Reducing overnight loads for freezers.

[Bill NC]

I was working on ways to reduce the overnight loads (when the grid is down) for a hybrid system that would have an AC coupled system added as a phase 2 project, and got to thinking of phase change materials.

I’ve often read about keeping freezers full, even if you just add plastic jugs filled with water. The frozen jugs of water would absorb the most heat when the ice goes through the phase change from solid to liquid. Unfortunately this happens at 32 deg and one does not ever want to let the contents of a freezer get this warm.

BUT, if you put just the right amount of salt in the jugs, the water/salt solution freezes just above the set point of the freezer, and thaws well below 32 d. 15% NaCl by weight should be a freezing point of about 15 deg f.

http://www.engineeringtoolbox.com/sodium-chloride-water-d_1187.html

One would want to access the freezers early in the day when the sun was shining.

Any thought?

Bill NC

[Mike90250]

I have a simple upright manual defrost freezer, and just put a timer on it, to disable it 2am - 9am, just to trim my overnight loads a bit. It (and the salt jug) still overall consumes the same amount of power, but I'm load shifting to some solar production hours for a load that my battery bank was not designed for.

[Bill NC]

I have a simple upright manual defrost freezer, and just put a timer on it, to disable it 2am - 9am, just to trim my overnight loads a bit. It (and the salt jug) still overall consumes the same amount of power, but I'm load shifting to some solar production hours for a load that my battery bank was not designed for.

Right. The overall load would not be reduced, just shifted to solar production hours. And, as you pointed out, a timer would be needed.

[J.P.M.]

I was working on ways to reduce the overnight loads for a hybrid system that would have an AC coupled system added as a phase 2 project, and got to thinking of phase change materials.

I’ve often read about keeping freezers full, even if you just add plastic jugs filled with water. This, however, works most efficiently when the temp in the freezer gets to 32 d f which is too warm. BUT, if you put just the right amount of salt in the jugs, the water/salt solution freezes just above the set point of the freezer, and thaws well below 32 d. 15% NaCl by weight should be a freezing point of about 15 deg f.

http://www.engineeringtoolbox.com/sodium-chloride-water-d_1187.html

One would want to access the freezers early in the day when the sun was shining.

Any thought?

Bill NC

I got a new freezer for free in 1982. Being somewhat curious about how things like heat loss, thermal capacity and thermal time constants, etc. work on a practical level, I fired up the freezer, put some water filled gallon size jugs in - nothing else, and had some fun. I added jugs and sort of kept track. This was before Kill-a- Watt meters, etc, so it was sort of hit/miss on results. Anyway, a couple of comments FWIW:

- It's OK if the water freezes - just use flexible containers, don't fill them all the way and leave the tops a bit loose.
- If you don't fill up the jugs, and allow the air to escape, they probably won't burst when the water freezes. H2O expands volumetrically about 6% or so when it freezes, as does H2O/brine.
- Ice has a specific heat of about .50 or so - not to be confused with something called the enthalpy of latent heat of fusion. Therefore, ice has about half the effective sensible heat capacity as water. However, that's still very approximately about 2X as effective on a mass basis as most other commonly available materials.
- On a thermal capacity per volume basis, water is still mostly king among commonly available materials. Ice is again about half as much capacity as water on a per volume basis, as is, to cite another example, solid concrete. The concrete may have practical limitations with respect to weight and some minor considerations about the rate of heat conduction through the concrete which may be an interesting conduction problem from an academic standpoint, but probably of little practical concern at this level. Concrete's probably out due to weight considerations alone.

- Something of an oversimplification: The energy savings realized by keeping jugs of water/ice or anything with a fair amount of thermal mass in a freezer are mostly the result of reduced cycling of the cooling equipment. Given a set temp., say +10 deg. F., and quasi steady state conditions - the door stays closed, the space around the freezer stays at sort of constant temp., etc., a freezer full of water, ice, lead, rocks, or anything else will gain as much heat as an empty one. The logic around any savings goes something like this: Putting stuff in the freezer increases something called the thermal time constant of the freezer. The more thermal mass, the longer the time constant. The result of that action is mostly that the full freezer's cooling equipment will cycle less and have fewer transient losses/wasted energy during startup/after shutdown. For example, suppose an empty freezer cycles 10 times in 24 hrs. and the compressor may run a total of 2 hrs. = 120 min.-->> 120/10 = 12 min./cycle. The same freezer full of ice will probably run for about for about the same 2 hrs = 120 min, but may cycle only 3 times in the same 24 hr. time period -->> 120/3 = 40 min. per cycle. The savings mostly come from having fewer cycles and thus fewer transient cycling losses, which may be quite a bit lower in modern equipment than that of say, 30 or so years ago.

- One possible, but probably impractical way to accomplish the same thing is to change the freezer thermostat's hysteresis or "dead band" - the temp. diff. between the temp. when the compressor starts and when it turns off. Say a freezer's thermostat tells the compressor to start when the freezer's interior temp. rises to + 12 degrees F. and continues to run until the temp. drops to +10 deg. F., then turn off. Now, if the thermostat logic is changed from those settings to say, 13 deg. turn on and 9 deg. turn off, to a first approx. you would assume the unit would cycle something like half as much. Probably not practical or even possible given the current state of things, but it may keep your jugs out of the freezer.

- In a similar way, I had a lot of self education farting around with the anticipator on the furnace in my first house in Buffalo. I could change the thermostat dead band and increase the cycle times. Doing so lowered the CH4 use a bit in the winter. I also added a lot of thermal mass to the dwelling, recalced' the building time constant several times and got lots of info and insights. That first house was a real test bed on energy conservation. FWIW, the law of unintended consequences came into play: One of the things I (slowly) learned was that my first wife could give Job lessons on patience.

Take what you want/need, leave the rest.

[Bill NC]

J P M:

I may have missed this in your post, but there is a lot of heat absorbed for water to change from ice at 32 deg to water at 32 deg. Or from frozen brine at 15 deg to liquid brine at 15 deg.

Bill NC

[sensij]

J P M:

I may have missed this in your post, but there is a lot of heat absorbed for water to change from ice at 32 deg to water at 32 deg. Or from frozen brine at 15 deg to liquid brine at 15 deg.

Bill NC

He did reference the enthalpy of latent heat of fusion early in the post, but yes, it looks like most of the post was with regard to heat capacity of a single phase material. Your idea to use a phase change material to hold a lower average temperature for the same heat exchange is a fine one (as long as the extra energy to "initialize" the system is available at some point), and I've participated in the design of laser systems that use it as a way to level out the temperature of the diodes. J.P.M.'s point about density change in the phase transition is one to keep in mind... don't fill the jugs all the way.

[J.P.M.]

J P M:

I may have missed this in your post, but there is a lot of heat absorbed for water to change from ice at 32 deg to water at 32 deg. Or from frozen brine at 15 deg to liquid brine at 15 deg.

Bill NC

See the part about latent heat of fusion. Once the temp. of the interior of the freezer and everything in it gets and stays below the freezing temp. of the liquid (the phase transition temp., or the min. phase transition temp. if such phase change happens over a temp. range), the enthalpy of fusion of any anything inside the freezer that can undergo a phase change plays no part in any steady or quasi steady state analysis. It's all about sensible heat capacities at that point.

[Bill NC]

See the part about latent heat of fusion. Once the temp. of the interior of the freezer and everything in it gets and stays below the freezing temp. of the liquid (the phase transition temp., or the min. phase transition temp. if such phase change happens over a temp. range), the enthalpy of fusion of any anything inside the freezer that can undergo a phase change plays no part in any steady or quasi steady state analysis. It's all about sensible heat capacities at that point.

I see that I was unclear in the opening post, which I just edited. My intent was to get freezing point of the brine solution to be just above the freezer set point so that the brine solution acted as a reverse battery of sorts. Storing the ability to absorb the amount of energy that it takes to melt the brine solution from a 15 deg solid to a 15 deg liquid.

Bill NC

[sensij]

Thinking more about this, although the phase change process would help overnight, implementing this satisfactorily may be more difficult than it seems. Let's say your solution has a freezing point of 15 deg C, and the thermostat on the freezer causes it to begin cooling at temps above 16 deg and stop at 14 deg. During the day, when the sun is out and you would like to be "charging" your reverse battery by freezing the solution, it is possible that it is "discharging", or still melting. Since the temperature in either condition is 15 deg, the thermostat has no trigger to turn on until the entire solution has melted and the freezer temperature begins responding again. The time of day that will happen passively is unlikely to be constant throughout the year.

That assumes the freezer is perfectly uniform in temp and the reality will be less simple than that, but it is an example of what you might need to consider as you are sizing the system and / or taking other steps to ensure that each day when the sun goes down you are starting with a fully frozen jug.

A more controlled implementation would force the freezer cooling cycle on each day at the appropriate time to fully freeze the jug, before the jug had fully melted. This isn't compatible with a temp controlled thermostat, unless you rely on the uneven temp distribution inside the freezer as some kind of indicator of how much melting has occurred (analogous to SOC of a battery).

[Bill NC]

Thinking more about this, although the phase change process would help overnight, implementing this satisfactorily may be more difficult than it seems. Let's say your solution has a freezing point of 15 deg C, and the thermostat on the freezer causes it to begin cooling at 16 deg and stop at 14 deg. During the day, when the sun is out and you would like to be "charging" your battery by freezing the solution, it is possible that it is "discharging", or still melting. Since the temperature in either condition is 15 deg, the thermostat has no trigger to turn on until the entire solution has melted and the freezer temperature begins responding again. The time of day that will happen passively is unlikely to be constant throughout the year.

That assumes the freezer is perfectly uniform in temp and the reality will be less simple than that, but it is an example of what you might need to consider as you are sizing the system and / or taking other steps to ensure that each day when the sun goes down you are starting with a fully frozen jug.

Good point.

I picked 15 deg F assuming that my freezers got down to 0 deg F.

The freezing point has to above the turn off set point AND below say 30 deg F.

Bill NC

[J.P.M.]

I see that I was unclear in the opening post, which I just edited. My intent was to get freezing point of the brine solution to be just above the freezer set point so that the brine solution acted as a reverse battery of sorts. Storing the ability to absorb the amount of energy that it takes to melt the brine solution from a 15 deg solid to a 15 deg liquid.

Bill NC

Try this: Assume this freezer operates in a constant temp. environment. Put a control surface around the freezer. If the interior of the freezer is less than the temp. outside the control surface, heat will cross the control surface and into the freezer. To counteract this heat gain, energy must be supplied to a refrigeration unit of some sort if something like a reasonably steady temp. is to be maintained inside the freezer. Regardless of the contents, or the thermal mass of those contents, over time, the total amount of energy crossing the control surface will be the same.

For this discussion. any savings on an electric bill, such as may be possible, will come from reducing the number of times a refrigeration unit cycles. Using phase change materials is one possible way to accomplish that goal. As a practical matter, it has a way to go.

Your idea of using the latent heat of materials is not a bad one - been around for a long time. Solar thermal storage using something called eutectic salts was the darling of some folks back in the day. The practical problems with using the latent heat of anything, at least as far as current refrigeration equipment is concerned is that the phase transition temp. tends to be more stable than most freezer thermostats' ability to hit a temp. that close, so the phase change material is often in one phase or the other, and thus not taking advantage of the freeze/thaw energy as much as needed. Kind of a tricky business to hit and probably more so to monitor. Also, sat. solutions of brine often tend toward non uniform concentrations when heated/cooled further complicating things.- see salt gradient solar ponds as an example.

Nice idea, and maybe someone has a reliable phase change storage medium, But I don't think it's an idea whose time has come yet.

[sensij]

I don't think the goal here is electric bill savings, but to time shift the load so the cooling demand occurs during solar generation periods instead of at night when the energy would come by discharging batteries.

The freezing point has to above the turn off set point AND below say 30 deg F.

Yes, I think as long as this condition is met, you can safely shut the freezer off over night. When you turn it back on during the day, your reverse battery will always be in "charging" state and you will get to the end of the day and be sure the frozen jugs are ready to go. The only remaining trick is to size the jugs so that by morning, they have not fully melted.

Edit: I would do that sizing with an otherwise empty freezer, which should be the worst case scenario.

Edit further: Actually, the condition that needs to be met is that the freezing point of the solution be above the turn on setpoint, not the turn off. If the setpoint is between the turn on and turn off setpoints, the condition of the ice at the end of the day is unknown.

[inetdog]

Good point.

I picked 15 deg F assuming that my freezers got down to 0 deg F.

The freezing point has to above the turn off set point AND below say 30 deg F.

Bill NC

If you want to maximize the amount of heat that goes into the phase change each day and use it to keep the overall temperature lower during the night, you should probably aim closer to (but above) 0F instead of going all the way up to 15F.

[Bill NC]

I don't think the goal here is electric bill savings, but to time shift the load so the cooling demand occurs during solar generation periods instead of at night when the energy would come by discharging batteries.



Yes, I think as long as this condition is met, you can safely shut the freezer off over night. When you turn it back on during the day, your reverse battery will always be in "charging" state and you will get to the end of the day and be sure the frozen jugs are ready to go. The only remaining trick is to size the jugs so that by morning, they have not fully melted.

Edit: I would do that sizing with an otherwise empty freezer, which should be the worst case scenario.

Edit further: Actually, the condition that needs to be met is that the freezing point of the solution be above the turn on setpoint, not the turn off. If the setpoint is between the turn on and turn off setpoints, the condition of the ice at the end of the day is unknown.

You're exactly right. That's actually what I meant to write. The turn on point would be warmer, the turn off point colder.

[paulcheung]

I have a freezer about 30 cu.ft. and a refrigerator ~26 cu.ft.; I program the timer for the freezer to turn on from 8AM to 4PM every day and also turn on at 9pm to 10pm for one hour in the night, I have put some solid ice on top of the basket as the members suggested, I have no problem at all.

The refrigerator turn on 8am to 8pm and also run two hours in the night from 11pm to 12 am and 3am to 4am. everything run fine, just the box milk won't last as long as before when it run straight through.