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Low Temperature Differential Stirling Engine

posted Nov 16, 2012, 6:08 PM by Sean M. Messenger   [ updated Dec 30, 2013, 5:39 PM ]
I had the distinct privilege of interning at Breedt Production Tooling & Design in the summer of 2012. Working at Breedt gave me intensive, hands-on experience in a professional workplace that many of my peers would envy. I worked alongside and learned from experienced engineers. 

I first heard of Breedt while talking to Brant DenHerder, a student that was on my FIRST Robotics team, at our club year-end party late June. He mentioned the great opportunity he had, how he got in contact with the owner, Mr. Andries Breedt, and how he was learning so much by doing an individual project. I did not have an internship for the summer, and Brant suggested that I may be able to join him -- in fact, I talked with Andries and started working at Breedt July 17, 2012 at 8:00 in the morning. Reflecting on that summer, I could not have imagined anything being a better use of my time.

Like Brant, I started work on my own design and personal project. After a few days talking with Brant, Andries, and other engineers, I decided to take a shot at making low temperature differential Stirling engine. The engine uses temperature differences between two surfaces to drive air in an enclosed cylinder. This convection in turn pushes a plunger that is connected to a flywheel. One up and down motion of the plunger translates into a full rotation in the flywheel. In effect, small temperature differences between the top and bottom of this cylinder can be used to spin, and keep spinning, a flywheel. This is nearly a perpetual motion machine -- except for the fact that it relies on some amount of heat from, say, a coffee cup, bag of ice (relatively making the opposing side "hot"), or sunlight exposure on one surface.

Over the next few weeks, I worked on researching the principles behind the engine, working on a design, and then fabricating the end product. The research turned up some interesting concept designs. According to Wikipedia, there are a few variations on how a Stirling engine uses heat differences -- the Alpha variant uses both hot and cool cylinders to act on the wheel at the same time from different locations, whereas the Beta type combines everything in one cylinder. 

                                     Beta Type
             Alpha Type                                              Beta Type

The low temperature differential version is most efficiently built using the Beta Type design, where the air flows between the top and bottom portion of the air-tight cylinder -- the top being between the two pistons, and the bottom being below both. We will call the top piston the power piston and the bottom the displacer piston. The heat difference drives the air, forcing the pistons to move. When the air is distributed with the hot air at the top, the momentum of the flywheel drives the pistons back to continue circulation. This process repeats, drawing energy from the temperature difference to drive the flywheel. Some designs have been able to work with under 3 degrees Fahrenheit temperature difference.

With this information, I went through an iterative design process to get an initial, full render as shown below. This was rendered on July 23, just about a week after I first started. Some parts are stand-ins, the screws are missing, but the holistic design is there. The concept is to have the large, flat cylinder at the bottom be representative of the Beta Type design, the main displacer piston and shaft in the middle while the power piston is behind the flywheel mount. The second render was done on August 20, right at the end of the entire project and after I had done a lot of the manufacturing. 
July 23 LTDS Render           August 20 LTDS Render
               23 July 2012                                                     20 August 2012

The first thing I made was the displacer cylinder that goes between the two plates. The shortest length I could find that was 6" in diameter was one foot from McMaster. That is pictured below on the right with a support cylinder inside of it (black UHMW material). In order to cut the plastic displacer cylinder with a very low tolerance, I needed a way to support the shape and keep in from deforming. That black UHMW piece was cut to fit inside the displacer cylinder so I could put the cylinder on the lathe and make a clean cut.

Support Cylinder  Support Cylinder

I found a scrap piece of white UHMW material that was pretty much the perfect size and shape to serve as the flywheel tower. I grabbed it, sanded some of the edges, drilled a hole a hair over 11/16", press fit two bearings in, and sanded it some more.

Sanding Flywheel Tower

There was some extensive machining on a manual Tree mill based on drawings from the design. Pictured below are the setup and workspace for machining the counterweight and displacer angle pivot. 

Workspace and Counterweight Angle Pivot Workspace
Counterweight Layout Angle Pivot Layout
                      Counterweight                                                        Angle Pivot

Finally, there are the two 1/8" thick aluminum plates that serve as the heat surfaces. Aluminum has a very high thermal conductivity, meaning heat transfers very easily through it -- say from steam on the bottom to the inner surface that heats the air inside the chamber. The displacer cylinder was intentionally made of plastic, a material with low thermal conductivity, and using plastic standoffs, in order to prevent heat from transferring from one plate to another. Everything on the plates except the countersinks and the circular slot were cut using a waterjet (which I operated). The hole sizes are not critical, as screws are threaded into the standoffs and not the plate itself, so the waterjet was perfectly accurate for our purposes. The plates were clamped down to the mill table, I used an indicator to set zero, and then drilled the slot with a circle command and power piston recession using a pocket command. The countersinks were all aligned by eye, and then the plates were sanded and buffed to a smooth, scratch-free surface finish.  

Plate Sanding Plate View

After putting the time in to clean it up, I decided to look into anodizing parts of the machine -- both Brant and the company, Breedt, had things they wanted anodized too, so I threw a few pieces in with the batch. Unfortunately, time was running out and I had to leave the pieces behind to get anodized as I returned to school for the start of my sophomore year. The final setup is shown below.

Final            Stirling Assembled

Over the course of this project, I had the opportunity to attend a CAMPS (Center for Advanced Manufacturing Puget Sound) meeting and briefly talk about my experience. I also went to a Boeing panel discussion at the Boeing 737 Plant in Renton on August 21. The Panel showcased a new curriculum, Core +,  to interested CTE teachers from 18 school districts. I explained my experience at Breedt, the opportunity I had, and how much I learned in such a short amount of time. Brant and I co-presented our experience from a PowerPoint presentation and got fantastic feedback and lots of interest from the teachers.

There were many challenges along the way, and because of my experience at Breedt, I have a hands-on background in machining, the design process, and how things become much more challenging than you may anticipate at the outset. I will remember this experience for the rest of my life, and know how lucky I was to have such a great individual, group of individuals, and company look after and help me along. It is this kind of experience that really helps one develop a passion for design, engineering, and manufacturing through direct exposure. This is something that is lacking in the modern education system -- yes, we learn about the principle, learn the design concepts, and get some experience working with tools (if you're lucky). But nothing like this. Working at Breedt showed me another perspective of my education, something that I could not have learned in a classroom setting and something more valuable, in many ways, than a college degree.

Started: 17 July 2012
Finished: 31 August 2012