Design of a 5th Generation District Heating Substation Prototype for a Real Case Study
Round 1
Reviewer 1 Report
First, there is a major issue because the literature is not referenced and, when an article is cited in the paper, a question mark appears instead of a number. So, it was difficult to check the literature review made in the present work.
This paper deals with the use of a reversible heat pump in a substation of a 5th generation district heating network. It is an interesting subject and the real case study to be implemented is presented. However there is a lack of scientific analysis in this paper and I suggest that this work needs to be improved before being resubmitted.
Here are my comments and suggestions for improvement.
1. Perhaps the title (with the word “Design”) is misleading because no calculation is made to design and size effectively the heat pump of the substation. It is more a configuration that is presented in the paper rather than a design with motivated choices of configuration and technology.
2. Line 59: what do you mean with "neutral networks" ?
3. Lines 62-63: you wrote “Natural sources can be low temperature geothermal energy or solar thermal collectors…” Don’t thermal collectors provide heat at a temperature level higher than low temperature of a 5GDHN ?
4. Lines 113-119: I think there is mistakes in the comment on the article of Gudmundsson et al. Indeed, you wrote “A particularly critical aspect of the 5GDHNs is the complexity of the substations at the users, which must include the presence of a reversible heat pump and a decentralized pumping system necessary to enable the dual configuration of heating and cooling the building.” I think that the heat pump does not always have to be reversible. It depends on the consumer. A consumer which needs heat produces cold and a consumer which needs cold produces heat. So they are prosumers. This is inherent to the operation of a heat pump. Sometimes, depending of the season for example, it happens that the need of the consumer changes: in this case, but only in this case, the heat pump needs to be reversible. For example a store with cold rooms will always need cold and the heat pump does not need to be reversible. See, for example, the block diagram of the network in the following reference: Applied Energy 291 (2021) 116672. You wrote “…which shows that the economic advantage of a simpler piping network in the case of a 5GDHN cannot counterbalance the high investment and operating costs of the heat pump” First, I don’t understand why you wrote “a simpler piping network in the case of a 5GDHN”. Moreover Gudmundsson shows that the 4GDHN centralized heat generation is more advantageous than 5GDHN end-users heat generation. So I think you should write “…in the case of a 5GDHN cannot counterbalance the high investment and operating costs of all the heat pumps of the end-users which are prosumers”.
5. In relation to my previous comment: “… the bidirectional pumping system, necessary to enable the prosumer role of the network customer…” (lines 532-533). I am not agree with this statement. A 5th generation district and cooling network relies on a great number of consumers capable of presuming heat and cold. A customer may needs only heat (with a heat pump in heating mode) or only cold (with a heat pump in cooling mode) or alternatively heat and cold (with reversible heat pump). My feeling is that the prototype is a reversible system to be able to test the two configurations (cooling or heating)…Is that true or do you really deal with a consumer needing sometimes heat at 65°C and sometimes cold à 6°C ?
6. Line 137: which kind of boilers (gas, biomass…)?
7. Line 146: “In addition, to decrease the utilization of fossil fuels …” So the boilers use fossil fuels?
8. Line 170: “costumers” should be consumers
9. Lines 199-201: How is the preheating temperature (40°C) selected?
10. Figure 2 is unclear.
11. Figure 3: you should put the prosumer behind the thermo-hydraulic circuit. Perhaps figure 6 could be gathered with figure 3 and figure 3 could then have two cases (heating configuration and cooling configuration). (Moreover, the heat exchanger was not shown on figure 6). In the same manner, I think paragraph 3.1 should be put at the end of paragraph 2.2.
12. Page 8: you should present the equations you used. You explained the choice of Tcond,out and Tevap,out. For sake of clarity you could give the average value of the July evaporative tower circuit temperature and the the average value of the January evaporative tower circuit temperature. How did you choose the value 2.5°C of the temperature difference in the evaporator (heating configuration) and 3.5°C in the condenser (cooling configuration)? Aren’t they small? What is the value of the mass flow in the two cases? What is the value of the power exchanged? What is the power of the heat pump?
13. Which equation of state is used in the CoolProp C++ library ?
14. Line 314: in paper dealing with such district networks mathematical optimization is often used. So the term “optimized” should be replaced by another expression because I think you don’t perform any optimization.
15. Figure 9 is not useful and is redundant with table 2.
16. Figure 7: it could be more convenient if temperatures were given in °C (as in the text).
17. As far as refrigerants are concerned, it would be interesting to discuss about regulation of these components. Hence, R134a R410a and R407c should perhaps be banned soon. The choice of R134a in a new generation of district heating could appear to be “anachronistic”. Why do you choice this fluid?
18. Table 2 could also present powers.
19. The readability of Figure 10 could be improved. How do you explain that the COP exhibits an optimum value (not reached yet for some refrigerants)? Shouldn’t the choice of preheat temperature be linked to this curve?
20. When you compare the substation with stand-alone heat pump solution do you perform a pre-heating in the two cases?
21. Line 547: the authors thank…(not tank !)
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The article is devoted to the investigation of heating networks of the 5th generation (5GDHN). The study aims for designing a substation prototype for a real case study in the city of Brescia, Italy. The prototype will be built and monitored to test it in operation mode to scale it up further on. The heat source for the 5GDHN will be evaporative towers. The first prototype aims for covering heat and cooling demands in their own buildings. Therefore, the evaporative towers were analysed for the year 2020. For running the heat pumps (HP) in the 5GDHN different refrigerants are being compared regarding differences in efficiencies depending on different evaporating temperatures in HP and GWP. Additionally, a comparison between air-to-water HP and the suggested water-to-water HP is laid out for COP and EER. For operation mode, a monitoring design is set up and explained. A brief and well-structured introduction into the development of district heating networks is given and most of the relevant literature is discussed. The structure of the paper is clear.
Q1. Please emphasise more strongly the unique selling points and the novelty of your work in a highly visible place of your work. That could contain a part of what can be provided to the local utility company and how do they benefit from the results or something else.
Q2. Since sustainability has a lot of dimensions, which one are you referring to in Line 142?
Q3. The selection of the refrigerant doesn’t seem so obvious to me, since the efficiencies aren’t very far apart, but R134a has a far higher GWP than R1234ze. Maybe you can discuss why GWP hasn’t such a big impact?!
Q4. Is it possible to generate equations for a fitted curve for the functions shown in Fig. 10? I believe that could generate a benefit for interested readers, since I understand them as universally valid?.
Q5. A detailed outlook in the conclusion section containing a sum up of all the things that has been done and which hasn’t i.e. looking into seasonal COPs, electricity for circulation pumps etc. would round your work up. Also, some of the implementation details, i.e. time schedule and so on could be interesting.
Found typos: Line 21 “The 75%”, Fig. 3 “Sistem”, Line 304, Line 547 “tank”
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments
1. Explain more comprehensively about what this research adds to the subject area compared with other published literature.
2. The references are not mentioned at the end of the paper. Also, there is no referencing in the manuscript.
3. In the literature review, a table can be added to compare different generations of the district heating.
4. Figure 14 and Figure 15 are presented after conclusion. Transfer them to the previous section.
5. Discuss the results of the paper by a sentence in the abstract quantitatively.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I thank the authors for their point by point responses to my comments and those of the other reviewers.
The article has been improved and is now suitable for publication.
Reviewer 3 Report
The paper is acceptable in the present from.
