Author Topic: Adding "ground source" to an exixting heat pump?  (Read 5969 times)

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PNW_Steve

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Adding "ground source" to an exixting heat pump?
« on: January 27, 2016, 03:47:59 PM »
Hey Everyone,

I have been looking at a small heat pump and found that in our climate electric heat strips are required for colder weather operation.

That grinds my gears! I want to buy a heat pump to replace resistance heating and I have to add resistance heating to the heat pump....

I looked at ground source heat pumps but could not find a reasonably priced unit in the 1-2 ton capacity.

I wonder how effective it could be to take a conventional split system and add an air/water heat exchanger to the air intake on the outdoor unit. Then circulate water from a 275 gallon tank buried in my yard through the heat exchanger.

Any thoughts?

DamonHD

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Re: Adding "ground source" to an exixting heat pump?
« Reply #1 on: January 27, 2016, 04:20:18 PM »
There are air-source heat pumps that will operate down to -25C.

What does your outside temperature get to?

Rgds

Damon

PNW_Steve

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Re: Adding "ground source" to an exixting heat pump?
« Reply #2 on: January 28, 2016, 09:48:04 AM »
There are air-source heat pumps that will operate down to -25C.

What does your outside temperature get to?

Rgds

Damon

Thanks for the suggestion Damon.

We see -5C pretty commonly and on occasion as cold as -15C. The heat pump that I have been looking at will work in these temps but only with the addition of resistive heat strips to warm the incoming air.

My question is whether I can substitute a ground sourced air/water heat exchanger for the heat strips and gain some efficiency?

If it works as I envision then wit would also be beneficial in hot weather to help cool the evaporator when running the heat pump in A/C mode.

DamonHD

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Re: Adding "ground source" to an exixting heat pump?
« Reply #3 on: January 28, 2016, 11:27:52 AM »
"Warming the incoming air" with resistive heating is mad and sounds a bit like warming your smokeless fuel stove with coal!

Eg see this post for a -20C spec unit:

http://www.ebuild.co.uk/topic/17704-system-are-ashps-really-as-bad-as-some-people-make-out/page__st__20#entry144115

Rgds

Damon

PNW_Steve

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Re: Adding "ground source" to an exixting heat pump?
« Reply #4 on: February 01, 2016, 08:27:28 AM »
I skimmed over that thread and did not see anything relevant. Did I
 miss something?

DamonHD

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Re: Adding "ground source" to an exixting heat pump?
« Reply #5 on: February 01, 2016, 10:23:39 AM »
The post I linked to discussed air temperatures similar to the ones you talk about, without any 'resistive heating' to my knowledge.

Rgds

Damon

PNW_Steve

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Re: Adding "ground source" to an exixting heat pump?
« Reply #6 on: February 01, 2016, 06:34:11 PM »
Ok. I skimmed over the thread and missed that.

What I am looking for is information regarding using groundwater to improve the efficiency and operating range of conventional heat pumps. My first thought is to use a water to air heat exchanger on the air intake for the outdoor unit.

southpaw

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Re: Adding "ground source" to an exixting heat pump?
« Reply #7 on: February 02, 2016, 06:58:37 AM »
By definition a 1 ton heat pump has the capacity to absorb as much heat as a ton of ice absorbs while melting. Therefore your heat pump would theoretically turn your 275 gallons of water into a block of ice in an hour. It would probably take a couple of hours because of the heat transfer from the tank to the ground. Not much surface area on a tank, thats why they use coils of pipe or vertical lengths of pipe for ground source heat pumps. If you could absorb enough heat from the ground source you would have better results submerging your outside coil completely in the water from your groundsource and using the power that runs your outside coil fan to run your circulating pump. Better efficiency would also be achieved because of the elimination of one heat exchanger. It would also increase your efficiency in air conditioning mode.

Frank S

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Re: Adding "ground source" to an exixting heat pump?
« Reply #8 on: February 02, 2016, 08:48:33 AM »
By definition a 1 ton heat pump has the capacity to absorb as much heat as a ton of ice absorbs while melting. Therefore your heat pump would theoretically turn your 275 gallons of water into a block of ice in an hour. It would probably take a couple of hours because of the heat transfer from the tank to the ground. Not much surface area on a tank, thats why they use coils of pipe or vertical lengths of pipe for ground source heat pumps. If you could absorb enough heat from the ground source you would have better results submerging your outside coil completely in the water from your groundsource and using the power that runs your outside coil fan to run your circulating pump. Better efficiency would also be achieved because of the elimination of one heat exchanger. It would also increase your efficiency in air conditioning mode.
  Southpaw I don't mean to contradict you but your explanation is not quite correct. thus requires a little more information when talking about converting 1 ton of water to ice and vice versa the the conversion figures are based on water in liquid form at 32 f to ice at 32f commonly used as both the melting and freezing points of water however under certain conditions water molecules may remain fluid at temps as low as -41f this is termed as super cooled water, which has no relevance here.
  The roots for refrigeration are in the ice making industry, and the ice manufacturers wanted an easy way of understanding the size of a refrigeration system in terms of the production of ice. If 288,000 Btu are required to make one ton of ice, divide this by 24 hours to get 12,000 Btu/h required to make one ton of ice in one day. This is the requirement for the phase change from liquid to solid to convert water at 0C (+32F) into ice at 0C (+32F). As a practical matter, additional refrigeration is required to take water at room temperature and turn it into ice.
To be specific, one ton of refrigeration capacity can freeze one short ton of water at 0C (32F) in 24 hours.
So, a ton of refrigeration is 3.517 kW. This is derived as follows:
The latent heat of ice (also the heat of fusion) = 333.55 kJ/kg = 144 Btu/lb
One short ton = 2000 lb
Heat extracted = 2000 x 144/24 hr = 288000 Btu/24 hr = 12000 Btu/hr = 200 Btu/min
1 ton refrigeration = 200 Btu/min = 3.517 kJ/s = 3.517 kW = 4.713 HP
I live so far outside of the box, when I die they will stretch my carcass over the coffin

southpaw

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Re: Adding "ground source" to an exixting heat pump?
« Reply #9 on: February 02, 2016, 10:02:11 AM »
Thanks for the correction Frank. I forgot the ton per day bit. I'm not a refrigeration guy, i just have bits of information rattling around in my head and sometimes they don't come out the way they went in.
There are a lot of factors which affect whether his idea will work, soil temp, soil type etc. but eventually he will end up with a block of ice unless he has a lot more surface area on his heat sink.
Glad there are knowledgeable people on this forum keeping misinformation to a minimum. Don't believe anything you read and only half of what you see. 

PNW_Steve

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Re: Adding "ground source" to an exixting heat pump?
« Reply #10 on: February 02, 2016, 12:12:38 PM »
Good info!!

Thank you.

I have a small stream that runs within about 20' of where the heat pump would be located. I wont say that it never freezes over but it has not frozen in the 5 years that we have owned the place. Perhaps I could divert from the stream?

Another option could be water pumped from the creek at the back of my lot. A much larger project as I will have a 40'+ change in elevation to overcome. Unless I come up with a "water powered" pump the energy used to pump the water could be a serious hit on any savings.

DamonHD

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Re: Adding "ground source" to an exixting heat pump?
« Reply #11 on: February 02, 2016, 02:09:06 PM »
Water source heat pumps can work well.

Rgds

Damon

southpaw

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Re: Adding "ground source" to an exixting heat pump?
« Reply #12 on: February 03, 2016, 05:30:11 AM »
Good info!!

Thank you.

I have a small stream that runs within about 20' of where the heat pump would be located. I wont say that it never freezes over but it has not frozen in the 5 years that we have owned the place. Perhaps I could divert from the stream?

Another option could be water pumped from the creek at the back of my lot. A much larger project as I will have a 40'+ change in elevation to overcome. Unless I come up with a "water powered" pump the energy used to pump the water could be a serious hit on any savings.

If it is a closed loop from the stream to the heat exchanger and back to the stream, gravity from the return line will cancel the energy required to lift the water and convection will actually assist circulation. The cold water exiting the heat exchanger is denser than the warm water entering it. You would only need to overcome pipe resistance.
Google "spiral pump"  It might fit your application.

PNW_Steve

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Re: Adding "ground source" to an exixting heat pump?
« Reply #13 on: February 05, 2016, 08:51:04 AM »
Good info!!

Thank you.

I have a small stream that runs within about 20' of where the heat pump would be located. I wont say that it never freezes over but it has not frozen in the 5 years that we have owned the place. Perhaps I could divert from the stream?

Another option could be water pumped from the creek at the back of my lot. A much larger project as I will have a 40'+ change in elevation to overcome. Unless I come up with a "water powered" pump the energy used to pump the water could be a serious hit on any savings.

If it is a closed loop from the stream to the heat exchanger and back to the stream, gravity from the return line will cancel the energy required to lift the water and convection will actually assist circulation. The cold water exiting the heat exchanger is denser than the warm water entering it. You would only need to overcome pipe resistance.
Google "spiral pump"  It might fit your application.

The little stream next to the house may be too small to support the spiral pump. I have looked at the idea and like it. I wonder if I could make the spiral pump play on the creek?

During the summer months I will have to lift the water close to 40'. Much less in the winter but the varying water levels may be a problem. Seasonal pics below:

southpaw

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Re: Adding "ground source" to an exixting heat pump?
« Reply #14 on: February 06, 2016, 06:34:29 AM »
Good info!!

Thank you.

I have a small stream that runs within about 20' of where the heat pump would be located. I wont say that it never freezes over but it has not frozen in the 5 years that we have owned the place. Perhaps I could divert from the stream?

Another option could be water pumped from the creek at the back of my lot. A much larger project as I will have a 40'+ change in elevation to overcome. Unless I come up with a "water powered" pump the energy used to pump the water could be a serious hit on any savings.

If it is a closed loop from the stream to the heat exchanger and back to the stream, gravity from the return line will cancel the energy required to lift the water and convection will actually assist circulation. The cold water exiting the heat exchanger is denser than the warm water entering it. You would only need to overcome pipe resistance.
Google "spiral pump"  It might fit your application.

The little stream next to the house may be too small to support the spiral pump. I have looked at the idea and like it. I wonder if I could make the spiral pump play on the creek?

During the summer months I will have to lift the water close to 40'. Much less in the winter but the varying water levels may be a problem. Seasonal pics below:
If the stream closer to the house is at a higher elevation than the creek you could use siphon effect to circulate the water eliminating the need for a pump.

PNW_Steve

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Re: Adding "ground source" to an exixting heat pump?
« Reply #15 on: February 06, 2016, 09:58:50 AM »
Good idea!

I am fairly certain that the stream has sufficient flow to accomplish this in the Winter. I am not so sure about the flow in the summer.

I was out looking things over yesterday with this application in mind. I picked a spot where I can place a small dam and measure/manipulate the flow.