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Demolite 11-15-2011 12:20 PM

Heat storage system (Prius-like insulated coolant reservoir)
2 Attachment(s)
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.

Piwoslaw 11-15-2011 03:16 PM

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:)

some_other_dave 11-15-2011 04:24 PM

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.


Demolite 11-15-2011 04:40 PM

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.

Frank Lee 11-15-2011 06:59 PM

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.

Ryland 11-15-2011 07:48 PM

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.

user removed 11-15-2011 08:20 PM

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.


user removed 11-15-2011 08:22 PM

You have nothing to worry about cracking anything engine wise.


user removed 11-15-2011 08:31 PM

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.


Demolite 11-16-2011 08:04 AM

1 Attachment(s)
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.

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