Motor vehicle coolant heat recovery

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I originally posted this on the thread about alternators, it was a bit ot but in response to PhilipR's mention of it in plumber's forum. (A link to that would be good, PhilipR!) Thought it made sense to make it a fresh topic so it can be more easily found for future reference.

I have long thought about the 70%-ish waste heat generated by the infernal combustion engine, and read with interest as people like dickjotec on the veg oil forum made good use of the heat generated by his ancient Lister to heat his house (having already provided electricity) and pre-heat the lard/bio-from-lard which the engine can (and does) run on.

Anyone who walks past a queue of traffic is aware of the huge volumes of hot air created by modern motor cars - those with air conditioning run the cooling fan(s) continually with the ac on. The size of radiators on modern motors says it all - the one on my old 110bhp Mercedes 3 litre diesel looks tiny in comparison with those on a modern 2 litre turbo-diesel family hatch. Exhaust heat loss is huge, too - on a warm day you can see the heat haze around a petrol engine's exhaust. Cats run at around 600C.

So has anyone captured any of this 'free' heat from a vehicle and used it for space heating or DHW? With fuel at £7 a gallon, most cars still average less than 45 miles on that amount so to think that 70%+ is wasted to the air as heat is frightening, if you care about the environment or your pocket. 

To tap into the cooling system is easy, storage of 150 litres of hot water slightly less so but hardly impossible and making use of the exhaust downpipe heat would be possible too, if a little more involved. I also like the idea of storing coolant in a vacuum flask to keep it hot overnight, as Saab did many years before the Toyota Prius/Pious demonstrated it was possible.

On a large turbo diesel vehicle with a 100kW engine (134hp) you could say that on an average longer journey 50kW is the average output from the engine. So if the engine is 30% efficient (theoretically more but not in practice) over that journey (kinetic energy output)  - then 35kW will be wasted as heat. Say that 40% (off the top of my head, does anyone know measurements?) of the waste heat is recovered by the coolant (the rest exhaust and radiation and convection) then that leaves 14kW which can be used from the cooling system. Call it an easy 10kW after losses from the pipework and insulated tank.

About 7kWh are required to raise 150 litres of water by 40C, ignoring losses. So in just under an hour water a 150 litre tank could be raised from 10C to 50C. I think bothering with extracting heat from the exhaust would be unecessary complication at first. Anyone fancy putting this into practice? Something like a Tam105 pump (as many veg oilers and biodiesel makers use) would transfer the 150 litres sufficiently quickly from the car tank to a house tank. A 15kW+ coil from an old copper cylinder inside the car tank could act as heat exchanger for the vehicle's coolant.

Instead of sending the engine coolant through the radiator as the thermostat opens it would be easy to divert it to the storage tank coil and with a thermometer in the tank a manual (or automatic) bypass to the radiator could be fairly easily plumbed. The tank and pipes would have to be outside the vehicle (for safety) and well insulated. Tank baffles wouldn't be necessary if the tank were always 95% full or empty then water slopping about wouldn't be an issue, I think? Smaller tanks for smaller cars, the spare wheel space could be the starting point for installation where no other under-body space was free - although between the axles would be ideal from a mass distribution point of view. Any tank would have to be closed off from the car interior. 4x4s and estate cars would most likely be the easiest to fit a tank to.

I like the idea of a hot bath or a warm house for 'free' after a long journey. Since waste veg oil powers the fleet, it's as green a heat as anyone this side of a hot spring would experience!

Other-Power suggested using a phase change material like petroleum based wax to reduce size and weight for same number of units. I thought it would create more complications than it would solve.

I look forward to hearing your thoughts...

[Other-Power]

Paraffin melts around 37oC apparently http://en.wikipedia.org/wiki/Paraffin

Specific heat capacity of 2.13 kJ/kG.K http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

Will ad some more later, we can crunch some numbers

Jon

[EccentricAnomaly]

Paraffin melts around 37oC apparently...

You can get paraffin waxes for all sorts of temperatures. E.g., http://www.rubitherm.com/english/pages/02a_latent_heat_pcms.htm. The higher the temperature of the phase change the better from the point of view of efficient extraction of energy though there is the problem that conduction in the solid phase is relatively poor.

[Other-Power]

One of my peers at uni did her dissertation on this, her paper is here http://www.mirarco.org/ercm/docs/kim_work.pdf

cheers

Jon

[Quakered]

A well-insulated tank containing 150 litres is going to weigh around 160 kilos or around 2 people. This will adversely impact MPG and therefore has a cost. As you mentioned, the only heat you could hope to capture is that in the radiator and this only kicks in when the engine is fully warmed up (after 10 minutes?) so would only be of any use on long journeys. Looking at the size of a car’s radiator, I would be stunned if it is losing 3 kilowatts and not be surprised if it was less than 2.. If this is true then it will take over 3 hours to warm your tank to 50C.

Great if the 3 hour journey is home so you can use your collected heat – but surely everyone will be more interested in a cup of tea than draining heat out of the car. If you 3 hour journey is to visit friends of go on holiday then the heat is wasted unless you also plan fitting a bath in the car….

The inefficiency in a 4 stroke (4 cylinder) engine is that only one cylinder is producing energy on each revolution. The other 3 are adsorbing energy….. Solve that problem and wealth beyond the dreams of aversive await you. Finding a reason to fit a huge hot water tank is unlikely to interest anyone other than Heath-Robinson! It could make an interesting safely feature to absorb impact energy….

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A 150 litre tank could be for the biggest vehicles - 4x4s and larger estates. It would weigh about the same as two tankfuls of fuel, and have some effect on economy - the least when on motorways and quieter main roads - ie longer journeys. For a Golf-sized machine, a 70 or 80 litre tank may be about right, so weigh barely more than a tank of fuel. If you're so concerned about 1 or 2 mpg, good tyres and a service can make that sort of difference.

As for how much heat the radiator loses, I think I will bypass one via a container with a coil heat exchanger in there and see just how long it takes to raise 40 degrees from around 5 or 10C. The water will remain hot for hours if the tank is well insulated, if not pumped out and used it would rapidly heat the engine from a cold start a few hours later. In winter and running on wvo as I do, that would make a lot of sense - even on fossil fuel a pre-heated or rapidly heated engine is brilliant when temperatures drop low.

If radiator losses are low, then it would make sense to investigate scavenging heat from the exhaust. As plenty have done this with their Lister CHPs, the know-how is already well tried and tested.

btw, I've been googling car radiator heat dissipation figures, around 4000 btu/min or around 70kW max seems to crop up quite commonly. With plenty of 150 kW engines about, that would seem to make sense, especially since many modern cars have their engines enclosed away from the airstream for aerodynamics and air con and gearbox rads often sit in front of them.

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Sorry 70 litres of water weighs the same as 93 litres of petrol so unless you are driving a Range Rover, your analogy that your water tank weights the same as 2 fuel tanks is off the mark. Worse on average most fuel tanks will be half full- your water tank will be full always. Assuming your Range Rover returns 20 mpg (very optimist) and your weight of water drops MPG by 3% (equally optimist) your 200 mile journey will consume an extra third of a gallon or £2 or 16kwh of electricity or over 40 of gas.

Doesn’t make sense to me…..

[Quakered]

PS You could save energy by cooking your meals on the cylinder head as you drive!

[M]

PS You could save energy by cooking your meals on the cylinder head as you drive!

Have you ever tried holding a frying pan still at 50mph in a straight line, let alone during all the swerving. My wife's excuse was that she couldn't see the road properly around the bonnet!

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Yes, cooking food is well documented. Have even heard of roadkill being butchered by the roadside, popped under the bonnet and if you remember to carry potatoes there's only the veggies to cook once home for the roast!!  

Yes, the weight of the water is a good bit more than petrol - as I wrote I was thinking of my veg oil which is considereably denser at about 0.92kg/l. Whatever, the weight is an issue, although I know from my fuel computer that carrying a 200kg load in the boot increases fuel consumption from 45mpg to 43mpg unless the road is a slow one over hilly terrain. I do drive with some sympathy for the extra weight, though - not accelerating up hills, using momentum to its best effect etc.

I suppose I can't get the thought out of my head that for every £100 of petrol or diesel people pour into their tanks, £65 and more is wasted - mainly as heat. It would be more efficient to scavenge heat from the exhaust and have a smaller water tank  - 50 litres perhaps - which is heated to 80C. That way you would carry the least water over the smallest distance. I'm still tempted to try - I didn't expect to be running on old fryer oil, let alone think it feasible with (relatively) modern diesel engines.

[Ivan]

If you could invent some ultra-high heat capacity fluid, then it would be very worthwhile. Actually, a much more productive approach would be the other way around (paradoxically) ie in cold winter months, using heat generated electrically or by the household boiler to preheat the car's cooling system. This would mean it was at operating temperature immediately, and therefore the engine would not be running rich (petrol engines) or spewing imcompletely combusted fuel (diesel engines) whilst cold.