Techno-Economic Analysis of the Hybrid Solar PV/H/Fuel Cell Based Supply Scheme for Green Mobile Communication
Round 1
Reviewer 1 Report
The presented paper presents the current problem of storing electricity obtained from renewable energy sources using hydrogen, which is an environmentally friendly and promising technology. The paper is presented at a sufficiently high level and may be of interest to specialists and researchers of the subject under consideration, however, as comments and recommendations, several points should be noted:
1. The authors conduct a review of systems with wind generators, but the lack of it in their installation is insufficiently substantiated.
2. What justified the choice of the HOMER program for calculations, and not any other?
3. Along with the absence of a wind generator, the absence of an auxiliary generator on fossil fuels in the absence of solar radiation for a long time is insufficiently substantiated.
4. The authors should describe in more detail the efficiency of converting electricity to hydrogen and back to electricity, as well as the losses in this process.
5. There is still a rechargeable battery in the system - it is necessary to justify the expediency of its use, since the authors cited the disadvantages of its use. Perhaps it makes sense to use a more capacious battery instead of additional rewarding a complex additional system with hydrogen (it is necessary to take into account the overall efficiency of this system).
6. All abbreviations and designations used in the text should be placed before the main text for a complete understanding of the content.
7. The authors should describe in more detail the design of the electrolyzer used, since this unit is of great interest. Also of interest are the designs of the gas cylinders used (long-term storage of hydrogen is very difficult) and fuel cells (conversion efficiency is very important).
8. The process of pumping into gas cylinders under high pressure is also interesting, since the process should be autonomous and automated - it is practically not described in the paper.
9. On page 7 and line 258 instead of "??" authors should indicate the numbers of the equations - apparently 5, 6 and 7.
10. It is necessary to justify the use of the selected storage battery - its type, service life, cost. Make comparisons with other types of batteries.
11. On page 10 and line 301, replace "??" to the number of the table - apparently 3. All references in the text to figures and tables must be located strictly before the location of the figures and tables themselves in the text.
12. For Figure 8, a short description should be made based on the resulting graphs.
13. For the formulas in (17), additional designations should be introduced instead of the "first", "second", ...
14. Authors should justify the choice of PV modules in more detail, since they are one of the main components of the system. The authors indicated the service life at 25 years, but there are technologies that increase the service life, which will have a positive effect on the economic component of the project, which the authors should also take into account and indicate in the work (Panchenko V., Izmailov A., Kharchenko V., Lobachevskiy Ya. Photovoltaic Solar Modules of Different Types and Designs for Energy Supply. International Journal of Energy Optimization and Engineering, V. 9, I. 2, 2020, 74-94, DOI: 10.4018 / IJEOE.2020040106). Also important is the efficiency of converting solar radiation into electrical radiation, where its value can be more than 20-25% (Panchenko V. Photovoltaic solar modules for autonomous heat and power supply. IOP Conference Series: Earth and Environmental Science 317, 2019, 012002, 9 p. doi: 10.1088 / 1755-1315 / 317/1/012002), which will also have a positive effect on the economics of the project and the authors should take this into account and indicate in the work.
15. The Figures 11 and 12 should be described in more detail and conclusions should be drawn from the values obtained.
16. Is it planned to assemble a prototype to compare the calculated values ​​with the experimental ones - such a comparison will be very interesting. Authors should add a subsection "Directions for further research", where indicate the planned research.
Limitation Analysis looks very appropriate and describes some of the nuances of the system. Comparison of the systems looks convincing, as it shows the analysis carried out by the authors of the work. The list of sources used looks solid and compelling. The sources used are relatively fresh and relevant, papers are published in renowned international journals.
Thus, after supplementing the paper according to the above comments and recommendations, the revised paper can be recommended for publication in the journal Sustainability.
Author Response
Reviewer#1
The presented paper presents the current problem of storing electricity obtained from renewable energy sources using hydrogen, which is an environmentally friendly and promising technology. The paper is presented at a sufficiently high level and may be of interest to specialists and researchers of the subject under consideration, however, as comments and recommendations, several points should be noted:
Reviewer#1, Concern#1: The authors conduct a review of systems with wind generators, but the lack of it in their installation is insufficiently substantiated.
Author response: We thank the reviewer for the very thorough and pertinent comments. The hybrid solar PV/wind turbine (WT) system is not feasible because of the low-speed wind graph in the selected location.
Author action: We have revised the manuscript as below:
Introduction: “The development of a solar PV/wind turbine (WT) powered isolated base station is studied in references [6, 27].”
[6] Alsharif, M.H.; Kim, J. Hybrid Off-Grid SPV/WTG Power System for Remote Cellular Base Stations Towards Green and Sustainable Cellular Networks in South Korea. Energies 2017, 10. doi:10.3390/en10010009.
[27]Singh, S.; Singh, M.; Kaushik, S.C. A review on optimization techniques for sizing of solar-wind hybrid energy systems. International Journal of Green Energy2016, 13, 1564–1578.
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Reviewer#1, Concern#2: What justified the choice of the HOMER program for calculations, and not any other?
Author response: Thank you very much for your insightful comment. Hybrid Optimization Model for Electric Renewables (HOMER) simulates the operation of a system by making energy balance calculations at each time step of the year. For each time step, HOMER compares the electric load demand in that time step to the energy that the system can supply in that time step, and calculates the flows of energy to and from each component of the system. For systems that include batteries and solar-wind generators, HOMER also decides in each time step whether to charge or discharge the batteries [R1]. HOMER performs these energy balance calculations for the considered system configuration. It then determines a feasible configuration i.e., whether it can meet the electric demand under the specified conditions, and estimates the cost of installing and operating the system over the lifetime of the project. The system cost calculations account for costs such as capital, replacement, operation and maintenance, fuel, and interest [R2]. The optimization step follows all simulations HOMER finds the least-cost combination of components that meet base station load [R3]. In the optimization process, HOMER simulates many different system configurations, discards the infeasible ones (those that do not satisfy the user-specified constraints), ranks the feasible ones according to total net present cost, and presents the feasible one with the lowest total net present cost as the optimal system configuration. HOMER follows a linear optimization model. HOMER Optimizer is a proprietary “derivative-free” optimization algorithm that was designed specifically to work in HOMER. The flow diagram of the HOMER software is shown in Figure 1.
Figure 1: Flow diagram of the HOMER optimization software.
[R1] T. Lambert, P. Gilman and P. Lilienthal, “Micropower System Modeling with HOMER”, Integration of Alternative Sources of Energy, Chapter 15, pp. 379-418, Dec 2006.
[R2] M. H. Alsharif, “A Solar Energy Solution for Sustainable Third Generation Mobile Networks”, Energies, vol. 10, pp. 429 (17 pages), Mar 2017.
[R3] HOMER Energy. [Online]. Available: https://www.homerenergy.com/products/pro/docs/3.11/optimization.html
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Reviewer#1, Concern#3: Along with the absence of a wind generator, the absence of an auxiliary generator on fossil fuels in the absence of solar radiation for a long time is insufficiently substantiated.
Author response: The hybrid PV/WT scheme provides better performance but is not a feasible solution due to the poor wind profile in the selected location. Additionally, the standalone DG and hybrid PV/DG have poor performance because of burning huge amounts of fossil fuel and high fuel transportation costs. As per your suggestion, we have revised the entire manuscript for presenting a clear view to the reader.
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Reviewer#1, Concern#4: The authors should describe in more detail the efficiency of converting electricity to hydrogen and back to electricity, as well as the losses in this process.
Author response: Thank you for pointing out this issue. Power to hydrogen to power (P2H2P) systems are characterized as the round trip efficiency (η) can be expressed as follows [R4].
The efficiency of the used electrolyzer ( ) is 85% and fuel cell (FC) efficiency ( ) is 75%. The original manuscript had 28 pages. To make the paper concise, we didn’t analyze the efficiency and losses in detail.
[R4] Dawood, F.; Shafiullah, G.; Anda, M. Stand-Alone Microgrid with 100% Renewable Energy: A Case Study with Hybrid Solar PV-Battery-Hydrogen. Sustainability 2020, 12. doi:10.3390/su12052047.
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Reviewer#1, Concern#5: There is still a rechargeable battery in the system - it is necessary to justify the expediency of its use, since the authors cited the disadvantages of its use. Perhaps it makes sense to use a more capacious battery instead of additional rewarding a complex additional system with hydrogen (it is necessary to take into account the overall efficiency of this system).
Author response: The operation of the network can be divided into the following two states: First, energy is supplied to the BS by the hybrid solar PV/H/FC supply system, where excess electricity is stored in the battery bank and hydrogen tank. Second, if the solar PV/H/FC energy is not sufficient due to the deficiency of solar resources or seasonal changes, the battery bank will supply backup electricity. Notably, the fully charged battery bank alone has the capacity to run the BS for about 40.6 hours. This power supply will continue until the minimum depth of discharge of the battery is reached for ensuring the 0% power outage.
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Reviewer#1, Concern#6: All abbreviations and designations used in the text should be placed before the main text for a complete understanding of the content.
Author response: According to your directions, we list the notations and symbols in Table 1.
Author action: We have revised the manuscript as below:
Table 1: Summary of the notations and symbols.
Notations Meaning Notations Meaning
BB
BS
BW
COE
DG
FC
GHG
NPC
PV
QoS Baseband
Base station
Bandwidth
Cost of energy
Diesel generator
Fuel cell
Greenhouse gas
Net present cost
Photovoltaic
Quality of service RE
WT
CO2
ESurplus
H2
PBS
P2H
P2H2P
RTotal
Renewable energy
Wind turbine
Carbon dioxide
Surplus energy
Hydrogen
BS power
Power to hydrogen
Power to hydrogen to power
Throughput
BS traffic
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Reviewer#1, Concern#7: The authors should describe in more detail the design of the electrolyzer used, since this unit is of great interest. Also of interest are the designs of the gas cylinders used (long-term storage of hydrogen is very difficult) and fuel cells (conversion efficiency is very important).
Author response: As per your suggestions, we have revised the manuscript by addressing the mentioned issues in Section 3 subsection 3.3.
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Reviewer#1, Concern#8: The process of pumping into gas cylinders under high pressure is also interesting, since the process should be autonomous and automated - it is practically not described in the paper.
Author response: Thank you for your valuable comments. In our future work, we will consider it.
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Reviewer#1, Concern#9: On page 7 and line 258 instead of "??" authors should indicate the numbers of the equations - apparently 5, 6 and 7.
Author response: As per your suggestion, we have revised the entire manuscript by fixing the ?? errors.
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Reviewer#1, Concern#10: It is necessary to justify the use of the selected storage battery - its type, service life, cost. Make comparisons with other types of batteries.
Author response: The battery bank is a backup device that acts as an energy buffer by compensating for the mismatch between the energy generation and the demand. On focusing the capital cost, and characteristics the ‘Trojan L16P’ battery model is considered in this work. Additional information can be found in Figure 2. The original manuscript had 28 pages. To make the paper concise, we didn’t make a comparison with other types of batteries. However, the Round trip efficiency, Lifetime throughput, Initial cost, Replacement cost, and OMC/Year of the selected battery (Trojan L16P) are shown in Table 3.
Figure 2: Details of the battery ‘Trojan L16P’.
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Reviewer#1, Concern#11: On page 10 and line 301, replace "??" to the number of the table - apparently 3. All references in the text to figures and tables must be located strictly before the location of the figures and tables themselves in the text.
Author response: We have revised the entire manuscript by fixing the ?? errors.
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Reviewer#1, Concern#12: For Figure 8, a short description should be made based on the resulting graphs.
Author response: Figure 8(a) represents the DC load profile of the macro cellular base station over 24 hours and Figure 8(b) represents the AC load profile (30 W AC lamp) which remains on from 6 PM to 6 AM. As per your suggestion, we have updated the description of Figure 8 to present a clear view to the reader.
Author action: We have revised the manuscript as below:
3.7. Methodology of the System: “Figure 8(a) represents the DC load profile of the macro cellular base station over 24 hours. This seasonal DC profile of the macro BS is derived from equation (1) for 10 MHz system bandwidth. Figure 8(b) represents the AC load profile (30 W AC lamp) which remains on from 6 PM to 6 AM.”
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Reviewer#1, Concern#13: For the formulas in (17), additional designations should be introduced instead of the "first", "second",
Author response: As per your suggestion, we have revised the manuscript by updating the designations of equation (17)
Author action: We have revised the manuscript as below:
Equation 17: “The joint contribution of energy from solar panels and fuel cells can surely fulfill BS's yearly energy needs to provide reliable power as presented in constraint (1). The limitation in constraint (2) assures that the yearly energy generated by the integrated renewable energy sources carries with its related losses and the annual BS consumption. For future usage, the quantity of surplus energy is stored in the hydrogen tank and battery bank as stated by the limitation constraint (3). The energy conserved also meets the power reliability limit. The limit constraint (4) means that the battery storage capacity shall neither exceed the limit nor be below the threshold level.”
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Reviewer#1, Concern#14: Authors should justify the choice of PV modules in more detail, since they are one of the main components of the system. The authors indicated the service life at 25 years, but there are technologies that increase the service life, which will have a positive effect on the economic component of the project, which the authors should also take into account and indicate in the work (Panchenko V., Izmailov A., Kharchenko V., Lobachevskiy Ya. Photovoltaic Solar Modules of Different Types and Designs for Energy Supply. International Journal of Energy Optimization and Engineering, V. 9, I. 2, 2020, 74-94, DOI: 10.4018 / IJEOE.2020040106). Also important is the efficiency of converting solar radiation into electrical radiation, where its value can be more than 20-25% (Panchenko V. Photovoltaic solar modules for autonomous heat and power supply. IOP Conference Series: Earth and Environmental Science 317, 2019, 012002, 9 p. doi: 10.1088 / 1755-1315 / 317/1/012002), which will also have a positive effect on the economics of the project and the authors should take this into account and indicate in the work.
Author response: The required number of solar PV panels (NPV) to produce a specific output (Pp) can be formulated as follows
N_PV=P_P/P_nom (1)
where Pmax is the maximum power of the solar PV panel. Table 1 represents the specification of the considered solar PV panel. As a consequence, 1 kW peak power solar array requires 4 independent Sharp solar modules as referred to in Table 1.
The capacity of the PV array is 4 kW for 10 MHz system bandwidth, hence the Sharp ND-250QCs module is a proper selection. According to the (1), the solar PV array consists of 16 Sharp ND-250QCs modules (4kW/250W=16), connected as 4 in a series and 4 in parallel to satisfy the requirements of the solar energy controller, in this paper which is known as energy management unit: (i) the open-circuit voltage of the solar energy controller should be higher than the open-circuit voltage of the solar PV array; and (ii) the current of the solar energy controller should be higher than the short circuit current of the solar PV array.
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Reviewer#1, Concern#15: The Figures 11 and 12 should be described in more detail and conclusions should be drawn from the values obtained.
Author response: As per your suggestion, we have revised the article by updating the conclusion and describing Figures 11 and 12 in more detail to present a clear view to the reader.
Author action: We have revised the manuscript as below:
Conclusion: “It is also found that the proposed system has the potential to decrease CO2 emissions up to -0.388 Kg per year (under 10 MHz system BW).”
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Reviewer#1, Concern#16: Is it planned to assemble a prototype to compare the calculated values with the experimental ones - such a comparison will be very interesting. Authors should add a subsection "Directions for further research", where indicate the planned research.
Limitation Analysis looks very appropriate and describes some of the nuances of the system. Comparison of the systems looks convincing, as it shows the analysis carried out by the authors of the work. The list of sources used looks solid and compelling. The sources used are relatively fresh and relevant, papers are published in renowned international journals.
Thus, after supplementing the paper according to the above comments and recommendations, the revised paper can be recommended for publication in the journal Sustainability.
Author response: In the future extension of this work we will develop a prototype system to ensure the effectiveness of the hybrid solar PV/H/FC-based green mobile communication. As per your suggestion, we have updated the manuscript by adding the future directions of this research work.
Author action: We have revised the statement as below:
Directions for Further Research:
At the current stage of the proposed research, developed hybrid solar PV and fuel cell-focused macro cellular network without energy sharing mechanisms have much room for being improved and extended in several interesting directions. Future extensions of this research work are summarized as below:
Developing a prototype system to ensure the effectiveness of the hybrid solar PV/H/FC-based green mobile communication.
Developing a generic algorithm and control system for sharing green energy across surrounding BSs and industry by maximizing the use of renewable energy in heterogeneous cellular networks.
Developing a cell zooming technique for running heterogeneous cellular networks based on the amount of green energy generated and the rate of incoming traffic.
For enhancing energy efficiency performance, renewable energy cooperation of cloud radio access network (C-RAN) heterogeneous networks paradigm may be examined.
For a better compromise between data rate and fairness, an efficient resource and energy scheduling technique can be implemented.
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
Please see attached file.
Author Response
No comments are found for reviewer 2.
Author Response File: Author Response.pdf
Reviewer 3 Report
I appreciate the work presented in this manuscript. The work carried out will be quite relevant while designing green mobile communication systems. However, there are few minor comments:
- Abbreviation needs to be defined properly and their uses required to be checked properly.
- There are few typos errors (like spellings, equation-table citations etc). That may be checked and rectified throughout the manuscript.
- What is Rtotal in equation (22) and (23)? Is this refers to power?
- In line 413, in Epv calculation, the size of pv should be 4 kW or 1 kW??
Author Response
Reviewer#3
I appreciate the work presented in this manuscript. The work carried out will be quite relevant while designing green mobile communication systems. However, there are few minor comments:
Reviewer#3, Concern#1: Abbreviation needs to be defined properly and their uses required to be checked properly.
Author response: We thank the reviewer for the thorough and pertinent comments. As per your suggestions, we have revised the entire manuscript by addressing the abbreviation in the proper position.
Reviewer#3, Concern#2: There are few typos errors (like spellings, equation-table citations etc). That may be checked and rectified throughout the manuscript.
Author response: We have updated the manuscript by fixing the spellings, and properly citing the equations, tables, and figures in the text.
Reviewer#3, Concern#3: What is Rtotal in equation (22) and (23)? Is this refers to power?
Author response: In Equations (22) and (23), RTotal represents the total achievable throughput of the wireless network. We have revised the manuscript by addressing the TTotal in the text.
Author action: We have revised the manuscript as below:
4.4. Energy Efficiency Modeling: “According to Shanon’s theorem on information capacity, overall achievable throughput performance (RTotal) of the wireless network at t can be represented as [45]”
Reviewer#3, Concern#4: In line 413, in Epv calculation, the size of pv should be 4 kW or 1 kW??
Author response: In line 413, the size of the solar PV should be 4 kW. We are sorry for the mistake in the previous submission.
Author action: We have revised the manuscript as below:
5.2.1. Solar PV Energy (Line 413):
“EPV = 4 kW × 4.59 × 0.9 × 365 Days/Year = 6,031.26 kWh/Year”
Author Response File: Author Response.pdf
