Abstract
A carbon dioxide (CO2) refrigeration system in a transcritical cycle requires modifications to improve the coefficient of performance (COP) for energy saving. This modification has become more important with the system’s more and more widely used applications in heat pump water heaters, automotive air conditioning, and space heating. In this paper, a single vortex tube is proposed to replace the expansion valve of a traditional CO2 transcritical refrigeration system to reduce irreversible loss and improve the COP. The principle of the proposed system is introduced and analyzed: Its mathematical model was developed to simulate and compare the system performance to the traditional system. The results showed that the proposed system could save energy, and the vortex tube inlet temperature and discharge pressure had significant impacts on COP improvement. When the vortex tube inlet temperature was 45 °C, and the discharge pressure was 9 MPa, the COP increased 33.7%. When the isentropic efficiency or cold mass fraction of the vortex tube increased, the COP increased about 10%. When the evaporation temperature or the cooling water inlet temperature of the desuperheater decreased, the COP also could increase about 10%. The optimal discharge pressure correlation of the proposed system was established, and its influences on COP improvement are discussed.
Highlights
In recent years, the ever-increasing menace of ozone layer depletion and global warming has caused a great deal of attention to be paid by environmental protection and energy-saving refrigerant insiders
When an expansion valve is used for isenthalpic expansion, the irreversible loss it causes is as high as 40%, which is higher than that of a compressor [3]
The coefficient of performance (COP) of a CO2 transcritical refrigeration system is usually low, and reducing cycle irreversible loss becomes an important way to improve the COP of a CO2 system
Summary
The ever-increasing menace of ozone layer depletion and global warming has caused a great deal of attention to be paid by environmental protection and energy-saving refrigerant insiders. The natural refrigerant CO2, which is a probable replacement for conventional refrigerants such as HCFCs (R22) or HFCs (R134a and R404A), is environmentally friendly (ODP (Ozone Depletion Potential) = 0, GWP (Global Warming Potential) = 1), nonflammable, and nontoxic. CO2 has a relatively high operating pressure due to its low critical temperature (31.1 ◦C) and high critical pressure (7.38 MPa): A transcritical cycle system is usually adopted. The coefficient of performance (COP) of a CO2 transcritical refrigeration system is usually low, and reducing cycle irreversible loss becomes an important way to improve the COP of a CO2 system. There are many ways to improve performance, such as a vortex tube, multistaging, an internal heat exchanger, or an expansion turbine and ejector [3,4,5,6,7,8]
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