This excerpt from the Stanford Emerging Technology Review (SETR) focuses on sustainable energy, one of ten key technologies studied in this new educational initiative. SETR, a project of the Hoover Institution and the Stanford School of Engineering, harnesses the expertise of Stanford University’s leading science and engineering faculty to create an easy-to-use reference tool for policy makers. Download the full report here and subscribe here for news and updates.
The development of sustainable energy, a vital strategic resource for nations, generally involves generation, transmission, and storage. In recent years it has also come to include carbon capture and its removal from the atmosphere. Energy mix and innovation are key to efforts to address climate change.
The most significant challenge to achieving sustainable energy is scale. Countries will need to source, manufacture, and deploy massive generation, transmission, and storage capabilities to meet global energy needs. Because global energy needs are vast, no single technology or breakthrough will be enough. Over-the-horizon challenges include decentralizing and modernizing US electricity grids and achieving greater national consensus about energy goals to enable strategic and effective research and development programs and funding.
Counting the cost
For the past few hundred years, fossil fuels have been the primary source of human energy consumption. However, humanity’s reliance on fossil fuels has released enough carbon dioxide and other greenhouse gases into the atmosphere to raise the specter of significant changes in climate around the planet over the next century.
One critical aspect of reducing the harm from increases in atmospheric CO2 is the development of more sustainable energy sources such as solar, wind, hydropower, and nuclear energy.
Global energy needs are large, growing, and hard to fathom. Numbers such as a billion cars, hundreds of millions of trucks, billions of tons of material to be stored or produced, billions of people to feed, and tens of millions of airplane flights per year characterize the scale of the problem. The excess CO2 in the earth’s atmosphere is similarly characterized by large numbers: tens of billions of tons of CO2 are produced from burning fossil fuels every year.
No single technology or breakthrough can possibly meet the world’s demands for energy. Success will require a combination of approaches that bridge present sources, consumption, and infrastructure to a more sustainable future. Further, energy technologies must be deployed over the planet on a scale commensurate with the number of people who will use that energy. The imperative to deliver energy at scale unavoidably places an emphasis on cost. High-cost technologies, whether new or old, and no matter how promising, cannot and will not be deployed on a wide scale.
As for the economic impact of sustainable energy, one analysis indicates that doubling the share of renewable energy as a fraction of the world’s energy consumption by 2030 would increase global GDP by up to 1.1 percent, or $1.3 trillion. These positive effects would be driven mostly by increased investment in renewable-energy deployment, which triggers ripple effects throughout the economy.Such a transition would also create twenty-four million jobs globally for people working in the renewable-energy sector.
Domestically, eliminating air pollution emissions from energy-related activities in the United States that use renewable energy sources would prevent more than fifty thousand premature deaths each year and provide more than $600 billion in benefits each year from avoided illness and death.
Creating and storing energy
Sustainable energy sources such as solar and wind are intermittent. Without long-duration energy storage, the electric grid is perhaps only 50 to 60 percent sustainable. Beyond that, storage is needed, and a variety of technological concepts are being researched:
- Gravity storage. Power generated in excess of demand can be used to pump water from lower to higher levels and recovered by letting the water flow back down through generators. Large multi-ton weights can be lifted hundreds of meters and then allowed to fall gently to recover energy.
- Thermal storage. This approach stores excess power in the form of heat, such as heating a large volume of salts to a very high temperature. When needed, that heat can be released to generate power.
- Mechanical storage. Energy can be captured in the form of compressed air or spinning flywheels and then released on demand to produce electricity.
None of these technologies is a silver bullet for long-duration energy storage—if they prove economically feasible, each will fill its own niche applications. The chief challenge of all these forms of long-duration energy storage is scalability.
Batteries can capture electrical energy and release it on demand. But although batteries may be a useful way for homeowners to store excess solar power for later use, at present they are too expensive and difficult to maintain for use as energy storage on a grid scale.
For example, consider the problem of storing the world’s electricity consumption for seventy-two hours; this problem sets the scale of the storage problem for sustainable electricity generation. Around 200,000 gigawatt-hours of battery storage would be needed, and at about 200 watt-hours per kilogram, requiring about a billion tons of battery.
Lithium-ion batteries are the best batteries available today for large-scale production, but they will not satisfy all our needs in long-duration energy storage. Such batteries need to be able to endure tens of thousands of capture-and-release cycles, retain charge over several tens of hours, and be made of inexpensive materials. Aqueous battery chemistries such as manganese-hydrogen batteries for long-duration energy storage are more promising from a cost perspective—key materials such as manganese are one-tenth the cost of nickel—and they have lower lifecycle costs due to reduced maintenance needs.
New fuels and carbon capture
Research on renewable fuels aims to create fuels that do not rely on extraction from the earth and whose burning does not release the carbon previously stored underground. Renewable fuels include combustible hydrocarbons such as biodiesel, which can be produced from animal fats or vegetable oils, and bioethanol produced from corn or algae. Hydrogen can be directly burned without releasing CO2. However, for most transportation applications, frequent refueling of hydrogen is impractical and to carry enough hydrogen, it is necessary to carry it as a liquid or a highly compressed gas. In these forms, the energy density of hydrogen is significantly lower—by a factor of four—than for hydrocarbon fuels, which means that a hydrogen tank for a car needs to be four times as large to provide comparable range.
Research into hydrogen storage is therefore vital if hydrogen is to play a meaningful role in the energy transition. These efforts focus on developing cost-effective hydrogen storage technologies with improved energy density that do not depend on liquefication or compression.
Fossil fuels have many advantages over other sources of energy. They essentially store solar energy captured eons ago in concentrated form and carry a significantly larger amount of energy per kilogram. Such energy, which can be released on demand and held in liquid form, is especially useful and convenient in road or airborne vehicles. Coal and natural gas continue to provide a large fraction of the world’s electricity. Despite their advantages, however, humanity’s reliance on fossil fuels has released enough carbon dioxide and other greenhouse gases into the atmosphere to raise the specter of significant changes in climate around the planet over the next century. Emission-free energy production will take decades to accomplish, and fossil fuels will be an appreciable (though declining) fraction of society’s mix of energy sources for some time to come. In the meantime, carbon capture and removal technology is advancing.
Carbon capture usually takes place at the source of emissions, such as the smokestack of a fossil-fuel-burning power plant. Technologies to capture CO2 at the source include liquid and solid materials that hold on to CO2 in large amounts and then are sequestered, and membranes that can separate CO2 from other gases. Research challenges include developing inexpensive materials for capturing CO2 rather than other gases, that are easy to handle and manage and require little energy in the regeneration process of releasing captured CO2 for recovery.
Carbon removal calls for taking CO2 directly from the atmosphere—also known as direct air capture (DAC)—at concentrations that are several orders of magnitude lower than at the smokestack for carbon capture. Engineered technologies for DAC rely on absorptive/adsorptive materials as in the case for source-point capture, though it is desirable for DAC materials to be optimized for use in low-concentration environments. Natural technologies may also be used for DAC, and research continues to determine the CO2-capturing role of plants that naturally consume CO2 such as trees, ocean kelp, and algae.
The future grid
The electric grid of the future will be far more decentralized and heterogeneous than the one of today. Sources of electricity will be more varied and geographically distributed as local power generation increases. Consumers of electricity will become more numerous as electrically operated systems displace systems powered by fossil fuels. Energy storage—virtually nonexistent in today’s grid—will have to be managed as well.
Demand for additional power will increase, requiring more generating facilities as well as more efficient use of existing power sources. Those demands will have to be better synchronized with timelines for generation and release of electricity from storage in ways that minimize CO2 production.
Addressing all these challenges securely is the goal of what is generally known as the “smart grid,” which will coordinate all these moving parts to increase efficiency, reliability, and resilience against attack or natural disaster.
