This item is only available as the following downloads:
1 ABSORPTION COOLING FOR DIFFUSION DRIVEN DESALINATION PROCESS By UDAY KIRAN MAHAK A LI A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTE R OF SCIENCE UNIVERSITY OF FLORIDA 2011
2 2011 Uday Kiran Mahakali
3 This work is dedicated to my parents and sister who have supported me in all my endeavors.
4 ACKNOWLEDGMENTS First, I would like to thank Dr. James F Kla usner, the c h air m an of my graduate com m ittee, for allowing m e to work under his guidance and for granting me the opportunity to be a part of his research team on the diffusion driven desalination process. I am grateful to Dr. Klausner for his constant guid ance and patience throughout the two years of my master degree studies I would also like to thank Dr. H A Ingley member on my graduate committee, for his technical assistance on the absorption cooling system. I also extend my thanks to Dr. S A Sherif for being part of my graduate committee and for his valuable time and attention. I would also like to thank all my friends who provided me with moral and academic support throughout my graduate studi es. I extend my special thanks to Dr. Fadi Al n a im a t for his crucial technical assistance. Above all, I would like to thank my family for their unwavering faith in me, love and support. Their blessings and encouragement gave me the motivation to achieve my goals.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 LIST O F KEY SYMBOLS ................................ ................................ ................................ 9 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 14 2 DIFFUSION D RIVEN DESALINATION ................................ ................................ ... 19 2.1 Description of Conventional DDD Plant ................................ ............................ 19 2.2 Diffusion Desalination Process with Ammonia Absorption Syst em ................... 22 2.2.1 Requirement of Ammonia Absorption System ................................ ......... 22 2.2.2 Description of DDD Plant with Ammonia Absorption System .................. 23 3 REFRIGERATION SYSTEM ................................ ................................ ................... 25 3.1 Conventional Vapor Compression Refrigeration ................................ ............... 25 3.2 Ammonia Absorption Refrigeration System ................................ ...................... 26 3.2.1 Principle of Operation ................................ ................................ .............. 27 3.2.2 Characteristics of Ammonia Absorption ................................ ................... 31 3.2.3 Factors Affecting COP of Ammonia Absorption ................................ ....... 32 3.2.4 Advantages of Ammonia Absorption Refrigeration System ..................... 32 4 MATHEMATICAL MODELLING OF DDD PROCESS ................................ ............. 34 4.1 Flow in the Diffusion Tower ................................ ................................ ............... 36 4.2 F low in the Condenser Tower ................................ ................................ ........... 37 4.3 Numerical Procedure ................................ ................................ ........................ 40 5 RESULTS AND DISCUSSION ................................ ................................ ............... 41 5.1 Diffusion Tower Analysis ................................ ................................ ................... 41 5.2 Condenser Tower Analysis ................................ ................................ ............... 45 6 INDUSTRIAL APPLICATION OF DDD PLANT WITH AAR S YSTEM .................... 56 7 CONCLUSIONS ................................ ................................ ................................ ..... 63
6 APPENDIX A ................................ ................................ ....................... 65 B CO G ENERATION PLANT DETAILS ................................ ................................ ..... 66 C NUMERICAL ANALYSIS RESULTS ................................ ................................ ....... 6 9 D INDUSTRIAL APPLICATION NUMERICAL ANALYSIS RESULTS ........................ 73 LIST OF REFERENCES ................................ ................................ ............................... 74 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 76
7 LIST OF TABLES Table page C 1 List of results from diffusion tower analysis ................................ ........................ 69 C 2 List of results from condenser tower analysis (without AAR system) .................. 70 C 3 List of results from condenser tower analysis (with AAR system) ....................... 71 C 4 List of results from condenser tower analysis ................................ ..................... 72 D 1 List of results from diffusion tower analysis ................................ ........................ 73 D 2 List of results from condenser tower analysis ................................ ..................... 73
8 LIST OF FIGURES Figure page 2 1 Typical diffusion driven desalination plant. ................................ ......................... 20 2 2 DDD plant with ammonia absorption refrigeration system. ................................ 24 3 1 Ammonia absorption refrigeration system. ................................ ......................... 28 4 1 Differential control volume for liquid/gas heat and mass transfer within diffusion tower. ................................ ................................ ................................ ... 34 4 2 Differential control volume for liquid/gas heat and mass transfer within counter current condenser ................................ ................................ .................. 35 5 1 Diffusion tower exit air temperature variation with air to feed water mass flow ratio ................................ ................................ ................................ .................... 43 5 2 Diffusion tower exit feed wat er temperature variation with air to feed water mass flow ratio. ................................ ................................ ................................ ... 43 5 3 Diffusion tower exit humidity ratio variation wit h air to feed water mass flow ratio. ................................ ................................ ................................ ................... 44 5 4 Condenser tower exit air temperature variation with fresh water feed to air m ass flow ratio. ................................ ................................ ................................ ... 48 5 5 Condenser tower exit fresh water temperature variation with fresh water feed to air mass flow ratio. ................................ ................................ .......................... 50 5 6 Condenser tower exit humidity ratio variation with fresh water feed to air mass flow ratio. ................................ ................................ ................................ ... 51 5 7 Fresh water production variation with fresh water feed to air mass flow ratio. .... 53 5 8 Percentage increase in fresh water production variation with fresh water feed to air mass flow ratio. ................................ ................................ .......................... 55 6 1 Variation of fresh water produced with ................................ ............................ 60 6 2 Variation of required area for diffusion and condenser towers with ................ 62 B 1 Co generation plant ................................ ................................ ............................ 66
9 LIST OF KEY SYMBOLS Cross sectional area m 2 Specific surface area m 2 / m 3 Wetted speci fic surface area m 2 / m 3 Specific heat kJ/kg Molecular Diffusion coefficient m 2 /s Diameter of packing bed m Air mass flux kg/m 2 s Enthalpy kJ/kg Latent heat of vaporization kJ/kg Mass transfer coefficient m/s Water mass flux kg/m 2 s Mass flux of fresh water produced kg/ m 2 s Molecular weight of vapor kg/kmol Mass flow rate kg/s Pressure kPa Power / Rate of energy kW Universal gas constant kJ/kg K Temperature o C or K Heat transfer coefficient W/m 2 K The fraction of exhaust air going into refrigeration system Height of packing material m
10 Greek letters Dynamic Viscosity Pa s Density kg/m 3 Critical surface tension of the packed bed N/m Liquid surface te nsion N/m Humidity ratio Relative Humidity Subscripts Air Absorber Ammonia absorption Refrigeration Cooling fresh water Condenser Cooling load Diffusion driven desalination Exhaust air Eva porator Fresh water Generator Air/Vapor mixture Gas side parameter based on the specific area of the packed bed Liquid/Vapor interface Inlet
11 Liquid Liquid side parameter based on the specific area of the packed bed Li quid side parameter based on the specific wet area of the packed bed Outlet Available for refrigeration system Saturated state Saline water
12 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science ABSORPTION COOLING FOR DIFFUSION DRIVEN DESALINATION PROCESS By Uday Kiran Mahakali August 2011 Chair: James F Klausner Major: Mechanical Engineering In order to make the diffusion driven desalination (DDD) process commercially more attractive the fresh water production from the process must be increased. This research investigate s the possible increase in the fresh water production from the DDD process when the conden ser fresh water inlet temperature is reduced. The reduction in the temperature of fresh water is achieved by a single effect ammonia absorption refrigeration system which can be run on waste heat like the DDD process. The heat and mass transfer analysis f o r the DDD process i s utilized and simulations a re carried out for the heated air / heated water case for different air and water mass fluxes. Simulations a re performed for two condenser feed water inlet temperatures of 25 o C and 1 o C, for the same diffusion tower outlet conditions. The results show an increase in the fresh water production A conceptual design of the DDD plant coupled with an ammonia absorption refrigeration system working on waste heat from a power plant is conceived. A parametric study i s m ade in which the DDD plant and the ammonia absorption refrigeration system a re powered by the waste heat available at 93 o C from a 271 MW solar combined cycle power plant. The of the ammonia absorption refrigeration system operating on the waste heat under the available conditions is calculated. Results
13 indicate that due to the low of the refrigeration system, there is a higher fresh water production when the enti re available waste heat is utilized by the DDD plant itself and the ammonia absorption refrigeration system is not used
14 CHAPTER 1 INTRODUCTION Water is an indispensable part of everyday human life. Fresh water is needed for the biological activities of living organisms. Agriculture requires fresh water for crop cultivation. Industries require fresh water for washing, cooling, fabrication and processing. Approximately, 70% the Earth's surface is covered by a water body and 30% is composed of land. Unfo rtunately, of water available on the Earth, 97% is salt water and only 3% is fresh water  The salt water has very limited use for mammalian life support. The 3% of fresh water is not completely available for ready use by mammals since 69% is trapped in the form of ice and 30% resides under the ground as ground water. The remaining 1% is available on the surface. Of this 1%, only 0.3% is present in the rivers and lakes, which can be easily used by mammals. The remaining 0.7% is either present in the atmo sphere as vapor or some other unusable form. Given the limited availability of fresh water and its role in mammalian life, it should be treated as a very precious resource. The increase in population of humans and the proportional increase in their need f or fresh water have led to shortage s of fresh water supply around the world. The scientific and engineering communities are actively developing new technologies to address this shortage. One such popular method is Desalination Many countries such as Saud i Arabia, United Arab Emirates, and Kuwait depend on desalination technologies to meet their fresh water requirements. According to an IDA Worldwide Desalting Plant I nventory Report  in 2009 approximately 1.7 billion gallons per day of fresh water wa s being produced by desalinati ng salt and brackish water in the desalination plants commissioned all over the world.
15 Desalination involves the conversion of the abundantly available salt and brackish water to consumable fresh water. This is achieved by remo ving the salt and minerals from the water. The desalination process sometimes yields sea salt as a by product. There are many different processes available for desalinating salt or brackish water  such as processes based on ph ase change, processes using membranes and processes based on modifying the chemical bonds. S ome of the commercially popular methods  used for desalination are Vacuum Distillation, Low Temperature Thermal Desalination, Multistage Flash Distillation, Multiple Effec t Distillation, Electrodialysis and Reverse Osmosis. Multistage Flash Distillation is the most widely used method while the Reverse Osmosis method is gaining in popularity. The L ow Temperature Thermal Desalination method is a relatively new one. Some other methods of desa lination are solar humidification dehumidification freezing, renewable energy powered conventional desalination among others Although Multi s tage Flash Distillation and Reverse Osmosis are commercially popular and reliable methods there are certain dis advantages For example, the rmal distillation is economically feasible only for very large scale production, typically more than 300 400 kL/day  Thermal distillation is very energy intensive and requires a high level of technical knowledge to build op erate and maintain efficient plants Reverse Osmosis technology is gaining in popularity as the cost for equipment has reduced in recent years due to mass production. However, the performance and longevity of a Reverse Osmosis plant is significantly depen dent on the pre treatment of the feed water which is expensive Also, there is a danger of contamination of the product due to the growth of bacteria on the membranes. High pressures are required to operate the
16 Reverse Osmosis plant and the high pressure s lead to an increase in frequency of plant shut down due to failures in the mechanical equipment providing the high pressures. In an attempt to find a lower cost desalination process, Humidification De h umidification (HDH) was studied by Bourouni et al.  According to Bourouni it is a simple and flexible process utilizing low grade heat energy. It is described as a process involving low initial cost, lower operation and maintenance costs. Many novel methods like the Multiple Effect Humidification proc ess, Mechanically Intensified Evaporation process which are based on the principle of HDH process, have been developed to carry out desalin ation. However, each of them ha s its own disadvantages. For example, the HDH process is unsuitable for the economical fresh water production on a large scale when compared against Reverse Osmosis or Multis tage Flash Distillation methods because it requires a very large area of land In order to overcome the short comings of the HDH desalination method mentioned above, a n innovative desalination method known as the Diffusion Driven Desalination (DDD) process was studied by Klausner et al.  The DDD process provides an economically feasible desalination method suitable for desalinating saline water o n a large scale. The DDD process, like the HDH method, can be run on low grade waste heat. In the DDD process, air is pumped into a diffusion tower and is made to contact saline or brackish water flowing in the opposite direction along the height of the tower. Heat transfer a nd mass transfer occur between the air and saline or brackish water and as a result only the fresh water vaporizes leaving behind salts and other minerals. T he air exits the diffusion tower humidified. This humidified air is then pumped
17 into another simila r tower known as the condenser tower. In this to wer, it is made to contact fresh water flowing in opposite direction, which is at a lower temperature than the air entering the condenser tower. Heat transfer occurs and the water vapor condens es out of the a ir stream and joins the fresh water flowing in the tower. Thus the mass of fresh water at the tower exit is greater than that at the tower inlet, the difference being the fresh water production. Also, the air exits the condenser tower dehumidified. The DDD process which has been described in detail by Li et al.  is presented in the chapter 2 of this report. It is known that as the temperature of air decreases, its capacity to hold water vapor also decreases and the water vapor condenses out. This is th e principle utilized in the condenser tower of the DDD process to extract fresh water from the air stream. Currently, the amount of fresh water that can be extracted from the air stream is limited by the temperature of the fresh cooling water being pumped into the condenser tower. This temperature is the ambient temperature of fresh water available, which is typically 25 o C. In the current work, this temperature is brought down to 1 o C, which is the lowest possible temperature for the safe circulation of wate r without forming ice. Utilizing the heat and mass transfer analysis available for the DDD process which is developed by Klausner et al.  the amount of fresh water produced when the fresh cooling water temperature is1 o C has been calculated for various mass flux combinations of air and water. Also, the increment in the amount of fresh water produced due to the reduction in the fresh cooling water temperature from 25 o C to 1 o C is also presented. The main advantage of the DDD process compared with commerc ially available processes is that it can run on low grade heat energy. Therefore, in an effort to keep up
18 with this advantage, the fresh cooling water is cooled to 1 o C by an Ammonia Absorption Refrigeration (AAR) plant, which can also be run on low grade e nergy. The results of a study involving a 271 MW solar combined cycle electric power generation plant together with the DDD system and an AAR plant running on shared waste energy from the power generation plant are presented in this work. The optimum ope rating condition for th e DDD process as part of this co generation plant is also discussed
19 CHAPTER 2 DIFFUSION DRIVEN DES ALINATION 2.1 Description of Conventional DDD P lant The c onventional diffusion driven desalination process has been developed by Kl ausner et al.  A laboratory scale facility is currently in operation at the University of Florida, Gainesville. A brief description of the operation of the DDD plant is given below. It is possible to run the DDD plant in three modes: 1. Ambient Air and Heated Water 2. Heated Air and Ambient Water 3. Heated Air and Heated Water In the Ambient Air and Heated Water mode, the air that is passed into the diffusion tower is at atmospheric temperature while the sea water that is passed into the diffusion tower is he ated to a temperature greater than that of the atmospheric air. In the Heated Air and Ambient Water mode, the air that is passed into the diffusion tower is heated to a higher temperature than that of the sea water that is passed into the diffusion tower, which is at atmospheric temperature. In the Heated Air and Heated Water mode, both air and water that are passed into the diffusion tower are heated to higher temperatures than the atmospheric temperature at which they are available. The following descrip tion on the operation of the DDD plant is based on the Ambient Air and Heated Water operating mode. Figure 2 1 shows a process flow diagram for a typical DDD facility. There are two towers and three fluid circul ation systems: Diffusion Tower Condenser Tow er Saline Water, Air/Vapor and Fresh Water.
20 Fig ure 2 1 Typical d iffusion d riven d esalination p lant. Sea water from the surface or from shallow depths is taken, as it is warmer than the water which is deeper. This water is pum ped into the water cooler which is a heat exchanger. The sea water is preheated in the water cooler by the fresh water discharge from the condenser It is further heated to a higher temperature using waste heat of the low pressure condensing steam from a t hermal power plant or from another source of waste heat. Waste heat can be utilized because the required feed water inlet temperature into the diffusion tower can be as low as 50 o C for the DDD process. This heated water is then sprayed in the diffusion tow er from the top. Simultaneously atmospheric air is forced into the diffusion tower from the bottom by a forced draft blower. A portion of the sea water evaporates and diffuses into the air rapidly. The evaporation in the diffusion tower is governed by the concentration gradient at the
21 of a low pressure drop and high surface area packing material The packing material is made from polypropylene. Polypropylene has a v ery low cost and is inexpensive to replace  A great portion of the heat and mass transfer occurring in the diffusion tower takes place in the packed bed. The sea water sprayed from the top of the diffusion tower travels downwards through the tower by m eans of gravity and passes over the packing material forming a thin layer of saline water on it. This film of water contacts the air which is flowing upwards in the diffusion tower facilitating the heat transfer between the saline water and air. The height and diameter of the diffusion tower is chosen so that the air entering the diffusion tower leaves it in a saturated condition. The portion of saline water that is not evaporated is now at a lower temperature due to evaporation and heat transfer with air. This saline water is collected at the bottom of the diffusion tower and dischar ged into a sea water reservoir. The saturated air exiting the diffusion tower is passed through a piping system and enters the counter current condenser tower from the bottom wh ere it is cooled and dehumidified. The condenser tower is a direct contact cond enser with counter current flow  As there is a large fraction of air/vapor mixture that is non condensable  direct contact condensation is more effective than film conde nsation as concluded by Bharathan et al.  In the DDD plant, a packed bed condensation approach is utilized in the direct contact condenser as it is found to be more effective than droplet direct contact condensation  The packing material in the c ondenser tower is similar to that in the diffusion tower. The fresh water is collected at the bottom of the condenser tower and is pumped by a cooler pump through the water cooler where it is cooled to a lower
22 temperature than the air/vapor mixture from th e diffusion tower exhaust. This co ol fresh water is sent into the fresh water storage tank. A c ertain mass of this fresh water in the storage tank is pumped by a fresh water pump and is sprayed at the top in the condenser tower. The remaining water in the storage tank is taken out as fresh water production. The saturated air/vapor mixture from the diffusion tower exhaust, which is at a higher temperature than the fresh water feed is forced from the bottom of the condenser tower. It meets the fresh water fee d in the condenser tower and most of the heat and mass transfer occurs within the packed bed. The water vapor in the air/vapor mixture condenses out due to lowering its temperature along the saturation line and the condensed fresh water is taken out along with the fresh water feed at the bottom of the condenser tower. The fresh water is collected and sent through the water cooler again to be used as fresh water coolant 2.2 Diffusion Desalination Process with Ammonia Absorption System 2.2.1 Require ment o f Ammonia Absorption System Thermal analysis of the DDD process suggests that one way to improve the fresh water production is to lower the fresh water temperature into the condenser as low as possible. The lowest possible temperature of the fresh water fe ed that is possible is just above 0 o C. Reducing the fresh water temperature further will result in freezing. So, a safe and stable operating fresh water feed temperature of 1 o C is chosen. In order to bring about the large reduction in th e fresh water tempe rature, a refrigeration system is required. In line with the objective of running the entire DDD plant on waste heat, an Ammonia Vapor Absorption Refrigeration System is used in place of the water cooler.
23 2.2.2 Description of DDD P lant with Ammonia Absorp tion System The functioning of the DDD plant remains exactly the same as that already described The waste heat from a combined cycle solar power plant is divided between the Desalination plant and the Ammonia Absorption Refrigeration system The fresh wat er sent into the condenser tower is passed through the evaporator of the a mmonia a bsorption r efrigeration system in order to reduce its temperature to 1 o C. The sea water, exiting the diffusion tower can be used as a coolant for the condenser or absorber i n the ammonia absorption refrigeration system. This way the sea water feed is preheated and it reduces the amount of heat to be added in the main feed water heater. Alternatively, the exhaust air from the condenser tower can also be used as a coolant. The process flow design depict ed in F igure 2 2 uses the latter scheme A choice between the two possibilities is made depending on the operating conditions. In some cases, even the fresh water that is produced and stored in the tanks can be utilized as a cool ant. Figure 2 2 shows a diagram of the DDD plant with an ammonia absorption refrigeration system, depicting the different components and fluids involved in the working of a DDD plant. A detailed description of the working of ammonia absorption refrigeratio n system is presented in the next chapter. Details regarding the completely solar combined cycle power generation plant are also discussed in this report
24 Figure 2 2 D DD plant with ammonia absorption refrigeration system.
25 CH APTER 3 REFRIGERATION SYSTEM There are various refrigeration methods available for rejecting heat at low temperature, such as Vapor Compression, Vapor Absorption, Gas C ycle, and Stirling C ycle. The Vapor Compression system is the most widely used system fo r refrigeration. The Gas cycle is not very efficient compared to Vapor Compression. The Stirling C ycle is too complex to be implemented at a competitive cost. The Vapor Compression system requires a compressor to function and the compressor requires high quality energy to run it. The objective of this study is to run the entire desalination plant on waste heat. Of these currently available refrigeration methods, the Vapor Absorption system can be run on waste heat commonly discharged by industrial users I t can also be run on solar energy. Its use with the DDD process will be investigated in further chapters. 3. 1 Conventional Vapor Compression Refrigeration A brief description of the conventional vapor compression refrigeration system is provided here in or der to emphasize the main differences in construction and operation between this cost effective and relatively efficient refrigeration system and the ammonia absorption system, which is the subject of importance in this thesis. The conventional vapor compr ession refrigeration system usua lly consists of a compressor, a condenser, an expansion device, an evaporator and a working fluid called refrigerant. The compressor requires mechanical energy to drive the shaft It converts this mechanical energy into an i ncrease in the pressure potential and thermal heat storage in the refrigerant It also helps to circulate the refrigerant through the entire system. The compressor can be a reciprocating or rotary type. The reciprocating type
26 provide s higher pressure rat io s than the rotary type but is less efficient and noisier The compressed refrigerant which is at a higher pressure and temperature enters the condenser from the compressor. The condenser is a heat exchanger. The refrigerant rejects heat to a coolant and th is brings down its temperature but the pressure is maintained almost the same. However, the refrigerant pressure might reduce to some extent in the condenser owing to flow losses. The condenser is typically a shell and tube type or finned tube type. The c oolant is usually air or water. The refrigerant then flows into an expansion device. The refrigerant loses its pressure as it expands across the device and thus the temperature significantly reduces. The rate of refrigerant flow in the system can be contro lled by this expansion device. It is typica lly a small orifice capillary tube or a thermostatic expansion valve. The cooling load take n by the refrigeration system i. e the amount of cooling depends on the rate of refrigerant flow. The minimum temperatur e that can be reached by using the system also depends on the type of refrigerant and the amount of expansion. There are various types of refrigerants available for different applications and to reach different temperatures. A refrigerant should be non tox ic, less damaging to the environment, economically viable and easily available. This low pressure and low temperature refrigerant enters into evaporator which is the refrigerated space. This component is a heat exchanger. 3.2 Ammonia Absorption Refrigerat ion System Ammonia has a great affinity for water. This property is the basis for the working of an ammonia vapor absorption system. The ammonia vapor absorption refrigeration ( V AR) system was invented in 1850 by Ferdinand P E Carre. It utilizes ammonia a nd water as operating fluids. It has been patented in the USA in 1860. In the early days,
27 machines based on this basic design were used in industrial refrigeration for storing food and making ice. In the 1950s a new vapor absorption refrigeration system us ing lithium bromide and water as operating fluids was introduced. As concluded by Horuz  the VAR system operating on lithium bromide and water is more efficient than that operating on ammonia and water However, the danger of crystallization and the impossibility of operating at sub zero temperatures due to the usage of water as a refrigerant render the lithium bromide VAR system unsuitable for the present application. The coefficient of performance is a measure of a cycle's ability to transfer heat between different temperatures. The coefficient of performance ( )  of a VAR is: ( 3.1 ) The for the system is the maximum possible performance that can be achieved and for the VAR system: ( 3.2 ) where, (K) is the temperature in the generator, (K) is the temperature in the absorber, (K) is the temperature in the condenser and (K) is the temperature in the evaporator of the VAR system 3.2.1 Principle of Operation In an ammonia water vapor absorption refrigeration system, ammonia is the refrigerant and water is the absorbent. The vapor absorption refrigeration system consists of an absorber, a solution pump, a re generator, a generato r, an analyzer, a rectifier, a condenser, a receiver, an expansion device and an evaporator as shown in F igure 3 1
28 Fig ure 3 1 Ammonia a bsorption r efrigeration s ystem. Pure ammonia in a gaseous sta te is sent into the absorber from the evaporator. In the absorber, the pure gaseous ammonia comes into contact with the water which absorbs ammonia. This absorption process is exothermic. However, the concentration of ammonia in water increases with decre asing temperature. Therefore in order to have maximum absorption of ammonia by water, the absorber temperature is maintained low
29 by removing the heat released due to the reaction by circulating cooling water. The following reaction occurs in the absorber: (3 .3 ) The strong solution of ammonia in water exiting the absorber is pumped into the generator via the re generator. Generally a c entrifugal pump or a diaphragm pump is used for this purpose. The energy consumed by this pump is very low compared to the heat energy supplied to the generator, usually on the order of 0.25%. The pump also raises the press ure of the Ammonia Water strong solution. This strong solution is passed through a re generator which is a heat exchanger so as to preheat the solution before entering into the generator. This reduces the amount of thermal energy supplied to the generator and helps in improving the syste m This strong solution at higher pressure is then sent into the generator which is also a heat exchanger. Here, the heat energy is supplied from sources like waste heat from a thermal power plant or a renewable energy source like solar energy. The temperature at which heat is supplied is generally above 85 o C. In the generator, the heat energy supplied is used to raise the temperature of the strong Ammonia Water solution. The solubility of ammonia in water decreases with increase in temperature and t he pure ammonia vapor separates out from the solution leaving a weak solution of Ammonia Water behind. Ideally only pure ammonia should leave the generator, but in practice the heat energy supplied also vaporizes some water. So, a mixture of Ammonia Water vapor leaves the generator. When water vapor is also carried into the evaporator, it reduce s the performance of the system. In order to remove the water vapor in the mixture, it is passed through an analyzer and then through a rectifier. The analyzer is pl aced on top of the generator and
30 it generally consists of a distillation column. It contains a number of horizontal plates along its length. When the Ammonia Water Vapor mixture enters the analyzer, it rises up and cools down. The boiling point of water be ing higher than that of ammonia, it condenses first and is collected in the bottom of the analyzer. The ammonia is still in gaseous phase. A stronger mixture of Ammonia Water vapor mixture exits the analyzer from the top and passes into the rectifier in wh ich further removal of water particles from the mixture occurs and nearly pure ammonia in the gaseous state exits the rectifier. The rectifier is a heat exchanger which is cooled externally by a coolant. This further reduces the temperature of the Ammonia Water vapor mixture, and water particles condense out along with few ammonia particles. This mixture is then sent into the analyzer and is collected at the bottom along with the analyzer exhaust. In both the rectifier and the analyzer, the weak Ammonia Wat er mixture drains down the system into the generator by gravity. This is mixed with the strong Ammonia Water mixture in the generator and further heated to generate more ammonia vapor. Finally, the weak Ammonia Water mixture is drained down from the genera tor by gravity and is sprayed into the absorber from the top. The weak mixture is passed through the re generator before being sprayed into the absorber, where it is pre cooled losing its heat to pre heat the strong Ammonia Water mixture. From the re gener ator, it is passed through a valve where it is further expanded lowering its temperature and finally into the absorber. The pure ammonia gas which is at a high pressure and temperature at the exit of the rectifier now enters into the condenser which is a h eat exchanger similar to that in the vapor compression system. In the condenser pure ammonia gas rejects heat to a coolant and it condenses into liquid. The pressure remains almost constant barring minor flow
31 losses. The liquid ammonia is then sent through an expansion device where it expands suddenly and significantly drops in pressure, there by further lowering its temperature. It is through this valve that the flow rate of ammonia and the amount of expansion is controlled which in turn determines the amo unt of cooling load and the minimum temperature that can be attained through the refrigeration system. The liquid ammonia which is at a very low pressure and temperature is then sent into the evaporator which is the space to be refrigerated. The ammonia ab sorbs heat from the refrigerated space and changes its phase into vapor. This low pressure but higher temperature gaseous ammonia is then sent into the absorber for absorption by water and the cycle repeats. 3.2.2 Characteristics of Ammonia Absorption The ammonia absorption refrigeration system has man y characteristics that make it suitable for use with low grade waste heat. Some of these characteristics are discussed here. The mechanical compression present in the conventional vapor compression refrigerat ion (VCR) system is replaced by chemical thermo compression in the VAR system. As mentioned earlier, the VAR system can run on waste heat unlike the VCR system which requires electrical energy to operate. The VAR system converts the vapor from the evapora tor exit back into a liquid using a method that only requires heat which is low grade energy compared to electrical energy which is required for the VCR system. To operate the VAR system, the source temperature can be relatively low, although a higher sou rce temperature improves the system The mechanical compressor from a conventional vapor compression refrigeration system is replaced by an absorber, a pump and a generator in the VAR system. The absorber and generator have no moving parts like the mechanical compressor utilized in th e VCR system. Also
32 the pump employed in the VAR system operates between lesser pressure differential compared to the compressor employed in the VCR system. A s imple apparatus such as re generator and a rectifier can be incorporated to improve the system pe rformance. A sub cooler can be placed in between the evaporator and absorber to cool the gaseous ammonia before entering the absorber there by improving the system performance. Multi stage vapor absorption refrigeration systems, which are relatively expens ive, are also available. They have a higher COP than single stage vapor absorption refrigeration systems. 3.2.3 Factors A ffecting COP of Ammonia Absorption The COP of the ammonia absorption refrigeration system is affected by the generator temperature ( ), evaporator temperature ( ) and condenser temperature ( )  These effects are presented below: The VAR system COP increases with an increase in the generator temperature ( ). The system COP increases as the evaporator te mperature ( ) increases and the system COP increases as the condenser temperature ( ) decreases. It is also to be noted that the cooling capacity increases as the condenser temperature ( ) decreases  3.2.4 Advantages of Ammonia Absorption Refrigeration System The ammonia absorption system offers certain advantages over the conventional vapor compression refrigeration system: It has no moving parts except the solution pump. So there is less wear and tear leading to less frequent maintenance and less shutdown time. This implies that the maintenance cost of the VAR system will be less compared to the VCR system. The
33 VAR system can be installed out doors or on roof tops as is done for the VCR system. It has a wide operating range of +5 o C to 55 o C as the freezing point of ammonia is 77 o C. The VAR system typically has a long life of at least 25 years. There are some advantages of using ammonia as a refrigerant  Ammonia has zero potential for global warming and ozone layer deplet ion unlike the CFCs. Ammonia has a higher latent heat of vaporization and hence is slightly more efficient as a refrigerant than CFCs. Therefore the system runs on lesser energy. Ammonia is cheaper than CFCs. Also, there exists tremendous amount of practic al experience in the handling of ammonia as a refrigerant. Ammonia has an unpleasant odor and this property of it makes it easy to detect any leaks in the system. There is an unlimited availability of ammonia which makes it an economical refrigerant. Howev er, there are few disadvantages of using ammonia as a refrigerant  : It is poisonous to humans if inhaled beyond a certain amount. If the concentration of ammonia in the air reaches 25 parts per million (ppm), it can cause headaches, nausea, and intense burning of the eyes, nose, throat, and skin. Ammonia is combustible, but it is very difficult to combust ammonia in air in the absence of a catalyst. Ammonia has an auto ignition temperature of 651 o C. Ammonia is a corrosive substance and it is incompatibl e with copper, which is the most common material used in the current refrigeration systems.
34 CHAPTER 4 MATHEMATICAL MODELLING OF DDD PROCESS The diffusion tower and the direct contact condenser are the main components of the DDD system. I nside the diffus ion tower, evaporation occurs when heated saline water is sprayed from the top leading to the form ation of a thin film of saline water o n the packing material and is contacted by a low humidity turbulent air stream flowing in the opposite direction to that of saline water The principles of h eat transfer and mass are utilized to quantify rate of evaporation of water and the subsequent increase in the humidity of the air. It is desired that for high production of fresh water from the DDD system, the humidity ratio of the air stream coming out of the diffusion tower should be as high as possible. Ideally, the air exiting the diffusion tower should be in a saturated state. The equations governing the evaporation and humidification processes in the diffusion tow er have been developed by Klausner et al.  It is based on a two fluid film model in which one dimensional conservation equations for mass and energy are applied to a differential control volume shown in Figure 4 1 Fig ure 4 1 Differential control volume for liquid/gas heat and mass transfer within diffusion tower. In the condenser tower cool fresh water contacts the high humidity ratio turbulent air stream in the packing material and the heat given up to the water film lea d s to the
3 5 condensation of water vapor and thus fresh water production Thus, inside the direct contact condenser, the fresh water mass increases and the humidity ratio of the air stream decreases The formulation for the direct contact condenser has been developed by Klausner et al.  A one dimensional two fluid conden sation model is used to represent the change in the humidity ratio of the air inside the direct contact condenser tower. The mass and energy conservation equations are applied to a differ ential control volume shown in Figure 4 2 Fig ure 4 2 Differential control volume for liquid/gas heat and mass transfer within counter current condenser The air temperature variation along the transverse direction in the conde nser tower is important for the condensation process  The local humidity based on the local transverse air temperature is averaged and the mean humidity is used in the one dimensional conservation equations. The following assumptions  a re made for the one dimensional model reported here: 1. The process is a t steady state. 2. Air and Water Vapor display perfect gase s behavior 3. The changes in kinetic and potential energies are negle cted 4. Water is to be pumped only against the gravitational force. 5. Heat lost to the surroundings is negligibly small
36 4.1 Flow in the Diffusion Tower For the mathematical modeling of the flow inside the diffusion tower, the conservation of mass principle is applied to the differential control volume shown in Figure 4 1 The conser vation equations are applied separately to both the liquid and vapor phases in the d ifferential control volume. U tilizing these conservation equations the convective law of mass transfer and the relationship between relative humidity ( ) and the humidity ratio ( ), the following first order ordinary di fferential equation for the gradient of the humidity ratio in the diffusion tower is obtained: ( 4 .1 ) Here, is the air mass flux, is the mass transfer coefficient, is the wetting area of packing, is the Universal gas constant, is the vapor molecular weight, is the system pressure and is the vapor saturation pressure at temperature The solution for Equation 4.1 yields the variation of humidity ratio along the height of the diffusion tower. By assuming tha t the energy convected from the liquid is approximately equal to that convected to the gas, the inte rfacial temperature ( is calculated using the Eq uation 4.2 : ( 4.2 ) Here, and are the heat transfer coefficients on the gas and liquid respectively. Applying conservation of energy to the liquid phase in the different ial control volume, a first order ordinary differential equation for the gradient of the liquid temperature ( ) is obtained: ( 4.3 )
37 Here is the water mass flux, is the overall heat transfer coefficient is the latent heat of vaporization, is enthalpy, is the specific area of packing mate rial and is the specific heat. This first order ordinary differential equation can be solved for yielding the water temperature distribution along the height of the diffusion tower. Similarly, applying conservation of energy to the air/vap or phase and neglecting the heat loss from air  the following equation is obtained : ( 4.4 ) Eq uation 4.4 is also a first order ordinary differential equation which yields the air/vapor mixture temperature through the diffusion tower. Together, the Eq uations 4.1, 4.3 and 4.4 constitute a set of coupled ordinar y differential equations that can be solved for the humidity ratio, water temperature and air/vapor mixture temperature variations along the height of the diffusion tower. These coupled set s of equations require closure relationships since a one dimensiona l formulation is used here. This implies that the overall heat transfer coefficient and gas side mass transfer coefficient are required. The heat transfer coefficients for the air and vapor are evaluated using the heat and mass transfer analogy presented b y Klausner et al.   is used to calculate the mass transfer coefficients, and in the diffusion tower. 4.2 Flow in the Condenser Tower The flow modeling in the counter current direct contact condenser is described in this section. The humidity ratio which is dependent on the air temperature is calculated using the relationship between the relative humidity and the humidity ratio utilizing Eq uation 4.5 :
38 ( 4.5 ) w here (kPa) is the total system pressure and (kPa) is the water saturation pressure corresponding to the local air temperature (kPa) can be calculated using the following empirical representation of the saturation line : ( 4.6 ) w here the empirical constants are: = 0.611379, = 0.0723669, = 2.78793e 7, = 6.76138e 7 and ( o C) is the temperature For the DDD application, the temperature range across any given cross section is small and so the area averaged humidity ratio can be approximated by in Equation 4.5  However, the relative humidity of air remains 100% during the condensation process. This condition implies that the absolute humidity is only a function of ai r temperature Taking this into consideration and differentiating Equation 4.5 and combining with Equation 4.6 the following first order ordinary differential equation shown in Equation 4.7, which expresses the gradient of humidity along the heigh t of the direct contact condenser is obtained ( 4.7 ) Applying the conservation of energy to the liquid phase in the differential control volume shown in Figure 4 2 the gradient of water temperature in the condenser tower can be expressed as : ( 4.8 )
39 By applying energy conservation to the gas phase in the differential control volume shown in Figure 4 2 the gradient of the air temperature in the condenser tower is expr essed as : ( 4.9 ) Similar to the evaporation model, a one dimensional approach i s used for the condensation model ( Equation s 4.7 and 4.8 ) and thus it requires closure relationships ( Equation s. 4.4 and 4.6 presented in the Appendix A are used to calculate the mass transfer and heat transfer coefficients In this model the gas side mass transfer coefficient is taken as (see Appendix A ). ( 4.10 ) Onda suggests that the effective packing diameter affects the mass transfer coefficient on the gas side and he recommended the use of 5.23 for the co efficient in Equation 4.10 for the cases where the effective packing diameters are larger than 15mm and 2.0 for those less than 15mm. In the DDD system, the packing material used in the direct contact cond enser has an effective packing diameter of 17mm, which is close to the limit reported by Onda. The use of 2.0 for the coefficient in Equation 4.10 is justified in Knight et al. . The fresh water production rate is calculated as follows: ( 4.11 ) Here, the subscripts respectively refer to the fresh water, condenser inlet and condenser outlet.
40 4.3 Numerical Procedure The numerical procedure to compute the exit humidity ratio, exit water temperature and exit air temperature from the diffusion tower and the condenser tower is p resented by Klausner et al.  and is detailed below: 1. Specify the inlet water temperature, air temperature and humidity. 2. Guess the exit water temperature. 3. Compute the temperature distributions and humidity distribution through the packed bed using Equation s 4.1, 4.3 and 4.4 4. Check whether the computed inlet water temperature a grees with the specified inlet water temperature and stop the computation if agreement is achieved, otherwise repeat from step 2. For the counter current direct contact condenser analysis, the numerical procedure to calculate the exit water temperature, e xit air temperature and exit humidity ratio, is reported here: 1. Specify the inlet water temperature, air temperature and bulk humidity. 2. Guess the exit water temperature. 3. Compute the temperatures and bulk humidity at the next step change in height using Equa tions 4.7, 4.8 and 4.9. 4. Proceed to new height and restart the computation from step 3 until the computed air exit temperature matches the specified air exit temperature. 5. Check whether computed inlet water temperature agrees with the specified inlet water temperature and stop the computation if agreement is found, otherwise repeat the procedure from step 2.
41 CHAPTER 5 RESULTS AND DISCUSSION The main aim of the discussion presented in this section is to investigate the improvement in the performance of the DDD plant due to the introduction of AAR system. T he AAR system reduces the fresh water inlet temperature into the condenser tower to 1 o C from 25 o C, and It is expected that there would be an increas e in the amount of condensation in the condenser tower. Th us, due to the increased condensation, there will be an improvement in the amount of fresh water produced by the DDD plant. As mentioned earlier, the DDD plant can be run in three modes namely, the heated air / ambient water mode, the heated air / heated w ater mode and the ambient air / heated water mode. In the current analysis, the heated air / heated water mode is chosen T he application of interest involves running the DDD plant with the waste heat energy available from a solar driven combined cycle pow er plant, and both heated air and heated water are readily available Also, a preliminary study which is not mentioned in this report, indicates that the heated air / heated water case yields the highest fresh water production among all the three cases fo r the same mass fluxes. 5.1 Diffusion Tower Analysis The equations that were developed in the theoretical modeling of the diffusion tower are solved using the numerical procedure that is described in the previous chapter. A Runge Kutta 4 numerical method w as used to solve the equations developed by Yi Li et al.  A code is written by Yi Li in Fortran 77 for the same. The same code is used to solve the heated air / heated water case. The coefficient correlation is fixed as 2.0 for this case as is justified by Knight et al.  In the
42 numerical analysis results presented t he specific area of packing is taken as 267m 2 /m 3 and the diameter of the packing material is taken as 0.018m. These are the specifications corresponding to HD QPACK, a commercial packing material manufactured by Lantec. Certain temperatures, such as the saline water inlet temperature and air inlet temperature into the diffusion tower, used in this numerical analys is are calculated from the energy balance of a 271 MW power plant describ ed in A ppendix B In the heated air / heated water case, the temperature of air is taken as 93 o C, since it is the temperature of exhaust air available from the combined cycle power p lant which has been mentioned in  It is assumed for the purpose of the numerical analysis that saturated air at ambient i.e. at 25 o C, is taken and heated to 93 o C. Therefore, the inlet humidity ratio of air at 93 o C entering the diffusion tower remains unchanged and is equal to the humidity ratio of saturated air at 25 o C i.e. 0.0201. The temperature of the saline water entering the diffusion tower of the DDD plant is assumed to be 80 o C for the reason that it is possible to heat the pre heated saline wate r, coming out of the steam turbine condenser at 35 o C, to 80 o C utilizing the energy available in the exhaust air stream, which is at 93 o C The diffusion tower analysis is performed in order to obtain the input data into the condenser tower like the tempera ture and humidity ratio of air entering the condenser tower. In the diffusion tower analysis, the numerical simulations were run for different air to feed water mass flow ratios for a particular feed water mass flux. Results are obtained in a similar way f or different feed water mass fluxes. A sample of t he numerical values thus obtained are tabulated and presented in the A ppendix C The results are
43 plotted and are presented here. The results have been verified against the results presented in Knight et al.  and it is observed that similarity exists in the trends observed in both the results. Fig ure 5 1 Diffusion tower exit air temperature variation with air to feed water mass flow ratio Fig ure 5 2 Diffusion tower exit feed water temperature variat ion with air to feed water mass flow ratio. 45 50 55 60 65 70 75 0.4 0.6 0.8 1.0 1.2 1.4 Exit Air Temperature(C) Air to Feed Water Mass Flow Ratio 0.5 1 1.5 2 2.5 3 Diffusion Tower m L (kg/m 2 s ) 44 45 46 47 48 49 50 51 0.4 0.6 0.8 1.0 1.2 1.4 Feed Water Exit Temperarure(C) Air to Feed Water Mass Flow Ratio 0.5 1 1.5 2 2.5 3 Diffusion Tower m L (kg/m 2 s)
44 Fig ure 5 3 Diffusion tower exit humidity ratio variation with air to feed water mass flow ratio Figure 5 1 represents the variation of the exit air temperature from the diffusion tower with air to feed water mass flow ratios, for different feed water mass fluxes. It is seen that the exit air temperature from the diffusion tower is decreasing with increase in the air mass flux, for a given feed water mass flux. In F igure 5 2 the variation of feed water exit t emperature with air to feed water mass flow ratio, for various feed water mass fluxes is shown. It is seen that there is initially a dip in the feed water exit temperature from the diffusion tower as the air to feed water mass flow ratio increases. Further increase in the air to feed water mass flow ratio causes a small increase in the feed water exit temperature. The plot shows that as the water feed mass flux increases, the feed water exit temperature from the diffusion tower decreases. The exit feed wat er from the diffuser cannot be used as a coolant for either the steam turbine condenser or the absorber in the ammonia water vapor absorption 0.050 0.070 0.090 0.110 0.130 0.150 0.170 0.190 0.4 0.6 0.8 1.0 1.2 1.4 Diffusion Tower Exit Humidity Ratio Air to Feed Water Mass Flow Ratio 0.5 1 1.5 2 2.5 3 Diffusion Tower m L (kg/m 2 s )
45 refrigeration system utilized in the DDD plant. This is because it is clear from the graph in F igure 5 2 that the feed water exit temperature from the diffusion tower is high, making it unsuitable for use as a coolant in the present case. However, energy required to heat the feed water for the diffusion tower can be reduced by re circulating this exit feed water. Fi gure 5 3 shows the variation in the exit humidity ratio from diffusion tower as the air to feed water mass flow ratio changes. A high exit humidity ratio is desired from the diffusion tower as it indicates better mass transfer. However, it should be noted that a high exit humidity ratio alone does not necessarily indicate a high fresh water production. It can be observed from the graphs that a change in water feed mass flux has no significant effect on t he exit humidity ratio of air f r o m the diffusion tow er. It can also be seen from the graphs that low air to feed water mass flow ratios yield higher exit humidity ratios. The arguments justifying the observed behavior of the different parameters presented in the F igures 5 1 5 2 and 5 3 have been mentioned in detail by Knight et al.  5.2 Condenser Tower Analysis The results for the condenser tower are discussed next Based on the results of exit humidity ratio from the diffusion tower, the rate of water vapor evaporated into the air stream is computed as : ( 5.1 ) where, is the mass flux of air, and are respective ly the humidity ratio of air at the inlet and at the outlet of the diffusion tower. From these results, exit humidity ratio
46 from the diffusion tower which yields the highest fresh water output is chosen and is considered for the analysis of the condenser t ower. The exit temperature, exit humidity and mass flux of air from the diffusion tower of the highest exit fresh water output case become the inlet temperature, inlet humidity ratio and inlet mass flux of air respectively, in the condenser tower analysis. Interestingly, the highest fresh water output is obtained for the lowest exit humidity case instead of the highest exit humidity case. This is due to the fact that the lowest exit humidity from the diffus ion tower is obtained for the highest air to feed w ater mass flow ratio and hence, a large amount of air is available to condense out the fresh water, there by yielding a high fresh water production. The condenser analysis is similar to the diffusion tower analysis. In the diffusion tower analysis presente d previously, the air to feed water ratio is varied for different feed water mass fluxes and the behavior of different parameters is observed. However, in the condenser tower analysis, the fresh water feed to air mass flux ratio is varied for different air mass fluxes and the variation of different parameters such as the condenser tower exit air and water temperatures and condenser tower exit humidity ratio are computed In addition, in the condenser tower analysis, the behavior of the different parameters is compared for two inlet feed water temperatures, the temperatures being 25 o C and 1 o C. As mentioned in the earlier section, the reason for comparing the two temperatures is, it is expected that by reducing the inlet feed water temperature in the condense r tower to 1 o C, more condensation would be possible leading to higher fresh water production. The choice of the fresh water feed inlet temperature is made as 25 o C because it is assumed that the fresh water at ambient is available at this temperature. The o ther feed water inlet temperature is chosen as 1 o C
47 as it is the lowest possible temperature below which water freezes and causes operational problems for the DDD plant. The same Runge Kutta 4 numerical method used for the diffusion tower analysis is utili is fixed as 2.0 for this analysis. The other parameters that were involved in the numeric al analysis are mentioned here: The specific area of packing is taken as 267m 2 /m 3 and the packing diameter is taken as 0.017m. Figures 5 4 A and 5 4 B show the varia tion of exit air temperature from the condenser tower when fresh water inlet temperature is 25 o C and 1 o C respectively, for different fresh water feed to air mass flow ratios. The variation in the exit air temperature is also shown for various air mass flux es. The change in the behavior of the exit air temperature for the two inlet feed water temperatures is also presented in the graphs. The exit air temperature from the condenser tower tends to remain nearly constant for large fresh water f eed to air mass f low ratio. It is observed that this constant temperature value in both cases is approximately equal to the respective inlet feed water temperatures i.e. 25 o C and 1 o C. However, in both cases, at low fresh water feed to air mass flow ratios, the exit air tem perature is elevated indicating poor heat transfer which can be attributed to the lack of a sufficient amount of fresh water for cooling The air mass flux is found to show a small influence on the exit air temperature from the condenser towe r for the 25 o C inlet water case. However, for 1 o C inlet water, the air mass flux has a significant influence at low feed water to air mass flow ratios.
48 Fig ure 5 4 Condenser tower exit air temperature variation with fresh water feed to air mass flow ratio Fresh wa ter inlet temperature is: A) 25 o C. B) 1 o C. It is desired that t he exit air temperature from the direct contact condenser be as low as possible because this air is intended to be used as a coolant for the condenser in the AAR system. The lower the coolant t emperature, the lower will be the temperature 20 25 30 35 40 45 0.0 2.0 4.0 6.0 8.0 10.0 Condenser Tower Exit Air Temperature(C) Fresh Water Feed to Air Mass Flow Ratio A 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower Fresh Water Inlet Temperature= 25 o C m a (kg/m 2 s ) 0 2 4 6 8 10 12 14 16 0.0 2.0 4.0 6.0 8.0 10.0 Condenser Tower Exit Air Temperature(C) Fresh Water Feed to Air Mass Flow Ratio B 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower Fresh Water Inlet Temperature= 1 o C m a (kg/m 2 s )
49 of the ammonia vapor cooled in the condenser and higher will be the efficiency of the AAR system. Figures 5 5 A and 5 5 B show the variation of the exit feed water temperature from the direct contact condenser to wer when fresh water inlet temperature is 25 o C and 1 o C respectively. The graphs show that the exit feed water temperature from the direct contact condenser decreases with an increase in the fresh water feed to air mass flow ratio. It can be observed that i n both cases, the exit feed water temperature decreases rapidly up to a fresh water feed to air mass flow ratio of 4.0, and the decrease is gradual with a further increase in the fresh water feed to air mass flow ratio. It can be observed from the graphs t hat, in both cases, the exit feed water temperature from the direct contact condenser tower is unaffected by the air mass flux in the condenser tower. In general, the reason for the decrease in the feed water exit temperature with increase in fresh water f eed to air mass flow ratio is that the amount of air available decreases and the amount of heat carried and so there is lower amount of heat being tran sferred to the feed water there by decreasing its exit temperature. Specifically, when the feed water tem perature is 1 o C, its exit temperature is desired to be as low as possible. This is because, in the DDD plant, the exit feed water from the direct contact condenser is cooled to 1 o C by the AAR system and is re circulated as inlet feed water to the condenser tower. Th e lower this exit temperature is, the less cooling load will be r equired from the AAR system. Figure s 5 6 A and 5 6 B show the variation of the exit humidity ratio from the direct contact condenser tower when fresh water inlet temperature is 25 o C and 1 o C
50 respectively, for different fresh water feed to air mass flow ratios. This variation is presented for the two fresh water feed inlet temperatures of 25 o C and 1 o C. Fig ure 5 5 Condenser tower exit fresh water temperature variation with fresh wat er feed to air mass flow ratio Fresh water inlet temperature is: A) 25 o C. B) 1 o C. 25 30 35 40 45 50 55 60 65 0.0 2.0 4.0 6.0 8.0 10.0 Condenser Tower Fresh Water Exit Temperature(C) Fresh Water Feed to Air Mass Flow Ratio A 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower Fresh Water Inlet Temperature= 25 o C m a (kg/m 2 s ) 0 10 20 30 40 50 60 0.0 2.0 4.0 6.0 8.0 10.0 Condenser Tower Fresh Water Exit Temperature(C) Fresh Water Feed to Air Mass Flow Ratio B 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower Fresh Water Inlet Temperature= 1 o C m a (kg/m 2 s )
51 Fig ure 5 6 Condenser tower exit humidity ratio variation with fresh water feed to air mass flow ratio. Fresh water inlet temperature is: A) 25 o C. B) 1 o C. In general, i t can be observed from the graphs that the exit humidity ratio from the condenser tower remains largely unaffected by larger fresh water feed to air mass flow 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.0 2.0 4.0 6.0 8.0 10.0 Condenser Tower Exit Humidity Ratio Fresh Water Feed to Air Mass Flow Ratio A 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower Fresh Water Inlet Temperature= 25 o C m a (kg/m 2 s ) 0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010 0.0 2.0 4.0 6.0 8.0 10.0 Condenser Tower Exit Humidity Ratio Fresh Water Feed to Air Mass Flow Ratio B 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower Fresh Water Inlet Temperature= 1 o C m a (kg/m 2 s )
52 ratio s. It is desired that the difference between the inlet and exit humidity ratios of air from the direct contact condenser tower is as high as possible. This is because, th is difference in the humidity ratio directly relates to the amount of fresh water that can be condensed It can be observed from the graphs that when the fresh water feed inlet t emperature is 1 o C, the exit humidity ratios are significantly lower, nearly 10 times less, than their corresponding values when the fresh water feed inlet temperature is 25 o C. T his implies that the difference between inlet and exit humidity ratios is signi ficantly higher when the fresh water feed inlet temperature is 1 o C than when the fresh water feed inlet temperature is 25 o C. Therefore, it can be concluded from the graphs that by reducing the fresh water feed inlet temperature to 1 o C from 25 o C, there can be a significant increase in the production of fresh water from the DDD plant. Figures 5 7 A and 5 7 B show the fresh water mass flux produced in the condenser tower when the fresh water feed inlet temperature is 25 o C and 1 o C respectively, for various fresh water feed to air mass flow ratios and also for different air mass fluxes in the condenser tower The rate of fresh water mass flux produced in the condenser tower is computed as, (5.2) where, is the mass flux of air, and are respectively the humidity ratio of air at the inlet and at the outlet of the condenser tower. As observed from the graphs, in both cases, there is no significant increase seen in the mass of fresh water produced with an increase in the fresh water feed to air mass flow ratio beyond 2.0. It is economical to operate the DDD system with the least amount of electric p ower. The electric power is
53 required in the DDD system to run the pumps and blowers which circulate air and water throughout the system. Fig ure 5 7 F resh water production variation with fresh water feed to air mass flow ratio Fresh water inlet temper ature is: A) 25 o C. B) 1 o C. 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.0 2.0 4.0 6.0 8.0 10.0 Mass flux of Fresh Waterr Produced in the Condenser Tower(kg/m 2 s) Fresh Water Feed to Air Mass Flow Ratio A 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower Fresh Water Inlet Temperature =25 0 C m a (kg/m 2 s) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.0 2.0 4.0 6.0 8.0 10.0 Mass flux of Fresh Water Produced in the Condenser Tower(kg/m 2 s) Fresh Water Feed to Air Mass Flow Ratio B 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower Fresh Water Inlet Temperature =1 0 C m a (kg/m 2 s)
54 The electric power consumed by the pumps varies directly with the mass of fluid it can pump. s that the DDD plant consumes least electricity for the amount of fresh water produced at a fresh water feed to air mass flow ratio of 2.0 Therefore, in keeping with the above argument, from the F igures 5 5 and 5 6 it can be concluded that it is economical to operate the condenser tower of DDD plant at a fresh water feed to air mass flow ratio of 2.0, since it yields the maximum fresh water production with least electricity c onsumption. The percent increase in water production by incorporating the AAR system is shown in Figure 5 8. It is observed from Figure 5 8 that for higher air mass fl uxes, there is an increase in the mass flux of fresh water produced from the condenser tower due to the reduction in the fresh water feed inlet temperature from 25 o C to 1 o C, for all corresponding operating conditions. It is also clear from Figure 5 8 that for fresh water feed to air ma ss flow ratios below 2.0, the improvement in the performance of the DDD plant due to the addition of AAR system is significantly higher than that for fresh water feed to air mass flow ratios above 2.0. Also, the increase in the performance of the DDD plant remains nearly constant beyond a fresh water feed to air mass flow ratio of 2.0. From the analysis presented in this section, it can be concluded that there is an improvement in the performance of the DDD plant due to the reduction of the fresh water feed inlet temperature into the condenser tower. Hence, the addition of a refrigeration plant, which in this case is the AAR plant, to the DDD plant to reduce the fresh water feed inlet temperature gives better performance. Whether or not the increased perform ance can be justified based on cost requires an economic assessment.
55 Fig ure 5 8 Percentage increase in fresh water production variation with fresh water feed to air mass flow ratio 0 10 20 30 40 50 60 70 80 0.0 2.0 4.0 6.0 8.0 10.0 % Increase in Fresh Water Production Fresh Water Feed to Air Mass Flow Ratio 0.7 1.4 2.1 2.8 3.5 4.2 Condenser Tower m a (kg/m 2 s)
56 CHAPTER 6 INDUSTRIAL APPLICATION OF DDD PLANT WITH AAR SYSTEM In this section, a preferred operating condition for the DDD plant with an AAR system is investigated where it is powered by the waste heat from a 271MW solar combined cycle power plant. The operating conditions for the 271 MW solar power plant are discussed in d etail in Appendix B The approach that is followed in arriving at a preferred operating condition is described is the mass flow rate (kg/s) of exhaust air from the combined cycle power plant that is diverted to the AAR system to supply heat energy to it and is the total mass flow rate (kg/s) of exhaust air from the combined cycle power plant. Therefore, de fines the fraction of exhaust air ( exhaust or waste energy ) that is utilized to power the AAR plant denotes the fraction of exhaust air that powers the DDD plant. The ambient saline water temperature and the ambient air temperature are taken as 25 o C T he ambient sink temperature is also taken as 25 o C. The aim of this analysis is to arrive at a preferred value of which yields the highest fresh water production with the available energy. The most efficient o perating conditions for the condenser tower were identified by Yi Li et al. [6 ] as : 1. The area s of the diffusion and condenser towers are assumed to be equal. 2. Mass flux of air in the diffusion and condenser tower s, = 1.5 kg/m 2 s 3. Mass flow ratio of fresh water fee d to air in the condenser tower, = 2 where, is the mass flux (kg/m 2 s) of fresh water in the condenser tower. Consider the AAR system, = kW ( 6. 1 )
57 is the amount of waste heat available for the AAR system, where is the specific heat of air at constant pressure (kJ/kg K ), is the exhaust temperature of air from the combined cy cle plant which is 93 o C and is the temperature to which this exhaust air can be cooled to. The value of depends on the temperature of the ammonia water mixture entering the generator of the AAR system and is taken to be approximately 5 o C greater tha n the temperature of the ammonia water mixture entering the generator of the AAR system Let the coefficient of performance of the AAR plant as defined in earlier sections be denoted by By definition of the cooling load , that ca n be achieved by the AAR system with the available input energy , is calculated as, kW ( 6. 2 ) Let denote the m ass flow rate (kg/s) of fresh water that can be cooled by an AAR system with a cooling capacity of Therefore, ( 6. 3 ) where, is the specific heat (kJ/kg K) of water, is the cooling fresh water t emperature entering the evaporator of the AAR system. Its value depends on the cooling fresh water temperature exiting the condenser tower which is influenced by the specified operating mass fluxes an d temperatures of the DDD plant. The value of is taken as 30 o C in the cases where the cooling fresh water exit temperature from the condenser tower is greater than 30 o C. This is because, it is expected that with the available sink temperature of 25 o C, the cooling fresh water exiting the condenser tower can be cooled to 30 o C, thereby reducing the load on the AAR system. In the other cases, is taken to be equal t o the temperature of the cooling fresh water exiting
58 the condenser tower. In the cases of being less than 30 o C, first an appropriate value of is guessed and simulation is performed The obtained value of is c ompared against the guess value and if they both are different, another value of is guessed and the procedure is iterated until the obtained value and guess value of are equal. is the inlet temperature of fresh water feed in the condenser tower. In the present application can be either 1 o C or 25 o C, depending on whether AAR plant is utilized or not. is calculated from Equation 6. 3 since all the other parameters involved are specified. Als o the fresh water mass flux, is known, where is the area (m 2 ) of the diffusion tower and the condenser tower. Therefore, the area of the towers is determined from knowing and Since, the air mass flux, is known, the mass flow rate (kg/s) of air flowing through the DDD system, is determined From the se variables the saline water mass flow rate (kg/s) , is determined as follows. kW (6.4) Here, is the amount of waste heat diverted to the DDD plant to supply heated feed water is the temperature to which the exhaust air can be cooled to and it de pends on which is the steam turbine condenser cooling water outlet temperature. is taken to be approximately 5 o C greater than The mass of saline feed water that can be heated to 80 o C utilizing is calculated using (6.5) where, is the specific heat (kJ/kg K) of saline water, is the saline water inlet tempe rature into the diffusion tower, which is equal to 80 o C in the present case. Since
59 the area of the diffusion tower and are known, the mass flux of the saline feed water into the diffusion tower, is calculated. Simulatio ns for value s of ranging from 0 to 1.0 are carried out based on the values of , and obtained from the above calculations. Based on the area of the towers, which depends on the parameter the mass flow rate of fresh water tha t is produced in the condenser tower is evaluated for each value of a s, kg/s ( 6. 6 ) For some larger values of the value of becomes negative indicating that there is no waste heat available for heating saline water feed. In such a case, the saline feed water temperature is taken to be equal to Also, since cannot be obtained by the appr oach mentioned previously, a value of the ratio is chosen. In this application, the ratio is taken as 1.0 for =0.9, 1.0. For =1, the entire waste heat is utilized for supplying cooling fresh water at 1 o C to the condenser t ower by the AAR system. In this case, the temperature of air entering the diffusion tower is equal to and the saline feed water temperature is equal to For =0, the entire waste heat is utilized for supplying heated air and heated water to the DDD plant. Therefore, the cooling fresh water temperature entering the condenser tower is taken to be equal to the sink temperature which is 25 o C. The value of is chosen as 1.0 in the simulation. A sample calculation has been performed by ta king the as 0.4, as 634 kg/s, and as 40 o C and as 35 o C. The remaining inlet conditions and the
60 results of the simulations for various values of are tabulated and presented in Appendix D The is taken as 0. 4 as a result of an approximate theoretical analysis performed on a single effect AAR system operating under the available conditions. The results of th e analysis are presented graphically below: Figure 6 1 shows the volume of fresh water produced in one day by the DDD plant, in US gallons, as varies from 0 to 1.0. Fig ure 6 1 Variation of fresh water produced with It is observed in Figure 6 1 that there is a maximum production of fresh water when is 0.There is also a local maximum occurring when is approximately 0. 6 It is observe d that the fresh water produced when is 0. 6 is significantly lower ( approximately 2.65 times ) than that when is 0. The condition = 0 virtually represents the absence of the AAR system. It can therefore be concluded from the graph in Figure 6 1 tha t the addition of AAR system, though improves the amount of condensation in the condenser tower and thereby the amount of fresh water produced 25000 75000 125000 175000 225000 0 0.2 0.4 0.6 0.8 1 Fresh Water Produced in1 day (US Gallons) = ( / )
61 from the DDD plant, it does not increase fresh water production when both the DDD plant and the AAR system are ru nning on shared energy from the same source. In other words, more fresh water can be produced from the DDD plant if the entire available waste heat from the source is utilized by the DDD plant alone without the AAR system. The reason for this is the low of the AAR system under the available conditions. It has been determined that under the same operating conditions, if an AAR system with a of approximately 4.0 can be designed, only then the fresh water production from the DDD plant with an A AR system at least equals to that from the DDD plant without an AAR system. It is estimated that it would be extremely difficult to achieve a of approximately 4.0 even with a multiple effect AAR system, under the available conditions. It is underst ood from the above discussion that an AAR system with a greater than 4.0 is required in order to obtain higher fresh water production rates from the DDD plant than that from a DDD plant without the AAR system. It is also observed from the numerical calculations of the above analysis, which are not included in this report, that as the increases the second highest fresh water production point shifts very slowly towards left. The variation of the area of the diffusion tower and the condenser tow er is shown in Figure 6 2 It is observed from the graph that the area of the diffusion and condenser towers is highest for = 1.0. As the increases, it is observed from the theoretical analysis that only the area corresponding to = 0, remains the same but the area corresponding to all other values of increases, with the area corresponding to = 1.0 being the highest
62 Fig ure 6 2 Variation of required area f or diffusion and condenser towers with 0 10 20 30 40 50 60 70 80 90 100 0 0.2 0.4 0.6 0.8 1 Area of the Diffusion and Condenser Towers (m 2 ) = ( / )
63 CHAPTER 7 CONCLUSIONS In this work, t he improvement in the performance of the DDD plant due to the reduction of condenser inlet fresh water temperature from 25 o C to 1 o C has been presented. A literature study is conducted to understand the DDD process and the ammonia absorption r efrigeration s ystem in detail which is included in this report. Simulations are written in FORTRAN 77 for the heated air / heated water case for the condenser fresh water inlet temperatures of 25 o C and 1 o C, for the same diffusion tower outl et conditions. The results of the simulations have been analyzed and it i s found that there is an increase in the fresh water production from the DDD plant due to reduction in the condenser fresh water inlet temperature. An ammonia absorption refrigeratio n system is chosen for cooling the condenser inlet fresh water from 25 o C to 1 o C, as it can run on waste heat. Waste heat from a solar combined cycle power plant is distribut ed between the DDD plant and an ammonia absorption refrigeration system and a theor etical analysis i s carried out. The analysis yielded that the highest fresh water production occurs when all heat is diverted to the DDD system. The important outcomes of this work are listed below: 1. For the heated air / heated water case reducing the con denser fresh water inlet temperature from 25 o C to 1 o C results i n an increa se in the fresh water production. 2. Utilizing the entire available waste heat for the DDD plant yields a higher fresh water production instead of distributing the energy between the D DD plant and a low ammonia absorption refrigeration system. 3. In order to obtain higher fresh water production by reducing the condenser fresh water inlet temperature, the of the refrigeration system must be great er than 4.0 in the present appl ication
64 It is recommended that no further studies for a joint DDD/AAR system are warranted. Economic feasibility does not appear to be achievable.
65 APPENDIX A *This equation has been modified
66 APPENDIX B CO GENERATION PLANT DET AILS DDD Diffusion Driven Desalination AAR Ammonia Absorption Refrigeration Figure B 1. Co g eneration p lant State Point 1: Here, the air at ambient condit ions enters the compressor. Mass flow rate of air = = 634 kg/s Inlet temperature of air = = 25 o C Inlet pressure of air = = 1.01 bar
67 State Point 2: Here, the air has been compressed and enters into the solar heating chamber. Pressure ratio = = 12.4 Pressure of air = = 12.524 bar Temperature of air = = 338.8 o C State Point 3: Here, the compressed air has been heated using solar energy and enters the gas turbine. Pressure of air = = 12.524 bar Temperature of air = = 990 o C State Point 4: Here, the hot compressed air is expanded in the gas turbine and the exhaust from it enters into the boiler or steam generator. The pressure is calculated assuming an isentropic expansion in the turbine. Temperature of air = = 500 o C Pressure of air = = 2.25 bar State Point 5: Here, part of the energy available in the exhaust stream of the gas turbine is utilized to heat water and produce steam. Assuming no heat loss in the steam generator and p erforming an energy balance on the steam generator as the co ntrol volume, we obtain the temperature of air exiting the steam generator. Temperature of air = = 93 o C State Point 6: Here, steam is generated in the steam generator and it enters the steam turbine. Mass flow rate of steam = = 80.28 kg/s Inlet pressure of steam = = 36 bar
68 Inlet temperature of steam = = 473 o C State Point 7: Here, the steam exits the steam turbine after expanding in it and enters the condenser. Pressure of steam = = 0.065 bar Temperature of steam = = 37.63 o C State Point 8: Here, the exhaust steam from the turbine has been condensed in the condens er and the condensate is drawn by the pump. Assuming ideal conditions in the condenser. Pressure of steam = = 0.065 bar Temperature of condensate = = 37.63 o C State Point 9: Here, the condensate which is drawn from the condenser has been press urized by the pump and this high pressure condensate, which is water, enters into the steam generator for conversion into steam. Pressure of condensate = = 0.065 bar Note: and are the inlet and exit temperatures of the cooling water, which in this case is the saline water that is sent into the DDD plant. Cooling water inlet temperature = = 25 o C Cooling water exit temperature = =35 o C
69 APPENDIX C NUMERICAL ANALYSIS R ESULTS Table C 1 List of results from diffusion tower a nalysis Exit Air Temp. Exit Water Temp. Exit 0.5 0.4 70.0848 50.4327 0.1572 0.5 0.6 66.6292 47.8207 0.1232 0.5 0.8 64.0220 47.2108 0.1024 0.5 1.0 61 .9745 47.5116 0.0889 0.5 1.2 60.2114 48.1898 0.0791 0.5 1.4 58.7795 48.9668 0.0724 1.0 0.4 67.7308 49.5627 0.1620 1.0 0.6 63.7433 46.8440 0.1273 1.0 0.8 60.8751 46.1214 0.1061 1.0 1.0 58.7279 46.2363 0.0926 1.0 1.2 57.0560 46.6460 0.0835 1.0 1.4 55.5665 47.1018 0.0757 1.5 0.4 66.4673 49.2259 0.1642 1.5 0.6 62.2439 46.4168 0.1293 1.5 0.8 59.3095 45.5847 0.1084 1.5 1.0 57.1497 45.5467 0.0949 1.5 1.2 55.4478 45.7726 0.0850 1.5 1.4 54.2679 46.04 24 0.0797 2.0 0.4 65.6556 49.0584 0.1654 2.0 0.6 61.3137 46.1739 0.1306 2.0 0.8 58.3221 45.2459 0.1092 2.0 1.0 56.1960 45.0900 0.0963 2.0 1.2 54.5095 45.1866 0.0859 2.0 1.4 53.1752 45.3338 0.0781 2.5 0.4 65.0900 48.968 7 0.1659 2.5 0.6 60.6759 46.0190 0.1312 2.5 0.8 57.6792 45.0110 0.1100 2.5 1.0 55.5550 44.7609 0.0969 2.5 1.2 53.9666 44.7629 0.0876 2.5 1.4 52.7715 44.8238 0.0812 3.0 0.4 64.6782 48.9225 0.1664 3.0 0.6 60.2212 45.9155 0.1 316 3.0 0.8 57.2418 44.8398 0.1110 3.0 1.0 55.1187 44.5130 0.0976 3.0 1.2 53.5444 44.4418 0.0880
70 Table C 2 List of r esults from c ondenser tower analysis (without AAR system) Inlet Air Inlet Water Inlet Exit Air Exit Water Exit T emp. Temp. Temp. Temp. 0.7 1.0 58.780 25.0 0.0724 37.2904 58.7517 0.0225 0.0349 1.4 2.0 58.780 25.0 0.0724 25.1181 46.8352 0.0110 0.0430 2.8 4.0 58.780 25.0 0.0724 25.0236 36.1076 0.0109 0.0430 4.2 6.0 58.780 25.0 0.0724 25.0145 32.4457 0.0109 0.0430 5.6 8.0 58.780 25.0 0.0724 25.0109 30.5990 0.0109 0.0430 7.0 10.0 58.780 25.0 0.0724 25.0096 29.4866 0.0109 0.0430 1.4 1.0 55.567 25.0 0.0757 38.351 4 55.5117 0.0297 0. 0645 2.8 2.0 55.567 25.0 0.0757 25.5727 46.4552 0.0140 0.0863 5.6 4.0 55.567 25.0 0.0757 25.1231 35.9795 0.0136 0.0869 8.4 6.0 55.567 25.0 0.0757 25.0771 32.3644 0.0136 0.0869 11.2 8.0 55.567 25.0 0.0757 25.0607 30.53 92 0.0136 0.0870 14.0 10.0 55.567 25.0 0.0757 25.0530 29.4392 0.0136 0.0870 2.1 1.0 54.268 25.0 0.0797 39.3027 54.1979 0.0353 0.0933 4.2 2.0 54.268 25.0 0.0797 26.2439 46.6485 0.0165 0.1327 8.4 4.0 54.268 25.0 0.0797 25.2817 36.1914 0.0156 0.1347 12.6 6.0 54.268 25.0 0.0797 25.1798 32.5154 0.0155 0.1349 16.8 8.0 54.268 25.0 0.0797 25.1433 30.6551 0.0154 0.1350 21.0 10.0 54.268 25.0 0.0797 25.1262 29.5337 0.0154 0.1350 2.8 1.0 53.175 25.0 0.0781 38.7108 53.0598 0.0355 0.1194 5.6 2.0 53.175 25.0 0.0781 26.6554 45.7156 0.0176 0.1695 11.2 4.0 53.175 25.0 0.0781 25.4241 35.7716 0.0163 0.1730 16.8 6.0 53.175 25.0 0.0781 25.2794 3 2.2389 0.0162 0.1734 22.4 8.0 53.175 25.0 0.0781 25.2269 30.4488 0.0161 0.1736 28.0 10.0 53.175 25.0 0.0781 25.2000 29.3678 0.0161 0.1737 3.5 1.0 52.772 25.0 0.0812 39.2925 52.6446 0.0389 0.1479 7.0 2.0 52.772 25.0 0.0812 27.3109 45.9445 0.0194 0.2162 14.0 4.0 52.772 25.0 0.0812 25.6206 36.0065 0.0175 0.2229 21.0 6.0 52.772 25.0 0.0812 25.4131 32.4068 0.0173 0.2236 28.0 8.0 52.772 25.0 0.0812 25.3383 30.5787 0.0172 0. 2239 35.0 10.0 52.772 25.0 0.0812 25.3001 29.4737 0.0172 0.2241 4.2 1.0 52.409 25.0 0.0824 39.3703 52.2576 0.0405 0.1762 8.4 2.0 52.409 25.0 0.0824 27.8048 45.8220 0.0207 0.2592 16.8 4.0 52.409 25.0 0.0824 25 .80 33 36.0294 0.0183 0.2691 25.2 6.0 52.409 25.0 0.0824 25.5426 32.4308 0.0180 0.2703 33.6 8.0 52.409 25.0 0.0824 25.4480 30.5997 0.0179 0.2707 42.0 10.0 52.409 25.0 0.0824 25.3980 29.4907 0.0179 0.2710
71 Table C 3 L ist of results from condenser tower analysis (with AAR system) Inlet Air Inlet Water Inlet Exit Air Exit Water Exit Temp. Temp. Temp. Temp. 0.7 1.0 58.780 1.0 0.0724 1.3221 54.3816 0.0023 0.0491 1.4 2.0 58.780 1.0 0.0724 1 .0268 28.7030 0.0022 0.0491 2.8 4.0 58.780 1.0 0.0724 1.0113 15.0891 0.0022 0.0491 4.2 6.0 58.780 1.0 0.0724 1.0084 10.4432 0.0022 0.0491 5.6 8.0 58.780 1.0 0.0724 1.0074 8.1010 0.0022 0.0491 7.0 10.0 58.780 1.0 0.0724 1.0068 6.6897 0.0022 0.0491 1.4 1.0 55.567 1.0 0.0757 3.8618 54.2327 0.0034 0.1012 2.8 2.0 55.567 1.0 0.0757 1.1378 29.0659 0.0028 0.1021 5.6 4.0 55.567 1.0 0.0757 1.0610 15.2865 0.0028 0.1021 8.4 6.0 55.567 1.0 0.0757 1.0470 10.5782 0.0028 0.1021 11.2 8.0 55.567 1.0 0.0757 1.0406 8.2029 0.0031 0.1021 14.0 10.0 55.567 1.0 0.0757 1.0376 6.7719 0.0028 0.1021 2.1 1.0 54.268 1.0 0.0797 9.4508 53.6830 0.0056 0.1555 4.2 2.0 5 4.268 1.0 0.0797 1.3213 29.9074 0.0032 0.1607 8.4 4.0 54.268 1.0 0.0797 1.1444 15.7355 0.0031 0.1608 12.6 6.0 54.268 1.0 0.0797 1.1113 10.8824 0.0031 0.1608 16.8 8.0 54.268 1.0 0.0797 1.0975 8.4332 0.0031 0.1608 21.0 10 .0 54.268 1.0 0.0797 1.0903 6.9569 0.0031 0.1608 2.8 1.0 53.175 1.0 0.0781 9.8455 52.2957 0.0060 0.2018 5.6 2.0 53.175 1.0 0.0781 1.5096 29.2423 0.0034 0.2093 11.2 4.0 53.175 1.0 0.0781 1.2392 15.3986 0.0033 0. 2095 16.8 6.0 53.175 1.0 0.0781 1.1884 10.6573 0.0033 0.2095 22.4 8.0 53.175 1.0 0.0781 1.1656 8.2631 0.0033 0.2095 28.0 10.0 53.175 1.0 0.0781 1.1542 6.8210 0.0033 0.2095 3.5 1.0 52.772 1.0 0.0812 13.2588 52.0631 0.0080 0.2561 7.0 2.0 52.772 1.0 0.0812 1.7606 29.9685 0.0036 0.2715 14.0 4.0 52.772 1.0 0.0812 1.3600 15.7913 0.0035 0.2718 21.0 6.0 52.772 1.0 0.0812 1.2832 10.9234 0.0035 0.2719 28.0 8.0 52.772 1.0 0.0812 1.2508 8.465 2 0.0035 0.2719 35.0 10.0 52.772 1.0 0.0812 1.2331 6.9830 0.0035 0.2720 4.2 1.0 52.409 1.0 0.0824 14.8097 51.6654 0.0092 0.3075 8.4 2.0 52.409 1.0 0.0824 2.0095 30.1804 0.0038 0.3300 16.8 4.0 52.409 1.0 0.0824 1.4839 15.9144 0.0037 0.3306 25.2 6.0 52.409 1.0 0.0824 1.3834 11.0084 0 .0037 0.3307 33.6 8.0 52.409 1.0 0.0824 1.3411 8.5302 0.0036 0.3308 42.0 10.0 52.409 1.0 0.0824 1.3183 7.0360 0.0036 0.3308
72 Table C 4. List of resu lts from condenser tower analysis Increase in % Increase in 0.7 1.0 0.0142 40.52 1.4 2.0 0.0061 14.27 2.8 4.0 0.0061 14.14 4.2 6.0 0.0061 14.13 5.6 8.0 0.0061 14.12 7.0 10.0 0.0061 14.12 1.4 1.0 0.0368 57.03 2.8 2.0 0.0157 18.23 5.6 4.0 0.0152 17.52 8.4 6.0 0.0152 17.45 11.2 8.0 0.0152 17.43 14.0 10.0 0.0151 17.42 2.1 1.0 0.0623 66.79 4.2 2.0 0.0279 21.05 8.4 4.0 0.0260 19.34 12.6 6.0 0.0259 19.17 16.8 8.0 0.0258 19.11 21.0 10.0 0.0258 19.08 2.8 1.0 0.0825 69.10 5.6 2.0 0.0398 23.48 11.2 4.0 0.0364 21.05 16.8 6.0 0.0361 20.79 22.4 8.0 0.0359 20.69 28.0 10.0 0.0359 20.65 3.5 1.0 0.1082 73.11 7.0 2.0 0.0552 25.55 14.0 4.0 0.0490 21.98 21.0 6.0 0.0483 21.59 28.0 8.0 0.0480 21.45 35.0 10.0 0.0479 21.38 4.2 1.0 0.1313 74.52 8.4 2.0 0.0708 27.34 16.8 4.0 0.0615 22.87 25.2 6.0 0.0604 22.36 33.6 8.0 0.0600 22.18 42.0 10.0 0.0598 22.09
73 APPENDIX D INDUSTRIAL APPLICATI ON NUMERICAL ANALYSI S RESULTS Table D 1. List of results from diffusion tower analysis Inlet Air Inlet Water Exit Air Exit Water Inlet Exit Temp. Temp. Temp. Temp. 0 1.5 1.5 1.0 93.0 80.0 62.4 53.4 0.0201 0.0797 0.1 43.0 1.5 28.7 93.0 80.0 --------------------0.2 18.9 1.5 12.6 93.0 80.0 --------------------0.3 10.9 1.5 7.2 93.0 80.0 --------------------0.4 6.8 1.5 4.6 93.0 80.0 6 2.8 66.1 0.0201 0.1472 0.5 4.4 1.5 2.9 93.0 80.0 62.4 61.7 0.0201 0.1293 0.6 2.8 1.5 1.9 93.0 80.0 62.3 58.0 0.0201 0.1111 0.7 1.7 1.5 1.1 93.0 80.0 62.4 53.8 0.0201 0.0836 0.8 0.6 1.5 0.4 93.0 80.0 63.1 53.5 0.0201 0.0500 0.9 1.5 1.5 1.0 93.0 35.0 48.5 42.1 0.0201 0.0271 1 1.5 1.5 1.0 40.0 35.0 35.7 34.4 0.0201 0.0229 Table D 2 List of results from condenser tower analysis I nlet Air Inlet Water Exit Air Exit Water Exit Area of Towers US gallons of fresh Temp. Temp. Temp. Temp. m 2 water produced 0 62.4 25.0 25.4 49.6 0.01006 92.93000 221530.92612 0.1 -------------------------3.70000 0.00000 0.2 -------------------------7.40000 0.00000 0.3 -------------------------11.09000 0.00000 0.4 62.8 1.0 1.1 46.3 0.00080 14.79000 68923.03823 0.5 62.3 1.0 1.1 44.6 0.00069 18.50000 81357.5 7666 0.6 62.3 1.0 1.1 39.8 0.00079 22.18000 83713.06224 0.7 62.4 1.0 1.2 32.4 0.00209 25.88000 72211.85920 0.8 63.1 1.0 1.1 23.0 0.00122 35.74000 59621.31179 0.9 48.5 1.0 1.1 14.4 0.00145 41.95000 36822.42561 1.0 35.7 1.0 1.1 11.3 0.00249 44.67000 31199.52009 Note: Higher air to sea water feed mass flow ratios were not considered.
74 LIST OF REFERENCES 1. Water: The Power, Promise, and Turmoil Nat ional Geographic Special Edition, Nation al Geographic Society, Washington D.C., November 1993. 2. A. A. Alawadhi Regional Report on Desalination GCC Countries, in: Proceedings of the IDA World Congress o n Desalination and Water Reuse Manama, Bahrain, (2002) 8 13. 3. Introduction to Desalination Techn ologies in Australia. Retrieved on May 20, 2011 from http://www.affa.gov.au/content/publication.cfm 4. K. Bourouni M.T. Chaibi and L. Tadrist Water d esalination by h umidification and d ehumidific ation of a ir: State of the a rt, Desalination, 137 (2001) 167 176. 5. J.F. Klausner, Y. Li, M. Darwish and R. Mei, Innovative d iffusion d riven desalination process J. of Energy Resources Technology, 126 ( 2004) 219 225. 6. Y Li Heat and Mass Transfer for the d i ffusi on driven desalination process, PhD d issertation Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, Florida, 32611, USA 2006. 7. J.F. Klausner, Y. Li and R. Mei, Evaporative h eat and m ass t ransfer for the d iffusion d r iven d esalination p rocess, J. of Heat and Mass Transfer, 42 (6) (2006) 528 536. 8. Y. Li, J.F. Klausner, R. Mei and J. Knight, Direct c ondensation in p acked b eds, International J. of Heat and Mass Transfer, 49 (2006) 4751 4761. 9. Y. Li, J.F. Klausner and R. Mei Performance c haracteristics of the d iffusion dr iven d esalination p (2006) 188 209. 10. D. Bharathan, B.K. Parsons and J.A. Althof, Direct Contact Condensers for Open Cycle OTEC Applications. National Renewable Energy Laboratory Repo rt SERI/TP 252 3108 for DOE Contract No. DEAC02 83CH10093, 1988. 11. I. Horuz A comparison between ammonia water and water lithium bromide solutions in vapor absorption refrigeration systems, Int. Comm Heat Mas s Transfer, Vol. 25, No. 5 (1998) 711 721. 12. Pongsi d Srikhirin, Satha Aphornratana and Supachart Chungpaibulpatana, A review of absorp tion refrigeration technologies, Renewable and Sustainable Energy Reviews, 5 (2001) 343 372. 13. G. Lorentzen, The use of natural refrigerants : a complete solution to the CFC/HC FC predicament, Int. J. Refri 9. Vol.18, No.3 (1995) 190 197.
75 14. J. Knight, Heat and m ass t ransfer within the d iffusion d riven d esalination process Department of Mechanical & Aerospace Engineering, University of Florida, Gai nesville, Florida, 32611, USA 2006 15. K. Onda, H. Takechi and Y. Okumoto, Mass t ransfer c oefficients between g as and l iquid p hases in p acked c olumns, J. Chem. Eng. Jpn., 1 (1968) 56 62.
76 BIOGRAPHICAL SKETCH Uday Kiran Mahakali was bo rn in Nellore India. Ud ay co m pleted his Bachelor of Technology in m echanical e ngineering from Jawah arlal Nehru Technological Unive rsity Hyderabad, Andhra Pradesh, India, in May 2009 after which he joined University of Florida to pursue his Master of Science degree in m echanic al e ngineering Uday st a rt e d working t o wards his maste r a t the University of Florida f rom the fall of 2009. Later, he got the opportunity to be a part of the d iffusion d riven d esalination process research team under the guidance of Dr. James F Klausner Upon completion of his m Uday plans to continue contrib u ting to the mechanical engineering industry and build on his knowledge and experience.