For the most part, energy leaves automobiles unused in the form of waste heat through the exhaust pipe – part of it could be recovered using thermoelectric modules. The automotive industry is actively working on the new technology.
Thermoelectric generators convert heat directly into electric energy. What the technology could be capable of in traffic and what must be done to make it suitable for use in the vehicle is currently the subject of investigation and testing.
Thermoelectrics – a leftover gone stale? On the contrary! The term refers collectively to all phenomena in electrically conductive materials in which electric potentials or currents produce temperature differences or heat flows and vice versa. But what can these almost 200-year-old principles offer today? Systems that use these effects are capable of converting heat into electrical energy – i.e. generate electric current – and transport heat with electric energy – i.e. cool or heat. As such, they permit the effective use of all kinds of heat sources, such as the heat dissipated from fuel energy.
The two most familiar effects are:
- The electrical effect of thermal gradients discovered by German physicist Thomas J. Seebeck in 1821: "Temperature difference produces an electric voltage."
- The thermal effect of electric current discovered by French physicist Jean Peltier in 1834: Electric current creates a temperature difference."
Although both effects always occur together, each can be used on its own with extreme precision. Depending on whether a temperature difference or an electric voltage is applied to thermoelectric materials, the respective counterpart is produced: electric energy or heat / cold.
Material research is opening up new perspectives
Even though both effects have been known for a long time and are already being used in practice – the modest level of efficiency of thermoelectric systems stood in the way of wide-scale industrial application for decades. Yet since the end of the last century, things have started to move on the thermoelectric front through the efforts being made in material research. Against the backdrop of new general conditions for mobility and traffic, this technology has attracted the interest of the automotive industry.
Thermoelectrics can help to secure mobility. Thermoelectric generators (TEGs) using the Seebeck effect convert heat directly into electric energy. As they use dissipated heat instead of mechanical energy, they generate electricity more or less for free and cut fuel consumption, CO2 emission and pollution. As a result, they significantly improve energy efficiency, environmental compatibility and the economy of forthcoming automobile generations. As they work at even small temperature differences, adapt flexibly to operating conditions and temperatures, have no moving parts, are robust, maintenance-free and scalable, they can be employed in the vehicle for many different purposes: e.g. for supplying the vehicle electrical system.
What we need are thermoelectric modules that meet the requirements of automotive engineering. Although the main focus is on the cost and efficiency of components – material price, availability and toxicity, module manufacturability and recyclability are important aspects too. On top of this, specifications and designs optimized for the planned use are essential to their introduction.
In particular, application in exhaust systems – through which approximately half of the heat energy lost is dissipated unused into the environment while promising the highest yields owing to the high temperatures involved – places exacting demands on the system as a result of dynamic engine operation. The automotive industry is currently in the process of conducting investigations and tests to find out what the technology is capable of in traffic and what is necessary to make it suitable for use in the vehicle.
IAV has been involved since 2006. Initially to get to know the technology and the market as well as to acquire the expertise necessary for developing and integrating such systems. To assess the significance of thermoelectrics for the automotive industry, possible applications are being identified on the basis of suitable heat sources in the vehicle. The focus is on all conceivable applications: From supplying the vehicle electrical system to powering autonomous systems, such as sensors or devices for monitoring drive and vehicle components.
At present, attention is centered on the geometric, mechanical and thermal aspects of integrating TEGs in the vehicle. One of the greatest challenges lies in integrating them into the heat flow – this has a major influence on the overall system's effectiveness. Prototypes and prototype testing in the laboratory (on test benches and in the vehicle) ensure that all requirements are met, e.g. in relation to the repercussions on combustion engine and exhaust-gas aftertreatment as well as in terms of efficiency and operating reliability. Work will shortly begin on the task of integrating them in the vehicle from an electrical and functional point of view.
Back to the leftovers: IAV shows how to prevent coffee ending up in the cup lukewarm and stale with a thermos jug developed by Fraunhofer Technology Development Group. A TEG uses a very small part of the content's thermal energy to measure its temperature and filling level. The measurement readings are indicated on a display on the jug and transmitted by wireless signal to a receiver, such as a PC in the staff kitchen. This then tells you how long the coffee's been made. What more could you want?
The thermos jug is one example of the potential available for supplying energy autonomously from ambient and lost energy. The project marks the start of collaboration between IAV and the Fraunhofer Technology Development Group in the field of autonomous and wirelessly networked systems. The part they could play in the vehicle is to be demonstrated within the scope of future joint projects.