Passive Refrigeration- Cool Ideas

"All good things are wild and free." -HDT



Zeer Pot

This blog has been haunting me. I'm averaging a post a week, but every bit of blogger advice I've come upon has said to write every day, or at least every other day. Since this is primarily the planning phase, there's not a whole lot of action to report. Most people like to see things getting done. So what am I supposed to write about EVERY DAY?! Don't get me wrong, I can ramble with the best of them, but the idea is to acquire readers- not turn them away. So how do I keep it interesting? When I think interesting, I think variety. Anyone who knows me will not be surprised. Random and eclectic are what I do best. So, going forward, I am going to try to write more often than once a week, and I'll be writing on whatever topics occupy my fancy at the moment. Of course, all of these will relate in some way shape or form to living sustainably.

Today's Topic: Passive Refrigeration.

Dan and I have decided that while we wait to begin our build, we will focus on better understanding and possibly testing out any of the concepts that we find particularly intimidating right now. The first one, which I spoke of in my last post, was the plumbing and heating of water. After much reading, speculating and conversing, we have tackled that obstacle....almost. We plan to do a three to five day hot water challege, meaning our water behavior will mimic that of the tiny house. Any hot water we want, we will heat on the stove, and showers will be taken using the camp shower. (Came in the mail a few days ago! Woohoo!) That should be a fun experiment.

The next challenge in line is building a solar power system. I am a firm believer that you can make something as simple or as complicated as you choose. We have been searching high and low to find the easiest DIY system that won't kill our wallets. As I mentioned last time, the biggest factor for the cost of your system is the amount of power you will need. The fridge would be the major power guzzling appliance, with laptops coming in at a far but still noteworthy second. It would be super wonderful if we could create our system for less than $2000, downright amazing if it were less than $1000. The easiest way to begin cutting that cost, is cutting power needs. This lead me to explore passive refrigeration, which introduced me to the world of passive cooling.

There are three common (there are others, but I've found more examples of these) types of passive cooling that can be applied to passive refrigeration:

1. Evaporative Cooling
2. Radiative Cooling
3. Earth to Air Heat Exchangers
   
   

For more information than you ever wished to know about these passive cooling methods, and others, check out this comprehensive study on the subject. I'll be borrowing their concise definitions for my top three.

EVAPORATIVE COOLING

“The process of evaporative cooling allows the cooling of air (incoming or exiting air) or of thermal masses (roofs, walls, ceilings). It uses the natural effect of evaporation to remove heat from the air. Sensible heat from the air is absorbed as latent heat necessary to evaporate water: arm dry air is changed to cool moist air - heat in the air is used to evaporate water. The amount of sensible heat absorbed depends on the amount of water that can be evaporated in the system.”

So, basically, when water evaporates, it releases heat. The more water evaporating, the more heat released. And what is cold? Simply the absence of heat. One thing to note about this method: it works best in applications where the relative humidity is low. So this would work great in the desert where the air is dry, but not so great in the steamy jungle.

RADIATIVE COOLING

“Radiative cooling is based on the heat loss by long-wave radiation emission from a body towards another body of lower temperature, which plays the role of a heat sink. In the case of buildings the cooled body is the building surface and the heat sink is the sky - since the sky temperature is lower, especially during night, than the temperatures of most of the objects upon earth. Sky temperature during summer nights can be <0°C, with clear summernight sky conditions even sky-temperatures of -10°C could be achieved.”

A great natural example is how cold a desert can get at night. The hot surface of the earth has a wide open view of the night sky, a massive heat sink. Since there are no building's or vegetation to block the view of this heat sink, great amounts of heat are absorbed into the night sky, leaving the desert thoroughly cooled.

EARTH TO AIR HEAT EXCHANGERS

“The basic principle for the use of air circulated earth to air underground heat exchangers is the seasonal thermal storage ability of soil, which results in a temperature delay compared to the outdoor temperature. This temperature difference makes possible to use the soil for cooling in summertime and for heating in wintertime. The heat exchange should only be applied in climates with big temperature differences between summer and winter and between day and night. The heat exchange can be applied for heating of supply air, cooling of supply air and heating and cooling of the supply air.”

Real life example? Ever notice that your basement seems to be the same temperature all year round? That's kind of what this is about. There are houses that are built half way into a hillside, or partially underground, sometimes called Earth-Integrated houses.  These structures are able to maintain relatively stable temperatures throughout the year, keeping it cool in the summer, and warm in the winter with little need for supplemental heating or cooling systems.

So how do these concepts translate into power-free fridges? Check out this article or just read my shoddy summary of it below...

Following the rules of evaporative cooling, you can create shelf fridge for less than $20! Simply by using a snap together plastic storage shelf, and draping it with soaked burlap material! Sounds crazy, right? Like how could that possibly make a difference and keep food fresh?! But it does! With ample air circulation (and assuming you're in dry air) this shelf can maintain an interior temperature cool enough to store produce for many days. Just keep the burlap wet!

Another example is the Zeer Pot. Mohammed Bah Abba of northern Nigeria created this power-free fridge using rules of evaporative cooling. Basically it is two clay pots, same shape but different sizes. The smaller pot is set inside a larger pot, then sand is packed in the space between the two. Wetting the sand, and then covering the Zeer pot with a wet cloth creates a small fridge. This simple invention has drastically improved the lives of thousands in Africa. To read more about his story click here.

Solar Oven

In order to create a radiative cooling fridge, you would use something you least expect: a solar oven! While the oven can get incredibly hot during the day and be used for cooking, at night, you can face it toward the sky, that big heat sink, and use it as cool storage. Some people are even able to make ice at night, which they then use during the day!

In regards to a method using the earth to air heat exchange concept, it's again, astonishingly simple. Dig a semi-deep hole in the ground, just larger than the container you wish to use for food storage. Lower the container into the hole, and then create a cover, like an earth-bag that will work like a big cork. The soil just a few feet deep maintains a cool temperature. For additional cooling, add a layer of ice.

In-ground cooling

I was really amazed that such simple laws of nature can be applied to create ingenious contraptions for storing and preserving fresh foods. Dan and I aren't sure if we would decide to go with one of these methods over the conventional fridge. There are going to be plenty of major transitions just by moving into the tiny house, we don't want the culture shock to kill us! Cost will be a major factor; if a solar system would be incredibly expensive because of the need to power a fridge, we may take a second, more serious, look. In the meantime however, you can bet we'll be experimenting! I think I'm going to give the Zeer pot style a try...would you give any of these a try?

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