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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