ENERGY, EXERGY AND ECONOMIC ANALYSIS OF A MICRO-CCHP SYSTEM by Ganesh Vinayak Doiphode A thesis submitted to the Department of Mechanical and Civil Engineering of Florida Institute of Technology in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering Melbourne, Florida May, 2019
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ENERGY, EXERGY AND ECONOMIC ANALYSIS OF A MICRO-CCHP SYSTEM
by
Ganesh Vinayak Doiphode
A thesis submitted to the Department of Mechanical and Civil Engineering of Florida Institute of Technology
in partial fulfillment of the requirements for the degree of
Master of Science in
Mechanical Engineering
Melbourne, Florida May, 2019
We the undersigned committee hereby approve the attached thesis, βENERGY, EXERGY AND ECONOMIC ANALYSIS OF A MICRO-CCHP SYSTEM,β by
Ganesh Vinayak Doiphode.
_________________________________________________ Hamidreza Najafi, Ph.D. Assistant Professor Mechanical Engineering
_________________________________________________ Troy Nguyen, Ph.D., P.E., ESEP Associate Professor Civil Engineering
_________________________________________________ Gerald J. Micklow, Ph.D., P.E. Professor Mechanical Engineering
_________________________________________________ Ashok Pandit, Ph.D., P.E., F.ASCE Professor and Head Department of Mechanical and Civil Engineering
iii
Abstract
Title: ENERGY, EXERGY AND ECONOMIC ANALYSIS OF A MICRO-CCHP
SYSTEM
Author: Ganesh Vinayak Doiphode
Advisor: Hamidreza Najafi, Ph. D.
Combined cooling, heating and power generation (CCHP) systems can be
utilized for commercial or multi-family residential buildings as efficient and reliable
means to satisfy building power requirements and thermal loads. In the present study,
a CCHP system consist of a Bryton cycle, an Organic Rankine cycle (ORC) and an
absorption Ammonia-water cycle is considered. A detailed model is developed via
MATLAB to assess the performance of the considered cycle from energy, exergy
and economic perspectives. Appropriate ranges for inputs are considered and the first
law efficiency, second law efficiency and ECOP of the cycle are determined as
77.17%, 33.18% and 0.31 respectively for the given inputs. Exergy destruction rates
are found to be greatest primarily in the generator and the absorber of refrigeration
cycle followed by the combustion chamber. The total exergy destruction rate in the
system is found as 5311.51 kW. The exergoeconomic analysis is performed using
iv
SPECO approach to evaluate cost flow rate equations of the complete system and its
individual components. Summation of capital investment cost rates and cost rates
associated with the exergy destruction for the whole system is found as $18.245 per
hour. Energy based cost of useful products is $2.14 per kW-h. A parametric study is
also performed to provide an understanding on the effect of total pressure ratio and
turbine inlet temperature of ORC on the performance of the system. A multi-
objective optimization using Genetic Algorithm is performed to maximize plant
energy efficiency and minimize the total cost flow rate of the whole system. A pareto
front of all possible optimized operating points is obtained. A suitable operating point
can be chosen making a trade-off between the two objective functions.
v
Contents
Contents .................................................................................................................... v
List of Figures ........................................................................................................ vii List of Tables ........................................................................................................ viii List of Symbols & abbreviations ........................................................................... ix
Acknowledgement ................................................................................................ xiii Chapter 1 Introduction ............................................................................................ 1
Chapter 2 CCHP System ....................................................................................... 10 1. CCHP Cycle Diagram and Description .............................................................. 10
a. Brayton Cycle ...................................................................................................... 10 b. Organic Rankine Cycle ....................................................................................... 12 c. Ammonia-Water Absorption cycle .................................................................... 12
2. Assumptions Made in Study ................................................................................ 13
Chapter 3 Thermodynamic Analysis .................................................................... 16 1. Mass and Energy Balances .................................................................................. 16 2. Exergy Definitions and Balances ......................................................................... 17
a. Physical & Chemical Exergy Destruction Rates for Each Component of Cycle ....................................................................................................................... 18
3. 2nd Law Efficiency ................................................................................................. 22 4. Properties of Working Fluids .............................................................................. 22 5. Validation of Working Model .............................................................................. 23
a. Identification of Exergy Streams ....................................................................... 27 b. Defining Fuel (F) and Product (P) For Each Component ............................... 27 c. Cost equations ..................................................................................................... 27
2. Energy specific costing ......................................................................................... 30
a. Exergoeconomic results ...................................................................................... 44 b. Energy Specific Results ...................................................................................... 46
πΏπΏπ»π»πΏπΏ Lower heating value (kJ/kg)
LMTD Log mean temperature difference
πππ»π»3 Ammonia
ORC Organic Rankine cycle
P1 Pump in ORC
P2 Pump in absorption cycle
ππππππ Refrigeration
R Recuperator
T1 Turbine in BC
T2 Turbine in ORC
V Throttle valve
VG Vapor generator
xiii
Acknowledgement
Thesis journey was a mix of intellect, fun, frustration and hard work but all that
made research super interesting. First and foremost, I would like to thank my thesis
advisor, Dr. Hamidreza Najafi at Florida Institute of Technology. He was there to
help me at every hurdle that I faced in research and writing. He steered me in the
right direction whenever I was deviating into the lost. He encouraged me not just to
do the research but to publish the research even before I graduate. He always insisted
me to attend conference, meet industry people and interact with them. Upon Dr.
Najafiβs recommendation, getting involved in ASHRAE activities and Brevard
Public Schoolβs energy auditing program proved really stimulating. I learnt a great
value from his interpersonal skills. I would also like to thank the Mechanical and
Civil Engineering department to provide me some financial support to complete my
thesis.
Finally, I must express my profound gratitude to my parents and my sisters for
believing in me. Me being in a different country, they constantly provided me
unfailing support and encouragement. I would also like to thank my girlfriend for
being there during mental breakdowns. Lastly, I would like to thank my late best
friend who always knew I have something in me.
1
Chapter 1 Introduction
Combined cooling, heating and power generation (CCHP) systems, owing to
their desirable characteristics, have been attracting a lot of attentions over the last
several years. Micro CCHP systems in particular are becoming more popular in
commercial and even residential sectors rapidly as they offer a reliable source of
energy to the building managers and end users. Micro CCHP systems improve
reliability in the event of natural disasters when long power outage episodes are
likely. In the state of Florida, where hurricane menace is expected, the power from
grid may be unavailable for days. In such cases, many critical facilities like old age
housing facilities, hospitals may find it very difficult to operate and may lose lives.
Presence of an in-house power generation system along with cooling and heating
capabilities can make the system self-reliant of such needs from the grid and can
sustain emergency power situations.
Many researchers studied various configuration of CCHP systems with the
ultimate goal of maximizing the performance and minimizing the cost of the system.
Different approaches have been also employed such as intercooling, reheating
2
turbine inlet air cooling and more in order to achieve a cost-effective design for the
system.
Ebaid and Al-hamdan [1] showed supplementary reheating decreases the
combined cycle efficiency, on the other hand supplementary heating significantly
increases the steam turbine cycle efficiency. Also, gas turbine pre-cooling improves
the gas turbine performance, but it has a less significant effect on the combined cycle
efficiency and the combined specific work output. Javanshir et al. [2] compared the
effect of working fluid properties on the combined Brayton/ORC cycle. They
considered twelve different working fluids and conclude that working fluids with
higher specific heat capacity produce higher net power output in a subcritical region.
Also, their economic analysis showed combined cycle requires significantly lower
total capital investment and levelized cost of electricity (LCOE) compared to the
regenerative Brayton cycle. Najjar and Abubaker [3] optimized thermo-economic
performance of cascade waster heat recovery system. They showed when the total
cost rate is minimized to 1.715 US$/s, net power and thermal efficiency decreased to
27,135 kW and 28.34% respectively.
A thorough assessment of a thermodynamic cycle requires both energy and
exergy analysis. While first law efficiency can provide an understanding of the
current performance of the system, the second law efficiency sheds light on the
irreversibilities and possible improvement opportunities. Several studies have been
conducted on the exergy and energy analysis of different types of CCHP systems.
3
Tuma et al. [4] formulated and discussed the equations for overall energy and
exergy efficiencies of a combined gas steam cycle plant with cogeneration. Bilgen
[5] developed an algorithm to carry out thermodynamic first and second law analyses
and engineering evaluation based on the levelized cost methodology and payback
period for the gas turbine cogeneration system. Huang et al. [6] formulated in detail
exergy balance equations for all the components of the combined system of STIG
cogeneration and forward-feed triple-effect evaporation process and showed that
exergy destruction is significant in the combustion chamber and maximum exergy
loss takes place in the stack. Authors concluded that with the vapor recompression,
system thermal efficiency of a combined system of STIG cogeneration and forward-
feed triple-effect evaporation process, is better than a system without vapor
recompression for a given steam injection ratio and feedstock mass flow rate. Pak et
al. [7] evaluated exergy flows of three different cogeneration systems to improve
power generation efficiency and concluded that increase in turbine inlet temperature
reduces exergy destruction in combustion chamber, also incorporating regenerator
reduces exergy destruction in waste heat boiler. Additionally, for a low heat demand,
highest exergetic power generation efficiency is achieved when dual-fluid cycle is
incorporated.
Some researchers incorporated the economic aspects of the CCHP cycles in their
analysis. Bejan et al. [8] showed that a true representative economic analysis must
be based on exergy and not energy. This is because energy analysis by itself does not
4
provide any information about usefulness of the energy transfers. Xia et al. [9]
performed thermo-economic analysis and optimization of CCP system consisting of
CO2 Brayton cycle (BC), an ORC and an ejector refrigeration cycle that produced
both power and refrigeration simultaneously to recover energy from engine waste
heat. Their Exergoeconomic analysis showed, increasing the BC turbine inlet
temperature, the ORC turbine inlet pressure and the ejector primary flow pressure,
lower average cost per unit of exergy product for combined system can be obtained.
However, increase in compressor pressure and compressor inlet temperature,
increases the average cost per unit of exergy product for combined system.
Guarinello Jr. et al. [10] performed exergy based thermoeconomic analysis to
determine the production cost of electricity and steam in STIG cogeneration system
located in industrial district Cabo (Pernambuco, Brazil) to provide thermal and
electrical needs. Thermodynamic exergy analysis performed on combined cycle
power plants in [11], [12], [13], [14], concluded that more than 80% of exergy
destruction takes place in combustion chamber and heat recovery steam generator.
Vandani et al. [15] performed comprehensive exergetic, economic and
environmental analysis for a combined cycle power plant that used natural gas and
diesel as fuels to show former fuel has better performance in terms of environmental
impact, contaminants, total annual cost of plant and exergy efficiency. Mohtaram et
al. [16] optimized exergy and thermal efficiencies using genetic algorithm and
performed parametric analyses of a combined absorption refrigeration and Rankine
5
cycle with ammonia-water as working fluid. Yang and Yeh [17] evaluated thermo-
economic performance optimization of an Organic Rankine Cycle utilizing exhaust
gas of a diesel engine, they showed raising turbine inlet temperature results in higher
optimal thermodynamic and economic performances. Also, compared optimal
economic conditions for refrigerants R245fa, R600, R600a, and R1234ze; and
concluded that R245fa performs most satisfactorily. Zhang et al. [18] presented novel
CHP system coupling biomass partial gasification and ground source heat pump
along with gas and steam turbine power generation. Authors studied exergetic and
exergoeconomic performance of the system and also performed parametric study on
several variables. Calise et al. [19] presented exergetic and exergoeconomic analyses
of solar-geothermal poly-generation system that provides electrical, thermal, cooling
energy, and producing fresh desalinized water from multi-effect distillation unit from
sea water. Xu et al. [20] compared two absorption-compression refrigeration cycles
with novel evaporator-subcooler and conventional evaporator-condenser, based on
energy, exergy, economic and environmental perspectives.
The previous studies showed the effectiveness of CCHP systems. One of the most
promising configurations for CCHP system consists of a main gas turbine cycle
followed by is using an organic Rankine cycle and an absorption refrigeration cycle.
An energetic analysis is performed on such a cycle by by Amin et al. [21]. Authors
found the energy efficiency of the plant to be around 77%. While the high energy
efficiency of the system makes it a promising configuration, no exergetic or
6
economic analysis have been conducted on the cycle to date. Furthermore, a
comprehensive optimization of the cycle is necessary in order to obtain the optimal
parameters for the cycle.
In the present study, a thorough energetic, exergetic and economic analysis is
performed on a micro CCHP system consist of a Brayton cycle, an Organic Rankine
cycle and an absorption-refrigeration cycle. The system is capable of generating
power, cooling effect and hot water and therefore is a good candidate to be used for
commercial and large residential buildings. The previous works on this particular
cycle have been focused on the energy aspect, however, the cost has not been
considered as a determining factor. Given the fact that the cost of the system can
make a significant impact on decision making process regarding implementation of
the system, in this thesis, an exergoeconomic study is performedto provide a clear
picture of the performance of the system to the designer or the end user. The results
of the exergetic analysis allows identifying irreversibilities and potential
improvement opportunities through the system. Exergoconomic analysis is used to
estimate the cost of each component of the system and also the cost of operation of
the whole cycle.
While maximizing the output and efficiency of the system is of great importance,
the system parameters must be set to minimize the cost simultaneously. Thus, an
optimization must be performed to achieve optimal system parameters. In past
decade, many researchers have studied co-generation systems and optimized it using
7
various optimization algorithms. Kavvadias and Maroulis [22] optimized a tri-
generation plant for economical, energetic and environmental performance using
multi-objective evolutionary algorithm. Wang et al. [23] constructed and maximized
a weighted objective function measuring energetic, economic and environmental
benefits of building cooling heating and power system using particle swarm
optimization algorithm. Ghaebi et al. [24] optimized tri-generation system for the
cost of total system product and found that objective system modification by 15%
after optimization. Hu and Cho [25] developed a probability constrained stochastic
multi-objective optimization model to optimize CCHP operation strategy for five
different cities namely Columbus, Minneapolis, San Francisco, Boston and Miami.
Wang et al. [26] optimized biomass BCHP system with thermal storage unit and
hybrid cooling system to minimize annual total cost using GA and combined it to the
case study in Harbin, China.
Najafi et al. [27] modelled solid oxide fuel cell- gas turbine hybrid system with
a multi-stage desalination unit and performed multi-objective optimization to
maximize exergy efficiency and minimize total cost rate of the system using genetic
algorithm. Authors found the optimal solution that led to exergy efficiency of 46.7%
and total cost of 3.76 USD/yr. Boyaghchi and Heidarnejad [28] performed single and
multi-objective optimization of a micro solar CCHP for summer and winter seasons
with objective functions being thermal efficiency, exergy efficiency and total product
cost rate. Authors found optimal results for summer as 28%, 27% and 17% for the
8
objective functions respectively, and in winter as 4%, 13%, and 4%. Many researcher
lately reviewed multi-objective optimization methods in tri, poly-generation CCHP
systems in power plant as well as in buildings applications.
Given the fact that any effort to increase the energy efficiency of the system will
result in higher total cost of the system, optimization of the system with respect to a
single objective will not provide a clear perspective regarding the optimal
performance of the system. Therefore, a multi-objective optimization has to be
performed. In this study, the CCHP system under consideration is optimized for
maximizing the first law efficiency and minimizing the total cost rate of the working
plant. A multi-objective optimization is performed using Genetic Algorithm and
Pareto front is generated which includes a series of optimal solutions each of which
is a tradeoff between the cost and the energy efficiency. The compiled results of this
thesis can be used to understand the complete system performance, from energy,
exergy and economic standpoints. The structure of this thesis document is briefly
reviewed as below.
Chapter 2 discussed the details of the system under observation. Necessary
system parameter values assumed to initiate the simulation and assumptions made in
the study are provided. Chapter 3 elaborates the thermodynamic modeling of the
system that includes energy and exergy relations used. The model is then validated
with the data available in the literature. Chapter 4, discusses the economic analysis
of the system. Details of SPECO approach and the necessary cost relating equations
9
are discussed in this chapter. Chapter 5 contains discussions on the optimization
algorithm used, the objective functions and the system variables under observation.
Simulation results and corresponding discussions are presented in Chapter 6.
Energy, exergy and the economic performance of the system is evaluated and
presented in this chapter. The results of the parametric study and optimization of the
system are also discussed in detail. Lastly, Chapter 7 enlists the important
conclusions drawn from the complete analysis.
Outcome of this thesis can be a value addition to the deeper understanding of
CCHP systems, importance of exergy and economic analysis and system
optimization in the design and decision-making process of micro-CCHP systems.
10
Chapter 2 CCHP System
The considered thermodynamic cycle as well as the components of the system
are described in this section.
1. CCHP Cycle Diagram and Description
Figure 1 shows the schematic diagram of the system proposed by Amin et al.
[15]. Complete system consists of three major parts, namely a Brayton cycle, an
Organic Rankine Cycle and an absorption refrigeration cycle.
a. Brayton Cycle
Ambient air gets pressurized in compressor 1, cools down through the intercooler
and flows into the compressor 2 where it gets further pressurized before flowing into
the combustion chamber. The intercooler is simply a heat exchanger which captures
heat content of the compressed air and transfer it to water that may be used for
domestic hot water applications. The combustion occurs in the combustion chamber
and high temperature and high-pressure combustion gases will rotate the turbine
which in turn rotates a generator and produce power. The flue gases that leaves the
Brayton cycle are at lower pressure and temperature but still has significant energy
content that can be harnessed to improve overall efficiency of the system.
11
Figu
re 1
: SC
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MA
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TH
E S
YST
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30]
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b. Organic Rankine Cycle
The flue gases that left the gas turbine flow through a heat recovery steam
generator (HRSG) which captures the remaining heat content of the flue gases and
evaporates the organic fluid. Toluene is used as the organic fluid for this study owing
to its desirable characteristics within the range of the operation of the considered
cycle. The HRSG consists of an economizer, an evaporator and a superheater.
Expansion of superheated steam in turbine produces power and turbine outlet stream
condenses in the condenser accordingly. The liquid Toluene is then pumped back to
the vapor generator. Even after expansion in the steam turbine, the ORC turbine
outlet temperature is marginally high, a recuperator placed before the condenser
boosts the efficiency of the plant and reduces the condenser load.
c. Ammonia-Water Absorption cycle
In order to provide cooling effect, a binary mixture of ammonia- water is used in
an absorption refrigeration cycle. The remaining energy of the flue gases that left the
HRSG is imparted to ammonia-water solution in the generator. In high temperature,
ammonia as the more volatile component of the mixture vaporizes and flows to the
condenser and liquid water returns to the absorber. The high-pressure ammonia loses
its high energy content in the condenser and its pressure regulates down through the
expansion valve. The low-pressure liquid ammonia flows in the evaporator where it
absorbs heat from the surrounding and produces cooling effect as chilled water. After
13
evaporation, saturated liquid ammonia reaches to the absorber where recombines
with water to produce the aqua ammonia solution. The strong ammonia-water
solution is pumped to the generator and the cycle repeats. Water is used for cooling
in the absorber and the condenser which gets preheated and may be used for domestic
hot water applications.
2. Assumptions Made in Study
Following are the important assumptions implemented in the analysis:
β’ All processes are assumed to be steady state.
β’ Both air and flue gases in Brayton cycle are considered as an ideal gas
mixture.
β’ Natural gas is used as fuel in the combustion chamber.
β’ All processes in Brayton cycle are adiabatic, except the combustion chamber.
β’ A constant isentropic efficiency is assumed for both compressors and turbine
of Brayton cycle.
β’ Condenser pressure in ORC is selected in a way that water can be used as
cooling agent.
β’ A constant isentropic efficiency is assumed for turbine and pump in ORC.
β’ All processes are considered adiabatic in ORC.
β’ Generator and evaporator outlet in refrigeration cycle are assumed to be
saturated vapor ammonia.
14
β’ Condenser outlet is assumed to be saturated liquid ammonia.
β’ Pressure losses in the pipes and all heat exchangers are negligible.
β’ All components are adiabatic in refrigeration cycle.
β’ A constant isentropic efficiency is considered for refrigeration pump and
pump in ORC.
Important parameters utilized to evaluate performance of the CCHP system
are listed in Table 1.
Table 1: CONSTANT PARAMETERS ASSUMED FOR THE CCHP SYSTEM ANALYSIS
Parameter Value unit Ambient Temperature 25 C Ambient pr. 1.01325 Bar Total pr. ratio 10 - Air mass flow rate 0.1 Kg/s BC compressor isentropic efficiency 85 % CC efficiency 95 % BC turbine isentropic efficiency 90 % BC TIT 800 C LHV 48,000 kJ/kg Intercooler pr. drop in BC 1 % ORC Pinch 10 C ORC TIT 350 C ORC TIP 25 Bar ORC condenser pr. 0.1 Bar ORC turbine isentropic efficiency 80 % ORC pump isentropic efficiency 70 % Recuperator Pinch 10 C Pr. drop in economizer 1 % Pr. drop in evaporator 1 % Pr. drop in superheater 1 % Pr. drop in recuperator 1 % Generator temperature 90 C Condenser temperature 40 C
15
Absorber temperature 20 C Evaporator temperature 2.5 C HX effectiveness 80 % Water temp rise in absorber 5 C
16
Chapter 3 Thermodynamic Analysis
1. Mass and Energy Balances
Mass and energy balance relations for each component of the cycle can be used
based on the first law of thermodynamics, as listed in Table 2.
Table 2: ENERGY RELATIONS USED FOR EACH COMPONENT OF THE SYSTEM
where οΏ½ΜοΏ½πππ is the capital investment cost rate of ππππβ component. οΏ½ΜοΏ½πΆπ€π€ and οΏ½ΜοΏ½πΆππ are the cost
rates associated with the work and heat transfer respectively. ππ and ππ represent usual
inlet and outlet flows. Capital investment cost rate is connected to capital investment
cost, ππππ, via following equation [30],
πΆπΆππππππππππ is taken to be 3 and efficiency of water heater (ππβππππππππππ) is taken high as 90%.