Ever sense solar power became available to provide home electricity more than 20 years ago, I've been fascinated by the possibilities of this technology. With our decision to build a small eco-resort on Bali, and the falling prices of solar panels, it seemed like the perfect opportunity. However, plopping solar panels on top of terra-cotta tiles, my roofing material of choice, would greatly effect the aesthetics and clash with traditional building designs used in the tropics. About this same time, I purchased a book on innovative eco-designs (Green Design by Marcus Fairs) and discovered an example of a solar tree that merged functionality with aesthetics (photo below).
The above example was designed by Ross Lovegrove and commissioned by a museum in Vienna, Austria. It's truly beautiful and serves both to generate electricity and provides public lighting; yet however, the design isn't easily duplicated by the average person. My criteria for building a solar tree was that the material should be locally available, inexpensive, and sustainable, which led me to bamboo. Bamboo from multiple perspectives is truly an amazing plant, able to grow a whopping one meter per day and possessing excellent strength to weight properties. However, bamboo is a member of the grass family and unlike most tree species doesn't exude resins and tannins that help protect the wood from decay; and thus, bamboo used in construction needs to have an anti-termite treatment.
Now that we decided on bamboo as the primary material for the solar tree, the solar panels would need to be as light as possible and we'd need to find a material to support the panels, essentially the leaves of the tree. Within the last five years many solar companies have begun manufacturing flexible light-weight solar panels, at a fraction of the weight of traditional panels, and are ideal for my use. Regarding the tree 'leaves' that would support the panels, we examined a range of materials including marine-grade plywood and eventually decided on 5mm thick acrylic sheets that under direct sunlight will last at least 10 years without maintenance. An additional requirement was that one solar tree should provide sufficient electricity to fully power at least one of our 5 x 7m luxury tents, including lights and ceiling fan, which requires about 500 watts/hour.
The Golden Ratio
A real sticking point was how we'd arrange the leaves that hold the solar panels in order to avoid higher leaves shading the lower solar cells. As a forester, I knew that tree species through millions of years have evolved to address this specific problem, maximizing their leaf area that can capture sunlight. As such, could nature serve as inspiration? Following that thought led to me stumble upon two long-held concepts - biomimicry and the Golden Ratio. Biomimicry is simply assessing a plant or animal's design to guide the development of a new invention or to redesign an existing device. One of the best known examples is Leonardo Da Vinci's drawing of a flying machine that he designed through countless hours studying the flight of birds. Could there be a formula in nature that we'd apply to maximize the electricity generation for our solar tree? In my ignorance, that question led me to find something astonishing-- nature's universal formula that controls the shape of an infinite number of organisms from tiny flowers to galaxies (see photos below).
Examples of the Fibonacci sequence represented in nature.
Known as the Fibonacci sequence, named for the man who in the 12th century first mathematically described this phenomenon in nature. Using the nautilus shell in the center of the photo as an example, Fibonacci found that the area of each subsequent compartment that the nautilus makes is the sum of the two preceding compartments with the sequence starting with 0, 1, 1, 2, 3, 5, 8, 13, 21, 34... The architecture of a vast majority of tree species follows a fibonacci sequence with each branch oriented 138 degrees from the preceding one. Armed with that tidbit of trivia, we were ready to design the solar tree. However, we still needed to decide on the total height of the solar tree and the height of where the leaves would start. Since this was our first prototype, we wanted the height of the bamboo poles to be easily managed by one or two people during installation and decided on a maximum height of four meters. For safety reasons, we thought that the minimum height of the leaves with solar panels should be two meters to ensure that nobody would bump their head while walking under the tree. I had purchased 15 flexible solar panels, each one capable of generating 30 watts that would sum to 450 wats of power under full sun. Two of the panels I wanted at the top-most center of the solar tree, while the remaining 13 panels would utilize a Fibonacci sequence, each panel supported by a bamboo pole and acrylic leaf forming a design similar to the figure below.
Given the high rainfall in our area of Bali, we needed a design that facilitated rainwater to run quickly off the leaf and solar panel. From my graduate forest ecology classes, I knew that trees in rainforests have adapted to deal with this same problem with their leaves having extended tips, often called 'drip tips' and our design emulated again nature (see below).
Before we began building the solar tree, we tested the design using SketchUp software to see if indeed a Fibonacci sequence would avoid overlap. With 13 bamboo poles and solar panels, applying a Fibonacci sequence means that the leaves will make five complete circuits around the tree. The SketchUp images below suggested that the design was valid and we began treating the bamboo poles with a borax solution and once dry created a subtle 90 degree bend in each pole.
We inserted a 3 mm steel rod 1.5m long into the top of each pole to provide additional strength with the rod protruding 30cm from the pole and welded a steel plate to the end of the rod, designed to hold the leaves securely to each bamboo pole using small bolts and nuts.
Ultimately, after working on this project off and on for two months, I needed to ask myself did I achieve the goal of producing a functional and esthetically pleasing solar power generator? Functional -- yes, it's producing 2.7 kilowatt (kw) per day, almost a megawatt of power each year. However, the price per kilowatt is 30 to 40% more expensive then just putting panels on a rooftop. From building this prototype, I've learned various short cuts in the construction process, which should decrease the total costs. What about the estethics of our solar tree? Well, beauty is in the eyes of the beholder; we certainly didn't achieve the same elegance of design as Lovegrove's solar tree. Nevertheless, there's an attractiveness to our tree, even more so if considering the green aspects of the design. For those that would like more details, feel free to contact me at firstname.lastname@example.org
I thank Carly Gayle for her assistance in the design and implementation of this project.