Solar Fuels as Versatile Energy Solutions

Gigi Lin in conversation with Prof. Tom Jaramillo

Fossil fuels currently supply 80% of the energy we consume today. They power human activities ranging from driving our cars to heating our homes to operating industrial plants. Although energy consumption in Europe and North America has largely stagnated in recent years, it has voraciously accelerated among developing countries, resulting in gigatons (Gt) of carbon dioxide (CO2) emissions per year and harmful implications for geopolitics and the environment.

To give a sense of magnitude, about 9.9 Gt of carbon (36 Gt of CO2) were emitted to the atmosphere in 2013, the highest record in human history [1]. Of the carbon emissions, 43% came from coal burning, oil 33%, gas 18%, cement 5.5%, and gas flaring, 0.6%. The two largest producers of CO2 emissions were China (28% of total CO2 emissions) and the United States (14%) [1].

I spoke with Professor Tom Jaramillo, an expert on the chemical transformations in renewable fuels production, about how solar fuels could contribute to a sustainable energy economy currently dominated by fossil fuels.

Photo by Dennis Schroeder, NRELAlgae can be engineered to produce lipids and oils from sunlight and carbon dioxide into jet fuel or diesel. Photo by Dennis Schroeder, NREL

First of all, what are “solar fuels?”

It’s very simple. Solar fuels refers to the very broad concept of using free energy from the sun and other plentiful resources in the environment – such as carbon dioxide from fuel emissions – to store energy in the form of chemical bonds, that is, as fuels. It falls under the even broader category of renewable fuels, which encompasses geothermal and wind sources.

Does that mean that any nondescript fuel can be classified as a solar fuel?

Any fuel can be classified as a “solar fuel” as long as it uses a renewable pathway to transform the sun’s energy into a chemical compound, which is more easily stored and used by our existing energy infrastructures. The fuels can be hydrocarbons, oxygenates, alcohols, and even hydrogen.

Currently, hydrogen fuel is produced at 60 megatons per year but from fossil sources. We want to find chemical pathways that produce hydrogen and other fuels from solar energy and other renewable sources to really put a dent in the energy problem. Of course, it is much more challenging to convert carbon dioxide into gasoline than it is to produce hydrogen renewably.

Because the term “solar fuel” itself is so broad, many different energy platforms are possible for producing solar fuels.

A woman in India makes “komaya”, a cow dung patty used for fuel. Both algae and cow dung can be considered solar fuels. Photo by Tevaprapas, distributed under a CC-BY 2.0 license

When discussing energy issues, environmental issues are often brought into the conversation. How have environmental concerns influenced interest in specific energy platforms used to produce solar fuels?

At the heart of the matter, cost still remains the most important factor in dictating the approach that people take in selecting a fuel pathway. It is more important than the environmental impact of the fuel emissions. Many people can’t afford to pay premium prices for fuels with better emissions records, especially since a fuel is a fuel. One might pay $400 for a cell phone with more functionality than a $99 model, but there is no utilitarian incentive for a consumer to purchase a more environmentally-friendly fuel that is more expensive than the de facto fuel, other than out of the goodness of his or her heart.

Author’s note: The cost of producing photovoltaic modules and wind energy for renewable energy production has declined over the last 30 years relative to that of wheat, a staple food product.

WheatPVComparisonTrends in renewable-energy costs and in petroleum costs from 1975–2010, both relative to wheat. The photovoltaic values refer to modules and do not include costs from installation [2].

As a result, most solar fuel production platforms are chosen based on the economics of producing the fuel, which is dependent on the efficiency and robustness of the system. One approach is to make a drop-in replaceable for a current fuel, such as jet fuel, diesel fuel, or another fuel already in existence. There is still an environmental gain from this approach, because one is sourcing the carbon dioxide from the atmosphere and producing net zero carbon emissions.

Another common approach is to make a fuel that is less conventional, such as hydrogen, because it is easier to transform water via electrolysis to hydrogen than it is to produce hydrocarbons from carbon dioxide. From an energy density perspective, there is no significant distinction between the fuels. A hydrogen-hydrogen bond may have a slightly different energy density than a carbon-hydrogen bond, but to a first-order approximation, these bonds are all great for storing energy.

Author’s note: One approach to evaluating the energy content of the fuels is to compare their higher heating values (HHV). The chart below provides a visual for comparing the HHV of several well-known fuels.

Comparison of higher heating values for several fuels. The higher heating value reflects the energy value of a compound [3].

What do you think is the greatest barrier to entry for solar fuels in the energy industry?

At the moment, the challenge lies in developing efficient and robust processes for producing solar fuels and also in making the processes intrinsically cheaper. However, even if the cost of raw materials were not a problem, currently no technology for solar fuels production exists that is both stable and efficient. The technology needs to be advanced on both fronts.

How can policymakers help?

Energy policies can be established to fund and develop the technology to its maximum potential. It’s possible. Bioethanol, a solar fuel produced from water, sunlight, and CO2, provides a great example of a fuel that has reached the limits of its growth potential in the energy market. The technology and market have been developed to the point at which the energy output is greater than the energy input, particularly if the feedstock is sugarcane. Already, 10% of bioethanol is blended into gasoline in the United States.

However, further increases in the contribution of bioethanol to energy production are limited by its competition with agriculture for arable land to grow the biomass feedstock. That is why a sustainable energy solution will require many different pathways and the contributions of energy sources like geothermal to contribute to the 80% slice of energy consumption that fossil fuels currently support.

Looking toward the future, what role do you envision for solar fuels in the energy industry?

The sweet spot for solar fuels resides in the transportation sector. As transportation needs continue to grow with the human population and economy, solar fuels can replace fossil-derived fuels in transportation vehicles. In the next 10 years, commercial processes may emerge that produce fuel from sunlight but the market will still be very small. In 20-30 years, renewables may account for a larger portion of the energy market, but that estimate is a fairly optimistic projection.

Developing countries will likely shape the trajectory of solar fuels, simply because their energy consumption is growing rapidly compared to the developed world. For example, China’s energy decisions will have the biggest impact on the direction of the world, because it is the most populous country and is developing the fastest.

 

Tom Jaramillo is an Associate Professor in the Chemical Engineering Department at Stanford University and an affiliate of the Precourt Institute for Energy. His research interest focuses on fundamental catalytic chemical-to-electrical and electrical-to-chemical energy conversion processes occurring on solid-state surfaces.

Gigi Lin is a 3rd year PhD candidate in ChemE at Stanford, researching the rheology of therapeutic protein aggregation in pharmaceutical environments. After graduation, she hopes to leverage her technical experience towards working on interdisciplinary environmental policy. 

 

References:

  1. “Global Carbon Budget.” Global Carbon Project.
  2. Newman, J. et al. Review: An Economic Perspective on Liquid Solar Fuels. J. Electrochem. Soc. 2012 159(10): A1722-A1729.
  3. Data taken from “Lower and Higher Heating Values of Fuels.” U.S. Department of Energy, Energy Efficiency & Renewable Energy: Hydrogen Analysis Resource Center.