Analysis Of Emission Characteristics In Photon Enhanced Thermionic Conversion

Solar energy is considered an environmentally friendly renewable energy.Photovoltaic(PV)solar cells and solar thermal converters are two conventional categories of technologies that have been used in solar energy conversion.Photon enhanced thermionic emission(PETE)is a new concept for solar electricity generation which combines photovoltaic and thermionic effects and utilizes solar photonic and thermionic energy simultaneously.Higher energy conversion efficiency can be achieved theoretically.In this paper,the design of a PETE performance test system and the experiments under different working conditions were carried out.The effects of space charge on the conversion efficiency of PETE were investigated.A vacuum PETE performance test system was designed and developed according to the design requirements.The components and frameworks of the vacuum chamber,vacuum system,illuminate system and parameter test system were described in detail.The amorphous silicon was used as the cathode and the I-V characteristic curves were obtained to analyze the performance of PETE system under different working conditions.The results showed that solar light and heat can increase the emission current simultaneously and the photocurrent increased with increasing cathode temperature.As heating the cathode from 250°C to 500°C,the photocurrent increased rapidly and eventually reached a plateau,and the thermal current increased slowly at low temperature but increased rapidly at higher temperature.When the temperature was lower than 450°C,the photocurrent was much larger than the thermal current and the impact of light was more significant.The photocurrent and thermal current had the maximum difference at 450°C,which was the best working temperature for cathode.However,the thermal current was greater than the photocurrent in low light conditions,which the cathode current was predominantly affected by temperature.The cathode current reached the maximum value when the gap width was 88μm at temperature of 480°C.The current dropped because of the ionization of cesium atoms between two electrodes when the gap width became smaller.When the gap width was larger than 200μm,reducing the interelectrode space had limited effect on promoting cathode current compared to increasing temperature.With different wavelength illuminations,the spectral responses of cathode were different,and the quantum efficiency under 300 nm illumination was the largest.The quantum efficiency increased with the raising temperature.The effects of space charge on electrons moving process was studied basing on the space charge theory.The results showed that charge carriers can form an electrostatic barrier above the vacuum level when carriers are moving between two electrodes in vacuum.Electrons that are emitted from the cathode can be reflected due to the barrier.The larger the gap width is,the larger the maximum motive is,the smaller the cathode net current is,and more obvious the space charge effect is.Reducing the interelectrode space to a few microns can weaken space charge effect effectively.A PETE energy balance analysis model was established based on the space charge effect,and the calculation formulas of output current,voltage and conversion efficiency were derived.The effects of the gap width,cathode temperature,electron affinity and anode work function on the efficiency and flux concentration were simulated in detail in the Matlab.There existed an optimal value for the cathode temperature and the electron affinity to attain the maximum efficiency when other things were invariable.When increasing the cathode temperature,the corresponding electron affinity needs to be enhanced to achieve maximum conversion efficiency.When the anode temperature was 400 K and the anode work function was 0.8eV,the optimal electron affinities for different cathode temperatures were obtained.The efficiency was proportional to the cathode temperature when the gap width was smaller than 4μm.Otherwise,the efficiency was inversely proportional to the cathode temperature.There was an optimal anode work function to maximize the efficiency.The efficiency increased linearly with the decreasing anode work function.However,if the anode work function was too small,the reverse current from the anode can become noticeable which reduced the efficiency rapidly.Therefore,the anode work function needs to be controlled within an appropriate range.