Thermionic energy conversion (TEC) using nanomaterials is an emerging field of research. It is known that graphene can withstand temperatures as high as 4600 K in vacuum, and it has been shown that its work function can be engineered from a high value (for monolayer/bilayer) of 4.6 eV to as low as 0.7 eV. Such attractive electronic properties (e.g., good electrical conductivity and high dielectric constant) make engineered graphene a good candidate as an emitter and collector in a thermionic energy converter for harnessing solar energy efficiently. We have used a modified Richardson–Dushman equation and have adopted a model where the collector temperature could be controlled through heat extraction in a calculated amount and a magnet can be attached on the back surface of the collector for future control of the space-charge effect. Our work shows that the efficiency of solar energy conversion also depends on power density falling on the emitter surface, and that a power conversion efficiency of graphene-based solar TEC as high as 55% can be easily achieved (in the absence of the space-charge effect) through proper choice of work functions, collector temperature, and emissivity of emitter surfaces. Such solar energy conversion would reduce our dependence on silicon solar panels and offers great potential for future renewable energy utilization.
In this paper, we present results authors published initially on the white light emission with broad band (330-465 nm)
excitation of the specially prepared nano-phosphor: Eu<sup>3+</sup>: ZnS which is capped with sodium methyl carboxylate and on
pure red-light emission from the nano-phosphor when capped with alpha methyl acrylic acid and prepared in a different
method. Then we discuss possible methods of future improvement of the white light emission from the nano-phosphor.
We then present the cost effective and energy efficient method of obtaining highest quality natural white light sources
using such nano-phosphor and blue or near UV blue light emitting diodes. The latter discussion includes the driving
circuit for the white LED and powering the LED by concentrated solar photovoltaics for both lighting and waste heat
energy storage for completely clean energy natural white lighting sources.
Graphene is a high temperature material which can stand temperature as high as 4600 K in vacuum. Even though its work function is high (4.6 eV) the thermionic emission current density at such temperature is very high. Graphene is a wonderful material whose work function can be engineered as desired. Kwon et al41 reported a chemical approach to reduce work function of graphene using K<sub>2</sub>CO<sub>3</sub>, Li<sub>2</sub>CO<sub>3</sub>, Rb<sub>2</sub>CO<sub>3</sub>, Cs<sub>2</sub>CO<sub>3</sub>. The work functions are reported to be 3.7 eV, 3.8 eV, 3.5 eV and 3.4 eV. Even though they did not report the high temperature tolerance of such alkali metal carbonate doped graphene, their works open a great promise for use of pure graphene and doped graphene as emitter (cathode) and collector (anode) in a solar thermionic energy converter. This paper discusses the dynamics of solar energy conversion to electrical energy using thermionic energy converter with graphene as emitter and collector. We have considered parabolic mirror concentrator to focus solar energy onto the emitter to achieve temperature around 4300 K. Our theoretical calculations and the modelling show that efficiency as high as 55% can easily be achieved if space-charge problem can be reduced and the collector can be cooled to certain proper temperature. We have discussed methods of controlling the associated space-charge problems. Richardson-Dushman equation modified by the authors have been used in this modelling. Such solar energy conversion would reduce the dependence on silicon solar panel and has great potential for future applications.
In this paper we considered in details of the energy exchanges that would take place when concentrated solar energy is focused normally onto a thermionic emitter of area equal to the area of focus with solar energy being incident parallel to the axis of the parabolic mirror. We then, using a simplified version of the equations, compute the power output from the thermionic energy converter with emitters of graphene on silicon carbide, assuming that with the advent of new work function engineering technology the work function of graphene can be modulated from 4.5 eV to 1.5 eV and also with pure monolayer graphene for which a new thermionic emission equation has been discovered by the authors. Our theoretical research shows that graphene being a high temperature material, it is quite possible to practically realize a solar thermionic energy converter with good conversion efficiency using a graphene-on-silicon carbide emitter.
For the first time we have derived an equation for the temperature (T) dependent work function (W(T)) containing terms up to fifth power of T which gives a modified Richardson-Dushman (MRDE) equation that fits excellently well the experimental data of thermionic current density, J vs temperature, T data for suspended monolayer graphene. It provides a unique technique for accurate determination of work function, W0, Fermi energy, EF0 at 0 K and surface density of charge carriers, ns of graphene. The corresponding values obtained for monolayer suspended graphene are: W<sub>0</sub> = 4.592 ± 0.002 eV, E<sub>F0</sub> = 0.203 ± 0.002 eV; n<sub>s</sub> = 3.16x10<sup>12</sup> cm<sup>-2</sup>. The model gives us unique method of determination of the Fermi energy of graphene as a function of temperature. The values of thermal expansion coefficient, α and surface density of charge, ns obtained with the use of the model are in excellent agreement with experiments. We also find that the model explains fairly well the J vs T data for carbon nanotubes, which is reported in a separate paper.
We have modified Richardson-Dushman equation considering thermal expansion of lattice and change of chemical potential with temperature in material. The corresponding modified Richardson-Dushman equation (MRDE) fits quite well the experimental data of thermo-electronic current density (J) vs T from carbon nanotubes. It provides a unique technique for accurate determination of work function Wo, Fermi energy, EFo at 0 K and linear thermal expansion coefficient of carbon nanotube in good agreement with experiment. From the value of EFo we obtain the charge carrier density in excellent agreement with experiment. We describe application of the equations for the evaluation of performance of concentrated solar thermionic energy converter (STEC) with emitter made of carbon nanotube for future applications.