This is a project to develop a thermoelectric generator that can produce 5 volts for charging any USB-compatible device from mobile phones to GPS units. The project is currently in the design/prototype phase and I am working in partnership with Adam Millat (LinkedIn). This post gives some background on thermoelectric materials, our motivation for the project, and showcases the two versions we have designed thus far.
Graduate school has offered me many interesting opportunities through coursework, seminar and research. One technology I repeatedly encounter is that of thermoelectric materials also known, strangely, as sunless solar cells. I find this nomenclature strange because the Seebeck, Thomson, and Peltier effects have been understood for quite some time (roughly 150 years or so) and their combined thermoelectric effects are very well understood.
In fact, people have been trying to exploit them for quite some time (warning: PDF doc). However, truly both technologies work along the same lines: a junction is created between dissimilar materials (one p-type and the other n-type) which, in the simplest possible terms, absorb radiation energy to move either electrons or so-called “holes”. There are important differences in each technology which are of interest (such as band gaps, useful wavelengths, and inherent inefficiencies) but these go beyond the scope of this design post. Perhaps an interesting topic for a future blog post, though.
Since my research in the areas of fluid mechanics and heat transfer is focused on small alternative energy systems, I have been learning quite a bit about the potential and limitations of thermoelectric materials. The quick facts are that they are dropping in cost, improving in efficiency, and can make a useful amount of power in a variety of situations. The hard truth is that it all comes down to maintaining a temperature difference (dT) across a thickness of only a few millimeters. This can be extremely challenging to accomplish and must be addressed through design. Furthermore, because of the low efficiency of many thermoelectric materials waste heat sources are the best for use.
Applications for the Technology
The Great Outdoors
Modern technology has begun to extend to the great outdoors in a big way. My experiences growing up and flying in rural Alaska have yielded a long list of uses for a small, portable power system. A well-designed thermoelectric generator (TEG) may fit the bill.
Modern technology has begun to extend to the great outdoors in a big way. My experiences growing up and flying in rural Alaska have yielded a long list of uses for a small, portable power system. A well-designed thermoelectric generator (TEG) may fit the bill.
Pilots, FV captains, sport fishermen, hunters, hikers, campers, and more are carrying technological tools with them into the wild. These include safety technology such as GPS devices and SPOT messengers. However, in more connected parts of the world people are even carrying cellular phones on wilderness outings. However, the weak point of all these devices is battery life. During extended outings, or unexpected survival situations, we may overly rely on our technology and drain the batteries when we need them most. A simple solution to this is, of course, to just carry extra batteries. However, that is not a long term sustainable solution and cold weather, moisture, or rough handling could destroy those backups as well. A more sensible solution would be to have an on-demand charging solution that is small, portable, robust and easy to use.
Photovoltaic (PV) charging solutions have proven popular in this regard; however there are two immediate shortcomings with that technology:
- PV panels tend to be fragile. They are susceptible to dust, mud, and can break easily.
- PV panels require the sun. They function best when set at an exact angle to the sun, something that is not always easily accomplished.
A small TEG, on the other hand, is robust and requires only a temperature gradient. Whereas PV’s require the sun, a TEG can utilize any number of different heat sources. For example, a fire, a running engine, or a camp stove. Even the sun could be used, allowing TEG’s to exceed the PV in potential usefulness.
The Developing World
Many of these applications also translate well to the developing world. While there are many sustainable solutions being investigated for use in the developing world, I believe that TEG’s should warrant a close look. They are simple, passive, and can be designed to be intuitive to use. I believe that for a technology to gain a foothold it must not be “gimmicky” or require the end user to build new habits or routines around it. I go into this concept more in-depth in my blog post on the possible role of TEG’s in the developing world. As we move forward with prototyping and testing our TEG, we will also be studying the developing world for unique patterns, routines, and habits that could allow a TEG to provide mobile technology charging.
Our Mobile Western World
The concept of a mobile charger that can top up your phone is extremely inviting. There is a market for these devices which is easily evidenced by running a web search for “mobile phone solar charger.” These devices can run anywhere between $20 and $100. The allure lies in being able to charge your phone anytime you are on the go outdoors, but it also lies in our desire to have neat, compact gadgets with a ‘wow’ factor. A small TEG that can work with the sun, off of an idling delivery truck, or the hood of a car could appeal to a wide-range of people.
These uses, along with the lack of an innovative solution on the market, inspired Adam and I to explore this technology more in-depth in the form of device design.
Version 1 Design: The Square Stamp
The Developing World
Many of these applications also translate well to the developing world. While there are many sustainable solutions being investigated for use in the developing world, I believe that TEG’s should warrant a close look. They are simple, passive, and can be designed to be intuitive to use. I believe that for a technology to gain a foothold it must not be “gimmicky” or require the end user to build new habits or routines around it. I go into this concept more in-depth in my blog post on the possible role of TEG’s in the developing world. As we move forward with prototyping and testing our TEG, we will also be studying the developing world for unique patterns, routines, and habits that could allow a TEG to provide mobile technology charging.
Our Mobile Western World
The concept of a mobile charger that can top up your phone is extremely inviting. There is a market for these devices which is easily evidenced by running a web search for “mobile phone solar charger.” These devices can run anywhere between $20 and $100. The allure lies in being able to charge your phone anytime you are on the go outdoors, but it also lies in our desire to have neat, compact gadgets with a ‘wow’ factor. A small TEG that can work with the sun, off of an idling delivery truck, or the hood of a car could appeal to a wide-range of people.
These uses, along with the lack of an innovative solution on the market, inspired Adam and I to explore this technology more in-depth in the form of device design.
Version 1 Design: The Square Stamp
The first design of our device focused, like many of my projects, on modularity; the ability for the device to expand and adapt without having to replace the whole unit.
On the front half, which is the side that attaches to the heat source, a dark aluminum plate fits into the recess. The dark color increases the absorptive properties when used to collect solar radiation. This plate sits flush with the surrounding plastic frame. The eight holes in the aluminum collector plate go through the device and connect the hot plate with the fins on the back side. It was created this way, because it was envisioned that the device could have the plate and fins easily switch out for other accessories.
The four holes in the corners house screws that hold the two plastic halves together. On the hot side, they are recessed and covered with neodymium disc magnets. These magnets allow the generator to “stick” to any hot metal surface for charging.
The geometry of the plate on the cold side is the same as the collector plate on the front. However, the plate is recessed on the rear and slotted to allow fins to mount in it. The fins were designed in this way to allow them to be replaced with improved shapes, sizes, or materials while retaining the original back plate. Additionally, we believe that the fins could be made with less material this way which saves resources. Finally, if a fin is damaged it can be replaced individually, reducing costs for the consumer.
The assembled result is shown above. While we plan on prototyping this design, there are a number of short comings with this initial effort. First, there is no way for the user to safely interact with the unit to remove it from the hot surface. This is not a huge problem when mounted on the hood of a warm car, but it is a problem if mounted to a hot wood stove.
Furthermore, proper cooling is a major concern for these devices. We believe that a set of heat sink fins will have trouble transporting enough heat out of the device. Even with the plastic frame serving as a buffer between the hot surface and the cool fins, natural convection will probably be insufficient.
However, undeterred we regrouped on a second design.
Furthermore, proper cooling is a major concern for these devices. We believe that a set of heat sink fins will have trouble transporting enough heat out of the device. Even with the plastic frame serving as a buffer between the hot surface and the cool fins, natural convection will probably be insufficient.
However, undeterred we regrouped on a second design.
Version 2 Design
Pictured above is our second design. It is based around effective cooling of the device and safe user interaction. At the right is the hot side. It again has magnets for mounting to ferrous surfaces such as warm cars, woodstoves, etc. The diameter of the hot side assembly is only 3 inches. This part of the new design is very similar to the v1 design. It is two identical plastic halves that house a TE device. The front plate is still dark metal to gather indirect heat energy from the sun.
The difference in this design is in the cooling system. Due to this system being the main differentiating factor, I won’t go into details of its design or function. This system is housed in the smaller diameter “barrel with fins” attached to the back of the TE unit. The fins here are smaller than before, but spaced wider to improve airflow. They are constructed out of aluminum and due to their smaller profile they can be economically machined from a solid billet to reduce waste. The diameter of this cooling system is 2 inches.
The housing of both the cooling barrel and the TE unit is white plastic to reduce radiated heat absorbance and provide a thermal break between the hot and cold sides of the unit. Additionally, the plastic on the cooling barrel serves as a convenient handling point for the user. The device can be easily and ergonomically applied and removed from surfaces here with no danger of being burned.
Wrap Up
Adam and I are very excited to be working on this fun side project over the summer. We are putting a lot of effort into having a working prototype by the start of the new school year at the end of September. I’ll keep this post updated with photos and videos as the project progresses.
