer and Zinn [41] conducted a theoretical and experimental study of expanders for CO2, including axial piston machines and gear machines. Measured energy efficiency reached 40 – 50% for axial piston machines, and 55% for gear machines. The higher efficiency of gear machines was somewhat unexpected, since these did not have any volume expansion (constant-volume machines). Important reasons for these results were lower friction losses and smaller clearances and leakage losses in the gear machine. Heyl and Quack [42,46] discussed various cycles with expanders, and showed the design and results of a free-piston expander/compressor concept. The machine had two-double-acting pistons, which were connected by a piston rod. Each piston divided the cylinder into a compression and expansion volumes. In order to achieve a balance of forces over the entire stroke, the expansion was conducted at full pressure, i.e. in a ‘square’ process in the pressure – volume diagram. Thus, only about 78% of the available expansion power could be recovered. The machine was intended as a second-stage compressor (from intermediate to high pressure), driven by the expansion work from high to low pressure. Nickl et al. [47] proposed the design principle of a rather simple second generation expander – compressor that provided a further 10% increase in COP compared with the first generation machine [42,46] and a 50% improvement over the same system with a throttle valve. They speculated that the discharge pressure of the main compressor could be further reduced. Hesse and Tiedemann [48] showed the possible use of a pressure wave machine for expansion work recovery in a CO2 system. The pressure wave machine could compress a part of the vapor from the evaporator outlet by using the expansion energy. Adachi et al. [49] showed a combined axial-piston compressor/expander unit with expansion ratio control means that could keep the high-side pressure at the optimum. Heidelck and Kruse [50] discussed a conceptual design for a CO2 expander based on a modified reciprocating (axial piston) machine. The expander needs mechanically controlled valves, and the authors showed a concept using a rotating control disc and slots similar to what is used in hydraulic machines. A design concept for a combined compressor – expander machine in one axial-piston unit was also outlined. Experiments on a modified hydraulic machine gave moderate efficiencies due to internal leakage inPDF Image | CO2 Vapor Compression Systems
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