Solar Fuels: An Alternative Way to Fuel the Future
Written by Elissa Yao
Edited by Carson Riar
"Nowadays, the limitation for solar energy is no longer how we can collect more energy but rather how we store it"
Using the energy from the sun to power our lives has been an idea in popular conscience since the 1950s with the birth of photovoltaics in Bell labs (Smithsonian Magazine, 2019). Over the decades, the efficiencies of solar panels increased from 4% at conception to up to 32% now (Farji, 2021). Nowadays, the limitation for solar energy is no longer how we can collect more energy but rather how we store it. A report from California Independent System Operator explained that over 305,241 megawatt-hours of solar and wind electricity have been wasted in the state because the demand for energy did not match the supply (Kennedy, 2017). That is enough energy to power 45,000 homes for a year. An adequate storage system for renewable energy generated needs to be found for the world to be able to transition to using predominantly solar energy. A holy grail of energy storage would be in the chemical bonds of fuels. Fuels are easy to trade, straightforward to store and we already have many systems that can use them such as automobiles, industrial manufacturing, air travel and more. Solar fuels are an alluring prospect to pursue because of several reasons: solar energy is abundant and the technology has become quite efficient in collecting that energy. Secondly, using solar energy for water splitting, producing hydrogen fuels would be a sustainable alternative to the methods for producing hydrogen fuel used today. Lastly, the environmental impact of current battery technology is extremely high, making an alternative form of storing the energy collected, much more attractive.
The celestial body sitting at the center of our solar system is an immense source of energy. Each year, the Earth receives 120,000 TW of solar radiation. The rise of solar photovoltaics globally can be attributed to their sharp declining costs since 2008 (Green, 2019). The most popular solar cells are made of polycrystalline silicon. Silicon, which acts as an intrinsic semiconductor is the key component of the ability for solar panels to utilize photovoltaics. Since 2008, the price of silicon-based solar cells has been dropping for several reasons. These reasons include the lowered cost of the other parts of the cell such as the glass, the metal conductors, the anti-reflective coating, and the reduced cost of polysilicon. Moreover, advancements in the engineering of techniques have reduced the amount of silicon required without sacrificing efficiency and the funding injections to reduce the cost of manufacturing. Polysilicon solar cells are reported to be around 18% efficient; the efficiency here is referring to “ultimate efficiency”, the percentage of photons absorbed on the semiconductor that is converted to useful energy (Farji, 2021). The Shockley–Queisser limit, a theoretical higher limit for the efficiency of solar cells is 33.7% and many photovoltaic cells (not made of silicon) have even been able to surpass this limit. As impressive as the engineering has become, pushing the efficiencies of solar cells higher and higher, the critical problem is that there is no easy way to store this energy. The mismatch of supply time (when the sun is out during the day) and demand time (usually evenings) means that large-scale energy storage is needed to use the energy collected. There have been tremendous strides forward developing solar cells in the last few decades and the development of how to store that electricity is struggling to keep up.
There are many ways that energy can be stored, each with its own advantages and challenges. A cutting-edge way to store the electricity generated from photovoltaics would be to make hydrogen fuel. Earlier this year, Alberta signed a $1.3 billion hydrogen farm followed by important discourse the future role of hydrogen fuel (Frangoul, 2021). A spokesman for the Canadian Association of Petroleum Producers said “Canada’s natural gas industry and oil industry has a significant role to play in a responsible energy future. The industry has the natural resources along with the talent, expertise and drive to deliver lower emissions intensive energy.” However, this is quite misleading since the method used to generate hydrogen fuel at this plant is very far from responsible. The method that is currently employed to generate hydrogen fuel is called “natural gas reforming” or “steam reforming” which generates the hydrogen fuel (H2) from methane (CH4). The reaction occurring is:
According to a life cycle analysis from 2000, natural gas reforming has substantial global warming potential (GWP) which looks at the impact a process might have on accelerating global warming . Excessive carbon dioxide emissions is one of the ways that a process might have a high GWP. To reform natural gas into hydrogen fuel, 1.188 kilograms of carbon dioxide are emitted per kilogram of hydrogen fuel made (Spath & Mann, 2000). Natural gas reforming is the most popular way to generate hydrogen fuel because of the low cost and the continuous growth of natural gas production in countries such as the US since 2005 (Enerdata Global Energy Statistical Yearbook 2021). Much of the environmental impact of hydrogen gas comes from the production of fuels since the by-product of burning hydrogen fuels is water.
Solar power offers a clean way to generate hydrogen fuel through water splitting in two possible ways. One method is electrolysis where electrical current is passed through water decomposing it into oxygen and hydrogen gas. Another method is photocatalytic water splitting which mimics the natural process of photosynthesis. The latter method has not yet met the minimum efficiencies for practical large-scale systems. The theoretical limit of efficiency for using solar photovoltaic energy through electrolysis is up to 90-95% (Jia, 2016). The generation is based on a simple electrochemical reaction:
The patent for the process of running this reaction was approved in the 1980s but has not been widely utilized due to the value of electricity often being higher than the fuel produced. Though now with high-efficiency solar panels, we are collecting more energy than we can use or store and that electricity can go towards producing a usable fuel that displaces fossil fuels. Already today, there are automotive vehicles on the road that run on a hydrogen fuel cell rather than an internal combustion engine such as the Toyota Mirai and the Hyundai Nexo. Companies like Ballard are adapting hydrogen fuel cell technology for larger-scale transportation such as trains, ships, buses and trucks. Interest in hydrogen fuels has been growing for many industries and if pursued correctly, could be the clean fuel alternative to fossil fuels.
Another problem that the development of solar fuels may seek to solve is reducing the environmental and geopolitical impact of producing batteries. Many of the sustainable energies available today such as hydropower and wind-generated electricity is stored in batteries. Unfortunately, the negative impacts of producing lithium-ion batteries, the most popular and widely used type of battery today, are massive and often overlooked as the world scrambles in transitioning to renewable energy. There are many environmental problems, many of which accompany the acquisition of raw materials such as lithium and cathode materials, predominantly cobalt and nickel. Lithium has spiked in demand as it finds its way into electronics and cars batteries and is only projected to be more sought after. Most of the lithium extraction occurs in the salt deserts of South America, where clean water is a precious resource to the locals. The water demand for lithium extraction is huge, using around 2273 L of water for every kilogram of lithium produced (da Silva Lima et al., 2021). Currently, the extraction process is reliant on chemicals such as hydrochloric acid which can contaminate the soil and water in the area. A life cycle assessment published in the Journal of Sustainable Mining explored the environmental impact of cobalt metal production. Cobalt metal is a crucial component of the cathode in lithium-ion batteries. They found that the largest impacts of cobalt extraction come from the energy of purification. The metal particles emitted during the process have harmful non-reversible impacts on the health of the environment that those who inhabit it.
The difference in energy density of fuels compared to batteries is tremendous with hydrogen fuels containing the equivalent of 33000 Wh/kg, while lithium-ion batteries tend to hold the range of 100-265 Wh/kg (da Silva Lima et al., 2021). In general, batteries have to improve substantially both in energy capacity and use earth-abundant, non-toxic materials to sustainably store renewable energy as the world transitions away from fossil fuels. Solar fuels may be able to slow the growing demand for batteries with hydrogen fuel cells that may replace the 540 kg lithium-ion batteries in electric vehicles such as the Tesla Model S.
The most substantial factor that holds back the use of hydrogen fuels cells today is the cost; a story that is familiar to every emerging renewable technology sector. Solar energy-generated fuels have the theoretical potential of utilizing energy infrastructure that already exists and building upon the advances already made. It is important when thinking about the future of energy to consider the entire picture rather than just the source or the inputs. Global solar and wind energy growth is exciting but only increasing production without considering sustainable storage is holding back the energy paradigm shift away from fossil fuels.
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