Analysis of different arrangements of combined cooling, heating and power systems with internal combustion engine from energy, economic and environmental viewpoints

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Analysis of different arrangements of combined cooling, heating and power systems with internal combustion engine from energy, economic and environmental viewpoints ⁎

Mohammad Mahdi Balakhelia, Mahmood Chahartaghia, , Mohammad Sheykhia, Seyed Majid Hashemiana, Nima Rafieeb a b

Faculty of Mechanical Engineering, Shahrood University of Technology, Shahrood, P.O.B. 3619995161, Iran Alma Mater Studiorum – Università di Bologna, Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Bologna, Italy

A R T I C LE I N FO

A B S T R A C T

Keywords: CCHP Internal combustion engine Zero-dimensional model Absorption chiller Net present value Carbon dioxide emission reduction

The aim of this study is comparing different arrangements of combined cooling, heating and power systems and presenting proper configuration from energy, environment, and economic viewpoints. The arrangements include two gas engines as prime movers, heat exchangers for the use of waste heat, and electric and absorption chillers to provide cooling. For system evaluation, a mathematical model called the zero-dimensional single-zone method is used for internal combustion engine and validated by the experimental data. The engine model can evaluate power output as well as heat transfer in the engine which can be recovered for heating and cooling demands. Afterward, four different arrangements for the combined cooling, heating and power, and an arrangement for the combined cooling and power have been proposed and these total five arrangements are compared with a conventional energy supply system to provide the same energy demand of the building. The arrangements are compared at various the engine rotational speed and air to fuel equivalence ratios and some main parameters such as primary energy savings and net present value are evaluated. The results reveal that by using electric chillers for cooling as well as utilizing the waste heat of prime movers for heating, the highest reduction in primary energy consumption and carbon dioxide emissions could be achieved in the range of 31 and 36%, respectively. On the other hand, from economic viewpoint, this arrangement has higher cost of fuel consumption and a longer payback period than other arrangements of combined cooling, heating and power systems based on cooling with an absorption chiller.

1. Introduction There are various ideas about energy saving, reducing greenhouse gas emissions and operating costs of thermal systems. Combined cooling, heating and power (CCHP) systems are one of the suitable ways to improve the thermal efficiency, reduce energy consumption and the operation costs, and cleaner production of air conditioning systems and power generation [1]. In fact, CCHP systems are the multiple productions of energy from one fuel source [2]. These systems consist of several components. Generally, CCHP systems consist of a prime mover for power generation, some cooling systems such as absorption and electrical chillers and a heat recovery system for use of wasted heat [3]. The use of combined heat and power (CHP) and CCHP systems in the building sector has been discussed in several studies. Dominkovic et al. [4] presented a CCHP system integrated to a pit

⁎

thermal energy storage for residential building applications. A biomass power plant system was used to generations of electricity and heat. Also, absorption chiller was utilized for cooling demands. The system power and heat capacities at different conditions were evaluated and economic analysis was performed. They showed the thermal energy storage could improve the overall power plant efficiency. Ebrahimi and Ahookhosh [5] investigated a CCHP system with a gas turbine, and an organic Rankine cycle (ORC). In their study, energy and exergy analyses of the system was performed and 37% fuel consumption saving was reported in the summer mode. Chahartaghi and Alizadeh- Kharkashi [6] analyzed a CCHP system containing polymer fuel cell, absorption chiller, and heat recovery system. They evaluated energy and exergy performances and showed that the greatest amount of exergy destruction occurs in the fuel cell. Wu et al. [7] presented a new configuration of a CCHP system with solar thermal system and ORC.

Corresponding author. E-mail address: [email protected] (M. Chahartaghi).

https://doi.org/10.1016/j.enconman.2019.112253 Received 7 August 2019; Received in revised form 31 October 2019; Accepted 1 November 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Mohammad Mahdi Balakheli, et al., Energy Conversion and Management, https://doi.org/10.1016/j.enconman.2019.112253

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Nomenclature

T up V VC Vd W Xb

General

A cross-section area (m2) Wiebe efficiency factor (–) aw ACO2 T CCHP carbon dioxide emission tax for CCHP system ($/year) ACO2 T Conv carbon dioxide emission tax for conventional system ($/year) ATCS annual cost savings ($/year) B cylinder bore (m) bmep brake mean effective pressure (kPa) COP coefficient of performance (–) Cost cchp fuel costs of the cchp system ($) Cost conv fuel costs of the conventional system ($) CostF natural gas purchase tariff ($.m−3) CostW electricity purchase tariff ($.kWh−1) Ctotal total capital investment cost ($) E electricity (kW) F cchp consumption fuel for CCHP system (kW) F conv consumption fuel for conventional system (kW) FCR fuel cost reduction percent (–) FV future saving investment cost ($) fmep frictional mean effective pressure (kPa) fr frequency of engine (Hz) H operating hours per year (hr/year) hc coefficient of convective heat transfer (W.m−2. K−1) hr coefficient of radiative heat transfer (W.m−2. K−1) imep indicated mean effective pressure (kPa) l connecting rod length (m) m mass of the working fluid in each element (kg) number of cylinder (–) Ncyl Wiebe efficiency factor (–) nw nr engine rotational speed (rpm) N engine rotational speed (rps) NPV net present value ($) P power out (kW) p pressure (kPa) PP payback period (year) PES primary energy saving (–) PV present value of the cash flow ($) heat transfer (kJ) Q Q̇ heat transfer rate (kW) r crank radius (m) s displacer stroke (m)

Temperature (K) piston velocity (m.\;s−1) volume (m3) clearance volume (m3) displacement volume (m3) work (kJ) Weber function

Greek symbols η ωgas Y γ μgas μCO2F μCO2W θ θs θd

efficiency (–) mean velocity of gases in the combustion engine (m.s−1) CO2 tax rate ($.gr−1) specific heat ratio (cp. c v−1) (–) dynamic viscosity of gases (kg·m−2 ·s−1) CO2 emission index for natural gas (gr .kWh−1) CO2 emission index for grid electricity (gr.kWh−1) crank angle (deg) crank angle at start of combustion (deg) crank angle of combustion duration (deg)

Subscript ac b cd CCHP Conv comb jw e ec ed ex El F gen grid hd HX ICE loss t w

absorption chiller boiler cooling demand combined cooling, heating and power system conventional system combustion cooling system power plant electric chiller electricity demand exhaust gas electrical fuel generator grid heating demand heat exchanger internal combustion engine loss to surrounding total wall of cylinder

CHP system. Asaee et al. [12] showed that using a CHP system with an ICE in Canada could achieve 13% annual energy savings and a 35% reduction in greenhouse gas emissions. Arbabi et al. [13] proposed two different combustion engines as the prime movers of the CHP system, after calculating the critical loads of a residential building, and the system was briefly evaluated for energy, exergy and economic points of view. Wang et al. [14] studied impact of ICE capacity at different working hours on payback period of CCHP system. They concluded that if the engine capacity reaches more than 100 kW, the payback period would be less than 4 years. Yang et al. [15] provided a sensitivity analysis of a CCHP system based on ICE with biomass and gas fuels. The exergy analysis was performed and the influence of fuel price on the exergy cost of the system was evaluated. They showed that the price of natural gas had more influence on unit product exergy cost compared to the biomass price. Alexi and Liakos [16] showed that with the use of ICE for CHP system in residential applications, it would be possible to achieve a 34%

They performed thermodynamic analysis and compared their system with conventional CCHP systems. The results showed that their new system could generate more electricity and its fuel consumption was less than conventional systems. Chahartaghi and Sheykhi [8] presented the modeling of the CCHP system with a Stirling engine and showed that the regenerator length is an effective parameter in determining the carbon dioxide emissions from the system. Also, in another analysis, Sheykhi et al. [9] presented a payback period of around 4 years for a CCHP system with a Stirling engine, absorption chiller and a heat recovery system. Onovwiona et al. [10] carried out a technical–economic assessment of a CHP system based on an internal combustion engine (ICE) for residential buildings. They revealed that selecting the proper capacity of the ICE engine, and electrical and thermal storage systems plays an important role in the performance of the CHP system for residential buildings. Meybodi and Behnia [11] presented a thermal-economic model with respect to carbon tax effects for choosing an appropriate ICE for the 2

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absorption chillers and their high investment cost, as well as higher volume and weight than absorption chillers leads to limit application of them in the buildings [26]. In the other hand, some of their advantage are low moving parts and therefore their maintenance cost is lower than absorption chillers. Also, they have not crystallization problem compared to Water-Lithium Bromide absorption chillers and so, these advantages may be reasons for more attention and consideration in the future. In most of the works about performance investigation of CHP and CCHP systems with different prime movers, usually the simulation of one particular arrangement of the system is done. Also, in most studies, the CCHP systems with ICE prime mover has not been compared and evaluated by all perspectives, such as reduced primary energy consumption, carbon dioxide emissions reductions, reduced fuel consumption cost, the factors that determine future investment cost (FV), the present value of the current cash flow (PV), as well as the net present value (NPV) and payback period for investment of system. In addition, in many of studies about CCHP systems with engine, the engine was not modeled by details and simple relations of heat balances was used to estimating the waste heat of engine. The aim of this study is presentation of a comprehensive research about CCHP system with ICE which can be complement of the previous works in some aspects. Therefore, in this paper, zero-dimensional single-zone model as a mathematical model is used to analyze the performance of an ICE and the model validation has been done with experimental results for two different types of engines. Then, two gasfired ICEs named EF7 engines are used as the prime mover for the CCHP system and five different arrangements of the CCHP systems with the gas engines are proposed. Also, different parameters such: fuel cost reduction (FCR), carbon dioxide (CO2) tax, carbon dioxide emission reductions (CO2ER), investment cost, operating and maintenance costs in different arrangements, annual total cost saving (ATCS), net present value (NPV), the present value of the current cash flow (PV), future investment cost (FV), and payback period for different arrangements have been evaluated. By comparison, different arrangements of CCHP systems from different perspectives can be presented and benefits of using these systems in building applications can be explained. In this way, the main innovations and contributions of this research are presented as below:

reduction in total annual energy consumption, in addition, pollutant emissions are significantly reduced with using this system. Zoghi et al. [17] presented a CCHP system including an ORC and a compression-absorption refrigeration system. The heat source was supplied by recovery of the waste heat of a diesel engine. Also, energy and exergy analyses of the system were presented and impacts of some main parameters such as generator temperature, ORC temperatures, pressure ratio of compressor on net power output and cost parameters. However, they did not presented details about engine modeling. Wang et al. [18] studied a CCHP system with ICE and two types of thermal energy storage (TES) system. Natural gas was burned in the ICE to power generation and exhaust gas as well as jacket water were utilized for supply of heater and absorption chiller. The engine performance was evaluated from empirical equations of efficiency and exhaust gas temperature based on nominal power of engine [19]. They performed thermodynamic models and evaluated the performance of the TES tanks. Also, the ratio of heating or cooling loads to power as well as primary energy ratio were estimated at various loads of building. The system performance was investigated for an actual case based on different ranges of power to heat. Fuentes-Cortes et al. [20] conducted a multi-objective optimization aimed at minimizing the annual cost and greenhouse gases emission of CHP system with ICE drives for use in residential complexes. They conducted their studies at two different residential complexes in Mexico, due to weather conditions and energy demand there. Jiang et al. [21] presented thermodynamic model of a new cycle of CCHP with dehumidification plant. They performed experimental tests for a large scale CCHP system in order to validation of the model. Also, an ICE was utilized for power generation, an absorption chiller was selected for cooling demands and the heat of water jacket was used in dehumidification plant. They showed that the higher power output of the ICE resulted in greater values of cooling capacity and generator heating load for the absorption chiller. But, the power of ICE has little effect on coefficient of performance (COP). In ICEs, the chemical energy of fuel inside the cylinder chamber is converted to mechanical and thermal energy after the combustion process. These engines have been recognized as the most commonly used prime mover in CCHP systems due to the simplicity of the settings, low maintenance costs, and high reliability [22]. In the CCHP systems, some devices such as absorption and adsorption chillers can recover the waste heat of prime movers [23]. Therefore, adsorption chillers can operate with combined systems in order to recover the waste heat of prime mover for supply of cooling demands [24]. However, their COP are lower than that for the absorption chillers [25]. Also, their cooling capacity is lower than

• Proposing different arrangements of the CCHP systems with the EF7 engine • Comparing different arrangements of the CCHP system with the •

conventional energy supply systems from the energy, environmental, and economic points of view Assessing the costs of investment and the annual cost savings for

Fig. 1. The first arrangement of the CCHP system. 3

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the building, and a combined cooling and power system is proposed. In all the above arrangements, the working conditions of the internal combustion engines are the same and the energy requirements for all the arrangements (the amount of fuel input to them) are equal.

different arrangements of the CCHP system

• Provide a payback period for each arrangement in different working conditions, with considering carbon dioxide emission tax • Presenting economic parameters including, net present value (NPV), •

present value of the current cash flow(PV) and future saving investment cost (FV) in order of estimating the operating performance of each arrangement Investigating the impacts of air to fuel equivalence ratio of engine on the performance of different arrangements

3. Modeling In this section, the modeling of engine as prime mover is presented and then the governing equations of CCHP system with different arrangements are proposed. Also, environmental and economic models are presented for evaluating of some operating parameters such as carbon dioxide emission reductions and payback period at different arrangements. The details of modeling are presented below.

2. Description of different arrangements In this section, different arrangements of the CCHP systems are presented, and the aim is to examine the potential for using each arrangement and provide a sensitivity analysis. In the first arrangement, a CCHP system has been proposed with two ICEs, two generators, one absorption chiller and one heat exchanger. As shown in Fig. 1, the power output of the two engines is fed into the power generators to supply the electrical power required for the building, as well as the exhaust heat output of the two engines is used for the heating of the building by utilizing a heat exchanger (HX) and the heat dissipated in the cooling system of the engines is used to supply the heat required by the absorption chiller. The cooling required by the building is provided by an absorption chiller. In the second arrangement (Fig. 2), instead of an absorption chiller, an electric chiller was used to provide cooling demands of the building. In this way, the power output of the first engine is used to supply the electrical energy required by the building to the electric generator and the electrical energy produced by the second engine is used to supply the required power for the electric chiller. Also, in this arrangement, the total heat output from the two engines enters the heat exchanger for the heating of the building. In the third arrangement (Fig. 3), the power output of the two engines is used to supply the electrical power required for the building, but only the exhaust heat from the first engine was used for heating, and the heat dissipated from the first engine cooling system and the heat dissipated in the cooling and exhaust system of the second engine is used to provide the required heat of absorption chiller and provides cooling to the building. In the fourth arrangement (Fig. 4), the power output of the two engines is used to supply the electric energy of the building, and only the heat dissipated from the cooling system of the first engine enters the absorption chiller for cooling and the heat dissipated by the exhaust gases of the first engine addition to the heat output from the second engine cooling system and its exhaust gases was used to provide heating for the building. Finally, in the fifth arrangement (Fig. 5), the total heat dissipated from both engines is used to the absorption chiller for cooling, and the power output of the two engines is used to produce electrical energy of

3.1. Engine modeling In this study, a mathematical model was used to simulate the internal combustion engine with utilizing the zero-dimensional model and developing of the single-zone method. The following equations for combustion engine modeling are presented. For each crank angle, the first law of thermodynamics in a combustion engine can be expressed using the following relationships [27]:

dU dQ dW = − dθ dθ dθ

(1)

dU dT = m Cv dθ dθ

(2)

dQjw dQ dXb = ηcomb LHVF mF − dθ dθ dθ

(3)

Also, the net output power of engine can be evaluated by [28]:

dW dV =p dθ dθ

(4) dU , dθ

dQ \; dθ \;

dW dθ

In Eqs. (1)–(4), and respectively represent the internal energy rate, input heat and net output power from the combustion dQjw dX dT engine at each crank angle, Also, dθ , ηcomb , LHVF , dθb , dθ and dV , are dθ temperature rate, combustion chamber efficiency, low heating value of fuel, burned mass fraction rate, waste heat rate of the cooling system and volume change rate of the combustion engine, respectively. On the other hand, m , Cv and p indicate the mass, specific heat capacity in the constant volume and the pressure (operating pressure) of the combustion engine, respectively. The calculated mass fraction used in Eq. (3) can also be calculated using the Weber function by Eq. (5) [27]. nw + 1

⎛ θ − θs ⎞ Xb = 1 − exp ⎡ ⎢−a w ⎝ θd ⎠ ⎣ ⎜

⎟

⎤ ⎥ ⎦

(5)

In Eq. (5), θs and θd are respectively the starting angle of ignition

Fig. 2. The second arrangement of the CCHP system. 4

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Fig. 3. The third arrangement of the CCHP system.

and the combustion time in terms of the angle of the crank. Also, a w and n w are the constants of the Weber function which these values for the spark-ignition engine are 5 and 2, respectively. The volume of the combustion engine cylinder at each angle can also be calculated using the following equation [13]

V (θ) = Vc +

πB2 (l + r − s ) 4

dT 1 dQ ⎞ 1 dV = T (γ − 1) ⎡ ⎜⎛ ⎟⎞ ⎛ − ⎛ ⎞⎛ ⎞⎤ ⎢ pV ⎝ dθ ⎠ ⎝ V ⎠ ⎝ dθ ⎠ ⎥ dθ ⎣⎝ ⎠ ⎦

Furthermore, the instantaneous pressure relation for the combustion engine is derived from differentiation of Eq. (10):

dp p dV p dT = ⎛− ⎞ ⎛ ⎞ + ⎛ ⎞ ⎛ ⎞ dθ ⎝ V ⎠ ⎝ dθ ⎠ ⎝ T ⎠ ⎝ dθ ⎠

(6)

1

γ=

Cp Cv

(9)

(12)

mF LHVF = W + Qt

Also, according to (13), the waste heat from the combustion engine is equal to the sum of the recyclable waste heat and the unrecyclable waste heat. So that recyclable waste heat includes recyclable heat in the cooling system and recyclable heat from exhaust gases in the combustion engine, as well as unrecyclable waste heat from the combustion chamber [29].

(7)

Relationships for gases constant as well as specific heat ratio can also be expressed by the following equations.

R = Cp − Cv (8)

(11)

According to Eq. (12), fuel energy is equal to the sum of the net output power and the waste heat from the combustion engine.

n Eq. (6), Vc , l , B , r and s represent the clearance volume, connecting rod length cylinder bore, crank radius and the distance between the crank axis and piston pin axis, respectively which the latter can be calculated by Eq. (7) [13].

s = r cos θ + (l 2 − r 2sin2 θ) 2

(10)

(8)

Qt = Qex + Qjw + Qloss

(13)

Below, Eq. (13) is expressed for each crank angle equal to sum of the recyclable and unrecyclable heat recovery rates:

(9)

dQjw dQt dQex dQloss = + + dθ dθ dθ dθ

In Eqs. (8) and (9), R and γ represent the gases constant and the specific heat ratio, respectively, Cp and Cv , respectively, shows the specific heat capacity at constant pressure and constant volume. In the following, by applying Eqs. (2), (4), (8), (9) in Eq. (1), the relation between the instantaneous temperature for each crank angle in the combustion engine can be expressed as follows [27]:

dQjw

dQex , dθ

(14)

and respectively, represents the recyclable In Eq. (14), dθ waste heat of the cooling system and the exhaust gases from the combustion engine, and dQloss expresses unrecyclable waste heat to the dθ surrounding environment.

Fig. 4. The fourth arrangement of the CCHP system. 5

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Fig. 5. The fifth arrangement of the CCHP system.

The recyclable heat recovery rate in the cooling system is obtained by using Anand's heat transfer model (15), as well as unrecyclable heat to the environment is calculated by using Eq. (16) [29]:

imep =

dQloss dXb = (1 − ηcomb ) mF LHVF dθ dθ

Qi̇ = Ncyl Qi

(15)

(17)

B T4

(18)

In Eq. (17)\;k gas and Nu indicate the thermal conductivity coefficient of the gas inside the cylinder and the Nusselt number, which can be calculated by the following relations [27]:

+ 7.38

·10−5T

−

1.25·10−8·T 2

(20)

In Eq. (19), Re is the Reynolds number, which is expressed by Eq. (21).

ρgas up B μgas

(21)

In Eq (21), ρgas , μgas , and up represent the gas density, gas dynamic viscosity, and piston speed, respectively. The dynamic viscosity of the gas inside the cylinder can be calculated using the following equation [27]:

μgas = 7.46·10−6 + 4.16 ·10−8T − 7.48·10−12·T 2

(27)

Vd fr = Ncyl . bmep . 2

bmep = imep − fmep

Qhd = Qex , I + Qex , II ηHX

(28)

Qcd = Qjw, I + Qjw, II COPac

(29)

Eed = PICE , I + PICE , II ηgen

(30)

3.2.2. Arrangement 2 For evaluating the heating, cooling and electrical loads, Eqs. (31)–(34) are performed based on energy balances for different components of arrangement 2 as below:

(22)

After calculating the amount of ICE work output and the heat transfer in each components of ICE in (kJ), Eqs. (23)–(27) can be used to calculate the ICE power output, mean effective pressures (mep) and heat transfer in various components of ICE in (kW), respectively [29]

PICE

(26)

3.2.1. Arrangement 1 The energy balance equations for arrangement 1 have been presented by Eqs. (28)–(30) for evaluating the heating, cooling and electrical loads.

(19)

Nu = b Re 0.7

Re =

fr , i = cool, ex , loss 2

(25)

The governing equations for modeling of the electrical power, and heating and cooling loads for each arrangement of the CCHP system with respect to the energy balance for each component are presented. In the following subsections, the mentioned equations are presented.

Tw4 ⎞

− hr = 4.25 × 10−9 ⎜⎛ ⎟ ⎝ T − Tw ⎠

k gas =

W Vd

3.2. General modeling of the combined cooling, heating and power system

k gas

6.19·10−3

=

In Eqs. (23)–(27) Vd , Ncyl , fr , pmax and N are the cylinder displacement volume, the number of cylinders, the combustion engine frequency, the maximum operating pressure in the combustion engine and the combustion engine rotation rate in seconds. In addition, bmep , imep and fmep , respectively, express the brake mean effective pressure, the indicated mean effective pressure and the frictional mean effective pressure.

(16)

In Eq. (15), Tw and ωgas are the temperature of the cylinder wall and the average velocity of the gases inside the cylinder. Moreover, hc and hr represent convective and radiative heat transfer coefficients. of the gases in the cylinder, the conduction heat transfer coefficient and the radiation heat transfer coefficient and they are calculated by the following relationships [27]:

hc = Nu

Vd

fmep = 0.061 + 1.167pmax + 4.9 × 10−6 N

dQjw

1 ⎞ = (hc + hr ) A (T − Tw ) ⎜⎛ ⎟ dθ ⎝ ωgas ⎠

∮ pdV

(23) (24) 6

Qhd = Qex , I + Qex , II + Qjw, I + Qjw, II ηHX

(31)

Qcd = EICE , II COPec

(32)

EICE , II = PICE , II ηgen

(33)

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Eed = PICE , I ηgen

3.4. Modeling of primary energy saving (34) The primary energy saving (PES) is a valuable indicator for estimating the decrease of primary energy consumption of the CHP and CCHP systems compared with conventional systems to supply the same amount of energy [30]. In this way, PES is given in Eq. (46). Also, in Eqs. (47) and (48), the amount of primary energy consumed by the CCHP and the conventional energy production system for supplying the same amount of energy is presented, respectively [31].

3.2.3. Arrangement 3 For arrangement 3, the energy balance equations for system components have been presented by Eqs. (35)–(38) in order to estimating the heating, cooling and electrical capacities of the system.

Qhd = Qex , I ηHX QHX , II = Qex , II ηHX

(35)

F PES = ⎛ ⎝

(36)

F CCHP

⎜

(37)

Eed = PICE , I + PICE , II ηgen

(38)

Qcd = Qjw, I COPac Eed = PICE , I + PICE , II ηgen

⎟

(46) (47)

Eed Qhd Q + Conv + Conv cd Conv ηeConv ηb ηe COPEl

(48)

3.5. Environmental modeling

(39)

Regarding the harmful effects of greenhouse gases, including carbon dioxide (CO2), this study evaluated the reduction of carbon dioxide emissions from the CCHP system compared with conventional energy production systems for buildings for the same amount of energy supply as an environmental factor. In the Eq. (49), the CO2 emission reductions is given. Also, in Eqs. (50) and (51), the amount of carbon dioxide released by the CCHP system and the conventional energy production systems for similar energy supplies are presented [31].

(40)

mCO2Conv − mCO2CCHP ⎞ CO2 ER = ⎜⎛ ⎟ 100 mCO2Conv ⎝ ⎠

3.2.4. Arrangement 4 The energy balance equations for arrangement 4 have been presented by Eqs. (39)–(41) which present the heating, cooling and electrical loads of the system.

Qhd = Qex , I + Qex , II + Qjw, II ηHX

− F CCHP ⎞ 100 F Conv ⎠

= mF LHVF

F Conv =

Qcd = Qjw, I + Qjw, II + QHX , II COPac

Conv

(49)

mCO2CCHP = μCO2F FCCHP (41)

mCO2Conv = μCO2W (Eed ) +

3.2.5. Arrangement 5 Also, for arrangement 5, the energy balance equations of system components have been presented by Eqs. (42)–(45) for evaluating the heating, cooling, and electrical loads of the system.

Qhd = 0

(42)

QHX = Qex , I + Qex , II ηHX

(43)

Qcd = (Qjw, I + Qjw, II ) + QHX COPac

(44)

Eed = PICE , I + PICE , II ηgen

(45)

(50)

μCO2F (Qhd ) ηbConv

+

μCO2W

(Qcd ) Conv COPEl

(51)

3.6. Economic modeling In this research, economic parameters such as fuel cost reductions (FCR), CO2 emission tax, investment cost, operating and maintenance costs in different arrangements, annual total cost saving (ATCS), net present value (NPV), present value of the current cash flow (PV), future saving investment cost (FV), and payback period for different arrangements have been analyzed. According to Eq. (52) the percentage of fuel cost reduction (FCR) for the CCHP system compared to the conventional energy supply systems is calculated for the same amount of energy supply. Moreover, in Eqs. (53) and (54), the cost of fuel for the CCHP and conventional system for supplying the same amount of energy is evaluated, respectively [9].

In Eqs. (28) to (45), Qhd , Qcd , Eed , ηHX , ηgen are heating load, cooling load, electrical load, heat exchanger efficiency and electrical generator efficiency, respectively. The efficiency of the heat exchanger and the electrical generator are considered to be 0.8 and 0.85, respectively. Also COPac and COPec are the coefficient of performance for the absorption and electrical chillers used in the system and their values for many conventional systems are 0.7 and 3, respectively.

Cost Conv − Cost CCHP ⎞ FCR = ⎛ 100 Cost Conv ⎝ ⎠

(52)

Cost CCHP = CostF FCCHP

(53)

⎜

⎟

3.3. Modeling of conventional energy supply system Fig. 6, present a design of traditional and conventional systems that are currently used to provide heating, cooling and electrical loads for buildings. As it is clear, in the conventional system, electric chiller is utilized for cooling, and for electricity and heating demands, are supplied by the grid and gas boiler, respectively.

Fig. 6. The conventional energy supply system for buildings. 7

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Cost Conv = CostW (Eed ) +

CostF (Qhd ) CostW (Qcd ) + Conv ηbConv COPEl

IR n ⎞ FV = D × ⎛1 + 100 ⎝ ⎠

(54)

where, FV and D, respectively, indicate the future value of the investment cost and the initial investment cost. By calculating the payback period of investment, a valuable indicator for the economic analysis of different arrangements of the CCHP systems can be achieved. In fact, the payback period of investment predicts the duration that the total initial investment cost can be recovered by the amount of profit from the system's savings. The payback period of the investment is given in Eq. (63) [35].

n order to deal with the harmful effects of greenhouse gas emissions, including carbon dioxide, the CO2 emission tax scheme has been implemented in many developed countries [32]. Thus, in this study, the annual tax on carbon dioxide emissions for the CCHP and conventional systems according to Eqs. (55) and (56) are presented, respectively [33].

ACO2 T CCHP = υCO2 . mCO2CCHP ·H

(55)

ACO2 T ConvυCO2 . mCO2Conv ·H

(56)

PP =

Also, the initial capital or investment cost of CCHP systems as one of the design factors has been considered. The investment cost for a CCHP system is equal to the total initial cost of the equipment, which can be calculated by using Eq. (57) [9]. CCHP Costinv =

∑ (Costinv )i i

∑ (CostMain )i . H . 365 i

Ctotal ATCS

(63)

4. Air to fuel equivalence ratio for the engine The air to fuel equivalence ratio (λ) is defined as the ratio of actual air to fuel ratio((AF )ac ) to that for stoichiometric condition ((AF )st ) and presented as [36]:

(57)

CCHP is the initial investment cost in dollars and the In Eq. (57), Costinv suffix i represents each of the equipment used in the CCHP systems. Also, the cost of operating and maintenance for the equipments of the CCHP systems has been used as an important factor in analyzing and comparing proposed arrangements. This cost can be calculated by Eq. (58) [29]. CCHP CostMain =

(62)

λ=

(AF )ac (FA)st = (AF )st (FA)ac

(64)

In a combustion process in the engine, for the rich mixture of air and fuel (with higher amount of fuel), λ < 1, for stoichiometric mixture, λ = 1 and for lean mixture λ > 1.

(58)

5. Validation of the engine model

CCHP CostMain ,

is the cost of operating and maintaining for In Eq. (58), each equipment in dollars per year, also H indicates daily operating time in hour. The net annual total costs saving (ATCS) is another factor which used to estimate and compare proposed CCHP systems with conventional energy supply systems. In Eq. (59), the net annual total costs saving is presented. This parameter is equal to the amount of savings in the annual fuel consumption and carbon dioxide emissions taxation in combined production systems when the annual cost of operating and maintenance of the CCHP system equipments is deducted [9]

In this section, the model which used to analysis of the performance of the ICE (zero-dimensional single-zone model) is validated. In order to validate the present model, the technical characteristics of two ICEs called the EF7 gas engine [37] and the gas engine at the University of Mississippi (Mississippi gas engine) [38] are used in accordance with Tables 1 and 2. Fig. 7 presents the power output for the present model and the experimental data [37] for the EF7 gas engine at different rotational speeds. Comparison of the results of the two models indicates the accuracy of the present model for analysis of the ICE. Furthermore, in Table 3 the results of the current model and other models for Mississippi gas engine as well as experimental data at the rotational speed of 1800 rpm are presented. Comparing the results of the present model with the experimental results and other models of previous studies in predicting the percentage of energy converted to the power output, cooling system, exhaust system and to the environment indicates the reliability of the current model.

ATCS = (Cost Conv + ACO2 T Conv )Annual − (Cost CCHP + ACO2 T CCHP )Annual − Co CCHP stMain

(59)

Where Cost CCHP and Cost Conv respectively represent the fuel consumption costs of the CCHP systems and conventional energy production systems. Also, ACO2 T CCHP and ACO2 T Conv represents, respectively, the amount of carbon dioxide emission tax on simultaneous production CCHP systems and conventional energy production systems and CostMain is the cost of service and maintenance in CCHP systems. Net present value expresses that the cash flow in the first year of a project will be worth more than the same cash flow in the year n (n > 1). This method takes into account the effect of time in the cash flow, in which costs are marked with a negative sign and savings with a positive sign. The present value of the cash flow (PV) is determined by the Eq. (60). Also, when determining the present value, a hypothetical interest rate (discount factor) is used which can be calculated by Eq. (61) [34].

PV = S × DF

(60)

IR −n ⎞ DF = ⎛1 + 100 ⎠ ⎝

(61)

6. Sensitivity analysis In this part, sensitivity analysis of five different arrangements of the CCHP system based on the two EF7 gas engines are presented. As previously described, the operating conditions of the two engines are the same in all arrangements, and all of them need to have the same primary energy (fuel) to supply energy. Table 4 contains the parameters required for the evaluation of PES , Table 1 The technical characteristics of the EF7 gas engine [37].

In Eqs. (60) and (61), PV is the present value of cash flow in n years, as well as DF and IR, represent discount factor and interest rate, respectively. The future value of capital investment cost for a project can also be calculated using the following equation [34]: 8

Parameter

Value

Number of cylinders Displacement volume

4 1645

Bore Piston stroke Compression ratio Speed range

78.6 85 11 1000–6500

Unit –

cm3 mm mm – rpm

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Table 2 The technical specifications of the Mississippi gas engine [38]. Parameter

Value

Number of cylinders Displacement volume

4 1820

Bore Piston stroke Compression ratio Speed range

84 82 8.5 1800

Table 4 Parameters required for the evaluation of, CO2 ER , FCR and ACO2 T . Unit

Parameter

–

Conv [9] COPEl

3

(–)

ηtConv [9]

0.8

(–)

ηeConv [9]

0.3

(–)

ηgen [9]

0.85

(–)

μCO2F [9]

220

(gr kWh−1)

μCO2W [9]

836

(gr kWh−1)

CostF [9] LHVF [9] CostW [9] YCO2 [39]

0.09 31650 0.13 0.00003

$ m−3 kJ m−3 $ kWh−1 $ gr−1

cm3 mm mm – rpm

value

Unit

Table 5 Initial investment and service and maintenance costs of the CCHP system equipments.

Fig. 7. Comparison the results of current model with experimental data for EF7 gas engine.

Device

Capital cost ($/kW)

Operating and maintenance cost ($/kWh)

Gas engine [40] Heat exchanger [40] Absorption chiller [40] Electric chiller [40] Generator [9]

1180 30 200 140 40

0.02 0.003 0.01 0.008 –

rising the engine rotational speeds, the energy savings for all arrangement will increase. According to Fig. 9, from the viewpoint of primary energy savings compared to conventional energy supply systems, because of the high COP of the electric chiller as well as recovery of the total heat dissipation of the engine for heating, the amount of PES for the second arrangement is higher than all other arrangements, and it is in the range of 31%. After arrangement 2, arrangement 4 has a larger share for supply of the building heating load than other arrangements, and only the dissipated heat of the cooling system of one engine is absorbed by the absorption chiller for cooling. Thus, due to the low COP of absorption chiller and the fact that the share of supply of heating demands of this arrangement is less than arrangement 2, the amount of energy savings for the arrangement 4 is lower than arrangement 2, but compared to other arrangements, it has a larger amount of primary energy savings. On average, the primary energy savings for arrangement 4 is about 29%. Furthermore, since, in the arrangements are of 1, 3 and 5, respectively, more heat dissipated from the two engines is used in the absorption chiller to provide cooling demands and less heat dissipation potential of the engines used for heating. In the arrangements of 1, 3 and 5, the amounts of primary energy savings are reduced and these values are in the range of 27%, 23% and 19% respectively. Fig. 10 indicates the effect of the EF7 gas engine rotational speed on the percentage of CO2 emission reductions (CO2ER) for different arrangements of the CCHP system. Due to the fact that the second arrangement has the highest primary energy savings, this arrangement can have the highest percentage of CO2ER, and in the following the arrangements of 4, 1, 3 and 5 have the highest percentage of CO2ER,

CO2 ER , FCR and ACO2 T . In addition, Table 5 shows the cost of investment and service and maintenance costs of the CCHP system equipments. Fig. 8 presents the effect of the rotational speed on the percentage of energy transferred to the cooling system, exhaust system, environment and power output of the EF7 engine. As the engine rotational speed increases, the time required to transfer the thermal energy from the engine to the cooling system decreases and the percentage amount of energy transferred to the cooling system decreases. As a result, the percentage of energy converted to the power output (thermal efficiency) and the percentage of energy converted to the exhaust system increases. Table 6 shows the cooling, heating and electrical capacities of five different arrangements of the CCHP system in different engine rotational speeds. As described about different arrangements of the system, the electrical load of the arrangements of 1, 3, 4, 5 is equal, and because in the second arrangement, one engine is used to supply the energy required by the electric chiller, and other engine can only provide the electrical energy of the building, in this case, the electrical load of this arrangement is half of the other arrangements. Also, the heating load in the second arrangement is higher than all arrangements, and then configurations of 4, 1 and 3 have the highest heating load, respectively. Fig. 9 illustrates the impact of the EF7 gas engine rotational speed on the percentage of primary energy saving (PES) for different arrangements of the CCHP systems. As shown in Fig. 8, with increasing the engine rotational speeds, the thermal efficiency increases, so with

Table 3 Comparison the results of current model with results of the other models for Mississippi gas engine.

Manufacturer’s data [38] Yun et al. [38] Arbabi et al. [13] Current model

Energy loss (%)

Jacket cooling efficiency (%)

Exhaust efficiency (%)

Power efficiency (%)

5–15 10 10.9 5.4

17–26 27 25 27

34–45 40 41.8 39.4

25–28 23 22.3 28.2

9

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Fig. 8. The impact of engine rotational speed on the percentage of energy converted.

utilizing all the potential of engine heat losses for heating load, the percentage of FCR of this arrangement is about 72% and is less than all the arrangements. Fig. 12 shows the investment costs for five different arrangements. In all the arrangements, the ICEs capacity is the same. Therefore, the costs of investment for two combustion engines are the same for all the arrangements. Since the capacity and type of cooling and heating equipments are different, their investment costs change and the total cost of the investment for each arrangement is different from the others. Because in the arrangement 5 the absorption chiller has the highest cooling capacity, the investment cost for this arrangement is about 256,000 $. Additionally, with regards to absorption chiller cooling capacity in the arrangement 4 is lower than the other arrangements, the investment cost for this arrangement is about 223,000 $. Also, the investment costs in arrangements of 3 and 2 are almost equal. In Fig. 13, the amount of total annual cost savings for each arrangement is shown for 9 h of daily work at 3000 rpm. In order to calculate the total annual cost savings, the sum of the annual fuel consumption cost savings and the carbon dioxide emission tax are subtracted from the annual cost of operating and maintenance of the system. According to Fig. 13, due to the high absorption chiller cooling capacity in arrangement 5, it has a larger share in fuel consumption cost saving, with an annual savings of about 35,000 $ per year. Therefore, as indicated in Fig. 13, the investment costs in the arrangements of 2 and 3

respectively. Consequently, the percentage of CO2ER for the arrangements of 2, 4, 1, 3, and 5 are in the range of 36%, 34%, 33%, 31%, and 28%, respectively. Fig. 11 shows the impact of the EF7 gas engine rotational speed on the percentage of fuel consumption cost savings or percentage of fuel cost reduction (FCR) for different arrangements of the CCHP system. As mentioned before, in arrangements of 5, 4, 3 and 1, the absorption chiller with using the engine thermal losses can provide cooling loads. In arrangement 5 the share of absorption chiller cooling load is higher than the other arrangements and after it, the arrangements of 3, 1 and 4, have more share in providing absorption cooling, respectively. On the other hand, conventional systems use electric chillers to provide the same amount of cooling load as well as use gas boilers to provide heating load and hot water demand. In addition, the grid electricity cost is much higher than the purchase cost of natural gas (Table 5). It can be concluded that in terms of saving in fuel consumption costs compared to conventional energy supply systems, the use of engine waste heat to provide of cooling load by absorption chiller saves more fuel cost in comparison with the use of waste heat to provide heating load. According to Fig. 11, arrangement 5 has the highest percentage of FCR and it is about 79% while the arrangements of 3, 1 and 4 have smaller share in providing the absorption chiller cooling load demand, and the percentage of FCR in these arrangements are about 78, 76 and 74%, respectively. Additionally, in arrangement 2, because of providing the cooling load by the electric chiller and

Table 6 Cooling, heating and electrical capacities of different arrangements of the CCHP system. nr (rpm)

Arrangement 1

Eed

̇ Qcd

Arrangement 2

̇ Qhd

(kW) 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

19.13 29.76 40.58 51.53 62.57 73.70 84.88 96.11 107.40 118.73 130.09

Eed

̇ Qcd

Arrangement 3

̇ Qhd

(kW) 23.60 32.54 40.89 48.83 56.48 63.88 71.09 78.13 85.03 91.79 98.46

23.83 38.03 52.71 67.74 83.02 98.50 114.14 129.93 145.84 161.85 177.96

9.56 14.88 20.29 25.76 31.28 36.85 42.44 48.06 53.70 59.36 65.04

̇ Qcd

Eed

Arrangement 4

̇ Qhd

(kW) 28.70 44.64 60.87 77.29 93.86 110.55 127.32 144.17 161.11 178.09 195.13

50.81 75.22 99.45 123.55 147.57 171.51 195.39 219.23 243.02 266.76 290.49

Eed

̇ Qcd

Arrangement 5

̇ Qhd

(kW)

19.13 29.76 40.58 51.53 62.57 73.70 84.88 96.11 107.40 118.73 130.09

10

31.94 45.85 59.34 72.54 85.54 98.36 111.04 123.61 136.07 148.44 160.75

11.91 19.01 26.35 33.87 41.51 49.25 57.07 64.96 72.92 80.92 88.98

19.13 29.76 40.58 51.53 62.57 73.70 84.88 96.11 107.40 118.73 130.09

Eed

̇ Qcd

̇ Qhd

40.28 59.16 77.79 96.25 114.59 132.83 150.99 169.09 187.12 205.09 223.04

0 0 0 0 0 0 0 0 0 0 0

(kW) 11.80 16.27 20.44 24.41 28.24 31.94 35.54 39.06 42.51 45.89 49.23

37.32 56.62 76.08 95.65 115.29 135.00 154.77 174.58 194.43 214.31 234.22

19.13 29.76 40.58 51.53 62.57 73.70 84.88 96.11 107.40 118.73 130.09

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Fig. 9. The impact of the engine rotational speed on the percentage of primary energy saving for different arrangements.

Fig. 10. The influence of the engine rotational speed on the percentage of carbon dioxide emission reductions.

Fig. 11. The impact of the engine rotational speed on the percentage of fuel consumption cost savings. 11

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Fig. 12. The investment costs for five different arrangements.

Fig. 13. The total annual cost savings for each arrangements.

equal to 33,993 $. As a result, from viewpoint of net present value, the most efficient choices are related to the arrangement of five, three, one, four, and two, respectively. In addition, at the end of the twentieth year, the 5th arrangement has the highest future cost of investment considering that it has the highest initial investment cost and the annual interest rate is equal for all arrangements. Fig. 14 illustrates the effect of engine rotational speed on the payback period of the investment with 9 h of work per day. Considering that the total annual cost savings for arrangement 5 is greater than the others, its payback period of the investment is shorter than the other arrangements and it is approximately equal to 7 years at 3000 rpm. With rising the rotational speed, the payback period becomes shorter because the saving rates increases. Moreover, in the following, arrangements of 3, 1, 4 and 2 have the shortest payback period, respectively. In the following, the impact of the air to fuel equivalence ratio of ICE on some economic parameters have been evaluated at 3000 rpm in Figs. 15 and 16. Fig. 15 presents the annual total cost savings (ATCS) for each arrangement in different equivalence ratios. By decreasing the equivalence ratio (tending to rich mixture) the engine working pressure rises and the engine output power increases. As the equivalence ratio decreases, the capacity of the components of the CCHP system increases. As mentioned, with decreasing the

Table 7 Net present value method related parameters. Parameter

value

Unit

Operating life of the CCHP system Interest rate

20 4

year (–)

are almost equal. Since in the arrangement 3, an absorption chiller is used for cooling, while in the arrangement 2, an electric chiller is used for cooling, the arrangement 3 has a higher saving on fuel consumption cost than arrangement 2, and the savings in the annual cost of the arrangement 3 is much higher than the arrangement 2. Therefore, the arrangement 3 is more favorable from the economic point of view. Table 7 presents the parameters needed to evaluate the economic criteria for NPV, FV, and PV. Also, in Table 8, the financial fitness of the proposed arrangement has been investigated. In this table, the future investment cost (FV), the present value of the current cash flow (PV) as well as the net present value (NPV) have been presented. Based on Table 8, the Arrangement 5 has the highest annual savings. Consequently, it is expected that the highest value of current cash flow (PV) is expected to belong to this system. Also, during the lifetime of the proposed CCHP systems, the net present value (NPV) for the 5th arrangement is equal to 222,980 $ more than other arrangements. On the other hand, the 2nd arrangement has lowest net present value and 12

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Table 8 Results of the net present value method. Arrangement 1

Arrangement 2

Arrangement 3

Arrangement 4

Arrangement 5

year

DF

PV($)

FV($)

PV($)

FV($)

PV($)

FV($)

PV($)

FV($)

PV($)

FV($)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1.000 0.962 0.925 0.889 0.855 0.822 0.790 0.760 0.731 0.703 0.676 0.650 0.625 0.601 0.577 0.555 0.534 0.513 0.494 0.475 0.456 NPV($)

−231,400 +24,676 +23,727 +22,804 +21,932 +21,085 +20,264 +19,495 +18,751 +18,033 +17,340 +16,673 +16,032 +15,416 +14,801 +14,236 +13,698 +13,159 +12,672 +12,184 +11,697 111,270

−231,400 −231,540 −231,680 −231,820 −231,960 −232,100 −232,230 −323,370 −232,510 −232,650 −232,790 −232,930 −233,070 −233,210 −233,350 −233,490 −233,630 −233,770 −233,910 −233,050 −234,190

−242,380 +19,559 +18,807 +18,075 +17,384 +16,713 +16,062 +15,452 +14,863 +14,293 +13,742 +13,216 +12,708 +12,220 +11,732 +11,284 +10,857 +10,430 +10,044 +9,658 +9,271 33,993

−242,380 −242,530 −242,670 −242,820 −242,960 −243,110 −243,250 −243,400 −243,550 −243,690 −243,840 −243,980 −244,130 −244,280 −244,420 −244,570 −244,720 −244,860 −245,010 −245,160 −245,310

−243,820 +27,094 +26,052 +25,038 +24,080 +23,151 +22,250 +21,405 +20,588 +19,799 +19,039 +18,307 +17,603 +16,927 +16,251 +15,631 +15,040 +14,448 +13,913 +13,378 +12,843 139,010

−243,820 −243,970 −244,110 −244,260 −244,410 −244,550 −244,700 −244,850 −244,990 −245,140 −245,290 −245,430 −245,580 −245,730 −245,880 −246,020 −246,170 −246,320 −246,470 −246,610 −246,760

−223,254 +21,585 +20,755 +19,947 +19,184 +18,444 +17,726 +17,053 +16,402 +15,774 +15,168 +14,585 +14,024 +13,485 +12,947 +12,453 +11,982 +11,511 +11,084 +10,658 +10,232 81,746

−223,254 −223,390 −223,520 −223,660 −223,790 −223,920 −224,060 −224,190 −224,330 −224,460 −224,600 −224,730 −224,870 −225,000 −225,140 −225,270 −225,410 −225,540 −225,680 −225,810 −225,950

−256,308 +33,920 +32,616 +31,346 +30,147 +28,984 +27,855 +26,798 +25,775 +24,788 +23,836 +22,919 +22,037 +21,191 +20,345 +19,569 +18,829 +18,088 +17,418 +16,748 +16,079 222,980

−256,308 −256,460 −256,620 −256,770 −256,920 −257,080 −257,230 −257,390 −257,540 −257,700 −257,785 −258,000 −258,160 −258,310 −258,470 −258,620 −258,780 −258,930 −259,090 −259,250 −259,400

Fig. 14. The effect of engine rotational speed on the payback period with 9 h of work per day.

equivalence ratios. As can be seen, in arrangements 1, 3, 4, 5 that use absorption chillers, with decreasing the equivalence ratio, from 1.1 to 0.9, the reductions payback periods are in the ranges of 1 to 1.5 years for the mentioned arrangements. For the arrangement 2, which uses an electric chiller to provide cooling demand, by decreasing the equivalence ratio, this arrangement, can achieve a high annual benefit and therefore, more decrease in payback period is occurred.

equivalence ratio, the capacity of the CCHP system increases and therefore, the annual total cost savings for all arrangements increases with decreasing the equivalence ratio. Since the cooling capacity of the electric chiller has more increment rate rather than that for the other arrangements, the rate of increase in ATCS for arrangement 2 is higher than that for the other configurations with decrease of the equivalence ration. Fig. 16 shows the payback period for each arrangement in different equivalence ratios. As can be seen, in arrangements 1, 3, 4, 5 that use absorption chillers, with decreasing the equivalence ratio, from 1.1 to 0.9, the reductions payback periods are in the ranges of 1 to 1.5 years for the mentioned arrangements. For the arrangement 2, which uses an electric chiller to provide cooling demand, by decreasing the equivalence ratio, this arrangement, can achieve a high annual benefit and therefore, more decrease in payback period is occurred. Fig. 16 shows the payback period for each arrangement in different

7. Conclusion In this paper the sensitivity analysis and the potential of applying different arrangements of the CCHP systems was examined. Although the prime mover conditions were the same in all arrangements, the results showed that applying different arrangements of the CCHP systems could provide suitable working conditions for these systems from different perspectives, including environmental and economic 13

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Fig. 15. The impact of equivalence ratio on annual total cost savings.

Fig. 16. The impact of equivalence ratio on payback period.

viewpoints. In summary, the results of this research are summarized below:

• If electric chillers are used to provide cooling load in CCHP systems

• • •

because these chillers use the power output of the engine to provide cooling load therefore, they have a relatively high COP. Also, in the arrangement 2, the engine waste heat is completely used for heating load. From the perspective of saving on primary energy consumption and reducing carbon dioxide, the arrangement 2 is much more suitable than the arrangements which utilize engine waste heat to provide cooling by using the absorption chillers. From the point of view of fuel consumption cost due to the high global prices of purchasing electricity comparing with natural gas purchasing cost, the use of all the engine waste heat potential is not suitable for heating, and it is better to use absorption chiller cooling systems. In terms of investment costs, CCHP systems with absorption chillers have higher investment costs than CCHP systems with electrical chiller, but because they have a larger amount of total annual cost saving, they will have a shorter payback period of investment. Among the proposed arrangements, the 5th arrangement has the

• •

highest net present value (NPV) of 222,980 $. Moreover, by considering that the 5th system has the highest annual savings rate and in addition, the discount rate is equal for all arrangements, this arrangement has the highest present value of the cash flow (PV). Furthermore, the 5th system, which has the highest initial investment cost, has the highest future saving investment cost (FV) compared to other proposed arrangements. With increasing of engine rotational speed, the payback period is continuously decreased for all of the arrangements and after 4000 rpm, payback period reduction rate is decreased. By changing the engine air to fuel equivalence ratio from 1.1 to 0.9, there can be a significant increase in annual profits (about 25%) for arrangements that use electric chillers for cooling (Arrangement 2). Also, in this case, the payback period is shorter than 3 years.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

14

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