Analyzing microcogeneration systems based on LT-PEMFC and HT-PEMFC by energy balances

Applied Energy 108 (2013) 82–91

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Analyzing microcogeneration systems based on LT-PEMFC and HT-PEMFC by energy balances Elio Jannelli a, Mariagiovanna Minutillo a,⇑, Alessandra Perna b a b

University of Naples ‘‘Parthenope’’, Centro Direzionale, Isola C4, 80143 Naples, Italy University of Cassino and Southern Lazio, Via G. Di Biasio 43, 03043 Cassino, Italy

h i g h l i g h t s  The performance of integrated systems based on HT-PEMFC and LT-PEMFC are compared.  A steam reforming unit is used to produce hydrogen rich gas for fuel cells feeding.  The analysis is performed by numerical models validated by using experimental data.  HT-PEMFCs offer higher system electric efficiencies but lower cogeneration efficiencies. HT-PEMFCs offer a more simple system architecture.

a r t i c l e

i n f o

Article history: Received 31 December 2012 Received in revised form 19 February 2013 Accepted 20 February 2013

Keywords: Reforming system High temperature PEMFC Cogeneration Numerical models Experimental activity

a b s t r a c t This paper focuses on the performance analysis of microcogeneration systems based on the integration between a reforming unit (RFU), consisting of a natural gas steam reforming, and a power unit, based on the PEM fuel cell technology. The analysis has been carried out considering, as power unit, three different PEM fuel cells: a low temperature PEM fuel cell with Nafion™ membrane (LT-FC) operating at 67 °C, a high temperature PEM fuel cell with a membrane based on polybenzimidazole material doped with phosphoric acid (HT-FC1) operating at 160 °C, and a high temperature PEM fuel cell that uses aromatic polyether polymers/copolymers bearing pyridine units doped with phosphoric acid as electrolyte (HT-FC2) operating at 180 °C. The study has been conducted by using numerical models tuned by experimental data measured in test benches developed at University of Cassino. For sizing the power units able to provide a maximum electric power of 2.5 kW (this size allows to satisfy the electric and thermal energy demand of an Italian household), two designing criteria have been considered. Results have shown that the integrated systems based on the HT-FCs are characterized by high electric efficiency (40%) and cogeneration efficiency (79%). Moreover, the thermal power recovered decreases with the stacks operating temperature, thus the highest cogeneration efficiency (80%) is obtained by the microcogeneration system based on low temperature fuel cells. However, the availability of high temperature heat makes the HT-FC an attractive solution for the cogeneration/trigeneration systems development. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, microcogeneration systems have received increasing attention because, by providing both useful electricity and heat with high efficiency, they can have a strategic role in the reduction of greenhouse gas emissions according to the European Union targets. Among the microcogeneration technologies, PEMFCs are considered an emerging alternative to combustion-based cogeneration systems because of their high power density, low operating tem⇑ Corresponding author. Tel.: +39 0815476792. E-mail address: [email protected] (M. Minutillo). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.02.067

perature, and fast start-up and shutdown. Thus, many studies and research activities have been focused on PEMFCs and their applications in the field of power generation [1–12]. PEMFCs function best with high purity hydrogen gas as the fuel source, but pure hydrogen is unlikely to be the fuel source in the near term due to technical and economic considerations in production and storage [13,14]. Thus, in order to overcome the lack of hydrogen infrastructure, the PEMFC can be fed with syngas produced by reforming conventional fuels such as natural gas, available in most metropolitan households [15]. However, this reformate gas contains small amounts of carbon monoxide (CO) which poisons the platinum anode catalyst [16– 19] impacting on the regular PEMFC operation [20]. Therefore, con-

E. Jannelli et al. / Applied Energy 108 (2013) 82–91

siderable efforts have been made to reduce the effect of CO, such as: (i) oxygen or air bleeding into the fuel, (ii) advanced purification of the synthesis gas (multi-stage preferential CO oxidation or separation membranes), (iii) developing CO-tolerant catalysts (e.g. PtRu/C, PtSn/C) [21]. All these mitigation methods have significant drawbacks and/or performance issues. For instance, the addition of oxidant to the fuel stream decreases the fuel utilization and compromises the safety, the additional fuel processing increases the system complexity and the costs, and the effectiveness of new CO tolerant catalysts is far from satisfactory. A different way to overcome the above mentioned limits is to increase the PEM fuel cell operating temperature at values higher than 100 °C (high temperature PEM fuel cell), because the adsorption of CO onto the catalyst sites is very reduced. This implies that the polymeric membrane has to be modified. Therefore, in the last years, a great attention has been addressed to the research of membrane materials. The major development can be summarized as follows [22–28]:  Modified PFSA membranes, by means of incorporating inorganic compounds (SiO2 and TiO2) which allow to improve the strength, thermal stability and water retention at high temperatures.  The use of sulfonated polyaromatic polymers and composite membranes (PEEK, PI, PSF and SPSF) which allow the realization of membranes less dependent on humidity than PFSA, assuring a good proton conductivity and a good cell performance.  The use of polybenzimidazole-based (PBI) membranes in which the water is replaced with another solvent for the proton transport with a higher boiling point (e.g. phosphoric acid and imidazoles).  The use of aromatic polyether polymers or copolymers that bear pyridine units. Aromatic polyether backbones are chosen for their high mechanical, thermal and chemical stability, while the incorporation of polar pyridine groups aids in the retention of phosphoric acid. By comparing with conventional PEMFCs, operating at around 80 °C (low temperature PEM fuel cell), the high temperature PEM fuel cells result to have [29]:  Accelerated reaction kinetics at the electrodes that could even allow platinum to be replaced by more economical catalysts.  A simpler water management because the presence of liquid water can be neglected.  A higher CO tolerance (e.g. >1% CO at 150 °C).  High temperature waste heat availability for cogeneration/trigeneration applications.  A simplified architecture of the integrated fuel processor-fuel cell system. This last advantage is very important to develop microcogeneration systems because a minor complexity and a better compactness can be obtained. As a matter of fact, high temperature PEM fuel cells, integrated with fuel processing systems, do not need large shift converters or selective oxidizers, due to their high CO tolerance [27] and the fuel cell reactants humidification system is not required because the proton transport in the membrane occurs without water dragging. By contrast, high temperature PEM fuel cells require cell voltages of over 0.7 V to achieve system efficiencies higher than low temperature PEM fuel cells, but their performance are currently still low [30,31]. In order to obtain system efficiencies higher than 40%, further research efforts have been recently made to improve the membrane and electrodes performance. In recent years several papers have focused on microcogeneration systems based on low temperature PEM fuel cell [32–35], but

83

only few studies concern the analysis of systems based on high temperature PEM fuel cells [36,37]. This paper focuses on the performance comparison of power systems based on low and high temperature PEM fuel cells in order to evaluate their ability of satisfying the requirements of high efficiencies and simple plant configuration needed for microcogeneration applications. The systems analysis has been performed by means of thermochemical models tuned by using experimental data. 2. The microcogeneration plant configuration and design The microcogeneration systems proposed in this paper are characterized by a reforming unit (RFU) and a power unit (PU). The reforming unit is a pre-commercial fuel processor for natural gas steam reforming [38] and produces 45 Nl/min of syngas. The hydrogen concentration is about 75% vol. (2 Nm3/h at full load), while the CO content is less than 1 ppm as declared by the manufacturer and verified in our test station. The power unit is based on PEMFC technology, both high and low temperature proton exchange membrane fuel cells (HT-FC and LT-FC). Therefore, two plant configurations of the microcogeneration system have been investigated:  RFU/LT-FC system  RFU/HT-FC system 2.1. RFU/LT-FC system configuration In Fig. 1 the plant configuration of the RFU/LT-FC system is depicted. The conversion of natural gas in a hydrogen rich stream for PEMFC feeding is carried out in two steps, a high temperature endothermic step that takes place in the steam reforming reactor (SR) in which the hydrocarbons are converted into a gaseous mixture of H2, CO, CO2, CH4 and unreacted H2O and a low temperature slightly esothermic step that occurs in the water gas shift reactor (WGSR) in which CO is reacted with H2O towards H2 and CO2. Because the shift reaction is equilibrium-limited, CO conversion is not complete and an additional step of CO removal is necessary in order to reduce the CO concentration at the value (<10 ppm) required by the low temperature PEM fuel cell. This is achieved by using a preferential oxidation reactor (PROX) where the syngas reacts with a controlled amount of air over an activated Ru catalyst (a novel Ru/Al2O3 catalyst) that allows to lower the CO content below 1 ppm. The heat required for the steam reforming reaction is supplied by a catalytic burner (CB) fed with the natural gas and the anodic exhausts from the PEMFC power unit. The flow rate and the composition of the anodic exhausts recirculated to the catalytic burner depend on the operating fuel utilization factor (or fuel stoichiometry) chosen for the PEM fuel cell stack. Moreover, the steam for the reforming reactions is produced by recovering the heat from the syngas cooling in two heat exchangers (HEX1 and HEX2). Due to the operating temperature of PROX and in order to prevent its catalyst from damage, the syngas is further cooled in HEX3 and dried in a drain trap (DRAIN). Finally, the syngas and the air, after passing throughout the humidifiers, are sent to the LT-FC where the electrochemical reactions occur. Water is used as stack cooling stream and the heat recovered is used for house heating purpose. 2.2. RFU/HT-FC system configuration With respect to the RFU/LT-FC lay-out, the plant configuration of RFU/HT-FC system, depicted in Fig. 2, is simplified.

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Fig. 1. Lay-out of the microcogeneration system RFU/LT-FC.

The reforming reactor, the water gas shift reactor and the heat exchangers (HEX1 and HEX2) operate as already described, while the PROX reactor and the reactants humidifiers are removed because they are not required for the HT-FC operation. The drain trap is replaced with a water knock-out drum (WKD) where the water content in the reformate is removed. The anodic exhaust from the HT-FC is recirculated to the catalytic burner, while the cathodic one, which has a high enthalpy content, is used for pre-heating the air for the catalytic burner (HEX3). Thus, in this configuration a great integration between the reforming unit and the power unit is realized. The HT-FC power unit is cooled by a heat transfer fluid (Paratherm MRÒ) and the heat recovered is used for house heating purpose. 2.3. Power unit design As power unit, three different PEM fuel cells were considered: (i) a low temperature PEM fuel cell with Nafion™ membrane (LTFC) operating at 67 °C, designed to be fed with reformate gas which contains H2, N2, CO2, CH4 and less than 10 ppm of CO; (ii) a high temperature PEM fuel cell with a membrane based on polybendimidazole material doped with phosphoric acid (HT-FC1) operating at 160 °C; (iii) a high temperature PEM fuel cell that uses aromatic polyether polymers/copolymers bearing pyridine units doped with phosphoric acid as electrolyte (HT-FC2) operating at 180 °C. The main characteristics and operating conditions of the fuel cells are reported in Table 1. These fuel cells have been investigated in two different test benches: the first one has been developed in order to characterize

the performance of low temperature PEM stacks (air or water cooled) under pure hydrogen or synthesis gas feeding [4,39,40], while the second one is able to characterize the behavior of high temperature PEM single cells by varying both operating parameters and feeding mixtures [41]. Fig. 3 shows the polarization curves measured for each fuel cell. The voltage of the LT-FC is an average cell voltage because the experimental activity has been conducted on a Ballard pre-commercial stack that operates in the range 0.05–0.2 A/cm2, whereas the polarization curves of the HT-FCs have been carried out on single cell stacks whose maximum current density is 0.8 A/cm2. The experimental data have been obtained by feeding the LT-FC with a syngas whose composition is that exiting the PROX reactor (see Fig. 1), and by feeding the HT-FCs with a syngas whose composition is that leaving the WKD component (see Fig. 2). The power units have been designed in order to provide a maximum electric power of 2.5 ± 0.2 kW, that is the size able to satisfy the electric and thermal energy demand of an Italian household; as a matter of fact, the typical electrical size of micro-CHP systems able to satisfy the energy demand profile of an Italian household is in the range of 1–10 kW e. In this power range the electric efficiency expected must be higher than 30% and the cogeneration efficiency higher than 70%. These values can allow to obtain a primary energy saving (PES)>0, as required by the European directive 2004/8/CE. Thus, for sizing the power units two designing criteria have been considered (see Fig. 3): (a) Case 1: fixed current density

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Fig. 2. Lay-out of the microcogeneration system RFU/HT-FC.

Table 1 Technical data of the analyzed PEMFCs. LT-FC

HT-FC1

HT-FC2

Design characteristics Membrane material Pt/C loading Membrane thickness (lm) Durability (h)

Nafion™ n.a. 178 40,000

Polybenzimidazole-H3PO4 0.7 mg/cm2 anode 1 mg/cm2 cathode 50–75 >20,000

Aromatic polyethers-pyridine – H3PO4 1 mg/cm2 anode 1 mg/cm2 cathode 65 >4000

Operating conditions Operating temperature, Tcell (°C) Temperature range (°C) Anode pressure, pan (atm) Cathode pressure, pcat (atm) Fuel stoichiometry, kan Air stoichiometry, kcat Anode humidification (%) Anode humidification (%)

67 55–70 1 1 1.2 2 100 100

160 120–180 1 1 1.2 2 – –

180 150–200 1 1 1.2 2 – –

(b) Case 2: fixed cell voltage (or cell efficiency) The values chosen as reference data correspond to the nominal operating point of the LT-FC, 0.2 A/cm2 and 0.676 V. Therefore, the HT-FC stacks have been designed to operate at maximum current density of 0.2 A/cm2, because this choice allows to satisfy the requirements of the cogeneration systems in term of electric power generated and high electric efficiency. In fact, by assuming for instance a current density of 0.8 A/cm2, the electric power produced by the power unit of RFU/HT-FC1 results of 1.6 kW (this value has been calculated considering the maximum hydrogen mass flow available from the reforming unit), and the cell efficiency is 0.23. These values are far from those required by the residential utility. Furthermore, the following assumptions have been adopted:

 The cell active area is equal to 200 cm2.  The maximum hydrogen mass flow is equal to 0.14 kg/h (the maximum hydrogen flow rate produced by the RFU).  The power unit consists of two stacks in order to satisfy the variable demand of electric load with high efficiency. Table 2 summarizes the stacks characteristics for case 1 and case 2.

3. System modelling The microcogeneration systems have been modelled by using a commercial thermo-chemical code. The modelling is based on the following assumptions.

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where ai,j is the number of atoms of the jth element in a mole of the ith species. Aj is defined as the total number of atoms of the jth element in the reaction mixture. The Lagrangian function (L) is:

1.2

LT-FC

HT-FC1

HT-FC2

1

k N X X L¼G  kj ni  ai;j  Aj

!

Cell Voltage (V)

t

j¼1

0.8

ð3Þ

i¼1

fixed cell voltage

0.6

where kj is the Lagrange multiplier. The partial derivatives of Eq. (3) are set equal to zero in order to find the extremum point:

0.4

@L ¼0 @ni

fixed current density

Eq. (4) creates the set of non-linear equations which are solved by an iteration technique. The equilibrium composition (the species considered are H2, CO, CO2, H2O, CH4, C4H10, C3H8, C2H6, C(s), N2) has been calculated for the given operating condition and, in order to evaluate the overall efficiency, the material and energy balances are also solved.

0.2

0 0.0

0.2

0.4

0.6

ð4Þ

0.8

Current density (A/cm2) Fig. 3. Experimental polarization curves of the fuel cells. LT-FC anode gas composition:75% H2, 20% CO2, 3% N2, CH4 2%, CO<1 ppm; HT-FCs anode gas composition: 75% H2, 24% CO2, 1% CO [39,41].

3.1. Reforming unit modelling  Steam reforming reactor (SR) and water gas shift reactor (WGSR). The steam reforming and the water gas shift processes are simulated assuming the chemical equilibrium (catalysts are employed to speed up the reactions) [42–44]. The equilibrium composition is solved by a non-stoichiometric formulation, the direct minimization of the Gibbs free energy for a given set of species, without any specification of the possible reactions which might take place in the system. In particular, considering the total Gibbs free energy of the system, Gt, defined as:

Gt ¼

N X ni  li

ð1Þ

i¼1

where ni is the moles number of species i, and li is their chemical potential, the problem is to find the values of ni which minimize the objective function Gt. The appropriate method, which is usually performed for the minimization of the Gibbs free energy problem, is the Lagrange multipliers. The constraint of this problem is the elemental balance, i.e. N X ni  ai;j ¼ Aj

j ¼ 1; 2; . . . ; k

ð2Þ

i¼1

Table 2 Design strategies for the PU of the microcogeneration system. LT-FC

HT-FC1

HT-FC2

Case 1 Cell current density (A/cm2) Cell efficiency (%) Cell voltage (V) Number of cells (for each stack) Number of stacks Stack power (kW)

0.2 0.364 0.676 46 2 1.24

0.2 0.356 0.659 46 2 1.21

0.2 0.335 0.620 46 2 1.14

Case 2 Cell voltage (V) Cell efficiency (%) Cell current density (A/cm2) Number of cells (for each stack) Number of stacks Stack power (kW)

0.676 0.364 0.2 46 2 1.24

0.676 0.364 0.17 55 2 1.26

0.676 0.364 0.12 78 2 1.26

 Catalytic burner (CB). This reactor is modelled considering the stoichiometric reactions of reactants which are converted in products according to a given fractional conversion factor.  Preferential oxidation reactor (PROX). This reactor is modelled as a stoichiometric reactor. The reformate is reacted with a controlled amount of air depending on the effectiveness factor of the catalyst employed. This means that the oxidation of hydrogen (and hydrocarbons species eventually present in the syngas) has to be also considered in the reactions set. 3.2. Power unit modelling Fuel cells modelling can be dealt with a different level of detail and rigor (from zero dimensional approach to three dimensions) and their usefulness depends on the application field. In particular the modelling issues can be viewed at cell level to improve the understanding of complex physical and chemical phenomena or at fuel cell system level to investigate the impact of the operating conditions on the overall system. In order to estimate the electric performance of the fuel cell, a zero-dimensional model, based on the empirical equation proposed by Kim [45], has been used to forecast the cell voltage. The thermal behavior of the fuel cell has been evaluated by using a thermal model based on a first law approach. The fuel cell voltage has been calculated by the following equation:

V cell ¼ V 0  b  lnðiÞ  R  i  m  expðn  iÞ

ð5Þ

2

where i is the current density (A/cm ), V0 (V) is the reversible cell potential, b (V) is the Tafel slope, R ( cm2) is the ohmic resistance, m (V) and n (cm2/A) are parameters that account for the mass transport overpotential. These coefficients have been calculated, for both LT-FC and HTFCs by using experimental data. The thermal power recovered from the fuel cell has been determined by the thermal analysis based on mass and energy balances [4]. Fig. 4 shows a schematic of the energy and thermal fluxes entering and exiting the fuel cell. The energy balance can be written as follows:

Pch;cell;in ¼ Pel;cell þ Pc;cell þ Pa;cell þ Pch;cell;out þ Pth;cell þ Ploss;cell

ð6Þ

where Pch,cell,in and Pch,cell,out are the chemical powers entering and exiting the anode side respectively (these are the product of the hydrogen mole flow rates at the inlet and outlet of the PEMFC and the HHV), Pel,cell is the electrical power generated, Pa,cell and Pc,cell are the net thermal fluxes associated with the anode and cathode

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Fig. 4. Schematic of the energy and thermal fluxes entering and leaving a fuel cell.

streams, Pth,cell is the thermal power recovered from the cell cooling, Ploss,cell is the heat loss from the cell external surface. Pa,cell and Pc,cell are calculated as:

Pa;cell ¼ ðRi Ui;a;out  Ri Ui;a;in Þcell

ð7Þ

Pc;cell ¼ ðRi Ui;c;out  Ri Ui;c;in Þcell

ð8Þ

where Ui,a,in and Ui,c,in are the sensible heats of the species i at the inlet of the anode and cathode side respectively and Ui,a,out and Ui,c,out are the sensible heats of the species i at the outlet of the anode and cathode side respectively. The heat loss from the cell external surface by natural convection and radiation is evaluated as follows:

Ploss;cell ¼ h  S  ðT cell  T room Þ þ r  e  S  ðT cell Þ4

ð9Þ

where S is the surface no insulated, Tcell is the cell temperature (K), Troom (K) is the ambient temperature, h is the convective heat transfer coefficient (equal to 9 W/m2 K as in [46]), r the Stefan–Boltzmann constant (5.67  108 W/m2 K4) and e the emissivity of graphite (equal to 0.5 as in [46]). Thus, the thermal power recovered can be calculated through the difference between the chemical power consumed by the PEM fuel cell and the all other terms, as:

Pth;cell ¼ Pch;cell;in  Pch;cell;out  Pa;cell  Pc;cell  Pel;cell  Ploss;cell

3.3. Model validation The model validation has been performed by means of experimental data measured in the test stations designed and realized at the Fuel Cells Lab of the Cassino University. The laboratory test facilities are equipped by a National Instruments Compact DAQ real-time data acquisition and a control system running under LabView™ software. Each bench commands the mass flow controllers that regulate each flow via two RS232 ports. Pressures and temperatures have been measured with a sampling frequency of 1 Hz. The reforming unit behavior has been investigated by measuring the catalytic reactors temperatures and the syngas compositions exiting the WGSR and the PROX reactor [47]. The compositions of the reformate gas have been analyzed by using a gas chromatograph (7890A system, Agilent Technologies Inc.,) able to operate under high sensitivity conditions. Table 3 reports the main assumptions for the operating conditions of the reforming

Table 3 Measured and predicted operating conditions of the reforming unit (RFU) [47].

Main operating conditions of RFU Fuel to SR (Nl/min) Water to SR (cc/min) Air to PROX (Nl/min) CB temperature (°C) SR temperature (°C) WGSR temperature (°C) PROX temperature (°C)

ð10Þ

The fuel cell model has been implemented into the thermochemical model of the microcogeneration system by using a FORTRAN block calculator in which the performance of the power unit has been calculated as:

Pel;PU ¼ Nstack  ncell  V cell  I

ð11Þ

Pth;PU ¼ Nstack  ncell  Pth;cell

ð12Þ

where I is the current intensity, Nstack and ncell are the number of stacks and of cells for each stack respectively.

Dry basis

Model

8.4a 22 1.6 805 ± 3 670 ± 4 265 ± 10 94 ± 6

8.4a 22 1.6 805 670

WGSR Measured

Syngas composition (vol.) H2 (%) 78.30 ± 0.4 CO2 (%) 17.80 ± 0.2 N2 (%) – CH4 (%) 3.60 ± 0.5 CO (%) 0.64 ± 0.06 a

Measured

PROX Calculated

Measured

Calculated

77.35 18.91 – 3.17 0.60

76.03 ± 0.3 18.10 ± 0.2 2.84 ± 0.2 3.03 ± 0.5 <10 ppm

74.92 19.87 2.82 2.38 0.55 ppm

Natural gas: CH4 88.9% vol, C2H6 6.8 vol.%, C3H8 3.1 vol.%, C4H10 1.2 vol.%.

E. Jannelli et al. / Applied Energy 108 (2013) 82–91

unit and the comparison between the measured and the calculated syngas compositions after the WGSR and the PROX reactor. Taking into account that a little variability in the chemical composition of the syngas is usually observed during regular operations, the thermo-chemical model developed is able to predict the reforming unit performance with a good agreement with respect to the experimental data. In order to estimate the coefficients of Eq. (5), a multiple regression technique has been applied to the measured data points (cell potential versus current density) reported in Fig. 3. Table 4 lists the empirical parameters of Kim’s equation for each PEM fuel cell while in Fig. 5 the experimental and calculated polarization curves for the low and high temperature PEM fuel cells are compared. Although the meaning of each coefficients should be physicochemical, it does not exist a single optimal solution and different combinations of equations parameters can describe the experimental data with reasonable accuracy [48].

1.2

1

Cell Voltage (V)

88

0.8

0.6

0.4

LT-FC exp HT-FC1 exp HT-FC2 exp

0.2

0 0.00

0.05

4. Energy balances and performance

Pel;PU _ NG;CB Þ  HHV NG þm þP

ð14Þ

_ NG;REF (kg/s) is the natural gas flow rate to the process, where m _ NG;CB (kg/s) is that sent to the catalytic burner, HHVNG (kJ/kg) is m its high heating value. In Fig. 6 the trend of the systems electric efficiency, calculated considering the designing criteria proposed (case 1 and case 2), are reported. The electric efficiency of the RFU/LT-FC system is the same for both cases, as shown in Table 2. On the contrary, the behavior of the RFU/HT-FC systems is very different when fixed cell voltage or fixed current density is chosen as sizing criterion for the power unit. In fact, it can observe that the system performance improves greatly by adopting the fixed cell voltage as designing criterion. At full load, the system electric efficiency of the RFU/HT-FC2 system varies from 0.37 (case 1) to 0.4 (case 2) while the gross electric power changes from 2.2 to 2.5 kW. Because the main scope in developing the microcogeneration system is to satisfy the electric and thermal demands with high efficiency without having more regard for weight and size, the designing criterion based on the cell voltage (case 2) has been adopted. The comparison between the performance in terms of electric and cogeneration efficiencies is shown in Fig. 7.

Table 4 Calculated values for empirical coefficients of the Eq. (5).

(V) (V) (X cm2) (V) (cm/A)

0.25

0.48

LT-FC

HT-FC1

HT-FC2

0.7018 0.0428 0.479 0 0

0.9489 0.0429 0.2488 0.3016 0.0001

0.9553 0.0557 0.3537 0.3583 0.178

RFU/LT-FC RFU/HT-FC1 RFU/HT-FC2

0.46

ð13Þ

el;PU th;PU gCHP ¼ _ _ NG;CB Þ  HHV NG ðmNG;REF þ m

V0 b R m n

0.20

Fig. 5. Fitting polarization curves to experimental data. For LT-FC, the anode gas composition is the same of the syngas out the PROX reactor. For HT-FCs, the anode gas composition is the same of the syngas exiting the WGSR.

System Electric Efficiency

P

0.15

Current density (A/cm2)

The developed numerical models have allowed to estimate and compare the performance of the microcogeneration systems in order to establish which system better satisfies the requirements of high efficiencies and more simple plant configuration, needed for microcogeneration applications. The electric and cogeneration efficiencies have been calculated according to the following equations:

gsys ¼ _ ðmNG;REF

0.10

LT-FC calc HT-FC1 calc HT-FC2 calc

Case 2

0.44

0.42

0.4

Case 1

0.38

0.36

0

0.5

1 1.5 2 Gross Electric Power (kW)

2.5

3

Fig. 6. Systems electric efficiency versus gross electric power (case 1 and case 2).

It is worth noting that the LT-FC allows to obtain high system electric efficiencies at low power (the efficiencies of the LT-FC system and HT-FC2 system are comparable) while, at intermediate and high loads, the integrated systems based on the HT-FCs are characterized by higher performance. Moreover, although the stack efficiencies of HT-FC1 are higher than those of HT-FC2 (as it is evident from the polarization curves reported in Fig. 5), the electric efficiencies of the RFU/HT-FC2 system are the highest in the whole operating range. This is due to the smaller flow rate of the natural gas sent to the catalytic burner linked to the higher temperature of the combustion air exiting the HEX3 exchanger. On the contrary, the RFU/LT-FC system has the better performance in term of cogeneration efficiency ranging from 0.78 to 0.80. The mass and energy balances and the performance of the microcogeneration systems at full load operation have been reported in Table 5.

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0.6

0.9

3

RFU/LT-FC RFU/HT-FC1 RFU/HT-FC2

0.8

2.5

CHP efficiency

CHP efficiency

0.6

0.5

Electric efficiency

0.5 0.45 0.4 0.4

0.3 0.2

RFU/LT-FC RFU/HT-FC1 RFU/HT-FC2

0.1 0

0

0.5

1

1.5

System electric efficiency

0.7

Thermal Power (kW)

0.55

2

1.5

1

0.5

0.35

0 2

2.5

3

0.3

0

0 .5

1

1.5

2

2.5

3

Gross Electric Power (kW)

Gross Electric Power

Fig. 8. Thermal power recovered versus gross electric power.

Fig. 7. Electric and cogeneration efficiency versus gross power.

3.5 Table 5 Mass and energy balances of the microcogeneration systems. RFU/LT-FC

RFU/HT-FC1

RFU/HT-FC2

Mass balances (kg/s10–4) Natural gas to SR Water to SR Natural gas to CB Air to PROX Air to CB Anode exhaust to CB Air to power unit Syngas to power unit Cooling stream to the power unit

1.12 3.5 0.068 0.36 9.82 3.37 27.2 3.8 656a

1.12 3.5 0.034 – 8.80 2.98 26.5 3.3 922b

1.12 3.5 0.029 – 8.64 2.98 26.5 3.3 894b

Energy balances (kW) Natural gas to CB (HHV) Natural gas to SR (HHV) Anode exhaust to CB (HHV) Heat from the cathode exhaust

0.373 6.13 1.96 –

0.188 6.13 1.78 0.101

0.159 6.13 1.78 0.116

System performance Gross Electric power (kW) Thermal power recovered (kW) Electric efficiency (%) Cogeneration efficiency (%) a b

Pth

Pa

Pc

Ploss

3 2.5

2.48 2.74 38 80

2.52 2.49 40 79

2.52 2.41 40 78

Water specific heat capacity 4.18 kJ/kg. Paratherm MRÒ specific heat capacity 2.7 kJ/kg.

It can be noted that the natural gas consumption in the catalytic burner of the RFU/HT-FC2 is the lowest, thanks to the higher heating value of the anode off-gas in comparison with that of the RFU/ LT-FC system (the hydrogen content is greater and the dilution effect of nitrogen is absent) and to the heat recovery from the cathode off-gas that allows to preheat the air sent to the catalytic burner at 160 °C (in the RFU/HT-FC1 system the air is preheated at 140 °C). Fig. 8 shows the thermal power recovered versus the gross electric power in the whole operating range of the proposed microcogeneration systems (the minimum stack power has been assumed of about 0.35 kW). The thermal power recovered, calculated by using Eqs. (9) and (12) in which the natural convection and radiation are considered only from the stack end-plates (the exchange surface area S is equal to 0.0818 m2), decreases when the stacks operating temperature increases.

kW

2 1.5 1 0.5 0

RFU/LT-FC

RFU/HT-FC1

RFU/HT-FC2

-0.5 Fig. 9. Thermal fluxes from the power units.

In order to improve the heat recovery, a more detailed evaluation of the thermal fluxes from the power units have been conducted, as shown in Fig. 9. It is worth noting that, while the thermal power available Pth,av (the sum of Pa, Pc, Ploss and Pth) is almost constant for all power units of the integrated systems, the net thermal fluxes, linked to the chemical species at the anode and cathode sides, grow as the stack temperature increases as well as the thermal losses, involving a lower thermal power recovered (Pth). However, the net thermal fluxes associated with the anode and cathode streams are not losses because their energy content is recovered in the integrated systems (the anode exhaust is sent to the catalytic burner and, in the RFU/HT-FCs, the heat available from the cathode off-gas is used to pre-heat the air to the catalytic burner), with a beneficial effect on the overall system electric efficiency. On the contrary, the heat loss (Ploss) from the stack endplates causes a decreasing of the thermal energy recovered. In order to evaluate the effectiveness of the RFU/PU systems for cogeneration purpose, a performance parameter, the Heat Recovery Ratio (HRR), has been defined as:

90

E. Jannelli et al. / Applied Energy 108 (2013) 82–91

1 0.9 0.8 0.7

HRR

0.6 0.5 0.4 0.3

RFU/LT-FC 0.2

RFU/HT-FC1 RFU/HT-FC2

0.1 0

0

0.5

1

1.5

2

2.5

3

Gross Electric Power (kW) Fig. 10. Heat Recovery Ratio (HRR) without insulation of the stack end-plates.

1 0.9 0.8 0.7

HRR

0.6 0.5 0.4 0.3

RFU/LT-FC RFU/HT-FC1 RFU/HT-FC2

0.2 0.1 0

0

0.5

1

1.5

2

2.5

3

Gross Electric Power (kW) Fig. 11. Heat Recovery Ratio (HRR) with insulation of the stack end-plates.

HRR ¼

P th Pth;av

power density and low emissions. However, the technical issues due to the water and thermal management and to the purity of feeding hydrogen have addressed the research to the development of high temperature PEM fuel cells able to overcome these limits. In this work the performance analysis and the comparison of cogeneration power systems, based on the integration of a reforming unit and a PEM fuel cells power unit, have been carried out by using a numerical model. The reforming unit (RFU) is a pre-commercial natural gas steam reforming system which produces 45 Nl/min (at full load) of syngas (the hydrogen concentration is about 75% vol. while the CO content is less than 10 ppm). The PEM fuel cells chosen as power unit are a low temperature PEM fuel cell (LT-FC), that uses a Nafion™ membrane, and two high temperature PEM fuel cells (HT-FC) characterized by a polybenzimidazole (PBI) based membrane (HT-FC1) and an aromatic polyether–pyridine based membrane (HT-FC2). The experimental activities, carried out in the test stations developed both for the reforming system and the PEM fuel cells, have allowed to tune and validate the numerical models created for predicting the performance of the integrated cogeneration systems. For sizing the power units, able to provide a maximum electric power of 2.5 ± 0.2 kW (this size allows to satisfy the electric and thermal energy demand of an Italian household), two designing criteria have been considered: (i) fixed current density; (ii) fixed cell voltage. Results have been pointed out that the systems performance improves greatly by adopting the fixed cell voltage designing criterion. By analyzing the trend of the electric and cogeneration efficiencies and the Heat Recovery Ratio (HRR) it has been found that the electric performance increases with the stacks operating temperature while the thermal performance decreases. In particular, the RFU/HT-FC2 system (the fuel cell uses aromatic polyether polymers/copolymers bearing pyridine units doped with phosphoric acid as electrolyte operating at 180 °C), shows the highest electric efficiency in the whole operating range, although the stack efficiencies are the worst. This is due to the optimal energy recovery from the cathode and anode off-gases. In conclusion, even if the performance of the proposed microcogeneration systems is quite similar, the advantages due to the system architecture simplification and to the availability of high temperature heat make the HT-FCs an attractive solution for the cogeneration/trigeneration systems development.

ð15Þ

In Fig. 10 the HRR versus electric power is depicted. For the RFU/ LT-FC system the HRR is very high in the whole operating range (it varies from 0.81 to 0.91), whereas in the systems based on the HTFCs, HRR values higher than 0.80 can be obtained only at high loads (electric power higher than 2 kW). An increase of the thermal energy recovered could be obtained by the insulation of the end-plates; in fact by assuming the adiabatic stack operation (a complete stack insulation), the HRR can be improved as shown in Fig. 11. It can observe that the heat recovery from the systems based on the HT-FCs rises at values higher than 0.80, even if the RFU/FC-LT system shows always the better behavior.

5. Conclusions Combined heat and power systems based on low temperature PEM fuel cells appear very promising for residential and commercial applications due to their high efficiency, rapid start-up, high

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