Heat Pumps In general 1

[Calador]

Hi,

I am currently researching the feasibility of using a seawater heatpump as it is like a giant replenishable heat reserviour. I have done some digging and there are so many articles out there that suggest that this is the way to go in terms of efficiency. I believe sea water heat pump should have the same principle as Ground source heat pumps (ill get to that later).

what baffles me is the fundamentals of how heat pump works. So i would like to get some view on my explanation of the cycle, whether it is right or wrong.

At the heat exchanged at the condenser: Say we are using ammonia as our refrigerant. Since the specific heat capacity fo ammonia and water is ~ same (4.7 vs 4.2), we can say that there is equal transfer in heat with the same flow rate. Simple enough as there is not change in phase. Past the expansion valve, we are talking about low pressure vapour/liquid changing its phase to gas in the evaporator (no change in temp). Abundant heat energy contantly coming from either the ground(10°C) or sea (10°C) as both sides have constant temperature.

Based on the fact that ammonia has a low boiling point (-33°C, low enthalphy of vaporization), I would therefore assume that even if the outdoor temperature (-3°C) is sufficiently large to trigger the change in phase and hence the continuation of the cycle.

With that in mind, I still cant justify the feasibility of having sea water heating/cooling or ground source heating/cooling. I must be missing the point here as this is a proven fact by many intelligent engineers out there. The thing is that, until i can satisfy myself that it works, i cant just drown this thought out.

Thanks for any guidance.

 

[chicopee]

If you had Classical Thermodynamics in college, you had to study refrigeration cycle. A heat pump operates on that same principle. The evaporator is exposed to a medium (sea water in your case) that is warmer than the low pressure side of the cycle thereby warming up the refrigerant slightly (if properly designed)into the superheat region for the compressor to operate properly. The condensor is exposed to a medium ( ambient air most likely) that is cooler than the high pressure of the cycle and that medium will get warm to heat up its space.
The heat gain from a heat pump is primarily from the heat of compression generated by the compressor and not so much from the enthaly gained on the low pressure side of the cycle.

 

[Calador]

that is what i am trying to get my head around. if you were to split the cycle into two parts, the low pressure and the high pressure, like you said, most of the heat comes from the fact that the refrigerant is changing phase and hence the heat generation in the high pressure side.

To complete the cycle, the high pressure liquid has to evaporate/vaporize to facilitate it being compressed again. During the change in phase, there is no temperature change in the medium.

So why do people say if it is more efficient as there is abundant heat supply coming from the sea when all it does is just to change the phase of the refrigerant.

If we substitute the refrigerant to something like water where the boiling point is higher, Yes that would work as the water will absorb whatever heat it gets from the sea and if the temperature gradient is higher (between the sea and the water in the cycle), it does show that there is Alot of energy in the sea. Q=mcT where m is close to infinite.

 

[MintJulep]

You seem to have a few misconceptions.

Firstly, there IS a phase change in the condenser. Think about what "condense" means.

Secondly, you seem to think that a refrigeration cycle is somehow self-sustaining. It is not. There is a compressor in there.

Thirdly, it requires LOTS of energy to change the phase of fluids. That's why the vapor compression cycle is so much better at moving heat energy around than single-phase systems.

Forthly, there is NO heat generation in the condenser (or the evaporator for that matter), only heat transfer.

 

[chicopee]

I dont exactly remember the numbers but for argument sake if the compressor requires 1KW of power, the heat exhange at the condensor will be a lot higher, let's say 4 KW. The ratio of 4:1 is not exact as it depends on the refrigerants but you get the idea.
What you have to also remember is that the life expectancy of a heat pump since it is also used during the summer for cooling is about half that of an air conditioner.

 

[willard3]

Read about "Carnot Cycle". Carnot cycle is pretty inefficient unless you don't pay for the heat source/sink.

Second law of thermo will acquaint you with "free" heat source/sink.

 

[Calador]

Read about "Carnot Cycle". Carnot cycle is pretty inefficient unless you don't pay for the heat source/sink.

As you might have guessed, i didnt take thermodynamics. I may have brushed it a little and thats probably why i vaguely recognise the graph. Thank you for pointing me in the right direction.

Firstly, there IS a phase change in the condenser. Think about what "condense" means.

My initial thought was that the phase change is happening in the compressor itself as in past the compressor, the refrigerant has already been pressurized and in the heated liquid region ready to transfer heat through the coils into the other closed loop system.

Secondly, you seem to think that a refrigeration cycle is somehow self-sustaining. It is not. There is a compressor in there

I am sorry if i mentioned i but am not quite sure where did i indicate or gave you that impression. Anyhoo....

Thirdly, it requires LOTS of energy to change the phase of fluids. That's why the vapor compression cycle is so much better at moving heat energy around than single-phase systems.

This i appreciate with the latent heat issues. What i initially had problems was acutally visualizing what happens within the coils e.g. the evaporator coil. Past the expansion valve, the liquid is low temperature low pressue. Assuming we have an infinite length of coil and a constant flow rate (which we obviously do), the fluid in the earlier part of the coil will undergo phase change and once it has changed its phase, the temperature starts to rise. since we have an infinite length of coil, the fluid's temperature will rise and be thermally equilibrium with the sea water which is at approx 10°C. With a short piece of coil, this will not happen unless the flow rate is slow. Up to that point, it was all about the fundamentals of thermodynamic and the rest is up to you to design the heat pump so that the flow rate will work best with the length of the coil/heat exchanger plate you have to work with.

Up to here, have i been straying away from what is actually happening or is my reasoning totally off the chart.

Forthly, there is NO heat generation in the condenser (or the evaporator for that matter), only heat transfer.

This is also something i dont quite agree. (at this point, I might very well be saying i found newton's first law)
if i were to imagine a bicycle pump; the piston being the compressor, the outlet(restricted opening) as the expansion valve and the tube being the coil. as the vapour is being compressed throughout the tube which is the whole of the condenser bit(compression takes place the moment the vapour leaves the compressor till it reaches the valve). So i believe that with respect to the flow rate, the flow rate has to be fast enough to be able to compress the vapour to liquid and still be within that cycle so that heat transfer can take place via the heat exchanger.

Does any of this make sense? or is my theory all wrong.

 

[MintJulep]

Does any of this make sense? or is my theory all wrong.

Pretty much the second part.

Wiki offers a decent description.

http://en.wikipedia.org/wiki/Vapor-compression_refrigeration

 

[IRstuff]

I think you missed a key point, which is elucidated in the link provided by MintJulep, that the condenser is a heat exchanger.

A refrigeration system has two heat exchangers, the primary one, which absorbs heat from the system being cooled, and the secondary one, wherein the waste heat absorbed by the primary exchanger is released to the environment or some other medium. In such a system, the absorbed heat must be transferred elsewhere, outside of the system, otherwise, it simply gets so hot that nothing else will happen.

TTFN

FAQ731-376

 

[sterl]

Heat is transferred from the refrigerant to the surrounding (Nominally air if this is a Comfort Heater) on the high pressure side of the system by the heat exchanger called "condenser"...The refrigerant is thus condensed.

Heat is transferred from a different set of surroundings (in this case the sea water) to the refrigerant by the heat exchanger called "evaporator"...The refrigerant is thus evaporated.

The difference in temperature between the target condition (in this case the interior air) and the consequential condition (temperature of the sea water) establishes the minimum pressure difference for the system based on the saturation pressure corresponding to the 2-temperatures. Of course the Ideal can only be achieved with infinitely large heat exchangers such that the temperature differences fluid to refrigerant are minimal. The compressor needs to develop as much pressure as it takes to overcome the effected fluid's temperature difference plus provide the temperature differences for the heat transfer for both heat exchangers.
The only concern of consequence on the low pressure side is not the boiling point (we build lots of systems to operate at 15 to 18 inches of Hg VACUUM on the low pressure side) but the freeze point of the refrigerant and the characteristics and transport of the lubricant that corresponds with both the refrigerant of choice and the style of the compressor. Once the lubricant starts travelling around the circuit, it can't get so cold or find a low velocity section of piping or heat exchanger that it won't return to the compressor from the cold side of the system....


Ammonia is a great refrigerant because it has relatively simple transport properties, is a cheap fluid and has a very high latent heat thus there is a minimum mass flow through the system and pipes can be quite small...Most of which doesn't make a lot of difference to a domestic oriented circuit because the piping is short and tranposrt details are controllable as per a factory manufactured unit; and the fluid charge is minimal.

In the meantime: Ammonia is toxic and corrosive to a variety of materials...And considerably more difficult to handle in a "service" sense than the halocarbon refrigerants.

Oh: and there are domestic unitary equipment out there that employ ammonia and water as a sorbent pair, that will far outperform any compression system.....

 

[KiwiMace]

Try this - online model is totally educational on heat pumps/ac. Play with the parameters and see what happens. I think i've posted this model before. Its great.

http://www.ornl.gov/~wlj/hpdm/doehpdm.shtml