Light From Heat Energy for Under $5

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We are two industrial design students in the Netherlands, and this is a quick technology exploration as a part of the Technology for Concept Design sub-course. As an industrial designer, it is useful to be able to methodically analyze technologies and gain a deeper understanding of them to make a well-substantiated decision for the implementation of a specific technologies in concepts.

In the case of this instructable, we are interested to see how efficient and low-cost TEG modules can be, and if they are a viable option for recharging outdoor accessories like power banks or flashlights with, for instance, a campfire. Contrary to battery power, heat energy by fire is something we can make anywhere in the wilderness.

Practical application

We were investigating the use of TEGs for the charging of batteries and powering of LED lights. We envision the use of TEG modules to, for example, charge a flashlight at the campfire so that it can be independent from grid energy.

Our investigation focuses on low-cost solutions we found on Chinese online retailers. At the moment it is difficult to recommend TEG modules in such a practical application as they simply have too little power output. Although there are highly efficient TEG modules on the market today, their price does not really make them an option for small consumer products like a flashlight.

Step 1: Parts and Tools

Parts

-Thermoelectric Module (TEG) 40x40mm (SP1848 27145 SA) https://www.banggood.com/40x40mm-Thermoelectric-Power-Generator-Peltier-Module-TEG-High-Temperature-150-Degree-p-1005052.html?rmmds=search&cur_warehouse=CN

-Tealights

-Breadboard

-Red LED

-Some wires

-Heatsink plaster/ thermal paste

-Scrap metal/heat sink (aluminium)

Tools

-Thermometer of some sort

-Soldering iron

-(digital) Multimeter

-Lighter

-Small Vise (or other object that allows you to put tealights under it)

Step 2: Working Principle & Hypothesis

How does it work?

Simply stated, a TEG (thermoelectric generator) converts heat into an electrical output. One side has to be heated and the other side has to be cooled (in our case the side with text has to be cooled). The temperature difference across the top and bottom sides will cause the electrons in both plates to have different levels of energy (a potential difference), which in turn creates an electrical current. This phenomenon is described by the Seebeck effect. It also means that when the temperatures on both sides become equal, there will be no electrical current.

As mentioned thermoelectric generators have been chosen to explore. We are using a SP1848-27145 type with a cost of under three euros per unit (including shipping). We are aware that there are more expensive and efficient solutions on the market, but we were interested in the potential of these ‘cheap’ TEGs.

Hypothesis

The website that sold the TEG modules had, what felt like, bold claims for the efficiency for converting electrical energy. We will take a small detour later on exploring these claims.

Step 3: Preparation and Assembly

Step 1: A simple heatsink was made by using scrap aluminium parts found in the workshop, these were attached to the TEG module by using thermal paste. However, other metals such as copper, brass, or messing will also work sufficiently for this setup.

Step 2: The next step involves soldering the first TEG’s negative lead to the second TEG’s positive lead, this ensures that the electrical current will be in series (meaning that the output of the two TEGs will be added up). With our setup, we were only available to generate about 1.1 volt per TEG. This means that in order to reach the 1.8 volts needed to light a red LED, a second TEG was added.

Step 3: Connect the red (positive) wire of the first TEG and the black (negative) wire of the second TEG to the breadboard in its respective places.

Step 4: Place a red LED on the breadboard (remember: the longer leg is the positive side).

Step 5: The last step is simple*, light the candles and place the TEG modules on top of the flame. You want to use something sturdy to put the TEGs on top of. This keeps them out of direct contact with the flame, in this case a vise was used.

Because this is a simple test, we have not spent much time to make proper enclosures or cooling. In order to ensure consistent results, we have made sure the TEG was positioned equal distance from the tealight for testing.

*When trying to repeat the experiment, it is advised to place the TEGs with heatsink in a fridge or freezer in order to cool them down. Make sure to remove them from the breadboard before doing so.

Step 4: Setup

Initial testing

Our initial test was quick and dirty. We placed the TEG module over a tea light and cooled the ‘cold end’ of the TEG using the aluminium enclosure of a tea light and an ice cube. Our thermometer (left) was placed in a small clamp (top right) in order to measure the temperature of the top of the TEG.

Iterations for final test

For our final test, we made several changes to the setup to ensure a more reliable result. Firstly we changed the ice cold water for a passive cooling by using a larger block of aluminium, this reflects the potential implementation more closely. Also a second TEG was added in order to achieve the desired result, which was to light the red LED.

Step 5: Results

Using the setup described will light a red LED!

How powerful is one TEG?

The manufacturer claims that the TEG can produce an open circuit voltage of up to 4.8V at a current of 669mA when subjected to a 100 degree temperature difference. Using the power formula P = I * V, it is calculated that this would be roughly 3.2 watt.

We set out to see how close we could come to these claims. Measuring around 250 degrees celsius at the bottom of the TEG and close to 100 degrees on the top end, the experiment shows quite a difference compared to the manufacturer's claims. The voltage stagnates around 0.9 volt and 150 mA, which is equal to 0.135 watt.

Step 6: Discussion

Our experiment gives us a good impression of the potential of these TEGs, as we can fairly say that their output is decent for a bit of fun and experimentation, but that the physics involved to properly cool these systems and generate a steady source of energy is far from feasible for a real-world implementation, when compared to other possible off-grid solutions like solar power.

There is definitely a place for TEGs, and the idea of using a campfire to power a flashlight does seem achievable; we are just severely limited due to the laws of thermodynamics. Because a temperature difference needs to be achieved, one side of the TEG needs (active) cooling and the other needs a constant heat source. The latter is not an issue in the case of a campfire, however the cooling needs to be so efficient that an active cooling solution will be needed and this is difficult to achieve. When considering the volume needed to make these solutions work, compared to existing battery technology, it is far more logical to choose a battery to power lights.

Improvements

For future experiments, it would be advised to acquire proper heatsinks (from a broken computer for instance) and apply them on both the hot and cool side of the TEG. This allows the heat to be more properly distributed and will make the waste heat on the cool side dissipate easier than a solid block of aluminium

Future applications of this technology
At the moment TEGs are primarily found in (environmentally friendly) technical products as a means of harnessing waste heat for energy. In the future this technology has the potential for much more. One interesting direction for the design of lighting products is that of wearables. Harnessing body heat could lead to battery-free lights that are easily mountable in clothing or on the body. This technology could also be applied in self powering sensors to allow for fitness monitoring products in more versatile packages than ever before. (Evident Thermoelectrics, 2016).

Step 7: Conclusion

In conclusion, as promising as the technology seems, the system requires an active cooling and a constant heat source to insure an even flow of electrical charge (in our case, sustained light). While our setup allowed for rapid cooling of the heatsinks using a fridge, this experiment would have been quite difficult to reproduce without any external electricity; the light would have been dead by the time the positive and negative sides reach the same temperature. While the technology is not very applicable at the moment, it's interesting to see where it will go considering the constant stream of new and innovative technologies and materials.

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    5 Discussions

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    gm280

    11 months ago

    I've said it before and will say it again, if you ever find a good way to convert heat to electricity, our entire Earth energy problems will be forever solved. Just think how many BTUs the Sun supplies to the Earth every day to warm it at least 30 degrees each and every day. Imagine the power we could generate. I say you guys need to keep studying and who knows, you may come up with something great one day. You never know.

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    Swansong

    11 months ago

    This would make a neat project to do for a science class :)

    1 reply
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    fim.ruttenSwansong

    Reply 11 months ago

    Absolutely, cheap and a simple way to get to grips with the scientific principles of TEGs!

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    hubi

    11 months ago

    Try to stack 2 TECs one over the other with thermal paste. Use an old Processor Heatsink for dissipation on the cool side, I have tried this way and cold light a 1W white LED with this and one tealight candle.

    1 reply
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    fim.ruttenhubi

    Reply 11 months ago

    Thats an interesting tip. Thanks! As the ceramic in these TEGs makes it difficult to maintain a nice temperature difference it would be interesting expriment stacking them for sure!