During the summer, the HeatPump blows cold air into the home, and the "ground source" water circulating thru the unit dispels the waste heat the unit makes back into the ground.

During the winter, the HeatPump blows hot air into the home, and the "ground source" water circulating thru the unit dispels the waste cold the unit makes back into the ground.
There is no "waste cold." The refrigeration cycle absorbs heat from the ground-source and moves it inside. The "waste heat" in summer is heat absorbed from inside the house that is being moved outside.

Not all heat pumps are "ground source." The majority are air-source. Think of it as installing a window unit backwards ... it's cooling the outdoors, moving heat inside.

Think of an air source heat pump as a window AC unit.

During the summer it dumps heat outside after taking it from the inside.
Refrigerant is boiled to cool the inside, then compressed and cooled to liquefy in the outside.

During the winter the unit is just 'turned around.'
The refrigerant is compressed and cooled giving up heat to the inside, and boiled on the outside.

In the winter, heat pumps transfer heat from the outdoor air to the indoor air. When the refrigerant boils in the coil, it absorbs heat (latent heat of vaporization). There is heat in the outdoor air even at brutally cold temps.

Everything you've read so far is dead-on as I understand it. But here's my two-bits' worth, FWIW.

You are quite right in that an air conditioner is a heat pump running just one direction, and a heat pump used to heat or cool is similar to an air conditioner, but can run both directions.

Refrigeration units depend on the compression and the expansion of gases to do their thing. Remember- evaporation is a cooling process. Refrigeration units work by taking a compressed gas and allowing it to evaporate in the right place- and of course that's in coils in your refrigerator, freezer, or air conditioner. Then they take the gas that filled the coils, compress it again, and repeat the cycle. The gas that they recover from the coils is warmer than when it was released into the coils, because it absorbed heat from area around it. And that's because energy always seeks to spread to whatever is around it.

So you have a bunch of heat to get rid of. The low pressure side of a system is cold, picking up heat; the high pressure side is hot, losing heat.

Rinse and repeat.

Energy is not being created here (and in fact a certain amount is lost because you need an outside source to force the compression) but it is being moved around. Energy on one side of a system is being harvested and moved to the other side of the system, where it can be allocated well. Good ground source heat pumps do this all pretty cheaply.

I hope I got this right ... :-)

M

O.K., so now that you all have explained how a heat pump operates, answer these questions.

The HP is touted as being very efficient, in that it provides more heat (watts) than the energy (electrical energy)used to generate it. When it operates in cooling mode, does it remove more heat (watts) than the amount of electrical energy used? What about a standard a/c?

V

There's no effective difference between a standard air conditioner and a heat pump running in cooling mode. Answer to both questions is yes.

And if it's a ground source heat pump it is orders of magnitude more efficient than an air source heat pump, because liquids are so much more dense than gases (water is orders of magnitude denser than air, that is, has so many more molecules in any given volume). Water has so much more energy in a given volume. Water from underground is in contact with a massive solar collector (the earth) and contains huge amounts of stored solar energy, which was free to collect, and yet is still cooler than the human comfort level by enough to make it efficient to cool the hot side.

These things work if they're well built and intelligently installed.

Cheap units, including many window-mount air conditioners, are probably nowhere near as efficient as they could-should be, but still work pretty well.

(I think ...)

:-)M

One other bonus to ground source is the heatsink is a rather constant temp around 50 degrees or so. You have the best of both worlds for heating and cooling.

I'm a little late to the party, but you may find this link useful:
http://www.etccreations.com/howdoesaheatpumpwork%3F

Also a comment to the reply above: "if it's a ground source heat pump it is orders of magnitude more efficient than an air source heat pump" - sorry, this is BS.

The efficiency of a heat pump depends on the temperature at which the coils are placed and the energy transfer between the air/water and the coils. The COP rating of the given unit will tell you how efficient it is at one specific temperature. Manufacturers have curves/tables that will tell you the efficiency for each air or water temperature.
A good water source Hp operating with 50-70F water can give COPs of around 4-5. A good air source HP operating at 50F can give a COP of... *5* also! (ref: Lennox XP19 specs).

Much of the cold winter, a water source heat pump will be more efficient because air temperatures are lower than the water temperatures. However, ground source closed loop systems often run with ground temperatures around freezing, so the air source will be about the same efficiency if the average daily temperature is around freezing too.

It's not cut and dried...

I would like to know where the ground temp is around freezing in a geothermal loop Tedsan?

"A good water source Hp operating with 50-70F water can give COPs of around 4-5. A good air source HP operating at 50F can give a COP of... *5* also! (ref: Lennox XP19 specs)."

I am very well aware that some heat pumps are capable of a COP of 5 at 50 degrees ambient temps. What is the COP of that Lennox XP19 at 0degrees? If the ground source has say after a long winter a water temp of even 40 degrees going into the pump with an ambient temp of 0 what would the COP be then?

Or the heat pump of your choice with an abient temp of 100degrees compared to a geothermal with a incoming water temp of say 60 degrees and an outside temp of 100?

Please explain what is BS I am confused. If you could keep the outside temp the same range as the geothermal operates at we wouldn't need a heat pump or air conditioner.

A high seer/COP heat pump may be more efficient at 40-50 degrees ambient temp than a ground loop geothermal since there is no water pumping needed but it is splitting hairs. I doubt there are many places that have an ambient temp in the 40s and 50's for very many days in a year. I have a few it is the ones in the 20s-30's and 80's-100's that are killer on my electric bill. I have many in the low and high extremes.

If that xp19 will perform the same as a 3 ton geothermal with 4 150' holes when its between 0-30 and 80-100+ PLEASE SOMEONE let me know before I drop the money on a ground loop. It would save me a bunch of money!!

Tedsan, thank you for your contribution here and on the other boards which address this subject.

I made the point that ground source heat pumps are orders of magnitude more efficient than are air source heat pumps, and I stand by that point.

For any given compressor-evaporator system liquids will be orders of magnitude more efficient than gases in effecting heat exchange. As I said before, systems have to be intelligently designed and installed -as anybody can take a good idea and screw it up in application. I've done it any number of times ... But certainly the available energy exchange in water (or in an anti-freeze loop) is much, much greater than in air.

Air has a density that's a mere fraction of water's- perhaps an 800th, depending on where the air is measured. Using water to carry off excess heat or to "wash off" ice is simply much, much more efficient than using air.

Mitigating circumstances: water is harder to move (costs more energy to transfer) than air, but on the other hand, the more you need energy from the air (i.e. the colder or hotter it is outside) the less suited it is to your purpose, because when you need to disperse energy from the high-pressure side the air is less dense, and can exchange heat less effciently, and when you need to draw energy from it to warm the high pressure side it's still not dense enough -gases just can't get very dense and still be gases! And, the molecules that are there are colder, just when you need them to be warmer. In other words, the denser air gets the colder it gets, and vice versa, just when you need the opposite.

Here in western Montana we've been "enjoying" sub-zero F temperatures and considerable winds, but our ground water, whether deliver by Mountain Water Co. or from my well, stays well into the 40's F range. We have less than six inches of accumulated snow, so there's not much insulation.

If you look at temperatures in terms of degrees Kelvin there's not much differenct between water that's freezing (at 32.4 F)and water that's 0 degrees fahrenheit: 255/273. In other words, at 0 degrees fahrenheit water stores 93% as much energy as it does at 32 F. Even if the ground temperature were 20 below zero F. a liquid circulating in it would carry 89% as much energy as a liquid circulating in ground at 32F.

My ground water at about 45 F or 280 F is only 1.0256 more capable of carrying energy than it would be if it were at the freezing point.

Certainly there are places where it doesn't make sense to invest in an expensive ground soure -the demand just doesn't justify it. But as a matter of physics -ground source kicks butt.

Now, my disclaimer: I am not an engineer, and it's probably true that I've missed something here, but I'll ask those who have trained as engineers to correct my mistakes.

What's wrong in my understanding? (and please keep in mind that I'm very limited in my time this week and might not get back to respond for days, if then.)

Thanks,

Mark

"I would like to know where the ground temp is around freezing in a geothermal loop Tedsan?"

Good question! It's quite non-intuitive, but explicable, and quite common, for closed loop systems. Here's what happens...
During the course of the heating season, energy is drawn from the ground around the loops. The energy gets replenished from ground further away from the loop. Depending on the thermal contact of the loops with the ground, the length of the loops and the conductivity of the surrounding earth, the rate of inflow of energy varies considerably.

In the ideal case, when the loops have been off for a long time, or been used extensively for cooling during the summer, the "near field" around the loops is typically 50+ F or warmer, giving a large amount of energy to extract for the heat pump's operation. During the winter, the longer the on-time of the heating system, the more heat is drawn from the ground. So the loops "see" lower and lower temperatures. If the system only runs an hour or two per day, and the factors above are sufficient, then the ground recharges back to the ambient ground temperature. However, the further into the season you get, the longer the on-time, and the less opportunity for the ground temperatures to recharge.

If the thermal conductivity of the ground is very high and you have lots of loop contact with the ground, then the recharge rate is fast, and the loop may never see ground temperatures of less than 40F. However, in most cases, a decision is made to balance the cost of many deep boreholes with the performance. You'll hear values from 100-250 ft per ton of capacity as typical ranges. The lower the number, the less the volume of ground to recharge the areas around the loops and the more the ground temperature will fall.

To further complicate matters, you have to space the boreholes far enough apart or they will start interacting. The cold from one bore reduces the heat available to recharge the volume around its neighbor. The wider the spacing the better, but typically 12-15 feet is seen as a reasonable compromise.

For all these reasons, if you look at the entering water temperature of most closed loop systems (that is, the temperature of the anti-freeze solution circulating through the closed loops) it will drop through the season, often down to freezing. The experienced geo installers I've worked with size their systems based on a 32F EWT to ensure that the system will still have adequate capacity at the end of winter.

These issues become progressively worse for larger installations, like institutional settings where you have several hundred bores under a parking lot, spaced on a 10-15 foot grid. Thermal modeling shows that bores on the inside of the grid show these seasonal heating/cooling problems progressively worse, the further from the edge of the grid. At a recent training session I attended by one of the leading geo consultants and college, they showed that their actual ground temperatures over a period of years were reaching 90-100F and increasing yearly because in institutional settings, usually there's a larger cooling load than heating. In this case, it's like having an air conditioner running all summer in 90-100F temperatures! It's great for the heating season, but lousy for cooling.

Sorry to be so verbose, I hope this description is useful. On to your other questions.
At 0F, that same system has a COP of 2.42. The problem is, the heat output has dropped from 46.7kBTU/hr at 50F to 17.8kBTU/hr at 0F, so you have to turn on your heat strips, say 15kw worth (about 51kBTU/hr). Since that's at a COP of 1, your effective COP drops substantially, to only a shade above 1. In the mid-atlantic region, only a few percent of our days are below 15F, so that doesn't too adversely affect the overall system performance. If you can get the balance point down to 25-30F, you can handle 80+% of the heating hours with only the heat pump, so your "seasonal COP" for these systems remains well above 2. The colder the climate, the worse, so I wouldn't recommend a conventional air source system where the heating degree days goes much above, say, 6000.

The same arguments apply to the 100F side. In this climate zone, the vast majority of the air conditioning is done at 75-85F, with maybe 10-20% above this. In very hot climates, the AC SEER definitely drops and the geothermal performs relatively better.

For all these reasons, I greatly prefer open-loop geothermal, where you get a relatively constant 50-60F water, year round. In my area, they don't like using them due to maintenance issues with mineral laden water, but if you don't mind the maintenance, you absolutely can't beat an open loop system and all the ground source heat pump sales info is right on.

The "BS" comment was about the "orders of magnitude" performance note. An order of magnitude is a factor of 10x. Orders of magnitude means hundreds, thousands or more times more.

"If you could keep the outside temp the same range as the geothermal operates at we wouldn't need a heat pump or air conditioner." - For open loop at 50-60F, definitely true. For closed loop, where the loops "see" ground temperatures in the low 30's most of winter and 70-80 during the summer, the air source heat pump does see these temperatures for the majority of the year. The GSHP is definitely more efficient when the outside temperatures drop below the 30's and above the mid 70's. Actually, in my GSHP system, the installer f'd up and my temperature loggers show ground temperatures in the low 20's by the time I'm in January, which really sucks. So for my system, I'd be better off a lot of the time with an air source system.

"If that xp19 will perform the same as a 3 ton geothermal with 4 150' holes when its between 0-30 and 80-100+ PLEASE SOMEONE let me know before I drop the money on a ground loop. It would save me a bunch of money!!"

600' of holes for a 3-ton system is great and your loops are likely to stay within 10F of the static ground temperature if the conductivity of the ground is reasonable. Few people around here are lucky enough to have that much. At $16-$20/foot for bores in my neighborhood, that's $12,000 just for the bores for a 3-ton system. I actually did a bunch of cost analysis for people around here who have houses that would be satisfied by a 3-ton system and the GSHP would have saved them maybe $500/year under good conditions. The installed costs were then about $10k for a top of the line air source and $24k for a mid-range GSHP. $14k delta is 28 years to make up the cost difference. Again, this is my climate, with about 5000 HDDs. The harsher your climate, the better the equation becomes for GSHPs.

All that said, the GSHP will be more comfortable during the winter because of no defrost cycles. This is a big pet peeve for air source systems. People hate the hourly blast of cold air while the system defrosts and I consider this one of the major stumbling blocks to their widespread adoption in moderate climates.

Mark - sorry for my initially snarky reply. I'm too used to trolls on discussion groups and my response was out of line.

You're correct about the heat exchange. If you have a square foot of liquid-liquid or liquid-ground contact, the energy transfer will be vastly greater than a square foot of air in contact with a heat exchanger. I think the issue that I have is that the overall transfer efficiency ends up being much more comparable in the common configurations.

Again assuming a typical closed loop system, you've got linear plastic pipe, a poor conductor, with relatively little surface area. This is stuck in a hole in the ground and, sometimes, backfilled with a moderate conductivity grout. Other times, it's just filled back in. The air source system has copper tubing bonded to a large area of aluminum fins, with large amounts of air being forced through the fins. So in one case, you've got a poor implementation with a great heat transfer agent and in the other you've got a great implementation of a poor heat transfer agent. When you're all done, it appears from the system specs, that all that really matters is the returning fluid temperature or the outside air temperature. I think the other factors come out in the wash.

I've often wondered about pumping losses, but I think the huge fan for the air source has to at least equal the typical pumps for ground source systems, so those two probably also neutralize one another.

Please don't get me wrong, my comments are limited to general theory and my climate zone. The colder the climate, the more the balance shifts in favor of ground source. And, any day of the week, I would choose a water source, open loop system over any other system out there if the price were right. But in this climate zone, with tight, well insulated homes, the energy usage is so low that a properly designed air source system works extremely well, requires little/no maintenance, and costs a fraction of a ground source heat pump. But I've heard that in some areas, you can get an open loop system, fully installed, for $10-$15k. At that price, why bother with air source?

All the best,
Ted