CONTENTS: • Introduction • Principle of Operation • Working Fluid for Vapor Absorption Refrigeration System (VARS) • Some Experimental Results for Different Fluid • Various Designs of VARS • Cost Analysis • Conclusions
INTRODUCTION • The basic aim of this presentation is to provide basic background and review existing literatures on VARS. • VARS is also belongs to class of vapor cycle is similar to the VCRS. • However, in VCRS mechanical work is required as input so it is called work operated cycle and unlike VCRS the required input in VARS is low grade thermal energy so, called heat operated cycle. • VARS is heat operated system then waste industrial heat or solar thermal energy can be used for it. • It helps to reduce problems related to global environmental, such as green house effect from CO2 emission from the utility plant.
INTRODUCTION cond.. • Electricity purchased from utility plant for VCRS can be reduced. • Another major difference is VCRS commonly used CFCs as refrigerants or working fluid which is cause of ozone layer depletion that will make VARS more prominent. • VARS use natural refrigerants such as NH3 and H2O. • COP of VARS is much lower than the VCRS. • Although VARS seem to be many advantages , VCRS still dominate all market sectors. • In order to promote the use of absorption system, further development is required to improve performance and reduce cost.
PRINCIPLE OF OPERATION • Working fluid in VARS is a binary solution consisting of refrigerant and absorbent . • Absorbent absorb refrigerant causing pressure to reduce and rejecting some amount of heat and make a stable solution. • from fig. Refrigeration is obtained by connecting two vessel left vessel containing pure refrigerant while right containing solution of refrigerant and absorbent.
This is basically an intermittent system means continuous refrigeration effect we can not get. • As the cooling process can not be produced continuously so we need a system which gives continuous refrigeration.
Mechanical work is less in VARS compared to VCRS because here pump is used instead of compressor. • However, a large amount of heat is required so the solution pump work is negligible compare to it. • Now COP of VCRS and VARS is given by COPVCRA = COPVARS = = • Since VARS uses heat energy its COP is much smaller then VCRS. • Comparing of system is not fully justify by only COP as mechanical energy is more expansive than thermal energy.
Sometimes a second law efficiency (i.e. Ratio of actual COP to that of Carnot COP) or exergetic efficiency. • It is seen that exergetic efficiency for VARS system is of the same order as that of a VCRS system. COPideal VARS = = = • Thus COP of ideal VARS increases as: • Evaporator temperature increases • Generator temperature increases • Heat sink (absorber + condenser) temperature decreases. • The COP of actual VARS is much smaller than ideal because of irreversibilities.
Working Fluid for Vapor Absorption Refrigeration System (VARS) Properties of Working Fluid (Refrigerant – Absorbent System): • Low viscosity to minimize pump work. • Low freezing point by which we can maintain the low evaporator temperature. • Thermal stability. • Irreversible chemical reaction of all kinds, such as decomposition, polymerization, corrosion, etc. Are to be avoided. • It must be completely miscible both in liquid as well as in vapour as well as in vapour phase. • In addition to above, two main thermodynamic requirements of the mixture are.
6. Solubility requirement: • The refrigerant should have more than Raoult’s law solubility in the absorbent (i.e. Solution is not ideal solution) so that a strong solution, highly rich in the refrigerant is formed in the absorber. 7. Boiling points requirement: • There should be a large difference in the NBP of the two substances at least 200oc, so that absorbent shows the negligible vapour pressure. • Thus almost absorbent free refrigerant is boiled off from the generator and the absorbent alone returns to the absorber. Different types of working fluid • A survey of absorption fluids provided by Marcriss suggested that there are some 40 refrigerants and 200 absorbents available.
Some Working Fluids Are: • Zellhoeffer et al. Determined the solubility of R21 and R22 in a number of solvents such as ether, esters, amides, and amines. They found that dimethyl ether of tetra ethylene glycol(DMG-TEG) is an extremely good solvent. • Arora et al. Chose DMG-TEG, isobutyl acetate, dimethylformamide (DMF) and diethyl formamide as absorbent for R22.
From the table we can see that DMF is the best absorbent because of low circulation rate. • by Arora et al. R22 + DMF gives high COP among them but due ozone layer depletion it is not environmental friendly and restricts its uses. • It is also costlier than NH3 and H2O. • Also a binary mixture using inorganic salt such as water +NAOH, and NH3 in combination with solid absorbent such as calcium chloride, sodium thiocynate, lithium thiocynate, lithium nitrate etc. Is BEING tried in VARS. • But the main problem is all absorbents are highly corrosive in nature and at high temperature and concentration the solution is prone to crystallization.
Some Experimental Results for Different Fluid • All fig. in this section is taken from the ref., Comparison of the performances of absorption refrigeration cycles by Z. CREPINSEK, D. GORICANEC, J. KROPE, Faculty of Chemistry and Chemical Engineering. • They use 10 working fluids to compare the performance of VARS . • Out of ten, only three working fluid is taken to be consideration to compare the COP and circulation ratio fvalues with different temperatures of the generator, evaporator, condenser, and absorber or compare the performance of 2 and 3 with 1 because 1 is most commonly used. • NH3 and H2O • NH3 and LiNO3 • NH3 and NaSCN
From fig. 1 NH3 and LiNO3 cycle a lower generators temperature can be used than for the others this is an important point for utilizing solar energy. Fig. 1 Comparison of the effect of COP values on generator temperatures Fig. 2 Comparison of the effect of circulation ratio values on generator temperatures
From fig. 3 NH3-NaSCN cycle gives better performance for lower temp and for higher temperature performance of NH3-H2O cycle is better. • Circulation ratio is greater for NH3-NaSCN cycle. • But we can not use NH3-NaSCN below -10oc because of crystallization. Fig. 4Comparison of the effect of circulation ratio values on evaporator temperatures Fig. 3 Comparison of the effect of COP values on evaporator temperatures
Increasing condenser temperatures cause a decrease in system performance. • From 20oc to 40oc NH3-NaSCN and NH3-LiNO3 cycles show better performance. For low condenser temperature NH3-NaSCN has highest COP and for high condenser temp. NH3-LiNO3 has high COP. Fig. 5 Comparison of the effect of COP values on condenser temperatures Fig. 6 Comparison of the effect of circulation ratio values on condenser temperatures
The effect of absorber temperature is similar to that of condenser temperature. • From the above discussion we can see that NH3-NaSCN and NH3-LiNO3 is suitable alternative for NH3-H2O. And also no analyser and rectifier are needed. Fig. 7 Comparison of the effect of COP values on absorber temperatures Fig. 8 Comparison of the effect of circulation ratio values on absorber temperatures
Briefly we can see the performance of HFC refrigerant such as R32, R125, R134a, R152a, and HCFC such as R22 and R124. Fig. 9 Variation of the COP with generator temperature, tg, for evaporator temperature of -5°C and cooling water temperature of 25°C
Various Designs of VARS • In this sction all fig. Are taken from the ref.P. Srikhirin et al. / Renewable and Sustainable Energy Reviews 5 (2001) 343–372 • Wastage of heat in condenser and absorber decrease with a solution heat exchanger and COP can be increased up to 60%. Fig. 10 A single-effect LiBr/water absorption refrigeration system with a solution heat exchanger (HX) that helps decrease heat input at the generator.
A double effect absorption system has a COP of 0.96 when the corresponding single-effect system has a COP of 0.6. Fig. 11 A double-effect water/LiBr absorption cycle. Heat released from the condensation of refrigerant vapor is used as heat input in generator II. This cycle is operated with 3 pressure levels i.e. high, moderate and low pressure.
Higher performance can be achieved with a single-effect absorption system with GAX stands for generator/absorber heat exchanger than double effect absorption system. • There is an additional secondary-fluid, which used for transferring heat between the absorber and the generator. Fig. 12 The dotted loop shows secondary fluid used for transferring heat from high the temperature section in the absorber to low temperature section in the generator.
Two parallel completely separated cycles using different kinds of working fluid. • This system consists of two single-effect absorption cycles using water/NH3 and LiBr/water. The NH3 system is driven by heat obtained from an external heat source. The heat reject from its absorber and condenser is used as a driving heat for the LiBr/water system. Fig. 13 Solar driven dual cycle absorption employs two different working fluids i.e. NH3/water and water/LiBr. Heat of absorption and condensation from NH3/water cycle are supplied to the generator of water/LiBr cycle.
An ejector is placed between a generator and a condenser of a single-effect absorption system. • LiBr/water is used as the working fluid. The ejector uses high-pressure water vapor from the generator as the motive fluid. Fig. 14 A combined ejector/absorption proposed by Aphornratana and Eames , was invented. High pressure refrigerant vapor from the generator enters the ejector as motive fluid to carry the refrigerant vapor from the evaporator.
For fig. 14 Experimental investigation showed that COP’s as high as 0.86 to 1.04 was found. • This system must be operated with a high temperature heat source (190 to 210°C) and acceptable surrounding temperature. • As the generator temperature is high, the corrosion of construction material may be problematic.
This cycle is a combined cycle between a steam jet heat pump and a single-effect absorption cycle. Fig. 15 A combined cycle proposed by Eames and Wu . The highest solution circuit temperature is maintained at about 80°C. So the corrosion problem is alleviated.
A steam jet system is used as an internal heat pump, which was used to recover rejected heat during the condensation of the refrigerant vapor from a single-effect absorption cycle. • The heat pump supplies heat to the generator of an absorption system. • The refrigerant vapor generated from the generator is entrained by the steam ejector and is liquefied together with the ejector’s motive steam by rejecting heat to the solution in the generator. • In this system the corrosion problem is eliminated as the solution maximum temperature is maintained at 80°C. • The driving heat (from an external source) is supplied to the steam boiler only at temperatures around 200°C. • The experimental COP of this system was found to be 1.03.
the refrigerant from the absorber can be transferred to the generator by an osmotic diffusion effect through the membrane without any mechanical pump. • The membrane should minimize heat transfer between the generator and the absorber. Fig. 16 A combined cycle proposed by Eames and Wu . The highest solution circuit temperature is maintained at about 80°C. So the corrosion problem is alleviated.
Fig. 17 A diffusion absorption refrigerator; DAR, schematic diagram is proposed. This system was once widely used as a domestic refrigerator as no electricity is required in its operation. NH3/water/auxiliary gas is charged in the machine as the working fluid.
It works on 3- fluid system third one is called auxiliary gas remains mainly in the evaporator thus reducing partial pressure of refrigerant to enable it to evaporate. • There is no need of solution circulation pump so neither electricity nor maintenance is required. • It can be easily used in remote areas where is no electricity • However, its cooling capacity is very low up to 50 W. • Hydrogen and helium is generally used as auxiliary gas. • A thermosyphone is used to lift the weak aqua form the generator. • Through out the system total pressure is constant (20.33 bar). And partial pressures of ammonia in evaporator are 1.516 bar at inlet and 2.36 bar at outlet.
Cost Analysis • presents some more general cost information on large tonnage (>100 tons) cooling equipment for space conditioning applications. Fig. 18 Chillers and auxiliary equipment costs - electric and absorption (Means, 1996).
To our knowledge, there is only one company (Yazaki,undated) currently manufacturing small tonnage (<20 tons) lithium bromide refrigeration equipment. • Currently, units are available in 1.3, 2, 3, 5, 7.5, and 10 ton capacities. • They require considerably more mechanical equipment for a given capacity than the conventional electric vapor compression equipment. Fig. 19 Simple payback on small absorption equipment compared to conventional rooftop equipment.