# Small water-to-water heat pump for off peak cold storage.



## diyer (May 13, 2009)

I am looking at storing cold in my thermal store and using it for air conditioning.  I am looking for recommendations. 

I have a Tarm pressurized system in an outside storage unit.  This runs a heat exchanger loop into an unpressurized 1500 gallon insulated milk tank. There is another loop for heat(forced air) and DWH. 

I currently use 2 window airconditioners during the summer.  They run continuously during the day.  I’m guessing that they are about 20K BTU’s total (about 1.5 tons).  If I find a water-to-water heat pump, I can run the hot side through my DWH superstore, continuing outside to the boiler loop(boiler off).  This would heat my water and dump excess heat outside.  The cold side would be put through the heat exchangers in the 1500 gallon tank.  I will be running Propylene Glycol in the system and would like to get down to 25F or so on the cold side. 

The cooler temperatures in the evening with some radiational cooling will allow me to harvest the cold “off peak”.  During the daytime, I would pump the cool out through a heat exchanger into the forced air system.  It would be sweet to add off peak electric rates for this if I can get them. 

I would also like to swap the hot/ cold loop and heat the tank with the daytime temps during spring and fall for use during the colder evenings. 

Does anyone know of a water to water heat pump in the 1 - 1 1/2 ton(12K - 18K BTU’s) range? 

Also, I was wondering what the best coil arrangement would be for this water cooled “chiller”.  I currently have a hot water coil in the up flow furnace plenum for heating.  I may want to find a chiller to put in above the current coil.  This would have the cold added at the top, flowing downwards, possibly plumbing the hot water coil in also. 

I don’t know how to add the condensate drain pan under the coils in the up flow.  It seems like the pan would block the air. 

Another concept would be to buy a downflow or side flow heat exchanger and place it before the furnace.  This could have the drain pan on the side of the furnace.  I could plumb the hot water coil, but it would be last in the cooled water loop to avoid condensation on it. 

I would really like to have the first solution (all coils in the furnace plenum).  This would be the same plumbing for heating and cooling.  Also with the extra cold water coil, I could get more heat out of it during the winter. 

All of my coils/plumbing is 1” PEX.  My Tarm boiler is in a storage container.  I am considering adding a solar exhanger to the side of the container.  This would add to my daytime heat gain for hot storage, and the radiational cooling during the cold storage season. 

This is really just a geothermal heatpump, but only using the diurnal ranges for short term storage. 

Thanks 
Carl


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## SolarAndWood (May 15, 2009)

That sounds like an extremely ambitious project with some engineering risk.  Keep us updated on how it works out.  We are in the middle of a house rebuild.  I have been looking at chilled beams to eliminate the need for AC but am very early in the process.


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## Redox (Jun 3, 2009)

Wow, where to begin....

There are a few manufacturers of small chillers that will do what you want but you aren't going to like the price.  Units like this are made by Florida Heat Pump, Water Furnace, McQuay and Climatemaster among others.  Most are made for the commercial market and are in the $1-2000/ton range.  There really isn't much to them but a couple heat exchangers and a compressor but the market is small and they have to charge hard for them.  It might be worth it if you can get good off-peak rates, but those have dried up around here after deregulation took place.  Right now, BGE is offering us a 22% discount on night time use, but you have to put up with much higher rates during the weekday.  I think those rates would have to drop back over 50% to make it worthwhile.  It makes a $200 window shaker look like a real bargain.

That being said, I think it is a great idea, but the details are going to be a killer.  Condensation issues are going to be your biggest problems as you want that to happen into a drain pan and not anywhere else.  If it drips off a vertical coil and into a drain, that's a good thing.  If it happens on anything else, it is a potential problem.  At the very least, you need to make sure there is a vapor barrier on all your insulation and make sure that absolutely everything is insulated.  That uninsulated circulator pump or valve is going to sweat a lot, especially with 25F glycol on the other side.  Wet insulation loses all its R-value and creates mold issues.

The best air side coil for chilled water is a vertical slab over a drain pan.  Anything that drips will go into the pan assuming it doesn' get blown off.  Horizontal won't work, but you might be able to mount the coil at an angle to form a slope or an A-shape arrangement.  A conventional heating coil is probably not going to be big enough as you need more surface area due to the lower delta T.  It also needs to be thicker and circuited differently if you actually want it to dehumidify.  Being on the east coast makes that kinda important!

Assuming you can work out those problems, I see several others.  One is insufficient heat rejection in the summer.  I don't think you are going to be able to dissipate enough heat, unless you are going to be running a bathhouse or something   :red: .  That 18000 BTU compressor is going to release about 25000 BTUs an hour and will heat up about 60 gallons of water to a nice hot shower temperature every hour (55-105F).  Your boiler will stay nice and warm over the summer, but you need to sink a LOT of heat either to the atmosphere or to ground if you want it to work.

Might be reading this wrong, but putting enough glycol in a 1500 gal tank to hit 25F is going to be a lot of glycol.  It isn't cheap and comes with other problems.  It also is an environmental headache if it ever leaks.  A more common method of thermal storage is an ice tank.  If you circulate cold glycol through coils in your water tank, you can get ice to form.  This uses less glycol and improves your storage ability.  Ice absorbs a lot of heat when it melts and your storage battery goes from about 500mbh up to almost 1800mbh (25-55F) in rough numbers.  If you skipped the glycol and only cooled it down to 40F (the practical limit without glycol), you will only store about 200mbh and this might not make it through the day without additional refrigeration.

Ice banked storage actually has a long history behind it.  The first movie theaters often used a tank of ice to get through a hot summer night back when refrigeration was really expensive.  Some large buildings are using it these days, but it isn't the norm and has many downsides and pitfalls.  Deregulation changed things overnight and some would contend that it causes even more global warming than a normal system due to the inefficiencies of the process.

But don't let me talk you out of it, though!  I have often contemplated building an ice tank under my deck and using a more conventional air-cooled compressor to do the ice making, but other projects keep getting in the way  :coolsmirk:.  In the meantime, I am trying something different here at the Redox Ranch (all 1/4 acre of it).  I signed up for TOU rates and am using the house itself as the storage device.  My thermostats go way down in the early AM when rates are cheaper and the outdoor temperature is lower.  Then about 10AM, they go up to about 80F or so which keeps the units off unless it is really hot out.  Even at 95F outdoors, an 80 degree dry house feels comfortable, especially with a few ceiling fans running.  When the rates drop back around 8PM, the unit goes back to a more normal temperature and the house cools off quickly because it is relatively dry.  So far, it is adding about $150 a month to the electric bills with 2500sf of house which is pretty good for our climate, but testing is ongoing.  I gotta have my AC!

In the end, you have to look at the numbers.  I think it might be a little ambitious to go for something that complicated in New Hampshire.  Your shorter cooling season makes the payback very long.

Speaking of long, I hope I haven't bored you, Carl.  I can go on at times...

Chris


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## Redox (Jun 3, 2009)

SolarAndWood said:
			
		

> That sounds like an extremely ambitious project with some engineering risk.  Keep us updated on how it works out.  We are in the middle of a house rebuild.  I have been looking at chilled beams to eliminate the need for AC but am very early in the process.



Interesting!

I had never heard about chilled beams until you mentioned it here.  It is apparently a European "thing" that is just now being introduced to the US.  They look like they would be a good idea in a hot dry climate, but they don't dehumidify.  This will reduce your heat load, but you still end up running something else for dehumidification which is an important part of air conditioning.

Years ago, there was a chiller manufacturer that introduced something called a valance cooler that sounds similar but was designed to sweat and dehumidify.  It looked almost like a strip of finned baseboard but would be several tubes wide.  It had a strip underneath that would collect condensate and carry it to a drain.  It seemed like a good idea at the time but didn't give you filtration and circulation and it never caught on.

Good luck with your project!

Chris


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## diyer (Jun 4, 2009)

Redox,

Thanks for the reply. 

The 1500 gallon tank is unpressurized and has heat exchangers in it.  The lower heat exchanger is the commercial ammonia exchanger for making ice cream.  The water in the heat exchanger/piping will have proplyene glycol in it, not the 1500 gallon tank.

I was planning on piping the hot side outdoors for the cool night air to gain efficiency.  There is no way my DWH would be able to freeze 1500 gallons.

I would like to also run this during the fall and spring (winter?) as heating.  The chiller coil and my hot water coil would be heated.  The source would be the 1500 storage.  I currently heat the tank and use the stored heat.  The heating starts degrading about 160F water temp.  I could run the hot water directly until the water temp dropped to about 160F, then turn on the heat pump to suck the stored heat out and put it into the heating coils.  It would give me longer times between burns.  Also the extra coil would let me run the water cooler than 160F without the heat pump running.

This does look like I am running into the "special equipment" category.  I am also looking at just keeping the storage tank at an average cool temp, not storing cold/ ice.  I could use a water to air heat pump which will run the tank temps up.  During the nighttime, I would run a pump to cool the tank back down.  This heat pump is more common.  I also can use the tank water for yard usage without wasting freezing water.  The coils for AC would be the smaller, usual freon coils.

I will be experimenting with the tank.  I will heat it up with the wood boiler and try cooling it with a pump during the evenings.  If I can get it to a reasonable temp, then the water to air heat pump may work.

Carl


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## Redox (Jun 6, 2009)

Okay, it's hard to wrap my head around what you are trying to do, so bear with me.

Heat pumps have a rather narrow range at which they are designed to operate.  If you feed a chiller 160F water, it is going to go into a massive overload and trip out.  The practical upper limit for a high temperature (AC) compressor is about 130F on the hot side and maybe 80F on the cold side.  Beyond that and you run into problems.  If you really need 160F water to heat your house, a heat pump isn't going to work very well for you.  If you can up the size of the heat exchanger in your air handler so that you can use 120F water, you might have a chance.  You could also sink the heat into a radiant floor or panels and use the heat pump to scavenge the remaining heat out of the tank.

One type of heat pump that is gaining in popularity is the swimming pool heat pump.  They started out as a regular heat pump with a water coil attached and have evolved into stainless and titanium versions to hold up to the pool water.  They are getting more popular down south as it makes more sense than burning oil or gas to heat a pool when it is 70 out.  They are also very expensive for what they are and generally only heat, not cool.  Most people aren't trying to cool their swimming pools. ;-) 

It would be possible to put something like a brazed plate heat exchanger on a conventional heat pump outdoor unit and pull off what you are trying to do relatively cheaply.  There's a few details that would need to be worked out, but lots of cheap equipment on Ebay that would do the job.

Chris


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## diyer (Jun 6, 2009)

Chris,

Thanks for staying with me.  I appreciate the info....



			
				Redox said:
			
		

> Okay, it's hard to wrap my head around what you are trying to do, so bear with me.
> 
> Heat pumps have a rather narrow range at which they are designed to operate.  If you feed a chiller 160F water, it is going to go into a massive overload and trip out.  The practical upper limit for a high temperature (AC) compressor is about 130F on the hot side and maybe 80F on the cold side.  Beyond that and you run into problems.  If you really need 160F water to heat your house, a heat pump isn't going to work very well for you.  If you can up the size of the heat exchanger in your air handler so that you can use 120F water, you might have a chance.  You could also sink the heat into a radiant floor or panels and use the heat pump to scavenge the remaining heat out of the tank.



The water temperatures could be handled with an aquastat or a mixing valve.  

Wikipedia says that "Standard ARI 330 ratings were intended for closed loop ground-source heat pumps, and assumes secondary loop water temperatures of 77°F for air conditioning and 32°F for heating. These temperatures are typical of installations in the northern USA."  

I have downloaded some of the climatemaster water to water heatpump info.  It seems to agree.  It will go way past 32F and 77F, but not very efficiently.


Below is a couple of pictures of my tank, one of the internal heat exchangers, and a sketch of my first plan.  I have added 600 feet of PEX heat exchangers also to this tank.

For air conditioning, I would run the water to water heat exchanger in the evenings when it is cool outside.  The pump to the outside water to air heat exchanger would take the heat away and cool the hot side in the cool evening air.  This would cool the thermal store.  This would be a heating cycle which would drive the thermal store to 32F (per wikipedia for heating).

In the morning, when the sun comes up, the outside water to air HX and pump would shut down.  As the afternoon goes on, the house would need some AC.  If the tank is 32F to maybe 40F, I would run the green bypass pump to avoid running the compressor.  If I had a hot hot day, and the thermal store heated up to 45F or so, the green bypass would shut off and the heatpump would turn on to airconditioning.  The brown loop pump would draw the cold water into the inside HX for AC.  This should work until the water hits 77F or so (wikipedia).

Winter time, the roles are reversed.  During the daytime, I could grab some sun on the outside heat exchanger and heat the tank with the heat pump.  During the evenings, the bypass pump would heat the house.  If the thermal store gets too cool, the bypass pump would turn off and the heat pump would heat from the thermal store.

I am adding more pumps to try to avoid running the heat pump compressor more than necessary.  I plan on using the heat pump only when it is most efficient (nighttime for AC, daytime for heating).  The thermal store will really only be storing the hot/cold for about 12 hours.



Yes, plan one has a requirement for a special heatpump, so plan 2 was using only a water to air heatpump.  I could put it outside to try to heat/cool the thermal store and only run water into the house for AC or heating.  Most of the water to air heatpumps are not rated for outside use.

Or...
If I put the water to air heatpump inside the house, I would be using the thermal store for the source.  I could try to cool/heat the thermal store with a water tower or other heat exchanger.  This would require some experimentation to see if this would work.


Rolling the last two water-to-air heatpump concepts would be one outside, and one inside.  The outside one would charge the thermal store during the most efficient times (storing cold when it's cool, hot when hot).  The other would run the AC/heat in the house, again with the thermal store temps at the more efficient levels.

I am planning on prototyping and running simulations on anything before I do it.  I need to make sure I get payback for any investments.  If it doesn't payback, I could just run a chest freezer to cool the thermal store at nights, and use my outdoor boiler for winter.

Carl


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