Digital Designs and Scientific Hardware - Open-Source Lab: How to Build Your Own Hardware and Reduce Research Costs - page 189

niques are prohibitively expensive for most applications. Fortunately, although the thermal
conductivity of polymers is generally orders of magnitude lower than metals, as long as the
polymer walls are made thin, the thermal resistance is negligible. So we made the walls thin
enough to enable high efficiencies to be possible using garbage bags. We measured an efect-
iveness of a prototype with 28-μm-thick black low-density polyethylene walls and counter-
low, water-to-water heat transfer in 2 mm channels. It was a respectable 72% and multiple
low-cost stages would provide the potential for very high effectiveness. This was simply awe-
some! We were able to get a garbage bag heat exchanger to outperform metal heat exchangers
on the market that cost hundreds of dollars. The potential for the technology is enormous
as heat exchangers are everywhere—refrigeration cycles, heat recovery, industrial processes,
vehicles, and conventional power plants. In addition, the potential to have low-cost heat ex-
changers opens up applications no one has ever considered because of costs, such as ultra-
low-cost appropriate technology for development [ 20 ]. In fact, David Denkenberger and I ori-
ginally became interested in heat exchanger design when we developed a simulation of a solar
water pasteurization system [ 28 ] . Our simulation showed that the lowest cost safe drinking
water could be made with a solar water pasteurizer if we could get the total system cost un-
der $25. We needed a good heat exchanger to make it work and those were expensive (e.g.
starting at $250). I vividly remember walking through a rough part of inner city Philadelphia
awkwardly carrying hundreds of dollars of conventional heavy-metal heat exchangers and
nervously hoping I was not mugged. I made it safely, and in retrospect, my worries were prob-
ably unfounded as there are not a lot of street thugs that would be able to put an accurate
value on heat exchangers. It was clear that any form of conventional heat exchanger design
was simply never going to work in the real world for this application, which demanded a rad-
ical approach to cost. The expanded polymer heat exchanger was just what we needed, we
Thad proved it could work, but there was one catch. The prototyping costs to make the heat ex-
changers despite the material costs being almost free—ran hundreds to thousands of dollars
for the necessary precision polymer laser welding. Enter Jansen's open design.
To develop a polymer laser welding system from scratch would have been extremely time-
consuming and difficult, but to build on free and open-source designs is much faster and in-
volves less effort. A group of my students including Rodrigo Periera Faria (from Brazil), Sai
Ravi Chandra Parasaram (from India) and from the U.S. Nick Anzalone and Thad Waterman
used Jansen's design as a starting point to develop an open-source polymer welding system as
shown in Figure 6.22 .