Last fall we were very fortunate to get funding from the Lemelson Foundation to support our tech development and business incubation work. Lemelson is very interested in supporting new inventors in the US and developing countries and helping with the diffusion of technologies that improve peopleâ€™s lives. Their grant is allowing us to form partnerships with companies and university groups designing for the bottom of the pyramid. Just a few weeks ago one such group from the Michigan State University came to work with us in Guatemala on solar refrigeration. Eric Tingwall, a mechanical engineer and freelance writer, writes about the team’s experience.
Mechanical Engineering team from Michigan State University at AIDG Guatemala working on a solar refrigerator
With unreliable or nonexistent electricity infrastructures, rural regions of developing countries often have high vaccine spoilage rates. With no way to keep vaccines cold over an extended period, these medicines sometimes must be administered within 48 hours from the time theyâ€™re put into a vaccine carrier for delivery.
On March 8, six mechanical engineering seniors from Michigan State University arrived at the AIDG Guatemala workshop as part of their work to address this problem. Working with the Appropriate Technology Design Collaborative, our goal is to develop an affordable, robust refrigerator that uses passive solar energy to maintain temperatures from 2Â°C to 8Â°C. The end goal is a set of clear manufacturing plans that will be posted online in an open-source format, free for anyone to access.
Building the solar collector
Since we wouldnâ€™t have electricity to power our fridge, previous Michigan State teams directed us away from compression refrigeration and pointed us to adsorption refrigeration. The Internet is stacked with academic knowledge on solar adsorption refrigeration, but our team has the goal of developing a system and assembly instructions that can be used by anybody. Our adsorption refrigerator design uses passive solar energy rather than electricity and has no moving parts. But before the adsorption process can be understood, it is necessary to understand the basics of refrigeration. All fridges operate on the principal that a liquid boiling to a gas takes heat away from its surroundings. It is also important to know that a liquid can be made to boil at a very low temperature by altering the pressure. While water boils at 100Â°C at atmospheric pressure, it can be boiled at 0Â°C under a very high vacuum.
A refrigerator operates by allowing the refrigerant to boil at temperatures as low as the desired cold space temperature. Eventually, the evaporated refrigerant must be condensed back to a liquid to repeat the cycle for continued cooling. While a typical home refrigerator relies on an electric condenser to turn the gaseous refrigerant back to our liquid, our system uses chemical processes to drive the evaporation and condensation of the refrigerant.
CAD drawing of solar fridge design
Previous Michigan State teams working on this project directed us to use ethanol as a refrigerant, since it is almost universally available and nontoxic. While pure ethanol may be difficult to find in developing countries, liquor with high alcohol content can be used in its place. As our ethanol boils, it is adsorbed by activated charcoal, a porous solid that is often used in fish tank and drinking water filters to remove impurities. In our refrigerator, the activated carbon allows the system to maintain a constant pressure as the ethanol evaporates. When the sun heats our solar collector and activated charcoal bed in the middle of the day, the heat causes the release of the adsorbed ethanol. The raised pressure and condenser cooling allow the ethanol to return to a liquid state so it can be evaporated again to cool the refrigerator.
Our trip to Guatemala was intended to be a learning experience â€“ an opportunity for our team to understand what materials were available in developing areas and what challenges would slow manufacturing. The team faced issues with material availability, manufacturing capabilities and power outages, but was still able to complete the build in less than three days. Our final product uses a simple cooler fitted to a system of copper pipes and connected to a steel solar collector, held in a wood frame. With just one day to test the device, we recorded our lowest temperature at 1.3Â°C.
Back in East Lansing, Michigan, we still have several weeks of work to refine the design, complete testing and compile our final instructions for manufacturing. There is also significant room for this design to evolve, especially in regards to cost. Currently, the device costs in the range of $400 to $500 for one-unit manufacturing. One area for substantial cost reduction is the activated charcoal. While the team currently buys activated charcoal at a cost of $200 per fridge, it can be made simply by burning coconut shells in a controlled environment.
Michigan State University Team and AIDG crew
The solar adsorption refrigerator project is sponsored by the Appropriate Technology Design Collaborative with the financial support of the Lear Corporation and Chrysler Foundation. The project is part of the Michigan State University Mechanical Engineering senior capstone class. The spring 2009 team members are Nabeel Aslam, Kevin McPhail, Ryan McPhee, Brent Rowland and Eric Tingwall, guided by faculty advisor Dr. Craig W. Somerton.
– Eric Tingwall