tesla500
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Why hot water heating?

Mon Nov 21, 2011 11:25 pm

Why did Nissan choose to use hot water based cabin heating? It seems to make much more sense to heat the air directly, and avoid wasting energy and time heating up the thermal mass of all that water.

My Pontiac Firefly electric conversion uses a 1.5kW ceramic direct-to-air heater, which is hot 10 seconds after turning the car on. The LEAF is a downgrade for me in this respect!

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GeekEV
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Re: Why hot water heating?

Mon Nov 21, 2011 11:34 pm

If I had to guess, I'd say it's so they can also capture some of the waste heat from the motor, inverter and battery pack. The water helps balance out what would otherwise be too inconsistent sources of heat.

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tesla500
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Re: Why hot water heating?

Mon Nov 21, 2011 11:39 pm

GeekEV wrote:If I had to guess, I'd say it's so they can also capture some of the waste heat from the motor, inverter and battery pack. The water helps balance out what would otherwise be too inconsistent sources of heat.
That makes sense, except the two cooling loops are separate. The motor/inverter loop cannot heat the cabin. In addition, the motor/inverter loop is usually so cold it couldn't produce any usable cabin heat. Feeling the temperature of the motor/inverter coolant after typical driving, there is no noticeable heating.

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hgoudey
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Re: Why hot water heating?

Tue Nov 22, 2011 12:34 am

Although admittedly casual speculation, I would guess that the choice of a water coupled loop is motivated by safety. They probably wanted to keep the 400 VDC high voltage and a hot heating element out of the passenger space because of potential fire and electrocution issues if something went wrong. Not that this couldn't be adequately designed for safety, I suspect they are just very conservative about such things.

I agree that functionally the water coupling is an inferior solution. It is travesty enough to fry precious Li watt-hours on an electric resistance heater, but at least one should be able to benefit from the instant heating potential. Instead we have to wait for heat just like an ICE car. I think small directed air heaters right in the vents to the windshield could potentially deliver faster and more efficient defogging of the glass.

I entertained the thought that the water loop was a way to scavenge waste heat dissipated from the motor/controller, but it doesn't make sense because the return temperature of the water after the heater core in the dash would likely exceed the desired temp to the drive system. In other words you can't get this heat without elevating the operating temperature of the drive system to the temperature of the return from the heater core, which is undesirable.

Howdy

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Ingineer
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Re: Why hot water heating?

Tue Nov 22, 2011 12:51 am

I think this is two-fold, one is indeed the safety concerns Howdy mentioned above, but the main reason was so they could re-use the standard off-the-shelf HVAC "block" they use in other cars without the need to engineer one specific to the Leaf.

The biggest disappointment is the lack of a heat-pump system. This would have enabled heating at a much lower energy cost, all for the addition of an $18 reversing valve (my cost at the local refrigeration supply house) and some software. Toyota has done it on the plug-in-Prius!

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cwerdna
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Re: Why hot water heating?

Tue Nov 22, 2011 2:28 am

Ingineer wrote: The biggest disappointment is the lack of a heat-pump system. This would have enabled heating at a much lower energy cost, all for the addition of an $18 reversing valve (my cost at the local refrigeration supply house) and some software. Toyota has done it on the plug-in-Prius!
Regarding the PiP, you sure about that on the production model? See http://priuschat.com/forums/prius-hybri ... ost1389049" onclick="window.open(this.href);return false;.

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vegastar
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Re: Why hot water heating?

Tue Nov 22, 2011 2:37 am

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.
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Stanton
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Re: Why hot water heating?

Tue Nov 22, 2011 8:04 am

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.
Now THAT'S interesting. I have to admit I didn't think I'd use the pre-heat/cool much, but I'm going to start doing it when it gets really cold (only in the AM when there's no "thermal" warming from sitting outside) because it's comfortable AND reduces energy usage on my way to work (probably due to what you described above).
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Re: Why hot water heating?

Tue Nov 22, 2011 9:26 am

tesla500 wrote:My Pontiac Firefly electric conversion uses a 1.5kW ceramic direct-to-air heater, which is hot 10 seconds after turning the car on. The LEAF is a downgrade for me in this respect!
I believe LEAF is 5 kW so maybe it would have to be three times the physical size and not integrate well.
5 kW vs 1.5 kW seems like a step up to me. I don't think the thermal mass of the coolent is all that huge to really get in the way.
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JimSouCal
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Re: Why hot water heating?

Tue Nov 22, 2011 9:33 am

Stanton wrote:
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.
Now THAT'S interesting. I have to admit I didn't think I'd use the pre-heat/cool much, but I'm going to start doing it when it gets really cold (only in the AM when there's no "thermal" warming from sitting outside) because it's comfortable AND reduces energy usage on my way to work (probably due to what you described above).
Water is like a thermal "flywheel" buffer. Makes sense to me... It's nice getting in the car with it warm. Why Mister LEAF seems to start charging when on ground power to accomplish this at the same time, well, that's in other threads....

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