A thermophotonic heat pump/heat engine
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1218-Z07-01
A thermophotonic heat pump/heat engine Jani Oksanen and Jukka Tulkki Department of Biomedical Engineering and Computational Science, Helsinki University of Technology, po.box. 2200, 02015 Finland
ABSTRACT We propose a solid state heat pump based on photon assisted heat transfer between two large-area light emitting diodes coupled by the electromagnetic field and enclosed in a semiconductor structure with a nearly homogeneous refractive index. Ideally the thermophotonic heat pump (THP) allows heat transfer and electricity generation at the Carnot efficiency, but in reality there are several factors that limit the efficiency. We present a numerical model that accounts for the most important losses of the thermophotonic heat pump to study the operating regimes and the fundamental limitations of the THP structure. The results show that the thermophotonic heat pump has potential to outperform heat pumps based on the thermoelectric effect especially for heat transfer across large temperature differences. In energy harvesting applications the performance of the THP is good for small power densities, but drops at high power densities unless the losses in the structure can be efficiently minimized. INTRODUCTION The operation of thermoelectric (TE) coolers, the most successful solid state heat pump to date, is based on the ability of electric current to transport heat along with electric charge and the absorption or generation of heat at the interfaces of dissimilar materials. Their drawback is the relatively low efficiency of commercially available devices, typically well below 10 % of the Carnot limit. New TE materials based on engineered nanostructures have been reported [1] but they still have a long route to applications [2]. The concept of electroluminescent cooling in semiconductor diodes was first suggested by Keyes and Quist in 1962 [3]. They noticed that the luminescence spectrum of a GaAs diode contained photons whose energy was larger than the energy corresponding to the applied voltage. The emission of these high energy photons was only possible if part of their energy was provided by lattice heat. The possibility of cooling by electroluminescence has since then been considered by various research groups [4], but light emitting diodes with the very high quantum efficiency needed for cooling have not yet been demonstrated. We propose a new solid state cooling device, the thermophotonic [5] heat pump (THP), that may overcome the main challenges of electroluminescent cooling and provide an alternative route to solid state cooling and heat energy to electric power – conversion. In particular, the THP structure allows separation of the hot and the cold parts of the device and consequently a very low thermal conductivity between them, which is not possible with the thermoelectric technology. In this paper we study the operation and limitations of the THP by developing a numerical model for the THP and present numerical simulation results obtained using the model.
THEORY The THP structure The thermophotonic
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