Automotive thermoelectric generator
An automotive thermoelectric generator (ATEG) is a device that converts some of the waste heat of 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 convert waste heat from an engine's coolant or exhaust into electricity. By reclaiming this otherwise lost energy, ATEGs decrease fuel consumed by the electric generator load on the engine. However, the cost of the unit and the extra fuel consumed due to its weight must be also considered.
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 semiconductors, 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.
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
- 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
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 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 increased pumping losses. 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 125 lb (57 kg), while for large trucks and SUVs, it can contribute up to 250 lb (110 kg) 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.
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. 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 testing its ATEG which utilized SiGe thermoelectric materials. Nissan ATEG produced 35.6 W in testing conditions similar to the running conditions of a 3.0 L gasoline engine in hill-climb mode at 60.0 km/h.
Clarkson University in collaboration with General Motors (GM) has designed an ATEG for a Sierra pick-up truck. The program was funded by the American Department of Energy (DOE) and New York State Energy Research and Development Authority (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, now the Advanced Technology division of Gentherm Incorporated and BMW of North America announced their intention to launch the first commercial ATEG in 2013. In January 2012, Car and Driver magazine named an ATEG created by a team led by Amerigon (now Gentherm Incorporated) one of the 10 "most promising" technologies. The Gentherm ATEG positions semiconductors between the exhaust stream and a cooled outer surface to produce electricity.
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