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Another take on off grid refrigeration...  RSS feed

 
Creighton Samuiels
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I know that I'm not the first to consider off grid refrigeration, nor the first to bring up vortex tubes, but I might be the first to do so in this particular context.  I have been contemplating something like this for a long time; how do I build a "cool room" with nested colder storage for foods that must be near or below freezing?  My first idea was to build an airtight & insulated box room in my basement, insulated on all four sides and the top, but not the bottom.  Basiclly a modern version of a root pantry, designed to benefit from the natural cold sink of the deep earth but protected from the heat of the rest of the house, particularly during mid-summer.  This is fine as far as it goes, but what about adding a deep freezer?  This creates the problem that the freezer would dump heat into the cool room's airspace, thus heating it up.  So I consider a trick I learned about while reading about sailboats, the keel cooled DC refrigeration compressor.  I could order a kit that would permit me to build my own refrigerated box, or just use a quality cooler, while placing the condensor and compressor outside of the cool room.  Great idea, and whatever "cold" leaked out of the little box would contribute to the cool, dry ideal environment for the rest of the pantry space.  The problem that I ran into is that these kits, designed for marine use, are incredibly expensive; most start about $1000, and that would be before I added in the solar array, batteries and charge controller to the budget.

But then I stumbled upon the concept of the vortex tube, on this very forum.  It seems so perfect, using compressed air power to drive an incrediblely simple heat separation/rejection device, while piping the cold output port into the small cooler inside the larger cool room; and piping the hot output port someplace else potentially useful, such as a preheat air-to-water exchanger before the water flows into my water heater, or outside to the greenhouse or chicken coop for a bit of warmth.

Of course, this would still require one or more methods of compressing that air to start with, and some pressure vessels to store it in.  For this I was going to use a small DC driven compressor & solar array, connected to PVC piping buried below the frost line using a trencher (perhaps encased in concrete?) in order to passively take advantage of geothermal cooling effects.  Alternatively, air can be compressed by a wind turbine driven compressor, or a micro-hydro driven compressor, or a spark or diesel engine driven compressor in place of a backup generator.  Regulation wouldn't be strictly necessary, since the output temp of a vortex tube can be controlled by adjusting the input pressure using an air regulator, but a simple thermostat switch powering an air solenoid would work great. 

Not only should this form of air powered refrigeration keep jugs of milk cold just fine; if the cool room is made large enough, it can be used as an escape from the heat of the day during the dog days of summer; just like us old people used to do at grandpa's farm.  (I remember playing board games in papa's basement under a single incandescent lamp from about lunchtime till about 3pm when we were staying there during July.  Papa didn't have air conditioning until he sold the farm, so papa's basement is where we kids would escape the heat & humidity.)  Not too much insulation would be necessary, because it would be useless beyond a point, since the air escapes anyway.  Place a deliberate air port on the top of the cool room, to let out the stratified (most warmed) air.  It should be low humidity, since compressed air is "dry" after cooling and passing through a water separator.

One downside that I can already see is that I would still need a battery array in order to go completely off grid, in order to power the lights, computer, ecetera.  While compressed air is particularly good for powered cooling in this way, compressed air is particularly poor at storing motive energy, since by cooling off the compressed air, motive energy is dissipated into the environment as waster heat. (this effect is exactly what we want for cooling, since expanding the room temp compressed air inside the vortex tube contributes to the cooling effect).

Does anyone else see problems with my plan that I can't see?  I've also considered buying a large, industrial CO2 tank or two, to use as "backup" air pressure for this system.  Of course, the cool room couldn't be used as a human occupied space if CO2 is used, since that might kill me; but instead of a cool room as the air dump, I could pipe the vent to the greenhouse for a bit of extra growth.

Thoughts?
 
Creighton Samuiels
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No commentary so far I see.  I was thinking about how the hot side output could be used by a mostly or fully off-grid homesteading family, and I realized that it could be used to make a food dryer simply by piping the hot air stream into a well sealed cooler on the outside of the cool room, and then ported outside or wherever from there.  It should be very effective use of the hot side air stream, because while it's not hot enough to actually "cook" anything with, the air flow is both hot to the human touch (above 110 degrees, but not likely hotter than 140; the bare minimum for safe, slow cooking) it is also extremely dry air.  For this reason, a wet hand in the air flow will feel cool for a bit, until the wetness is dried away and there is no longer enough moisture to protect the human skin from the heat.  Might be a fine way to make jerky, or dried fruit snacks, etc.

During winter, the hot side air stream could be piped into the master bedroom and to an air stone placed on top of the mattress, under the winter quilt, to preheat the bed before the wife climbs into it.



 
Creighton Samuiels
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Something like this would be an *outstanding* way to put a variable micro-hydro site to excellent use.  The length of air line pipe from the hydro site to the house would serve as both the geo-thermal cooling as well as storage volume.  Buried PVC pipe of 1 inch or less would be great, particularly if a pre-cooler were used to make sure that the compressed air flow wasn't above 90 degrees before it enters the PVC line, and a set of foot-values were inline of the buried line in the event of a leak or rupture.  The first several feet of the air line could just be weighted down so it stays below the flow of the water to pre-cool the air flow.  If the hydro site flow was good enough,  The compressed air from this hydro site, and any over-charging from PV panels that need to be dumped to a dump load anyway, could be used for food refrigeration for an entire intentional community.  If the community happens to be near cave country, and can locate an underground cavity (via deep penetrating radar, perhaps?) that can be drilled into with a well drill and then sealed back with a pressure pipe, then the community might just end up with enough pressure storage to run their fridges for days on end.
 
Creighton Samuiels
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After running some real numbers, I have found that the storage volume required to meet the normal 3-days-of-darkness rule commonly expected for completely off grid power systems would be impractically large for most homesteads; as well as cost a great deal of money regardless of the construction method or materials.  Focusing on simply making a 24 hour period following a good full sun day is much more practical, but still of considerable site size and expense.  The (somewhat risky to human health if installed in the main house) backup method of using an industrial high-pressure tank of liquid CO2 would likely work well, but would also likely be used too often to justify the safety risks over just owning a gasoline powered air compressor as a 'backup generator'.  (If the cool room were installed inside an outbuilding, perhaps close to the greenhouse, the risks of using CO2 as a cryogenic fluid might be manageable, but the outbuilding would have to remain pad-locked to keep small children from entering without adult supervision.)  So I have decided that the best backup refrigeration solution is still a spark-ignition engine driven air compressor.

That said, I have thought of a method of significantly reducing the amount of storage required to keep the primary refrigerated space to a regulated temperature overnight.  This is a two step process...

First off, an East-West solar array or a solar tracking mount would be required, in order to flatten out the power curve, to allow the DC air compressor to run continuously for as long as possible, directly from the solar array's output.  The DC air compressor would be sized to provide 10-25% more compressed air volume, at whatever pressure that our vortex tubes can consistently produce sub-freezing output temperatures, across the range of input air temperatures that we would expect during the hottest part of the summer.  Also, there would be an insulated volume ahead of the main regulated refrigerator volume, the purpose of which is to house plastic containers of water, to absorb the sub-freezing BTU's of cold before that air flow reaches the main refrigerator volume, which should then be just above freezing.  This creates a 'cold' energy storage volume, that while it can be used as a normal freezer, is primarily intended to store cold for the overnight cycle.  While the solar array is producing enough power to drive the DC air compressor directly, the solenoid is held open to allow continuous air flow through the vortex tube, freezer volume, main regulated refrigerator volume, into the general airspace of the cool room, and then finally out of the cool room via a flapper vent near the top of the cool room wall.  Ideally, the thermostat in the main refrigerator space would only turn off the solenoid if the air flow into the main refrigerator volume drops below freezing; which would imply that the water/ice containers in the freezer volume have completely frozen, and thus that energy storage method has maxed out.  Better, and probably cheaper overall, would be to have enough water/ice available in the freezer volume that the DC air compressor could not freeze all the water during a single daytime cycle, even of the sunniest of days.  Since the DC compressor is capable of outrunning our vortex tube on a continuous basis, over the course of a sunny daytime cycle our ice containers would be mostly frozen and a great deal of our air storage tank volume would likely be pretty well stuffed as well.

As the sun starts to set (or a sun-scattering storm rolls in) our solar array output would drop below what the DC compressor requires to run, we want to shut off that DC compressor because we don't want it drawing down our DC batteries.  Once this occurs, the thermostat in the regulated refrigerator volume should truly regulate, and switch open the solenoid only when the refrigerator space requires cooling.  This will open the (still pressure regulated) compressed air flow to our vortex tube, and push cold air through the system until the thermostat is happy again.  As this process continues overnight, the air pressure in our storage tanks might drop below our regulator settings.  If this happens, the volume of air flow through the vortex tube will decrease, as well as reduce the ability of the vortex tube to cool the air flow.  Eventually the incoming air pressure will decrease to the point that the vortex tube can no longer keep the airflow temperature below freezing, and the ice containers will begin to surrender their stored cold back into that airflow as it passes onto the main refrigerator airspace.  So long as there is still ice remaining in the freezer space, and a little bit of pressure left in our air storage tanks, and enough DC power to run the thermostat and solenoid; our main regulated refrigerator space can be maintained within the critical range of 33-39 degrees.  However; once the ice all melts, the air tanks reach parity with the atmosphere, or the batteries can't maintain minimum voltage; backup power will be required to bridge the gap till the next sunny day.

This kind of system is also rather expandable.  While the freezer & refrigerator volumes will have to be decided in the design & build stage; the actual amount of water/ice container storage within the freezer volume can be adjusted on the fly; and additional air storage volumes can be added as the will & finances permit.  Expansion of either or both would reduce the need for the backup compressor, and thus further reduce a homestead's economic & ecological footprint.  As a side note, I believe that as peak oil is followed by peak propane, a rather large number of used propane tanks are going to enter the secondary market.  With fewer & fewer propane customers, more and more of these decommissioned propane tanks will become available to be used as compressed air storage tanks.

Additionally, there are many effective ways to compress air using renewable energy.  If one lives in a regularly windy area, such as the middle plains states, a wind driven air compressor can be expected to provide plenty of air pressure overnight to significantly reduce minimum air storage needs, perhaps to even eliminate the need for a backup air compressor completely.  Likewise if a particular homestead has a year-round micro-hydro site on their property; as only a fraction of the compressed air energy is required overnight, as compared to during the daytime.  This is both due to the water/ice energy storage method, as well as due to the natural variation of cooling demands that can be expected from daytime to nighttime, pretty much anywhere between 30 degrees and 45 degrees of latitude.  Close to the equator, this day-night method might not perform well; and close to the poles it might be unnecessary.
 
R Scott
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Lots of the amish have wind driven compressors, hooked up to old propane or anhydrous tanks, often 10,000 gallons or more total.  Enough to run a large shop off grid without fuel.  I don't think any of them use vortex tubes, though...
 
wayne fajkus
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There are small ice machines on the market. I think they make 8 pounds in one hour. They don't keep them frozen though. They simply make it.

This seems like a great option for an offgrid situation. One hour running time, and the ice placed into a cooler chest. If this lasts even 12 hours, then your only using electric for 2 hours a day to keep everything cool.

A very basic solar system should keep up since there's plenty of in between time to recharge battery bank.
 
Creighton Samuiels
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wayne fajkus wrote:There are small ice machines on the market. I think they make 8 pounds in one hour. They don't keep them frozen though. They simply make it.

This seems like a great option for an offgrid situation. One hour running time, and the ice placed into a cooler chest. If this lasts even 12 hours, then your only using electric for 2 hours a day to keep everything cool.

A very basic solar system should keep up since there's plenty of in between time to recharge battery bank.


I've thought about this one too.  And it might very well work better.  Would need a chute to move the ice into the cooler, but 8 pounds per hour of ice is right on a gallon of ice per hour of sunshine.  If you use an East-West array intended to flatten out the peak power curve, the need for energy storage would be greatly reduced, since the energy storage would be in the ice itself.  And of course, ice is portable.  I'd like to know where you found such a small ice machine, so I could determine if I could run it directly from the array.

But I also wanted to mention that I have a test vortex tube, and it does get pretty cold.  But not only that, I've discovered that there are "cabinet coolers" that use this technology, intended to actively cool industrial control cabinets.  No need to reinvent the wheel here, just a judicious reapplication of existing technology.

Still, a solar powered ice maker wouldn't have the energy storage for the overnight cycle kept under pressure, so that would be a plus.
 
wayne fajkus
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I found them. 26 pound, not 8 pound.

www.amazon.com/gp/aw/d/B00INXG9MY/ref=mp_s_a_1_1?ie=UTF8&qid=1492739524&sr=8-1&pi=AC_SX236_SY340_FMwebp_QL65&keywords=counter+ice+maker&dpPl=1&dpID=41GtKx%2BAhFL&ref=plSrch
 
wayne fajkus
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Amazon search. Counter ice maker. I don't know how to link it
 
R Scott
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That is how I plan to make my off grid fridge.  Basically an old school ice box with an ice maker installed on top of the ice container.  It would run only during the day after the batteries are topped off.
 
Amit Enventres
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I got this here cabinet in my house. 5 months it averages 40 degrees. It's on the equator-facing wall. It ain't no deep freeze, but my goal will be to use a combo of the vortex tube and stuff listed here: http://www.resilience.org/stories/2006-11-22/passive-cooling/ to see if I can't get it to fridgy temp for another 4-7 months. Deep freezes are so efficient I don't know if I'll ever try improving on that one. I think ours costs like $20 in electricity/yr and I can't even tell when it's on the motor is so quiet and the exhaust not hot. I wish you luck though. I will let y'all know how the cabinet turns out though.
 
Creighton Samuiels
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wayne fajkus wrote:I found them. 26 pound, not 8 pound.

www.amazon.com/gp/aw/d/B00INXG9MY/ref=mp_s_a_1_1?ie=UTF8&qid=1492739524&sr=8-1&pi=AC_SX236_SY340_FMwebp_QL65&keywords=counter+ice+maker&dpPl=1&dpID=41GtKx%2BAhFL&ref=plSrch


That one is 26 pounds of ice per day, not per hour.  That is just about a pound per hour of runtime.  A good East-West array could expect to maintain the minimum power output to run this device's compressor for 6 hours to 12 hours, but certainly not more than that.  While that is not a bad idea in it's own right, 6 to 12 pounds of ice per day isn't going to replace a full sized refrigerator during summertime.  I wonder if anyone makes a DC icemaker?  Perhaps an under-counter unit would be better suited for such a thing.
 
R Scott
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The icemaker on top of a soda fountain machine can make 400 lbs a day on 700-1000 watts. But they cost $2-3k
 
Joseph Johnson
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Power
115V/60Hz
Ice Making Current
1.5 A
Ice Harvest Current
2.0A

I like this idea for another project, Helping out my evap cooler. Anyone know how many watts/hr the thing uses based on the listed amps?
 
R Scott
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Watts = volts x amps

So, 172.5 watts in ice making, 230 watts in harvest mode.
 
Michael Cox
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Seems to me like a really really well insulated box/room with a standard electric refrigeration unit would be simpler and need less energy to run? These proposals all seem to have lots of potential points of failure.
 
Creighton Samuiels
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Michael Cox wrote:Seems to me like a really really well insulated box/room with a standard electric refrigeration unit would be simpler and need less energy to run? These proposals all seem to have lots of potential points of failure.


Such a box would get incredibly hot and humid.  Refrigerators pump heat from the inside to the outside of their box, and produce a bit of extra heat doing it; so if you cage the refrigerator inside another insulated box, you are just dumping heat into that box and achieving nothing.  A split unit, with the compressor & condenser units on the outside of the bigger box would work fine, and they are available as custom (usually marine) units; but they cost thousands of dollars just for the compressor, condenser & evaporator.
 
m c nestor
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Years ago, when the children were young and we were still in Ohio, we visited an Amish family (friends) that had moved two counties away. They had had a large pond dug for the purpose of harvesting ice in the winter. The ice/spring house was on the edge house-ward. Noah (the father) had managed to extricate the freezer section of an old refrigerator, minus Freon, and had  slipped it into the cinder block wall. He planned on keeping his milk, etc. there when it was cold.
When we moved to Virginia we had three acres and the old spring house was still standing. The original owners sold milk from their cows and would keep it cold by flooding the storage area. It finally caved into itself and only the frogs and copperheads made any use of it.
 
Michael Cox
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Creighton Samuiels wrote:
Michael Cox wrote:Seems to me like a really really well insulated box/room with a standard electric refrigeration unit would be simpler and need less energy to run? These proposals all seem to have lots of potential points of failure.


Such a box would get incredibly hot and humid.  Refrigerators pump heat from the inside to the outside of their box, and produce a bit of extra heat doing it; so if you cage the refrigerator inside another insulated box, you are just dumping heat into that box and achieving nothing.  A split unit, with the compressor & condenser units on the outside of the bigger box would work fine, and they are available as custom (usually marine) units; but they cost thousands of dollars just for the compressor, condenser & evaporator.


That is what I was suggesting. You could try ripping the guts out of chest freezer to make your own room sized unit.
 
B Beeson
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Check out coolbot:

https://www.storeitcold.com/coolbot-homepage-test-1-0/

It controls a cheap window AC unit to cool a room down to 35F. With enough insulation, operating offgrid would only need a few hundred Watts for the smaller AC units.

 
Creighton Samuiels
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Michael Cox wrote:
Creighton Samuiels wrote:
Michael Cox wrote:Seems to me like a really really well insulated box/room with a standard electric refrigeration unit would be simpler and need less energy to run? These proposals all seem to have lots of potential points of failure.


Such a box would get incredibly hot and humid.  Refrigerators pump heat from the inside to the outside of their box, and produce a bit of extra heat doing it; so if you cage the refrigerator inside another insulated box, you are just dumping heat into that box and achieving nothing.  A split unit, with the compressor & condenser units on the outside of the bigger box would work fine, and they are available as custom (usually marine) units; but they cost thousands of dollars just for the compressor, condenser & evaporator.


That is what I was suggesting. You could try ripping the guts out of chest freezer to make your own room sized unit.


I'm afraid that wouldn't work, mostly because the inside & outside skins of the chest freezer are the evaporator and condenser, and there is no practical way to separate them without damaging the unit.  You'd have to start with a split system, which are available, but are usually designed for expensive yachts, so they are more expensive than other solutions.
 
Creighton Samuiels
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B Beeson wrote:Check out coolbot:

https://www.storeitcold.com/coolbot-homepage-test-1-0/

It controls a cheap window AC unit to cool a room down to 35F. With enough insulation, operating offgrid would only need a few hundred Watts for the smaller AC units.



Yes, I am aware of coolbot, and it would work pretty well for making a chilled "root cellar", but there are a couple of minor problems that I foresee that are solved rather smoothly by the vortex tube method.  First, while the coolbot will chill the room rather effectively, it depends upon a very small amount of air exchange to accomplish this goal.  As a result, the air inside the cool room would grow "stale", and there are a few issues that can develop without some degree of regular, & controlled, air exchange; including mold growth, or insect infiltration.  The air vortex tube solves for this by always using fresh, filtered & "dry" air from the air compressor; and by maintaining a small positive air pressure.  Without much possibility of air infiltration with moisture in it, the possibility of mold growth is very limited.  And an insect infiltration can be killed simply by switching to pure co2 for a couple days, driving out the available oxygen, and thus killing everything that depends upon oxygen to live.  The second problem that I can foresee is long term maintenance; as the vortex tube is both simple enough for a reasonably skilled machinist to repair or replicate in the event of damage to the unit, it's only moving part is the working fluid, which is the compressed air itself.  So not only is there no compressor to wear out, we don't care if the refrigerant leaks out, because it's only air.  Granted, this only moves the maintenance problem from a refrigerant compressor to an air compressor, but there are many ways to build an air compressor; and even if TEOTWAWKI were to occur, a DC motor and an air compressor are much easier technologies to replicate in a world made by human hands.  Even if desirable, chemically replicating any version of freon is beyond my art, and beyond anyone that I personally know; while repairing or building an air compressor or a hand wound DC motor is not.  In the end, I guess it is still a matter of preference; and one could both have a coolbot running on AC when available as well as a vortex tube unit there to help.
 
Peter VanDerWal
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I know this is an old thread, but I thought I'd point out that while Vortex tubes are rather simple devices, they are horribly inefficient. Compressing air is also horribly inefficient, so you end up with horribly inefficient squared. 

Just about any idea you can come up with would be better than this, well except maybe a candle in a flower pot.  That makes a lousy heater and even a worse refrigerator.

I would use night sky radiant cooling to cool off a large tank of water.  In some areas this alone would provide enough cooling for much of the year.  When it's not enough I'd use the tank of water as a heat sink for a heat pump and use that to cool down the room, and circulate the water in the tank through your radiant coolers to disperse the heat.
 
Something must be done about this. Let's start by reading this tiny ad:
The $50 and Up Underground House Book by Mike Oehler - digital download
https://permies.com/wiki/23442/digital-market/digital-market/Underground-House-Book-Mike-Oehler
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