Heat storage system (Prius-like insulated coolant reservoir)
 

Hello, this is my first post here on EcoModder, even though I've been a member for quite a while now. I'm from Finland and the winters here are quite cold. Temperatures of -20 C (-4 F) and colder are common in the wintertime.

I started working on a heat storage device for my Fiat Uno and would like to share my ideas and maybe get some good feedback. The Uno is a Italian supermini-class car and its 1100 cc engine doesn't warm up as fast as I would hope.

The heat storage system consists of a 10 liter (2.6 gal) coolant reservoir packed in a styrofoam insulation box with a couple of pumps. Coolant transfer is done in insulated hoses that run from the engine bay into the trunk. The engine + heater holds about 4 liters(1+ gal) of water while the whole original cooling system capacity is 6 liters (1.6 gal). The system uses waters high specific heat to store energy for warming an engine faster during cold starts. Works best when used with a core heater but results should be seen even without.


But how does it work?

As the thermostat opens, the IN pump in the reservoir starts to suck hot coolant into the reservoir while the OUT pump is pumps coolant from the bottom of the reservoir out into the water pump. For a while this works like a radiator, taking heat from the engine, but unlike a radiator the valuable heat is not blown away into the atmosphere but stored for future use. When the reservoir reaches a given temperature (75+ C/ 170+ F) it starts to pump slowly just to keep the temperature hot.

As the car is parked after a drive the engine starts to cool rapidly, but the coolant in the insulated box does not. If the engine is fully warmed up and the trip is long enough, temperatures at the time of parking in the reservoir are about 80-85 C (~180 F).

The next time the car is started the temperature of the engine and the coolant have dropped significantly. The temperature of the coolant in the reservoir depends on the time of parking but as my tests have shown, after a 8 hour work shift the temperature is about 55-60 C. This depends on the insulation. The cold engine starts to receive hot coolant from the reservoir which is mixed into the cold coolant (to make temperature rise consistant over entire engine and prevent cracking). The pumping stops when the engine and reservoir temperatures are the same. The cycle starts over when the thermostat opens.

The system is still in a beta stage with temperature probes being calibrated and the best locations for input and output hoses to the engine under consideration.

So what do you think? :)

I couldn't post images, maybe you can see the attachments.

WOW! :eek:
I believe that you are the first DIYer to do this successfully:thumbup:
My comments:

• 10 liters is quite heavy for a car like the Uno. You'd probably feel the extra weight in city driving.
• Could the 10-liter tank fit into the engine bay? This would not only reduce the length of plumping needed, but also any heat which escapes through the insulation would warm the engine bay.
• Maybe less water and more insulation? The 2nd-gen Prius has a thermos for coolant which is only 3 liters, iirc.
• Do you have any kind of insulation in the engine bay? Maybe a hood blanket when parked?
• I assume that your grille is blocked.

And welcome to EcoModder:)

What's the control system like? The simplest one would probably be a valve that is open the whole time the car is on. That way you get warmed-up coolant right away, and the coolant is always circulating (and presumably getting re-warmed after a little time) until you shut the car off. Might be interesting with longer periods of engine-off coasting.

The concerns about thermal stress cracking the block are probably worth worrying about, especially if you just have the valve open or closed. Let's see. Perhaps a small valve opening at first, followed up within minutes by full opening? Or perhaps seconds?

Sorry, random thoughts bouncing around here on the subject.

-soD

Thanks, the more water = more energy. 10 liter isn't actually enough :(. Energy is needed to heat the engine block too. The engine weight is about 100 kg which is mostly iron that has specific heat capacity of about 1/9 of water. Not calculating the oils and such materials.

I would have put more, but the plumbing to the back needs to be of small diameter to stop heat loss. => The pumps cant generate enough pressure to pump all of the water though in time.

The hood is factory insulated... There is also a small factory belly pan. The tank could fit into the engine bay but the box is about 30 liters in size and i would have to make the insulation thinner to make it fit too. There is also the possibility of the styrofoam catching fire when the engine is hot.

The control system as of now is still me ;). I can see the temperatures on my gauge and control the pumps manually. I have planned to put a Attiny 85 to do the job for me but I first need to experiment. The pumps run at the same time for now so i don't get a lot of pressure or level change in the reservoir.

As to the cracking issue, the hot water is mixed with the cold water before getting into the block. The picture might be a bit misleading.

The weight increase is still less than 2% of the cars weight so I think it won't affect my mileage seriously. This is also a "luxury" mod because I can start driving almost straight away without waiting for the windows to thaw. And running rich for a few minutes less will surely have an effect.

Nice work! :thumbup:

I wouldn't worry about cracking; for one the temp differential isn't that great and for another cracking comes from a big thermal shock of the sort when cold water hits a very hot part- it doesn't seem to work that way the other way around (hot water on cold part).

Since you have the room I'd suggest more blanketing of the engine compartment. That's what I've done to good effect.

How about filling your heat storage tank with something like paraffin wax instead of water then having a closed coil of tubing in the wax, as I understand it paraffin wax holds a lot more heat in the phase change of going from a solid to a liquid and the temps that you are looking at seem about right for wax heat storage.
I also think that if there is space that it would be worth having the storage tank in the cabin of the car instead of in the trunk, that way any heat loss is heating and defrosting the inside of the car and the trunk has less insulation then the cabin.

Run a T connection to both of your heater hoses. Run the hoses from the T connectors to your insulated reservoir. Install a mechanical or electronic heater control valve in either the supply of return line (makes no difference and may prove unnecessary).

When you start your engine the heater supply and return hoses are not controlled by the thermostat and your water pump provides the pressure to move the coolant. When you turn the engine off you can shut off the coolant flow to the reservoir with the control valve. It may not even be necessary to kill the coolant flow as it will stop when the water pump stops moving coolant.

On restart open the valve (if installed) which allows the stored hot coolant to flow directly into the engine which gives you faster warm up time (even faster with the thermostat closed). As long as your reservoir is capable of handling the cooling system pressure it will work automatically and coolant will flow since the thermostat will be closed until the engine gets to operating temperature.

Not meant to be a criticism of the excellent idea of the OP, just a simpler system. It would eliminate the necessity to add additional pumps and the system would work using the flow created by your water pump.

regards
Mech

You have nothing to worry about cracking anything engine wise.

regards
Mech

The heater hoses move coolant the instant you start your engine. They are placed on opposite ends of the engine block so you might try just a reservoir connected by T connectors to the heater supply and return hoses. I would bet you could make it a completely passive system with no need for any valves or pumps. The heater hoses bypass the thermostat and will not affect it's operation in any way since they already bypass the radiator. It might even make your heater system work instantly after the engine and the rest of the cooling system have become cold from sitting!

Great Idea, truly innovative!

I always admired the ingenuity of the Finnish people. I also collect their weapons from the WW2 era. My hat is always off in Salute to the toughness of the Finnish people.

regards
Mech

That's a good idea. It would have to have a electronic valve to monitor the temperature in the tank so it wouldn't circulate if it's too cold. It would mean a bit slower heat output to the cabin as hot coolant to the heater would come through the cold engine block at startup. I modified the "blueprints" a bit. I like drawing in PaintCAD. :p

As such, coolant would only flow when:

-Temperature of tank hotter than engine
-Engine is at normal operating temperature (thermostat is open)

This would rob some hot coolant from the radiator, but given the flow speed and size of the interior, I don't know if it will be noticeable.

I would rather have the ability to heat the coolant in the tank with household current in the case where the vehicle sat long enough for the tank to cool off completely. A hot water heater element would work fine with household current. I also like Ryland's idea of a coil of copper tubing inside the insulated vessel.

regards
Mech

Another reason that I see for having just a copper coil in the heat storage tank is that thermal expansion of the coolant is going to over fill your over flow bottle next to the radiator if you have 10 litters/2.6 gallons more fluid in a system that is designed for around 3 litters total.

The wax would eliminate the possibility of a bad leakage in case something goes wrong. How much does it expand when it melts? Heat conducting might be a problem also, the coil might have to have fins, and I'm not sure about how evenly the heat spreads in the wax. The heat needs to move fast, if it takes too long the efficiency will suffer.

Coolant expansion might be a problem. With the pump system the tank would have been almost non-pressurised and it would be isolated from the engine when pumps were not operating.

Electric heating is an option and could be wired to the existing block heater wiring. The system must be operational and not hinder the heating system even if there is no electricity available though.

If there's an existing block heater I don't see the point of heating up all that extra with electricity.

Keeping Engine Coolant Hot
Phase-change material for down-sizing cooling system
Schatz heat battery

Larger bottle or just lower cold coolant level. As long as the hose is not above the coolant level you have no problem.

regards
Mech

Counterpoint;

If you heat the coolant in an insulated container, you will retain that heat for longer periods of time. A normal block heater is fighting the heat being radiated to the atmosphere. At 40 below (doesn't matter centigrade and fahrenheit are the same at 40 below).

Even if you have the luxury of a heated garage, insulated heat retention vessel will be much better at preventing heat and electrical energy losses.

I really like this idea, especially where it really gets cold, like places where your temp gauge drops to dead cold in 15 minutes. I would like to try it on my Maxima but there are space issues and the heater hoses are buried behind the rear cylinder bank. Hehe maybe the F150 if I keep it. Aluminum blocks really cool off quick, so the F150, which is cast iron won't cool off as fast as the Max which is all aluminum.

regards
Mech

I've often wondered if I could steal a prius bottle and retrofit it into my cobalt, heck retrofit one or two into every vehicle.

Not sure how I would coordinate its operation though.

Well in my case I have a grille block and an engine blanket so the engine compartment is kind of like an upside down, open box. In the case of the F150 the engine "blanket" is 1" foil faced foam, foil facing down. Couple that with me only plugging it in for an hour before use, and I doubt there is just too much electricity and heat being wasted in addition to the benefit of not adding weighty, voluminous equipment to the vehicle.

That said, I like the storage Thermos but feel it is more efficiently used as a means of utilizing waste heat, not a place to pump more electrical energy into.

And it would compliment everything you are presently using, either as a totally passive system or with outside additional heating capacity. Personally I like the simplest passive system.

regards
Mech

Frank and you both have this right, IMO. No worries at all. I think the whole thing can be totally passive on the heater hose circuit without pumps or controls. If you want an external heat source for really cold overnights, you can place a tank-type heater in a loop on your tank system, on a timer.
It won't heat your engine of you place it right, just your tank.
TANK TYPE ENGINE COOLANT BLOCK HEATER 1500W DIESEL | eBay

Passive system (no extra pumps) with one electric valve is minimum I think.
About heat-storage - probably best would be to put second heater core into insulated container and fill container with wax (keep some air room for expansion)

If you add electric pump then you can release that heat before starting a car. But first you can probably get away without it.

What a great idea! I'm sure BMW did something similar but they used a material similar to the bricks you get in overnight electric storage heaters to store the heat. This might give you a more compact setup.

Thanks for the good ideas! I'm currently fabricating parts for my next attempt at a "passive" system. It does still have a valve controlled by a microcontroller based on two temperature sensors. Anyone have ideas on cheap electric valves that can take the pressure of a cooling system (1-2 bar max.)? I have some car vacuum solenoids, but I'm yet to test how they hold up to coolant and pressure. They are also quite small so they may resist flow. As for heat storage, I'm still staying with water. With no additional material in between the heat transfer is faster and this is key to the efficiency of the system. It's also cheaper to make. ;)

"Bricks?" What sort of bricks?

Search ebay for "Electric Solenoid Valve".
I found some:
eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices
eBay - New & used electronics, cars, apparel, collectibles, sporting goods & more at low prices

Yeah, I looked at those valves too but it says max water temp 95 C so it might not last too long as coolant temperatures stay quite near the max limit. I'll have to see if i can salvage a old electric heater valve from some car at the breakers.

Second is max 100 C so should be ok?

the bricks used in these heaters
Storage Heaters Advice

pic of the BMW latent heat store

bmwfans.info/parts/catalog/E39/Sedan/Europe/523i-M52/LHD/M/2000/march/browse/heater_and_air_conditioning/latent_heat_accumulator/

We have discussed this at some length on the Insight forum as well.

Insight Thermos Modification - Insight Central: Honda Insight Forum

I went as far as to get the special prius three way valve and pump and a stainless steel 2-3l thermos but never installed it. I added a 12v heating element into my thermos so you could boost the coolant temp with a very low current heater (even a solar panel on back shelf) as the the mod is not limited to winter use. I even knocked up few bits on the bench and bought some paraffin wax granules. i think the wax does expand a bit so you would have to leave a bit of room for that.

I think the thermos needs to be in the engine comp and hoses as short as possible, only one pump is reqd. There is a diagram of the prius system on the Insight thread.

I like the phase change fluid/wax idea and if you used a vacum bowl or wide neck flask you could incorporate a large surface area heat exchanger into it before soldering on a cover/end plate and insutalting that.

I was going to use simple pic12F683 and a couple of I2C temp sensors to monitor thermos internal temp and engine block temp and control pumping/valve with that.

Please post some pics of your setup.

I agree that using a phase change material would probably be a good idea but the phase change of paraffin takes place at too low a temperature (around 90 degrees F). I would look for something with a higher phase change temp.

Still, it's a great idea!

"Heater valve" ?
 

Looking at that image, are you sure there's a OEM valve in the heater line? I haven't seen one in use for decades because of their tendency to fail (usually in the "closed" position) or leak coolant into the cabin. I've had various older cars which have done either or both (and no heat when there are snowdrifts around is "no fun")

Modern cars tend to use flaps to control airflow through/around the heater matrix. This means your "heater blocked" bypass setup is unlikely to be needed.

Thanks for posting the ideas though. I've been toying with something similar to achieve fast warmup of both the engine and its WVO feed.

Does the Prius suck water from the block to the thermos or does it simply use a larger water capacity?

(FWIW I've noticed that manufacturers have tended to try and reduce water capacity over the years and I always assumed it was to achieve faster warmup - less water = less specific heat and modern designs recover vented coolant rather than dumping it.

Poor man's remote heater valve idea. Get an aftermarket heater valve, normally manually actuated via push/pull cable. Combine with an electric door lock actuator.

Many of those lock actuators use a 12V DC motor and rack and pinion gearing.

Yeah I'm sure there is a valve. It's part of the heater matrix and I have actually removed it once to clean it up when it got stuck. The reservoir is not in the car right now, as I'm working on it. For the valve I'm using a vacuum solenoid from a old Pontiac. I'm not sure if its going to last long, but we'll see. Also I bought some larger diameter hose. I wish I could fit the reservoir in the engine bay.

The Prius reservoir looks cool. If i had the money and the time, I'd look into making a vacuum insulated canister, but styrofoam will have to do for now. I'm sorry I don't have any actual pictures of my system; like all of my mods it's not very well documented. I don't have a garage, and it's been snowing the whole week so my projects tend to move along quite slowly. I'll try to get pictures when I start working on the passive version.

High temp wax is available ;) I think I bought some 80-90C stuff

What a neat idea. I would let the cooling system pump do the work, and use something like a wafer check valve, that opens at quite low pressure, so would open when the engine is started & running (and coolant is flowing), and would not permit flow when it's not running, keeping the heated water in the hot water tank by preventing thermo siphoning. I get that the wax phase change is a good energy storage & retrieval system, but initially, just using water to prove the system, would work quite well as well. Good point about the expansion of the fluid. Either a coil and tank system, or just get a bigger coolant reservoir. I'd go for the larger reservoir because it's easier. insulating the whole engine might also be an option. something like Rockwool or Kaowool (mineral fibre lagging material) would work just fine. This would place a higher demand on the cooling system though, as the block itself is not being cooled directly.
so... What is your progress on this?
I like the idea of putting it in the cabin of the car, probably under the drivers seat! Could be a slightly warmer car in the morning. The plumbing would run through the firewall and inside the centre console.
Keep us posted on progress.

OK. Sounds like the bypass is necessary then. :(

Thermos do some quite large containers for about $50-75 (nissan range for catering) They might be worth looking into but they won't take any pressure - using one as secondary heat store would work though. (phase change media and a coil of coolant plumbed through the lid springs to mind).

Where? And is it flammable?

What type of insulation are you going to use for the hot reservoir?

There is a product that's a zero pressure product. You actually run a special radiator cap that allows direct venting to the overflow tank. The coolant is installed straight with NO water. its also essentially a lifetime product, Its used in some over the road trucks.

Its like 30 bucks a gallon. But if its permanent.......

While the wax has cool factor its going to raise the price, and you'll have the issue of how fast you can get the heat back out of it. A storage tank would be faster for quick warmup. (I think...)

Great idea !
Dave