EFFICIENCY AND EMISSIONS STUDY OF A RESIDENTIAL MICRO–COGENERATION

SYSTEM BASED ON A STIRLING ENGINE AND FUELLED BY DIESEL AND ETHANOL

by

Nicolas Farra

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

Copyright © 2010 by Nicolas Farra

ii

Abstract

Efficiency and Emissions Study of a Residential Micro–Cogeneration System Based on a

Stirling Engine and Fuelled by Diesel and Ethanol

Nicolas Farra

Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

2010

This study examined the performance of a residential micro–cogeneration system

based on a Stirling engine and fuelled by diesel and ethanol. An extensive number of engine

tests were conducted to ensure highly accurate and reproducible measurement techniques.

Appropriate energy efficiencies were determined by performing an energy balance for each

fuel. Particulate emissions were measured with an isokinetic particulate sampler, while a

flame ionization detector was used to monitor unburned hydrocarbon emissions. Carbon

monoxide, nitric oxide, nitrogen dioxide, carbon dioxide, water, formaldehyde, acetaldehyde

and methane emissions were measured using a Fourier transform infrared spectrometer.

When powered by ethanol, the system had slightly higher thermal efficiency, slightly lower

power efficiency and considerable reductions in emission levels during steady state

operation. To further study engine behaviour, parametric studies on primary engine set

points, including coolant temperature and exhaust temperature, were also conducted.

iii

Dedication

To my mother. This one’s for you.

iv

Acknowledgments

I extend my most sincere thank you to Murray J. Thomson for his guidance and support

through the duration of my research project. I would like to express gratitude to all of my

colleagues in the Combustion Research Laboratory. In particular, Tom Tzanetakis, who

played an instrumental role in this work by providing much needed help in the emission

testing of the Stirling engine, but also for offering his high degree of all–around technical

expertise time and again, along with his friendship. I would also like to thank Amir A.

Aliabadi for his comprehensive and original Master’s thesis study on the Whispergen Stirling

engine. Much appreciation is extended to Eric Schutte of Whisper Tech New Zealand for his

help with the Stirling engine, particularly with understanding its inner workings and

debugging technical issues. Thank you to Sheila Baker for help in acquiring equipment for

the experimental setup. Financial support for this project was provided by NSERC. Lastly,

this work would not have been possible without the loving support of my family; my father,

Antoine Farra, my mother, Angèle Farra and my sister, Natalie. My sister deserves special

recognition for kindly proofreading this thesis, and for always pulling me through the tough

times.

v

Table of Contents

Abstract ................................................................................................................................... ii

Dedication .............................................................................................................................. iii

Acknowledgments ................................................................................................................. iv

Table of Contents ................................................................................................................... v

List of Tables ......................................................................................................................... ix

List of Figures ........................................................................................................................ xi

Nomenclature ...................................................................................................................... xiv

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

1.1 Motivation .................................................................................................................... 1

1.2 Objectives .................................................................................................................... 2

2 Literature Review .............................................................................................................. 4

2.1 Cogeneration ................................................................................................................ 4

2.2 The Stirling Engine ...................................................................................................... 5

2.2.1 Ideal Thermodynamic Cycle ............................................................................ 5

2.2.2 Non–Ideal Behaviour and Relation to Engine Performance ............................ 7

2.2.3 Engine Configurations ..................................................................................... 9

2.2.4 Emissions ....................................................................................................... 11

2.2.5 Commercial Devices and Applications .......................................................... 11

2.3 The Whispergen DC Micro–Combined Heat and Power System .............................. 12

vi

2.4 Ethanol ....................................................................................................................... 15

2.4.1 Production Process ......................................................................................... 16

2.4.2 Fuel Properties ............................................................................................... 17

2.4.3 Emissions ....................................................................................................... 21

2.5 Second Generation Biofuel Pathway ......................................................................... 23

2.5.1 Lignocellulosic Biomass ................................................................................ 24

2.5.2 Lignocellulosic Ethanol Production ............................................................... 24

2.5.3 Future Outlook ............................................................................................... 26

3 Experimental Methodology ............................................................................................ 27

3.1 Engine Installation ..................................................................................................... 27

3.1.1 Development of the Fuel Delivery System .................................................... 31

3.1.2 Data Acquisition Tools .................................................................................. 32

3.1.3 Engine Operating Characteristics .................................................................. 34

3.2 Energy Efficiency ...................................................................................................... 36

3.3 Particulate Matter Collection ..................................................................................... 37

3.3.1 Isokinetic Sampling ....................................................................................... 38

3.3.2 Exhaust Sampling System .............................................................................. 39

3.3.3 Testing Procedure .......................................................................................... 41

3.4 The Flame Ionization Detector .................................................................................. 42

3.4.1 Principle of Operation .................................................................................... 42

3.4.2 The California Analytical HFID Heated Total Hydrocarbon Analyzer ......... 43

3.4.3 Exhaust Sampling System .............................................................................. 44

3.4.4 Testing Procedure .......................................................................................... 46

3.5 The Fourier Transform Infrared Spectrometer .......................................................... 48

vii

3.5.1 Principle of Operation .................................................................................... 48

3.5.2 Quantitative Techniques ................................................................................ 52

3.5.3 The Thermo Scientific Nicolet 380 FTIR Spectrometer ................................ 55

3.5.4 Calibration of the FTIR Spectrometer ........................................................... 57

3.5.4.1 Definition of Species for the Calibration Model ............................. 58

3.5.4.2 Development of the Calibration System .......................................... 59

3.5.4.3 Mixture Preparation and Potential Issues with Species ................... 61

3.5.4.4 Testing Procedure ............................................................................ 64

3.5.4.5 Partial Least Squares Calibration Model ......................................... 65

3.5.4.6 Limitations of Model ....................................................................... 66

3.5.5 Exhaust Sampling System .............................................................................. 68

3.5.5.1 Testing Procedure ............................................................................ 69

4 Results and Discussion .................................................................................................... 70

4.1 Engine Operation ....................................................................................................... 70

4.1.1 Reproducibility .............................................................................................. 71

4.2 Energy Efficiencies .................................................................................................... 72

4.2.1 Reproducibility .............................................................................................. 76

4.3 Particulate Emissions ................................................................................................. 77

4.4 Unburned Hydrocarbon Emissions ............................................................................ 79

4.4.1 Reproducibility .............................................................................................. 81

4.5 Exhaust Species Emissions ........................................................................................ 83

4.5.1 Reproducibility .............................................................................................. 87

4.6 Coolant Temperature Study ....................................................................................... 88

4.7 Exhaust Temperature Study ....................................................................................... 90

viii

5 Conclusions and Recommendations .............................................................................. 94

5.1 Conclusions ................................................................................................................ 94

5.2 Recommendations ...................................................................................................... 96

References ............................................................................................................................. 97

A Diesel Fuel Specification ............................................................................................... 103

B Labview Program .......................................................................................................... 105

C PLS Calibration Model for the FTIR .......................................................................... 108

D Engine Performance with Diesel .................................................................................. 116

E Engine Performance with Ethanol .............................................................................. 121

F Statistical Analysis of Particulate Samples ................................................................. 127

ix

List of Tables

2.1

2.2

2.3

2.4

3.1

3.2

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

A.1

C.1

D.1

Residential Micro–Cogeneration Systems ......................................................................5

Cogeneration Systems Based on Stirling Engines ........................................................12

Specifications of the Whispergen DC Micro–CHP System ..........................................13

Comparison of Diesel and Ethanol Fuel Properties ......................................................20

Operating Stages of the Whispergen System ................................................................36

Summary of the Partial Least Squares Calibration Model ............................................66

Engine Parameters with Associated Standard Deviations .............................................71

Engine Parameters with Associated Standard Deviations for Multiple Tests ...............72

Energy Outputs and LHV Efficiencies ..........................................................................74

Energy Outputs and LHV Efficiencies for Multiple Tests ............................................76

Particulate Sampling Conditions and Relevant Parameters ..........................................77

Particulate Emissions Data ............................................................................................77

CO and NO Emissions ..................................................................................................86

CO and NO Emissions for Multiple Tests.....................................................................87

Energy Outputs and LHV Efficiencies for Coolant Temperature Tests .......................88

Engine Parameters for Exhaust Temperature Tests ......................................................90

Energy Outputs and LHV Efficiencies for Exhaust Temperature Tests .......................91

CO and NO Emissions for Exhaust Temperature Tests ................................................92

Diesel Fuel Specification.............................................................................................104

Standards for the Partial Least Squares Calibration Model ........................................110

HHV Efficiencies for Multiple Tests ..........................................................................120

x

E.1

E.2

E.3

F.1

F.2

HHV Efficiencies for Multiple Tests ..........................................................................126

HHV Efficiencies for Coolant Temperature Tests ......................................................126

HHV Efficiencies for Exhaust Temperature Tests ......................................................126

Statistical Analysis of the Diesel Particulate Sample ..................................................128

Statistical Analysis of the Ethanol Particulate Sample ...............................................128

xi

List of Figures

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

3.10

3.11

3.12

3.13

3.14

3.15

The Ideal Stirling Cycle ..................................................................................................6

Hypothetical Stirling Engine ..........................................................................................7

Non–Ideal Pressure–Volume Diagrams .........................................................................8

Piston–Cylinder Configurations ...................................................................................10

The Whispergen DC Micro–CHP System ....................................................................13

Schematic of the Whispergen System ..........................................................................14

Burner Assembly ..........................................................................................................14

Generalized Ethanol Production Process ......................................................................17

Experimental Setup of the Whispergen System ...........................................................28

Detailed Schematic of the Whispergen System ............................................................28

Experimental Setup of the Electrical Components .......................................................30

Schematic of the Fuel Delivery System........................................................................32

Thermodynamic Model of a Cogeneration System ......................................................36

Particulate Sampling System ........................................................................................40

Schematic of the Particulate Sampling System ............................................................40

Particulate Sampling Probe...........................................................................................41

The CAI Model 600 HFID Heated Total Hydrocarbon Analyzer ................................44

Schematic of the FID Exhaust Sampling System .........................................................45

FID Exhaust Sampling System .....................................................................................46

Testing Procedure for the FID ......................................................................................46

Schematic of the Nicolet 380 FTIR Spectrometer ........................................................48

Michelson Interferometer .............................................................................................50

The Thermo Scientific Nicolet 380 FTIR Spectrometer ..............................................55

xii

3.16

3.17

3.18

3.19

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

B.1

B.2

C.1

C.2

C.3

C.4

C.5

C.6

The Specac Cyclone C2 Gas Cell .................................................................................56

FTIR Calibration System ..............................................................................................60

Schematic of the FTIR Calibration System ..................................................................60

Schematic of the FTIR Exhaust Sampling System .......................................................68

LHV Efficiencies of the System Run on Diesel ...........................................................73

LHV Efficiencies of the System Run on Ethanol .........................................................73

Diesel Total Unburned Hydrocarbon Emissions ..........................................................80

Ethanol Total Unburned Hydrocarbon Emissions ........................................................80

Diesel Total Unburned Hydrocarbon Emissions for Multiple Tests ............................82

Ethanol Total Unburned Hydrocarbon Emissions for Multiple Tests ..........................82

Diesel Exhaust Species Emissions................................................................................83

Ethanol Exhaust Species Emissions .............................................................................84

Coolant Temperature Behaviour...................................................................................89

LHV Thermal Efficiencies for Coolant Temperature Tests .........................................89

Exhaust Temperature Behaviour ..................................................................................91

CO and NO Emissions for Exhaust Temperature Tests ...............................................92

Front Panel of the Labview Program ..........................................................................106

Block Diagram of the Labview Program ....................................................................107

Absorbance Spectrum of a 100 ppm Nitrogen Dioxide Sample over Time ...............109

Normalized Absorbance of Species in the Spectral Range of 1100–4000 cm–1

.........109

TQ Analyst Results for Carbon Dioxide ....................................................................112

TQ Analyst Results for Water ....................................................................................112

TQ Analyst Results for Carbon Monoxide .................................................................113

TQ Analyst Results for Methane ................................................................................113

xiii

C.7

C.8

C.9

C.10

D.1

D.2

D.3

D.4

D.5

D.6

D.7

E.1

E.2

E.3

E.4

E.5

E.6

E.7

E.8

TQ Analyst Results for Nitric Oxide ..........................................................................114

TQ Analyst Results for Nitrogen Dioxide ..................................................................114

TQ Analyst Results for Formaldehyde .......................................................................115

TQ Analyst Results for Acetaldehyde ........................................................................115

Fuel Flow ....................................................................................................................117

Air Flow ......................................................................................................................117

Fuel–Air Equivalence Ratio .......................................................................................118

System Temperatures..................................................................................................118

Power Output ..............................................................................................................119

Thermal Output ...........................................................................................................119

HHV Efficiencies........................................................................................................120

Fuel Flow ....................................................................................................................122

Air Flow ......................................................................................................................122

Fuel–Air Equivalence Ratio .......................................................................................123

System Temperatures..................................................................................................123

Power Output ..............................................................................................................124

Thermal Output ...........................................................................................................124

HHV Efficiencies........................................................................................................125

HHV Thermal Efficiencies for Coolant Temperature Tests .......................................125

xiv

Nomenclature

A Absorbance

a Absorptivity

b Path length

c Concentration

f Frequency

g Acceleration due to gravity

H Elevation

J Momentum flux ratio

hA

Specific enthalpy of air

hF

Specific enthalpy of fuel

hW

Specific enthalpy of waste

mf Mass fraction

mA

Mass flow rate of air

mF

Mass flow rate of fuel

mW

Mass flow rate of waste

Nmax Maximum shaft speed

P Pressure

PC Collection pressure

PE Extraction pressure

QL

Heat loss rate

QR

Heat recovery rate

Qin Heat input during isothermal compression

Qout

Heat output during isothermal expansion

Ah

Fh

Wh

Am

Fm

Wm

LQ

RQ

inQ

outQ

xv

Qregen

Heat transfer to and from regenerator

S Entropy

T Temperature

V Volume

VDC DC voltage

VDC, max Maximum DC voltage

v Velocity

vC Collection velocity

vE Extraction velocity

w Wave number

Wnet

Net power

Greek Symbols

ηenergy

Combustion efficiency

ηenergy

Energy efficiency

ηpower

Power efficiency

ηthermal

Thermal efficiency

ρ Density

σ Standard deviation

Abbreviations and Acronyms

AVG Average

CAI California Analytical Instruments

CHP Combined heat and power

CI

CLS

Compression ignition

Classical least squares

energy

power

thermal

regenQ

netW

combustion

xvi

DAQ Data acquisition

DTGS Deuterated triglycine sulfate

FH

FID

Fume hood

Flame ionization detector

FTIR Fourier transform infrared (spectrometer)

HHV Higher heating value

IC Internal combustion

LHV Lower heating value

MCT Mercury cadmium telluride

NOx Oxides of nitrogen, NO and NO2

PI Performance index

PLS Partial least squares

PM Particulate matter

ppm Parts per million (volume)

RMSE Root mean squared error

SS Steady state

SI Spark ignition

UHC Unburned hydrocarbon

UHP Unique Heated Products

1

Chapter 1

Introduction

1 Introduction

1.1 Motivation

The accumulation of greenhouse gases in the atmosphere is one of the primary factors

known to accelerate global climate change. According to the International Energy Agency, in

2008 81.3% of the world’s energy supply comprised of fossil fuel sources, which contributed

to 99.6% of all carbon dioxide emissions [1]. Thus, immediate action is required to

significantly reduce emissions from their current levels in order to properly address this

global issue. A proposed solution is to alleviate the effects of global climate change by

complementing fossil fuels with renewable forms of energy, such as biomass–derived fuels

known as biofuels. Biofuels are derived from recently dead biological materials and can

potentially have zero net greenhouse gas emissions, as their feedstock consumes carbon

dioxide throughout its life through the process of photosynthesis. Although biofuels have

been used throughout history, fossil fuels have ultimately replaced their use, because of their

relative abundance and high energy content. However, renewed interest in biofuels has been

generated due to an increase in the cost of fossil fuels, issues with long–term supply and fears

of global climate change.

One particular technology that can be used to reduce the effects of global climate

change is cogeneration. Cogeneration systems, which feature the production of more than

one useful form of energy from a single source of fuel, represent only seven percent of

1

2

national electricity production in Canada. This low percentage is attributed to low energy

prices and the regulatory policies associated with the production and sale of surplus

electricity [2]. However, cogeneration systems offer great potential as they can be used in a

wide variety of applications, ranging from use in transportation to stationary applications

typically in the industrial or residential sector. The use of cogeneration results in an increase

in energy efficiency and a decrease in fuel costs. These features make cogeneration an ideal

option for use in residential applications with the added benefit of on–site power

consumption, avoiding transmission losses. Micro–cogeneration systems based on Stirling

engines are particularly well–suited for use in the residential sector as they have very

competitive efficiencies in low power applications (< 20 kW). Since Stirling engines rely on

an external heat source, they can be powered by a variety of fuels. Also, greater control over

the combustion process can be achieved with external combustion, leading to significant

reductions in exhaust emissions when compared to typical electricity generating systems.

Biofuels are useful in external combustion–based devices such as the Stirling engine,

as they can be used to displace fossil fuels in stationary combined heat and power

applications. While conventional biofuel production requires high–quality agricultural land

and results in competition with the food supply, advantages are particularly clear when

considering second generation biofuels derived from non–food–based sources and grown on

agriculturally marginal lands. Lignocellulosic ethanol is one such biofuel that offers great

potential, but its production requires further development and reductions in cost to make it

economically feasible [3]. Alternative production processes can be used to yield more

economical cellulose–based fuels. For instance, pyrolysis oil can be produced by thermally

cracking biomass and rapidly condensing the product vapours and aerosols into a liquid [4].

Although this process generates low energy content biofuels that are not fully distillable and

feature undesirable combustion characteristics such as high water and solids content, low

emission levels could be attained with the particular mode of combustion in a Stirling engine.

1.2 Objectives

The focus of the current research was to compare the performance of a residential

micro–cogeneration system based on a Stirling engine and fuelled by diesel and ethanol. This

3

was accomplished by performing energy analysis, as well as particulate, unburned

hydrocarbon, carbon monoxide, nitric oxide, nitrogen dioxide, formaldehyde, acetaldehyde

and methane emission measurements. This experimental study was conducted on the

Whispergen DC micro–combined heat and power system, originally designed for use with

diesel fuel. Engine operation with ethanol required the development of a new fuel supply

system to compensate for ethanol’s much lower heating value. In addition, the engine was

optimized for use with ethanol through modifications to various engine parameters including

initial fuel flow, maximum fuel flow and glow plug duration. Lastly, parametric studies on

primary engine set points, including coolant temperature and exhaust temperature, were

conducted to further understand their effect on engine behaviour with respect to efficiency

and emissions.

Through the implementation of a highly oxygenated fuel as an alternative fuel for the

Stirling engine, the ultimate objective was to power the engine with more complex biofuels,

such as pyrolysis oil, through the development of a spray–based burner. A preliminary

investigation has been conducted on the combustion behaviour and emissions of pyrolysis oil

by the Combustion Research Group at the University of Toronto [5]. It must be stressed that

operation with ethanol does not directly demonstrate the behaviour that is to be expected

through the use of complex, high molecular weight biofuels. However, some of ethanol’s fuel

properties, which include its high autoignition temperature, low energy content, highly

oxygenated chemical structure and corrosive nature, will help reveal what effect certain

conventional biofuel characteristics have on engine performance and exhaust emissions.

4

Chapter 2

Literature Review

2 Literature Review

2.1 Cogeneration

Cogeneration, or combined heat and power (CHP) technology, is the production of

more than one useful form of energy from a single fuel source. This mode of operation

differs greatly from conventional fossil fuel–based electricity generating systems, as it

involves the utilization of both heat and power. As a result, cogeneration systems exhibit an

increase in the efficiency of energy conversion, which is accompanied with a net reduction in

greenhouse gas emissions and lower costs associated with fuel consumption [6]. Smaller–

scale cogeneration used in the residential sector is referred to as micro–cogeneration. Micro–

cogeneration technology typically incorporates an internal combustion (IC) engine, a Stirling

engine, a fuel cell system or a micro–turbine system. Current commercial micro–

cogeneration systems for single family applications produce 0.5–6.0 kW of power and 1.5–

15.0 kW of heat. The heat is utilized for space and hot water heating, while the power can be

used directly or it can be net metered, meaning that it can be used to supplement the grid’s

electrical supply. Depending on the application, cogeneration systems can be used to meet

electrical demands, thermal demands or both. Table 2.1 lists existing systems along with their

cogeneration technology, nominal power output and nominal thermal output.

4

5

Table 2.1: Residential Micro–Cogeneration Systems [6–8]

Manufacturer Unit Cogeneration Technology Power [kW] Heat [kW]

Senertec Dachs IC engine 5.5 12.5

Freewatt Warm Air IC engine 1.2 3.3

Whisper Tech Whispergen DC Stirling engine 0.8 5.5

Ebara Ballard LIFUEL Fuel cell 1.0 1.5

2.2 The Stirling Engine

Primarily used in CHP applications, Stirling engines are reciprocating external

combustion engines. Unlike IC engines, combustion in a Stirling engine does not occur

within the engine’s cylinders but rather in a chamber adjacent to the engine block itself. As a

result, a wide range of energy sources can be used as the source of thermal energy, including

conventional fossil fuel–based fuels such as gasoline or diesel, and renewable energy sources

such as biomass, solar energy and process heat. Since combustion takes place outside of the

engine, this results in a well–controlled continuous combustion process. Thus, emissions

from Stirling engines can be ten times lower than IC engines with catalytic converters,

making them comparable to modern gas burner technology. Furthermore, the sealed

operating chambers of the engine result in low wear and long maintenance–free operating

periods. However, despite their high costs, renewed development in Stirling engines is in

progress because of their high level of reliability, fuel flexibility, quiet operation and their

ability to achieve high efficiency, low emissions and good performance at partial loads [6, 9].

2.2.1 Ideal Thermodynamic Cycle

The Stirling engine and its corresponding engine cycle, the Stirling cycle, were

invented by Robert Stirling in 1816. The engine operates on a closed cycle, meaning that the

working fluid is enclosed within the engine’s cylinders and is thus completely independent of

the combustion process. Figure 2.1 shows the pressure–volume and temperature–entropy

diagrams of the Stirling cycle, which consists of four reversible gas processes. The working

fluid undergoes isothermal expansion (1–2), constant volume regenerative heat rejection (2–

3), isothermal compression (3–4) and constant volume regenerative heat addition (4–1) [10].

6

The cycle utilizes a regenerator, which is a thermal storage device that absorbs heat from the

working fluid during one part of the cycle and supplies heat to the working fluid during

another part of the cycle. The regenerator is usually a stainless steel or ceramic mesh. For

optimal engine performance, the regenerator is typically designed for maximum heat transfer

and minimum flow loss [11]. [6,7,8] [6,9] [10] [11]

The Stirling cycle requires intricate drive mechanisms and an engine arrangement

consisting of a heater, regenerator and cooler connected in series. A hypothetical engine,

shown in Figure 2.2, is constructed to simplify the engine setup and help illustrate the

underlying thermodynamic processes. The diagram consists of two pistons and a regenerator.

Initially, the working fluid is in the control volume shown on the left, at the maximum cycle

temperature and pressure (State 1). In the isothermal expansion process 1–2, the left piston

moves outward reducing the system pressure, while the external heat source provides the

system with enough heat to keep the temperature constant during expansion.

In the process 2–3, the system undergoes constant volume regenerative heat rejection.

Both pistons move to the right at the same rate to keep the volume constant, while the

working fluid transfers heat to the regenerator, resulting in minimum cycle temperature and

pressure. During the isothermal compression process 3–4, the right piston moves inward

increasing the system pressure, while the working fluid rejects heat to the cooling medium to

keep the temperature constant during compression. In the process 4–1, the system undergoes

constant volume regenerative heat addition. Both pistons move to the left at the same rate,

Figure 2.1: The Ideal Stirling Cycle [10]

S

T

1 2

4 3

regenQ

regenQ

(b) T–S Diagram

1

2 4

3

P

V

inQ

outQ

(a) P–V Diagram

7

while heat is transferred from the regenerator to the working fluid, resulting in maximum

cycle temperature and pressure and thus completing the cycle.

2.2.2 Non–Ideal Behaviour and Relation to Engine Performance

Although the ideal Stirling cycle achieves the Carnot efficiency, inefficient heat

transfer, material limitations and the presence of friction have a parasitic effect on engine

performance. As the operation of a Stirling engine relies primarily on heat transfer, materials

must be chosen with a high thermal conductivity so that high heat transfer rates can be

obtained between the working fluid and various engine components, including the

regenerator, heater and cooler. Since heat transfer rates are finite, Stirling engine

performance falls short in comparison to ideal operation. Another limiting characteristic is

the engine’s susceptibility to fluid friction, as the working fluid is repeatedly shuttled

between the hot space, the regenerator and the cold space. This creates a pressure drop

through each of the aforementioned components, resulting in a net reduction in power output.

The flow friction is particularly significant in the regenerator, typically designed as a high

Figure 2.2: Hypothetical Stirling Engine [10]

1–2

Regenerator

Control Volume

2–3

3–4

4–1

outQ

inQ

8

density mesh for maximum heat transfer rates. In addition, the seals and bearings within the

engine result in working fluid leakage, which further reduces the engine’s power output.

A more realistic representation of the compression and expansion processes is

illustrated in Figure 2.3(a). The dotted lines indicate processes that are adiabatic (no heat

transfer) rather than isothermal (constant temperature). In reality, cylinder walls in variable

volume spaces do not provide a sufficient heat transfer medium, resulting in little heat being

transferred to the working fluid in the compression and expansion spaces. This makes the

process adiabatic rather than isothermal, leading to a reduction in engine power up to 40%.

However, working fluid contained in the unswept volume of the pistons, referred to as dead

volume, is compressed and expanded nearly isothermally. As a result, the actual compression

and expansion processes are somewhere between the original and dotted lines, indicating that

the loss of work is not as great as the purely adiabatic case. Another significant effect

associated with dead volume is that an increase in the percentage of dead volume with

respect to the total engine volume is accompanied with a linear decrease in the engine’s

power output. In typical Stirling engine design, total dead volume can be up to 58% of the

total volume [9]. Dead volume is typically associated with the engine’s internal regenerators,

the engine’s clearance volumes and both the hot–end and cold–end heat exchangers.

In Figure 2.3(b), the importance of an effective regenerator is illustrated by a

pressure–volume diagram. Processes 4–1 and 2–3 assume perfect regeneration, whereas

processes 4–1′ and 2–3′ show the more practical behaviour associated with an imperfect

Figure 2.3: Non–Ideal Pressure–Volume Diagrams [9, 12]

(a) Adiabatic Processes

2′

1

2 4

3

P

V

inQ

n

outQ

4′

(b) Imperfect Regenerator

1

1′

3 3′

2 4

P

V

inQ

outQ

9

regenerator. Additional heat input is required to go from 1′–1, while additional cooling is

required to go from 3′–3. The additional energy input required for an imperfect regenerator

results in a reduction in efficiency and emphasizes the importance of high heat transfer rates

in a Stirling engine [12].

One of the most crucial factors affecting engine performance is the choice of working

fluid. An ideal working fluid has a high thermal conductivity for high heat transfer rates and

a low viscosity for minimal fluid friction. A low heat capacity on a volumetric basis is also

important because it allows a small amount of energy to produce a large change in

temperature for a given volume. The most common working fluids include hydrogen, helium,

air and nitrogen. Hydrogen is the best working fluid as it has the lowest viscosity, the highest

thermal conductivity and a low volumetric heat capacity. However, there are safety issues

associated with its use, as hydrogen’s high flammability and high diffusion rates in metals

make storage and containment difficult and potentially hazardous. Although the viscosity of

helium is twice that of hydrogen, helium is also a very good working fluid due to its inert

nature, high thermal conductivity and lower volumetric heat capacity compared to hydrogen

[13]. However, nitrogen and air are typically used in Stirling engines despite their poor heat

transfer and fluid friction properties due to their availability and low cost [14].

2.2.3 Engine Configurations

Stirling engines can be classified by their piston–cylinder configuration, mode of

operation and drive mechanism. The piston–cylinder configuration refers to how the piston

and cylinders are arranged. This typically involves one of three designs, alpha, beta or

gamma. Alpha–type engines have two pistons in separate cylinders which are connected in

series by a cooler, regenerator and heater. The main drawback is that both pistons have to be

sealed in order to contain the working fluid. Alternatively, beta–type and gamma–type

engines have one piston and one displacer piston: the displacer piston is used to move the

working fluid between the hot space, regenerator and cold space, but does not have to be

directly coupled to the engine’s power piston. Alternatively, it can be connected to the

crankshaft through mechanical linkages. While the beta–type engine has the piston and

displacer in the same cylinder, the gamma–type engine has the piston and displacer in

separate cylinders [11].

10

Figure 2.4: Piston–Cylinder Configurations [14]

Mode of operation typically refers to whether the engine is single–acting or double–

acting. In single–acting engines, the working fluid is in contact with only one side of the

piston. On the other hand, double–acting engines have the working fluid in contact with both

sides of the piston, meaning that these engines have multiple working spaces. In Figure 2.4,

three of the aforementioned piston–cylinder configurations are outlined, as well as a four–

cylinder double–acting alpha–type engine. In this case, the cylinders are interconnected such

that the expansion or hot space of one cylinder is connected to the compression or cold space

of another cylinder by a regenerator and a transfer port. This allows for multiple cylinder

arrangements that are capable of dramatically increasing the engine’s power output.

A final parameter that needs to be considered when designing a Stirling engine is its

inherent drive mechanism, which is responsible for coupling the engine’s pistons and

enabling power production. Kinematic drives based on mechanical linkages are most

commonly used in Stirling engines, including mechanisms that incorporate either a slider

crank, rhombic drive or swash plate. Alternatively, free piston drives can be used to move the

pistons by working fluid pressure variations rather than by mechanical linkages, thereby

reducing the amount of sealing required with the absence of a piston rod [11].

Alpha

Beta

H = Heater

C = Cooler

R = Regenerator

P = Piston

D = Displacer

Four–Cylinder Double–Acting Alpha

Gamma

R

D R

R

R D

H

P P

P

P P P

P

P

C H

C

H

C

H

C

H

C

H

C

H

C

11

2.2.4 Emissions

Since the Stirling engine features external continuous combustion, this allows greater

control over the combustion process and a substantial reduction in emissions when compared

to conventional cogeneration systems based on IC engines. Combustion chamber design is

one tool that is typically used to increase exhaust gas residence times in an effort to reduce

unburned hydrocarbon (UHC) emissions. Also, exhaust gas recirculation can be used to

suppress the formation of nitrogen oxides (NOx) by limiting the maximum temperature to

below 1400°C [6]. For fuels that are not fully distillable, the air used for combustion can be

preheated to achieve higher combustion efficiency. While numerous performance studies

have been conducted on Stirling engine–based cogeneration systems, only a handful have

focused on smaller–scale cogeneration units.

Onovwiona et al. [6] have reported the emission characteristics of a cogeneration unit

developed by SOLO, which features a natural gas burner and produces 2–9 kW of power and

8–24 kW of heat. Emission levels were fairly low with 80–120 mg/m3 of NOx, 40–60 mg/m

3

of carbon monoxide (CO) and trace levels of UHC and soot emissions. An efficiency and

emissions study has also been conducted by Aliabadi et al. [15] on a residential Stirling

engine–based cogeneration system developed by Whisper Tech. The system features a No. 2

diesel burner and produces 0.6–1.1 kW of power and 5.5–7.0 kW of heat. Emissions were

also found to be low with 158 mg/m3 of NOx, 21 mg/m

3 of CO, 0.65 mg/m

3 of soot and a

trace amount of UHC emissions. The engine was also operated using biodiesel, which

resulted in similar emission levels.

2.2.5 Commercial Devices and Applications

A variety of manufacturers have incorporated Stirling engines as a central component

in their cogeneration systems. Table 2.2 describes recently developed systems along with

their drive mechanism, working fluid, fuel type, nominal power output and power efficiency

based on the fuel’s lower heating value. It should also be noted that most of these units are

capable of operating with a range of fuels. In addition, these engines differ greatly when

considering the total number of cylinders used, the mean cycle pressures of the working fluid

and the specific type of drive mechanism used to couple the engine’s pistons. The

12

considerable variation in Stirling engine design results in a wide range of systems with

respect to scale and performance.

Table 2.2: Cogeneration Systems Based on Stirling Engines [6, 8, 16, 17]

Manufacturer Unit Drive

Mechanism

Working

Fluid

Fuel Power

Output

[kW]

Power

Efficiency

[%]

DTE Energy ENX 55 Kinematic Hydrogen Natural gas 55 30

SOLO Stirling 161 Kinematic Helium Natural gas 9 24

Stirling DK SD4E Kinematic Helium Wood chips 35 28

Sunpower EG–1000 Free piston Helium Propane 1 32

Whisper Tech Whispergen DC Kinematic Nitrogen Diesel 0.8 12

2.3 The Whispergen DC Micro–Combined Heat and Power System

Whisper Tech Limited is a New Zealand firm that has developed micro–cogeneration

systems based on Stirling engines for use in small–scale applications. The two main product

lines that are offered include an on–grid system fuelled by natural gas and an off–grid system

that can be powered by either diesel or kerosene. While both systems generate both heat and

power, the on–grid unit is specifically targeted for residential applications and is capable of

exporting any unused electricity back to the grid. On the other hand, the off–grid unit is much

more versatile, as it can be used in residential, marine, on–road and remote applications.

The Combustion Research Group at the University of Toronto has acquired a diesel

fuel–fired Whispergen DC micro–CHP system. The specifications of the system are detailed

in Table 2.3. The system and its schematic are illustrated in Figure 2.5 and Figure 2.6,

respectively. The system consists of a burner, a Stirling engine, an alternator and an

electronics enclosure (controller). The burner is composed of a series of shells designed to

transfer heat from the exhaust gases to the incoming air. This heat transfer, along with

radiation from the flame, preheats the air to approximately 500°C. The burner features

continuous premixed combustion with a single swirl evaporator, meaning that the fuel is not

injected as a spray but rather it enters an evaporator where it is heated, vapourized and then

mixed with air prior to combustion (Figure 2.7). Ignition is achieved using a glow plug, an

13

element used to preheat the burner during the cold start of the engine. A refractory ceramic

part is placed within the burner to achieve high radiant heat transfer, as well as to provide

insulation. The engine has a four–cylinder alpha–type double–acting configuration with

nitrogen pressurized to 2.8 MPa as the working fluid. The pistons are made of alloy steel and

are sealed using PTFE lip seals backed with O–rings. The engine’s hot–end heat exchangers

are made of high–temperature stainless steel for corrosion resistance, while the cold–end heat

exchangers are made of copper for high heat transfer rates. Also, the engine’s internal

regenerators consist of a stainless steel mesh [8]. [6,8,16,17] [18]

Table 2.3: Specifications of the Whispergen DC Micro–CHP System [8]

Feature Specification

Prime mover Four–cylinder Stirling engine pressurized to 2.8 MPa with nitrogen

Engine configuration Double–acting alpha–type with kinematic wobble yoke mechanism

Heat output 5.5 kW (nominal)

Power output 0.8 kW (nominal)

Fuel No. 2 diesel

Fuel consumption 1 L/hour (maximum)

Exhaust temperature 80°C (nominal)

Size 450 (W) × 500 (D) × 650 (H) mm3

Figure 2.5: The Whispergen DC Micro–CHP System

14

Figure 2.6: Schematic of the Whispergen System

(a) Burner Schematic [18]

Fuel

Air

Air Mixture

Swirl Generator

Glow Plug

Evaporator

Burner Shell

(b) Schematic of Fuel/Air Mixing

(c) Evaporator

Figure 2.7: Burner Assembly

Burner Shell

Assembly

Evaporator

Glow Plug

Air

Fuel

Air

Burner

Engine

Block

Alternator,

Rectifier

Exhaust

Heat

Exchanger

Shell and Tube

Heat Exchanger

Water Flow

Inverter

Battery

Controller

Computer

Exhaust Flow

Coolant Flow

15

The cylinders are repeatedly heated and cooled to produce expansion and contraction

of the working fluid, resulting in piston motion. Preliminary testing of this engine has

revealed a shaft speed of 1200–1500 rpm [14]. A wobble yoke mechanism uses the linear

motion to rotate the alternator, thus producing AC electricity. This mechanism was chosen

since it produces very low piston side loads, incorporates pre–lubricated single degree of

freedom bearings and is easy to manufacture. The AC electricity produced by the alternator

is converted to DC through the use of a rectifier [8, 19]. Next, the electrical output is stored

in a 12 V DC deep cycle battery, which can then be converted to 120 V of AC power using

an inverter to power auxiliary devices. A shell and tube heat exchanger is used to load the

thermal output, as heat is removed from the system by running cold water through the shell.

The engine coolant passes through the tubes in counterflow after it extracts heat from both

the engine block and the exhaust heat exchanger. The engine is equipped with an exhaust

oxygen sensor, responsible for maintaining a fixed fuel–air equivalence ratio (0.55–0.60),

and an exhaust temperature sensor, which maintains the exhaust temperature set point

(480°C). The engine is also equipped with additional sensors, outputs of which are logged by

the engine’s software. [8,19]

2.4 Ethanol

Ethanol is a colourless and flammable alcohol with a chemical formula of C2H5OH. It

is typically produced by fermenting either starch from corn or sugar from sugarcane. Ethanol

is used in the manufacturing of pharmaceuticals and cosmetics, as well as in the production

of alcoholic beverages. Although ethanol is used in a wide variety of industrial applications,

it has a number of properties that make it suitable for use as a biofuel. Advantages include its

volatility and its renewable, less polluting nature compared to diesel fuel. Concerns over

ethanol production are due to its relatively high cost, competition with the food supply,

differing studies on its net energy gain and the use of high–quality agricultural land.

However, a distinction is made between sugarcane–based ethanol and corn–based ethanol, as

sugarcane–based ethanol has substantially higher net energy yields per hectare and much

lower greenhouse gas emissions associated with production [20]. Alternatively, ethanol

production can involve the use of biochemical processes on biomass to produce

16

lignocellulosic ethanol. The main benefits of this fuel include its derivation from non–food–

based sources and that it can be grown on agriculturally marginal lands. Despite these

benefits, production must be made economically feasible through further development in

various micro–biological processes [3].

The chemical structure of ethanol has a polar fraction due to its hydroxyl radical and

a non–polar fraction due to its carbon chain. This makes ethanol miscible in both polar and

non–polar substances, resulting in its use in a variety of applications [21]. Various studies

have been conducted on the utilization of ethanol in both spark ignition (SI) and compression

ignition (CI) engines. Ethanol is particularly well–suited for use in SI engines due to its

resistance to knock, which allows for greater compression ratios and increased power

production. It is typically used as an additive to enhance the octane number of the fuel. It has

also been used in flexible–fuel vehicles, either directly as a fuel or blended with gasoline.

ASTM D4806 and ASTM D5798 are standard specifications developed by the American

Society for Testing and Materials outlining the use of ethanol as a fuel in SI engines [22, 23].

Ethanol has also been blended with diesel for use in CI engines, primarily due to its highly

oxygenated nature which has the potential of reducing particulate emissions. [22,23]

2.4.1 Production Process

Ethanol can be produced from a variety of crops, including corn, wheat, sugarcane,

sugar beets and potatoes. The feedstock must have a large fraction of sugar–based

components. Figure 2.8 reveals the series of stages required for ethanol production,

specifically pre–treatment, hydrolysis, fermentation, distillation and purification [24].

Commercial production of ethanol involves either wet milling or dry milling. It should be

stressed that the differences in wet milling and dry milling are largely based on the initial

processing of the feedstock and that the actual process of converting the sugar–based

components into ethanol is quite similar. Wet milling involves the separation of the grain

kernel into its component parts, which include germ, fibre, protein and starch. The starch

component is used to produce ethanol, while other fractions are used to produce byproducts,

such as corn oil and corn gluten meal. Alternatively, the dry milling process involves

grinding the grain kernel into flour [25].

17

The pre–treatment stage is responsible for converting the feedstock into a suitable

form. Following the pre–treatment stage associated with dry milling or wet milling, process

water is added to the mixture. The mixture is then heated and the starch in the mixture is

hydrolyzed to glucose and other sugars by various enzymes, including alpha–amylase and

glucoamylase [26]. This process is referred to as enzymatic hydrolysis, as enzymes are used

to convert the starch into simple sugars. This is followed by fermentation, an anaerobic

process that uses microorganisms, such as yeast, to convert the glucose (C6H12O6) and other

sugars to ethanol and carbon dioxide [27]. Equation 2.1 represents this chemical reaction.

2526126 CO2OHHC2OHC

><

(2.1)

Distillation is subsequently used to remove a substantial amount of water from the

mixture, followed by purification to obtain the desired grade of ethanol. A dehydrating agent,

such as calcium oxide, is typically used to yield pure ethanol, while a denaturing agent is

added to the ethanol to render it undrinkable. For the dry milling production process, grain

and soluble fractions that remain after the distillation stage can be used to produce

agriculturally useful byproducts, such as dried distillers grains with solubles [25]. As well,

some of the carbon dioxide is also captured for use in carbonated drinks and other

applications. Various studies that have reported negative net energy gains have ignored the

energy yields of numerous byproducts associated with ethanol production [28].

2.4.2 Fuel Properties

Ethanol has been used in both SI and CI engines, blended with either gasoline or

diesel fuel. Since this study examines the performance of a residential micro–cogeneration

system powered by ethanol and diesel, the fuel properties of ethanol will be compared to that

of diesel. The specific fuels used in this study are No. 2 diesel fuel and pure ethanol. Fuel

Figure 2.8: Generalized Ethanol Production Process [24]

Ethanol Crop

Pre–treatment

Hydrolysis

Fermentation

Distillation

Purification

Water Water

18

certification tests have been conducted on the No. 2 diesel fuel by the Alberta Research

Council. The tests have been established by the American Society for Testing and Materials

and include the determination of chemical structure (D5291), density (D4052), heat of

combustion (D4809), kinematic viscosity (D445) and the distillation curve (D86). A

summary of the test results are presented in Appendix A.

The fuel properties expected to have a substantial effect on engine performance

include energy content, ignition characteristics, heat of vapourization, volatility and chemical

structure. The differences in these fuel properties and their potential effects will be discussed

below. In addition, properties that affect safety and storage will also be addressed. These

include viscosity, hygroscopicity, corrosiveness, flammability limits, flash point and density.

Most of the diesel fuel properties discussed below are specific properties of the particular

blend of diesel used in this study, whereas others are reported general properties. ASTM

D975 is the standard specification that is used for No. 2 diesel fuels [29]. General fuel

properties are also used for ethanol, as the fuel in question is produced using a conventional

process which yields pure or 100% ethanol. Table 2.4 is provided at the end of this section to

summarize the most significant differences in the fuel properties of diesel and ethanol.

The energy content of a fuel has a substantial effect on both the engine’s power

output and its fuel consumption. It is typically quantified using the heat of combustion, a

measure of the heat release associated with the combustion of a fuel. If the water formed

exists as a gas, the heat of combustion is termed lower heating value (LHV), but if the water

formed exists as a liquid, the heat of combustion is termed higher heating value (HHV). In

this study, the lower and higher heating values for No. 2 diesel fuel are approximately 42.8

MJ/kg and 45.6 MJ/kg, respectively. The comparative values for ethanol are much lower at

26.9 MJ/kg and 29.7 MJ/kg, respectively [30]. The lower heat of combustion of ethanol

results in a substantial increase in fuel consumption. For instance, to convert an engine

powered by diesel to ethanol while maintaining the same amount of power output, the fuel

consumption must be increased by a factor of 1.5–1.6 on a mass basis.

Ignition characteristics of a fuel are critical for the development of a stable flame. A

method of assessing ignition quality is by using the Cetane number, a measure of a fuel’s

ignition delay in CI engines. A higher Cetane number indicates a shorter time period from the

injection of fuel to the start of ignition. No. 2 diesel fuels have Cetane numbers ranging from

40–50, while ethanol has a Cetane number in the range of 5–15 [31]. This suggests that diesel

19

fuel has a shorter ignition delay period when compared to ethanol. An alternative method of

quantifying ignition behaviour is to consider the fuel’s autoignition temperature, which is the

lowest temperature at which a fuel will spontaneously ignite. The autoignition temperatures

of diesel and ethanol are 204–260°C and 365–425°C, respectively [32]. Fuels with high

Cetane numbers have low autoignition temperatures, signifying that diesel has better ignition

characteristics, as previously mentioned. Another property that can affect ignition quality is a

fuel’s heat of vapourization, the amount of heat required to vapourize a fuel. Diesel fuel has a

heat of vapourization in the range of 225–600 kJ/kg, while the heat of vapourization of

ethanol is approximately 837 kJ/kg [32]. It is well–established that ethanol’s high

autoignition temperature and heat of vapourization pose difficulties in achieving flame

stability during the cold start of an engine.

Volatility, the tendency of a fuel to vapourize, is a critical property with regard to

combustion behaviour. The boiling point of a substance is typically used to characterize fully

distillable fuels, whereas a distillation curve is generated for fuels that are not fully

distillable. The distillation characteristics of the No. 2 diesel fuel used in this particular study

are shown in Appendix A. Diesel fuels that meet ASTM D975 have a 90% distillation

fraction between 282 and 338°C, while ethanol, a fully distillable fuel, has a boiling point

between 78 and 79°C [29, 32]. Fuel volatility is critical in the operation of systems based on

premixed combustion, as these systems rely on the complete evaporation of fuel. [29,32]

Depicted in Table 2.4, the chemical composition of the fuels also has a significant

effect on combustion. The high oxygen content associated with ethanol, 35% on a mass

basis, can result in more complete combustion, which is generally accompanied with a

substantial reduction in particulate emissions and potential reductions in exhaust species

emissions. However, ethanol use in conventional engines has previously been shown to cause

increases in certain exhaust emissions, particularly aldehydes [33, 34]. This occurs as the C–

C bond in ethanol is weaker than the C–OH bond, resulting in a series of chemical reactions

that lead to acetaldehyde formation [35]. The high oxygen content is also responsible for the

relatively low energy content of ethanol. In addition, the lack of aromatic hydrocarbons in

ethanol can lead to a further reduction in particulate emissions. [33,34] [35]

Kinematic viscosity is a measure of a fluid’s resistance to flow. It is an important

parameter, particularly in CI engines, as it can severely impact the operation of filters, pumps

20

and injectors. According to ASTM D445 at a temperature of 40°C, No. 2 diesel fuels have

kinematic viscosities in the range of 1.9–4.1 mm2/s, while ethanol has a kinematic viscosity

of 1.1 mm2/s [31, 36]. The addition of ethanol to diesel is beneficial for CI engines, as it

lowers fuel viscosity. However, this is not a significant issue in the operation of engines

based on external combustion. [31,36]

A noteworthy problem associated with ethanol is its hygroscopic nature, its ability to

attract and absorb water. The absorption of water decreases the fuel’s heating value and

increases its autoignition temperature, leading to increased fuel consumption and difficulties

with ignition, respectively. The presence of water is also responsible for accelerating

corrosion, which can have a particularly damaging effect on fuel injection systems. For this

reason, corrosion inhibitors are typically used as an additive to prevent damage to certain

metals and elastomeric components found in fuel injection systems [31].

Properties that affect safety and storage should be of primary concern in the

evaluation of fuels. The flammability limits of a fuel are the minimum and maximum

concentrations of combustible vapour in air, capable of propagating a flame with sufficient

ignition energy. The flash point is the lowest temperature at which a liquid has sufficient

vapour pressure to produce a flammable mixture in the air above the liquid. The flammability

limits of diesel and ethanol on a volumetric basis are 0.6–5.6% and 3.3–19%, respectively.

Ethanol has a flash point of 13°C, while No. 2 diesel fuels typically have flash points in the

range of 64–75°C [31, 32]. However, there is typically even greater variability associated

with the flash point of diesel fuel. Ethanol’s much larger flammability limits and significantly

lower flash point demand more stringent storage requirements when compared to diesel fuel.

Fortunately, ethanol is biodegradable, rendering it harmless after disposal. Furthermore, a

fuel’s density could also affect energy density and fuel consumption. However, the densities

of diesel and ethanol are quite comparable at 0.83 kg/L and 0.79 kg/L, respectively [32].

Table 2.4: Comparison of Diesel and Ethanol Fuel Properties [29, 30, 32]

Fuel LHV

[MJ/kg]

HHV

[MJ/kg]

Autoignition

Temperature

[°C]

Distillation/Boiling

Temperature

[°C]

C–H–O

Composition

[Mass %]

Diesel 42.8 45.6 204–260 282–338 86–14–0

Ethanol 26.9 29.7 365–425 78–79 52–13–35

21

The fuel properties discussed above were deemed to be the most significant properties

in the comparison of diesel and ethanol. However, there are additional properties that are

particularly relevant to diesel use, which include pour point, cloud point, ash content and

sulphur content.

2.4.3 Emissions

Numerous studies have been conducted on the utilization of ethanol in a variety of

combustion applications, with a particular emphasis on emission reductions. Ethanol

typically is blended with either diesel for use in CI engines or with gasoline for use in SI

engines. Additives are often used in ethanol blends to enhance solubility and increase shelf

life. The use of ethanol is generally accompanied with reductions in both particulate and CO

emissions. However, effects on UHC and NOx emissions can vary greatly depending on the

mode of combustion, the type of engine and the specific operating conditions.

Rakopoulos et al. [37] experimentally studied the engine performance and exhaust

emissions of a six–cylinder direct ignition Mercedes–Benz mini–bus diesel engine using

either 5% or 10% blends of ethanol with diesel fuel on a volumetric basis. The engine was

operated under a variety of conditions, with changes in both engine load and speed as part of

a parametric study. Powered by the ethanol–diesel blends, there was a slight increase in both

brake specific fuel consumption and brake thermal efficiency when compared to the diesel

fuel case. The study also showed that the blend containing 10% ethanol reduced particulate

emissions by 35–68% and CO emissions by 6–15%. This reduction was attributed to the

oxygen content of ethanol, which assists the combustion process in locally rich zones.

Interestingly, there was a notable increase in UHC emissions, approximately 12–30%. It was

suggested that ethanol’s higher heat of vapourization caused slower evaporation, poorer fuel–

air mixing and increased flame penetration, which led to flame quenching. Additionally, NOx

emissions were very similar between the ethanol–diesel blends and the standard diesel fuel.

Huang et al. [38] have previously tested a single–cylinder direct ignition diesel

engine with various ethanol–diesel blends, up to 30% ethanol on a volumetric basis. When

compared to standard diesel fuel, operation with the ethanol–diesel blends resulted in an

increase in UHC emissions and a decrease in smoke emissions. While these results were

similar to the experimental study mentioned above, CO emissions varied greatly with engine

22

load. The ethanol–diesel blends had higher CO emissions at high engine loads, but lower CO

emissions at low engine loads. Furthermore, NOx emissions between fuels varied with

operating conditions but were generally comparable.

It is evident that the performance and emission characteristics of CI engines fuelled

by ethanol–diesel blends are strongly influenced by an engine’s operating conditions.

Unfortunately, it has been observed that the use of ethanol in CI engines is associated with

increases in certain exhaust emissions, particularly UHC emissions. This is partly attributed

to ethanol’s poor ignition characteristics, confirmed by both its low Cetane number and high

autoignition temperature. As a result, ignition improvers are typically added to ethanol–diesel

blends in an effort to reduce ignition delay and cyclic variability, resulting in a notable

decrease in exhaust emissions [37].

There has been a substantial amount of research into the performance and emissions

behaviour of SI engines fuelled by ethanol–gasoline blends. One of the primary benefits

associated with ethanol include its higher octane number relative to gasoline. This provides

greater resistance to knock and allows for higher compression ratios, thus improving engine

performance. Yoon et al. [39] studied the performance and emissions of a four–cylinder SI

engine powered by pure ethanol and an ethanol–gasoline blend consisting of 85% ethanol

and 15% gasoline on a volumetric basis. The utilization of pure ethanol rather than gasoline

reduced CO and NOx emissions by 35% and 25%, respectively. This was also accompanied

with a substantial reduction in UHC emissions. It was suggested that ethanol’s high oxygen

content resulted in more complete combustion, thus reducing exhaust emissions. The

reduction in NOx emissions was specifically attributed to ethanol’s high oxygen content and

heat of vapourization, leading to oxygen enrichment and a lean effect in the mixture, causing

a decrease in flame temperature.

Costa et al. [21] conducted a performance and emissions study of a four–cylinder SI

engine fuelled by hydrous ethanol (mass content of 6.8% water) and a ethanol–gasoline blend

containing 78% gasoline and 22% ethanol on a volume basis. The study demonstrated that

hydrous ethanol reduced both CO and UHC emissions by a considerable margin relative to

the ethanol–gasoline blend. However, this was accompanied with a substantial increase in

NOx emissions, attributed to ethanol’s faster flame speed. Combined with the more advanced

ignition timing, this resulted in higher peak pressure and temperature. Lastly, the specific fuel

23

consumption of hydrous ethanol was approximately 54% higher than that of the ethanol–

gasoline blend, due to ethanol’s much lower heating value.

Certainly, there are benefits associated with the utilization of ethanol in IC engines.

However, this experimental study will examine the performance of a residential micro–

cogeneration system based on external premixed combustion. As such, emissions behaviour

will not be directly comparable to that of CI and SI engines. However, ethanol’s fuel

properties pose similar problems in external combustion engines. A primary issue is the cold

start of an engine due to ethanol’s high autoignition temperature and heat of vapourization.

This leads to difficulties in achieving a stable flame and a substantial amount of exhaust

emissions during start–up. Although ethanol’s fuel properties have a similar effect on an

external combustion engine, the far greater control over the combustion process allows for

greater stability and a marked decrease in exhaust gas emissions.

2.5 Second Generation Biofuel Pathway

First generation biofuels derived from corn, sugarcane and other food–based crops are

readily available and are viewed as an intermediate step to reduce greenhouse gas emissions

[40]. However, these traditional biofuels have a number of severe disadvantages related to

their feedstock. Since current costs are much higher than conventional fossil fuel–based

fuels, biofuels require substantial subsidies to make them competitive and viable options.

Higher costs are associated with the low net energy yield of typical annual crops, the energy–

intensive nature of crop production and the use of high–quality agricultural land. The

agricultural land used to harvest the feedstock results in low productivity and high fertilizer

usage, limiting the potential reductions in greenhouse gas emissions that are expected

through biofuel use [3]. In addition, there is a great deal of concern with first generation

biofuels, as their production is based on the allocation of food–based crops. This is a

controversial issue as it leads to competition with the food supply and creates excess demand

for food–based crops, further increasing their costs.

Second generation biofuels are derived from lignocellulosic biomass feedstock which

includes agricultural and wood residues, herbaceous energy crops, short–rotation coppice and

organic waste. Compared to traditional biofuels, these biofuels can be grown on less valuable

24

agricultural land and can offer economic benefits, as well as higher net energy yields per

hectare. Perennial crops and grasses are examples of feedstocks that offer higher net energy

yields in comparison to traditional biofuels. Overall, biofuels based on lignocellulosic

biomass are most attractive due to their derivation from non–food–based sources, their higher

overall energy conversion efficiencies compared to traditional biofuels and their ability to be

grown on agriculturally marginal lands in a variety of conditions [3, 40]. [3,40]

2.5.1 Lignocellulosic Biomass

Lignocellulosic biomass consists of carbohydrate polymers (cellulose and

hemicellulose), lignin and a relatively small fraction of acids, salts, minerals and extractives.

Cellulose is the principal component, typically around 40–60% of the dry biomass. It is a

linear polysaccharide polymer composed of very long chains (up to approximately ten

thousand) of glucose monosaccharide units. The orientation of the linkages and the hydrogen

bonding associated with the multiple hydroxyl groups on each glucose unit result in a very

rigid cross–linked material, which is very difficult to hydrolyze. The hemicellulose fraction,

which makes up approximately 20–40% of the dry biomass, contains shorter chains of highly

branched sugars, including galactose, glucose, mannose, xylose and arabinose, as well as a

small amount of non–sugar–based acetyl groups. Compared to cellulose, hemicellulose is

relatively easy to hydrolyze due to its amorphous, branched nature and its solubility in alkali.

The third and final main component of lignocellulosic biomass is lignin (10–25%), a

complex polymer of phenylpropane and methoxy groups, responsible for connecting all cells

within the biomass and adding a great degree of strength to the polymer. Since lignin is not a

sugar–based component, it is a residue associated with ethanol production [24, 27].

2.5.2 Lignocellulosic Ethanol Production

The generalized production process for ethanol was presented in Section 2.4.1.

Compared to the conventional process of ethanol production, lignocellulosic ethanol

production has a higher cost and a greater degree of complexity associated with most stages.

For lignocellulosic ethanol production, pre–treatment refers to the actions that are taken to

convert the biomass into a more suitable form that is more accessible for further biological or

chemical treatment. Depending on the type of feedstock, a particular set of processes are used

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in the pre–treatment stage to maximize yield and minimize cost in subsequent stages. In

general, the steps taken in this stage include washing and chipping of the feedstock, removal

of the lignin fraction and hydrolysis of the hemicellulose fraction. The hemicellulose

hydrolysis is classified as a pre–treatment procedure, while the hydrolysis of the cellulose

fraction occurs in the hydrolysis stage. The pre–treatment stage incorporates a variety of

chemical and physical treatments, including dilute acid hydrolysis and steam explosion.

Dilute acid hydrolysis uses sulphuric, hydrochloric or nitric acid to hydrolyze the

hemicellulose fraction, while steam explosion uses high pressure steam to make the biomass

more accessible to enzymes in the hydrolysis stage [24].

Hydrolysis without a pre–treatment stage results in yields below 20%, whereas

hydrolysis following pre–treatment is associated with yields exceeding 90%. The hydrolysis

stage is responsible for hydrolyzing the cellulose fraction by enzymatic hydrolysis, dilute

acid hydrolysis or concentrated acid hydrolysis. Enzymatic hydrolysis uses a complex

mixture of enzymes, typically referred to as the cellulase enzyme, while acid hydrolysis uses

acids similar to those used in the hemicellulose hydrolysis process. The use of enzymatic

hydrolysis rather than acid hydrolysis is considered one way to reduce costs associated with

ethanol production in the future. Following hydrolysis, acids that were formed in the pre–

treatment and hydrolysis stages remain in the mixture. Since acids inhibit the fermentation

process, some of these acids are recovered and recycled while those that remain are

neutralized with the addition of lime. This is followed immediately by a fermentation process

that uses a variety of microorganisms, including yeast, bacteria or fungi, to convert six

carbon atom sugars termed hexoses (galactose, glucose and mannose) and five carbon atom

pentose sugars (xylose and arabinose) into ethanol via a series of reactors [24]. However, the

fermentation of pentoses is problematic and requires the development of new enzymes to

improve yield [41]. Finally, conventional distillation and purification processes are employed

to yield the desired grade of ethanol. The lignin that remains as a residue is used to produce

electricity, heat and byproducts, such as high octane hydrocarbon fuel additives [24].

While lignocellulosic ethanol offers great potential, a major concern is its inherent

cost. The high cost is primarily associated with the numerous pre–treatment processes, the

use of a variety of acids and enzymes, the low conversion efficiencies and the sheer number

of reactors required for ethanol production. It is evident that there is a need for more efficient

pre–treatment technology and new microorganisms that yield higher conversion efficiencies

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in the hydrolysis and fermentation stages. As well, a crucial step for reducing costs

associated with lignocellulosic ethanol production is the integration of several conversions in

fewer reactors. Different levels of process integration are possible, such as a combination of

the hydrolysis and fermentation stages, or a co–fermentation process for both pentoses and

hexoses. In addition, further development in micro–biological processes is required to make

production economically feasible [3].

2.5.3 Future Outlook

Recent trends indicate a move from first generation biofuels derived from food–based

crops to second generation biofuels derived from lignocellulosic biomass feedstock. This is

partly attributed to additional funding that is being supplied for lignocellulosic ethanol

projects by government agencies, such as the United States Department of Energy [42].

According to a recent study by Sandia National Laboratories and General Motors

Corporation, researchers have found that 90 billion gallons of ethanol could be produced

annually by 2030, replacing a third of expected gasoline usage. The study showed that 75

billion gallons would be produced from lignocellulosic biomass feedstock, while 15 billion

gallons would be generated from food–based sources[43]. A variety of companies have

invested resources for lignocellulosic ethanol production with the intention of developing

commercial–scale plants. Demonstration plants have been developed by several companies,

including Ontario–based Iogen, the China Resources Alcoholic Corporation based in China

and the US–based Verenium. Many more plants are currently in development, including the

construction of a demonstration plant in Denmark by Biogasol [42]. Recently, BP has

acquired Verenium’s cellulosic biofuels business with the intent of moving towards

commercialization [44].

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Chapter 3

Experimental Methodology

3 Experimental Methodology

3.1 Engine Installation

The experimental setup of the Whispergen DC micro–combined heat and power

(micro–CHP) system is displayed in Figure 3.1. This installation incorporates a burner, a

Stirling engine, an alternator, an electronics enclosure (controller), a shell and tube heat

exchanger and other components associated with the Whispergen system. The basic

operation of these components was presented previously in Section 2.3, along with a

relatively simple schematic of the experimental setup in Figure 2.6. Figure 3.2 illustrates a

much more detailed schematic of the system to provide additional information on the primary

cooling system (coolant flow), the se