How does a GSHP work?

Discussion in 'General Discussions' started by ChrisJ, Aug 25, 2011.

  1. ChrisJ

    ChrisJ Active Member Forum Leader

    I wish I could give a better explanation of how my heat pump works. Of course I've read the general stuff, but what I would like to be able to explain is what happens between the water coming in from the ground and the compressor/refridgerant process. Is there a way to tell that to someone who says to me "I still don't get it", because to be honest, I don't get it exactly either.
    ChrisJ
     
  2. WF_Inc

    WF_Inc Member

  3. I will try and keep it simple.

    There is heat all around us, constantly. It is much easier and cheaper to "borrow" heat that is readily available than to produce it from zero. Example: electric house or hot water heating is very expensive. If you don't believe me, try it some time.

    Two of the most common sources of heat are the air itself and water or fluid flow.

    Whether we are dealing with air to air or water source heat pumps, air conditioning works on the principle that refrigerant gasses will "boil" or evaporate at certain temperatures and when they do so they absorb heat from that medium.

    So when you blow 78 degree air through your refrigerant coil, when the ac unit is running, a metering device will meter the "correct" amount of refrigerant into the refrigerant section of the coil, and as that refrigerant "boils" or flashes off at the point where it enters an evaporator or water coil, (drop in pressure) heat is absorbed and heat added or superheated.

    You had a circulating refrigerant, it is now gas; you have added the heat from the house, for example, as well as a little heat from the compressor, and so we say the refrigerant is superheated. The compressor compresses it, and it becomes pretty hot. That hot gas either goes to an outdoor condensor coil (if you have an air to air system) or the gas goes to one section of a water coil when you have geo.

    In a water coil one section is for refrigerant gas; the other section is for water flow. The two paths are totally separated. Water goes in one side. Water goes out the other side. In cooling mode, water comes in say 70 degrees; it may leave at 80 or 85 degrees carrying away the heat of compression. In terms of the gas side, in cooling mode gas comes in very hot, heat transfer from water flow cools the gas and gas changes state back into liquid to start the cycle all over again.

    Lets talk about winter heating. In the summer we removed heat from the house. In the winter all the heat that we need MUST come from liquid or water or air flow. In an air to air heat pump, the outdoor coil outside your bedroom window becomes the evaporator. Refrigerant is metered into it. It goes from liquid to gas absorbing heat from the air itself (even in 40 degree temperatures) That is why it "feels" cool when you put your hand over it. That superheated gas is compressed by a compressor and the hot gas goes to the evaporator where a blower blows air from inside the house across the coil condensing the gas back into liquid where the cycle begins over again. Water going into the coil may be 55 degrees. Water leaving may be 45 degrees, or something else.

    The air blowing into your house will vary somewhere between 86 - 96 degrees usually.

    In the case of geo, the heat is coming from either water from the ground or in closed loop systems, the circulating liquid circulating in the piping buried in your ground or water. In closed loop systems even though it is cold outside, there is still heat in the ground and we are collecting that "free" heat and using it. In winter correct water flow is very important.

    For new people the hardest thing to grasp is that there is still plenty of heat around even at 40 degrees, because we are accustomed to thinking 75 is warm, less than 75 degrees is cold. Not so.

    I could get more technical, but that is the simpler explanation.
     
  4. ChrisJ

    ChrisJ Active Member Forum Leader

    HVAC Tech thank you, all great info!

    "In a water coil one section is for refrigerant gas; the other section is for water flow. The two paths are totally separated."

    Within the water coil I think is where things are happening that I don't quite understand.

    In heating mode, within the water coil, the refrigerant changes from liquid to gas. The entering 42* water is sort of helping to superheat the gas. Thats why it leaves at 37*. Does the refrigerant get cold when it changes to a gas, thats why the water leaving is colder?

    I'll have to read your post a few dozen times, but I think I am starting to get it.

    @ WF_Inc, Sorry that link was more of the same general info I've been reading for 2 1/2 yrs. Any links or articles of a more technical nature?

    Thanks, ChrisJ
     
  5. zach

    zach Member Forum Leader

    Chris

    Think of it in terms of the flow of heat. Heat ALWAYS flows downhill.....from hot to cold.

    Take a look, beginning on page 25 of this document:
    http://www.hydronmodule.com/images/unit_literature/EMHL1002.pdf

    The 42* water is giving up heat to the colder liquid refrigerant(heat moves from hot to cold) and "boiling" it back to a vapor so it is again ready for the compressor;this is why the water cools to 37*. Just before this in the vapor/compression cycle, the refrigerant leaves the bottom of the condensor(coil the air blows over in heating) as a high pressure/high temp liquid. It hits the throttling valve which reduces its pressure and temp(pv=nrt). It then gets "boiled" via the heat from the 42* water as stated earlier. Loop fluid goes back out and extracts heat from the warmer ground.

    Read about the vapor compression cycle. This is all about phase change and heat flow. Look here:
    Google Images
     
    Last edited: Aug 25, 2011
  6. short reply so as not to confuse

    Others are pointing you in the right direction.

    We are borrowing or taking or stealing heat from that enterring 42 degree water you just mentioned. The action of the refrigerant does this. Typically if you remove x % of that heat then it will drop to 37 or something like that. That is referred to as delta T or the difference between the two temperatures. In the example you gave, the delta t is 5 degrees. A more typical number might be 12 - 19 degrees. Manufacturers indicate what they want to see..

    Remember, water underground remains relatively constant year round, (though this depends on the geographical area), thus warmer, especially when you go deeper than 15 feet. Think about this. If you have a closed loop system, even though the ground is relatively cold, we normally have sunlight shining on the ground many days of the year. (I am speaking about average homes, not Antartica). This reheats the ground.
     
  7. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I always tell my customers to think in terms of the heat pump they already own.....their air conditioner. It absorbs heat from their relatively cool (72*) house and combines it with the out door air (through refrigeration process) so the air blowing out of the top of their ac condenser is actually hotter than the outside air even though it collected heat from cooler air. An air source heat pump can reverse the process and collect heat from outside and discharge it inside. A ground source heat pump does away with the visible outdoor heat exchanger and moves it underground.

    Refrigeration "process" is sponge in the row boat stuff- absorb, here; wring out, there.
    So when refrigerant flashes from liquid to vapor it absorbs heat. When it is condendensed (turned back to liquid) we discharge (wring out) heat. The discharged heat is carried (combined with) away by water or air. Where we discharge the heat determines whether we are heating or cooling the home.

    Hope that helps.
    j
     
  8. ChrisJ

    ChrisJ Active Member Forum Leader

    Thanks everyone,

    It's a little hard fathom that there is heat in 30* water, I understand the efficiency is less, but still.

    Now I can hopefully explain it to the guy who did our excavating. He is coming back for another little project. Ever since he started the trenches for the loop field he's said "I don't get it".

    Thanks, ChrisJ
     
  9. Looby

    Looby Member Forum Leader

    Water with NO heat has a temperature of absolute zero -- that's
    about -460°F. ("No heat" is the definition of absolute zero.)

    Now, imagine starting with a block of super-cold ice -- is it hard
    to fathom that you'd have to burn some fuel to heat it to 30°F?
     
  10. zach

    zach Member Forum Leader

    Chris

    In your head, separate the concepts of heat and temperature. Heat is energy, temperature is a measure of heat/energy. As Looby said, heat is contained in matter down to absolute zero (I always remembered this as -273 Kelvin).

    The loop fluid need only be at a temperature greater than the temperature at which refrigerant boils/vaporizes. Someone correct me but this vaporization temp is likely in the low 20's at the pressue inside the heat pump circuit. So, you are moving heat from the loop fluid to the refigerant via the 2nd(i think) law of thermodynamics(hot to cold). All you need is a delta T between the two. Obviously, the greater the delta T the quicker and likely more efficiently the heat is transferred.

    Think about air source heat pumps. They will not work as the outdoor temp drops. Why? Because the outdoor temp no longer contains enough heat energy to boil/vaporize the refrigerant. Delta T is going the wrong way.

    Refrigerant needs to be vaporized as a compressor does not like liquid refirgerant. Plus, think about the "extra" heat absorbed by the refrigerant as it is vaporized. I think this is called latent heat of enthalpy. Or, the extra heat energy needed to turn a liquid to a gas. And latent heat of fusion which is the "extra" energy needed to turn liquid to solid.

    The magic of a heat pump.
     
  11. stuartrivchun

    stuartrivchun New Member

    GSHP are electrically powered systems that tap the stored energy of the greatest solar collector in existence: the earth. These systems(i.e.Ground source Heat pumps) use the earth’s relatively constant temperature to provide heating, cooling, and hot water for homes and commercial buildings The heat pump system uses solar energy stored in the earths crust. Energy is transferred to and from the earths surface by solar radiation wind and rainfalls.Ground sourced heat pumps simply move energy from one place to another. This energy is transferred over the condenser and into the distribution circuits, where hot water is circulated through the under floor circuits and in turn heat the property. For more detail visit;
    Geothermal Basics: How Geothermal Energy works, How Heat Pump Works, Types of Ground loops
     
  12. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Sorry I am late to this.

    The magic is in the change of state of a compound. To change from a solid to a liquid or a liquid to a gas requires energy. Adding or taking away if one goes from gas to liquid to solid.

    A water glass filled with ice and topped off with water will show a temperature of 32*F or 0*C. The glass contains the same substance in two states, solid and liquid. How many BTUH will it take to melt all the ice to water is the magic. I think Mr. Carrier's guy called this latent heat.

    One can run the same thoughts on liquid to gas. That is shown on Mr. Carrier's commissioned chart. It has taken me twenty years to get the chart, out of thirty five years doing this kind of work.

    What I am trying to teach is how a heat pump can concentrate low quality heat into home comfort. I am sure I missed.

    Mark
     

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