Energy

For the past 14 years, the MIT Energy Conference — a two-day event organized by energy students — has united students, faculty, researchers, and industry representatives from around the world to discuss cutting-edge developments in energy.
Under the supervision of Thomas “Trey” Wilder, an MBA candidate at the MIT Sloan School of Management, and a large team of student event organizers, the final pieces for the 2020 conference were falling into place by early March — and then the Covid-19 pandemic hit the United States. As the Institute canceled in-person events to reduce the spread of the virus, much of the planning that had gone into hosting the conference in its initial format was upended.
The Energy Conference team had less than a month to transition the entire event — scheduled for early April — online.
During the conference’s opening remarks, Wilder recounted the month leading up to the event. “Coincidently, the same day that we received the official notice that all campus events were canceled, we had a general body Energy Club meeting,” says Wilder. “All the leaders looked at each other in disbelief — seeing a lot of the work that we had put in for almost a year now, seemingly go down the drain. We decided that night to retain whatever value we could find from this event.”
The team immediately started contacting vendors and canceling orders, issuing refunds to guests, and informing panelists and speakers about the conference’s new format.
“One of the biggest issues was getting buy-in from the speakers. Everyone was new to this virtual world back at the end of March. Our speakers didn’t know what this was going to look like, and many backed out,” says Wilder. The team worked hard to find new speakers, with one even being brought on 12 hours before the start of the event.
Another challenge posed by taking the conference virtual was learning the ins and outs of running a Zoom webinar in a remarkably short time frame. “With the webinar, there are so many functions that the host controls that really affect the outcome of the event. Similarly, the speakers didn’t quite know how to operate it, either.”
In spite of the multitude of challenges posed by switching to an online format on a tight deadline, this year’s coordinating team managed to pull off an incredibly informative and timely conference that reached a much larger audience than those in years past. This was the first year the conference was offered for free online, which allowed for over 3,500 people globally to tune in — a marked increase from the 500 attendees planned for the original, in-person event.
Over the course of two days, panelists and speakers discussed a wide range of energy topics, including electric vehicles, energy policy, and the future of utilities. The three keynote speakers were Daniel M. Kammen, a professor of energy and the chair of the Goldman School of Public Policy at the University of California at Berkeley; Rachel Kyte, the dean of the Tufts Fletcher School of Law and Diplomacy; and John Deutch, the Institute Professor of Chemistry at MIT.
Many speakers modified their presentations to address Covid-19 and how it relates to energy and the environment. For example, Kammen adjusted his address to cover what those who are working to address the climate emergency can learn from the Covid-19 pandemic. He emphasized the importance of individual actions for both the climate crisis and Covid-19; how global supply chains are vulnerable in a crowded, denuded planet; and how there is no substitute for thorough research and education when tackling these issues.
Wilder credits the team of dedicated, hardworking energy students as the most important contributors to the conference’s success. A couple of notable examples include Joe Connelly, an MBA candidate, and Leah Ellis, a materials science and engineering postdoc, who together managed the Zoom operations during the conference. They ensured that the panels and presentations flowed seamlessly.
Anna Sheppard, another MBA candidate, live-tweeted throughout the conference, managed the YouTube stream, and responded to emails during the event, with assistance from Michael Cheng, a graduate student in the Technology and Policy Program.
Wilder says MBA candidate Pervez Agwan “was the Swiss Army knife of the group”; he worked on everything from marketing to tickets to operations — and, because he had a final exam on the first day of the conference, Agwan even pulled an all-nighter to ensure that the event and team were in good shape.
“What I loved most about this team was that they were extremely humble and happy to do the dirty work,” Wilder says. “Everyone was content to put their head down and grind to make this event great. They did not desire praise or accolades, and are therefore worthy of both.” More

Nuclear energy provides about 20 percent of the U.S. electricity supply, and over half of its carbon-free generating capacity.   
Operations of commercial nuclear reactors produce small quantities of spent fuel, which in some countries is reprocessed to extract materials that can be recycled as fuel in other reactors. Key to the improvement of the economics of this fuel cycle is the capture of gaseous radioactive products of fission such as 85krypton.
Therefore, developing efficient technology to capture and secure 85krypton from the mix of effluent gasses would represent a significant improvement in the management of used nuclear fuels. One promising avenue is the adsorption of gasses into an advanced type of soft crystalline material, metal organic frameworks (MOFs), which have extremely high porosity and enormous internal surface area and can incorporate a vast array of organic and inorganic components.
Recently published research by a multidisciplinary group that includes members of MIT’s Department of Nuclear Science and Engineering (NSE) represents one of the first steps toward practical application of MOFs for nuclear fuel management, with novel findings on efficacy and radiation resistance, and an initial concept for implementation.
One fundamental challenge is that the mix of gasses produced during fuel reprocessing is rich in oxygen and nitrogen, and existing methods tend to collect them as well as the part-per-million quantities of krypton that represent the highest risk. This reduces the purity of the collected 85Kr and increases the waste volume. Moreover, existing krypton extraction methods rely on costly and complex cryogenic processes.
The group’s study, published in the journal Nature Communications, evaluated a series of ultra-microporous MOFs with different metal centers including zinc, cobalt, nickel, and iron, and found that a copper-containing crystal, SIFSIX-Cu, showed good promise.
To harness its favorable combination of radiation stability and selective adsorption, while also minimizing the volume of waste, the team proposed a two-step treatment process, in which an initial bed of the material is used to adsorb xenon and carbon dioxide from the effluent gas mixture, after which the gas is transferred to a second bed which selectively adsorbs krypton but not nitrogen or oxygen.
“If one day we want to treat the spent fuels, which in the U.S. are currently stored in pools and dry casks at the nuclear power plant sites, we need to handle the volatile radionuclides.” explains Ju Li, MIT’s Battelle Energy Alliance Professor of Nuclear Science and Engineering and professor of materials science and engineering. “Physisorption of krypton and xenon is a good approach, and we were very happy to collaborate with this large team on the MOF approach.”
MOFs have been seen as a possible solution for applications in many fields, but this research marks the first systematic study of their applicability in the nuclear sector, and the effectiveness of different metal centers on MOF radiation stability, notes Sameh K. Elsaidi, a research scientist at the U.S. Department of Energy’s National Energy and Technology Laboratory and the paper’s lead author.
“There are already over 60,000 different MOFs, and more are being developed every day, so there are a lot to choose from,” says Elsaidi. “The selection of one for 85Kr separation during reprocessing is based on several essential criteria. During our long search for porous materials that can meet these criteria, we found that a class of microporous MOFs called SIFSIX-3-M can efficiently reduce the volume of nuclear waste by separating 85Kr in more pure form from the other nonradioactive gasses. However, in order to be useful for practical separation of 85Kr, these materials must be resistant to radiation under reprocessing conditions.
“This is a first look at candidates that can meet the criteria. I feel very lucky to be working with Ju and [MIT NSE postdoc Ahmed Sami Helal] as we start to evaluate whether these materials can be used in the real world. This project was a very good example of how collaborative work can lead to better fundamental understanding, and there’s a lot down the road that we can do together,” adds Elsaidi.
Helal notes, “Studying the effect of high-energy ionizing radiation, including β-rays and γ-rays, on the stability of MOFs is a very important factor in determining whether the MOFs can be used for capture of fission gasses from used fuel. This work is the first to investigate the radiolytic stability of MOFs at radiation doses relevant to practical Xe/Kr separation at fuel reprocessing plants.”
Developing a practical adsorption process is a complex task, requiring capabilities from multiple disciplines including chemical engineering, materials science, and nuclear engineering. The research leveraged several specialized Institute resources, including the MIT gamma irradiation facility (managed by the MIT Radiation Protection Program) and the High Voltage Research Laboratory, which was used for beta irradiation measurements with assistance from Mitchell Galanek of the MIT Office of Environment, Health and Safety.
Those efforts, in conjunction with X-ray diffraction studies and electronic structure modeling, “were fascinating and helped us learn a lot about MOFs and build our understanding of non-neutronic radiation resistance of this new class of materials,” says Li. “That could be useful in other applications in the future,” including detectors.
In addition to MIT and the National Energy Technology Laboratory, collaborators on the project included the Pacific Northwest National Laboratory (Praveen Thallapally), the University of Pittsburgh (Mona Mohamed), and the University of South Florida (Brian Space and Tony Pham). Programmatic funding was provided by the U.S. Department of Energy’s Office of Nuclear Energy, with additional support from the National Science Foundation. Computational resources were made available via an XSEDE Grant and by the University of South Florida.

Topics: Nuclear science and engineering, Materials Science and Engineering, Research, Collaboration, Energy, Nuclear power and reactors, School of Engineering, Department of Energy (DoE), National Science Foundation (NSF), Emissions, DMSE More

The following letter was sent to the MIT community today by President L. Rafael Reif.
To the members of the MIT community,
I am delighted to share an important step in MIT’s ongoing efforts to take action against climate change.
Thanks to the thoughtful leadership of Vice President for Research Maria Zuber, Associate Provost Richard Lester and a committee of 26 faculty leaders representing all five schools and the college, today we are committing to an ambitious new research effort called Climate Grand Challenges.
MIT’s Plan for Action on Climate Change stressed the need for breakthrough innovations and underscored MIT’s responsibility to lead. Since then, the escalating climate crisis and lagging global response have only intensified the need for action.
With this letter, we invite all principal investigators (PIs) from across MIT to help us define a new agenda of transformative research. The threat of climate change demands a host of interlocking solutions; to shape a research program worthy of MIT, we seek bold faculty proposals that address the most difficult problems in the field, problems whose solutions would make the most decisive difference.
The focus will be on those hard questions where progress depends on advancing and applying frontier knowledge in the physical, life and social sciences, or advancing and applying cutting-edge technologies, or both; solutions may require the wisdom of many disciplines. Equally important will be to advance the humanistic and scientific understanding of how best to inspire 9 billion humans to adopt the technologies and behaviors the crisis demands.
We encourage interested PIs to submit a letter of interest. A group of MIT faculty and outside experts will choose the most compelling – the five or six ideas that offer the most effective levers for rapid, large-scale change. MIT will then focus intensely on securing the funds for the work to succeed. To meet this great rolling emergency for the species, we are seeking and expecting big ideas for sharpening our understanding, combatting climate change itself and adapting constructively to its impacts.
You can learn much more about the overall concept as well as specific deadlines and requirements here.
This invitation is geared specifically for MIT PIs – but the climate problem deserves wholehearted attention from every one of us. Whatever your role, I encourage you to find ways to be part of the broad range of climate events, courses and research and other work already under way at MIT. 
For decades, MIT students, staff, postdocs, faculty and alumni have poured their energy, insight and ingenuity into countless aspects of the climate problem; in this new work, your efforts are our inspiration and our springboard. 
We will share next steps in the Climate Grand Challenges process later in the fall semester.
Sincerely,
L. Rafael Reif More

Today, Russia’s economy depends heavily upon its abundant fossil fuel resources. Russia is one of the world’s largest exporters of fossil fuels, and a number of its key exporting industries — including metals, chemicals, and fertilizers — draw on fossil resources. The nation also consumes fossil fuels at a relatively high rate; it’s the world’s fourth-largest emitter of carbon dioxide. As the world shifts away from fossil fuel production and consumption and toward low-carbon development aligned with the near- and long-term goals of the Paris Agreement, how might countries like Russia reshape their energy-intensive economies to avoid financial peril and capitalize on this clean energy transition?
In a new study in the journal Climate Policy, researchers at the MIT Joint Program on the Science and Policy of Global Change and Russia’s National Research University Higher School of Economics assess the impacts on the Russian economy of the efforts of the main importers of Russian fossil fuels to comply with the Paris Agreement.
The researchers project that expected climate-related actions by importers of Russia’s fossil fuels will lower demand for these resources considerably, thereby reducing the country’s GDP growth rate by nearly 0.5 percent between 2035 and 2050. The study also finds that the Paris Agreement will heighten Russia’s risks of facing market barriers for its exports of energy-intensive goods, and of lagging behind in developing increasingly popular low-carbon energy technologies.
Using the Joint Program’s Economic Projection and Policy Analysis model, a multi-region, multi-sector model of the world economy, the researchers evaluated the impact on Russian energy exports and GDP of scenarios representing global climate policy ambition ranging from non-implementation of national Paris pledges to collective action aligned with keeping global warming well below 2 degrees Celsius.
The bottom line: Global climate policies will make it impossible for Russia to sustain its current path of fossil fuel export-based development.
To maintain and enhance its economic well-being, the study’s co-authors recommend that Russia both decarbonize and diversify its economy in alignment with climate goals. In short, by taxing fossil fuels (e.g., through a production tax or carbon tax), the country could redistribute that revenue to the development of human capital to boost other economic sectors (primarily manufacturing, services, agriculture, and food production), thereby making up for energy-sector losses due to global climate policies. The study projects that the resulting GDP increase could be on the order of 1-4 percent higher than it would be without diversification.
“Many energy-exporting countries have tried to diversify their economies, but with limited success,” says Sergey Paltsev, deputy director of the MIT Joint Program, senior research scientist at the MIT Energy Initiative (MITEI) and director of the MIT Joint Program/MITEI Energy-at-Scale Center. “Our study quantifies the dynamics of efforts to achieve economic diversification in which reallocation of funds leads to higher labor productivity and economic growth — all while enabling more aggressive emissions reduction targets.”  
The study was supported by the Basic Research Program of the National Research University Higher School of Economics and the MIT Skoltech Seed Fund Program.

Topics: Joint Program on the Science and Policy of Global Change, MIT Energy Initiative, Climate change, Alternative energy, Energy, Environment, Economics, Greenhouse gases, Carbon dioxide, Research, Policy, Emissions, Russia, Technology and society More

An essential component of any climate change mitigation plan is cutting carbon dioxide (CO2) emissions from human activities. Some power plants now have CO2 capture equipment that grabs CO2 out of their exhaust. But those systems are each the size of a chemical plant, cost hundreds of millions of dollars, require a lot of energy to run, and work only on exhaust streams that contain high concentrations of CO2. In short, they’re not a solution for airplanes, home heating systems, or automobiles.
To make matters worse, capturing CO2 emissions from all anthropogenic sources may not solve the climate problem. “Even if all those emitters stopped tomorrow morning, we would still have to do something about the amount of CO2 in the air if we’re going to restore preindustrial atmospheric levels at a rate relevant to humanity,” says Sahag Voskian SM ’15, PhD ’19, co-founder and chief technology officer at Verdox, Inc. And developing a technology that can capture the CO2 in the air is a particularly hard problem, in part because the CO2 occurs in such low concentrations.
The CO2 capture challenge
A key problem with CO2 capture is finding a “sorbent” that will pick up CO2 in a stream of gas and then release it so the sorbent is clean and ready for reuse and the released CO2 stream can be utilized or sent to a sequestration site for long-term storage. Research has mainly focused on sorbent materials present as small particles whose surfaces contain “active sites” that capture CO2 — a process called adsorption. When the system temperature is lowered (or pressure increased), CO2 adheres to the particle surfaces. When the temperature is raised (or pressure reduced), the CO2 is released. But achieving those temperature or pressure “swings” takes considerable energy, in part because it requires treating the whole mixture, not just the CO2-bearing sorbent.
In 2015, Voskian, then a PhD candidate in chemical engineering, and T. Alan Hatton, the Ralph Landau Professor of Chemical Engineering and co-director of the MIT Energy Initiative’s Low-Carbon Energy Center for Carbon Capture, Utilization, and Storage, began to take a closer look at the temperature- and pressure-swing approach. “We wondered if we could get by with using only a renewable resource — like renewably sourced electricity — rather than heat or pressure,” says Hatton. Using electricity to elicit the chemical reactions needed for CO2 capture and conversion had been studied for several decades, but Hatton and Voskian had a new idea about how to engineer a more efficient adsorption device.
Their work focuses on a special class of molecules called quinones. When quinone molecules are forced to take on extra electrons — which means they’re negatively charged — they have a high chemical affinity for CO2 molecules and snag any that pass. When the extra electrons are removed from the quinone molecules, the quinone’s chemical affinity for CO2 instantly disappears, and the molecules release the captured CO2. 
Others have investigated the use of quinones and an electrolyte in a variety of electrochemical devices. In most cases, the devices involve two electrodes — a negative one where the dissolved quinone is activated for CO2 capture, and a positive one where it’s deactivated for CO2 release. But moving the solution from one electrode to the other requires complex flow and pumping systems that are large and take up considerable space, limiting where the devices can be used. 
As an alternative, Hatton and Voskian decided to use the quinone as a solid electrode and — by applying what Hatton calls “a small change in voltage” — vary the electrical charge of the electrode itself to activate and deactivate the quinone. In such a setup, there would be no need to pump fluids around or to raise and lower the temperature or pressure, and the CO2 would end up as an easy-to-separate attachment on the solid quinone electrode. They deemed their concept “electro-swing adsorption.”
The electro-swing cell
To put their concept into practice, the researchers designed the electrochemical cell shown in the two diagrams in Figure 1 in the slideshow above. To maximize exposure, they put two quinone electrodes on the outside of the cell, thereby doubling its geometric capacity for CO2 capture. To switch the quinone on and off, they needed a component that would supply electrons and then take them back. For that job, they used a single ferrocene electrode, sandwiched between the two quinone electrodes but isolated from them by electrolyte membrane separators to prevent short circuits. They connected both quinone electrodes to the ferrocene electrode using the circuit of wires at the top, with a power source along the way.
A power source creates a voltage that causes electrons to flow from the ferrocene to the quinone through the wires. The quinone is now negatively charged. When CO2-containing air or exhaust is blown past these electrodes, the quinone will capture the CO2 molecules until all the active sites on its surface are filled up. During the discharge cycle, the direction of the voltage on the cell is reversed, and electrons flow from the quinone back to the ferrocene. The quinone is no longer negatively charged, so it has no chemical affinity for CO2. The CO2 molecules are released and swept out of the system by a stream of purge gas for subsequent use or disposal. The quinone is now regenerated and ready to capture more CO2.
Two additional components are key to successful operation. First is an electrolyte, in this case a liquid salt, that moistens the cell with positive and negative ions (electrically charged particles). Since electrons only flow through the external wires, those charged ions must travel within the cell from one electrode to the other to close the circuit for continued operation.
The second special ingredient is carbon nanotubes. In the electrodes, the quinone and ferrocene are both present as coatings on the surfaces of carbon nanotubes. Nanotubes are both strong and highly conductive, so they provide good support and serve as an efficient conduit for electrons traveling into and out of the quinone and ferrocene.
To fabricate a cell, researchers first synthesize a quinone- or ferrocene-based polymer, specifically, polyanthraquinone or polyvinylferrocene. They then make an “ink” by combining the polymer with carbon nanotubes in a solvent. The polymer immediately wraps around the nanotubes, connecting with them on a fundamental level.
To make the electrode, they use a non-woven carbon fiber mat as a substrate. They dip the mat into the ink, allow it to dry slowly, and then dip it again, repeating the procedure until they’ve built up a uniform coating of the composite on the substrate. The result of the process is a porous mesh that provides a large surface area of active sites and easy pathways for CO2 molecules to move in and out.
Once the researchers have prepared the quinone and ferrocene electrodes, they assemble the electrochemical cell by laminating the pieces together in the correct order — the quinone electrode, the electrolyte separator, the ferrocene electrode, another separator, and the second quinone electrode. Finally, they moisten the assembled cell with their liquid salt electrolyte.
Experimental results
To test the behavior of their system, the researchers placed a single electrochemical cell inside a custom-made, sealed box and wired it for electricity input. They then cycled the voltage and measured the key responses and capabilities of the device. The simultaneous trends in charge density put into the cell and CO2 adsorption per mole of quinone showed that when the quinone electrode is negatively charged, the amount of CO2 adsorbed goes up. And when that charge is reversed, CO2 adsorption declines.
For experiments under more realistic conditions, the researchers also fabricated full capture units — open-ended modules in which a few cells were lined up, one beside the other, with gaps between them where CO2-containing gases could travel, passing the quinone surfaces of adjacent cells.
In both experimental systems, the researchers ran tests using inlet streams with CO2 concentrations ranging from 10 percent down to 0.6 percent. The former is typical of power plant exhaust, the latter closer to concentrations in ambient indoor air. Regardless of the concentration, the efficiency of capture was essentially constant at about 90 percent. (An efficiency of 100 percent would mean that one molecule of CO2 had been captured for every electron transferred — an outcome that Hatton calls “highly unlikely” because other parasitic processes could be going on simultaneously.) The system used about 1 gigajoule of energy per ton of CO2 captured. Other methods consume between 1 and 10 gigajoules per ton, depending on the CO2 concentration of the incoming gases. Finally, the system was exceptionally durable. Over more than 7,000 charge-discharge cycles, its CO2 capture capacity dropped by only 30 percent — a loss of capacity that can readily be overcome with further refinements in the electrode preparation, say the researchers. 
The remarkable performance of their system stems from what Voskian calls the “binary nature of the affinity of quinone to CO2.” The quinone has either a high affinity or no affinity at all. “The result of that binary affinity is that our system should be equally effective at treating fossil fuel combustion flue gases and confined or ambient air,” he says. 
Practical applications
The experimental results confirm that the electro-swing device should be applicable in many situations. The device is compact and flexible; it operates at room temperature and normal air pressure; and it requires no large-scale, expensive ancillary equipment — only the direct current power source. Its simple design should enable “plug-and-play” installation in many processes, say the researchers.
It could, for example, be retrofitted in sealed buildings to remove CO2. In most sealed buildings, ventilation systems bring in fresh outdoor air to dilute the CO2 concentration indoors. “But making frequent air exchanges with the outside requires a lot of energy to condition the incoming air,” says Hatton. “Removing the CO2 indoors would reduce the number of exchanges needed.” The result could be large energy savings. Similarly, the system could be used in confined spaces where air exchange is impossible — for example, in submarines, spacecraft, and aircraft — to ensure that occupants aren’t breathing too much CO2.
The electro-swing system could also be teamed up with renewable sources, such as solar and wind farms, and even rooftop solar panels. Such sources sometimes generate more electricity than is needed on the power grid. Instead of shutting them off, the excess electricity could be used to run a CO2 capture plant.
The researchers have also developed a concept for using their system at power plants and other facilities that generate a continuous flow of exhaust containing CO2. At such sites, pairs of units would work in parallel. “One is emptying the pure CO2 that it captured, while the other is capturing more CO2,” explains Voskian. “And then you swap them.” A system of valves would switch the airflow to the freshly emptied unit, while a purge gas would flow through the full unit, carrying the CO2 out into a separate chamber.
The captured CO2 could be chemically processed into fuels or simply compressed and sent underground for long-term disposal. If the purge gas were also CO2, the result would be a steady stream of pure CO2 that soft-drink makers could use for carbonating drinks and farmers could use for feeding plants in greenhouses. Indeed, rather than burning fossil fuels to get CO2, such users could employ an electro-swing unit to generate their own CO2 while simultaneously removing CO2 from the air. 
Costs and scale-up
The researchers haven’t yet published a full technoeconomic analysis, but they project capital plus operating costs at $50 to $100 per ton of CO2 captured. That range is in line with costs using other, less-flexible carbon capture systems. Methods for fabricating the electro-swing cells are also manufacturing-friendly: The electrodes can be made using standard chemical processing methods and assembled using a roll-to-roll process similar to a printing press. 
And the system can be scaled up as needed. According to Voskian, it should scale linearly: “If you need 10 times more capture capacity, you just manufacture 10 times more electrodes.” Together, he and Hatton, along with Brian M. Baynes PhD ’04, have formed a company called Verdox, and they’re planning to demonstrate that ease of scale-up by developing a pilot plant within the next few years.
This research was supported by an MIT Energy Initiative (MITEI) Seed Fund grant and by Eni S.p.A. through MITEI. Sahag Voskian was an Eni-MIT Energy Fellow in 2016-17 and 2017-18.
This article appears in the Spring 2020 issue of Energy Futures, the magazine of the MIT Energy Initiative. 

Topics: MIT Energy Initiative, Chemical engineering, Climate, Energy, Environment, Sustainability, Alumni/ae, School of Engineering, Carbon, Emissions, Carbon nanotubes, Pollution, Greenhouse gases, Research, Startups, Innovation and Entrepreneurship (I&E) More

For the mining industry, efforts to achieve sustainability are moving from local to global. In the past, mining companies focused sustainability initiatives more on their social license to operate — treating workers fairly and operating safe and healthy facilities. However, concerns over climate change have put mining operations and supply chains in the global spotlight, […] More

Over the past decade, the cost of solar photovoltaic (PV) arrays has fallen rapidly. But at the same time, the value of PV power has declined in areas that have installed significant PV generating capacity. Operators of utility-scale PV systems have seen electricity prices drop as more PV generators come online. Over the same time period, many coal-fired power plants were required to install emissions-control systems, resulting in declines in air pollution nationally and regionally. The result has been improved public health — but also a decrease in the potential health benefits from offsetting coal generation with PV generation.
Given those competing trends, do the benefits of PV generation outweigh the costs? Answering that question requires balancing the up-front capital costs against the lifetime benefits of a PV system. Determining the former is fairly straightforward. But assessing the latter is challenging because the benefits differ across time and place. “The differences aren’t just due to variation in the amount of sunlight a given location receives throughout the year,” says Patrick R. Brown PhD ’16, a postdoc at the MIT Energy Initiative. “They’re also due to variability in electricity prices and pollutant emissions.”
The drop in the price paid for utility-scale PV power stems in part from how electricity is bought and sold on wholesale electricity markets. On the “day-ahead” market, generators and customers submit bids specifying how much they’ll sell or buy at various price levels at a given hour on the following day. The lowest-cost generators are chosen first. Since the variable operating cost of PV systems is near zero, they’re almost always chosen, taking the place of the most expensive generator then in the lineup. The price paid to every selected generator is set by the highest-cost operator on the system, so as more PV power comes on, more high-cost generators come off, and the price drops for everyone. As a result, in the middle of the day, when solar is generating the most, prices paid to electricity generators are at their lowest.
Brown notes that some generators may even bid negative prices. “They’re effectively paying consumers to take their power to ensure that they are dispatched,” he explains. For example, inflexible coal and nuclear plants may bid negative prices to avoid frequent shutdown and startup events that would result in extra fuel and maintenance costs. Renewable generators may also bid negative prices to obtain larger subsidies that are rewarded based on production. 
Health benefits also differ over time and place. The health effects of deploying PV power are greater in a heavily populated area that relies on coal power than in a less-populated region that has access to plenty of clean hydropower or wind. And the local health benefits of PV power can be higher when there’s congestion on transmission lines that leaves a region stuck with whatever high-polluting sources are available nearby. The social costs of air pollution are largely “externalized,” that is, they are mostly unaccounted for in electricity markets. But they can be quantified using statistical methods, so health benefits resulting from reduced emissions can be incorporated when assessing the cost-competitiveness of PV generation.
The contribution of fossil-fueled generators to climate change is another externality not accounted for by most electricity markets. Some U.S. markets, particularly in California and the Northeast, have implemented cap-and-trade programs, but the carbon dioxide (CO2) prices in those markets are much lower than estimates of the social cost of CO2, and other markets don’t price carbon at all. A full accounting of the benefits of PV power thus requires determining the CO2 emissions displaced by PV generation and then multiplying that value by a uniform carbon price representing the damage that those emissions would have caused.
Calculating PV costs and benefits
To examine the changing value of solar power, Brown and his colleague Francis M. O’Sullivan, the senior vice president of strategy at Ørsted Onshore North America and a senior lecturer at the MIT Sloan School of Management, developed a methodology to assess the costs and benefits of PV power across the U.S. power grid annually from 2010 to 2017. 
The researchers focused on six “independent system operators” (ISOs) in California, Texas, the Midwest, the Mid-Atlantic, New York, and New England. Each ISO sets electricity prices at hundreds of “pricing nodes” along the transmission network in their region. The researchers performed analyses at more than 10,000 of those pricing nodes.
For each node, they simulated the operation of a utility-scale PV array that tilts to follow the sun throughout the day. They calculated how much electricity it would generate and the benefits that each kilowatt would provide, factoring in energy and “capacity” revenues as well as avoided health and climate change costs associated with the displacement of fossil fuel emissions. (Capacity revenues are paid to generators for being available to deliver electricity at times of peak demand.) They focused on emissions of CO2, which contributes to climate change, and of nitrogen oxides (NOx), sulfur dioxide (SO2), and particulate matter called PM2.5 — fine particles that can cause serious health problems and can be emitted or formed in the atmosphere from NOx and SO2.
The results of the analysis showed that the wholesale energy value of PV generation varied significantly from place to place, even within the region of a given ISO. For example, in New York City and Long Island, where population density is high and adding transmission lines is difficult, the market value of solar was at times 50 percent higher than across the state as a whole. 
The public health benefits associated with SO2, NOx, and PM2.5 emissions reductions declined over the study period but were still substantial in 2017. Monetizing the health benefits of PV generation in 2017 would add almost 75 percent to energy revenues in the Midwest and New York and fully 100 percent in the Mid-Atlantic, thanks to the large amount of coal generation in the Midwest and Mid-Atlantic and the high population density on the Eastern Seaboard. 
Based on the calculated energy and capacity revenues and health and climate benefits for 2017, the researchers asked: Given that combination of private and public benefits, what upfront PV system cost would be needed to make the PV installation “break even” over its lifetime, assuming that grid conditions in that year persist for the life of the installation? In other words, says Brown, “At what capital cost would an investment in a PV system be paid back in benefits over the lifetime of the array?” 
Assuming 2017 values for energy and capacity market revenues alone, an unsubsidized PV investment at 2017 costs doesn’t break even. Add in the health benefit, and PV breaks even at 30 percent of the pricing nodes modeled. Assuming a carbon price of $50 per ton, the investment breaks even at about 70 percent of the nodes, and with a carbon price of $100 per ton (which is still less than the price estimated to be needed to limit global temperature rise to under 2 degrees Celsius), PV breaks even at all of the modeled nodes. 
That wasn’t the case just two years earlier: At 2015 PV costs, PV would only have broken even in 2017 at about 65 percent of the nodes counting market revenues, health benefits, and a $100 per ton carbon price. “Since 2010, solar has gone from one of the most expensive sources of electricity to one of the cheapest, and it now breaks even across the majority of the U.S. when considering the full slate of values that it provides,” says Brown. 
Based on their findings, the researchers conclude that the decline in PV costs over the studied period outpaced the decline in value, such that in 2017 the market, health, and climate benefits outweighed the cost of PV systems at the majority of locations modeled. “So the amount of solar that’s competitive is still increasing year by year,” says Brown. 
The findings underscore the importance of considering health and climate benefits as well as market revenues. “If you’re going to add another megawatt of PV power, it’s best to put it where it’ll make the most difference, not only in terms of revenues but also health and CO2,” says Brown. 
Unfortunately, today’s policies don’t reward that behavior. Some states do provide renewable energy subsidies for solar investments, but they reward generation equally everywhere. Yet in states such as New York, the public health benefits would have been far higher at some nodes than at others. State-level or regional reward mechanisms could be tailored to reflect such variation in node-to-node benefits of PV generation, providing incentives for installing PV systems where they’ll be most valuable. Providing time-varying price signals (including the cost of emissions) not only to utility-scale generators, but also to residential and commercial electricity generators and customers, would similarly guide PV investment to areas where it provides the most benefit. 
Time-shifting PV output to maximize revenues 
The analysis provides some guidance that might help would-be PV installers maximize their revenues. For example, it identifies certain “hot spots” where PV generation is especially valuable. At some high-electricity-demand nodes along the East Coast, for instance, persistent grid congestion has meant that the projected revenue of a PV generator has been high for more than a decade. The analysis also shows that the sunniest site may not always be the most profitable choice. A PV system in Texas would generate about 20 percent more power than one in the Northeast, yet energy revenues were greater at nodes in the Northeast than in Texas in some of the years analyzed. 
To help potential PV owners maximize their future revenues, Brown and O’Sullivan performed a follow-on study focusing on ways to shift the output of PV arrays to align with times of higher prices on the wholesale market. For this analysis, they considered the value of solar on the day-ahead market and also on the “real-time market,” which dispatches generators to correct for discrepancies between supply and demand. They explored three options for shaping the output of PV generators, with a focus on the California real-time market in 2017, when high PV penetration led to a large reduction in midday prices compared to morning and evening prices.
Curtailing output when prices are negative: During negative-price hours, a PV operator can simply turn off generation. In California in 2017, curtailment would have increased revenues by 9 percent on the real-time market compared to “must-run” operation.
Changing the orientation of “fixed-tilt” (stationary) solar panels: The general rule of thumb in the Northern Hemisphere is to orient solar panels toward the south, maximizing production over the year. But peak production then occurs at about noon, when electricity prices in markets with high solar penetration are at their lowest. Pointing panels toward the west moves generation further into the afternoon. On the California real-time market in 2017, optimizing the orientation would have increased revenues by 13 percent, or 20 percent in conjunction with curtailment.
Using 1-axis tracking: For larger utility-scale installations, solar panels are frequently installed on automatic solar trackers, rotating throughout the day from east in the morning to west in the evening. Using such 1-axis tracking on the California system in 2017 would have increased revenues by 32 percent over a fixed-tilt installation, and using tracking plus curtailment would have increased revenues by 42 percent.
The researchers were surprised to see how much the optimal orientation changed in California over the period of their study. “In 2010, the best orientation for a fixed array was about 10 degrees west of south,” says Brown. “In 2017, it’s about 55 degrees west of south.” That adjustment is due to changes in market prices that accompany significant growth in PV generation — changes that will occur in other regions as they start to ramp up their solar generation.
The researchers stress that conditions are constantly changing on power grids and electricity markets. With that in mind, they made their database and computer code openly available so that others can readily use them to calculate updated estimates of the net benefits of PV power and other distributed energy resources.
They also emphasize the importance of getting time-varying prices to all market participants and of adapting installation and dispatch strategies to changing power system conditions. A law set to take effect in California in 2020 will require all new homes to have solar panels. Installing the usual south-facing panels with uncurtailable output could further saturate the electricity market at times when other PV installations are already generating.
“If new rooftop arrays instead use west-facing panels that can be switched off during negative price times, it’s better for the whole system,” says Brown. “Rather than just adding more solar at times when the price is already low and the electricity mix is already clean, the new PV installations would displace expensive and dirty gas generators in the evening. Enabling that outcome is a win all around.”
Patrick Brown and this research were supported by a U.S. Department of Energy Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award through the EERE Solar Energy Technologies Office. The computer code and data repositories are available here and here.
This article appears in the Spring 2020 issue of Energy Futures, the magazine of the MIT Energy Initiative.  More

Over the past decade, the cost of solar photovoltaic (PV) arrays has fallen rapidly. But at the same time, the value of PV power has declined in areas that have installed significant PV generating capacity. Operators of utility-scale PV systems have seen electricity prices drop as more PV generators come online. Over the same time period, […] More