The Sky Is Always Colder

Do Not Feed The Refrigerators!

Is your refrigerator running? Well, then you might want to turn it off and let it chill.
Yes, I mangled the punchline of the old joke, but I wasn't joking. Instead, I ask why your refrigerator is running at all.
Whether your refrigerator is on its way out to the junkyard, or you want to save money, but the cost of feeding that twenty year old beast is holding your wallet hostage, there is a superior alternative to the newest ENERGY STAR refrigerator, but nobody is selling it. If you are living off-grid, or you are preparing to survive when SHTF, there is a refrigeration device that requires no electricity or fuel of any kind, and you probably haven't read about it in prepper magazines or related online forums. If you are going green and trying to reduce your carbon footprint, there is a refrigeration solution that doesn't require a large investment in equipment and power and that produces zero emissions. This is permaculture at its finest.
It is not a zeer pot or any kind of evaporative cooler. it is not an ice house or a spring house. It is not an icey ball. It does not use solar or geothermal. In fact, it uses no source of external heat. This technology was refined decades ago, but given the almost complete lack of available information about this particular application it is unlikely that you have learned about it before the publication of this article. It is also possible that you might not believe it unless you do the math.
This article discusses the science, construction and operation of a refrigeration device that can maintain sub-freezing temperature without any external power. Optional enhancements that require minimal power are proposed for convenience.

How much should refrigeration cost?

The newest efficient electric refrigerators advertise a rated energy consumption of approximately 296kWh/yr for a 10 cubic foot, upright, top-freezer refrigerator,^[1] which will cost approximately $40 (plus tax) annually at the national average of 13.5 cents per kilowatthour.^[2] That might not seem very expensive, but the actual cost will depend on many factors, including actual cost of electricity, climate, ventilation, usage pattern, regular maintenance, etc. Your actual cost might be significantly higher. If the power goes out, you lose. If you live without access to grid power, you lose.
A modern propane refrigerator will burn an average of 10 gallons of propane monthly^[3] for a total of $23 per month or $276 annually at $2.30 per gallon.^[4] That's a large chunk of change. Propane prices also fluctuate. Less than a year ago the national average for residential LPG was $4/gal. If you have a propane refrigerator you are at the mercy of the market. Good luck buying propane during a state of emergency. Propane refrigerators are also relatively expensive to buy, though not quite as expensive as the DC refrigerators that are advertised for those with solar power.
If you plan to use solar power and an efficient DC refrigerator, such as those made by Sun Frost and SunDanzer, you'll make a large upfront investment for the more expensive refrigerator and for extra solar panels and additional battery capacity to handle the load, and you'll have additional expenses to maintain and replace that capacity.
How much should it cost to operate a refrigerator? The answer is zero. As this article will demonstrate, a refrigerator doesn't need any power to refrigerate. However, automated thermostatic control to prevent your cucumbers from freezing might cost you a few watthours daily. In other words, we've been spending too much money to keep our perishables from perishing.
There are a few designs for passive refrigeration out there, but they all have usability issues. Most of the designs, including the zeer pot, the evaporative cooling cabinet and similar inventions employ evaporative cooling. The drawback with evaporative cooling is that effective cooling is highly dependent on ambient temperature and humidity. It is unlikely that evaporative cooling will lower the temperature of a container to less than a few degrees below ambient, and it works even less effectively in humid climates.
Other designs rely on cold outdoor temperatures. Unless the climate is adequately cold throughout the year, this strategy requires a large cold store for seasonal cooling to be useful for warmer weather, and the cold store probably won't cool the cabinet below freezing. Back in the mid eighties Sun Frost developed a passive refrigerator^[5] with a heat pipe that would vent heat to a cold environment. The refrigerator also had a powered refrigeration system, so it wasn't completely passive. The model was discontinued.
There are other designs. The majority use a heat source, such as sunlight, which isn't very effective at night or on cloudy days.
One seasonally cooled design^[6] came very close to solving the puzzle. That design uses a thermosyphon^[7] between a radiator and a cold store of 300 gallons of water, which freezes seasonally and continues to cool the cabinet when the weather warms. By the way, 300 gallons of water weighs over one ton. That design was so close! Apparently, the designer didn't understand Earth's energy budget or optics. Had he understood, the ton of water would have been completely unnecessary, and this author would not be planting this epic seed.

Radiative Cooling

Other people have almost solved the puzzle.^[8] However, there seems to be a lot of resistance to the idea that it could work, that you can connect a refrigerator to the wheels of nature and freeze your peas and carrots using only the energy in your peas and carrots, until you eat them. One only needs to do the math to realize that it will work.
Several backyard experimenters have played with the concept of radiative cooling.^[9][10][11] Even children have done related science fair projects. The second place winners of the 2012 California State Science Fair aimed to maximize the effects of radiative cooling for use in an unpowered refrigerator and achieved a very respectable 15 degrees below ambient,^[12] which surpassed the results of other backyard tinkerers in available online reports. Though the project summary didn't specify the temperature scale, certain facts would suggest that it was 15°C. The science fair experiment studied the effectiveness of different types of covers. The project summary doesn't indicate experimentation with other factors.
These experiments have shown that it is possible to cool an object below ambient temperature during the night, but the experiments stop there. The science has been applied to cooling buildings, which requires the removal of a lot more energy than cooling a refrigerator. Why has nobody applied the science to refrigeration for food preservation?
The joke is that the invention that is detailed in the present article was patented by Argonne National Laboratories on behalf of the United States Government back in 1986,^[13] and nobody told you about it. Let me repeat that. Your Uncle Sam owned the patent, but he didn't tell you about it. Conspiracy? Maybe. The patent claims that the invention can achieve up to 80°C below ambient! To cool a freezer to -10° only requires a 40° difference from 30°C. Of course, Argonne's model included exotic materials for the cover that insolates the radiator from the atmosphere. The model presented in this article will demonstrate that a difference of 40°C is easily achievable. The Argonne patent only mentions a use for passively making ice in warm climates, but the spirit of the patent encompasses common refrigeration. It's as though they were hiding something in plain sight. One must wonder why. The patent expired in 2005, so the invention belongs to the public.

Very Basic Thermodynamics

One of those pesky laws of thermodynamics says that heat only moves from warmer to colder unless work is done to move heat from colder to warmer. There's no such thing as free energy. Many have tried, many have failed, and many unscrupulous and deluded characters have claimed otherwise. There is no free lunch. There is no loophole in the laws of thermodynamics.
The loophole is that we don't need a loophole. We just need to do some t'ai chi and use the force.
Most of us have been making stuff hot to make stuff cold. How ridiculous is that? I repeat, we have been making stuff hot to make stuff cold. In a standard vapor-compression refrigerator we burn fossil fuels, to produce electricity, to run a motor, to run a compressor, to heat a fluid, to expand the fluid, to cool the fluid, to remove heat from the refrigerator contents. In a propane refrigerator, burning the propane is more direct, with the same result of heating stuff to cool stuff.
Why are we burning money to make stuff hot to make stuff cold? Is it logical? Does it make any sense if there is an alternative? Is there an alternative? Yes, there is. Then why are we burning stuff to cool stuff? Are you mad yet?
Instead of making something hot to make something cold, let's go with the flow and make something cold by connecting it to something even colder.
"That's heresy!" you say. "Well, why didn't I think of that?"
We just need something colder than what we want to cool. What is always colder than your freezer and always there? Did you go to the local planetarium for a third grade field trip? Think, Outer space! But how do we connect our refrigerator to outer space?
What are the three methods of heat transfer? Conduction, convection and radiation. There is no solid medium between your refrigerator and outer space, so we can rule out conduction. The fluid atmosphere is likely warmer than your refrigerator needs to be, so let's rule out convection. So what about radiation? Well, there's an app for that!
The atmosphere is relatively transparent to the infrared radiation of objects at earthly temperatures. If it weren't so we would be crispy critters from the daily solar radiation. This energy balance could work for us, to free us from burning stuff to keep our milk from curdling. It's almost as though it was designed this way for us by intelligent beings, game developers or something.
An object with a given surface area with a specific emissivity at a specific temperature will radiate a specific amount of heat. The object also will absorb a certain amount of radiated heat from its surroundings. If the object absorbs more heat than it radiates then its temperature will increase. If the object radiates more heat than it absorbs then the object will become colder. That's what we want. Is it not? So let's apply some basic science.
Of course, there are no perfect insulators or conductors, so there will be losses. We don't need superconductors. Ordinary materials will suffice. As long as the net radiated output is greater than the losses the beer will get colder. The sky is always colder!

Enough with the Suspense, Already!

It's rather simple, if you think about it. Connect your refrigerator to outer space. To accomplish this feat, you'll need a cabinet, a thermosyphon and a radiator. That is all that you need to freeze your buns.
Most refrigerators use a compressor to force a refrigerant to move heat from inside the cabinet to outside the cabinet. That force requires power, but force is entirely unnecessay. Heat rises. Let's cooperate with thermodynamics and start by putting the cabinet below the radiator.
We could fill the plumbing with a liquid and rely on the thermosiphon effect to move the unwanted heat. However, all of those molecules would get in the way, and the process would be quite slow. Let's look at a regular refrigerator again.
The refrigerant, usually freon, exists within the refrigerator in two states. The compressor pressurizes the freon gas to a superheated gas, which is then forced through a condenser coil, which removes heat from the superheated gas and condenses the gas to liquid freon. The liquid is then forced through an expansion valve, where it exits into the evaporator coil at a lower pressure and flash evaporates a portion of the liquid, cooling the mixture to below the temperature of the refrigerator compartment, which cools the compartment by removing heat, which evaporates the liquid part of the mixture. The resulting gas again enters the compressor to complete the cycle.
Can we move heat without the compressor? Actually, we can. If the radiator is colder than the freezer and if the refrigerant will evaporate at the temperature of the refrigerator and condense at the temperature of the radiator we can move heat very efficiently. The sky is always colder! Therefore, the radiator will always be colder. What magical medium will do this for us? We have several options. We could use anhydrous ammonia, which is rather toxic and not readily available, thanks to the clandestine labs. We could use water, but that would require an almost perfect vacuum, which is very difficult to achieve without some fancy equipment. We also could look to the standard vapor-compression refrigerator and the aforementioned Argonne patent and use R134a freon, which is readily available and will work at a more easily achieved pressure. This is our thermosyphon.

Heat Is The Enemy

The radiator must be colder than the cabinet. The sky is always colder! To keep the radiator cold, the radiator must be insulated from atmospheric convection, and it must be shielded from radiation from warmer objects in the environment. Argonne and others gave us solutions to both problems.
To solve the convection problem isolate the radiator from the atmosphere with a vacuum and a cover that is relatively transparent to infrared radiation. Thin HDPE film, such as painter's plastic will suffice. A key point in the Argonne patent is that thermally isolating the radiator from the cover with the vacuum also retards thermal equalization between the cover and radiator, reducing condensation on the cover when the radiator is below the dewpoint, which would reduce radiative output. Preventing condensation allows much colder temperatures at the radiator and much colder chicken breasts.
For the radiation shield the optimum geometric shape is the 3D compound parabolic reflector.^[14] In addition to shielding the radiator from terrestrial objects, the compound parabolic reflector will limit the radiator's exposure to a smaller, colder slice of the sky pie.^[15] Cold is good!
Also regarding heat, direct sunlight could be an app killer. At 34.5°N, from early October to early March the sun will be below the visible horizon of a 35° compound parabolic reflector. On the longest day of summer the radiator will be exposed to six hours of concentrated direct sunlight. If you paint the radiator with 95% reflective paint, even with the effective thermal diode, i.e. the thermosyphon, that is still a lot of heat that can conduct from the radiator to the cabinet, which could spoil your spinach. Also, UV from direct sunlight will shorten the lifespan of practical radiator covers.
For every problem there must be a solution. Let's use a retractable cover to shield the radiator from direct sunlight. It will also be useful for sheltering it from rain, so that we don't need to climb up on the roof with a mop after a storm. Shielding the radiator from direct sunlight for six hours on the longest day of the year gives a minimum of 18 hours of net radiated output daily, and we don't need even that much to keep our ice cubes icey.
Yes, the sky fridge will work during the day, as long as we keep the radiator out of direct sunlight. Diffuse solar radiation will be less than 100 Watts per square meter on a clear day at solar noon near the summer solstice and far less (under 70 W·m^-2) when the sun is lower in the sky; clouds could bump that up to 250W·m^-2. However, the inverse concentration ratio of our compound parabolic reflector will reduce that to approximately 31.5%, 79W·m^-2 on an overcast day and, since we'll protect the radiator from direct sunlight, under 22W·m^-2 on a clear day. With 95% reflective paint on the radiator the loss in the visible spectrum will be from under 1W·m^-2 on a clear day to 4W·m^-2 on a cloudy day. Because the paint has high infrared emissivity the paint will absorb a majority of the infrared radiation, but the infrared radiation in diffuse sunlight will be negligible. Thus, the combined losses of diffuse sunlight will be tolerable as long as we put it into the refrigerator's thermal budget.
How do you turn it on? When the radiator is exposed to the sky, without direct sunlight, the fridge is on. Since we're not using any power, we'll just leave it on as long as the radiator isn't in direct sunlight and as long as it isn't raining. As long as it's on the freezer temperature will drop, until it reaches a stagnation temperature, when the net radiated output equals the losses.
We'll want a thermostat to protect our pickles from freezing. Instead of turning off the refrigerator with the thermostat, let's arrange it like most standard refrigerator-freezers. Whereas there are a few models with separate compressors for refrigerator and freezer compartments, most models only cool the freezer and then vent cold air from the freezer to the refrigerator. We'll do the same.
Alternatively, we could arrange separate radiators for the freezer and fridge. However, that would require another heat pipe and another evaporator. That's just too much work. Also, there will be some intervals when there is no net output at the radiator, due either to direct sunlight or weather, and it might not be possible to turn it on when we need it. Instead, we let the freezer get as cold as it will go and then regulate the refrigerator to keep our cold cuts cold cut.
With this arrangement, the temperature of the freezer compartment will be well below freezing throughout the year, even in warmer climates. Warmer and more humid climates will require larger radiators to achieve similar temperatures, but the same principle applies for all earthly climates.

Putting It Together

The construction of our sky fridge is simple enough to be made by anyone who has taken a basic shop class and even anyone with enough determination to practice with basic tools. We have some decisions to make about how we're going to build it. The three aspects that might require the most consideration are the shape of the evaporator coil of the thermosyphon, what we use for a cabinet and how we construct the radiator.

The Evaporator Coil

The evaporator coil of the thermosyphon will be in direct contact with the interior of the freezer. The evaporator cools our pork chops when our pork chops heat the liquid refrigerant in the evaporator. The heat of the frozen and unfrozen frozen foods boils the liquid refrigerant to its gaseous state, and the gas must carry the heat to the condenser (the radiator). Because the refrigerant is not forced from the bottom to the top of the thermosyphon with a compressor we need to ensure that the heat can go where we want it to go, so the shape and orientation of the evaporator coil are important.
Heat rises, so let's cooperate with our refrigerant. If the coil turns downward at any point the gaseous refrigerant that our popsicles worked so hard to produce will be trapped at the top of the turn, until conduction takes over. It would be a major fight between liquid, gas and solid, and we wouldn't get much work done.
Let's avoid that mess with a continuously upward sloping evaporator coil. With this arrangement, the less dense gaseous refrigerant can move up from the evaporator, and the denser condensate from the radiator can move down with less fighting between the two factions. Surely there will be some scuffles between them, but they'll generally play along nicely. Cap the evaporator coil at the low end.

The Cabinet

The cabinet requires some careful consideration. The cabinet must keep our tenderloins comfortable and fit the design of the thermosyphon that will make it cold in the first place. In other words, we need an insulated cabinet that will fit an appropriately shaped coil, which will function as the evaporator of the thermosyphon.
One might jump to the thought of using a residential refrigerator and either reusing the built-in coil or fitting the cabinet with another coil. However, you would be very lucky to have or even find one that would be suitable. The cabinet probably is insulated adequately. However, because the vapor-compression refrigerator is compressed the evaporator coil is probably not shaped suitably.
Furthermore, access to the coil to either verify its suitabilty or to replace it is problematic, because the coil might be in the wall of the cabinet. Trying to get to it will likely compromise the insulation. Unless you're a refrigeration technician, you probably don't want to try it. There are other problems with trying to repurpose a refrigerator. It just isn't worth it.
So what now, Kung Pao? Let's build our own cabinet. Arrange it however you like, but unless you need an upright to fit it into your kitchen you might want to seriously consider a top-load, side-by-side chest refrigerator-freezer. It just makes more sense.^[16]
Make your frame with 4x4 rails and stiles. Sandwich 2.5" foam board insulation between 1/4" inch interior and exterior plywood panels, leaving an air gap between the insulation and the exterior panels. Make the wall between the fridge and freezer in a similar manner, but leave no air gap between the insulation and either panel, to prevent condensation between the insulation and the cold panels. Seal the interior with epoxy to protect the interior surface from moisture. Finish it with insulated hinged or sliding doors.
And viola! You have a 6.23 cubic foot fridge and a 6.23 cubic foot freezer, and with the room temperature at 25°C and the freezer at -30°C (and it will get that cold) the cabinet losses are less than 10 Watts.

The Compound Parabolic Emitter

The compound parabolic emitter (CPE) consists of the radiator, its 3D compound parabolic housing and reflector and an infrared-transparent cover that is thermally isolated from the radiator by a vacuum. The construction of this assembly poses a few challenges.
Unless you have a 3D printer, the parabolic dish is one of the most difficult shapes to make accurately. A compound parabolic dish might be more difficult. Search the web, and you will find many tutorials for drawing a parabola and making a parabolic trough. There are a couple of methods for making parabolic dishes, but the methods are cumbersome. The first involves making a tool that is used to carve a plug out of a mixture of mud and straw. The second involves cutting circles from styrofoam, stacking the shapes, sanding the interior parabolic shape and using the result as a female mold. Let's not try either of these methods. However, the latter leads to a simpler solution.
After we plot the compound parabolic shape^[17] and determine the radius of the circle at each interval, we can use a table saw or a jig saw to cut discs from plywood, stack the discs, aligned with a dowel through the centers, and smooth the outer surface with joint compound, to form a male plug with the desired shape for the inner surface of our compound parabolic dish. We then wrap the plug with wax paper to aid separation of the dish from the plug and then wrap it with reflective mylar, which will become the reflector. Glass it, separate it, and we have a beautiful compound parabolic dish with a reflector.
It is slightly more complicated. The complications include the radiator, the cover and the vacuum.
The cover will need to be replaced once in a blue moon, due to weathering, so unless we want to replace the entire dish at the same time, the cover should be removeable and replaceable. This entails making the dish an assembly of two pieces. Flanges on the adjacent parts will make it easier to seal and assemble the parts. We can add a larger disc or square to the plug for each part before filling and subsequent glassing. Just remember to factor the thickness of the flanges into the calculations of the radii of the adjacent parts of the housing to maintain the compound parabolic shape. Reinforcment at the flanges might or might not be necessary. A rubber gasket between adjacent flanges and rubber washers between the housing and bolt heads and between the housing and nuts should suffice to both seal the junction and to cushion the fiberglass from the moderate force that will be necessary to adequately mate the adjacent flanges.
The radiator is a flat metal disc that must be thermally attached to the top of the thermosyphon. Its skyward surface is painted with a paint with high emissivity in the infrared and high reflectivity in the visible, for desirable surface properties. Krylon 1502 flat white will do. Since the radiator will be kept out of direct sunlight and since it will be in vacuum, the paint will probably outlast the dish and the reflector. Nevertheless, we'll split the dish into a third part for ease of assembly. This will require another flange on each of the adjacent parts of the housing. This time we'll only need to compensate for the thickness of the flange above the radiator, because the compound parabolic shape terminates at the radiator; the housing below the radiator will house the radiator only and will not contribute to reflection.
There also needs to be a way to pull a vacuum from between the radiator and cover after each assembly. This will require a bulkhead on the portion of the housing between the radiator and cover. Because it would adversely affect the spectral properties of the emitter to put the bulkhead through the reflector, it is more desireable to put the bulkhead through an appendage on the exterior of the housing with only a pinhole through the reflector for pulling a vacuum from the cavity.
There is one important factor of the CPE design that this article hasn't addressed yet at length. The size of the radiator will affect both the available cooling power and the size of the CPE housing.
We'll do the math in another article, but let's look at an example. If we determine that we need a radiator with an area of .4 square meter (4.3 square feet), the radius of the radiator will be 36cm (14"), and the CPE with a 35° viewing angle will be 1.4m (55") tall and 1.25m (49") in diameter at the top. Though this is comparable to the size of an ordinary central-air air conditioner, buiding this monstrosity DIY poses a few problems. First, the plug for the CPE would be humongous, which would require a lot of material for the plug and make construction of both the plug and the housing very cumbersome. We also must consider the wind load of something this large and the extra engineering to deal with it.
Fortunately, this is not an app killer. We can reduce the height of the CPE, have a slightly larger footprint, use less material for the plug, use a comparable amount of material for the housing and have the same radiator area. We do this by making an array of smaller CPEs.
In the above example, let's split the radiator into an array of six CPEs. Each radiator will have a radius of 14.6cm (5.75"), and each CPE will have an upper diameter of 50.9cm (20") and a height of 57.2cm (22.5"). By staggering the CPEs, leaving an in inch between adjacent CPEs, the footprint of the array will be approximately 154.25cm x 127cm (60.75" x 50.19"). For a 27% larger footprint we have a 59% decrease in height. The disc of each radiator can be cut from a 12"x12" piece of sheet metal with a small margin, which is great for us Americans.

The Thermosyphon

Construction of the thermosyphon is straightforward. The thermosyphon, which includes the evaporator coil, thermally connects the cabinet to the radiator and must be insulated between the two parts of the system. It will require an R134a low-side charging port, for charging it with a standard charging kit. Unless the charging kit includes a vacuum port, we'll also need a vacuum port to evacuate the thermosyphon prior to charging it with freon.
Connect the evaporator coil to the charging and vacuum port assembly with refrigerator coil or appropriately rated copper pipe. Connect the other side of the charging port to the CPE radiator or, if using a CPE array, to the manifold which connects to each radiator in the array. When the assembly is complete, perform a static pressure test.
Evacuate the thermosyphon with a vacuum pump. Charge the thermosyphon to an appropriate pressure for the desired freezer temperature. The evaporator needs to boil the refrigerant at the desired temperature. The boiling point of the refrigerant depends on the pressure. Of course, it will be necessary to compensate for the temperature of the evaporator at the time of charging. Consult an R134a temperature chart.

Let's See Some Numbers

Up to this point, this article has made some claims that you might find hard to swallow. The point was to keep it simple without bogging down the reader with the math. The math will be covered in a future article, but let's clarify everything with an example.
I have applied three different models for radiative cooling: Berdahl(2012), Berdahl-Martin and a combination of Smith(2009) and Berdahl. The results of all three models agree very closely, with a maximum difference in net output of one Watt for a sample system with a radiator area of .4m^-2. The following results are for a system with a 13 cubic foot cabinet and .4m^-2 radiator under present local daytime weather conditions using Smith+Berdahl.
On October 22, 2014, at 13:00MST, in Snowflake, Arizona, it is currently 21°C with 35% humidity, a dewpoint of 4°C and clear skies. The calculated losses, including CPE losses (per Smith), cabinet and plumbing losses, are 34W. The sun is below the visual horizon of the radiator, so let's add .4W for diffuse solar radiation to our losses. At -18°C the calculated net output at the radiator is 21.8W, afer losses. The calculated stagnation temperature is -33°C. That'll keep your chicken tits fresh.

Spread The Word

There are millions of refrigerators in the world, and they're all burning something. What does this cost us? How many tons of pollutants do these power-hungry critters release into our atmosphere? I haven't found a number, and I don't care to guess.
There are millions of people without refrigerators. How much does this cost them, when it could cost so little?
I haven't completed our sky fridge. I have the tools. I bought the materials. I have the desire. Certain obstacles have been in my way. Other projects have been in the way. There's a cabinet where there should be a sky fridge, and our kitchen cabinets are 200 miles away awaiting transport. As soon as these obstacles are out of the way, I will complete our sky fridge. Until then, we have an icebox.
Take this seed. Plant it. Let it grow and reproduce. Let it multiply. Let it spread far and wide. Share it with friends, family, neighbors, colleagues, acquaintances and strangers. Build it. Help others to build it. Refine it. Solve the problems that I might have missed. Let's see how far we can take this idea. Let's see how far we can go.

References

[1] ENERGY STAR Certified Residential Refrigerators
[2] EIA Electricity Data
[3] Propane Fridge Comparisons
[4] Weekly U.S. Propane Residential Price
[5] Cold Weather Passive Refrigeration | Sun Frost Blog
[6] Daystar: The Four Mile Island Icebox
[7] Heat pipe - Wikipedia
[8] Halfbakery: Cosmic Background Refrigeration
[9] Night Sky Cooling
[10] First Results From THE BOX
[11] using a solar cooker as a radiant refrigerator at night.
[12] Maximizing the Effects of Radiative Cooling for Use in a Non-Electric Refrigerator
[13] US 4624113
[14] Optics for Passive Radiative Cooling
[15] Amplified Radiative Cooling Via Optimized Combinatons of Aperture Geometry and Spectral Emittance Profiles of Surfaces and The Atmosphere, J.B. Smith, 2009
[16] Chest Fridge
[17] Optic of Solar Concentrators, Segal, 2010