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Options for heat

3109 Views 14 Replies 8 Participants Last post by  major
I've been thinking about how to keep my EV ('94 Saturn SC2) heated in the winter. I like the idea of using the original heating components, just heating the coolant and using the normal controls.

I know I don't want to use cold outside air to mix in with the heater core to adjust them temperature. I plan to recirculate air only and use a variable speed water pump to adjust how much heat I get in the cabin.

An electric water heater is the simplest option, but it directly impacts range and I want to avoid this at all costs.

A gas/diesel/propane water heater would be compact and can effectively extract most of the energy from the fuel. However it goes against the purist's all-electric spirit and it requires periodic refilling.

Then I had a crazy brainstorm: Wax. Yes, wax. Standard everyday paraffin candle wax. No, I don't plan to burn it, I plan to melt it. I plan to melt about 50 lbs of candle wax while plugged in charging and extract the heat out of it during my commute.

The 50-lbs is a SWAG based on how much energy it takes to melt that much wax, about a 1KW electric heater running for 1 hour. I would extract all of that heat from the wax at about 160-180 degrees, whatever the melting point is for the wax I use. When the wax all solidifies the temp would then drop down below the wax's melting point. Hopefully my trip will be done before that and I will plug the car back in to charge and melt the wax again.

The plan would be to take a basic car radiator, enclose it in a plastic tank sized just right to hold 50-lbs of melted wax, which I figure to be about 8 gallons or so. The wax is a great insulator, so I want most of the melted wax close to radiator's fins or the wax will "freeze" onto the fins and the heat from the rest of the melted wax will not be able to get to the radiator's fins to heat the water to heat the heater core to heat the cabin air and ultimately to heat me. The radiator needs to be large enough to have 8 gallons of melted wax in a snug tank. Wax expands as it melts so the tank has to be able to flex a little. Thick insulation around the tank should keep heat loss minimal.

I would have an electrical heater in-line with the coolant system as well and as the car was charging the heater would heat the water up above the wax's melting point and the water pump would transfer that hot coolant to the radiator to melt the wax. The heater's thermostat could be set to 200 or so, like a nice warm engine.

In the morning the driver can turn on the car's heating system to heat the cabin up before disconnecting the charger and the electric heater. Then the heater only has to keep the cabin warm for the drive.

I know I can just carry an additional battery along with me that would hold 1 KWh or so of electric heat for the same weight as the wax, radiator, tank, pump and heater. Heck, I could probably carry 2 batteries! But what is the fun in that? Using wax as a "heat battery" just seems more cool to me, or should that be hot? :)

Wax would cost about $1.50 a pound, but I'm thinking of just recycling every old candle I can find instead. More eco-friendly to recycle old wax, I guess. I should be able to use the car's original radiator. I'm thinking of pouring the wax onto the radiator to solidify and make a mold around it in the shape I want for the tank. Then I can use something else to form a tank, a poured plastic resin, maybe? I can make a custom, sealed tank around the radiator and only leave the radiator's inlet and outlet. Next I would cure the tank, connect hoses and insulate the heck out of it.

If I go to all of this trouble it makes sense to just embed the heating elements inside the tank, right onto the aluminum radiator. Then I wouldn't even need to run the pump when I want to heat it up. I should embed the temperature probe for the thermostat in there as well, on the outside of the tank, to measure the wax's temperature and turn the heater of and on.

If I need extra heat on the trip I can run the heater off of the traction battery just as well, and I can do that also if I pull a generator-trailer for long trips, as I have dreams of doing one day.

So what do you think? Could it work?

Thanks for your input.

-- Paul
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What is the specific heat capacity of wax compared to water?

I would have thought that if you are going to use wax as a heat storage and transfer medium and then devise ways to prevent the solidifaction of the wax from preventing further heat transfer then surely preheated water would be as good?

You could have an insulated water tank, heated while charging, and then the water can be circulated, in a controlled fashion, to the heater matrix.

An alternative may be oil. Oil filled radiators are quite effective so using oil wouldn't have any need for anti-freeze or anti-corrosion additives.
I would have to agree with Woodsmith on this one, using oil as the heat storage medium seems like a better way. I have a detached garage without a furnace and I use a small propane heater to heat up the space, at the same time I turn on a 1500W oil heater that I use to maintain heat in the space. The oil heater won't heat the space from cold but it does a great job of giving off heat for a long time even after you turn it off.
The goal is to use the phase transformation of the wax to store the heat, the Heat of Fusion. Waxes and salts have been used this way for a long time.

I just did a little more research and Wikipedia lists the heat of fusion of paraffin wax as about 200 J/g, so to get 1KWh of heat (3,600,000 J) would require 18 Kg, or about 40 pounds of Wax. That is just the heat of fusion. If the melting point is 170 degrees it would take 1KWh of electric heat to melt solid wax at 169.9 degrees and raise it to 170.1 degrees.

For comparison, it would only take 0.5KWh to heat 40 pounds of water from just above freezing (32.1 degrees) to just below boiling (211.9). The heat of fusion is what stores the heat, just like how ice absorbs so much heat when it melts. The great thing about the wax is that it melts up at a temperature that works well for this application, 150-170 degrees or so. Also, it is cheap, environmentally safe, and easy to work with.
I did a quick read on wikipedia, looks interesting. Give it a try and let us know how it works. I was aware of the energy required to make the change while heating, but blissfully ignored the fact that energy would be released going the other way. I guess it comes down to being able to efficiently get the heat out of that system.

What happens if you are shopping or go somewhere and park where you can't recharge your heat will go away over time and you could be left with a chilly car for the drive home?
In theory that sounds good, and will be if you can make it work for you.

If you can make it work then there will be a lot more areas that you can apply it to. I hope it works for you.:)

I last heard about that process as a building wall board that contained waxes designed to absorb solar gain during the day and release it during the night. I'm not sure where that is up to and if it is commercially available yet.

For simplicty though, oil or water systems would be easier for most people to set up.
If the tank is well insulated the heat wouldn't go away, at least not the tank and the wax. The car and the rest of the heating system would, of course. But when it was turned back on the heat from the wax could heat the car up relatively quickly. And it is always still an option to turn the heater on using the traction battery. You would only want to heat it up to below the wax's melting point; no use using the car's energy to melt the wax. But that temp would still be plenty to keep the interior warm, just at the expense of range.

The Schatz "Heat Battery" does same kind of thing, but uses molten salts. It stores energy denser than the wax, about 50% more. It is also more expensive and I think this idea is easier and cheaper.

I've read about the wax in the wallboard idea too. For that application the idea is to get a wax with a melting point down around 72 degrees. The same principle though.
I think there are also more traditional ways that you can "pre-heat" the car off line voltage before starting your trip. Later on when I have more programing time I plan to have temperature sensors connected to the Arduino that will turn on heaters if the temperature is below a certain point at a certain time before I would be leaving.

It would really be a bunch of if statements in the code, if there is an A/C connection, if the temp is below a certain value at a certain time heaters turn on, I think it would take a lot less energy from the pack to maintain heat than to heat the fluid and the car from a cold state. (that's what I'm hoping atleast, since thats how I will be implementing my heating system)
u could heat up some large stones like they use to put in your bed... to keep warm... :D:D,

Then again, if you get scented may be worth it.... (just teasin):p
Just thinking out loud but if you object to burning diesel or other fossil fuels; have you considered burning vegtable oils in place of diesel? They are basically the same thing and very similar energy density. Any place diesel can be used, so can veg oil.
A wax heater-I wonder why nobody's used this before? Any idea how much durability your radiator will require for the wax during the melting and expansion phases?

Also, are you considering placing the radiator near the motor/controller area in order to capture the waste heat from that as well, or are you concentrating specifically on melting the wax during the Charge cycle? If you could capture the waste heat coming off the motor and controller (with a vent to re-route the heat after the wax reaches heat saturation so the components remain cool) you might save/release heat all day long while running the vehicle...
I don't know how far you are going on each trip, but I use an oil heater in
my car.
It is on a timer set to come on about 1 hour or so before I leave for work.

I can drive for 5 to 8 miles without gloves and have a clean windshield, while it gives off heat from the hot oil. A lot less mess than heating wax.

I used to work with wax a lot in my investment casting shop. It doesn't give off as much heat as oil. The wax cools from the outside in. The oil cools uniformly from the bottom up. In my wax press, heat transfer oil was used to heat the wax.
15 gals of oil to heat 1 gallon of wax, to 120 degrees. Not that efficient..

Once the sun is up, the car stays warm for more driving around town.

I put it in the back seat area and then remove it in the spring.

It has worked well for over 3 years.
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Re: radiator stress. I don't think that will be too much of an issue. Wax is not exactly hard, even cold. When it gets warm, near the melting point, it is very soft, so when it expands as it becomes liquid it will be able to push the mushy wax out of the way. As long as the "tank" is flexible, it should be fine.

I decided that a fiberglass tank would work best. I could lay the radiator on it's side with something flat below the fins and pour liquid wax in to fill up the spaces between the fins. It would be best if the radiator is very warm for this part. I would make the main heat-exchanger part just about a solid layer of wax. Then I could coat it all around with another 1/4" inch or so of wax. Then I would use the wax itself as a mold to wrap the fiberglass around to build a tank. Just like a "lost wax" method, except I'd keep the wax. :)

I also thought I could just line the radiator with electric heating elements. Then the radiator itself would be the heater and I wouldn't have to run the pump while melting the wax to store the heat. The aluminum is a great thermal conductor so the radiator would transfer the heat evenly to the whole tank. I'd have to run the probe for whatever thermostat I would use inside the "tank" as well. It would all be in one simple unit.

Re: oil heat and my trip. I am trying to get set up for a 1-hour commute to work. I would charge at work and then drive an hour home. I need more than a warm car to make it that far. I need something to generate heat. 40-lbs of wax can hold as much heat as running a 1-kw electric heater for an hour. That should suffice. If I do run low on heat and I have the battery capacity I can just wire the "heater-radiator-molten wax storage tank" up to the traction battery and heat that way. I just need to be sure the thermostat I use works on DC as well as AC. Or I can use the old-fashioned manual thermostat - a toggle switch and my finger. :)
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Oh yeah, capturing waste heat. I'd love to be able to capture that heat but I'd like to keep the controller well below 170 degrees.

I'm not sure how to best cool the motor. I think I'm going to go with forced air cooling. I'd build a little shroud around the air-inlet part of the motor, by the brushes. I'm running a Warp 7 motor. A fan would push ambient air (from the cowl maybe) through the motor and the warm air would exit the drive end as normal. I guess you could duct that end of the motor and run that air through a heat exchanger, or even pipe it right into the cabin, though I think the brushes give off ozone or something, right? I guess I could filter out the dust. I don't know what it would smell like.

I figure the losses from driving the fan would be more than made up by the increased efficiency of the motor, not to mention motor life. The other nice thing about that is that I plan to use some plastic panels to smooth out the underside of the car to reduce drag. That would mean less cool ambient air available to the motor in the engine compartment. Forced air cooling would alleviate that problem. I'd want to cool the controller that way as well, and the DC/DC converter.
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I've been thinking about how to keep my EV ('94 Saturn SC2) heated in the winter.
Hi Paul,

I like your idea about using phase change material. Good luck with it. Here is another idea. Just heat the part you really want to be warm instead of the whole cabin. I got an email with this link this morning.

I never saw this before. I do own a driver and flashlight which use that battery. It is very small and lightweight. Remarkable it can keep that jacket warm for 6 hours. And it is 12V, so could be adapted to the aux battery in the car.

I know :( It won't keep windows defrosted or your face and fingers warm. But interesting jacket. They probably have electric clothing for motorcycle riders. Good luck with the wax.

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