Monday, May 23, 2011

Bringing 3D Printing to Bolivia

I'm currently in Cochabamba, Bolivia as a volunteer for the Solar energy company Energetica, writing reports on the social-economic impact of their work on communities. On the side, I am also designing a 2-axis "Solar Tracker" for some of their smaller >150W systems. But not just any simple Solar Tracker. This is going to be a 3D printed, Arduino-powered Solar Tracker. And hopefully my prototype will be the first of many prototypes designed, printed and sold in Bolivia, and the Grand-daddy of a cheap reliable Solar Tracker.

Here are a few screenshots of the Motor case and Pivot which I designed with Google Sketchup (fig.1) and then simulated the print with Pleasant 3D to double-check the print quality (fig.2)

My lifelong interests of the problems of poverty and development as well as engineering and design collide quite neatly in the field of 3D printing. In places where it is difficult to create complicated parts quickly and cheaply, 3D printing fills the void. In a country where demand for relatively expensive durable items is fickle, on-demand printing is more economically viable. In a country where resources are scarce and a simple part means something very important to be working, this is economically valuable.

To prove this point, designing and manufacturing a cheap reliable solar tracker seemed like a challenge that a Makerbot or other 3D printer could handle. Ideally, it would have to be less than 100 dollars, last more than 15 years, be weather-resistant and consume less power than it would conceivably 'create'. In addition, it would have to be easy to print, easy to build and easy to install.

There are multiple ways of orienting panels towards the sun: a sensor tracks the position of the sun, a table of sun positions for every day of the year, or a formula that uses the current time and location. Sensors are prone to fail and cannot deal with oddities like...a cloud going past. It can become confused. A table of sun positions is a big table...bigger than the memory bank of the average Arduino microcontroller. However, using a clock, loading the geographical location and programming a formula could be much more reliable in all circumstances. The only drawback is that the system has to be active at all times. However, simplicity and reliability are first and foremost in such a design.

Lucky for me (a useless coder at best), there are a few codes out there that can calculate the position of the sun. Mowcius has developed an excellent open-source code that does everything we will need.

However, there are a few electronic and engineering issues that are beyond calculation and need to be resolved.

Can a stepper motor hold a 16 Kg, 150W panel?
-After searching high and low for good (yet inexpensive) stepper motors that had a holding torque of more than 200mN*cm, I realized that simply balancing the panel with a counterweight would do the trick. But this is a theory, and not practice. (I'll scan a drawing of my design soon)

How weather resistant is ABS plastic?
-Apparently, not very.  After thinking about coating the plastic with a UV resistant paint, I realized that the entire system will probably be shaded by the panel! It remains to be seen what other problems would crop up after 15 years of sand, wind, indirect sunlight, dry-air, high altitude...etc. But the big problem can be mitigated.

Will the marginal increases in power generation cover the inevitable consumption by the tracker?
-According to our Wikipedia, 2-axis Solar Tracking Systems have an estimated 36% increase over fixed systems .... capturing almost 100% of direct and indirect light. (Grabs a napkin and pen) So, for a 150W system going from 74% to 100% is a mean increase of 54W. It is much more difficult to calculate energy output because of seasonal and daily changes in solar outputs. An Arduino Uno (5V) with a motor shield(5V), an RTC and 2 12V, 0.35amp Motors would consume ... 22V times ( 0.35(2) + 0.4 amps) = 24W ...

Decreasing these figures so we're not eating half the output increases (and probably the entirety of it in the morning and evening) is the next challenge. Preliminary research says its possible to sleep the Arduino or even turn it off (esp. since i have an RTC running in the background). The big question is the power consumption of the motors to hold the panel in position. In a perfect design, the motor would simply MOVE the panel and the structure would lock it until it needed to move it again. This is not important at the moment.

What about a once-in-a-decade Windstorm?

Again, that is a question for future prototypes. Keeping it simple and reliable is going to be a challenge!

Now that I've finished designing the 3D models, I need them printed. There are no Makerbotter's in Bolivia, so I'm hoping to crowdsource the printing until I can convince Energetica to buy a printer. If not this project is going to be stalled indefinitely.


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