Optimized Dimensioning and Operation Automation for a Solar-Combi System for Indoor Space Heating. A Case Study for a School Building in Crete
Round 1
Reviewer 1 Report
1) Paper seems to be a "case study" that should be added in a title, because results cannot be generally used for other locations and buildings
2) Stratification thermal energy storage is commonly used in Europe, so it's hard to see element of novelty in a proposed solution
3) Comparing actual system (oil) with solar+biomass is a very specific selection of possible to apply variants, so the reason should be well described.
4) See articles about difference between actual and theoretical energy consumption in schools - conslusion that now "the building’s heating loads were 92% covered on annual basis with the diesel-oil 269 burner and the recorded annual diesel oil consumption" should be revised
5) Why only heating is going to be supplied by the system?
6) What do authors suggest to do with heat produced in summer? All disadventages of this system must be discussed.
7) What is payback time of a proposed solution?
Author Response
Please see the attached file.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper presents an approach to the control of a so-called solar-combi heating system. The technical concepts are not new and in the domain of professional activities rather than of the scientific research. The only novelty claimed by the authors is the operation algorithm though which the control of the heating system is achieved, investigated by theoretical means. To be considered for publication, the issues listed below should be addressed:
- In line 37-39, a 30% overall energy efficiency may apply to exceptional cases, in fact even inefficient gas or heat pump systems have much higher efficiency.
- The presentation of different types of solar collectors in lines 43-94 is very long and partially out of scope, so it could shortened and focused on the considered technology.
- In line 74 it is not the ‘reflectance’ but the ‘(thermal, or far infrared) emissivity’.
- In lines 93-94, the mentioned total efficiency of 85% should be explained, and hopefully not obtained by summing pears and apples, that is electric and thermal efficiencies, but rather combining these in terms of primary energy.
- In line 144, even if it is a common concept, give a definition of ‘levelized energy cost’ in the introduction.
- The use of a biomass heater is claimed, but very little is written on its characteristics, it efficiency, its inertia, the fuel (both biomass pellets and (bio?)diesel are mentioned afterwards) and the coupling with the solar system. This should by even briefly analyzed somewhere between sections 1.2 and 1.3.
In lines 183-184 give some details on hybrid cooling (what do you mean?) and substation of diesel oil heater (the existing one renewed? A new one? Is this or a pellet burner integrated with the solar system?).
In line 189, I’d write ‘relatively cool summer’.
The sentence in lines 267-269 is questionable: there are so many uncertainties that the calculated 92% coverage is not substantiated.
Symbols listed in line 320 with smaller and larger fonts of the same letter T are not easy to be distinguished. Please use different symbols here and in the following.
In section 3.4 or somewhere before it should be briefly explained how the thermal inertia of the building, which is subjected to highly intermittent loads and gains, is modelled. In this regard, considerations on intermittent operation given in Section 4 could be anticipated.
In line 458, a thermal heat power (but ‘lower calorific value’ seems a more appropriate term) of 5792 kWh/kg is given for pellets, much higher than usual values around 5-5.2 kWh/kg of commercial products. Please explain or recalculate.
Please add to Table 5 the column for the case with 3 tanks described in lines 466-470.
The English language is fair, yet a thorough check is recommended. A non-exhaustive list of specific suggestions is given below:
- In line 18, ‘It is seen that’ instead of ‘It is seen, that’.
- In lines 71-72, maybe ‘around 80%’ instead of ‘at the range of 80%’.
- In line 79, ‘raised’?
- In line 82, ‘by convection’ instead of ‘from convection’.
- In line 88, ‘in which high temperatures’ instead of ‘that high temperatures’.
- In lines 138-139, ‘with selective coating’ instead of ‘of selective coating’.
- In lines 151-152, as well as in line 166 and elsewhere when occurring, ‘the considered building’ instead of ‘the under consideration building’.
- In line 158, ‘at the appropriate temperature’ instead of ‘in the appropriate temperature’.
- In lines 171-172, ‘funded by’ instead of ‘funded from’.
- In line 182, ‘with LED technology’ instead of ‘of LED technology’.
- In line 192 and line 197, ‘measured by’ instead of ‘measured from’.
- In lines 210-211, delete ‘variation’; moreover, ‘measured by’ instead of ‘measured from’.
- In line 211, ‘mast’?
- In line 245, check ‘& occupant’.
- In line 449, ‘with selective’ instead of ‘of selective’.
Author Response
Please see the attached file.
Author Response File: Author Response.docx
Reviewer 3 Report
The paper contains an interesting case study, however, I recommend major revisions for the following reasons:
-The grammar is very poor. Even the title itself contains three major grammatical errors: 1- Optimization Dimensioning, should this read "Optimized Dimensioning"?, 2- Automization, should be "Automation", and 3- "Spaces Heating", should be "Space Heating". This last error was found throughout the paper. I highly recommend getting a native English speaker to carefully proofread this paper.
-The topic doesn't appear to be especially novel as solar space heating is a concept that has been around for a very long time. If you rely on solar, then you need a backup fuel source, making every solar heating system a "Solar Combi". The fact that it's biofuel is almost inconsequential as biofuel would have the same basic characteristics as any other fuel. Please focus on what might make this novel. Are there any synergistic benefits in using the two fuels together?
-The paper contains almost no mathematical description of the model. The simulation is meant to be one of the major contributions of this work, but readers are given almost no detail as to the math contained in the simulation model.
-The authors allude to "Optimization Dimensioning". To me, I was expecting some sort of design optimization contribution and I really didn't see this covered in any detail.
-For a place like Crete, how much sense would solar space heating really make? During the summer, how much of that heat is wasted because there is no need for space heating in the summer in Crete?
-The author's compute an LCOE of $0.15/kWth, which seems very high to me. Can the author's compare this to an LCOE of a conventional, gas-fired heating system. I'm guessing this would be much lower, on the order of $0.02-0.04/kWhth.
-Neglecting heating loads when the building is not in operation doesn't make much sense. Wouldn't there still be some minimum set point that the building needs to achieve? This may be lower than when the building is in operation, but you can't just neglect the whole system entirely during these periods.
My assessment is that this paper is very close to the "reject" threshold, but I will give it one more chance, only if the authors are able to successfully respond to some of these major concerns.
Author Response
Please see the attached file.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
My previous remarks were considered and a manusctipt was improved.
Author Response
Please see my answers to your comments in the attached file.
Author Response File: Author Response.docx
Reviewer 2 Report
The authors replied to most reviewers' remarks. While still being a case study with not much of scientific innovation, it now presents results of some interest and given in a generally correct and coherent way.
Just consider that:
- Combined steam-gas turbine electric generation systems with relatively small size and adjustable energy output with efficiency in excess of 60% are commercially available, and spreading, and plants with efficiency lower than 40% are progressively shut down all over Europe, so long term economic analyses should take care of a probable, progressive retrofit and improvement of the energy system. For the time being, some official data on the average efficiency of the local energy system is probably available, and should be used.
- The term 'specific heat capacity' used for pellets may be incorrect since it is generally referred to heat transfer resulting in temperature changes rather than that heat coming from combustion processes, the term 'lowest calorific value' is indeed the most used one.
Author Response
Please see my answers to your comments in the attached file.
Author Response File: Author Response.docx
Reviewer 3 Report
I believe the authors have significantly improved the article and have at least responded to my concerns to the best of their ability. I'm okay with publishing the article.
Author Response
Please see my answers to your comments in the attached file.
Author Response File: Author Response.docx
