Long- and Short-Term Comparative Analysis of Renewable Energy Sources
Round 1
Reviewer 1 Report
The authors analyze the power generation, stability and reliability of three types of renewable energy sources - solar, wind, hydro - with data collected over a period of several years in a local town. They give details on the three operating systems and present their data over long (yearly to entire measuring period) and short term (daily with measurements every 15 min down to every second for varying days). This lead to a dense collection of data. Apart from general conclusions commonly expected for these sources of power generation, the authors focus on (abrupt) changes and possible generation of higher harmonics and related distortion in the supplied energy.
Overall, this article can make a contribution to better understand the joint influence of diverse power sources on the connected network regarding stability and predictability of supplied energy. Aspects of technology and economics are to some degree addressed and balanced. The manuscript is well written and apart from some points raised below comprehensive. Therefore, I recommend publication in Energies after adressing the following comments and suggestions:
Text / Discussions
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(a) The abbreviation THD has not been properly introduced. The term and its relevance for this study as well as the standards regulating maximum THD content should be placed in the introduction. In relation to the final results presented in table 5, the difference in the units THDu and THDi should be clarified.
(b) Regarding Fig. 18(c), you mention that the observed spikes are due to other loads and sources connected to the same network. Is there any more specific indication, which and/or how many system components caused these spikes? Can you estimate an upper limit of the number of other sources connected after which standards would be violated frequently?
(c) Given the high stability of hydro power, you often only briefly discuss results on this power generation source without showing the respective graphs. Hence, why not discuss hydro power briefly, but all aspects together in a separate section? By shifting this, the discussions around solar and wind power become more concise and less distracted by mentioning of the hydro power source. This discussion could be further extended: How does hydro power generated from a location at/in the ocean differ from your results? How strongly does the power generation from hydro plants influence the stability of other RES?
(d) In lines 262-267 you discuss a critical situation regarding THD. Can you give an estimate for an upper limit of the number of other sources operating in a small region for which THD standards would be still secure?
(e) Regarding the results presented in Figs. 22 and 23: Should power generation sources be turned off when operating at low power in order to avoid THD? Do power sources under this conditions not contribute to significantly to THD in other components of the network?
Figures & Tables / Presentation
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(f) Fig. 1: It would be helpful, if a straight line would separate actual data from projected data.
(g) Table 2: It's the only table in which the units of parameters are given in brakets after naming the parameter. Tables 1-4 should have the same style.
(h) Table 1: Nominal current and power don't have the 'N' in the subscript. Please amend.
(i) Tables 3 & 4 could be summarized into a single table to make the presentation more compact.
(j) In Figs. 20-23, it is not clear what the different colors mean. Are those different measurements in time? In relation with table 5, which color is connected to the data indices 1, 2, 3?
Author Response
Please see the attachment
Author Response File: 
Author Response.docx
Reviewer 2 Report
Review:
The English style need an improvement by a native speaker, see: long first sentence line 10ff.
Keyword: What does harmonic distortion mean – sounds for me unusual line 24
Line 27: It is not the limited natural resource …. It is the willing of the environmentalists and now the politicians to change the energy politics with a focus on renewables.
There are a number of authors available (e.g. Werner Sinn, IFO, Germany), who explain that this change is possible but connected with a number of problems. In this sense, your work helps to identify one problem, the lack of continuously supply of energy.
Line 38: Figure 1: colours are misunderstand able; please change to different types of lines.
Line 49,50: sentence must be improved, not clear what you mean.
Lines 80.81: The paper will end with discussion and conclusion… What you have written in 284 are the results. Discussion means to take other literature to compare your results … please do so.
Conclusions (from line 316)
-It sounds more like a summary.
--Please add some information as results from your discussion such as:
--Is there more related research recommended?
--What kind of consequences are to define on national and EU – level from your results??
-- Some further directions about future energy politics in Europe (you know: e.g. Space for Windpower become in Germany limited, Hydropower is limited to a very few projects. Solar can be installed more on buildings…. But finally Solar is expensive not less effective …
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
Major concerns
The authors should explain why these energy sources located nearby a small city Olsztynek are worth studying.
In the introduction, the authors should highlight the differences in renewable energy consumption within countries. See, in this regard, (see, in this regard, The effect of financial development on renewable energy consumption. A panel data approach, Renewable Energy, Volume 147, Part 1, March 2020, Pages 330-338).
In the introduction, the authors should present the contribution to the extant literature.
The results of the current study are not compared with previous and similar studies. This seems to be a serious concern in the study.
The authors should highlight the implications of their findings and the contributions to the extant literature.
Recommended readings:
The effect of financial development on renewable energy consumption. A panel data approach, Renewable Energy, Volume 147, Part 1, March 2020, Pages 330-338, https://doi.org/10.1016/j.renene.2019.09.005
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 4 Report
energies-841565: Long- and Short-Term Comparative Analysis of Renewable Energy Sources
This is an interesting paper that compares three exemplary renewable energy sources, photovoltaic cells power station providing 1 Mega Watt (MW) of power, a wind farm producing 2MW of power, and a hydroelectric power station using two large water ’Kaplan’ turbines producing.
Introduction
Provides background to the study, mentioning conventional energy sources hard and brown coal, petroleum or natural gas. Then mentions renewable energy sources hydropower, wind and solar. Authors report on percent share of energy consumption with estimation of future renewable energy sources (RES) in terms of highest shares in global primary energy consumption (Figures 1 and 2). As a consequence, the power network models must reflect distributed sources and consumers instead of models assuming simple unidirectional energy flow. As stated by the authors “Such a change can have both a positive and a negative influence on the network and end-users, especially in the field of power quality”.
Figure 1. Global primary energy consumption by energy source (below 2018 - historical data, above 2018 – projections.
Figure 2. Absolute values and percentage shares of energy sources in global consumption
Authors then introduce more about the quality of electrical supply is defined by the power quality and reliability, i.e. continuity of supply. Some indices describing voltage and current waveforms have been established in norms, e.g. EN 50160 [3], in order to measure power quality. Authors cite a number of previous studies in this area. Then the authors describe the aims of their present study “However, there are no papers which present analysis and comparison of three most types of renewable energy sources. This paper aims in such analysis which can help to draw more general conclusions supported by real measurements. The paper is based on measurements taken during 4 years in power stations representing three types of renewable energy sources (hydropower, wind and solar). They have similar power ratings and are located within short distance of each other.
Authors then describe the layout and sections of their paper.
Section 2. Characteristics of renewable energy sources used for the analysis. Authors state the location of the three power stations (All energy sources under consideration are located nearby a small city Olsztynek (53.585968N, 83 20.277693E) in Warmia-Masuria Province in northeastern Poland).
Authors then describe the PV power station connected to the network through cable line equipped with two transformers 110 kV / 15 kV having power ratings of 16 MVA and 6 three-phase 20 kW inverters (SYMO 20.0-3-M) which transform energy collected from 4000 (43 x 88 + 3 x 72) monocrystalline PV panels having rated power equal to 250 W. Thus, the total nominal power is equal to 1 MW.
The PV panel parameters are given in Table 1 and a simplified electrical diagram of the PV power station and its connection with the power system is shown in Figure 3.
The authors then describe the 2MW wind farm in a similar manner, using Table 2 to describe parameters and figure 4 as the schematic diagram. Likewise, authors describe the hydroelectric power station using table 3 (synchronous), table 4 (asynchronous) generators with electrical connections schematic in figure 5.
Section 3 Energy production: is devoted to comparison of energy production of renewable energy sources using long-term measurements spanning a few years taken in the power stations. The analysis shows seasonal changes and trends (shown in Figures 6-8). The high season for PV is also a low season for the wind farm so these sources complement each other.
Figure 9 shows “Annual energy production per 1 MW of installed power” which is the best way of comparing systems of different sizes and shows energy efficiency which is three times lower in the case of the PV power station.
Section 4 (Long term RES analysis) includes statistical analysis evaluating average loading of systems reflecting differences in nominal power usage. Relative usage of both the photovoltaic power station and the wind farm varies significantly (Figures 10 and 11) whilst the variation is much lower for the hydroelectric power station (Figure 12) due to manual control of output power and low variability of hydrological conditions (i.e. water level). The changes of average power based on measurements taken every 15 minutes are higher for the wind farm (Figure 14) than for the photovoltaic power plant (Figure 13). The changes for hydroelectric power plant were the lowest (Figure 15) – usually less than 2%.
Section 5 (Short term RES analysis)
The energy production in the case of hydroelectric power stations depends only on water stream set by the plant staff and except to abnormal situations when the dam water level is very low, the power can be regarded as constant, so no data included for hydroelectric. The generation of the photovoltaic power station depends strongly on weather conditions and contrary to averaged waveforms can be easily
observed in short-term waveforms (Figures 16a and 17a). The wind farm is also significant but not as high as for the photovoltaic power station – (from comparing Figures 16a and 17a with 18a and 19a, respectively).
Section 6. Power quality RES analysis
Authors point out the use of power electronic devices such as inverters, which are a crucial part of photovoltaic and wind power stations (i.e three-phase convertors). Hydroelectric power stations are not problematic in the same way so are not included. Higher harmonics content in current waveforms depends strongly on the energy generation level and actual power in relation to nominal power. The section is described and discussed in accordance with the standards which regulate application of inverters in renewable power stations, which for the nominal conditions means that the content of higher harmonics in waveforms cannot be higher than 5%. The exemplary measurement results presented in Figures 20 and 21 confirm that both the photovoltaic and the wind power station operation is up to standard. With the help of figures 22 and 23, authors explain the problems of higher harmonics, especially the fifth harmonic resulting in breach of the limit imposed by standards in the case of the network voltage. Further, it may bring about increased losses in a transmission system, transformer overloads or disturbances in sensitive load operation. Authors point out that the problem of higher harmonics arises if the working point is far below nominal power (Figures 22 and 23). Table 5 presents THD coefficients for different power levels.
Section 7 is the Discussion. Here the authors define various useful variability coefficients and apply them to the three RE systems (Table 6).
Section 8 is the conclusions comparing long- and short-term behaviour and covers the periodicity of production depending on geographical location and different high seasons in each case, suggesting how sources can complement one another. Also authors point out the efficiency of the three systems and that these should be taken into account when the payback period is estimated. The long-term analysis has confirmed high repeatability of annual energy generation patterns of all three sources. Short-term analysis confirms that the PV and the wind power stations are highly variable depending on weather conditions and that the variation can influence the network voltage and power quality in the supply network (sags and swells). Analysis of voltage and current waveforms for different source power levels leads to the conclusion that for powers close to the nominal the current THD coefficients are very low. The variability indices presented in the paper allow us to compare renewable energy sources. Also, they state that “The photovoltaic system demonstrates the highest variability both in short- and long-term perspective. However, the wind farm is a source with the highest variability of average 15-minutes power – almost two times higher than for the other sources”.
Authors should state what THD means (total harmonic distortion) the first time it is mentioned and should not assume that all readers will know.
Likewise for MVA and MW and how these relate? Not all readers of Energies will be familiar with these and similar terms.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Round 2
Reviewer 3 Report
The authors have improved significantly the paper according to my recommendations (and not only). I consider that the paper can be published in this form.
Reviewer 4 Report
Authors have addressed my concerns. I have no further comments
