Round  1

Reviewer 1 Report

Very similar results have been obtained and presented in the following papers, which should be mentioned in the references:

Two-Phase liquid cooling for electric vehicle IGBT power modulethermal management

PCIM Europe 2017, 16 – 18 May 2017, Nuremberg, Germany

                            Comparison of the Heat Transfer Capabilities of Conventional Single- and Two-Phase Cooling Systems for an Electric Vehicle IGBT Power Module          

          Itxaso Aranzabal   ;         Iñigo Martínez de Alegría   ;         Nicola Delmonte   ;         Paolo Cova   ;         Iñigo Kortabarria  

IEEE Transactions on Power Electronics

2019, Page(s):4185 - 4194

Author Response

We sincerely appreciate that you had been assigned your valuable time to review our manuscript.

Point 1: Very similar results have been obtained and presented in the following papers, which should be mentioned in the references:


Two-Phase liquid cooling for electric vehicle IGBT power module thermal management

PCIM Europe 2017, 16 – 18 May 2017, Nuremberg, Germany

Comparison of the Heat Transfer Capabilities of Conventional Single- and Two-Phase Cooling Systems for an Electric Vehicle IGBT Power Module          

Itxaso Aranzabal; Iñigo Martínez de Alegría;  Nicola Delmonte; Paolo Cova; Iñigo Kortabarria  

IEEE Transactions on Power Electronics, 2019, Page(s):4185 - 4194

Response 1:

We have added those papers in our manuscript.

2.2. Expected benefits

…If inverters and converter are integrated into the refrigerant cooling system referring to previous researches [7,8], rRefrigerant cooling should reduce these four heat exchangers into two: a radiator for the engine cooling and a condenser for the refrigerant cooling as shown in Figure 2(b).

References

Aranzabal, I.; Martinez de Alegria I.; et al. Two-phase liquid cooling for electric vehicle IGBT power module thermal management, 2017 11th IEEE International Conference on Compatibility, Power Electronics and Power Engineering (CPE-POWERENG), 2017, No. 16896418.

Aranzabal, I.; Martinez de Alegria I.; et al. Comparison of the Heat Transfer Capabilities of Conventional Single- and Two-Phase Cooling Systems for an Electric Vehicle IGBT Power Module, IEEE Trans. Power Electron. 2019, 34(5), 4185 – 4194.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper presented a new idea to cool the motor by combining the AC system in the EV. The concept is very interesting. However, the paper should consider and address the following suggestions before publication. 

1) The cooling performance by water jacket was selected as a baseline. In order to compare, I think the inlet temperature of water and refrigerant should be at same level. In this paper, the temperature of the refrigerant is significant lower than the water. The author was recommended to explain the reasons.

2) Fig. 1 showed that the refrigerant came from the receiver. For a typical AC system in the EV, the temperature and pressure of the refrigerant here is of low temperature and high pressure, but not low as the author expected. If the sub-cool was 5 ℃, the temperature and the pressure of the liquid refrigerant (R134a) could be, say 1MPa and 40 ℃, respectively. So the testing conditions should consider this.

3) Following the comments 2, as for the refrigerant used to cool the jacket, the pressure did not change too much if we neglect the pressure drop in the jacket. (It always reasonable, because the pressure drop is very small when compared with the pressure of the liquid refrigerant). That will cause a big problem to the compressor

Author Response

We sincerely appreciate that you had been assigned your valuable time to review our manuscript.

Point 1: The cooling performance by water jacket was selected as a baseline. In order to compare, I think the inlet temperature of water and refrigerant should be at same level. In this paper, the temperature of the refrigerant is significant lower than the water. The author was recommended to explain the reasons. 


Response 1: To help understanding, the inlet temperature of water and refrigerant at initial condition and during motor operation is listed in Table A. The inlet temperature of water depends on water temperature which is controlled by circulation system in this experiment. On the other hand, the inlet temperature of refrigerant depends on room temperature at initial condition. Once motor operation starts, the inlet temperature of refrigerant depends on the target temperature of 0, 10, 20°C which is controlled by compressor operation.

Table A Inlet temperature of water and refrigerant

As shown in Figure 6, if the inlet temperature of water and refrigerant is at same level of 20°C, the coil temperature after the 30 minutes rated operation become same level as well.

The reason why the temperature of the refrigerant is significant lower than the water in Figure 5 is to show the maximum difference of cooling performance between water cooling and refrigerant cooling.

We have revised the explanation of Figure 5 as follows:

 4.2.1. Experiment 1

Figure 5 shows that refrigerant cooling kept the coil temperature lower than water cooling. Comparing the highest temperature of water cooling of 60°C and the lowest temperature of refrigerant cooling of 0°C, the difference of the coil temperatures extended up to 53°C after 30-minute motor operation at rated torque. The water cooling of 60°C is the highest water temperature The upper limit of water temperature was determined from the highest temperature allowed for water cooling in the specification. The refrigerant cooling of 0°C is At the same time, compressor ability determines the lowest refrigerant temperature of refrigerant cooling, which is determined by compressor ability. When the water temperature was set at 20°C to adjust the initial coil temperature, refrigerant cooling even showed the higher cooling performance by 12°C than water cooling.

Point 2: Fig. 1 showed that the refrigerant came from the receiver. For a typical AC system in the EV, the temperature and pressure of the refrigerant here is of low temperature and high pressure, but not low as the author expected. If the sub-cool was 5 ℃, the temperature and the pressure of the liquid refrigerant (R134a) could be, say 1MPa and 40 ℃, respectively. So the testing conditions should consider this.

 Response 2: A thermal expansion valve (TXV) is attached at an inlet of the motor housing to decrease refrigerant temperature. To clarify TXV attachment, we have added TXVs in Figure 2(b)   

 Please see the word file

Point 3: Following the comments 2, as for the refrigerant used to cool the jacket, the pressure did not change too much if we neglect the pressure drop in the jacket. (It always reasonable, because the pressure drop is very small when compared with the pressure of the liquid refrigerant). That will cause a big problem to the compressor

 Response 3: In this simulation and experiments, refrigerant cooling is compared with water cooling under the assumption that pressure is reduced sufficiently at the TXV installed at the inlet of the motor housing and liquid refrigerant is completely vaporized at the motor outlet.

In practice, a mechanical or electronic expansion valve controls the flow rate so that the liquid refrigerant does not flow into the compressor.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper compares utilizing refrigerant (from the vehicle AC system) with conventional water cooling for cooling an electric motor.  The authors showed significant benefit from using the refrigerant cooling compared to water cooling, which was primarily the result of the lower temperature of the coolant.  It does also appear that some benefit is being realized due to phase transition, as apparent in figure 5 but not discussed.

The biggest concern this reviewer has about utilizing refrigerant cooling for an electric motor is the parasitic power draw due to the compressor operation.  The authors did not address this aspect at all.  Vehicle AC systems draw a very large amount of power, and while there’s no doubt the performance would increase dramatically, it would likely be at the cost of significantly decreased vehicle range.  It would be good to see this aspect at least acknowledged because parasitic power is one reason refrigerant cooling is not more widely utilized.

Another concern regarding the use of refrigerant is the amount that would be required to cool the entire system.  How much more refrigerant is required to cool the entire vehicle’s systems compared to a typical AC system?  Have the authors considered the environmental impacts of using such a large amount of refrigerant with regards to disposal or unintended release (i.e. vehicle collision)?

In section 2.2 The authors mention both PHEVs and EVs and it is unclear what is being compared.  It almost looks like a PHEV is being compared to an EV (if so, the comparison is not valid in the context), but there is no internal combustion engine shown in Figure 2a.  If 2a is actually an EV, why the complicated cooling scheme?  Why does 2a require oil and 2b does not?  This section is at best confusing, and at worst misleading.  Figure 2 also appears to propose using refrigerant to cool the inverter and converter as well, yet it is not mentioned anywhere else in the paper.

Table 1: Many of the entries in the table appear as symbols such as triangles or circles.  Please replace these symbols with text.

While this reviewer did not have issues following the language in the paper, it is recommended to have an edit by a native English speaker as there are multiple typos and grammar issues.

Author Response

We sincerely appreciate that you had been assigned your valuable time to review our manuscript.

Point 1: The paper compares utilizing refrigerant (from the vehicle AC system) with conventional water cooling for cooling an electric motor.  The authors showed significant benefit from using the refrigerant cooling compared to water cooling, which was primarily the result of the lower temperature of the coolant.  It does also appear that some benefit is being realized due to phase transition, as apparent in figure 5 but not discussed. 


 Response 1: The benefit of phase transition is to decrease refrigerant temperature and to keep it cold. That is, the high temperature liquid refrigerant is partially vaporized at the TXV by the pressure reduction to cool itself. Further, since the refrigerant temperature is kept constant during phase transition, it is considered to be suitable to cool parts uniformly. We have revised manuscript to refer to the benefit of phase transition as follows:

4.2.1. Experiment 1

…The coil temperature was proportional to the temperature of fluid through the water jacket as shown in Figure 6., which This result suggests shows the higher cooling performance of that the refrigerant cooling. performance should be superior to the water cooling performance due to the compressor control, which sets the temperature lower than ambient temperature for refrigerant cooling. The compressor achieved this result by keeping the refrigerant temperature below the ambient temperature for its phase transition causing endothermic reaction through the TXV.

Point 2: The biggest concern this reviewer has about utilizing refrigerant cooling for an electric motor is the parasitic power draw due to the compressor operation.  The authors did not address this aspect at all.  Vehicle AC systems draw a very large amount of power, and while there’s no doubt the performance would increase dramatically, it would likely be at the cost of significantly decreased vehicle range.  It would be good to see this aspect at least acknowledged because parasitic power is one reason refrigerant cooling is not more widely utilized.

 Response 2: As you pointed out, refrigerant cooling has no merit if you simply compare the power consumption of an electric pump for water cooling and a compressor for refrigerant cooling. However, we think it is possible to reduce the power consumption of compressor by optimally controlling its operation rate.

We refer to the point in manuscript as follows:

2.2. Expected benefits

…The essentials to realize this ideal system are: the greater cooling ability of the compressor; the distribution control of refrigerant; and the compressor control to minimize the power consumption regulating its operation rate to minimize the deterioration of the electric mileage.

Point 3: Another concern regarding the use of refrigerant is the amount that would be required to cool the entire system.  How much more refrigerant is required to cool the entire vehicle’s systems compared to a typical AC system?  Have the authors considered the environmental impacts of using such a large amount of refrigerant with regards to disposal or unintended release (i.e. vehicle collision)?

 Response 3:

In the case that one motor is added to the A / C system as shown in Figure. 1, the required amount of refrigerant becomes 1.3 times.

To estimate the required amount, the following conditions are assumed:

-coil loss is about 3 kW

-25% of refrigerant is distributed to the motor at flow control valve

However, instead of increasing the amount of refrigerant, it is possible to improve the cooling performance by increasing the size of condenser or the displacement of compressor.

Point 4: In section 2.2 The authors mention both PHEVs and EVs and it is unclear what is being compared.  It almost looks like a PHEV is being compared to an EV (if so, the comparison is not valid in the context), but there is no internal combustion engine shown in Figure 2a.  If 2a is actually an EV, why the complicated cooling scheme?  Why does 2a require oil and 2b does not?  This section is at best confusing, and at worst misleading.  Figure 2 also appears to propose using refrigerant to cool the inverter and converter as well, yet it is not mentioned anywhere else in the paper.

 Response 4:

We compare Outlander PHEV cooling system with an ideal cooling system of PHEV using refrigerant cooling. Internal combustion engine is NOT illustrated in Figure 2.  “EV cooling” meant the cooling part of the EV system of PHEV.

Outlander PHEV has two cooling systems: oil cooling for the front motor and the generator and water cooling for the rear motor and invertors. In the ideal refrigerant cooling system, oil cooling as well as water cooling is replaced by refrigerant cooling.

To clarify Figure2 intention, we have revised manuscript as follows:

2.2. Expected benefits

Following tThree expected benefits from replacing water cooling system by refrigerant cooling are: benefits are expected from refrigerant cooling:

•    Fewer components for PHEV

•    Greater cooling uniformity

•    Higher cooling performance

First of all, integration of EV cooling systems with an the A/C system should reduce components. Figure 2(a) shows the current cooling system of Outlander PHEV. In front of a vehicle, Ffour heat exchangers are located in the front of a vehicle: two radiators (one for an engine not illustrated in Figure 2 and another for waterEV cooling of EV system); one oil cooler for an EV oil cooling of the front motor and the generator; and one condenser for the A/C. If inverters and converter are integrated into the refrigerant cooling system referring to previous researches [7,8], rRefrigerant cooling should reduce these four heat exchangers into two: a radiator for the engine cooling and a condenser for the refrigerant cooling as shown in Figure 2(b).

 Point 5: Table 1: Many of the entries in the table appear as symbols such as triangles or circles.  Please replace these symbols with text.

 Response 5: We have replaced all symbols with text as follows:

(We have deleted “cost” row since refrigerant cooling cost is strongly dependent on system structure and the system has not set yet.)

  Please see the word file   

  Please see the word file

Point 6: While this reviewer did not have issues following the language in the paper, it is recommended to have an edit by a native English speaker as there are multiple typos and grammar issues.

 Response 6: We have had our English transition team check the manuscript.

Author Response File: Author Response.docx