Heat Pump Theory

[chillerman2006]

Air-source heat pumps

Heat pumps use the same vapor-compression cycle as the refrigeration systems
described above, but they have additional components that enable them to pump
heat in either direction, such that the same equipment unit can provide cooling
or heating. The two added components that make heat pumps fully reversible are:

• A 4-way reversing valve, which can reverse the direction of flow
  around the refrigerant loop while maintaining the same direction of flow through
  the compressor (the next two diagrams show how this is done).
• A bi-directional expansion valve, which is able to meter the flow of
  liquid refrigerant in either direction (for purposes of illustration, the
  diagrams below show two expansion valves and two bypass valves, but modern heat
  pumps often incorporate all of this functionality in a single valve).
• 
  An air-source heat pump in cooling mode acts
  identically to an air conditioner. Refrigerant vapor exits the compressor at a
  temperature in the range of 120-140°F, which is warmer than the outside air
  temperature. As a result, it spontaneously loses heat when it enters the outdoor
  coil, causing it to condense. The refrigerant leaves the condenser as a liquid,
  still under high pressure. The expansion valve lets through just as much
  refrigerant liquid as can be completely vaporized by the indoor coil. The
  pressure drop through the expansion valve vaporizes some refrigerant and lowers
  its temperature to 40-50°F. As a result, it spontaneously gains more heat, which
  vaporizes the rest of the refrigerant liquid. The low-pressure refrigerant vapor
  leaves the indoor coil, goes through a U-bend in the reversing valve, and
  returns to the compressor, where the cycle begins again.
  
  The reversing valve can be switched to heating mode
  such that the high-pressure output of the compressor is directed toward the
  indoor coil, which now acts as a condenser where the refrigerant gives up its
  latent heat to the room. It is then expanded in the reverse direction (compare
  with Figure 6) and vaporized in the outdoor coil, where it gains latent heat
  from the outside air. The refrigerant vapor then goes through a U-bend now on
  the other side of the reversing valve, and returns to the compressor where the
  cycle begins again.
  
  
  To summarize, the indoor and outdoor coils are where the refrigerant changes
  phase, gaining or losing latent heat through evaporating and condensing. The
  compressor drives the refrigerant around the loop and creates the high-pressure
  and high-temperature conditions that enable it to condense as a liquid. The
  expansion valve meters the flow of liquid refrigerant from the
  high-pressure/temperature side to the low-pressure/temperature side of the loop,
  such that it all will be vaporized in whichever coil is acting as the
  evaporator. The reversing valve determines which coil is on the ""high side"
  (condensing) or "low side" (evaporating) of the loop. Following the natural
  tendency of spontaneous heat flow, the high side loses heat to its surrounding
  environment, and the low side gains heat from its surrounding environment.
  
  
  An air-source heat pump can be installed either as a "packaged" unit, where
  both coils are in a weatherproof enclosure, typically mounted on flat roofs
  (Figure 8), where they supply warmed or cooled air to the rooms immediately
  below them, or as a "split" system, where the indoor coil is contained within
  the building's air handling system (Figure 9). Packaged units are more common in
  commercial and institutional buildings, while split systems are more common in
  residential applications.
  
  
  Packaged terminal heat pumps also can be installed as "through the-wall"
  units, where they are fitted into a sleeve that passes through an exterior
  building wall. These typically are noisier than rooftop packaged units, since
  the compressor and fan are located in the room; they are commonly found in
  hotels and motels.
  
  
  During the winter heating season, air-source heat pumps cease to be effective
  when the outside air temperature falls below 25-35°F. To handle such conditions,
  they are supplied with a supplemental heating system - usually electric
  resistance strips to further warm the building supply air after it leaves the
  indoor coil (Figure 9).
  
  
  During the heating season, moisture in the air outside may freeze on the
  outdoor coil if its surface temperature drops below 32°F. Therefore, when
  outside temperatures fall below about 40°F the heat pump will periodically enter
  a defrost cycle, during which the reversing valve intermittenly sends hot
  refrigerant through the outdoor coils for periods lasting anywhere from two to
  ten minutes. During a defrost cycle, the electric heater is used to warm the
  indoor supply air, but its temperature still may fall below skin temperature,
  causing a "cold blow" sensation. This is not a problem with geothermal heat pump
  units, which do not require defrost cycles even in cold weather, due to the
  stable ground loop temperature.
  
  
  Water-source heat pumps
  
  
  Water is a much more efficient heat energy transfer medium than air, due to
  its much higher specific heat. Using pumps or fans with comparable efficiencies,
  it takes four times less energy to move a given quantity of heat with water than
  with air. Furthermore, due to the higher density of water, a piping conduit
  takes up less space than an air duct with the same heat moving capacity.
  Therefore water is the preferred heat distribution medium for large, multi-story
  buildings.
  
  
  Conventional water-source heat pumps use a fossil-fuel-fired boiler as a heat
  source during the winter and an evaporative cooling tower to reject heat during
  the summer. This is sometimes referred to as a boiler/tower system; it is also
  known as a "California system", since this concept is thought to have originated
  in that state. The water loop temperature is maintained between 60 and 90°F.
  When the loop temperature falls below 60°F, the boiler adds heat, and when the
  loop temperature exceeds 90°F, the cooling tower rejects heat.
  
  
  As shown in Figure 10, there is a common water loop connected to all the heat
  pump units distributed throughout the building, which themselves can have a
  variety of configurations, including horizontal, vertical, or console. The water
  loop can be integrated with the sprinkler system to reduce cost.

[chillerman2006]

Some more info to take into consideration when designing a system

R's chillerman

[chillerman2006]

Evening All

what is the main issue's with ASHP's and defrosting ?

and what is the best way to overcome them ?

also what is the pro's/con's of the attached type of coil ?

R's chillerman

[Brian_UK]

Nice summary CM, any credit to the original author would be appreciated.

[chillerman2006]

Nice summary CM, any credit to the original author would be appreciated.

Original Post - Credit to : Geo4VA

R's chillerman

[Greengrocer]

As you have stated the heating capacity of all ASHP systems reduces as outside ambient air temps get lower – just when you need it most. As such the chosen system must be selected to meet the design heating load at the design winter conditions allowing for the capacity reduction. If sized for the design heating load at say -10c an air to air heat pump system will be oversized at any outside air temperature above the design condition – see graph below.
Some manufacturers provide heating & cooling combination ratings or tables. These can be plotted onto a graph where the effect of the defrost cycle can be easily seen as a pronounced dip in capacity at ambient temperatures of 10 to 7°c or lower. I found an excellent document on the University of Strathclyde website which is based on actual use of Sanyo CO² Air to water heat pump. The link below discusses aspects of the defrost cycle – bare in mind the system in question is an air to water heat pump system. Similar results / experiences can be expected for Air to Air heat pumps.


http://www.esru.strath.ac.uk/EandE/Web_sites/10-11/ASHP_CO2/modelling_defrost.html



As ambient air temps drop below 10°c / 50°F the outdoor coil starts to operate below freezing. As a result any airborne moisture will freeze on the outdoor coil of an Air to Air heat pump. To remove the frost the ASHP system goes into defrost mode & reverses its operation (cooling). This puts hot gas into the outdoor coil (now acting as a condenser instead of an evaporator) & melts the accumulated frost. Once the outdoor unit is defrosted the systems returns to the heating mode. The control of defrost operation is by temperature / pressure sensors &/or timers – or a combination of all. To counter the defrost operation in Air to Air systems it is normal to either shut down the indoor unit fan (cold draft prevention) or, to energise an electric resistance heater to temper the air leaving the indoor coil (now in cooling mode). These electric heaters can also be operated to supplement the heat pump heating capacity as it drops off in colder ambient temperatures. The electric heaters are normally controlled via outside ambient thermostats which will inhibit their operation until a certain ambient temperature has been reached. Multiple electric heaters and outdoor air thermostats can be used to provide staged supplemental heating. The electric heaters can also be used as an emergency source of heating should the heat pump system fail. Consideration should also be given to the amount of condensate water that will be discharged from the outdoor unit when in defrost mode. Supplemental electric heating is normally only available as an option in larger ducted single package heat pumps (rooftop type). 1 to 1 Splits & VRV/VRF type systems normally employ the cold draft prevention method when defrosting occurs & do not use any additional electric heating.


The pictures you have attached are of USA style outdoor units i.e. square casing, “L” shaped coil with a top discharge fan. These are normally mounted at ground level or perhaps on a flat roof. However, they are not that easy to mount on cantilevered wall brackets. Most smaller capacity European style outdoor units are shallower (depth), have 1 or 2 side discharge fans and can be easily mounted on wall brackets, on the ground or on a flat roof. Multiple outdoor units can also be installed “back to back” or stacked one on top of the other if space is restricted. Some people call them condensing units or condensers which is not strictly true since they operate as a condenser in cooling mode & as an evaporator in heating mode. I prefer to use the term “Heat pump outdoor unit”. Having said, & just to confuse the issue a little, you can also get indoor mounted versions which have a ducted fan arrangement. An “indoor outdoor unit” if you will.

[chillerman2006]

Hi Greencrocer

Good link/info mate, cheers

R's chillerman

[MikeHolm]

CM, all the info i have accumulated about Air to water HPs, tells me that to defrost effectively keep lots space around the Evap to prevent the ice from becoming a glacier. It is not really possible with a split like a Mits or Daikin but if the coil were on an angle, drainage could be enhanced and there are some European units that do this.

And very important is to have enough buffer water to take heat from.... to complete the defrost. I have seen some sensors that were great at detecting frost (and were used for other reasons) but i have never seen this type on refrig systems. It is another thing to research. It would be great if we could accurately sense the presence of frost and prevent a defrost coming on when not needed or to bring it on earlier if needed. I don't know how accurate, over time, current defrost mechanisms are or put another way, could we save power overall if there was more accurate detection.

Green Grocer, in the USA and Canada, there are very few air to Water HPs. Almost all are air to air or GSHPs (which mostly heat air) and there is almost always an electric resistance back up coil in the plenum of the air handler.

[chillerman2006]

Hi Mike

maybe look into 'magnehelic gauges' mate

they were used to initiate a defrost on transport refrigeration units

and were very effective, but not sure if still used as out of that game for long time now

R's chillerman

[mad fridgie]

A heat pump is basically no different to any other refrig system, (all have the 4 main components), how ever what is different is the massive changes in the evap and cond loads (a normal refrig system for example generally has quite a stable evap load in compassion) So you first have to manage the instability of the system.
Most isseus arise from the application, of lack of understanding of the application, both by the client and the contractor. ASHP are not boilers, and should not be compared as a like for like application.
Defrost is just part and parcel of air source heat pumps, there are different methods to reduce or aid the defrost issue, how ever this normally involves increased cost, or noise, again these issues should be covered with in the application/ design.
In my opinion in semi cold climates, where whole heating is required, ASHP should only be used on slow acting underfloor heating systems, where ebbs and fall can be absorbed by the thermal mass of the concrete floor

[chillerman2006]

A heat pump is basically no different to any other refrig system, (all have the 4 main components), how ever what is different is the massive changes in the evap and cond loads (a normal refrig system for example generally has quite a stable evap load in compassion) So you first have to manage the instability of the system.
Most isseus arise from the application, of lack of understanding of the application, both by the client and the contractor. ASHP are not boilers, and should not be compared as a like for like application.
Defrost is just part and parcel of air source heat pumps, there are different methods to reduce or aid the defrost issue, how ever this normally involves increased cost, or noise, again these issues should be covered with in the application/ design.
In my opinion in semi cold climates, where whole heating is required, ASHP should only be used on slow acting underfloor heating systems, where ebbs and fall can be absorbed by the thermal mass of the concrete floor

Morning MF

From what I read of others posts and governments plans to subsidise hp's here

I need to really get my head around these

Would you say uk climate is suitable for ashp and if so is that just floor heating

Or will it be good for hot water & radiator central heating

Is there any good books/net links on the ashp's

and water system design

R's chillerman

[chillerman2006]

CM, all the info i have accumulated about Air to water HPs, tells me that to defrost effectively keep lots space around the Evap to prevent the ice from becoming a glacier. It is not really possible with a split like a Mits or Daikin but if the coil were on an angle, drainage could be enhanced and there are some European units that do this.

And very important is to have enough buffer water to take heat from.... to complete the defrost. .

Hi Mike

is the use of heater bars not an option

not seen them mentioned on ashp's before

but often see them on walk in freezers even when they have hot gas defrost

wonder how the cost of running heater bars, balances out to useing the water

you have heated once and will need to re-heat

R's chillerman

[mad fridgie]

1 Go back to basic refrigeration, what determines efficiency? This is fundamental.
2 What temps do you want running around your rads?

add 1 and 2 together what do you get.

I am not saying that you could not use rads, but you would have teach the end user that the rads are not hot or it will not be cheap to run if he wants it hot, you can not have both.

Your DWH, this can be looked into in a different way, and depends upon how and why it is applied.

[chillerman2006]

Hi MF

1. mass flow of refrigerant & compression ratio

2. in the buffer tank 50*c mine is set to (boiler is 60*c)

Hmm, 1 + 2 dont add up now for sure

To achieve water flow temps of 60*c

Your compression ratio would have to be too wide

Then efficiency is lost and ratio will get wider the lower the ambient falls

See what you mean now

R's chillerman

[Greengrocer]

Morning MF

From what I read of others posts and governments plans to subsidise hp's here

I need to really get my head around these

Would you say uk climate is suitable for ashp and if so is that just floor heating

Or will it be good for hot water & radiator central heating

Is there any good books/net links on the ashp's

and water system design

R's chillerman

We have been installing air to air heat pumps in commecial buildings for 30 odd years. However, Air to water heat pumps in domestic buildings is a relatively new development brought about by the rise in cost of gas / electric.
Many manufacturers offer systems for domestic application. However, key to getting the application right is matching the system as a whole to the type of house. Like most things its cheaper to install a system when a house is built. Retrofitting a system into a house is always more expensive.
Some systems are available that generate higher water temps so these can used with existing radiators. However, in the main, to achieve the best energy efficiencies & lowest running costs HP systems with lower water temps are the better which are ideal for an underfloor heating systems.
Its horses for courses. Start with the house. Insulate where ever possible, then select the correct Heat Pump system to suit e.g. UF system or existing rads. Add in solar etc as required.
Best bet is to get yourself on one or two manufacturers training courses to get the low down and knowledge. Alternatively read up on anything you can get your hands on eg. http://www.daikinheating.co.uk/default.jsp

A word of warning though. There are some real pitfalls & horror stories out there if you get it wrong. Just search this forum.

[MikeHolm]

Hi Mike

maybe look into 'magnehelic gauges' mate

they were used to initiate a defrost on transport refrigeration units

and were very effective, but not sure if still used as out of that game for long time now

R's chillerman

We use "magnehelic" gauges for precise gas flow measurement when setting up commercial boilers with power burners such as Weishaupt or Riello (not natural draft), so I am familiar with some of the products.

The main goal in the ASHP design is to increase the annual cold temp COP to help in the stability issues the Mad talks about and if solar can be included, it is possible that there could be summer power consumption could be down to 5% of normal ASHP usage. I would like to fool the system into thinking it is -5 outside when it is -15 without the use of much direct electrical consumption. If we can do that, we will be miles ahead.

[MikeHolm]

Hi Mike

is the use of heater bars not an option

not seen them mentioned on ashp's before

but often see them on walk in freezers even when they have hot gas defrost

wonder how the cost of running heater bars, balances out to useing the water

you have heated once and will need to re-heat

R's chillerman

Heater bars would be my last choice as they are direct resistance (if I am thinking of the same thing as you are).

[chillerman2006]

Hi Mike

you certainly got an interesting project mate and can see your gradually overcoming the obsticles, if I am following you here, as long as you got sun, regardless of ambient you can increase suction pressure foc and following all your (everyones) posts here, good insulation coupled to underfloor heating, am starting to see the theory coming together

If you can get 10*c heat increase in the evap enclosure, that would be good here where I am as rarely drops below...... -5*c and normaly above freezing, the bit that I am at 35*c is where I place water off, but then I am looking at it with water flow rates for heat rejection(chillers) not heat recovery

What temperature are you aiming for ? and I presume the more passes the better allowing for stratification in the tank to gradually take place and cop to remain high as long as possible ?

R's chillerman

[chillerman2006]

We have been installing air to air heat pumps in commecial buildings for 30 odd years. However, Air to water heat pumps in domestic buildings is a relatively new development brought about by the rise in cost of gas / electric.
Many manufacturers offer systems for domestic application. However, key to getting the application right is matching the system as a whole to the type of house. Like most things its cheaper to install a system when a house is built. Retrofitting a system into a house is always more expensive.
Some systems are available that generate higher water temps so these can used with existing radiators. However, in the main, to achieve the best energy efficiencies & lowest running costs HP systems with lower water temps are the better which are ideal for an underfloor heating systems.
Its horses for courses. Start with the house. Insulate where ever possible, then select the correct Heat Pump system to suit e.g. UF system or existing rads. Add in solar etc as required.
Best bet is to get yourself on one or two manufacturers training courses to get the low down and knowledge. Alternatively read up on anything you can get your hands on eg. http://www.daikinheating.co.uk/default.jsp

A word of warning though. There are some real pitfalls & horror stories out there if you get it wrong. Just search this forum.

Hi Greencrocer

thanks for info and link again mate, I will keep chipping away at this for probaly the next year prior to taking a course, as need to understand enough to make the most of a course

R's chillerman

[chillerman2006]

Heater bars would be my last choice as they are direct resistance (if I am thinking of the same thing as you are).

Hi Mike

yes same mate, direct resistance

R's chillerman

[anup]

We are the manufacturer of ASHP to water. For defrosting we use a combination of Temp controller to measure ambient temp, cyclic timer and solenoid valve.
As we know there will be temp drop of 5-8 deg. C in air across evaporator, as soon as the ambient temp reaches at 7-8 deg. C, our temp. controller activates the solenoid valve so that compressor discharge, instead of Condenser diverts to Evaporator for 3-5 minutes. The cyclic timer is used to determine the timing of Compressor Discharge Diversion". If the temperature is continuously below 8 deg. C, then the cyclic timer makes this diversion after 15 to 30 min i.e. SET time based on the location.
We found it very effective, low energy consumption than electric heated bars.
We have installed this mechanism in 41 systems, having capacities (input) from 1 TR to 25 TR compressor at a single location.