Technologies for R744 to become mainstream option, Gustav Lorentzen 2012

By Janaina Topley Lira, Jun 28, 2012, 15:25 3 minute reading

Professor Pega Hrnjak gave the keynote presentation at the 10th IIR Gustav Lorentzen Conference, 25-27 June, in which he stressed natural refrigerants have “serious potential to become mainstream option”. To increase the number of successful CO2 applications, compressor and heat exchanger improvements are needed. In this first article R744.com looks at some results from sessions dedicated to experimental analysis of CO<

According to Professor Hrnjak, “the easiest way to implement natural refrigerants is to: reduce technology gaps so that natural refrigerant systems look and act like systems with conventional refrigerants; and select the right applications - not all applications are good for every refrigerant.” 
 
“For CO2 it is important to improve efficiency because people perceive CO2 as a low efficient fluid, but it is rather a matter of how much money or work we put into CO2. We can make CO2 equally efficient,” said Hrnjak.
 
Technologies needed to advance the position of major natural refrigerants: HC, NH3, and CO2, P. Hrnjak
 
To reduce costs and improve performance of CO2, work recovery options like ejectors, expanders and similar devices will be critical, explained Professor Hrnjak in the opening presentation of the 10th Gustav Lorentzen Conference on Natural Working Fluids, that took place in Delft, in the Netherlands. A large part of Professor Hrnjak’s presentation was dedicated to discussing charge reduction. R744 has low sensitivity to pressure drop, as a result of which a high-pressure drop will not result in a high temperature drop, meaning that microchannel heat exchangers are ideal.
 
Overall, professor Hrnjak concluded that we should be looking for ways to make the cycle closer to Carnot, at lower expense, using multistage compression, expanders, subcoolers, ejectors and internal heat exchangers.
 
“It is important not to think the same way as before. If you make the system the same way as a conventional system you are bound to fail… we always need to think one step ahead to think where we want to go,” said Hrnjak.
 
Modelling and experimental study of an ejector for a transcritical CO2 refrigeration system, A. Bousziane, A. Bensafi, P. Haberschill
 
In the paper presented by A. Bouziane an ejector for a CO2 transcritical refrigeration system is suggested to improve the performance. CO2 suffers large expansion losses and higher irreversibility during the gas cooling process and ejectors offer a potential solution to improving the steady state performance of CO2, thanks to their low cost, absence of moving parts and ability to handle two-phase flow without damage.
 
Replacing expansion valves, ejectors expand a high pressure substance, such a refrigerant to entrain a slightly higher pressure substance as a result of which the pressure of the refrigerant at compressor suction is higher than the pressure of evaporator. 
 
The investigation of the CO2 transcritical ejector included a simulation using a one dimensional constant-area-ejector flow model, following which a test model was constructed. Results showed that:
 
  • The primary flow rate has a great effect on the primary flow rate
  • The critical compression ratio is the optimal compression ration of an ejector therefore it is necessary to operate the system at the corresponding critical compression ratio and the maximum entrainment ratio.
 
Experimental study of a triple-passage motive nozzle ejector in a small R744 heat pump, K. Banasiak, A. Hafner, T.M. Eikevik
 
In this presentation different supply configurations of the motive nozzle channels were looked at. The experiments showed that the coefficient of performance (COP) depends on the motive nozzle supply strategy. Therefore to optimise CO2 system operation the motive nozzle feeding management is proposed as an option.
 
Simulation of shock waves in supersonic flow of CO2 through a converging-diverging nozzle of transcritical ejector refrigeration system, M. S. Berana and M. Nakagawa
 
Refrigerants can undergo single phase or two phase flow processes and shock waves can degrade the performance of the refrigeration cycle by lowering the kinetic energy of an accelerating flow. This reduces the energy-conversion efficiency of converging-diverging nozzles of ejector refrigeration systems. The paper presented simulations of the behaviour of shock waves using a model for non equilibrium flow. Results show that pseudo-shock waves occur in liquid-dominated two-phase flow, and relatively stronger dispersed shock waves occur in vapour dominated flow. Shock waves intensify with increasing divergence angle given the same inlet condition and with increasing supercritical inlet entropy given the same nozzle.

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By Janaina Topley Lira

Jun 28, 2012, 15:25




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