unnurbj__rnsdottir2004

Use of Geothermal Energy for Cooling

Unnur Bjrnsdttir

FACULTY OF E NGINEERING - U NIVERSITY OF I CELAND June 2004

Use of Geothermal Energy for CoolingUnnur Bjrnsdttir

A thesis submitted in partial fulllment of the requirements for the degree of magister scientarium

Faculty of Engineering University of Iceland June 2004

PrefaceThis thesis has been written under the guidance of associate professor Fjla Jnsdttir and Halldr Plsson Ph.D in mechanical engineering, both with the faculty of mechanical engineering at the university of Iceland. Their assistance is hereby greatly acknowledged. Acknowledgements are also due to my moderator, Kristinn Ingason with VGK Engineering of Reykjavk, Iceland and Professor Pll Valdimarsson, with the faculty of mechanical engineering, has given great help in using the EES program. Reykjavik Energy for sponsoring the work and VGK Engineering for various support. Unnur Bjrnsdttir

Faculty of Engineering, University of Iceland Reykjavk, May 2004.

AbstractThis is a thesis on using geothermal heat as an energy source for refrigeration systems. Various refrigeration cycles using heat energy are generally reviewed, with a special focus on absorption refrigeration systems. Thermodynamic models are presented for two prevalent kinds of absorption systems. These models are used for simulating the systems and performing analysis of their relationships between internal parameters of the system and their inuence on the COP. Various comparisons of the systems in consideration are made, for instance with regard to sensitivity of COP and costs. The work is wrapped up with a case study of a shrimp processing plant.

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ContentsPreface . . . . Abstract . . . . List of Figures List of Tables . 1 Introduction 2 Refrigeration Cycles using Geothermal Energy 2.1 Absorption Refrigeration . . . . . . . . . . . . . . . 2.1.1 Water - Lithium Bromide Systems . . . . . 2.1.2 Water-Lithium Bromide Two-stage System 2.1.3 Advanced Systems . . . . . . . . . . . . . . 2.1.4 Ammonia - Water Systems . . . . . . . . . 2.1.5 Ammonia - Water Two-stage System . . . . 2.1.6 Generator-Absorber Heat Exchange Cycles 2.2 Adsorption Refrigeration . . . . . . . . . . . . . . . 2.3 Steam Jet Refrigeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii . v . viii . xi 1 5 5 7 9 9 11 12 13 14 15 19 19 21 26 33 33 33 35 36 36 36 38

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3 Thermodynamic Models 3.1 Absorption Refrigeration . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Water-Lithium Bromide Systems . . . . . . . . . . . . . . 3.1.2 Ammonia-Water System . . . . . . . . . . . . . . . . . . . 4 Comparison and Analysis 4.1 Comparison of heat driven cycles . . . . . . . . . . . . . . . . . . 4.2 Comparison of Absorption Systems . . . . . . . . . . . . . . . . 4.3 Absorption Refrigeration vs. Vapour Compression Refrigeration 4.4 Sensitivity Analysis of COP . . . . . . . . . . . . . . . . . . . . . 4.4.1 LiBr-Water model . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Ammonia-Water model . . . . . . . . . . . . . . . . . . . 4.5 Ammonia-water model: Discussion . . . . . . . . . . . . . . . .

viii 4.6 Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6.1 Investment cost . . . . . . . . . . . . . . . . . . . . . . . . 4.6.2 Operational cost . . . . . . . . . . . . . . . . . . . . . . . 40 40 40 43 47 50 51 55

5 Case Study 6 Conclusions and Further Work Bibliography A Figures B EES models

List of Figures2.1 2.2 2.3 2.4 2.5 2.6 2.7 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 4.1 4.2 4.3 4.4 4.5 5.1 Water-lithium bromide absorption refrigeration cycle with typical temperatures. . . . . . . . . . . . . . . . . . . . . . . . . . . . Two-stage water-lithium bromide absorption refrigeration cycle. Ammonia-water absorption refrigeration cycle with typical temperature values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ammonia-water absorber double-effect system with two rectiers in the second stage. . . . . . . . . . . . . . . . . . . . . . . . Generator-Absorber heat exchange cycle. . . . . . . . . . . . . . Adsorption refrigeration cycle. . . . . . . . . . . . . . . . . . . . Steam jet ejector refrigeration cycle. . . . . . . . . . . . . . . . . . Heat exchange, showing the pinch point. . . . . . . . Water-lithium bromide absorption refrigeration cycle. Heat exchange, condenser. . . . . . . . . . . . . . . . . Heat exchange, evaporator. . . . . . . . . . . . . . . . Heat exchange, absorber. . . . . . . . . . . . . . . . . . Heat exchange, generator. . . . . . . . . . . . . . . . . Heat exchange, solution heat exchanger. . . . . . . . . Ammonia-Water absorption refrigeration cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 10 12 13 14 15 17 20 21 23 24 25 26 27 27 34 37 38 39 39 44 51 52 52

Evaporation temperatures for ammonia and water. . . . . . . . . COP sensitivity - LiBr-water model. . . . . . . . . . . . . . . . . COP sensitivity - Ammonia-water model. . . . . . . . . . . . . . COP as a function of evaporating temperature . . . . . . . . . . Steam input temperature as a function of evaporating temperature, ammonia-water model. . . . . . . . . . . . . . . . . . . . . . Flow diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A.1 COP vs. effectiveness of the solution heat exchanger, shx . . . . . A.2 COP vs. Tpinch,absorber . . . . . . . . . . . . . . . . . . . . . . . . . . A.3 COP vs. cooling water temperature for absorber, T17 . . . . . . .

x A.4 COP vs. evaporator temperature, Te . . . . . . . . . . . . . . . . . A.5 COP vs. condenser temperature, Tc . . . . . . . . . . . . . . . . . A.6 COP and evaporation temperature as a function of generator temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.7 Minimum generator temperature as a function of evaporation temperature for different cooling water temperetures. . . . . . . 52 52 53 53

List of Tables2.1 2.2 2.3 2.4 3.1 3.2 3.3 3.4 4.1 4.2 4.3 4.4 4.5 4.6 5.1 5.2 5.3 Absorbent pairs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pros and cons of water-lithium bromide system. . . . . . . . . . Benets and drawbacks of water-ammonia system. . . . . . . . Adsorbent pairs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermodynamic state point summary. . . . . . . . . . . . . . . . Model inputs and outputs. . . . . . . . . . . . . . . . . . . . . . . Thermodynamic state point summary . . . . . . . . . . . . . . . Model inputs and outputs. . . . . . . . . . . . . . . . . . . . . . . Comparison between different methods of using heat for cooling, (Noeres et al., 1999). . . . . . . . . . . . . . . . . . . . . . . . Comparison of an Ammonia Absorption Plant and a Lithium Bromide Absorption Plants. . . . . . . . . . . . . . . . . . . . . . Comparison between different types of absorption cycles. . . . . Comparison of an ammonia absorption plant and a vapour compression plant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Capital cost of water-LiBr absorption refrigeration plants compared to vapour compression, (Means, 1995). . . . . . . . . . . . Energy cost of 100 kW water chiller. . . . . . . . . . . . . . . . . Refrigeration capacity of the cooling system in the shrimp processing plant in Hsavk, (Bergsteinn Gunnarsson., 2004). . . . . Sizes of components in refrigeration system. . . . . . . . . . . . Total energy and cooling water usage of the absorption refrigeration system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 9 11 16 22 22 28 29

34 35 35 36 40 41

43 44 45

AcronymsCOP LiBr EES N H3 : : : : Coeffecient of performance Lithium bromide Engineering equation solver Ammonia

Nomenclatureh : Q : W : m : T : x : v : P : cp : Q : UA : entalphy Heat ow Work mass ow Temperature Mass fraction Specic volume Pressure Specic heat vapour quality Heat exchanger size

Chapter 1 IntroductionThe subject of this thesis are refrigeration systems where geothermal heat is used as energy source with a focus on absorption refrigeration systems. Geothermal heat one of Icelands most important energy source after hydro power. Geothermal energy is a renewable energy source and geothermal heat processing is sustainable if thermal advection to the geothermal areas is in balance with the processing. About 60 TWh/year of the heat that ows to the surface is useable, and there of, around 14-15 TWh have been used, (Orkustofnun, 2003). The rest is not economically feasible for use, because it is either not concentrated or not hot enough. Geothermal areas are divided into hightemperature and low-temperature areas. The water from low-temperature areas is mostly used for tap water and for heating houses. The temperature of low-temperature areas is usually about 100 C, at a depth of 1 km. The hightemperature areas are connected to volcanic zones and the temperature is usually more than 150 C, at a depth of 1 km. Water from high-temperature areas usually cannot be used directly as tap water without problems with corrosion and precipitation. Therefore, the high- temperature areas are used for producing electricity and also for heating houses. In Iceland about 85% of all the residences are heated with geothermal heat, mostly from the low-temperature areas. Hot water for heating has been produced there since 1928, (Wu, 1992). Geothermal heat is also used for geothermal horticulture, swimming pools and electricity production. However, the efciency of high-temperature energy in electricity production is only about 10-15%, (Orkustofnun, 2003), which is why it is economical to have heating plants integrated with electrical power plants and use the residual heat for heating. Some geothermal areas cannot be harnessed, for instance those containing Ice-

2

Introduction

lands best known natural treasures. An example hereof, are the erupting hotsprings in the Geysir area. Geothermal areas can also be subglacial and therefore technically difcult to utilise. Geothermal heat is found in several countries of the world even though it is not used in all of them. The largest quantities of geothermal energy are found in The Philippines, Italy, Indonesia and New-Zealand. The earliest geothermal electric production plant dates from 1904 in Italy, (Wu, 1992). Refrigeration is used for many purposes. Fridges and freezers can be found in most homes and in hot climates, air conditioning is important. Refrigeration is dened as any process of heat removal, or process of reduction and maintenance of a specic temperature of a space or material, below the temperature of the surroundings. Refrigerators are cyclic devices and use working uids called refrigerants. The performance of refrigerants is expressed in terms of the coefcient of performance (COP). Several refrigerants can be used and their enviromental friendliness varies. Refrigeration plants need driving energy for a continuous or periodic working process. The most common refrigeration system is the Vapour-Compression System. That system has a high COP (about 4), (Dossat, 1980) and uses electricity. This system is adequate for most refrigeration applications and is used for both small and large systems and also for various temperatures. They are simple, inexpensive, reliable and practically maintenance free. Improvements in efciency were made in the 1980s, (Braven et al., 1993). These improvements were partly driven by governmental regulations in the USA. Despite this, energy crises have forced various industries to seek new methods in refrigeration. At the present time, geothermal heat is not used for refrigeration in Iceland like it is in some other countries. The rst widely reported occurance of use of geothermal heat for cooling was an air conditioning system in a Hotel in New Zealand, (Reistad, 1980). In Mexico, several experiments have been conducted using both ammonia-water and ammonia-lithium nitrate absorption coolers operated by low temperature geothermal energy, (Best et al., 1990). Mexico possesses large amounts of geothermal brine at temperatures which are to low to enable electricity production and also the ambient temperature is relatively high. Insufcient cooling is a nationwide problem in Mexico, annually causing a great deal of spoiled food. In Turkey, some examinations of using geothermal heat for air conditioning have been conducted by using LiBr-water absorption refrigeration, (Kececiler et al., 2000). There, the focus is on the hot springs in the Sivas area, but the energy cannot be used efciently for electricity generation because the temperature is too low. There is a chiller plant in Germany which uses steam jet refrigeration, dating from 1997, (Noeres et al., 1999). This plant uses thermal heat as power input

3 (143 C, 3 bar) and is connected to a district cooling system. It is more common to use solar thermal energy or waste heat with absorption and adsorption refrigeration, but the technology is similar and therefore it is possible to use geothermal energy if it is accessible. In Iceland, little research has been performed on the possibilities of using geothermal energy in refrigeration. Comparison of geothermally driven ammoniawater absorption refrigeration and normal vapour compression systems has been performed in Reykjanes, (Ragnarsson, 1976). Research on freeze-drying with geothermal heat and absorption refrigeration has also been performed, (Gunnarsson, 1989). By using geothermal heat directly as an energy source for refrigeration systems it is possible to reduce exergy loss. Exergy is the work potential of energy, also called the available energy. During the conversion of energy, exergy is lost via heat loss. By using geothermal energy to produce electricity and then drive refrigerators, the energy goes through many transformations: thermal energy mechanical energy electric energy mechanical energy. While converting heat energy into work, only about 20% - 40% are used, the rest is lost, (Valdimarsson, Pll, 2003). Therefore, electric energy is a much more rened energy form than the thermal one, and consequently more feasible for general use, making it more valuable economically. Sometimes it is also possible to use residual heat or low-temperature waste heat. Energy shortage is a real challenge in some areas, making the use of new energy sources a necessity. Vapour compression refrigeration systems use a lot of energy and can be polluting. Cooling with geothermal heat is therefore a viable and exciting option. Many options are viable because various cycles use both high and low temperature heat. The goal of this project is to examine the possibilities of using geothermal heat for cooling, that is, to operate air conditioning systems, industrial refrigeration systems and other cooling systems. Various refrigerating cycles and their possible utilisation are examined and compared with regard to cost and performance. For this purpose, the cycles are modelled in a computer program called Engineering Equation Solver (EES) (Klein, S.A. and F.L. Alvarado, FChart Software, 2004) and compared with reference systems. These systems will also be modelled for different temperature inputs. The main research contribution of this project into the area in discussion is the following: A detailed examination of more than one method of utilisation of geothermal energy for cooling and a local feasibility assessment for employing these methods in Iceland.

4

Introduction

The remaining four chapters of the thesis are organized as follows: In chapter 2 the background of various refrigeration cycles using geothermal energy is described. The functionality of the cycles is discussed, along with the main components, advantages and disadvantages of a system based on each cycle. Thermodynamic models that are in focus in this thesis are described in chapter 3. Corresponding thermodynamic equations are stated and the cycles described in more detail. Chapter 4 is centered on comparison of the the models of chapter 3 with regard to performance and cost. A brief comparison is also made of the methods of chapter 2. In chapter 5 is a case study. Finally, chapter 6 gives conclusions and further work.

Chapter 2 Refrigeration Cycles using Geothermal EnergyThe most common refrigeration cycle using thermal energy is the absorption cycle which has been known for more than hundred years. This cycle is known in a few forms, using different uid working pairs. A working pair consists of two chemicals, a refrigerant and a transport medium. Ammonia or water are used as refrigerants and water, lithium bromide or lithium chloride are used as a transport medium. Both single and two-stage systems are known. Another system is the adsorption refrigeration which has developed rapidly in recent years but has not yet been put into mass production. This system is also known in a few forms, using various adsorbent beds and various adsorbent pairs. An adsorbent pair consists of a solid and a liquid. Many adsorbate/adsorbent pairs can be used, giving different results. A less known cooling process is the steam jet ejector chiller technology. In this process, a mechanical compressor of a compression chiller is replaced by a steam jet ejector and water ideally as refrigerant. The steam jet systems can both be open or closed processes. The absorption, adsorption and steam jet systems are described further below.

2.1 Absorption RefrigerationThe absorption cycle is a process by which a refrigeration effect is produced through the use of two uids and heat energy, rather than electrical input as in vapour compression cycles. Small quantities of electricity are used. Different

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Refrigeration Cycles using Geothermal Energy Absorbent Lithium bromide (LiBr) Water (H2O) Lithium Cloride (LiCl) Lithum nitrate (LiN O3 ) Refrigerant Water (H2 O) Ammonia (N H3 ) Water (H2 O) Ammonia (N H3 )

Table 2.1: Absorbent pairs.

types of thermal energy are used, for instance, waste heat, gas burners and solar energy but geothermal energy is not so commonly used. As stated earlier, the rst refrigeration system using geothermal energy was an absorption system used for air conditioning in a hotel in New Zealand, (Reistad, 1980). This system was devised in 1966. In 1859 Ferdinand Carre of France got a patent for the ammonia-water refrigeration machine (engel and Boles, 1998). Absorption refrigeration systems are also economically feasible if there is a source of inexpensive thermal energy available such as geothermal energy, solar energy or waste heat from cogeneration or process steam plants. These systems use very little electricity and are often classied as heat driven systems. Absorption systems are similar to vapour compression systems, except that the compressor has been replaced by an absorber, a pump, a generator, a heat exchanger and an expansion valve. They have one major advantage over compression systems: liquid is compressed instead of a vapour. A relatively larger power input per unit mass ow is needed to compress a gas than to pump a liquid between the same pressure differences. They are also simpler, less expensive and operate with less noise and maintainace. But the coefcient of performance is much lower. The COP for the vapour compression system is dened as: COPvc = useful refrigeration rate / input work rate. The COP for the absorption system is dened as: COPar = useful refrigeration rate / rate of heat addition at the generator. Therefore, direct comparison is not fair. However, absorption systems are usually not economical unless inexpensive thermal energy is available, that is, geothermal or industrial waste heat. Here after follows a list of various absorption refrigeration cycles.

2.1 Absorption Refrigeration

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2.1.1 Water - Lithium Bromide SystemsThis cycle is the simplest and most widely used of the absorption cycles. In the water-lithium bromide system, water is the refrigerant and LiBr is the transport medium. Hence, cooling below 0 C is not possible. These systems are ideal for air conditioning. Few companies produce air conditioning systems with LiBr-water as a working pair but almost all of the units burn gas to produce steam. This system is widely used in large buildings such as churches, hospitals and schools, using direct red systems. Producers for these systems are mostly Japanese and American companies. The absorption heat pump industry is based on LiBr-water technology that was pioneered in the U.S. in the late 1950s, (Braven et al., 1993). Today, the U.S. absorption industry is back in business. The U.S. industry was leading until the market "dried up" in the late 1970s, partically due to political decisions in the early 1970s. The Japanese manufacturers developed the double effect absorption chiller, which has higher efciency, (Herold et al., 1996). These systems have a few advantages over other refrigeration systems. There are no moving parts so there is little maintenance. The process is steady, that is, it is in balance . The system can use waste heat and can be used for both heating and cooling. In comparison to ammonia-water systems, it has one major advantage. The absorbent is non-volatile so there is no absorbent mixed with the refrigerant leaving the generator and a rectier (a rectier is really a separator) is not required in the system. But there are also disadvantages. Because water is the refrigerant, cooling under 0 C is not possible and the system is therefore not suitable for use in applications where the evaporator heat temperature is below 0 C. These systems need large quantaties of cooling water and consist of rather large units which take space and are both expensive and rather complex. Lithium bromide is not completely soluble in water under all the conditions likely to occur in the system so special precautions must be taken in the design to avoid conditions that will allow precipitation and crystallization of the absorbent. As shown in Figure 2.1, there are eight basic units in the water-LiBr system. The refrigerant (water-vapour) exits the generator and enters the condenser. In the condenser, the vapour is condensed on the surface of a cooling coil. Refrigerant liquid accumulates in the condenser. From the condenser, the high pressure liquid refrigerant passes through a expansion valve before it enters the evaporator. The pressure is lower in the evaporator than in the condenser. It drops in the expansion valve and is additionally inuenced from the ab-

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Refrigeration Cycles using Geothermal Energy

Condenser T=80C

Generator

T=40C Expansion valve T=2C T=40C T=30 T=2C T=50C

T=60C

T=90C

Heat exchanger T=30

Pump Evaporator Absorber

Figure 2.1: Water-lithium bromide absorption refrigeration cycle with typical temperatures.

sorber in the evaporator. The refrigerant boils on the surface of the chilled water coil in the evaporator as it ows in. The refrigerant then evaporates at 3-4 C. Heat is removed from the recirculating water which is chilled. This water can then be used for air cooling. Then the refrigerant vapour ows to the absorber. In the absorber, the LiBr solution is mixed with the refrigerant. Solubility is temperature dependent and therefore the solution is cooled with cooling water. The refrigerant is absorbed by the concentrated LiBr solution as it ows through the absorber coils. The cooling water removes heat from the condensation and dilution. The higher the concentration of LiBr solution, the lower the saturated vapour pressure of the solution; thus, the solution tends to absorb the refrigerated vapour. The dilution is then pumped through a heat exchanger before entering the generator. The heat medium (hot water or steam) enters the generator and heats the dilute, and the boiling process drives the refrigerant vapour and droplets of concentrated solution to the separator. Finally, refrigerant vapour goes through to the condenser and concentrated LiBr solution is precooled through the heat exchanger before owing into the absorber. The pressure in the system is less than atmospheric.

2.1 Absorption Refrigeration Benets No moving parts Heating and cooling Steady process Can use waste heat Absorbent is non - volatile Drawbacks Minimum heat is > 0 C Need lots of cooling water Large - space requirements Expensive

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Table 2.2: Pros and cons of water-lithium bromide system.

2.1.2 Water-Lithium Bromide Two-stage SystemTwo-stage lithium bromide systems require a higher temperature of the heat source than single stage systems, about 180-190 C. Nonetheless, the heat input is 30% - 40% lower than that of single systems assuming the same output (Sther, 1999). They also need less cooling water, about 20%. The two-stage systems are often driven by steam or direct red with natural gas. These systems have high investment costs and because of the need for higher temperature, they can seldomly use waste heat. Thus, they seem to be more expensive to run, even though they have higher COP than single systems or COP about 1.4 Two-stage systems have a higher number of components, such as, high-pressure generator, low-pressure generator, high-temperature heat exchanger, low- temperature heat exchanger, drain heat exchanger, a solution pump, and a refrigerant pump. The double effect system parallel ow is shown in Figure 2.2. Few versions are available: parallel ow, serial with a high desorber rst and serial with a low desorber rst. These systems have different (good) capacities and COPs.

2.1.3 Advanced SystemsThe cycles discussed above represent absorption chillers from various manufacturers that are currently on the market with water LiBr as a working pair. In addition, there are cycles that have been conceived as solutions to some particular thermal management challenges. Each of the cycles has a particular niche application which makes it desirable, but they also have limits. These cycles generally use water as a refrigerant and LiBr as an absorbant. These cycles are the following:

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High heat exchanger High generator

Pump 2 Low condenser

Expansion valve

Low-generator Expansion valve Expansion valve Absorber Pump 2

Low heat exchanger

Evaporator

Expansion valve

Figure 2.2: Two-stage water-lithium bromide absorption refrigeration cycle.

Half-effect cycle When the temperature of the heat source is less than the minimum temperature to drive a single-effect cycle, a half-effect cycle can be used. The half-effect cycle has two absorbers, high and low, and two generators. But the COP is only half of that of a single-effect machine or about 0.35 for the LiBr-water machine. Triple-effect cycle The triple-effect cycle is in development by several manufacturers. It is possible that it could reach a COP of 1.5 with little increase in initial costs. Triple effect implies higher temperature of the heat input. Resorption cycle The resorption cycle is different, it has two solution circuits where the second circuit replaces the condenser and the evaporator. This cycle only has a COP of about 0.55.

2.1 Absorption Refrigeration Benets No moving parts Heating and cooling Steady process Can use waste heat Environmentally friendly Drawbacks Absorbent is volatile Need lots of cooling water Large - take space Expensive Smells bad

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Table 2.3: Benets and drawbacks of water-ammonia system.

2.1.4 Ammonia - Water SystemsThe oldest absorption system is the ammonia-water system with ammonia as a refrigerant and water as a transport medium. With ammonia as refrigerant the cooling range is -60 -+5 C. These systems are used in domestic refrigerators and in commercial and industrial systems. This system can be used for air conditioning refrigerators and freezers. Most products require natural gas to burn as a fuel to generate heat but some can also use geothermal heat. Producers are few, seeing as some have gone bankrupt in recent years, but come from all around the world for instance India, The Netherlands and the USA. ( e.g the oldest company Borsig GMbh with more than 80 years of experience has gone bankrupt), (Borsig GmbH, 2003) This system has some of the same advantages and disadvantages as the waterlithium bromide system as seen in Tables 2.2 and 2.3. But with ammonia as refrigerant, it is possible to cool under 0 C. Ammonia is relatively environmentally friendly despite its smell, but poisonous to living organisms, so it must not be disposed of into the enviroment. But with this pungent smell, the ammonia is self alarming and even a small leak is noticed. In the other hand, ammonia is solvable in water so any normal leaking test with bubbles does not work. An ammonia-water absorber works very similarly to a water-LiBr absorber but because the absorbent (water) is reasonably volatile, a rectier is needed to separate the absorbant from the refrigerant. When the refrigerant vapour (ammonia) leaves the generator it contains appreciable amounts of water. If the water vapour is passed through the condenser and enters the evaporator, the evaporator temperature will rise and the refrigerating effect will be reduced. The rectier removes the water vapour from the mixture. In ammonia systems, the pressure is higher than in the LiBr system. Therefore, a smaller pipe diameter is required.

12Condenser

Refrigeration Cycles using Geothermal EnergyGenerator Rectifier T=35C T=100C T=30C T=100C Heat exchanger T=130C T=100C

Expansion valve T=-40C

T=30C Expansion valve T=30C T=20C T=20C

Evaporator

T=-35C Absorber

Pump

Figure 2.3: Ammonia-water absorption refrigeration cycle with typical temper-

ature values.

2.1.5 Ammonia - Water Two-stage SystemThese systems are similar to Water-LiBr two-stage systems but have more components and are therefore slightly more complex. This system is also known in more than one version. There are 26 different two-stage congurations. However, the performance potential of only a few hereof have actually been investigated: Double-effect Ammonia-Water systems The term double-effect refers to a conguration in which a certain quantity of heat is used twice to generate refrigerant vapor. It is a special case of a twostage system. It is different from the double-effect water-LiBr system. It has two pressure levels while the LiBr system has three. It is also more complicated and has more components.These systems are also known in more than one version, but there are two basic types: double-condenser, double-effect and double-absorber, double-effect. Figure 2.4 shows one type of a doubleabsorber double-effect system. Another type are triple-effect cycle, but it is known in three versions: the three condenser cycle, the three absorber cycle and the two absorber/two condenser


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cycle or kangaroo cycle. The COP is higher than of single-stage systems, or about 0.7-1.4.Condenser Generator

Rectifier 1

Rectifier 3

Rectifier 2

Heat exchanger Heat exchanger

Heat exchanger

Expansion valve

Pump

Expansion valve

Expansion valve

Absorber 1 Evaporator Absorber 2

Figure 2.4: Ammonia-water absorber double-effect system with two rectiers

in the second stage.

2.1.6 Generator-Absorber Heat Exchange CyclesThe generator-absorber heat exchange (GAX) cycle can either be considered as a single-stage or two-stage cycle. It uses ammonia-water as a working pair. Single-stage because it has only one pump, absorber, genarator, condenser and evaporator like the single stage system. But because the absorber heats the generator, it can also be considered as two-stage or a double effect system. The COP is about 1.4. This cycle is known in more complicated versions such as branched GAX, or BGAX. The BGAX boosts the thermal performance by recycling a portion of the ammonia/water solution before it reaches the generator. A branched cycle can have COP up to 2. GAX cycle system has been manufactured using gas as energy supply.

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Condenser

Qr

Generator

Rectifier Qint

Absorber

Evaporator

Figure 2.5: Generator-Absorber heat exchange cycle.

2.2 Adsorption RefrigerationAdsorption refrigeration began attracting interest during the energy shortage of the 1970s,(Lu et al., 2001). Adsorption refrigerators can be used in nonelectried areas, such as in cryogenics and in automotive air conditioning. Solid adsorption refrigeration has a few advantages over the traditional vapor compression refrigeration:

1. The working cycle can be driven by a low-temperature heat source so it can use waste heat. 2. The refrigerant adsorbate employed in refrigeration is environmentally friendly (does not reduce the ozone layer or contribute to greenhouse effects). 3. The main component of an absorbent bed refrigerator is xed, and other parts do not move much, so it can be applied in movable or vibrational surroundings.

2.3 Steam Jet Refrigeration

15

This system is not without aws. Its cycle is non-continuous, the cycle time is long and the cooling capacity is low. The COP is also very low, about 0.4. The performance af an adsorption cooling system is highly dependent on the dynamic behavior of the adsorber and the various interactions between the adsorber and the outside heating/cooling uids. These systems need very high efforts in construction and are therefore expensive.QHS Adsorbers QCS Condenser Adsorbed vapour Expansion valve Evaporator Desorbed vapour

Figure 2.6: Adsorption refrigeration cycle.

Adsorption refrigeration cycles rely on the adsorption of a refrigerant gas into an adsorbent at low pressure and subsequent desorption by heating. The adsorbent is driven by heat and acts as a "chemical compressor". The adsorbent can adsorb a large amount of gas in ambient temperatures and desorb it at a higher temperature (about 100 C). In desorption, the liquid adsorbent vapourizes. In general, there are two broad categories of adsorptive systems, intermittent and continuous. The intermittent systems include solar-powered, daily-cycled systems. The continuous cooling systems have multible beds.

2.3 Steam Jet RefrigerationThe steam jet ejector was rst used in 1901 by LeBlanc in France and Parsons in England (ASHRAE, 1983). This refrigeration cycle is similar to the vapour-

16

Refrigeration Cycles using Geothermal Energy Adsorbent Silica gel Silica gel CaCl2 Activated carbon NaX Activated carbon Refrigerant Water (H2 O) Ammonia (N H3 ) Ammonia (N H3 ) Ammonia (N H3 ) CO2 CO2

Table 2.4: Adsorbent pairs.

compression cycle but instead of a mechanical compressor device, a steam ejector is used to compress the refrigerant to the condenser pressure level. By continuous vapourization of a part of the water in the evaporator at a low pressure, cooling is produced. A steam jet refrigeration system consists of an evaporator (or ash chamber), a steam-jet ejector, a condenser, and a two-stage ejector non-condensable pump. The COP depends strongly on the recooling water temperature. The process consists of two cycles, the motive medium cycle and the refrigerant cycle. An absolute pressure corresponding to the desired chilled water temperature is maintained in the ash(mixing) chamber. Water to be chilled enters the chamber continuously and a small portion is evaporated. It takes about 2,500 kJ/kg to evaporate water and the removal of this much heat with the vapour cools the liquid to its equilibrium pressure at the absolute pressure (of the tank). This water ash, or boiling at low pressure creates the natural refrigeration of the remaining water in the chamber. Vapour is compressed by the steam jet booster to an absolute pressure where steam and vapour can be condensed with the availible condensing water supply. The chilled water is pumped from the ash chamber and circulated to the process. Temperatures below 0 C can be reached, if wanted.

2.3 Steam Jet Refrigeration

17

Steam jet ejector

Steam generator

Condenser

Evaporator

Heat source

Figure 2.7: Steam jet ejector refrigeration cycle.

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Chapter 3 Thermodynamic Models3.1 Absorption RefrigerationIn this study, the focus is on absorption refrigeration. The two most common absorption cycles were chosen for further investigation and their models are described in this chapter. The cycles that have been presented can be modelled as steady-state systems. The models are based on a balanced mass ow and heat ow. These balances form a system of equations for each component of the system. For the components that have a single inlet and single outlet, the mass balances are trivial. The equations are solvable for certain input values to obtain values for the output variables. These systems are mostly based on heat exchangers. There are three ways to specify the size of heat exchangers: set the pinch, giving the efciency or using the size and heat transfer coefcient (UA). Of these methods, setting the pinch is the simplest. The pinch is dened as the minimum temperature difference between the hot and cold curves or as: Tpinch = Th2 Tc2 (3.1) where Th2 is the hot uid temperature outlet and Tc2 is the cold uid temperature inlet of a heat exchange seen in Figure 3.1. The effectiveness of the heat exchanger is the ratio of the actual heat transfer to the maximum possible heat transfer, (Holman, 1997). Effectiveness of a counterow heat exchanger where the cold uid is the minimum uid, that is the cold uid has lower C, where C = m Cp is given as: = Tc1 Tc2 Th1 Tc2 (3.2)

where Th1 is the hot uid inlet temperature and Tc1 is the cold uid outlet temperature as seen on Figure 3.1.

20

Thermodynamic Models

T Th1 Tc1

Th2 Tpinch Tc2 A

Figure 3.1: Heat exchange, showing the pinch point.

The third and most unstable method (due to the use of logarithm) is setting the size. In heat exchangers, the heat ow is given by: Q = U AF Tm (3.3)

where U is the heat transfer coefcient, A is the heat transfer area, F is a correction factor and Tm is the log mean temperature difference and dened as: Tm = (Th1 Tc1 ) (Th2 Tc2 ) ln (Th1 Tc2 ) (Th2 Tc2 ) (3.4)

The NTU effectiveness method is a comparible method to the log mean temperature method. Mass ows and any heat transfer or work interactions contribute to the energy balance for the energy owing into or out of the system. The COP for these systems is dened as: COP = Qe Qg + Wp Qe , Qg Wp Qg (3.5)

where Qe is the refrigeration capacity, Qg is the heat input in the generator and Wp is work input of the pump. To solve the equation system the Engineering Equations Solver program (EES) is used. EES uses built in thermodynamic properties of ammonia-water solutions and lithium bromide-water solutions. All these systems are simplied. For instance, pressure drops in pipes are not assumed in the calculations. Models for each system are listed in Appendix B.


21

3.1.1 Water-Lithium Bromide Systems

Qc 12 11 2 9 7 1

Qg 17 18

8

3 14 13 Qe Qa 16 4

10 5

6

Wp 15

Figure 3.2: Water-lithium bromide absorption refrigeration cycle.

The LiBr model is more stable than the ammonia model due to uid properties. LiBr systems have been the most tested and built. The input/output variables are listed in Table 3.2 and thermodynamic state points are in Table 3.1. Following are descriptions of single parts in the model shown in Figures 2.1 and 3.2. Condenser The refrigerant arrives in the condenser as steam at high temperature T1 . At this stage it is almost pure water. The steam is condensed to water and leaves at temperature T2 . The condenser is cooled by water which enters at a heat T11 and leaves at a heat T12 . The heat exchange can be seen in Figure 3.3. There it is shown that the temperature drops fast down to condensing temperature and then stays constant until the uid has condensed completely. Effects of the desuperheating (T1 T2 ) is left out for simplication. The energy balance for the condenser is given by: Qc = m1 (h1 h2 ) Qc = m11 (h12 h11 ) (3.6) (3.7)

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Point 1 2 3 4 5 6 7 8 9 10

State Superheated vapour Saturated liquid water Vapour-liquid water state Saturated water vapour Saturated liquid solution Subcooled liquid solution Subcooled liquid solution Saturated liquid solution Subcooled liquid solution Vapour - liquid solution state

Table 3.1: Thermodynamic state point summary.

Inputs mref [kg/s] m11 [kg/s] m13 [kg/s] m15 [kg/s] m17 [kg/s] T11 [ C] T13 [ C] T15 [ C] T17 [ C] U Ac [kW/ C] U Ae [kW/ C] U Aa [kW/ C] U Ag [kW/ C] Outputs m [kg/s] COP Qc [kW] Qe [kW] Qa [kW] Qg [kW]

Pump ow rate External heat transfer uid ow rate, condenser External heat transfer uid ow rate, evaporator External heat transfer uid ow rate, absorber External heat transfer uid ow rate, generator External heat transfer uid inlet temperature, condenser External heat transfer uid inlet temperature, evaporator External heat transfer uid inlet temperature, absorber External heat transfer uid inlet temperature, generator Heat exchanger size, condenser Heat exchanger size, evaporator Heat exchanger size, absorber Heat exchanger size, generator Refrigerant and solution loop ow rates System performance Heat tranfser rate, condenser Heat transfer rate, evaporator Heat transfer rate, absorber Heat transfer rate, generatorTable 3.2: Model inputs and outputs.

3.1 Absorption Refrigeration where hx is the enthalpy at point x and Qc = U Ac Tmc The mass ow balance gives: m1 = m2

23

(3.8) (3.9)

T

T1

T2 T11 T12

AFigure 3.3: Heat exchange, condenser.

Expansion valve 1 In the expansion valve 1 the pressure drops and hence the temperature drops to evaporator temperature. The enthalpy is in balance, that is: h2 = h3 (3.10)

Expansion valve 2 In the expansion valve 2 the pressure drops. The enthalpy is in balance, that is: h9 = h10 (3.11)

Evaporator After going through an expansion valve, the water enters the evaporator at temperature T3 . In the evaporator the water evaporates at a low pressure and takes heat from the chilled water in the heat exchange pipe. The steam leaves at a temperature of T4 . Water to be chilled enters at a temperature T13 and leaves at a temperature of T14 . The heat exchange can be seen in Figure 3.4. There it can be seen that the evaporating temperature T3 is the same at the outlet T4 . This water can be used to cool air for air conditioning. The energy balance for the evaporator is: Qe = m4 (h4 h3 ) (3.12)

24 Qe = m13 (h13 h14 ) Qe = U Ae Tme where, m3 = m4 = m1

Thermodynamic Models (3.13) (3.14) (3.15)

T T13 T14 T4 T3

A

Figure 3.4: Heat exchange, evaporator.

Absorber Steam enters the absorber at a temperature T4 . The lithium-bromide solution absorbs steam from the evaporator in the absorber. The absorber is cooled by water. The cooling water enters at a temperature T15 and leaves at temperature T16 . The solution leaves at a temperature of T5 . The heat ow balance for the absorber gives: Qa = m4 h4 + m10 h10 m5 h5 Qa = m15 (h16 h15 ) Qa = U Aa Tma The mass ow balance gives: m5 = m4 + m10 The LiBr balance gives: m5 x5 = m4 x4 + m10 x10 (3.20) (3.19) (3.16) (3.17) (3.18)


25

T T10 T16 T5 T15 AFigure 3.5: Heat exchange, absorber.

Generator In the generator the solution is heated by hot water or steam to remove the water. The steam or hot water used to heat the generator is at a temperature T17 and leaves at temperature T18 . If steam is used it will condense in the generator and leave at the same temperature. The solution enters the generator at temperature T7 ; the solution is heated in the generator and the water evaporates and ows out at a temperature T1 , but the solution is heated further until it leaves at a temperature T8 . Either the LiBr balance in the generator or the absorber is needed to decide the LiBr balance in the cycle. The heat ow balance gives: Qg = m 1 h 1 + m 8 h 8 m 7 h 7 Qg = m17 (h17 h18 ) Qg = U Ag Tmg The mass ow balance gives: m7 = m1 + m8 The LiBr balance gives: m7 x7 = m1 x1 + m8 x8 (3.25) (3.24) (3.21) (3.22) (3.23)

Solution Heat Exchanger The lithium bromide water solution enters the heat exchanger at a temperature T6 and is heated up and leaves at a temperature T7 . The lithium bromide enters

26


T T17 T18 T8 T1

A

Figure 3.6: Heat exchange, generator.

the heat exchanger at a temperature T8 and is cooled and leaves at a temperature T9 . The effectiveness of the heat exchanger is an input variable. The heat ow balance gives: h7 = h6 + The mass ow balance gives: m6 = m7 m8 = m7 (3.27) (3.28) m8 (h8 h9 ) m6 (3.26)

Pump The efcieny of the pump is a input variable. The work input of the pump is given by: Wp = m5 (h6 h5 ) (3.29) The mass ow balance gives: m5 = m6 (3.30)

3.1.2 Ammonia-Water SystemWhen using the ammonia-water absorbent pair, the refrigerant solution consists of an ammonia/water mixture. This solution is unstable. That makes the modelling more difcult than with the LiBr model. Other and more inputs must be decided to stabilize the model. The input/output variables are listed in Table 3.4 and thermodynamic state points are in Table 3.3.


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T T8 T9 T7 T6

AFigure 3.7: Heat exchange, solution heat exchanger.

Qc 14 13 1

Qg Qr 11 19 20 12

2 9

7

8

3 Qa 16 15 Qe 18 17 Wp 4 10 5

6

Figure 3.8: Ammonia-Water absorption refrigeration cycle.

Following are clarications of singular parts of the model shown in Figures 2.3 and 3.8

28 Point 1 2 3 4 5 6 7 8 9 10 11 12

Thermodynamic Models State Saturated vapour Vapour-liquid state Vapour-liquid state Vapour-liquid state Saturated liquid solution Subcooled liquid solution Vapour-liquid solution state Saturated liquid solution Subcooled liquid solution Subcooled liquid solution Saturated vapour Saturated liquid solution

Table 3.3: Thermodynamic state point summary

Condenser The refrigerant arrives in the condenser as steam at high temperature T1 . At this stage it is almost pure ammonia. The steam is condensed to liquid and leaves at temperature T2 . The condenser is cooled by water which enters at a temperature T13 and leaves at a temperature T14 . The system has two pressure stages. The high pressure stage in the system is decided by the condenser temperature Tc which is an input. The heat ow balance gives: Qc = m1 (h1 h2 ) Qc = m15 (h14 h13 ) The mass ow balance gives: m1 = m2 (3.33) (3.31) (3.32)

Expansion valve 1 In the expansion valve 1 the pressure drops and because of that, the temperature drops. The enthalpy is in balance as: h2 = h3 The mass ow gives: m2 = m3 (3.35) (3.34)


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Inputs m13 [kg/s] m15 [kg/s] m17 [kg/s] m19 [kg/s] T13 [ C] T15 [ C] T17 [ C] T19 [ C] Tpinch,absorber [ C] Tpinch,generator [ C] Te [ C] Tc [ C] Qe [kW] Outputs mref [kg/s] m [kg/s] COP Qc [kW] Qa [kW] Qg [kW] U Ac [kW/ C] U Ae [kW/ C] U Aa [kW/ C] U Ag [kW/ C]

External heat transfer uid ow rate, condenser External heat transfer uid ow rate, evaporator External heat transfer uid ow rate, absorber External heat transfer uid ow rate, generator External heat transfer uid inlet temperature, condenser External heat transfer uid inlet temperature, evaporator External heat transfer uid inlet temperature, absorber External heat transfer uid inlet temperature, generator Pinch of absorber Pinch of generator Evaporator temperature Condensder temperature Heat transfer rate, evaporator Pump ow rate Refrigerant and solution loop ow rates System performance Heat transfer rate, condenser Heat transfer rate, absorber Heat transfer rate, generator Heat exchanger size, condenser Heat exchanger size, evaporator Heat exchanger size, absorber Heat exchanger size, generatorTable 3.4: Model inputs and outputs.

30


Expansion valve 2 In the expansion valve 2 the pressure drops. The enthalpy is in balance as: h9 = h10 The mass ow gives: m9 = m10 (3.37) Evaporator After going through an expansion valve, the liquid ammonia enters the evaporator at a temperature T3 . In the evaporator the ammonia evaporates at a low pressure and takes heat from chilled water in the heat exchange pipe. The steam leaves at a temperature of T4 . The low pressure in the system is decided by the evaporator temperature Te wich is a input to the system. The evaporator heat ow Qe is also an input. The heat ow balance gives: Qe = m4 (h4 h3 ) The mass ow balance gives: m3 = m4 = m1 (3.39) (3.38) (3.36)

Absorber Steam enters the absorber at a temperature T4 and ammonia-water solution enters at T10 . The ammonia-water solution absorbs ammonia steam from the evaporator in the absorber. The absorber is cooled by water. The cooling water enters at heat T17 and leaves at temperature T18 . The solution leaves at a temperature of T5 . The heat ow balance gives: Qa = m4 h4 + m10 h10 m5 h5 The mass ow balance gives: m5 = m4 + m10 The ammonia balance gives: x5 m5 = x4 m4 + x10 m10 (3.42) (3.41) (3.40)

Generator In the generator the solution is heated by hot water or steam to remove some of the ammonia. The steam used to heat the generator is at a temperature T19 and is condensed in the generator and therefore leaves at the same temperature.


31

Hot water could also be used, but it would leave at temperature T20 . The size of the generator is decided by setting the pinch point. The heat ow balance gives: Qg = m 1 h 1 + m 8 h 8 m 7 h 7 Qg = m19 (h19 h20 ) The mass ow balance gives: m7 = m1 + m8 The ammonia balance gives: m7 x7 = m1 x1 + m8 x8 (3.46) (3.45) (3.43) (3.44)

Heat Exchanger The ammonia water solution enters the heat exchanger at a temperature T6 and is heated up and leaves at a temperature T7 . The hotter solution enters the heat exchanger at a temperature T8 and is cooled and leaves at a temperature T9 . There is no mixing of uids in the heat exchanger so the ammonia strength does not change. The size of the heat exchanger is an input variable. The heat ow balance gives: h7 = h6 + The mass ow balance gives: m6 + m8 = m7 + m9 (3.48) m8 (h8 h9 ) m6 (3.47)

Rectier The solution leaving the generator still contains some water so it arrives at the rectifer at a temperature T11 and then the ammonia leaves at a temperature T1 and goes to the condenser and the solution goes back to the generator at temperature T12 . The heat ow balance gives: Qr = m11 h11 m12 h12 m1 h1 The mass ow balance gives: m1 + m12 = m11 (3.50) (3.49)

Pump The pump increases the pressure slightly. The efciency of the pump is an

32 input variable. The pump work is given by: Wp = m5 (h6 h5 ) The mass ow balance gives: m5 = m6


(3.51) (3.52)

Chapter 4 Comparison and AnalysisIn this chapter, the models discussed in Chapter 2 are compared with regard to performance, (COP) and cost. Different types of absorption systems are compared briey. The models described in Chapter 3 are compared numerically using the EES program, with a more detailed analysis of the ammonia-water model than the water-LiBr model. A comparison is also made to a single stage vapour compression system.

4.1 Comparison of heat driven cyclesTable 4.1 shows a rough comparison of different heat driven systems. Absorption refrigeration is much more common than adsorption and steam jet refrigeration, although adsorption refrigeration has been reserched more in recent years and is becoming a more attractive option in many ways. Energy crisis has driven research for alternative choices in refrigeration. These systems are mostly used in air conditioning, apart from ammonia absorption and some types of adsorption, depending on cooling media.

4.2 Comparison of Absorption SystemsAs mentioned before, the two basic absorption cycles are the single effect waterLiBr cycle and single effect ammonia-water cycle. These cycles are used in systems that have been manufactured for some time. The main difference between the LiBr and the ammonia cycles is that LiBr is a water chiller, but ammonia is for freezing. The evaporation temperatures for ammonia and water are shown in Figure 4.1, and the main differences between the LiBr and ammo-

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Comparison and Analysis Heat Source Cooling [ C] [ C] COP Capital cost USD/kWc 80-160 430-1400 380-1600 100-250

Single effect cycle LiBr Single effect cycle N H3 Adsorption Steam Jet *depends on cooling media

70-115 100-160 55-95 65-180

4-10 C

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