Operating conditions and thermodynamic bounds of dual radiative heat engines

We propose a unified description of dual radiative heat engines (RHEs), consisting of two facing optoelectronic components (diodes) and capable of generating electrical power from heat. They can operate in three regimes depending on the applied biases, namely in thermoradiative-negative electroluminescent (TRNEL), thermoradiative-photovoltaic, or thermophotonic regimes (TPX, consisting of a light-emitting diode and a photovoltaic cell). They have access to operating conditions that are unachievable by single RHEs such as thermophotovoltaic systems: at the radiative limit, TRNEL devices can reach the Carnot efficiency for any bandgap, while TPX devices achieve large power outputs by means of electroluminescent enhancement. The influence of nonradiative losses is also investigated, revealing the importance for dual engines to operate close to the radiative limit to clearly outperform single engines. Expressions of the maximum power output and related efficiency achieved by dual engines at the radiative limit are derived analytically, and reveal that the power output of TPX engines is not bounded. A comparison to usual efficiency bounds also highlights the impact of thermalization losses, and the subsequent interest of spectral filtering to extend the operating region. This work provides a common framework and guidelines for the study of RHEs, which represent a promising solution for reliable and scalable energy conversion.