Study On The Influence Mechanisms Of Near-field Thermophotovoltaics And Its Application In Solar Thermal Power Generation

Limited by the blackbody radiation limit,the output power capability of conventional thermophotovoltaics is weak.It has been shown that by narrowing the gap between the emitter and the thermophotovoltaic cell to a near-field scale--smaller than their characteristic wavelengths,the photon tunneling effect occurs and the output power density of the thermophotovoltaic cell is increased by up to two orders of magnitude.Thermophotovoltaic technology that exploits the photon tunneling effect is also called near-field thermophotovoltaic technology,which is a new means of thermoelectric conversion.We first explore the theoretical limiting performance of near-field thermophotovoltaic based on indium arsenide(In As)thermophotovoltaic cells at a 10 nm gap in terms of the dielectric properties of the emitter material.It is verified that the ideal emitter with optimal performance is always a plasmonic material with Drude lineshape.Further,a comparative performance of real materials shows that indium tin oxide(ITO)and doped silicon perform optimally.In order to verify the thermal radiation properties of these two plasmonic materials in the near-field gap,a near-field thermal radiation experimental apparatus was built and their enhanced radiative heat transfer rate in the near-field gap was observed.A Hall effect tester revealed that the carriers of ITO exhibit significant temperaturedependent properties at high temperatures,and simulations predicted that these properties would significantly affect the near-field radiative heat flux.The mechanism of ITO’s temperature-dependent carrier mobility on the near-field thermophotovoltaic performance is further discussed.The modes number distributed at resonance frequency is reduced at high temperatures due to the intensification of collisions between free electrons and other particles.The surface plasmon polaritons(SPPs)excited by the resonance of free electrons with photons are severely weakened.More modes are shifted to the infrared band,especially in the far-infrared band,which are more likely to couple with the surface phonon polaritons(SPh Ps)excited by the thermophotovoltaic cell.As a result,the output power density and conversion efficiency are significantly reduced.It is also found that the ITO samples prepared by the sintering method have a small temperature coefficient,which can effectively reduce this effect.To further enhance the performance of 100 nm gap near-field thermophotovoltaics,a2 D grating structure remitter is proposed.In In As cells,there is also a free carrier loss mechanism,nonradiative recombination,which leads to the energy of free carriers being transferred to phonons in a direct or indirect way and converted into thermal energy.After taking into account the temperature-dependent properties of the nonradiative recombination,it is found that as the cell temperature increases,the enhanced nonradiative recombination leads to a significant decrease in the open-circuit voltage and thus a sharp decrease in the nearfield thermophotovoltaic performance,although the red-shift of the bandgap results in a larger short-circuit current.A high-performance near-field thermophotovoltaic model was developed after an in-depth exploration of the influence mechanism of various factors.Then it is applied to the solar thermal power generation,and a theoretical model of near-field solar thermophotovoltaic is built.By comparing with the ideal concentrated photovoltaic,the disadvantages of this theoretical model are pointed out and the key means to improve the performance are indicated.Finally,based on the concept of solar energy cascade utilization,a hybrid system of concentrated photovoltaic and near-field thermophotovoltaic is first proposed.By means of a wavelength splitter,the fullspectrum solar energy is splitting into two parts: the short-wave part is converted by the concentrated photovoltaic,while the long-wave part is converted by the near-field thermoelectric photovoltaic.After selecting the appropriate splitting wavelength,the hybrid system exhibits excellent solar energy conversion performance.