Cold Weather Impact on EV range

[lphe]

Some Prius owners preheat the power split device to reduce the fluid friction when it's cold. It would be a significant boon if you could preheat the reduction gears in the Model S.

How do Prius owners preheat the power split device?

The following chart shows the transmission fluid temperature for a 60 mile commute in the winter at -5 F (blue line) and in the summer at 68 F (red line). In the winter, the temperature never gets above 100 F even with the ICE running to help warm it up. The transmission fluid still has significant viscosity at 100 F. So throughout the entire commute in the winter, transmission friction is a significant drag on mileage. I wonder what happens with pure EVs.

 

[jerry33]

How do Prius owners preheat the power split device?

They take some heating cables and wrap it around the transaxle. I've never done it myself as I don't live where it's cold enough to bother with. Just partially blocking the front grill was enough (use the foam pipe covers).

 

[lphe]

They take some heating cables and wrap it around the transaxle. I've never done it myself as I don't live where it's cold enough to bother with. Just partially blocking the front grill was enough (use the foam pipe covers).

I have attempted blocking the grille with foam pipe covers. I monitored all the temperatures in the car for my commute when it was -10 F with and without the covers. The pipe foam covers had absolutely no effect at that temperature. All the temperatures (transmission, cylinder head, heater core, motor, etc.) were identical for the two commutes.

 

[lphe]

The last item that has a big impact on EV range is cabin heating. This plot shows temperatures for my 60 mile commute at -2 F. The car isolates the coolant in the heater core from the ICE coolant when the ICE is off and uses an electric heater to heat that coolant, if necessary.

The red line is the ICE temperature. The blue line is the heater core temperature. The purple line is the temperature inside the car. I deliberately turned off the heater until the ICE had a chance to warm up to avoid wasting energy from the battery to heat the cabin.

When the ICE turns off, both the red line (ICE temperature) and blue line (heater core temperature) drop. The thing to note is how quickly the blue line drops, relative to the red line, when the coolant in the heater core is isolated from the ICE coolant. The car is taking in the cold outside air and warming it up by passing it through the heater core. It takes a lot of of energy to heat below 0 F outside air to 72 F. That's why the blue line drops so quickly. If I recirculate inside air then the red and blue line will basically coincide after the car warms up. But unfortunately, then the windows ice up from the humidity. If the outside temperature is above 10 F or so, then the red and blue lines also basically coincide even when recirculation is disabled. When it is below 0 F, it simply takes a tremendous amount of energy to heat outside air to cabin temperature.

There must be a more efficient way of heating the cabin than exclusively using cold outside air. Unfortunately, the recirculate button is either all or nothing. I would think they would make the climate control systems in a car more intelligent. The car should draw in only enough cold outside air to keep the humidity inside the car low enough to prevent the windows from icing up.

 

[kennybobby]

This is some excellent testing and a great presentation of the data that you have done, you are asking and trying to answer the technical questions that the curious folks want to know. Thank you for sharing.

i use a small space heater on AC power to preheat the cabin in winter, kinda kludgy but it helps knock off the kold.

 

[spottyq]

The last item that has a big impact on EV range is cabin heating. This plot shows temperatures for my 60 mile commute at -2 F. The car isolates the coolant in the heater core from the ICE coolant when the ICE is off and uses an electric heater to heat that coolant, if necessary.

The red line is the ICE temperature. The blue line is the heater core temperature. The purple line is the temperature inside the car. I deliberately turned off the heater until the ICE had a chance to warm up to avoid wasting energy from the battery to heat the cabin.

(snip)

Do you have any idea from where does the saw pattern from the ICE temperature and heater core comes from at the beginning of the graph, with the cabin heater set to off ?

I also wanted to thank you for logging and sharing all this data (sorry, I don't have rep points to give you ;-) ) !

 

[lphe]

Do you have any idea from where does the saw pattern from the ICE temperature and heater core comes from at the beginning of the graph, with the cabin heater set to off ?

I also wanted to thank you for logging and sharing all this data (sorry, I don't have rep points to give you ;-) ) !

The car is pulling in cold outside air through the heater core to warm it up and direct it into the cabin. That quickly cools down the heater core. When the ICE is off, the heater core is isolated from the ICE coolant. The electric heating element is then used to heat the coolant isolated in the heater core. The outside air quickly cools the heater core down. Before the electric heater comes on to heat the heater core, I restart the ICE. The car sends the hot coolant from the ICE into the heater core to warm it back up. It takes a tremendous amount of energy to heat subzero cold outside air.

 

[spottyq]

The car is pulling in cold outside air through the heater core to warm it up and direct it into the cabin. That quickly cools down the heater core. When the ICE is off, the heater core is isolated from the ICE coolant. The electric heating element is then used to heat the coolant isolated in the heater core. The outside air quickly cools the heater core down. Before the electric heater comes on to heat the heater core, I restart the ICE. The car sends the hot coolant from the ICE into the heater core to warm it back up. It takes a tremendous amount of energy to heat subzero cold outside air.

So, if I understand you correctly, you were purposefully turning the ICE on and off each 5min or so to maximize overall (gas+electricity) energy efficiency ?

 

[lphe]

So, if I understand you correctly, you were purposefully turning the ICE on and off each 5min or so to maximize overall (gas+electricity) energy efficiency ?

Yes--I don't want to waste energy from the battery to heat the car. When the heater core cools down, I let the ICE warm it back up rather than drawing power from the battery to warm the heater core with the electric heater. I need to run the ICE anyway for my 60 mile commute. I might as well use all of its waste heat to keep me warm and save the energy in the battery for EV. There is a switch in the car to choose hybrid or EV operation.

 

[spottyq]

Yes--I don't want to waste energy from the battery to heat the car. When the heater core cools down, I let the ICE warm it back up rather than drawing power from the battery to warm the heater core with the electric heater. I need to run the ICE anyway for my 60 mile commute. I might as well use all of its waste heat to keep me warm and save the energy in the battery for EV. There is a switch in the car to choose hybrid or EV operation.

Ok, it makes sense. Thanks again !

 

[Panoz]

A future Tesla owner begs to differ...

After seeing a video from a Chevy Volt owner, I do not agree that Tesla was wasteful in using resistance heating strips for cabin heat. The Volt uses a heat pump as you describe, and the car was unable to heat the cabin for one owner above 27 degrees on a cold Minnesota morning. A $100,000 car better keep me warm! Heat pumps have a limit to the amount of heat they produce, so I applaud Tesla's decision for resistance heating. The 2016 Volt has some change to the heating to make it better, so I conclude Tesla did the right thing.

IMHO, the vast majority of the increase results from the heating of the battery. The main evidence for this is that turning off the heat (for the cabin) makes a big difference in consumption. If we could turn off the heat to the battery (not a good idea to do routinely) I believe the difference would hardly be noticeable. Why do you claim "there is no heater for the battery in the car." There certainly is!

Are you able to prewarm your car for your return trip? Is that part of the trip included in your data? If you don't prewarm, my experience is that a huge amount of energy comes from the battery initially (or from the wall if you do prewarm). Even when you do prewarm, the car must still be maintained at a livable temperature of the battery, if not yourself, and the battery is out in the cold compared with the passengers, so it is significant.

The air conditioning load is minor compared with heating because the Model S uses a heat pump for that, but amazingly not for heating, so all heat is wastefully generated using resistance strips, one for the battery and one for the cabin. Tesla should make this easy fix, since knowing that your range is strongly temperature dependent, to an unknown degree, is the biggest single cause of range anxiety. Data such as you show here is a significant aid to reducing uncertainty, and hence anxiety, so thanks! However, keep in mind that you are not allowed to interfere with the management of the battery in a safe and healthy temperature range.

 

[doctorwho]

Heat pumps are more efficient at low temperatures if they use (ironically) CO2 as a coolant. This requires higher pressures though (apparently). I'm buying a domestic heat pump for a sub-alpine house and so we have settled on a CO2 heat pump

 

[dgpcolorado]

Heat pumps are more efficient at low temperatures if they use (ironically) CO2 as a coolant. This requires higher pressures though (apparently). I'm buying a domestic heat pump for a sub-alpine house and so we have settled on a CO2 heat pump

As I understand it, the effectiveness of a heat pump primarily depends on the reservoir of heat from which one is transferring it. In a household system it is usually a ground loop that benefits from the relatively constant temperature below ground even with cold weather above. This keeps the temperature difference between the house and the ground relatively narrow.

By contrast, a heat pump in a car uses the outside air as the heat source. If the outside air temperature (OAT) is too low it becomes difficult to transfer the heat from the outside to the inside of the car and the efficiency decreases to the point that it doesn't work anymore. Quite different from a house heat pump with a relatively stable ground temperature to draw from. While a more effective heat pump system might lower the useful OAT at which it could work, there are diminishing returns as the OAT drops. Eventually it becomes necessary to revert to something like like resistance heating as a backup.

 

[doctorwho]

Not all heat pumps are ground source. They probably need to be in the US or at least the parts of it that get very cold but if you use CO2 as the refrigerant you can have effective air exchange heat pumps at pretty low temperatures. Eventually, yes we would need a resistive element but most of the effective heating much of the year in much of the world could be done with a heat pump

 

[eloder]

I've never seen much point in a heat pump in EVs. By the time the heat pump stops working is about the same time when you actually start needing heat above heated seats / heating steering wheel.

 

[dgpcolorado]

I've never seen much point in a heat pump in EVs. By the time the heat pump stops working is about the same time when you actually start needing heat above heated seats / heating steering wheel.

That is my experience as well! But my impression is that people who live in mild climates tend to use cabin heat in situations where you or I wouldn't even think about it. So they benefit from heat pumps for their "cold" drives in 40ºF weather. I suppose it depends on each person's personal comfort zone.

 

[dgpcolorado]

Not all heat pumps are ground source. They probably need to be in the US or at least the parts of it that get very cold but if you use CO2 as the refrigerant you can have effective air exchange heat pumps at pretty low temperatures. Eventually, yes we would need a resistive element but most of the effective heating much of the year in much of the world could be done with a heat pump

In looking at the information in that link, it refers to water heating. The house heat pump systems used here are intended for large-scale space heating. Hence, the huge advantage of a ground loop for the heat source (since the ground never gets very cold and has a lot of thermal mass).

The FAQ about that system addresses the temperature question:

10. At what temperatures will the Sanden "Eco®" Hot Water Heat Pump System operate effectively? The Sanden HWHP will operate effectively between minus 10 and plus 43 degrees C, without the need for an electric booster element...

That seems to be roughly equivalent to current automobile air source heat pumps, which don't work well below -10ºC and need backup resistance heating. Perhaps the CO[SUB]2[/SUB] coolant system would be able to improve the efficiency somewhat, as you suggest, even if the effective temperature range is about the same. That could have some value. I'm not sure what coolant the current auto heat pumps use.

For moderately cold temperatures even the current automotive heat pump systems would be an improvement over resistance heating. But they won't help with very cold temperatures.

 

[doctorwho]

I can see why a heat pump would be an issue in some parts of Europe and the US in winter but for most trips most of the time an efficient reverse cycle airconditioner would be adequate at much lower power consumption

 

[RDoc]

I continue to think that a small diesel powered auxiliary heater that would warm the battery and interior is the best solution for winter use. It would be relatively inexpensive, solve most of the winter range problem, and emit less CO2 than resistance heating and about the same or less than a heat pump in the majority of cases.

 

[doctorwho]

I continue to think that a small diesel powered auxiliary heater that would warm the battery and interior is the best solution for winter use. It would be relatively inexpensive, solve most of the winter range problem, and emit less CO2 than resistance heating and about the same or less than a heat pump in the majority of cases.

In that case I'd prefer a fireplace and a nice log fire in the car:biggrin: