stopcrazypp
Well-Known Member
In a house, maybe not, but we are talking about a car. There's a limited amount of space in the car to put a unit (as pointed out above, for a liquid cooled pack, both heat and cooling may be required at the same time).
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Stop saying it's not possible just because Tesla hasn't done it yet.
It is possible to install two compressors to allow pack cooling and cabin heating, or a plumbing/valve system that does both with one compressor. But to what end?
Assume:
Heater output of 10kBTU constant
250 W/mile drive only @ 65 MPH
10kBtu/ 3412 BTU/kw = 2.93kW for heat
65 MPH *250W/mi = 16.25 kW for motion
19.18 kW total
85% of power used for motion
Optimal heat pump case,
COP of 3
No aux heat
2.93kW/3 = 0.98 kW
17.2 kW total
94% used for motion
10% efficiency gain
So in the optimum situation for the heat pump, you get 10% higher efficiency.
This is useful if:
1. You need to make it 10% further to get to a charger/ destination
2. You like saving money/ being the most efficient
How much money?
Difference in heater power usage 2.93 kW-0.98 kW = 1.95 kW per hour
1.95 *2 hours a day *5 days a week * 4 weeks a month * 4 months a year * 5 years * $0.12 per kwH = $188
So if you live in cold (but not too cold) climate, and commute a long way (33k/yr) you save $188 over 5 years.
I'm guessing the extra plumbing, software and such costs more than $188. Even if it is free, if any of that extra plumbing requires an out of warranty (or even in, based on value of time), there is a net loss on the system.
So I would say, yes, a heat pump is more efficient, a heat pump is possible, but a heat pump is not worth it unless you need to squeeze the most range out of your battery pack.
stopcrazypp
Well-Known Member
Huh? Where did I say it was impossible (for example they can lose the frunk if necessary)? My point is just that the resistive heater might be "better" taking everything into account given the halving of capacity when in lower temperatures.
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10% more range? I'll take it. But you're still missing the obvious. Real life isn't a roadtrip. Try getting stuck in rush hour in city traffic twice a day, all of a sudden your HVAC costs become relatively huge compared to propulsion, not to mention cold startup costs, where your heater was running at 6kW for a significant fraction of time. Now range may not be an issue, but it's plain and simple a giant energy hog. Bad for the environment, bad for your electric bill. I've seen real consumption from the wall approaching 1200Wh/mi in these scenarios (can't rely on trip meter anymore, which is already reading ridiculously high)
LoL Rick
Like Buttah
It certainly can be done. Nissan did it with the Leaf, as have others.
My theory on why Tesla didn't do it is that the Model S was designed in 2009-11 when Tesla was a tiny startup with very limited resources. Many automotive suppliers refused to work with Tesla either because the lot sizes were too small or they were afraid of not getting paid. Things have changed since then, but I'm guessing that switching to a heat pump just hasn't risen high enough on the priority list to get done yet. After all, the battery is huge and not that many people complain about it. But supposedly Model 3 is a game changer and they are now getting tier 1 suppliers so maybe it will happen on the next refresh. Maybe Model 3 will even get a heat pump.
My theory on why Tesla didn't do it is that the Model S was designed in 2009-11 when Tesla was a tiny startup with very limited resources. Many automotive suppliers refused to work with Tesla either because the lot sizes were too small or they were afraid of not getting paid. Things have changed since then, but I'm guessing that switching to a heat pump just hasn't risen high enough on the priority list to get done yet. After all, the battery is huge and not that many people complain about it. But supposedly Model 3 is a game changer and they are now getting tier 1 suppliers so maybe it will happen on the next refresh. Maybe Model 3 will even get a heat pump.
If its cold enough out that cabin heat is needed it is highly unlikely that the drivetrain needs cooling beyond what is available from ambient air. If for some bizarre reason that situation did occur, the resistance heat could be used to heat the cabin while the A/C is used to cool the drivetrain.
I suppose the worst case is supercharging in winter after a leg on the freeway while waiting in the car (drinking a frozen coffee while wearing shorts). Not enough data to calculate if fan induced airflow would be sufficient.
Another thought on heat pumps in general. Domestic ones rely on the fact that if the outdoor unit freezes up, the system can shut off the outdoor fan and kick on the defrost coils. In an automotive situation, the airflow over the unit would prevent the ability to defrost, unless louvers are added to block airflow (more complexity/ cost), or the pack has enough surplus heat to divert to the coils to melt them (in which case, the more efficient thing would be to route the pack heat to the cabin).
Again, I like heat pumps, just working through the issues involved in their use for this application.
Well, me too. Aside from the need for simultaneous heating and cooling in some conditions, the largest issue, it seems to me, is the huge overcapacity required to get the net COP into the worthwhile range (remember, even a COP of *3* could save you only 10%) in cold weather. I'd love to see a heat pump that was *only* at 50% of its rated cooling capacity when heating from -20F -- does one of the people claiming that have a datasheet to share? In the real world it looks like it'll be more like 20% or even less.
That overcapacity doesn't just mean wasted space in the frunk and more cost. It also means a tremendous degree of short-cycling when cooling. That's uncomfortable for the passengers, very inefficient (quoted COP figures are *steady state* and ignore all startup costs), and very hard on the equipment -- so bad for reliability.
A moderately sized unit (perhaps 1.5X what's needed for the cooling load) combined, probably, with the exact same resistive heating capacity used today seems like the most likely solution in the real world.
SmartElectric
Active Member
Um. You have a "P" 90D, the 90D had 10% more range than your car, but you still bought the P90D. Clearly, you don't care about range as much as you protest... hmmm.
This thread is interesting. New points after 8 pages.
I never considered the de-ice cycle occasionally required.
Agree with above - it would likely be sized to match cooling requirements and still require equal resistance heat. But what would that provide as far as COP?
I replaced straight AC with heat pumps at home and the outdoor unit is 1.7x larger.
I never considered the de-ice cycle occasionally required.
Agree with above - it would likely be sized to match cooling requirements and still require equal resistance heat. But what would that provide as far as COP?
I replaced straight AC with heat pumps at home and the outdoor unit is 1.7x larger.
SmartElectric
Active Member
Thanks, good point!
Now, let's do a real highway drive at 78MPH
78 MPH * 300W/mi = 19.9 kW for motion for 65 miles
22.43 kW total
89% of power used for motion
So, I can drive faster, get to my destination sooner, and use only 5% more energy heating as a percentage of my drive.
Tesla's choices.
1. Focus on aerodynamics to reduce the cost of driving high speed in the winter when the air is more dense.
2. Big battery pack so you drive long distances with energy sapping electronics and climate controls
3. Heat and cool the battery pack to maintain capacity over long term
Vs. Nissan Leaf
1. Small battery pack
2. Climate control system with heat pump to save a few kWh over a two hour maximum drive
3. Do not thermally control pack and replace packs due to poor longevity
One of these is not like the other.
If you go fast enough, you don't need a heater, super-maximum-plaid mode (hold down T on display, them enter "SR71")
MDMGSO47
Member
"Why does Tesla use a Resistance Heater instead of Heat Pump"
Simple. Resistance heaters are ohm run! (;>)
Simple. Resistance heaters are ohm run! (;>)
Well, in cooling mode you'll get whatever the rated COP of the heat pump is, minus any losses from short-cycling. In heating mode, you're going to get the weighted average of 1 and the COP of the heat pump at any given input temperature, according to how many BTU come from each unit, like so (hypothetical but not unrealistic numbers - see Ductless Minisplits May Not Be As Efficient As We Thought for a useful discussion):
Assumptions:
- Nominal cooling output of heat pump is 12,000BTUh
- 10,000BTUh required to heat the cabin at 5F external temperature
- Output of heat pump at 5F is 6,000BTUh
- COP of heat pump at 5F is 1.5
Sounds about right to me.
When I bought it I didn't know I my consumption would be 1200Wh/mi on cold days.
jlarmstr
Member
Not to mention louder, when my AC is working hard the compressor really rumbles same would happen in reverse I suspect.
SmartElectric
Active Member
So you have a short commute then?!?
Otherwise you have less range than my Smart car! 90kWh divided by 1.2kWh per mile is less range than I get in my Smart Electric Drive. Your numbers have a certain truth to them...
Yes plus traffic, but you have the same problem any time you're doing "normal life" and not a roadtrip. Drive somewhere, stop, cold soak, start driving + huge consumption, stop, cold soak, repeat. Work to gym to grocery store, etc, etc.
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