+++++THESIS - An Absorption Chiller in a Micro BCHP Application - Model Based Design and Performance Analysis

8/8/2019 +++++THESIS - An Absorption Chiller in a Micro BCHP Application - Model Based Design and Performance Analysis

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Copyright Declaration

I hereby declare that I am the sole author of this thesis.

I authorize Carnegie Mellon University, Pittsburgh, Pennsylvania to lend this thesis to other

institutions or individuals for the purpose of scholarly research.

I authorize Carnegie Mellon University, Pittsburgh, Pennsylvania to reproduce this thesis by photo

copying or by other means, in total or in part, at the request of other institutions or individuals for the

purpose of scholarly research.

Copyright 2006 by Hongxi Yin


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Acknowledgment

It has been a long journey to complete my Ph.D. thesis with the objective of making myself more

capable of dealing with the increasing complexity of building-related technical issues. The scientific

research in the Intelligent Workplace (IW) starts my academic career and a brand new professional

practice. In the future, I shall see myself as an engineered architect, who could help the building

industry create healthy, efficient, and economical and ultimately sustainable environments.

I wish to express my sincere appreciation and gratitude to my advisor, Professor Volker Hartkopf, for

his invaluable vision, support, and encouragement. His enthusiasm and inspiration were essential to

the success of this research, and his wisdom and insights will serve as a source of ideas for my future

endeavors.

Let me extend my profound gratitude to Professor David Archer who has played a pivotal role in this

thesis. He has far exceeded his duty as an advisor, a loyal colleagues and an enthusiastic partner in this

endeavor. Furthermore, and more importantly, he has given me a deep understanding of building

energy systems, and has also implanted his rigorous method of thinking and effective way of working.

I would like to thank Mr. Zhang Yue, CEO of Broad Air Conditioning Co., and his colleagues for their

generous support, diligent work, and warm cooperation over the past several years. Mr. Zhang Yue

spent much time on the design, test, and commercialization of this chiller. His strong motivation and

ability to convert scientific research into commercial products is one of the essential lessons he taught

me.

It gives me great pleasure to thank Professor David Claridge of Texas A&M University for providing

valuable suggestions and clarifications and Professor Richard Christensen of Ohio State University for

his careful review of the draft and his constructive critique of this work.

I also voice my appreciation to Nancy G. Berkowitz for her diligent guidance on writing skills and

editing efforts. Above all are these life-long experiences that are important for my future endeavors. I

am indebted to my colleague and lovely wife, Ming Qu, who gave me unconditional support and took

the responsibility for caring for our baby, Ryan, who fills us with joys every day. This thesis is also

dedicated to my parents in their confidence, their high expectations, and their hearty blessing.


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Table of Contents

Copyright 2006 by Hongxi Yin.........................................................................................................i

Acknowledgment.................................................................................................................................ii

List of Figures...................................................................................................................................viii

List of Tables...................... .................. ................. .................. .................. .................. .................. ....... x

Abstract...............................................................................................................................................xi

1 Introduction....................................................................................................................... 1

1.1 Background and Motivation ............... .................. .................. ................. .................. ........... 2

1.1.1 CHP Systems ....................................................................................................................3

1.1.2 BCHP Systems..................................................................................................................3

1.1.3 Heat Utilization.................................................................................................................4

1.2 Overview of Absorption Chiller Technology................ .................. .................. ................... . 5

1.2.1 Absorption Cycle Analysis ................ .................. .................. ................. .................. ........ 6

1.2.2 Absorption Refrigeration Working Fluids ................. .................. ................. .................. .. 8

1.2.3 Absorption Refrigeration Operating Conditions............ .................. .................. ............... 9

1.2.4 Absorption Chiller Cycle Modifications............. ................... .................. ................... ...... 9

1.3 Research Objectives........................ .................. .................. .................. .................. ............ 11

1.4 Research Approach ................ .................. .................. .................. .................. .................. ... 12

1.4.1 The Planning and Installation of Experimental Equipment ................ .................. .......... 12

1.4.2 The Test Program and Experimental Data ................ ................. ................. ................. ... 13

1.4.3 The Development of Computational Performance Model .................. ................... ......... 13

1.4.4 The Analysis of the Experimental Data ................... .................. ................... .................. 14

1.5 Current Absorption Chiller Modeling Studies .................. .................. .................. .............. 14

1.5.1 Absorption Chiller Modeling Approaches ................. .................. .................. ................. 14 1.5.2 The Insufficiencies of Current Absorption Chiller Modeling Studies ................... ......... 15

1.6 The Comprehensive Performance Model and its Applications........................ ................. .. 16

1.6.1 The Chiller Model Description ................. ................... .................. ................... .............. 16

1.6.2 Applications of the Chiller Performance Design Model....... .................. ................. ....... 18

1.6.2.1 Preliminary Design Computations.................. ................. .................. ................. ........ 18


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2.5 Further Information from Chiller Testing ................. .................. .................. .................. .... 54

3 Chiller Design and Performance Model........................................................................ 55

3.1 Flow Diagram ................. ................. .................. ................. .................. ................. ............. 55

3.2 D hring Chart Representation ................ .................. .................. .................. .................. .... 57

3.3 T-Q Diagram.......................................................................................................................59

3.4 Calculation Procedure............ .................. .................. .................. .................. .................. ... 59

3.4.1 Mass Balance ................ .................. .................. ................. .................. .................. ......... 60

3.4.2 Energy Balance .................. ................. .................. .................. .................. .................. .... 60

3.4.3 Thermodynamic Property and Equilibrium Relations ................ .................. ................. . 61

3.4.4 Heat Transfer Models............... .................. .................. .................. .................. ............... 61

3.4.5 Overall Heat Transfer Coefficient Model ................. .................. .................. .................. 62 3.4.6 Mass Transfer Models......... .................. ................. .................. .................. .................. ... 65

3.4.7 Model Assumptions ............... .................. ................. .................. .................. ................. . 65

3.5 Model Steps ................. .................. ................. .................. .................. ................. ............... 66

4 Model-based Experimental Data Analysis.................................................................... 69

4.1 Analytical Method .................. .................. .................. .................. .................. .................. .. 69

4.1.1 Statistical Analysis Procedure................. .................. ................... .................. ................. 70

4.1.2 Absorption Cycle at Design Condition.................. .................. .................. .................. ... 72

4.1.3 Overall Deviation............. .................. .................. .................. .................. .................. ..... 74

4.2 Model Analysis .................. .................. .................. .................. .................. .................. ....... 75

4.2.1 Analysis of Cooling-Load Variation ................. ................. .................. .................. ......... 75

4.2.2 Performance Curve ................. ................. ................. ................. ................. ................. ... 77

4.2.3 Flow Rate Variations....... .................. .................. .................. .................. .................. ...... 79

4.2.4 Temperature Variations ................ ................. .................. ................. .................. ............. 81

4.2.5 Composition Variations............. .................. .................. ................. .................. ............... 82 4.2.6 Vapor Quality Variations..... .................. ................. .................. ................. .................. .... 83

4.2.7 Heat Transfer Area Variations.................. .................. ................... .................. ................ 84

4.2.8 Deviation Variations.................... .................. ................. .................. ................. .............. 85

4.2.9 Analysis of Other Test Data ................... .................. ................... .................. .................. 86


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5 Contributions and Areas of Future Research............................................................... 87

5.1 Contributions ................. ................. .................. ................. .................. ................. .............. 87

5.2 Areas of Future Research.............. ................... .................. ................... .................. ............ 89

5.2.1 Extended Chiller Model for Multi-Heat Resources ................. ................... .................... 89 5.2.1.1 Hot Water Absorption Chiller ................ .................. ................. .................. ................ 90

5.2.1.2 Natural Gas Absorption Chiller...................... .................. .................. .................. ....... 90

5.2.1.3 Exhaust Gas Absorption Chiller................ .................. ................. .................. ............. 91

5.2.2 System Integration and Application.......... .................. .................. ................... ............... 91

5.2.2.1 Chiller Performance Tables for Building Simulation Tools.................. .................. .... 92

5.2.2.2 Cost Model..................................................................................................................92

References ............................................................................................................................... 93

Appendix 1A ........................................................................................................................... 97

Appendix 2A ......................................................................................................................... 102

Appendix 2B ..........................................................................................................................118

Appendix 3A ......................................................................................................................... 130

Appendix 4A ......................................................................................................................... 150

Acronyms .............................................................................................................................. 194


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List of Figures

Figure 1-1: Gross estimation of annual rejected heat in the U.S., 2004 ................. ................... .............. 2

Figure 1-2: Conceptual Diagram for System Integration in Buildings....... .................. .................. ......... 3

Figure 1-3: Schematic diagram of BCHP systems.................... ................... .................. .................. ........ 4

Figure 1-4: Basic vapor compression chiller cycle... ................... .................. ................... .................. ..... 7

Figure 1-5: Basic LiBr absorption chiller cycle............. ................... .................. ................... ................. . 7

Figure 1-6: Typical two-stage parallel flow absorption chiller configuration ................ .................. ..... 10

Figure 1-7: Typical two-stage series flow absorption chiller configuration ................. .................... ..... 11

Figure 2-1: Absorption chiller installed in the IW.................... .................. ................... .................. ...... 22

Figure 2-2: Schematic diagram of the absorption chiller............. .................. .................. .................. .... 23

Figure 2-3: Structure of the absorption chiller............. .................. ................. .................. ................... .. 25

Figure 2-4: Configuration of the lower vessel ................. .................. ................... .................. ............... 26

Figure 2-5: Configuration of the upper vessel ................. .................. ................... .................. ............... 28

Figure 2-6: Configuration of cooling tower........... .................. ................... .................. .................. ....... 31

Figure 2-7: Simplified flow diagram of the chiller test system ................. ................... ................... ...... 33

Figure 2-8: Site views of the absorption chiller test system ................. ................... ................... ........... 34

Figure 2-9: Control and instrumentation structure....................... .................. ................. .................. ..... 36

Figure 2-10: Absorption chiller monitoring software .................. .................. .................. .................. .... 37

Figure 2-11: Test system monitoring software.................... .................. .................. .................. ............. 38

Figure 2-12: PI&D diagram of the absorption chiller............... ................... .................. ................... ..... 39

Figure 2-13: Typical start-up of the chiller test system .................. .................. ................... .................. 47

Figure 2-14: Steady-state operation of the chiller under design load condition ................. ................... 48

Figure 2-15: Steady-state operation of the chiller under design load condition ................. ................... 49

Figure 2-16: Chiller performance under various load conditions.................... .................. .................. .. 53

Figure 2-17: Chiller power consumption under various load conditions.................... ................. .......... 53

Figure 2-18: Comparison of chiller performance .................. ................... .................. ................... ........ 54

Figure 3-1: Simplified flow diagram for chiller model .................. .................. .................. .................. . 56

Figure 3-2: D hring chart at design condition........... ................... .................. ................... .................. .. 58

Figure 3-3: T-Q diagram for the heat transfer components.................. .................. ................... ............. 59

Figure 3-4: Steps in the use of the performance model ................. .................. ................... .................. . 67

Figure 3-5: Structure of the design model ................. ................. .................. ................. ................... ..... 68

Figure 3-6: Structure of performance model ................ ................... .................. .................. .................. 68


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Figure 4-1: Data analytical procedure flow diagram ................. .................. .................. .................. ...... 70

Figure 4-2: Absorption cycle at design load condition ................ .................. .................. .................. .... 73

Figure 4-3: D hring chart at 55% design load condition............. .................. ................... .................. ... 76

Figure 4-4: Absorption cycle variations with load changes........ ................... .................. .................. .... 77

Figure 4-5: Chiller performance curve under various load conditions ................. .................. ............... 78 Figure 4-6: Heat transfer load on each component under various load conditions.......................... ...... 78

Figure 4-7: Steam supply flow rate under various load conditions ................ ................. .................. .... 79

Figure 4-8: Sorbent solution flow rate under various load conditions............... .................. .................. 80

Figure 4-9: Sorbent solution split ratio under various load conditions............. .................. .................. . 80

Figure 4-10: Refrigerant regeneration rate under various load conditions ................ .................. .......... 81

Figure 4-11: Refrigerant vaporization temperature under various load conditions .................. ............. 82

Figure 4-12: Sorbent solution composition changes under various load conditions ................. ............ 82

Figure 4-13: Refrigerant vapor quality leaving the LTRG under various load conditions ................. ... 83 Figure 4-14: UA changed for the 5 major components under various load conditions ................. ........ 84

Figure 4-15: Surface contact area changes under various load conditions ................. .................. ......... 85

Figure 4-16: Overall and weighted deviations under various load conditions ................. .................. ... 86

Figure 5-1: Simplified HTRG configurations for natural-gas-driven absorption chiller................... .... 91


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List of Tables

Table 1-1: Power generation equipment rejected heat temperature ranges................. .................. ........... 5

Table 1-2: Water-LiBr absorption chiller thermal energy types and temperature ranges ..................... ... 5

Table 2-1: Component names and corresponding abbreviations ................. ................. .................. ....... 23

Table 2-2: Specifications of the absorption chiller .................. .................. ................... .................. ....... 25

Table 2-4: Control points of the chiller......... ................... .................. .................. ................... ............... 41

Table 2-3: Instrumentations of the chiller test systems....... .................. ................... .................. ............ 42

Table 2-5: Input and primary output of the test program............ .................. .................. .................. ..... 45

Table 2-6: Measurement data of the chiller under design condition............ .................. ................... ..... 50

Table 2-7: Comparison of chiller performance under design conditions.......... ................... .................. 51

Table 2-8: Primary measurement for chiller input and output................ .................. .................. ........... 52

Table 3-1: Chiller model state point descriptions ................. ................... .................. ................... ......... 57

Table 3-2: Physical features of heat and mass transfer components.... .................. .................. .............. 63

Table 3-3: Heat and mass transfer correlations used in the performance model ................... ................ 64

Table 4-1: Measured values and model calculations for 100% and 55% of design load conditions ..... 71

Table 5-1: Heat transfer features of the HTRG of different heating media ................. ................... ....... 90


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Abstract

Developments in absorption cooling technology present an opportunity to achieve significant

improvements in microscale building cooling, heating, and power (BCHP) systems for residential and

light commercial buildings that are effective, energy efficient, and economic. However, model based

design and performance analysis methods for micro scale absorption chillers and their applications

have not been fully developed; particularly considering that thermal energy from a wide variety of

sources might be used to drive the chiller in a residential or light commercial building. This thesis

contributes important knowledge and methods for designing and integrating absorption chillers in

BCHP systems that reduce energy consumption, decrease operational costs, and improve

environmental benefits in residential and light commercial buildings.

To be more specific, this thesis contributes the development and application of absorption chiller and

the computational model in the following areas:

1) establishment of a unique experimental environment and procedures for absorption chiller

tests under various conditions

2) conduct of a comprehensive testing program on a microscale absorption chiller

3) construction of a comprehensive chiller model based on the pertinent scientific and

engineering principles adapted to the design of a chiller and to the analysis of extensive,

detailed test data obtained from the test program4) analysis of the measured data, refinement of the model, and improvement of the chiller design

on the basis of the data analysis process

The model is now being used as a tool to adapt the chiller to various heat sources and sinks and to

carry out performance simulations of micro BCHP system.


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1 Introduction

In the United States, residential and commercial buildings more than 107 million households (2001)

[1] and 71.7 billion square feet of commercial floor space (2003) [2] account for more than one-third

of the total energy consumption of the country. Significant energy efficiency improvements in heating,

ventilation, air conditioning and refrigeration (HVAC&R) systems for residential and light

commercial buildings might be achieved by the application of microscale heat-driven absorption

chillers for space and ventilation air cooling.

Absorption chillers are key components in a building cooling heating and power (BCHP) system to

cool space in buildings. They can be driven directly by the thermal energy and heat recovered from

various sources, including power generation equipment and solar receiving devices. The combination

of heat recovery equipment and heat-driven absorption chillers provides significantly increased overall

energy efficiency. Most of todays heating and cooling technologies for buildings, however, are not

designed to make use of rejected heat. Performance modeling studies of heat-driven absorption chillers

are accordingly limited, contributing to the difficulty of preparing and applying building simulation

programs for BCHP system design and performance analysis.

This thesis contributes important knowledge and methods for designing and integrating absorption

chillers in BCHP systems that reduce energy consumption, decrease operational costs, and improve

environmental benefits in residential and light commercial buildings.

The gap between experiment and simulation is closed in this thesis because of the availability of a

unique microscale absorption chiller and an associated experimental setup. By developing and

applying a numerical performance model, a refined understanding of a particular chiller and its

operation can provide improved design and modeling tools for heat-driven absorption chillers in

general. The approach developed in this thesis will allow developers to simulate the interaction of the

BCHP components as a system along with its interactions with:

power and other energy supply systems

electricity grids

indoor air conditions

various load profiles


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The modeling tool will also allow engineers to assess different operating strategies of such a system to

find the most economic operating conditions, based on the idealized nonlinear systems with only a few

degrees of freedom.

1.1 Background and Motivation

In the United States, approximately two-thirds of the energy of the fuel used to generate electricity is

wasted as rejected heat. Annually, 28.8 to 34.0 quadrillion Btu of thermal energy are rejected to the

atmosphere, lakes, and rivers from power generation, building equipment operations, and industrial

processes, Figure 1-1, [3, 4].

Figure 1-1: Gross estimation of annual rejected heat in the U.S., 2004

Nationaltotal energyconsumption(99.74 Quads)

Electricity(14.2 Quads)

Power generation(40.77 Quads)

Transportation(27.79 Quads)

HVAC, lighting, and others(4.84-5.54 Quads)

HVAC, lighting, and others(2.81-3.21 Quads)

Manufacturing processes(16.94-19.06 Quads)

Power productionwaste heat(24.5 -26.5 Quads)

Residential sector waste heat(1.38-2.08 Quads)

Commercial sector waste heat(0.8-1.2 Quads)

Industrial sector waste heat

(2.12-4.24 Quads)

National totalwaste heat(28.8-34.02 Quads)

Industrialsector (21.18 Quads)

Residentialsector (6.92 Quads)

Commercialsector (4.02 Quads)

Rejected heat from power generation can be used for building operations. Renewable energy sources

(such as solar thermal energy to drive absorption chillers and boilers) combined with advanced

distributed electric energy generation can also be used in buildings. Figure 1-2 illustrates the system

integration concepts that Volker Hartkopf put forward for the first time [5], for the opportunities of

simultaneously achieving energy conservation, using renewable resource, and deploying distributed

electricity generation technologies. The building of the future is conceived as a power plant (BAPP)

that would generate more energy on site than is brought to it in the form of non-renewable resources.

The surplus of energy (power, heating, and cooling) could export to the utility grids or neighboring

buildings.


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Figure 1-2: Conceptual Diagram for System Integration in Buildings

SystemIntegration

Resource conservation:Energy, water,material, and so forth

Distributed generation:engine generator,gas turbine, and fuel cell

Renewables: solar, wind, bio-gas,day-lighting, natural ventilation, passive/active heating/cooling

Source: Volker Hartkopf [5]

1.1.1 CHP Systems

Combined heating and power (CHP) systems are based on the concept of producing electrical energy

and recovering rejected heat for useful purposes. Compared with conventional power plants, CHP

systems can improve overall energy efficiency from 30% to 70% or more. CHP is effective in large-

scale industrial plants, hospitals, university campuses, and urban district energy systems. Recent

developments in small-scale power generation, heat recovery, and heat-driven refrigeration

technologies make possible the installation and effective operation of CHP in residential and small

commercial applications.

1.1.2 BCHP Systems

In BCHP systems, the electrical energy generated on site is used to meet the demands of lighting and

electrical equipment. The rejected heat in power generation is used to provide space ventilation,

cooling, heating, dehumidification, and domestic hot water for the building, Figure 1-3.

Various technologies can be used to configure a BCHP system. The power generation equipment, as

illustrated at the top of the figure, could be a steam turbine, combustion turbine, reciprocating spark

ignition, Diesel engine, or fuel cell. These power generators produce power and reject heat in variousquantities at various temperatures that can be used for the building operation. Heat recovery

exchangers/boilers, absorption chillers, and desiccant dehumidifiers are equipment that can deliver

heating, cooling, or ventilation to the building space. As indicated in Figure 1-3, the thermal input can

also be provided directly from solar thermal receivers. Finally, a capable, robust control system is

needed to integrate the operation of all equipment to meet the needs of the building and its occupants


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and to achieve the full benefits of system efficiency and economy. Heat-driven absorption chiller

technology plays a prominent role in making use of the reject as well as solar energy, for space and

ventilation air cooling, and thus in the design and operation of overall BCHP systems.

Figure 1-3: Schematic diagram of BCHP systems

Traditionally, CHP systems with power generation capacities below 500 kW are categorized as

microscale systems. With the development of compact, microscale absorption chillers, more reliable,

lower-emitting reciprocating engines, and high-temperature fuel cell power supplies, BCHP is feasible

for packaged systems in residential and light commercial buildings having power requirements less

than 15 kW. This introduction of micro-BCHP systems presents many technical and commercial

challenges, but the production of heat-driven absorption chillers and their integration in BCHP

systems can assist the nation in

increasing energy efficiency

integrating renewable forms of energy

eliminating transmission and distribution costs and losses increasing reliability by combining distributed with centralized utility power supplies

1.1.3 Heat Utilization

Table 1-1 illustrates the temperature range of rejected thermal energy from typical power generators

and heat recovery units. Among them, a solid oxide fuel cell (SOFC) gives the highest exhaust gas


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temperature for heat recovery and utilization. The hot water temperature from solar collectors varies

with the type of collector. Solar collectors with parabolic trough reflectors can generate hot water up

to 180 oC; integrated compound parabolic collectors, ICPCs, 140 to 160 oC; flat plate collectors, 65 to

90 oC.

Table 1-1: Power generation equipment rejected heat temperature ranges

No. Power Generation Equipment, Waste Stream Temperature ( oF) Temperature ( oC)

1 Solid Oxide Fuel Cell Exhaust 1300 700-8002 Reciprocal Engine Exhaust 1100-1200 600-650

3 Molten Carbonate Fuel Cell Exhaust 1100 600

4 Gas Turbine Exhaust 950-1000 510-540

5 Microturbine Exhaust 450-600 230-3156 HRSG Exhaust 350 175

7 Reciprocal Engine Jacket Water 180-200 80-958 Phosphoric Acid Fuel Cell 180 80

9 Solar Thermal Collector 150-250 65-180

Table 1-2 shows typical temperature ranges for the heating medium to drive a water-lithium bromide

(LiBr) absorption chiller [6]. A single-stage hot-water-driven chiller can use heat at a temperature as

low as 75 oC. Tables 1-1 and 1-2 show that an absorption chiller can be found to use heat from a wide

range of sources. Because of its higher thermal efficiency, this study focuses on a two-stage absorption

chiller and its appropriate sources of rejected heat.

Table 1-2: Water-LiBr absorption chiller thermal energy types and temperature ranges

No. Heat-driven Absorption Chiller Type Pressure (kPa) Temperature ( oC)

1 Direct-fired fossil fuel (natural gas, oil, LPG etc.) - 1,000 1,8002 Double-stage exhaust gas - 400 - 600

3 Single-stage exhaust gas - 230 - 350

4 Double-stage steam 400 1,000 144 - 180

5 Single-stage steam 100 - 400 103 - 1336 Double-stage hot water 350 1,100 140 - 200

7 Single-stage hot water 40 - 200 75 - 1208 Other fuel/steam/hot water/exhaust gas Same as above Same as above

1.2 Overview of Absorption Chiller Technology

An absorption chiller is a machine that, driven by heat, produces chilled water for space and

ventilation air cooling. Little or no mechanical energy is consumed in an absorption chiller, and little

or no electric power is required. A great variety of hot media, gases and liquids, over a broad range of

temperatures above ambient can be used. The chiller must also reject an amount of heat equal to that

provided in driving it plus that absorbed in producing the chilled water. Ammonia-water (NH 3-H2O)


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absorption refrigeration technology has been used for more than 150 years. As a refrigerant, ammonia

has high latent heat and excellent heat transfer characteristics, but its toxicity has limited its use in this

technology.

Since 1945, water-LiBr absorption chillers have achieved widespread use. This trend reached its peak in the 1960s, and then diminished in the late 1970s. The technology has since revived in Asia, because

the rapidly increasing electricity demand has limited the application of electrically driven vapor

compression chillers. The sales data of a leading absorption chiller manufacturer, presented in

appendix 1A, shows several new developments in the current absorption chiller market. Today, water-

LiBr absorption chiller technology is returning to the United States with the increasing application of

CHP systems.

In the past three years, heat-driven water-LiBr absorption chillers have been used widely both in large

commercial buildings combined with advanced power generation equipment and in individual houses

driven directly by fossil fuels or by other heat sources. The cooling capacity of chillers can vary from

greater that 1,000 refrigeration ton (3,561.85 kW) to as low as a microscale, 4.5 refrigeration ton (16

kW). This thesis will focus on microscale water-LiBr absorption chiller research, development, and

demonstration in residential and light commercial applications.

1.2.1 Absorption Cycle Analysis

A chiller produces chilled water by removing heat from it and transferring this heat to a vaporizing

refrigerant. The process is illustrated in Figure 1-4 for a conventional vapor compression chiller and

in Figure 1-5 for an absorption chiller. In both, the refrigerant liquid flows into an evaporator,

evaporates at a reduced pressure and temperature, and absorbs heat from chilled water flowing in a

tube through the evaporator. In the vapor compression process, the refrigerant vapor is compressed

and condensed at a high-pressure and temperature, transferring heat to cooling water or to the

surroundings in a condenser. The high-pressure condensed refrigerant is then returned through the

expansion valve to a low-pressure evaporator, once again to absorb heat from the chilled-water flow.


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Figure 1-4: Basic vapor compression chiller cycle

Condenser

Evaporator

from chilled water

expansionRefrigerant

valve

Heat rejected

Work

Compressor

Heat absorbed

T

P

to cooling water

In the absorption process shown in Figure 1-5, the refrigerant vapor from the evaporator is absorbed at

low pressure into a sorbent solution in the absorber. Heat is released as the refrigerant vapor is

absorbed. This heat is removed by cooling water flowing through the absorber. The sorbent solution

is then pumped to the regenerator, where refrigerant vapor is driven from the sorbent solution by the

addition of heat at high temperature and pressure. The refrigerant vapor is condensed at high pressure

and temperature with the removal of heat to ambient or to cooling water. The liquid refrigerant is

returned to the evaporator through the expansion valve.

Figure 1-5: Basic LiBr absorption chiller cycle


valve

expansionSolution

valve

from chilled water Heat absorbed

pumpSolution

T

P

Condenser

Heat rejectedto cooling water

Evaporator Absorber


Regenerator

Heat input


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This basic absorption chiller cycle shown in Figure 1-5 is similar to the traditional vapor compression

chiller cycle in Figure 1-4 in that

refrigerant vapor is condensed at high pressure and temperature, rejecting heat to the

surroundings refrigerant vapor is vaporized at low pressure and temperature, absorbing heat from the chilled

water flow

The chiller cycles differ in that

the pumped circulation of a sorbent solution replaces the compression of the refrigerant vapor

The energy, work, required by the pump is significantly less than that required by the

compressor

heat must be supplied in the regenerator to release refrigerant vapor at high pressure for

condensation, and heat must be removed from the absorber

From the standpoint of thermodynamics, the vapor compression chiller is a heat pump, using

mechanical energy and work, to move heat from a low to a high temperature. An absorption chiller is

the equivalent of a heat engine absorbing heat at a high temperature, rejecting heat at a lower

temperature, producing work driving a heat pump.

1.2.2 Absorption Refrigeration Working Fluids

An absorption chiller requires two working fluids, a refrigerant and a sorbent solution of the

refrigerant. In a water-LiBr absorption chiller, water is the refrigerant; and water-LiBr solution, the

sorbent. In the absorption chiller cycle the water refrigerant undergoes a phase change in the

condenser and evaporator; and the sorbent solution, a change in concentration in the absorber and

evaporator.

Water is an excellent refrigerant; it has high latent heat. Its cooling effect, however, is limited to

temperatures above 0o

C because of freezing. The sorbent, LiBr, is nonvolatile, so a vapor phase inthe absorption chiller is always H 2O. The sorbent solution, water-LiBr, has a low H 2O vapor pressure

at the temperature of the absorber and high H 2O vapor pressure at the temperature of the regenerator,

facilitating design and operation of the chiller. The advantage of the water-LiBr pair includes its

stability, safety, and high volatility ratio. It has no associated environmental hazard, ozone depletion,

or global warming potential.


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1.2.3 Absorption Refrigeration Operating Conditions

The choice of the refrigerant, water, and sorbent, water-LiBr solution, along with the designation of a

chilled-water outlet temperature and cooling-water inlet temperature determines the operating

temperatures and pressures in the evaporator, absorber, regenerator, and condenser of the LiBr

absorption chiller as illustrated in Figure 1-5.

In the evaporator, low operating temperature and pressure are required to vaporize refrigerant

to absorb heat from the chilled water.

In the absorber, the cooling-water temperature determines the composition of the sorbent

solution so that it absorbs the refrigerant vapor, as required, at the pressure determined by the

evaporator.

In the regenerator, the pressure is that of the condenser. An elevated value is required to

condense the refrigerant vapor at the temperature of the cooling water. The temperature in the

absorber is that required to vaporize the refrigerant from the sorbent solution.

The low operating pressure in the evaporator and absorber requires high equipment volume and a

special means for reducing pressure loss in the refrigerant vapor flow. Preventing the leakage of air

into the evaporator and the absorber is one of the main issues in operating an absorption chiller. A

special purge device removes air and other noncondensable gases, and an external vacuum pump is

used periodically to maintain low operating pressure. The high operating pressure in the regenerator

and condenser requires the use of heavy-walled equipment and a pump to deliver the sorbent solutionfrom the low-pressure absorber to the high-pressure regenerator. Crystallization, the deposition of

LiBr from the sorbent solution at high concentrations and low temperatures, can block the sorbent

flow and cause the chiller to shut down. Controls are usually necessary to prevent crystallization.

1.2.4 Absorption Chiller Cycle Modifications

Several modifications can be made in the basic absorption chiller cycle to reduce the heat required to

operate the chiller and to reduce the extent of heat transfer surface incorporated in the machine.

Countercurrent heat interchange can be arranged between the two sorbent solution flows

connecting the low-temperature absorber and the high-temperature regenerator. This

interchange can significantly reduce the heat quantities involved in the operation of both; less

heat will need to be supplied to the regenerator, and less heat will need to be removed form the

absorber.


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The refrigerant vapor leaving the high-temperature and -pressure regenerator can be used to

vaporize an equal quantity of refrigerant from the sorbent solution in a second regenerator

operating at a lower temperature and pressure. This second stage of regeneration reduces the

heat requirement of the absorption chiller by a factor approaching 2.

Heat transfer between the vaporizing refrigerant and the chilled water in the evaporator can befacilitated by recirculating the refrigerant liquid over the heat transfer surface, reducing the

temperature difference and the heat transfer area.

Figure 1-6: Typical two-stage parallel flow absorption chiller configuration

Low-temp.regenerator

High-temp.heat exchanger

Low-temp.heat exchanger

Refrigerantcombiner

pumpRecirculation

Solutioncombiner

splitter Solution

T

P


valve

expansionSolution

valve

pumpSolution

Condenser

LTRGHeat to


Evaporator Absorber


Condenser


Regenerator

Heat input

The revised flow diagrams illustrating these absorption chiller flow diagrams are shown in Figures 1-6

and 1-7. The flow of the sorbent solution from the absorber to the two regenerators can be either

parallel or in series. In a parallel flow arrangement, the dilute solution from the absorber is pumped to

both the high-temperature and the lower-temperature regenerators in parallel, as shown in Figure 1-6.

Concentrated solutions from both regenerators are recombined and returned to the absorber. In a

series flow arrangement, the solution from the absorber is first pumped to the high-temperature, high-

pressure regenerator; and the partially concentrated sorbent solution then flows to the lower-pressure,

lower-temperature regenerator, as shown in Figure 1-7.


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Figure 1-7: Typical two-stage series flow absorption chiller configuration

Low-temp.regenerator

High-temp.heat exchanger

Low-temp.heat exchanger

Refrigerantcombiner

pumpRecirculation

T

P


valve

expansionSolution

valve

PumpSolution

Condenser

LTRGHeat to


Evaporator Absorber


Condenser


Regenerator

Heat input

A parallel flow configuration has several advantages over the series flow configuration. The sorbent

solution flow in each heat interchanger is only half that of the series flow configuration. In general,

the parallel configuration has a lower heat input requirement than the series flow configuration.

1.3 Research Objectives

The objective of this research is to develop methods for the effective design and evaluation of

absorption chiller-based micro-BCHP systems that reduce energy consumption, decrease operational

costs, and improve environmental benefits in residential and light commercial buildings. The methods

demonstrated in the thesis can be widely used in building energy system design and evaluation; they

can also be broadly applied in an absorption chiller and other BCHP system equipment design, and in

system integration. The analytical methods also provide the basis for diagnosing and optimizing the

operation of absorption chiller-based micro-BCHP systems.

Four research areas are involved in this work on microscale absorption chiller system evaluation and

performance simulation:


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1) establishment of a unique experimental environment and procedures for absorption chiller

tests under various conditions

2) conduct of a comprehensive testing program on a microscale absorption chiller

3) construction of a comprehensive chiller model based on the pertinent scientific and

engineering principles adapted to the design of a chiller and to the analysis of extensive,

detailed test data obtained from the test program

4) analysis of the measured data, refinement of the model, and improvement of the chiller design

on the basis of the data analysis process

The model is now being used as a tool to adapt the chiller to various heat sources and sinks and to

carry out performance simulations of micro BCHP system. In both its theoretical and practical aspects,

this study contributes important knowledge for the development and application of micro-BCHP

systems in residential and light commercial buildings. The improvements in BCHP system analyticalmethods lay the groundwork for developing of overall BCHP system performance assessment tool; the

practical progress in microscale-BCHP system experiment and evaluation setups establishes the

threshold for an efficient and integrated microscale building energy supply, distribution, and delivery

system. These contributions are made possible by close cooperation in research and development

(R&D) with a leading manufacturer; in turn, some of the research achievements of this study have

been promptly incorporated into the emerging technology and product.

1.4 Research Approach

To achieve the research objectives, this thesis focuses on equipment installation and test, model

development, data analysis, and system simulation of a microscale, steam-driven, two-stage LiBr

absorption chiller for an energy supply system in Carnegie Mellon University (CMU)s Robert L.

Preger Intelligent Workplace (IW). Experimental data and a computational model are the two basic

components of this work. The experience gained provides the framework for other BCHP component

studies and system integration. The research has been carried out in the following several steps: some

in parallel, others sequentially:

1.4.1 The Planning and Installation of Experimental Equipment

A microscale BCHP energy supply system (ESS) has been designed for the IW, a 6,500 ft 2 office

environment at CMU, to provide power and space cooling heating, and ventilation. As the first stage

in realizing this overall system, a 16kW steam-driven water-LiBr absorption chiller was installed in

the south section of the IW. This chiller


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is driven by steam, reducing summer electrical peak demands and leveling the year round

demand for natural gas and other fuels

is flexible in adapting to thermal recovery equipment associated with various prime movers

provides a cooling capacity and compactness appropriate for residential, small commercial, and

institutional buildings

incorporates a cooling tower to reject the heat from its operation as required

The chiller was installed together with its auxiliary steam and chilled-water supply, and test load

systems in the IW. A web-based chiller automation system (CAS) was also installed to operate the

chiller with its auxiliary systems, monitor the overall system status, and collect the experimental data.

In this test-bed the absorption chiller was also integrated into the IW and campus chilled-water system,

so when the test was over, the chiller could provide chilled water to the IW and the campus.

Experiments were carried out under a broad range of system operating parameters.

In this work, both equipment testing and mathematical model simulation of the chiller were combined

to provide a detailed understanding of the equipment, to analyze the test data, to discover possible

chiller design improvements and modifications, and to provide a method to design and evaluate

overall BCHP systems.

1.4.2 The Test Program and Experimental Data

The chiller was tested by varying six operating parameters in turn: the chilled-water return temperatureand flow rate, the cooling-water supply temperature and flow rate, and the steam pressure. In the test

program, only one parameter was adjusted at a time, and the others were kept at design conditions.

Additional sensors were installed in the chiller beyond those provided by the manufacturer to operate

the chiller and its auxiliary system to calculate chiller performance such as the coefficient of

performance (COP) and cooling capacity, and to observe chiller internal conditions. Experimental data

obtained from 11 temperature sensors in the chiller were used to verify the predictions of the

performance model.

1.4.3 The Development of Computational Performance Model

On the basis of scientific and engineering principles and the specific configurations of the chiller, a

detailed computational performance model was constructed to evaluate the chiller performance under

various operating conditions. This model was developed for the chiller to further refine the


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understanding of the principles of the chiller, to analyze the experiment data from the test program, to

assist in the equipment design, and to evaluate the performance of BCHP systems.

The basic equation types incorporated in the model include: mass and energy balances,

thermodynamic property relations, thermal and phase equilibrium relations, and heat and mass transfer coefficient correlations. The variables in these equations are the operating conditions pressures,

temperatures, compositions, and flows throughout the chiller. The model includes 416 variables and

409 equations. If seven operating conditions are specified, the model can be solved and all the

operating conditions throughout chiller can be calculated.

1.4.4 The Analysis of the Experimental Data

To assess the performance data collected, an analytical method was developed that minimizes the

deviations between the experimental measurements and the model solutions. Several modelassumptions were adjusted to improve the agreement between the experimental measurements and the

model calculations. These adjustments significantly improved the agreement between the calculated

and measured variables.

1.5 Current Absorption Chiller Modeling Studies

The microchiller performance model is one of the major efforts of this research. The literature for

absorption chiller model studies has been reviewed; the existing model studies are categorized and

summarized in the following sections.

1.5.1 Absorption Chiller Modeling Approaches

In the past decades, computer models have been developed to investigate the performance of various

water-LiBr absorption chiller cycles. Among these models, some [8, 9] are system specific for

particular machines, flow configurations, and working materials. Others [10, 11, 12] are generic to

handle various potential absorption cycles with one modularized model. The system specific models

are performance models aimed at simulating a specific design and investigating its performance under

various operation conditions; the generic models are aimed at exploring novel absorption cycles and

evaluating their performance under various boundary conditions.

The advantage of system specific or performance models is that the model simulates the configuration

of absorption chiller systems in detail. Thermodynamic cycle, heat, and mass transfer characteristics

can be investigated on the basis of the physical details of the chiller. In these studies the simulation


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instrumentations greatly restrict their accessibility for the experiments. The small cooling capacity of a

microscale chiller, however, makes it possible to provide a test cooling load and to simulate a wide

range of operation conditions for the chiller.

Third, the model validation method has been simplified in the past studies. The deviations betweenthe experimental and the performance simulation results for the COP and the cooling capacity at a

single given operational condition are used to judge the overall quality of the model.

Finally, the available packaged absorption chiller models lack the flexibility to be integrated into

building simulation tools to support the design and analysis of absorption chiller-based BCHP systems.

The work reported in this thesis addresses these insufficiencies.

1.6 The Comprehensive Performance Model and its Applications

In this work, a steady-state performance model has been developed for the Broad BCT16 absorption

chiller to further refine the understanding of the principles of this chiller, to analyze the experiment

data from the test program, to assist in the equipment design, and to evaluate the performance of

BCHP systems.

1.6.1 The Chiller Model Description

In the model, the absorption chiller is composed of the following components:

an evaporator: a countercurrent two-phase coiled tube heat exchanger an absorber: a countercurrent two-phase coiled tube mass and heat exchanger

two regenerators: one high temperature, one intermediate temperature: well mixed, two-phase

boiling coiled tube heat exchangers

a condenser: a countercurrent heat exchanger

two plate heat interchangers: countercurrent single-phase heat exchangers

two tube and shell heat recovery exchangers: countercurrent single-phase exchangers three pumps: a sorbent pump, a refrigerant pump, and a chilled-water pump

associated spray nozzles, trap, valves, and pipe fittings

The cooling tower associated with this chiller includes the following components:

a countercurrent plate column two-phase mass and heat exchanger


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a cooling-water pump an air fan

The complete steady-state chiller model is composed of the following nonlinear algebraic equations

applicable to each of the above chiller and cooling-tower components:

two mass balances, water and LiBr

an energy balance

thermodynamic property relations for stream enthalpies as a function of pressure, temperature,

and composition

phase equilibrium relations among pressure, temperature, and compositions of the coexisting

phases

the appropriate heat transfer (and for the absorber and cooling tower, mass transfer) relations correlations of overall heat and mass transfer coefficients, U and K, for the respective

components based on their specific design and operating conditions, (see chapter 3)

work computations for the pumps and fan

These equations involve, as variables, the properties pressure, temperature, composition, and flow

of all the phases present in and flows among the chiller components. The completed chiller model

interrelates variables of all these equations based on the configuration and the flow diagram, of the

chiller. In general it has been assumed that:

The properties of a stream leaving a component to an interconnected component are those of

eithera liquid or a vapor, thus the quality of the stream is either 1.0 or 0.0

There is no pressure loss and no heat loss/gain in the lines connecting the components

Tthe sorbent solution charged to the chiller has a concentration of 55% LiBr. Once the chiller

operates under design conditions, the concentration difference of the sorbent solutions flow in

and out of the high-temperature regenerator is roughly at 5%; that of the intermediate

temperature regenerator is approximately 4%. Dilute sorbent is distributed to the two

regenerators in approximately equal quantities.

The completed chiller model involves 416 variables and 409 nonlinear algebraic equations. Solving

the model and determining values for all the chiller variables therefore requires specifying values for

seven operating parameters. In this work, the specified operating parameters are: the chilled water


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data, and chiller performance are presented. The chiller internal control principles and the system

operation instructions are presented in appendixes 2A and 2B, respectively.

Chapter 3, Computational Model describes the framework of the performance model within which

the absorption chiller component modules are developed. It provides an in-depth presentation of thegoverning equations and modeling assumptions. The computational and numerical issues are

addressed in the various stages of the absorption chiller component modeling in appendix 3A; the

source code of the performance model is attached in appendix 3B.

Chapter 4, Model-based Data Analysis assesses the model calculations and experimental data

accuracy and reliability to learn how to validate the model as well as improve the equipment designs.

The analysis results presented regard the test programs that vary for five operating parameters: chilled-

water supply temperature and flow, cooling-water supply temperature and flow, and steam supply

pressure. When analyzing the experimental data, opportunities to improve the accuracy of the model

became apparent. Consequently, the adjustments to model assumptions significantly improved the

agreement between the calculated and the measured variables.

Chapter 5, Contributions and Areas for Future Research summarizes the contributions of this

thesis and suggests future areas for research and the issues involved, including: extension of the

validated steam-driven absorption chiller model to several other heat sources: hot water, natural gas,

and exhaust gases. The chiller performance models can be integrated and evaluated into overall BCHP

system configurations on an annual basis.


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2 Chiller Test System and Program

As a first step in providing an energy supply system for CMUs IW, a 16kW, steam-driven, two-stage

absorption chiller was installed together with an auxiliary steam supply and a variable load for the

chiller test and performance evaluation. A web-based data acquisition and control system wasdeveloped to operate the chiller and its auxiliary equipment while storing and displaying the test

measurement data. The chiller was tested at various operating conditions in accordance with a test

program. In the future, the chiller and its control system will be incorporated in the cooling system of

the IW and connected with the campus chilled-water supply system.

2.1 Absorption Chiller

2.1.1 System Descriptions

The absorption chiller installed in the IW is a steam-driven, two-stage, water-LiBr, parallel-sorbent-

flow series-cooling-water flow chiller with a cooling tower. This chiller, provided by Broad Co., has a

16kW rated cooling capacity. It is the smallest absorption chiller available in the existing market and

the only steam-driven absorption chiller of such capacity in the world.

Figure 2-1: Absorption chiller installed in the IWFigure 2-1 shows the absorption chiller installed on

a platform adjacent to the IW. The chilled-water

supply and return, steam supply, condensate return,

power, and city water lines connect with the chiller at the bottom left. Figure 2-2 is a schematic flow

diagram recreated from the manufacturers brochure

for a commercial natural-gas direct-fired chiller; this

flow diagram shows all the heat and mass transfer

components, pumps, and pipe fittings. It also

indicates the design values for temperatures

throughout the chiller. The measurement and

control features of the chiller will be discussed inconjunction with a detailed process and

instrumentation (P&I) diagram in the section that

follows. The components and parts indicated in

Figure 2-2 are listed in Table 2-1.


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Figure 2-2: Schematic diagram of the absorption chiller

Table 2-1: Component names and corresponding abbreviations

Abbreviation Name Abbreviation NameABS Absorber EVP Evaporator

BPHX By-pass heat exchanger HTRG High-temperature regenerator CHSV Cooling/heating switch valve HRHX Heat recovery heat exchanger

CHWBPV Chilled-water by-pass valve HTHX High-temperature heat exchanger

CHWP Chilled-water pump LTHX Low-temperature heat exchanger COND Condenser LTRG Low-temperature regenerator

CT Cooling tower RBPSV Refrigerant by-pass solenoid valveCTOF City-water overflow RP Refrigerant pumpCTWS City-water switch RPH Refrigerant pump heater

CWBPV Cooling-water by-pass valve SF Steam filter CWDD Cooling-water drain device SP Solution pump

CWDV Cooling-water detergent valve ST Steam trap

CTF Cooling-tower fan SV Steam valve

CWP Cooling-water pump


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The absorption chiller in Figure 2-2 consists of five major and four minor heat transfer components,

three pumps, a cooling tower, an automatic inert gas purge device, and the associated valves and pipe

fittings. Specifically, the five major components are:

an evaporator, a countercurrent two-phase heat exchanger an absorber, a countercurrent two-phase heat and mass exchanger

a high-temperature regenerator (HTRG), a well-mixed, two-phase, boiling heat exchanger a low-temperature regenerator (LTRG), a well-mixed, two-phase boiling heat exchanger

a condenser, a countercurrent heat exchanger

The four minor components are:

a high-temperature heat interchanger (HTHX), a countercurrent, single-phase heat exchanger

a low-temperature heat interchanger (LTHX), a countercurrent, single-phase heat exchanger

a heat recovery heat exchanger (HRHX), a countercurrent, single-phase heat exchanger

a refrigerant by-pass heat exchanger (BPHX), a countercurrent, single-phase heat exchanger

The three pumps are:

a solution pump (SP), a variable-speed pump

a chilled-water pump (CHWP), a single-speed pump

a refrigerant pump (RP), a single-speed pump

The cooling tower (CT) includes:

a countercurrent vertical plate column; a two-phase, mass and heat exchanger

a cooling-water pump (CWP); a single-speed pump

a cooling-tower fan (CTF); a three-speed air fan

associated valves and drain devices

Other associated components include:

an automatic gas purge device (AGPD)

associated valves, spray nozzles, and pipe fittings


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Figure 2-3 : Structure of the absorption chiller

The physical arrangement of the absorption chiller is shown in Figure 2-3. The main body of the

chiller consists of two sealed vessels: the upper one at an elevated pressure, the lower vessel at a high

vacuum. The upper vessel includes the HTRG, the LTRG, and the condenser. The lower vessel

includes the absorber, the evaporator, the BPHX, the LTHX, and the HTHX. The flows of sorbent

solutions, refrigerant, and cooling water penetrate the vessel walls in pipes between the two vessels.

The high vacuum in the lower vessel is maintained by the AGPD and a manual vacuum pump

independent of the chiller. The chilled water and cooling water are circulated by the CHWP and the

CWP, respectively. The inclusion of the cooling tower enables chiller installation where cooling water

may be unavailable.

Table 2-2: Specifications of the absorption chiller

Name Quantity UnitCooling capacity 16 kWChilled-water return temperature 14 oC

Chilled-water supply temperature 7 oC

Chilled-water flow rate 2 m 3/h C h i l l e d w a t e r

Chilled-water pump head 8 mH 2O

Rated steam pressure, absolute 0.7 mPaSteam pressure limit, absolute 0.9 mPa

S t e a m

Maximum steam consumption 24 kg/hPower voltage 220 VPower frequency 60 Hz

P o w e r

Maximum power consumption 1 kWWater-LiBr sorbent solution mass 65 KgWater-LiBr sorbent concentration 55 %

S o l u t i o n


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Table 2-2 lists the chiller specifications from the manufacturer; these are the only published

performance data for this unique chiller. A test program was developed to investigate chiller

performance and to provide additional measurements of chiller operating conditions. The chiller

specification data are useful in evaluating the results of the chiller tests. The chiller working principles

are described in the following sections.

2.1.2 Evaporator and Chilled-Water Pump

The evaporator of the chiller, shown in Figures 2-2 and 2-4, occupies the lower vessel. The evaporator

tube bank comprises two parallel tubes spiraling 18 times from the bottom to the top of the coil. Water

refrigerant is distributed evenly over the tubes in the bank by nozzles spraying water from the

condenser. Water that was not evaporated in the first pass collects in the refrigerant tray at the base of

the evaporator and is recirculated by the refrigerant pump. The refrigerant vaporizes in the evaporator

at low pressure, about 0.8-1.0 kPa, and low temperature, about 3-4 oC. The vaporization absorbs heat

from the chilled water flowing through the evaporator coil, cooling this flow from 14 oC to 7 oC.

Figure 2-4: Configuration of the lower vessel


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At a constant flow rate of 2 m3/h and a head of 8 mH 2O to overcome the pressure loss, the evaporator

functions as a countercurrent, two-phase heat exchanger. The steam flow to the HTRG is adjusted to

maintain a constant refrigerant level water tray reservoir; a low level requires an increase in the steam

flow to provide more refrigerant. The chiller control system is discussed in appendix 2.A

2.1.3 Absorber and Solution Pump

The absorber, shown in Figures 2-2 and 2-4, maintains the low operating pressure required in the

evaporator. It is a spiral tube bank, consisting of two tubes spiraling from the bottom to the top. The

coil surrounds the evaporator but is separated from it by a chevron separator to prevent carryover of

refrigerant liquid. Concentrated water-LiBr sorbent solution is distributed evenly over the tubes of the

absorber coil by nozzles spraying sorbent from the two regenerators, cooled in the HTHX and the

LTHX. The water refrigerant vapor from the evaporator passes through the chevron separator, enters

the absorber, and is absorbed in the water-LiBr sorbent flowing 5 m 3/h over the coil. The heat released

by the sorption of the refrigerant in the sorbent is transferred to the cooling water flowing in the tubes

of the coil, increasing its temperature of 30 oC. The cooling water circulates to the condenser and then

to the cooling tower of the chiller where the sorption heat is rejected to the surroundings by

evaporation. The concentrated sorbent solution becomes dilute by absorbing the refrigerant vapor. The

dilute sorbent solution, collected in the solution reservoir at the bottom of the lower vessel, is pumped

back to the HTRG and LTRG with pressure about 10 kPa and 100 kPa, respectively, either in series or

in parallel by the solution pump for regeneration.

2.1.4 High-Temperature Regenerator

The water-LiBir sorbent solution, diluted by absorbed water refrigerant vapor, is pumped in the Broad

chiller to the two regenerators in parallel: the HTRG and the LTRG. In each regenerator, the

refrigerant water vapor added to the sorbent in the absorber is removed by evaporation at elevated

temperature and pressure. Approximately equal quantities of sorbent solution are fed to each

regenerator controlled by a flow restriction device in the pipe leaving the solution pump. In the

HTRG, steam in a coil is used to boil off refrigerant vapor from the sorbent. The temperature and

pressure of the refrigerant vapor produced in the HTRG is high enough to generate an approximately

equal quantity of refrigerant vapor from the sorbent in the LTRG operating at a lower temperature and

pressure. The driving heat provided to the HTRG is thus cascaded and used twice. This makes the

absorption cycle a two-stage process. The generation of additional refrigerant from a given heat input,

improves significantly the cycle performance.


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The design of the HTRG differs depending both on the heating medium, gas, or liquid, and on its

temperature. Many forms of thermal energy can be used in the HTRG to drive a two-stage absorption

chiller, such as steam, hot water, exhaust gas, natural gas, oil, and liquid pressurized gas. In this

section, only the steam-driven HTRG is discussed; other kinds of heat sources - natural gas, hot water,

and exhaust gas - are discussed in the sections that follow.

The water-LiBr sorbent, reconcentrated in the regenerators, returns to the absorber through flow

restrictions that assist in maintaining appropriate liquid levels to submerge the heat transfer coils in the

regenerators. The solution pump frequency is adjusted to maintain a constant level in the HTRG.

Figure 2-5: Configuration of the upper vesselBoth the HTRG and the LTRG use water vapor as

a heat resource; they have similar functions and

structure. The heat transfer process includes

condensation inside the tubes and boiling on the

outer surface of these tubes.

The configuration of the upper vessel for the

absorption chiller installed in the IW is similar to

that of a natural gas direct-fired absorption chiller

of the same capacity shown in Figure 2-5. The

combustion chamber and convection chamber of

the natural-gas-fired HTRG are replaced by a

spiral tube bank in the steam-driven HTRG to

vaporize water refrigerant from the water-LiBr

sorbent.

The major part of the HTRG is a spiral tube bank with three parallel tubes spiraling eleven rounds

from the top to the bottom. Steam supply flows in parallel through the tubes from top to bottom. The

dilute sorbent solution is pumped into the HTRG from the bottom of the tank, and the concentrated

sorbent solution leaves the HTRG from the bottom of the tank at a distant point. The vigorous mixing

resulting from the boiling in the regenerator minimizes sorbent concentration differences in the HTRG.

While mass transfer is involved as water diffuses to and is evaporated from the sorbent-vapor

interface, the vigorous mixing minimizes mass transfer resistance. The HTRG thus functions as a

well-mixed two-phase boiling heat exchanger.


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At design conditions, the HTRG requires a steam supply at 0.7 mPa; the maximum steam supply

pressure is 0.9 mPa, and the maximum flow rate is 24 kg/h. An elevated pressure, typically at a

saturated vapor pressure of 100 kPa, is maintained in the HTRG to provide a condensing temperature

of about 100 oC.

2.1.5 Low-Temperature Regenerator

The LTRG is a staggered tube bank with 14 parallel tubes circulating once around. Vapor from the

HTRG enters at one end of each parallel tube, and condensate leaves the other end of the tubes and

enters the condenser. One end of each tube is connected to the HTRG, the other, to the condenser. The

refrigerant, water, and vapor from the HTRG passes through the LTRG tubes and transfers the heat of

condensation to the sorbent solution surrounding the tube bank. The dilute sorbent solution enters the

LTRG on the top; the concentrated sorbent solution leaves from the bottom. Refrigerant vapor is

boiled off; the dilute sorbent solution is concentrated. Similar to the HTRG, the boiling process in the

LTRG is violent; bubbles stir the sorbent solution. The concentration of the sorbent in the LTRG is

therefore nearly uniform, close to the exit value, and mass transfer is not a limiting process. Similar to

the HTRG, the LTRG functions as a well-mixed, two-phase boiling heat exchanger.

The LTRG has a lower boiling temperature and pressure than the HTRG. At design conditions, a

medium pressure, typically at a saturated vapor pressure of 10 kPa, is maintained to provide an

evaporating temperature of about 45 oC. The LTRG has no solution level control like the HTRG, but

the maximum solution level is measured in the LTRG to prevent crystallization in the LTHX. Thedetails of the control principles are discussed in appendix 2.A.

2.1.6 Condenser

The condenser and the LTRG are housed in the same vessel with the HTRG, and they operate at the

same intermediate pressure. The condensate from the LTRG flashes into the condenser operating at

intermediate pressure. The condenser then condenses both the vapor produced in this flashing and the

water vapor from the LTRG, transferring heat into cooling water flowing into the condenser coil. This

condensate is returned to the evaporator.

The condenser is a spiral copper tube bank with three parallel tubes spiraling three rounds from the

bottom to the top. The cooling water flowing from the absorber enters the condenser from the bottom

and leaves the condenser to the cooling tower at the top. The liquid condensed from the vapor as a


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film on the surface of tube bank drips down to a drain pan that separates the condenser from the LTRG.

The condenser functions as a two-phase, countercurrent heat exchanger.

2.1.7 Heat Recovery Devices

In Figure 2-2, the four minor heat transfer components in the chiller are used to recover thermal

energy by heat exchange between the various refrigerant, sorbent, and steam condensate streams. All

these exchangers are single-phase, countercurrent heat exchangers that recover heat from a hot stream

and deliver it to a cold stream. One is the LTHX, and the other is the HTHX. These interchangers

reduce the heat requirements of the regenerators and the cooling requirement of the absorber.

In the chiller, the temperature of the condensate leaving the HTRG is high enough to be used to

preheat the dilute solution from the LTHX before it enters the LTRG. A heat recovery exchanger

between the steam condensate and the sorbent stream entering the LTRG reduces the heat requirementof the LTRG and the temperature of the steam condensate, avoiding its flashing in the condensate tank.

A heat recovery exchanger between the water refrigerant leaving the condenser and the sorbent pool in

the absorber, called the by-pass heat exchanger (BPHX), increases cooling in the evaporator. Broad

terms it an elbow-heat exchanger. In the elbow, the liquid refrigerant condensed from the condenser

releases a small amount of heat to the dilute solution in the absorber.

2.1.8 Cooling Tower

A cooling tower is widely used to dissipate reject heat from a water-cooled air-conditioning system to

the surroundings. This Broad absorption chiller has a built-in cooling tower, as shown in Figures 2-2

and 2-6. Its compact design facilitates chiller installation and operation. The cooling water in the

chiller flows in series through the absorber, the condenser, and then through the cooling tower. This

arrangement provides for a minimum operating temperature in the absorber that is required to achieve

a low chilled-water temperature; the high flow in both the absorber and the condenser provides for

high heat transfer coefficients in these components. The recirculating cooling water flows down

vertical plates in countercurrent contact with upward-flowing ambient air.

Evaporation of a small portion of the water flowing downward through the cooling tower reduces its

temperature; makeup water added to the cooling tower replaces that evaporated. The air temperature is

also reduced, but the humidity increases markedly. Thus, the cooling tower functions as a two-phase

countercurrent heat and mass exchanger.


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As illustrated in Figure 2-6, the cooling tower attached

to the chiller comprises spray nozzles, vertical PVC

plates, a PVC mist collector, a cooling-water tank, a

cooling-water pump, a cooling-water by-pass valve,

and a cooling-air fan along with devices for water

drain and city-water supply and detergent addition.

The major components of the cooling tower are the

PVC vertical plates (a heat and mass transfer medium)

that increase water/air contact surface as well as the

duration of contact. The closely packed vertical PVC

plates are spaced with staggered bars installed below

the spray nozzles in the air path. At design conditions,

the cooling water is distributed from the top of thetower through spray nozzles at a temperature of 35.5oC. The speed of the cooling tower air fan is varied to

maintain the cooling-water supply to the chiller at 30oC.

Figure 2-6: Configuration of cooling tower

Air inlet

City water

PVC Plates

PVC mist

Spray

CTF

Water tank

CWBPV

To chiller

Air outlet

From chiller

CWP

Drain

CWDV

CTWS

CWDD

collector

nozzles

As illustrated in Figure 2-6, the cooling tower attached to the chiller comprises spray nozzles, vertical

PVC plates, a PVC mist collector, a cooling-water tank, a cooling-water pump, a cooling-water by-

pass valve, and a cooling-air fan along with devices for water drain and city-water supply anddetergent addition. The major components of the cooling tower are the PVC vertical plates (a heat and

mass transfer medium) that increase water/air contact surface as well as the duration of contact. The

closely packed vertical PVC plates are spaced with staggered bars installed below spray nozzles in the

air path. At design conditions, the cooling water is distributed from the top of the tower through spray

nozzles at a temperature of 35.5 oC. The speed of the cooling-tower air fan is varied to maintain the

cooling-water supply to the chiller at 30 oC.

2.1.9 Vacuum System

The pressure of the evaporator and the absorber is significantly below atmospheric pressure, and air

can leak into the absorption chiller. Corrosion can also occur in the chiller, generating another

noncondensable gas, H 2. Air and other noncondensable gases in the evaporator and absorber can

seriously reduce the rate of heat and mass transfer pr

+++++THESIS - An Absorption Chiller in a Micro BCHP Application - Model Based Design and Performance Analysis

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