BMW and LG Chem Trump Tesla in Battery Thermal Management

2 years ago by George Bower 42

LG Chem's Holland, Michigan Facility

LG Chem’s Holland, Michigan Facility

Some BMW i3s Head Out For Delivery

Some BMW i3s Head Out For Delivery

We saw in a previous article that both Tesla and GM use glycol liquid cooling in their batteries. We need more cooling as we go to higher charging rates and these liquid cooled systems have a serious drawback: low heat transfer rates because of the poor heat transfer characteristics of liquid glycol.

What if there was a way to fix this problem and at the same time simplify and potentially reduce the cost of the whole vehicle’s thermal management system?

Direct Expansion – DX – is the trick and it uses the same refrigerant as used in your vehicle’s air conditioning system to cool the battery directly.  The liquid glycol is eliminated in the battery pack. DX offers cooling rates 3-4 times higher than liquid glycol.

A sketch of a direct expansion battery cooling concept is shown below:

bmw lg slide 6Tesla superchargers are charging at 120 kw. Audi has pledged 150 kw. SAE J1772 specs allow for charging power to 240 kw. These higher charging powers demand more and more cooling from the vehicle’s TMS.

With 150 kW charging rates, the cooling system will have to dissipate approximately 10 kW of heat to protect the battery pack cells, plus a high heat load on the charger power electronics – approximately 7 kW.  DX would be a big winner.

DX allows higher charging power, reduced complexity, potentially reducing costs and has the addedsafety advantage of getting liquids entirely out of the battery pack.

We saw from a previous article that Tesla has 4 different cooling loops.

bmw lg slide 7

The Chevy Volt has 5 different cooling loops. Radiators and coolant tanks everywhere! This new system would simplify things. We could drop the number of loops to only 3 in the Tesla.

bmw lg slide 8

Enter BMW i3

As it turns out, BMW already has such a system in the i3. Per the words of Munro and Associates at 2 minutes into this You Tube video: “This is the easiest heating and cooling system I’ve taken apart in 30 years.”

Here is a freeze frame of the refrigeration cooling tubes being installed in the bottom of the aluminum battery tray at 28 seconds into this BMW Group video. The batteries are then placed on top of the cooling tubes.

Courtesy BMW Group

Courtesy BMW Group

The following shot is from the Munro and Associates You Tube video at 2 minutes.

Courtesy Munro and Associates

Courtesy Munro and Associates

Here are the batteries in their aluminum cases that sit on the refrigeration tubes:

Courtesy Munro and Associates

Courtesy Munro and Associates

Here is a nice shot of the refrigerant connections and tubes.

Courtesy Munro and Associates

Courtesy Munro and Associates

Enter LG

LG Chem also has a prototype of such a system and a patent on a system that incorporates the DX cold plate concept.

LG will be in charge of many of the systems on the new Chevy Bolt.

bmw lg slide 13

One could argue that the Chevy Bolt is really made by LG chem. LG also makes refrigeration systems so a blending of the 2 technologies is simple for them.

The following 2 figures are from the LG patent showing the direct expansion cold plate and the thin aluminum plates that reside between the prismatic cells.

bmw lg slide 14


bmw lg slide 15Simpler, better heat transfer, and lower cost. What’s not to like?

Will we see this DX cold plate cooling scheme in the new Chevy Bolt and the Tesla Model 3?  We predict yes.

About the authors: George Bower is a mechanical engineer and a Prius and Volt owner.

Keith Ritter is a mechanical engineer and a licensed professional engineer with over 35 years experience in heating ventilating and air conditioning systems

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42 responses to "BMW and LG Chem Trump Tesla in Battery Thermal Management"

  1. David Murray says:

    The only thing not to like is the fact that anything with refrigerant in it is prone to leaking due to the extreme pressure. Much easier to prevent leaks using a low-pressure liquid coolant.

    1. Anon says:

      And glycol can be used to both cool AND heat batteries on the same loop, over time. Refrigerant implies needing an additional heat system, which adds complexity and cost.

      1. Mart says:

        Tie the refrigerant to a heat pump, and it can both heat and cool.

      2. Rob says:

        Heat pumps can reverse the flow so heat is added instead of removed

    2. Priusmaniac says:

      Only the high pressure side, the hot radiator side, is prone to leaks, not the low pressure cooling side. In the case of Tesla the cooling gas could simply flow free right through the battery box between the individual cylindric cells. That would spare any piping in the battery and improve the cell contact surface with the cooling gas. It would come in on one side, cools all the cells and go out on the other side sucked by the compressor pump. The battery would be lighter, with no piping in it and there would be more cooling potential for the case of a supercharger fast fill up at 500 KW or 1000 KW.
      Only one disadvantage, when the battery is opened to intervene on cells, the coolant gas would have to be purged before operations. Although a car battery is normally not touched while in the car and opening it to access the cells remain an extremely rare operation. It is also only done in dedicated factories so this inconvenience is not really one actually.

    3. jerryd says:

      That and it costs much more.
      And only cools the lower part of the battery mostly and what is transferring the battery to the bottom plate?
      It’ll need something.
      Best cooling is just air, cooled if needed by the A/C unit.
      Nor is glycol that bad a coolant, only 5-15% worse than water depending on the mix.
      Note the thickness of the Alum battery box along with th e large alum frame and I’m starting to see why the i3 is so heavy for a CF EV, heavier than several steel cars by several hundred lbs.

      1. Brendan says:

        The i3 weighs 1100kg’s I don’t know any cars that size that weigh that little and the battery is 180kg not to bad I thought

  2. Jeff Songster says:

    looks great… I’m wondering if this is at all close to the thermal system they designed into the eNV200. It is tied into ac apparently.

    1. vdiv says:

      No, from what we’ve seen the e-NV200 battery has a small evaporator and a blower fan inside the battery housing so the cells are still air-cooled, albeit with chilled air.

  3. Anthony says:

    I think I remember someone from GM (WOT?) saying a year or two ago about how the battery tray design was going to win out (that was in the GM Spark EV). Did the Spark EV use DX?

  4. Bill Howland says:

    “What’s not to like?”.

    Well its somewhat analogous to the system used in the Tesla Roadster, which sends refrigerant to an evaporator near the battery, which then chills a glycol solution.

    This system is “direct” in the sense that the battery is “INSIDE” the evaporator, and is piped, just as refrigerators from 85 years ago were…. You put the Ice Cream INSIDE the (direct expansion) evaporator.

    But in these systems where there is one compressor running multiple evaporators, refrigerant oil miscibility at least has to be considered, along with Oil return to the compressor. I’m sure it works fine, as it did in the Tesla, but the issue has to be confronted by watching routing and sizing of lines so that oil is returned gradually at all flow rates and therefore doesn’t return as one huge slug to blow out the compressor.

    I’ve seen one Carrier 7G8 system (50 hp), when the refrigeration tech kept ‘solving’ the low oil problem by just adding more to the compressor crankcase; then a few days later a huge slug of oil returned through the suction line and , since oil is incompressable, the piston came through the side of the crankcase (ruined).

    So this system is fine, and ‘proven’, since refrigerators and freezers have used this system for around 120 years at least. Much better than having to send a ship out to find an iceburg and saw off a section of ice.

    I’m going to argue the GM Volt system is still far superior, since although there is a chilled water loop (as most big commercial buildings use) since, in moderate weather when not that much cooling is needed, the water may be cooled just by a low-energy fan.

    During moderate weather, the Volt’s charging efficiency was very very high, even at a relatively high 3.3 kw charging rate – you’d just hear a small fan directly cooling the water circulating through the battery; whereas my Roadster HAD to run the air conditioner which meant the condenser fan had to remove not only the battery heat, but also the heat-of-compression of the compressor. This is a substantial percentage of the cooling energy required, and you can prove it to yourself by using a plain old bicycle pump. After pumping up a tire, the base of the pump is very hot. This is not Friction generated heat, it is the unavoidable heat-of-compression which must be removed, the majority of which happens at the condensor just prior to liquification of the hot gas. That is why, even in moderate weather, billows of heat would come out of the condenser fans just below the windshield. Even in moderate weather, and at a very slow charge rate for the roadster (6 kw, when the peak was 16.8 kw), it took 67 kwh to put around 50 kwh into the battery. or around 75% efficiency (which, 6-10 kw charging rate was the PEAK roadster charging efficiency – faster or slower charging rates were even worse).

    Whereas, the volt would take around 11.2 kwh to get 9.5 kwh, which is around 85% efficiency, a huge improvement over the Roadster, and important to people who live in areas with confiscatory electricity rates.

    1. alohart says:

      During moderate weather, the i3’s battery pack cooling system needn’t run at all. Even in Honolulu’s warm climate, our i3’s battery pack cooling system has run only twice while charging during the past year at the same 3.3 kW charging rate as the Volt. Maybe the i3’s battery pack being under the floor more exposed to the outside compared with the Volt’s battery pack being confined in the car’s central tunnel allows charging heat to escape more easily from the i3’s battery pack without active cooling.

      The battery pack cooling system’s goal is to remove enough heat to keep the battery cell temperatures below a certain level. While a heat pump certainly uses more instantaneous power than a fan motor, the Volt’s cooling system also requires an electric pump which is not needed with a heat pump system. Also, refrigerant is much colder than glycol coolant, so it removes heat much faster. This means that a heat pump needn’t run as long as a fan and pump to remove the same amount of heat, so a heat pump cooling system might not use more total power than a fan and pump cooling system.

      The Idaho National Laboratory measured battery pack charging efficiencies for various EV’s. At the maximum charging rate, the Volt’s charging efficiency is 88.5% while the i3’s is 93.8%. I was unable to determine whether the battery pack cooling systems were active during charging efficiency tests. A description of the testing conditions stated that the temperature during charging would be the lesser of 120º F or the maximum temperature allowed by the manufacturer. I would think that at 120º F, the battery pack cooling system would be active. But even if they were not active, the i3’s cooling system could be 5% less efficient than the Volt’s and still be as efficient charging overall.

      1. Bill Howland says:

        IEVV acting up, cant type,. refrig shouldn’t be colder for hi eff since it lowers cop. Also it might be the newer I3 may have batteries that simply can get hotter or have somewhat lower ESR. Under what environment circumstances can an I3 have 94% EFF?

        On another blog, tom generalized, saying bat cell is .002 ohm. but when I looked up the data sheet in 1 place it said <= .045 ohm and another it said 'calculated resistance' of 0.110 ohm. But the data sheet from 'industrialpanasonic' didn't specify the conditions for either value, nor did tom answer my question as to ESR under 4 charge/discharge states, perhaps because the manufacturer keeps the info too close to the vest. So there's too many variables in play here.

        If the authors want to say that an I3 in general charges more efficiently than a TESLA, I'd be inclined to agree, but there was almost no detail or set conditions in this article, which I'd think the engineers would want to rectify.

        As far as 'low pressure leaks' are concerned, the system will only be at 'low pressure' if it is pumped down prior to deactivation. Since multiple liquid line solenoids are required, it might, although during pump down there would be no refrigeration/airconditioning in the car period, since the compressor would need to scavenge the evaporator. But in operation if we're talking about r134a then we'd be talking 70 psi on a hot day (saturated suction temperature of 69 deg F) . Depends on how much superheat they plan for. Of course if the battery is really hot then the suction pressure could go higher, but without the skimpiest details of the system I'm not going to generalize. So there's still room for leaks. Of course, Prius Maniac's proposed CO2 'low pressure' side could be easily 400 psig. But you can't use CO2 again because of the low critical temperature of it..

        Now my Roadster may have been inefficient due to it having very early LI batteries in it, in fact they were so inefficient that as they got dead their ESR would greatly increase and the air conditioner would have to come on WHILE DISCHARGING to get rid of all the extra heat which wouldn't be there if the battery was more fully charged and its 'dynamic Equivalent Series Resistance' was lower.

        So again, with vague generalities no hard and fast conclusions can be drawn.

        1. Priusmaniac says:

          It could be interesting to take the problem from the other side. Checking what a battery box can resist to, determine what it would take to improve that and in parallel see what gas system can be applied once the reasonable stress limits have been set.

      2. SparkEV says:

        Considering DCFC is 90% to 95% efficient, 93% efficiency of BMW i3 would need close to 100% efficiency for electronics, which isn’t likely. I suspect the 93% quoted is for DCFC, and more realistic L2 is about 85%.

        2015 SparkEV has similar battery and cooling as Volt, and I measure ~92% for DCFC, ~80% for L1. I haven’t measured L2, but I suspect it’ll be similar to Volt as it uses similar 3.3kW charger (same as Volt?), cooling (2014 was different with bottom plates), LGChem batteries (2014=A123).

        Below is the link to the data I used to find 92% DCFC efficiency for SparkEV.

        InsideEV won’t let me post multiple, so you’ll have to find “Spark EV efficiency” blog post to see the data for L1.

  5. SJC says:

    “Tesla superchargers are charging at 120 kw.”
    The reports were that quick charging does not shorten battery life. I find that hard to believe.

    1. Big Solar says:

      Me too. Hey I thought this was an article about Donald Trump.!?

      1. Bill Howland says:

        It must be nice to see your surname in every headline article. It must infuriate the New York Times when, even our ally, the Germain Foreign Minister, says “Putin is not the problem, its that FOOL in Turkey!”.

        So when Trump aligns his views with Putin, it must make the other republicans-in-name-only super-upset. Its getting to the point where even Europe doesn’t buy it anymore.

        1. Jim says:

          If he is serious about more US jobs (which seems to be the case), it will be interesting to see how he feels about EVs. Manufacturing EVs, batteries, solar panels, etc, could bring lots of jobs to the US.

          I don’t expect to hear much before the general election, since some Republicans still think EVs are just for tree huggers. It’s not just a far left thing either, as the moderates on both sides see the potential.

          The left likes to discount the right, but to give some credit, many of the right are going solar and/or EV for the benefit of being energy independent.

          1. SparkEV says:

            Trump is socialist like Sanders. When politicians talk about bringing jobs, that’s the clear mark of socialist. Socialists create jobs via government. The jobs they bring back are to use inferior and/or more expensive American labor, just like how North Korea boasts of being self sufficient. Government can only create government jobs.

            Government can help the industry by removing barriers. What the non-socialist would say is that they will remove existing barriers to industry so that companies are more likely to hire in US. With millions of pages of regulations, there’s lots of stuff they can chop.

            I don’t hear much about this anymore from any candidate. All I hear is “we’re going to protect (inferior/expensive) American workers” and “we’re going to create (government) jobs”.

            1. Jacked says:

              Of course there are the expensive German workers who live with far greater government involvment in industry than in the US. It seems to work out great for Germany.

              Same with Japan.

              In fact most nations known for manufacturing prowess are MORE socialist than the US, not less, so your theory is absurd.

    2. Bob says:

      One aspect of battery degradation is charging, but some knowledgable guys found out it is a function of the time spend charging more than the kwhs charged. Therefore the faster charging the better. (not considering over-heating during charging)

  6. Priusmaniac says:

    While we are shaking the tree of cooling, has anyone considered the potential of a thermoacoustic cooling system for a battery?
    I mean, a thermoacoustic cooler needs a cavity (battery container), a stack made of individual plates (the individual cells), a cold source (the radiator) and the stationary acoustic wave generator. The gas in the battery becomes air or helium, the speaker is fed with electricity just like a compressor pump. The extra from such a system would be that cooling power would be even higher and the battery heat,while driving, could be recuperated in the form of electricity by running the thermoacoustic cooler in reverse as a generator. A kind of battery heat powered rex returning the normally lost energy as extra electricity to the motor.

    1. LZL says:

      Whoa…a space-age device in a car? Sounds expensive.

      1. mr. M says:

        They are expected to become cost competitive within the next few years.

        1. martinwinlow says:

          What…, like fuel cells were? MW

          1. Priusmaniac says:

            Thermoacoustic is interesting, hydrogen fuel cells are not, that set them completely apart.
            But thermoacoustic is also the most reliable heat pump that one could bring to Mars or other places since it has no moving parts but a loud speaker. You can heat directly with electric resistors but on Mars it is freezing cold, so that would take a lot of electricity. Thermoacoustic heat pumps allow to heat a station there with much less electricity. In the same time it is the most lightweight system that would allow to cool methane and Oxygen to the cryogenic temperatures needed for return rocket propellants.

  7. Liquid glycol doesn’t not have “poor” thermal transfer. What a load of s***.

    There are advantages and disadvantages to both systems, but I predict that the liquid solution is superior at ultimately removing the heat.

    How that liquid is cooled is by a refrigerant.

    BMW kept the refrigerant and eliminated the glycol.

    1. Priusmaniac says:

      A hand under a hose of cold water or a hand under the hose of a carbon dioxide fire extinguisher, what do you chose to cool it down fast and deep? Check it out and you have your answer.

    2. Bill Howland says:

      Yeah Tony I agree…. Too many broad generalizations and not much specificity for 2 guys who have 70 years of engineering experience between them. I invited them to add some corroborating data to their article.

      As it is, there’s much more data in the comments by the commenters.

    3. Tech01x says:

      Yes, the writing of this article is suspect. Why the emphasis on the glycol? The actual thermal transfer agent is water and glycol helps it not freeze at 0 degrees C. Here is a paper that compares a water cooling system (R718) against R134a and others.

      From the paper:
      “According to Table I, R718 is the best refrigerant as ODP, GWP, safety and mass specific refrigeration capacity are taken into consideration. When COPs are compared, R718 is the third best for the specific operating conditions selected for Table I. This study finds a set of combinations that include evaporator temperature, condenser temperature and polytrophic efficiency, at which R718 gives the best COP.”

      I would love to see substantiation on this claim by the authors: “DX offers cooling rates 3-4 times higher than liquid glycol.”

      1. JakeY says:

        Very good point. While I appreciate that the authors are experts in their fields this article lacks any sort of quantitative analysis.

        Even accepting the 3-4x number in a certain benchmark, in this specific application there is enough complexity and differences in how the cooling system is designed that the heat removal rate of the overall system may not necessarily be better.

    4. Pushmi-Pullyu says:

      Tony Williams said:

      “There are advantages and disadvantages to both systems, but I predict that the liquid solution is superior at ultimately removing the heat.”

      Well, said, Tony.

      It is disappointing that InsideEVs has chosen to run not one, but two, articles which do an apples-to-oranges comparison between Tesla’s battery TMS (Thermal Management System, and some other EV’s TMS, then conclude the other is “better” merely because of one single factor among several.

      Here is one way in which a glycol system is clearly better than a refrigerant system: The glycol system only needs a water pump to circulate the coolant. The refrigerator system needs a condenser. Anybody who knows much about home energy consumption knows that a refrigerator’s condenser consumes a lot of energy when it runs. Contrariwise, a simple water pump doesn’t use much.

      Now, it’s true that a refrigerant system can transfer heat faster per square inch of exposure to hot objects. But this can be compensated for simply by increasing the surface area of tubing that the glycol is run thru.

      Here’s another way Tesla’s TMS is better: A refrigerant system must run at a much higher pressure. We all know how frequently a car’s A/C loses pressure and needs a costly refill/repair. Will refrigerant-based battery thermal management systems prove to be any better?

      Bottom line: A refrigerant based TMS is “better” in terms of space… but only at the expense of being more expensive, consuming more energy, and being more prone to needing costly repairs.

      Overall, I think it’s very difficult to justify the claim that a refrigerant-based battery TMS is “better” than a glycol-based one. Looking at the overall picture, I think the opposite is true.

  8. pjwood1 says:

    Heating is more the issue than cooling. Tesla batteries can get hot, on a warm day, fresh off the highway after 280 miles of driving. Still, supercharging goes fast. They otherwise have to be run very hard, to even get to the new “Ready” state, for “Max Power”, as featured on the P85D and P90D. This tells me the greater efficiency of refridgerant wouldn’t pay a big dividend.

    Heat, from GM’s coolant loop that travels directly through the exhaust manifold, is a better innovation, to me. They upped the kw of the resistance heater, which was crude, but since the car has an engine and Volt 1 would open its thermostats (for a river of hot coolant) after only a tenth of one gallon of gas, Volt 2 should also be that much faster in engine mode.

    A car that doesn’t comfortably warm up the way people expect is a much bigger threat to the OEMs chasing PHEV/BEV, in my opinion.

  9. Djoni says:

    One other advantage is this system should be lighter, since gas is lighter and the A/C compressor is already in the car.

    It seems superior in just about every way.

    1. Pushmi-Pullyu says:

      Or, looking at the same factor from the other direction: Since the refrigerant has less mass, it can’t absorb as much total heat, which means the condenser in the refrigerator system has to run that much harder, and use that much more energy, to do the same amount of cooling as a glycol-based system.

      Again, you have to take a pretty narrow viewpoint to conclude the refrigerator system is “better” than the glycol cooling system.

      There are various reasons why the Tesla Model S, although it is a large heavy car, is almost exactly as energy efficient as the lighter Leaf, when measured by the same EPA driving test cycle. The superior energy efficiency of the Tesla Model S battery pack’s thermal management system is one reason for that.

  10. techguy says:

    Bob do you have a link to those “knowledgeable guys”? I want to read about their findings

  11. Steven says:

    With glycol and a leak, the fluid will leak down to the point of the leak and you could “limp home” as long as the glycol doesn’t boil.

    With R134a (refrigerant) it will leak down to the point of atmospheric pressure. I can’t imagine the car would let you drive anywhere at that point.

    But what do I know?

  12. Get Real says:

    It also should be noted that aside from Battery TMS, Tesla and at least Kia scavenges waste heat off the drive motor/inverter for cabin heating and as such this makes for a more efficient use of this otherwise wasted heat.

  13. HVACman says:

    A few technical points in response to comments above:

    Re: refrigerant vs glycol heat transfer. ASHRAE (American Society of Heating Refrigerating, and Air Conditioning Engineers) Fundamentals Handbook, Chapter 4 shows that the heat transfer coefficient in a water-based heat exchanger is about 200-300 BTU/hr/ft2/deg. F. Chapter 5 shows that two-phase heat exchangers such as a refrigerant evaporator is between 600-1000.
    Parker-Hannifin makes heat transfer devices for high-power electronics and utility-scale PV/wind turbine system inverters – very similar to what is required for cooling large EV battery packs and high kW EV power electronics. Their own tests show that a two-phase refrigerant-based plate system has a heat transfer coefficient three times higher than for liquid water-cooled plates in the same configuration. They now market such systems, as they can cool higher power electronics with simpler systems using direct refrigerant evaporator-based cooling plates than with chilled water-cooled plates.

    And glycol, when added to water, reduces heat transfer even more. Per ASHRAE, pure water conductivity is 0.34 BTU-ft/hr/ft2/deg. F, but a 50/50 polyethylene glycol/water mixure is only 0.22.

    Another point – because of the isothermal nature of boiling heat transfer, all that heat transfer is “latent” and happens at the same temperature (think boiling water – goes from 100% liquid to pure steam vapor at 212 deg. F). Compare this to using liquid glycol to remove heat. It is all “sensible” heat transfer – it warms up as it absorbs heat, so each square foot of heat transfer surface becomes less effective.

    Final point – When using glycol, it has to be cooled somehow. It is cooled in a refrigerant evaporator heat exchanger. To cool the glycol to, say 55 degrees, the refrigerant has to evaporate at 45 degrees. It is a lot simpler and more efficient to just route the refrigerant directly into the plate and make the plate several degrees colder, with fewer parts and less effort (no secondary pumps, coolant tank, glycol or secondary heat exchanger.)

    The plate-type battery pack TMS with direct-expansion refrigerant cooling is where some, such as BMW, already are. LG is seriously exploring it. Smaller, lighter, cheaper, and more efficient than glycol. We think this will be an industry trend.

    – Keith

    1. bill howland says:

      That’s all true, but GM knows all of the above also, and still decided on an indirect system.

      I still would like to see some power in/power out comparisons under varying conditions. This is really the only thing that matters, along with the ‘leakproofness’ of their refrigerant fittings. They don’t look like anything I’ve seen on this side of the Atlantic.

      To me, its amazing that BMW worries about this pretty trivial thing, yet has unbelievably Horrible efficiency, Noise, and Power Robbing of their Range Extender.

      Everyone always says its good enough. But there have been UNBELIEVABLY *ZERO* tests with the cabin heater on HIGH. This just has to take another 10 horsepower of very inefficiently used gasoline power to heat the cabin. And 34 hp to push the car through the added inefficiency of the power conversions – when you take another 10 away, there ain’t much puffing left.

  14. Pushmi-Pullyu says:

    How disappointing that an article comparing the battery pack TMS (Thermal Management System) of different EVs does not take into consideration which system is more energy efficient. That’s doubly disappointing when one of the authors is a “licensed professional engineer with over 35 years experience in heating ventilating and air conditioning systems”.

    Tesla is leading the EV revolution in technology, and every other EV maker is struggling to catch up. It’s rather silly, not to mention completely wrong-headed, to state or imply a refrigerant-based TMS is “better” just because it has a higher heat transfer rate per square inch. That can be compensated for simply by using either more square inches of exposure to the battery cells, or a higher flow rate of coolant, or both. When it comes to scaling up such systems to handle a larger battery pack, using more glycol/water in a water cooling system is much, much less expensive, and much more energy-efficient, than using more refrigerant in a refrigeration system.

    It is, again, surprising that an article written by two engineers, one of which has experience with heating and cooling systems, fails to note that Tesla’s battery pack TMS must handle much higher heat loads during sustained acceleration and Supercharger charging than the competing systems they’re comparing it to. The Model S has a higher top speed and is capable of faster charging than those other cars, so claiming refrigerant-based systems are better at handling total heat transfer is ignoring reality pretty firmly.

    Let’s look at all the important engineering factors, rather than only the single one this article considers:

    Energy efficiency: Glycol/water cooling wins

    Size/compactness: Refrigeration wins

    Heat sink/thermal mass of the coolant: Glycol/water wins

    Manufacturing cost: Glycol/water cooling wins

    Pressure required: Glycol/water cooling wins

    Maintenance cost: Glycol/water wins