EnergyPlus simulation of single coil twin fan air conditioning system

A review of existing simulation software, computerized cooling coil models, and ventilation stream control was completed to establish the current state of the art.. EnergyPlus was the

ENERGYPLUS SIMULATION OF THE SINGLE-COIL

TWIN-FAN AIR CONDITIONING SYSTEM

CLAYTON C MILLER (B.S., M.A.E., University of Nebraska, United States)

ACKNOWLEDGEMENTS

I would like to acknowledge my parents for taking me to so many Nebraska State Geography Bees when I was a kid so that I would dream of one day traveling to other countries The support from all my family and friends has been substantial throughout this process

Prof Chandra Sekhar of NUS, my advisor, has been a major asset in this process through his guidance and support I would like to acknowledge the assistance from Dr Uma, Brent Griffith, Fred Buhl, and Alice Goh as well

I am grateful to the United States Department of State and the Fulbright Scholar program for supporting me for the first nine months of this project The Fulbright program is a beacon of light

in the world and promoting mutual understanding and respect amongst cultures is something that will always be a part of my life I must mention also the support of NUS and the School of Design and Environment

Clayton C Miller

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS i

TABLE OF CONTENTS ii

SUMMARY iv

LIST OF TABLES v

LIST OF FIGURES vi

LIST OF SYMBOLS AND ABBREVIATIONS viii

Chapter 1: INTRODUCTION 1

1.1 BACKGROUND 1

1.2 OBJECTIVES OF THE STUDY 2

1.3 ORGANIZATION OF THESIS 2

Chapter 2: LITERATURE REVIEW 4

2.1 SINGLE COIL TWIN FAN (SCTF) SYSTEM 4

2.2 AIRSIDE SYSTEM MODELING 7

2.3 COMPUTERIZED COOLING COIL MODELS 12

Chapter 3: RESEARCH METHODOLOGY 15

3.1 SIMULATION APPROACH 15

3.2 MODEL IMPLEMENTATION 18

3.3 MODEL VALIDATION 26

Chapter 4: DEVELOPMENT OF ENERGYPLUS AIR TERMINAL UNIT MODEL 29

4.1 CALCULATION METHODOLOGY 31

4.2 MODULE DEVELOPMENT 35

Chapter 5: TERMINAL UNIT MODEL VALIDATION 37

5.1 THREE ZONE ANALYTICAL VERIFICATION MODEL 37

5.2 BCA ZERO ENERGY BUILDING (ZEB) COMPARISON 42

Chapter 6: CONCEPTUAL DEVELOPMENT OF COMPARTMENTED COOLING COIL MODEL 53

6.1 ANALYSIS OF COIL CONTROL 53

6.2 CONCEPTUAL COMPARTMENTED COIL CURVE FIT MODEL 56

Chapter 7: CONCLUSION AND RECOMMENDATIONS 59

7.1 SUMMARY OF RESULTS 59

7.2 FUTURE WORK 59

REFERENCES 61

LIST OF PUBLICATIONS 63

Appendix A: DOCUMENTATION OF EXISTING ENERGYPLUS CALCULATIONS 65

Appendix B: ASHRAE TOOLKIT DETAILED COOLING COIL MODEL 75

Appendix C: ALTERNATIVE SIMULATION APPROACHES 83

Appendix D: AIRTERMINAL:DUALDUCT:VAV:OUTDOORAIR 85

Appendix E: INPUT DATA FILE (.IDF) FOR THREE ZONE ANALYTICAL COMPARISION 95

Appendix F: BCA VAV FRESH AIR FLOWRATE DATA 115

Thesis directed by Assoc Professor S Chandra Sekhar

The goal of this thesis was to describe the development of a building energy simulation to show the potential energy performance of the newly-developed Single-Coil Twin-Fan (SCTF) air conditioning system The objectives of this project were to develop a customized module of the energy efficient SCTF system, validate the model using industry-accepted simulation verification methods, and implement the module in a program with the capability to produce an energy

simulation to show the potential performance of a typical building under a range of operating conditions and climates Due to unique features such as a compartmented cooling coil and

decoupled ventilation and recirculated air streams to the zone level, it is not possible to effectively simulate the SCTF system in any mainstream modeling application A review of existing

simulation software, computerized cooling coil models, and ventilation stream control was

completed to establish the current state of the art EnergyPlus was the simulation software chosen and two main modifications were made to the source code: decoupling of the ventilation air stream to the zone level and creation of a new dual duct terminal unit to control the separate air streams An analytical and empirical verification of these modifications was completed using a three zone theoretical model as well as data from an installed system in the BCA Zero Energy Building (ZEB) It was found that the new terminal unit model performed as expected in the analytical model and was correlated with the empirical data from the ZEB Comparison between simulated and measured air flow rates showed less than a 2% error for the time period selected, however, the overall shape of the two curves was dissimilar This observation was attributed to the differences in the way the system was controlled in the simulation as compared to actual installed system A conceptual development of the compartmented cooling coil was outlined and future effort in the modeling of the SCTF system was suggested including a more rigorous

validation process and inclusion of reheat capabilities

LIST OF TABLES

Table 1: Three Zone Analytical Verification Model Details 37

Table 2: Three Zone Analytical Model Internal Loads 38

Table 3: Three Zone Analytical Model Scenarios 39

Table 4: Approximate Occupancy of VAV 2-1 Based on Measured Data 51

Table 5: Measured vs Simulated Fresh Air Amount 52

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LIST OF FIGURES

Figure 1: SCTF Topology Diagram and Psychrometric Performance (Sekhar et al., 2004) 5

Figure 2: Air and Coolant Flow Arrangement of the Compartmented Coil (Sekhar et al., 2004) 6

Figure 3: EnergyPlus Program Schematic (EnergyPlus, 2009) 8

Figure 4: Connections between the Main HVAC Simulation Loops and Sub-Loops (EnergyPlus, 2009) 9

Figure 5: EnergyPlus Zone Equipment (EnergyPlus, 2009) 10

Figure 6: Schematic of AirTerminal:SingleDuct:VAV:Reheat Unit (EnergyPlus 2009) 11

Figure 7: Historical Development of Common Computerized Cooling Coil Model 14

Figure 8: SCTF Simulation Scope Scenarios 17

Figure 9: EnergyPlus Ventilation System Diagram (EnergyPlus, 2009) 19

Figure 10: Proposed SCTF Ventilation System 19

Figure 11: EnergyPlus VAV Dual Duct Air System Component Arrangement 20

Figure 12: SCTF System Component Arrangement and Discrepancies 21

Figure 13: Supply Airside SCTF Components and Controller EnergyPlus Node Diagram 24

Figure 14: Demand Airside SCTF Components and Controller EnergyPlus Node Diagram 25

Figure 15: Waterside SCTF Components and Controller EnergyPlus Node Diagram 26

Figure 16: Model Verification Method (Neymark and Judkoff, 2002) 27

Figure 17: AirTerminal:DualDuct:VAV:OutdoorAir Topology and Field Inputs 31

Figure 18: Ventilation and Flow Rate Calculation Flowchart 32

Figure 19: Calculation of Control Action Type 33

Figure 20: Recirculated Air flow Rate Calculation 34

Figure 21: Reheat Option Calculation Flow Chart 35

Figure 22: Three Zone Analytical Verification Model 38

Figure 23: Analytical Model Occupancy Profile 39

Figure 24: VAV Single Duct - Design Day System Flowrates 40

Figure 25: SCTF System - Design Day System Flowrates 41

Figure 26: System Outdoor Air Flowrate Comparison 42

Figure 27: BCA Academy and Systems (BCA, 2010) 44

Figure 28: ZEB Second Storey Floor Plan (BCA, 2010) 45

Figure 29: ZEB AHU 2-1 (BCA, 2010) 46

Figure 30: AHU 2-1 VAV Terminal Unit (BCA, 2010) 47

Figure 31: VAV 2-1 Fresh Airflow Functional Testing Results 48

Figure 32: AHU 2-1, VAV 2-1 Example Operation 49

Figure 33: Actual VAV Box Control for ZEB 50

Figure 34: Measured vs Simulated Fresh Air Flowrate for VAV 2-1 on Aug 5, 2010 51

Figure 35: Prototype Compartmented Cooling Coil Control 54

Figure 36: Recirculated Air Stream Bypass Damper (Sekhar et al 2004) 55

Figure 37: Implemented Compartmented Cooling Coil Control 55

Figure 38: Compartmented Cooling Coil Curve Fit Model Information Flow 57

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LIST OF SYMBOLS AND ABBREVIATIONS

IDD Input Data Dictionary for EnergyPlus (.idd file extension)

IDF Input Data File for EnergyPlus (.idf file extension)

SCTF Single Coil, Twin Fan Air Conditioning System

environmental quality and thermal comfort in humid climates while being the most energy

efficient One such innovation, the Single-Coil, Twin-Fan system (SCTF) was developed at the National University of Singapore as a dynamic solution to the need for improved ventilation and thermal comfort control while reducing the overall energy and greenhouse gas impact of the HVAC system

The SCTF air conditioning system was developed at NUS as a new method of arranging a typical system by splitting the air stream supplied to each space within a building into two paths while still using a single, compartmented coil to simultaneously condition both This arrangement can reduce the airflow in either air stream when indoor or outdoor air conditions will allow and this can significantly reduce energy consumption while still maintaining proper comfort conditions and indoor environmental quality through adequate ventilation (Sekhar et al., 2004) Much of the energy savings can be attributed to the variable control of the ventilation air in tropical climates such as Singapore, where removal of humidity is a major issue

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In this thesis, the building simulation environment EnergyPlus was utilized to produce a detailed energy model of a typical office building that utilizes the SCTF system EnergyPlus is a next- generation, modular simulation program designed to model the performance, energy consumption and pollutant production of a building (Crawley et al., 2001) It was developed by the U.S

Department of Energy as a tool for building designers and operators to predict the energy

performance of a building in order to make design or operations decisions It was designed so that individuals could develop new “modules” that describe the energy consumption characteristics of innovative types of HVAC technologies that could be added to the existing EnergyPlus code in order to simulate those systems

1.2 OBJECTIVES OF THE STUDY

The individual objectives of this study were as follows:

 Identify the deficiencies within various building simulation engines with regards to modeling the unique components and configuration of the SCTF system

 Formulate mathematical models of the unique SCTF system components based on

previous research and information

 Implement the modeling strategies into the chosen energy simulation engine

 Analytically validate the implemented terminal unit module using a theoretical three-zone building with the SCTF system

 Compare the actual control of an installed system at the Zero Energy Building (ZEB) to the module in order to identify discrepancies and further development opportunities

1.3 ORGANIZATION OF THESIS

Chapter 2 focuses on a literature review of the topics of whole building energy simulation using EnergyPlus, the SCTF system development and case studies, and popular cooling coil models

used in common energy simulation programs Chapter 3 breaks down the methodology process in

which the SCTF system was analyzed for applicability into EnergyPlus and identifying

modification opportunities in existing modules were identified Chapter 4 goes into detail in the

development of a new terminal unit model within EnergyPlus which models outdoor air and

recirculated air separately Chapter 5 focuses on the validation and simulation results of this air

terminal unit model Chapter 6 gives an overview of the conceptual development of a

compartmented cooling coil model based on empirical data Finally, Chapter 7 aggregates the

conclusions and future recommendations for SCTF simulation in the EnergyPlus environment

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Chapter 2: LITERATURE REVIEW

A review of the literature was completed for this thesis in order to investigate previous efforts to develop and define the physical description of the Single-Coil Twin-Fan system and the

performance models, which can be used to predict its energy consumption The specific features

of the SCTF system and the challenges in terms of modeling were investigated individually and methods of verification were reviewed as well The module development and technical

information for EnergyPlus from the U.S Department of Energy (DOE) was reviewed to

understand how a new mathematical model could be inserted into the existing source code,

compiled, tested, and verified for the successful completion of a whole building energy

simulation In addition, a review of past efforts to model cooling coils in computerized simulation was completed to provide background for a compartmented cooling coil model

2.1 SINGLE COIL TWIN FAN (SCTF) SYSTEM

In order to understand how the Single-Coil Twin-Fan system can be modeled in the EnergyPlus simulation environment, the development of the system was studied through the work of various researchers Sekhar, Tham and Cheong were the first to investigate the unique concept of

decoupling the recirculated and outdoor air fresh air streams and conditioning them separately using a single, compartmented cooling coil (Sekhar et al., 2004) Their initial proof-of-concept study defined the air conditioning and distribution topology, major system components, and the ability to independently control the temperature and humidity of two different airstreams Figure 1

is taken from this study and it shows the topology diagram of the SCTF system as developed for the first prototype The figure also illustrates the psychrometric process of the two air streams as they are conditioned and supplied through the system

Figure 1: SCTF Topology Diagram and Psychrometric Performance (Sekhar et al., 2004)

The study covered the conceptual framework of the system, a detailed analysis of the

air-conditioning and air distribution method, and described a series of seven experiments in which a prototype was used to condition two office space chambers These experiments were designed to demonstrate the system‟s ability to address varying combinations of thermal and ventilation loads

in different zones The results of this study validated the ability of the system‟s configuration to effectively condition and distribute air while maintaining both zones within the acceptable limits

of thermal comfort and ventilation while reducing the cooling coil energy consumption by an

estimated 12% This installation was also the first application of a compartmented cooling coil and the basics of implementation were covered in this initial study Figure 2 was taken from the report and shows a schematic of the unique orientation of the compartmented cooling coil

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Figure 2: Air and Coolant Flow Arrangement of the Compartmented Coil (Sekhar et al., 2004)

Subsequent research projects were undertaken to more specifically analyze the performance of the SCTF system, initially Maheswaran and Sekhar developed a fin efficiency method which is a departure from traditional methodology of assuming constant heat transfer coefficients across the whole of a coil (Maheswaran and Sekhar, 2004) Elimination of these types of assumptions was meant to bring solutions closer to real world conditions and give models the ability to replicate the performance of more complex coils

Next, research was completed which focused on establishing a mathematical model of the

compartmented coil configuration (Maheswaran et al., 2006) The goal of this study was to indentify a method of obtaining the fundamental heat and mass transfer coefficients for the

compartmented coil to be used in the coil sizing and selection process as well as a means of predicting its performance characteristics Additionally, the performance of the compartmented coil was studied through the concept of fin efficiency using a simplified numerical model which was evaluated using a Monte Carlo simulation approach (Maheswaran and Sekhar, 2007)

Throughout the process of each of these investigations, data was collected from the installed

prototype and used in the formulation of the theoretical mathematical model A more detailed aggregation of the data from these studies was presented by Maheswaran in a doctorial thesis (Maheswaran, 2005) A study by Bin and Sekhar outlined detailed, numerical CFD modeling of the compartmented cooling coil (Bin and Sekhar, 2007) This effort was furthered through a study which outlined a three dimensional model for a diffuser specific to the SCTF‟s decoupled outdoor and recirculated air streams (Bin and Sekhar, 2007)

In 2007, a real-world system was installed in an office building at the National University of Singapore A study was completed which gathered operations data from the installed system and concluded that “the SCTF system is able to provide adequate ventilation in a typical large office premises, based on “demand ventilation” and “demand cooling” in the individual occupied zones.” (Sekhar et al., 2007) A review of air conditioning systems in tropical climates was also completed by Sekhar which included the SCTF system as an option for optimal control and dehumidification (Sekhar, 2007) In addition, a review of one of the industry‟s most prominent ventilation standards, ASHRAE Standard 62.1 was completed in order to put into context the usual ventilation requirements that the SCTF system must uphold (ASHRAE/ANSI, 2007) The Singapore Standard 553: Code of Practice for Air-Conditioning and Mechanical Ventilation in Buildings was also covered to understand the local

requirements (SPRING, 2009)

2.2 AIRSIDE SYSTEM MODELING

HVAC systems performance prediction has been the focus for a significant amount of research throughout the years and especially since computing and information technology has become ubiquitous and relatively cheap Most of the early simulation programs incorporated some form or airside systems modeling An overview of the approaches used to model such systems over time

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was completed by Wright and it includes explanations for the two main categories of component simulation: Empirical and First Principle (Wright, 2010)

The purpose of the study of the SCTF system was to create the ability to simulate the performance

of the system in a detailed whole building energy simulation program EnergyPlus (Crawley et al., 2001) was selected as a suitable platform for this purpose based on its developer-friendly modular structure and flexible approach to modeling central air systems An outline of the decision to use EnergyPlus can be found in Chapter 3 EnergyPlus is an integrated simulation environment in which the zone, system, and plant are solved simultaneously using fundamental heat balance

principles The EnergyPlus source code is essentially a collection of hundreds of different

FORTRAN modules that contain the algorithms necessary to simulate many different complex heating, cooling, ventilation, water, and lighting systems as well as various auxiliary energy

consuming end uses The modules interact with each other during a simulation in a hierarchical fashion that is governed by the integrated solution manager Figure 3 was taken from the

EnergyPlus User Manual and it shows the relationship between the major simulation driver

routines and the different system module categories (EnergyPlus, 2009)

Figure 3: EnergyPlus Program Schematic (EnergyPlus, 2009)

At the system level, the airside can be simulated using the HVAC Air loop Module (Fisher et al., 1999) The Air Loop is a system-level object that describes the individual components that make

up the airside conditioning and distribution system, establishes the relationships each component has with each other, and divides them into the supply and demand section categories Figure 4,

which was taken from the EnergyPlus Engineering Reference, illustrates how the building

systems are simulated on a system level The program solves for the zone air temperature by

prediction of the value at the beginning of the simulation based on previous runs, performs a

simulation of each system type on both the demand and supply sides, and then corrects the value based on the simulated system response This process is repeated until the difference between

predicted and corrected zone conditions has met a certain convergence value The airloop

arrangement for VAV systems was reviewed as part of this study as many of its modeling

considerations can be applied to the SCTF system (Yasutomo et al., 2003)

Figure 4: Connections between the Main HVAC Simulation Loops and Sub-Loops (EnergyPlus,

2009)

The SCTF system is primarily unique in the way it conditions the air in a centralized manner

using a compartmented cooling coil and how it regulates two decoupled air streams in a dual duct

Zone equipment is simulated in EnergyPlus within the Air Loop orientation through the use of interchangeable modular terminal units that are sized and operated based on the zone information Figure 5, taken from the EnergyPlus User Manual, shows the interchangeable options available within the Air Loop concept

Figure 5: EnergyPlus Zone Equipment (EnergyPlus, 2009)

At the zone level, the existing air distribution system and air terminal units within EnergyPlus were reviewed to determine functionality and potential modification – especially the variable air volume dual duct unit (EnergyPlus, 2009) The current AirTerminal:DualDuct:VAV object simulates a hot and cold deck, variable volume supply air system by alternating the flow of each air stream according to the needs of the zone thermostat The

AirTerminal:SingleDuct:ConstantVolume:Reheat object models the performance of a single duct terminal unit with reheat Figure 6, taken from the EnergyPlus User Manual, shows the diagram

of the AirTerminal:SingleDuct:VAV:Reheat object currently in EnergyPlus

Figure 6: Schematic of AirTerminal:SingleDuct:VAV:Reheat Unit (EnergyPlus 2009)

Module development for the EnergyPlus source code is outlined in the EnergyPlus Programming Standard which outlines basics in Fortran development, EnergyPlus naming conventions, program variables, module structure and interaction, and code documentation (EnergyPlus, 2009) The EnergyPlus level subroutine calling tree was analyzed and used in the development and

debugging of the modified source code (EnergyPlus, 2009) Multiple EnergyPlus development processes for research purposes were also reviewed including a reports by Stadler outlining the implementation of onsite electricity generation (Stadler et al., 2006), Ihm on development of a thermal energy storage model (Ihm et al., 2004), and Sailor regarding creation of a green roof model (Sailor, 2008) An especially applicable implementation study of a VRV systems modeling

in EnergyPlus was reviewed due to the similarities of the objectives with this study in terms of an innovative system type being simulated (Zhou et al., 2008)

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Documentation pertaining to existing EnergyPlus calculation algorithms and source code module structure can be found in Appendix A The information analyzed and structured in this appendix was used in the necessary modification analysis to determine what additions would be made in the project processes

2.3 COMPUTERIZED COOLING COIL MODELS

The process for development of options to simulate the performance of a compartmented cooling coil required a literature review of the existing conventional cooling coil performance models that have been implemented into computerized code throughout the last 40 years

The first category of these types of models is a based on the Log Mean Temperature Difference (LMTD) solution (Elmahdy and Mitalas, 1977) This method predicts the performance of the coil and leaving air and water conditions for all three possible coil conditions: “all wet”, “all dry”, and

“partially wet-partially dry” Further studies went into more detail regarding the confirmation of these coil condition phenomenon (Elmahdy and Biggs, 1979) Elmahdy and Biggs also

investigated the efficiency of extended surfaces or fins on a heat exchanger and the methods in which this phenomenon could be mathematically modeled (Elmahdy and Biggs, 1983) The computerized versions of these algorithms were adapted for the MODSIM/HVACSIM+ program (Clark, 1985) and the detailed simulation model for EnergyPlus

The second heat exchanger modeling type which was implemented in a forward model is based

on the NTU/effectiveness relationship of heat and mass transfer (Kays and London, 1964) This type of coil model calculates the number of transfer units (NTU) that are a function of the heat transfer coefficient of the coil and the minimum stream capacity The NTU value is then used to calculate the effectiveness of the coil as a heat exchanger that can then be used to determine the coil leaving conditions The Kays and London model was used as a basis for cooling coils in the

HVAC2 Toolkit: Algorithms and Subroutines for Secondary HVAC Systems Energy Calculations (Brandemeuhl, 1993) and TRNSYS Simulation Program (TRNSYS, 1990) which in turn provided components of the EnergyPlus simple model An overview of the calculation procedure of the HVAC2 Toolkit cooling coil model can be found in Appendix B An improved version of the HVAC2 Toolkit model was created for the purposes of implementation in EnergyPlus for the simple cooling coil modeling option (Chillar and Liesen, 2004) This detailed calculation flow was studied to determine applicability in a forward model of the compartmented cooling coil

More advanced dynamic forward model versions were also developed in order to simulate coils in advanced feedback controller, control strategies, and diagnostic methods (Zhou and Braun, 2007) These models can simulate the transient behavior of cooling and dehumidifying coils over time and are usually more computationally intensive

In order to model a cooling coil in EnergyPlus, there are two different options: simple and

detailed geometry methods (EnergyPlus, 2009) The simple method uses the least number of user inputs to approximate the leaving air and water temperatures and energy consumption and is a simulation program-optimized combination of the algorithms from the Elmahdy/Mitalas and Kays/London models The detailed model uses inputs such as area, number of tubes, fin spacing and other geometry-based inputs and was based on the Elmahdy and Mitalas algorithm

Another standard type of model is the curve fit algorithm that uses empirical manufacturer or experimental data to create a data-driven (inverse) model This type is common in many

manufacturer coil selection and energy simulation programs A curve fit model developed for the water to air heat pump for EnergyPlus was analyzed as a potential strategy for approximating the performance of the compartmented coil (Tang, 2003)

Figure 7 shown shows the historical relationships between the reviewed computerized cooling coil models including the ones found in EnergyPlus

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Figure 7: Historical Development of Common Computerized Cooling Coil Model

Chapter 3: RESEARCH METHODOLOGY

The research methodology process for this project is centered on the formulation of mathematical models to simulate the operation of the SCTF system in the real world After a thorough

investigation of the previous efforts in simulation of airside and waterside systems in other

computerized models, a simulation approach was developed and implemented in the EnergyPlus source code This implementation was then empirically validated by comparing the predicted performance of the SCTF system to that of collected data from an installed system at the Building and Construction Authority‟s Zero Energy Building (ZEB) in Singapore

3.1 SIMULATION APPROACH

Through a review of the conventional cooling coil model literature and the existing research completed on the SCTF system a set of possible project direction scenarios was formulated and analyzed to determine which overall strategy to take for this project

The first scenario outlines a study focused on the airside development of the SCTF system in EnergyPlus The entire solution would be built in EnergyPlus and decoupling of the outdoor air and recirculated air would be developed from the outdoor air system to the terminal unit A new forward model dual duct unit would be developed and any deficiencies in the outdoor air system would be investigated and modified for the SCTF system orientation This approach would modify EnergyPlus to have the ability to simulate any centralized dual duct DOAS system and not just the SCTF system In this scenario, the compartmented cooling coil would be approximated using two coil models and a conceptual development plan would be presented for future research and implementation into EnergyPlus

The second scenario focused on development of a forward physical or data-driven inverse model

of the compartmented cooling coil, implementation of this model in the more modular simulation

The last scenario included was an implementation of the forward physical model developed in previous research of the compartmented cooling coil (Maheswaran, 2005) The main objective of this approach would be to create a computerized version of the detailed fundamental model presented in the research with row-by-row sequential control volume analysis across both

compartments accounting for the variable heat transfer coefficients and fin efficiencies that have

to be considered on such a complex coil This model would be computationally intensive and the first application would be most appropriate in a FORTRAN or MATLAB modeling environment Less complex versions of this fundamental model could then be fine-tuned for whole building energy simulation analysis programs like EnergyPlus Appendix C contains the basis of

development for this computerized mathematical model

The decision was made early in the process to pursue the first scenario The main focus of the project was to enhance EnergyPlus for the benefit of the thousands of real world users who utilize this free modeling engine for design and operations The second scenario was not considered optimal because it didn‟t address this concern The third scenario was rejected because while the mathematical model developed in previous research is deemed appropriate for advanced coil simulation and selection, it is not suitable for hourly energy simulation programs due to a long computation time and its need for inputs and coil characteristics not available within the

EnergyPlus environment

Figure 8 demonstrates the three possible scenarios considered and details regarding types of tasks

to be accomplished

Figure 8: SCTF Simulation Scope Scenarios

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3.2 MODEL IMPLEMENTATION

Through investigations during the literature review process and analysis between the SCTF system and a conventional dual duct VAV system, three key objectives were identified with regards to EnergyPlus modification and addition:

1 Modeling of the decoupled outdoor air and recirculated air streams in a centralized air distribution system

2 Zone-by-zone flow control of the two airstreams based on individual ventilation and thermal load requirements

3 Performance prediction of the compartmented cooling coil

These discrepancies between existing and proposed simulation capabilities are elaborated upon in this section A detailed overview of the SCTF modeling approach was outlined in a conference paper by the author (Miller and Sekhar, 2010)

Decoupling of Outdoor and Recirculated Air Streams

Currently, in order to simulate a ventilation air system in EnergyPlus, the model is defined as shown in Figure 9 in which an “Outdoor Air Mixer” is an integral component of the outdoor air system, and in a centralized air distribution system, the supply air must be distributed to the individual zones in a mixed air condition Setting the OA Mixer at 100% ventilation air can simulate traditional single duct DOAS systems and then a zone-conditioning unit must be added

in each zone to compensate for the additional conditioning load

Figure 9: EnergyPlus Ventilation System Diagram (EnergyPlus, 2009)

In the proposed SCTF model, the ventilation and relief air streams are not mixed until the air loop equipment and this feature is a fundamental deviation from existing air loop simulation

methodology This difference can be seen below in Figure 10

Figure 10: Proposed SCTF Ventilation System

Zone-by-Zone Flow Control at the Terminal Unit

With respect to the air loop configuration and terminal unit design, it was identified that the SCTF system most closely resembles the component arrangement of the centralized, dual duct VAV system

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Figure 11: EnergyPlus VAV Dual Duct Air System Component Arrangement

Figure 11 illustrates that an existing mixed air dual duct system is comprised of three major

categories of components: the outdoor air system, cooling and heating coils and fans, and the air distribution system The outdoor air system is designed to regulate the system-wide ventilation air requirements and mix the appropriate amount of recirculated return air The mixed air is then

conditioned in either the hot or cold deck and supplied via mixing boxes at the zone level that regulate based on the zone air temperature setpoint

The SCTF system conditions and distributes the air without mixing the ventilation and

recirculated air and due to its main application so far in humid, tropical climates, it only cools each air stream Figure 12 shows the orientation of the SCTF system and key discrepancies from a conventional mixed air system

Figure 12: SCTF System Component Arrangement and Discrepancies

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The primary difference between the SCTF system and a conventional system is that the

recirculated air is not mixed with the outdoor air before being conditioned and supplied to each zone In this way, the system is able to decouple the ventilation load from the thermal

requirements and control it based on the occupancy of the space or a set ventilation requirement The return airstream is divided into relief air and recirculated air at the outdoor air system and then conditioned and supplied based on the thermal load requirements of the space The airflow in each stream is controlled by its own variable speed fan, which allows decoupled control of the system resulting in energy savings An advantage of this system orientation is better control when

a space has a high ventilation load requirement but a low thermal load requirement, as in the case

of a fully occupied conference room on a mild day The outdoor air stream is kept at full load capacity to accommodate for the high ventilation requirement while the recirculated air stream can be kept to a minimum or even off if there is little cooling requirement In a mixed air system, when the ventilation requirement is the dominating load it is not possible to regulate the airflow based on the thermal load and overcooling can occur or reheat may need to be installed to

maintain thermal comfort

Performance Prediction of the Compartmented Cooling Coil

Another key difference of the SCTF system is the compartmented cooling coil This coil

conditions both air streams with a single continuous water coil arrangement The unique nature of the compartmented cooling coil poses several challenges with regards to simulation as compared

to a conventional cooling coil Previous research has identified several of these differences

(Maheswaran et al., 2006):

 Different off coil conditions are delivered by the coil and different heat transfer rates exist for the two different air streams

 The geometry and characteristics such as fin spacing, water tube lengths, materials, face area, etc across the two compartments can vary according to the load requirements

 The coil is to be controlled using the off-coil conditions of one of the air streams which results in float in the off-coil conditions of the other air stream

These issues were the main motivation for a series of experiments conducted on a set of

compartmented cooling coils in previous research in order to determine the fundamental heat and mass transfer coefficients and boundary conditions with the goal of formulating a mathematical model (Maheswaran et al., 2006) The resultant fundamental forward model that was developed was based on variable heat transfer coefficient calculations as a function of the coil fin surface temperature None of the existing cooling coil models within EnergyPlus have the ability to simulate the unique considerations outlined above or the leaving air and water conditions of either compartment In the scope chosen, the compartmented cooling coil was approximated through the use of two separate cooling coils Conceptual development of future cooling coil models is

outlined in Chapter 6

As outlined in Scenario #1, the main objectives of the approach were to develop the new dual duct air terminal unit (AirTerminal:DualDuct:VAV:OutdoorAir) and test the outdoor air system to determine if the approach of splitting the return air before the relief air input of the outdoor air system would result in balanced and stable airflow rates throughout the EnergyPlus air loop Demand and supply airside diagrams of this arrangement are shown in Figure 13 and Figure 14

It should be noted that the two air-side nodes that symbolize the passing of air to and from

outdoor air conditions are shown as the Outdoor Air Inlet Node bring ventilation air into the building and the Relief Air Outlet Node which is exhausting a portion of the recirculated air The waterside diagram in Figure 15 illustrates the separate cooling coils and the control and supply of chilled water on the waterside

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Figure 13: Supply Airside SCTF Components and Controller EnergyPlus Node Diagram

Figure 14: Demand Airside SCTF Components and Controller EnergyPlus Node Diagram

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Figure 15: Waterside SCTF Components and Controller EnergyPlus Node Diagram

3.3 MODEL VALIDATION

After the development of the mathematical solution of a new air terminal unit and the

implementation and debugging of the source code for the

AirTerminal:DualDuct:VAV:OutdoorAir object, options for validation and verification of the

modifications were completed A framework for building model validation is outlined in Figure

16 taken from 2009 ASHRAE Fundamentals Handbook (ASHRAE, 2009) The framework was part of a review done for the International Energy Agency Building Energy Simulation Test and diagnostic method for heating, ventilating, and air-conditioning equipment models -HVAC BESTEST (Neymark and Judkoff, 2002)

Figure 16: Model Verification Method (Neymark and Judkoff, 2002)

According to earlier work by Judkoff, a more detailed explanation of the types of accuracy

evaluation is as follows (Judkoff, 1983):

 Empirical Validation – in which calculated results from a program, subroutine, or

algorithm are compared to monitored data from a real building, test cell, or laboratory experiment

 Analytical Validation – in which outputs from a program, subroutine or algorithm are compare to results from a known analytical solution or a generally accepted numerical method for isolated heat transfer mechanisms under very simple and highly defined boundary conditions

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 Comparative Testing – in which a program is compared to itself or to other programs that may be considered better validated or more detailed and, presumably, more physically correct

For the purposes of this study, it was decided to first pursue the Analytical Validation from the standpoint that the decoupling of the outdoor and recirculated air could be calculated using inputs from the simulation and performed using a less complex simulation method such as a spreadsheet

A three zone analytical model was used to complete this process with a comparative simulation of both the SCTF system and a conventional system The three-zone model was based on the

conventional building model used by the EnergyPlus development team for demonstration of new technologies within the simulation engine

An empirical validation was completed using measured data from an installed SCTF system This was not the most optimal empirical validation scenario in that many aspects of the system

performance can‟t be controlled and quality of the data may be questionable due to the normal calibration and installation issues that occur outside of a controlled laboratory These aspects were taken into consideration during the analysis of the results and it is suggested that future work focus on validation according to data measured in a more controlled environment – especially when it comes to validation of a compartmented cooling coil model Data from the month of August 2010 was collected for this purpose as previous months had multiple calibration issues and system configuration problems that were addressed during this project The data from the week of August 1-7 was focused on as the quality of the sensor data for that time period was deemed the best The data was collected from the installed BMS system that was accumulating data points at 1-3 minute intervals since the building was completed in 2009 This data was collected after a series of commissioning activities were undertaken; an overview of the

commissioning process completed to increase the sensor data value is outlined in Chapter 5.

Chapter 4: DEVELOPMENT OF ENERGYPLUS AIR TERMINAL UNIT MODEL

In response to the tasks outlined in Chapter 3, a series of improvements to the HVAC Air Loop simulation within EnergyPlus was completed which allow for the capability to simulate the SCTF system The feature covered in this Chapter is with respect to modeling the decoupled outdoor air and recirculated air streams that are applicable to any type of centralized dedicated outdoor air systems (DOAS)

Currently within the Air Loop system in EnergyPlus, outdoor air for ventilation is provided solely through the AirLoopHVAC:OutdoorAirSystem object, which is a subsystem on the supply side This object describes the components and controllers that precondition and modulate the OA based on either a minimum flow rate for the system, which is derived from design conditions, or a dynamic flow rate based on occupancy The Outdoor Air system currently requires that an

OutdoorAir:Mixer component be present within the arrangement This object mixes the mandated amount of ventilation air with recirculated air to meet a mixed air temperature setpoint Details of the existing EnergyPlus Outdoor Air system model can be found in Appendix B

The needs of the SCTF system require that the two air streams remain unmixed and supplied through a dual duct arrangement This arrangement was constructed using the existing EnergyPlus air loop components and branch structure by inserting a return air splitter in the recirculated air stream before the outdoor air system This return branch was split off into the recirculated air stream while the remainder was used as an input into the outdoor air system The outdoor air system control was overridden to provide 100% outdoor air into the OA stream and exhaust all of the return air that is passed to it This orientation was tested for errors in node agreements and, while none were found, the system simulation wasn‟t stable or accurate using a conventional Dual Duct VAV Terminal unit due to the existing model‟s expectations of a hot and cold air stream inputs

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The determined solution for this control issue was to develop a new type of air terminal unit that was the key component in the demand-side of the Air Loop and an alternative to the existing ventilation control in the supply-side Outdoor Air System object This unit was designed to set the airflow setpoints of each airstream according to the individual requirements of each zone A summation of the zone-by-zone airflow rates for each stream was then used to set the flow for the supply-side primary air system by following these control arrangements:

 Outdoor Air (OA) Stream Flowrate Control - The terminal unit was designed to set the airflow of the of the OA stream at the zone level based on the zonal ventilation

requirements which are defined and calculated within the module The summation of these zonal flow rates controls the outdoor air stream on the supply side of the Air Loop

 Recirculated Air (RA) Stream Flowrate Control - The RA stream controller within the terminal unit sets the flowrate of recirculated cooling air stream in order to meet the zone temperature setpoint

 Terminal Reheat Control - If the thermostat calls for heating when the RA stream is fully closed then the Reheat water/steam/electric coil is activated until the zone air setpoint is met The terminal unit is modeled without a reheat coil by leaving the associated fields blank The reheat control component was planned as part of the new terminal unit design but was not focused on in this study due to its lack of use in tropical climates

The new dual duct air terminal unit was designed to first size the system based on the maximum combined airflow of the two streams using zone thermal load calculations and ventilation

requirements defined at the zone level The user then specifies the outdoor air control action type and calculation method These choices stipulate how the terminal unit sets the outdoor airflow at each timestep

4.1 CALCULATION METHODOLOGY

The EnergyPlus object included a number of inputs related to the topology and calculation

requirements for the system Figure 17 shows the node arrangements of the object and the

EnergyPlus input fields needed for simulation

Figure 17: AirTerminal:DualDuct:VAV:OutdoorAir Topology and Field Inputs

The calculation procedure for Dual Duct system control is contained within the

“DualDuctHVAC.f90” module of the EnergyPlus source code The existing source code for this module was modified and amended to simulate the unique nature of decoupled ventilation and recirculated air streams The first step in the calculation procedure was to determine the zone level ventilation requirements at design condition and for each time stamp This process is done by the user inputting information regarding whether the zone outdoor air should be calculated based on Flow/Person, Flow/Area, the Sum or Max of those two calculations, or the Zone Exhaust air flow rate Figure 18 illustrates the calculation procedure the terminal unit model completes in order to size the unit‟s maximum total zone airflow and the design ventilation requirement.