Experimental And Theoretical Study Of Thermionic Energy Conversion For Concentrating Solar Power

Solar thermionics,which is a direct heat-to-electricity generation technology,has a potential of converting concentrated solar radiation into power with a theoretical efficiency of 60%-70%.Thermionic emission can be not only driven by high temperature thermal energy,but improved by high frequency solar photon excitation(larger than cathode's bandgap).However,the mechanisms of thermalized electron emission and transport is unclear,as well as the thermodynamics of solar thermionics.It is necessary to conduct a systematic research on(photon-enhanced)thermionic conversion to deeply understand thermionic energy transfer process.Thermalized electron behavior in thermionics has been investigated to reveal the electron emission and transport characteristics.High temperature and low surface barrier are required for efficient emission of thermalized electrons.It is found that the cathode(i.e.,heat source)temperature can regulate the barriers of electrodes and interelectrode gap in cesium vapor thermionics.Due to the dynamic adsorption/desorption of cesium on the surface of the cathode,the surface barrier is increased as the cathode's temperature increases,leading to a nonlinear variation trend of thermionic electrons(increased initially,followed by a decrease,but then again increased).The interelectrode gap can cause a change in the probability of the collision between electrons and cesium,as well as the probability of field-induced cesium ionization.Furthermore,it is observed that optical radiation would increase thermionic current,which is expressed as photoexcited thermionic emission,i.e.,cesium acts as a medium for electron transport.A maximum thermionic power of~21.7 mW/cm~2 is attained with a barium tungsten cathode,while the conversion efficiency is~1.74%at the cathode temperature of 1200?and an interelectrode gap of 22?m.A thermionic short-circcuit current of~160 mA/cm~2 is achieved at the cathode temperature of 1200?,which is~11 times higher than that of 900?.The thermionic current at an interelectrode gap of 22?m is~21 times higher than that of 198?m.A term of thermionic-to-electricity efficiency is defined to illustate the probability of electron energy arrived at anode converted to electricity,where the rest is converted to heat.The thermionic-to-electricity efficiency increased from 6%to 11%as the cathode temperature increased from 900?to 1200?.This is explained that the difference between cathode and anode Fermi levels increases,which is resulted from the faster increase rate of cathode's work function than that of anode.The thermionic-to-electricity efficiency increased from 11%to 19%as the cathode temperature increased from 900?to 1200?,which is attributed to an increase of interelectrode barrier and an increase of difference between cathode and anode Fermi level.The mechanism of photon-enhanced thermionic emission is explored to understand the photon-electron interaction in thermionic power.It is observed that PETE current is higher than TE current,which confirms that both of thermal excitation and photoexcitation contribute to PETE.When the interelectrode gap is<130?m,cesium ionization effect is significant and thus the output current increases as the gap increases.On the other hand,when the gap is>130?m,collision effect between thermionic electrons and cesium is more significant as the electron transition time increases,leading to a decrease of thermionic current.The energy loss during the exchange between photoelectrons and lattice decreases as the cathode temperature increases,resulting in an increase in PETE electrons.Moreover,the photoexcited electrons increases as the intensity of incident light increases,giving rise to an increase of PETE electrons.The energy of photoexcited electrons increases as the wavelength of incident light increases,leading to an increase of PETE electrons that have enough energy to overcome the surface barrier of cathode.A thermodynamic assessment of solar thermionic conversion is conducted to identify the essence between thermionics and photon-enhanced thermionics.A thermodynamic model(i.e.,energy,entropy and exergy)is derivated and system parameters on solar thermionic performance wass investigated.The roles of photoexcitation and thermalization are identified,as well as the exergy flow and losses in thermionic conversion process.The combination of photovoltaic effect and thermionic effect facilitates the thermionic emission exergy ratio up to 62.36%for solar photon-enhanced thermionic conversion,which is much higher than that of thermionic conversion(51.44%)at a concentration ratio of 500.Temperature-entropy diagrams with quantitative analysis are proposed for the thermodynamic processes of conventional thermionic and photon-enhanced thermionic conversion.As for conventional thermionic conversion,the working electron fluid is thermalized from and cycles back to the Fermi level of the cathode.As for photon-enhanced thermionic conversion,the electron fluid is thermalized from the conduction band while it cycles back to the valance band of the cathode.The energy level difference(i.e.,chemical potential)leads to full conversion of the photoexcited energy to electricity.Finally,a solar dish thermionic-Stirling system is designed to analyse the effect of solar radiation distribution on thermionic power generation.It is indicated that the output power varies monotonically with solar radiation throughout a day.The peak power efficiency of the thermionic combined system ranges from 34%to 37%at a concentration ratio of 500and a mirror area of 100m~2,while the annual power generation is about 119MWh.