Heat pump limitations

[Bhatsantosh]

Can someone please reply what is the main reason for heat pump having limitations of not achieving temperature more than 65-70 degree Celsius??? what are the other limitations of heat pump in industrial applications

 

[Drazen]

you need a course in thermodynamics and than all will be clear, it cannot be described in common language, you need to understand Carnot's cycle to go further with it.

 

[EnergyProfessional]

+ 1 for Drazen. Also ask the manufacturer. They are built for different temperatures. But ultimately the limit is the lift (the pressure or temperature difference between condenser and evaporator).

The refrigerant properties determine the temperatures and pressures the pump operates under.

in theory with a different refrigerant you could get 100°C (condenser), but then you need an evaporator stream of 40°C. the problem is, when it is 40°C outside we don't need heating....

 

[willard3]

You should find an Engineer who knows about this. This is a web forum, not a thermo class.

 

[Drazen]

As far as I know, heat pumps with 65-70 Celsius are offered by some manufacturers and they only serve as least evil for retrofit projects.

If there is existing large heating network that is not planned for renewal, using such pumps to replace old boilers can be feasible solution if proper calculations prove it. of course, their efficiency is not comparable to ordinary heat pump regime, so they are generally unacceptable for new projects.

 

[chicopee]

Actually the Carnot cycle will not explain much, on the other hand, the refrigeration cycle, being part of the Thermo program, in conjunction with the type of refrigerant used and the construction features of a refrigeration system to which a heat pump belongs should answer your question.

 

[dbill74]

All the above.
Heat pumps work via the "vapor compression refrigeration cycle"; in short this evaporates/condenses a refrigerant in the respective sides of the cycle. I just found this page this morning and it will help explain the physics.
Basically, temperatures you are able to get out of a heat pump are limited by the physical characteristics of the refrigerant used.

 

[getitdone91]

Isn't the "vapor compression refrigeration" or "refrigeration" cycle a version of a theoretical Carnot cycle? Hence, reviewing a Carnot cycle to understand condensing and evaporating limits (pressure and temperature) of a given refrigerant would help the OP understand the thermodynamics of a heat pump.

 

[dbill74]

Yes, the refrigeration cycle is a version of the Carnot cycle. The thing about the Carnot cycle is it is a theoretical ideal. However we don't live in an ideal world, thus we have real thermodynamic cycles like the refrigeration and power cycles which are based on Carnot cycle.

On the page I linked previously there are graphs showing a curve that looks like a bell, this curve is a saturation line and represents when the refrigerant is completely gaseous or liquid with the space under the curve being a mix of gas and liquid. What the graphs do not show is actual temperature and pressure values. This is because the bell curve moves based on the physical characteristics of the refrigerant. Manufacturers can also play with the pressure of the refrigerant to move the cycle curve up and down on the bell curve; or they can get really fancy and add things like hot gas bypass completely changing the shape of the curve (but that would be a topic for another thread).

In a Carnot cycle there would is no limit to the temperature of a heat pump which does not answer OP's question.

 

[getitdone91]

I see what you are saying. I was viewing it in an over-simplified manor.

 

[Drazen]

on contrary, theoretical, ideal cycle is ultimately necessary to understand basic concepts, than it is much easier to develop further understanding of specific instances.

mr carnot himself did not live in ideal world, but he managed to abstract principles which allow us to have overall understanding now.

skipping general, basic theory while dedicating to some isolated detail of the problem is no. 1 reason for costly mistakes which are often irreparable in economically feasible way.

 

[willard3]

The important thing to remember about the Carnot cycle is that the theoretical efficiency is in the 30% range. From an Engineer's standpoint, this is lousy efficiency.

 

[EnergyProfessional]

willard3: all depends on operating temps. Use sun surface temperature on the one side and space cold on the other and efficiency improves

 

[alianapaul]

Heat pump usages the electricity so sufficient electrical connection required to heat pump.