ABSTRACT
Light has a dual nature (particle and wave). Solar technology exploits the particle nature of light and converts light to electricity by the charge separation method. Carbon nanotubes (CNTs) have been highlighted for their potential to enhance the efficiency of solar energy conversion into electricity because they possess exceptional thermo-mechanical and optical properties. Despite this breakthrough, there are challenges to CNTs achieving widespread usage. In this paper, obstacles plaguing the solar energy field, the manufacturing of CNTs and recent progress concerning utilization are reviewed. A summary of current challenges and prospective future research directions for employing CNTs is presented.
INTRODUCTION
Since the Industrial Revolution, energy consumption has risen dramatically. Much of this energy is supplied by fossil fuels, such as coal or natural gas. Natural gas is used for everyday purposes, such as heating a home, cooking food, starting an engine, and generating electricity. However, in the last ten years most conventional natural gas sources in America and Europe have been exhausted because of advances in the drilling industry, emerging technology, and collisions between energy demands and environmental goals. Thus, prices for natural gas and other fuels have been rising steadily and other forms of energy have become more attractive as a result. Solar energy has received much attention in recent years as a clean and renewable energy source that is cheaper than energy from fossil fuels, such as coal or natural gas, but it has been difficult to determine how best to funnel and transform it into a sustainable power source. Solar panels have been a significant innovation in energy technology, but they are less efficient than they could be because of wasted heat-energy that is lost while converting the sun’s energy into electricity. New technology made possible by CNTs offers a solution that could bring solar panels into the lead in energy technology.
Solar cells review and challenges
Solar cells consist of semi-conducting material like silicon which has had other differently charged elements added to create an internal electric field. When the photons from the sunlight hit the solar panel, they knock electrons off the silicon atoms (Raza et al, 2017 Fig. 1). These excited electrons can then be pulled into an external circuit as electricity. However, not all of the solar radiations that touch the solar panel get turned into electricity because only certain wavelength photons carry the right amount of energy to eject the valence electrons (Fig. 2) For the rest of the wavelengths that cannot perform this action, that energy is lost and unable to be used in a solar panel to generate electricity. This mismatch in energy levels is one of the biggest issues that limits solar panel efficiency. This, along with other longstanding problems such as the resistance that electrons face when passing through conducting materials, has limited peak efficiency of solar energy panels (Raza et al, 2017).
Figure 1
Solar Cells
Note: Solar cells are composed of two layers of semiconductor material with opposite charges. Sunlight hitting the surface of a cell knocks electrons loose, which then travel through a circuit from one layer to the other, providing a flow of electricity.
Figure 2
Spectrum of sunlight
Note: Spectrum of sunlight used by silicon solar panels.
Carbon nanotubes review
Carbon nanotubes are cylinders that consist of rolled up sheets of single layers of carbon atoms, also known as graphene (Fig 3). Graphene is a water-based nano-fluid that consists of nano-sized particles dispersed in the base fluid (water, ethylene glycol, ionic liquids etc.). Research on the thermo-physical and rheological properties of various nano-fluids and their heat transfer characteristics have shown that these materials can work as solar spectrum splitters and can either absorb, reflect, or transmit specific ranges of the solar spectrum. This characteristic of nano-fluids make them a potential candidate for many solar applications (Sajid et al, 2020).
Appeal of carbon nanotubes
Researchers have addressed the complication of wasted heat energy in current solar cells by adding a film of carbon nanotubes to harness the heat from the radiation that is not absorbed by solar panels and turning that into electricity. Thermal radiation, like the kind released by solar cells, is broadband; converting sunlight into electricity is only efficient if the emissions are in a narrow band. Wafer-thin carbon nanotubes have been developed that can absorb the broadband waste heat and channel it into narrow bandwidth photons that can be easily converted to electricity. Thus, instead of going from heat to electricity, the nanotube film makes the conversion process more efficient by taking that energy from heat to light to electricity.
Another appeal of carbon nanotubes is their potential to dramatically improve energy storage and renewable energy. They can be easily mass produced and can offer a breakthrough in energy potential and a solution for many applications such as composite materials for large metallic parts, highly efficient transistors, nano-inks, biotechnology, and space elevators (Rathinavel et al, 2021).
CNTs challenges
Despite several benefits, CNTs are also associated with some disadvantages that affect their production and application. The performance of the developed and used CNTs in the solar system is under the influence of various factors including their types such as single wall CNTs versus multi wall CNTs, their operating conditions, concentration, and the systems in which they are applied (Ghalandari et al, 2020). Difficulties in manufacturing carbon nanotubes have hindered their widespread commercial use, specifically issues with precision and the separation of different types during manufacturing. Carbon nanotubes are essentially molecular sheets of carbon atoms, which are arranged in hexagonal lattice or honeycomb arrangement. These sheets are organized into layers and manipulated into cylindrical shapes. CNTs can be designed and arranged in many ways and their arrangement can impact their conductivity.
The manufacturing of carbon nanotubes typically yields a mixture of different types. As they have a tendency to clump together, separating them apart has been a big challenge for mass scale manufacturers due to their small size. Single-walled carbon nanotubes (SWCNTs) are made up of a single layer of graphene. Multi-walled carbon nanotubes (MWCNTs) are an extension of single-walled and are made up of multiple layers of graphene which better insulates their thermal and chemical properties compared to single-walled tubes. Double-walled carbon nanotubes (DWCNTs) are a combination of single-walled and multi-walled with thermal, chemical and conductivity properties and that are intermediate compared to the other two (Fig 3a). Depending on how the hexagon lattice is rolled affects how the hexagonal pattern spirals along the tube. There are three basic types of patterns that form—armchair, zigzag and chiral (Fig 3b). The final design determines if the nanotube acts as a metallic or semiconducting material. It is only when carbon nanotubes are grouped by purity (metallic or semi-conducting) that they can be used effectively (He et al, 2013).
Figure 3a and 3b
Carbon Nanotube
Note: Conceptual diagrams of SWCNTs vs DWCNTs and Carbon nanotube structures of armchair, zigzag, and chiral configurations.
TECHNIQUES
Over the past few years, there have been breakthroughs in how to effectively isolate carbon nanotubes after manufacturing which has helped to advance their benefits. Some methods use chemical reactions to modify the nanotubes to force them to separate. Cresol-based solvents have been used to separate nanotubes and to form them into a thick, moldable gel. This solvent system makes the nanotubes behave essentially like polymers which makes the once hard to process carbon nanotubes as common as plastic (Raza et al, 2017).
Dr. He and coworkers at Rice University have discovered a technique that uses vacuum filtration to produce large mono-domain films of aligned SWCNTs that are densely packed as much as 1×106 nanotubes in a cross-sectional area of 1 μm2 (Fig 4). This method allows for nanotubes to be synthesized in multiple ways while controlling the thickness of the films to a few nanometers. These types of breakthroughs will allow us to go from bulk manufacturing, where all the nanotubes are tangled together, to isolated mass scale manufacturing of carbon nanotubes (He et al, 2016).
Figure 4
Fabrication and Characterization of Wafer-scale Mono-domain Films of Aligned CNTs
Note: a. A CNT suspension goes through a standard vacuum filtration system. For spontaneous CNT alignment to occur, the filtration speed must be kept low and the CNTs must be well dispersed in the suspension. b. A wafer-scale, uniform CNT film is formed on the filter membrane. c. Optical image of the produced film after being transferred to a transparent substrate by dissolving the filter membrane. d-f. SEM image (d), a high-resolution SEM image (e) and a top-view TEM image (f) of the film, showing strong alignment and high density. g. A high-resolution cross-sectional TEM image, showing a high cross-sectional areal density of ~1 x 106 µm-2. h. Angular distribution of CNTs within a 1 cm2 area of film, with a standard deviation of 1.5o, determined by SEM image analysis. ij. The film is opaque to light polarized parallel to the CNT alignment direction and transparent to light polarized perpendicular to the alignment direction on a macroscopic scale (i) and a microscopic scale (j). Note also that the film can be easily patterned using conventional photolithography techniques (j).
One of the most active areas of CNT research is around its use in solar paneling. Gao and associates have reported a method to improve solar panel efficiency by up to 80% by having the nanotubes capture the unused potential of infrared heat from the sun. Current panels are capturing only light and converting that into electricity. Building upon the above-described vacuum filtration approach, they designed an array of cavities patterned to a film of aligned carbon nanotubes which are able to absorb and channel thermo-photons and emit them as light. Essentially, they are converting heat into a form that the solar panel can then convert into electricity (Gao et al, 2019).
As previously discussed, another problem with solar panels is that they lose efficiency when they are too hot. Li and coworkers at King Abdullah University of Science and Technology have developed a system that keeps the solar panels cool with no moving parts or excess energy drain. They have developed a polymer that contains calcium chloride. When the material is exposed to humid air, it absorbs moisture and expands in size. In their latest research, they combined the gel polymer with carbon nanotubes to reverse this cycle and release the trapped water. When the gel was applied to the back of a solar panel, it was able to absorb moisture from the humid air at night and then slowly release it during the hottest parts of the day. The team saw panel temperatures reduce by 10 ℃ which improved efficiency by up to 20%. Since this cooling technology is easy to adapt to different scales, it can fulfill the requirements of many applications because water vapor is everywhere. The technology could be made as small as several millimeters for electronic devices, hundreds of square meters for a building, or even larger for passive cooling of power plants (Li et al, 2020).
RESULTS AND DISCUSSION
Most of the previously discussed techniques are still in the early stages of development. Mass production of CNTs for commercial use is still a little ways away, but the future appears bright. The study by Raza and colleagues using Cresol-based solvents anticipates CNTs being a major breakthrough in solar power once widely available. They predict that as much as 40 percent of India’s total power supply could come from solar energy by 2022.
The Rice University team headed by Dr. He was able to use their vacuum filtration method to produce photodetectors lined with wafer thin sheets of SWCNTs. When compared with previously produced photodetectors using the same device architecture, the new CNT laced units enhanced the responsiveness by a factor of three. This may be due to the removal of less responsive metallic tubes. They believe that this technology can go much further as the design becomes fully optimized.
The 80% efficiency estimation by Dr. Gao and colleagues is only theoretical at this time, but is an exciting possibility compared to the current estimates. The consensus among many in the field is that current solar panel technology tops out around 30 percent efficiency due to their inability to harness the broad spectrum of available light and waste heat-energy. Gao’s team has developed aligned CNTs that can remain thermally stable up to 1600 ℃ and thermal emitters with a large photonic density of states (PDOS) over a large spectrum range longer than 4.3 µm (Gao et al, 2019).
As previously mentioned, non-organic solar cells can only use a small spectrum of light, but according to Gao and coworkers, “A closely packed bundle of aligned metallic or doped semiconducting SWCNTs can be described as a uniaxial anisotropic medium with effective permittivities along the tube axis and in the perpendicular plane. The conductivity due to free carriers along the nanotubes causes a metallic optical response. In contrast, the nanotubes are insulating in the perpendicular plane. This extreme anisotropy leads to a broadband hyperbolic dispersion, which spans a majority of the mid-infrared range, a significant portion of the spectrum of interest for selective thermal emitters.” (p. 1603) (Fig. 5) It is believed that this approach can revolutionize the industry and, if their efficiency predictions are true, then solar energy could become the leading source of energy consumption of modern societal needs worldwide. This approach coupled with others like the group at Abdullah University will allow for massive increases in the capture of waste-heat energy while limiting the damage to cells from high temperature exposure.
Figure 5
Fabrication and Characterization of Macroscopically Aligned SWCNTs as a Mid-infrared Hyperbolic Material
Note: (a) Schematic diagram of the experimental setup for thermal emission and reflectivity measurements at temperatures up to 700 ℃. Samples were heated using a PID-controlled resistive heater surrounded by ceramic spacers under high vacuum <10-5 Torr. A tantalum film was used to support samples and block the direct thermal emission from the heater. A zinc selenide (ZnSe) window provides optical access for the light collection and analysis using a microscope and a Fourier transform infrared (FTIR) spectrometer, respectively. (b) A scanning electron micrograph and a transmission electron micrograph (inset), demonstrating a perfect alignment and high packing density. (c) Polarization attenuation spectra in a wide frequency range, from the THz/far-infrared range to the visible range, at room temperature. (d) The dielectric constants parallel and perpendicular to the SWCNT alignment direction, with an ENZ frequency in the mid-infrared range.
CONCLUSION
The future of solar energy seems to be very bright. Many experts believe that the use of solar energy in the United States will grow exponentially over the next 30 years. Carbon nanotubes look to play a major role in enhancing the efficiencies of solar panels through the capture of waste heat-energy, broadband light spectrum absorption, as well as cooling and protection of panels from harsh environmental factors. CNT technology is in its infancy and there’s no telling the effects it could have going forward.
It’s an exciting and hopeful technological moment when it comes to renewable energy. Our planet faces the greatest threat in human history from rising temperatures due to carbon emissions. Solar power may play a major factor in the reduction of human induced climate change, but the current technology can only take us so far. The manufacturing limitations of CNTs are a true roadblock, but one that seems entirely possible to overcome through current innovations mentioned in this paper that will hopefully lead to universal commercial production in the near future.
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