Electricity from waste heat

Direct conversion of thermal energy into electrical energy using a solid-state device

Author: Dr. Rolf Siegel, bionic surfaces, Würzburg (as of September 2021)

According to the German Federal Ministry for Economic Affairs and Energy (BMWi), approximately 200 terawatt hours (TWh) of thermal energy are generated annually as a result of waste heat, particularly in the metal and chemical industries and in the production of glass, ceramics and paper.

Only a fraction of this abundant form of energy has so far been used by means of combined heat and power (CHP). Due to the steam lines, turbines and generators required for this purpose, this type of energy conversion is indirect, costly and complex.

In terms of a direct conversion of thermal energy into electrical energy, the company - bionic surfaces -, Würzburg, Germany, has developed a technically simple solid-state device and has applied for a patent.

The energy converter resembles a plate capacitor, with all the materials used being abundant, non-toxic, thermally stable and precisely matched in terms of energy: For example, one plate is made of graphite, the other of magnesium, and instead of a dielectric, a semiconductor, for example iron(III) oxide, i.e. rust, contacts the plate surfaces.

The term "energetically matched" as well as the mode of operation of the energy converter presented here will be vividly described using the energetic positions of water: Sun warms the sea - clouds form, migrate and collide with a mountain range, it rains - rainwater collects in rivers and, using its kinetic energy by means of hydroelectric power plants, returns to the sea.

The analogy shows the different forms of energy: Low potential energy of water in the sea, high potential energy of water in mountains, and utilization of the energy difference by means of hydroelectric power plants.

Electrons as energy carriers

Instead of water, electrons are the energy carriers in the energy converter now described in more detail. Using the terms of solid-state physics, the analogy can then be translated as follows:

Sun heats the sea = electromagnetic radiation, e.g. heat (radiation), acts on a material with a large work function, e.g. a graphite plate,;
Clouds form, migrate and collide with a mountain range, it rains = high-energy, "hot" electrons leave the (graphite) material, enter the (energetically higher lying) conduction band of an n-type semiconductor and, if there is still enough ballistic energy, come into contact with an energetically lower lying material, for example a magnesium plate;
the rainwater collects in rivers and returns to the sea using its kinetic energy by means of hydroelectric power plants = graphite plate and magnesium plate are connected via current collectors and a consumer.

To date, the solid-state device for direct conversion of thermal energy into electrical energy has been developed to 'technology readiness level' TRL 5. Graphite is used as the low energy material (= sea), iron(III) oxide Fe2O3 (= clouds) is used as the n-type semiconductor material, and magnesium (= mountains) is used as the high energy material. If graphite of a 1 cm² device is heated to approx. 70°C, an open terminal voltage VOC of 1.3 volts can be tapped at a short-circuit current ISC of 1.2 mA. - Even at room temperature, a quartz clock can be operated, see figure.

The thermal stability of the materials used (melting point graphite 3,700°C, Fe2O3 1,560°C, magnesium 650°C) allows the solid-state device to be used for energy conversions in the temperature range between 30 and 500°C. As mentioned, the materials are non-toxic, abundant (even in Germany) and also inexpensive.