Suppose you lived in a hot and muggy climate and wanted to build a new house that was comfortable and energy efficient. You came to me, a builder, and asked how we could dehumidify the house, and I told you to put a waterfall in your living room.



After you had fired me, I chased after you and convinced you to let me explain. Yes, it does seem impossible that flowing water could take moisture out of the air, but this is no ordinary waterfall. It has a special ingredient and it operates under one of nature’s most powerful principles, the Second Law of Thermodynamics.



Read on for the science behind this phenomenon and a real-world example of a home-based waterfall in action.



Think back to your last (and probably distant) physics class and you will recall that in an isolated system, energy will tend to move toward entropy. This principle is what activates and reactivates the work that living things get from gradients.



This is the “surfing for free” that I wrote of in my last article, and the living world is rife with examples. Mine was of a tree pulling water from the soil by a gradient started by evaporation in its leaves.



The tree has a complementary gradient that aids this evaporative engine, and it is also based on the Second Law. It is osmosis. Within the cells of the tree’s roots, for example, are selective membranes, letting some molecules, like water, pass through while blocking others, like salts.



When certain minerals reach high concentrations, water will flow into the cell to dilute the higher concentrations and reach an equilibrium on either side of the membrane. The plant uses this to extract water out of the soil, sometimes overcoming tens of atmospheres of load to do so.



There are lots of other examples. Soaring birds take advantage of convection currents or thermals, drifting larvae ride on similar fluid gradients initially stirred by winds created by temperature gradients.



Even prairie dogs have gotten wise and construct their homes with openings at different heights in order to set up a house breeze, courtesy of the Bernoulli effect.



Perhaps the most important energy gradient to all of us personally happens in our bodies at the cellular level. In a previous article I mentioned a few key components in natural energy systems. One is the standard unit of energy, adenosine triphosphate (ATP), which is a sugar-phosphate molecule that both plants and animals make and use to power their biological processes.



The power comes from the breaking off of one of the phosphates from ATP, thereby releasing the stored energy in the chemical bond. This energy is then used to do other things in the cell, like building proteins. Add another phosphate to the newly stripped adenosine diphosphate (ADP) and you are back in business with the original power unit, ATP. This is going on now, as you read this, millions of times in each cell in your body.



What’s fascinating to me about ATP is how it is made, and its manufacture depends on a gradient. Whether in the mitochondria of animals or the chloroplasts of plants, the molecule is made by the closest thing that nature has to a turbine, the ATP synthase, a spinning rotor embedded through the membrane of either organelle. This rotating enzyme cluster is powered by hydrogen ions (protons) flowing into the mitochondrion, like water passing through a millwheel. The spinning generates energy to make our standard power unit, ATP. The rotor can go in reverse as well, this time powered by ATP from the inside, pumping protons out of the mitochondrion.



This gradient of hydrogen ions on either side of a membrane is maintained by the organelle. The mitochondrion, for instance will pump up the concentrations of ions outside in order for them to rush through the ATP synthase rotor, just as a child at the beach refills her plastic waterwheel to make it spin. Again, physical things may tend to equilibrium, but living things are busy maintaining dis-equilibrium, because that’s what does all the work.



Our waterfall is in good company, therefore, and it, too, is going to save you energy by surfing the Second Law. It, too, is going to take advantage of concentration differences, and molecules traveling from higher to lower concentrations and pressures.



Its secret ingredient is calcium chloride added to the water. Calcium chloride, a salt, is a dessicant, just like table salt and the grain of rice that you put in the shaker to keep it flowing freely. It is hygroscopic, meaning it attracts water molecules from the air by absorption or adsorption.



What we have here is a liquid dessicant system, a wet flowing feature that can suck the water out of your indoor air for a lot less energy. These systems have been used in the commercial and manufacturing sector for many years, reducing ice formation, mold growth and other consequences of moist air. Unlike standard compressed air conditioners they don’t have to work to lower air temperatures enough to precipitate water out and then use more energy to raise them back up for comfortable living.



The waterfall idea comes from a clever group of students and faculty at the University of Maryland who recently used it in a model green residence, turning a functional mechanism into an aesthetic feature. For their efforts they were awarded second place for their LEAF House in the 2007 U.S. Department of Energy’s Solar Decathalon.



As in all of our examples, there is a point when energy must be used to keep the dis-equilibrium going. The mitochondrion must pump more ions out, the bird must flap to reach the updraft, the child must refill the waterwheel. In this case, the dessicant becomes diluted with the water it has harvested from the air, the gradient stops and the excess water must be purged out of the system.



The LEAF House team, as is typical in these systems, solved this by heating the water to evaporation and venting this outside, a so-called regeneration phase of the system. To reduce their energy costs, they used solar hot water heaters to do the job rather than electricity. With the dessicant concentrated again, it is ready to do more work pulling water out of the air.



As in many passive and semi-passive climate control systems, the efficiency devil is in the details. Its Coefficient of Performance (COP), or ratio of output to input will depend on many different factors like temperature ranges, contact surfaces and flow volumes. It is, however, a cyclical, gradient-powered and mostly passive air conditioning system and it does a great job of employing one of nature’s most powerful principles.



In my next article I will write about an architect who has designed and built two award-winning buildings that put many of these thermodynamic principles to very useful work indeed.



Tom McKeag teaches bio-inspired design at the California College of the Arts and University of California, Berkeley. He is the founder and president of BioDreamMachine, a nonprofit educational institute that brings bio-inspired design and science education to K12 schools.



Waterfalls - CC license by Wetsun and Pear Biter