A Review Paper on Thermodynamic Analysis of Vapour Absorption System Driven by Low Grade Energy

IJSTE - International Journal of Science Technology & Engineering | Volume 4 | Issue 2 | August 2017 ISSN (online): 2349-784X

A Review Paper on Thermodynamic Analysis of Vapour Absorption System Driven by Low Grade Energy Satyam Tiwari M. Tech Scholar Department of Automobile Engineering RJIT BSF Academy, Tekanpur, India

Anand Baghel Assistant Professor Department of Automobile Engineering RJIT BSF Academy, Tekanpur, India

Abstract This paper provides a review on various Vapour Absorption Refrigeration technologies. And study on different working fluids used in VAS, and improvement of Vapour Absorption process are discussed. The effect of various parameters on coefficient of performance of Li-Br water vapour absorption system also discussed and analyzed with reference to its use in automobiles for air conditioning. This review paper also illustrates various advantages of using VAS by means of waste/low grade heat.

Keywords: Vapour Absorption, Lithium Bromide, Automobile, Coefficient of Performance, Low Grade Heat ________________________________________________________________________________________________________ I.

INTRODUCTION

In present fossil fuel reserve of the world is continuously decreasing, at current consumption rate fossil fuels will be exhausted within 50 years & use of non-conventional energy resources is gaining high importance. Also a thorough literature review has been done from many international papers that chlorine in CFCs molecules (for refrigeration) cause depletion of ozone. As CFCs molecules reach stratosphere, they are dissociated by the sunlight into active chlorine compounds which in turn attack ozone, generating a chain reaction has forced to look for alternative system i.e., vapour absorption system. Also an important point to mention absorption refrigeration systems is the continuing substitution of chlorinated fluorocarbons (CFCs) by alternative refrigerants, according to the Montreal Protocol, signed in 1987 by 46 countries and revised in 1990 to protect the ozone layer.

Vapour absorption system in general can be classified into two types according to different working fluids 1) Absorption Refrigeration (LiBr-H2O, NH3-H2O, LiCl-H2O) 2) Adsorption Refrigeration (CaCl2-NH3, H2O-Silica gel, H2O-Zeolite, Activated charcoal-methanol) Basically H2O-LiBr or NH3-H2O combination are generally used.NH3-H2O system can be widely used in low temperature applications since freezing point of NH3 is -77°C. Use of NH3-H2O system is found to be (8-10) times costlier than the conventional refrigeration system. In the 1950’s, a system using lithium bromide/water as the working fluid was introduced for industrial applications, cycle efficiency & operating characteristics of an absorption systems. The main feature of the system is that H2O being the refrigerant it cannot work below 50°C because there is chance of water being frozen also H2O-LiBr combination works at temperature above 5°C because of probability of water freezing at low temperature. So this system can be applied for comfort air-conditioning system where temperature requirement for water as secondary refrigerant is above 60°C. The aim of this paper is to provide basic background and review existing literatures on absorption refrigeration technologies. A number of absorption refrigeration systems and research options are provided and discussed. It is hoped that, this paper should be useful for any newcomer in the field of refrigeration technology.

II. PRINCIPLE OF OPERATION The absorption type of air-conditioning equipment works on two basic principles: a salt solution absorbs water vapour and the evaporation of water causes cooling. In this particular discussion, the absorption cooling cycle is appropriate since it is used in gasfired air conditioners. Most gas-fueled air-conditioning equipment use a solution of lithium bromide (LiBr) in water. Lithium bromide is a colorless, salt like compound that dissolves in water, even to a greater extent than does common salt.

A solution of lithium bromide and water can absorb still more water. Note that, in Figure 2.1, a tank of absorbing solution (tank B) is connected with a tank of water (tank A). The air in the system is almost completely evacuated. The partial vacuum aids the evaporation process. Water vapor is drawn from the evaporator to the absorber. Evaporation of the water in the evaporator causes the water remaining in it to cool about 10ºF (5.5ºC). The evaporator effect in the evaporator is greatly hastened if the water is sprayed through several shower-bath sprinkler heads.

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A Review Paper on Thermodynamic Analysis of Vapour Absorption System Driven by Low Grade Energy (IJSTE/ Volume 4 / Issue 2 / 001)

Fig. 2.1: Lithium Bromide system of refrigeration

A coil of pipe through which a material such as water passes can be placed within the shower of evaporating water. The water entering the coil of pipe at 55ÂşF (12.8ÂşC) will be cooled to about 45ÂşF (7.2ÂşC).

Since the absorber (B), shown in Figure 2.1, continually receives water, it would soon overflow if the excess water that comes to it as water vapor was not removed. To avoid overflow, the solution that has absorbed water is pumped to a generator (C). In the generator, the solution is heated directly by a natural gas flame. A steam coil may heat it indirectly. The steam is made in a gasfueled boiler. When the solution is heated, some of the water evaporates and passes into the condenser (D). The concentrated solution, that remains is sprayed back into the absorber (B). Here, it again absorbs water vapour that comes from the evaporator.

Water vapor in the condenser (D) is cooled by a separate coil of pipe through which water passes. The condensed water is returned to the evaporator (A). Careful engineering is needed to make the system work well and economically. Attention must be given to temperatures, pressures, and heat transfer in all parts of the system. Practical machines with very few moving parts have now been developed.

Coefficient of Performance of an absorption refrigeration system is obtained from: đ??śđ?‘œđ?‘œđ?‘™đ?‘–đ?‘›đ?‘” đ?‘?đ?‘Žđ?‘?đ?‘Žđ?‘?đ?‘–đ?‘Ąđ?‘Ś đ?‘œđ?‘?đ?‘Ąđ?‘Žđ?‘–đ?‘›đ?‘’đ?‘‘ đ?‘Žđ?‘Ą đ?‘’đ?‘Łđ?‘Žđ?‘?đ?‘œđ?‘&#x;đ?‘Žđ?‘Ąđ?‘œđ?‘&#x; COP= đ??ťđ?‘’đ?‘Žđ?‘Ą đ?‘–đ?‘›đ?‘?đ?‘˘đ?‘Ą đ?‘“đ?‘œđ?‘&#x; đ?‘Ąâ„Žđ?‘’ đ?‘”đ?‘’đ?‘›đ?‘’đ?‘&#x;đ?‘Žđ?‘Ąđ?‘œđ?‘&#x;+đ?‘Šđ?‘œđ?‘&#x;đ?‘˜ đ?‘–đ?‘›đ?‘?đ?‘˘đ?‘Ą đ?‘“đ?‘œđ?‘&#x; đ?‘?đ?‘˘đ?‘šđ?‘?

The work input for the pump is negligible relative to the heat input at the generator, therefore, the pump work is often neglected for the purposes of analysis.

III. SUMMARY OF LITERATURE REVIEW A number of experimental and theoretical studies of Vapour absorption air-conditioning systems have been done in the past.

With rapid energy demand growth worldwide, human being have to face more energy scarcity and environmental issues. The conventional technologies to produce useful energy product, such as electricity normally results in considerable environmental pollution and non-renewable resource depletion. These interrelated challenges solved through the use of energy conversion enhancement, waste heat recovery and renewable energy resources utilization. [1] Selection of Liquid Refrigerant Investigations and experiments found that calcium chloride is the cheapest and most readily available desiccant, but its vapor pressure at a given temperature is relatively high, and its unstable conditions depending on inlet air conditions and desiccant concentration in solution limit its widespread use. Lithium chloride is the most stable desiccant with advantageously low vapor pressure, but its cost is slightly higher compared with others. The cost and vapor pressure of lithium bromide are intermediate.

Triethylene glycol is the earliest used desiccant in liquid desiccant dehumidification systems, but the liquid residence caused by its high viscosity make the system operation unstable. Besides, triethylene glycol has a very low surface vapor pressure, which causes some of them to evaporate into the air flowing into the conditioned areas. Owing to the difference of the purity of the metalsalt, the surface vapor pressure of the liquid desiccants often varies. Experimental measurement is an important method but not a universal method due to the capital support. Lithium chloride and calcium chloride are the universal desiccants mostly used to get cost-effective mixture in different weight combinations in the open literature [2-4]

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A Review Paper on Thermodynamic Analysis of Vapour Absorption System Driven by Low Grade Energy (IJSTE/ Volume 4 / Issue 2 / 001)

Solar Vapour Absorption System Li and Sumathy [5] experimentally studied a solar powered absorption air conditioning system of lithium bromide-water solution as the refrigerant fluid. Their experimental results showed that using a partitioned hot-water storage tank is necessary to enhance the reliability of the system and achieve a continuous process operation.

Atmaca and Yigit [6] showed that evacuated, selective surface solar collector is the best option for the effective operation of their solar-power absorption cooling system. Their results showed that solar power absorption cooling system requires a high performance collector.

Vapour Adsorption system In 1929, Hulse invented an adsorption refrigeration system, which was powered by high temperature combustion gas of dimethylmethane with silica gel–SO2 working pairs to meet the needs of railage refrigeration in France. Zeolite–methanol Tchernev, 1999 [7], silica gel–water Suzuki, 1993 [8], activated carbon–ammonia Vasiliev et al., 1996 [9]; Critoph 1994 [10] and Rane et al., 1999 [11], Substitution of petroleum-based combustion fuels in the 1970’s affected the application of absorption refrigeration, but, at the same time, new opportunities arose, such as usage of solar energy to operate this system Costa [12] A limitation of the pair water–lithium bromide is the difficulty to operate at temperatures lower than 0ºC. Besides, lithium bromide crystallizes at moderate concentration, and, at high concentration, the solution is corrosive to some metals and is of high cost Horuz [13] Vapour Absorption System in Automobile Huangfu et al. [14] designed and developed an experimental prototype of an integrated thermal management controller (ITMC) for internal-combustion-engine-based cogeneration system. Based on the principle of variable conductance heat pipe (VCHP), the authors presented the concept of ITMC for I.C Engine based cogeneration system application.

Singh and Mahala [15] “Analysis of modified single effect lithium-Bromide water absorption system using exhaust gases of I.C. Engine” Meunier [16] discussed adsorption air conditioning for automobiles as a very challenging possibility for adsorption cooling. The author states that car air conditioning is an ideal solution for sorption systems to be competitive from the global warming point of view, even with low coefficient of performance (COP).

Jiangzhou et al. [17] presented an adsorption air conditioning system used in internal combustion engine locomotive driver cabin. The system consists of an adsorber and a cold storage evaporator driven by the engine exhaust gas waste heat, and employs zeolite–water as working pair.

Single Effect and Double effect LiBr-H2O Vapour Absorption System Analysis of single and double effect system are done by Anand and kumar [18] and Kaushik, Arora [19] and COP of the systems are calculated. Anand and kumar [18] carried out the irreversibility analysis of single effect and series flow double effect systems for condenser and absorber temperatures 37.8℃, evaporate temperature 7.2℃, and generator temperature 87.8℃, for single effect and 140.6℃, for double effect systems. They did not attempt to find the optimum generator temperature for the operation of a series flow double effect system in nearly 60-70% greater than single effect COP. Lee and sheriff (2001) [20] presented the second law analysis of double effect lithium bromide absorption chillers or a chilled water temperature 7.22℃, and cooling water temperature 29.4℃, and 35℃, and computed COP and exergetic efficiency. In the studies of double effect double system, the effectiveness value of heat exchangers considered for analysis has not specified.

Moreover their results are valid for water cooled systems only.

Mishra et al. [21], presented an application of thermo economic theory to the economic optimization of a single effect H 2O/LiBr vapour absorption system aimed at minimizing its overall operation and amortization cost. Appropriate optimum design configuration was obtained by means of sequential local optimization of the system, carried out by unit by unit.

Ouaskit and Fathi [22] presented two modifications in single effect cycle. By these modifications in absorption refrigeration machine functioning with LiBr-H2O which determines energy and mass statement of each cycle element supposed in the permanent regime. In this research work they determined COP of solar single stage absorption refrigerator as a function of the temperature in the different component of cycle.

NH3- H2O Vapour Absorption System Since the invention of an absorption refrigeration system, water NH 3 has been widely used for both cooling and heating purposes.

Both NH3 (refrigerant) and water (absorbent) are highly stable for a wide range of operating temperature and pressure. NH 3 has a high latent heat of vaporization, which is necessary for efficient performance of the system. It can be used for low temperature applications, as the freezing point of NH3 is -77°C. Since both NH3 and water are volatility, the cycle requires a rectifier to strip away water that normally evaporates with NH3. Without a rectifier, the water would accumulate in the evaporator and offset the system performance. There are other disadvantages such as its high pressure, toxicity, and corrosive action to copper and copper

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A Review Paper on Thermodynamic Analysis of Vapour Absorption System Driven by Low Grade Energy (IJSTE/ Volume 4 / Issue 2 / 001)

alloy. However, water/NH3 is environmental friendly and low cost. Thermodynamic properties of water/NH3 can be obtained from [23–25].

IV. GAPS IDENTIFIED FROM LITERATURE REVIEW From study of above literature some points found Maximum of experiments regarding solar absorption driven vapour absorption system are LiBr-H2O with LiBr (absorbent) and H2O (refrigerant).The main feature of the system is that H2O being the refrigerant it cannot work below 50ºC because there is chance of crystallization. So this system can be applied for comfort air-conditioning system where temperature requirement for water as secondary refrigerant is above 60ºC. It has also been found that in simple VAS system by changing some of the parameters can make practical VAS which not only work effectively but improve Coefficient of Performance (COP) of the system.

V. CONCLUSION From review of literature following conclusions can be drawn:  Vapour absorption system is the cheapest and environmental friendly system can be used,  Vapour absorption system can be run by low grade energy,  Vapour absorption system can be a good substitute of Vapour compression system for domestic, industries and in automobile.  COP of Single effect vapour absorption system is less than 1 and lies between 0.6 to 0.8 whereas COP of Double effect system lies between 0.9 to 1.5,  For better working of vapour absorption system, the crystallization phenomena should be carefully study.

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