|
|
| Fig. 1: Mining areas targeting materials critical for renewable energy technology and infrastructure. [18] (Source: Wikimedia Commons) |
The great majority of scientists agree that human activity has significantly contributed to climate change and global warming. [1] Though the awareness of severe health risks related to climate change declines in the scientific literature, factors related to anthropogenic global warming continue to be a profound health risk to current and future humans. [2] It is extremely likely that observed warming of surface temperatures between 1951-2010 is due to the corresponding increase in greenhouse gas (GHG) levels; furthermore, surface temperatures are projected to continue to rise over the 21st century as heat waves and droughts will increase in length and intensity. [3]
Anthropogenic greenhouse gas (GHG), or gases that absorb and emit radiant energy in the thermal infrared range, levels are now higher than ever, as there is now an unprecedented amount of carbon dioxide in the Earth's atmosphere. This increase is unprecedented, at least over the course of the past 160,000 years, and is believed to have profound climatic consequences. [4] Other primary GHGs include water vapor, methane, and ozone, but rising CO2 emissions are of the greatest concern as the gas continues to get funneled into the atmosphere unabated. A reduction in GHG levels would combat, or at least prolong, future high-temperature catastrophic climate risks. [5]
The largest source of CO2 is fossil fuel combustion. [6] An estimated 94% of CO2 emissions around the world are due to the sale and use of gas, oil, and coal. [7] In order to effectively curtail rising rates of greenhouse gases while also limiting global warming to a mere 2°C, a rapid-phaseout of fossil-fuel-related emissions is needed. [8] Thus, as a movement to commit to clean energy, or energy that comes from zero CO2 emitting sources, continues to gain prominence throughout the world, the question stands: what is the most cost-effective, promising source of clean energy?
Renewable energy sources are those that are continually replenished by nature. This includes the sun, wind, water, and plants, all of which have the potential to be used as "clean" energy sources in the future. [9] As fossil fuels continue to burn, not only are they shuttling mass amounts of CO2, but they will eventually run out. Thus, limited fossil fuels and production rates also trammel the growth potential for the global economy. [10] As such, there is a need to prepare for alternative forms of energy, specifically energy derived from renewable, or clean sources. Utilization of solar, nuclear, wind, and hydroelectric power appears to be the future of energy. In fact, government projections show that, for the first time in recorded history, the US is on track to produce more electricity from renewable energy than it is from coal. [11]
Furthermore, in spite of significant obstacles caused by the COVID-19 pandemic, the world is projected to add 4 percent to its capacity to generate electricity from renewables in 2020 and add 10 percent the subsequent year. [12] Specifically, in the United States, the total demand in power generation has remained constant in recent years, and it is expected that the share of renewable energy generation will increase from 15 to 18 percent between 2016 and 2025. [13]
More clean alternatives such as hydrogen fuel cells show future potential. At first glance, hydrogen fuel cells seem quite promising as they are reliable, environmentally friendly, and fuel efficient. Nevertheless, at the end of the day, they are very expensive, difficult to store, and highly flammable. Though costs of hydrogen fuel cells are decreasing, many energy executives continue to note the precarious future of a hydrogen-based economy in relation to other factors such as consumer behaviors and technology costs. [14]
The transition from fossil fuels to renewables as a source of energy is not necessarily a smooth one. The Clean Power Plan of 2015 had originally proposed that the US carbon power plant carbon emission target will be reduced by 32% from 2005, further bolstering support for alternative forms of energy. However, in 2019, the law was struck down by the Trump administration and replaced by a more flexible policy aimed at combating GHG emissions not by a cap, but rather through efficiency improvements at power plants and energy generating stations. [15] The United States has also since withdrawn from the Paris Agreement, a global pact negotiated by 196 nations which pledged to limit the increase in global average temperature to below 2°C. [16] Hence, as recent climate policies in the United States have been fickle and inconsistent, more clean energy solutions that can appeal to the economics, environmental concern, and vision of the future must be pursued.
One creative idea that has been explored recently is to harness the energy released by the combustion of some transition metals. This proposal regarding recyclable metal fuels implies that there may be carbon-neutral methods that could meet consumer energy demands. In order for it to be seen as a potential solution to the many other proposed alternatives, however, the process must be clean, recyclable, and physically feasible. It is true that metal powders are energy carriers that could theoretically store sources of clean, renewable energy in large quantities while also being easily transportable. As metal fuels are burned, they produce clean zero-carbon energy in the form of heat while their only product is a metal oxide. Though ostensibly promising, there are a great deal of considerations that must go into assessing the possibility of investing in clean metal fuels.
For example, in order to sustain a truly "clean" cycle, these metal oxides must be able to be reduced back to their original forms by another clean zero-carbon power source in a process known as electrolytic metal-fuel cycling. [17] In theory, metals can react with different oxidizers including, but not limited to oxygen, sulfur, carbon dioxide, and water. Often, due to ease of access and availability, the choice of reacting agent is elemental oxygen. Reactive metals such as iron and aluminum carry high energy densities that can easily be utilized through their reactions with oxygen. This gives reason to the fact that, when extracted from the Earth, both aluminum and iron are mostly found in their oxidized forms and need to be reduced in order to produce the pure metal. Nevertheless, that is where many key issues arise related to metal oxidation as a "recyclable fuel." If there is no effective energy source that is both profitable and environmentally conscious by which metal oxides can be reduced into their pure form, metal fuels are unlikely to become a large- scale renewable energy source. And, if there were such sources of energy, would the most effective use be in metal oxidation or in other mechanisms that mitigate CO2 rates in the atmosphere?
Recyclable metal fuels could also impose a significant environmental cost as a threat to global biodiversity. Mining occurs all around the world, and 8% of mining areas coincide with Protected Areas, 7% with Key Biodiversity Areas, and 16% with Remaining Wilderness. [18] As such, Fig. 1 shows that further utilization of metal mining will significantly exacerbate current mining threats to biodiversity. Furthermore, overuse of many common metals could lead to an increased likelihood of the effects of toxic metal ions or noxious smoke to the surrounding environment. Thus, it is imperative that both their environmental benefits of metal fuels, as well as their costs, be explored in greater depth if they are to be seriously investigated further as a clean energy alternative.
|
||||||||||||||||||||
| Table 1 : Kilocalorie (Cal) per dollar analysis of Al, Mg, Ti, Zn metals. [19,20] |
To simply put into perspective the large quantities of metal needed in order to sustain global energy demand for simply one year, an energy per dollar analysis of four common metals was conducted. [19] Data regarding the dollar value cost per unit mass of pure metal was acquired through a geological survey report tracking the fluctuations of metals prices through the year 2010. [20] In Table 1, the metals were analyzed based on their costs per kg in 2010 U.S. dollars.
World energy consumption totaled 5.84 × 1020 Joules in 2020. [21] Thus, by evaluating the most cost-effective metal from Table 1 in aluminum, it would take at least 1.88 × 1013 kg of aluminum to satisfy world energy consumption demands. This calculation assumes that the extraction of energy from aluminum combustion is a perfectly efficient process, so, in reality, it is likely that more aluminum is required to meet global energy standards.
The yields calculated are importantly theoretical yields, but they still show the underlying problems with metal fuels as a primary source of clean energy. Once the metal is oxidized, there will need to be efficient, large-scale appliances put into place in order to reduce the metals so that they may be used again. No calculations were made regarding the efficiency of extracting fuel from each respective metal, and it is likely that the technologies associated with each process requires varying levels of cost and labor. There is currently no source of clean energy that could transform the metals back into their purified form at such a level. Further investigation is required regarding the ability to store, convert, and utilize this energy at a consumable level. In fact, in order for aluminum fuel to be competitive with a $50 barrel of oil, the reduction process by electricity of the aluminum oxide must be priced at $26 MWh-1 or less. [22] Thus, among the four elements analyzed, aluminum, the most abundant metal on the Earth's crust, produces the greatest amount of heat energy per 2010 U.S. Dollar, but it is still not a potential source of clean energy on a global scale.
Energy reform is among the most important yet complicated forms of policies at the state, local, and international levels. Ultimately, there will be a day when all of the fossil fuels in the world will no longer be in the ground. Whether that will be in a few decades from now or in a few centuries does not matter. What matters are the logical, calculated steps we take in the next few years to address the global energy crisis.
Thus, as nations begin to find ways to move away from fossil fuels, it does not seem that recyclable metal fuels are a viable alternative source of clean energy in the immediate future. Though their combustion may be able to produce large amounts of energy, there is currently no clean source of reducing the metal fuels back into their original state as pure metals. Moreover, accelerating the mining, refining, and combusting of metals could have long-lasting detrimental effects on the environment that may counteract any real environmental benefits.
© Alexander Bhatt. 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] N. Oreskes, "The Scientific Consensus on Climate Change," Science 306, 1686 (2004).
[2] C. D. Butler, "Climate Change, Health and Existential Risks to Civilization: A Comprehensive Review (1989-2013)," Int. J. Environ. Res. Public Health 15, 2266 (2018).
[3] A. Dai, "Increasing Drought Under Global Warming in Observations and Models," Nat. Clim. Change 3, 52 (2013).
[4] P. M. Vitousek, "Beyond Global Warming: Ecology and Global Change," Ecology 75, 1861 (1994).
[5] M. L. Weitzman, "GHG Targets as Insurance Against Catastrophic Climate Damages," J. Public Econ. Theory 14, 221 (2012).
[6] "Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2018," United States Environmental Protection Agency, April 2020.
[7] C. Le Quéré et al., "Global Carbon Budget 2017," Earth Syst. Sci. Data 10, 405 (2018).
[8] J. Lelieveld et al., "Effects of Fossil Fuel and Total Anthropogenic Emission Removal on Public Health and Climate," P. Natl. Acad. Sci. USA 116, 7192 (2019).
[9] Renewable Energy, 2nd Edition, ed. by G. Boyle (Oxford University Press, 2004).
[10] D. J. Murphy and C. A. S. Hall, "Energy Return on Investment, Peak Oil, and the End of Economic Growth," Ann. N. Y. Acad. Sci. 1219, 52 (2011).
[11] B. Plumer, "In a First, Renewable Energy Is Poised to Eclipse Coal in U.S.," New York Times, 13 May 20.
[12] S. Reed, "Renewable Power Grows Strongly, Despite the Pandemic," New York Times, 10 Nov 20.
[13] X. Xu et al., "Global Renewable Energy Development: Influencing Factors, Trend Predictions and Countermeasures," Resour. Policy 63, 101470 (2019).
[14] "World Energy Issues Monitor: 2017," World Energy Council, April 2017.
[15] L. Friedman, "E.P.A. Finalizes Its Plan to Replace Obama-Era Climate Rules," New York Times, 19 June 19.
[16] L. Friedman, "U.S. Quits Paris Climate Agreement: Questions and Answers," New York Times, 4 Nov 20.
[17] P. Julien and J. M. Bergthorson, "Enabling the Metal Fuel Economy: Green Recycling of Metal Fuels," Sustain. Energy Fuels 1, 615 (2017).
[18] L. J. Sonter et al., "Renewable Energy Production Will Exacerbate Mining Threats to Biodiversity," Nat. Commun. 11, 4174 (2020).
[19] S. H. Fischer and M. C. Grubelich, "Theoretical Energy Release of Thermites, Intermetallics, and Combustible Metals," Sandia National Laboratories, July 1998.
[20] "Metal Prices in the United States Through 2010," U.S. Geological Survey, Scientific Investigations Report 2012-5188, 2013.
[21] "Statistical Review of World Energy: 2020," British Petroleum, June 2020.
[22] K. A. Trowell et al., "Aluminum and Its Role as a Recyclable, Sustainable Carrier of Renewable Energy," Appl. Energ. 275, 115112 (2020).
