“Energy policy” and “clean energy” may be political hot buttons this year, but the technological realities and challenges to achieving energy and environmental goals are seldom discussed. There is strong public sentiment that the U.S. should decrease our reliance on fossil fuels because of concerns about pollution, global warming, ecosystem damage, and energy security. Although a domestically abundant energy source, coal power is imputed as being a major contributor to smog, acid rain, and global warming. High-profile accidents associated with coal mining and coal ash management have further damaged coal’s reputation. Grass-roots campaigns to replace coal as a major source of electricity claim that wind, solar, and geothermal power could replace retired coal capacity.

In 2011, 42% of the electricity generated in the U.S. was from coal, according to the Energy Information Administration. Although coal generation for 2012 is projected to fall 15 percent, coal is still expected to represent a significant percent of the nation’s generating capacity through 2035. Reducing reliance on coal faces challenges beyond policy and market economics. What are the technical constraints of the U.S. electric generating system, what role does coal power play, and how can we further incorporate renewable energy sources?

To understand the technological challenges, it is helpful to understand the roots of U.S. electrification – how our electrical system evolved, and the legacy of coal power within that system. Modern coal-fired power generation is a vestige of Thomas Edison’s 1882 Pearl Street Station, which was the first coal power station to provide electricity to residents of New York City. With the introduction of alternating current (AC) by Westinghouse Electric, by the early 1900s power could be successfully transmitted over long distances, allowing centralized stations to deliver power to population centers connected by high voltage transmission lines. Although turn-of-the-century electric power came from numerous smaller generators, improvements to steam power turbine technology coupled with economies-of-scale further encouraged the consolidation of electric power.

Transmission technology improved as well during the early part of the 20th century. Increased voltage capacity enabled power to be more efficiently carried for longer distances. Utility companies were able to interconnect multiple plants, allowing the most efficient plants to deliver power to a wide area, provide backup power, and further reduce electricity costs. The use of coal for electric generation expanded rapidly to support soaring electricity demand during World War II, and the transmission grid was built to take advantage of centralized power generation. The legacy of that growth and consolidation is that large coal power plants still dominate the U.S. electric system, and the infrastructure for carrying high-voltage power (see Figure 1) from smaller, distributed sources is lacking.

The electric system necessitates a real-time balancing of demand with generation. Appliances, air conditioning, and the power to feed our increasingly wireless lives require electric generation be delivered to meet that demand. Failing to meet electric demand results in blackouts with severe economic consequences – think Northeast Blackout of 2003.

The electric generated by all sources – coal, nuclear, wind, solar, geothermal, biomass, natural gas, etc. – must be delivered by the wires, buses, transformers, substations, and ancillary equipment that comprise the transmission grid. These components have physical constraints, including thermal limits (related to sag of the transmission wire) and voltage stability (related to the ability to prevent sudden voltage dips that can lead to failures). Sometimes a specific power plant is needed to maintain electric reliability, as was the case for five of First Energy’s coal plants that were scheduled to retire but are required to continue operating for voltage support. The electric system relies on redundancies including operating reserve (excess capacity) to ensure reliability.

Baseload generation currently provides the backbone for the electric grid. Baseload is the minimum level of electric demand over 24 hours, such as during late evening or early morning and is served by plants that provide steady and low-cost power with few unscheduled outages. Nuclear and coal have predominately served as baseload plants because they operate most efficiently at full, steady output and are slow to ramp up or down. Geothermal and hydropower have also been used in certain areas as baseload power. Hydropower with pumped storage is a flexible energy source able to serve sudden spikes in demand, such as during hot summer days (peak demand). Natural gas turbines, which can quickly ramp up or down to follow electric load, have been a preferred source of peaking power.

Load-following or intermediate demand plants provide power in between off-peak and peak hours, which is when solar and wind power have had the most use. Intermittent or diurnal sources such as wind and solar have been widely considered unsuitable for baseload generation because of their variability. In other words, you can’t count on them to meet demand 24x7. Energy storage may help bridge the gap for intermittent generating sources. Success with baseload solar power is promising, while other energy storage technologies are still under development.

So why can’t we just use wind and solar when available, supplement with current energy storage capabilities, and use quick-start resources such as natural gas turbines as needed? The problem lies with transmission constraints. While some studies have shown that load shifting using energy storage could help eliminate minimum generation constraints, these technologies have not reached wide-scale deployment and transmission infrastructure is lacking to fully support distributed renewable generation.

Regional differences in available electric generating sources compound the problem. While some states such as California generate only a small percentage of power from coal, in other states including Kentucky and Indiana, over 85 percent of electricity generation is from coal. Hydropower sites are abundant in the Pacific Northwest, but relatively few installations exist in some areas of the U.S.. As a result, regional transmission system operators responsible for balancing load and maintaining electric reliability face a range of technical challenges. What works in the Northeast will not work in Texas. Each system has to find a way to incorporate renewable sources given the existing generating fleet, existing transmission infrastructure, and planned improvements.

So we have an electric system based on large, centralized baseload plants that run (nearly) continuously and power that must be delivered in real-time by a transmission grid that needs modernization. To increase the complexity of this high-wire balancing act, increasing numbers of plug-in electric vehicles (EVs) are projected to hit the roadways. While electrification of transportation will help decrease reliance on fossil fuels, where will the power for those EVs come from? In some areas of the country, the answer right now is coal.

Retiring older coal plants that operate off-peak can occur without impacting electric reliability, and is evidenced by the slate of recent retirement announcements. But replacing baseload coal generation with alternative power sources will be more difficult. Some people see repowering with natural gas as the solution, as carbon emissions from natural gas generation are 45 percent less than coal per megawatt-hour. Natural gas generation could serve as baseload generation, but opposition to hydraulic fracturing spurs concerns about future supply and potential price spikes. Permitting and constructing new nuclear plants is fraught with difficulties, partly due to opposition from environmental groups and ensuing cost overruns.

Some envision smart grid technologies and transmission upgrades completely eliminating the reliance on baseload “must-run” generation, with an electric system powered mostly by renewable sources. Because renewable sources tend to be much smaller than coal-fired power plants, and located in areas that may not have sufficient transmission access, simply replacing coal for renewables is not straightforward. To reach 80 percent “clean energy” – including combined cycle natural gas generation as clean – would require the replacement of 35 percent of summer generating capacity (see Figure 2, coal + petroleum). The technological scale of such build out (over 370 gigawatts) is astounding. That would require about 185,000 2-megawatt wind turbines or over 700 large (500-megawatt) solar farms. Considering that even solar and wind projects have faced local opposition, this is a tall order.

Of course, the solution does not have to be an either-or situation. Perhaps the “all-of-the-above” approach must necessarily include coal power in the near future. Newer coal plants have advanced pollution controls and far lower emissions than older coal plants, and provide the bridge between the legacy transmission system and the electric grid of the future. What is clear is that an “all-of-the-above” energy policy must first consider transmission planning and improvements, demand-side management, and energy storage as the first steps in reinventing our electric system.