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DaveEV
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Re: Why hot water heating?

Tue Nov 22, 2011 12:44 pm

jkirkebo wrote:
vegastar wrote:For really cold weathers a heat pump alone would not work. The best would be a heat pump and the current system. Up to 0ºC (32ºF) a heat pump is enough and very efficient, below that it would be necessary a resistive heating element.
Modern heat pumps work well down to at least 0F/-18C. For example the newest Toshibas has a COP of between 2,07 and 2,25 at -15C (5F). So you'd need about 2kW input to get 4.5kW output in 5F weather. A big improvement IMHO. The 5kW resistive element should be retained of course, it can get colder than -40 (C or F) over here some places.
Definitely with blended mode operation of the PiP the need for a heat-pump is less unless your drive is within the 15 mi EV range and you are able to keep speeds and acceleration rate low in cold weather.

There's definitely a bit of complexity to use a heat-pump - no matter what you still need to retain the resistance heater:

1. The condenser tends to ice up so you need to run it in reverse to defrost it periodically - Think of driving through rain in low, but not freezing temps - water will collect on the HVAC condenser and then freeze since you are pulling heat from it. To de-ice the condenser to you need to run the HVAC in AC/mode (pump some heat back out cooling inside air) which means unless you are ready to be blasted by freezing air, you better have backup heat. Driving when it's snowing you'd have the same issue - condenser will quickly clog up requiring it to be de-iced frequently.
2. As mentioned, efficiency drops off at low temps (a lot of the reason is energy used by de-icing activity), so you may want/need resistance heat for backup.
3. Heat pumps typically aren't able to generate as much of a temperature differential and will require more volume of air for high efficiency, so when warming up the car unless you like cold air blowing around you're going to have to run resistance heat at the same time, anyway.

Most of these issues could probably be worked around by automatically blending in resistance heat based on environmental sensors. You'd also want to be able to figure out how to detect when the condenser is iced up accurately so you only have run in de-ice mode as little as possible.

tesla500
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Re: Why hot water heating?

Tue Nov 22, 2011 3:09 pm

drees wrote: There's definitely a bit of complexity to use a heat-pump - no matter what you still need to retain the resistance heater:

1. The condenser tends to ice up so you need to run it in reverse to defrost it periodically - Think of driving through rain in low, but not freezing temps - water will collect on the HVAC condenser and then freeze since you are pulling heat from it. To de-ice the condenser to you need to run the HVAC in AC/mode (pump some heat back out cooling inside air) which means unless you are ready to be blasted by freezing air, you better have backup heat. Driving when it's snowing you'd have the same issue - condenser will quickly clog up requiring it to be de-iced frequently.
2. As mentioned, efficiency drops off at low temps (a lot of the reason is energy used by de-icing activity), so you may want/need resistance heat for backup.
3. Heat pumps typically aren't able to generate as much of a temperature differential and will require more volume of air for high efficiency, so when warming up the car unless you like cold air blowing around you're going to have to run resistance heat at the same time, anyway.

Most of these issues could probably be worked around by automatically blending in resistance heat based on environmental sensors. You'd also want to be able to figure out how to detect when the condenser is iced up accurately so you only have run in de-ice mode as little as possible.
Automotive heat pumps are certainly practical. The EV1 had a heat pump (plus resistance heat of course).

Defrosting is usually done once the temperature of the refrigerant exiting evaporator is some amount below ambient, indicating the coils are frozen. On home heating heat pumps, the outdoor fan is stopped so the coils can heat and melt the ice. In a vehicle, the airflow from vehicle motion may impede this heating, so a closeable front grill (like the Volt has) may be required to stop the airflow during defrost. Alternately, if you were clever and ran the radiator fans backwards at the right speed, you could possibly stop the air flow that way, without adding much cost.

The COP of a heat pump should theoretically never be less than 1, so it should always be more efficient than resistance heat. The amount of heat available will be less at low tempratures however. Sizing the heat pump properly is critical.

There's misinformation going around that heat pumps won't work below 0C. During a cold snap to -10C last year (yeah, that's not "cold"), my home heat pump had no problem keeping the house at 20C. The auxiliary electric resistance heat was only ever used during defrost.

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Re: Why hot water heating?

Tue Nov 22, 2011 4:02 pm

sounds like heat pumps in cars are complicated..

tesla500
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Re: Why hot water heating?

Tue Nov 22, 2011 5:33 pm

Herm wrote:sounds like heat pumps in cars are complicated..
Nothing compared to an internal combustion engine.

GaslessInSeattle
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Re: Why hot water heating?

Tue Nov 22, 2011 6:01 pm

vegastar wrote:One reason to heat the water and not the air directly is to store the heat when pre-heating the cabin from the wall. I notice that when I pre-heat the cabin, the heater only starts to draw energy from the battery after 10-15 minutes of driving, while the air from the outside is still heated from the hot water.
You might be on to something there! I think Nissan needs to come up with an explanation for how they expect the heating system to be used by us. I fear I'm over thinking it and may be working against it's design but for the life of me can't figure out why it's designed the way it is... if preheating is really adequate to save the bulk of the energy lost to cold whether driving from the CC then I should stop messing with it and just preheat every time I go out and let it do it's thing. Just about everything else about the car has been very carefully thought through, it's hard to believe there isn't a good reason for it being designed the way it is.
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Re: Why hot water heating?

Tue Nov 22, 2011 9:01 pm

GaslessInSeattle wrote: if preheating is really adequate to save the bulk of the energy lost to cold whether driving from the CC then I should stop messing with it and just preheat every time I go out
Why would you not want to preheat?

tesla500
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Re: Why hot water heating?

Tue Nov 22, 2011 10:26 pm

Herm wrote:
GaslessInSeattle wrote: if preheating is really adequate to save the bulk of the energy lost to cold whether driving from the CC then I should stop messing with it and just preheat every time I go out
Why would you not want to preheat?
Preheating when you're plugged in is a no-brainer. The cost of the energy is miniscule, and it comes straight from the wall. Preheating when you're not plugged in is a different story. If you want heat quickly, you really have to, but you'll loose a little range.

GaslessInSeattle
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Re: Why hot water heating?

Tue Nov 22, 2011 11:26 pm

Herm wrote:
GaslessInSeattle wrote: if preheating is really adequate to save the bulk of the energy lost to cold whether driving from the CC then I should stop messing with it and just preheat every time I go out
Why would you not want to preheat?
I've been more concerned about range than comfort and haven't really been aware that preheating heats up a reserve in the system, that will motivate me to preheat more ahead of time... though to get defrost to work recirculate shuts off and outside air comes in... seems like the heat dissipates pretty quickly... I'll see how it goes with longer preheats.
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TimLee
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Re: Why hot water heating?

Wed Nov 23, 2011 12:23 am

drees wrote:
There's definitely a bit of complexity to use a heat-pump - no matter what you still need to retain the resistance heater:

1. The condenser tends to ice up so you need to run it in reverse to defrost it periodically - Think of driving through rain in low, but not freezing temps - water will collect on the HVAC condenser and then freeze since you are pulling heat from it. To de-ice the condenser to you need to run the HVAC in AC/mode (pump some heat back out cooling inside air) which means unless you are ready to be blasted by freezing air, you better have backup heat. Driving when it's snowing you'd have the same issue - condenser will quickly clog up requiring it to be de-iced frequently.
2. As mentioned, efficiency drops off at low temps (a lot of the reason is energy used by de-icing activity), so you may want/need resistance heat for backup.
3. Heat pumps typically aren't able to generate as much of a temperature differential and will require more volume of air for high efficiency, so when warming up the car unless you like cold air blowing around you're going to have to run resistance heat at the same time, anyway.

Most of these issues could probably be worked around by automatically blending in resistance heat based on environmental sensors. You'd also want to be able to figure out how to detect when the condenser is iced up accurately so you only have run in de-ice mode as little as possible.
Excellent input drees.
Everything about the Leaf 2011 design is driven by two things. Cost and the limited amount of time they had to rapidly get the product to market. Adding heat pump capability to the AC on the Leaf could have been done. But as you point out, it is quite a bit more complicated than just an $18 reversing valve and a bit of software. Nissan could have done it, probably for $200 to $300 dollar net cost, but I don't think they really had time. And if Toyota didn't include it in the production plug-in Prius, it may be that for the OEM vehicle manufacturer it just doesn't meet a sufficient threshold of Benefit versus Cost.

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DaveEV
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Re: Why hot water heating?

Wed Nov 23, 2011 1:04 am

TimLee wrote:And if Toyota didn't include it in the production plug-in Prius, it may be that for the OEM vehicle manufacturer it just doesn't meet a sufficient threshold of Benefit versus Cost.
No mass-market plug-in on the road has it - not the Volt, not the iMiev, not the Focus EV... All those engineers surely couldn't be overlooking something simple to improve range in the cold when they all know that cold weather performance can be a big issue for plug-ins.

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