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Our current approaches to producing solar energy are not practical on a commercial or industrial scale, and if it is to survive in the energy industry new methods must be utilized. The methods discussed in this article are alternative approaches and improvements to substantially boost the efficiency of solar cells, and if further developed, boost the efficiency of any power generation system that produces waste heat.
Thermophotovoltaic (TPV) systems essentially produce electricity the same way regular photovoltaic (PV) cells do (i.e. solar panels), except for one vital difference; thermophotovoltaics utilize infrared light (IR) instead of visible light. Infrared light is a bandwidth of light below (lower in energy) the visible spectrum, and constitutes most of the emitted light in thermally radiative processes. In other words, anything that is hot enough to emit visible light (i.e. red hot) is also emitting a significant amount of infrared light (IR). Currently, most PV cells operate in bandwidths within the visible spectrum, however, there are many PV cells that are able to operate within the IR spectrum.
As seen in Figure 1, TPV systems consist of three major components.
- Photovoltaic Cell
The absorber is a material that is designed to absorb as much incident light as possible, from as many bandwidths as possible: infrared, visible, UV etc. The purpose of the absorber is to capture as much energy as possible, causing the absorber to heat up and re-emitting light in the IR range. Since the material must be able to handle high temperatures, refractory metals are often used. The emitter and the absorber are typically the same material, however, it varies depending on the emission spectra of the absorber. Tungsten, for example, is a great candidate as it has both a high absorption spectra and a good IR emission spectra, in addition to having a melting point of ~3400°C. Filters can also be employed between the absorber and the PV cell to selectively allow specific wavelengths of light through, and reflect the rest back to the absorber. This is helpful in regulating the unwanted light being absorbed by the PV cell, which can heat up the PV cell causing a decrease in efficiency. Although the filters seem great in theory, they are not perfect and can sometimes lead to unnecessary energy losses.
The Sun emits a wide array of wavelengths, yet typical PV cells can only optimize in a small bandwidth, typically within the visible spectrum. As a consequence, only a small portion of the available radiant (light) energy is used. This is a result of the Shockley-Queisser limit, which essentially puts a cap on the maximum efficiency of any solar PV cell at ~ 33.7 percent. However, the PV cells in TPV systems do not directly use solar light and, if treated as an ideal system (no energy losses), can reach a theoretical maximum of ~80 percent. With that being said, a recent study has only achieved efficiencies of up to 25 percent , however, this is a massive improvement from earlier years and still holds promise for improvements. The photovoltaic cells for TPV systems must optimize in energy conversion within the IR range. Some examples of effective IR-optimized PV cells typically consist of germanium, indium gallium arsenide antimonide, or PbSe nano-crystals[3, 4].
TPV systems are incredibly appealing for a few reasons. TPVs don't rely on sunlight to operate but they rely on heat. This is important because the majority of energy produced in commercial/industrial power plants is wasted as heat. For example, coal and natural gas-fired power plants are roughly 30% and 40% efficient, respectively. TPVs, if employed as waste-heat converters, could boost these efficiencies, utilizing the waste heat and could, theoretically, produce close to as much energy as the main turbine itself. Additionally, these systems could be implemented in vehicles, planes, spacecraft, satellites, etc. as compact reliable energy systems, reliant on either concentrated solar energy or heat from any fuel source.
Nanotechnology: Upconversion, Downconversion, and Quantum Cutting
TPVs, although promising, require very high operating temperatures and if designed for solar specifically (STPVs), they would require solar energy to be intensely concentrated, making it somewhat impractical. However, with recent advances in the research of the phenomena known as upconversion, downconversion, and quantum cutting have allowed scientists to alter the spectrum of light incident with the solar cell, proposing a more refined approach to overcoming the Shockley-Queisser limit.
Upconversion and downconversion are most often observed in a series of 15 elements known as lanthanides (Ln). The electron configurations in lanthanides are able to occupy the 4f orbital, which gives them very unique chemical properties. Oxysulfides, when doped with lanthanides (i.e. Ln₂O₂S), are able to produce a phenomenon known as scintillation. Scintillation is the emission of visible light from high energy particles. This phenomenon is commonly used in high energy particle detectors. For example, gadolinium oxysulfide is doped with terbium(Tb) and is able to produce visible photons when irradiated with X-rays. Although lanthanides do possess this desirable ability to down-convert photons, their luminescent properties aren't as promising. This is why lanthanides are typically used as doping agents to utilize their downconverting properties while avoiding their inability to re-emit light.
Upconversion (UC) is an energy conversion process in which low energy photons (i.e. IR light) are absorbed successively by an atom. This sequential absorption of photons allows the atom to use the two (or more) low energy photons to become excited, or in other words, to cause its electron to jump into the next energy level. It does this by exciting the electrons into a longer-lived virtual intermediate state in between each photon absorption. This higher energy level would typically be unachievable unless a single photon with the combined energy of the two lower energy photons was absorbed, however, using the vibrational energy within the crystal lattice of the material, the atom can absorb phonons (quantized compressional energy, i.e. vibration) from the lattice to help excite the electron. The atom will then become excited and consequentially unstable, causing the electron to decay back down to a lower energy state. When this decay occurs, the electron releases a single photon proportional to the difference in energy between the two energy levels, and since it required two IR photons to get there the emitted photon will be equal to the combined energy of the two absorbed IR photons. This means the emitted photon will be within the visible spectrum and thus usable by the photovoltaic cell to produce electricity, form otherwise unusable light.
Downconversion and Quantum Cutting
Down-conversion (DC) is a process in which a photon of a higher energy, such as UV light, is turned into a lower energy photon, such as visible or IR light.
The incident high energy photon, known as the pump photon, is passed through a nonlinear crystal. This pump photon is then split into two lower energy photons, known as the idler and signal photons, in which the sum of the signal and idler photons is equal to the energy of the pump photon (due to the conservation of energy). The diagram (top right) visually explains this process. With the conversion of high energy photons into several lower energy photons, we are able to input light that would otherwise be unusable and output photons that a PV cell can absorb and turn into useful energy.
Implementing the Technology
As stated before, typical photovoltaic cells are only able to absorb bandwidths within the visible spectrum, so being able to convert photons that are energetically above and below the visible spectrum into visible photons is extremely useful in improving the energy efficiency of solar cells. The diagram above depicts how a system utilizing upconversion and downconversion would be set up. On the top of the solar cell (left), the incident UV light that is out of range for the PV cell to use would be absorbed by this layer, where downconversion would take place producing lower energy visible photons. Incident visible and infrared photons would not be high enough energy to cause downconversion, thus passing through this layer and into the PV cell. Infrared light that is not utilized by the PV cell is able to heat up the PV cell, in which an upconverting layer (right), accompanied by a reflecting layer, can be applied underneath the PV cell to convert the waste infrared light into visible light, emitting it back to the PV cell as useable light.
Although this technology is still in a research and development stage, it holds significant promise. One of the most desirable traits of this technology is its capability to be added to already existing solar panels. This saves commercial solar operations the need to completely retrofit their energy plants with contemporary solar panels, and instead simply apply a thin layer to the surface of the solar cell.
Solar energy is the most abundant and naturally used energy source for life on Earth. For as long as humans persist on Earth so shall solar energy, and with the minimal consequences for harnessing such energy, it would be foolish to not invest in and optimize solar technologies.