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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]

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Abstract

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This study demonstrates comparative energy and exergy analysis of an advanced biomass energy

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based integrated multigeneration system with two innovative embodiments of thermoelectric

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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

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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

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thermal system and the influence of operating conditions on energy and exergy efficiencies of

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multigeneration system and organic Rankine cycle are investigated. The present results show that an

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increase in combustion temperature improves the work rate by turbines and thermoelectric generators

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appreciably. In addition to this, the overall energetic and exergetic efficiencies of system 1 are found

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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

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in embodiment 2 is improved from 8023 kW to 8908 kW. The highest amount of exergy destruction

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(91.5%) occurs in Gas turbine of system 1 followed by the organic Rankine cycle 1 (6.64%) of

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system 2. The proposed system has superior and unique features as compared to conventional

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systems with thermoelectric generation.

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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

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prod

product

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t

turbine

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TEG

Thermo Electric Generator

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0

environment state/ (reference)

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1, 2,….57

state points

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Acronyms

80

COP

coefficient of performance

81

HCCI

homogeneous charged compression ignition engine

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HEX

heat exchanger (solar)

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LiBr-H2O

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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

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renewable energy resources, which are replenished naturally after use. The energy from sun is

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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combustion chamber at a higher pressure. The conveyors are used to transfer biomass from one

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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

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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

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combustor. The rotation speed of the screws controls the mass flow rate of biomass to the combustor.

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The costs for handling and preparing the material can be about 10-20% of the total plant costs [31].

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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

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turbine 1.

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2.2 Organic Rankine cycle 1 (ORC1)

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The ORC working fluid with high critical temperature can use waste heat more efficiently [32] . The

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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

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temperature 380K. This exiting working fluid transfers heat to Lithium bromide/water solution

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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.

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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

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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 ‚\ƒ„ 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

hŠƒe ' = h= + h‹

312

The energy balance applied to the electrolyzer can be defined as:

313

hŠƒe ' ℎŠƒe ' + f X = h= ℎ= + h‹ ℎ‹ + ŒX[[

314

The entropy balance across the electrolyzer can be written as

315

  hŠƒe ' ℎŠƒe ' + 

= h= Ž= + h‹ Ž‹ + W‘’’

316

The exergy balance of electrolyzer can be defined as

317

hŠƒe ' ℎŠƒe ' + ” X = h= ℎ= + h‹ ℎ‹ + 9& [e + 9& [e,X[[

1

†‡ ˆ‡