|
|
| Fig. 1: Microscopic or nanoscopic particles suspended in glass or plastic absorb ultraviolet (the purple arrow) or infrared (the dark red arrow) light while allowing visible light to pass through (the remaining arrows). Allowing visible light to pass through makes the system transparent, leaving the view from indoors unhindered. (Source: A. Mangu) |
Providing electricity to densely populated urban areas accounted for 31% of US carbon emissions in 2012. [1] Therefore, providing energy to cities without producing greenhouse gas emissions is a crucial part of the sustainability problem in the US. Producing the energy elsewhere and transporting into the city is also problematic, however, due to the lack of available infrastructure and the limits of electrical transmission lines. One possible solution, solar energy, is problematic because of the limitations of conventional solar technology. Conventional solar cells cannot function efficiently in downtown areas due to lack of available roof space (relative to the amount of power needed for the dwellings and offices). [2] Silicon solar cells need as much surface area as possible to collect sunlight and cannot be operated on a cloudy day at peak efficiency. Scientists have been making progress in this problem by developing transparent solar cells that harvest light in non-visible parts of the electromagnetic spectrum and that can be coated onto windows to form a large network of power supplies. [3,4] One example of such technology is the luminescent solar concentrator, which can develop truly transparent windows that produce electricity and can revolutionize how we power skyscrapers.
Optimizing transparent luminescent solar concentrators is still an active area of research, so specific designs may vary from research group to research group. The principle of how they generate electricity, however, is common across different devices. Microscopic or nanoscopic dye or semiconducting particles are suspended in transparent glass or a type of plastic. [5] Light at some nonvisible wavelength, usually infrared or ultraviolet light (see Fig. 1) is absorbed by the suspended particles, exciting some electrons to a higher quantum state. When the electron falls back to a lower energy level, as in other materials, the particle will re-emit some light. In normal quantum mechanics, the electron would return to the previous state and emit light of the same wavelength. However, in LSC devices, the absorbing particles are chosen such that they emit light of a different wavelength than the wavelength absorbed. These materials are said to exhibit a Stokes shift. [4] The new wavelength is usually chosen to be in the deep-infrared part of the spectrum, approximately 1100 nm - 1500 nm (see Fig. 2). This new wavelength is constrained to stay in the transparent medium by total internal reflection. The medium then guides the light to the edges of the glass, where small conventional photovoltaic cells can be placed to collect this wavelength of light very efficiently. [2]
|
| Fig. 2: A diagram of the different parts of the electromagnetic spectrum. LSCs absorb either ultraviolet or near infrared light (infrared light near the visible red part of the spectrum) and re-emit deeper infrared light (infrared light farther away from visible light) within the device to be absorbed by conventional PV cells. (Source: Wikimedia Commons) |
The amount of usable energy (i.e. electrical energy) that can be harvested by this technology depends on the efficiency of the device created. For this context, we will use a recent estimate of the Power Conversion Efficiency (PCE), defined as the ratio of the incident power (in the form of solar illumination) to the output electrical power. [4] The highest reported power conversion efficiency has been 7.1%. [4] An important consideration in performing this calculation is that the light incident on windows comes from all directions after bouncing off other surfaces. The measured quantity most closely resembling this incident light is the Global Horizontal Irradiance (GHI). This quantity is highly dependent on the location of the window on earth. Using data from Modesto, CA, the average GHI from 2009 is reported at 4.92 kWh/m2/Day. [6]
Performing the calculation,
| P | = | 4.92 kWh m-2 day-1 × 0.071 × 3.6 × 106 Joules kWh-1 |
| = | 1.258 × 106 Joules m-2 day-1 |
We can use this figure to estimate the energy produced if this technology was placed on the windows on an apartment complex in Modesto. In order to see whether this technology will be able to be feasibly incorporated into a real use case scenerio, we will use data from a controlled apartment complex built by the Department of Energy to monitor the status of zero-net energy technologies. [7] The consumption data is well recorded in the report for this apartment complex, as well as the living conditions. For the purposes of this calculation, we will estimate the height of the apartment at 4 stories, approximately 13.2 meters tall. Estimating the size of the block to be 100 m by 30 m, the total side surface area comes to 3,432 m2. Now we calculate the energy produced if 100% of the side area on this model is used to produce energy with luminescent solar concentrators:
| PEmpire State | = | 1.258 × 106 Joules m-2 day-1 × 3.432 × 103 m2 |
| = | 4.317 × 109 Joules day-1 |
How does this compare to the need of the building? Data from the study done on this apartment building (set in Davis, CA) gives the total energy consumption for the year to be approximately 1.968 GWh. [7] This is equivalent to 1.941 × 1010 Joules day-1. [7] This crude estimate, therefore, indicates that at this efficiency, Luminescent Solar Concentrators can account for 22% of the energy demand of this model apartment complex, which means this technology can lead to a huge reduction of demand from the grid.
To be clear, this calculation is limited in accuracy by many factors. The yearly consumption of energy was estimated based on a sum of monthly consumption data to within 5% accuracy. The area of the building was not reported, so the area of the building was estimated based on pictures. This technology would not be placed on every square meter of the side of the building. This technology would be most effective on skyscrapers in downtown areas with almost 100% of the side area dedicated to transparent windows. These figures were used in these calculations because they were the best reported figures that could be used in this type of calculation. However, if we consider residential apartments in downtown areas with large window surface area in California, this estimate tells us that there is potential in reducing the use of fossil fuels in providing residential energy.
This technology is limited, however, by the area of the actual device produced. A large window with this technology will produce less efficiently than multiple smaller windows, each with PV cells. [4] This presents a challenge in implementation because the need for many small devices creates a high cost barrier to implementation. However, with increased efficiency, this technology can help significantly reduce the energy needs of buildings in urban centers throughout the world.
© Anudeep Mangu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] K. S. Fu, M. R. Allen, and R. K. Archibald, "Evaluating the Relationship between the Population Trends, Prices, Heat Waves, and the Demands of Energy Consumption in Cities," Sustainability 7, 15284 (2015).
[2] A. Extance, "What Watts From Yonder Window Flow?" IEEE Spectrum 55, No. 2, 27 (February 2018).
[3] Y. Zhao et al.," Near-Infrared Harvesting Transparent Luminescent Solar Concentrators, Adv. Opt. Mater. 2, 599 (2014).
[4] p. Moraitis, R. E. J. Schropp, and W. G. J. H. M. van Sark, "Nanoparticles for Luminescent Solar Concentrators - A Review," Opt. Mater. 84, 636 (2018).
[5] M. Sharma et al., "Near-Unity Emitting Copper-Doped Colloidal Semiconductor Quantum Wells for Luminescent Solar Concentrators," Adv. Mater. 29, 1700821 (2017).
[6] A. Nottrott and J. Kleissl, "Validation of the NSRDB-SUNY Global Horizontal Irradiance in California," Solar Energy 84, 1816 (2010).
[7] A. German et al., "West Village Student Housing Phase I: Apartment Monitoring and Evaluation," U.S Office of Energy Efficiency and Renewable Energy, June 2014.
