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Thermoeconomic assessment of an absorptionrefrigeration and hydrogen-fueled diesel powergenerator cogeneration system

Mario David Mateus Herrera 1, Felipe Raul Ponce Arrieta 2,Jose Ricardo Sodre*

Pontifical Catholic University of Minas Gerais, Department of Mechanical Engineering, Av. Dom Jose Gaspar, 500,

30535-901 Belo Horizonte, MG, Brazil

a r t i c l e i n f o

Article history:

Received 13 October 2013

Received in revised form

3 January 2014

Accepted 7 January 2014

Available online 1 February 2014

Keywords:

Hydrogen

Thermoeconomic assessment

Diesel engine

Ammoniaewater absorption refrig-

eration system

* Corresponding author. Tel.: þ55 31 3319 49E-mail addresses: mario_mateus_h@hotm

jrsodre@pq.cnpq.br (J.R. Sodre).1 Tel.: þ55 31 3319 4910.2 Tel.: þ55 31 9614 5380; fax: þ55 31 3319 4

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2014.01.0

a b s t r a c t

The thermoeconomic assessment of a cogeneration application that uses a reciprocating

diesel engine and an ammoniaewater absorption refrigeration system for electrical power

and cold production from hydrogen as fuel is presented. The purpose of the assessment is

to get both exergetic and exergoeconomic costs of the cogeneration plant products at

different load conditions and concentrations of hydrogenediesel oil blends. The exhaust

gas of the reciprocating diesel engine is used as an energy source for an ammoniaewater

absorption refrigeration system. The reciprocating diesel engine was simulated using the

Gate Cycle� software, and the ammoniaewater absorption refrigeration system simulation

and the thermoeconomic assessment were carried out using the Engineering Equation

Solver software (EES). The results show that engine combustion is the process of higher

exergy destruction in the cogeneration system. Increased hydrogen concentration in the

fuel increases the system exergetic efficiency for all load conditions. Exergy destruction in

the components of the ammoniaewater absorption refrigeration system is increased with

increasing load due to the rise of heat transfer. At intermediate and high loads energy

efficiency is increased in the power system, and low values of unit exergetic cost and

competitive specific exergoeconomic costs are noticed. The cogeneration system operation

at intermediate and high engine loads was proven to be feasible.

Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

Cogeneration plants with reciprocating engines are widely

used because of their cost effectiveness, mobility and high

11; fax: þ55 31 3319 4910.ail.com (M.D.M. Herrera)

910.2014, Hydrogen Energy P28

efficiency. Reciprocating diesel engines have historically been

the most popular type of thermal engine for small and large

power generation [1].Cogeneration plants with reciprocating

engines had been studied by several authors [2e5]. The

objective of this work is to perform a thermoeconomic

, felipe.ponce@pucminas.br (F.R.P. Arrieta), ricardo@pucminas.br,

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 5 9 0e4 5 9 9 4591

assessment of a cogeneration system for production of elec-

tric power and cold at different loads of a diesel power

generator fueled by blends of diesel oil and hydrogen at

different concentrations. The purpose of the assessment is to

get both exergetic and exergoeconomic costs of the cogene-

ration system. The engine exhaust gas is used as an energy

source to produce cold in an ammoniaewater absorption

refrigeration system.

The energy recovery from the diesel engine exhaust gas in

the generator of the absorption refrigeration system improves

fuel utilization efficiency. A similar quantity of thermal energy

could be recovered from the engine coolant. The fundamental

differences between these waste energy sources are the

temperature of the heating fluid and their fluid flow rate. The

temperature of the engine exhaust gas can be several hundred

degrees, but the temperature of engine coolant is much lower

and usually restricted to a maximum temperature close to

110 �C. The performance of the coolant-driven system is

limited by the coolant flow rates; however, it would be

possible to utilize the thermal energy as a source for providing

domestic hot water [6].

Hydrogen (H2) is one of the most promising alternative

fuels [7] due to its clean burning characteristics and high

performance delivery [8]. The use of hydrogen as a fuel in

internal combustion engines reduces pollutants such as car-

bon monoxide and unburned hydrocarbons. If the generation

of hydrogen fuel can be done using renewable energy sources,

and other challenges such as storage and transport of

hydrogen can be solved, the replacing of fossil fuels with

renewable sources using hydrogen as an energy carrier may

help to mitigate the effects of carbon dioxide emissions [9].

However, hydrogen cannot be used as a sole fuel in a

compression ignition engine because the compression tem-

perature is not enough to initiate the combustion due to its

higher self-ignition temperature as required in the compres-

sion ignition engine [8]. So, hydrogen has can be seen as the

perfect fuel for energy systems. It can be used in combustion

devices or fuel cells without any carbon emissions, and min-

imal emission of other pollutant gases.

The use of hydrogen in a pre-mixed homogeneous charge

compression ignition engine presents ignition timing control

problems, but the use of pre-mixed hydrogen in the intake air

of conventional compression ignition engines does not pre-

sent this problem. The direct injection of hydrogen allows for

much better control of engine operation. Consideration must

be given to the control of injection timing and duration, as

these variables heavily influence factors such as the rate of

pressure rise and maximum combustion pressure. Direct in-

jection offers the possibility to control and limit excessive

mechanical load [9].

In a dual fuel engine, the main fuel is inducted or injected

into the intake air stream with combustion initiated by diesel

oil. Most of the energy is obtained from diesel oil, while the

rest of the energy is supplied by hydrogen. The hydrogen

operated dual fuel engine has the property to operate with

lean mixtures at part load and no load, which results in NOx

reduction, with an increase in thermal efficiency, thereby

reducing fuel consumption [8].

Some authors studied the effects of hydrogen blends at

different proportions on combustion and emissions of diesel

engines. Gatts et al. [10] performed an experiment of incom-

plete combustion of gaseous fuels of a heavy-duty diesel en-

gine fueled by hydrogen and natural gas.Wu andWu [11] used

the Taniguchi method to determine the optimal combinations

of concentrations for a diesel engine blend using H2 and

cooled exhaust gas recirculation (EGR) at the inlet port. It was

concluded that hydrogenediesel co-fueling solved the draw-

back of lean operation of hydrocarbon fuels such as diesel oil,

which is hard to ignite and results in reduced power output, by

reducing misfires, improving emissions, performance, fuel

economy, and also combustion noise [12]. Due to enhanced

combustion, cylinder gas temperature is high with hydrogen

fuel enrichment [13].

The relatively low exhaust temperature produced by diesel

engines can make them unattractive for cogeneration pur-

poses, but the waste heat from the engine, when coupled with

an absorption refrigeration unit, can be used for environment

control purposes, or for improving the efficiency of the pro-

cess [6]. Diesel engines have not generally been adopted for

cogeneration systems because a substantial fraction of the

heat rejected is at too low temperature for any recovery

compared with other cycles. Heat rates are relatively low for a

diesel engine compared to a steam or gas turbine, but diesel

engines are better suited to a high ratio of electric power to

steam. A stationary power plant does not suffer some of the

limitations of a mobile power plant. In the former, it would be

possible to incorporate equipment to overcome some of the

stated difficulties [6].

On the other hand, absorption refrigeration technology has

been used for over 100 years, and today the technology has

made it an economic and effective alternative. The electricity

cost and environmental problems have made this cycle

attractive for residential and industrial applications. In an

absorption refrigeration system, a binary solution consists of a

refrigerant and an absorbent as the working fluids. The per-

formance of an absorption refrigeration system is critically

dependent on the chemical and thermodynamic properties of

the working fluid [14].

Absorption refrigeration systems differ from vapor

compression refrigeration systems due to the utilization of

thermal energy source instead of electric energy. The basic

components of an ammoniaewater absorption refrigeration

system are an absorber, a pump, a condenser, an evaporator,

and a generator. The purpose of the generator is to provide a

stream of vapor solution and a weak solution of refrigerant;

the purpose of the pump is to raise the pressure of the strong

solution leaving the absorber. The evaporator, the condenser,

and the absorber have the purpose to transfer or reject energy

from the vapor stream [15]. The evaporation process in the

absorption refrigeration system is important because in the

case of binary mixtures such as ammoniaewater, vapor pu-

rification is required.

The absorption refrigeration system is of high interest

because it can produce higher cooling capacity than the vapor

compressor system and it can be powered by other sources of

energy (like waste heat from engine exhaust gas) rather than

electricity. This system does not deplete the ozone layer and,

hence, it poses no danger to the environment. Experimental

studies of absorption refrigeration systems have shown that a

high temperature heat source has not been utilized efficiently.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 5 9 0e4 5 9 94592

Recent analyses included the second law of Thermodynamics

to provide better understanding of the thermal performance

characteristics of each of the system components. This facil-

itates the detection of a component with high energy and

exergy dissipation or irreversible losses [16].

2. Cogeneration system description

Fig. 1 shows the schematics of the absorption refrigeration

system coupled with the diesel power generator (see Table 1

for specifications), which was used for the exergetic and

exergoeconomic assessment. The absorption refrigeration

system uses a binary mixture as a combination of absorbent

and refrigerant. Binary mixtures for an absorption system

must be completely miscible in both the liquid and vapor

phases [17]. Ammonia refrigerant and water absorbent are

highly stable for a wide range of operating temperature and

pressure. Ammonia has a high latent heat of vaporization,

which is necessary for efficient performance of the system.

In the absorption refrigeration system, ammonia is the

refrigerant and water is the absorbent. This system used a

double rectifying column with a second heat exchanger. The

generator generates the ammonia vapor purified and sepa-

rates the binary solution of water and ammonia by causing

the ammonia to vaporize with a rectifying column.

According to Fig. 1, a strong-liquid solution, with a large

concentration of ammonia refrigerant leaves the absorber at

state 1. This solution is pumped to the condensing pressure,

and preheated in the heat exchanger to reduce heating at state

3. The heated strong solution enters the rectifying column.

The column produces a weak liquid solution with a low con-

centration of ammonia refrigerant at the bottom, at state 4,

and nearly pure ammonia vapor at the top, at state 7. The

solution with a low concentration of ammonia refrigerant

enters the solution heat exchanger, and flows through the

expansion valve to enter the absorber.

The ammonia refrigerant is sent to the condenser at state

7, which condenses it to sub-cooled liquid at state 8 (Fig. 1).

Fig. 1 e Schematics of the absorption refrigeration system

coupled to the diesel power generator.

The liquid enters the heat exchanger to cool at state 9 and

then it enters the expansion valve at state 9. The ammonia

leaving the expansion valve at state 10 enters the evaporator,

where the liquid phase vaporizes to absorb the refrigerant

load in the system. The refrigerant is further heated in the

heat exchanger prior to being absorbed in the weak-liquid

solution in the absorber at state 12, and then it returns to

state 1 to begin another cycle [17].

3. Methodology

The thermoeconomic assessment of a cogeneration plant that

uses a diesel power generator and an ammoniaewater

refrigeration system for electrical power and cold production

from diesel oil and hydrogen blends as fuel was developed in

four steps, as explained below. The technical specifications of

the diesel power generator are presented in Table 1. The en-

gine was simulated with varying load from 10 kW to 30 kW

and with hydrogen concentrations in the fuel of 0%, 25%, 35%

and 50% on energy basis.

3.1. Diesel power generation simulation

The diesel power generator was simulated using the Gate

Cycle� software for different operating loads and hydro-

genediesel oil blend concentrations. The Gate Cycle� is a

parametric model where the diesel power generator test vari-

ables and parameters are correlated in two tables of variable

parameters, in which, for the different loads and fuel blends,

there is a set of values for each test run. With the diesel power

generator Gate Cycle�model, it is possible via interpolation to

get the resultsofall variablesandparametersatany loador fuel

blendwithin the test limits. For the simulation the lowheating

value and density for both diesel oil and H2, of 43,000 kJ/kg

(835.3kg/m3)and119,961kJ/kg (0.081kg/m3), respectively,were

considered.As an example, a prediction of the results achieved

with the diesel power generator Gate Cycle� model is pre-

sented in Fig. 2. Themodel was adjusted by experimental data

on fuel consumption, intakeairmassflowrate andexhaust gas

temperature obtained from Refs. [18,19].

3.2. Ammoniaewater absorption refrigeration systemsimulation

The ammoniaewater absorption refrigeration system simu-

lation was developed in the Engineering Equation Solver (EES)

software. Themodel developed includes themass, energy and

Table 1 e Diesel engine specifications.

Parameter Value

Type 4 stroke, naturally aspirated

Injection type Direct

Number of cylinders 4

Compression ratio 17.0:1

Total displacement (L) 3.922

Bore � stroke (mm) 102 � 120

Rated power (kW) 44

Crankshaft speed (rev/min) 1800

Fig. 2 e Sample of the diesel power generator simulation results in the Gate Cycle software.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 5 9 0e4 5 9 9 4593

entropy balances in each component of the system. As a

result, the irreversibilities of the components, the efficiency of

the generator, the heat transfer in the condenser, evaporator,

absorber and heat exchanger, and the pump power are

calculated, as well as the coefficient of performance (COP) of

the system. The ammoniaewater absorption refrigeration

system data used in the simulation are presented in Table 2. A

detailed description of the simulation model can be found in

Herrera [20].

3.3. Cogeneration plant performance characterization

The performance index used for characterization of the

cogeneration system is the exergetic efficiency of the diesel

power generator. In addition, for the ammoniaewater ab-

sorption refrigeration system, the parameters analyzed are

coefficient of performance, exergetic efficiencies of the whole

system and the generator alone, and the exergy destruction.

Table 2 eAbsorption refrigeration system data usedfor simulation.

Parameter Value

Reference temperature (�C) 30

Condenser temperature (�C) 37

Generator temperature (�C) 127

Evaporator temperature (�C) 7

Absorber temperature (�C) 38

Reference pressure (kPa) 101.32

System high pressure (kPa) 1378.95

System low pressure (kPa) 206.84

Pump efficiency (%) 75

The specific fuel consumption of the diesel engine (SFC, in

kg/kWh), given by the ratio between the total fuel mass flow,

diesel oil plus hydrogen ðm$ fuel in kg=hÞ and the output power

ðW$

in kWÞ, is expressed by the following equation:

SFC ¼ m$

fuel

W$ (1)

The exergetic efficiency of the diesel power generator (hexDE)

is given by the ratio between the output power from the diesel

power generator and the total exergy supplied with the fuel

ðE$

xfuel; in kWÞ. This includes the diesel mass flow

ðm$ D; in kg=sÞ and its specific exergy (exD, in kJ/kg), and the

hydrogen mass flow ðm$ H2; in kg=sÞ and its specific exergy

ðexH2; in kJ=kgÞ. The exergetic efficiency hex

DE is calculated as:

hexDE ¼ W

$

E$

xfuel

¼ W$

m$

D$exD þm$

H2$exH2

(2)

The coefficient of performance (COP) of the ammonia-

water absorption refrigeration system is given by the ratio

between the total cold produced ðQ$

C; in kWÞ and the energy

input from the exhaust gas ðQ$

G; in kWÞ plus the power

consumed by the pump ðW$

P; in kWÞ. The total cold produced

is computed considering the refrigerant mass flow

ðm$ R; in kg=sÞ and its specific enthalpy at the inlet (h10, in kJ/

kg) and the outlet (h11, in kJ/kg) of the evaporator (see Fig. 1).

The exhaust gas energy considers the gas mass flow

ðm$ G; in kg=sÞ and its specific enthalpy (hG, in kJ/kg). The

power consumed by the pump considers the refrigerant mass

flow and the specific enthalpies at the pump inlet (h1, in kJ/

kg) and outlet (h12, in kJ/kg) (see Fig. 1). The COP is calculated

by the equation:

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 5 9 0e4 5 9 94594

COP ¼ QC$ $ ¼ h11 � h10�

$ $� (3)

$

QG þWP mG=mR :hG þ ðh2 � h1Þ

The exergetic efficiency of the ammoniaewater absorption

refrigeration system (hexARS) is given by the ratio between the

total exergy of the produced cold ðE$

xC; in kWÞ, the total

exergy of the exhaust gas at the refrigerator system generator

inlet ðE$

xG; in kWÞ and the work consumed by the pump. The

total exergy of the produced cold is computed considering the

refrigerant mass flow and its specific exergy at the evaporator

inlet (ex10, in kJ/kg) and the outlet (ex11, in kJ/kg) (see Fig. 1).

The total exergy of the exhaust gas considers the gas mass

flow and its specific exergy (exG, in kJ/kg). The exergetic effi-

ciency of the absorption refrigeration system is computed by:

hexARS ¼ E

$

xC

E$

xG þW$

P

¼ ex11 � ex10�m$

G=m$

R

�$exG þ ðh2 � h1Þ

(4)

The exergetic efficiency of the refrigeration system gener-

ator ðhexARSG

Þ is givenby the ratio between the exhaust gas exergy

and the exergy gain by the mass flows in the generator

ðm$ 3;m$

4;m$

7; all in kg=sÞ (Fig. 1) due to their respective specific

exergies (ex3, ex4, ex7, kJ/kg), and it is computedby the equation:

hexARSG

¼ m$

4$ex4 þm$

7$ex7 �m$

3$ex3

E$

xG

(5)

The exergy destruction ðI$

; in kWÞ of each component (i) in

the absorption refrigeration system is computed using the

Fig. 3 e CHP plant pro

Guy-Stodola theorem, considering the generated entropy

ðS$

gen; in kW=KÞ and the assumed dead state temperature

(T0 ¼ 300 K) by the equation:

I$

i ¼ T0$S$

gen (6)

The generated entropy is computed from application of the

entropy balance at steady state in each component of the

absorption refrigeration system.

3.4. Cogeneration plant thermoeconomic assessment

In order to perform the thermoeconomic assessment, it is

useful to define a productive or causal structure, the coun-

terpart to the physical structure used to calculate the system

energy and the exergy flows [21]. For the thermal scheme of

the cogeneration plant presented in Fig. 1, the productive

structure developed is shown in Fig. 3, which is a schematic

representation of the plant based on the Fuel-Product concept.

To calculate the unit exergetic cost and the specific exer-

goeconomic cost, a model based on application of the Struc-

tural Theory of Thermoeconomic analysis was developed [22].

The thermoeconomicmodel is amathematical representation

of the productive structure of a system [23]. Exergy balances

were applied to each system component, as shown in Fig. 1.

The aim is to calculate the unit exergetic cost and the specific

exergoeconomic cost of each stream in the productive struc-

ture, mainly the net electrical power and cold produced by the

system at the different load and fuel mixtures simulated. The

ductive structure.

Table 4 e Considered investment cost.

Equipment Components US$

Diesel power generator Engine and its auxiliaries 12,555.00

Electric generator 1395.00

Absorption refrigeration

system

Generator 2441.20

Condenser 2082.20

Evaporator 2527.00

Absorber 2986.88

Heat exchanger 1 1163.16

Heat exchanger 2 1163.16

Valve 1 300.00

Valve 2 300.00

Pump 653.40

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 5 9 0e4 5 9 9 4595

unit exergetic cost (k*) is defined by the correlation between

the total stream exergy cost ðE$

x�; in kWÞ and the total stream

exergy ðE$

x; in kWÞ. The specific exergoeconomic cost (c*) is

defined by the correlation between the stream exer-

goeconomic cost ðC$ �; R and the total stream exergy. That is:

k� ¼ E$

x�

E$

x(7)

c� ¼ C�

E$

x(8)

The assumptions for the thermoeconomic assessment are

shown in Table 3. The investment cost of the cogeneration

plant is presented inTable4 [24]. Thediesel oil price considered

in the calculations was 0.4 R$/L. This is the value of the

commercialization price for thermal power generation estab-

lished by the Brazilian Ministry of Finance [25]. The hydrogen

price used was 1.8 US$/g referred by the Global Hydrogen

Incorporated in May 2012 [26]. More details about the ther-

moeconomic assessment can be consulted in Herrera [20].

3.5. Model integration

The input data from the engine used for the simulation in the

Gate Cycle� software is shown by Table 5. The data includes

the engine exhaust gas temperature, diesel oil and hydrogen

mass flow rates at different engine loads and hydrogen con-

centrations. Additional data used for the Gate Cycle� simu-

lation is presented in Table 6. The results from the Gate

Cycle� calculations are:

- The mass, energy, and entropy balances, as shown by

Fig. 2;

- The estimated chemical composition of the exhaust gas;

- The mechanical power, the electric power, the specific fuel

consumption (Eq. (1)) and the exergetic efficiency (Eq. (2)).

All this information can be obtained with the Gate Cycle�model, via interpolation, at any engine load between 0 kWand

30 kW and for any hydrogen concentration in the range from

0% to 50%. The estimated chemical composition, temperature,

pressure, mass flow rate, specific enthalpy and specific en-

tropy are used for calculation of the specific and total exergy of

the engine exhaust gas. The methodology employed for

calculation of the exhaust gas exergy is that presented by

Lozano and Valero [27].

The engine exhaust gas exergy is the same adopted at the

inlet of the absorption refrigeration system. This information

together with the one shown in Table 2 are the input data for

Table 3 e Assumptions for exergoeconomiccalculations.

Parameter Value

Annual operation time (h) 6000

Interest rate per year (%) 10

Life time (year) 20

Maintenance factor per year (%) 5

Amortization factor per year (%) 12

the absorption refrigeration system simulation model elabo-

rated in the EES software. The output data obtained with this

model include mass, energy, entropy and exergy balances for

each equipment of the refrigeration system. Temperature,

pressure, specific enthalpy, specific entropy, specific exergy

and mass flow rate are also obtained for each stream of the

system shown in Fig. 1. Other calculated data are: heat

transfer rate at low temperature (produced cold) and high

temperature (dissipated heat), pump power, COP (Eq. (3)) and

the exergetic efficiency (Eq. (4)) of the refrigeration system. All

information calculated by the EES absorption refrigeration

system model was obtained at any engine load between 0 kW

and 30 kW, and hydrogen concentrations in the range from 0%

to 50%.

The results from the Gate Cycle� model used for the

thermoeconomic assessment are: mechanical and electric

power, total fuel exergy, exergy variation of the engine cooling

water flow and total exergy of the exhaust gas. The results

from the EES absorption refrigeration system simulation

model used for the thermoeconomic assessment are: total

exergy of each stream of the system, total exergy of the heat

transfer rate and pump power. The information presented in

Tables 3and 4 complement the input data used for the ther-

moeconomic assessment. The results of the thermoeconomic

calculations are: fuel, product, exergy destruction, unit exergy

consumption and exergy efficiency of all components, and

unit exergetic cost and exergoeconomic cost for each stream

shown in Fig. 3. The unit exergetic cost and the exer-

goeconomic cost of streams 20 (produced electric power) and

50 (cold of the cogeneration system) are very influential in the

results.

4. Results and discussion

The effect of hydrogen concentration in the fuel on the engine

exergetic efficiency with varying load is illustrated in Fig. 4.

When engine load and hydrogen concentration in the fuel are

increased the engine exergetic efficiency is also increased.

That is a direct consequence of the best fuel conversion effi-

ciency achieved. The diesel engine used in the analysis pro-

vides a significant decrease of specific fuel consumption until

about half of its rated power (44 kW), then the SFC calculated

from the data of Table 5 only shows a slight reduction for

higher load power. That is the reason for the marked increase

of the exergetic efficiency observed until around 20 kW of load

Table 5 e Experimental test results [20].

Engineload(kW)

100% diesel oil 75% diesel oil þ 25% H2 65% diesel oil þ 35% H2 50% diesel oil þ 50% H2

Exhaustgas

temp (�C)

Dieseloil flow

rate (kg/h)

Exhaustgas

temp (�C)

Dieseloil flow

rate (kg/h)

H2 flowrate(kg/h)

Exhaustgas

temp (�C)

Diesel oilflow rate(kg/h)

H2 flowrate(kg/h)

Exhaustgas temp

(�C)

Diesel oilflow rate(kg/h)

H2 flowrate(kg/h)

0 143.01 1.91 135.22 1.91 e 145.52 1.77 e 136.45 1.83 e

10 224.09 3.37 210.99 3.21 0.317 211.54 3.09 0.444 207.64 3.02 0.634

20 324.17 5.17 300.48 4.70 0.472 296.72 4.64 0.661 297.22 4.22 0.944

30 447.79 7.15 411.77 6.37 0.656 409.21 6.37 0.919 409.40 5.99 1.312

Fig. 4 e Variation of engine exergetic efficiency with load

power and hydrogen concentration in the fuel.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 5 9 0e4 5 9 94596

power, with more modest increase of the exergetic efficiency

for higher loads (Fig. 4).

In Fig. 5 it is observed that the absorption refrigeration

system exergetic efficiency is decreased with increasing en-

gine load, which is a result from the rise of the irreversibilities

in the refrigeration system components caused by the in-

crease of heat transfer due to higher engine exhaust gas

temperature [18]. On the other hand, the use of hydrogen in

the fuel increases the refrigeration system exergetic effi-

ciency. The increased engine exhaust temperature achieved

with increasing load [18] producesmore cold in the absorption

refrigeration system (Fig. 6). A constant COP of 0.605 was ob-

tained for all load range investigated.

In Fig. 7, the exergetic efficiency of the absorption refrig-

eration system generator is shown for diesel oil and for the

fuel with 35% of hydrogen concentration. It is observed a

reduction of the exergetic efficiency of the refrigeration sys-

tem generatorwith the increase of engine load. That is a result

of higher entropy generation (or irreversibilities) caused by

heat transfer in the generator at higher temperature differ-

ence between the engine exhaust gas and the refrigeration

system working fluid.

The exergy destruction (or irreversibilities) in the refriger-

ation system components is shown by Fig. 8 for different en-

gine loads using diesel oil as fuel. The increase of exergy

destructionwith engine load finds the same explanation given

for the decreased exergetic efficiency of a component (see

Fig. 7). The highest exergy destructions are noticed in the

evaporator and the absorber. These trends have been

observed for any concentration of the ammoniaewater pair.

The unit exergetic cost of the produced cold is shown by

Fig. 9. For the load range investigated the unit exergetic cost is

lower than when hydrogen is used in the fuel, in comparison

with the use of diesel oil only. On the other hand, the positive

effect of the increased engine efficiency with increasing en-

gine load [19] is not enough to cancel the negative effect of the

Table 6 e Additional data for simulation.

Parameter Value

Air inlet temperature (�C) 30.00

Diesel oil inlet temperature (�C) 30.00

Cooling water inlet temperature (�C) 80.00

Atmospheric pressure (kPa) 101.32

Engine shaft speed (rev/min) 1800.00

Fractional mechanical losses 0.05

Fractional electrical losses 0.05

Power factor 0.80

increased irreversibilities due to the higher temperature dif-

ferences in the refrigeration system component resulted from

the increased engine exhaust gas temperature [18]. As a

consequence, the unit exergetic cost of the produced cold is

increased with increasing engine load.

As it is seen in Fig. 10, the unit exergetic cost of the pro-

duced power is reduced with increasing engine load and

Fig. 5 e Variation of absorption refrigeration system

exergetic efficiency with engine load power and hydrogen

concentration in the fuel.

Fig. 6 e Variation of the cold produced by the absorption

refrigeration system with engine load power.

Fig. 8 e Variation of exergy destruction of absorption

refrigeration system components with engine load power.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 5 9 0e4 5 9 9 4597

increasing hydrogen concentration in the fuel. The specific

exergoeconomic cost of the produced cold is lower when

hydrogen is used in the fuel (Fig. 11). On the other hand, even

though more cold is produced at higher engine loads (see

Fig. 6), the specific exergoeconomic cost of the produced cold

is increased as a consequence of more irreversibilities being

produced due to higher temperature differences in the

refrigeration system components.

The specific exergoeconomic cost of the produced power is

presented in Fig. 12. In addition, it can be seen the current

prices of residential electricity rates practiced by the local

energy company. From this information, it is viable to

compare the effect of fuel prices on the cost of the produced

power. Also, the considered price of diesel oil is lower than

that of hydrogen. At these conditions, it is feasible to use

diesel oil only in the cogeneration system, instead of fuels

containing hydrogen. Considering the prices of residential

rates without taxes, both diesel and the fuels containing H2

Fig. 7 e Variation of generator exergetic efficiency with

engine load power and hydrogen concentration in the fuel.

can be competitive in the existing scenario, but, for these

fuels, the cogeneration system must be operated at interme-

diate or high engine loads. In general, the use of hydrogen as a

partial replacement to diesel oil has been proved to be a useful

strategy to increase the system exergetic efficiency (Figs. 4 and

5) and reduce the costs of the produced cold (Figs. 9 and 11).

5. Conclusions

The use of increased hydrogen concentration in the fuel in-

creases the exergetic efficiency of both the engine and the

absorption refrigeration system. At low loads the cogenera-

tion system achieves lower performance indicators when

compared with the system performance at intermediate or

high loads. When the engine load increases, the exergetic ef-

ficiency of the absorption refrigeration system is decreased

due to increased irreversibilities in the refrigeration system

components. The highest exergy destruction (or irreversibil-

ities) in the refrigeration system takes place in the evaporator

Fig. 9 e Variation of produced cold unit exergetic cost with

engine load power and hydrogen concentration in the fuel.

Fig. 11 e Variationofproducedcoldexergoeconomiccostwith

engine load power and hydrogen concentration in the fuel.

Fig. 12 e Variation of produced power exergoeconomic cost

with engine load power and hydrogen concentration in the

fuel.

Fig. 10 e Variationofproducedpowerunitexergeticcostwith

engine load power and hydrogen concentration in the fuel.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 5 9 0e4 5 9 94598

and in the absorber. The produced power unit exergetic cost

and exergoeconomic cost are reduced when the engine load

and the hydrogen concentration in the fuel are increased. The

irreversibilities in the refrigeration system components in-

crease the unit exergetic cost and exergoeconomic cost of the

produced cold when the engine load rises. With the consid-

ered fuel prices, the cogeneration system ismore feasible to be

operated at intermediate or high engine load.

Acknowledgments

The authors thank ANEEL/CEMIG GT-292 research project,

CNPq and FAPEMIG for the financial support to this work.

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