Transcritical Carbon Dioxide Refrigeration as an Alternative to Subcritical Plants: An Experimental Study

Transcritical Carbon Dioxide Refrigeration as an Alternative to Subcritical Plants: An Experimental Study

Adriana Greco (Università di Napoli Federico II, Italy), Ciro Aprea (Università di Salerno, Italy) and Angelo Maiorino (Università di Salerno, Italy)
DOI: 10.4018/978-1-4666-8398-3.ch008
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Carbon dioxide (R744) is as a valid alternative to classical substances such as HFCs used in vapour compression plants. A transcritical refrigeration cycle is needed because the critical temperature of carbon dioxide is usually lower than the ambient temperature. In this chapter the performances of a transcritical cycle have been evaluated with a prototype R744 system working as a classical spit-systems to cool air. An experimental analysis has been carried out on the effect of: refrigerant charge, internal heat exchanger, heat rejection pressure on the energetic performances of the transcritical plant. An experimental analysis of a hybrid trans-critical refrigerator-desiccant wheel system has been carried out in order to improve the COP. The experimental transcritical cycle has been examined in comparison with a classical vapour compression plant working with the R134a.
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The traditional refrigerants, i.e. CFCs and HCFCs, were banned by the Montreal Protocol because of their contribution to the disruption of the stratospheric ozone layer. The Kyoto Protocol indicated that most hydrofluorocarbons have large Global Warming Potentials (GWPs). And therefore provide a non-negligible, direct contribution to global warming if leaked to the atmosphere.

As a consequence, considerable research effort was devoted to the development of environmentally safe refrigeration systems using natural working fluids. CO2, was considered as a promising alternative of HFCs. Carbon dioxide is a component of the atmosphere. It is obtained from atmospheric air by fractionation and, thus, it would have no impact on global warming, apart from for the energy consumption associated with the fractionation process itself.

CO2 has many excellent advantages in engineering applications, such as no toxicity, no inflammability, a higher volumetric capacity that enables compact systems, low pressure ratio, better heat transfer properties, complete compatibility with normal lubricants, easy availability, lower price and no recycling problem.

The critical temperature of CO2 (31.1 °C) is usually lower than typical heat rejection temperatures of air-conditioning and heat pump systems (Lorentzen & Petterson, 1993; Lorentzen,1994,1995). This results in a trans-critical vapour compression cycle in lieu of a conventional one in water heating and comfort cooling and heating. In the transcritical cycle the condenser has been replaced by a gas cooler. The latter operates above the critical pressure, whereas the evaporator operates under that.

The transcritical systems have strong potential in two sectors: i) automotive air-conditioning and ii) heat pumps. Application of a trans-critical CO2 cycle to domestic water heating systems has advantages over conventional systems in terms of power consumption and heating efficiency. The performance of trans-critical CO2 cooling system, however, is lower than that of conventional air-conditioners, due to large expansion losses and high irreversibility during the gas-cooling process.

Therefore, many researchers analyzed the performance of the trans-critical CO2 refrigeration cycle, in order to identify opportunities to improve the energy efficiency of the system.

The present research work is concentrated on an optimization of cycle and components and on an experimental analysis in the differential heating and cooling applications.

The major contributions of the present work are:

Key Terms in this Chapter

TEWI: Total Equival Warming Impact, an index hat takes into account both direct ad indirect contribution of a refrigeration plant to global warming.

Internal Heat Exchanger: R744 at the outlet of the gas cooler exchanges heat in the internal heat exchanger with R744 flowing out the evaporator.

R744: CO 2 used as a refrigerant fluid.

Hybrid Transcritical Refrigerator-Dessiccant Wheel: A transcritical cycle coupled with a desiccant wheel for desiccant cooling.

Transcritical Cycle: Refrigerant cycle that works above the critical temperature.

Exergetic Analysis: Thermodynamic analysis base on an exergy balance.

Optimal Refrigerant Charge: Refrigerant charge of R744 that maximizes the COP.

Optimal Heat Rejection Pressure: Pressure at the outlet of the gas cooler that maximizes the COP.

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