Potential of Proton-Exchange Membrane Fuel-Cell System with On-Board O₂-Enriched Air Generation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Line 34-44

It is necessary to indicate the numerical value of activation, ohmic and concentration losses.

 

Line 45-49

It is necessary to indicate how much activation losses increase with decreasing O2 concentration.

 

Line 70-84

An increase in O2 concentration can lead to oxygen corrosion of fuel cells. This must be indicated in the article, as well as the recommended O2 concentration.

 

Line 155, 188

It is necessary to clarify the dimension g/hr/cm2. In the proposed version, g/hr/cm2 is g×cm2/hr. Perhaps the authors meant (g/hr)/cm2 or g/(hr/cm2).

 

Line 155 Formula (1)

It is necessary to clarify the dimension of the formula. If current density in A/cm2, Faraday constant in C/mol, and molecular weight of H2 in g/mol we get the value in (A×C×g)/(cm2×mol2).

 

Line 232 Figure 2.

The top part of Figure 2a is uninformative.

 

Line 250

It is necessary to indicate the value of the operating pressure coefficient (π1) when using a supercharged fuel cell, as well as how much the air temperature increases.

 

Line 273 Figure 4.

The authors suggest a boost pressure in the second stage of 4 bar. This is very high pressure. In this case, the air temperature will increase to 260...280°С. To cool the air to a temperature of 80°C, a large consumption of hydrogen H2 is required. This will reduce the energy efficiency of the installation.

 

Line 265-267

It is necessary to indicate the power of the second electric compressor, which provides air compression. This is due to additional energy losses for the operation of the installation.

 

Line 319 Figure 5.

The top part of Figure 5b is uninformative.

 

 

Line 325 Figure 6.

I recommend that the authors rename the figure as

Balance of plant (BOP) efficiency as a function of current density and O2 molar fraction

 

Line 325 Figure 6.

The bottom part of Figure 6 is uninformative.

 

Line 389-412

It is necessary to add what additional power is needed to operate the installation.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Author Response

Reviewer 1

 

Line 34-44

It is necessary to indicate the numerical value of activation, ohmic and concentration losses.

Answer: While we acknowledge the reviewer's suggestion to provide more detailed information about the voltage losses associated with each region of the polarization curve, providing quantitative information on that regard is not simple due to the inherent interrelationship among activation, ohmic, and concentration losses in PEMFCs. Moreover, these losses are significantly influenced by the specific design and operating conditions of the cell. However, we agree that the introduction would benefit from a clearer explanation of these losses and the voltage ranges where they typically occur. To address this feedback, we have modified the text indicated by the reviewer, incorporating relevant information about ranges from Reference 13, specifically from the figure provided in this response:

 

 

Previous version

 

Line 34-44. In fuel cells, the polarization curve assesses the voltage-current density relationship [11], offering valuable insights into the energy generation and efficiency. It is vital to understand the potential losses that escalate with higher current densities. In the case of PEMFC, analysing the polarization curve yields essential information about activation, ohmic, and concentration losses [12], which significantly impact the system performance. Activation losses arise from slow electrochemical reactions at fuel cell electrodes, influenced by electrode materials, catalyst properties, operation temperature, and gas properties [13]. Conversely, ohmic losses stem from the electrical resistance encountered by the protons in the electrolyte membrane and by the electrons in the current collectors [14]. Finally, concentration losses arise due to the depletion of reactants, particularly O2 at the cathode [15], becoming prominent when mass transfer limitations occur [16].

 

Current version

 

Line 34-47. Polarization curves are vital in fuel cell performance analysis, highlighting the voltage-current density relationship and energy efficiency [11]. Particularly in PEMFC studies, these curves allow to assess activation, ohmic, and concentration losses, each varying with cell design and operation [12, 13]. Activation losses arise from slow electrochemical reactions at fuel cell electrodes, influenced by electrode materials, catalyst properties, operation temperature, and gas properties [14]. Such losses are characterized by a sudden voltage drop at low current density conditions, reaching values between 0.9 and 0.7 V in this region. Conversely, ohmic losses stem from the electrical resistance encountered by the protons in the electrolyte membrane and by the electrons in the current collectors [15]. This linear trend appears from the end of the activation losses area to a value around 0.6-0.4 V. Lastly, concentration (or transport) losses, occurring from 0.6-0.4 V, are identified a sudden voltage decrease in a narrow range of current density. Concentration losses arise due to the depletion of reactants, particularly O2 at the cathode [16], becoming prominent when mass transfer limitations occur [17]

Line 45-49

It is necessary to indicate how much activation losses increase with decreasing O2 concentration.

 

Answer: Acknowledging the relevance of the information highlighted by the reviewer, the authors have amended the manuscript accordingly. An explanatory sentence has been added on line 45 to incorporate the suggested details,

 

Previous version

 

Line 45-50. O2 concentration plays a vital role in determining the extent of activation [17] and concentration losses [18]. Activation losses are aggravated by the hindrance of electro-chemical reactions caused by the low partial pressure of O2 during low current conditions [19]. Moreover, a decrease in O2 concentration intensifies concentration losses by impeding reactant supply to the electrodes, resulting from mass transfer limitations between the gas channel and the catalytic layer of the cathode [20].

 

Current version

 

Line 48-56. O2 concentration plays a vital role in determining the extent of activation [18] and concentration losses [19]. Activation losses are aggravated by the hindrance of electro-chemical reactions caused by the low partial pressure of O2 during low current conditions [20]. A substantial reduction in activation losses, reaching 92%, has been reported following an increase in O2 concentration from 24% to 100. This improvement translates to enhanced efficiency across the entire range of current densities [24]. Moreover, a decrease in O2 concentration intensifies transport losses by impeding reactant supply to the electrodes, resulting from mass transfer limitations between the gas channel and the catalytic layer of the cathode [21].

 

Line 70-84

An increase in O2 concentration can lead to oxygen corrosion of fuel cells. This must be indicated in the article, as well as the recommended O2 concentration. “Degradation mechanisms of proton exchange membrane fuel cell under typical automotive operating conditions”

 

Answer: Acknowledging the reviewer's concern about the impact of increased O2 concentration on fuel cell degradation, the authors have chosen not to elaborate extensively on this matter, as detailed in reference [30]. This decision is based on the premise that appropriate fuel cell design can mitigate such degradation, at least at the cell level. However, understanding the importance of this aspect in fuel cell research, the corresponding paragraph has been revised to highlight that elevated O2 concentrations may accelerate degradation through certain mechanisms, underscoring the need for careful consideration in fuel cell design. Consequently, the authors have refrained from specifying a range of working O2 concentrations, recognizing that such parameters are contingent upon the specific application and characteristics of each fuel cell.

 

Previous version

 

Line 79-81. However, it also presents challenges and potential drawbacks, including oxidative stress and elevated thermal control requirements [30]

 

Current version

 

Line 85-91. However, it also presents challenges and potential drawbacks, including elevated thermal control requirements and oxidative stress [31]. Elevated O2 levels can induce oxidative stress in high-temperature conditions [31] and promote the formation of high-energy species leading to degradation, particularly if O2 permeates to the anode [30]. Nevertheless, these drawbacks can be effectively mitigated through meticulous cell design and appropriate thermal management [31].

 

Another aspect is related to the corrosion of the structural components, mostly the bipolar plates. This aspect may be a concern depending on the kind of materials used: currently, graphite is the most extended material, but metal bipolar plates are being developed to reduce material and manufacturing costs. For such materials, corrosion induced by the higher O2 concentration may be a limiting factor for the implementation of the proposed concept. The following sentence has been added on that regard:

 

Yan Wang [12] found that the corrosion effects on metallic bipolar plates materials can be significantly high in the cathode side, so increased levels of O2 concentration may limit the implementation of such materials.

 

 

Line 155, 188

It is necessary to clarify the dimension g/hr/cm2. In the proposed version, g/hr/cm2 is g×cm2/hr. Perhaps the authors meant (g/hr)/cm2 or g/(hr/cm2).

Answer: The authors acknowledge the potential confusion caused by the previously used nomenclature and have revised the representation of these dimensions to g/(hr/cm2) for clarity and unambiguity.

 

Line 155 Formula (1)

It is necessary to clarify the dimension of the formula. If current density in A/cm2, Faraday constant in C/mol, and molecular weight of H2 in g/mol we get the value in (A×C×g)/(cm2×mol2).

 

Answer: The authors are grateful to the reviewer for identifying the error in the equation, which resulted in incorrect notation and units. Following this observation, Equation 1 has been corrected to

Moreover, Equation 2 has been changed following the same concept as before.

Line 232 Figure 2.

The top part of Figure 2a is uninformative.

 

Line 319 Figure 5.

The top part of Figure 5b is uninformative.

 

Line 325 Figure 6.

The bottom part of Figure 6 is uninformative.

 

Answer: The authors initially standardized the y-axis and x-axis limits across various subplots in each figure to minimize interpretative errors. However, upon implementing the reviewer's suggestion, it was observed that the axes are distinct enough to preclude rapid reading errors, while significantly enhancing graph visibility. Consequently, Figure 2 has been modified accordingly, and similar adjustments have been applied to Figures 5 and 6, as recommended by the reviewer.

 

Figure 2

 

 

 

Figure 5

 

 

Figure 6

 

 

Line 250

It is necessary to indicate the value of the operating pressure coefficient (π1) when using a supercharged fuel cell, as well as how much the air temperature increases.

 

Answer: The authors concur with the reviewer's suggestion regarding the importance of specifying the working pressure range of the supercharged cells and the gas temperature at the output of the compression process. Accordingly, the manuscript has been updated with an additional sentence to address this aspect.

Figure 3b and the corresponding text have been updated to incorporate the variable π, thereby characterizing the compression ratio for any supercharged fuel cell. This modification ensures a clear distinction from the compression ratio of the first compression stage in the proposed system (π₁), avoiding any potential confusion.

Current version

 

Line 278-280. In supercharged fuel cells, π is in range from 1.2 to 2.2 [38]. This variation suggests that, depending on the compressor efficiency, the air temperature at the heat exchanger inlet could exceed 250 °C.

 

Line 273 Figure 4.

The authors suggest a boost pressure in the second stage of 4 bar. This is very high pressure. In this case, the air temperature will increase to 260...280°С. To cool the air to a temperature of 80°C, a large consumption of hydrogen H2 is required. This will reduce the energy efficiency of the installation.

 

Answer: The authors agree with the reviewer's observation regarding potentially high temperatures in the second compression stage. Indeed, despite starting from ambient temperature after the first exchanger, the air can reach temperatures as high as those indicated by the reviewer in extreme cases. However, it has to be considered that hydrogen’s high specific heat capacity (14.3 kJ/(kgK) compared to air's 1 kJ/(kgK)), allow for comparable thermal load in the H2 and air streams for most of the conditions. This enables the H2 stream to cool the air to the necessary temperature, with a limiting condition that the H2's exit temperature from the exchanger does not exceed the air's inlet temperature, which has been already considered in the energy balance calculations performed in the study. It is important also to note that the primary role of the two heat exchangers introduced in the oxy-fuel cell BOP is to elevate the temperature of H2 for more efficient expansion in the turbine, rather than cooling the air, which could alternatively be achieved with water-to-air heat exchangers fed by the fuel cell stack cooling circuit.

 

However, in scenarios where large air flows are needed for significant O2 enrichment, it must be acknoweledged that the cooling capability provided by the hydrogen might be insufficient due to the low ratio between hydrogen and cathode mass flows. In order to take into account such situation, a third (conventional) heat exchanger has been incorporated into the BOP description to ensure that the air temperature at the membrane inlet does not exceed 80 ºC in any condition, thereby preserving membrane integrity.

 

To prevent misunderstandings and clarify the presence of a mechanism to maintain the membrane inlet temperature within safe limits, figure 4 has been revised to include this third exchanger, and a corresponding explanation has been added to the manuscript.

 

Previous version

 

Figure 4

 

 

Line 269-273 The output of this compression is cooled down again in Heat Exchanger 2, bringing it to a temperature of around 80◦C, close to the optimal operating temperature for both the polymeric O2-N2 separation membrane and the fuel cell stack itself.

 

Line 274-279 In the H2 stream, the first component is a supply valve from the 700 bar tank in the vehicle such as in the standard system, although controlled to a higher pressure (πH2). Subsequently, the expanded, low-temperature H2 is used as a cooling fluid for the two heat exchangers mentioned above. This helps to raise the H2 temperature before it arrives at the turbine side of the turbocharger, where it is expanded to provide the power required for the first air compression.

 

Current version

 

Figure 4

 

 

Line 297-303. The output of this compression is cooled down again in Heat Exchanger 2, with the objective to bring it down to a temperature of around 80◦C, close to the optimal operating temperature for both the polymeric O2-N2 separation membrane and the fuel cell stack itself. To safeguard the N2 membrane's operation in circumstances where the H2 stream is unable to effectively cool compressed air to 80°C, a third exchanger is employed (utilizing water as the coolant) to ensure that the air enters the separation membrane at the specified temperature.

 

Line 307-314. In the H2 stream, the first component is a supply valve from the 700 bar tank in the vehicle such as in the standard system, although controlled to a higher pressure (πH2). The low-temperature H2, characterized by its high specific heat capacity of 14.3 kJ/(kgK) compared to air's 1 kJ/(kgK) at 25 ºC, functions as a cooling medium for the first two heat exchangers. The principal objective of these exchangers is to increase the H2 stream temperature. This helps to raise the H2 temperature before it arrives at the turbine side of the turbocharger, where it is expanded to provide the power required for the first air compression.

 

Line 265-267

It is necessary to indicate the power of the second electric compressor, which provides air compression. This is due to additional energy losses for the operation of the installation.

 

Answer: the authors acknowledge that the electric compressor is a major consumption in the proposed BOP architecture. However, it has to be highlighted that this consumption is already accounted for in the information depicted in Figure 5 (left), which represents the combined consumption of both compressors. Additionally, it has to be considered that the current calculations impose that the power balance in the shaft connecting the H2 turbine and compressor 1 is maintained, with a mechanical efficiency of 97%. Consequently, the difference between the compressor power in Figure 5 (left) and the H2 turbine recovered power in Figure 5 (right) presents directly the power consumed by the second compressor. This point has been highlighted in the new wording of the manuscript:

 

Previous version:

Lines 299-306: “Figure 5 shows power consumed by both compressor stages together with power recovered in the turbines present in the H2 stream and in the membrane outlet (retentate) stream. As it was already analyzed in figure 2, to achieve high levels of O2-enrichment it is necessary to significantly increase the membrane inlet mass flow rate. This implies a proportional increase in the power consumed since, as mentioned above, the pressure ratio and efficiency of the compressors are fixed as a first approximation. It should be noted, that in the case of working with standard air (21% O2 molar fraction), there would be no power consumed since it is assumed that the system would operate as an atmospheric fuel cell.”

 

Current version:

Lines 371-378: “Figure 5 shows the combined power consumed by both compressor stages together with power recovered in the turbines present in the H2 stream and in the membrane outlet (retentate) stream. It must be considered that the power for the first compressor stage is provided by the power produced in the H2 turbine, with a mechanical efficiency of 97% in the turbocharger shaft. Therefore, the difference between the compressors consumption and the H2 recovery represents the power consumed by the electrical compressor, which is the most significant energy consumption for most of the conditions.

 

As it was already analyzed in figure 2, to achieve high levels of O2-enrichment it is necessary to significantly increase the membrane inlet mass flow rate. This implies a proportional increase in the power consumed since, as mentioned above, the pressure ratio and efficiency of the compressors are fixed as a first approximation. It should be noted, that in the case of working with standard air (21% O2 molar fraction), there would be no power consumed since it is assumed that the system would operate as an atmospheric fuel cell.”

 

 

Line 325 Figure 6.

I recommend that the authors rename the figure as Balance of plant (BOP) efficiency as a function of current density and O2 molar fraction

 

Answer: The authors agree with the recommendation to rename the figure, and it has been changed to:

“Balance of plant (BOP) efficiency as a function of current density and O2 molar fraction”

 

In addition, the following line has been added to the Nomenclature in the Acronyms section:

 

“BOP Balance of plant”

 

Line 389-412

It is necessary to add what additional power is needed to operate the installation.

 

Answer: The second point in the conclusions section has been reworded to highlight that the main power consumption in the proposed BOP is represented by the electrical compressor.

 

Previous version:

Lines 396-400: Higher levels of O2-enrichment imply the need to supply the O2-N2 separation membrane with a mass flow significantly higher than the useful (permeate) O2-enriched generated stream. This reverses the reduction of cathode mass flow seen at the stack level, significantly increasing the compressor power consumption and limiting the potential advantages of the system.

 

Current version:

Lines 474-481: Higher levels of O2-enrichment imply the need to supply the O2-N2 separation membrane with a mass flow significantly higher than the useful (permeate) O2-enriched generated stream. This reverses the reduction of cathode mass flow seen at the stack level, significantly increasing total air mass flow introduced to the system compared to a traditional fuel cell system. As a consequence, the new proposed concept has an additional consumption of power, which is mostly concentrated in the second stage of compression, driven by an electric motor.

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript theoretically evaluated the potential of a system including polymeric membranes for O2-N2 separation to produce O2-enriched air. Generally speaking, this manuscript is well-written and covers a topic of interest to the journal’s readership. Therefore, I would like to recommend its publication after a minor revision. Some suggestions below should be carefully considered.

 

1. How to validate and verify the present method? Could authors provide a comparison between the present against the benchmark solutions?

2. Please clearly mention the weak points of former works (identification of the gaps) and describe the novelties of current work.

3. The assumptions of the present method are critical for its application. Could authors clearly state the assumptions involved in the preset method?

4. In Table 1, how to choose the base values and range scopes? Are there any criteria for choosing these values? It would be better if the authors could add some interpretations.

5. More details should be provided in Section 2. Approach to fuel cell system analysis for the reader to understand the present method. 

Author Response

Reviewer 2

 

This manuscript theoretically evaluated the potential of a system including polymeric membranes for O2-N2 separation to produce O2-enriched air. Generally speaking, this manuscript is well-written and covers a topic of interest to the journal’s readership. Therefore, I would like to recommend its publication after a minor revision. Some suggestions below should be carefully considered.

 

1. How to validate and verify the present method? Could authors provide a comparison between the present against the benchmark solutions?

 

Answer: The authors acknowledge the importance of the need to validate the methods and hypothesis followed along any research study and compare them to other works in the literature. However, the authors would like to highlight that the main purpose of the current work is to explore if on-board oxygen-enriched air generation using polymeric membranes is a viable option for a hydrogen fuel cell concept. For this reason, relatively simple and well stablished equations have been used to represent the effects of the fuel cell and air management component. This idea has been expressed more clearly in the first paragraph of section 2:

 

Previous version:

Lines 146-152: In this section, the methodology used to estimate the performance of the balance of plant components as a function of the O2-enrichment is presented. First, the main two components of the system (the fuel cell stack and the O2-N2 separation membrane) are detailed. Then, the proposed balance of plant for on-board O2-enriched air generation is presented and compared with a standard PEMFC balance of plant, including the hypotheses associated to some specific components such as heat exchangers, compressors and turbines included in the system.

 

Current version:

Lines 161-178: In this section, the methodology used to estimate the performance of the balance of plant components as a function of the O2-enrichment is presented. It has to be highlighted that the current work intends to determine if on-board oxygen-enriched air generation using N2-O2 separation membranes is a viable option for this concept. For this reason, the operation of the balance of plant elements has been represented by simple relationships to achieve a general approximation to the potential energy balance and efficiency achievable of the concept without specific details of the design of each subcomponent. In this sense, the main two components of the system (the fuel cell stack and the O2-N2 separation membrane) are characterized by data extracted from the literature from existing components representative of the technologies state-of-the-art at production stage. Then, the proposed balance of plant for on-board O2-enriched air generation is presented and compared with an atmospheric PEMFC balance of plant, including the hypotheses associated to some specific components such as heat exchangers, compressors and turbines included in the system. Particularly, the heat exchangers are represented by a certain cooling efficiency, guaranteeing the energy balance between both flow streams feeding each component, while compressors and turbines are characterized by constant isentropic efficiency values representative of the average operation of radial turbomachinery components extracted from the literature.

 

 

2. Please clearly mention the weak points of former works (identification of the gaps) and describe the novelties of current work.

 

Answer: While previous cited works have explored the impact of oxygen concentration in the fuel cell stack operation, to the best of the authors’ knowledge there is no previous work that focuses on the method to obtain the oxygen-enriched cathode stream from ambient air, and its impact on the system efficiency. This is the research gap that is intended to be covered by the current work. This has been further highlighted in the Introduction section of the updated manuscript:

 

Previous version:

Lines 85-115: From these findings, it is concluded that a higher O2 concentration in the cathode stream increases the electrical power produced (at equal H2 consumption), improving the fuel cell stack performance as an individual element. To achieve a certain level of enrichment, O2 can be supplied from a pure O2 source or directly obtained from the atmosphere using N2/O2 separation techniques. For on-board generation, the most typical techniques encompass chemical separation methods, such as absorption on molten salts or the utilization of Mixed Ionic-Electronic Conductors, which operate at extremely high temperatures, or cryogenic distillation techniques necessitating temperatures below -150 ◦C [32]. It should be noted, however, that the extreme temperatures associated with these air enrichment techniques are outside the working range of a typical PEMFC, and can induce damage during its operation [33]. An alternative separation technique, which does not require altering the air temperature but rather its pressure, could involve the utilization of polymeric membranes for O2 separation. This technology offers lower energy consumption than other techniques when intermediate levels of O2 purity is required [35]. Such polymeric membranes are based on a hollow fiber system, where air passes through small channels, enabling the selective permeation of O2 molecules while blocking other gases [36].

 

In the case of on-board O2-enrichment, it is crucial to consider the energy consumption associated with the separation of O2 from atmospheric air. The utilization of current polymeric membrane technology implies maintaining a minimum pressure ratio between the inlet of the membrane and the permeate (i.e. the O2-enriched stream) flow. Consequently, this entails the generation of a certain boost pressure, which is lost during the O2 separation process, with the fuel cell stack working in atmospheric conditions. Moreover, a significantly higher inlet mass flow rate must be supplied to the membrane compared to the desired O2-enriched stream, with the excess mass flow primarily comprising N2, not directly useful for the current application. The ratio in between permeate stream mass flow and the inlet air mass flow is defined as the stage cut ratio. Both of these factors, related to the membrane selectivity, contribute to an increase in the compressor power consumption. Therefore, in order to assess the real potential of on-board O2-enrichment for fuel cell systems, it is necessary to carefully study the gases management systems to reduce their net consumption and improve their integration in the overall balance of plant.

 

Current version:

Lines 97-131: From these findings, it is concluded that a higher O2 concentration in the cathode stream increases the electrical power produced (at equal H2 consumption), improving the fuel cell stack performance as an individual element. To achieve a certain level of enrichment, O2 could be supplied from a pure O2 source or directly obtained from the atmosphere using N2/O2 separation techniques. For on-board generation, the most typical techniques encompass chemical separation methods, such as absorption on molten salts or the utilization of Mixed Ionic-Electronic Conductors, which operate at extremely high temperatures, or cryogenic distillation techniques necessitating temperatures below -150 ◦C [32]. It should be noted, however, that the extreme temperatures associated with these air enrichment techniques are outside the working range of a typical PEMFC, and can induce damage during its operation [33]. An alternative separation technique, which would not require altering the air temperature but rather its pressure, consists on the utilization of polymeric membranes for N2-O2 separation. This technology, currently used for industrial N2 production, offers lower energy consumption than other techniques when intermediate levels of O2 purity are required [35]. Such polymeric membranes are based on a hollow fiber system, where air passes through small channels, enabling the selective permeation of O2 molecules while blocking other gases [36]. However, to the best of the authors’ knowledge, no previous work has analyzed the potential of such membranes in the context of fuel cell systems operation.

 

In order to determine the potential of on-board O2-enrichment with polymeric membranes for fuel cell systems, it is crucial to consider the energy consumption associated with the separation of O2 from atmospheric air. In this sense, it has to be considered that in order to achieve a proper N2-O2 separation, such membranes need to maintain a minimum pressure ratio between the inlet of the membrane and the permeate (i.e. the O2-enriched stream) flow. Consequently, this entails the generation of significantly higher pressure upstream the separation membrane than the one arriving the fuel cell stack. Moreover, a significantly higher inlet mass flow rate must be supplied to the membrane compared to the desired O2-enriched stream, with the excess mass flow primarily comprising N2, not directly useful for the current application. The ratio in between permeate stream mass flow and the inlet air mass flow is defined as the stage cut ratio. Both of these factors, related to the membrane selectivity, contribute to an increase in the compressor power consumption. Therefore, in order to assess the real potential of on-board O2-enrichment for fuel cell systems, it is necessary to carefully study the gases management systems to reduce their net consumption and improve their integration in the overall balance of plant.

 

 

3. The assumptions of the present method are critical for its application. Could authors clearly state the assumptions involved in the preset method?

 

 

Answer: A new subsection has been included at the end of section 2 to summarize the main hypotheses behind the calculations. The wording for this section is also included below:

 

2.4. Summary of main hypotheses

 

Considering the information and discussion included in the previous points, the main hypotheses of the study are summarized below:

 

• For all operating conditions, the sensitivity of the fuel cell stack polarization curve with respect to the oxygen molar fraction is assumed to be equal to the data obtained from the study by Fournier et al [XX].
• The hydrogen consumption is directly calculated from the anode reaction stoichiometry. In the cathode, the total oxygen mass flow is the same regardless the oxygen molar fraction achieved, while this value is adapted as a function of the current density assuming a constant oxygen excess factor.
• The separation membrane operates at a constant temperature of 80ºC and pressure ratio (nominally 4). The ratio of inlet to permeate (O2-enriched) mass flows (stage cut) is extracted from the operating map of a commercial system (model NM C05 of the UBE Corporation).
• In the retentate flow (nitrogen-enriched) a pressure drop of 5% is assumed for all working conditions. This stream is directed to an electric turbine9, characterized by a constant isentropic efficiency. Instead, in the permeate (oxygen-enriched) stream, the outlet pressure is assumed to be equal to 1 bar. For this reason, the operation of the oxygen-enriched concept will be compared to an atmospheric fuel cell, also working with 1 bar at the cathode inlet, so that the pressure effect on the fuel cell stack performance is equivalent.
• In order to reach the necessary pressure upstream the separation membrane, a two-stage boosting system with intermediate cooling is used. A cooling efficiency of 90% is assumed for this intermediate cooling.
• The hydrogen stream is assumed to be provided at 5 bar controlled by an expansion valve at the hydrogen tank outlet. This stream is heated using the energy available in the cathode stream after compression, increasing the exergy available in the flow. Afterwords, the 5 bar heated hydrogen stream is directed to a turbine included in a turbocharger, providing the power for the first compression in the cathode stream with a mechanical efficiency of 97%. Instead, the second compression is directly driven by an electrical compressor, representing the main energy consumption in the proposed system.
• The isentropic efficiency of each compressor stage is assumed to be equal and independent on the working conditions. In the same sense, both nitrogen and hydrogen turbine efficiencies are also constant and equal. The selected ranges for turbine and compressor efficiencies are extracted from typical values of radial turbomachinery currently used in automotive applications in the peak efficiency area.

 

4. In Table 1, how to choose the base values and range scopes? Are there any criteria for choosing these values? It would be better if the authors could add some interpretations.

 

Answer: In response to your comment, we have updated Table 1 to include references for the source of each range scope. Regarding the base values, we have selected them to represent the mean value of the usual working range for each parameter. This approach provides a representative starting point for the analysis. To clarify this, the following comment has been added to the text:

 

Previous version

 

(which we will call the base case described in the next section)

 

Current version

 

(which are defined using values close to the average values within the usual working range for each parameter, and will be called base case in the following section)

 

5. More details should be provided in Section 2. Approach to fuel cell system analysis for the reader to understand the present method.

 

Answer: As previously stated, a new section has been added to summarize and clarify the main hypothesis and simplifications around the calculations. We hope that this addition helps to improve the readability of the section and provide the details required for future readers to properly understand the work done.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript titled "Evaluating the potential of a proton-exchange membrane fuel-cell system with on-board O2-enriched air generation" provides interesting results obtained from a basic theoretical study of O2 enrichment on fuel cell performance. The manuscript is well-written and organized. The authors have used clear language and provided a thorough discussion of their findings. However, in terms of research methodology and supporting the research findings, the manuscript can be improved by performing relevant experiments or a 1D system modeling including the dynamic performance characteristics of all the sub-systems, such as, compressor and turbine efficiency variation. 

Author Response

Reviewer 3

 

The manuscript titled "Evaluating the potential of a proton-exchange membrane fuel-cell system with on-board O2-enriched air generation" provides interesting results obtained from a basic theoretical study of O2 enrichment on fuel cell performance. The manuscript is well-written and organized. The authors have used clear language and provided a thorough discussion of their findings. However, in terms of research methodology and supporting the research findings, the manuscript can be improved by performing relevant experiments or a 1D system modeling including the dynamic performance characteristics of all the sub-systems, such as, compressor and turbine efficiency variation.

 

Answer: The authors would like to show appreciation for the comments received. For that, to clarify, a paragraph has been added in the Conclusion section.

 

The authors express their gratitude for the insightful feedback provided. In response to the reviewer's comments, the primary aim of this paper is to explore the potential of O2-enriched PEM fuel cell technology from an energy efficiency perspective. The findings emphasize the promise of this technology, particularly in terms of fuel cell and BoP efficiency, warranting further investigation both theoretically, through the development of a bespoke model for cell operation analysis, and experimentally. To emphasize that our findings lay a foundation for promising future research, the conclusions section has been revised and restructured. This revision aims to highlight that the identified research tasks represent new avenues for future studies, aligning with the directions suggested by the reviewer.

 

Previous version

 

Considering this, the following future research actions are proposed: 413

• Perform an experimental analysis regarding the combined influence of O2 molar fraction (in the range of 21-40%) and O2 excess ratio (from 1 to 2) in the cathode stream. This would allow to identify the benefits from O2-enrichment in the cell performance at higher current density levels, as well as evaluate the potential for cathode O2 excess ratio reduction.
• Evaluate other O2-enrichment separation membrane technologies with higher selectivity and analyze the potential to reduce the membrane working pressure and/or the ratio of inlet to permeate mass flows.

Integrate the fuel cell and the membrane in a one-dimensional model of the complete balance of plant to study the sizing of the compressor and turbine elements, including preliminary maps to better assess the efficiency as a function of the working operating conditions.

 

 

Current version

 

Line 494-511. Based on the results of this study, the following research areas are proposed for further investigation:

 

• Conduct an experimental analysis to investigate the effects of O2 molar fraction (21-40%) and O2 excess ratio (1-2) in the cathode stream. The aim is to ascertain the advantages of O2 enrichment on cell performance at increased current densities and assess the feasibility of reducing the cathode O2 excess ratio. This research will also aid in developing a 1D fuel cell model with the capacity of analyzing reactive species depletion, thereby enhancing system efficiency.
• Evaluate other O2-enrichment separation membrane technologies with higher selectivity and analyze the potential to reduce the membrane working pressure and/or the ratio of inlet to permeate mass flows.
• Integrate the fuel cell and the membrane in a one-dimensional model of the complete balance of plant to study the sizing of the compressor and turbine elements, including preliminary maps of these components to better assess their efficiency as a function of the operating conditions.

 

These research initiatives are directed towards advancing knowledge and development in O2-enriched PEM fuel cell technologies, with the goal of develop more efficient, sustainable, and cost-effective systems.

 

Author Response File: Author Response.pdf