Heat pumps. Are they a a solution in a cold climate?

[Grumage]

There's a massive push towards ' air source heat pumps ' currently going on here in the UK.  However I remain sceptical about the concept. I wonder if our experts could shed some light on the subject please? Obviously they work by taking ambient temperature air from the environment and by reversing the refrigeration concept give this heat to us. ( simple layman's understanding )  My problem comes when we have an air temperature of less than zero degrees Celsius or minus 32 degrees Fahrenheit. surely there is no heat available to use in the process?  Presumably we are now driving the compressor, generating heat by mechanical means at a much lesser efficiency than say using it in a resistance heating element?

Cheers Grum.

[Chet K]

Grum
Yes here too ( encouraging their use)
We have an associate who is heavily involved in the Net Zero housing movement Globally , last we spoke he was mentioning
The advancements in the air handlers feeding the heat pumps
Working well below zero (Fahrenheit)
Good excuse to call him for an update..

Will report back ( it's actually a rock solid tech being improved all the time
Respectfully
Chet K

[Jose Antonio]

The quality of the system will depend on how much you have in your budget and the type of system you want. Here is a high-level view of some background information that should get you looking in the right direction. I will start off with a brief overview of a system.

There are four main parts to a system and they are as follows:


Compressor: Moves refrigerant throughout the system and creates the required pressure differential needed to drive the refrigeration process. Today's compressors utilize VRF (variable refrigerant flow) and that means that the RPM of the motor can be modulated effectively providing for variable system capacity (btu/hr).


Condenser: (Heat Energy Rejection) Most common are air side coils and their job is to drive a Change-of-State by the process of condensing. Here heat energy is removed from the hot compressed gas to drive it to a liquid. An abbreviated explanation of the function of the condenser would be to De-Superheat - Condense - Subcool all in that order. (Note: I will not be reviewing superheat/subcooling here but it is a critical process of refrigeration)


Metering Device: This device creates a pressure differential in a system through the process of restricting flow. Most metering devices on heat pumps are of the electronic type and provide precision control. Some systems (low-end) still utilize a capillary tube or a fixed orifice. Regardless, we need a method to control the amount of refrigerant into our evaporator and in the process create a pressure drop.


Evaporator: (Heat Energy Absorption) Most common types are air-type and their job is to drive the absorption process. Heat energy transfers through the material the evaporator is made from (normally copper/aluminum) and into the refrigerant. The more heat I can get my evaporator to absorb the more energy I can transfer. Because a compressor can't compress a liquid, the evaporator is required to operate with some level of Superheat.

Be aware that the entire refrigeration process, and it doesn't matter if we are talking about ultra-low temperature applications or residential/commercial systems, is about transferring energy from one place to another. Simplified we can say we absorb heat energy from inside the house and reject it to the outdoors (AC) OR absorb energy from outdoors and reject it indoors (heat pump). The fundamental driver in the process is the ability of a refrigerant to change state (liquid to gas and gas to liquid).

A heat pump will contain the essential parts noted above (and more) to provide heat to a dwelling by picking-up heat in the environment and transferring it to another environment. Theoretically, unless we are at absolute zero there is always heat energy available.

The general process of a air-source heat pump utilizing a fan coil is as follows:

The outdoor fan drives air across the outdoor coil that contains a refrigerant whereas the temperature of the refrigerant is lower than the ambient air around the coil. This creates a condition for heat from the air to transfer to the refrigerant circulating in the coil. This heat saturated refrigerant is compressed to a higher pressure/temperature (vapor) and sent over the indoor coil.

At the indoor fan coil air is circulated across the coil containing the hot vapor and much like the process of heat absorption, heat rejection requires that the ambient air around the coil is cooler than the refrigerant temperature. In this scenario, heat from the refrigerant is transferred to the air stream. The result is a warm liquid refrigerant that loses some of its heat. This warm liquid heads back to the outdoors to start the process all over again.

That is a general description. Be mindful that different manufactures may use different components in different locations but the fundamental process is the same.


There are different types of heat-pumps available to the residential market, and they are:

Air-source - The most common, and popular, seen and utilized in residential and commercial applications. Affordable and easy to install or replace an existing ducted system with one of these. Ductless splits are very popular in Europe  however they are not as powerful as a ducted system. Today's refrigerant driven heat pumps utilize compressor VRF (Variable Refrigerant Flow) technology. This technology varies the speed of the compressor based on btu load and has been a real game charger in the heat pump market.

Air-source systems can operate down to 10 Deg.F (-12.2 Deg.C) and still deliver nameplate btu's. It should be noted that the heat pump will have to enter Defrost Mode periodically and that means it needs to run. The only downside to an air-source heat pump is that air is an insulator  in Air-Conditioning mode to melt off the ice on the condenser. This is important to understand because if you purchase the ductless system, the indoor fan shuts off during the defrost mode or else it would distribute cold air. This leads to uneven temperatures.

On ducted systems it is highly recommended that electric strip heaters be added to the air-handler. Then in the defrost mode the heaters energize, warming the air keeping space temperature steady. Another important note is when utilizing electric strip heaters they will utilize a good amount of energy. As a real general example utilizing some rules of thumb. A 1200 square foot house will average a 3-ton system (36,000 btu/hr) and utilize close to a 10kw electric strip heater for the system. This heater, when operating, will run at around 40 amps (give or take) so you need to have the electrical infrastructure to handle this.

One unspoken aspect of air-source systems is that air is an insulator and as good as they are they are not nearly as superior as the other types below. Out of all the heat pumps I would say the ductless splits provide the most uneven heat in a space. Great for small apartments and used exclusively now in small IT closets. A ducted system (heat pump) would look similar to a gas hot air system. Ducted is much better for air distribution but be mindful that the air temperature coming out of a heat pump is not as warm as a gas furnace.


Water-Source - If you live in a building that provides water for systems then certainly a water-source heat pump would be the way to go. Water can transfer large amounts of energy at very low cost, is versatile in its applications, and unlike air-source does not require a defrost mode. Water-source systems can even incorporate a Heat Recovery module that will allow you to heat your domestic water to about 120 Deg.F (48.8 Deg.C). These systems have all the benefits of air-source including ductless applications.


Geo-thermal - If you have the space for geo-wells then this is certainly an option. Of all systems this is the most expensive to have installed and you want to make sure your contractor really knows what he is doing. Obviously, specific (earth) conditions are needed for superior conductivity. Sandy soil is an issue because of its insulating qualities and areas with excessive rock are no better. Geo-wells can be of various types but my preference is for the vertical well installation. Just like water-source, you can use a geo-system to provide domestic hot water but they are really superior when supplying water to a radiant ceiling or floor. 


Odds & Ends

None of what I wrote above means anything if the structure you live in is drafty and has poor insulation. It is not just about the efficiency of the appliance but a big part is how well you can stop energy transfer (Summer gains/Winter losses). A structure will lose/gain energy from its interior surfaces on an exterior face, this is why steam radiators have been historically located under windows.

If you are looking for a superior efficient system of the warm-air type I would suggest a high efficiency natural gas system. A gas 90+Efficiency system is unbelievably efficient and can be coupled with a central AC system.

Thoughts on electric heat

If it wasn't for the price/Kw varying across areas and it being high. Electric heat in my opinion would be the most efficient and rendore oil, gas, refrigeration (heat pumps) irrelevant. Electric heat is 100% efficient and heating elements can be modulated to provide tremendous turndown ratios and high quality elements outlast mechanical equipment. As an example, electric baseboard can be added to a house and these elements are modulated with SCR controls to provide precision heat based on a process driver like outside air or indoor temperature. Adding to that, we would now be able to efficiently zone a structure room to room and have a thermostat in every zone. Just imagine what we can do with a Figuera,  B&L or Tariel device driving a system that provides heating and cooling. Possibilities are endless.

Hope this helps.

-JA

[Verpies]

My problem comes when we have an air temperature of less than zero degrees Celsius or minus 32 degrees Fahrenheit. surely there is no heat available to use in the process?

If it wasn't you asking the question, I would not post in this thread.

Your question/statement is false.  Let me revise it for you so it becomes true and hopefully no further explanation is necessary:

"...when we have an air temperature of zero degrees Kelvin or minus 273.15 degrees Celsius, there is no heat available to use in any extraction process."

[Paul-R]

Water-Source - If you live in a building that provides water for systems then certainly a water-source heat pump would be the way to go. Water can transfer large amounts of energy at very low cost, is versatile in its applications, and unlike air-source does not require a defrost mode. Water-source systems can even incorporate a Heat Recovery module that will allow you to heat your domestic water to about 120 Deg.F (48.8 Deg.C). These systems have all the benefits of air-source including ductless applications.

In my view, this is the answer, if implementable.

If a builder is starting with a cleared site, put in a massive water tank of the septic tank type and build upwards. This tank, insulated, will take all the building's shower, bath and washing/dishwashing machine water and the coil. If the sump is warm, the efficiency will go up and the thermodynic curves will show it.

The water could deliberately heated by circulating the water through matt black tubes on a sunny roof.

Retrofitting is less easy but possible and of course, if there is a garden, the tank could be dug in there.

[Grumage]

If it wasn't you asking the question, I would not post in this thread.

Your question/statement is false.  Let me revise it for you so it becomes true and hopefully no further explanation is necessary:

"...when we have an air temperature of zero degrees Kelvin or minus 273.15 degrees Celsius, there is no heat available to use in any extraction process."

Dear Verpies.

Your sentiments in the above post are greatly appreciated. 

However although the weather conditions here in NE Wales aren't particularly great they could hardly be akin to outer space!!

Cheers Grum.

Whilst posting I would just like to Thank Jose for his highly detailed and very informative reply to my original question.

Cheers Grum.

[Chet K]

Grum
Spoke briefly with associate about his net zero project in northern US
He had just arrived from out of town ,he briefly mentioned that LG company out of Korea was best choice .

Arranged another meeting with him tomorrow for more information .
Respectfully
Chet

[Frederik2k1]

There are also heat pumps for which -25°C is not a problem. The flow temperature in the building is very important if there is a water-bearing heating system (I am only speaking from German experience here, water-bearing systems are installed everywhere here, not ducted systems). The electrical energy consumption of the heat pump increases with every Kelvin of temperature difference between the source (mostly outside air) and the sink (the water-bearing heating system). As a rule, the efficiency of the heat pump decreases by 2-2.5% per Kelvin temperature difference.

This means that it is usually very important to keep the flow temperature in the house of the heating system low. How low it can be lowered depends on the insulation of the house and the nature of the heating system. Old radiators require high flow temperatures in order to deliver a certain heat output at all. There are newer, so-called low-temperature radiators, which emit a certain heat output at a flow temperature of just 35°C. Actively ventilated radiators are even better and underfloor heating systems that can be operated with a flow temperature of 28°C are even better.

So it is definitely possible to use a heat pump in existing buildings. There is a large study by Frauenhofer ISE Germany where exactly this was investigated in field tests in real buildings over years. If I remember correctly, the core statement of the study is that a heat pump always makes sense and is feasible if the energy requirement of the building does not exceed 150 kWh/m² and the flow temperature of the water bearing heating system is not higher than 55°C.

See here: https://www.ise.fraunhofer.de/content/dam/ise/de/downloads/pdf/Forschungsprojekte/BMWi-03ET1272A-WPsmart_im_Bestand-Schlussbericht.pdf  At least the summery is in english. I cannot copy it here, the PDF is copy protected. At the end of the study you can even see the buildings investigated. It is the most popular study on this topic in Germany.

[TinMan]

I was just only reading about a week ago, about a guy here in Oz that has invented a new type of heat pump that uses ambient air as the gas, and his prototype is over 400% efficient.
You can install it yourself with ease, as you don't need to gas the system up, as it does it itself when you switch it on, as it draws air in from the environment, and exhaust the air back out into the environment.
It also is reverse cycle, where it can cool your home as well, simply by reversing the motor/compressor.

I thought i saved it to my favorites, but apparently not.
But it is a very simple device.
The biggest problem he is having, is fighting against the big manufactures, who claim it would result in a big loss of income in both gas suppliers and installers.
The system is also very easy to make, and very cheap to make.
Another case of--we can't save the people money, and be a big loss for multi million dollar companies.

I will see if i can find it again, and post it here, but it is very simple to build--yes, you could DIY.

It works like this--
The motor/compressor unit is housed in a liquid (lite oil) filled compartment, which also has the inlet tubing in it, so as it can also utilize the heat generated by the motor/compressor unit to heat the incoming air-every bit of waste heat is used. So the compressor draws in the ambient air, which is then compressed in the internal condenser inside the home, which causes it to heat up. You then draw the heat off of that condenser via the fan, and then the cooled gas is then either vented back out into the environment, or if that exhaust gas is warmer than the outside air temperature, it is recycled back through the compressor to start the cycle over again.

To operate in reverse cycle, you simply reverse the direction of the compressor.
This then draws in the ambient air in through a sealed water evaporating unit, and then into the internal condenser, which is under a vacuum state, and the air cools as it moves from a high pressure state, to a lower pressure state.
We see this effect with the standard workshop compressor, where the inlet pipe to the compressor gets very cold, and the exhaust pipe from the compressor gets very hot.
This heat pump works well with air, as the pressures at which it operates are very high- up to 400psi, but the flow rate is very low, which makes it energy efficient.
The 1Kw unit has a heating capacity of 4Kw at an outside air temperature of 2*C.

Brad

[Vidura]

Hi all. interesting topic. Some time ago I had an idea of making heat-pumps AC and refrigerators more efficient by recovering the mechanical energy of the pressurized medium. replacing the release valve by a turbine or air-motor mounted on the same axle as the compressor. Doing some research on prior art I found that there where a couple of patents several decades ago. The oldest source was for an car air conditioning system using air as medium, but it seems there where some issues with icing the conduits in humid climatic conditions. But the basic concept is quite valid and promising. Of course there are losses in the recovery, but if only say 50% of the energy could be redirected to the prime mover the COP of the device would be doubled. Therefore a heat-pump could have a very reasonable performance despite of unfavourable weather conditions.

[lota]

Hello Tinman,
this pump already existed in 1844. The connecting rod of the compressor had a small 90° angle. This caused a delay and the heat could be transported.
Greeting
Lota

https://secon-gmbh.com/produkte/luftkaelteanlagen/
https://vhkk.org/page/vortrag/pdf/Das_Kaeltemittel_Luft.pdf