Hydronic cold storage?

[BioHazard]

It's almost that time of year again when I drag out all of the window air conditioners for my house. Aside from the massive electrical bill, the noise, and the ugly things sticking out of my windows, they're great. 

This is my first year on the "time of use" power plan, so, when I need air conditioning so does everybody else and the cost for power is at a premium. At night; however, my price is cut by 2/3. I was thinking about building a large cold storage tank where I could chill water at night, and use that to cool during peak electric periods.

Firstly I would need a centralized A/C system, not a bunch of scattered winow units. My thinking is to use the existing ductwork and blower fan for my forced air natural gas furnace. In order to cool the house, I would need to mount a heat exchanger (car radiator?) in the return air plenum and pump cold water through it.

Is this a sound idea? I believe that is how larger HVAC systems work but I don't really know.

What I'm left with figuring out is a water/cold storage tank, and some sort of compressor driven chiller. For the hot side of the compressor I would like to use a few thousand gallon pond in my yard as a large cooling tank. Water normally stays below 70 degrees in the summer.

The major questions I have in my mind are:
How much cold liquid do I need to store, and at what temperature?
WHAT cold liquid should I store? Would there be benefits to using an antifreeze mixture to keep the liquid below  the freezing point of water, or simply let it freeze into a big block of ice every night?
Is there much difference between storing, say, 200 gallons of liquid at 50 degrees, vs 100 gallons of liquid at half that temperature?

Just thinking outloud here. Any thoughts are welcome.

[Ronmar]

Well the rules for BTU apply here.  1600LB of water at 50F would absorb 32000 BTU getting to 70F  800# of water would absorb the same 32000 BTU for a 40F temp rise.  The problem with the lower temp medium, is the same as with storing heat with a higher temp medium.  The greater the temp difference between it and it's environment, the greater the loss directly from the storage vessel.  So for a given insulation, the larger mass would absorb less from it's surroundings than the colder/smaller volume, so get closer to absorbing that full 32KBTU from your source.  I have not studied ice characteristics at all, so I don't know if there is any gain by going below 32F.  It is more of an issue to deal with from an expansion/contraction standpoint.  I have seen an industrial system that does what you talk about.  At night, it chills water to freezing at lower power rates.  During the day, it circulates water thru pipes in the ice/chillwater tank to the cooling network in the building.  There is another benefit from chilling at night.  The condenser coils will be more efficient at dissipating the heat to the cooler night air than they would in the daytime.  Having the storage inside the house would help, as any heat absorbed would be from the house, but again condensation would need to be planned for and managed.

Inside the central heat system would probably work to distribute the cool air, but you need to account for condensation at the point the chill is applied.  IE, there needs to be a drip pan under the radiator to collect and dispose of the condensate from the air.   

[Crofter]

It takes a lot of space and expensive container and insulation to store in water only a few dollars worth of energy. If you can contend with the expansion when freezing occurs there is 144 times as much energy stored per pound in the change of state as there is in each pound degree temperature change of liquid water. I think it would have an awfully long pay back period at todays interest rates.

[mbryner]

Biohazard, I don't know where you live specifically, but it's only mid-March and you're already thinking about A/C so I have to assume you live somewhere in the south.....   Wait, I just looked at your profile: NW Oregon.    Hmm, I'm guessing your climate is not as hot in summer as SW Oregon where we are.  Why not just install a high efficiency heat pump?    It seems for all the hassle of making your own system, you'd come out ahead.   (Who am I to talk though, I'm the one w/ cooling earth tubes and a couple thousand gallon heat storage tank under the house.)   What's the insulation like in your house?   Any way of increasing your insulation and using passive cooling at night?

[Thob]

Off the top of my head:

I like the idea of charging a bank of batteries from the cheap electricity at night and using an inverter to power stuff during the day (when the rates are high).  It depends on the difference in rates and efficiencies of chargers and inverters, but in some cases it might make sense.  It allows you to keep your noisy window units, plus only cool the room you're currently using (instead of the entire house).  A battery bank also lends itself to being charged by solar/wind/other renewable source, and using the grid to make up any difference or occasional peak demand.  You could use one of the battery backup type inverters with a built in charger for this, use a timer to shut off the incoming AC during the day and turn it back on to recharge at night.

Ultimately, storing cold in ice is somewhat like a battery, only with limited use.  An electrical charge in a battery can be used for a lot more stuff.  For example, it can also run lights, the refrigerator, etc.  Whatever uses high priced electricity during the day can use it from the battery (within the capacity of the battery and inverter).

Freezing ice will definitely keep the storage area smaller, but it's kinda hard to pump ice around thru the radiator.  You would need a separate cooling loop with antifreeze to pump thru the radiator and back thru another heat exchanger in the ice/cold storage.  OR - just freeze a big block of ice in one room and use the furnace blower to circulate air from that room to the rest of the house.  Cover/uncover the ice depending on how much cooling you need.

[Tom Reed]

Once upon a time churches used to be cooled this way. They used a small refrigeration system to freeze a block of ice and then used the energy stored in the ice on Sunday to keep the congregation cool. A heat exchanger frozen in the block with an anti-freeze loop should pull the heat out well. Make sure you've good good drainage for the heat exchanger in the house.

[Ronmar]

It takes a lot of space and expensive container and insulation to store in water only a few dollars worth of energy. If you can contend with the expansion when freezing occurs there is 144 times as much energy stored per pound in the change of state as there is in each pound degree temperature change of liquid water. I think it would have an awfully long pay back period at todays interest rates.

Wow!  144 times...  I know there is a huge transfer from liquid to gas, but had never studied the liquid to solid transfer numbers.  I do recall the system I refered to earlier was for a small office building, but the cold tank was not all that large.  They did not go over how they contend with expansion and contraction from the ice...

So I am envisioning something like a chest type freezer with some flexible foam liner and a flexible bladder/liner inside.  Then lowered into this, several sets of freezer evap coils linked to a much larger compressor with external condenser or even a ground loop.  A heatex coil/loop would be in the bottom of the tank to add heat from the home during the daylight hours. 

Another option might be to not freeze the entire tank.  A large industrial ice maker draws water from the insulated tank to make loose cube ice that it then dumps back into the tank during the night.  During the day, the water in the bottom of the tank conducts the heat from the cooling loop heatex coil in the bottom of the tank to the ice, melting it to maintain the near freezing temp of the water untill the ice is exhausted.  No problems with the expansion from freezing with loose cubes.  No issues with possible damage to the chiller/evap coils suspended in a water tank being repeatedly frozen/thawed.  It could also be built from mostly off the shelf parts, with only some modifications to the icemaker to remote or ground source the condenser...

Ice also solves another issue I didn't go over in my first post, quality of heat or cold.  With a water tank chilled to 40F, you will have a good delta to start with pumping 70F room air thru your radiator, but as the day progresses and the water warms, it will be slower and slower to remove a given ammount of heat from the house as the temperature differential gets less and less.  A tank of icewater is going to stay very near 32F untill all the ice is expended... And as more and more ice melts, it will expose more and more water to more ice surface area to absorbe more heat as the day progresses.   

[LowGear]

Why not just install a high efficiency heat pump?

Now we all know the chick magnet factor is much lower on this off the shelf solution but if you got a hamburger flipping job to pay for it you'd be ahead on time too not to mention all that room to store really important stuff.  And it might co-gen in the winter to heat the place.

Casey

[Ronmar]

But that still dosn't solve the problem of cutting your bill with the implimentation of peak usage metering.  A heat pump would be more efficient, but a BTU of cooling is still a BTU of cooling and it has a cost.  A unit sized for a house is still going to have a significant electrical consumption.  The cold storage thought places the heavy cooling loads in the off-peak zone...

[LowGear]

I've never lived around air conditioning - cooling.  How much does it cost to run one of those window units in KWH?  What are the two rates you're faced with?



Casey

[Crofter]

Here is a link to a no moving parts refrigerator using a huge block of ice as a thermal flywheel. In our climate it will freeze enough ice to give you year around refrigeration with no energy imputs ( well none you have to pay for)     http://fourmileisland.com/IceBox.htm    I have had it bookmarked for quite a while. There are some enticing ideas a fellow could use if he did not have to fit into conventional systems and codes. If you have to comply by satisfying a mortgage holder or an insurance company or building code they dictate that you must do it in a manner that puts money in their pockets. They hold hands together but not yours.

Water has some amazing properties as a heat exchange medium if you can turn them to your advantage. It's other change of state from a liquid to a gas or vice versa exchanges nearly a thousand times the energy compared to a simple temperature change as a liquid.

[BioHazard]

Why not just install a high efficiency heat pump?    It seems for all the hassle of making your own system, you'd come out ahead.   (Who am I to talk though, I'm the one w/ cooling earth tubes and a couple thousand gallon heat storage tank under the house.)   What's the insulation like in your house?   Any way of increasing your insulation and using passive cooling at night?

If that's your answer you're on the wrong website! My insulation isn't great, but not terrible either. House is about 15 years old with 2x6 walls filled with fiberglass. It doesn't get terribly hot here, but I am VERY intolerant of heat and I just can't let my living quarters get above about 70. I do also plan on installing an attic/whole house vent because my house does like to stay hot after the temp drops at night.

I've never lived around air conditioning - cooling.  How much does it cost to run one of those window units in KWH?  What are the two rates you're faced with?

My units are about 500w each, and they normally run almost constantly throughout the hot parts of the day. Times 5. Usually sounds like there is a helicopter about to take off in my house.  Peak power rates are approximately 12 cents, "mid" peak is only 6 cents, and "off" peak is only 4 cents. The industrial "time of use" rates are even cheaper and I'm also going to need to come up with some fort of cooling for my new light industrial shop.

I like the idea of an ice (cube) machine, now I guess I need to calculate just how much ice I might need.

Rather than hooking this to my main air duct system, I may just run some PEX with cold water to individual rooms with smaller heat exchangers. That way I don't need to cool the whole house. I could certainly modify the system to make heat in the winter too....

[BioHazard]

While we're talking about it, what if I put some kind of water misting system on my roof, and then capture that water later for irrigation? Got a huge southern exposure of black asphalt shingles that love to soak up the heat. Would it help cool my house if I water cooled the roof?

[mbryner]

If that's your answer you're on the wrong website!

I figured I might get some flack for that! 

what if I put some kind of water misting system on my roof, and then capture that water later for irrigation?

You'll probably lose most of the water to evaporation.   The whole house fan and attic vents are also a great idea.  Especially the solar powered attic fans.

[deeiche]

rm /