Automotive Thermoelectric Generators
Encyclopedia
An automotive thermoelectric generator (ATEG) is a device that converts waste heat
in an internal combustion engine
(IC) into electricity using the Seebeck Effect. A typical ATEG consists of four main elements: A hot-side heat exchanger
, a cold-side heat exchanger, thermoelectric materials, and a compression assembly system. ATEGs can be classified into two categories depending on their hot-side heat exchanger: exhaust-based and coolant-based. The exhaust-based ATEGs convert the waste heat from the exhaust in an IC engine into electricity. Alternately, coolant-based ATEGs use the engine coolant’s waste heat to generate electricity.
, while the heat exchangers are metal plates with high thermal conductivity
.
The temperature difference between the two surfaces of the thermoelectric module(s) generates electricity using the Seebeck Effect. When hot exhaust from the engine passes through an exhaust ATEG, the charge carriers
of the semiconductors within the generator diffuse from the hot-side heat exchanger to the cold-side exchanger. The build-up of charge carriers results in a net charge, producing an electrostatic potential
while the heat transfer drives a current. With exhaust temperatures of 700°C (~1300°F) or more, the temperature difference between exhaust gas on the hot side and coolant on the cold side is several hundred degrees. This temperature difference is capable of generating 500-750 W of electricity.
The compression assembly system aims to decrease the thermal contact resistance between the thermoelectric module and the heat exchanger surfaces. In coolant-based ATEGs, the cold side heat exchanger uses engine coolant as the cooling fluid, while in exhaust-based ATEGs, the cold-side heat exchanger uses ambient air as the cooling fluid.
and quantum well
technologies could increase efficiency up to 15% in the future.
The efficiency of an ATEG is governed by the thermoelectric conversion efficiency of the materials and the thermal efficiency of the two heat exchangers. The ATEG efficiency can be expressed as:
ζOV = ζCONV х ζHX х ρ
Where:
ζOV : The overall efficiency of the ATEG
ζCONV : Conversion efficiency of thermoelectric materials
ζHX: Efficiency of the heat exchangers
ρ : The ratio between the heat passed through thermoelectric materials to that passed from the hot side to the cold side
Decreased fuel consumption also results in increased fuel economy. Replacing the conventional electric generator with ATEGs could ultimately increase the fuel economy by up to 4%.
The ATEG’s ability to generate electricity without moving parts is an advantage over mechanical electric generators
alternatives.
Since the exhaust has to flow through the ATEG’s heat exchanger, kinetic energy from the gas is lost, causing the flow of the exhaust to decrease. This is referred to as back pressure
, which reduces the engine’s performance.
To make the ATEG’s efficiency more consistent, coolant is usually used on the cold-side heat exchanger rather than ambient air so that the temperature difference will be the same on both hot and cold days. This increases the radiator’s size since piping must be extended to the exhaust manifold. It also adds to the radiator’s load because there is more heat being transferred to the coolant.
ATEGs are made primarily of metal and, therefore, contribute a significant weight to the vehicle. An ATEG designed for small cars and trucks weighs about 125lbs, while for large trucks and SUVs, it can contribute up to 250lbs to the vehicle. The added weight causes the engine to work harder, resulting in lower gas mileage.
Cost is a prevalent issue in ATEGs. Thermoelectric generators with higher efficiencies require higher quality, more expensive thermoelectric materials. With the thermal cycling and vibration of the vehicle, the generator’s longevity is a concern. Although high quality thermoelectric materials could produce more electricity, the cost of replacing them could outweigh the savings in fuel economy.
In 1963, the first ATEG was built and reported by Neild et al.. In 1988, Birkholz et al. published the results of their work in collaboration with Porsche
. These results described an exhaust-based ATEG which integrated iron
-based thermoelectric materials between a carbon steel hot-side heat exchanger and an aluminium cold-side heat exchanger. This ATEG could produce tens of watts out of a Porsche 944
exhaust system.
In the early 1990s, Hi-Z Inc designed an ATEG which could produce 1 kW from a diesel truck exhaust system. The company in the following years introduced other designs for diesel trucks as well as military vehicles
In the late 1990s, Nissan Motors
published the results of its ATEG which utilized SiGe
thermoelectric materials. Nissan ATEG produced 35.6 W in testing conditions similar to running conditions of a 3.0 L
gasoline engine in hill-climb mode at 60.0 km/h.
Clarkson University
in collaboration has designed an ATEG for a GM
Sierra pick-up truck. The program was funded by the American DOE
and NYSERDA. The published literature of this ATEG explained its ability to produce 255 W at a vehicle speed of 70 mph.. In 2006, scientists in BSST and BMW
of North America announced their intention to launch the first commercial ATEG in 2013.
Waste heat
Waste heat sometimes called Secondary heat or Low-grade heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine, running on a source of high-temperature heat...
in an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
(IC) into electricity using the Seebeck Effect. A typical ATEG consists of four main elements: A hot-side heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
, a cold-side heat exchanger, thermoelectric materials, and a compression assembly system. ATEGs can be classified into two categories depending on their hot-side heat exchanger: exhaust-based and coolant-based. The exhaust-based ATEGs convert the waste heat from the exhaust in an IC engine into electricity. Alternately, coolant-based ATEGs use the engine coolant’s waste heat to generate electricity.
Operation principles
In ATEGs, thermoelectric materials are packed between the hot-side and the cold-side heat exchangers. The thermoelectric materials are made up of p-type and n-type semiconductorsP-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
, while the heat exchangers are metal plates with high thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
.
The temperature difference between the two surfaces of the thermoelectric module(s) generates electricity using the Seebeck Effect. When hot exhaust from the engine passes through an exhaust ATEG, the charge carriers
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
of the semiconductors within the generator diffuse from the hot-side heat exchanger to the cold-side exchanger. The build-up of charge carriers results in a net charge, producing an electrostatic potential
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
while the heat transfer drives a current. With exhaust temperatures of 700°C (~1300°F) or more, the temperature difference between exhaust gas on the hot side and coolant on the cold side is several hundred degrees. This temperature difference is capable of generating 500-750 W of electricity.
The compression assembly system aims to decrease the thermal contact resistance between the thermoelectric module and the heat exchanger surfaces. In coolant-based ATEGs, the cold side heat exchanger uses engine coolant as the cooling fluid, while in exhaust-based ATEGs, the cold-side heat exchanger uses ambient air as the cooling fluid.
Efficiency
Currently, ATEGs are about 5% efficient. However, advancements in thin-filmThin-film transistor
A thin-film transistor is a special kind of field-effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, since the primary application of TFTs is in liquid...
and quantum well
Quantum well
A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...
technologies could increase efficiency up to 15% in the future.
The efficiency of an ATEG is governed by the thermoelectric conversion efficiency of the materials and the thermal efficiency of the two heat exchangers. The ATEG efficiency can be expressed as:
ζOV = ζCONV х ζHX х ρ
Where:
ζOV : The overall efficiency of the ATEG
ζCONV : Conversion efficiency of thermoelectric materials
ζHX: Efficiency of the heat exchangers
ρ : The ratio between the heat passed through thermoelectric materials to that passed from the hot side to the cold side
Benefits
The primary goal of ATEGs is to reduce fuel consumption. Forty percent of an IC engine’s energy is lost through exhaust gas heat. By converting the lost heat into electricity, ATEGs decrease fuel consumption by reducing the electric generator load on the engine. ATEGs allow the automobile to generate electricity from the engine's thermal energy rather than using mechanical energy to power an electric generator. Since the electricity is generated from waste heat that would otherwise be released into the environment, the engine burns less fuel to power the vehicle's electrical components, such as the headlights. Therefore, the automobile releases fewer emissions.Decreased fuel consumption also results in increased fuel economy. Replacing the conventional electric generator with ATEGs could ultimately increase the fuel economy by up to 4%.
The ATEG’s ability to generate electricity without moving parts is an advantage over mechanical electric generators
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
alternatives.
Problems
The use of an ATEG presents new problems to consider:Since the exhaust has to flow through the ATEG’s heat exchanger, kinetic energy from the gas is lost, causing the flow of the exhaust to decrease. This is referred to as back pressure
Back pressure
Back pressure refers to the resistance to a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving, such as piping or air vents, against its direction of flow....
, which reduces the engine’s performance.
To make the ATEG’s efficiency more consistent, coolant is usually used on the cold-side heat exchanger rather than ambient air so that the temperature difference will be the same on both hot and cold days. This increases the radiator’s size since piping must be extended to the exhaust manifold. It also adds to the radiator’s load because there is more heat being transferred to the coolant.
ATEGs are made primarily of metal and, therefore, contribute a significant weight to the vehicle. An ATEG designed for small cars and trucks weighs about 125lbs, while for large trucks and SUVs, it can contribute up to 250lbs to the vehicle. The added weight causes the engine to work harder, resulting in lower gas mileage.
Cost is a prevalent issue in ATEGs. Thermoelectric generators with higher efficiencies require higher quality, more expensive thermoelectric materials. With the thermal cycling and vibration of the vehicle, the generator’s longevity is a concern. Although high quality thermoelectric materials could produce more electricity, the cost of replacing them could outweigh the savings in fuel economy.
History
Although the Seebeck effect was discovered in 1821, the use of thermoelectric power generators was restricted mainly to military and space applications until the second half of the twentieth century. This restriction was caused by the low conversion efficiency of thermoelectric materials at that time.In 1963, the first ATEG was built and reported by Neild et al.. In 1988, Birkholz et al. published the results of their work in collaboration with Porsche
Porsche
Porsche Automobil Holding SE, usually shortened to Porsche SE a Societas Europaea or European Public Company, is a German based holding company with investments in the automotive industry....
. These results described an exhaust-based ATEG which integrated iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
-based thermoelectric materials between a carbon steel hot-side heat exchanger and an aluminium cold-side heat exchanger. This ATEG could produce tens of watts out of a Porsche 944
Porsche 944
The Porsche 944 is a sports car built by Porsche from 1982 to 1991. It was built on the same platform as the 924, although 924 production continued through 1988. The 944 was intended to last into the 1990s, but major revisions planned for a 944 S3 model were eventually rolled into the 968 instead,...
exhaust system.
In the early 1990s, Hi-Z Inc designed an ATEG which could produce 1 kW from a diesel truck exhaust system. The company in the following years introduced other designs for diesel trucks as well as military vehicles
In the late 1990s, Nissan Motors
Nissan Motors
, usually shortened to Nissan , is a multinational automaker headquartered in Japan. It was a core member of the Nissan Group, but has become more independent after its restructuring under Carlos Ghosn ....
published the results of its ATEG which utilized SiGe
SiGe
SiGe , or silicon-germanium, is a general term for the alloy Si1−xGex which consists of any molar ratio of silicon and germanium. It is commonly used as a semiconductor material in integrated circuits for heterojunction bipolar transistors or as a strain-inducing layer for CMOS transistors...
thermoelectric materials. Nissan ATEG produced 35.6 W in testing conditions similar to running conditions of a 3.0 L
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
gasoline engine in hill-climb mode at 60.0 km/h.
Clarkson University
Clarkson University
-The Clarkson School:The Clarkson School, a special division of Clarkson University, was founded in 1978 as a unique educational opportunity. The School offers students an early entrance opportunity into college, replacing the typical senior year of high school with a year of college...
in collaboration has designed an ATEG for a GM
General Motors
General Motors Company , commonly known as GM, formerly incorporated as General Motors Corporation, is an American multinational automotive corporation headquartered in Detroit, Michigan and the world's second-largest automaker in 2010...
Sierra pick-up truck. The program was funded by the American DOE
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...
and NYSERDA. The published literature of this ATEG explained its ability to produce 255 W at a vehicle speed of 70 mph.. In 2006, scientists in BSST and BMW
BMW
Bayerische Motoren Werke AG is a German automobile, motorcycle and engine manufacturing company founded in 1916. It also owns and produces the Mini marque, and is the parent company of Rolls-Royce Motor Cars. BMW produces motorcycles under BMW Motorrad and Husqvarna brands...
of North America announced their intention to launch the first commercial ATEG in 2013.