Phase-change material for down-sizing cooling system
 

Old article from Autospeed:
http://autospeed.com/A_110772/cms/article.html


The article describes the device as an intercooler although it is really a temporary heat-sink.
The idea is a phase-change material -or PCM- (in this case a petroleum wax) absorbs heat from the charge when on-boost and 'gives it back' to the charge when off-boost.

This only works because when the charge air is turbo'd the temperature (due to compression) raises passed the wax's fusion temperature (ie. when it turns to liquid).

Since many of the solutions today in tackling fuel economy are about evening out the peaks of a cycle (whether it be hybrids, or fly-by-wire lean-burn), then it occurred to me that the same ought to be true for cooling.

I think I read somewhere that the Prius already uses PCMs to store coolant heat for quick-up-to-temp restarting. So I was wondering, if this idea is true for intercoolers on turbo'd road cars (not race as they are on-boost over 50% of the time) then could a similar advantage be gained by employing a small tank of PCM in-line with the coolant circuit?

Unlike the example in the article the use of PCM for block cooling would not be a simple heat-sink, but would be an intermediary, with the wax itself in liquid-state pumped into a specially designed radiator. On current liquid-cooled engines the mass of the fluid has the effect to even-out the temperature load across the driving cycle (ie. it is sinking variations in heat, not only at local hot-spots on the head, but from a moment of hard-driving) - This is why it is not an immediate effect for an electric rad-fan to switch on if you start pushing it.

The intermediate-PCM stage would allow several effects.
1. It could mean a much smaller mass of coolant in the system (although what there was would need to circulate quicker or have higher heat-transfer characteristics).
2. Because the specific heat of the PCM is equivalent to many times that of the same mass of water, the length of time that the engine would be able to be pushed hard for without overheating would be much longer. Looked at another way, if a driver is driving more-or-less normally then the size of the radiator (and it's aero impact) gets smaller.

So, I'm not saying this idea is workable for a mass-produced car of today (unless it's hybrid) due to it having limitations for continued heavy driving. But for the eco-minded driver it is, I believe, a realistic proposition - Remember, just like in the intercooler example in the article the driver is borrowing heat-capacity now to be paid-back later.

The North American, NHW20 model (2004-09) has a thermos that holds hot coolant. No phase-change heat storage.

The current ZVW30 model has a heat exchanger behind the catalytic converter to tap exhaust heat and rapidly heat up the block.

Bob Wilson

Thanks Bob.

Any other opinions (or ridicule) on my idea for using PCMs - by anyone?

IMHO, downsizing a cooling system is not a good idea in general. Current cooling systems do have a phase change temperature regulation. If you look at a modern coolant recovery tank cap they are designed to vent at a given pressure. As your car engine over heats water turns to steam and builds pressure. When the cap vents it allows more steam to be generated. Provided your coolant doesn't drop below a certain level your engine cooling system will not exceed boiling point.

Using PCMs to store engine heat has been researched. It's called a Schatz heat battery.

Good thread.

" . . many of the solutions today in tackling fuel economy are about evening out the peaks of a cycle"

is always worthy of consideration.

Thanks for the link to the previous EM discussion on these materials.
From the way I see it the simple way get the engine up to normal temp quickly is to use an exhaust heat-exchanger. I don;t suppose it takes much longer that using a thermos or a PCM, but it's got to be more simple and less bulky/lighter.

What can't be done any other way is allow for a smaller radiator opening (ignoring adjustable grill blocks). Using a PCM as an intermediate heat-sink should, under all but the most extreme driving-cycle situations, result in the need for a substantially smaller radiator and its aero penalty.

Anyone has good examples to show how to set up exhaust heat exchanger?
Basically you need to wrap copper pipe around exhaust and coolant flows through that spiral? How many wraps around exhaust?
How much is it possible to make warmup time quicker?

That autospeed idea seems really good to use it as heat battery - to preserve waste heat that in the morning you get engine warm sooner.

UP!
Nobody has not done exhaust heat exchanger?
What about phase-change heat-battery?

IIRC, Volvo experimented with phase change materials in an insulated container, heated by engine coolant. Something about it in the early 1980's in Popular Science or Popular Mechanics.

They abandoned the idea due to size and weight and the insulation materials available had to be too thick to make it practical.

But with advances in insulation technology, the overall size and weight can be reduced while making the amount of PCM large enough to be useful.

For a vehicle used every day, there needs to be enough PCM with good enough insulation to keep it liquid at least 24 hours. Another thing is the vehicle must be driven long enough each time to "recharge" the PCM.

If you don't run long enough to fully heat up the PCM tank, then it's just wasted weight.

I'd be wary of adding any weight to my car, but an exhaust heat exchanger sounds like a great idea, if you could overcome some practical problems. I'd be interested in how Toyota overcame some of them - specifically, you don't want the exhaust heat exchange to be part of the cooling loop once the engine is up to temperature... however, simply changing how the thermostat functions (instead switching from exhaust heat exchanger to radiator loop once up to temperature) would not work, as the liquid in that part of the loop would quickly reach astronomical temperatures. Instead, you would need to somehow decouple the heat exchanger from the exhaust at some point.

Use a two path exhaust pipe with flaps to route the heat through the exchanger or not. Another possibility is have the heat exchanger near the exhaust pipe and use air heated by the exhaust pipe to blow through the coolant heat exchanger.

That would need something like an "air stove" around part of the pipe, similar to the old heat risers that used to be ducted to the air filter housing with carbueretors. Probably would need to cover more of the exhaust pipe. A length of double wall pipe, with the gap sealed at the ends and a fan blowing circulating air around through the space between the pipes and the coolant heat exchanger would allow for cutting off the heat.

Water mist injection on my diesel seems to really help keep the electric cooling fans off.
The water pump uses a tiny amount of power compared to running a fan.

I've always wondered about using sodium acetate as a heat sink for car engines... Phase-changes to liquid when hot, but stays liquid when cooled down, so it could be used to bring an engine partially up to temperature when you trigger phase-change back into solid state (occurs at some 50 degrees C).

Got anything proven to work?

So far with water injection I can keep my electric fans off with my diesel. Driving around town when I use to need my fans the most I need the least water, maybe about 2gph around town moving at slow speed. When stopped less than 1gph will keep it cool.
I didn't expect water injection to be any use at all at low speed or when stopped.

Read the article a while back, nice idea for a niche application.
For everyday operation wouldn't the first step be to remove all heat sensative equipment from the engine bay, like battery & electronics, then completely seal & insulate the engine bay so the entire engine including coolant is a heat sink. Have a cool air intake, coolant in & out controlled by thermostat & exhaust out.

If you insulated the engine bay completely (you'd need to manufacture new cowling to pipe the air exiting the rad) the header and exhaust heat would quickly raise the temps around the engine and then the coolant system would need to deal with quite a considerable additional thermal load. And even if the car was driven gently the extra need to radiate heat via the coolant through the radiator would not do anything but bad things for efficiency, and may still boil the coolant.

Water injection for cooling Diesels is nice, but the water still has to be carried, even if the consumption is quite low. (I assume the 1gph stated above is 1 gallon an hour, which still adds up).
The idea of having such a massive overhead, in terms of sizing a cooling circuit and radiator for sustained full-on driving in the hottest & driest climates is, I guess, what car mnftrs have to build for, but it would be nice if, like a hybrid-electric car around town 'borrows' power from its batteries, the cooling system could borrow heat-sinking capacity at the moment when the driver needs it (say to climb a long hill) against a later time when the engine is working much less hard.... That was my original thinking and I still have little doubt it would make no difference at all to all but the most lead-footed of driver.

For a large vehicle like what I have, I am going to try to put a used 40 gallon plastic side fuel tank under there and use that for holding water. I would only fill it to capacity before a long trip because 40 gallons of water will weigh something like 330 pounds.
On the open road I would be using about 2 gallons of water for every gallon of water.
40 gallons of water to 25 gallons of fuel on a road trip would make a pretty good match.

A diesel car could get away with using far less water.

Entirely agree, but wouldn't it be easier to seal off the entire engine bay area, and just add and appropriate cooling air exhaust at the rear bottom of the engine bay to open and release heat, as well as allow airflow through the radiator, than it would be to add another complete cooling system as a heat sink.
Eg when warming up all vents closed, engine warms up very quickly, once warm, vents open as much as required, then when stopped and shut down, all vents close and retain engine heat, when re started and running vents open as much as required.

I see your point about working with what you've got rather than adding further components to the cooling circuit (as I suggest), but a few things occur to me:

• As well as having to fabricate an airtight space under the bonnet, you need to fab at least two actuated vents (air-in & air-out).
• Especially when stationary or going slow, your cooling air will need larger (and perhaps extra) electric fans to force enough air through. This is because you've strangled the free air flow in the engine bay.
• The manifold (header) and top of the exhaust would need to be thermally separate from your controlled ducted airflow (no idea how you'd do that). If not, because normally the header relies on radiating some portion of the hundreds of degrees it will reach into the free air of the engine bay. Either way, this large additional heat dissipation needs to be factored into your alternative design.

The thing is there's nothing like the amount of thermal capacity, even in all the air 'contained' in an engine bay, as can be held in a few kilos of thermal salts (PCMs). And I'm not convinced your alternative 'plumbing' would even be as simple.

Not the air I'm talking about keeping hot, it's 200kg of engine, steel, alloy & fluids etc.how many kilos of salt and plumbing will you need to match that thermal mass?
Insulate & seal the bonnet, stop the heat venting out the top, the firewall is usually pretty well insulated to the cabin anyway, insulate the side guards, the bottom doesn't so much need to be insulated as properly sealed.
When driving the rear bottom vent is opened to let air out, and the normal radiator opening is left open to let air in, as much as required by the conditions, when parked, close lower vent and front grille area, no need for additional fans.
Once you stop convection, the heat stays in, and any added insulation minimises the conductive losses.
I think if you pull out some numbers on it you will find you will need to fill the entire trunk with atleast 50kg of salt, with all the associated equipment, just to get engine from ambient cold to 40C.
Remember the salt can't give up all it's heat, it can only equalise the engine temp.
Let's say you have 50kg salt at 100c and it stores twice the heat of steel etc.
You have 200kg of engine at 10C, best you are going to get is 55C across everything, but remember as the two bodies get closer in temp, the rate of heat transfer slows down, so you will get it to 40C fairly quickly, but to get that last 15C might take half an hour of pumping coolant through.
If you can keep the engine hot enough, say 40C by the end of a working day, then you just, open your vents, get in and drive away.
If I had to deal with the extreme cold issues, that's what I would look at first.
Edit: Just thought, has anyone just tried using one of those cheap car covers to see what difference in heat loss from engine bay area is, just trapping a layer of air may actually be quite effective for a few hours?

quote: "I think if you pull out some numbers on it you will find ....
Let's say you have 50kg salt at 100c and it stores twice the heat of steel etc."


PCM (salt hydrates / eutectics) - storage density: 93 kWh/m3
Water (as just sensible heat) - storage density: ~8 kWh/m3

And Iron fairs much worse than Water by weight (water is actually very good)
Water has a Specific Heat of 4190 J/kg.degC
Cast Iron has a Specific Heat of 540 J/kg.degC


Remember the great thing with PCM salts, etc is that they can tap the large amount of energy needed for phase change (The latent heat).

So from above 200kg of iron holds about the heat of 25.7kg of water.
So we'll be generous and say a biggish engine block with water has a heat capacity of equivalent to 30kg of water. And the PCM can hold over eleven times as much heat once molten.
Unless I'm mistaken we find that just 2.6 kilos of our pcm can hold the equivalent heat of your entire engine.


Also, the PCM salt can give up very near to its entire heat as it solidifies/freezes (as most of the thermal capacity is the latent heat) providing a salt is chosen that melts just below the ideal coolant temp.

So whether you're trying to keep coolant heat stored for fast warm up later on or overnight (as you're idea is focused on), or you're trying to cut aerodynamic overheads required by a large radiator and opening (as is my starting point), phase-change materials are the way to go.

Ok, I'll pay you that,
I hadn't gone back to look at the actual numbers.
But I still think the complexities & return are still going to take it down the path of diminishing returns.
Unless the salvaged heat is somehow converted to useable energy, then the overwhelming issue is to still remove the heat generated during normal operation as it is currently a waste product for ICE's, this will still mean radiator & cooling system cannot be downsized to any great degree.

Read:
Engine Cooling System with a Heat Load Averaging Capability
to see why it can.

Note that:
"
- Under typical driving conditions, an engine generates only about 30% of available power for 90% of the time.
- The remaining 10% generally [near peak output] is used for accelerating or climbing steep inclines.
"

So with a sympathetic driver at the wheel (who understands that he will be 'borrowing' heat capacity for an overtake, etc against driving with a light foot later) I would estimate that an 80% reduction in rad-size/grill-opening would not be unrealistic for anywhere but the most hot, arid environments.