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Intermittent Adsorption Refrigerator  RSS feed

 
Marcos Buenijo
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It's possible to build fairly simple intermittent adsorption refrigerator similar in principle to the intermittent absorption ice maker described here: http://knowledgeableideas.blogspot.com/2013/05/a-solar-ammonia-absorption-icemaker.html . Note that the adsorption chiller uses water as the refrigerant, and the system operates at a high vacuum. Also, there is a solid desiccant (zeolite or silica gel) that takes in the refrigerant vapor to drive the cooling process.

I have experimented with this process. I was able to rapidly boil a small quantity of water down to 30F (I believe I accidentally added some salt to the water which lowered its freezing point). All I did was expose water to a dry desiccant within a vacuum chamber. After I evacuated the chamber and shut off the vacuum pump, then the water continued to boil as the desiccant absorbed the water vapor in the chamber to reduce pressure. I was also able to verify that generating a high vacuum is not difficult when the vessels are relatively small as the forces are smaller. Also, I found that steam should be used to assist in evacuating air from the system. Since steam is less dense than air, then the vacuum pump should take suction at a low point in the system. The procedure I used is to provide hot water to the system, then when the vacuum pump is applied the hot water will flash to steam as the pressure drops. The steam will help displace air from the system while the vacuum pump is applied.

The benefit of this set up is primarily that zero electricity is required. In fact, there is no energy input required other than heat. Also, the efficiency of these systems can be quite good. Personally, I recommend using a small batch loaded biomass furnace to regenerate these systems as a surprisingly small amount of fuel can be used. In the adsorption chiller, the combination of (1) the very high heat of vaporization of water, and (2) the higher temperatures of a refrigerator (vs a freezer), will allow for a small amount of water refrigerant to work well. An evaporator with a total volume of only one gallon will provide a cooling of about 8000 btu, and this is similar to the cooling provided by a standard modern refrigerator (less freezer) over a 24 hour period. Super insulation will go a long way to reducing the cooling further, and in fact it should easily cut the water (and energy) required in half. The procedure is to heat the desiccant with a small batch TLUD furnace. The water vapor driven off the desiccant will condense in the copper coil condenser provided, then the water will drip down into the evaporator inside the refrigerator. Once the furnace runs out of fuel, then the system cools down and starts re-adsorbing the water vapor from the system thereby chilling the water in the evaporator and cooling the refrigerator in the process. Place the evaporator within a mass of water that can be partially frozen during operation (*), and this would provide an excellent thermal mass.

In principle, it is possible to regenerate the system by placing the desiccant vessel inside a large solar oven, and this might be practical where solar insolation is very high and regular. I suggest adding a system much like an automatic green house window opener that will force the system shut as temperature rises during the day, then open up automatically at night to let the system vent for cooling.

I suggest zeolite as the best adsorbent. One needs about 4 times as much zeolite as water. Since zeolite is 7 pounds per gallon, then one volume of water would require roughly five volumes of zeolite.

* - The freezing point of the water can be lowered by adding a salt or other substances. This can allow the evaporator temperature to dip below freezing for better cooling results, and this may be necessary to prevent freezing which might block water vapor exchange. It is crucial for good results that all air is evacuated from the system. Again, use steam combined with a good vacuum pump (I actually used a crappy vacuum pump which is a testament to the value of using steam to displace the air out of the system). It's also possible to displace the air only with steam, but the entire system needs to be heated to high temperature to do this. This might be done by connecting the evaporator to the desiccant vessel with a high temperature tube, then containing the entire apparatus inside a solar oven. Let the steam vent at a low point in the system through a small opened vent. Cap the vent quickly while it is venting, then allow the system to cool. A vacuum will form. The evaporator can then be placed inside the refrigerator as the tubing will allow for moving this component easily relative to the desiccant vessel. Note that most small flex tubing can handle a high vacuum well.
 
R Scott
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I gotta see this!!! No, really. Do you have pictures or video?

They also mention having multiple units staged so one could be cooling while the other is regenerating. It wouldn't be hard to make two and alternate which one gets fired.

I wonder if you can scale this to the point you can use it as AC in a small cabin? It is easy to stay cool in the desert with a swamp cooler, but swamp coolers don't work IN the swamp. I see this as a potential solution for hot humid off-grid!!!
 
Marcos Buenijo
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R Scott wrote:I gotta see this!!! No, really. Do you have pictures or video?

They also mention having multiple units staged so one could be cooling while the other is regenerating. It wouldn't be hard to make two and alternate which one gets fired.

I wonder if you can scale this to the point you can use it as AC in a small cabin? It is easy to stay cool in the desert with a swamp cooler, but swamp coolers don't work IN the swamp. I see this as a potential solution for hot humid off-grid!!!


Sorry to disappoint, but there are no pics or video as I did only crude testing. Still, the fact that my digital thermometer showed -2 degrees Celsius for the water demonstrates that I was able to evacuate virtually all the air from the system with a poor vacuum pump by using steam displacement (I found this more interesting than anything else). Also, the water hit that temperature fairly quickly (of course, there was no heat entering the water at the time - a real world system would see some heat input that would make it difficult to reach such low temperatures - then again, this only means the system would be working properly to cool things down). The most valuable lesson I learned during this project is that IT'S EXPENSIVE to do this kind of testing. Seriously, you get nickle and dimed to death! I have no doubt WHATEVER that I can devise an air conditioning system powered by a small biomass furnace, and with heat recovery for water heating and what not. However, I can't afford its development costs. That's ok, as I learned something with what little I performed. The refrigerator system is certainly viable, especially when excellent insulation is used.

If you are referring to the Sortech (sortech.de) units when you write "They", then know that they are using silica gel or zeolite. They have found a way to crystallize zeolite directly onto heat exchanger surfaces. This makes for excellent heat transfer rates. Yes, they switch back and forth between two independent adsorbent systems (one is being heated while the other takes over the task of adsorption), and they're making chilled water under 40F in the process. Yes, this can be used for a/c, and they are using it for a/c (they have 5 ton systems that can be ganged together). However, for best results it requires a high temperature heat source. Otherwise, there is a lot of electricity required to cool the condenser to a sufficiently low temperature (cooling water pumps and fans). A furnace would solve that problem. Sortech has devised systems powered by solar heated water. Unfortunately, the relatively low temperature there makes it difficult to see good performance without a lot more electricity consumption. If the heat source is at a sufficient temperature, then things get a lot simpler (and the condenser heat can be harvested for water heating or even biomass fuel drying). Description of basic process: http://www.solair-project.eu/142.0.html

I was restricting my consideration primarily to an intermittent adsorption (i.e. heat powered) refrigerator. It is also possible to provide a/c with this process. I am interested primarily in systems simple enough to be fabricated and maintained by individuals without specialized tools and training. I believe this technology is a candidate. However, development is necessary to arrive at a viable configuration, and this takes time and money.
 
Marcos Buenijo
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Some might find my first post hard to follow. Basically, I'm making the assertion that a refrigerator can be had using the same general configuration as the ammonia absorption ice maker described in the link I provided. Rather than using ammonia and calcium chloride as the refrigerant and absorbent, I am suggesting the use of water and zeolite. This system would operate under a high vacuum. Silica gel might also be used, but I think zeolite will provide superior results as it has a higher affinity for water vapor and has a surprisingly high bulk density. If one desires to construct a solar powered model, then silica gel might be preferable as it is a little easier to regenerate (i.e. slightly lower temperatures).

The testing I did involved the use of vacuum equipment to verify for myself that the process works. The results of my testing was favorable (yes, it works quite well). However, I did nothing beyond basic testing. Perhaps the most important thing I found is that steam can be used to leverage a vacuum pump when removing air from a system.

I'll go ahead and share another thing I found during my testing. I verified that small magnetic drive fluid pumps work very well under a high vacuum. These pumps use a hermetic system where the impeller of the pump is driven by magnetic force (i.e. no drive shaft). These pumps are highly efficient due to removing the restriction otherwise presented by a seal on the drive shaft. These pumps are made by the millions in China for solar water heating purposes, many are DC models, they are inexpensive, and they are highly reliable. These are the best option available for pumping water at low pressures and high flow rates. Note that I considered using one of these pumps as a chilled water pump that distributes the refrigerant directly to fan coil units as opposed to cooling the chilled water indirectly with a heat exchanger. This approach would take suction from the bottom of the refrigerant vessel (provide a few feet of heat for the pump suction), then send the water through the system to return to the vessel through a check valve (keep a few psi positive pressure in the system). This approach eliminates a heat exchanger while also providing colder chilled water than otherwise.
 
Marcos Buenijo
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The performance of these adsorption chillers are pretty good. They show a thermal Coefficient of Performance (COP) of 0.6 - 0.8 which means that the cooling achieved is 60-80% of the heat applied to the system. One of the limitations of performance is a consequence of the intermittent operation of the system. After the adsorbent is heated for regeneration, then it must be cooled to drive adsorption. Well, the heat stored in the adsorbent dumped during this process is not normally put to use. In principle it's possible to regenerate some of this heat back into the system. However, a simple system shouldn't bother with this.

I have described in other posts (see thread entitled "Desiccants for Thermal Storage") an open desiccant cooling system based on zeolite. I started considering this approach after experiencing first hand some of the problems in working with fairly large vacuum systems. Also, the prospect of devising a control system for a continuous system is rather daunting. If someone can come up with a practical and cost effective intermittent system, then this seems the most practical way to make use of this technology at the micro scale. Sure, we can wait for a commercial system, but I wouldn't hold your breath. Also, while fascinating, a more practical system can be had using photovoltaics to power air conditioning as an opportunity load. Even a freezer or refrigerator could be powered as an opportunity load with a phase change material used as a thermal mass to carry the system through to the next cycle (water bottles in the refrigerator, and salt water bottles in the freezer). Still, I like the potential for this technology to provide a low tech solution for food preservation (the closed systems), and HVAC for small modest homes (the open systems).

OPEN SYSTEM: I'll describe an open system for the reader. These can be used for both space heating and space cooling. The benefit is the lack of pressure vessels. Everything is open to atmosphere.
(1) Space Cooling - A desiccant such as zeolite is used to dry air. This is done by bringing in hot humid outside air into the home through a desiccant bed. The air increases in temperature measurably during this process due to the heat of adsorption. The hot dry air is cooled with a heat exchanger using cool air vented from the home. The now cool dry air can be used effectively for evaporative cooling (i.e. a swamp cooler). The desiccant bed has to be regenerated periodically (such as daily). I had speculated in other posts that this can be done by sending the hot exhaust gases from a furnace (mixed with additional air to get the temperature down) through the desiccant bed. I had also suggested the possiblity of operating a wood gas engine system daily for battery charging, then directing the engine exhaust and cylinder cooling fan exhaust through the desiccant bed for regeneration. Note that the steam driven off the desiccant bed during heating can be put to use in heating applications such as water heating. Another possibility is using solar heated air. Silica gel has been shown to be regenerated efficiently with solar heated air at 160-180F).
(2) Space Heating - If one has set up a system for space cooling, then it's possible to reconfigure the system for space heating. Cold air from outside can be sent into the home first through the heat exchanger for preheating. The warm dry air then passes through an evaporative cooler (i.e. a wet pad), then the cool humid air can enter the home through the desiccant bed to pick up heat of adsorption. A side benefit is the ability to increase the humidity of the air during dry winters.

NOTE: After regeneration, the desiccant bed must be flushed for several minutes with clean air to remove noxious gases and to cool the desiccant.

In theory, the cooling (or heating) capacity of these systems is roughly equal to the heat supplied during regeneration. However, since this process is not 100% efficient, then the process falls short. A good system should provide roughly 2/3 of this capacity. So, if one desires to provide a constant cooling rate of 1/2 ton (6000 btu/hour), then roughly 215,000 btu of heat is required (roughly 30 pounds of dry wood). Interestingly, a small wood gas engine system operated daily would provide this heat with about 40 pounds of dry wood (while providing about 7 KWh of electricity after all losses are considered - alternator, battery, inverter). This seems highly efficient to me. A modest off grid home can do well on 7 KWh of electricity even with 2-3 KWh dedicated to the small blower fan(s) required to support this HVAC system. Using efficient fans powered by small permanent magnet DC motors will go a long way.
 
Marcos Buenijo
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Note that existing silica gel and zeolite adsorption chillers use conventional finned heat exchangers that are packed with small beads of the desiccant. A fine wire mesh is used to contain the desiccant material. Sortech has used epoxy with high thermal conductivity to adhere the desiccant to heat exchanger surfaces for better heat transfer. Currently, they are crystallizing zeolite directly onto heat exchanger surfaces.

Sortech was able to make inexpensive large vacuum vessels by using the heat exchangers inside as part of the internal support structure. This allowed them to make fairly large vacuum vessels with thin sheet metal. I speculate that a packed bed of zeolite might provide some internal support for a vacuum vessel, but this is pure speculation. One should experiment along these lines to see if some forms of bulk zeolite can handle the forces. Perhaps this can allow a vessel otherwise not suitable for a high vacuum application to be used? Perhaps this can allow an intermittent system to provide an a/c application for a very modest space? A problem in regenerating such a large vessel might be seen. Perhaps a number of smaller vessels can be used to increase heat transfer surface area. Cooling can also present a problem. A structure similar to a fire tube boiler might be used here where the bed is penetrated by many tubes to increase heat transfer and distribute heat more evenly to the desiccant within. These tubes can also provide an internal support structure to help resist the forces caused by the vacuum. Perhaps a wire mesh matrix that is connected to the tubes can be used to further distribute heat to the zeolite? This kind of system might make an intermittent air conditioning application possible, but wow one would need a large vessel! Let's assume 6000 btu/hour of cooling over a 24 hour period (144,000 btu). This seems suitable for a modest and well insulated off grid home using passive cooling techniques. We need to evaporate 144 pounds of water to achieve this cooling effect, or about 17 gallons. Therefore, the system would require on the order of 85 gallons of zeolite, or roughly 11.3 cubic feet. Maybe this isn't so impractical a prospect after all? Note that a packed bed is not good for seeing high cooling rates, but I believe a large packed bed will allow for a low rate that would be ideal for a modest off grid home. The main problem is building the thing. This approach to air conditioning would require the least electricity than any other system I've considered, so it's an interesting prospect.

R Scott, what do you think?

NOTE: For space heating the zeolite vessel requires a supply of water vapor. Air in the home might be passed through the vessel tubes for heating. Since the system is closed, then an evaporator containing the water must be connected to the system and exposed to temperatures sufficient for evaporation. So, this would not work well for heating in a very cold climate. The water should be exposed to temps greater than 40F. Interestingly, this system is a heat pump, and can be configured to provide more heat than supplied by the furnace. The steam driven off the desiccant during regeneration can be used in water heating to provide heat. Then, during adsorption, the system can take heat from outside air (or ground) to evaporate the water and supply the desiccant with water vapor for more heating. This has been demonstrated in a pilot system in Germany for a COP of 1.35.

NOTE: The size of the system I suggested earlier is probably rather large for a truly modest and functional off grid living space. A super insulated cabin with passive cooling strategies should get by with a lot less. In fact, it is possible to provide several evaporator vessels. Those not in use can be insulated to slow evaporation and thereby use the system for spot cooling elsewhere. For example, while sleeping an evaporator might be placed near the bed and a small fan can blow cool air. I'm only brain storming here, but imagine cylindrical vessels as evaporators that are surrounded by an insulated shroud. Small fans might be used to force air across the cylinders for cooling (between the insulated shroud and cold evaporator vessel) and ducting can be used to direct the cold air flow for personal spot cooling. Also, consider a single long pipe surrounded with an insulated shroud. It might be possible to place tees on the length and tap it with small fans for personal spot cooling along its length. Also, a system might use both an open and a closed desiccant cooling system where the open system is used for air drying, and the closed system for personal spot cooling and food refrigeration. This would allow the size of each system to be a lot more manageable. For example, an open desiccant bed might be suspended above a vacuum vessel. A furnace could heat the lower vacuum vessel to a high temperature for good regeneration, then the still hot gases will be directed through the open desiccant bed. The steam driven off the vacuum vessel is directed through a copper condenser coil contained in an insulated water storage tank (i.e. water heater), and the condensate drains by gravity back into the evaporators. A modest blower fan could send air through the system at a low rate for air drying and also to cool the lower vacuum vessel to remove the heat of adsorption. The best part about this prospect is really getting electricity requirements as low as possible. Go with an intermittent ammonia absorption freezer as well to really get electricity use down.
 
Marcos Buenijo
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Just a quick disclaimer: I'm doing brainstorming here which entails speculation. Basically, I'm bored. I don't consider the prospect of an intermittent adsorption air conditioning system to be practical. The vaccum vessels required would be too large. I do, however, consider the prospect of an open system (described previously) to be promising. Also, intermittent adsorption and absorption systems for food refrigeration is a viable prospect where extreme insulation is used.

 
Marcos Buenijo
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Just making a post on a possible means to provide extreme insulation for a freezer or refrigerator (i.e. more speculation). I am aware of dry perlite under vacuum used to insulate cryogenic tanks. I have wondered about this prospect for insulating a freezer or refrigerator. In particular, I have wondered if a vessel might be surrounded with the dry perlite, contained with a durable plastic seal, and then draw a high vacuum on the plastic. Perhaps one can make their own vacuum panels with this process? The heat transfer rates through this kind of insulation is listed as 20 times lower than polyurethane foam.
 
Dale Hodgins
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Marcos Buenijo wrote: The heat transfer rates through this kind of insulation is listed as 20 times lower than polyurethane foam.


This seems awfully high ! Is this an R value rating ? This would indicate R 120 per inch. I wonder if it reflects just radiant or just conductive or some other measure. If an inch of this equals 20 inches of polyurethane, I want it in the attic.
 
Marcos Buenijo
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Dale Hodgins wrote:This seems awfully high ! Is this an R value rating ? This would indicate R 120 per inch. I wonder if it reflects just radiant or just conductive or some other measure. If an inch of this equals 20 inches of polyurethane, I want it in the attic.


I wish I had more information, but I don't. I understand R value to be the inverse of thermal conductivity. So, a high R value (good insulation) indicates a low thermal conductivity. This insulation is used in cryogenic tanks, so it clearly is effective. I was only musing on the possibility of forming insulating panels with this material. If someone were to figure a way to build such panels cheaply, then using it for insulation in a home would be great. Perhaps there is potential as perlite is not expensive.
 
Abe Connally
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why would perlite in a vacuum be more effective than just a vacuum?

dry perlite has excellent insulation value, but more on the lines of R2-3 per inch.
 
Marcos Buenijo
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Abe Connally wrote:why would perlite in a vacuum be more effective than just a vacuum?

dry perlite has excellent insulation value, but more on the lines of R2-3 per inch.


I don't expect perlite under vacuum is better than a vacuum. I suspect perlite is used as a filler to help resist the forces. Otherwise, perhaps the costs involved would be higher. Of course, I am speculating here.

NOTE: There is a good, brief discussion on vacuum insulated panels on wikipedia: http://en.wikipedia.org/wiki/Vacuum_insulated_panel . The discussion there notes the performance of vacuum panels to be on the order of only 5 times that of conventional insulation (per unit thickness). It is also noted that the price of conventional materials is much lower. So, while an interesting prospect, perhaps it is best to just beef up insulation using conventional materials rather than pursue vacuum insulation in the micro setting.
 
Abe Connally
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Marcos Buenijo wrote:I don't expect perlite under vacuum is better than a vacuum. I suspect perlite is used as a filler to help resist the forces. Otherwise, perhaps the costs involved would be higher. Of course, I am speculating here.


oh, I see. Yeah, that makes sense. Use something like perlite as an internal structure. That should be pretty easy to make. Just take a plastic bag, fill with perlite, and suck the air out.

I was thinking a lot about this concept last night. Some evacuated tubes for the solar collector and a well insulated chest freezer would make a good test system. Do you have any links to data for what is needed in terms of temperatures to get down to -10C with zeolite/water?

As part of a wood gas CHP system, it might be really good, too.
 
Marcos Buenijo
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Abe Connally wrote:oh, I see. Yeah, that makes sense. Use something like perlite as an internal structure. That should be pretty easy to make. Just take a plastic bag, fill with perlite, and suck the air out.

I was thinking a lot about this concept last night. Some evacuated tubes for the solar collector and a well insulated chest freezer would make a good test system. Do you have any links to data for what is needed in terms of temperatures to get down to -10C with zeolite/water?

As part of a wood gas CHP system, it might be really good, too.


Good thinking. Perhaps small bags of dry perlite might be vacuum sealed into modules that can serve as insulation. Now that might be worth testing. I was considering placing a plastic vessel inside a larger one, filling the annular space with perlite, sealing the top of the annular space, then drawing vacuum. It would be easy to measure the insulation value with testing. I have the tools to do this, so I suppose I'll have to check it out with a small system to see if it's worth the trouble. A large vessel like this could be practical for long term bulk frozen food storage. An ammonia cylinder could be placed inside as the evaporator.

I considered the zeolite/water system only for a refrigerator system to maintain internal temps in the mid to high 30's F. Go with ammonia refrigerant for lower temperatures (i.e. a freezer). I don't expect it to be practical to try and get temperatures below freezing with zeolite/water. Note that I achieved it only because the small quantity of water I used in the test (literally only half a cup) (1) has some lithium bromide mixed in with it, and (2) it was not exposed to any heat source. Zeolite will adsorb water vapor very slowly as the temperature of the water approaches freezing simply because the density of the water vapor at these low temperatures is extremely low.
 
Marcos Buenijo
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I deleted this comment. I will be using it to start another thread entitled "Continuous Ammonia Absorption Air Conditioner".
 
Marcos Buenijo
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Just a comment on this technology used to make an air conditioning system. Water is clearly preferable to ammonia as a refrigerant for many reasons. The problem is working with vacuum vessels and in efficiently heating and cooling the adsorbent. The best option for the latter problem in my opinion is to pack finned heat exchangers with the adsorbent beads. Heat by sending steam through the copper tubing, and cool by sending cooling water. The steam can be from a steam generator coil placed in a small furnace and can use thermosiphon easily (*). Connect the steam with short sections of high temp flex hose on barbed fittings as these will pop off on excess pressure (a relief valve). You want steam pressure to rise during heating as this will allow good regeneration, and the rising temperature is used as a control for the system (lets it know when it's time to switch). The cooling water requires a high flow rate pump (see the mag drive pumps), and these would be a good fit for a chilled water pump as well. A single fan coil unit seems a good choice for a small system.

(*) The principle here is that the heat exchanger is above the steam generator. The steam generator coil is partially filled with water, then connected to the heat exchanger. When heated, the heat exchanger will fill with steam and the steam will condense in the copper heat exchanger tubing, then the water will drip back down to the steam generator to be reheated. As the adsorbent becomes progressively more dry during regeneration, then heat transfer rates will fall. This will cause the steam to condense at a lower rate, and the steam pressure will rise. This causes the steam temperature to rise which increases heat transfer rates back up to a new equilibrium. At a certain point corresponding to a certain steam temperature/pressure, the system will be sufficiently regenerated to shift over. A thermostat can be used here.
 
Marcos Buenijo
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The idea on dry perlite under vacuum used for insulation should be tested. I will try some simple testing and share with the forum, but I don't know when I can get to it. What I intend to do is find a couple of cylindrical containers of suitable size where one is slightly larger than the other, fill the annular space with dry perlite, seal the top of the annular space, then put my vacuum pump on it. I will place a quantity of ice within and see how long it takes to thaw (that is, to exceed 32F). This can be used to determine the heat transfer rate across the insulation. If that shows good results, then I will vacuum seal dry perlite in freezer bags and use them to insulate a larger vessel.

I'm thinking that with superior insulation, then a small intermittent absorption/adsorption freezer or refrigerator can be practical. Consider that a good 10 c.f. chest freezer consumes about 1 KWh of electricity each day (3412 btu of energy). With a COP of about 1.5, this equates to roughly 5000 btu of cooling. If the insulation of vacuum panels such as the vacuum perlite is 5 times superior as I've seen listed, then this suggests 1000 btu per day. Assuming a COP of 0.50 for the ammonia absorption, then this suggests 2000 btu of fuel would be required per day to regenerate (or roughly 1/4 pound of bone dry wood - a TLUD the size of a large soup can and packed with dry sticks could be used).

There are really all kinds of possibilities assuming the insulation value can be sufficiently high. For example, a cabin with walls packed with bags of vacuum sealed perlite is an interesting prospect. Under these conditions a small absorption air conditioning system might be practical. Then again, it also could make it practical to provide a/c with photovoltaics.
 
Abe Connally
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perlite is often sold as a soil amendment (nurseries) or building component for concrete (check hardware stores)

I often use it for insulation in small projects, and it works really well: http://velacreations.com/food/animals/bees/104-solar-wax-melter.html
 
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I guess more than one of us has the same idea... or close enough anyway. I started talking about the same thing in another thread. I obviously should have started that here. I think I may ask to have some of those message moved here. You seem to have more knowledge about this stuff than I do and so I hope you don't mind me asking some (lots of) questions.

First some background thought. When I first started thinking about off grid and what power needs I would have. It came down to... a freezer. Lights would be nice and so would entertainment, but a freezer would be a must. My thought was a freezer with two thermostats, one for when the sun was out, set really low and one for battery use set to -10C. The inverter would come after that so that it would not draw anything when the freezer wasn't running. Someone suggested using a fire to run an absorption chiller, but I felt that I wanted to be able to go away for a week without having to worry about it. I was reminded of this fridge, and wondered if the concept couldn't be extended to make a freezer. They use 300 gallons of water that they freeze in the winter that supplies all the fridge needs for the whole year. That would not work for me even for fridge needs let alone freezer needs because I live on the west coast. But an ammonia chiller, fired by solar or wood could get water cold enough to keep a freezer frozen for some time. A cooler/fridge could be run from that. My thought is that a cooler only needs to be able to run a few days without someone being there as things start to go bad after that, but if we have a freezer anyway that lasts longer that doesn't matter.

So, salt water can have a freezing point as low as -21.1C. I don't know if the energy to melt with salt is as high as fresh water or if another anti-freeze would work better. Using a phase change storage mass just makes sense.

The icyball stuff just looks and sounds too dangerous for anything that may involve someone other than me at some point (maybe me too). Also, I don't know if one charge would be enough to freeze my ice block and even if it could, the heat transfer would not be that great unless the working ball was immersed... which means it would have to then stay there till thaw out. But I am thinking ammonia as a working fluid in continuous unit. Then the working fluid could go through the ice block in a tube... would the action of making ice possibly break that tubing either by crushing or by shearing? The fourmileisland rig above seems to get away with it.

Would I be able to (easily) scavenge parts from an RV fridge? How hard is it to remove the working fluid? How much could I extend the tubing to the chill coils? I am guessing that everything would have to remain "level" with the original way it was set up and that the chill coils would have to have lines that still allowed gravity to work on the fluids.

Now the heating end. How much does the temperature of the heater affect the chiller temperature. My thought was that less heat would still want to get as cold, but not as fast. That so long as I got the water/ammonia warm enough to boil the ammonia off the unit would work.

Is there a maximum temperature before things go bad/blow up/whatever? My thought would be to use a wood fire for as long as it took to get to freezing the mass, then use solar to help keep it there. A temperature sensor could tell me if things were getting too warm (max temp is -10C, so the alarm would have to be set lower than that, I would think just above thaw). I could then use fire to recharge, but that recharge should be good for at least a few weeks, a month preferably. Would it hurt the unit to keep heating it once the mass was as low as the chiller could get anyway? I am thinking both from a wood fire and a solar POV.

My thought is that in place of turning things off and on to keep a steady temperature, using mass would be better. I am thinking an air tube from this super freezer could be used to also keep a fridge/cooler at about 4C.

Does any of this make sense?

 
Marcos Buenijo
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Hi Len. I do a lot of brainstorming on this forum. Some of my ideas could be practical in some settings, but most are not. I don't have a problem with this as it can be educational to think without such constraints. However, I will emphasize practicality at times, and I think this is important. With respect to a freezer (which I also consider a must in an off grid setting), I take the position that a conventional unit is the most practical option. I do like the idea of powering the unit with a PV array using opportunity loading and the phase change thermal mass that you noted. This combined with a large array seems a practical possibility. The idea there is to use the thermal mass to carry the system over night, and use a large array to get some cooling even during inclement weather. This makes sense if you plan to be away for long periods as a larger array normally used to support systems in addition to the freezer can be dedicated solely to the freezer during these times. Augmenting insulation might also be done. Salt water (that is, aqueous NaCl) is probably not the best option, but it would work. Other solutions would probably be better like a glycol solution or other aqueous salts like calcium chloride. The idea there is of course to minimize the demands on a battery system which I consider the Achilles heel of off grid power systems. It may be possible to do away with the battery by using a capacitor in its place as a voltage buffer for the inverter - if PV panels of the right voltage can be found, then this might work well - but you can manipulate the voltage with a simple circuit if necessary. A modest capacitor pack will start large motors, and I made a post recently where a 2 pound pack is used to start a car several times with a single charge... you don't get a lot of energy, but it delivers it quickly with little voltage droop - perfect for starting motors. http://www.permies.com/t/30468/energy/Super-capacitors

I will consider the details of an ammonia absorption freezer if you are set on this, but I don't consider it a practical alternative. I DO consider an absorption/adsorption chiller to be a good prospect for off grid air conditioning, and I prefer the prospect of using a solid adsorbent with water refrigerant.

Another possibility is to use a wood gas engine system to intermittently power a vapor compression cycle for freezing a thermal mass. The most efficient system would use the engine to drive the compressor mechanically. However, I still don't consider this to be terribly practical.




 
Marcos Buenijo
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Len, since I'm emphasizing practicality at the moment, then I should note that I consider a conventional off grid PV system as the most practical way to power a freezer off the grid. That is, a system that uses a battery/controller/inverter system. I have a special disdain for relying on batteries or sophisticated electronic devices, so I tend to consider ways around them. However, they really are the best thing going for energy storage.


 
Nick Raaum
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Marcos,
Have you ever considered running a stirling engine as a cooler or even simpler an open loop single piston expansion cooler with air as working medium? I've not looked into the realities of it, but seems conceptually simple enough. Probably need to run multiple stages to achieve desired cooling temp, but don't see why it couldn't be done with a slow speed high volume simple to construct approach. Any thoughts on the idea?
 
Len Ovens
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Marcos Buenijo wrote:

I do a lot of brainstorming on this forum. Some of my ideas could be practical in some settings, but most are not. I don't have a problem with this as it can be educational to think without such constraints. However, I will emphasize practicality at times, and I think this is important.

I don't have a problem with that at all. I am quite far from the practical point anyway.

With respect to a freezer (which I also consider a must in an off grid setting), I take the position that a conventional unit is the most practical option. I do like the idea of powering the unit with a PV array using opportunity loading and the phase change thermal mass that you noted. This combined with a large array seems a practical possibility. The idea there is to use the thermal mass to carry the system over night, and use a large array to get some cooling even during inclement weather. This makes sense if you plan to be away for long periods as a larger array normally used to support systems in addition to the freezer can be dedicated solely to the freezer during these times. Augmenting insulation might also be done.

That is along the lines of my original thought. However, most standard freezers can not have their insulation augmented because they use the outside surface as a heat sink (to dissipate heat). On the plus side, compressor chillers are easy to come by and are not as picky about component placement. An in window A/C unit might provide all the parts needed. On the downside, a good chiller using a compressor will have a MTTF of about 10 years. An absorption chiller may run for 50. Though I will note that there are some old compressor fridges that have run just as long, so it can be done. I do not know what part of operation is hardest on the compressor, running or starting (or even sitting). Intermittent operation would help things to last longer if starting was the wear point.

Salt water (that is, aqueous NaCl) is probably not the best option, but it would work. Other solutions would probably be better like a glycol solution or other aqueous salts like calcium chloride.

I would like to stay away from glycol... particularly around food... if it leaked it may not be noticeable either by smell or taste (worse, it may improve taste). However there are many things that work as an anti-freeze and I am not stuck on anything (or even against anything, I would use glycol if I was sure of the separation from food). Still using a phase change mass as the battery instead of lead acid would seem to me to be longer lasting and more reliable.

The idea there is of course to minimize the demands on a battery system which I consider the Achilles heel of off grid power systems. It may be possible to do away with the battery by using a capacitor in its place as a voltage buffer for the inverter - if PV panels of the right voltage can be found, then this might work well - but you can manipulate the voltage with a simple circuit if necessary.

All inverters manipulate the voltage

A modest capacitor pack will start large motors, and I made a post recently where a 2 pound pack is used to start a car several times with a single charge... you don't get a lot of energy, but it delivers it quickly with little voltage droop - perfect for starting motors. http://www.permies.com/t/30468/energy/Super-capacitors

I agree that I want to use batteries as little as possible.... maybe better to say I want to rely on batteries as little as possible. I may feel that going to bed when the sun sets is ok, but I am sure the rest of the family will want lights and computers. There will have to be some battery storage I think, and it will have to last some days. How they will get charged is another question. I have some solar panels now, but would like to be able to harness power from other things as they are available. Wind, heat, manual... even genset if need be. I have inherited a gen set and I may need it for building. I don't know how long the little gas motor will last, but the generator itself should last about forever if I can find something else to drive it (Lister anyone?) All that to say, my experience with UPS systems has not been good, they seem to have let me down when I most needed them even when tested as little as a week before. So batteries yes, rely on them? No. I can always fire up a gas lantern if I must have light. (or a candle or oil lamp)

It sounds like you have had some fun with super caps. How is their holding power? My experience with electrolytics has been that they leak. Even with the drain resistor (safety discharge) removed they seem to be flat pretty quick (old CRTs on the other hand, seem to hold a charge forever). I can see using them along with solar panels for motor starting, but not for long term storage. Maybe you have more current experience that differs.


I will consider the details of an ammonia absorption freezer if you are set on this, but I don't consider it a practical alternative. I DO consider an absorption/adsorption chiller to be a good prospect for off grid air conditioning, and I prefer the prospect of using a solid adsorbent with water refrigerant.

Another possibility is to use a wood gas engine system to intermittently power a vapor compression cycle for freezing a thermal mass. The most efficient system would use the engine to drive the compressor mechanically. However, I still don't consider this to be terribly practical.


My major consideration is longevity. The compressors I have seen are all one sealed unit. They can be spun only with the motor they come with. I am guessing though, that there are lots of different compressors out there if I knew where to look. If I could use an automotive compressor that could be turned directly with whatever I happen to have turning at the moment, that may be a start. Buying parts like that would likely cost more than buying a unit ready made, but perhaps it may be possible to have it last longer... or at least make it repairable.

It is interesting to note that even though it seems practical to use a compressor system, many RVs still come with a more expensive absorption unit. Is that so it can be powered more than one way? Propane is easier to carry than more batteries? (that is weight is more important)

Air conditioning (honestly) is not interesting to me. This is not only from selfishness, but it would be quite hard for me to evaluate such a system as our climate is more stable than most. Just keeping a high amount of mass within the building envelope will probably keep things cool enough to be comfortable. On the other hand.... something like this would be interesting to me. We do go camping enough for something like this to be useful... and it would allow my fridge and freezer to be more physically separated

The reason I am more interested in a freezer as I said, is that it is a must have... at least right now. Perhaps in the future we will learn other ways of preserving things (we is my family, not man kind) and will be able to get by without. My interest in something that cools with only heat, is that heat is easy to make by various means. However, if the phase change mass is big enough (and the insulation is good enough), that starts not to matter so much.

The TED talk mentioned above seems to indicate there are lots of things that could be used besides ammonia in absorption mode, I am wondering if that means just at the temperatures he is interested in or if there are others that would do freezing applications (that are not toxic/corrosive).
 
Len Ovens
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Marcos Buenijo wrote: A modest capacitor pack will start large motors, and I made a post recently where a 2 pound pack is used to start a car several times with a single charge... you don't get a lot of energy, but it delivers it quickly with little voltage droop - perfect for starting motors. http://www.permies.com/t/30468/energy/Super-capacitors


Well that answers some of my questions There is leakage. This not a huge problem as it seems as they are still able to give lots of current. I understand a bit more what you mean by "manipulate the voltage"... most inverters are made for batteries which do not like going below a certain voltage and so they are designed to drop out relatively early to protect the battery. The caps could give useful power much lower because current draw could increase for constant power use. This would balance out some of the lower capacity of the caps compared to batteries. His use of a "small" solar panel for trickling the booster to keep it topped shows that while there is leakage, it is not much. It is just that the booster doesn't have that much over all capacity to begin with and in the case of the starter motor it can only get enough starting power down to 10v or so. Unfortunately, some of the other electronics in the car are even less forgiving... things like injectors and fuel pumps seem to have a higher voltage dropout... the spark is not so hot either as some of the newer circuits don't run the coil at double as they did in days past... gotta sell the new battery sooner and all.

All in all, I think that all of our battery sizing is based on the quirks and failures of the flooded lead acid battery:
- at half of their life, they have half the life... or half the charge
- they don't like being over charged
- each cell has an innate voltage
- they don't like being under charged, even deep cycle batteries don't do well if drained beyond 50%. So even brand new, a 100 amp hour battery can only supply 50 amps for an hour before needing to be recharged.

Considering these things, I think super caps could be considered as only needing 1/4 the storage of a flooded lead acid battery to be equivalent. Perhaps even less with an inverter designed to work with a wider range of input voltage. The same inverter techniques could be used do provide various DC voltages as well.
 
Len Ovens
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Another question... along the lines of the post above about using Stirling engine or other style compressor, is there a compressor that could be home made that would work without a leak problem? I think my question is along the lines of can I get a long lasting (30 years plus) compressor either by buying or building... Can I get something for heavier duty and run it lighter duty and get it to last longer. I am not sure of how I would do that: spin it slower, run at lower pressure... whatever. I guess I am asking what it is that wears them out... maybe in a lot of cases it is just the starter cap
 
Marcos Buenijo
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Len, just a few thoughts. I'm speculating, but a suitable mechanical compressor might be had in automotive a/c compressors. These are available as piston or scroll units. Using a dc electric motor as an opportunity load on a PV array seems interesting, and this would eliminate the inverter. Optimizing the life of these units can be done by running at lower speeds and ensuring good lubrication. A piston unit tends to run most efficiently at low speed, and I expect the lower speed there will allow a piston unit to last longer than the scroll - but again, I'm speculating.

I like the prospect of an ammonia absorption freezer, but the time and cost required in development would be prohibitive. Sure, a commercial unit could be had, but it would require modification to fuel with biomass - otherwise, it seems a waste to have the system rely on a commercial fuel source. Note that a small wood gasifier furnace can be devised to operate at a low and continual rate for long periods, so I think it's a reasonable prospect to devise an absorption freezer to be fueled with wood. I think vapor compression is the better prospect for a freezer. I prefer the prospect for absorption/adsorption refrigeration systems for air conditioning in the off grid setting because providing a/c for a home would consume too much electricity or mechanical energy otherwise required to drive the compressor. Using heat is a different matter. Since I'll be finally locating to the east Texas region, then a/c is a special consideration.
 
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Nick Raaum wrote:Marcos,
Have you ever considered running a stirling engine as a cooler or even simpler an open loop single piston expansion cooler with air as working medium? I've not looked into the realities of it, but seems conceptually simple enough. Probably need to run multiple stages to achieve desired cooling temp, but don't see why it couldn't be done with a slow speed high volume simple to construct approach. Any thoughts on the idea?


I have considered this before. The main problems I perceived at the time was the need to eliminate moisture in the air, the low performance of the cycle, and the high quantity of mechanical energy required. I consider the last problem as the more serious. Personally, I'm pretty much set on finally devising an adsorption chiller fueled by a wood gasifier furnace, and preferably using the heat from a micro piston steam engine as part of a CHP system fueled by biomass.

Nick, if you did not check out the following thread: http://www.permies.com/t/32176/solar/Solar-Steam-Power-System, then take a look... I think you will be interested. I consider all things for interest, but personally, I've been set on micro CHP with a piston steam engine fueled by biomass for a long while - there is a good chance I'll be in a position to start a serious project later this year with another individual. There is a lot of potential in small scale steam power with piston engines for use in distributed micro to medium scale systems. Small scale steam has an undeserved bad reputation in my opinion.
 
Len Ovens
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Marcos Buenijo wrote:Len, just a few thoughts. I'm speculating, but a suitable mechanical compressor might be had in automotive a/c compressors. These are available as piston or scroll units. Using a dc electric motor as an opportunity load on a PV array seems interesting, and this would eliminate the inverter. Optimizing the life of these units can be done by running at lower speeds and ensuring good lubrication. A piston unit tends to run most efficiently at low speed, and I expect the lower speed there will allow a piston unit to last longer than the scroll - but again, I'm speculating.

I was wondering about that, car A/C seems to work very well even at idle, but still at HW speeds. An AC motor may be better if we are using some unit to play with the voltage of a capacitor pack anyway. 3 phase motors are very simple having only two easy to replace bearings. A 3 phase starter/controller could run right off the chopped DC. A dc motor would speed up and slow down with voltage ok, but at low speeds would draw much more power for the same useful output... if I have my theory right...


Since I'll be finally locating to the east Texas region, then a/c is a special consideration.

Yes, AC is much more important there. There are some parts of my own province that are the same, but here on the coast we have that big ocean mass, 29C is about as high as it gets.
 
Marcos Buenijo
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Len Ovens wrote:Well that answers some of my questions There is leakage. This not a huge problem as it seems as they are still able to give lots of current. I understand a bit more what you mean by "manipulate the voltage"... most inverters are made for batteries which do not like going below a certain voltage and so they are designed to drop out relatively early to protect the battery. The caps could give useful power much lower because current draw could increase for constant power use. This would balance out some of the lower capacity of the caps compared to batteries. His use of a "small" solar panel for trickling the booster to keep it topped shows that while there is leakage, it is not much. It is just that the booster doesn't have that much over all capacity to begin with and in the case of the starter motor it can only get enough starting power down to 10v or so. Unfortunately, some of the other electronics in the car are even less forgiving... things like injectors and fuel pumps seem to have a higher voltage dropout... the spark is not so hot either as some of the newer circuits don't run the coil at double as they did in days past... gotta sell the new battery sooner and all.

All in all, I think that all of our battery sizing is based on the quirks and failures of the flooded lead acid battery:
- at half of their life, they have half the life... or half the charge
- they don't like being over charged
- each cell has an innate voltage
- they don't like being under charged, even deep cycle batteries don't do well if drained beyond 50%. So even brand new, a 100 amp hour battery can only supply 50 amps for an hour before needing to be recharged.

Considering these things, I think super caps could be considered as only needing 1/4 the storage of a flooded lead acid battery to be equivalent. Perhaps even less with an inverter designed to work with a wider range of input voltage. The same inverter techniques could be used do provide various DC voltages as well.


I don't consider capacitors as useful for appreciable energy storage. The main purpose in the configuration I considered is to provide just enough storage to buffer voltage transients, such as those seen when starting a motor. Really, the practical choice is to use a battery system for any off grid PV system, but it's interesting to me to consider ways around it. Hopefully, batteries will get a lot better. Perhaps using a smaller modern battery like lithium iron phosphate makes sense where the size and cost of the battery can be lessened with strategies discussed here (like phase change thermal mass in a freezer, and other opportunity loading of a PV array).
 
Connor Macreno
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You may want to see this:
http://www.storeitcold.com

A unit for easy conversion of a window A/C to cool a walk-in refrigerator and plans for building a fridge.

These units can get below freezing (not good freezer temperature range but low enough for phase change).

I've been thinking about a walk-in fridge inside a large root cellar, with a small freezer (actually a set of small freezers) inside the fridge.
If the freezer(s) are closer to the cooling unit you should be able to keep them freezing/close to freezing while keeping the majority of the refrigerator slightly above freezing. Using a phase change medium (thinking regular ice for this) around an insulated bin/bins should allow the bin(s) to be cooled to proper temperature with very little energy and maintain acceptable temperature for an extended period with no cooling.

I'm not specifically recommending the ColdBot, just the idea of incremental cooling, particularly where space is less off an issue.
A root cellar, using earth contact (and possibly additional minimal cooling and dehumidification) is the first zone, then refrigeration, then sub-zero; cooling each progressive stage will heat the previous stage, but the energy needed to compensate for this should be relatively small, and the time that goods remain cold should be greatly increased.

I'm also thinking the root cellar could (ideally, with new construction or major remodel) be directly under the kitchen with a stairwell into the cellar and a dumbwaiter to raise goods to the counter. I'd probably have a cart for moving goods from the freezer and fridge to the dumbwaiter.
 
Connor Macreno
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This picture and the ColdBot were the inspiration behind my thinking.


I'll hopefully be trying for this in an area where I can use the basic design linked to earlier in the thread from fourmileisland so I'll need little or no energy for the refrigerator.
Cellar.png
[Thumbnail for Cellar.png]
 
Mario Lazetti
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This thread is really incredible. Lots of knowledge. Been trying to figure out strategies for a 8000 sqft industrial warehouse w 18' ceilings and this has provided A LOT of them. Thanks so much for the great discussion.
 
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