[KC2TZB]

Although I took delivery of my (leased) 2012 volt in June, and haven't driven it in winter, that many report the Volt gets significantly lower miles-per charge with the cabin heater running.
What I don't understand is why GM would elect to use a resistive heating element (which are very inefficient) when they already have an electric A/C compressor that can be easily reversed and used as a heat pump.

One post here mentioned that the heater uses ~5000W full blast, whereas the A/C uses about 1500W full blast.
If the compressor was used as a heat pump, I would expect the power draw to be 1/8th of the A/C power draw to achieve the same temp difference (i.e. 10 degree increase with heat pump vs. 10 degree decrease with A/C)

Opinions? Comments?
Did the engineers at GM miss this one? Or is there a very specific reason they went with the current heating system?

 

[saghost]

First of all, both of those numbers are low. The electric heater has a nominal 6.5 kW rating, and I've seen the A/C pull close to 5kW (including the fans and idling power electronics.)

More to the point, though: The Volt is largely evolved from the EV1, with a big dose of 2-mode hybrid. The EV1 used a heat-pump for cabin heating. So while GM's engineers have never explained their reasoning, it wasn't a casually missed bet. My guess is that it's because of high humidity low temperature conditions - you can't both heat and dehumidify at the same time with a heat pump (you get a lower relative humidity from heating the air, of course, but when that hot wet air hits the cold windshield...) But it's just a guess.

 

[DonC]

Saghost makes a great point about the EV1. Hadn't thought of that.

Heated seats work well and they're so much more efficient than any form of heating that would heat the air that the most obvious move would be to heat all four seats.

 

[frankydude]

... And the steering wheel! It would be convenient to be able
To have the steering wheel heated. That would not cost much
and it would be energy efficient.

I agree wit kc2bsz: improving the design of the heating
system by the use of a heat pump - let's add a reversing valve-
would help a lot in colder weather.

Francois
B2653
2.3kW grid tied solar

 

[Cheezmo]

Nissan seems to be on the right track here. They already have heated seats and steering wheel and promote that as a more efficient way to heat. And, they are reportedly moving to a more efficient heat pump heating system in the MY2013.

I think it will come to the Volt too eventually.

 

[Steverino]

All EV's would benefit from more efficient cabin heating. So I expect cabin heating to be an area of innovation in the years to come as manufacturers look for ways to provide winter heat with less battery loss.

Heating the seats and steering wheel definitely reduce the need for heating the air. The air does not need to be heated if you body feels warm. For those who have ever enjoyed radiant heat floors, you know that forced air heating is not as comfortable. A similar effect in the Volt can be achieved by heating the car seat (how about a heated rear seat option with switchs?). A heater steering wheel would be comfy. Of course the window fogging is still an issue. I have *not* had good results with RainX Interior anti-fog.

I think part of the lower winter mileage may also be caused by resistance to movement (thickened oil, lube), as well as the ICE coming on to heat the battery pack due to exterior temps below 26 degrees F. The Volt does take advantage of the ICE heat in that situation to also heat the cabin.

Perhaps any excess heat from the ICE could be stored in a phase change material for gradual release while the ICE is off further reducing the need for resistance heating, if not the need for a fan? http://en.wikipedia.org/wiki/Phase-change_material

 

[jsmay311]

I think part of the lower winter mileage may also be caused by resistance to movement (thickened oil, lube), as well as the ICE coming on to heat the battery pack due to exterior temps below 26 degrees F.

The engine does NOT turn on in cold weather to heat the battery. It turns on to help heat the cabin.

There is no physical interaction between the engine and battery coolant loops. The battery uses its own energy to heat itself.

 

[Neromanceres]

Heat pumps and air conditioners work on the carnot heat cycle. Basically these are not heating or cooling units but move energy inside or outside the car. This is why they can be efficient. Technically a resistive heater is 100% efficient because all of the energy is converted to heat. But a heat pump doesn't generate heat. It moves it.

The problem is it takes energy to move energy. For an AC unit you are trying to move energy from an environment that is ~70F to an environment that is ~100F. (a 30 degree differential). So the carnot cycle works well in this case. But for a heater in a car you need to move energy from an environment that could be -22F to an environment of ~70F (a 90 degree differential). To move energy across this big of a differential in a car with a heat pump is likely not efficient (if even possible) and at such a large difference resistive heating might even be more efficient.

I do suppose at some point a heat pump could be used when the cabin only needs a little heating (ambeint temps above ~30F) and then a resistive heater kicks in a lower temps (or the engine kicks on to generate the heat when the temperature is low enough that a heat pump is no longer efficient).

 

[Steverino]

The engine does NOT turn on in cold weather to heat the battery. It turns on to help heat the cabin.

There is no physical interaction between the engine and battery coolant loops. The battery uses its own energy to heat itself.

Hmm. Why do you suppose the ICE needs to start up to only heat its own coolant if you are using battery only to propel the car?

 

[saghost]

Hmm. Why do you suppose the ICE needs to start up to only heat its own coolant if you are using battery only to propel the car?

The conventional wisdom is that it's to ensure the car has enough heating power for the defroster to keep the windows clear. GM hasn't been forthcoming on the actual reason.

 

[hvacman]

Here's a link to the complete explanation to the Volt's heating and cooling systems:

http://gm-volt.com/2010/12/09/the-chevrolet-volt-coolingheating-systems-explained/

If the cooling system were a simple one-zone system, it would be a no-brainer to make it a heat pump. We do it all the time in the HVAC industry. You just change the expansion valve to a two-way reversible expansion valve and add a defrost cycle system to defrost the outdoor coil when ice builds up on it (low outdoor air temp with high humidity conditions). Air-to-air Heat pump technology has progressed to where they can now efficiently heat down to zero degrees F with a COP of about 1.8 (1.8 watts of heat out per watt energy in). At milder temps, the COP's can be as high as 4-5.

Air-source heat pumps don't work at all below zero deg. F, so even with the best heat pumps, an EV would necessarily require a supplemental heating system, either engine-based (if EREV) or electric-resistance based, for extreme conditions.

My instinct is that they kept the Volt's compressor system cooling-only because the Volt cooling system is already asked to do so much. It must be able to cool the battery via a water-cooled circuit, cool the cabin via an air-based circuit, or both. Multi-zoning really complicates the refrigerant flow patterns. Adding reversible refrigerant flows with the multi-zone concept can be done (refer to Mitsubishi's Multi-City variable-refrigerant-flow (VRF) system but it is very complex and probably would have taken more development time than GM had for gen-1.

http://www.mitsubishipro.com/en/professional/overview-and-benefits)

If the Gen 2 or 3 Voltecs can simplify/eliminate battery cooling, then heat pumps will probably become the standard cabin heating/cooling practice, paired with heated seats/heating coil from the engine cooling system as the final heating stage.

 

[Steverino]

The conventional wisdom is that it's to ensure the car has enough heating power for the defroster to keep the windows clear. GM hasn't been forthcoming on the actual reason.

I found this posted by Scottf:

http://gm-volt.com/forum/archive/index.php/t-8210.html

"At ambient temperatures of approximately 25° F (–4° C) or below, the ICE will operate to assist in warming the coolant used to warm the vehicle’s cabin. After the coolant temperature reaches approximately 150° F (65° C), the ICE will turn off and remain off (as long as the battery state of charge is sufficient) until the engine coolant temperature drops to approximately 104° F (40° C), at which time the ICE will restart. This function will continue as long as the ambient temperature is approximately 25° F (–4° C) or less while driving. After the ambient temperature rises above approximately 25° F (–4° C), the vehicle will resume normal electric operation (as long as the battery state of charge is sufficient). "

He got that from a GM Techlink article (PDF): http://www.sandyblogs.com/techlink/Jan%202011%20Techlink%20F.pdf

I mistakenly thought there was a heat exchanger being used between the ICE and the battery TMS. That's why I love this forum, it's educational

Since cooler weather is approaching, some may be interested in this related discussion about the use of Fan, Eco, and Comfort http://gm-volt.com/forum/showthread...-low-temperature-quot-ERDTLT&highlight=ERDTLT

Excellent link to the heating cooling system diagram's hvacman!

 

[Voltarian]

Maybe in 2015

http://delphi.com/about/news/media/pressReleases/pr_2012_04_25_006/

 

[frankydude]

As Voltarian writes, it is planned for the 2015 MY.
There is an SAE technical paper that can also be downloaded directly from delphi's website:
http://delphi.com/pdf/techpapers/2012-01-1050.pdf
The paper describes more in-depth technical details of the design of the system.
The abstract is also interesting:
The Unitary HPAC (Heat Pump Air Conditioner) System has been developed to enable a heat pump system in
passenger vehicles. Unitary HPAC uses technology of reversing the coolant instead of refrigerant to distribute heat from
where it is generated to where it is needed. Integrating this system in a plug-in hybrid vehicle reduces the energy required
by the heating and air conditioning system, reducing the grams of CO2 per mile by up to 25%.

So, I conclude that it was not included in Volt generation 1 because the engineering for an heat pump wasn't ready for
prime time / production. I think we might see this get included in generation 2. (I hope for, as this is really an enhencement
for cold climate Volts).

Francois
B2653
2.3kW grid tied solar.

 

[Bob G]

The resistive heater sucks battery like nothing else! It is definitely on my short list of disappointments with my Volt. I don't understand why, with waste heat available from the battery, inverter, and motor, as well as an air conditioning pump that could also be used as a heat pump, and gasoline that could be used in a combustion heater, the Volt uses a simple resistive heating element instead.

Last winter, my AER went from 24 miles one day to 30 the next simply by changing the heat from "comfort" to "fan only" + seat heat + warm coat, hat and gloves. I don't know if these results are typical (because I have been afraid to turn on the heater ever since), but maybe I'll develop the courage try it again this winter.

If a heat pump is available when my lease expires in 2015, it will be a strong selling point for a new Volt.

 

[KC2TZB]

Hi all,

Some good points mentioned here.

While I tend to agree that a heat pump may not be great in colder <25F temps, the ICE will turn on regardless to heat the cabin.
I think that the heat pump would still work best for ~90% of volt owners who do not regularly see temps below 25F for more than a total of a few weeks in winter.

What GM could do is to have a resistive heater in addition to the heat-pump to supplement it in extreme conditions.

I know that with the A/C on 75degrees in "eco" mode, I can easily achieve ~50 miles of evenly mixed city/highway driving when ambient temps are about 95F

So it stands to reason that the heat pump, which is at least 8 times as efficient, would be able to heat the battery, and cabin to 75F from 35F ambient temps and I would still be able to achieve close to 50 miles per charge. Maybe they can use it to heat some of the other oils/lubricants as well! (although at greater cost/complexity)

And what about a peltier cooler/heater to keep the car cool/warm once the compressor has brought the cabin temp within an acceptable range? Even though it's not as efficient, you can use 200W to run the peltier vs. cycling the compressor on and off.