evaporative cooling fridge test

So I decided to do a test in my basement. ambiet is 68 -70F, near the concrete floor its closer to 65. I put up a small box with a wet towel on both sides with a fan blowing air through. I cant seem to get below 59-60F. Is it because the air is already humid enough or is the temperature too low that the differential isnt as great? Any advice would be helpful. Thanks!

Rob

Check out the ZEER pot. http://en.wikipedia.org/wiki/Pot-in-pot_refrigerator

If the dew point in your region is high, then you're not going to get good results in any configuration. However, there is a means to raise the evaporation rate and enhance the cooling effect by drying the air (in effect, lowering the dew point). This can be done with a desiccant like silica gel (crystal cat litter is a cheap source). I like any idea that doesn't consume electricity. I don't know how well it will work, but I'm curious to know the results of placing a ZEER pot inside an insulated vessel along with a mass of desiccant like silica gel. Since desiccants heat up as they adsorb water, the container of desiccant would have to protrude out of the vessel to allow this heat to dissipate. It should be possible to dry the air enough to support a fairly high evaporation rate and a much lower dewpoint. What I am imagining is a large pot of silica gel with a sealed lid and a perforated base that could be used to seal the top of the insulated vessel that contains the ZEER pot. Water vapor would be drawn into the pot to be adsorbed in the silica gel. The silica gel would heat up and this heat would be dissipated out of the system. Heat would be continually drawn out of the system as long as the silica gel is not too wet, not too hot, and the ZEER pot not too cold. Periodically the silica gel would have to be heated to remove the water. Just place it on a hot surface like the barrel of a rocket mass heater.

ADDENDUM: It should be simpler to place porous clay pots filled with water inside the insulated vessel. These pots and the water contained within them would be cooled by the evaporation. Water could easily be added to the system by interconnecting the pots with tubing, and adding a single fill line. Perhaps the pots could be placed on a top shelf to allow for maximum storage beneath, and this would provide a natural convection current. It should also provide a nice thermal mass.

NOTE: I just considered this idea off the cuff. So far it seems viable to me. Anyone please chime in and burst my bubble if necessary.

OK I figured it out. The incoming air was not warm or dry enough so I was getting minimal impact. Too bad, I need some form of no electricity or low electricity cooling in my basement for food storage. A root cellar would be awesome, but I dont have the ability to do that.

http://www.gasrefrigerators.com/faq/how-gas-refrigerators-work

this is the cycle of the gas powered refrigerator....but the gas is only used to create heat. One could cobble up some unit which cools on the same principle using any heat source.

When i consider all the parts to this i think likely it would not be much cheaper to build than to buy an existing unit and alter it to suit your available heat source. The heat needed to evaporate is less than 100C (212F) so solar would do nicely to power during daylight hours (think in terms of solar powered ovens you see). Quite possibly you could scrounge around for parts in auto junk yards or buy a used gas powered fridge.

I thought about buying a used compressor for a fridge and just building a larger insulated chamber. I dont need 35F, so 45F would probably work just fine. Im experimenting with frozen soda bottles atm as well.

you did not specify how large the refridgerated space should be, if you want root cellar size then you would need more than if you are cooling a bowl. You also didnt say if it was year round storage or after harvest. Lastly, you did not say what the climate was you lived in, i know if i were using area of my basement as root cellar i could cool it with snow well into the spring thaw.

I have had many conversations with non english speakers, the abreviation "u" for you is something which is easily handled

The specific error message is: "u" is a silly English abbreviation; use "you" instead

Rob Sigg wrote:I thought about buying a used compressor for a fridge and just building a larger insulated chamber. I dont need 35F, so 45F would probably work just fine. Im experimenting with frozen soda bottles atm as well.

Perhaps the most practical and efficient system would be to build a highly insulated enclosure around a standard small freezer such that the condenser is exposed to ambient. Water bottles are contained in the chest freezer for freezing to provide a large thermal mass. Do some cutting on the freezer and add a small fan to circulate air through the frozen water bottles, and use a thermostat if you want to get fancy (these thermostat fans already exist on the market). The compressor for this freezer is configured as a diversion load on the solar PV system that you have. Of course, you're going to have to get more panels, but the battery system need not be enlarged. Furthermore, this configuration will protect your existing battery system from heavy discharge. The setpoint on diversion load controllers can be adjusted in many models. Ideally, you could set the voltage such that the freezer comes on during an absorption charge. If the panel wattage is sufficient, then it should be possible for the compressor on both freezers (the thermal mass freezer and your regular freezer) to operate while also maintaining absorption on the battery, and during particularly sunny days you could take the battery to float. In this configuration the battery losses are greatly minimized. Adding a thermal mass to your regular freezer in the form of multiple small bottles of salt water of the proper concentration can minimize its cycling and further enhance these effects.

HI Marcos, that is pretty much what Ive been thinking. I got the thermal mass already and even though I probably wont get another panel I am getting another 4 batteries to double my capacity.

Rob Sigg wrote:HI Marcos, that is pretty much what Ive been thinking. I got the thermal mass already and even though I probably wont get another panel I am getting another 4 batteries to double my capacity.

If you're thinking on getting more batteries, then you're probably not thinking along the same lines as I. The primary purpose of the approach I outlined is to minimize the long term expense of the battery by allowing for a smaller battery system and by greatly minimizing its discharge (so it will last longer). It will also increase the output from the panels by avoiding many battery losses. The simplest configuration that would illustrate this concept is to power only a single large chest freezer from a PV system. The freezer is loaded with a thermal mass in the form of bottles of salt water of a concentration to achieve a melting point of 5-10F which my research suggests is acceptable for a deep freezer (just stay below 15F). The thermostat on the freezer is set at the lowest setting which is well below 0F. It's not terribly difficult to put enough thermal mass in the freezer to hold its temperature below 15F for several days of literally zero solar incidence. Consider that one c.f. of ice absorbs about 8000 btu as it melts, and that a typical chest freezer consumes about 1.2 KWh of electricity to achieve this cooling effect. The presence of a large thermal mass of salt water combined with lowering the thermostat setting will cause the compressor to operate continually until most of the thermal mass is frozen, and this will not discharge the battery if the compressor is placed on a diversion circuit. This is similar to storing several KWh of electricity in a battery, but without the hassle and expense associated with a large chemical battery. Panels are comparatively less costly than batteries when considered over the long term (by a long shot). Another interesting fact is that large chest freezers do not consume much more electricity than small units because the surface area/volume ratio decreases as the units get larger. This means a slightly larger freezer buys extra space for the thermal mass without consuming very much more electricity. However, the battery losses alone that can be avoided by going to this configuration will more than compensate for the additional electricity that would otherwise be consumed by a larger freezer. It also avoids multiple cycling of the compressor throughout the day which may increase its life. I also recommend adding additional insulation to a large chest freezer and keeping the condenser well cooled. I've done some calculations, and it's clear that the performance of a typical freezer can be dramatically improved by doing this.

While the original discussion centered around making a cooler, the same principles apply - just use regular water in that case (not salt water), raise the thermostat setting in the freezer, make a large highly insulated enclosure for your cooler, and circulate air in the cooler through the thermal mass with a small fan place on a thermostat.

Again, the main benefit of this approach cannot be appreciated without considering costs over the long term. You will save many thousands of dollars in the long run by being able to minimize the size of a battery system and by greatly minimizing its discharge.

NOTE: I don't know exactly how to configure the system for this, but I believe it's worth investigating.

I think I got it now. You are storing energy in ice instead of batteries? I like your approach, but the reason Im getting more batteries is because I dont have any more room for panels currently. My goal is to not discharge the batteries much lower than 80% on a daily basis and get them to absorption most of the time.

I have insulated my freezer, but I do like your comment on keeping the condenser as cool as possible. Im going to look at mine more closely because I agree standard chest freezers can be made more efficient. At what point does adding more insulation lose its ability to prevent heat into the cooled area? Is it based on thickness/r value or something else?

How do you get the right salinity level with common kitchen tools?

I had purchased a new mini fridge the other day, since my old one ceased to cool. Anyway, this site helped me make the right pick: http://www.minifridge24.com/

Hope it helps you guys as well.

Rob Sigg wrote:I think I got it now. You are storing energy in ice instead of batteries? I like your approach, but the reason Im getting more batteries is because I dont have any more room for panels currently. My goal is to not discharge the batteries much lower than 80% on a daily basis and get them to absorption most of the time.

I have insulated my freezer, but I do like your comment on keeping the condenser as cool as possible. Im going to look at mine more closely because I agree standard chest freezers can be made more efficient. At what point does adding more insulation lose its ability to prevent heat into the cooled area? Is it based on thickness/r value or something else?

How do you get the right salinity level with common kitchen tools?

Exactly, water is a lot cheaper than batteries and it lasts a helluva lot longer.

Note: The absorption stage of charging is particularly inefficient (especially near the end), and a larger battery system requires more energy to complete absorption. If the existing battery system is not making it through absorption on a regular basis, then a larger battery system is certainly not going to make it (all else equal).

Getting the right salinity for water added to a freezer is straightforward. You'll have to do some research and experimentation to find the quantity of salt by weight necessary to add to a known volume of water to get the desired freezing/melting point. Mix it up well, place it in a chest freezer for freezing, then remove it when it's frozen and measure the temperature as it melts by placing a temperature probe in the water/ice slush. Once you find the right proportion of water/salt, then you can mix any quantity desired by keeping this same proportion. Also note that the size of the bottles and their placement in the freezer may affect the results. Trial and error is the way to go, but once it's set is should be good to go. If you decide to try this for your freezer, then please take data beforehand to determine how often your freezer cycles in its current form for comparison.

If you want extreme insulation, then polyeurethane is among the best. Styrofoam is also good. You just want to add as much as practical. The best freezers I've heard of have 4-6" of polyeurethane. As far as cooling the condenser, it seems reasonable to me to just make sure the condenser is open to air flow and stays clean.

* Discussion: An idealized system that uses the thermal mass principle would put the compressor on a timer that cuts on right about the time of day that solar incidence is highest, then cuts off at the end of the day to prevent the compressor from cycling at night . When the timer switches on each day, then the compressor should come on immediately since the thermostat is set on the lowest setting. If properly designed, then the compressor will stay on until all the salt water that melted during the previous night is refrozen. A properly sized array should accomplish this during the first half of most days thereby leaving the latter part of the day dedicated mainly to taking the battery through an absorption charge. This approach will leave a near fully charged battery ready for light loads during the evening.

Marcos, Ive been putting more thought into the salt water idea. Wouldnt we want to have a higher melt point so the water would stay colder longer? I new to this concept, so sorry to be stupid about it!

Regarding your discussion point. I pretty much did what you suggested with running my freezer only during peak times. At the time I didnt have enough thermal mass in the freezer to keep it all the way through the night to the next day without the temps going too close to 32, I was getting alot of moisture build up as a result, which made a mess. Id like to mess with this more once I get more thermal mass. Its a tricky experiment since I have over $500 worth of meat in the freezer and dont want to risk ruining it!

Rob Sigg wrote:Marcos, Ive been putting more thought into the salt water idea. Wouldnt we want to have a higher melt point so the water would stay colder longer? I new to this concept, so sorry to be stupid about it!

Regarding your discussion point. I pretty much did what you suggested with running my freezer only during peak times. At the time I didnt have enough thermal mass in the freezer to keep it all the way through the night to the next day without the temps going too close to 32, I was getting alot of moisture build up as a result, which made a mess. Id like to mess with this more once I get more thermal mass. Its a tricky experiment since I have over $500 worth of meat in the freezer and dont want to risk ruining it!

Imagine a cooler loaded with six packs of beer. Ideally, we want a beer to be as cold as possible, but not frozen. When you take ice out of a freezer, its temperature is about 0F (or roughly whatever the temp in the freezer is at). When you put your ice in the warm cooler, the temperature of the ice rather quickly rises to 32F as it cools off the inside of the cooler, then it stays at 32F. The ice will maintain 32F as it continues to keep the inside of the cooler cold until most of the ice has melted. So, keeping beer cold (but not frozen) was done with ice at a melting point of 32F. Similarly, keeping the temperature inside a freezer a little below 15F requires ice with a melting point well below 15F (I suggest 5F). NOTE: Freezers should be maintained below 15F.

Why is salt water as a thermal mass superior to alternatives? If you place one pound of meat, ice, or anything else into the freezer, then it will provide a thermal mass. Let's consider ice. One pound of ice absorbs about 140 btu of heat as it melts. However, it only does this at 32F! Therefore, ice is not useful for maintaining proper freezer temps. It will provide some thermal mass at freezer temps, but not much. The heat capacity of water is such that 1 btu of heat is required to raise its temperature by 1F. So, from 0 to 15F a pound of water will absorb 15 btu to help moderate a temperature rise in the freezer. By contrast, a pound of salt water provides the same 15 btu, but also provides latent heat of roughly 130 btu (and does so at optimal freezer temps!). So, salt water is about 10 times better than water, and far better than adding more meat which provides no latent heat and has low heat capacity.

Some numbers to consider: A typical modern freezer consumes about 1 KWh of electricity to provide a cooling capacity of 5000 btu (just a very rough thumb rule). Now, an efficient 25 cubic foot freezer consumes about 1.5 KWh each day, so it loses heat at a rate of about 7500 btu each day. How much salt water must be added to maintain freezer temps for 48 hours with ZERO electricity? Answer: about 2(7500) / 130 = 112 pounds. This is roughly 30 two liter bottles of salt water, or about two cubic feet of water. Now, these bottles take up more space than two cubic foot (it's probably around 5 cubic feet). So, adding the bottles would reduce the freezer capacity to 20 cubic feet. In my opinion, this is a good compromise that makes a lot more sense than increasing battery capacity.

So basically the saltwater is aborbing heat sooner than standard water, therefore preventing that heat from building up on other higher melt objects?

Rob Sigg wrote:So basically the saltwater is aborbing heat sooner than standard water, therefore preventing that heat from building up on other higher melt objects?

I've never seen it put quite like that, but I think it's a good way to see it (sort of like a sacrificial anode in a cathodic protection system)... the salt water ice will melt before the food thaws to keep the food not only frozen, but also at optimal temps. It also minimizes the cycling of the compressor, and it can also reduce battery losses if properly configured.

Well at least Im getting it lol

look into "brine coolers", most of them use isolated loops, so that the exchanger is outdoors, but a few work like standard evaps. Tough on metal parts tho.

http://www.coolerado.com/products/how-coolerado-works/

You can prob get a standard evap to work, you just need better airflow, and less saturation. Use cheesecloth, or order some cooler pads air can actually pass thru. Temps matter less than rel humidity.

the best way is to actually use the foil backed foam , and a room cooler , check out this little box, that decouples the thermistor in a room ac, so that it can cool a complete walk in, comm freezer.

Manufacturer site http://www.storeitcold.com/

www.amazon.com/CoolBot-Walk-In-Cooler-Controller-conditioner/

Hitachi and sanyo make "split" ac units with the evap outside, and here is a comm unit

http://www.koldpack.net/

and here is the cool , new heat to chill unit from britain

http://www.firechill.com/products/

thanks ill check them out!

Rob S : A common reason for siting the farmers barn where it was sited was adequate water for livestock Springs coming out of the ground were prized, protected by a spring house and development of a spring was carefully considered and discussed before actually proceeding. In the days before wind mill driven pumps the lose of a spring was a tragedy.

Before a barn was sited a spring house would be built to help monitor the springs temps and flow rate thru the year, I have seen spring houses Not connected to a barn or farmers house that looked rather like outhouses with concrete foundations rather porous and 1 or 2 concrete settling basins , though most were built very low to the ground to aid in allowing the spring to keep the interior as cool as possible.

Starting our conversation from there, it is the temperature of the floor not the air temp that was your limiting factor on cooling your chill-box.

It is a common sight for sump pump basins to have the remains of a pulley set above them to allow for suspension and retrieval of small milk cans from as far down as 5' below the cellar floor.

The 1st thing a Farmers milk parlor had to have, after the needs of the livestock were met was an open or preferably closed water bath with flowing ground water to chill down the twice daily production of milk !

With a delay in reaching the warmest ground and ground water temps into the fall when night time temps are starting to drop, much could be planned for and done in chilling food stuff and carrying it forward several days till marketing day

The astute town dweller who had his own spring house would have better and safer food storage than his neighbor !

Check your local records for local ground and ground water temps, they probably stopped checking 50 years ago but global warming will not have made a noticeable difference yet !

These are more mind sets than skills but todays farmer would be lost with out pumps and refrigeration, only the Amish and Menionites ( local spelling ) retain these mind sets ! hope this helps Allen L.

So sitting it on the floor was bad? I should have elevated? Please explain. Im familiar with spring houses, I live in Amish country and all of my neighbors are either Mennonite or Amish, some have pumps but in this area alot of them are turning to solar.

Rob ; I am sure that the temp. you cooled your pot box down to was actually a little higher than the temp of your floor,unless you set out to insulate your box from the concrete. Compare getting an good replacement air flow in a root cellar requiring many feet of pipe buried in the ground, so that a dry root cellar needing the fewest number of air exchanges is good but a running ground water bath can cool down its container and temper a large room so that 'coming in from outside' chills you. A spring house can be built to allow you running ground water in the mid fifties, at that temp there is less moisture in the air. Being above ground It is liable to freezing esp. because it is hard to keep most insulations dry. I have seen spring houses with 3 roofs with air spaces to help keep interior cool. Back in the thirties, well before my time , a general store with electricity would have a simple chest type beverage cooler and your cola,grape nehi, root beer would be sitting in water to make the drinks almost float , the fact that you opened from the top and reached down into a well insulated box that was always full made this type of cooler much more efficient than any thing you see today, I last saw one of these in the '70s! - google chest refrigerators just to see what's out there! - tonight my thoughts seem to jump around a lot hope this still makes sense to you ! Allen L.