Tidal Stream vs. Wind Energy: The Value of Cyclic Power When Combined with Short-Term Storage in Hybrid Systems
Round 1
Reviewer 1 Report
This is indeed a nice work to have a comparative study between two renewable forms of energy. I would recommend this manuscript for publication in the Energies, however I have some minor comments. Please look at the attached pdf for more details.
Comments for author File: 
Comments.pdf
Author Response
The authors would like to thank the reviewer for their useful feedback. Please see our responses below in red:
- In the authorship list the affiliation of 3rd author "Zoe Goss" whose subscript "3" description is
missing, please check that. CORRECTED - Kindly put the references in a proper order and in proper form, since corresponding to some references the key information is not available, for example references from 33 - 40, not able to find out/ access those articles.
References corrected so that websites have a url and date of access. All journal papers now include doi, and books include publisher information. Pugh date corrected from 2006to 1996. Additional references added include Clarke et al., Rashid et al., Simonsen et al., Met Office.
- Please write the full form of the terms CapEx/OpEx in the manuscript, you have used them from
the abstract but haven't defined any where in the manuscript.
Changed text in abstract to ‘capital and operating expenditure’.
Changed the text in section 4.2, (i.e. where CapEx and OpEx first appear in the main body of the paper) to read: ‘The economic viability of these systems will be
dependent on the capital expenditure (CapEx) and operating expenditure (OpEx) of the tidal stream and wind turbines and associated infrastructure.’
Reviewer 2 Report
The paper presents work on tidal stream vs. wind energy, the author has modelled and simulate this case and results look fine, I like to ask the author to do some revision to improve the paper, my comments are:
-Abstract should include clearly the objective and maybe the expected outcome.
-Introduction looks fine, I think that the author missed a good number of papers recently published in this topic, please check.
-I like to see more details of the modelling and modelling procedures.
-As the paper is based on modelling concept, it would be advisable to compare this work with other published work.
-Finally as mentioned above, also the abstract should show clearly the contribution of this work.
Author Response
The authors would like to thank the reviewer for their useful feedback. Please find our responses below:
-Abstract should include clearly the objective and maybe the expected outcome.
Text added to abstract to be more specific on the objectives of the paper: ‘This study quantifies the technical, economic and environmental performance of hybrid systems that use either a tidal stream or wind turbine, alongside short-term battery storage and back-up oil generators. The systems are designed to partially displace oil generators on the island of Alderney, located in the British Channel Islands.’
The outcomes of the paper clearly outlined in the abstract: ‘The tidal stream turbine provides four power generation periods per day, every day. This relatively high frequency power cycling limits the use of the oil generators to 1.6 GWh/year. In contrast, low wind resource periods can last for days, forcing the wind hybrid system to rely on the back-up oil generators over long periods, totalling 2.4 GWh/year (50% higher). For this reason the tidal hybrid system spends £0.25 million/year less on fuel by displacing a greater volume of oil, or £6.4 million over a 25 year operating life, assuming a flat cost of oil over this period. The tidal and wind hybrid systems achieve an oil displacement of 78% and 67% respectively (the same as the reduction in carbon emissions). For the wind hybrid system to displace the same amount of oil as the tidal hybrid system, two additional wind turbines are needed. The ability of the battery to store excess turbine energy during high tidal/wind resource periods relies on opportunities to regularly discharge stored energy. The tidal hybrid system achieves this during slack tides. Periods of high wind resource outlast those of high tidal resource, causing the battery to often remain fully charged and excess wind power to be curtailed. Consequently the wind hybrid system curtails 1.9 GWh/year, whilst the tidal turbine curtails 0.2 GWh/year. The ability of the tidal stream turbines to reduce curtailment, fuel costs and carbon emissions may provide a case for implementing them in hybrid systems, if these benefits outweigh their relatively high capital and operating expenditure’.
-Introduction looks fine, I think that the author missed a good number of papers recently published in this topic, please check.
References added include Rashid et al., Bahaj A et al, Clarke et al, Simonsen et al.
In general, an extensive literature review was completed to include the main research in this field. Whilst additional literature exists on the topic (as will always be the case), we believe that we have included all novel research related to hybrid energy systems relevant to tidal energy, totalling 43 references.
-I like to see more details of the modelling and modelling procedures.
Methods section split into 2 sections; ‘Hybrid system overview’ and ‘Hybrid system modelling’.
Figure 3 added to illustrate the different operating schedules of the hybrid systems. This is accompanied by the following text: ‘At times when renewable power generation exceeds demand and the battery has spare capacity, excess turbine power is used to charge the battery. This is illustrated in Figure 3a, where system components that are being used are shown in black, and components that are redundant are shown in grey. Arrows show the direction of power flow between components. If the battery is already fully charged during these periods, the excess turbine power is wasted through a dump load (Figure 3b). This has the same result as curtailing the turbine, in that the turbine energy is not used. When the turbine power falls below the demand, the battery is used to meet the shortfall if it is sufficiently charged (Figure 3c). In this case the tidal/wind turbine box appears dashed, to indicate that the turbines could be either generating or not in this scenario. If the battery is not sufficiently charged during these periods, the oil generators are used (Figure 3d) to meet demand’.
-As the paper is based on modelling concept, it would be advisable to compare this work with other published work.
Findings from similar work have been summarised in the Introduction:
Bryden et al [15] conducted an investigation into the use of a tidal stream turbine and flywheel to achieve firm power. This work highlighted the effectiveness of short term storage to help achieve this goal, as a result of the cyclic nature of tidal stream energy supply. Clarke et al. [16] also investigated the ability of tidal stream to provide firm power, concluding that a complimentary technology such as pumped storage or phased operation of a conventional hydro power plant is needed. Barbour et al. [17] investigated the optimal properties of a storage system to provide either demand-matching or baseload output when combined with a tidal stream turbine. Results demonstrate that the system can provide significant benefits over a tidal stream turbine operating in isolation, such as reduced curtailment in transmission constrained applications. The techno-economic viability of a hybrid system at Digby Neck in the Bay of Fundy, Canada was investigated by Manchester et al. [18]. The system used a 0.9 MW wave energy converter, a 0.5 MW tidal stream energy converter and an energy storage system. The study investigates the financial case for increasing battery storage duration, which reduces curtailment to increase revenue from energy sales, but increases the capital cost of the battery.
It is not possible to compare this tidal vs. wind hybrid system study with other work as this comparison has not been made before. This is the novelty of the work, as highlighted in the introduction.
-Finally as mentioned above, also the abstract should show clearly the contribution of this work.
This has been addressed, as demonstrated above.
Reviewer 3 Report
This article investigates the performance of hybrid electricity systems used to displace oil on the island of Alderney, located in the British Channel Islands. The model hybrid systems use either a tidal stream or wind turbine, alongside battery storage and back-up oil generators. I have the following suggestion to improve the article:
Please insert references to all the equations that are not original.
The quality of the article can be further solidified by adding recent and relevant references. I suggest adding more references to enrich the article. For example:
The author states "Most renewable energy technologies are non-dispatchable, so energy storage can be used to help meet demand during low renewable energy resource periods" I suggest strengthening the sentence by adding relevant recent studies as references like:
- (2019). Design, Economics, and Real-Time Optimization of a Solar/Natural Gas Hybrid Power Plant (Doctoral dissertation, The University of Utah).
The author further states " Combining intermittent technologies such as wind and solar pv can help reduce fluctuations in overall renewable power production, where wind power tends to be greatest at night and during winter months, whilst solar pv power is greatest during summer days. " I suggest strengthening the sentence by adding relevant recent references like:
- (2019). Dynamic simulation, control, and performance evaluation of a synergistic solar and natural gas hybrid power plant. Energy Conversion and Management, 179, 270-285.
Author Response
The authors would like to thank the reviewer for their useful feedback. Please see our responses below:
This article investigates the performance of hybrid electricity systems used to displace oil on the island of Alderney, located in the British Channel Islands. The model hybrid systems use either a tidal stream or wind turbine, alongside battery storage and back-up oil generators. I have the following suggestion to improve the article:
Please insert references to all the equations that are not original.
Reference added to Equation 2. All other equations referenced.
The quality of the article can be further solidified by adding recent and relevant references. I suggest adding more references to enrich the article.
References added include Rashid et al., Bahaj A et al, Clarke et al, Simonsen et al.
For example:
The author states "Most renewable energy technologies are non-dispatchable, so energy storage can be used to help meet demand during low renewable energy resource periods" I suggest strengthening the sentence by adding relevant recent studies as references like:
(2019). Design, Economics, and Real-Time Optimization of a Solar/Natural Gas Hybrid Power Plant (Doctoral dissertation, The University of Utah).
This statement is already referenced to Friedrich et al. (2017). The authors do not deem it necessary to add further references here, ultimately because it is not a contentious statement. The statement is widely accepted as truth, so we feel that the reviewer’s suggestion to reference a PhD thesis is not the correct approach to take. Since reviewer 1 and 2 did not raise an issue here, we assume this to be acceptable.
The author further states " Combining intermittent technologies such as wind and solar pv can help reduce fluctuations in overall renewable power production, where wind power tends to be greatest at night and during winter months, whilst solar pv power is greatest during summer days. " I suggest strengthening the sentence by adding relevant recent references like:
(2019). Dynamic simulation, control, and performance evaluation of a synergistic solar and natural gas hybrid power plant. Energy Conversion and Management, 179, 270-285.
Reference added.
