Heat is a by-product of all energy conversion processes, whether it is converting fuels into energy or energy into work. This loss of energy in waste heat costing industries billions of dollars every year (ENER-G-ROTORS, 2016). Increasing Global populations, Climate change, dwindling supplies of fossil fuels and increased reliance on electrical power are factors driving the increased need to implement renewable and clean energy. Many household appliances/systems produce waste heat which can be reclaimed and used for practical purposes.
A Thermoelectric Generator (TEG) can be used as a portable power generator which harnesses waste heat (from appliances such as wood/pellet stoves) and produce electrical energy which can be stored in a Lithium Battery. This system could be used as a portable power source for charging a mobile phone or similar, lithium battery operated device.

Step 1: Parts & Tools

Parts

Heatsinks x 2 (Copper) (can be recovered from old/broken computer systems and other machinery.)
TEG Module
Electric Fan (can also be recovered from electrical appliances or computers).
DC-DC Converter (MT3608)
Lithium Charger/protector
Lithium Battery (18650)
Battery holder (18650)
1N4004 or similar Diode
Thermal Conductive paste
Heat resistant insulation tube
Screws and other

Tools

Screwdriver
Soldering iron
Multimeter
Heat Gun (for testing)
Etc

Step 2: Technical Information & Working Principle

TEG

A TEG converts waste heat directly into electrical energy using the Seebeck effect. One side of the TEG is exposed to waste heat whilst the other side is cooled creating a temperature difference. Heat energises the semiconductors which in their turn create a positive and negative electron flow (respectively P- & N- channel).

The generator consists of two heat sinks; the first conducts heat to one side of the TEG Module, the second heat sink, assisted by a fan, removes heat cooling the other side of the module and maintaining a heat differential. The varying current and voltage produced are converted to a stable voltage and then stored in a battery.

Buck converter

To convert the low voltage outputted by the TEG to a stable useable voltage (5v), we use the MT3608. This is a constant frequency current mode step-up converter available from various eastern websites.

Lithium battery and charger

We use a lithium-ion battery and lithium battery charger board to store the generated energy. We had both laying around, bit if you do not have them in your drawer, they are easy to acquire from your favourite website.

Step 3: Preparation of Parts

We need to assemble all parts step by step to prevent releasing the magic blue smoke!

TEG

The first thing we need to do is check what the cold and what the hot side of the TEG is. If your supplier did not give you information about this, it is very easy to find out yourself. Just apply a bit of heat on one side (with your heat gun) and at the same time measure the voltage coming out of the TEG with your multimeter.

Now that you know in what direction the heat is going, you can start mounting the TEG between the two heatsinks. Don't forget to put a little bit of heat paste in between, and tighten up the TEG equally on each side.

Now test it again with your multimeter and the heat gun to check if it is still operational.

Lithium charger

Now let's connect all the parts for the battery charger. Connect the battery holder to the appropriate connections on the battery charger board. DON'T PLUG IN THE BATTERY YET while soldering everything!

Step 4: Assembly

The next step is connecting the DC-DC converter to the TEG. Before we are going to do that, we need to set the voltage on the DC-DC converter by adjusting the trim pot. Connect the converter board to a stable power supply between 2-5v and adjust the pot while at the same time measuring the voltage at the output with your multimeter.

If the voltage is set you can solder it between the TEG and the fan. To test it out, just heat up the hot side of your build. The fan should spin after <2 minutes.

Step 5: Last Step!

In the last step, we solder the battery charger to the 5v output of the DC-DC converter. Place the battery in the holder and heat up the TEG. After a while, a red LED should light up meaning that the battery is charging!

Step 6: Improvements & Future Uses

Practical applications of Thermoelectric generators are limited by the inefficiency of the modules and the cost of the materials used. The power (Watts) produced by the TEG is dependant on:

The amount of heat flux which can move through the module, The temperature difference between the hot and cold sides. (Tecteg, 2016)

High-efficiency Thermoelectric (TE) materials need to be good electrical conductors and poor thermal conductors in order to maintain a high heat differential and eliminate the backflow of heat. Because of conflicting material characteristics, researchers have been unable to produce a thermoelectric material with high enough efficiencies for many practical applications (WÜSTENHAGEN, 2016).

Improvements in nanotechnologies will allow development of more effective materials. Eventually, these will be produced at a competitive cost, making the TEG modules of the future more efficient over a range of high and low temperatures and cost effective. Low-temperature waste-heat technologies “really are where the industry is going,” Mark Taylor, an analyst at research firm New Energy Finance (Kho, 2016).

Future applications of TEG include;

Automotive:

Regulatory authorities are ensuring future automobiles must have improved fuel efficiency. Using TEGs to convert waste heat from exhausts into useable energy can improves the mileage of cars and can reduce greenhouse gas emissions. Consequently, nearly all manufacturers of automobiles are working to incorporate TEGs within their vehicles within the next two years (Evident Thermoelectrics, 2016).

Industrial Waste Heat Recovery:

Waste heat recovery units are being used internationally however they require high, regular temperatures. TEGs are smaller, solid-state, thermally, chemically and structurally stable and are well suited to medium and high-temperature ranges (Evident Thermoelectrics, 2016).

Wearables:

Wearable technologies are increasing in demand but are restricted by size and battery life. With improved TE materials TEGs will be able to sufficiently convert body temperature into useable electrical power (Evident Thermoelectrics, 2016).

Consumer Products:

Converting waste heat energy, from appliances within the home, and storing the electrical energy will increase the efficiency within the home and cut electricity costs. Energy stored can also be used as a back-up in case of power cuts/shortage. This has particular implication in areas affected by natural occurrences such as earthquakes or flooding which cause power cuts (Evident Thermoelectrics, 2016).

Self-Powered Sensors:

Sensors, such in machinery, aeroplanes, medicine and robots often have to be located where it is not possible to be wired to an electricity source or battery. TEGs can be used to convert low temperature into electrical energy which can be used directly by the device (Evident Thermoelectrics, 2016).

Step 7: References

ENER-G-ROTORS,. (2016). Waste Heat. Ener-g-rotors.com. Retrieved 20 December 2016, from http://www.ener-g-rotors.com/waste-heat/

Evident Thermoelectrics,. (2016). Nanoscale Materials - Evident Thermoelectrics. Evidentthermo.com. Retrieved 22 December 2016, from http://evidentthermo.com/

Kho, J. (2016). Electricity from Waste Heat. MIT Technology Review. Retrieved 21 December 2016, from https://www.technologyreview.com/s/411230/electricity-from-waste-heat/

MPowerUK,. (2016). Battery and Energy Technologies - Stirling Engine. Electropedia. Retrieved 21 December 2016, from http://www.mpoweruk.com/stirling_engine.htm

Tecteg,. (2016). How Thermoelectric TEG Generators Work - Tecteg Power Generator.com. Tecteg Power Generator.com. Retrieved 20 December 2016, from http://tecteg.com/how-thermoelectric-teg-generators-work/

WÜSTENHAGEN, V. (2016). The Promise and Problems of Thermoelectric Generators (1st ed., pp. 1-2). O-flexx. Retrieved from http://www.o-flexx.com/fileadmin/media/documents/chip_silver_edition_-_The_promise_and_problems_of_thermoelectric_generators.pdf