Tuesday, December 05, 2006

Passive Cold

Passive heating is pretty broadly understood. Passive cooling is less well known, though there are several different and viable methods. But most people think that "passive" and "ice" or "refrigeration" are just not possible. Not so.

A true passive solar refrigerator is possible--using either anhydrous amonia or water as the refrigerant--but it certainly isn't simple. The point of this post is to consider modern adaptation of the ancient techniques for making ice in the desert. Iranian architect Nader Khalili first piqued my interest in this topic, and I think that it has potential for application in modern, sustainable architecture.

The two key concepts to understand are evaporative cooling and radiative cooling. It takes energy for water to evaporate, so when it does so it cools the remaining standing water and the air into which it evaporates. And when water (or anything else) is exposed to the night sky, it cools through radiative cooling. Combine these two, and it is possible to make ice on a dry desert night, even if temperatures only drop to the low 60's. A few illustrations will help:



Figure 1: The tradtional Iranian and Indian means of making "night ice" is to place a shallow pan of water in a pit, insulated from the warm ground, and exposed to the night sky. Radiative and Evaporative Cooling combine to form a thin layer of ice.


Figure 2: The same concept from above can be modified to create a "cold box," where the cold from the ice tray cools the content of a box.


Figure 3: Or, on a larger scale, this concept could be applied to refrigerate an entire root cellar by cooling it at night to a temperature below what is possible in a traditional root cellar (where the minimum temperature is that of the surrounding earth).

6 comments:

halcyon said...

Passive houses (booth warmed and cooled) has been a big area of study in Europe.

Some links:

http://www.passiv.de/
http://www.passivhaus-info.de/
http://www.iea.org/dbtw-wpd/Textbase/work/2004/cooling/presentations.htm
http://www.unige.ch/cuepe/html/plea2006/proceedings.php

halcyon said...

Passive houses (booth warmed and cooled) has been a big area of study in Europe.

Some links:

http://www.passiv.de/
http://www.passivhaus-info.de/
http://www.iea.org/dbtw-wpd/Textbase/work/2004/cooling/presentations.htm
http://www.unige.ch/cuepe/html/plea2006/proceedings.php

Big Gav said...

George Monbiot is a big proponent of the Passivhaus concept too.

On the ice box topic, I'm not sure if you saw this article recently about the zeer pot but its interesting to see this technology has been in use in Africa for a long time:

http://permaculturetokyo.blogspot.com/2006/11/passive-cooling.html

Anonymous said...

Very Vool, Jeff. I had no idea.

Thanks!

Janene

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