Tuesday, January 15, 2019

Not from Concentrate

In 212 BC, the Greek scholar Archimedes invented a death ray that used an army’s worth of mirrors to reflect the sun onto a single point, concentrating sunlight enough to set enemy warships on fire.  Two thousand years later, scientists are trying to use a similar design to create alternative fuels.

Of course, you can’t fit an army in a research laboratory, so scientists at the University of Minnesota are using a solar simulator, instead.  The solar simulator reflects light from seven high-powered light bulbs and concentrates it to between 1000 and 3000 times the sun’s radiance.  The energy in this concentrated “sunlight” is then used to drive chemical reactions.

The solar simulator at the University of Minnesota

In a study published last month in Energy, members of the Solar Energy Lab used the sunlight to convert methane, a natural gas, into a mixture of hydrogen and carbon monoxide called synthesis gas.  Synthesis gas can then be used to make most products we have historically gotten from petroleum, like liquid fuels.

Solar liquid fuels solve one of the major problems with current alternative energy: storage.  Most current processes, like photovoltaic cells and wind turbines, create electricity, which is difficult and expensive to store for use at other times.  With this method, energy from the concentrated sunlight is stored in the chemical bonds of the methane-based fuel, increasing the amount of energy that methane can produce.  In this form, solar energy can be collected in one place when the weather is nice and used six months later in the middle of a snowstorm.  Solar-based fuels also make more efficient use of all the energy in the solar spectrum than do photovoltaic cells, reduce greenhouse gas emissions, and can be used in current car and jet engines without buying new electric vehicles.

The recent paper demonstrates that a specific process, solar chemical-looping methane reforming, has the potential to be an efficient and commercially viable way of storing solar energy.  The team at UMN were able to demonstrate that the chemical reaction that creates synthesis gas can be run using readily available materials with consistently high efficiency, suggesting that this could be a viable method of storing solar energy on a commercial scale. “One of the big goals of this work was to take something that’s primarily been studied at a small scale and show that we’re able to scale it up while still being able to get steady and repeatable performance and demonstrate high efficiencies,” said Jesse Fosheim, co-author of the study. “Efficiency is going to correlate directly with cost, so the more efficient we’re able to make the process, the cheaper it’s going to be.”    

“Their work has established a new benchmark against which other approaches to solar chemical looping reforming will now be compared,” said Dr. Luke Venstrom, Assistant Professor at Valparaiso University, who was not involved in the study.  “Their work sets up the next step, which would be to evaluate the techno-economics of a large-scale plant based on their performance projections.”

Economics is currently a dark cloud for solar chemical processes. Although the actual process of creating solar-derived fuels is relatively inexpensive, building plants capable of producing fuels on a commercial scale requires a large influx of upfront capital, making it a potentially high-risk investment for governments and industry investors. Unfortunately, Venstrom sees it unlikely that the cost of solar chemical plants will come down without putting money into a larger scale plant first. “Large-scale demonstrations reduce risk for investors and drive costs down the learning curve. Laboratory work can only take a technology so far,” he said.  Fosheim agreed. “A pilot-scale plant would show that this technology can work at these larger scales and also give engineers and scientists a test bed to figure out the problems that we don’t encounter at the laboratory scale to really work out the kinks.”

Fortunately, the German Aerospace Center (DLR) has recently debuted a solar simulator that operates on the megawatt-scale (100 times greater than the simulator at the University of Minnesota).  Although it isn’t a full-scale plant using real sunlight, the simulator will hopefully allow researchers to begin testing these processes beyond the benchtop to make the transition to solar energy easier.  For Fosheim, this, not incinerating ships, is what drew him to engineering. “Engineers’ goal is to use math, science, and technology to solve problems.  In today’s day and age, one of the most pressing problems we have is how to change our energy infrastructure such that it’s clean, renewable, and sustainable moving into the future.”