increasing efficiency of open loop GT heat pump

stevehull · 03-02-13, 08:56 AM

Years ago I helped a friend in Colorado rig up a simple AC system for those hot days in the mountains. Via cheap black pipe, we brought down water from a spring up the hill, ran it through an old car radiator in the cabin, dumped "hotter" water into a downhill pond and used a simple box fan to blow across the cold radiator.

The water temp was about 45 F, used about 1 gallon per minute and did a simply fantastic job

. It also provided a source of water for the cabin . . . , but no so good in winter

Currently, I am using ground source (open loop) heat pumps to heat and cool. My water temp is in the 50's.

The idea is to put a precooler in front of the "A" coil using water that will shortly be used to go through the GT heat pump.

A few calculations show that I can reduce the inlet air temperature by about 5-10 degrees F, minimally change the water temperature (perhaps 1 degree F) and increase the moisture extraction of the "A" coil by precooling the inlet air. Water flow is about 4 GPM.

The cost is an old radiator, some PVC pipe and a bit of time . . .

Before I start, has anyone tried this? I have heard that ClimateMaster offers this as an option, but calls to a local dealer indicate he has not heard of it.

Steve

randen · 03-02-13, 03:48 PM

Yes Steve
The contractor that installed my Geothermal did that precooling with the ground loop water on a previous install. He did mentioned it was a success.

I had tried that inplace of airconditioning my shop. I thought that the loop temp would be cool enough for airconditioning. (10 degree C). Now it did work but not nearly as well as an air-conditioner. The air that past though the radiator didn't cool enough to reduce the moisture or relative humidity. The water did pour out but the cooled air of about 20 Deg C was still 100% relative humidity at that temp. The result was the shop was cooler but you still felt very clammy. The radiator needs much cooler temps to reduce the moisture content of the air to be conditioned.

Randen

stevehull · 03-02-13, 05:17 PM

Randen,

The temp of the water 10 C is plenty cold to dehumidify. You may have had the blower on at too high a speed.

Lowering the fan speed decreases the velocity of air across the coil and results in a greater extration of humidity.

Try it next summer!

Steve

MN Renovator · 03-02-13, 05:22 PM

I don't think it is. You need the temperature of the refrigerant(in this case we are talking about water as the medium) to be colder because the air should really be at ~10C to dehumidify effectively. You aren't going to get the air anywhere near there, even with a huge coil with any decent amount of CFM to actually cause a latent heat drop. I'm seeing this being almost purely a sensible heat drop while the water all stays in the air. ...and not much of one. Your COP would probably be pretty bad, even moreso if you are using a high wattage pump instead of an efficient one, same goes for the power usage of the blower.

randen · 03-02-13, 09:04 PM

MN Renovator wrote:
Your COP would probably be pretty bad, even moreso if you are using a high wattage pump instead of an efficient one, same goes for the power usage of the blower.

Actually the thought of the 85watt loop circulation pump with the lowspeed fan air handler was the reason to try. But it didn't provide the comfort I was looking for and the homemade heat-pump was already started and would be installed for heat/cooling capibilities upon completion.

Randen

AC_Hacker · 03-03-13, 09:18 AM

Sounds to me like the "A coil" being mentioned is actually part of a vapor compression system, so lowering the air temp prior to that coil might well be a good idea.

The vapor compression machine would still do the heavy lifting with respect to de-humidification, but there would be less lifting for it to do.

-AC

jeff5may · 03-03-13, 01:44 PM

This trick would have the same effect as increasing the size of your refrigerant-to-water hx in your heat pump.
On the air side, the pre-cooler would aid the a-coil in getting the incoming return air close to its dew point, if not actually condensing any moisture. The a-coil would then do a much better job at removing the remaining latent heat due to the reduced airflow the precooler would create due to added resistance.
on the water side, the sensible heat the precooler removes from the air would be added to your incoming loop water. Since the temperature increase would be small, it would have very little impact on the COP of the heat pump. The added heat in the water would be very close to the removed heat in the air, but the reduction in delta-t on the a-coil might gain you a few percent cooler air.
As many have learned, there is no substitute for a larger heat exchanger. The human perception of comfort doesn't always factor in well with the math until you get into the fringes of "normal" operation. With this trick, you should be able to pick up lots of added comfort, especially on those hot, muggy days. The added precooler would help lay waste to all that extra water in the air, maybe even allowing you to raise the thermostat a couple degrees. Or you could bask in the cold, dry wind coming from the vent.

MN Renovator · 03-03-13, 11:40 PM

What are the limits of the latent/sensible ratio? I'd imagine there is the point where you can cool the air down enough to where it will freeze the coil and then no air flows, then your vapor compression freezes back to the compressor and liquid refrigerant slugs back to the compressor and kills it. I had my house at 85 degrees(was away for the weekend), decided that 55 degrees was plenty warm to run the air conditioner outside if the inside was 85 since I figured the split would be about 70 degrees. Ran fine for 2 hours and the air flow was heavy and cold as expected after hour 3, I noticed there was very little airflow so I shut it off, just as I expected I froze the coil and the refrigerant lines back to the compressor were frozen. Haven't killed the compressor but with further testing later I determined that 60 degrees is the minimum while the inside is 75 or warmer. Sure cools quickly and the condensate line sure is pouring out the water. I'd give raising the fan speed in the furnace blower a shot, but it's maxxed out already(as it should be for a PSC or shaded pole motor for maximum efficiency. ..just wasn't designed for low ambient and it isn't really worth the cost or effort kitting it for low ambient.

In a nutshell, find a way to be sure you aren't going to freeze your coil by dropping the temperature of the incoming air too much. I'm also curious what the limits are for how much latent you can really get out. I think most air conditioners are a 0.8 ratio in normal operation, I've read articles that say that nothing really exists that can do better than 0.5 other than dedicated dehumidifiers that are negative since they heat the air, which is beside the point of cooling a building at the same time.

03-02-13, 08:56 AM	#1
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	increasing efficiency of open loop GT heat pump Years ago I helped a friend in Colorado rig up a simple AC system for those hot days in the mountains. Via cheap black pipe, we brought down water from a spring up the hill, ran it through an old car radiator in the cabin, dumped "hotter" water into a downhill pond and used a simple box fan to blow across the cold radiator. The water temp was about 45 F, used about 1 gallon per minute and did a simply fantastic job . It also provided a source of water for the cabin . . . , but no so good in winter Currently, I am using ground source (open loop) heat pumps to heat and cool. My water temp is in the 50's. The idea is to put a precooler in front of the "A" coil using water that will shortly be used to go through the GT heat pump. A few calculations show that I can reduce the inlet air temperature by about 5-10 degrees F, minimally change the water temperature (perhaps 1 degree F) and increase the moisture extraction of the "A" coil by precooling the inlet air. Water flow is about 4 GPM. The cost is an old radiator, some PVC pipe and a bit of time . . . Before I start, has anyone tried this? I have heard that ClimateMaster offers this as an option, but calls to a local dealer indicate he has not heard of it. Steve __________________ consulting on geothermal heating/cooling & rational energy use since 1990

03-02-13, 05:17 PM	#3
stevehull Steve Hull Join Date: Dec 2012 Location: hilly, tree covered Arcadia, OK USA Posts: 826 Thanks: 241 Thanked 165 Times in 123 Posts	Randen, The temp of the water 10 C is plenty cold to dehumidify. You may have had the blower on at too high a speed. Lowering the fan speed decreases the velocity of air across the coil and results in a greater extration of humidity. Try it next summer! Steve __________________ consulting on geothermal heating/cooling & rational energy use since 1990

03-03-13, 09:18 AM	#6
AC_Hacker Supreme EcoRenovator Join Date: Mar 2009 Location: Portland, OR Posts: 4,004 Thanks: 303 Thanked 723 Times in 534 Posts	Sounds to me like the "A coil" being mentioned is actually part of a vapor compression system, so lowering the air temp prior to that coil might well be a good idea. The vapor compression machine would still do the heavy lifting with respect to de-humidification, but there would be less lifting for it to do. -AC __________________ I'm not an HVAC technician. In fact, I'm barely even a hacker...

03-03-13, 01:44 PM	#7
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	This trick would have the same effect as increasing the size of your refrigerant-to-water hx in your heat pump. On the air side, the pre-cooler would aid the a-coil in getting the incoming return air close to its dew point, if not actually condensing any moisture. The a-coil would then do a much better job at removing the remaining latent heat due to the reduced airflow the precooler would create due to added resistance. on the water side, the sensible heat the precooler removes from the air would be added to your incoming loop water. Since the temperature increase would be small, it would have very little impact on the COP of the heat pump. The added heat in the water would be very close to the removed heat in the air, but the reduction in delta-t on the a-coil might gain you a few percent cooler air. As many have learned, there is no substitute for a larger heat exchanger. The human perception of comfort doesn't always factor in well with the math until you get into the fringes of "normal" operation. With this trick, you should be able to pick up lots of added comfort, especially on those hot, muggy days. The added precooler would help lay waste to all that extra water in the air, maybe even allowing you to raise the thermostat a couple degrees. Or you could bask in the cold, dry wind coming from the vent. Last edited by jeff5may; 03-03-13 at 01:51 PM..

03-02-13, 03:48 PM	#2
randen Uber EcoRenovator Join Date: Apr 2011 Location: Strathroy Ontario Canada Posts: 657 Thanks: 9 Thanked 191 Times in 129 Posts	Yes Steve The contractor that installed my Geothermal did that precooling with the ground loop water on a previous install. He did mentioned it was a success. I had tried that inplace of airconditioning my shop. I thought that the loop temp would be cool enough for airconditioning. (10 degree C). Now it did work but not nearly as well as an air-conditioner. The air that past though the radiator didn't cool enough to reduce the moisture or relative humidity. The water did pour out but the cooled air of about 20 Deg C was still 100% relative humidity at that temp. The result was the shop was cooler but you still felt very clammy. The radiator needs much cooler temps to reduce the moisture content of the air to be conditioned. Randen

03-02-13, 05:22 PM	#4
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	I don't think it is. You need the temperature of the refrigerant(in this case we are talking about water as the medium) to be colder because the air should really be at ~10C to dehumidify effectively. You aren't going to get the air anywhere near there, even with a huge coil with any decent amount of CFM to actually cause a latent heat drop. I'm seeing this being almost purely a sensible heat drop while the water all stays in the air. ...and not much of one. Your COP would probably be pretty bad, even moreso if you are using a high wattage pump instead of an efficient one, same goes for the power usage of the blower.

03-02-13, 09:04 PM	#5
randen Uber EcoRenovator Join Date: Apr 2011 Location: Strathroy Ontario Canada Posts: 657 Thanks: 9 Thanked 191 Times in 129 Posts	MN Renovator wrote: Your COP would probably be pretty bad, even moreso if you are using a high wattage pump instead of an efficient one, same goes for the power usage of the blower. Actually the thought of the 85watt loop circulation pump with the lowspeed fan air handler was the reason to try. But it didn't provide the comfort I was looking for and the homemade heat-pump was already started and would be installed for heat/cooling capibilities upon completion. Randen

03-03-13, 11:40 PM	#8
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	What are the limits of the latent/sensible ratio? I'd imagine there is the point where you can cool the air down enough to where it will freeze the coil and then no air flows, then your vapor compression freezes back to the compressor and liquid refrigerant slugs back to the compressor and kills it. I had my house at 85 degrees(was away for the weekend), decided that 55 degrees was plenty warm to run the air conditioner outside if the inside was 85 since I figured the split would be about 70 degrees. Ran fine for 2 hours and the air flow was heavy and cold as expected after hour 3, I noticed there was very little airflow so I shut it off, just as I expected I froze the coil and the refrigerant lines back to the compressor were frozen. Haven't killed the compressor but with further testing later I determined that 60 degrees is the minimum while the inside is 75 or warmer. Sure cools quickly and the condensate line sure is pouring out the water. I'd give raising the fan speed in the furnace blower a shot, but it's maxxed out already(as it should be for a PSC or shaded pole motor for maximum efficiency. ..just wasn't designed for low ambient and it isn't really worth the cost or effort kitting it for low ambient. In a nutshell, find a way to be sure you aren't going to freeze your coil by dropping the temperature of the incoming air too much. I'm also curious what the limits are for how much latent you can really get out. I think most air conditioners are a 0.8 ratio in normal operation, I've read articles that say that nothing really exists that can do better than 0.5 other than dedicated dehumidifiers that are negative since they heat the air, which is beside the point of cooling a building at the same time.