Automotive Thermoelectric Generator

The world had now focused more on how to reduce pollution, global warming, greenhouse effect. In order to find a solution for these, there is a lot of research and development had been going on in the field of renewable energy or alternative source of energy apart from conventional energy production from coal and natural gas. The main reason is to keep our natural resources safe and to reduce pollution by combustion of them. Here, the automotive industry also had a significant part in reducing pollution as most automotive vehicles are run by combustion of fuel. Now, there is a great revolution in the automotive industry with the introduction of electric vehicles, which are now becoming more popular as they do not harm the environment. We also had a part of the world where a lot of research is going in the field of Thermoelectric generator, which produces electricity from the waste heat generated by a machine.

What is a Thermoelectric Generator?

A thermoelectric generator produces electricity when a temperature gradient occurred between two dissimilar materials causing a hot and cold junction. There is a voltage developed called Seebeck voltage. The thermoelectric generator works on a principle called Seebeck Effect which is discovered by Thomas Seebeck in 1821.

These thermoelectric generators are solid-state devices integrated circuits which had made in keeping the number of thermoelectric modules in series to develop more voltage and connected parallel in order to increase the thermal conductivity. When a temperature gradient happened between the hot and cold sides of two materials, then there will be a flow of charge carriers which results in the production of electricity.

These thermoelectric generators are mainly affected by the Peltier effect, Thomson effect, and Seebeck effect. In 1834, Jean Charles Athanase Peltier discovered a reverse theory of the Seebeck effect that if a current passes from an external source through a circuit of two metals, it cools one junction and heats the other, known as the Peltier effect. These thermoelectric modules are thermocouples that are made up of p-type and n-type semiconductors. These are all connected in series by a metal conductor or strip.

In thermoelectric generators, when the hot side of the n-type and p-type semiconductor increases enough heat for the movement of the electrons in the n-type and holes in the p-type semiconductor will move to the other side of it where a metal strip exists. Now, there will be a creation of charge carriers which leads to the development of voltage, which varies along with the variation in temperature. In the next diagram, you can see how the semiconductors, metal strips, and where the movement of charge carriers occurs. 

In general, the materials used for semiconductors in thermoelectric generators are bismuth (Bi2Te3) telluride, lead telluride (PbTe), and Silicon germanium (SiGe). The main application of TEG will be space, automobile, medical, electronic industries, etc.

Now, we will discuss the role of TEG in the Automobile industry, where these are called Automotive Thermoelectric Generators (ATEG).

What is an ATEG?

ATEG is a thermoelectric generator that converts the waste heat generated by the automotive parts like the engine, exhaust, etc., into useful electricity, which reduces the load of the engine and increases fuel efficiency.

How will an ATEG be placed in an Automotive vehicle?

ATEG requires heat and cold temperature surface, so, in a vehicle, these are placed near the engine or exhaust where there will be an enormous amount of heat that gets wasted without any use. This will serve as the heating surface of the ATEG, and the cold surface will be a heat exchanger or sink, which will be the other side of a TEG. Below is a thermoelectric generator in BMW X6.

Why do we need an ATEG in a vehicle?

For an internal combustion engine, only 25% of the energy was useful for mechanical energy, and 40% was wasted through the exhaust gases from the total combustion of fuel. The heat emitted from the system will be as high as 100 degrees Celsius to 800 degrees Celsius. This heat energy is getting wasted and also reduces the efficiency of the internal combustion engine.

Research by the U.S Department of Energy on how much fuel can be saved if we use a vehicle with ATEG says that around 5% of fuel consumption can be saved, which is a very big number when seen from an industry point of view and below is the table from that research.

These values clearly show the impact of usage by an ATEG-equipped vehicle both in cost-benefit and a reduction in pollution, global warming, and emission of co2 by automobiles where, which decreases fuel combustion before.

Values are looking great!

So, what is stopping ATEGs from not used in the automobile industry?

Type of semiconductor material used for the TEG and each material is suitable for only a certain limit of temperature like Bi2Te3 for low-temperature range <250 °C, P-TAGS&N-PbTe for a medium temperature range of 250 °C – 5oo °C and skutterudite materials like P-CeFeRuSb12 & N-CoSb3 for high temperatures range of 500 °C – 700 °C. Sufficient space under the vehicle to mount the ATEG in a vehicle. Power density is also a concern for ATEG, where it has to achieve 10kW heat energy from the exhaust, given that most of the current system efficiency is less than 10%. Also, another concern where the car can produce heat at more than 1000oc during very high speeds, which is a very high temperature for the TE materials and finally leads to their damage.

Thermoelectric generators that are designed for the automobile industry are less reliable due to the continuous process of heat-to-energy conversion, where there are a lot of parts that get damaged in the long run.

Research on ATEGs:

1913 – L.E. HALE published a paper named Thermo Electric Battery, For Motor Vehicles, in which thermoelectric cells based on a thermocouple wire powers lighting and ignition systems, this is meant to eliminate the alternator in a vehicle.

1961 – 1963: Bauer and Tomarchio conducted research at Clarkson University, USA, using a Pb-Te-based thermoelectric material to recover the heat in automotive vehicles and gave the conclusion that speeds below 20mph require more sophisticated materials in order to replace the alternator.

1950 – Goldsmid and Ioffe both had brought development in the bismuth telluride-based thermoelectric material but these are adopted in automobile generators after 1987.

1987 – Hi-Z Technology, Inc, funded by the US Department of Energy, conducted research on which location of a vehicle would give more efficient output for TEG and concluded as Exhaust gases.

1988 – Birkholt U developed an ATEG and tested it on Porsche 944, which had a peak power of 58W at 800°C and FeSi2 was used as a thermoelectric material.

1994 – Hi-Z again conducted a test on Cummins NTC 325 & NTC 30 engines by installing a 1kW ATEG. They obtained the power of 1.68kW at 1700 rpm for a 300hp engine. For this, they used 72 TE modules based on bismuth telluride with 4.5% efficiency.

1998 – Nissan Motor for a gasoline engine using SiGe thermoelectric modules and obtained a power of 35.6W at 60km/h.

1999 – Nissan again conducted for two different engines and obtained 193 W output.

2004 – 2012: Under US, Department of Energy BMW, FORD, Genterm had collaborated for ATG research and installed ATEG in BMW X6, Lincoln MKT at the exhaust system for the US06 drive cycle. They found 1.2% fuel efficiency at 110kmph and the maximum peak power obtained was 605W by BMW X6 at 125kmph.

2008 – Renault and Volvo worked on a project named RENOTER to build an ATEG for Diesel and gasoline cars.

2011 – 2015: The US Department of Energy, with the collaboration of Genterm, BMW, and Tenneco with an aim to achieve 5% efficiency, ended up achieving 1.2& fuel efficiency for FORD F350 with installed ATEG. During this, there is an increase in co2 emission with installed ATEG in BMW X3. Whereas Tenneco achieved 2kW for an army vehicle using only 20% of heat wastage.

Below is the table of funding for ATEG in different countries from the US Department of Energy’s presentation. During all this research, companies had manufactured different types of ATEG like cylindrical, flat, exhaust, etc.; some are shown below

In India, Researchers at International Advanced Research Center for Powder Metallurgy and New Materials (ARCl) developed an ATEG which can generate a 200W output power for 350°C exhaust gases with acceptable backpressure in the heat exchanger, which is used for capturing more heat and the cold side is maintained by aluminum cold plates. It can also use for ship boat engines. Below is an image shows of ATEG.

Conclusion: TEG is used in the space industry for generating electricity by radioactive materials, which is very successful, whereas Automobile thermoelectric generators are facing issues in various parts for decades but the researchers are able to show a growth in the output from every passing year and also some researchers are creating simulation models to understand more about the system before going into practice. In the coming years, researchers are able to produce an efficient ATEG that can actually implement in-vehicle and can bring to production.