Why hot water heating?

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!

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.

GeekEV said:

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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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.

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

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!

-Phil

Ingineer said:

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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Regarding the PiP, you sure about that on the production model? See http://priuschat.com/forums/prius-hybrid-ev-alt-fuel-news/97989-2012-toyota-prius-plug-in-15-mile-electric-vehicle-range-87-mpge-32-000-a-10.html#post1389049" onclick="window.open(this.href);return false;.

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.

vegastar said:

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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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).

tesla500 said:

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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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.

Stanton said:

vegastar said:

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.

Click to expand...

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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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....

cwerdna said:

Ingineer said:

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!

Click to expand...

Regarding the PiP, you sure about that on the production model? See http://priuschat.com/forums/prius-hybrid-ev-alt-fuel-news/97989-2012-toyota-prius-plug-in-15-mile-electric-vehicle-range-87-mpge-32-000-a-10.html#post1389049" onclick="window.open(this.href);return false;.

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Wow, you are right, according to Doug, they scrapped it for the Production models. I studied it thoroughly on the demos, it was a nice setup, and it's definitely not for the reasons Doug is claiming; Weight/Complexity? Weight is total B/S, it's just basically an aluminum reversing valve with a small solenoid. Complexity might be a partial truth; the real reason is probably to save a few $.

After all, in a car that will probably only drive 15-20 minutes on Electric, the cabin pre-heat is enough to keep it warm until the ICE kicks in. But in a BEV like the Leaf, it would be a great benefit!

-Phil

It was because the LEAF is not a ground-up EV as Nissan would like everyone to believe. They use many parts form ICE cars and trucks including the heat which is simply an ICE system with an inefficient and slow tank heating system. This is the EXACT same system used in an ICE conversion for years when one is too lazy or it is too difficult to use another type of heater. A heat pump would have been vastly better, far more efficient, and lower weight with fewer parts and not to mention faster. At the very least they could have designed it so it would not always come on when it's not needed, it's like a double hit on efficiency.

Ingineer said:

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!

Click to expand...

And where is this project on your growing and interesting list of potential improvements for development?! I'm guessing that labor for installation might be the biggest piece of this mod because opening and restoring the refrigerant loop requires equipment not found in most home garages. On a personal note, all of your efforts are applauded and of interest to this LEAF owner. Thanks for contributing!

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.

vegastar said:

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.

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The Think had a electric heating element like many EVs. it would heat in 10 seconds. That could be used in conjunction with a heat pump. Nissan also blew it by not making heated seats standard in all cars or at least an option, at a couple hundred watts total it would beat using the heater in many cases.

vegastar said:

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.

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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.

jkirkebo said:

vegastar said:

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.

Click to expand...

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...

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In automotive applications?

edatoakrun said:

jkirkebo said:

vegastar said:

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.

Click to expand...

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...

Click to expand...

In automotive applications?

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I don't see why automotive applications would be any different ? You can use more or less the same components.

Those new Heat Pumps that work well in sub freezing temperatures are very expensive and I don't know if they are possible in a space restricted area as an automobile. Of course if possible they should be used as long as the price can be kept low.

jkirkebo said:

I don't see why automotive applications would be any different ? You can use more or less the same components.

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It's not. It's all about how good your compressor is: Can you create enough suction to boil the refrigerant at the low ambient temperature? Can you also create enough pressure so the discharge gas is hot enough (via heat of compression) to do useful heating?

The limiting factor is usually the discharge temperature. As outside temperature drops the compressor is doing more and more work to keep the gas hot, and at some point if becomes difficult to keep things operating efficiently. The larger the the temperature difference, the larger the portion of energy the compressor contributes, and the COP drops like a stone.

A heat pump with electric element assist is the best option IMHO. The water in the system does provide some pre-heat buffer which is nice, but I'm not sure that was really the full intention plus it's extra weight you might not really need to carry. Get as far as you can with the heat pump and boost it with a direct-to-air heater element. Bonus points if you can scavenge any waste heat off of the motor/inverter system as input for the heat pump (would probably backfire in the cooling season, though!)
=Smidge=