Why don't we heat homes using air conditioners?

[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.

 

[pallidin]

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. 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.

I see.
I recuse myself from this disussion.
Best of luck to all.

 

[Jocko Homo]

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

What?!

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...

You still haven't rebutted this... at all!

 

[russ_watters]

Best of luck to all.

You too.

I recuse myself from this disussion.

I think that's a good idea, but more importantly, while dropping the fight, I still highly recommend you go back and read some of the evidence and discussion posted - read it for real, not as if people are trying to trick you to win a game. If you drop the fighting stance, you may yet be able to start learning. And don't worry - this really isn't about winning or losing, so I don't much care if you learn you are wrong and come back to admit it. The learning is what is important.

...and FYI, while it may feel clever to say you'd bet your paycheck on this issue, I actually do bet my paycheck (and my clients' money!) on this issue every day!