Download PDF The Bridge: 50th Anniversary Issue January 7, 2021 Volume 50 Issue S This special issue celebrates the 50th year of publication of the NAE’s flagship quarterly with 50 essays looking forward to the next 50 years of innovation in engineering. How will engineering contribute in areas as diverse as space travel, fashion, lasers, solar energy, peace, vaccine development, and equity? The diverse authors and topics give readers much to think about! We are posting selected articles each week to give readers time to savor the array of thoughtful and thought-provoking essays in this very special issue. Check the website every Monday! Entering the Solar Era: The Next 50 Years of Energy Generation Wednesday, December 23, 2020 Author: Rebecca Saive It would be absurd today if anyone attempted to launch a product using chlorofluorocarbons (CFCs). Yet less than 50 years ago, the use of CFCs was entrenched in industry standards for numerous products, such as aerosols and refrigerants. With the 1987 Montreal Protocol, 197 countries agreed to phase out CFCs to halt the growing destruction of the ozone layer (EPA 2020), and the atmosphere is recovering from this human activity fallout (Merzdorf 2020). This story suggests that it is possible to overcome environmental problems on a global level and bring Earth back to a healthier state. Moving Away from Fossil Fuels As happened with CFCs, the next 50 years will see a shift of mindset regarding fossil fuels, although their phaseout poses a greater challenge than CFCs, as fossil fuels have long been perceived as the foundation of modern prosperity. However, the commercial technology is already available to transition to a fossil-fuel-free society, with economic drivers favoring renewable energy sources in a multitude of use cases (e.g., Pyper 2019). One recent analysis finds that “it is already cheaper to build new renewables including battery storage than to continue operating 39 percent of the world’s existing coal fleet” (Bodnar et al. 2020, p. 6). The future energy grid will be a mix of renewables—solar, wind, hydro, and geothermal—heavily dependent on the major local resource. Five countries already boast at or near 100 percent renewable electricity generation: Albania, Costa Rica, Iceland, Norway, and Paraguay, leveraging their advantageous geography of highlands, mountains, and rivers to generate electricity from hydropower. FIGURE 1 Comparison of renewable and fossil global energy potential. For renewables, the amount of energy is shown per year; for fossil sources the total reserve is displayed. Solar energy received by emerged continents only, assuming 65% losses by atmosphere and clouds. TWy = terawatt-year. Adapted from Perez and Perez (2009). The abundance of solar outstrips all other resources by many orders of magnitude (figure 1; Perez and Perez 2009), and, combined with its ubiquity, will position it as the dominant global energy source, particularly as electrification spreads. In many countries, the cost of solar-generated electricity is already on par with the cost of traditional electricity generation. This has led to several decades of global, exponentially increasing solar power production per year. However, maintaining current adoption rates will not displace enough fossil fuel use to stay below catastrophic CO2 emission levels. To enhance the solar adoption rate, breakthrough technologies are required to catapult us into the era of solar energy. Implementation Direct power will be possible through high-performance solar cells that capture available irradiance almost loss-free. This will be accomplished through advanced engineering of the solar-harvesting device itself and of the microenvironments surrounding the device. One possibility for direct power is a combination of microconcentrators (Domínguez et al. 2017) with multijunction solar cells (Geisz et al. 2020) and effectively transparent contacts (ETCs) (Saive et al. 2016). As shown for a solar-powered electric car in figure 2, multijunction solar cells (C) capture nearly all wavelengths with optimal efficiency by employing different absorber layers optimized for different parts of the irradiance spectrum. Triangular cross-section silver front electrodes (e.g., ETCs) ensure optimal light capture and electric current extraction (D). External to the device, microconcentrators (B) funnel light to microscale solar cells. FIGURE 2 Schematic of future solar-powered electric cars. (A) Solar cells are seamlessly integrated with car exterior. (B) Schematic of microconcentrators funneling light (top arrows) onto microsolar cells. (C) Principle of multijunction solar cells: layers optimized for a narrow-wavelength regime converting sunlight efficiently into electricity. The rainbow colors denote the different parts of the solar spectrum. (D) Triangular cross-section microscale silver contacts (effectively transparent contacts) allow for low-loss light capturing and electricity extraction. Through light concentration, solar cells work more efficiently, to some extent analogous to any thermodynamic process that runs more efficiently given a higher temperature difference between the hot and cold reservoirs. As an added benefit of such micro- or nanostructures, antisoiling properties can be readily integrated (Quan and Zhang 2017), keeping solar-powered vehicles clean. Impacts For individual use, portable items such as mobile phones, computers, and most importantly automobiles (figure 2) will be powered both directly and indirectly by solar cells. Buildings can be fully integrated with solar cells (Meinardi et al. 2017; Peng et al. 2011) and smart energy monitors, allowing them to be a net energy source rather than a sink most of the year. Semitransparent and colorful solar cells integrated into smart windows and façades can transform any building surface into aesthetically pleasing solar power plants, and luminescent solar concentrators offer additional freedom for novel designs and functionality (Aghaei et al. 2020; Einhaus and Saive 2020; Needell et al. 2018). Around the world, massive solar power plants providing gigawatts of energy will become widespread. Densely populated countries will integrate solar with agriculture, optimizing both crop and electricity yield (Dinesh and Pearce 2016; Dupraz et al. 2011; Weselek et al. 2019). New power plants will increasingly employ bifacial solar cells that capture light on front and rear surfaces (Guerrero-Lemus et al. 2016), capitalizing on the power in ground-reflected (albedo) light (Russell et al. 2017). Moreover, with the emergence of privatized space travel, solar harvesting need not remain terrestrial. Prototypes of space solar power projects have been demonstrated (Kelzenberg et al. 2018) and likely will be employed within the next 50 years. When the Sun Doesn’t Shine and the Wind Doesn’t Blow But the sun does not shine during the night, nor provide enough energy during the winter in all areas of the world. Solar and wind energy often complement each other, but what Germans call Dunkelflaute—the simultaneous absence of wind and sun—poses a risk to electricity supply. If space solar generation does not become a viable workaround, several solutions are available: energy storage, smart appliances, and internationally interconnected electric grids. Nowadays, batteries easily buffer daily electricity variations, and their steadily increasing performance has led to commercially available electric cars with a 400-mile range (Crider 2020). Further developments will allow electric cars to provide stability to the grid by offering decentralized storage through their batteries. With smart software and electricity pricing—and perhaps autonomous driving protocols—electric cars will recharge when renewably generated electricity is abundant and cheap, and discharge during demand peaks. A car owner might even profit if instantaneous electricity trading prices are applied—although the likelihood of individually owned and operated vehicles 50 years from now will strongly diminish (indeed, as autonomous, community-owned cars become more prevalent, individual driving may be viewed the same way we now regard horseback riding). Smart appliances would also both augment the electric grid and help buffer fluctuations on a timescale of seconds to hours: refrigerators, laundry machines, air conditioners, and others could easily run whenever there is an oversupply of electricity, thus abating curtailment concerns. Nevertheless, pervasive smart grids do not solve the issue of seasonal variations of solar resources and the needs of energy-dense industrial processes. These will require either long-term storage—ideally in the form of energy-dense chemical fuels—or intercontinentally connected electric grids. The sun is always shining and the wind is always blowing somewhere. Conclusion Solar energy breakthroughs will occur at every level of society, seamlessly integrated and perfectly normal to the next generations. Optimistically, renewables may even eventually enhance international peace and stability. Diminishing the necessity of fossil fuels might settle at least some territorial conflicts, enabling most countries to become energy-independent. Moreover, the indiscriminate way the sun distributes its power to both developed and less developed countries may lead to increased wealth and independence in third world countries. References Aghaei M, Nitti M, Ekins-Daukes NJ, Reinders AH. 2020. Simulation of a novel configuration for luminescent solar concentrator photovoltaic devices using bifacial silicon solar cells. Applied Sciences 10(3):871. Bodnar P, Gray M, Grbusic T, Herz S, Lonsdale A, Mardell S, Ott C, Sundaresan S, Varadarajann U. 2020. How to Retire Early: Making Accelerated Coal Phaseout Feasible and Just. Basalt CO: Rocky Mountain Institute. Crider J. 2020. Tesla Model S Long Range Plus exceeds 400 miles of range, EPA confirms. CleanTechnica, Jun 16. Dinesh H, Pearce JM. 2016. The potential of agrivoltaic systems. Renewable and Sustainable Energy Reviews 54:299–308. 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Weselek A, Ehmann A, Zikeli S, Lewandowski I, Schindele S, Högy P. 2019. Agrophotovoltaic systems: Applications, challenges, and opportunities – A review. Agronomy for Sustainable Development 39(4):35. Rebecca Saive is an assistant professor of applied physics and nanophotonics at the University of Twente. FIGURE 1 Comparison of renewable and fossil global energy potential. For renewables, the amount of energy is shown per year; for fossil sources the total reserve is displayed. Solar energy received by emerged continents only, assuming 65% losses by atmosphere and clouds. TWy = terawatt-year. Adapted from Perez and Perez (2009). FIGURE 2 Schematic of future solar-powered electric cars. (A) Solar cells are seamlessly integrated with car exterior. (B) Schematic of microconcentrators funneling light (top arrows) onto microsolar cells. (C) Principle of multijunction solar cells: layers optimized for a narrow-wavelength regime converting sunlight efficiently into electricity. The rainbow colors denote the different parts of the solar spectrum. (D) Triangular cross-section microscale silver contacts (effectively transparent contacts) allow for low-loss light capturing and electricity extraction.  Data and statistics, International Energy Agency (https://www.iea.org/data-and-statistics?country=WORLD &fuel=Energy%20supply&indicator=Total%20primary% 20energy%20supply%20(TPES)%20by%20source)  Caltech Space Solar Power Project, https://www.spacesolar.caltech.edu/  There are plans to connect Southeast Asia and Australia, Europe, and North Africa (so Europe can get solar energy from the Sahara)(Patrick 2020). In principle it is technically feasible (just as there are intercontinental communication fibers and oil pipelines). See also https://en.wikipedia.org/wiki/European_super_grid (including sources). About the Author:Rebecca Saive is an assistant professor of applied physics and nanophotonics at the University of Twente.