# Maximizing Efficiency: Exploring the Possibility of Self-Running Heat Pumps

• I
• some bloke
In summary, the conversation discusses the efficiency and workings of heat pumps, specifically ground source heat pumps. These pumps use simple physics to create elegant solutions, claiming to be 300-400% efficient in producing heat for every unit of electricity used. However, the idea of using a thermoelectric system to harness electricity from the heat generated by the pump is not viable due to the limitations of heat engines. This would violate the second law of thermodynamics.
some bloke
TL;DR Summary
now I want to make it completely clear that I am not talking of the dread PM here! I want to know whether a heat pump can use some of it's heat to generate the electricity needed to run itself!
So heat pumps are an interesting thing which I've recently discovered, and it has led to curiosity about how they work and what you can do with them. I hope to make my own property at some point, so a heat pump will doubtless come up then as I would like it to be an eco-home.

Ground source heat pumps work by taking cold liquid or gas, running it underground in pipes to gain a few degrees from the ambient ground temperature, then compressing it to amplify the temperature, using a heat exchanger to claim the thermal energy for your hot water or heating, and then they decompress the now cool liquid and recycle it to gather more heat from underground. They're an excellent example of using simple physics to create elegant solutions.

They claim that for each unit of electricity used, the pump produces 3-4 units of heat, giving it a 300-400% efficiency (of energy you pay for, obviously, not the total energy in).

This implies to me that if you could use, say, a thermoelectric system which is 50% effecient, you could harness that 1 unit of electricity from the 3-4 units of heat and run the pump, and keep 1-2 units of thermal energy for heating the house up. The whole system would run for as long as the liquid can be warmed up to take heat from the ambient surroundings, and the bearings don't give out.

I am assuming that the issue is that methods for getting electricity from heat are not efficient enough to make them viable - is this correct?

davenn and Delta2
I think if you get rid of the idea of it "feeding itself", the question becomes could you extract enough power to run the pump/compressor if you had a turbine across the same thermal reservoirs ## T_H ## and ## T_C ##.

You would have to show that the pump system (accounting) for its efficiency consumes well under theoretical limit of useful work that can be extracted from the turbine across ## T_H ## and ##T_C##.

I would say the answer to that depends on ## T_H ## and ##T_C##. Theoretically it seems ok (not violating the Second Law as far as I can tell) but economically, probably not.

EDIT:
Just to be clear: I'm telling you how you "could" run the pump system without external input of electricity, not how you could run it without external input of power.

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some bloke said:
If you could use, say, a thermoelectric system which is 50% effecient, you could harness that 1 unit of electricity from the 3-4 units of heat and run the pump
The amount of electricity you can get from heat depends on the [absolute] temperature ratio between the heated reservoir and the environment.

If the heat pump is 300 to 400 percent efficient that demands a temperature ratio close to 1:1. (e.g. 4:3 or 5:4). That in turn means that a heat engine using the temperature difference can be no more than 33% or 25% efficient. Real world losses mean that you cannot even do that well.

TANSTAAFL

If you could tack an ideal heat engine onto an ideal heat pump and thereby increase the net efficiency of your heat pumping efforts, you would have the makings for a PMM. So this discussion is definitely treading on thin ice.

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davenn, berkeman, erobz and 3 others
some bloke said:
Summary: now I want to make it completely clear that I am not talking of the dread PM here! I want to know whether a heat pump can use some of it's heat to generate the electricity needed to run itself!
That statement is a contradiction that indicates you are misunderstanding what a PMM is...because that's exactly what you are describing.

A PMM is simply a device that violates one or more laws of thermo. Yours would violate, at least, the 2nd one.

Dale and Delta2
jbriggs444 said:
If the heat pump is 300 to 400 percent efficient that demands a temperature ratio close to 1:1. (e.g. 4:3 or 5:4). That in turn means that a heat engine using the temperature difference can be no more than 33% or 25% efficient.
In other words, the efficiency equations are inverses of each other.

jbriggs444
some bloke said:
Summary: now I want to make it completely clear that I am not talking of the dread PM here! I want to know whether a heat pump can use some of it's heat to generate the electricity needed to run itself!

I am assuming that the issue is that methods for getting electricity from heat are not efficient enough to make them viable - is this correct?
Exactly. There's a theoretical maximum to how much heat a heat engine can turn into useful work. There's always some heat left over that must be discarded.

The engine-pump system you described, if it worked as you envision, would violate the second law of thermodynamics because the system would move heat from a cold reservoir to a hot reservoir without requiring any energy input. In other words, it would look like heat spontaneously moved from a cold object to a hot object.

russ_watters and Delta2
This in no way means that there are sometimes not simple solutions. Co-generation (electric power plants that use peoples homes as the "cold resevoir" make very good sense in certain climates) in fact is nearly a free lunch by circumstance. But you always have a one-way flow from which to extract.
One cannot pump the water back up the hill and have energy left over.

jim mcnamara and russ_watters
I think we've adequately covered the issue here. Thread locked.

davenn

## 1. Can a heat pump run itself without any human intervention?

Yes, a heat pump can run itself without any human intervention. Heat pumps are designed to automatically turn on and off based on the temperature settings and can run for long periods of time without needing any adjustments.

## 2. How does a heat pump know when to turn on and off?

A heat pump has a built-in thermostat that senses the temperature in the room. When the temperature drops below the set point, the heat pump will turn on and start heating the room. Once the desired temperature is reached, the heat pump will turn off.

## 3. Can a heat pump be programmed to run at certain times?

Yes, most heat pumps come with programmable settings that allow you to set specific times for the heat pump to turn on and off. This is especially useful for when you are away from home or want to save energy during certain times of the day.

## 4. Do heat pumps require maintenance to run themselves?

While heat pumps are designed to run themselves, they do require regular maintenance to ensure they are running efficiently. This includes changing air filters, cleaning the outdoor unit, and scheduling professional maintenance checks at least once a year.

## 5. Can a heat pump run itself in extreme weather conditions?

Most heat pumps are designed to run in a wide range of temperatures, but extreme weather conditions can affect their performance. In very cold climates, a heat pump may struggle to maintain the desired temperature and may need additional backup heating. In very hot climates, a heat pump may have a hard time cooling the air and may need to run for longer periods of time.

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