Why don't we heat homes using air conditioners?

[TurtleMeister]

Yes. I like to use the train analogy. Imagine a train bringing coal to an electrical power plant. The energy contained in the coal is far, far greater than the energy that the train used to bring it. Yet, we don't say that the train is 100000% efficient, because it didn't create the coal...it simply moved it.

The heat pump performs the same function as the train. The energy is there, it simply brings it to where we want it.

Edit: an electric conveyor delivering chopped wood is yet another example :)

Good analogy.

I've come up with a thought experiment that has helped me understand the difference between the electric heater and the heat pump. Imagine two rooms, room A and room B, separated by an insulated wall. Both rooms are also insulated from the outside. There is a heat pump in the wall that separates the two rooms. Initially, both rooms have the same temperature.

Now, we want to increase the temperature in room A. So we turn on the heat pump. As the temperature increases in room A, it decreases in room B. But as the temperature difference increases we find that the average temperature of both rooms remains the same. So the electrical energy used by the heat pump is NOT being converted into heat energy.

However, if we have the same setup, except this time we use a space heater to heat room A, we find that as the temperature increases in room A, the temperature in room B remains the same. The average temperature of both rooms increases. So the electrical energy being used by the space heater IS being converted to heat energy.

 

[cjl]

Good analogy.

I've come up with a thought experiment that has helped me understand the difference between the electric heater and the heat pump. Imagine two rooms, room A and room B, separated by an insulated wall. Both rooms are also insulated from the outside. There is a heat pump in the wall that separates the two rooms. Initially, both rooms have the same temperature.

Now, we want to increase the temperature in room A. So we turn on the heat pump. As the temperature increases in room A, it decreases in room B. But as the temperature difference increases we find that the average temperature of both rooms remains the same. So the electrical energy used by the heat pump is NOT being converted into heat energy.

However, if we have the same setup, except this time we use a space heater to heat room A, we find that as the temperature increases in room A, the temperature in room B remains the same. The average temperature of both rooms increases. So the electrical energy being used by the space heater IS being converted to heat energy.

Well, almost...

The energy used by the heat pump will indeed go into heat energy, and the average temperature of both rooms will increase. However, the average temperature of both rooms will increase much less for a given delta t in the room being heated than it would with a resistive heater.

 

[TurtleMeister]

Well, almost...

The energy used by the heat pump will indeed go into heat energy, and the average temperature of both rooms will increase. However, the average temperature of both rooms will increase much less for a given delta t in the room being heated than it would with a resistive heater.

Yes, that is true. But the nice thing about thought experiments is that you can imagine such things as "perfect heat pumps". And the point of the thought experiment was to show the main difference between heat pumps and space heaters. Another reality of heat pumps, that I haven't quite figured out yet, is that the greater the difference in temperature (between room A and room B) the less effective the heat pump becomes. Eventually you will reach a temperature difference where the space heater becomes more effective than the heat pump. Does it have anything to do with the refrigerant, and the temperature difference between the evaporator and condenser coils?

 

[Lsos]

The difference in temperature is the equivalent of asking the train with the coal to go up a hill. The greater the temperature difference, the higher the hill. Eventually you get to the point where you're just better off screwing the train and digging for coal on the spot, or exploring other options.

 

[OmCheeto]

Cause it's just a fluid going around in a circle.

I didn't think you could have a refrigeration cycle with strictly a fluid. Should this be; "Cause it's just a fluid/gas going around in a circle." ?

I suppose a radiator/cooling system in a car is strictly fluid. But I don't think they refer to them as refrigeration units, even though they remove heat.

hmm... http://en.wikipedia.org/wiki/Refrigeration" ?:

Refrigeration is a process in which work is done to remove heat from one location to another.

Bah!

That means I'm a refrigerator...

 

[Antiphon]

The car radiator is convective cooling only. In my book, to be heat pump you'd have to involve a thermodynamic cycle resembling Carnot. Unlike a radiator, such a thing can move heat from a colder into a warmer area.

 

[OmCheeto]

The car radiator is convective cooling only. In my book, to be heat pump you'd have to involve a thermodynamic cycle resembling Carnot. Unlike a radiator, such a thing can move heat from a colder into a warmer area.

Not according to the all knowing, all seeing wiki:

http://en.wikipedia.org/wiki/Convection" is the movement of molecules within fluids (i.e. liquids, gases) and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids.

bolding mine

I don't know what planet you are from, but all of the automotive radiators* on my planet are solid.

*mostly made out of either copper or aluminum. both very good thermal conductors.

 

[russ_watters]

I didn't think you could have a refrigeration cycle with strictly a fluid. Should this be; "Cause it's just a fluid/gas going around in a circle." ?

Note the definition of "fluid" in your next post...

A car radiator utilizes both conduction and convection: Conduction is what transfers energy to/from the two fluids and the radiator and convection which mixes/equalizes temperature inside the fluid due to turbulence.

 

[Antiphon]

Not according to the all knowing, all seeing wiki:


bolding mine

I don't know what planet you are from, but all of the automotive radiators* on my planet are solid.

*mostly made out of either copper or aluminum. both very good thermal conductors.

I meant the heart of the radiator system of course which is a circulating convective fluid loop.

 

[OmCheeto]

Note the definition of "fluid" in your next post...

Gads!

I stand corrected.

 

[cjl]

Yes, that is true. But the nice thing about thought experiments is that you can imagine such things as "perfect heat pumps". And the point of the thought experiment was to show the main difference between heat pumps and space heaters. Another reality of heat pumps, that I haven't quite figured out yet, is that the greater the difference in temperature (between room A and room B) the less effective the heat pump becomes. Eventually you will reach a temperature difference where the space heater becomes more effective than the heat pump. Does it have anything to do with the refrigerant, and the temperature difference between the evaporator and condenser coils?

First off, even a perfect heat pump/air conditioner actually requires some energy, and will add that to the heat leaving the hot side. The amount of energy added to the warmer room per unit of energy used by an ideal heat pump is equal to the (absolute) temperature on the warm side divided by the difference in temperature between the warm and the cold side. This also shows that as the temperature difference increases, the maximum efficiency goes down.

As for why the pump becomes less effective (physically) with larger temperature differences? You're pretty much right about it being the temperature difference between the evaporator and condenser coils. To maintain a larger temperature difference between evaporator and condenser, the pressure difference between the two must be increased, which means that the compressor must work harder.

 

[TurtleMeister]

The difference in temperature is the equivalent of asking the train with the coal to go up a hill. The greater the temperature difference, the higher the hill. Eventually you get to the point where you're just better off screwing the train and digging for coal on the spot, or exploring other options.

Another good analogy Lsos.

As for why the pump becomes less effective (physically) with larger temperature differences? You're pretty much right about it being the temperature difference between the evaporator and condenser coils. To maintain a larger temperature difference between evaporator and condenser, the pressure difference between the two must be increased, which means that the compressor must work harder.

I'm thinking now that it's not just the temperature difference, but also the temperature of the cold side. The colder it is, the less heat energy there is to move. Which makes it more difficult. Similar to pulling a vacuum.

Edit:
On second thought, the vacuum analogy is probably not right. However, having less heat energy available to move may still have an effect on the effectiveness of the heat pump.

 

[pallidin]

Stop already.
A purely resisitive heating element is far more efficient than a two-stage mechanism.
A 1000 watt air conditioner in reverse is LESS able to produce useful heat as a 1000 watt resistive space heater.
Not to mention the fact that you don't have to have part of the resistive heater outdoors.

 

[russ_watters]

Stop already.
A purely resisitive heating element is far more efficient than a two-stage mechanism.
A 1000 watt air conditioner in reverse is LESS able to produce useful heat as a 1000 watt resistive space heater.
Not to mention the fact that you don't have to have part of the resistive heater outdoors.

Nope, nope, nope. Read the specs on any heat pump.

[though I don't know what you mean by a "two-stage mechanism".]

Try this one: http://www.residential.carrier.com/products/acheatpumps/heatpumps/performance.shtml

It has a cooling SEER of 16.5 and a heating HSPF of 9.5. That means:
In heating mode, it produces 2.8 watts of heat for every watt of input power.
In cooling mode, it produces 4.8 watts of "cool" for every watt of input power.

 

[Lsos]

A 1000 watt air conditioner in reverse is LESS able to produce useful heat as a 1000 watt resistive space heater.

Maybe you're right. However, an air conditioner in reverse doesn't PRODUCE heat, it merely MOVES it. And if your goal is to have a warm home, in a lot of cases it reaches this goal far more effectively.

 

[pallidin]

Maybe you're right. However, an air conditioner in reverse doesn't PRODUCE heat, it merely MOVES it. And if your goal is to have a warm home, in a lot of cases it reaches this goal far more effectively.

OK, that's my contention!
Doesn't it take MORE energy to "move" heat than to simply produce it outright??
(In a winter condition that is)

Come on guys, anyone that knows me knows I respect PF and am very rarely argumentative.
But this whole idea seems intuitively ludicrous.
Or maybe I'm just brain dead.

@Russ: the "two-stage mechanism" I was referring to is with regards to the fact that a compressor is used.

 

[Jocko Homo]

OK, that's my contention!
Doesn't it take MORE energy to "move" heat than to simply produce it outright??
(In a winter condition that is)

Come on guys, anyone that knows me knows I respect PF and am very rarely argumentative.
But this whole idea seems intuitively ludicrous.
Or maybe I'm just brain dead.

@Russ: the "two-stage mechanism" I was referring to is with regards to the fact that a compressor is used.

I really must be missing something 'cause there's something that seems totally obvious to me but every seems to keep missing it...

The heat pump can't be less efficient (loosely defined) than the "resisitive heater" because heat is the necessary by-product of the heat pump (aside from the noise, I guess, but I think the energy in that is trivial)...

For example, let's compare a heat pump using 1000 W of energy to a resistive heater using the same amount of energy. Obviously the resistive heater is going to produce 1000 W of heat energy. However, the heat pump will also produce that much heat energy (as its waste by-product, otherwise heat pumps can be perfectly efficient) plus whatever heat it was pumping from whatever reservoir it was connected to. Therefore, the heat pump can't be any worse than the resistive heater, hence my originating post...

Why aren't all electrically heated homes using heat pumps?

 

[pallidin]

OK...

Envision two "igloo's" in Antartica, one has a 1500 watt reverse air-conditioner mounted in a window, the other igloo has a 1500 watt resistive space heater on the floor.

I would bet my next paycheck that the resistive heater will warm my igloo much, much better.

 

[Jocko Homo]

OK...

Envision two "igloo's" in Antartica, one has a 1500 watt reverse air-conditioner mounted in a window, the other igloo has a 1500 watt resistive space heater on the floor.

I would bet my next paycheck that the resistive heater will warm my igloo much, much better.

Without insult, that is a vacuous answer!

Never mind that your example didn't even necessitate the Antarctic homes being "igloos," it's a naked appeal to common sense. However, since I've never actually heated my home using my air conditioner, my common sense isn't worth very much in this scenario. I didn't ask whether you would heat your home with an air conditioner, I asked why don't you? In other words, why doesn't the heat pump heat as well as a resistive heater?

Incidentally, you may need to prepare to give up your next pay cheque!

 

[pallidin]

A resistive heater (or a resistive infrared heater) is the most efficient electrically heat producing product on earth. Nothing else even comes close. Using electricity that is.

 

[russ_watters]

OK...

Envision two "igloo's" in Antartica, one has a 1500 watt reverse air-conditioner mounted in a window, the other igloo has a 1500 watt resistive space heater on the floor.

I would bet my next paycheck that the resistive heater will warm my igloo much, much better.

You would most definitely lose that bet.

Consider this analogy: which system will provide more heat for your house:

1. A 1000w resistive heater.
2. A 1000w pump bringing a gallon of oil per minute to your furnace, where it is burned.

 

[pallidin]

You would most definitely lose that bet.

Consider this analogy: which system will provide more heat for your house:

1. A 1000w resistive heater.
2. A 1000w pump bringing a gallon of oil per minute to your furnace, where it is burned.

We are talking about reverse air-conditioners and their comparability to a resistive space heater.

 

[russ_watters]

Do you understand the concept of an analogy? The purpose of this one is for you to make sure you recognize that situations exist where you can get more heat out than the electrical energy you put in. So far you haven't been trying to learn how a heat pump works but instead have been arguing based on COE.

 

[pallidin]

Do you understand the concept of an analogy? The purpose of this one is for you to make sure you recognize that situations exist where you can get more heat out than the electrical energy you put in. So far you haven't been trying to learn how a heat pump works but instead have been arguing based on COE.

I fully understand that, russ, but that scenario is a "special circumstance"
Much like my "igloo example" is a special circumstance.

ANY heat pump specifically relies on environmental conditions; resistive heating does not.

 

[Jocko Homo]

A resistive heater (or a resistive infrared heater) is the most efficient electrically heat producing product on earth. Nothing else even comes close. Using electricity that is.

Okay pallidin... If you're so sure of your conclusion then please point out the flaw in the reasoning I stated earlier:

The heat pump can't be less efficient (loosely defined) than the "resisitive heater" because heat is the necessary by-product of the heat pump (aside from the noise, I guess, but I think the energy in that is trivial)...

For example, let's compare a heat pump using 1000 W of energy to a resistive heater using the same amount of energy. Obviously the resistive heater is going to produce 1000 W of heat energy. However, the heat pump will also produce that much heat energy (as its waste by-product, otherwise heat pumps can be perfectly efficient) plus whatever heat it was pumping from whatever reservoir it was connected to. Therefore, the heat pump can't be any worse than the resistive heater, hence my originating post...

If you can't spot the flaw in this logic then you should consider why it is...

 

[Jocko Homo]

Do you understand the concept of an analogy? The purpose of this one is for you to make sure you recognize that situations exist where you can get more heat out than the electrical energy you put in. So far you haven't been trying to learn how a heat pump works but instead have been arguing based on COE.

What does COE mean in this context?

 

[Jocko Homo]

I fully understand that, russ, but that scenario is a "special circumstance"
Much like my "igloo example" is a special circumstance.

ANY heat pump specifically relies on environmental conditions; resistive heating does not.

...but you haven't explained what makes it a special circumstance. What about those circumstances that makes the heat pump not heat the home as well as the heater?

Heat pumps rely "on environmental conditions" but you haven't explained what it is about those conditions that prevent the heat pump from heating as well as the heater. I've already explained why the heat pump should heat better. Can you give an actual counter example as to why the heater will heat better?

 

[russ_watters]

ANY heat pump specifically relies on environmental conditions; resistive heating does not.

Of course: the COP drops as the outside temperature drops. But as stated, it never drops below 1. In fact, if it were below 1, that would violate COE! (Conservation Of Energy). Caveat: below a certain temp the refrigerant will no longer change state and it will cease to function. Presumably if someone were to decide to use a heat pump in antarctica they'd design it so it could actually function.

 

[Jocko Homo]

Of course: the COP drops as the outside temperature drops. But as stated, it never drops below 1. In fact, if it were below 1, that would violate COE! (Conservation Of Energy). Caveat: below a certain temp the refrigerant will no longer change state and it will cease to function. Presumably if someone were to decide to use a heat pump in antarctica they'd design it so it could actually function.

Ah, your caveat is fascinating!

In the case where the refrigerant no longer changes state, the heat pump simply becomes a resistive heater. It might lose a little bit of efficiency dumping heat to the outside as it continues to pump the refrigerant but I think that loss will be very little, much like the loss due to noise...

The solution to this problem is obvious though... use a different refrigerant!

 

[pallidin]

Of course: the COP drops as the outside temperature drops.

My space heater wins. Cheque please.

Oh, that's only a buck two-ninety-five to your favorite charity.

 

[Drakkith]

My space heater wins. Cheque please.

Oh, that's only a buck two-ninety-five to your favorite charity.

Only if you take your standard home AC unit out to an igloo. Design it so that you have the right coolant and I'm betting it will work.

 

[pallidin]

Not as good as a resistive space heater. Show me ANY evidence otherwise.

 

[russ_watters]

My space heater wins.

Again, the COP of a heat pump never drops below 1, so no, your space heater does not win.

This game is really tiring. You're being argumentative and making no effort at all to learn how these things actually work, trying to contrive a scenario where an electric might win while ignoring the regular usages where they never do. What you're doing is like saying walking is faster than driving because my car won't start in Antarctica. It's irrelevant and purposely evasive/argumentative.

Not as good as a resistive space heater. Show me ANY evidence otherwise.

Logic has been posted that you refuse to think about and evidence has been posted that you refuse to look at. What's left to do? Perhaps you should email Carrier and tell them their performance spec is wrong.

 

[Mapes]

OK, that's my contention!
Doesn't it take MORE energy to "move" heat than to simply produce it outright??
(In a winter condition that is)

Come on guys, anyone that knows me knows I respect PF and am very rarely argumentative.
But this whole idea seems intuitively ludicrous.
Or maybe I'm just brain dead.

Not brain dead, but your intuition is leading you astray. And it's easy in this context; the idea of a heat pump moving more energy than it consumes is really mind-blowing, and it confuses a lot of people. I've been enjoying following this thread, and just wanted to duck into get a piece of your paycheck.

A mechanical device must obey the Laws of Thermodynamics. It's a common thought experiment to consider the workings of a reversible heat engine (a Carnot engine) to determine how efficient a heat engine can possibly be. Now let's try to do the same thing with a heat pump. We want to see how efficient such a pump could be in principle, so we'll assume that the mechanism is carefully built and optimized and generates very little entropy. However, we must obey the First and Second Laws: total energy must be constant, and total entropy can't decrease.

Consider a heat pump operating between two reservoirs at 200 K and 300 K. Assume that it reversibly moves 600 J per cycle (look at an air conditioner if you doubt that one part of a machine can be colder that a cold region and another part hotter than an adjacent hot region, and can subsequently transfer energy up this temperature gradient). As a result, 600 J of energy and 3 J/K of entropy leave the cold reservoir per cycle. This corresponds to 600 J of energy and 2 J/K of entropy entering the hot reservoir. But this isn't possible on its own, because then total entropy would decrease and the Second Law would be violated. (This is, of course, why colder objects never heat hotter objects.) So we supply 300 J of work to our electricity-driven heat pump, which is used to heat the hot reservoir, adding 300 J and 1 J/K. In the end of each cycle, the heat pump has used 300 J to move 600 J.

In practice, the heat pump will generate entropy due to mechanical inefficiencies and temperature gradients in the mechanism. But I hope this convinces you that your first statement above is not universally correct.

(Source: a thousand textbooks.)

 

[Drakkith]

Not as good as a resistive space heater. Show me ANY evidence otherwise.

From wikipedia on Heat Pumps:

When used for heating a building on a mild day of say 10 °C, a typical air-source heat pump has a COP of 3 to 4, whereas a typical electric resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place.

Note that the heat pump is more efficient on average in hotter climates than cooler ones, so when the weather is much warmer (in a desert city or southern city)the unit will perform better than average COP. Conversely in cold weather the COP approaches 1. Thus when there is a wide temperature differential between the hot & cold reservoir's the COP is lower (worse).

When there is a high temperature differential on a cold day, e.g., when an air-source heat pump is used to heat a house on a very cold winter day of say 0 °C, it takes more work to move the same amount of heat indoors than on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will approach 1.0 as the outdoor-to-indoor temperature difference increases for colder climates (temperature gets colder). This typically occurs around −18 °C (0 °F) outdoor temperature for air source heat pumps. Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice. In other words, when it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater than to strain an air-source heat pump.

Geothermal heat pumps, on the other hand, are dependent upon the temperature underground, which is "mild" (typically 10 °C at a depth of more than 1.5m for the UK) all year round. Their COP is therefore normally in the range of 4.0 to 5.0.