Author: Jack Darrah
Greetings from the newest new guy! It’s been a busy 2014 as we progress toward a close-looped prototype and the physical realization of what has only been heretofore a model on a computer screen. For me personally, it’s been a plunge into the depths as I continually learn about the intricacies--and originality--of SunChill™’s design as well as the system’s current and future challenges. There was also that pair of 15-plus-hour flights to Mozambique to mull it all over.
Here are some highlights from the depths, 2014:
What’s gone well:
In short, where there was nothing, now there is something—and it’s something that resembles the original idea (lower case, no Platonic commitments here). Although we’re not quite there yet, it has been satisfying to see the assembled prototype up and running. With two iterations thus far, we’re definitely moving in the right direction, each time refining and improving the system so it is will operate more robustly in the developing world. TheSunChill™ cycle may be science, but the work of designing and building is art. Yes, even piping design. Art.
If you ever choose to build a piping system, here’s what you may learn: thread tape is one of the greatest inventions of all time--much like toilet paper, it’s simplicity and usefulness cannot be overstated. Waterproofing is a non-trivial exercise and just because you hear the pump running doesn’t mean anything is flowing. Also, vapor-lock. It’s a thing.
Still, each prototype has been successful in teaching us ways to improve the system. For example, our first iteration relied on high-temp flexible tubing, which, while easy to work with, was expensive, not available in the developing world, and prone to vapor lock. Additionally, we had been trying to integrate passive temperature control like those found in anti-scalding home hot water systems, into the flow system. Both these have since been discarded: the former for the sake of sourcing materials locally in Mozambique and the latter due to the system’s low pressure and low flow constraints.
Enter: the second iteration and the glories of PVC. While PVC pipe only works at our high temperatures because of our low pressures, it’s ubiquity and cost made it a natural replacement for the system, with some creative jerry-rigging—also something of an art, by the way. We also decided to use a thermal cutoff for temperature control for it’s low cost and simplicity.
Of course, these are only some of the changes time and insight have wrought on our system thus far. As things currently stand, we’ve rebuilt the whole prototype and introduced brine to the system, both major milestones for the year. The current iteration, however, still employs a makeshift of our 3-D printed precipitation heat exchanger (lovingly referred to in the lab as the “PHX”) and an solar collector emulator that allows us to mimic typical solar cycles in Mozambique and thereby optimize the system’s performance without relying on the sun, whose 24 hour time scale is something of a burden to R&D prototyping. Shifting to our printed PHX and solar collector will be the primary technical goals of early 2015. How they work with our membrane, very much the heart of our system, too remains to be seen.
What’s been challenging:
With any new technology, all things are challenging, but our PHX has been a thing unto itself. Taking a cue from the hexagonal wax cells made by bees, we had set our geometry by the end of summer; a geometry, which, it turns out, offers a generously high surface area to volume ratio (seriously, our first prints had a ratio 4000 times too high) as well as a printer-amenable geometry. The design also features a staggered hourglass shape to facilitate airflow, improving its ability to dump heat to the environment. Unfortunately, despite a promising geometry, we experienced some setbacks due to a faulty 3D printer and a rather dubious customer support team—no names named.
Unable to make our first print set up work, we eventually shifted to another 3D printer and have been running prints ever since. When I asked Sean, our grad student and resident printer extraordinaire, for some words for the blog, he simply replied, “Nothing to say that’s worthy of the blog, besides the fact that I can use the printer like a boss.” So there’s that. And he’s not wrong: the printer has been running more or less smoothly, enabling us to focus on the post-print processing, which involves a chemical bath to ensure proper welding at the print’s seams, a difficult thing to achieve even with a heated build chamber.
It’s taken some time to hone this post-print processing given the scale of our parts, which require separate self-contained segments to be fitted then processed, and our PHX’s internal features, which are complex yet integral for providing the required cooling. We’ve recently validated that our process is sound by printing a full scale PHX component that is completely water-proof at operating pumping pressure. Now, it’s just a question of repeatability and producing the prints to the scale we need. The PHX is without a doubt the most complex and interesting of our system’s parts, so optimizing it is a welcome challenge for 2015.
At the end of November we took a second trip to Mozambique, this time to scope out the nuts-and-bolts required to rebuild and deploy SunChill™. We took field temperature measurements at Mozambique Organicos—baby corn, mostly, a popular vegetable in South African supermarkets, go figure—wandered around a couple villages and towns looking for parts and ate a whole bunch of Matapa, a local food of boiled cassava leaves that is one of the best things going, food-wise, anywhere.
e took field temperature measurements to determine the transient conditions of the crops as they moved from the field to the industrial refrigerator, which is just a traditional air cooled room. The findings ended up confirming our initial intuition from simple heat transfer (you know, being thermal engineers and all): pre-cooling could hasten the removal of field temperatures from the crops, eventually reducing spoilage of each harvest, and lending credence to the idea that SunChill™ could have a potentially positive impact.