Here comes the sun

Here comes the sun

Daniel Nocera
Scientists have launched a revolutionary low-carbon process based on solar power. Emerging economies may lead the way in creating the infrastructure to use it effectively

The year was 1898 when civic planners and engineers gathered in New York City to discuss the greatest problem confronting society: horse manure. At the cusp of the industrial revolution, the population was expanding, horse buggies were the preferred mode of transportation and horse manure was accruing in the cities. Extrapolation of the transportation needs of the growing population yielded an alarming conclusion. As Eric Morris writes in ACCESS (no. 30 Spring 2007) "The situation seemed dire" with estimates "that by 1950 every street in London would be buried 9 feet deep in horse manure" and in New York City "horse droppings would rise to Manhattan’s third-story windows." Conference attendees were "stumped by the crisis" and accordingly decided to dissolve their 10-day conference after 3 days because "the urban planning conference declared its work fruitless." I doubt most readers have ever heard about the "horse manure crisis" because science and engineering was developing a number of technological "disruptions" that were to change the course of our society. By 1846, the Canadian geologist Abraham Gesner had discovered a distillation process that refined kerosene from coal, and by 1856 the first oil refineries had been constructed. At the same time, numerous engineers were investigating engines that could be fueled by hydrocarbons, and in 1885 Karl Benz developed the Benz Patent Motorwagen in Mannheim Germany, a vehicle acknowledged to be the first practical automobile powered by an internal combustion engine. Thus by 1898, when the civic planners and engineers from around the globe met in New York City,  they didn’t realize that science and engineering had already been set on a path of "disruption" a decade earlier, disruption that would avoid the impending societal crisis of horse manure.

The role of science and technology in our future

A revolutionary #lowcarbon process based on #solarpower was launched. We need a #disruption for a new #energy model

The parallels between the 1898 horse manure crisis and today’s discussion of the energy crisis are uncanny. Today, a growing world population, especially in the developing world, is fueling an accelerated demand for energy. The global population is expected to increase from the current 7.3 billion to 9.7 billion by 2050, and in addition to these 2.5 billion new inhabitants of the planet, 3 billion people in the emerging world seek a rising standard of living. Because energy consumption scales directly with a country’s gross domestic product, their energy use will only increase dramatically as they modernize. Extrapolating the energy demand attendant to this growth in population and the increase in the aspirations and needs of billions of people in emerging economies, the rate of worldwide energy consumption is predicted to double by mid-century and triple by the end of the century. If energy need is met with our current energy infrastructure, the atmospheric carbon dioxide concentration will likely double and even triple within the 21st century. Not unlike the 1898 horse manure conference, considerable anxiety surrounds a decision to maintain an energy path based on carbon, a path that will result in burying ourselves, this time not in horse manure, but in CO2. While the consequences of this increase cannot be predicted precisely, what is certain is that we are disturbing the planet on a scale never experienced before. The situation seems dire. But it is not widely realized that once again, science and engineering have initiated a course of "disruption" to mitigate the impending societal crisis due to carbon.
The scientific research community has set its sight on a solar-based renewable energy supply for the global future. The last decade of solar energy research and technology has delivered astounding discoveries that sets the stage for a paradigm shift in our global energy infrastructure. Shifts have occurred along 2 lines: generation and storage. With regard to the former, new photovoltaic materials and processing techniques have led to unprecedented efficiencies for the generation of electricity from sunlight. Who could have imagined less than a decade ago that a solar photovoltaic material operating at efficiencies in excess of 20% could be prepared by simple precipitation of a semiconducting material from solution? This is indeed the case for perovskite solar cells that combine organic and inorganic compositions. Their high efficiency is stunning in view of how cheap they are to produce and how simple they are to manufacture, although they are not without challenges for implementation (the highest efficiency materials are moisture sensitive and rely on lead, a heavy metal). An especially impactful advance in solar generation will be roll-to-roll manufacturing on flexible substrates. Even here significant advances have been made in recent years with amorphous silicon (a-Si) and with the potential offered by materials that combine elements from the periodic table in the Groups of III and V (commonly called “3-5” photovoltaics). Nonetheless, it is fair to say that none of these advances are essential, as none of these scientific advances have been as disruptive as the sea change in manufacturing. Owing to a significant Chinese commitment to the production of crystalline silicon (c-Si), prices are in the mid-dollar per watt range for a solar module. Accordingly, there has not been much need for discovery in solar generation, as c-Si (which has also had reported efficiencies of 25.6%) is already cheap enough for the widespread implementation of solar power. But that has not yet happened, and the question is why. The simple answer, in view of the Chinese commitment to c-Si production, is that it is not a generation issue but rather one of storage. Once solar energy is stored, it becomes a useful commodity. Accordingly, energy storage is the single most critical challenge to the widespread implementation of renewable energy.

22 countries had enough capacity at end-2015 to meet more than 1% of their electricity demand 50 GW of solar energy added in 2015, at global level (REN21)

The real challenge: energy storage

Renewable energy storage assumes different targets depending on whether it is for transportation, and, if not, whether energy distribution is centralized over a grid as is the case for mature markets, or decentralized, as is more possible in emerging markets.  Energy storage in the transportation sector has been driven by lithium-ion (Li-ion) batteries, though there has been some innovation in battery materials with the Li-ion paired with oxides of cobalt, manganese or nickel (and combinations of these metals) or as a titanate or iron phosphate. The disruption in Li-ion has come in a commitment to large scale manufacturing, which is having a major impact in driving the explosive growth of the electric vehicle market. Often the promise of Li-ion batteries in the electric vehicle (EV) sector is applied to large-scale stationary (grid) applications, which require much lower price points than those of the transportation sector. "Learning curve" extrapolations applied to Li-ion often make it appear to be a viable option for large-scale storage technology. However, such extrapolations are likely to be overestimates as there will be materials limitations (not just in lithium but the other metal oxides as well) at large scale. A better situated technology for grid storage are redox flow batteries (RFBs), which effectively are rechargeable "fuel cells" in which a dissolved electroactive species flows through an electrochemical cell that reversibly converts chemical energy directly to electricity. The RFB is a powerful technology option because energy density and power density are separated, and hence a versatile storage technology for the grid operator. Price points for redox flow batteries are well below Li-ion batteries, and some RFBs have been acquired by large engineering companies for commercial scale-up to megawatt (MW) storage capacities. At the same time, RFBs are also a viable option for energy storage on microgrids in emerging energy markets. It is important to realize that RFBs have much lower current and energy densities than Li-ion—hence RFBs are confined to large-scale stationary energy storage whereas Li-ion are ideal for the transportation sector. Batteries are not well suited for the terrawatts-equivalent of energy storage needed by mid-century because of their limited energy densities. In a battery, the electrons must reside on atoms, and thus the stored energy is limited by the physical density of materials. With lithium as one of the lightest elements in the periodic table, and hence with one of the lowest physical densities, stored energy in the form of electrons within batteries has already approached a ceiling. Society has intrinsically understood this limitation. Although batteries were known since the turn of the 18th century, hydrocarbon-based fuels were immediately adopted in the 20th century to power industrialization. The energy density of a liquid fuel is 50-100 times greater than that of a battery, and consequently the future of energy storage will not change as the large scale storage of energy storage will necessarily be in the form of chemical fuels. Discovery during the last decade has set off on a path of disruption to convert a fossil fuels industry to a solar fuels industry.

The sun: A source of carbon-neutral energy

The simplest process to store solar energy in the form of a carbon-neutral fuel is to use the sun to split water into hydrogen and oxygen. Recombining the hydrogen and oxygen converts the stored solar energy back in a useful form (electricity through fuel cells) when and where it is needed. In this renewable fuels cycle, no carbon dioxide is produced and there is no loss of water since it is the product of hydrogen and oxygen recombination. In less than a decade, remarkable advances have been made in this area of renewable energy. Catalysts have been created from earth abundant elements to perform the water splitting reaction under simply engineered (and thus inexpensive) conditions. When integrated directly with silicon (i.e., the artificial leaf) or indirectly by wiring to a silicon photovoltaic, solar-to hydrogen efficiencies of greater than 10% have been achieved, with 15% efficiencies on the horizon. The challenge of using hydrogen as a fuel is the lack of a widespread infrastructure for its use. In this regard, the rise in popularity of natural gas may well be a driver for a hydrogen infrastructure, as hydrogen can be generated by combining natural gas with water in a process called reforming. As natural gas becomes more prevalent, the price of a gas gallon equivalent of hydrogen is approaching $1.50 USD, and point-of-use hydrogen becomes feasible with on-site reforming as yet to be achieved in a cost-effective way, thus alleviating the need for a hydrogen distribution infrastructure. It is to be noted that, in the reforming process of natural gas with water, carbon dioxide is produced in addition to hydrogen. Thus, it is a short jump to solar-driven water splitting as the carbon neutral version of methane reforming to produce hydrogen.
Natural photosynthesis confronts the same challenges society does in storing hydrogen. Photosynthesis also uses the sun to split water to hydrogen and oxygen. To circumvent the hydrogen storage challenge, it combines the hydrogen from water splitting with carbon dioxide to produce carbohydrate or some other form of biomass. On an electron equivalency basis, the production of the carbohydrate stores only less than 1% more energy than water splitting. Thus, the solar energy storage in photosynthesis is achieved by water splitting; the carbohydrate is nature’s method of storing the hydrogen released from the water splitting reaction. Even here, advances in science have been astounding. Using the tool of synthetic biology, organisms have been engineered to breathe in the hydrogen from water splitting and then combine it with carbon dioxide to produce biomass in excess of 10% solar-to-fuels efficiencies. Realizing that the best growing crops achieves a biomass efficiency of 1% reveals the extraordinary accomplishment of science in the last decade. Even more striking, the organisms have been further engineered to bypass biomass and directly synthesize liquid fuels at 5 to 7% efficiency. Thus, science has shown that we, as a society, can far surpass the solar energy process of nature that drives our planet.

227 GW of global Solar PV energy capacity in 2015 (REN21) 3.8 millions estimated direct and indirect jobs in solar energy sector (PV and Heating/Cooking)

We need a change in our energy model

A ledger of science and technology advances in just the last decade demonstrates that we can generate solar energy in a cost effective manner with silicon. Batteries can store renewable energy and meet our transportation and grid-based electricity needs. The sun can be stored in the form of the chemical fuels of hydrogen, biomass and liquid fuels, and at efficiencies that put society on a genuine path forward to a restructuring of a fuels industry based on fossil-fuels to one based on solar energy. The disruption delivered by science and technology for historic change to the global energy infrastructure now exists. So why isn’t it being adopted? In short, the last century has seen a massive investment in an energy infrastructure that has been paid off. Thus, there is no discovery that can supplant this energy infrastructure in mature markets. This is where emerging economies offer hope for leading global society to a new energy infrastructure. In the absence of massive investment, it will be easier for emerging economies to leap frog the established energy infrastructure and adopt new renewable energy innovations and technologies. In either case, in emerging or mature energy markets, there must be a societal imperative for changing our energy infrastructure that extends beyond near term costs. Unfortunately, the price of burying ourselves in carbon dioxide is not in society’s current equation for change. When it does become part of the equation, science and technology will be ready with the enabling and disruptive paradigm shift to deliver the Sun to the people of our planet as their direct energy supply.

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