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Analysis and assessment of biomass energy-based multigeneration system with thermoelectric generator Shahid Islam, Ibrahim Dincer, and Bekir S. Yilbas Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01749 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017
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Energy & Fuels
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Analysis and assessment of biomass energy-based multigeneration system with thermoelectric generator
3
Shahid Islam*,a,b Ibrahim Dincer1,a and Bekir Sami Yilbas2,b a
4 5 6 7 8
Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada b Center of Excellence Renewable Energy and Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran, 31261, Saudi Arabia. *Email:
[email protected] 9
Abstract
10
This study demonstrates comparative energy and exergy analysis of an advanced biomass energy
11
based integrated multigeneration system with two innovative embodiments of thermoelectric
12
generators. These two systems uniquely integrate the configurations of thermoelectric generators for
13
better performance. System 1 incorporates integration of the thermoelectric generators at the exhaust
14
of Gas Turbine. In system 2, thermoelectric generators are considered at the inlet of organic Rankine
15
cycle 1. The heat rejected at low temperature interface of the thermoelectric devices generates
16
electricity through organic Rankine turbine 2. Some of the electricity produced by thermoelectric
17
generators is used to operate an electrolyzer to produce hydrogen. The utilization of thermoelectric
18
device developed in system 2 enhances the energetic and exergetic efficiencies of overall
19
multigeneration system and organic Rankine cycle 1. Also, the net amount of work output by organic
20
Rankine cycle 1 of system 2 is improved greatly. Furthermore, the design configurations of the
21
thermal system and the influence of operating conditions on energy and exergy efficiencies of
22
multigeneration system and organic Rankine cycle are investigated. The present results show that an
23
increase in combustion temperature improves the work rate by turbines and thermoelectric generators
24
appreciably. In addition to this, the overall energetic and exergetic efficiencies of system 1 are found
25
to be 73.4% and 33.3%, respectively, whereas for system 2 these efficiencies are enhanced in the
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order of 74.5% and 34.02%, respectively. The net amount of work output by organic Rankine cycle 1
27
in embodiment 2 is improved from 8023 kW to 8908 kW. The highest amount of exergy destruction
28
(91.5%) occurs in Gas turbine of system 1 followed by the organic Rankine cycle 1 (6.64%) of
29
system 2. The proposed system has superior and unique features as compared to conventional
30
systems with thermoelectric generation.
31
Keywords: Biomass, Multigeneration, Energy, Exergy, Efficiency, Thermoelectric generator.
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Nomenclature
34
c
compressor
35
cost rate
36
CRF
cost recovery factor
37
exergy rate (kW)
38
ex
exergy specific (kJ/kg)
39
GT
Gas Turbine
40
h
specific enthalpy (kJ/kg)
41
LHV
lower heating value (kJ/kg)
42
K
conductivity, thermal (W/m2K)
43
L
semiconductor length
44
m
mass flow rate (kg/s)
45
P
pressure (kPa)
46
Q
heat rate (kW)
47
entropy generation rate (kW/K)
48
s
specific entropy (kJ/kgK)
49
T
temperature (K)
50
v
specific volume (m3/kg)
51
W
work rate (kW)
52
Z
figure of merit
53
Greek Letters
54
55 56
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density (kg/m ) energy efficiency exergy efficiency
57
∅
58
Subscripts
59
a
absorber
60
bio
biomass
61
ch
chemical
62
CI & M
capital investment and maintenance cost
63
cond
condenser
64
dest
destruction
65
eva
evaporator
exergy to energy ratio of fuel
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gen
generator
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gt
gas turbine
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HE
heat exchanger
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hw
hot water
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n
semiconductor “n” type
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P
pump
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p
semiconductor “p” type
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ph
physical
74
prod
product
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t
turbine
76
TEG
Thermo Electric Generator
77
0
environment state/ (reference)
78
1, 2,….57
state points
79
Acronyms
80
COP
coefficient of performance
81
HCCI
homogeneous charged compression ignition engine
82
HEX
heat exchanger (solar)
83
LiBr-H2O
84
ORC
organic Rankine cycle
85
PTSC
parabolic trough solar collector
solution of lithium bromide-water
86 87
1. Introduction
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The natural sources of energy like sunlight, wind, geothermal, biomass, rain and tides are treated as
89
renewable energy resources, which are replenished naturally after use. The energy from sun is
90
considered as sustainable and a renewable source. In addition to this, solar thermal systems can
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provide power indirectly. Biomass is a biological material which is largely extracted from living or
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dead matter available on the earth [1]. Many researchers have studied biomass energy based co-
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generation systems for numerous industries like; palm oil, rice, wood, sugar, and paper [2] .
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Multigeneration processes capabilities to yield high energy efficiencies, reduced operating
95
expenditures and minimized pollutants [3]. The low efficiency of a single renewable energy based
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systems can be improved by conserving low grade heat through unique integration of sub-systems.
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Garcia et al. [4] performed techno-economical overview of variety of biomass samples and found
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competitive results for almond shell and olive stone.
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Kalinci et al. [5] conducted exergoeconomic analysis and asseseed the performance of
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different gasification systems for the production of hydrogen and power. Ozturk and Dincer [6]
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assessed an integrated coal gasification multi-output system based on solar energy to produce
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power, hydrogen, oxygen, heating, cooling and hot water. Riaza et al. [7] studied ignition and
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combustion characteristics of small particles of biomass and coal. In addition to this, Al-Ali and
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Dincer [8] reported 10% increase in the exergy efficiency in the event of shifting system from single
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generation to multigeneration. Soltani et al. [9] developed a biomass energy-based multigeneration
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system and reported an increase of 49% and 12% in energy and exergy efficiencies, respectively.
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In general, the multi-generation system uses one or more energy sources to produce many
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useful outputs. Multi-generation systems not only mitigate environmental impact and cost but also
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increase efficiency and sustainability. There is a huge connection between the qualities of the
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available energy for multigeneration with the reference environment. Renewable energy based
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multigeneration systems produce better efficiency and these are sustainable [10]. Al-Sulaiman et al.
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[11] conducted a research on energy efficiency of electrical system and found that energy efficiency
113
can be enhanced to 94% in case of trigeneration.
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Islam et al. [12] investigated a multigeneration system energetically and exergetically based
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on solar energy and found an increase of 4.5% in energy efficiency and 5.1% increase in exergy
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efficiency of PV, through innovative cooling configuration. They investigated the overall energy and
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exergy efficiencies including and excluding thermoelectric generators and found these to be 50.6%
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and 39.8%, and 51.33% and 40.32%, respectively. The configuration of thermoelectric devices was
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not included in that study. Bicer and Dincer [13] combined solar and geothermal resources to
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produce hydrogen, electricity, heat and cooling and evaluate its performance energetically and
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exergetically.
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Islam and Dincer [14] studied four embodiments of a cogeneration system and reported an
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increase of 10% in exergy efficiency in the event of shifting system from single generation to
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multigeneration. Islam et al. [15] designed and investigated a solar energy-based multigeneration
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system for off-grid areas to compensate electricity through homogeneous charged compression
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ignition (HCCI) engine. It is important to develop an advanced electrolyzer to fulfill the growing
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demand of hydrogen. The hydrogen production through electrolysis is practicable approach without
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consumption of fossil fuels. Farrukh et al. [16] studied a solar and biomass based integrated system
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including organic Rankine cycles and absorption chiller. In their study they found that the overall 4 ACS Paragon Plus Environment
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energy and exergy efficiencies of combined solar and biomass system in the order of 66.5% and
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39.7%, respectively, whereas, both efficiencies for biomass and solar system alone were found to be
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64.5% and 37.6% and 27.3% and 44.3%, respectively.
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Taheri et al. [17] developed biomass based multigeneration system and reported overall
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energy and exergy of the system to be 39.5% and 25.7%, respectively. Moreover, the energy and
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exergy efficiencies associated with the ORC were found to be 21.7% and 52%, respectively.
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Lythcke-Jorgensen et al. [18] modelled and optimized a flexible multi-generation system (FMG) on
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the basis of combined heat and power plant and concluded that the operational uncertainties should
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be taken into account while designing such a system. Akrami et al. [19] studied energetic, exergetic
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and exergoeconomic analyses of a multigeneration system and found overall energy and exergy
140
efficiencies in the order of 34.98% and 49.17%.
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The systematic process integration is very important in designing flexible multigeneration
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systems [20]. The economic value of a multigeneration system can be improved through careful
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design and integration of sub-systems [21]. The cost optimal solution of a new office building can be
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found through designing a multigeneration system [22].
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Thermoelectric generators carry huge potential to generate electricity through waste heat.
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Their low efficiency is dominated by other features like design simplicity, easy to operate and no
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(zero) emissions. These characteristics make them selective option for the generation of electricity.
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Yilbas and Sahin [23] studied the aftermath of configuration of thermoelectric device on performance
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of refrigeration cycle. Coefficient of performance (COP) of power and refrigeration cycle is
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enhanced by integrating thermoelectric device between ambient and condenser. Yilbas et al. [24]
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investigated that cold junction temperatures improve the performance of thermoelectric generators.
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Ali and Yilbas [25] modified the pin material configuration for increased efficiency and high power
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output Power. Yazawa and Shakouri [26] optimized the efficiency of thermoelectric devices through
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thermal contact. The variation in efficiency and power produced by thermoelectric devices
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corresponding to the geometric configuration has been conducted in past [24-25].
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Saudi Arabia is one the biggest crude exporter in the world and also consumes more oil than
157
any other country for electricity generation. Its consumption is a minimum of 900,000 barrels a day
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at peak periods of the year to meet its electricity demand which is equivalent to more than $16bn per
159
year based on recent oil prices [29]. The most recent International Energy Agency states reflect that
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the country is investing in renewable energy sources to increase their crude export [29]. The specific
161
objectives of this research are to evaluate the best integration of thermoelectric modules with
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biomass based multigeneration system and to determine the energy and exergy efficiencies of overall 5 ACS Paragon Plus Environment
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system and its subsystems. This newly proposed multigeneration system is specific, new and
164
unique which compares the integration of two different configurations of thermoelectric devices
165
with the same subsystems. Upto the best knowledge of authors, multigeneration systems with
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thermoelectric device have been proposed but their best integration has not been studied yet. In
167
addition to this, a parametric study is conducted for the system 2 to determine the sensitivity of
168
energy and exergy efficiencies of overall system against variations in operating conditions.
169
2. System description
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Biomass energy-based Gas Turbine is used to develop two configurations of multigeneration system
171
incorporated with thermoelectric generators as presented in Fig. 1 and Fig. 2, respectively. This
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newly developed system produces electricity, hydrogen, space heating, cooling and hot water. The
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electricity is generated for remote locations (particularly off grid areas) of Saudi Arabia where date
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palm waste is available in abundant [30], heat for industrial use, cooling for cold storage of food, hot
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water for hot water supply to the domestic/industrial users and hydrogen for storage. The hydrogen
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can be sold out to chemical or petrochemical industries. Biomass composed of dry olive pits and air
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are combusted in combustion chamber to run Gas Turbine. The proposed system consists of seven
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subsystems, namely; biomass cycle, thermoelectric generator, two organic Rankine cycles (ORC),
179
Lithium bromide driven absorption chiller, heat exchanger 3 and an electrolyzer.
180
2.1 Gas turbine cycle (GT)
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The air at ambient pressure enters the compressor which compresses the air and delivers to
182
combustion chamber at a higher pressure. The conveyors are used to transfer biomass from one
183
handling station to another and finally to the combustor. Solid fuel belt conveyors, scraper conveyors
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and elevators are the most frequently used. The type of handling equipment required depends on fuel
185
quality and the lay out of the plant. The biomass combustion plant is usually incorporated with a fuel
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silo, which smooths out the fuel flow. The screw feeders are used to feed the biomass to the
187
combustor. The rotation speed of the screws controls the mass flow rate of biomass to the combustor.
188
The costs for handling and preparing the material can be about 10-20% of the total plant costs [31].
189
Biomass and compressed air are mixed and combusted in combustion chamber to produce gases with
190
high temperature and pressure. Then hot pressurized gases enter Gas Turbine to generate electricity.
191
The heat of the exhaust of the Gas Turbine is recovered through heat exchanger 2 to drive ORC
192
turbine 1.
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2.2 Organic Rankine cycle 1 (ORC1)
196
The ORC working fluid with high critical temperature can use waste heat more efficiently [32] . The
197
efficient operation of the ORC depends heavily on two factors, working conditions of the cycle
198
and the thermodynamic properties of the working fluids therefore, to ensure an efficient ORC, the
199
isobutane is chosen as circulating fluid based on its suitability with proposed operating conditions
200
and thermodynamic properties like high critical temperature. Moreover, isobutane is readily available
201
and it is highly efficient in terms of heat transfer [33]. The ORC fluid isobutane leaves condenser 1 at
202
302K and enters in heat exchanger 2 (HE#2) where it changes phase due to the heat of exhaust of
203
Gas Turbine. Then isobutane enters turbine 1 at 493K to generate electricity and exits at state 30 with
204
temperature 380K. This exiting working fluid transfers heat to Lithium bromide/water solution
205
through the generator of absorption chiller for cooling purpose. Heat recovered from the condenser 1
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between states 35 and 36 is used for space heating. Finally, pump 5 recirculates isobutane and the
207
loop is completed.
208
2.3 Absorption chiller
209
The availability of absorption systems depends on absorber combination and the refrigerant. Most
210
widely used systems are either ammonia water or lithium bromide/water [34]. The absorption chiller
211
for space cooling are mostly driven by Lithium bromide /water [35]. The main components of
212
absorption chillers are; generator, pump and absorber. The hot isobutane leaving turbine 2 is the
213
source of heat which drives the generator of absorption chiller.
214
2.4 Thermoelectric generators
215
Thermoelectric generators are integrated with multigeneration system with two different
216
configurations as presented in Fig. 1 and Fig 2. In system 1, a fraction of exhaust gases coming out of
217
Gas Turbine is extracted at state 23 and diverted to the high temperature side of thermoelectric
218
generator. The ORC 2 working fluid R113 is circulated across the low temperature junction to
219
conserve the waste heat and generate additional electricity as shown in Fig. 1. In system 2, a fraction
220
of isobutane is extracted and passed through high temperature side of thermoelectric generators as
221
depicted in Fig. 2. The ORC2 working fluid R113 absorbs the waste heat and generates electricity.
222
The thermoelectric generators produce electricity with no pollution and no moving parts are involved
223
during their operation.
224
2.5 The organic Rankine cycle 2 (ORC2)
225
The working fluid Trichlorotrifluoroethane (R113) is selected to recover waste heat of thermoelectric
226
generators. The working fluid R113 is selected in this study as it recovers low grade heat
227
efficiently [36]. R113 enters turbine 2 at state 53 with high enthalpy and exits at state 54 after 7 ACS Paragon Plus Environment
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228
producing power through turbine. Low grade heat of R113 is recovered at condenser 2 and supplied
229
to the water of electrolyzer. Pump 4 recirculates working fluid to complete the cycle.
230
2.6 TEG driven electrolyzer
231
The hot water of condenser 2 between states 55 and 56 is used to heat up the water circulating
232
through electrolyzer to favor the process of hydrogen production. High pressure hydrogen is
233
produced by breaking the water molecules in the electrolyzer. Stream of water and oxygen is
234
inducted to oxygen separator where oxygen is collected and stored for the use in industry. A typical
235
electrolyzer is chosen for the production of hydrogen amount of about 10772.64 m3/year [37]. Table
236
3 tabulates the operating parameters of typical electrolyzer.
237
2.7 Heat exchanger 3
238
The low grade heat of exhaust gases is extracted through water between states 28 and 29. This hot
239
water is used to meet the domestic hot water requirement. Finally, exhaust gases leave heat
240
exchanger 2 at state 27 at 301.4K and ambient pressure.
241
3. Analysis and assessment
242
The Engineering Equation Solver is selected to perform energy and exergy analysis of all
243
components alone and hence overall system. Wang et al. [34] used the Engineering Equation Solver
244
(EES) to perform thermodynamic analyses of combined cooling heating and power system. The
245
following lists the assumptions made to investigate performance and sensitivity of the proposed
246
system.
247
•
The temperature T₀=298K and pressure P₀=101.325 kPa are taken as dead state properties of the multigeneration system.
248 249
•
The operating conditions of the system are of steady state.
250
•
Negligible or no changes occur in both kinetic and potential energies.
251
•
Both R113 and water are treated as actual.
252
•
The ideal gas properties are chosen for air to perform analysis.
253
•
The pumps and turbines are considered adiabatic.
254
•
The isobutane and R113 are selected as working fluid for ORC1 and ORC2, respectively.
255
•
All turbines and pumps are assumed to be adiabatic.
256
•
The isentropic efficiencies of 85% are taken for all pumps and turbines.
257
•
The exhaust gases leaving the gas turbine are treated as air [16].
258
•
Typical combustion efficiency is assumed to be 88% [38].
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Energy & Fuels
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The proposed system is modelled on the basis of mass, energy, entropy and exergy
260
balance equations. The balance equations related to mass, energy, exergy and entropy of the
261
proposed systems are presented here. Moreover, energetic and exergetic performances of sub-
262
systems, and multigeneration system with irreversibility rate are also discussed.
263
3.1 Gas turbine cycle
264
Biomass and compressed air enter the combustion chamber at states 17 and 18, respectively, as
265
shown in Fig. 1 and Fig. 2. The detailed specifications of dry olive pits are tabulated in Table 1.
266
Table 2 tabulates the composition of biomass used in this study. General chemical reaction equation
267
of biomass can be written as
268
+ + 3.76 → ! + " + #
269
The low heating value of dry biomass in kJ/kg can be determined as )**,***,-**,.**/
(1)
0 --6,.**,-**,.** 123.45
270
$%&' =
271
The exergy to energy ratio (7 is used to determine the chemical exergy of fuel (89: as [39]
272
7 = ;
273
The exergy-to-energy ratio for biomass derived fuel with composition A B C D can be provided as
274
[39].
275
7 = 1.0401 + 0.1728 + 0.0432 + 0.2169 K1 − 0.2062 M A A C A
276
The exergies associated with chemical and physical states combusted gases can be found as [40]:
277
89NO,-PQ∑ S S
(2)
-,,-.
(3)
?@5
B
C
D
B
(4)
(5)
TU ,VW3 ∑ S X S
278
89YO,-PQO1Z /O3 /W3 [1Z /[3
279
The exit temperature\-] , of compressor can be determined as
(6)
Sa1
^ Sb \-6 K 1_ M T ^1`
280
\-] =
281
where k and N represents the specific heat ratio and compressor isentropic efficiency, respectively.
282
The temperature of exiting gases from Gas Turbine can be found as
283
Wc3 Wc1
^c3
= K^ M
(7)
bd Sa1 S
(8)
c1
284
where e is the isentropic efficiency of Gas Turbine.
285
The work done by the gas turbine can be determined as
286
fe = h* ℎ* − h- ℎ-
287
The rate of exergy destruction of Gas Turbine can be calculated as
(9)
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288
Exlmno,po = h* 89* − h- 89- − fe
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(10)
289 290
3.2Thermoelectric generator
291
The balance of energy can be applied to low and high temperature junctions of thermoelectric
292
generator as
293
h ℎ − h) ℎ) = hq ℎq − hq ℎq + fWrs
294
The balance of exergy across thermoelectric devices can be expressed as
295
tmno,uvw h 89 − h) 89) = hq 89q − hq 89q + fWrs + Ex
296
3.2.1 Maximum work yielded by thermoelectric devices
297
The maximum electric power achieved through thermoelectric generators and high figure of merit
298
fluctuate with the variation in the operating temperature. Sahin and Yilbas [41] calculated the
299
maximum amount of work output by thermoelectric as
300
xyz{ |Wc
=
-/} c ~Wz
(11)
(12)
(13)
}-,} W
301
where = Wc which is the ratio of temperatures and \ represents average temperature based
302
figure of merit. The equation 14 can be used to find the thermal conductivity “K” as follows
303
=
304
Thermal conductivity and material of TEG for calculations of “p” and “n” type semiconductors are
305
taken 1.2 W/m2K and bismuth telluride, respectively. The efficiency of the thermoelectric devices
306
depends on electronic conductivity and figure of merit. Therefore, bismuth telluride is selected as
307
it has high figure of merit=1, high electronic conductivity and suitable operating temperature
308
[42].
309
3.3 TEG driven Electrolyzer
310
The balance of rate of mass flow across the electrolyzer can be applied as [43]:
311
he ' = h= + h
312
The energy balance applied to the electrolyzer can be defined as:
313
he ' ℎe ' + f X = h= ℎ= + h ℎ + X[[
314
The entropy balance across the electrolyzer can be written as
315
he ' ℎe ' +
= h= = + h + W
316
The exergy balance of electrolyzer can be defined as
317
he ' ℎe ' + X = h= ℎ= + h ℎ + 9& [e + 9& [e,X[[
1
