[Mid-Term] "Inducing Transpiration" by Jesse Crupper
This project was born out of a response to a list serve email that I received from the Northwest Ecobuilders Guild. The original post showed beautiful imagery of the newest, "greenest" walls, each more breathtaking than the last. A few days later, another email from the Guild entitled "green walls are NOT 'green.' " The premise being that these walls require so much energy, much more than being grown on the ground and more than they will offset in reduced cooling/heating loads. These arguments are not new and are fairly well documented. The surprise to me came from the signature at the bottom, a colleague of mine from the landscape department, whom I had worked with in the Ecological Design Center. A challenge from a fellow student gave me the pretext for my final product this summer, a passive living wall.
The most typical and successful models for a green wall system are based on hydroponics. A nutrient rich water solution is pumped through the system and dripped or misted directly on the roots, requiring no conventional soil medium for their growth. This water is then collected at the bottom and recirculated. After thoroughly researching these methods I came to the conclusion that I would either need to collect water on the roof, and slowly let it drip down the wall, or I would need to make the water move against gravity. Given our site, a small trailer, I quickly realized that the former is not an option.
Soon I was reliving fourth grade, watching water travel up a paper towel, moving water from one glass to another using capillary action. Later, I was in eighth grade, relearning how a tree works. Transpiration is a powerful form of evaporation. The tree starts water up with capillary action, meanwhile the leaves allow water to evaporate causing a negative pressure to suck water up. An oak tree can create a pressure of 75 ppi, and now I must emulate it.
Leading up to this point, I have been conducting material experiments to try and figure out the best way to move water up a wall. The basic idea is that an absorbent fabric will wick water up from a basin. At certain points up the wall are pockets containing the plant growing medium, a mixture of perlite, slow-release fertilizer, and a super-absorbent polymer. The super-absorbent polymer serves 2 purposes. First it creates a reservoir for the plant living in it, and secondly it allows the fabric to use it as a step. Gravity limits the height a liquid will move up through capillary action, so this creates a new base point for the fabric at each level. Further, the plant pulls water up creating even more incentive for the water to travel up the wall.
The site, a metal container(green), the toolshed for the community gardens. my wall will go on the western side, facing the federal courthouse(grey). This side is protected by a retaining wall(seen on right), which will offer protection, and keep evaporation to a minimum.
Kitchen turned Lab
Rendered on site
Although not the proper location, this rendering tries to put things in perspective
Moving on from this point, I will continue to experiment with the stepping method for water, described earlier. When the system is proven, I would like to then explore how to push the wall in several directions. First, literally giving the wall more shape, curving it out from the wall, or potentially into it. Secondly, the materials used this summer are all within the means of a college graduate student, so some theoretical research into more advanced materials could have this wall reaching for the sky!