Chemistry
Thermodynamics of Refrigeration
In the olden days winter was the best time of the year for cold drinks, sorbets and ice creams because ice was readily available. Without a thorough understanding of thermodynamics our communities might have to be brought their ice in an ice wagon just like in the olden days. The laws of thermodynamics were developed by scientists and used when processes of the Industrial Revolution were developed. The physics of heat transfer are applied in order to transfer heat away from the portion of a refrigerator or air conditioning that needs to continually cool the air. Now the physics of heat transfer are used for sophisticated machinery such as jet airplanes and in power plants.
The history of thermodynamics has been briefly reviewed in this paper along with the first two laws of thermodynamics. The First and Second Laws of Thermodynamics are crucial for an understanding of refrigeration. The refrigeration system is essentially a reverse heat engine (hot to cold instead of cold to hot). Refrigeration is not only important in order to keep our groceries cooled at home until they are ready to be eaten or served. It is an important concept for energy efficiency in industrial processes. Industries are interested in energy efficiency more than ever before due to rise in cost of energy sources plus an understanding of the negative impacts wasted energy can have on the environment.
This literature review was conducted by using online computer databases such as EBSCOhostcom and Questia.com. The search words used included ‘thermodynamic analysis’ ‘thermodynamics’ ‘refrigeration cycle’ ‘refrigeration.’ Definitions are included in the text because most of the definitions can also be written mathematically so the two forms for the definitions are offered at the same time.
Thermodynamics of Refrigeration: Physics
A medical doctor, John Gorrie, received the first U.S. patent on a reverse heat engine in 1851.The technology for home refrigeration was ready for sale in 1913. The first refrigerator’s trade name was the ‘Domelre;’ the factory was located in Chicago. Then in 1915 Alfred Mellowes invented the self-container type refrigerator for home use. He called it the ‘Guardian Frigerator.’ Mellowes sold the design and manufacturing process in 1918 to General Motors Corporation and the name became ‘Frigidaire.’ This was an important patent for General Motors to hold in developing many of its other products.
Access to cold food storage is not the only purpose of refrigeration. The thermodynamic concepts of refrigeration are essential in industry in order to determine the amount of energy needed, energy available, or energy wasted in a system. Developing practical and accurate ways to measure these parameters are part of the research discussed. Good measurements of energy (measured as heat) aid in the design of more efficient systems. More efficient systems save energy, save money, and help power plants ensure electricity even during times of peak demands. The concept of a reverse heat engine for refrigerators has been developed and expanded upon for other varied and diverse purposes. The concept of the reverse heat engine is firmly based on the concepts of the First and Second Laws of Thermodynamics.
Thermodynamics
The first law of thermodynamics is known as the Law of Conservation of Energy and it states that ‘Energy can be neither created nor destroyed.’ So energy is conserved or one could say that “energy is the capacity to do work or to supply heat, (energy) can neither be created nor destroyed; it can only be converted from one form of energy into another” (Gürses & Ejder-Korucu, 2012, p. 3). In other words the change in the Energy of the system plus the change in the Energy of the surroundings is equal to zero. Therefore Internal Energy (U) has a two-way path which designates the conservation of energy in Figure 1 which is a portion of a concept map of ‘The Conservation of Energy.’ Internal energy passes by work and by heat to other types of energy. This can be written as a mathematical equation.
Where, U = internal energy, q= heat, w= work;
then dU=dq+dw
The equation can also be understood as the change in internal energy (U) is the system change from the initial state (i) of energy and the final state (f) of energy.
ΔU = Uf – Ui
Next to the right hand bottom corner of the concept map is the pathway of Internal Energy to Energy which also displays conservation of energy inputs and outputs. The energy can be in units of heat (Joule or Calorie) which are measurements of kinetic and potential energy and unit so temperature (°C, °F, K). These are all connected to the right hand corner of the map which describes that energy can be transferred (under appropriate conditions) between higher and lower temperatures which is very important in making refrigeration work. On the left side of the concept map in Figure 1 the inputs and outputs to Enthalpy are depicted; these include the chemical processes important to making refrigeration work.
Heat transfer denotes the movement of heat from one object to another. Heat always moves to a colder (lower) temperature which can take place in phase such as vaporization and condensation, melting and freezing, and sublimation. If no work is done to reverse the process, then usually the temperature rises. Enthalpy is a change of energy when part of the Internal Energy is not equal to the input because some of the energy is lost to the surroundings of the system. Enthalpy is a thermodynamic system which takes into account pressure and volume.
When W = work, P = pressure, and V = volume
then W = -PΔV.
Figure 1. The Conservation of Energy (Gürses and Ejder-Korucu, 2012, p. 3). See Appendix for whole concept map.
Standard reaction enthalpies are temperature dependent. Here is how the first law of thermodynamics is respected in the process of refrigeration. The internal energy (U) must undergo a phase change because work must be done on the system to force the internal energy to flow from a cold region to a hot region. A refrigerant such as fluorinated hydrocarbons (such as Freon-12) is the medium where the phase change takes place. The amount of pressure is lowered and raised so that the phase changes from evaporation to condensation.
For refrigeration an electric motor is used to convert electrical energy into mechanical energy which is used to power a pump. The purpose of the refrigerator pump is to expand gas, as the gas expands it becomes cold which is endothermic energy. Endothermic processes have a positive enthalpy change. Refrigerators work like a reverse heat engine. A refrigerant is the medium where phase change happens.
The Second Law of Thermodynamics states generally that the entropy of a closed or an isolated system cannot decrease. Often it is even more simply worded as entropy of systems (such as the universe) increases. Lord Kelvin offered a classical definition “No (heat) engine whose working fluid undergoes a cycle can absorb heat from a single reservoir, deliver an equivalent amount of work, and deliver no effect.” Another way to understand the second law of thermodynamics is to think of in terms of the disorder of the system (also known as entropy) and in terms of potential energy because in a closed system (no energy can enter or leave the system) the potential energy will always be less than the initial amount of energy. The change between initial energy and potential energy can be described as an infinitesimal amount named ‘ds.’ The change (difference between initial and potential energy) can be described by the two mathematical equations which describe gain or loss.
ds = cv ln(T2/ T1) + R ln(ρ1 / ρ2) or ds = cp ln(T2/ T1) - R ln(p2/ p1)
where
ds=entropy change
cv=specific heat capacity at a constant volume process
cp=specific heat capacity at a constant pressure process
T=absolute temperature
R=individual gas constant
ρ=density of gas
p=absolute pressure
Designs for Refrigeration/Cooling for Better Plant Energy Efficiency
A learning tool in the form of a portable air-conditioning experimental apparatus has been built by Abu-Mulaweh who is in the Mechanical Engineering Department at Purdue University. The apparatus (Figure 2) was designed and built in order to help students better understand the processes of air-conditioning and the refrigeration cycle. Not only that the apparatus is useful in the laboratory in teaching other basic concepts such as “heat transfer, thermodynamics, and heat exchange” (Abu-Mulaweh, 2009, p. 157). The interesting characteristic about this particular device is that it is equipped with “an intuitive user interface” which “allows data acquisition by a computer (using LabView)” (Abu-Mulaweh, 2009, p. 157). A computer hook-up is not necessary though, when theoretical thermodynamic processes are being demonstrated on the portable air-conditioning experimental apparatus.
Figure 2. Air-conditioning experimental apparatus.
(Abu-Mulaweh, 2009, p. 246)
Carnot Cycle
A Carnot Cycle is considered the mot efficient type of heat exchange system theoretically no heat is gained or lost in the Carnot system. Figure 3 is a schematic of the classical Carnot Cycle. The measurement of the Carnot efficiency is calculated as the
[(TH – TC)/ TH] * 100 % = Carnot efficiency
where
TH equals the temperature of the volume of isothermal expansion
TC equals the temperature of the volume at isothermal compression
W equals work
Q equals heat
The system described above is taking place in an adiabatic system, a system where heat cannot enter or leave. The amount of entropy is equal to zero, therefore it is a theoretical system which is not physically possible. So the best way to think of a Carnot efficiency numerical value is - putting a limiting value on the fraction of heat that cannot do work in the system (as defined by the Second Law of Thermodynamics).
Thermodynamics
The classical Carnot heat engine
Figure 3. Classical Carnot Heat Engine.
(Eric Gaba, 2006, public domain)
The Carnot cycle has some important characteristics to remember because so much refrigeration is based on the reversible Carnot cycle and a lot of research is designed by exploring adaptations of the Carnot cycle to solve real physical problems. Figure 3 is a concept map of the Carnot cycle which is a theoretical concept which can be notice right away by the oval which is the second one in the row from the right. The oval represents entropy and the notation next to the oval states that (the system) has constant entropy which cannot happen in the real world.
Chen (et al., 2011) conducted research to find the optimal piston speed ratio for an irreversible Carnot refrigerator in order to evaluate the amount of heat leakage in a system. “Expressions of cooling load and coefficient of performance (COP) of the Carnot refrigeration cycle as well as heating load and coefficient of performance (COP) of the Carnot refrigeration cycle as well as heating load and COP of the heat pump cycle are derived for a fixed cycle period” (Chen et al., 2011, p. 105). The researchers succeeded in developing a model to determine optimal piston speed ratios which act as “theoretical guidelines for the designs and operations of practical refrigerators and heat pumps” (Chen et al., 2011, p. 111).
Figure 4. The Carnot Cycle concept map.
(Nave, R., n.d., http://hyperphysics.phy-astr.gsu.edu)
Brayton Cycle
Zhang (et al., 2012) have used thermodynamic optimization in order to evaluate an open regenerated Brayton cycle for refrigeration-heat pump cycle. The Brayton cycle is also called the Joule cycle; it was developed by George Brayton (1830 – 1892). Air refrigeration cycle can use net work input when the Brayton cycle is driven in reverse (with a gas as the working fluid). (See Figure 5) Zhang (et al., 2012) adjusted the mass flow rate in order to maximize the performance for an open regenerated inverse Brayton cycle. The researchers concluded that “the cooling load, heating load and the power input increase with the increase in the compressor inlet relative pressure drops” (Zhang et al., 2012, p.93). The researchers were using finite thermodynamics; the practical importance is for designing ways to use energy as efficiently as possible.
When the optimization is performed with the maximized again by properly allocating the fixed flow area among the compressor inlet and the expander outlet. The coefficient of performance of refrigeration can be maximized again by properly allocating the fixed heat conductance inventory among the hot and cold side compressor pressure ration. (Zhang et al., 2012, p. 101)
In other words the ability to control the ratio of the hot:cold side compressor pressure was possible by appropriately controlling the fixed heat conductance inventory.
Figure 5. Closed Brayton Cycle where C is the compressor, T the turbine assembly, w is a high-temperature heat exchanger, and low-temperature heat exchanger ~ mechanical load, e.g.electric generator
Kumar and Khaliq (2011) developed a way to analyze “industrial waste heat recovery based ejector vapor compression refrigeration system” (p. 192). The researchers considered the ratio of energy (total) to exergy (available energy). The motivation for this research is the problem during peak use of refrigeration and air conditioning to power suppliers. In particular peak demand from electric powered air conditioning systems can disrupt energy supply and even damage power supply. Since waste heat from flue gasses of plants can measure from 400° to 500° C this heat should be used as an energy supply instead of released into the environment. The research evaluates a heat recovery process based on using an ejector (EJ) instead of throttling valve to reduce the irreversibility of expansion device which controls the vapor compression refrigeration cycle. The researchers concluded that by combining evaluation of energy and exergy using thermodynamic methodology is useful in evaluating “how efficiently the energy resources are used from the quality point for view for better design, analysis and performance improvement of power and refrigeration systems” (Kumar & Khaliq, 2011, p. 199)
Thermodynamics and green engineering
Researchers are developing solar and other green engineering alternatives based on thermodynamics to run refrigerators and generators. Morrison (1999) calls the new types of designs “alternative thermodynamic cycles” (p. 62). Morrison (1999) reports that the Department of Energy has estimated that “1,954 Giga Watt (GW)/hours of primary energy would be saved over a 30-year period if domestic refrigerator energy efficiency were improved by 30 percent” (p. 63).
Klein and Reindl (2005) evaluated the use of solar for refrigeration in order to determine if it could work in refrigeration systems. They measured the coefficient performance (COPsys) and defined as “the ratio of refrigeration capacity to input solar energy” (p. S30). Interestingly they concluded that “this definition might not be the most relevant metric for a solar refrigeration system because the fuel that drives the system during operation, solar energy, is free” (Klein & Reindl, 2005, p. S30). They reported some positive news that if “capital cost and efficiency of tracking solar collectors can be significantly reduced” solar refrigeration will become practical for use in large-scale refrigeration applications.
Conclusions
The articles cited in this paper demonstrate that the uses for refrigeration are varied, diverse and necessary from a cost perspective. Not only that, no matter what type of fuel source, heat transfer or phase medium is being used practical use of refrigeration can only be understood by having a good understanding of thermodynamics. The First and Second Laws of Thermodynamics were presented here because they are the two laws that best act as a foundation for the examples given. The Laws of Thermodynamics are theoretical but they are essential in order to design workable processes which will solve practical problems.
References
Abu-Mulaweh, H. I. (2009). Development and performance validation of portable air-conditioning experimental apparatus. International Journal of Mechanical Engineering Education, 37(2), 144-158.
Ahmet Gürses and Mehtap Ejder-Korucu (2012). A View from the Conservation of Energy to Chemical Thermodynamic, Thermodynamics - Fundamentals and Its Application in Science, Ricardo Morales-Rodriguez (Ed.), ISBN: 978-953-51-0779-8, InTech, Available from: http://www.intechopen.com/books/thermodynamics-fundamentals-and-its-application-in-science/a-view-from-the-conservation-of-energy-to-chemical-thermodynamic
Chen, L. G., Feng, H. J., & Sun, F. R. (2011). Optimal piston speed ratio analyses for irreversible Carnot refrigerator and heat pump using finite time thermodynamics, finite speed thermodynamics and direct method. Journal of The Energy Institute, 84(2), 105-112.
Dinḉer, İ and Kanoğlu, M. (2010). Refrigeration Systems and Applications.2nd Ed. Sussex, UK: John Wiley & Sons, Ltd.
Gürses, A. and Ejder-Korucu, M. (2012). A View from the Conservation of Energy to Chemical Thermodynamic, Thermodynamics. Chapt.1 in Fundamentals and Its Application in Science, Ricardo Morales-Rodriguez (Ed.), ISBN: 978-953-51-0779-8, InTech, Retreived November 7, 2012 from http://www.intechopen.com/books/thermodynamics-fundamentals-and-its-application-in-science/a-view-from-the-conservation-of-energy-to-chemical-thermodynamic
Klein, S. A., & Reindl, D. T. (2005). Solar Refrigeration. ASHRAE Journal, 47(9), s26-s30.
Kumar, R. R., & Khaliq, A. A. (2011). Exergy analysis of industrial waste heat recovery based ejector vapour compression refrigeration system. Journal of the Energy Institute, 84(4), 192-199.
Li, Z. J., Zhu, W. F., & Chen, J. P. (2010). Optimal performance of irreversible absorption refrigerator with four heat sources. Journal Of The Energy Institute, 83(1), 56-62
Morrison, G. (1999). Stirling renewal. Mechanical Engineering, 121(5), 62.
Pearson, A. (2012). Defrost Cylcle. Downsides of Refrigeration. Refrigeration Applications. ASHRAE Journal, October: p. 94
Riffat, S. B., & Su, Y. H. (2001). A novel absorption refrigeration cycle using centrifugal reverse osmosis. Journal Of The Institute Of Energy, 7466-69.
Smith, J.M., Van Ness, H., and Abbott, M. (2005). Introduction to Chemical Engineering Thermodynamics. 7th Ed. (The McGraw-Hill Chemical Engineering Series). New York, NY: McGraw-Hill Companies, Inc.
Zhang, W., Chen1, L., & Sun, F. (2012). Thermodynamic optimisation for open regenerated inverse Brayton cycle (refrigeration/heat pump cycle). Journal Of The Energy Institute, 85(2), 86-95.
Zhang, W., Chen1, L., & Sun, F. (2012). Thermodynamic optimisation for open regenerated inverse Brayton cycle (refrigeration/heat pump cycle). Journal Of The Energy Institute, 85(2), 96-102.
Apendix 1
The Conservation of Energy Concept Map (Gürses and Ejder-Korucu, 2012, p. 3).
