https://www.sfu.ca/~mbahrami/ENSC%20461/Project/Some%20papers%20for%20CO2%20Ref/Fundamental%20process%20and%20system%20design%20issues%20in%20CO2%20vapor%20compression%20systems.pdf

Fundamental process and system design issues in CO2 vapor compression systems

Energetic and exergetic analyses as well as optimization studies of a transcritical carbon dioxide heat pump system are presented in this article. A computer code has been developed to obtain sub-critical and super-critical thermodynamic and transport properties of carbon dioxide. Estimates for irreversibilities of individual components of the system lead to possible measures for performance improvement. The COP trends indicate that such a system is more suitable for high heating end temperatures and modest cold end temperatures. Expressions for optimum cycle parameters have been developed and these correlations offer useful guidelines for optimal system design and for selecting appropriate operating conditions.

/pdf/optimization-of-a-transcritical-co2-heat-pump-cycle-for-1ldwye9m25.pdf

Optimization of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications

Heat transfer of supercritical carbon dioxide cooled in circular tubes was investigated experimentally. The effect of mass flux, pressure, and heat flux on the heat transfer coefficient and pressure drop was measured for four horizontal cooling tubes with different inner diameters ranging from 1 to 6 mm. The radial distribution of the thermophysical properties (i.e. specific heat, density, thermal conductivity and viscosity) in the tube cross-section was critical for interpreting the experimental results. A modified Gnielinski equation by selecting the reference temperature properly was then developed to predict the heat transfer coefficient of supercritical carbon dioxide under cooling conditions. This proposed correlation was accurate to within 20% of the experimental data.

In-tube cooling heat transfer of supercritical carbon dioxide. Part 1. Experimental measurement

This paper presents natural refrigerants as the ideal, environmentally friendly refrigerants and the ultimate solution to the problems of ozone depletion and global warming. HFC refrigerants are currently the leading replacement for CFC and HCFC refrigerants in refrigeration and air-conditioning systems. However, they are equally foreign to nature like CFCs and HCFCs, consequently, strong basis for the need to embrace the use of natural refrigerants as replacement for the halocarbon refrigerants was provided. This paper also analyses potentials of various natural refrigerants and their areas of application in refrigeration and air-conditioning systems. Natural refrigerants especially hydrocarbons and their mixtures are miscible with both mineral oil used in R12 and poly-ol-ester oils used in R134a systems. Also, with exception of ammonia, they are fully compatible with all materials traditionally used in refrigeration systems. Finally, this paper has revealed that natural refrigerants are the most suitable long time alternatives in refrigeration and air-conditioning systems.

Ozone depletion and global warming: Case for the use of natural refrigerant – a review

Halogenated hydrocarbons with high ozone depletion potential (ODP) were banned under Montreal Protocol (1987) due to their detrimental effects on ozone layer that shields the planet against ultraviolet radiations. The greenhouse gases (GHG) used in modern refrigeration, air conditioning, and heat pumping systems, are under Kyoto Protocol (1997)’s time-barred permission period. In order to reduce the depletion of ozone and reverse the climate change effects, the European Union legislation (2014) and Paris Accord (2016) are strongly emphasizing the phasing out of the use of harmful synthetic refrigerants. Choice of natural refrigerants makes no net addition of the greenhouse gases (GHG) in the environment. To retrofit and modify existing cooling and heating systems using natural refrigerants, extensive investigations are in progress worldwide. This work reviews timeworn, current and the next-generation refrigerants using Refrigerant Qualitative Parametric (RQP) quantification model to assist the refrigerant choice decision process. It is based on the ratio of arithmetic sums of actual parametric values of refrigerants normalized to equivalent ideal values. This model can help in choosing alternative refrigerants to replace CFCs by HCFCs or HFCs provisionally and finally replacing HCFCs or HFC to low GWP and ODP synthetic and natural refrigerants. A set of 16 refrigerants, both natural and synthetic, as an example, is computed for the standard Vapour Compression Cycle (VCC) based on the proposed model using REFPROP (NIST- 23 standard). The techno-economic parametric values of chosen refrigerants are taken from cited literature, ASHREA safety standards and international environmental legislations, laws and protocols. This paper reports the environment benign natural (CO2, NH3, HCs) and a few synthetic (R-152a, R-1234yf) refrigerants to be the optimal options.

Natural and synthetic refrigerants, global warming: A review

The ideal refrigeration or heat pump cycle for a given purpose is defined by the boundary conditions of the application and is completely independent of the refrigerant used. The real cycle should approach the theoretical ideal as closely as practically possible. The thermodynamic and heat transfer properties of the refrigerant are important in this respect. Natural substances such as ammonia, propane and carbon dioxide are often better than the present halocarbons in this regard. By using simple methods of safety it is possible to use these three natural fluids for practically all conventional refrigeration and heat pump systems.

G Lorentzen

Papers

The use of natural refrigerants: a complete solution to the CFC/HCFC predicament