Thermionic and thermoelectric energy conversion

Even today, thermal-to-electrical energy conversion is mainly performed by thermomechanical engines activated by hot fluids, such as Stirling engines or turbines. Although such converters are characterized by a high efficiency (roughly >20% at temperatures >400°C), they have several limitations in terms of bulky size, reduced scalability (efficiency depends on size), and frequent maintenance of moving mechanical parts. Conversely, solid-state converters are preferable due to their small volume, up- and down-scalability (efficiency does not depend on size), and long lifetime, because of their operational static nature. Among them, thermionic and thermoelectric solid-state converters are receiving increasing attention from both scientific and industrial communities. In recent years, their applicability has extended from the long-standing sectors (nuclear and aerospace) to novel sectors (concentrated solar energy, energy recovery from energy storage, and metallurgical processes), boosting research and development activities on the related materials and concepts. In this chapter the main historical development and recent results, the physical principles and limitations, and the production strategies and innovative concepts will be described and discussed.