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Running an absorption refrigerator using evacuated tube and parabolic trough

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I've looked high and low, and I can't seem to find any examples of this on a small scale, home use type of setup. Industrial applications of solar powered absorption cooling have been around for years. Does anyone have any experience, or know of anyone who has tried to scale it down? My very general idea would be to place an evacuated tube in a parabolic trough, with the collector end heating the area that would normally be heated by the propane flame. Then at night, switch to a bio-gas flame if it is a refrigerator, or just let it sit with  nothing happening if it is a freezer and gets cold enough during the day to maintain freezing temps over night. I'm not an engineer, just a hobbyist, so be gentle. Lol
Posts: 376
Location: SW Missouri
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Type solar ammonia ice maker into Google and you will get a wealth of information your looking for. Including fit projects if you can source the anhydrous.  Does not require an evacuated tube. Good luck
Posts: 6
Location: Portland, OR
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Non-electric forms of refrigeration are actually a great interest of mine, as well as refrigeration generally.  I really can't say for sure if you're idea will work, but I could take a stab at it!  

First thing to understand, is that an RV absorption refrigerator (Diffusion Absorption) calls for heat whenever the refrigerators thermostat calls for it.  A propane flame is ignited which heats the generator (boiler) portion, and also heats the bubble pump (if it is separately plumbed).  The size of the burner was specifically engineered for the refrigerator, such that the generator reaches an appropriate temperature.  Not so hot or the water in the generator will boil excessively and waste heat in the rectifier and condenser, and the bubble pump may cease to run correctly, but not so cool or flow rates internally will slow, and the bubble pump again cease due to inactivity.  Getting near this temperature would make your project more of a success.  There's definitely wiggle room, so don't worry!

One way to determine this, might be to enable the electric heating element portion it probably has, in in the case of small hotel refrigerators, the only heat source it has.  I would monitor both the power consumption (it will be high!), and the generator temperature.  This might be best done by strategically placing thermocouples on the generator.  You might have to remove the gas flame flue to do this, but make sure you reinstall for accurate readings.  I'm guessing they run at somewhere between 100 to 150 Celsius.  A bit of research online will also help here.

I think temperatures like this should be possible with an evacuated tube, especially in a parabolic trough.  Something else to consider, is the need to keep these refrigerators well leveled.  A slight angle can cause serious problems and early failure.  You would have to find a way to circulate the heat between the heat collector and the generator.  I have a suggestion.

Unless you plan on rotating the parabolic trough around the stationary evacuate tube, you can forget about adjusting for the best angle.  This thing is arranged for best overall sun exposure and fixed in position.  The evacuated tube is presumably angled to face south (if you're in the northern hemisphere), and the generator is elevated above, perhaps behind a wall to protect the fridge from direct sun exposure.  Heat would be carried from the tube by a simple two-phase thermosiphon, probably a copper pipe with water sealed inside under a vacuum.  Water carries heat extremely well, so it is probably the best refrigerant for the application.  It will be in a positive pressure when heated up, but the pressure shouldn't be getting too terribly high (take precautions and understand this nonetheless).  The issue now is to get the heat to the generator effectively.  A simple straight pipe is not best here.  I would instead, remove the flue and carefully wrap copper tubing as many times and as tightly to the generator as possible.  You want good thermal contact.  This will be difficult, so wrapping the coil with a thin piece of sheet metal and tightening it around the coil with tubing clamps or wire might be better.  Some kind of thermal paste or putty mashed into the coils could be a plus.  The top of the coil is soldered or brazed to the top end of the copper pipe in the evacuated tube, and the bottom of the coil runs to the bottom of the tube (or just enters the top of the pipe and extends to near the bottom).  Wrap the whole assembly in high temp fiberglass insulation.

So when the pipe heats up the few ounces of water (you'd have to figure out an appropriate amount), it vaporizes and travels to the top of the coil where it dissipates the heat and drives the refrigerator.  The vapor condenses and falls back to the bottom of the pipe to pick up more heat.  I'm guessing one evacuated tube won't be nearly enough, so you could plumb several together in parallel.  One of the limiting factors with this design is getting enough surface area in the generator coil (the condenser) to effectively heat it.  Nonetheless, I think it could work!  Maybe make an inside and outside condenser coil.

Sorry for the long post; it was fun to write though!  I have many more suggestions if you want to go down this road.  If this device worked, the collectors could even be placed on the roof, with lines running down to the kitchen below by using a two-phase geyser pump.  The fridge will over-cool on very sunny days, so provisions would need to be made for that.

Good luck.  Build it!  
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Location: Greybull WY north central WY zone 4 bordering on 3
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Suggest looking at a different but similar process.  Here is one I tripped over here on permies that shows real promise.  It beats most of the problems inherent in active solar thermal collectors and does refrigeration as a side affect.   The problem with the normal liquid filled active solar collector panel is moving pieces and need for outside power. There are a few systems that will run without but they are sensitive and fickle. Otherwise it only works if the system has the condition of being able to have the collector as the lowest point with the storage tank above it and the place of heat use higher still it will function as a thermo-siphon system without a pump. But that condition is rare. Usually the desired location for the collector panel is above everything else and there is no system to reliably and durably push heat down with no moving parts and no outside power source.

Batch box ice maker

It is intended as a batch box ice maker for locations with no power. It has no moving parts and it should definitely drive the heat downward. It can't over heat. It also gives the possibility of giving not only hot water but free refrigeration. Now it has some problems. The 3 biggest are 1. it is a batch box design so it has a limited run time and it only does 10lbs of ice worth of cooling per cycle. Because of heat transfer characteristics and efficiency it should produce way more heat than cold but that is also limited by the batch nature of the design. 2. The ammonia refrigerant used in it is toxic which is why it is not commonly used in home situations.  And it involves a fairly large volume of refrigerant.  3. The batch box needs to be open to radiate heat between batches.

Now here is what I am thinking as modifications.

1. To solve a major chunk of the toxicity danger if the ammonia pipe were enclosed in a water pipe or tank any time the line was inside the house the danger could be greatly reduced. Small leaks the water would chemically absorb greatly reducing the risk and simple electronic leak detection would provide warning. In the event of a big leak if the water was there to absorb a lot of it and then the tank was vented to the outside the risk could be reduced but not eliminated.

2. To solve the batch cycle problem several steps would be needed.
A. Add a reflective shutter between 2 heat absorption pipes. By hinging it up or down it would be possible to reflect the sun at either pipe as it came from the stationary reflector. Still a moving part but at least a really simple moving part and outside the sealed system not inside.

B. Each pipe system would need 2 check valves added, a return line through a refrigeration evaporator and an expansion device. The refrigeration outlet line from the refrigeration tank would need a dip tube going right to the bottom of the liquid refrigerant tank so instead of chilling the cold box it chilled whatever followed the expansion device.

3.  The door needs to be open.  
C. Because of the batch box nature of the cold chamber in this design needs to radiate heat for a bit from its open door before making ice as cold storage. To deal with this ideally the box should have high thermal mass and the ability to store cool/cold. So lets make it a small cistern that is designed to freeze solid at times. The water would help provide protection from a leak and provided the final bit of cooling for the refrigerant tanks.

So the system would push the heat into a heat storage tank built like the one in this video starting at 1 hour and 10 minutes and running through 1 hour and 29 minutes. Work with the stratification. If you are interested in home construction techniques or passive solar watch the rest of the video as this is a different way of looking them.

Home design/ hot water storage system.

Then the refrigerant would go to the cold tank. At the inlet to the refrigerant tank would be the first check valve. That tank would radiate the heat to the water in the cold cistern around it. Now the question is how do you cool the cool cistern? Lets add 2 other technologies to this. Lets add a heat pipe to carry the heat up. A heat pipe is a separate sealed system that moves large quantities heat with no moving parts. In this case because it would be installed vertically with the heat on the bottom it could be simply constructed without an internal wick and has the advantage of acting like a check valve against heat movement back into the system. Then tie this into a radiant cooler aimed directly at the sky built like a cone shaped solar cooker. This would be located on the north side of the building to avoid getting sunlight into the cone. By aiming the cone at the night sky it is possible for it to cool 20 degrees below ambient on clear sky nights. It it was built as a thermal chimney too it would at least act as a thermal chimney and should cool to at least ambient on cloudy or smokey nights. See the following link for this information. About in the middle of the article it discusses using the solar cooker for cooling at night.

solar colletor / radiant refrigerator

For the refrigerant to get back to the heating element pipe when regenerating a dip tube to pull the liquid refrigerant from the very bottom of the tank to the next expansion device and then to the evaporator to pull the heat from the fridge freezer.  Then past the second check valve and back to the big solar hot pipe.

For the fridge/freezer if you built it as a walk in fridge with a second room inside that, that cycled freezing or fridge and used that space to store wheat, rice, flour, pasta etc that has possible bug problems it would serve to isolate the freezer more and add thermal mass. Then inside the 2nd room put the real freezer. I know from running a camping cooler inside a camping cooler that the inner cooler doesn't hardly begin to warm up till all the ice is almost gone in the outer. By layering this the inner cold should be able to be stretched over longer periods.

A back up electrically driven system might still be needed for the heat of the summer but most of the year it should cool I would think.

Okay where is my thinking going wrong? Am I missing something in the science?
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