Prospects and requirements for thermophotonic waste heat energy harvesting

Thermophotovoltaic (TPV) power generators offer great possibilities for thermal energy conversion when thermal sources with temperatures nearing or exceeding 1000 K are available. While the power density of conventional TPV systems is generally determined by Planck's law in the far field, their fundamental performance is known to be dramatically affected by near field effects between the thermal emitter and the photovoltaic cell. Another potentially disruptive enhancement to the performance may be reached by transforming the thermal emitter to exploit electroluminescence. Taking advantage of an electroluminescent emitter as the source of radiation fundamentally alters the thermodynamics of the system. This allows boosting the achievable power densities by orders of magnitude, and also provides access to electroluminescent coolers, thermophotonic (TPX) heat pumps, and TPX power generation devices that can outperform both TPV and thermoelectric heat engines, especially at the low-grade waste heat (LGWH) temperature range (300–500 K) containing in total the majority of recoverable energy. In reality, functional TPX devices are yet to be demonstrated experimentally, due to several material and design bottlenecks. Here, we discuss the thermodynamics, ideal characteristics and advantages of TPX heat engines, and quantify how non-idealities such as non-radiative recombination, optical and resistive losses affect their performance. Our results suggest that, at LGWH temperatures, TPX heat engines start to outperform the best TPV systems when reaching quantum efficiencies of the order of 90%; beyond this threshold, TPX systems become increasingly efficient and powerful.