A perspectives of the heat pumps

[Moreno]

Do I understand it correctly, that if outside temperature is -20 C and we intend to heat premises to +20 C, using heat pump solely, the maximum theoretical efficiency going to be around 700%?

https://www.engineeringtoolbox.com/heat-pump-efficiency-ratings-d_1117.html

If yes, then why heat pump practical efficiencies are still much lower than theoretical values? What is the issues with technologies? Why Otto or Diesel engines work much closer to theoretical values than heat pumps?

Also, how practical would be air-air heat pumps designed for apartments in buildings where use of ground waters is hardly an option? What would be their size and cost at temperatures -20 C and +20 C outside and inside? I can imagine they would need to process a giant amount of cold air to retrieve sufficient amount of thermal energy from it to heat a large apartment? What is actually the directions of development in this field?

Could there be a heat pump which is brought in action by burning fuel rather than electricity? If yes, would it be still as efficient as an electric one?

Edited March 3, 2018 by Moreno

[studiot]

Heating the room air to 20C alone will not raise the room temperature to 20C as a whole.

Look carefully at the link you quoted and their example refers to heat supplied to room, not air temperature.

There are pros and cons to both air to air and air to water heat pump systems.

[Moreno]

11 minutes ago, studiot said:

Heating the room air to 20C alone will not raise the room temperature to 20C as a whole.

Look carefully at the link you quoted and their example refers to heat supplied to room, not air temperature.

There are pros and cons to both air to air and air to water heat pump systems.

So, if we need raise room temperature to +20 C how efficiencies change than?

[Endy0816]

I don't *think* COP is multiplied by 100% like Carnot efficiency is for heat engines. Calculated 7.325. Almost the exact inverse formula though.

Diesels benefit from being able to largely use the environment to reset their cycles. Hot exhaust gasses are cooled and made usable again. Does wonders for efficiency.

 

May want to price out electricity and fuel costs, then go looking at the different models. Generally here we'd be looking at BTU's removed(or added in this case) based on room/house size. Joules or maybe KWH might be the equivalent there.

[Frank]

6 hours ago, Moreno said:

Also, how practical would be air-air heat pumps designed for apartments in buildings where use of ground waters is hardly an option? What would be their size and cost at temperatures -20 C and +20 C outside and inside? I can imagine they would need to process a giant amount of cold air to retrieve sufficient amount of thermal energy from it to heat a large apartment? What is actually the directions of development in this field?

Could there be a heat pump which is brought in action by burning fuel rather than electricity? If yes, would it be still as efficient as an electric one?

Needing ground source heat is a big problem - tends to be expensive no matter where it is, worse where space is limited.  It would be nice if the city could supply it from a nearby basin or river.

Air heat pumps tend to equal energy put in as heat at the bottom end of their usefulness which is above the lowest outdoor temperatures in cold regions, so that other means are necessary to supplement when temperatures are coldest.  Basically, we need two systems, again expensive.

There are heat pumps that run off of fuels (natural gas) from whispergen, honda and others, but they all have their issues - noise, vibration, oil changes, expense or efficiency...

I'd love to find a solution to this, looking at solar heat as potential since there is a lag between coldest temperatures and shortest days, but it's a cumbersome system.

 

Edited March 3, 2018 by Frank
missing word

[studiot]

10 hours ago, Moreno said:

So, if we need raise room temperature to +20 C how efficiencies change than?

 

Would you rather believe the salesman's exaggerated claims or those of a proper engineer?

 

The truth is the advertisers play fast and loose with the temperatures Th and Tc.

Tc is indeed the mean outside air temperature where the pump is located.

But Th is not the room temperature.

Th is the temperature for the heat pump fluid, within the heat pump itself.

So the efficiencies you are being shown is just for the heat pump are are indeed true theoretical maxima.

But it takes more than a heat pump to heat a room. It takes a complete heating system.

This means that there is a heat exchanger of some sort within the room which has to be above room temperature to supply heat.
The greater the heat demand of the room the greater the required temperature difference, above room temperature.

In turn this heat exhanger is heated by the heat pump fluid which has to be at a yet higher temperature.

Then that fluid has to be piped from the heat exchanger itself to the heat exchanger and no insulation is perfect.
So the fluid leaving the heat pump has to be at a yet higher temperature still to arrive at the heat exchanger at the correct temperature.

 

So the Th produced by the heat pump has to be higher than the room temperature by three increments, each of which has to be carefully controlled to minimise inefficiency.

 

Bottom line the efficiency of the heat pump is not the efficiency of the heating system.

 

Having said all that I installed an air to water hat pump heating system in 2016 and am happy to discuss it performance with anyone who has a genuin interest.

 

[Moreno]

What do you think about heat pumps and air conditioners based on principally new technologies, such as optic or magnetic cooling? Can they ever become commercially viable and surpass the liquid-based refrigeration in all the principal accounts?

[studiot]

1 hour ago, Moreno said:

What do you think about heat pumps and air conditioners based on principally new technologies, such as optic or magnetic cooling? Can they ever become commercially viable and surpass the liquid-based refrigeration in all the principal accounts?

 

Was that truly your response to my post and offer?

 

Did you not even want to correct my spelling mistakes in the last line?

Edited March 3, 2018 by studiot

[Moreno]

1 hour ago, studiot said:

 

Was that truly your response to my post and offer?

 

Did you not even want to correct my spelling mistakes in the last line?

No it wasn't. It was a general question.

[Moreno]

7 hours ago, studiot said:

 

Would you rather believe the salesman's exaggerated claims or those of a proper engineer?

 

The truth is the advertisers play fast and loose with the temperatures Th and Tc.

Tc is indeed the mean outside air temperature where the pump is located.

But Th is not the room temperature.

Th is the temperature for the heat pump fluid, within the heat pump itself.

So the efficiencies you are being shown is just for the heat pump are are indeed true theoretical maxima.

But it takes more than a heat pump to heat a room. It takes a complete heating system.

This means that there is a heat exchanger of some sort within the room which has to be above room temperature to supply heat.
The greater the heat demand of the room the greater the required temperature difference, above room temperature.

In turn this heat exhanger is heated by the heat pump fluid which has to be at a yet higher temperature.

Then that fluid has to be piped from the heat exchanger itself to the heat exchanger and no insulation is perfect.
So the fluid leaving the heat pump has to be at a yet higher temperature still to arrive at the heat exchanger at the correct temperature.

 

So the Th produced by the heat pump has to be higher than the room temperature by three increments, each of which has to be carefully controlled to minimise inefficiency.

 

Bottom line the efficiency of the heat pump is not the efficiency of the heating system.

 

Having said all that I installed an air to water hat pump heating system in 2016 and am happy to discuss it performance with anyone who has a genuin interest.

 

But it looks (according to diagram above) that even if Th is 40 C (which is 2 times above the room temperature we intend to have in a room) COP theoretical maximum is still above 5 at Tc -20 C, what is not that bad.

[studiot]

Well it looks like you and I are the only ones interested in mundane things like heat pumps, rather than unattainable nonsense at the supposed end (or beginning) of the universe.

 

 

So is 40C enough?

 

It really depends on what you want to achieve.

You can't start with the heat pump ideal chart, you need to define objectives first.

Air to air pumps have a higher operating COP than air to water because they may operate at a lower Th.

However I use mine to also supply DHP (domestic hot water) and 40C would be nowhere near hot enough.

The salesman claimed my pump has a COP of "over 4"", but he won't provide figures for what the outside temperature is.

 

There is another thing about air source - the air humidity plays a large role in performance and high humidity (as we have) can reduce COP of the pump.

But overall the system is producing cost savings compared to the gas boiler it replaced.

 

 

[Frank]

17 hours ago, studiot said:

Having said all that I installed an air to water hat pump heating system in 2016 and am happy to discuss it performance with anyone who has a genuin interest.

 

Hey Studiot,

What sort of ground source do you use?  Was it expensive?

 

[studiot]

Quote

 

Frank said

 

23 hours ago, studiot said:

Having said all that I installed an air to water hat pump heating system in 2016 and am happy to discuss it performance with anyone who has a genuin interest.

 

Hey Studiot,

What sort of ground source do you use?  Was it expensive?

 

I really don't know how to answer this, but I am happy to discuss the capital cost which was higher for the hatpump ( ) than for a replacement gas boiler.

Ground source would have been in some ways better in some ways  but I worked out that I would need 100 metres of buried pipe and there is not easily enough room in my back garden for that.

 

The picture empahsises the point about insulation, the black insulated pipes on the wall convey the ouput hot water from the heatpump to indoor heatstores.

 

[Moreno]

32 minutes ago, studiot said:

 

I really don't know how to answer this, but I am happy to discuss the capital cost which was higher for the hatpump ( ) than for a replacement gas boiler.

Ground source would have been in some ways better in some ways  but I worked out that I would need 100 metres of buried pipe and there is not easily enough room in my back garden for that.

 

The picture empahsises the point about insulation, the black insulated pipes on the wall convey the ouput hot water from the heatpump to indoor heatstores.

 

So, does it work well throughout the year on air alone?

[studiot]

1 hour ago, Moreno said:

So, does it work well throughout the year on air alone?

Not as well as the salesman promised, but perhaps as well as the engineer expected.

You can see the end of last week's snow melting away around the base.

[Frank]

4 hours ago, studiot said:

 

I really don't know how to answer this, but I am happy to discuss the capital cost which was higher for the hatpump ( ) than for a replacement gas boiler.

Ground source would have been in some ways better in some ways  but I worked out that I would need 100 metres of buried pipe and there is not easily enough room in my back garden for that.

 

Right.  It would have been a vertical loop if anything and cost the same as a lower end car to install.  There has to be a better way.

You already had a radiant heating system, so that's a plus.  Forced air needs an even hotter hot side which eats into efficiency.

That's where the confusion came in, usually, around here, it's air to air heatpump or water to air and air to air doesn't cut it.  Glossed over both the spelling and the reversed order...

 

[Frank]

Use solar thermal panels instead of a ground loop.  Is there enough solar energy during cold, short days?  The system below seems to rely upon air temperature as well as solar heat, plus an electric heater...

SunPump pumps new life into solar thermal heating : TreeHugger: https://www.treehugger.com/clean-technology/sunpump-solar-powered-heat-pump-sort.html

 https://www.sunpump.solar/

 

[studiot]

frank,

 

Yes all systems are a balance of compromises.

I think a 50m borehole would cost more than a small car in Somerset. And it would go below local river level, which would involve issues from the river /water authority.

Thank you for the link. Interesting. 

There are other ways to add in a solar boost.

Modern heat stores have additional internal coils for any available preheat source. This is the cheapest method.

 

 

[Frank]

Googling "heat stores" came up with the idea that electricity is often cheaper at night (it is here) so solar to heat-pump has that disadvantage.

Shower drainwater came to mind as a preheat source (can also simply preheat incoming cold water as in a powerpipe).  Other sources may have diminishing returns.

I've always liked the idea of efficient combined water and space heating.

 

[Moreno]

On ‎3‎/‎3‎/‎2018 at 1:49 AM, Frank said:

There are heat pumps that run off of fuels (natural gas) from whispergen, honda and others, but they all have their issues - noise, vibration, oil changes, expense or efficiency...

Do they have moving parts? If not, then where noise and vibration come from? How their efficiencies and power is compared to electrical heat pumps? If they are less efficient, then why? 

What can be done to increase heat pump efficiencies radically at lower temperatures? For example, could use of propane instead of Freon be helpful? 

[Endy0816]

Boilng point of propane is only about 12-15 degrees lower. Flammability is also a concern. It is on the refrigerant list though, R-290.

https://en.m.wikipedia.org/wiki/List_of_refrigerants

 

Better to look at more parameters than raw efficiency. Cost is a big one, safety a close second.

Most everything does have drawbacks to use. Leidenfrost effect for liquids with lower boiling points. Solids can't be pumped or blown as easily.

 

[Frank]

19 hours ago, Moreno said:

Do they have moving parts? If not, then where noise and vibration come from? How their efficiencies and power is compared to electrical heat pumps? If they are less efficient, then why? 

What can be done to increase heat pump efficiencies radically at lower temperatures? For example, could use of propane instead of Freon be helpful? 

The honda one is an ICE, no surprise.  The whispergen is a sterling cycle engine.  Regular heat pumps are electric motors.  All of them mechanically compress gas into liquid, so they all have moving parts.  There is an absortion cycle which runs off of heat so no moving parts (except the fluid), but efficiency is low.  Unless there is a "free" heat source, it's hardly worth it (COP <2 IIRC).

 

Efficiency is good with the mechanical heat pumps if the "waste" heat is used for space heating.  But they still burn fossil fuels.

 

[Moreno]

I've read claims that heat pumps which are based on adsorption process are more efficient. Is it true? If yes, does it apply to any outside temperatures?

[Endy0816]

This paper talks about them a bit. Looks like COP tends to be lower, though can also see lower maintenance and operating costs.

 

[Endy0816]

Can I ask where your interest lies with this?

Are you wanting something better for personal use or just wanting to talk about theoretical ways we might improve COP?

 

Generally my experience is that there's always some form of resistance to our objectives(whatever they might be). Never seen anything that dictates how much there has to be though, so may be able to improve things until it is arbitrarily small(if still not zero). Heat exchangers, better materials, better collection/utilization of waste heat, etc.