CO2 Transcritical Cycle——Heat Pump and Refrigeration
Background
From an environmental perspective, CO2 is a very attractive refrigerant with zero ODP and a GWP of 1. It is a naturally occurring substance and abundant in the atmosphere. From its pure thermodynamical properties, CO2 is not very well suited as a refrigerant. However, CO2 has several unique thermophysical properties:
• Very good heat transfer coefficient
• Relatively insensitive to pressure losses
• Very low viscosity
In practical applications, the CO2 systems deliver very high performance, the main reasons being better heat exchange, very low pumping power when CO2 is used as a secondary fluid, and in cold climate the possibility of operating with a very low condensing pressure in the winter.
The efficiency of systems with CO2 depends more on the application and the climate than with other refrigerants. For all refrigerants, there is a decline in system efficiency with increasing condensing temperatures, and CO2 is among the refrigerants with the steepest drop. The good thermo-physical properties of CO2 can compensate to some extent, but there is a limit. CO2 has high energy content at higher temperatures, and when this heat can be reclaimed for heating sanitary water or similar application, the efficiency of the total system becomes very high.
CO2 is a high-pressure refrigerant where high operating pressures are required for efficient operation. During standstill, the ambient temperature can reach and exceed the critical temperature and the pressure can exceed the critical pressure. Hence systems are typically designed to withstand pressures up to 90 bar, or sometimes even equipped with a small standstill condensing unit to keep pressures low. At the same time, CO2 has a low compression pressure ratio (20 to 50% less than HFCs and ammonia), which improves volumetric efficiency. With evaporation temperatures in the range of -55 ºC to 0 ºC, the volumetric performance of CO2 is, for example, four to twelve times better than that of ammonia, which allows compressors with smaller swept volumes to be used. The triple point and critical point of CO2 are very close to the working range. The critical point may be reached during normal system operation. During system service, the triple point may be reached, as indicated by the formation of dry ice when a liquid containing parts of the systems are exposed to atmospheric pressure. Special procedures are necessary to prevent the formation of dry ice during service venting.
Unlike most other refrigerants, CO2 is used in practice in three different refrigeration cycles:
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Subcritical (cascade systems)
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Transcritical (CO2-only systems)
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Secondary fluid (CO2 used as a volatile brine)
The technology used depends on the application and the intended location of the system. There are a several applications where CO2 is attractive and already widely used today:
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Industrial refrigeration. CO2 is generally used in combination with ammonia, either in cascade systems or as a volatile brine
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Food/retail refrigeration
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Heat pumps
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Transport refrigeration
Transcritical CO2 Refrigeration and heat pump test-beds
The heat pump water heater and refrigerated display cabinet testing facilities have been designed and built in order to obtain experimental data on the cycle layouts presented in the previous section, aiming at a high flexibility of operation. Each test rig consists of a refrigerant loop designed to withstand the high pressures needed for operation with R744 (up to 14 MPa), which is provided with the required thermal and cooling loads by a thermal load management system operating with water to water and air to air as secondary fluid. These facilities also feature a set of pressure, temperature and mass flow meters and a power analyzer, which are managed by a PC with specifically developed data acquisition software. The single subsystems are thoroughly described in the forthcoming paragraphs.
The main equipment that makes up the experimental plant developed to carry out the evaluation of carbon dioxide as refrigerant working in a trans-critical cycle (Figure 2) is: a 2.5 kW semi-hermetic single-stage vapour compressor, a double stage expansion system with an liquid receiver between stages, concentric counter current gas-cooler and evaporator and internal heat exchanger, as shown in layout of the system loop, The special of the heat pump system, which the double stage expansion system consists of a pressostatic expansion valve (back-pressure) that is employed for the first expansion and an manual expansion valve for the second. The first expansion stage, performed by the pressostatic expansion valve, allows the gas-cooler outlet pressure to be controlled, and the second enables us to control the evaporating process by means of the manual expansion valve whose pressure sensors are placed at the inlet of the evaporator. The liquid receiver is placed after the first expansion stage in order to regulate the mass of refrigerant in the plant.
The figure shows a sketch of the refrigerated display cabinet experimental plant. Basically there is one single-stage semi-hermetic reciprocating compressor, an oil separator, a micro-channel air gas-cooler and evaporator, a liquid receiver, an electronic expansion valve, and an electronic back pressure valve. To study the influence of the flash vapor a by-pass with an auxiliary expansion valve has been mounted. The refrigerant state at the outlet of the expansion device is in a two-phase condition, provided that the fluid crosses the saturated liquid line during the isenthalpic expansion process. For this reason some fraction of the refrigerant flow enters the evaporator in vapor state not having a cooling effect. Using the flash gas by-pass the performances of trans-critical carbon dioxide cycle increase improving the low pressure side of the system because the evaporator is fed with liquid only.