The Long-Term Usage of an Off-Grid Photovoltaic System with a Lithium-Ion Battery-Based Energy Storage System on High Mountains: A Case Study in Paiyun Lodge on Mt. Jade in Taiwan
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis manuscript details the long-term use of an off-grid photovoltaic system and lithium-ion battery energy storage system at Paiyun Villa in Yushan, Taiwan. By analyzing energy data over four years, the authors proposed a method to assess the health status of lithium-ion battery systems and discussed power system aging issues and improvement projects. The results provide valuable empirical references for long-term energy solutions in alpine environments. However, there are some areas that need improvement in the paper, and the following are some detailed comments.
1. When comparing the cost models of different battery systems, it is recommended to add an in-depth discussion of the results to analyze why lithium-ion batteries have more advantages in economic and environmental performance in long-term use.
2. In the data analysis method section, the data processing and analysis steps should be described in more detail to ensure that other researchers can repeat the experiment.
3. The background of the application of lithium-ion batteries in alpine environments can be further enriched. For example, a description of the impact of alpine environmental characteristics on battery systems can be added.
4. The conclusion section needs to summarize the research findings more comprehensively and put forward specific engineering suggestions.
Comments on the Quality of English LanguageModerate editing of English language required.
Author Response
I appreciate the valuable comments from the reviewer.
As I started to modify the manuscript accordingly, the revised manuscript became rich, and I learned much new knowledge through the revision process.
I hope the revised manuscript can benefit the scientific and engineering communities.
The responses are listed point by point below.
- When comparing the cost models of different battery systems, it is recommended to add an in-depth discussion of the results to analyze why lithium-ion batteries have more advantages in economic and environmental performance in long-term use.
Modified.
A more detailed discussion as shown below is inserted on page 23 of the manuscript.
“Lead-acid batteries were invented in 1859 by French physicist Gaston Planté. It is still one of the most widely utilized battery systems worldwide [124]. However, excessive charging will cause lead-acid batteries to emit hydrogen, oxygen, and some toxic gases. This process is known as “gassing,” and toxic gas emissions will result in air pollution. The sulfuric acid gases can react with the exposed metals, leading to corrosion problems. Also, lead (Pb) pollution and exposure caused heavy-metal environmental and health problems. Some lead compounds are extremely toxic. Long-term exposure to even tiny amounts of lead compounds can result in brain and kidney damage, hearing issues, and learning problems in children [125]. Poorly managed tailing and gangue disposal processes can carry out water, soil, and transport pollution problems [126]. On the other hand, Li-ion batteries possess no gassing phenomenon during normal working, indicating no on-site toxic air pollution, corrosion, or heavy metal problems. Hence, Li-ion batteries become eco-friendly replacements for lead-acid batteries.”
- In the data analysis method section, the data processing and analysis steps should be described in more detail to ensure that other researchers can repeat the experiment.
Modified.
In the preprint version, I found data processing annoying, so I just discussed the data directly. But after being reminded by the reviewer, I think it is necessary to talk about it. After all, there are indeed many difficult points worthy of discussion, and it can also allow readers to see how to deal with large-scale data.
To describe the data processing, a new figure (Fig. 3) is designed, and a subsection entitled “Data processing” with the following descriptions has been added to the manuscript (on page 5).
“Data processing is crucial before data analysis. The data obtained from BMS is stored in a CSV (comma-separated values) file every day. There are about 1400 CSV files in the data storage. The time duration between data points is about 15 seconds. The accuracy of data and time plays an important role in the data processing. If the time is obtained from the standalone BMS without network calibration, the timeline of the data should be checked first. In this case, the time was obtained based on the passive device (i.e., a crystal Oscillator), and there was no network connected for time calibration once a day or once a week. After drawing the first and last CSV file, a huge time shift of about 15000 seconds was found.
It's difficult to deal with 1400 CSV files one by one manually, e.g., using Microsoft Excel or some lightweight scientific drawing software. In this work, MathWorks MATLAB was used. The first step was to combine the data in 1400 CSV files, i.e., putting all datasets in a single database. Then, drawing a daily colormap to see the time shift condition. Because of the daily routine operation of the ESS, the data is predicted to exhibit a roughly daily pattern on a large time scale. The time shift issue can be solved by adjusting the time until the data exhibits a roughly daily pattern. As shown in Fig. 3(a), the time shift caused an obvious pattern shift of total voltage. In Jan. 2017, the red region began at about 8 o’clock, while in Jul. 2020, the red region began at about 12 o’clock. The time shift was a continuous process, exhibiting a parallelogram of red region. The red line on the left side of the parallelogram can serve as an indicator of time shift. The more sloping the red line is, the more serious the time shift is. Some data on the right side were shifted to the left side (indicated by the red rectangle). The time shift issue can be solved by linear adjustment as shown in Fig. 3(b). The red region becomes a rectangle, indicating a roughly daily pattern. A three-dimensional (3D) plot figure for showing the variation of total voltage is given in Fig. 3(c).
Other technical suggestions for efficient large-scale data processing are given. (1) Using parallel computing technique. In this study, 12 cores in the CPU were used simultaneously for data calculations. (2) Use programmable drawing software. There are about 1400 figures in this work. Writing a program to perform batch drawing, instead of drawing figures one by one manually.”
- The background of the application of lithium-ion batteries in alpine environments can be further enriched. For example, a description of the impact of alpine environmental characteristics on battery systems can be added.
Modified.
A subsection entitled “Alpine environmental impacts on battery systems” with the following descriptions has been added to the manuscript (on page 12).
“The impact of alpine environmental characteristics on battery systems can be evaluated from the perspective of two major physical quantities, i.e., temperature and pressure. In terms of temperature, if the battery is placed outdoors, it may be too low, making it difficult to charge and discharge the battery. Outdoors, there is also the problem of excessive temperature difference between day and night, which will cause battery life to be shortened. If the battery is placed indoors, in a well-insulated environment, and combined with the heat generated by the operation of other equipment, the battery temperature may be moderate, and there will be no problem of excessive temperature difference between day and night. For example, the ESS in Paiyun Lodge is located indoors. In terms of pressure, the air pressure in high mountains is low, which will cause expansion problems for soft-packed batteries. Another point is the opening pressure of the battery safety relief valve. Since the valve opening pressure is designed for use on flat ground, moving to a mountain with lower outside air pressure, and the external pressure drops may cause the valve to open early.”
- The conclusion section needs to summarize the research findings more comprehensively and put forward specific engineering suggestions.
Modified.
The research findings are given in more detail in the first three paragraphs. The forward-specific engineering suggestions are provided in the 4th paragraph.
The entire section of the conclusions has been modified as follows (on page 24).
“In Li-ion battery analysis, a data processing method has been developed for dealing with large-scale datasets. Four major daily operation patterns are classified based on the conditions of the solar and backup energy. The low C-rate distribution (CR < 0.15 h−1) made the battery system to avoid from excessive voltage differences among battery cells and heat accumulation on the current collectors, improving system stability and expanding the life of the battery system. The temperature distributed within the recommended range of operation temperature (15–35â—¦C) for more than half of the time, meaning that temperature variation were in normal region and won’t affect the system too much. Also, the temperature difference ∆T of all time is almost under 5â—¦C, indicating that the battery system is in good temperature uniformity. Overall, the operation temperature and C-rate are close to the minimum of Arrhenius plot, indicating a slower aging rate and longer life of the battery system. Accumulated capacity (energy) is an important quantity for judging the
health of batteries. Compared to the data of LFP batteries with cycle life about 2000–4000 cycles, the battery system is far away from EOL with an estimated remaining capacity of more than 95%. The Li-ion batteries installed in Paiyun Lodge remain healthy and can continue to be used.
The electric power improvement of Paiyun Lodge has been accomplished, making a more durable power system, a more stable battery system, and a more comprehensive EMS. For the Li-ion battery system, faulty battery cells are replaced, and active balancers for shrinking the voltage differences of battery cells are installed. For the power system, over-installation of the hybrid solar inverters is applied for system maintenance on high mountains. Six hybrid solar inverters are installed, when one down, the rest five can still work with load increase by 20%. The power of the 6th PV array is reconnected, and bout 16.7% solar capacity is recovered. For the EMS, the self-developed cloud EMS has been installed, providing remote monitor of entire energy input and output of the off-grid PV ESS. On the other hand, the engineering issues on high mountains are given, including (1) alpine worker carrying, (2) demolition and transportation of old and abandoned facilities, (3) material preparation, occupational safety, quality control, safety and health facilities, (4) risks and obstacles. We believe that after the power improvement project, the reoptimized off-grid PV ESS can provide green electricity for Paiyun Lodge and continue to serve climbers. Above all, this study gives engineers and researchers a fundamental understanding of a real case of long-term usage of off-grid PV ESSs and engineering on high mountains.
The simple cost model demonstrates that Li-ion batteries (instead of lead-acid batteries) are better choice for long-term application on high mountains. Owing to the extra moving cost, the cross of the cost curves of two battery systems occurs in the second two years (3–4 years), and the Li-ion battery becomes a good choice since the third two years (5–6 years). On the other hand, the continued low price of Li-ion batteries in the market will cause the replacement of lead-acid batteries more quickly in the future.
EMS plays an important role in future research. The old power system possesses no EMS, i.e., no energy data obtained. It’s hard to answer the energy usage problem accurately. With the installation of EMS in the new power system, we can gather the energy input and output data of the entire power system, evaluating the entire energy input and output accordingly. PV- and diesel-generated energies, energy consumption, charge and discharge energies of the battery system can be resolved accurately. The durability of over-installed hybrid inverters and the effect of balancers for long-term usage of the battery system can also be confirmed. As the annual energy data is obtained, we can realize whether the capacities of PV and ESS are sufficiently large or not. If the PV and ESS capacities are not sufficient, we will enlarge the capacities accordingly. Our goal is to largely reduce GHG emissions by using renewable energies.”
Reviewer 2 Report
Comments and Suggestions for AuthorsIn this work, the author reported a detailed case study of the off-grid photovoltaic system with lithium-ion battery-based energy storage system in Payiun Lodge on Mt. Jade in Taiwan. Specifically, the authors analyzed the 4-year data collected from the system in terms of daily operation patterns, C-rate, temperature, and accumulated energy distributions. The status and the encountered problems have been listed. Meanwhile, some advice for further improvement has been proposed. In addition, the author also analyzed the cost using a simple model. Generally speaking, the reported work could provide useful information and insights for future development of grid energy storage systems. In this regard, I am in favor of its publication with the minor modification noted below.
1. When carrying out the cost analysis, the author claims that the lifetime of lead-acid batteries is 2 years. It is too short. Usually lead-acid batteries can last 3-5 years and some even up to 10 years. The cost analysis should be revised with necessary literature cited.
Author Response
I appreciate the valuable comments from the reviewer.
As I started to modify the manuscript accordingly, the revised manuscript became rich, and I learned much new knowledge through the revision process.
I hope the revised manuscript can benefit the scientific and engineering communities.
The responses are listed point by point below.
- When carrying out the cost analysis, the author claims that the lifetime of lead-acid batteries is 2 years. It is too short. Usually lead-acid batteries can last 3-5 years and some even up to 10 years. The cost analysis should be revised with necessary literature cited.
I agree with the opinion that “Usually lead-acid batteries can last 3-5 years and some even up to 10 years.”
The service life of lead-acid batteries is highly related to the usage conditions, such as C-rate and DoD (depth of discharge).
For a car starter battery, the lead-acid batteries are often used at a high C-rate (over 3C for less than 1 second) and very low DoD (less than 1%), and the service life is 2~5 years [REF: J. Power Sources 42 (1993) 1, DOI: https://doi.org/10.1016/0378-7753(93)80132-9 ].
For a grid-scale energy storage system, the lead-acid batteries are often used under a low C-rate (less than 0.25 C) and at a low DoD (less than 50%), and the service life can even expand to 10~15 years [REF: J. Energy Storage 15 (2018) 145, DOI: https://doi.org/10.1016/j.est.2017.11.008 ].
However, for lead-acid batteries in Paiyun Lodge, the C-rate was low (less than 0.25 C) and the DoD was very high (close to 100%), the service life was reduced to 1~3 years. According to the datasheet from Yuasa, the higher the DoD, the smaller the cycle number [REF: https://www.yuasabatteries.com.au/media/y5mhhm0t/yu109-2102a-yuasa-rec80-12.pdf ]. A battery used in 100% DoD may possess 1/3 service life as the battery used in 50% DoD.
The manager of Paiyun Lodge told us that the lead-acid batteries were usually replaced every two years. He didn’t know what is DoD. He told us: “When the battery is close to a very low voltage or the system is shutting down, he will turn on the diesel generator.” This description indicates that the batteries were used close to 100%. Hence, the service life is extremely reduced. And in the original model, I admitted his statement, using 2 years as a replacement period.
In combination with the reviewer’s comment, we have made a more general description of the battery service life in the model, giving readers sufficient knowledge to decide which service life should be chosen for their own case.
In the manuscript, a new paragraph is added (page 23).
“In the above example, the service life is chosen as 2 years, based on the past battery replacement durations in Paiyun Lodge. Generally, the service life of lead-acid batteries might range from 2–10 years, which is highly related to the usage conditions, such as C-rate and DOD. For a car starter battery, the lead-acid batteries are often used at a high C-rate (over CR = 3 h−1 for less than 1 second) and very low DOD (less than 1%), and the service life is 2–5 years [122]. For a grid-scale energy storage system, the lead-acid batteries are often used under a low C-rate (CR < 0.25 h−1) and at a low DOD (less than 50%), and the service life can even expand to 10–15 years [123]. However, for lead-acid batteries in Paiyun Lodge, the C-rate was low (CR < 0.25 h−1) and the DOD was very high (close to 100%), the service life was largely reduced to roughly 2 years.”
Reviewer 3 Report
Comments and Suggestions for AuthorsThis article investigates the profitability and compares the different battery chemistries of a photovoltaic system with green energy storage in a lithium-ion battery storage system. The study was conducted at Payiun Lodge in Taiwan.
While the article provides a comparison between lithium-ion and acid batteries, a more impactful analysis for developers and manufacturers would be a comparative exploration of various lithium-ion chemistries (e.g., LFP, NMC, LTO). However, the final decision regarding the specific content remains with the editor.
My comments about the manuscript:
1. Use a maximum of 5 shorted keywords.
2. The introduction should include a methodology figure/scheme.
3. In a few figure, for consistency, please ensure the font matches the Palatino Linotype typeface and size used in the main body of the text.
4. For improved readability and clarity, consider including an abbreviation table listing all defined terms used within the manuscript.
5. Table 2 can be rotated 90 degrees.
6. Figures 7, 8, and 9 are too large for inclusion in their current format. Please revise and resubmit the figures in accordance with the publication's size requirements.
7. Line 559 - 1183 USD/kWh in 2010 – satatista reference is missing
8. A review of the bibliography is recommended to ensure the inclusion of the most recent articles relevant to the research topic.
Author Response
I appreciate the valuable comments from the reviewer.
As I started to modify the manuscript accordingly, the revised manuscript became rich, and I learned much new knowledge through the revision process.
I hope the revised manuscript can benefit the scientific and engineering communities.
The responses are listed point by point below.
- Use a maximum of 5 shorted keywords.
Corrected. ( 6 -- > 5 )
The number of keywords has been corrected from 6 ((1) Lithium-ion battery, (2) battery analysis, (3) power improvement, (4) high mountain, (5) cost analysis, (6) Paiyun Lodge) to 5 ((1) Lithium-ion battery, (2) power improvement, (3) high mountain, (4) cost analysis, (5) Paiyun Lodge).
- The introduction should include a methodology figure/scheme.
Modified.
A methodology scheme (Fig. 2) is added, and the following description is inserted for describing (page 3).
“The research methodology scheme of this work including three parts is shown in Fig. 2. In the first part (Sec. 2), the energy data of the Li-ion battery system for four years is analyzed. Analysis of such massive data is not an easy task (more than 7 million rows). Especially, the data is recorded under the actual operation instead of regular data from cycle life tests in the laboratory. A method is established for analyzing the energy data during operation and estimating the health of the Li-ion battery system, including daily operation patterns as well as C-rate, temperature, and accumulated energy distributions. In the second part (Sec. 3), the status and problems of the old electric power system of Paiyun Lodge are described. The engineering status of the electric power system is reported, such as (1) reform and optimization of existing Li-ion battery cabinets, (2) PV inverter system reformation and optimization, (3) reorganization of distribution boxes and power line adjustment, (4) a self-developed cloud energy management system (EMS) is installed to remotely monitor the off-grid PV ESS. In the third part (Sec. 4), a simple cost model is built for comparing the cost between lead-acid and Li-ion battery systems. This model reasonably explains that the expensive Li-ion batteries can compete with the cheap lead-acid batteries for long-term usage on high mountains. Above all, this study gives engineers and researchers a fundamental understanding of a real case of long-term usage of off-grid PV ESSs on high mountains.”
- In a few figure, for consistency, please ensure the font matches the Palatino Linotype typeface and size used in the main body of the text.
Modified.
Great! I appreciate that the reviewer gave critical information about the font “Palatino Linotype.” It’s very important for me to adjust this issue.
The text objects in all figures have been checked and modified to Palatino Linotype typeface, and the font size has also been modified.
- For improved readability and clarity, consider including an abbreviation table listing all defined terms used within the manuscript.
Modified.
An abbreviation list and a symbol list are added at the end of the manuscript.
- Table 2 can be rotated 90 degrees.
Modified.
Table 2 has been rotated 90 degrees. To achieve this goal, some texts are replaced by symbols.
- Figures 7, 8, and 9 are too large for inclusion in their current format. Please revise and resubmit the figures in accordance with the publication's size requirements.
Modified.
Figs. 7, 8, and 9 (become Figs. 9, 10, and 11 in the revised version) have been revised and resubmitted in accordance with the publication's size requirements.
- Line 559 - 1183 USD/kWh in 2010 – satatista reference is missing
Corrected.
The missing reference (from BloombergNEF) has been inserted.
- A review of the bibliography is recommended to ensure the inclusion of the most recent articles relevant to the research topic.
Modified.
11 most recent articles relevant to the research topic have been added. These articles are listed below.
(1) Adv. Mater. Technol. 8 (2023) 2200459
(2) Energies 17 (2024) 1019
(3) J. Mar. Sci. Eng. 12 (2024) 843
(4) J. Energy Storage 88 (2024) 111567
(5) J. Energy Storage 87 (2024) 111508
(6) J. Energy Storage 86 (2024) 11132
(7) J. Radiat. Res. Appl. Sci. 17 (2024) 100927
(8) Process Saf. Prog. 43 (2024) 357
(9) Renew. Sustain. Energy Rev. 197 (2024) 114412
(10) Sci. Adv. 12 (2023) eadg5135
(11) Sustain. Energy Grids Netw. 38 (2024) 101330
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors have thoroughly revised the manuscript, which is suitable for publication in its current form.
Comments on the Quality of English LanguageModerate editing of English language required
Reviewer 3 Report
Comments and Suggestions for AuthorsEven if it is not easy to follow the changes, all questions raised have been answered. I will accept the revised version as it is.
