# Heat recovery from car radiator.

by questionsarewis
 P: 1 Trying to see if it is viable to capture waste heat from radiator of cars(s) to pre-heat water heater and or radiant floor. Off grid homeowner here and every little bit of energy counts! Enclosed garage. R-50 SIP panel "house" to pull nose of car into, Solar-powered fan to draw air over a metal fin system filled with glycol alcohol. Heated fluid is drawn towards water heater and or radiant storage tank. Simple system, am I as dumb as I think I am?
 Sci Advisor PF Gold P: 2,015 I don't know what a Sip paneled house is. My thought is that the radiator doesn't store enough heat to make it worthwhile (only a large van or suv has a radiator with even 1 gallon capacity, you can figure out the stored heat). What you want is to suck the heat out of your engine block, that's a big hot mass especially if it's a V8. You could install an aftermarket electric water pump to circulate coolant through the block after you turn the vehicle off. Is it worthwhile? Calculate the heat stored in the block, subtract the energy spent in fans and pumps, and then you'll know.
 PF Gold P: 8,961 Welcome to PF, questionsarewis. While what you want is certainly possible, Marcus has made the main point. Is it worth it? I have an idea of how to achieve it fairly easily and cheaply, but I don't know how effective it will be. And keep in mind that it will only work for a very short time, and only when the car has been driven recently. Here goes: Build a heat exchanger, perhaps from a water heater tank or similar, and fill it with the same water/antifreeze mix that you use in your vehicle (in the same quantity that the vehicle holds EDIT: no, there should be more than the vehicle holds). There should be 2 hoses from it long enough to reach the car, with a thermometer mounted in one of them. Put shut-off valves near the ends, to prevent draining. Insert a diverter valve to a tap on your lower radiator hose, and another shut-off-valved tap just after it (between it and the rad). When you park the vehicle after a drive leave it running. Immediately connect the hose with the thermometer to the first tap, and the other hose to the second. Open the shut-off valves in the hoses and switch the diverter valve to the 'flush' position. Let the engine water pump force the coolant into the heat exchanger. It should force the cool stuff that was already in the exchanger into the second tap. When the thermometer drops to room temperature, you've completed the transfer. Shut the motor off, switch the diverter valve back to the 'drive' position, close the shut-off valves, and disconnect your hoses. I don't know how much you can get in the way of recycled heat, but there might be enough to do something useful like fill a dishwasher. Put some filtration in the system to prevent contaminating your coolant, and make very sure that your diverter valve hook-up doesn't leak. It's still up to you to figure out whether or not it's worthwhile.
PF Gold
P: 2,015

## Heat recovery from car radiator.

Hi Danger,
Good to cross paths with you again!
questionsarewis, I like to calculate things. I assume you live in a cold climate, otherwise you wouldn't be so energy-hungry in the first place. It's snowing here today, so I'll use my numbers as an example.

Imagine you have a 4500 lb SUV with a V8 since anything smaller probably doesn't have enough heat to bother with. The engine block and heads (which are accessable to the coolant) probably weigh around 400 lbs and reach a temp, we'll say, of 250F. The frame, chassis, body, suspension, etc. maybe weigh 3000 lbs and when I drive home from work they are 20F. I pull into my 50F garage. The ratio of heat lost from the garage over heat given to the garage is
3000*(50-20) / 400*(250-50) = 1.12
and when you add the heat lost by opening up the garage door to pull in, your vehicle is an overall drain on the household heating budget. You do even worse by inefficiently draining the heat from the radiator.

Conclusion: in the winter, you gain more heat by leaving the SUV in the driveway!
PF Gold
P: 8,961
 Quote by marcusl Good to cross paths with you again!
Likewise. Good explanation of the efficiency there.
I just thought of a further complication with the system that I came up with. There's no way it will work the way I was thinking of, because I forgot to account for the vehicle thermostat. It will close and shut off coolant flow through the engine, and thus through the heat exchanger, when the temperature drops below 230 or whatever. I also don't know what effect depressurizing the coolant system will have upon the temperature of the emerging fluid.
This is why I usually try to work something out properly before posting.
 Sci Advisor PF Gold P: 2,793 As the others have pointed, the juice just isn't worth the squeeze. However, you might be interested in looking into a micro-cogeneration unit. This could provide electricity and heat for your home, at upwards of 90% efficiency (compared with perhaps 30-40% with a traditional home). Just a thought...
 PF Gold P: 8,961 Hmmm.... (this is sort of an edit to my last post)... I suppose that you could build your connections for my system by drilling and tapping into opposite sides of the thermostat housing to bypass it. But, again, it's not worth it.
 P: 131 There have been some cars (Cadillac I think) that have experimented with this (sorta). Some have a "thermos" in them so that you don't have to run the car to have heat right away. There has also been some experiments with storage devices in the drive train for mechanical energy as you brake to gain more acceleration (alot like a hybrid, except mechanical energy is stored as such). So if anything, I would maybe add a thermos type device so that you don't have to warm up your car and will have instant heat!!
 Sci Advisor PF Gold P: 2,793 wxrocks, I don't think this was the OP's idea; the idea was to use energy from a warm parked car to heat a house. Developing the idea and my suggestion of micro-cogen, I wonder whether a system using your car's engine as a source of electricty and heat for your home could be made to work. Some kind of docking station would obviously be required with the necessary fuel, water and power connections, but my first impressions would be that the power and heat outputs are probably not that far off what's required for the average home. Russ, any basic general figures on power and heat consumptions for average domestic households in western countries?
 P: 131 I understand that -- but a problem we all face is thinking globally with energy savings. It is nice to at least think a few steps farther down the line in energy savings. The gas you save by not running the car to warm up could then be used to heat the house. If the car is not running -- I just cant see any gain in taking the heat from a car -- there are much better ways to save energy.
 Sci Advisor PF Gold P: 2,793 I think global issues with energy savings should be going a bit further than not warming up a Cadillac before you take it out...! wxrocks, my proposal was not for a car which isn't running. My idea was to have the car actually run, in the garage, to power and heat your home.
 PF Gold P: 8,961 Not sure about efficient heat transfer, but at least if it's running you can also use the exhaust as a source. As for electricity, just build a roller rig like a chassis dyno with a generator hooked up.
 Sci Advisor PF Gold P: 2,793 I was thinking about this as a serious proposal. Not just exhaust heat, why not jacket water and any intercooler water heat too? And why have a chassis dyno where it could be feasible to incorporate a PTO or alternator into new vehicles designed to be used as a CHP installation? I'd really like to see this as a proposal. Who was asking about final year uni projects?
 Sci Advisor PF Gold P: 2,015 Won't a reciprocating IC engine be inefficient compared to burning natural gas? I'm not just waving hands. The gas furnace has been optimized over 150 years to solve your this problem, and is 90 to 92% efficient (modern two stage furnace) in heating your house. The car does badly because it solves a different problem, namely producing mechanical power while minimizing heat production. Better would be connecting the driveshaft to a heat pump and extracting heat from the (cold) outside to warm the interior, in addition to capturing the engine's waste heat. However, a) it would suck to switch the driveshaft each morning and evening and b) heat pumps operating in this mode are inefficient so I bet even this approach loses fairly significantly. Unless you are making free solar or wind energy (both good ideas), I find it hard to believe you can improve on the utility industry's century or more of optimization.
 Sci Advisor HW Helper PF Gold P: 2,860 Hi Question. I wonder if you're still lurking? Anyway, welcome to the board. If you were to have a storage tank of water inside your car such that you could essentially eliminate the radiator and store all the heat inside this tank you might get enough energy to make it worth while, but as you'll see, there's not a tremendous amount of energy here. Let's say we have a tank with 100 lbm of water (about 12 gallons, give or take). Let's say you heat that water using your engine to 210 F, and then you're able to use that heat down to 90 F. The dU for this is 120 Btu/lbm. So for 100 lbm, you get 12,000 Btu. This equates to roughly 3.5 kW hrs. Where I live, electricity costs roughly $0.07, so you can save the equivalent of 24.5 cents worth of electricity with a 100 lbm tank. You'd need 400 pounds to save$1. That might actually be worth it if you can save that much energy every day, or if you spend significantly more for energy. Now ask, "Can the vehicle produce that much heat?" Think about how much power it takes to drive your car, and then estimate the heat loss through the radiator. That's how much you'll be able to capture given 100% efficiency in capturing that heat. I'll come back to this question if you'd like, or perhaps someone else would like to dig into it.
PF Gold
P: 2,793
 Quote by marcusl Won't a reciprocating IC engine be inefficient compared to burning natural gas? I'm not just waving hands. The gas furnace has been optimized over 150 years to solve your this problem, and is 90 to 92% efficient (modern two stage furnace) in heating your house. The car does badly because it solves a different problem, namely producing mechanical power while minimizing heat production. Better would be connecting the driveshaft to a heat pump and extracting heat from the (cold) outside to warm the interior, in addition to capturing the engine's waste heat. However, a) it would suck to switch the driveshaft each morning and evening and b) heat pumps operating in this mode are inefficient so I bet even this approach loses fairly significantly.
As far as heat recovery goes, yes, the efficiency will be less than a modern gas boiler. However, if you treat the electricity generated as a byproduct (ostensibly 35% of the fuel input), and offset this against the cost, then I reckon you'd definitely be in with a fair scheme by which to offset the heat and electricity costs required for the average home, particularly if you use the same natural gas that your boiler would. I suppose the heat pump would work in certain climates, but I still like the idea of cogeneration, or trigeneration for warmer climates.
PF Gold
P: 8,961
 Quote by brewnog I was thinking about this as a serious proposal. Not just exhaust heat, why not jacket water and any intercooler water heat too? And why have a chassis dyno where it could be feasible to incorporate a PTO or alternator into new vehicles designed to be used as a CHP installation?
Oh, I was just thinking if someone wanted to cobble something together at home rather than a commercial product. And I actually meant the exhaust heat in addition to that of the water jacket. If it were a commercial unit, I'd also consider using a turbocharger as the water pump to avoid parasitic losses on the fan belt.
Mentor
P: 21,886
 Quote by brewnog As far as heat recovery goes, yes, the efficiency will be less than a modern gas boiler. However, if you treat the electricity generated as a byproduct (ostensibly 35% of the fuel input), and offset this against the cost, then I reckon you'd definitely be in with a fair scheme by which to offset the heat and electricity costs required for the average home, particularly if you use the same natural gas that your boiler would. I suppose the heat pump would work in certain climates, but I still like the idea of cogeneration, or trigeneration for warmer climates.
Generally, cogen is looked at from the other way around: the heat is a biproduct of the electricity generation. You don't lose much electricity by adding cogen, but if you want to use a heating boiler to generate electricity, you lose a lot of heat.

An electric generator on an IC engine with little in the way of extra goodies to increase its efficiency may be 35% efficient at making electricity, by a decent stock residential boiler or gas furnace is upwards of 95% efficient at heating your house.

So to answer marcus's question: If your primary goal is generating power and you have 65% of your energy going up a stack, it may make sense to throw an aft-end boiler on it to recover the heat. If your primary goal is generating heat, it never makes sense to throw a generator onto your boiler.

Regardless...
 I was thinking about this as a serious proposal. Not just exhaust heat, why not jacket water and any intercooler water heat too? And why have a chassis dyno where it could be feasible to incorporate a PTO or alternator into new vehicles designed to be used as a CHP installation? I'd really like to see this as a proposal.
So would I. I'm not convinced it would be worthwhile, but it could be. At the very least, it could make for a nice emergency backup system. I'm too lazy to do any calculations, but I'm reasonably certain a car engine could easily provide both the heat and electricity for the decent sized house at barely above idle power.

More generally, cogen is something that larger commercial buildings tried in the '80s and somewhat surprisingly, it has turned out to not be economically viable. One client of mine, a 750 unit condo building in Philly, is getting rid of their cogen plant, which at one time was a highly regarded test-case in Philly. It is maintenance intensive and needs a well-trained engineering staff to run it, but worse than that, deciding when to run it is not a simple economic decision.

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