High COP system?

[TimSR2]

I'm going to take a flyer here..

Many posts have been made here  about efficiency in extracting waste heat from electric generators. As we all know, any internal combustion engine is a better source of heat than it is of electric power. It occurs to me that heat pump systems with a COP or 3.0 to 1 are easily attainable.  So it should be possible to triple the waste heat extracted, by using refrigeration principles, either self powered, or from grid.

As the COP of a heat pump is affected greatly by the available heat sink source temperature, a dump tank for waste heat could be easily substituted for a ground loop, at much lower installation cost.

Am I seeing aliens behind a post here,  or am I on to something?

[glort]

I have thought about this very thing at length myself.

Reverse cycle AC units put out more heat than the wattage of the power they use.

As far as heating goes, they drop off the curve rapidly at around 3-5 oC from memory.  I thought about blowing the exhaust through the condenser of an AC unit in the cool weather to bump the efficiency as well as using the engine heat. for an extra boost.

I have been searching the net for anyone that has set up a car AC unit with an IC motor but so far haven't found a single example. I'm surprised I haven't found at least a couple of people that have done it.
I believe all that would need to be done is put a TX valve in the system and a car aircon could be used for heat or cooling.  It would seem that driving a compressor directly would be a lot more efficient than a generator and then the electric AC unit. 

[mobile_bob]

interesting topic that many of us have considered for some time now...

here are my concerns, fwiw

lets put the prime mover in an insulated box, so that we can capture all the waste heat
(or nearly so for the sake of discussion)

using the 1/3rule, in that 1/3 of the fuels btu's are converted to mechanical energy
the other 2/3rds are coolant and exhaust heat
(again for the sake of discussion)

lets assume the first third is a kwatt of mechanical energy, this leaves the other 2/3rds or 2 kwatts as harvestable waste heat

now we can recover that heat via heat exchangers, and maybe get 75% or  1500watts heat recovered from the available 2kw of heat, thus leaving only 500watts to recover via other means? or just let it go?

heat exchangers generally have little to go wrong, require little maintenance and are a fraction of the cost of a heat pump? 

even if we use a heat pump we probably must also use some form of heat exchanger(s)

so considering the amount of heat available in the box is finite at 2kwatt, we cannot get more out than was placed in the box.

so maybe we get a conversion efficiency of 90% using the heat pump unit, for a recovery of maybe 1800watts of the available 2000watts?

the difference between 1500 watts recovered and 1800watts recovered is only 300watts, and the recovery came at a rather significant first cost and ongoing cost.  the heat pump is going to require some electrical power to do its thing too.

i think the reason we don't see this scheme typically is because if we use a higher efficiency heat exchanger the gap narrows to the point that it just isn't econcomically viable?

now all this predicated on the use of a water cooled prime mover...

it might work out much differently for an aircooled prime mover where the cooling air is much harder to harvest simply using a heat exchanger, and much easier to recover with a heat pump?

it might also be that as sizing goes up the scheme becomes more viable?  maybe the relationship is not linear?

i think the idea that a heat pump is 3 times more efficient at making heat than the power it uses to produce it, is based on the fact that the heat source is near infinite (at least in well designed systems) or at least in comparison to the heat available from a prime mover which is very finite.

perhaps someone can educate me otherwise, but this is my understanding of the topic

bob g

[BruceM]

This is like the wind turbine powered car... it won't work.

Heat pumps have high efficiency only because they are MOVING heat from a practically unlimited source (earth, body of water or air). 

The heat of the engine is neither unlimited nor free, and moving it via a low pressure, low power circ pump will be much more efficient than via a heat pump refrigerant system.

[veggie]

Good point BruceM,

I recently inquired about a heat pump system for the home.
The numbers were impressive....at first.
The fellow showed how efficient the heat pump unit was compared to natural gas. $40 per month cheaper than a Natural gas furnace.
However, he conveniently left out the part where a 1HP grundfoss circulation pump was required to run 24/7 for circulation into the ground loop.
That adds $45/mo to my electric bill.  
This is the main reason why off-grid solar systems rarely make use of heat pumps. It's a 24/7 drain on the battery bank.

Not exactly the same setup as TimSR2 was suggesting, but each time I look into heat pumps I find a cost in the form of electrical requirements and capital outlay that I cannot reconcile.

veggie

[Ronmar]

Never liked the term heat pump.  IMO "Heat Concentrator" better describes the process.  The whole point of a heat pump is to concentrate and multiply low grade heat from a source into a usefull form, and they are very efficient at doing that.  Engine waste heat is already in a very usefull form, so running it thru a "concentrator" will involve loss, as no conversion is 100% efficient...  Way easier to move it around than to try and convert it:)

They say on a hot day, as high as 10HP can be consumed by an auto A/C.  I don't have a 10HP electric motor lying around or I would have tried it by now:)

[glort]

I'm really not following where some of you are going with this.

A reverse cycle AC unit will output around 3 times the BTU's put into it.  How is that not efficient? 
Yes, it does pull heat out of the air to supplement the electrical ( or mechanical) energy going in but what is the big deal with that?

Veggie, I think you must be talking about something different to what I'm thinking of. I have never seen a circ pump on an AC unit yet.  The system circulates refrigerant  and there is no ground loop. They work on air only.


The  AC units I am talking about  all draw air through a HE. It would be very easy to set the Box you are talking about Bob so all the heat from the engine and exhaust was drawn through that HE and then converted, efficiently, to heat through the AC compressor etc.  You could duct the exhaust through the condenser very easily and you could also use an air cooled motor if enclosed in your hypothetical but entirely practical box.
I have a garden watering system Mist Nozzle on my AC outdoor units which I turn on on real hot days.  The improvements in the lowering of the output air inside are measurable well and truly.  Putting heat from an IC engine back through the HE is going to have the same effect when the thing is in heat mode.

One thing I tried last winter which worked pretty well was ducting heat from my roof back into the house.  I just got some ducting and put it in the peak of the roof cavity and put a blower fan on the end sucking  the air into the house through the manhole.
Got a surprising amount of warmth on a sunny day mainly from the warmed roof tiles radiating the heat in the ceiling.  Probably picked up some escaping heat from the house but it's pretty will insulated so I doubt there was a lot came from that.

It was some extra free warmth that was quite effective on sunny days so all good.

[Cornelius]

I think veggie talks about a ground to air, or ground to water heat pump, where the latter are the most efficient kind, reaching a COP of over 5 easily. A circulation pump to circulate coolant in the ground loop are required, but 1HP sounds excessive; 100-200W would more likely suffice. A ground heat pump are preferred in climates where several months have below freezing temperatures...

I have an air to water heat pump with a COP of 4,6 @ 0deg. C., which are 5 years old now, so why you guys talk about COP of 3 i don't get. That's atleast 30 years old technology....

[mobile_bob]

maybe i am missing something here, but i don't think so...

the OP related a question about harvesting the heat from the generators burning fuel

i took that to mean the heat source to be the engines waste heat, perhaps the generators heat
and maybe if it is all in a box, the drive systems waste heat.

all of these heats are a finite souce

if for instance we burn a gallon of diesel per hour and produce 10kwe
and all the waste heats equate to about 20kw equivalent

then the absolute maximum heat we can recover from within the box is 20kw equivalent
no matter how efficient the method of harvest is...

this precludes any other heat source being taken into account.

i think once an analysis is done, any system that require mechanical active heat recovery is going to be less efficient than a well engineered passive recovery system... that again based on the assumption we are only going to harvest what heat is available within the confines of the box.

most well engineered passive heat recovery methods (coolant and exhaust heat exchangers) can achieve over 80% efficiency without much problem or relative cost...

now an active mechanical system "might" get close to 100% heat recovery from the box, however that last 20% of recovered heat comes at substantial cost, and it is likely that cost will be higher than the value of the last 20% recovered heat.

several years ago, i posited this question to a ME who's specialty is thermal engineering, he having been employed by a very well known entity that has a vested interest in all things thermal.

my thinking at the time was putting an aircooled diesel genset in such a box and using a common A/C compressor system from a car to remove all waste heat generated in the box to be moved to the point of use (in my case the house).

it just could not be made to map out in such a way as to be economically viable, however

he put me in the direction of passive recovery, which i thought to be the common plate or tube exchangers, but rather his direction led me to explore "heat pipe" technology.

my thinking is a "heat pipe" based system, coupled to a small recirculation pump to move the heat into a concrete slab or other thermal mass "might" well be viable.

so i pass that along for consideration

bob g

[TimSR2]

Ok thanks all.. I get it now. Ronmar's 'heat concentrator' idea made it very clear.