An Alternative Refrigeration System For Automotive Applications By Shannon Marie McLaughlin A Thesis Submitted to the Faculty of Mississippi State University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Mechanical Engineering in the Department of Mechanical Engineering Mississippi State, Mississippi August 2005
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An Alternative Refrigeration System For
Automotive Applications
By
Shannon Marie McLaughlin
A Thesis Submitted to the Faculty of Mississippi State University
in Partial Fulfillment of the Requirements for the Degree of Master of Science
in Mechanical Engineering in the Department of Mechanical Engineering
Mississippi State, Mississippi
August 2005
An Alternative Refrigeration System For
Automotive Applications
By
Shannon Marie McLaughlin Approved: ________________________________ ________________________________ Dr. Pedro Mago Dr. Louay M. Chamra Assistant Professor Associate Professor Mechanical Engineering Mechanical Engineering (Major Professor) (Committee Member) ________________________________ ________________________________ Dr. B. Keith Hodge Dr. Steven R. Daniewicz Professor Professor Mechanical Engineering Mechanical Engineering (Committee Member) (Graduate Coordinator) ________________________________ Dr. Kirk H. Schulz Dean and Professor Bagley College of Engineering
Name: Shannon McLaughlin
Date of Degree: August 6, 2005
Institution: Mississippi State University
Major Field: Mechanical Engineering
Major Professor: Dr. Pedro Mago
Title of Study: AN ALTERNATIVE REFRIGERATION SYSTEM IN AUTOMOTIVE APPLICATIONS
Pages in Study: 46
Candidate for Degree of Master of Science
The air conditioning systems currently utilized in automobiles are the vapor
compression systems. This type of system has many disadvantages: the refrigerant used
is not environmentally friendly, the compressor is in competition with the engine coolant
system, and the compressor uses a significant portion of the engine power. A waste heat
driven absorption refrigeration system is one alternative to the current systems that could
address these problems. The absorption refrigeration system uses solutions for the
absorbent-refrigerant pair that do not harm the environment. This investigation includes
a theoretical analysis of the feasibility of absorption air conditioning system in
automotive applications. Also, a comparison of the power requirements of the proposed
system and the vapor compression system is performed.
ACKNOWLEDGMENTS
I would like to dedicate this research to my husband, Dino, and my children,
Alyssa, Breanna, and Dino III. I would also like to thank my major professor, Dr. Pedro
Mago, for his help throughout my time at Mississippi State and my advisory committee
1.3 Working Fluid for Absorption Refrigeration Systems
The performance of an absorption refrigeration system is vitally dependent on the
chemical and thermodynamic properties of the working fluids used. A fundamental
requirement of the absorbent/refrigerant combination is that, in liquid phase, they must
have a margin of miscibility within the operating temperature range of the cycle. The
mixture should also be chemically stable, non-toxic, and non-explosive. There is no ideal
working pair suitable for every application because several of the requirements of a pair
are contradictory. A summary of the requirements for working fluids is as follows:
11Refrigerant
• high specific enthalpy of evaporation • favorable pressure range • low pressure difference
Refrigerant and absorbent • high thermal and chemical stability • good solubility of refrigerant in the absorbent • low specific mass flowrate • low heat of mixing • low specific heat capacities • large difference in boiling temperatures
The most important criteria for a working pair are thermal and chemical stability and
good solubility of the refrigerant in the absorbent. The molecular basis for good
solubility and thermal stability can be found with flouridized alcohols (Nowaczyk and
Steimle, 1992).
Over the years, many working fluids have been studied. Over 40 refrigerant
compounds and 200 absorbent compounds are available (Srikhirin et al., 2001). The
absorption process has been known for more than a century, but except for ammonia
(NH3)-water and water-lithium bromide (LiBr), other working pairs are rarely used.
Water-NH3 has been widely used for both cooling and heating purposes since the
absorption refrigeration system was invented. Both NH3, the refrigerant, and water, the
absorbent, are highly stable for a wide range of operating temperatures and pressures.
Since ammonia and water are volatile, the cycle must include a rectifier to strip away
water that normally evaporates with NH3 (Srikhirin et al, 2001). Without the rectifier, the
water would accumulate in the evaporator and degrade system performance. The
12disadvantages to water-NH3 such are its high pressure, toxicity, and corrosive action to
copper and copper alloys. However, water-NH3 is environmentally friendly and cheap.
The use of LiBr-water for absorption refrigeration systems began around 1930.
Two outstanding features of this pair are the non-volatility of LiBr and the extremely
high heat of vaporization of water. However, using water as a refrigerant limits the low
temperature application to that above 0°C. Since water is the refrigerant, the system must
be operated under vacuum conditions. At high concentrations, the solution is prone to
crystallization. It is corrosive to some metals, and is expensive. Some additive may be
added to the pair as a corrosive inhibitor or to improve heat-mass transfer performance
(Srikhirin et al, 2001).
Many studies have been performed on the applicability of the Freon refrigerants,
R21 and R22, for refrigeration purposes using as the absorbents dimethyl formamide
(DMF) and tetraethylene glycol dimethyl ether (DMETEG). A comparison of their
working performances was also studied (Kumar et al, 1991). These studies are important
for refrigeration of foodstuffs, vegetables, and fruits in tropical regions using low
generator temperatures. These absorbents paired with Freon refrigerants are not
especially useful for the automobile industry.
Water-NH3 mixtures have been quiet extensively investigated. However, for
smaller capacity applications, like automobiles, ammonia has the obvious drawbacks that
it is toxic and that small leaks will cause an unacceptable odor. In contrast to this, carbon
dioxide, CO2,–acetone has the following advantages:
13• harmless • greatly reduced compression ratio compared to conventional refrigerants • compatibility with common machine building materials and oils • available • simple operation and service • no “recycling” required • low cost
The only disadvantage is the high operating pressure of pure CO2 (Groll, 1997).
1.4 Alternate Refrigeration Systems
The Stirling cycle and the pulse-tube unit were the alternate systems studied.
Among the many refrigeration cycles, the Stirling cycle, found in Figure 1.4, is selected
as one of the promising candidates because of its use of non-CFC’s, simplicity, and high
thermal efficiency. The Stirling cycle cooler is a free-piston, linear-motor driven device.
The internal running surfaces are supported by gas bearings, so no contact wear takes
place. The entire unit is sealed and it is capable of continuous modulation and of
maintaining high efficiencies down to low loads, which means that it adapts easily to
cooling needs and keeps performing with high efficiency even at low demand, (Kim et
al., 1993).
Figure 1.4 Stirling schematic
14 Stirling cycle refrigerators are not yet available on a commercial basis. They may
well represent the future direction towards a super efficient, solar powered refrigerator.
There are revered characteristics that make Stirling engines impractical for use in many
applications, including automotive. Because the heat source is external, some time is
required for the engine to respond to changes in the amount of heat being applied to the
cylinder because of the time required for the heat to be conducted through the cylinder
walls and into the gas inside the engine. This means that
• The engine requires some time to warm up before it can produce useful power.
• The engine can not change its power output quickly.
These shortcomings all but guarantee, that the Stirling engine won't replace the internal-
combustion engine for automotive use.
The pulse-tube refrigeration unit offers a viable alternative to systems that
currently require chlorofluorocarbon (CFC) or hydro-chlorofluorocarbon (HCFC)
working fluids. The pulse-tube refrigeration unit uses helium, which is nontoxic to
humans and harmless to the environment, as the working fluid. Pulse-tube refrigerators
can be operated over a wide range of temperatures. With only a single moving part, the
pulse-tube refrigerator offers the potential for greater reliability than the Stirling cooler
(Radenbaugh, 1990). These systems can be used in numerous space and commercial
refrigeration applications, including food refrigerator/freezers, laboratory freezers, and
freeze dryers. Pulse-tube refrigerators can also be used to cool detectors and electronic
devices.
15Pulse-tube refrigeration, a variation of the Stirling engine, is new compared to
other refrigeration cycles. The pulse-tube refrigerator was first developed in mid-1960
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APPENDIX A
46 This is the set of equations input into the computer program in order to find the enthalpies, and rate of heat transfers (Moran et al., 2000).