How can heat exchangers have COP>1

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In summary, the conversation discusses the concept of a heat pump and its efficiency in transferring heat from the outside air to the inside of a house. The COP (coefficient of performance) of a heat pump can be greater than 1, meaning that it can produce more heat energy than the mechanical energy put in. This is due to the fact that a heat pump is "focusing" the heat in a smaller volume of air. However, the efficiency of a heat pump is dependent on the temperature difference between the inside and outside air. It is also noted that if the electricity used to power the heat pump comes from fossil fuels, the overall energy and CO2 emissions savings may not be significant. The conversation also explores the possibility of using heat
  • #1
Low-Q
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Hi,

I am building a new house. This house will be heated by a heat exchanger. I have been told that heat exchangers have a COP of 3-4 in practice (depending on inside and outside temperature, and the cooling medium).

What I do not understand, is the COP>1 part. In order to "charge" the house with heat, by extracting heat from outside air, it will require energy input to the pump. The house is leaking heat back to the outside air, so the heat exchanger must run in order to keep the same temperature in the house.

The house is insolated well, but that would also mean that a normal electric oven must add small portions of energy to keep the air inside at the same temperature...

I simply cannot understand how input energy is greater than output energy, and that heat exchangers does not require the same energy to sustain the temperature insde as a conventional electric oven does.

Can someone explain this in detail?

br.

Vidar
 
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  • #2
You mean a heat pump, not a heat exchangers.

Since a heat pump doesn't make heat directly, it just moves it from one place to another, it can have a COP (which is different than the efficiency) of greater than 1. Consider this: if you are pumping water, the energy input and efficiency are determined by flow rate and pressure. But what if you were more interested in the fact that the water was very warm compared to what you need. Then the energy you get out of the water and the pumping energy aren't directly related to each other anymore and you can get more heat energy out than mechanical energy put in. Same idea with a heat pump or air conditioner (which also has a COP of at least 3).
 
  • #3
russ_watters said:
You mean a heat pump, not a heat exchangers.

Since a heat pump doesn't make heat directly, it just moves it from one place to another, it can have a COP (which is different than the efficiency) of greater than 1. Consider this: if you are pumping water, the energy input and efficiency are determined by flow rate and pressure. But what if you were more interested in the fact that the water was very warm compared to what you need. Then the energy you get out of the water and the pumping energy aren't directly related to each other anymore and you can get more heat energy out than mechanical energy put in. Same idea with a heat pump or air conditioner (which also has a COP of at least 3).
OK, so the heat pump is just moving heat from the outside air to the inside at "no" cost except the energy required to maintain the pressure difference between hot and cold zone?
Is it correct to say that a heat pump is "focusing" the heat in the enormous volume of heat containing air at a small point (The heat pump itself)?

But wouldn't the potential difference in temperature be the same as the energy required to run the pump? If so, I am a little confused - still...Vidar
 
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  • #4
Low-Q said:
OK, so the heat pump is just moving heat from the outside air to the inside at "no" cost except the energy required to maintain the pressure difference between hot and cold zone?
Is it correct to say that a heat pump is "focusing" the heat in the enormous volume of heat containing air at a small point
It is taking some of the thermal energy of a larger mass of cooler matter (the ground) and transferring it to a smaller mass of warmer matter (the air in your house). If that is what you mean by "focusing", yes.
(The heat pump itself)?
The heat pump does the work needed to transfer the thermal energy. The second law of thermodynamics says that heat only flows on its own from hotter bodies to cooler bodies. To make it flow the other way, you must do work.
But wouldn't the potential difference in temperature be the same as the energy required to run the pump? If so, I am a little confused - still...
The greater the temperature difference, the greater the work (input energy) that must be done in order to move a given quantity of heat from the outside to the inside.

Incidentally, if your heat pump has a COP of only 3 or 4, you are not going to be saving much energy or reducing CO2 emissions overall if you use electricity that is generated by burning fossil fuels. A coal fired power plant is about 1/3 efficient: one unit of electricity output for three units of heat from burning fossil fuel. And there are losses from transmission that reduce that electrical energy a little further in getting it to your house. So if your COP is 3 you will be getting heat in your house that is about the same as if you burned fossil fuel in a furnace. If you are getting your electricity from a hydro, wind or solar source it is a different matter.

AM
 
  • #5
Does it exist "power plants" using heat pump technology? I mean, if it is possible to harness the heat from thin air and then "focus" that heat energy to make it more efficient for a heat-to-mechanical work engines (turbines or similar), wouldn't it be possible to generate electric power that is greater than the energy required to run the pump? In my mind, that should not be possible, but if the energy is present already in the air, it might be possible after all?

Vidar
 
  • #6
No. Efficiency of a heat engine and heat pump are inverses of each other. So if a heat pump has a cop of 4, then the best a heat engine could do for converting back to mechanical power is 25%, for no net gain.
 
  • #7
Ok. Thanks. Now I understand much better.
 
  • #8
russ_watters said:
No. Efficiency of a heat engine and heat pump are inverses of each other. So if a heat pump has a cop of 4, then the best a heat engine could do for converting back to mechanical power is 25%, for no net gain.
This thread is old, but I have one question that I came up with here the other day. If a heat pump have a COP of 4, and I also have a heat pump with COP 2, would that last heat pump become a heat engine with 50% efficiency that is being able to be "self powered" by the COP 4 heat pump? Or is the calculations wrong regarding the relationship between COP and efficiency.

Br.

Vidar
 
  • #9
Any idea about post 8?

My guess is that the big area in the ground cools down as the "self running" system runs. The ground will be supplied with heat from surrounding groundwater or soil.

Is it possible to increase the efficiency of a heat engine this way - as questioned in post 8?

Anyone?

Vidar
 
  • #10
The performance of an ideal heat pump and heat engine is dictated by the temperature difference produced/utilized. So the DT produced by a 4:1 COP heat pump will be much, much too small to run a 50% efficient heat engine. If you try to increase the DT out of the heat pump, the COP will go down. If you try to use a lower DT to run the heat engine, the efficiency will go down.

You can google for the efficiency equations and play with them -- they are not hard to use.
 

1. How is it possible for a heat exchanger to have a COP greater than 1?

The coefficient of performance (COP) is a measure of the efficiency of a heat exchanger, and a COP greater than 1 means that the heat exchanger is producing more heat output than the amount of energy input. This is possible because heat exchangers do not create energy, they simply transfer it from one source to another. Therefore, a COP greater than 1 can be achieved by utilizing an external energy source, such as electricity, to drive the heat transfer process.

2. What factors influence the COP of a heat exchanger?

The COP of a heat exchanger is influenced by a variety of factors, including the design and type of heat exchanger, the flow rates and temperatures of the fluids being exchanged, and the efficiency of the heat transfer surfaces. Additionally, the type of energy source used to power the heat exchanger can also impact the COP.

3. Are there any limitations to achieving a COP greater than 1 in a heat exchanger?

While a COP greater than 1 is theoretically possible, in practice, there are limitations that prevent heat exchangers from achieving this level of efficiency. These limitations include losses due to friction, thermal resistance, and other inefficiencies in the heat transfer process. Additionally, the energy source used to power the heat exchanger may have its own limitations.

4. Can a heat exchanger with a COP greater than 1 violate the laws of thermodynamics?

No, a heat exchanger with a COP greater than 1 does not violate the laws of thermodynamics. The first law of thermodynamics states that energy cannot be created or destroyed, only transferred. A heat exchanger with a COP greater than 1 is simply transferring energy from one source to another, using an external energy source to drive the process.

5. How can a heat exchanger with a COP greater than 1 be beneficial?

A heat exchanger with a COP greater than 1 can be beneficial in situations where a high level of heat transfer efficiency is needed, such as in industrial processes or in heating and cooling systems. It can also help to reduce energy costs and improve overall system efficiency. Additionally, advancements in heat exchanger technology have made it possible to achieve COPs greater than 1, making them a more viable and sustainable option for energy transfer.

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