Confirmed: Cybertruck Will Have a Heat Pump

[Bill906]

When it gets cold enough heat pumps stop working so there has to be another source of heat. Rumor has it that this is the inverter. I can't tell you how it works because I don't know but I can point out a couple of things from which we can deduce how it probably works. Refer to the following sketch:

The items A, a, B, b, C and c are solid state switches which can be "gated" on and off electronically. When driving A, b and c would be turned allowing current from the battery to flow into the coil at 10 o'clock and out of the coil ar 2 and 6 o'clock. This results in a magnetic field directed at 10 o'clock. Those switches are then gated off and B, a and c are turned on. This results in current flowing into the coil at 2 o'clock and out of the coils at 6 and 10 resulting in a field oriented towards 2 o'clock. Finally those are turned off and C a, and b turned on giving a field oriented towards 6 o'clock. The sequence then repeats with the result that one has a magnetic field rotating in a clockwise direction in steps of 120 °. In fact there are more than just 3 coils and the field rotates in smaller steps.

The resistive power dissipated by a switch (or any other device) is the product of the voltage across the switch with the current through it. When any of the switches are turned off the voltage across it is high and only a small leakage current flows through them so that the power dissipated is small. When one is turned on there is a large current through it but the voltage across it small and, again, the power dissipated in it is small. But the switch has to get from the off state to the on state and in doing so it passes through states where both current and voltage are intermediate and power is dissipated. Engineers strive hard to get the switch from off to on and back to off as fast as possible to minimize the energy (integral of voltage times current with respect to time) lost to heat. So one strategy for getting heat from the inverter, the one that would be used when moving and more battery heat is required than supplied by the battery's internal resistance, would be to slow the switching speed (apply a gating waveform with a more gradual rise time).

When trying to pre-warm the battery (car not moving) a possible strategy is to turn all the transistors on part way. No current flows through the motor coils under these conditions - just through the transistors which will get warm. Yet another strategy might be to turn on one capital letter transistor and the other two small letter transistors. Current then flows through the transistors (which get warm) and through the coils which also get warm (the motor is also on a glycol loop). All the energy going to the motor gets converted to heat in this arrangement as the motor is not turning and none gets converted to mechanical energy.

Thus my guess is that one or the other or some combinations of these techniques is used for supplemental heat in the Y and will be in the CT when it's too cold for the heat pump.

I^2R is I^2R whether the current is flowing through a resistor or transistor so this method is just as efficient as a resistive heater. What other form could the battery energy be converted to?

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[Bill906]

Great job on the inverter diagram. I work for a company that produces inverters for the industrial world. I do not know if this is true or not in vehicle manufacturing regulations, but in the industrial world we offer a feature on our variable frequency drives (VFD’s) aka inverters called Safe Torque off. When the VFD is commanded to be in a safe state it prevents the transistors from being gated. I would guess (again I do not know) that when the vehicle is in park or neutral it would also have to be in a state where the inverter cannot produce current.

On the efficiency part, yes, current flowing through a conductor creates heat regardless of if it’s a dedicated electric heater, or the transistors in the inverter. If you created the heat in the inverter you would have to transport it from the inverter to the cabin and battery. You would lose some heat in the transfer and also have to power the coolant pumps and possibly the heat exchanger compressor.

I think a dedicated electric heater would be a better design and is my guess as to how Tesla does it. I would think the cost, weight and volume used by an electric heater would be negligible compared to the losses of moving the heat from the inverter. But I do not know for sure and I am always the first to admit I could be wrong.

 

[ajdelange]

I would guess (again I do not know) that when the vehicle is in park or neutral it would also have to be in a state where the inverter cannot produce current.

When the car is in the garage (conditioning/preheat) the parking brake is applied. This would prevent unintended motion but as I envision things there is no rotating magnetic field and hence no motion. Plus the current to each phase is monitored so that there is an additional way to monitor that things are safe in this regard.

On the efficiency part, yes, current flowing through a conductor creates heat regardless of if it’s a dedicated electric heater, or the transistors in the inverter. If you created the heat in the inverter you would have to transport it from the inverter to the cabin and battery. You would lose some heat in the transfer and also have to power the coolant pumps and possibly the heat exchanger compressor.

Whenever the inverter is operating it produces heat which in warm/hot weather must be carried away thus the transistors always have been on cold plates in glycol loops with the battery, motor stators and, in the induction motors, the rotors. There was an electric battery heater on this same loop and a separate electric cabin heater on a separate loop. I do not believe inverter, motor or battery heat could be transferred to the cabin. What's new is that it can through a more sophisticated valving and pump arrangement and that the heat pump can now collect heat from the ambient air.

Sure there will be losses in moving this heat around but where does that "lost" heat go? Into the cabin thus these are not losses. Yes, the pumps consume energy but where does it go? Into warming of the glycol so it is not lost. The electrical energy that runs the compressor motor gets converted to heat in the exhaust gas which is collected by the high side heat exhanger and is thus added to the loop going to the cabin heat exchanger and battery so it is not lost either.

I think a dedicated electric heater would be a better design and is my guess as to how Tesla does it.

Tesla has come to the opposite conclusion and has stopped using the electric heater approach.

I would think the cost, weight and volume used by an electric heater would be negligible compared to the losses of moving the heat from the inverter.

Cost, weight, volume and losses are certainly apples, oranges, lemons and pomegranites. What Tesla was interested in was the trade between the extra cost weight and volume of the redesigned heat management system (if any - I wouldn't be surprised to find it lighter and cheaper) and the added range it lends. As we noted above the things you are thinking are losses aren't really losses and the COP of the heat pump (3-4) surely beats the COP of an electric heater (1) thus as much as 10% range improvement is realized.

 

[Bill906]

A.J. I really enjoy having intelligent discussions like this. Thank you.

First don't get me wrong. I agree with using a heat pump system to take heat from places that have excess heat and pumping it to areas that need it. I find that to be brilliant. My argument is not that an electric heater is better than using a heat pump. Heat pumps do not work in all situations. The heat pump system needs a heat source. If one is not readily available we need to add one. My argument is that the supplemental heat source would be more cost effective if it was a dedicated electric heater placed strategically in the heating system (In the cabin air vents, or in the battery compartment) instead of turning the inverter into an electric heater and pumping that heat to where it's needed. I do not think the cost savings, weight savings, or space (volume) savings of not having a dedicated heater would be significant enough to outweigh the losses of turning the inverter into a heater and having to pump it to areas that need it. There would be losses. My use of the word "cabin" is the space the passengers occupy. I do not consider the inverters being in the cabin. Some heat will be lost in the spaces the inverters reside and I suspect on a cold day some of that heat will make it to the outside. You will also lose energy in the coolant pump and/or heat pump compressor. If you supply 100Wh of energy to a coolant pump it will not produce 100Wh of heat energy. Some energy will be turned into mechanical energy some will be converted to heat. The mechanical energy would not be needed if the heat source was in the cabin instead of in the inverter. I also believe having a dedicated heater for the example given (-20°F parked outside but plugged in) would be better because heating up the inverter, the coolant and heating system would take time. A dedicated heater would be putting heat where it is needed almost immediately.

You stated "Tesla has come to the opposite conclusion and has stopped using the electric heater approach. " I am not sure we know this. Definitely not with the CT as it is still being designed. As far as I know there are no schematics or other diagrams of the model Y's electrical or heating systems in the general public. I do not know if a supplemental electric heater is in the Model Y. An electric heater could be incorporated into some part in the system (an example, an electric heater could be incorporated in the cabin blower module) and not be obvious to someone outside of Tesla.

I am really excited to get a CT. But I am paying close attention to any information on how it will perform in cold temperatures. I live in the northern Midwest. The CT will be my only vehicle. I now live 280 miles from where I was born and raised and go home often to visit family. The distance between me and my family is probably the single reason I do not have an electric vehicle. Yes, some of the more recent EV models have a range greater than 280 miles. But what about wanting to go home for Christmas and it's below 0°F? I've seen reports that some EV's can lose over 40% of their battery range in cold weather. This is why I'm actively engaging in this discussion. It's the first (hopefully of many) discussion I've seen related to the CT and cold weather performance.

Thank you again for your time in this discussion. I do enjoy it.

 

[Dids]

A.J. I really enjoy having intelligent discussions like this. Thank you.

First don't get me wrong. I agree with using a heat pump system to take heat from places that have excess heat and pumping it to areas that need it. I find that to be brilliant. My argument is not that an electric heater is better than using a heat pump. Heat pumps do not work in all situations. The heat pump system needs a heat source. If one is not readily available we need to add one. My argument is that the supplemental heat source would be more cost effective if it was a dedicated electric heater placed strategically in the heating system (In the cabin air vents, or in the battery compartment) instead of turning the inverter into an electric heater and pumping that heat to where it's needed. I do not think the cost savings, weight savings, or space (volume) savings of not having a dedicated heater would be significant enough to outweigh the losses of turning the inverter into a heater and having to pump it to areas that need it. There would be losses. My use of the word "cabin" is the space the passengers occupy. I do not consider the inverters being in the cabin. Some heat will be lost in the spaces the inverters reside and I suspect on a cold day some of that heat will make it to the outside. You will also lose energy in the coolant pump and/or heat pump compressor. If you supply 100Wh of energy to a coolant pump it will not produce 100Wh of heat energy. Some energy will be turned into mechanical energy some will be converted to heat. The mechanical energy would not be needed if the heat source was in the cabin instead of in the inverter. I also believe having a dedicated heater for the example given (-20°F parked outside but plugged in) would be better because heating up the inverter, the coolant and heating system would take time. A dedicated heater would be putting heat where it is needed almost immediately.

You stated "Tesla has come to the opposite conclusion and has stopped using the electric heater approach. " I am not sure we know this. Definitely not with the CT as it is still being designed. As far as I know there are no schematics or other diagrams of the model Y's electrical or heating systems in the general public. I do not know if a supplemental electric heater is in the Model Y. An electric heater could be incorporated into some part in the system (an example, an electric heater could be incorporated in the cabin blower module) and not be obvious to someone outside of Tesla.

I am really excited to get a CT. But I am paying close attention to any information on how it will perform in cold temperatures. I live in the northern Midwest. The CT will be my only vehicle. I now live 280 miles from where I was born and raised and go home often to visit family. The distance between me and my family is probably the single reason I do not have an electric vehicle. Yes, some of the more recent EV models have a range greater than 280 miles. But what about wanting to go home for Christmas and it's below 0°F? I've seen reports that some EV's can lose over 40% of their battery range in cold weather. This is why I'm actively engaging in this discussion. It's the first (hopefully of many) discussion I've seen related to the CT and cold weather performance.

Thank you again for your time in this discussion. I do enjoy it.

There are heat pumps that work down to -80f. I am not saying Tesla is using them but it is possible. https://en.m.wikipedia.org/wiki/Thermoelectric_cooling
As long as the cold side has more heat than the minimum operating temp than it is just a question of volume to concentrate the heat up the hill.

 

[ajdelange]

The heat pump system needs a heat source. If one is not readily available we need to add one. My argument is that the supplemental heat source would be more cost effective if it was a dedicated electric heater placed strategically in the heating system (In the cabin air vents, or in the battery compartment) instead of turning the inverter into an electric heater and pumping that heat to where it's needed.

My observation would be that it can't be more cost effective than not adding an extra heat source as one apparently isn't needed. The cabin heat exchanger(s) are on glycol loops which effectively transfer heat from motor, battery and inverters to that heat exchanger or to the outside heat exchanger when it is necessary to dump than heat overboard. You aren't turning the inverter into a heat source. It is already a heat source - an undesired one in most cases. All they are doing is backing off all the hard work they have done to minimize the heat produced by the inverters. An X going down the road at 60 mph is pulling about .270*60 = 26 kW from the battery. If the inverter is 97% efficient there is half a kW wasted heat to be disposed of and the valves switch to send it to the battery to warm it if it needs warming or to the radiator if it needs to be disposed of. If more heat is needed by the battery then reduce the waveform slope to lower the inverter efficiency to 90% and now you have 1.6 kW (5500 BTU/h) to send wherever it is needed. This is one of the cleverest bits of engineering I have ever encountered. Just to be clear, the X, which I used as an example because I know it takes 16 kw at 60, does it the old way and dumps the inverter heat overboard even when it could be used for cabin heating thus hurting cool weather range. Cabin heating, and emergency battery heating come from separate electric heaters.

Why would we not want to do this?

I do not think the cost savings, weight savings, or space (volume) savings of not having a dedicated heater would be significant enough to outweigh the losses of turning the inverter into a heater and having to pump it to areas that need it.

I hope you will do a little more thinking about this. I'm not sure you are fully on board with the fact that the inverter does not get turned into a heater that has to be attached to pumps and plumbing to get it's heat where it is wanted. It IS a heater already plumbed into a system that efficiently sends its heat to where it is wanted (battery, overboard or cabin).

There would be losses.

No more than there already is but those are miniscule. Think about a W/W heatpump system in a house. The pipes run from the heat pump to the water to air exchangers and the vast majority of the heat gets to the heat exchanger. The pipes are insulated and even though the runs may be long very little heat is lost. Were it the impressive COPs these systems deliver wouldn't be realized. By contrast in the Y the plumbing runs are very short. Losses, which we will redefine to mean any heat that goes anywhere other than where you want it too are very small. That is, evidently, an important part of the octovalve design which appears, in the Munroe video, to be a big lump with ports connected by short hoses to loads and sources. I guess the valves and pumps are inside.

My use of the word "cabin" is the space the passengers occupy. I do not consider the inverters being in the cabin. Some heat will be lost in the spaces the inverters reside and I suspect on a cold day some of that heat will make it to the outside.

I hope I have covered that in the new definition of loss and yes, some heat will go other than where you hope it will. I think your problem with the concept is that you think that is much more than it actually will be. The heat producing components are on cold plates inside the motor housing. Is heat from the transistor die going to flow through its case into the interior air of the inverter housing, through the cover of the housing and out into whatever the surrounding space is or through the much lower thermal impedance path to the glycol? The system is designed such that it is the latter though of course a watt or 2 goes elsewhere. Surely your company use heat sinks/cold plates in its designs?

You will also lose energy in the coolant pump...If you supply 100Wh of energy to a coolant pump ...

Again think about what you know about a house heating system. A 3 ton hydronic system in a house typically uses a 1/35 or 1/25 HP circulator (20 - 30 W). The heating system in a car doesn't need to move anything near 36,000 BTU/h. Perhaps 1/3 to 1/4 of that so the pump is going to be very small. 100 Wh would run such a pump for hours and hours.

If you supply 100Wh of energy to a coolant pump it will not produce 100Wh of heat energy. Some energy will be turned into mechanical energy some will be converted to heat.

And what happens to the rest? Does the pump make a lot of noise? Does it give off light. Radiation of any other kind? No. All that energy gets converted to heat. The only forces the circulating fluid experiences is friction and that associated with turbulent flow in the heat exchangers which increases intermolecular friction. Both of these simply warm the glycol and all that energy (except a tiny bit) is just where it is wanted: in the glycol.

The mechanical energy would not be needed if the heat source was in the cabin instead of in the inverter.

If 1 kW of heat was needed in the cabin we could get that from 900 W dissipated in the transistors plus the 100W running the pump (if the pump had to be that big). Note that I do not include any losses here because I have trouble believing they would be more than a couple of watts. If you used an electric heater with a pump it could be a 900 W heater with a 100 W pump. Were this heater directly in the air you could use a 900 W heater but to get the extra 100 W you would need a 100 W blower.

You will also lose energy in the heat pump compressor.

Yes you do but once again the loss is small - so small that when a refrigerant cycle is analyzed it is assumed that the compression and expansion are isentropic (no heat lost or gained). IOW all the electrical energy delivered to the compressor appears in the form of heat energy in the exhaust gas which gets transferred to the glycol in the condensing heat exchanger. Again, think about it a bit. Were there appreciable heat loss in the compressor it would not deliver the COP it does and, thus, not deliver the range extension that is the reason it is used in the first place. As a final comment on the compressor it would be turned off when the OAT is below the boiling point at the low side pressure because we don't want to be slugging the compressor with liquid.

I also believe having a dedicated heater for the example given (-20°F parked outside but plugged in) would be better because heating up the inverter, the coolant and heating system would take time. A dedicated heater would be putting heat where it is needed almost immediately.

Certainly a separate electric heater based system would have a smaller thermal mass than the inverter/glycol loop and so heat faster but in the parked outside but plugged in scenario why do you care?

You stated "Tesla has come to the opposite conclusion and has stopped using the electric heater approach. " I am not sure we know this.

Well we don't of course (Sandy Munroe teardown) seems to point in that direction in the Y. Why would they go backwards in the CT?

As far as I know there are no schematics or other diagrams of the model Y's electrical or heating systems in the general public. An electric heater could be incorporated into some part in the system (an example, an electric heater could be incorporated in the cabin blower module) and not be obvious to someone outside of Tesla.

Right to Repair laws have forced Tesla to publish parts lists and diagrams. One will presumably appear in short order for the Y and you will be able to check this out. For now you will either accept that the preponderance of the evidence and common sense suggest that there is no such heater or you won't.

ample, an electric heater could be incorporated in the cabin blower module) and not be obvious to someone outside of Tesla.

Yes, some of the more recent EV models have a range greater than 280 miles. But what about wanting to go home for Christmas and it's below 0°F? I've seen reports that some EV's can lose over 40% of their battery range in cold weather.

Are there no charging opportunities between where you live and where your parents live? It is possible that range of a BEV drop to 40% of its EPA range (headwind, high speed, heater on full bore, wet or snow covered road, uphill, towing) but it would, I think, have to be close to a perfect storm to get that low. There are obviously things you can do to prevent such poor performance such as slow down and wait for better weather. The cars have excellent consumption modeling software on board which will give you a good prediction of margin at destination. Using this one simply stops and charges if he needs to.

Thank you again for your time in this discussion. I do enjoy it.

No problem there. I am more than happy to share what I know and what I think. You have to take it with a grain of salt though because there is not a lot of hard data available from Tesla.

 

[curtishibbs]

There may only be a 7% difference in the heat content of 20 degree air vs 0 degree air, but the ability to extract that heat depends on the temperature difference between the refrigerant and the air. The absolute heat content doesn't really matter that much.

 

[ajdelange]

Well, no. It only depends on the ratio of the temperatures of the source air to the sink air. Theoretically. For a source at 0 °C, for example, to pump heat to room temperature, the Second Law says you cannot build a machine with a COP better than 1/(1 - 270/293) = 14.65. But practically speaking, heat pumps working a temperature difference like that deliver COP of around 4 or 5. That's still good. Though. We can get 4 or 5 kW of heat for each kW going to run the compressor, pump(s) and fan(s). If the temperarture of the air drops to -10 °C there is less heat in it but, more importantly, the theoretical best COP drops to 9.8. Whereas we could theoretically extract 1 kW by spending 68W we now would have to expend 103. That's quite a bit more but as long as the air temperature is above 0 K we can get what little heat is contains out of it but a greater expense of compressor input power. Theoretically. You notice I slip this is over and over again. Now what I think you are concerned about are the practical implications of this. If I have outside air at -5 °F and operate my R134a heat pump at a low side pressure of 22 psia (pardon the switch to imperial - the p-H chart I have for R134a is imperial) I've got a problem becuase R134a boils at 0 °F and -5 ° air won't boil it. If, OTOH, I lower my low side pressure to 15 psia R134a will boil at -10 °F and -5° air will boil it. But the heat flow per unit if communicating area between the refrigerant side and outside depends on the temperature difference so that if I lower the pressure still further to get a 10 ° ∆T instead of 5 I will be able to extract twice as much heat. The alternative to lower evaporation temperature is, of course, more common area i.e. a longer heat exchanger and so there are clearly trades to be made. For a given refrigerant and compressor we want low side and high side temperatures to be within given bounds and we can't just build an install a heat exchanger of arbitrary length. In general practice it seems that people consider heat pumps to run out of steam at about -10 °F though they still deliver heat below that though the COP may be small.

 

[ldjessee]

I think it was either one of the Sandy Munro videos or the 3rd Row Podcast that had Elon Musk and Sandy Munro on it that there is a heater loop to help get the heat pump 'start' was discussed.

 

[ajdelange]

It's been made pretty clear in earlier posts that the heat pump will not provide sufficient heat if the air is cold enough and so some other source of heat is necessary. That is, of course, the battery. Whether the battery's energy is used to warm the inverter transistors or a separate resistive element is, of course, unknown but the transistors are already on the appropriate glycol loop and the idea is so bloody clever that one has to hope that that is how they do it.

Now another matter is thick compressor oil caused by cold and related to this is the fact that refrigerant dissolves in that oil when it is cold. To solve both of these problems a "sump heater" is often installed in compressor sumps but this is a relatively small heater, often thermostatically controlled, there to heat the sump oil, not provide part of the heating load. Whether these are used in automotive systems or not I don't know but it does seem to fit with the "to help get the heat pump 'start' ' notion. Another aspect of eased starting is being sure the compressor stays stopped long enough for pressures to equalize around the refrigerant loop but that's a matter of a simple timer.

 

[Bill906]

Hi AJ, thanks for the detailed explanation you gave me earlier. Sorry for the long delay replying. Things have been busy.

After reading your post I did a lot of thinking and have come to the conclusion that my estimates on losses are most likely a lot higher than reality. When not having hard data I typically am very conservative on what the gains are compared to losses. I now think I may have been overly conservative.

I did forget that the energy turned into mechanical energy would eventually be turned into heat energy. Guess I was thinking Tesla was using the frictionless surface used so often in Physics 101 problems.

I guess I was unclear in what I meant when I said "Turn the inverter into a heater". I am aware inverters produce heat in normal operation (or any operation when electrical current is flowing through them). What I meant by "turn the inverter into a heater" was to use it exclusively as a heat source and not as it's original, primary function, an inverter. Which is what would happen in the scenario I presented - parked, plugged in, preheating cabin, batteries etc. I'm not saying how I think of it is right or wrong, just explaining what I meant. I was calling the device by the name that best matches it's primary function. When the inverter is used to supply/control power to the motor it's an inverter where heat is a byproduct of that. If you are firing the transistors for the sole purpose of creating heat and not for motor control, that is what I meant by turning the inverter into a heater. I do see where my choice of words could be confused.

When you asked "Why would we go backwards with the CT" I again, think my point is getting misunderstood. Taking heat energy from sources that have excess and pumping that energy to places that need it is brilliant. The complex system of moving heat like this is awesome. Getting rid of that would be going backwards but that is not what I'm arguing. My argument was adding a resistive heater to only be used when no other heat source is available. In the case presented (-20°C parked but plugged in) I doubt the heat pump would be much better than 1 COP. So heat pump would not be any better than resistive heater (at least not significantly). My thought it that using a resistive heater ONLY in the case were no other heat source is readily making heat. I'm not seeing that as "going backwards", more of supplementing the move forward.

I guess another main reason I don't like using the inverter as a heater when it's not being used as an inverter is that seems like someone who builds a house somewhere that only has electric power. No gas, oil etc. for heating and cannot justify installing heat pump for one reason or another. If you had to build that house, would you install an actual electric home heating system like base board heat, or would you just heat the house by leaving the electric oven on. It is a heater. Again, not saying I'm right or wrong. Just explaining my reasoning for what I think.

I do think using the inverter as a heat source even when the inverter isn't being used as an inverter may be the way Tesla went. But putting in small simple heaters for supplemental use only is also a possible solution they may have considered.

Even plugged in I would say the delay in heating the cabin because of the time required to heat up the thermal mass of the heating system would be a factor to consider. On a very cold day like the -20°C I suggested I would think it would easily take 5 minutes or more just to get the coolant etc. from -20°C to a usable temperature for warming up the cabin. (Guessing AT LEAST to 50°C on the coolant.) I currently remote start my ICE vehicle and run it for 15 minutes on cold days. On very cold days I typically run it through a second cycle. (Automatically shuts off the engine after 15 minutes). I'm not saying this is a definite reason to use supplemental resistive heaters, just saying I wouldn't rule this point out as insignificant.

When I first looked at Tesla vehicles (Model S) and heard of the Supercharger network I checked and at that time there was not a charger between my house and my parents. Now there are two. I currently do the trip with minimal stopping. A few times I've done the 5 hour trip non-stop. (Yes I realize that sitting in a chair for 5 hours straight is not a healthy thing to do, ). This is of course the last big hurdle of the EV vs ICE argument. Time required to fuel. My parents are getting up in age. A year ago dad had a minor stroke (luckily he pulled through with no lasting effects). But when it happened, my sister called me and I was packed and on the road in 15 minutes. If I had to sit at a charging station for 30 minutes halfway through my trip, wondering how dad is doing.... that would have made me very angry that I chose EV vs ICE. With the 500+ mile range that Tesla is claiming on the CT, I feel much better about that decision. You've got to remember 500 miles is ideal. From what I've heard you typically only charge to 100% if/when needed. I believe typical day to day driving Tesla recommends not going over 80% If I needed to make an emergency trip 280 miles away, my battery would at best be at 80% or 400 miles range. If in the winter, from reports on the internet, that could lower range by worst case scenario of 40%. Yes, I cold drive slower, maybe wrap a blanket around me instead of using heat, but neither of those things are needed with an ICE vehicle. I am a fast but safe driver. I realize that if often considered a contradiction but I have received many speeding tickets in my life, and I have never been in an accident. One might say I'm lucky, but lucky for roughly 35 years of driving? The point being, I don't consider driving slower being an option.

Sorry so long and wordy. I think the isolation of the pandemic makes me more talkative when I get a chance. And as always, thank you for sharing your time, knowledge and understanding.

 

[ajdelange]

I did forget that the energy turned into mechanical energy would eventually be turned into heat energy. Guess I was thinking Tesla was using the frictionless surface used so often in Physics 101 problems.

Stick with the thought in the first sentence. In analyzing/designing these systems we assume the compression operation is not only lossless in terms of friction but that it is isentropic i.e. that no heat is lost or gained. But feel the compressor. It can feel warm or cold depending on what is going on and, as Sandy Munroe pointed out they can be noisy too. Feeling hot or cold implies heat loss or gain meaning that the process in fact isentropic and noise implies energy loss in another form. I'm sure you have observed that the suction line of systems and sometimes the compressor too are insulated but this is as often as not to keep them from "sweating" (condensing water from the air) or to keep them quiet as it is to improve efficiency.

I guess I was unclear in what I meant when I said "Turn the inverter into a heater"....

The important concept is that those six transistors can be used as an inverter (driving up hill) or rectifier (coasting down hill) by switching the legs differentially. In either of those modes heat is produced as a consequence of current flowing through the transistors and the stator windings. If that heat is not wanted it must be dumped overboard and is thus wasted. The switching is thus done in such a way as to minimize the heat production. If heat is wanted all one has to do is switch them in such a way that losses go up. This can be done by switching less rapidly or switching on in common mode. The latter would be done in the stationary (preheat in the garage) scenario so that the stator gets warm but there is no rotating magnetic field.

My argument was adding a resistive heater to only be used when no other heat source is available. In the case presented (-20°C parked but plugged in) I doubt the heat pump would be much better than 1 COP. So heat pump would not be any better than resistive heater (at least not significantly).

The counter argument is that it wouldn't be any worse and that the transistors are in no practical way inferior to a separate electric heater so the electric heater is not needed.

My thought it that using a resistive heater ONLY in the case were no other heat source is readily making heat. I'm not seeing that as "going backwards", more of supplementing the move forward.

But other heat sources are available (transistors, stators) and demonstrably render the vehicle more efficient overall. and less expensive to build. Knowing this (and they do know this from experience) and continuing to use the less efficient, more expensive technology (separate resistive heaters for cabin and battery) would be, IMO, retrograde.

I guess another main reason I don't like using the inverter as a heater when it's not being used as an inverter is that seems like someone who builds a house somewhere that only has electric power. No gas, oil etc. for heating and cannot justify installing heat pump for one reason or another. If you had to build that house, would you install an actual electric home heating system like base board heat, or would you just heat the house by leaving the electric oven on. It is a heater. Again, not saying I'm right or wrong. Just explaining my reasoning for what I think.

Well let's suppose you have a house in Quebec where it gets pretty darn cold in the winter and you do install a heat pump. In fact lots of people do this. Knowing that it is going to get colder there than the heat pump can handle these systems always have backup which is usually in the form of an electric heating coil in the plenum. Given the origin of most of the population they love good food so tend to have gas ovens/stoves. Now let's suppose that the power goes off for 10 days (which it has been known to do). What would they use for backup? In fact the answer is almost universally a fireplace but suppose you don't live actually live there in the winter and are too lazy to drain the pipes. Who is going to tend to the fire? What kind of backup do you install for this case? Well you don't install any because you already have one - the stove/oven. It is more than sufficient to keep the house warm even in sub 0 weather. So my answer is yes, I'd use the oven/stove as a backup in such a situation and in fact did do exactly that one winter when the e-heat controller failed in my heat pump system during a long cold snap. There are other approaches to the problem available up there a common one of which is a supplemental free standing (i.e. can be moved from place to place as long as there is a gas outlet) gas heater with a battery operated themostat. I think you are offended by the idea of using the stove because it is intended to be used for cooking - not as a backup heat source though in fact it can be used as one. If I am right, you would prefer the special unit designed to be a heater.

The Tesla system is more like the system I have in Virginia which is a W/W heatpump. It never gets so cold here that backup is required for that reason but, of course, backup is required if the heat pump fails which it does every month or so. We get backup from our hot water heater. It is a "combi" unit with two output loops one of which goes to the domestic hot water load but the other can be switched to auxilliary coils in the air handlers. Thus it is, as are the inverter/rectifiers in the car, installed primarily to provide hot water for showers, the dish washer etc. But it is already plumbed into the house and can serve perfectly well as a backup heat source. Why not use it for that?

I do think using the inverter as a heat source even when the inverter isn't being used as an inverter may be the way Tesla went. But putting in small simple heaters for supplemental use only is also a possible solution they may have considered.

I think we can be very sure that it was considered carefully.

Even plugged in I would say the delay in heating the cabin because of the time required to heat up the thermal mass of the heating system would be a factor to consider. On a very cold day like the -20°C I suggested I would think it would easily take 5 minutes or more just to get the coolant etc. from -20°C to a usable temperature for warming up the cabin.

There is a video out there in which the presenter has a contest between an X and a Y to see which can defrost and warm the cold soaked car faster. To my surprise the Y wins.

Time required to fuel.

There is no way BEV will ever beat ICE on that. For nearly all practical use one can keep his battery between 40 and 60% and maximize it's life. In my current X that would imply 70 miles per day which was about 3 times my daily use when I was working and much more than my daily use now (especially with COVID). In the CT that would be 100 miles. But also note that 60% charge implies 175 mi range (assuming you don't want to go below 10%) in the X and 250 mi in the CT for the call in the middle of the night scenarios.

 

[TyPope]

I would bet I spend more time refueling my current truck than I will the CT. See, I'll spend approximately 5 seconds a day plugging in my CT where I spend 20 minutes a week refueling my F350. Now trips, that'll be a little different. Let's see, we currently spend about 35 minutes every about 250 miles with restroom stops/fuel stops. That's an average. We don't get fuel EVERY stop but almost every stop. I have four girls so... Anyway, if I can get the same 250 miles with the CT and IF I can recharge those 250 miles in 35 minutes, it would be a wash. It would be really cool if it recharged faster but personally, I'm looking forward to taking naps while the kids watch something on the big screen.

 

[ajdelange]

While ABRP defaults TriMotor consumption at 485 Wh/mi I have trouble believing it will be that high. The range extended X plus is rated at 258 which is 76% of ABRP's default for it of 340. Applying that same factor to ABRPs number for the CT would give 370 Wh/mi for the CT and that seems reasonable. The CT isn't going to be that much bigger/heavier than an X. Multiplying 370 by 250 miles says that you'd need to take on 92.5 kWh to replenish 250 miles range. At a 150 kWh charger that would take 37 min. if the charger delivered 150 kW the whole time. The chargers do taper but as the TriMotor battery will probably be about 200 kWh a 90 kWh charge is only 45% SoC and so you might be charging from 20% to 65% or 10% to 55% and your taper would not be that much so that I'd think 37 min is a reasonable estimate. Exceptional circumstances can, of course, arise but we have found our average super charging stop to be of about that duration which isn't really that different from how long we used to stop for petrol (fill up, rest room, burger, stretch legs, try to get the dogs to pee...). Those here (the majority) that haven't driven a BEV may find this hard to believe but traveling in one is pretty much the same, from the refueling perspective, as with an ICE car. Your options for places to stop and do it are fewer but so are available stops with gas if you want to stay on or near the freeways. Many of the CT's are on or near the freeways.

At home refueling is dramatically different. You never go to the gas station.

The time to think of refueling as a BEV shortcoming is past.

 

[ajdelange]

It dawned on me this morning that not all the transistors in a Telsa are in the inverter. There are others in the charger rectifier circuit too. These become an additional possible heat source when the vehicle is connected to shore power as when one wants to prewarm the car in the morning which clearly could be done without using battery energy. Depending on the versatility of the octavalve heat from the inverter could be sent to the battery and heat from the rectifier to the cabin or conversely. All speculation, of course.

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