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1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 122 ADSORPTION REFRIGERATION SYSTEM FOR AUTOMOBILES AN EXPERIMENTAL APPROACH Peethambaran K M1 , Asok Kumar N2 , John T D3 1, 3 Professor, Department of Mechanical Engg., Govt. College of Engineering – Kannur, Kerala, India 2 Professor, Department of Mechanical Engg., College of Engineering – Trivandrum, Kerala, India ABSTRACT The use of waste heat for refrigeration and air conditioning purposes have been accepted by people and various systems have been developed and proven attractive but its implementation in real applications is still limited. The adsorption system is advantageous in small scale systems if compared with absorption systems especially for the handling of the system and the cost. Adsorption refrigeration and heat pump cycles rely on the adsorption of a refrigerant gas into an adsorbent at low pressure and subsequent desorption by heating the adsorbent. The adsorbent acts as a “chemical compressor” driven by heat. As it makes use of heat to pressurize the refrigerant, this system can be used in various situations which enable waste heat recycling like in factories and automobiles. The objective of this work was to compare various adsorbent-refrigerant pairs and find the best pair, which would give maximum COP and will be cheap and easily available. The adsorbent- refrigerant pairs considered for the present study were silica gel-water, silica gel-methanol, zeolite- methanol, zeolite-water, activated carbon- ammonia, and activated carbon- methanol. Experiments were carried out to analyse the adsorption nature of these pairs. The variation of adsorption capacity with temperature was analysed. It is concluded that the silica gel - water is the best among the pairs compared in terms of coefficient of performance. It is also found that water attains its saturation point on zeolite quickly followed by water on silica gel and ammonia on carbon. Key words: Adsorption System, Adsorbent-Refrigerant Pairs, Waste Heat Refrigeration. 1. INTRODUCTION Technological innovations have lead to various appliances, which have helped us lead lives that are more comfortable. The implementations of air conditioning systems in automobiles have INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 5, Issue 2, February (2014), pp. 122-132 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2014): 3.8231 (Calculated by GISI) www.jifactor.com IJMET © I A E M E
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 123 helped create more comfortable travelling. The expanding population and the energy crisis have brought serious problems to the world environment and to sustainable development. The electric driven vapour compression refrigeration system has faced a challenge as CFC s and HCFC s are not favourable to the environment. The use of waste heat for refrigeration and air conditioning purposes have been accepted by people and various systems have been developed and proven attractive but its implementation in real applications is still limited. Electric driven air conditioning systems have reached a COP of over 4, while absorption systems are usually in the range of 1.1-1.25. The adsorption system is advantageous in small scale systems if compared with absorption systems. For exhaust heat utilization, a solid adsorption system is possibly the best system for refrigeration purposes. A large part of the energy from the fuel that is burnt gets wasted through the exhaust gases. If a system that uses all the energy from the exhaust of an engine to run its air conditioning system can be designed, it could be very well accepted. Adsorption refrigeration cycles rely on the adsorption of a refrigerant gas into an adsorbent at low pressure and subsequent desorption by heating the adsorbent. The adsorbent acts as a “chemical compressor” driven by heat. When the adsorber is cooled, the adsorbate gets adsorbed onto the adsorbent. While the adsorber is heated in the next cycle, this adsorbate gets desorbed at high temperature. With the use of a pressure vessel and a check valve its pressure value can be increased. The rest of the refrigeration system remains the same as that of a vapour compression system. Exhaust heat recycling is gaining prominence these days because of the increased stress on fuel consumption and also because it helps to reduce pollution levels to an extent there by making it environmental friendly. The major disadvantage of general adsorbtion refrigeration system is that the COP of the system is comparatively lower than the conventional vapour compression system. In future, there can be scope for improvement of the same so as to be used in vehicles. Objective of this experimental study is to have a general idea in selection of adsorbent-refrigerant pair, to compare various adsorbent-refrigerant pairs and find the best pair, which would give maximum COP and will be cheap and easily available. 2. SCHEME The first consideration in any refrigeration system is deciding the capacity of the system. In this work, it is opted for a small capacity system to test its effectiveness. For the desired cooling effect, the best adsorbent refrigerant pair is to be chosen. For this reason it is decided to compare the working of following adsorbent-refrigerant pairs namely silica gel-water, silica gel-methanol, zeolite-methanol, zeolite-water, activated carbon- ammonia, activated carbon- methanol. In the event of doing estimation, we came to know that adsorption capacity was to be found by conducting experiments at different temperatures. Then graphs connecting the adsorption capacities and temperatures are plotted. Then the theoretically best pair is found out. 3. LITERATURE SURVEY 3.1 Adsorption Refrigeration System An adsorption refrigeration system driven by a heat source is a closed sorption process . There are two main processes inside the system: refrigeration and regeneration. The refrigerant is vapourised in the generator (or evaporator) and adsorbed by a solid substance with a very high microscopic porosity. In the regeneration process, the adsorbent is heated until the refrigerant desorbs and goes back to the evaporator, which now acts as a condenser. There are several pairs of refrigerant/absorbent such as water/zeolite, methanol/activated carbon. The system is not as widely
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 124 used as the absorption system. However, this application can be integrated with the low temperature solar collector or the exhaust of automobiles. The adsorption cycle is illustrated in Fig.1 and proceeds as follows. Fig 1. Thermodynamic cycle for adsorption The four basic processes involved in the cycle are: (i) Heating And Pressurisation During this period, the adsorber receives heat while being closed. The adsorbent temperature increases, which induces a pressure increase, from the evaporation pressure up to the condensation pressure. This period is equivalent to the “compression” in compression cycles. (ii) Heating, Desorption And Condensation During this period, the adsorber continues receiving heat while being connected to the condenser, which now superimposes its pressure. The adsorbent temperature continues increasing, which induces desorption of vapour. This desorbed vapour is liquefied in the condenser. The condensation heat is released to the second heat sink at intermediate temperature. This period is equivalent to the "condensation" in compression cycles. (iii) Cooling And Depressurisation During this period, the adsorber releases heat while being closed. The adsorbent temperature decreases, which induces the pressure decrease from the condensation pressure down to the evaporation pressure. This period is equivalent to the "expansion" in compression cycles. (iv) Cooling, Adsorption and Evaporation During this period, the adsorber continues releasing heat while being connected to the evaporator, which now superimposes its pressure. The adsorbent temperature continues decreasing, which induces adsorption of vapour. This adsorbed vapour is vaporised in the evaporator. The evaporation heat is supplied by the heat source at low temperature. This period is equivalent to the "evaporation" in compression cycles.
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 125 3.2 Utilizing Waste Heat There are three potential uses for waste heat in a vehicle: cabin heating, cabin cooling, and electricity generation, the last of which could be used for heating and cooling. Heating is already performed efficiently, compactly, and economically by routing engine coolant through a small finned tube heat exchanger (HEX) in the cabin air duct. The only drawback is the long delay (5 min or more) during frigid weather between engine start-up and effective cabin heating and defrosting. Fig 2. Uniform temperature heat recovery or ‘double effect’ heating Fig 3. Temperature variation through adsorbers, HTF heater & HTF cooler for ‘thermal wave’ regeneration Lambert and Jones  reviewed the current state of the art in adsorption heat pumps. Research groups in the United States, Italy, France, China, and Japan have concentrated their efforts on devising improvements to the all-critical adsorbers, with the primary goal of improving efficiency (COPC), which requires increasing the percentage of recycled heat. Several investigations agree in identifying the two most important parameters that must be maximized in order to increase COPC: the ratio of adsorbent (‘live’) mass to non-adsorbent (‘dead’) mass, and the NTU of the heat exchanger. According to Lambert and Jones, some previous designs suffer from a low ‘live’–‘dead’ mass ratio, the first of the two critical governing parameters identified above. 3.3 Commonly Used Adsorption Materials (i) Silica Gel : It is an amorphous form of SiO2, which is chemically inert, nontoxic, polar and dimensionally stable (< 400°C) (ii) Zeolites : These are natural or synthetic aluminum silicates which form a regular crystal lattice and release water at high temperature. These are polar in nature.
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 126 (iii) Activated Carbon : They are highly porous, amorphous solids consisting of microcrystallites with a graphite lattice. They are non-polar and cheap. But they are combustible. 3.4 Adsorption Cycle as Applied to an Automobile With a single adsorber, cooling is intermittent, which is undesirable because it wastes much of the continuous supply of exhaust heat. Therefore, at least two adsorbers are needed for an automobile. Multiple adsorbers beyond two can improve COPC by permitting incrementally more effective ‘thermal wave’ regeneration but add volume and mass, decreasing SCP. Thus, a compromise must be struck between SCP and COPC to satisfy constraints on both. COPC must be high enough to ensure adequate cooling even for the worst-case scenario of a subcompact car idling for an extended duration (i.e. traffic jam), since it has the largest ratio of cooling load to exhaust heat. Maintaining an already surge-cooled cabin at a comfortable temperature requires 1.7 kW cooling. Assuming that a realistic 80 per cent of the 3.5 kW available exhaust heat can be extracted (2.8kW), the required COPC = 1.7 kW÷2.8 kW≈0.60, which can be accomplished with uniform temperature ‘double-effect’ heating. For a given configuration, SCP and COPC are inversely proportional. However, both SCP and COPC are directly proportional to NTU and inversely proportional to the fraction of ‘dead’ mass. Thus, the fundamental objectives are to maximize NTU and to minimize dead mass. 4. FUNCTIONAL REQUIREMENTS The amount of refrigerant in the full reservoir is mr,reservoir = Qcool,reservoir ÷ ∆hevap Compact and mid-size cars would require 20 and 40 per cent more refrigerant than the subcompact (hybrid) car examined above. 4.1 Required Amount of Adsorbent Three adsorbers, instead of two, are employed to take advantage of the fact that minimum 400 °C exhaust (at idle) can rapidly heat one adsorber, permitting the other two to be cooled for twice as long at half the rate . A cooling rate that is half the heating rate incurs half the ∆THTF-ads so that the adsorbent can be cooled closer to ambient and adsorb more refrigerant. Fig 4. Temperature versus time for the adsorbent in the three adsorbers
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 127 At any instant, one adsorber is heated while two are cooled. Cycle duration is set at 10 min and is divided into thirds. Each adsorber is heated for one-third of the cycle (∆t,heating = 3.33 min = 200 s) and cooled for the remaining two-thirds of the cycle (∆t,cooling = 6.67 min = 400 s). Their phase angles are evenly spaced at 0, 120, and 240, so, at any given instant, one adsorber is being heated while two are being cooled. The amount of refrigerant that must be expelled from each adsorber during its heating phase is mr = Q˙cool×∆theating ÷ ∆hevap, the minimum practical adsorption temperature is Tads,min = 65°C high enough above the foreseeable Tamb = 50°C to permit adequate heat rejection during cooling phase. At 65°C, the maximum adsorption capacities of various refrigerants from the corresponding adsorbents are given in the table. In addition, the maximum adsorber temperature is 200°C at which the adsorbers would be completely depleted of the refrigerants. therefore the amount of adsorbent required to hold the calculated amount of refrigerant is given by mads = mr ÷ (MFmax−MFmin) 5. CALCULATION OF ADSORPTION CAPACITY OF VARIOUS REFRIGERANT- ADSORBENT PAIRS To find the mass of adsorbent required for adsorbing the given mass of refrigerant, maximum and minimum adsorption capacity has to be calculated. For this, experiments to analyse the adsorption nature of various refrigerants on the various adsorbents is conducted. A known mass of adsorbent is taken in a crucible. The vacuum desiccator is filled with the refrigerant up to its neck and the crucible containing the adsorbent is placed in it. After placing the compounds in the desiccator, a vacuum is created inside it using a vacuum pump. Then the setup is kept aside. After 24 hours, the mass of the adsorbent is again measured using the high precision weighing balance. The increase in mass of the adsorbent gives the mass of refrigerant adsorbed on it. This experiment is repeated for all adsorbent-refrigerant pairs at different temperatures. The high temperature is obtained by keeping the desiccator in a water bath. Weight of the empty crucible = W1 (kg) Weight of the crucible with adsorbent = W2 (kg) Weight of the crucible with adsorbent after 24 hours = W3 (kg) Mass of the adsorbent taken, W4 = W2 - W1 (kg) Mass of the refrigerant adsorbed in adsorbent, W5 = W3 - W4 (kg) Adsorption capacity = W5 / (W5 + W4) From the measured masses, the adsorption capacity is obtained by calculating the mass fraction of the refrigerant in adsorbent. Then the graphs connecting adsorption capacity and temperature are plotted and maximum and minimum adsorption capacities are noted from these graphs.
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 128 0 5 10 15 20 25 30 0 20 40 60 80 100 Adsorptioncapacity(%) Temperature (oC) Silica Gel - Methanol 0 5 10 15 20 25 30 35 40 0 20 40 60 80 100 Adsorptioncapacity(%) Temperature (oC) Silica Gel - Water 0 5 10 15 20 0 20 40 60 80 100 Adsorptioncapacity(%) Temperature (oC) Zeolite- Methanol
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 129 Fig 5. Adsorption capacity versus Temperature plots 6. WORKING OF THE FABRICATED SYSTEM The exhaust heat driven adsorption refrigeration system is basically a refrigeration system which contains all the parts of a conventional refrigeration system. In this the main highlight is that it does not require a compressor which draws power from the engine. Here it is replaced by a chemical 0 5 10 15 20 25 30 35 0 20 40 60 80 100 Adsorptioncapacity(%) Temperature (oC) Zeolite- Water 0 5 10 15 20 25 30 0 20 40 60 80 100 Adsorptioncapacity(%) Temperature (oC) Carbon - Ammonia 0 10 20 30 40 50 60 0 20 40 60 80 100 Adsorptioncapacity(%) Temperature (oC) Carbon - Methanol
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 130 compressor, which is the adsorption chamber. The compression process is effected by desorption of refrigerant vapours from the adsorbent. All the parts involved in the system are designed suitably to attain the proper cooling effect or heat transfer by selecting the proper materials. Detailed design drawings are made for each component. The machining processes are selected suitably to meet the requirements and the components are fabricated accordingly. The adsorption chamber is made of mild steel (AISI 1040) of thickness 2 mm with an outer diameter of 10.16 cm and 30 cm in length. The chamber is provided with a flange of 6 mm thickness and 15.24 cm diameter. The inlet and exit plenums are made with mild steel (AISI1040) of thickness 2 mm with an outer diameter of 10.16 cm and length 4 cm with a flange of thickness 6 mm and a diameter of 15.24 cm. Two asbestos gaskets discs of 15.24 cm diameter and 4 mm thickness is placed between the plenums and the chamber. Asbestos cement is chosen as the material of the gasket as it has several of the required properties which make it ideally suited material for this application. 9 copper tubes of 1.27 cm outer diameter are placed within the chamber for the exhaust gas to flow through. The chamber is packed and compacted with adsorbent particles in the space enclosed between the chamber and the copper pipes. The condenser is used to condense the refrigerant vapours coming out from the adsorption chamber. The condenser coil is made of copper as it has high thermal conductivity of around 386 W/mK , high thermal diffusivity of around 112.34 x 10-6 ,low specific heat of 383 J/kgK which helps in quick and proper heat transfer and ensures proper condensation. The copper tube of 7.9 mm diameter is cut out to a length of 4m and then bent at lengths of 50 cm. The evaporator is used to cool the cabin of the automobile by transferring heat to the refrigerant, which evaporates and provides the required cooling. The evaporator is made of copper tubes of 7.9 mm diameter. The overall length of the evaporator is 2m. A blower is placed behind the evaporator coils so as to cool the cabin. The exhaust gases are allowed to pass through the adsorber chamber. The temperature of the adsorber rises slowly. This heating is continued for 200 seconds. As heating is continued the refrigerant gets desorbed and the pressure inside the adsorber chamber begins to rise. When the pressure gauge indicates the required pressure (Pcond,in), the outlet valve is opened slowly and the refrigerant, goes slowly to the condenser. The valve is opened very slowly and when the pressure falls well below Pcond,in , it is closed. After 200 seconds, the cooling phase begins. The exhaust gas supply to the adsorber is cut. Compressed air is blown over the adsorber chamber in order to cool it. When the adsorber chamber gets cooled the temperature falls and more refrigerant gets adsorbed. Because of this the pressure inside the chamber falls. When the pressure reaches the evaporator pressure, the refrigerant inlet valve, which connects the adsorber and evaporator coil, is opened to allow more refrigerant to flow in at a controlled rate. Because of the increase in flow of the refrigerant, amount of refrigerant adsorbed also increases. At a particular stage, the adsorbent will become saturated. The pressure inside the chamber becomes steady and then starts to rise. The inlet valve is then closed stopping the flow of the refrigerant. The adsorbed refrigerant is then released during the heating phase. The refrigerant from the adsorber condenses in the condenser at a temperature of 65ᵒC. This condensed refrigerant is then expanded through the thermostatic expansion valve and evaporated in the evaporator at a corresponding evaporator pressure and temperature of 5ᵒC. This produces the cooling effect. The pressure at which the adsorber is to be opened to the condenser during the heating phase is known as condenser inlet pressure (Pcond,in). It is almost equal to 25 kPa for water, 2948 kPa for ammonia, 101 kPa for methanol. at these pressures the condenser valves have to be opened. The pressure at which the valve connecting the evaporator and adsorber chamber is to be opened is called evaporator outlet pressure (Pevap,out). It is almost equal to 0.8 kPa for water, 291 kPa for ammonia, 10 kPa for methanol.
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 131 7. CONCLUSIONS The theoretical as well as experimental comparison of various adsorbent-refrigerant pairs namely silica gel-water, silica gel-methanol, carbon-ammonia, carbon-methanol, zeolite-methanol, zeolite-water was conducted. The most adsorbent zeolite can adsorb 36, 30 wt % of water and methanol respectively at room temperature and pressure. But the cost of zeolite is very high and it is scarce in availability. Activated carbon has much greater affinity for methanol compared to zeolite. It comes to around 55 wt% for methanol and 62 wt% for ammonia. This adsorptivity can be increased by coating it with CaCl2. Silica gel on the other hand has conductivity similar to that of zeolite, exhibiting a greater affinity for methanol to about 55wt%. Water is non-toxic, non-flammable, non-polluting, stable, and has the highest latent heat among common substances. However, its vapour pressure is very low requiring a large condenser and evaporator. Moreover, operating at sub atmospheric pressure invites air ‘poisoning’. Operating the evaporator at just a few degrees above the freezing point requires precise control. Ammonia is toxic, flammable in some concentrations (16–25 per cent), non-polluting, stable, and has the second highest latent heat among common substances. Methanol is toxic, highly inflammable, non polluting, unstable beyond 393 K, and has the third highest latent heat among common substances and ‘poisoning’ by air is a possibility. From the results, it can be seen that silica gel-water gives the maximum COPc almost equal to 6.39. This is followed by zeolite-water, which has a COPc of 6.21, and carbon-methanol, which has a value of 6.12. So from COPc point of view it can be seen that silica gel- water is the best among the pairs compared. Now from the various time study graphs that were plotted, it can be seen that water attains its saturation point on zeolite quickly followed by water on silica gel and ammonia on carbon. REFERENCES  P. Somasundaram, S. Shrotri and L. Huang, 1998, Thermodynamics of adsorption of surfactants at solid-liquid interface, International Union for Pure and Applied Chemistry, Vol. 70, No.3, pages 621-626.  M. A. Lambert and B. J. Jones, 2006, Automotive Adsorption Air Conditioners Powered by Exhaust Heat Part 1&2.  R. Z. Wang, 2001, Adsorption refrigeration research in Shanghai Jiao Tong University, Renewable and Sustainable Energy Reviews 5, pages 1-37.  Boatto P, Boccaletti C, Cerri G, and Malvicino C, 2000, Internal combustion engine waste heat potential for an automotive absorption system of air conditioning. Part 1:tests on the exhaust system of a spark ignition engine. Proc. IMechE, Part D: J. Automobile Engineering, pages 979–982.  Satish M. Manocha, 2003, Porous Carbons, Sadhana Vol. 28, Parts 1&2, February/ April 2003, Pages 335-348.  Alfred Clark, The Theory of Adsorption and Catalysis, Academic Press Inc., 1970 Edition.  S.J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity. Academic Press Inc., 1991 Edition.  Marc A. Anderson and Alan J. Rubin, Adsorption of Inorganics at Solid-Liquid Interface, Ann Arbor Science, 1981 Edition.  Paul N Cheremisinoff and Fred Ellerbusch, Carbon Adsoprtion Hand Book, Ann Arbor Science, 1980 Edition.  P. K. Nag, Engineering Thermodynamics, Tata Mcgraw Hill Publishing Company Limited, 1993 edition.
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME 132  K. Mahadevan and K. Balaveera Reddy, Design Data Hand Book(in SI and metric units) For Mechanical Engineers, CBS Publishers And Distributors, Third Edition 2002.  C. P. Kothandaraman and S. Subramnyam, Heat And Mass Transfer Data Book, New Age International Private Limited, Fifth Edition 2004.  www.eia.doe.gov, US Department of Energy, Energy Information Administration, Washington, DC.  Anirban Sur and Dr.Randip.K.Das, “Review on Solar Adsorption Refrigeration Cycle”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 1, Issue 1, 2010, pp. 190 - 226, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.
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