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| Fig. 1: Pumped Hydroelectric Storage (Source: Wikimedia Commons) |
The advantages of renewable energy sources, such as wind and solar energy, are clear and indisputable. These clean sources of power cut down on carbon emissions, save billions of gallons of water every year, and reduce pollution and smog. For instance, electricity generated from wind turbines has zero carbon emissions; consequently, it is estimated that one million kWh of electricity generated from wind power can save up to 600 tonnes of carbon dioxide emissions. [1] In terms of saving water, it takes 615 gallons of fresh water to cool off a nuclear power plant to generate electricity for an average home per day. [2] In a sharp contrast, solar panels and wind turbines need magnitudes of lower amount of water, saving a ton of water for human consumption and farming. One of the questions that immediately rise in switching to these renewable energy sources is their unpredictability. In terms of meeting demand, it may be okay when there is excess of wind and solar production; however, a flip side of this question is more urgent and challenging: what happens when the production falls short?
The current solution to the problem is grid-scale energy storage. While there are many different forms of grid-scale energy storage, 97% of energy storage in the United States is by pumped hydroelectric storage, which stores energy in the form of gravitational potential energy of water. [3] During times of excess production, electricity is used to pump water to high elevation. During times of excess demand, water is released to re-generate electricity. While pumped hydroelectric storage is currently the most cost- effective and the most efficient design in storing intermittent energy forms, it is geographically limited, as shown in Fig. 1. It requires geographical elevation and water source. Thermal energy storage may be an alternate to pumped hydroelectric storage. Thermal energy storage proposes storing electricity as heat. During times of excess production, solar and wind energy will be used to heat extremely low-cost raw materials like carbon blocks. During times of demand, thermophotovoltaic heat engine will be used to convert heat back to electricity using photons. [4] If successful, thermal energy storage may resolve the spatial challenges met by grid-scale energy storage systems at an even lower cost and comparable efficiency.
While many agree that solar and wind are cleaner, safer, and less expensive forms of energy than fossil fuel, they are nonetheless intermittent energy sources and their abundance varies with weather. Making them more readily available and accessible regardless of the location remain challenging; however, thermal energy storage may be the next step in storing intermittent energy sources in a more scalable, environmentally friendly, and accessible way.
© Anika Kim. 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] R. Saidur et al., "Environmental Impact of Wind Energy," Renew. Sustain. Energy Rev. 15, 2423 (2011).
[2] L. Castillo, W. Gutierrez, and J. Gore, "Renewable Energy Saves Water and Creates Jobs," Scientific American, 7 Aug 2018.
[3] "Pumped Storage Report," National Hydropower Association, 2018.
[4] Y. Nam, et al., "Solar Thermophotovoltaic Energy Conversion Systems with Two-Dimensional Tantalum Photonic Crystal Absorbers and Emitters," Sol. Energy Mat. Sol. C. 112, 287 (2014).
