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Performance analysis of 5 kW PEMFC-based residential micro-CCHP with absorption chiller
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 e n e r g y 4 0 ( 2 0 1 5 ) 1 0 6 4 7 e1 0 6 5 7
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Performance analysis of 5 kW PEMFC-based residential micro-CCHP with absorption chiller Xi Chen a,b, Guangcai Gong a,*, Zhongmin Wan b,**, Liang Luo c, Junhua Wan b a
College of Civil Engineering of Hunan University, Changsha 410082, China College of Information & Communication Engineering, Hunan Institute of Science and Technology, Yueyang 414006, China c College of Physics and Electronics, Hunan Institute of Science and Technology, Yueyang 414006, China b
article info
abstract
Article history:
A novel residential micro-combined cooling heating and power system (CCHP) incorpo-
Received 4 May 2015
rating a proton exchange membrane fuel cell (PEMFC) stack, a single effect absorption
Received in revised form
chiller and accessories is proposed. The proposed CCHP system can provide electric power,
22 June 2015
hot water and space heating/cooling for family demand simultaneously. A steady-state
Accepted 28 June 2015
mathematic model of overall system is developed, and validated by reference data. For
Available online 21 July 2015
parametric analysis, the effects of operating parameters (i.e.: inlet gas temperature and pressure, fuel cell operating temperature and current density) on system performance are
Keywords:
analyzed, especially, the relationship of stack and absorption chiller performance is dis-
PEMFC
cussed. Furthermore, the performances of CCHP system in summer and winter are
Absorption chiller
compared. The maximum efficiency of CCHP system can reaches 70.1% in summer, while
Combined heating cooling and
82% in winter.
power
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Parametric study System efficiency
Introduction In recent decades, the atmospheric pollution caused by extensive economy development in China is deteriorating. The living condition in most of big cities including Beijing is influenced heavily by the hazy weather. Aiming at decreasing the fossil consumption, reducing the carbon emission, fuel cells as one of the clean energies has been widely used. As a prime mover of CCHP system, fuel cell generates power with accessory products of water and waste heat. Thus, it is very
suitable for residential micro-CCHP application and helpful to improve energy utilization efficiency and living condition. Currently, the research on micro-CCHP integrated with fuel cells is widespread, especially on PEMFC and SOFC-based system. By virtues of higher efficiency, lower noise and pollution, the PEMFC is more competitive than SOFC in microCCHP applications [1]. Gigliucci [2] conducted an experimental study on a 4 kW PEMFC-based residential CHP system, the electric efficiency and total efficiency can reach 18% and 50%, respectively. The system was improved by recovering waste heat from steam reformer and optimizing steam to carbon
* Corresponding author. Tel.: þ86 13973123865; fax: þ86 731 88823082. ** Corresponding author. Fax: þ86 730 8648870. E-mail addresses: [email protected] (G. Gong), [email protected] (Z. Wan). http://dx.doi.org/10.1016/j.ijhydene.2015.06.139 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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ratio (S/C), total efficiency increased to 85%. Radulescu [3] tested a 4.5 kW PEMFC-based CHP system, analyzed the influences of current on fuel cell electric efficiency, steam reformer efficiency and system electric efficiency. The system average efficiency is less than 40%, due to the high electric losses, low steam reformer efficiency and bad cooling water pipeline. Chu [4] carried out a particular experiment on PEMFC in a small scaled CHP system, compared the polarization curves of fuel cell stack before and after being activated and performances of hydrogen and natural gas feed fuel cell stack, pointing out the former places emphasis on generating power, the later generating heat. Giacoppo [5] investigated a microCHP system with HT-PEMFC, proposed a power decay factor to evaluate the effects of inlet gas humidity on stack performance, and compared the influences of different fuels on stack performances. Wang [6] developed a PEMFC-based CHP system model, discussed influences of key parameters (i.e.: inlet gas flow, cooling water flow, system temperature and power load) on system efficiency. Furthermore, Wang [7] studied the performance of improved CHP system with steam reformer, the total system efficiency reached 80.6%, the relative errors between simulation and experimental results were less than 4.4%. Arsalis [8] designed a 1 kW HT-PEMFCbased residential micro CHP system which was simplified on the fuel process subsystem and discussed the influences of S/ C, fuel cell operating temperature, combustor temperature and hydrogen stoichiometry on system efficiency. Briguglio [9] conducted an experiment on a LT-PEMFC-based residential CHP system and made evaluation on system performance. A peculiar gas humidification and stack cooling method, socalled “direct water inject” was applied in this system, recovering most of the waste heat produced by the fuel cell. Behzad [10] proposed a 30 kWe HT-PEMFC-based residential CHP model, and compared with LT-PEMFC-based system, results indicated that the power generating efficiency and total efficiency of proposed system were both higher. System parametric study was also conducted. However, the CHP system cannot meet the cooling demand of family in summer, thus the CCHP system is introduced which is able to provide electric power, heating and cooling simultaneously. Currently, most of CCHP studies are focus on the integration of CHP system and chiller (i.e.: electric chiller, absorption chiller and adsorption chiller). Liu [11] proposed an IRSOFC-based CCP system with adsorption chiller. Waste gas from SOFC was combusted to provide heat to adsorption chiller. The effects of decision parameters (i.e.: fuel flow, circulation ratio, fuel utilization factor, mass of adsorbent and inlet air temperature) on system efficiency were discussed. The total efficiency could reach 77% (electric and cooling efficiency took up 62% and 15%, respectively). Yu [12] presented a CCHP system model incorporating SOFC and double-effect absorption chiller, analyzed the influences of current density and fuel utilization factor on fuel cell efficiency, cooling and total efficiency. The system maximum efficiency could reach 80%. Velumani [13] proposed a commercial hybrid CHP system composed of SOFC, micro turbine and single-effect absorption chiller. The waste gas from SOFC is combusted in micro gas turbine to generate power, and the waste heat from turbine is utilized by absorption chiller for cooling. The overall system could generate 230 kW power and 55 kW cooling capacity.
Vadiee [14] presented a commercial greenhouse system based on PEMFC which provide power and heat for the whole system, investigated the influences of operating temperature and air stoichiometry. The results showed that a 3 kW fuel cell system was able to cover 25% and 10% of the usual power and heat demands of a 1000 m2 commercial house. Yu [15] studied the effects of key parameters on the performance of SOFCbased CCHP system under different operating mode. Ma [16] developed a novel CCHP system integrated with SOFC-GT in which waste heat was recovered by ammonia-water mixture. The system parametric study is performed and total efficiency is above 80%. Wang [17] presented a micro-CCHP system which includes DFFC, boiler and double-effect absorption chiller. The effects of fuel cell equivalence ratio and fuel utilization factor is discussed under different operating mode. Chiang [18] performed a parametric study of CCHP system, especially discussed the influences of generator temperature and solution concentration on chiller COP, however, the relationship between the heat source steam and chiller performance was not analyzed. The thermodynamics evaluation on CCHP system based on first and second law of thermodynamics is performed in a number of studies. Barelli [1] compared system efficiency and exergy efficiency of PEMFC and SOFC-based micro-CCHP system, the results indicated that PEMFC-based CCHP is more efficient under atmospheric pressure and low temperature. Xie [19] proposed a 1 kW micro-CHP model using Aspen Plus software, discussed influenced of key parameters on system efficiency and exergy efficiency, analyzed main source of energy and exergy loss. The maximum system energy and exergy efficiency was obtained in a certain parameter range. Currently, most of parametric studies are focus on fuel cell based CHP system and SOFC-based CCHP system. However, less analysis on residential PEMFC-based CCHP system combining absorption is conducted, especially in the influences of fuel cell stack on lithium bromide absorption chiller performance. In this paper, a novel residential microCCHP model is proposed, including PEMFC, single-effect lithium bromide absorption chiller and accessories, and the system model is validated partially by reference data. The main purpose is to investigate the influences of key parameters on the performances of fuel cell stack, chiller and whole system under steady-state operating condition in summer/ winter operating mode.
System layout A configuration of proposed CCHP system is shown in Fig. 1. It consists of a PEMFC stack, a lithium bromide absorption chiller, a heat exchanger and a hot water tank. The electric power generated by PEMFC stack is inverted to AC, and supplied to residence. Besides, the waste heat of PEMFC is recovered by cooling water and admitted to heat exchanger, producing hot water restored in hot water tank. The CCHP system operates in summer/winter mode. In summer, a portion of hot water is used as domestic hot water to meet the daily demand of family such as cooking and shower, and the rest is supplied to absorption chiller for cooling. In winter, it is utilized for space heating instead.
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Fig. 1 e Schematic of CCHP system based on PEMFC and absorption chiller.
System model
ENernst ¼ 1:299 0:85$103 $ðT 298:15Þ h i þ 4:31$105 $T$ ln pH2 $p0:5 O2
Fuel cell stack Fuel cell is a complex system referring to electrochemistry, thermodynamics, heat and mass transfer etc. The numerical model is developed on the following assumption: 1) The hydrogen purity is 100% and it is reacted completely in fuel cell. Air with 21% oxygen and 79% nitrogen is assumed. 2) The concentration and temperature of gas in the stack are uniform. 3) The inlet gas temperatures of hydrogen and air are accordant and set to be constant as 348, 353, 358 K. 4) The operating condition of stack is accordant with single fuel cell. 5) The heat loss in the stack is negligible. The fuel cell stack model is based on [20e22]. In PEMFC, hydrogen and air are supplied to the anode and cathode side, respectively. By means of catalyst layer, the hydrogen is spited into protons and electrons. The protons permeate through the membrane and reach the cathode, while the electrons have to pass the load circuit before arriving cathode. The protons and electrons react with the oxygen modules producing water and waste heat. The chemical reactions can be given as follows:
Anode reaction: H2 / 2Hþ þ 2e
(1)
Cathode reaction: 2Hþ þ 2e þ 1/2O2 / H2O þ heat
(2)
Overall reaction: H2 þ 1/2O2 / H2O þ heat
(3)
The output voltage of single fuel cell can be defined as follows [21]: Vcell ¼ ENernst Vact Vohm Vconc
(4)
where ENernst is Nernst potential (open circuit potential), the calculation formula under the standard condition (298 K, 1 atm) is [21]:
(5)
Vact is activation polarization loss. It plays a leading role in electric loss at low current density and can be expressed as follow [21]: Vact ¼ x1 þ x2 $T þ x3 $T$ln CO2 þ x4 $T$lnðIcell Þ
(6-a)
where xi is empirical coefficient determined by thermodynamics, dynamics, electrochemistry equations, can be given as: DGec DGe x1 ¼ 2F ac nF
(6-b)
x2 ¼
h i a i R h R ln nFAk0c ðCHþ Þð1ac Þ CH2 O c þ ln 4FAk0a CH2 ac nF 2F
(6-c)
x3 ¼
Rð1 ac Þ ac nF
(6-d)
R R þ x4 ¼ 2F ac nF
(6-e)
where DGec (DGe) is standard state free energy (gas state) of chemisorptions (J mol1); ac is chemical activity parameter for cathode; n is number of equivalents involved in a reaction; A is cell active areas (cm2); k0a , k0c are the intrinsic rate constants for the anode and cathode reactions, respectively (cm s1); CHþ ; CH2 O are the concentration of proton, water at anode/ cathode and gas interface (mol cm3); CH2 is the concentration of hydrogen at anode and gas interface (mol cm3); R is gas constant, 8.3143 (J mol1 K1). In this model, the values of xi can be assigned as follows: x1 ¼ 0.948, x2 ¼ 0.00312, x3 ¼ 7.6 105, x4 ¼ 1.93 104 [22]; CO2 is oxygen concentration on cathode catalyst layer (mol cm2); Icell is current in fuel cell (A). Vohm is ohmic polarization loss caused by electrons and protons transmitting procedure in fuel cell, given as: Vohmic ¼ Icell $Rint
(7-a)
where Rint is the internal resistance (U) affected by electrolyte membrane, can be expressed as:
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Rint ¼
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rM l A
(7-b)
where l is membrane thickness; rM is the membrane specific resistivity for the flow of hydrated protons (U cm); A is cell active area (cm2), can be given as [21]: 2 2:5 Icell T 181:6 1 þ 0:03 Icell þ 0:062 A A 303 rM ¼ Icell l 0:634 3 A exp 4:18 T303 T
(7-c)
where l is a semiempirical parameter representing the effective water content of the membrane ranging from 0 to 22. Vconc is concentration polarization loss caused by concentration variation of reactant gases in catalyst layer, given as [24]: Vconc ¼
RT J $ln 1 nF Jmax
(8)
J is current density (A cm2); n is number of equivalents involved in a reaction. A sketch of fuel cell stack is described in Fig. 2. Hydrogen and air are supplied to anode and cathode of fuel cell, respectively, generating power and heat through electrochemistry reaction. The role of cooling water is to guarantee the operating temperature keeping on setting value, and recover waste heat. The current density is considered to be one of key input parameters to control the operating condition of fuel cell. Assuming the operating condition of each single fuel cell is uniform, the output voltage of stack can be given as: Vst ¼ Vcell $N
(9)
where Vcell is the output voltage of single fuel cell, N is the number of single fuel cell. As the prime mover of the system, the PEMFC stack model is designed according to Table 1. The electrical, thermal power and efficiency of PEMFC stack can be given as follows [23]: Pel ¼ Vst $Icell
(10)
Pth ¼ N$ð1:25 Vcell Þ$Icell
(11)
1:25 Vcell 1 ¼ SH2 1:25
Stack parameters (unit)
Value
Number of single fuel cell Operating temperature (K) Inlet gas pressure in anode (atm) Outlet gas pressure in anode (atm) Inlet gas pressure in cathode (atm) Outlet gas pressure in cathode (atm) Hydrogen stoichiometry [13] Air stoichiometry Inlet gas temperature (K) Current Density (mA) Active area (cm2) Volume flow rate of hydrogen (lpm) Volume flow rate of air (lpm)
75 358e368 1e3 1e3 1e3 1e3 1.15 2.5 348e358 0e1000 200 200 650
Gas supply system Before being supplied to the fuel cell stack, the hydrogen and air gases are 100% humidified, pre-heated and compressed to operating condition by stack cooling water and compressor. 1) Cost for gas humidification The heat for gas humidification can be equal to the cost of translating 298 K water to saturated steam at inlet gas temperature, the calculation formula can be given as: Phumid ¼ DhH2 O fH2 O
air
þ fH2 O
(14)
H2
where DhH2 O is the enthalpy difference between 298 K water 1 and saturated steam at inlet gas temperature (kJ mol ); fH2 O air ; fH2 O H2 are the mole flows of steam in humidified air and hydrogen, respectively, and affected by the saturate steam pressure at operating temperature (mol$ s1 ). 2) Cost for gas pre-heating The cost for gas pre-heating from ambient temperature to inlet gas temperature can be given as: Phg ¼ m_ air $cair þ m_ H2 $cH2 $ðTin Tambi Þ
Vcell 1 Vcell ¼ hel ¼ mf 1:25 SH2 1:25 hth ¼ mf
Table 1 e PEMFC stack parameters and values used in the analysis.
(12) Vcell 1 1:25
(13)
where 1.25 (V) is the equal voltage of lower heating value (LHV) of hydrogen [23]; mf is the fuel utilize factor; SH2 is hydrogen stoichiometry.
where m_ is the mass flow rate (kg s ) of inlet gas; Tin is inlet gas temperature; Ta is ambient temperature (K); cair, cH2 are specific heat capacity of air and hydrogen at normal temper1 ature and pressure (kJ$ kg $ K1 ), respectively. 3) Cost for gas compression As the fuel cell stack is operated at high pressure, the inlet gas is compressed by compressor. The power demand to operate the stack at elevated pressure is significant (the compressor efficiency is assumed to be 100%), can be calculated as follows [24]: Pcompress ¼ cp DTgas m_
DTgas ¼ T2 T1 ¼ T1 Fig. 2 e Schematic of PEMFC stack.
(15)
1
(16-a) ! g1 p2 g 1 p1
(16-b)
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g ¼ cp cv
(16-c)
where cp and cv are the specific heat of inlet gas at constant pressure and volume (kJ kg1 K1), respectively; DTgas is the temperature rise of inlet gas after being compressed (K). p1, p2 are the gas pressures before/after being compressed, respectively (atm); g is the ratio of cp and cv.
Lithium bromide absorption chiller [25e29] The single-effect waterelithium bromide (LiBr) absorption chiller consists of four main components: generator, condenser, evaporator, absorber [25], as is shown in Fig. 3. The operating principle can be described as: the LiBreH2O solution is heated by heat source (cooling water of stack) at highpressure and temperature in generator, being separated into refrigerant vapor and concentrated LiBreH2O solution (strong LiBreH2O solution). The refrigerant vapor is condensed into refrigerant liquid flow at high pressure in condenser, and then supplied into evaporator. In evaporator, the refrigerant liquid flow is evaporated at a low pressure and temperature, removing the heat from chilled water. Then the vapor from evaporator is absorbed in the absorber, by the strong LiBreH2O solution from generator. The diluted LiBreH2O solution (weak LiBreH2O solution) is pumped into generator for the next cycle. In general, the cooling load of residential CCHP system is less than 10 kW. Considering the thermal power of fuel cell stack in the proposed system, the RXZ-11.5 type small absorption chiller produced by Zhejiang Lianfeng Group is integrated for cooling. Its general design parameters are shown in Table 2. The absorption chiller model is based on the assumption as follows: (1) The temperature in main components is stable. (2) The heat source temperature is lower than fuel cell operating temperature for 5 K. (3) The temperature of strong LiBreH2O solution in the generator outlet is lower than heat source for 5 K.
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Table 2 e Absorption chiller design parameters. Parameters
Value
Cooling capacity (kW) Input temperature of chilled water tc1(K) Output temperature of chilled water tc2(K) Input temperature of cooling water tw(K) Output temperature of cooling water tw2(K) Input temperature of heat source th1(K) Output temperature of heat source th2(K)
7 288 283 305 309 358 353
(4) The LiBreH2O solution in generator and absorber outlet is saturated. (5) The working medium leaving condenser and evaporator is saturated. (6) The heat and pressure loss of piping and components are negligible. The mass and energy balance equations of components in absorption chiller can be given as follows X X X
_ in ðmÞ
X
m_ in $Xin m_ in $hin
_ out ¼ 0 ðmÞ
X X
(17)
m_ out $Xout ¼ 0
(18)
m_ out $hout þ Q_ ¼ 0
(19)
The energy balance equations of key components in absorption chiller are as follows: QG þ Gw h7 ¼ Mh'3 þ ðGws MÞh4
(20)
Qc þ Mh3 ¼ Mh'3
(21)
QE þ Mh3 ¼ Mh'1
(22)
QA þ Gws h2 ¼ Mh'1 þ ðGW MÞh8
(23)
where QG is the thermal power supplied (from heat source) to generator (kW); Gws is the mass flow rate of weak solution
Fig. 3 e Single-effect absorption chiller circle.
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interring the generator; M is the steam generating rate (kg s1); h7 (h4) is the mass specific enthalpy of weak solution entering (leaving) the generator (kJ kg1); QC is the heat removed by cooling water in condenser (kW); h3 and h'3 are the mass specific enthalpy of refrigerant water and steam, respectively (kJ kg1); QE is the cooling capacity of evaporator; h'1 is the mass specific enthalpy of evaporator outlet vapor (kJ kg1); QA is the heat released when strong solution is absorbing vapor in the absorber (kW); h2 is the mass specific enthalpy of weak solution in absorber outlet (kJ kg1); h8 is the mass specific enthalpy of strong solution entering the absorber after being heat exchanged; qG is the specific heat load of generator (kJ kg1). The cooling capacity of absorption chiller is defined as: QG QE ¼ h'1 h3 qG
(24)
The coefficient of performance (COP) is defined as: COP ¼
QE h'1 h3 ¼ QG qG
Fig. 5 e Waste heat flow chart.
residence after being inverted, on the other hand, the waste heat is utilized to produce hot water to meet the family demand, the rest is used for cooling. In this paper, the actual loads of cooling, heating and power of family in daily life are not considered, just focus on the system energy utilization efficiency. The energy efficiency of CCHP system is defined as:
(25) hCCHP ¼
Residential micro-CCHP system The simulation model of CCHP system is developed using Matlab/Simulink software, as described in Fig. 4. The system is composed of fuel cell stack, lithium bromide absorption chiller and accessories. The input parameters are as follows: input gas temperature, input gas pressure, operating temperature, current density; The output parameters include cooling capacity, COP and system efficiency. The waste heat from fuel cell stack is utilized for inlet gas pre-heating and humidification, hot water supply, cooling and space heating, as shown in Fig. 5.
System efficiency Summer: The electric power and heat are generated by fuel cell stack. On one hand, the electric power is supplied to
Qhw þ QE þ Pel Qin
(26)
where Qhw is the power consumption of domestic hot water (kW); Pel is electric power (kW); Qin is the input power of fuel (kW); hth and hel are the thermal and electric efficiency of fuel cell stack, respectively. The power consumption of domestic hot water can be given as: Qhw ¼
Vhw $m$103 $ðThw Tambi Þ$cw 1 day
(27)
where Vhw is the volume of hot water demand for each people (m3); m is the number of family members; Thw is the temperature of domestic hot water (K). Winter: On account of there is no cooling demand for family in winter, the CCHP system works as CHP system actually. The electric power generated by fuel cell stack is supplied to residence, at the same time the waste heat is recovered for domestic hot water and space heating.
Fig. 4 e PEMFC-based CCHP system simulation diagram.
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The energy efficiency of CHP system is defined as: hCHP ¼
Qhw þ Qth þ Pel Pth þ Pel ¼ Qin Qin
(28)
Model validation Fig. 6a and b shows the performance of fuel cell stack at different inlet gas temperature. The simulation results have a good agreement with the reference data [30]. However, the differences between simulation results at different inlet gas temperature are less than reference data [30] due to the actual voltage loss caused by fuel cell in series is neglected. The relationship between single fuel cell output voltage and operating temperature at 600 (mAcm2) is shown in Fig. 6c. The fluctuation of cell voltage in Ref. [30] is due to the effect of operating temperature on moisture content in membrane, which is neglected in the simulation to simplify the model and consequently the linear results is obtained. The errors of simulation results and reference data [30] are analyzed in Table 3 indicating that the two agrees well, thus the fuel cell stack model is reliable.
Results and discussion To conduct parametric study, the CCHP system simulation model has been developed using Matlab/Simulation software. In this paper, the effects of key parameters including inlet gas temperature and pressure, fuel cell operating temperature and current density on CCHP system performance are analyzed. Similarly, the performances of CHP system operated in winter are also discussed. In general, the inlet gas and operating temperature are set at 348 K and 363 K, respectively. The inlet pressure and current density are fixed at 1 atm and 800 mAcm2.
Fig. 7 e Effect of operating temperature on fuel cell stack power.
The effect of operating temperature on fuel cell stack performance Several studies on the effect of operating temperature on PEMFC performance are conducted [30,32,33]. It is shown in Fig. 7 that the electric and thermal power of fuel cell stack is influenced by the operating temperature at different inlet gas temperature. When the inlet gas temperature is 384 K, with the increasing of operating temperature from 358 K to 368 K, the electric power drops monotonously from 7.16 to 6.95 kW, decreasing by 2.9%. Moreover, the thermal power increases from 7.84 to 8.05 kW, increasing by 2.7%. Comparing with the thermal power, electric power is more sensitive to the variation of operating temperature. When the inlet gas temperature is 353 or 358 K, the variation tendency is similar with before. As a result, aiming at setting the thermaleelectric ratio
Fig. 6 e Comparison of simulation results and reference on fuel cell performance.
Table 3 e Comparison of output voltages between simulation result and reference data [30]. Operating temperature (K) 358e368
Inlet gas temperature (K)
Average value of simulation (V)
Average value of reference [30] (V)
Average relative error
Maximum relative error
348 353 358
0.656 0.678 0.691
0.655 0.678 0.690
0.14% 0 0.14%
0.9% 1.6% 1.7%
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properly, the appropriate operating temperature should be fixed in view of the actual electric and thermal demand of consumer.
Effects of current density and operating temperature on absorption chiller In the proposed system, the cooling capacity of absorption chiller is affected indirectly by the fuel cell current density, as given in Fig. 8a. The cooling capacity increases with current density and reaches maximum value of 7.65, 8.37, 7.76 kW at 1000 mAcm2 with the operating temperature at 358, 363, 368 K, respectively. The reason is the thermal power of fuel cell increases with the raising of current density, the more waste heat recovered from fuel cell, the more cooling capacity offered by chiller. Fig. 8b indicates that the cooling capacity and COP are also influenced by the operating temperature. The cooling capacity fluctuates from 4.43 to 4.26 kW at different temperature, reaching maximum value of 4.8 kW at 363 K. The reason is that the waste heat supplied to absorption chiller drops quickly with the growth of cost on gas humidification. Meanwhile, with the growth of operating temperature, the specific thermal load of generator is decreased by the increase of solution circulation ratio. The two different decreasing
tendencies results in the maximum cooling capacity at 363 K. Furthermore, chiller COP increases with the raise of operating temperature monotonously, due the decrease of specific thermal load of generator which is caused by the raise of heat source temperature relative to the operating temperature (the specific thermal load of evaporator is constant, cooling water inlet temperature is set to be 305 K).
Effect of current density on system efficiency The energy efficiency of CCHP system is influenced significantly by the variation of current density in summer and winter, as given in Fig. 9. In summer, when the current density increases from 100 to 400 mAcm2, the hCCHP grows dramatically to from 53.1% to 67.4%, then increases to maximum value of 68.3% at 641 mAcm2, finally drops to 64.4% at 1000 mAcm2, attaining average at 66.3%. The fluctuation is due to the dominating effect of fuel cell electric power on hCCHP. In addition, the contribution of thermal power on hCCHP is weakened by absorption chiller (COP ¼ 0.7), though thermal power is proportionally linked with current density. Comparing with summer operating mode, hCHP in winter achieves the maximum value of 80.5% at 1000 mAcm2, increasing by 17.9%, and mean value is 77.3% increasing by 16.6%. The improvement is due to the heat loss in cooling is saved in winter.
Effects of inlet gas temperature and operating temperature on system efficiency As shown in Fig. 10, the hCCHP and hCHP at different inlet gas temperature are analyzed. When operating temperature reaches 362 K, the hCCHP with inlet gas temperature at 348, 353, 358 K achieves maximum value of 68.1%, 68.2%, 68.5%, respectively. Meanwhile, the effect on hCHP is similar. To some extent, high inlet gas temperature is helpful to improve the system efficiency, however, the improvement is too little. It can be found that the effect of operating temperature on system efficiency is obviously. Some relevant study is conducted. Arsalis [8] studied the influence of operating temperature on CHP system efficiency. Behzad [18] studied the
Fig. 8 e Effects of current density and operating temperature on cooling capacity.
Fig. 9 e Effect of current density on system efficiency.
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study on the effect of inlet gas pressure on output voltage of PEMFC. Meanwhile, fuel cell is the prime mover of CCHP system, the effect of inlet gas pressure on CCHP system should be discussed. As shown in Fig. 11, in summer, the hCCHP with inlet gas pressure at 1, 2, 3 atm, reaches maximum value of 68.3%, 70.1%, 69.7%, respectively. Obviously, the hCCHP with inlet gas pressure at 2 atm is the highest, hCCHP at 3 atm is a little bit lower than 2 atm, while much higher than 1 atm. There are two reasons: on one hand, Nernst potential at low inlet gas pressure grows quickly, however it is saturate at high inlet gas pressure, on the other hand, the power cost on gas compression at 3 atm is almost 50% higher than 2 atm. In winter, the relationship is similar with that in summer, and the maximum hCHP at 2 atm reaches 82%. Therefore, the optimum inlet gas pressure of fuel cell stack is 2 atm in both summer and winter. Fig. 10 e Effects of inlet gas temperature and operating temperature system efficiency.
relationship between operating temperature and output voltage and gross electric efficiency of micro-cogeneration system. In this paper, the work is focus on the effect of operating temperature on hCCHP with the influence of absorption chiller. The heat source temperature of chiller connected with operating temperature plays a significant role connecting performances of fuel cell stack and absorption chiller, affecting total system efficiency. As shown in Fig. 10, hCCHP fluctuates with the growing of operating temperature, and achieve maximum value of 68.1% at 362 K due to the oscillation of cooling capacity influenced by the heat source capacity and temperature, and the electric power dropping of fuel cell. In winter, with the growth of operating temperature from 358 to 368 K, the hCHP decreases monotonously from 81.7% to 73.5%, the highest efficiency is obtained at 358 K. It can be seen that low operating temperature is helpful to improve the CHP system efficiency in winter.
Effect of inlet gas pressure on system efficiency The performance of fuel cell is obviously affected by inlet gas pressure. Wang [31] conducted experimental and simulation
Fig. 11 e Effect of inlet gas pressure on system efficiency.
Conclusion In this paper, a residential micro-CCHP system driven by PEMFC is developed, supplying electric power, hot water, space heating and cooling to residence in summer and winter. The system incorporates a fuel cell stack, a single-effect watereLiBr absorption chiller and accessories. The influences of key parameters (i.e.: inlet gas temperature, inlet gas pressure, operating temperature and current density) on thermal and electric power of fuel cell and system efficiency are studied, the conclusions are as follows: (1) The operating temperature of fuel cell stack has obvious influence on the performance of CCHP system. In summer, hCCHP fluctuates with the increasing of operating temperature, reaching maximum value of 68.1% at 362 K. In winter, the hCHP drops monotonously with the increasing of operating temperature. The effects of heat source temperature relative to the operating temperature on cooling capacity and COP of absorption chiller are the key reasons responsible for the different system performances in summer and winter. (2) The current density plays an important role on the performance of CCHP system, its effects are different in summer and winter season. In summer, hCCHP oscillates with the increasing of current density, achieving maximum value of 68.3% at 641 mAcm2, attaining average at 66.3%. In winter, the maximum and average hCHP increase by 17.9% and 16.6%, respectively than in summer. The better performance of CHP system in winter is due to the better waste heat utilization than CCHP system in summer. (3) High inlet gas temperature is helpful to the performance improvement of CCHP system. However the improvement is too little. Thus the low inlet gas temperature could be adopted to save the energy cost on gas pre-heating. (4) The increasing of inlet gas pressure is able to improve the CCHP system efficiency. The system efficiency at 2 atm is higher than 1, 3 atm due to the saturation of Nernst potential at high inlet gas pressure and the power cost of compressor.
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Acknowledgment This work is supported by National Natural Science Foundation of China (Grant no. 51378186); The Key Technology Research and Application of Department of Construction in Hunan Province, China (No. KY201111); Science and Technology Support Program of National “Twelve-Five” (No. 2011BAJ03B07) and National Natural Science Foundation of China (No. 51376058).
references
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Nomenclature C: concentration, mol cm2 F: Faraday's constant, 96,485 C mol1 G: mass flow rate, kg s1 h: mass specific enthalpy, kJ kg1 I: current, A J: current density, mAcm2 _ mass flow rate, kg s1 m: N: number of single fuel cell n: number of equivalents involved in a reaction p: pressure, atm P: power, kW Q: heat or power, kW R: resistivity or gas constant, Uor 8.3143 J mol1 K1 S: stoichiometry T: operating temperature, K V: overpotential or voltage or volume, V or m3 X: concentration
Greek letters h: efficiency x: empirical coefficient m: fuel utilization factor Subscripts and superscripts act: activation ambi: ambiance A: absorber or active area of fuel cell, cm2 cell: fuel cell conc: concentration C: condenser or concentration, mol m3 elec: electronic el: electric E: evaporator gh: gas pre-heating G: generator humid: humidification hw: hot water th: thermal i: species in: inlet ohm: ohmic out: outlet prot: proton st: fuel cell stack ws: weak solution
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Available online at www.sciencedirect.com
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Performance analysis of 5 kW PEMFC-based residential micro-CCHP with absorption chiller Xi Chen a,b, Guangcai Gong a,*, Zhongmin Wan b,**, Liang Luo c, Junhua Wan b a
College of Civil Engineering of Hunan University, Changsha 410082, China College of Information & Communication Engineering, Hunan Institute of Science and Technology, Yueyang 414006, China c College of Physics and Electronics, Hunan Institute of Science and Technology, Yueyang 414006, China b
article info
abstract
Article history:
A novel residential micro-combined cooling heating and power system (CCHP) incorpo-
Received 4 May 2015
rating a proton exchange membrane fuel cell (PEMFC) stack, a single effect absorption
Received in revised form
chiller and accessories is proposed. The proposed CCHP system can provide electric power,
22 June 2015
hot water and space heating/cooling for family demand simultaneously. A steady-state
Accepted 28 June 2015
mathematic model of overall system is developed, and validated by reference data. For
Available online 21 July 2015
parametric analysis, the effects of operating parameters (i.e.: inlet gas temperature and pressure, fuel cell operating temperature and current density) on system performance are
Keywords:
analyzed, especially, the relationship of stack and absorption chiller performance is dis-
PEMFC
cussed. Furthermore, the performances of CCHP system in summer and winter are
Absorption chiller
compared. The maximum efficiency of CCHP system can reaches 70.1% in summer, while
Combined heating cooling and
82% in winter.
power
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Parametric study System efficiency
Introduction In recent decades, the atmospheric pollution caused by extensive economy development in China is deteriorating. The living condition in most of big cities including Beijing is influenced heavily by the hazy weather. Aiming at decreasing the fossil consumption, reducing the carbon emission, fuel cells as one of the clean energies has been widely used. As a prime mover of CCHP system, fuel cell generates power with accessory products of water and waste heat. Thus, it is very
suitable for residential micro-CCHP application and helpful to improve energy utilization efficiency and living condition. Currently, the research on micro-CCHP integrated with fuel cells is widespread, especially on PEMFC and SOFC-based system. By virtues of higher efficiency, lower noise and pollution, the PEMFC is more competitive than SOFC in microCCHP applications [1]. Gigliucci [2] conducted an experimental study on a 4 kW PEMFC-based residential CHP system, the electric efficiency and total efficiency can reach 18% and 50%, respectively. The system was improved by recovering waste heat from steam reformer and optimizing steam to carbon
* Corresponding author. Tel.: þ86 13973123865; fax: þ86 731 88823082. ** Corresponding author. Fax: þ86 730 8648870. E-mail addresses: [email protected] (G. Gong), [email protected] (Z. Wan). http://dx.doi.org/10.1016/j.ijhydene.2015.06.139 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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ratio (S/C), total efficiency increased to 85%. Radulescu [3] tested a 4.5 kW PEMFC-based CHP system, analyzed the influences of current on fuel cell electric efficiency, steam reformer efficiency and system electric efficiency. The system average efficiency is less than 40%, due to the high electric losses, low steam reformer efficiency and bad cooling water pipeline. Chu [4] carried out a particular experiment on PEMFC in a small scaled CHP system, compared the polarization curves of fuel cell stack before and after being activated and performances of hydrogen and natural gas feed fuel cell stack, pointing out the former places emphasis on generating power, the later generating heat. Giacoppo [5] investigated a microCHP system with HT-PEMFC, proposed a power decay factor to evaluate the effects of inlet gas humidity on stack performance, and compared the influences of different fuels on stack performances. Wang [6] developed a PEMFC-based CHP system model, discussed influences of key parameters (i.e.: inlet gas flow, cooling water flow, system temperature and power load) on system efficiency. Furthermore, Wang [7] studied the performance of improved CHP system with steam reformer, the total system efficiency reached 80.6%, the relative errors between simulation and experimental results were less than 4.4%. Arsalis [8] designed a 1 kW HT-PEMFCbased residential micro CHP system which was simplified on the fuel process subsystem and discussed the influences of S/ C, fuel cell operating temperature, combustor temperature and hydrogen stoichiometry on system efficiency. Briguglio [9] conducted an experiment on a LT-PEMFC-based residential CHP system and made evaluation on system performance. A peculiar gas humidification and stack cooling method, socalled “direct water inject” was applied in this system, recovering most of the waste heat produced by the fuel cell. Behzad [10] proposed a 30 kWe HT-PEMFC-based residential CHP model, and compared with LT-PEMFC-based system, results indicated that the power generating efficiency and total efficiency of proposed system were both higher. System parametric study was also conducted. However, the CHP system cannot meet the cooling demand of family in summer, thus the CCHP system is introduced which is able to provide electric power, heating and cooling simultaneously. Currently, most of CCHP studies are focus on the integration of CHP system and chiller (i.e.: electric chiller, absorption chiller and adsorption chiller). Liu [11] proposed an IRSOFC-based CCP system with adsorption chiller. Waste gas from SOFC was combusted to provide heat to adsorption chiller. The effects of decision parameters (i.e.: fuel flow, circulation ratio, fuel utilization factor, mass of adsorbent and inlet air temperature) on system efficiency were discussed. The total efficiency could reach 77% (electric and cooling efficiency took up 62% and 15%, respectively). Yu [12] presented a CCHP system model incorporating SOFC and double-effect absorption chiller, analyzed the influences of current density and fuel utilization factor on fuel cell efficiency, cooling and total efficiency. The system maximum efficiency could reach 80%. Velumani [13] proposed a commercial hybrid CHP system composed of SOFC, micro turbine and single-effect absorption chiller. The waste gas from SOFC is combusted in micro gas turbine to generate power, and the waste heat from turbine is utilized by absorption chiller for cooling. The overall system could generate 230 kW power and 55 kW cooling capacity.
Vadiee [14] presented a commercial greenhouse system based on PEMFC which provide power and heat for the whole system, investigated the influences of operating temperature and air stoichiometry. The results showed that a 3 kW fuel cell system was able to cover 25% and 10% of the usual power and heat demands of a 1000 m2 commercial house. Yu [15] studied the effects of key parameters on the performance of SOFCbased CCHP system under different operating mode. Ma [16] developed a novel CCHP system integrated with SOFC-GT in which waste heat was recovered by ammonia-water mixture. The system parametric study is performed and total efficiency is above 80%. Wang [17] presented a micro-CCHP system which includes DFFC, boiler and double-effect absorption chiller. The effects of fuel cell equivalence ratio and fuel utilization factor is discussed under different operating mode. Chiang [18] performed a parametric study of CCHP system, especially discussed the influences of generator temperature and solution concentration on chiller COP, however, the relationship between the heat source steam and chiller performance was not analyzed. The thermodynamics evaluation on CCHP system based on first and second law of thermodynamics is performed in a number of studies. Barelli [1] compared system efficiency and exergy efficiency of PEMFC and SOFC-based micro-CCHP system, the results indicated that PEMFC-based CCHP is more efficient under atmospheric pressure and low temperature. Xie [19] proposed a 1 kW micro-CHP model using Aspen Plus software, discussed influenced of key parameters on system efficiency and exergy efficiency, analyzed main source of energy and exergy loss. The maximum system energy and exergy efficiency was obtained in a certain parameter range. Currently, most of parametric studies are focus on fuel cell based CHP system and SOFC-based CCHP system. However, less analysis on residential PEMFC-based CCHP system combining absorption is conducted, especially in the influences of fuel cell stack on lithium bromide absorption chiller performance. In this paper, a novel residential microCCHP model is proposed, including PEMFC, single-effect lithium bromide absorption chiller and accessories, and the system model is validated partially by reference data. The main purpose is to investigate the influences of key parameters on the performances of fuel cell stack, chiller and whole system under steady-state operating condition in summer/ winter operating mode.
System layout A configuration of proposed CCHP system is shown in Fig. 1. It consists of a PEMFC stack, a lithium bromide absorption chiller, a heat exchanger and a hot water tank. The electric power generated by PEMFC stack is inverted to AC, and supplied to residence. Besides, the waste heat of PEMFC is recovered by cooling water and admitted to heat exchanger, producing hot water restored in hot water tank. The CCHP system operates in summer/winter mode. In summer, a portion of hot water is used as domestic hot water to meet the daily demand of family such as cooking and shower, and the rest is supplied to absorption chiller for cooling. In winter, it is utilized for space heating instead.
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Fig. 1 e Schematic of CCHP system based on PEMFC and absorption chiller.
System model
ENernst ¼ 1:299 0:85$103 $ðT 298:15Þ h i þ 4:31$105 $T$ ln pH2 $p0:5 O2
Fuel cell stack Fuel cell is a complex system referring to electrochemistry, thermodynamics, heat and mass transfer etc. The numerical model is developed on the following assumption: 1) The hydrogen purity is 100% and it is reacted completely in fuel cell. Air with 21% oxygen and 79% nitrogen is assumed. 2) The concentration and temperature of gas in the stack are uniform. 3) The inlet gas temperatures of hydrogen and air are accordant and set to be constant as 348, 353, 358 K. 4) The operating condition of stack is accordant with single fuel cell. 5) The heat loss in the stack is negligible. The fuel cell stack model is based on [20e22]. In PEMFC, hydrogen and air are supplied to the anode and cathode side, respectively. By means of catalyst layer, the hydrogen is spited into protons and electrons. The protons permeate through the membrane and reach the cathode, while the electrons have to pass the load circuit before arriving cathode. The protons and electrons react with the oxygen modules producing water and waste heat. The chemical reactions can be given as follows:
Anode reaction: H2 / 2Hþ þ 2e
(1)
Cathode reaction: 2Hþ þ 2e þ 1/2O2 / H2O þ heat
(2)
Overall reaction: H2 þ 1/2O2 / H2O þ heat
(3)
The output voltage of single fuel cell can be defined as follows [21]: Vcell ¼ ENernst Vact Vohm Vconc
(4)
where ENernst is Nernst potential (open circuit potential), the calculation formula under the standard condition (298 K, 1 atm) is [21]:
(5)
Vact is activation polarization loss. It plays a leading role in electric loss at low current density and can be expressed as follow [21]: Vact ¼ x1 þ x2 $T þ x3 $T$ln CO2 þ x4 $T$lnðIcell Þ
(6-a)
where xi is empirical coefficient determined by thermodynamics, dynamics, electrochemistry equations, can be given as: DGec DGe x1 ¼ 2F ac nF
(6-b)
x2 ¼
h i a i R h R ln nFAk0c ðCHþ Þð1ac Þ CH2 O c þ ln 4FAk0a CH2 ac nF 2F
(6-c)
x3 ¼
Rð1 ac Þ ac nF
(6-d)
R R þ x4 ¼ 2F ac nF
(6-e)
where DGec (DGe) is standard state free energy (gas state) of chemisorptions (J mol1); ac is chemical activity parameter for cathode; n is number of equivalents involved in a reaction; A is cell active areas (cm2); k0a , k0c are the intrinsic rate constants for the anode and cathode reactions, respectively (cm s1); CHþ ; CH2 O are the concentration of proton, water at anode/ cathode and gas interface (mol cm3); CH2 is the concentration of hydrogen at anode and gas interface (mol cm3); R is gas constant, 8.3143 (J mol1 K1). In this model, the values of xi can be assigned as follows: x1 ¼ 0.948, x2 ¼ 0.00312, x3 ¼ 7.6 105, x4 ¼ 1.93 104 [22]; CO2 is oxygen concentration on cathode catalyst layer (mol cm2); Icell is current in fuel cell (A). Vohm is ohmic polarization loss caused by electrons and protons transmitting procedure in fuel cell, given as: Vohmic ¼ Icell $Rint
(7-a)
where Rint is the internal resistance (U) affected by electrolyte membrane, can be expressed as:
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Rint ¼
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rM l A
(7-b)
where l is membrane thickness; rM is the membrane specific resistivity for the flow of hydrated protons (U cm); A is cell active area (cm2), can be given as [21]: 2 2:5 Icell T 181:6 1 þ 0:03 Icell þ 0:062 A A 303 rM ¼ Icell l 0:634 3 A exp 4:18 T303 T
(7-c)
where l is a semiempirical parameter representing the effective water content of the membrane ranging from 0 to 22. Vconc is concentration polarization loss caused by concentration variation of reactant gases in catalyst layer, given as [24]: Vconc ¼
RT J $ln 1 nF Jmax
(8)
J is current density (A cm2); n is number of equivalents involved in a reaction. A sketch of fuel cell stack is described in Fig. 2. Hydrogen and air are supplied to anode and cathode of fuel cell, respectively, generating power and heat through electrochemistry reaction. The role of cooling water is to guarantee the operating temperature keeping on setting value, and recover waste heat. The current density is considered to be one of key input parameters to control the operating condition of fuel cell. Assuming the operating condition of each single fuel cell is uniform, the output voltage of stack can be given as: Vst ¼ Vcell $N
(9)
where Vcell is the output voltage of single fuel cell, N is the number of single fuel cell. As the prime mover of the system, the PEMFC stack model is designed according to Table 1. The electrical, thermal power and efficiency of PEMFC stack can be given as follows [23]: Pel ¼ Vst $Icell
(10)
Pth ¼ N$ð1:25 Vcell Þ$Icell
(11)
1:25 Vcell 1 ¼ SH2 1:25
Stack parameters (unit)
Value
Number of single fuel cell Operating temperature (K) Inlet gas pressure in anode (atm) Outlet gas pressure in anode (atm) Inlet gas pressure in cathode (atm) Outlet gas pressure in cathode (atm) Hydrogen stoichiometry [13] Air stoichiometry Inlet gas temperature (K) Current Density (mA) Active area (cm2) Volume flow rate of hydrogen (lpm) Volume flow rate of air (lpm)
75 358e368 1e3 1e3 1e3 1e3 1.15 2.5 348e358 0e1000 200 200 650
Gas supply system Before being supplied to the fuel cell stack, the hydrogen and air gases are 100% humidified, pre-heated and compressed to operating condition by stack cooling water and compressor. 1) Cost for gas humidification The heat for gas humidification can be equal to the cost of translating 298 K water to saturated steam at inlet gas temperature, the calculation formula can be given as: Phumid ¼ DhH2 O fH2 O
air
þ fH2 O
(14)
H2
where DhH2 O is the enthalpy difference between 298 K water 1 and saturated steam at inlet gas temperature (kJ mol ); fH2 O air ; fH2 O H2 are the mole flows of steam in humidified air and hydrogen, respectively, and affected by the saturate steam pressure at operating temperature (mol$ s1 ). 2) Cost for gas pre-heating The cost for gas pre-heating from ambient temperature to inlet gas temperature can be given as: Phg ¼ m_ air $cair þ m_ H2 $cH2 $ðTin Tambi Þ
Vcell 1 Vcell ¼ hel ¼ mf 1:25 SH2 1:25 hth ¼ mf
Table 1 e PEMFC stack parameters and values used in the analysis.
(12) Vcell 1 1:25
(13)
where 1.25 (V) is the equal voltage of lower heating value (LHV) of hydrogen [23]; mf is the fuel utilize factor; SH2 is hydrogen stoichiometry.
where m_ is the mass flow rate (kg s ) of inlet gas; Tin is inlet gas temperature; Ta is ambient temperature (K); cair, cH2 are specific heat capacity of air and hydrogen at normal temper1 ature and pressure (kJ$ kg $ K1 ), respectively. 3) Cost for gas compression As the fuel cell stack is operated at high pressure, the inlet gas is compressed by compressor. The power demand to operate the stack at elevated pressure is significant (the compressor efficiency is assumed to be 100%), can be calculated as follows [24]: Pcompress ¼ cp DTgas m_
DTgas ¼ T2 T1 ¼ T1 Fig. 2 e Schematic of PEMFC stack.
(15)
1
(16-a) ! g1 p2 g 1 p1
(16-b)
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g ¼ cp cv
(16-c)
where cp and cv are the specific heat of inlet gas at constant pressure and volume (kJ kg1 K1), respectively; DTgas is the temperature rise of inlet gas after being compressed (K). p1, p2 are the gas pressures before/after being compressed, respectively (atm); g is the ratio of cp and cv.
Lithium bromide absorption chiller [25e29] The single-effect waterelithium bromide (LiBr) absorption chiller consists of four main components: generator, condenser, evaporator, absorber [25], as is shown in Fig. 3. The operating principle can be described as: the LiBreH2O solution is heated by heat source (cooling water of stack) at highpressure and temperature in generator, being separated into refrigerant vapor and concentrated LiBreH2O solution (strong LiBreH2O solution). The refrigerant vapor is condensed into refrigerant liquid flow at high pressure in condenser, and then supplied into evaporator. In evaporator, the refrigerant liquid flow is evaporated at a low pressure and temperature, removing the heat from chilled water. Then the vapor from evaporator is absorbed in the absorber, by the strong LiBreH2O solution from generator. The diluted LiBreH2O solution (weak LiBreH2O solution) is pumped into generator for the next cycle. In general, the cooling load of residential CCHP system is less than 10 kW. Considering the thermal power of fuel cell stack in the proposed system, the RXZ-11.5 type small absorption chiller produced by Zhejiang Lianfeng Group is integrated for cooling. Its general design parameters are shown in Table 2. The absorption chiller model is based on the assumption as follows: (1) The temperature in main components is stable. (2) The heat source temperature is lower than fuel cell operating temperature for 5 K. (3) The temperature of strong LiBreH2O solution in the generator outlet is lower than heat source for 5 K.
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Table 2 e Absorption chiller design parameters. Parameters
Value
Cooling capacity (kW) Input temperature of chilled water tc1(K) Output temperature of chilled water tc2(K) Input temperature of cooling water tw(K) Output temperature of cooling water tw2(K) Input temperature of heat source th1(K) Output temperature of heat source th2(K)
7 288 283 305 309 358 353
(4) The LiBreH2O solution in generator and absorber outlet is saturated. (5) The working medium leaving condenser and evaporator is saturated. (6) The heat and pressure loss of piping and components are negligible. The mass and energy balance equations of components in absorption chiller can be given as follows X X X
_ in ðmÞ
X
m_ in $Xin m_ in $hin
_ out ¼ 0 ðmÞ
X X
(17)
m_ out $Xout ¼ 0
(18)
m_ out $hout þ Q_ ¼ 0
(19)
The energy balance equations of key components in absorption chiller are as follows: QG þ Gw h7 ¼ Mh'3 þ ðGws MÞh4
(20)
Qc þ Mh3 ¼ Mh'3
(21)
QE þ Mh3 ¼ Mh'1
(22)
QA þ Gws h2 ¼ Mh'1 þ ðGW MÞh8
(23)
where QG is the thermal power supplied (from heat source) to generator (kW); Gws is the mass flow rate of weak solution
Fig. 3 e Single-effect absorption chiller circle.
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interring the generator; M is the steam generating rate (kg s1); h7 (h4) is the mass specific enthalpy of weak solution entering (leaving) the generator (kJ kg1); QC is the heat removed by cooling water in condenser (kW); h3 and h'3 are the mass specific enthalpy of refrigerant water and steam, respectively (kJ kg1); QE is the cooling capacity of evaporator; h'1 is the mass specific enthalpy of evaporator outlet vapor (kJ kg1); QA is the heat released when strong solution is absorbing vapor in the absorber (kW); h2 is the mass specific enthalpy of weak solution in absorber outlet (kJ kg1); h8 is the mass specific enthalpy of strong solution entering the absorber after being heat exchanged; qG is the specific heat load of generator (kJ kg1). The cooling capacity of absorption chiller is defined as: QG QE ¼ h'1 h3 qG
(24)
The coefficient of performance (COP) is defined as: COP ¼
QE h'1 h3 ¼ QG qG
Fig. 5 e Waste heat flow chart.
residence after being inverted, on the other hand, the waste heat is utilized to produce hot water to meet the family demand, the rest is used for cooling. In this paper, the actual loads of cooling, heating and power of family in daily life are not considered, just focus on the system energy utilization efficiency. The energy efficiency of CCHP system is defined as:
(25) hCCHP ¼
Residential micro-CCHP system The simulation model of CCHP system is developed using Matlab/Simulink software, as described in Fig. 4. The system is composed of fuel cell stack, lithium bromide absorption chiller and accessories. The input parameters are as follows: input gas temperature, input gas pressure, operating temperature, current density; The output parameters include cooling capacity, COP and system efficiency. The waste heat from fuel cell stack is utilized for inlet gas pre-heating and humidification, hot water supply, cooling and space heating, as shown in Fig. 5.
System efficiency Summer: The electric power and heat are generated by fuel cell stack. On one hand, the electric power is supplied to
Qhw þ QE þ Pel Qin
(26)
where Qhw is the power consumption of domestic hot water (kW); Pel is electric power (kW); Qin is the input power of fuel (kW); hth and hel are the thermal and electric efficiency of fuel cell stack, respectively. The power consumption of domestic hot water can be given as: Qhw ¼
Vhw $m$103 $ðThw Tambi Þ$cw 1 day
(27)
where Vhw is the volume of hot water demand for each people (m3); m is the number of family members; Thw is the temperature of domestic hot water (K). Winter: On account of there is no cooling demand for family in winter, the CCHP system works as CHP system actually. The electric power generated by fuel cell stack is supplied to residence, at the same time the waste heat is recovered for domestic hot water and space heating.
Fig. 4 e PEMFC-based CCHP system simulation diagram.
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The energy efficiency of CHP system is defined as: hCHP ¼
Qhw þ Qth þ Pel Pth þ Pel ¼ Qin Qin
(28)
Model validation Fig. 6a and b shows the performance of fuel cell stack at different inlet gas temperature. The simulation results have a good agreement with the reference data [30]. However, the differences between simulation results at different inlet gas temperature are less than reference data [30] due to the actual voltage loss caused by fuel cell in series is neglected. The relationship between single fuel cell output voltage and operating temperature at 600 (mAcm2) is shown in Fig. 6c. The fluctuation of cell voltage in Ref. [30] is due to the effect of operating temperature on moisture content in membrane, which is neglected in the simulation to simplify the model and consequently the linear results is obtained. The errors of simulation results and reference data [30] are analyzed in Table 3 indicating that the two agrees well, thus the fuel cell stack model is reliable.
Results and discussion To conduct parametric study, the CCHP system simulation model has been developed using Matlab/Simulation software. In this paper, the effects of key parameters including inlet gas temperature and pressure, fuel cell operating temperature and current density on CCHP system performance are analyzed. Similarly, the performances of CHP system operated in winter are also discussed. In general, the inlet gas and operating temperature are set at 348 K and 363 K, respectively. The inlet pressure and current density are fixed at 1 atm and 800 mAcm2.
Fig. 7 e Effect of operating temperature on fuel cell stack power.
The effect of operating temperature on fuel cell stack performance Several studies on the effect of operating temperature on PEMFC performance are conducted [30,32,33]. It is shown in Fig. 7 that the electric and thermal power of fuel cell stack is influenced by the operating temperature at different inlet gas temperature. When the inlet gas temperature is 384 K, with the increasing of operating temperature from 358 K to 368 K, the electric power drops monotonously from 7.16 to 6.95 kW, decreasing by 2.9%. Moreover, the thermal power increases from 7.84 to 8.05 kW, increasing by 2.7%. Comparing with the thermal power, electric power is more sensitive to the variation of operating temperature. When the inlet gas temperature is 353 or 358 K, the variation tendency is similar with before. As a result, aiming at setting the thermaleelectric ratio
Fig. 6 e Comparison of simulation results and reference on fuel cell performance.
Table 3 e Comparison of output voltages between simulation result and reference data [30]. Operating temperature (K) 358e368
Inlet gas temperature (K)
Average value of simulation (V)
Average value of reference [30] (V)
Average relative error
Maximum relative error
348 353 358
0.656 0.678 0.691
0.655 0.678 0.690
0.14% 0 0.14%
0.9% 1.6% 1.7%
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properly, the appropriate operating temperature should be fixed in view of the actual electric and thermal demand of consumer.
Effects of current density and operating temperature on absorption chiller In the proposed system, the cooling capacity of absorption chiller is affected indirectly by the fuel cell current density, as given in Fig. 8a. The cooling capacity increases with current density and reaches maximum value of 7.65, 8.37, 7.76 kW at 1000 mAcm2 with the operating temperature at 358, 363, 368 K, respectively. The reason is the thermal power of fuel cell increases with the raising of current density, the more waste heat recovered from fuel cell, the more cooling capacity offered by chiller. Fig. 8b indicates that the cooling capacity and COP are also influenced by the operating temperature. The cooling capacity fluctuates from 4.43 to 4.26 kW at different temperature, reaching maximum value of 4.8 kW at 363 K. The reason is that the waste heat supplied to absorption chiller drops quickly with the growth of cost on gas humidification. Meanwhile, with the growth of operating temperature, the specific thermal load of generator is decreased by the increase of solution circulation ratio. The two different decreasing
tendencies results in the maximum cooling capacity at 363 K. Furthermore, chiller COP increases with the raise of operating temperature monotonously, due the decrease of specific thermal load of generator which is caused by the raise of heat source temperature relative to the operating temperature (the specific thermal load of evaporator is constant, cooling water inlet temperature is set to be 305 K).
Effect of current density on system efficiency The energy efficiency of CCHP system is influenced significantly by the variation of current density in summer and winter, as given in Fig. 9. In summer, when the current density increases from 100 to 400 mAcm2, the hCCHP grows dramatically to from 53.1% to 67.4%, then increases to maximum value of 68.3% at 641 mAcm2, finally drops to 64.4% at 1000 mAcm2, attaining average at 66.3%. The fluctuation is due to the dominating effect of fuel cell electric power on hCCHP. In addition, the contribution of thermal power on hCCHP is weakened by absorption chiller (COP ¼ 0.7), though thermal power is proportionally linked with current density. Comparing with summer operating mode, hCHP in winter achieves the maximum value of 80.5% at 1000 mAcm2, increasing by 17.9%, and mean value is 77.3% increasing by 16.6%. The improvement is due to the heat loss in cooling is saved in winter.
Effects of inlet gas temperature and operating temperature on system efficiency As shown in Fig. 10, the hCCHP and hCHP at different inlet gas temperature are analyzed. When operating temperature reaches 362 K, the hCCHP with inlet gas temperature at 348, 353, 358 K achieves maximum value of 68.1%, 68.2%, 68.5%, respectively. Meanwhile, the effect on hCHP is similar. To some extent, high inlet gas temperature is helpful to improve the system efficiency, however, the improvement is too little. It can be found that the effect of operating temperature on system efficiency is obviously. Some relevant study is conducted. Arsalis [8] studied the influence of operating temperature on CHP system efficiency. Behzad [18] studied the
Fig. 8 e Effects of current density and operating temperature on cooling capacity.
Fig. 9 e Effect of current density on system efficiency.
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study on the effect of inlet gas pressure on output voltage of PEMFC. Meanwhile, fuel cell is the prime mover of CCHP system, the effect of inlet gas pressure on CCHP system should be discussed. As shown in Fig. 11, in summer, the hCCHP with inlet gas pressure at 1, 2, 3 atm, reaches maximum value of 68.3%, 70.1%, 69.7%, respectively. Obviously, the hCCHP with inlet gas pressure at 2 atm is the highest, hCCHP at 3 atm is a little bit lower than 2 atm, while much higher than 1 atm. There are two reasons: on one hand, Nernst potential at low inlet gas pressure grows quickly, however it is saturate at high inlet gas pressure, on the other hand, the power cost on gas compression at 3 atm is almost 50% higher than 2 atm. In winter, the relationship is similar with that in summer, and the maximum hCHP at 2 atm reaches 82%. Therefore, the optimum inlet gas pressure of fuel cell stack is 2 atm in both summer and winter. Fig. 10 e Effects of inlet gas temperature and operating temperature system efficiency.
relationship between operating temperature and output voltage and gross electric efficiency of micro-cogeneration system. In this paper, the work is focus on the effect of operating temperature on hCCHP with the influence of absorption chiller. The heat source temperature of chiller connected with operating temperature plays a significant role connecting performances of fuel cell stack and absorption chiller, affecting total system efficiency. As shown in Fig. 10, hCCHP fluctuates with the growing of operating temperature, and achieve maximum value of 68.1% at 362 K due to the oscillation of cooling capacity influenced by the heat source capacity and temperature, and the electric power dropping of fuel cell. In winter, with the growth of operating temperature from 358 to 368 K, the hCHP decreases monotonously from 81.7% to 73.5%, the highest efficiency is obtained at 358 K. It can be seen that low operating temperature is helpful to improve the CHP system efficiency in winter.
Effect of inlet gas pressure on system efficiency The performance of fuel cell is obviously affected by inlet gas pressure. Wang [31] conducted experimental and simulation
Fig. 11 e Effect of inlet gas pressure on system efficiency.
Conclusion In this paper, a residential micro-CCHP system driven by PEMFC is developed, supplying electric power, hot water, space heating and cooling to residence in summer and winter. The system incorporates a fuel cell stack, a single-effect watereLiBr absorption chiller and accessories. The influences of key parameters (i.e.: inlet gas temperature, inlet gas pressure, operating temperature and current density) on thermal and electric power of fuel cell and system efficiency are studied, the conclusions are as follows: (1) The operating temperature of fuel cell stack has obvious influence on the performance of CCHP system. In summer, hCCHP fluctuates with the increasing of operating temperature, reaching maximum value of 68.1% at 362 K. In winter, the hCHP drops monotonously with the increasing of operating temperature. The effects of heat source temperature relative to the operating temperature on cooling capacity and COP of absorption chiller are the key reasons responsible for the different system performances in summer and winter. (2) The current density plays an important role on the performance of CCHP system, its effects are different in summer and winter season. In summer, hCCHP oscillates with the increasing of current density, achieving maximum value of 68.3% at 641 mAcm2, attaining average at 66.3%. In winter, the maximum and average hCHP increase by 17.9% and 16.6%, respectively than in summer. The better performance of CHP system in winter is due to the better waste heat utilization than CCHP system in summer. (3) High inlet gas temperature is helpful to the performance improvement of CCHP system. However the improvement is too little. Thus the low inlet gas temperature could be adopted to save the energy cost on gas pre-heating. (4) The increasing of inlet gas pressure is able to improve the CCHP system efficiency. The system efficiency at 2 atm is higher than 1, 3 atm due to the saturation of Nernst potential at high inlet gas pressure and the power cost of compressor.
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Acknowledgment This work is supported by National Natural Science Foundation of China (Grant no. 51378186); The Key Technology Research and Application of Department of Construction in Hunan Province, China (No. KY201111); Science and Technology Support Program of National “Twelve-Five” (No. 2011BAJ03B07) and National Natural Science Foundation of China (No. 51376058).
references
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Nomenclature C: concentration, mol cm2 F: Faraday's constant, 96,485 C mol1 G: mass flow rate, kg s1 h: mass specific enthalpy, kJ kg1 I: current, A J: current density, mAcm2 _ mass flow rate, kg s1 m: N: number of single fuel cell n: number of equivalents involved in a reaction p: pressure, atm P: power, kW Q: heat or power, kW R: resistivity or gas constant, Uor 8.3143 J mol1 K1 S: stoichiometry T: operating temperature, K V: overpotential or voltage or volume, V or m3 X: concentration
Greek letters h: efficiency x: empirical coefficient m: fuel utilization factor Subscripts and superscripts act: activation ambi: ambiance A: absorber or active area of fuel cell, cm2 cell: fuel cell conc: concentration C: condenser or concentration, mol m3 elec: electronic el: electric E: evaporator gh: gas pre-heating G: generator humid: humidification hw: hot water th: thermal i: species in: inlet ohm: ohmic out: outlet prot: proton st: fuel cell stack ws: weak solution
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