Increasing the Energy Efficiency of an Internal Combustion Engine for Ship Propulsion with Bottom ORCs
Round 1
Reviewer 1 Report
- The authors should describe in detail the heat sources and heat sink available. They say that IEC waste rejected to the environment is in the form of exhaust gases (about 3.1 MW available at 400°C) and cooling circuits (2.4 MW in LT and HT circuits). In order to best follow the analysis, the exhaust gases flow rate and HT and LT fluid flow rates are required as well as temperatures of HT and LT circuits. Furthermore, the heat sink available should be presented.
- The selection of the working fluids should be justified or a more extensive working fluid screening must be carried out.
- The configuration of the waste heat valorization system should be presented and justified. Why not try to use all the heat sources in an unique system? Which is the minimum recommended exhaust gases temperature for the waste heat recovery?
- The authors are fixing too much items in the exhaust gases waste heat recovery. Why? Which are the basis of these assumptions?
- For HT circuit, the authors use isobutene as working fluid for an ORC cycle. They should justify the working fluid selection. Why the authors do not make use of LT circuit jointly with HT?
- The optimal cycle configuration and the working fluid selection should be carried out jointly.
Author Response
Reviewer #1: The authors should describe in detail the heat sources and heat sink available. They say that IEC waste rejected to the environment is in the form of exhaust gases (about 3.1 MW available at 400°C) and cooling circuits (2.4 MW in LT and HT circuits). In order to best follow the analysis, the exhaust gases flow rate and HT and LT fluid flow rates are required as well as temperatures of HT and LT circuits. Furthermore, the heat sink available should be presented.
Additional information about the heat source and the heat sink has been added in the text, better specifying temperatures and mass flow rates.
The selection of the working fluids should be justified or a more extensive working fluid screening must be carried out.
The selection of the working fluids has been better commented, introducing also some additional references to justify the limited set of fluids considered.
The configuration of the waste heat valorization system should be presented and justified. Why not try to use all the heat sources in a unique system? Which is the minimum recommended exhaust gases temperature for the waste heat recovery?
The coupling of the different ORC configurations to the ICE has been more explicitly described in the text. In the additional information about the heat source, the considered exhaust gases temperature, equal to 400°C, has been reported, too. In the literature, various examples of Two-pressure-levels ORC can be found, and the adoption of two different fluids for the two different heat source is often recommended. See, for instance, Reference [34].
The authors are fixing too much items in the exhaust gases waste heat recovery. Why? Which are the basis of these assumptions?
In the comparison of the four working fluids considered for the exhaust gases energy recovery system, the conditions of the thermal oil have been fixed, jointly with the maximum and minimum pressures regarded as acceptable for the cycle, in order of obtaining a homogeneous comparison of the four fluids. In the detailed design of the heat exchangers (Chapter 5), as well as in the off-design analysis of the ORC regenerated cycle (Chapter 6), these limitations have been relaxed.
For HT circuit, the authors use isobutene as working fluid for an ORC cycle. They should justify the working fluid selection. Why the authors do not make use of LT circuit jointly with HT?
Isobutene shows good performance in comparison with other hydrocarbons and refrigerants at maximum cycle temperature of about 90°C, see Reference [44], as has been highlighted in the text. The LT circuit in Figure 2 is at a temperature of about 50°C, which has been regarded as too low for feeding a thermodynamic cycle with interesting efficiency.
The optimal cycle configuration and the working fluid selection should be carried out jointly.
In principle, this is true. Nevertheless, a different approach has been chosen for the paper, the aim of which is not a wide review of possible working fluids, but a realistic evaluation of the energy efficiency increase of a ship propulsion engine, through the adoption of an ORC-based energy recovery system.
Wider reviews of possible working fluids can be found in the literature, and some of them are shown in the References. It is worth noting that, for the typical hot source temperature of this application, the fluids considered (in particular toluene and MDM) always obtain efficiencies close to the maximum achievable value, with small gaps with respect to the best performance, if any. Obviously, different fluids are better suited for lower hot source temperature, like the isobutene introduced in the Two-pressure-levels recovery system of this paper.
Reviewer 2 Report
Dear Authors,
The article concerns a very current problem related to both environmental pollution and the efficiency of modern ships' propulsion systems. The authors proposed a very interesting approach to increasing efficiency by using heat, which in the classic solution is dissipated into sea water and air. The article presents the test results for various configurations of the ORC cycle and the corresponding resultant simulations. Additionally, economic analyses were carried out, which indicate that the proposed solutions are currently profitable.
After making the suggested modifications, the manuscript is recommended for publication
Suggested modifications:
30 It is ; “The today's cost survey of fossil fuels is constantly evolving”
Probably should be : “The today's cost survey of fossil fuels is constantly evolving”
33 A reference to the MARPOL convention, especially to the Annex VI will be desirable.
39 LNG-Liquefied Natural Gas
76 Reference “[4] Wärtsilä Italy, personal communication, 2015”. It cannot be recognized as a serious source of data. Wärtsilä Italy should give permission and authorize for publication of such data.
95 On the Figure 2 there are used many symbols without any description. On the other side. there are used some abbreviations without explanation in the text: WHR, IM, RIC, RIT, REINT, UM, HT, LT It will be properly describe air circuit, sea water circuit, HT CIRCUIT as HT FRESH WATER CIRCUIT and similar LT FRESH WATER CIRCUIT (Fresh water is demineralised water)
97 Should be : Figure 2
98-107 It is difficult understand what kind of diagram was described. It concern cooling system of Wärtsilä 6L50DF engine or any other.
137 Reference “[4] Wärtsilä Italy, personal communication, 2015”. It cannot be recognised as a serious source of data. Wärtsilä Italy should give permission and authorize for publication of such data.
138 temperature of hot surface – temperature of what or where? Exhaust gases ?
139 • mass flow rate of hot source - What is the “hot surface”
140 • evaporating temperature: 250° C- of what or where ?
141 • maximum pressure: 15 bar – of what or where?;
142 • condensing pressure: 0.3 bar – of what or where
145 numbers 1,2,3,4 on the Figure 3 are not described in the text.
180-186 probably refers to the Figure 3. Similarly as 138-142, description of the parameters are ambiguous.
189 3.2. Functional parameters of the ORC at ICE part load- as a whole chapter
How have been done simulation for different loads. Following text in the chapter 3, the simulations were done according ORC circuit presented on the Figure 2. So where is connection with ICE –please clarify this aspect.
215- Illegible text
218 Illegible text
273 Fig. 9 should be Figure 9
Author Response
Reviewer #2: Dear Authors,
The article concerns a very current problem related to both environmental pollution and the efficiency of modern ships' propulsion systems. The authors proposed a very interesting approach to increasing efficiency by using heat, which in the classic solution is dissipated into sea water and air. The article presents the test results for various configurations of the ORC cycle and the corresponding resultant simulations. Additionally, economic analyses were carried out, which indicate that the proposed solutions are currently profitable.
After making the suggested modifications, the manuscript is recommended for publication
Suggested modifications:
30 It is ; “The today's cost survey of fossil fuels is constantly evolving”
Probably should be : “The today's cost survey of fossil fuels is constantly evolving”
Done
33 A reference to the MARPOL convention, especially to the Annex VI will be desirable.
Done
39 LNG-Liquefied Natural Gas
Done
76 Reference “[4] Wärtsilä Italy, personal communication, 2015”. It cannot be recognized as a serious source of data. Wärtsilä Italy should give permission and authorize for publication of such data.
The previous document “Wärtsilä Italy, personal communication, 2015” has been replaced with another Wärtsilä’s document titled "Wärtsilä 50DF-Product Guide-2019", available on line.
95 On the Figure 2 there are used many symbols without any description. On the other side. there are used some abbreviations without explanation in the text: WHR, IM, RIC, RIT, REINT, UM, HT, LT. It will be properly describe air circuit, sea water circuit, HT CIRCUIT as HT FRESH WATER CIRCUIT and similar LT FRESH WATER CIRCUIT (Fresh water is demineralised water)
Done
97 Should be: Figure 2
Done
98-107 It is difficult understand what kind of diagram was described. It concerns cooling system of Wärtsilä 6L50DF engine or any other.
It has been specified that Figure 2 concerns the cooling system of Wärtsilä 6L50DF engine.
137 Reference “[4] Wärtsilä Italy, personal communication, 2015”. It cannot be recognised as a serious source of data. Wärtsilä Italy should give permission and authorize for publication of such data.
The previous document “Wärtsilä Italy, personal communication, 2015” has been replaced with another Wärtsilä’s document titled "Wärtsilä 50DF-Product Guide-2019", available on line.
138 • temperature of hot surface – temperature of what or where? Exhaust gases?
139 • mass flow rate of hot source - What is the “hot surface”
140 • evaporating temperature: 250° C- of what or where?
141 • maximum pressure: 15 bar – of what or where?;
142 • condensing pressure: 0.3 bar – of what or where
The paragraph has been modified, better specifying the thermodynamic parameters considered.
145 numbers 1,2,3,4 on the Figure 3 are not described in the text.
Done
180-186 probably refers to the Figure 3. Similarly, as 138-142, description of the parameters is ambiguous.
Done
189 3.2. Functional parameters of the ORC at ICE part load- as a whole chapter. How have been done simulation for different loads.
We have better specified the methods of analysis at different partial loads in paragraph 3.2.
Following text in the chapter 3, the simulations were done according ORC circuit presented on the Figure 2. So where is connection with ICE –please clarify this aspect.
The link between ORC and ICE has been clarified in various point in the text.
215- Illegible text
Done
218 Illegible text
Done
273 Fig. 9 should be Figure 9
Done
Reviewer 3 Report
The keywords are sufficient.
The introduction to the issue is properly written.
The paper is well arranged into sections.
The cited references are sufficient.
I recommend the paper for publication in Applied sciences. I have no comments and remarks.
Author Response
Reviewer #3: The keywords are sufficient. The introduction to the issue is properly written. The paper is well arranged into sections. The cited references are sufficient. I recommend the paper for publication in Applied sciences. I have no comments and remarks.
Thank you for your consideration.
Round 2
Reviewer 1 Report
I recommend that in optimization studies you try to minimize the limitations (fluids, operating conditions ...) and, if you have to impose them, justify them adequately.
You are ruling out a heat source with a higher temperature than condensation. Although it may not economically improve the system, an adequate justification is expected, and more in an optimization work.
Author Response
Reviewer 1 – 2° Round
I recommend that in optimization studies you try to minimize the limitations (fluids, operating conditions ...) and, if you have to impose them, justify them adequately
The choices concerning working fluids and the limitations of the simulation parameters have been better justified in the text, introducing some remarks previously shown in the rebuttal only.
You are ruling out a heat source with a higher temperature than condensation. Although it may not economically improve the system, an adequate justification is expected, and more in an optimization work.
The following justification has been introduced in the text: The regulation system of the engine cooling guarantees, for all engine loads, a constant temperature in the high temperature fresh water circuit, while the temperature of the low temperature fresh water circuit does not exceed 50°C, but it strong depends on the engine load and on the see water temperature. Therefore, an energy recovery from the LT circuit has to be regarded as not feasible in practice.
