Clean energy sources are diverse in their approaches, technological maturity, level of adoption, and future potential. As individual technologies develop and enter the marketplace, complex "energy ecosystems" are emerging. Power generation and distribution infrastructure must increasingly integrate inputs from various sources and incorporate storage capabilities to meet peak demand. The most successful efforts in transitioning to clean energy also involve "innovation ecosystems" of public and private entities and a broad commitment to the needed R&D, policies, regulations, incentives, and legislation.

Clean energy technologies can be implemented at any scale. Rooftop solar panels and smart systems that optimize energy use are popular choices for individual homes and office buildings, as are purchase agreements with public utilities to buy a certain percentage of electric power from renewable sources. Currently, a number of college campuses, office parks, and factories generate 100% of their own energy from clean sources or have made commitments to do so. At the public utility scale, renewable energy sources have become steadily more cost competitive with fossil fuels—the three least expensive sources are utility-scale solar photovoltaics (PV), onshore wind, and gas combined-cycle generation.

Incentives for implementing clean energy vary by location. Looking to the future, United States-based oil companies have begun to invest in wind and solar power as the costs of these energy sources have come down. However, abundant fossil fuel reserves have dampened enthusiasm for developing alternative energy sources in the U.S. This is in stark contrast to Europe, where fossil fuels are more expensive and environmental regulations are stricter.

Below is an expansive look at the clean energy landscape as it exists now, as well as the various paths it can take into the future (see “Scope and Scale”).


Wind power generation capacity grew 9% between 2017 and 2018, continuing a growth trend that started in 2001. In 2018, wind power generation (275 billion kilowatt-hours, kWh) in the U.S. almost caught up with hydroelectric power (292 billion kWh). The U.S. Department of Energy predicts that by 2020, 36 states will generate more than 113.43 GW of wind power, and that this capacity will reach 404.25 GW across 48 states by 2050. A growing percentage of this power will come from offshore installations along both coasts, the Gulf of Mexico, and the Great Lakes.

Capacity currently stands at about 600 gigawatts (GW) worldwide. About 4% of this capacity (23 GW) comes from offshore installations, but offshore wind power could clear the 100 GW mark worldwide by 2030. The levelized cost of onshore wind energy generation in 2018 was $29–$56 per megawatt-hour (MWh) and offshore was $92 per MWh, compared with $41–$74 for gas combined cycle or $60–$143 for coal.

The U.S. offshore wind market lags behind Europe in developing the supply chain, streamlining regulatory processes, and improving power transmission, according to Kyle Kingman, president of Offshore Power, LLC. Developing these capabilities took some time in Europe, but Kingman believes the U.S. will learn from Europe’s experience and benefit from cross-market availability.

Developments such as floating offshore turbines could be especially useful off the U.S. West Coast, where the ocean is too deep for seafloor-mounted turbines. Floating turbines could generate more than 10 times the wind energy than the region currently uses each year.

Advanced wind turbine generator technologies (e.g., direct-drive permanent magnet generators and various superconducting technologies) show promise for reducing weight and electricity costs and increasing capacity. However, robustness and reliability remain issues, as do capital costs, efficiency, and transportation costs to the installation site. Several companies offer computational modeling services that help turbine operators anticipate component wear rates and in-situ sensors that catch excessive vibration and efficiency losses at an early stage, both of which can prevent downtime and costly unscheduled repairs.

Scope and Scale

Here is a breakdown of current and future clean energy milestones in the United States.


  • Wind and solar power reach 10% of electricity production in the U.S.
  • Annual sales of electric cars top 100,000
  • Utility-scale battery energy storage capacity in the U.S. grows seventeenfold from 2008 levels
  • Unsubsidized utility-scale solar energy costs fall 86% and wind energy costs fall 67% from 2009 levels, becoming competitive with conventional generation technologies in certain situations


  • U.S. wind turbine generating capacity averages 2.43 MW per turbine
  • Burbo Bank Extension wind farm off the coast of England consists entirely of 8 MW turbines
  • Energy use in the U.S. declines 1.1% from 2008 levels, despite a population growth of more than 20 million


  • 65 American cities have committed to 100% renewable energy sources, and another six cities have already met this goal
  • The U.S. House Committee on Energy and Commerce calls for zero greenhouse gas pollution by 2050
  • California’s Alamitos battery storage project breaks ground and will offer 100 MW/400 MWh peaking capacity
  • A tidal turbine is installed on the Memorial Bridge between Portsmouth, New Hampshire, and Kittery, Maine


  • The U.S. Department of Energy predicts that 36 states will generate more than 113.43 GW of wind power


  • The U.S. is expected to add more than 15 GW of solar PV capacity annually


  • Offshore wind power could clear the 100 GW mark worldwide


  • Wind power capacity in the U.S. could reach 404.25 GW across 48 states
  • Utility-scale battery storage capacity in the U.S. could reach 40 GW, up from less than 10 GW in 2018


Solar power represents another growth area for power generation—it shares membership in a very exclusive group of low-carbon energy technologies with the potential to reach a very large scale. As of early 2019, total installed solar photovoltaic (PV) capacity in the U.S. reached 67 GW—a 10% year-over-year increase. This figure is expected to more than double over the next five years, and by 2024, the U.S. is expected to add more than 15 GW of PV capacity annually.

Solar PV devices are likely the best-known means of solar electric power generation, but other technologies are gaining ground, especially at the utility scale. However, because the service lifetimes of large-scale power generating facilities generally span decades, photovoltaics and concentrating solar power (CSP) technologies are expected to dominate solar power generation until at least 2050.

Advanced solar PV devices (for example, triple-junction devices based on stacks of epitaxial III-V compounds) have achieved 43% efficiency. This could be close to the limit of multijunction designs, however, as electrical and optical losses are expected to offset power gains from cells with more than four junctions.

CSP technologies are at a moderate-to-high level of maturity (see “Beyond Photovoltaics”). CSP power generators can produce anywhere from a few kilowatts to more than 50 MW using a variety of technologies at various stages of development. Challenges include developing cost-effective energy storage methods and power block and receiver technology. Land use and environmental considerations for solar collection fields, costs of developing and manufacturing components, power transmission, and regulatory issues are all areas of concern.

The 2018 levelized cost of rooftop residential solar PV energy generation was $160–$267 per MWh, compared with $73–$145 for the community scale and $36–$46 at the utility scale, in contrast to $41–$74 gas combined cycle or $60–$143 for coal. Solar thermal towers with storage fall within the $98–$181 per MWh range. Solar PV technology is less expensive than CSP by itself, but CSP plus thermal energy storage is cheaper than PV plus batteries. Both combinations are still more expensive than fossil fuel plants when considering required energy dispatchability, but CSP-thermal costs are expected to come down as new technologies increase operation and energy storage temperatures, reducing the amount of fluid necessary to produce a given amount of heat.

Beyond Photovoltaics

“The world is moving into a new era where renewable energy is rapidly growing,” says Guangdong Zhu, a senior researcher at the National Renewable Energy Laboratory (NREL) who focuses on large-scale energy storage technologies as part of the Concentrating Solar Power and Geothermal Technology programs.

Utility-scale batteries that provide backup power once a day can be expected to last 5–10 years, according to Zhu, requiring multiple expensive replacements over the typical 30-year life span of a power plant. Materials issues, including global lithium supplies and a lack of mature recycling technologies, are also causes for concern.

Zhu and his NREL colleagues are developing large-scale thermal energy storage (TES) with concentrating solar power (CSP). “You’re still using heated fluids to run the turbines, so you’re just replacing the fossil fuel burner with solar energy,” he says.

The initial capital outlay and operating costs make these systems most effective at the power-plant scale. The U.S. currently has solar-thermal installations that generate 2 GW of power, all located in the California-Nevada-Arizona grid, according to Zhu.

Molten salts are especially promising for solar-thermal storage, as they are chemically stable and have high heat capacities. Mature molten-salt technology runs at about 560°C, and upcoming technology is expected to raise this to over 700°C, according to Zhu. Solids such as ceramic particles can raise storage temperatures up to 1,100°C, which could provide several days’ worth of power backup.

The first power tower plant in the U.S. to use molten salt technology, the 110-MW Crescent Dunes CSP plant near Tonopah, Nevada, is currently operational. Two salt tanks (one hot, one cold) provide 10 hours of energy storage capacity and are expected to last for the lifetime of the plant.

Combining solar concentrators with naturally occurring thermal heat sources could also boost efficiency. Hybrid solar-geothermal systems, still in the early research stages, could be a way forward to seasonal energy storage. Solar-heated fluids injected into subsurface reservoirs in the summer could be drawn on in the winter. However, the chemical and physical effects of injecting large quantities of fluids into subsurface formations must be taken into consideration.

Zhu believes other solar technologies are on the horizon. NREL research and Google’s Malta project focus on pumped thermal electricity storage, which uses electricity to heat molten salts during low-demand periods and releases heat to generate electricity during high-demand periods. Another technology option, supercritical CO2, is more energy-dense than steam and could operate at a higher temperature to drive turbines someday. Because the CO2 is in a closed loop, it will not be released to the atmosphere, according to Zhu.

Energy storage

Because the peak generation times of intermittent power sources cannot be synchronized with periods of peak customer demand, energy storage at different scales, (large-scale in particular), will be increasingly important in switching power generation to intermittent sources. Utility-scale battery storage capacity in the U.S. is expected to grow from its current total of less than 10 GW to 40 GW in 2050, driven in part by growth in the solar and wind power sectors.

However, efforts to plan, build, or operate electric power infrastructure seldom take electricity storage into account. Barriers to adaptation include a lack of awareness as well as regulatory, policy, and financial limitations. Although the U.S. has no strong federal policy on energy storage, several states, notably California, have instituted their own mandates for utilities to procure energy storage capabilities.

A REEEM comparison of five energy storage technologies showed each at a high degree of technological readiness (89–97 on a scale of 100) and intellectual property readiness (80–100, reflecting the regulatory environment). Lithium-ion batteries led in market demand (84), ahead of compressed air energy storage (62) and hydrogen (59). Consumer awareness and demand were relatively strong (72–80) for all except hydrogen (55). Societal acceptance (low concerns over health, safety, materials scarcity, and environmental issues) was strongest for hydrogen (94), flow batteries (92), and supercapacitors (92), leading compressed air energy storage (84) and lithium-ion batteries (78).

Batteries are the most mature of the large-scale electric energy storage technologies. Large-scale battery storage, including for public utilities, and—on a somewhat smaller scale—vehicles, is limited by short service lifetimes, high replacement costs, and internal energy losses.

Developments such as floating offshore turbines could be especially useful off the U.S. West Coast, where the ocean is too deep for seafloor-mounted turbines.

As solar and wind energy deployment grow, energy storage could replace an increasing fraction of peaking capacity. At the single-building scale, half of all orders for rooftop solar panels in Germany now include a battery storage system. At the utility scale, four-hour battery storage systems could meet peaking capacity needs in every region in the U.S.

A recent NREL report considered the example of the impact that energy storage could have in replacing fossil fuel power with solar PV. In 2017, fossil fuels provided 261 GW peaking capacity of the U.S. total generating capacity of 1,187 GW, but under current U.S. grid conditions and demand patterns, only some 28 GW of this peaking capacity could be practically converted to four-hour energy storage. Increasing solar deployment to 10% of national electricity demand, coupled with four-hour storage, could increase this to 50 GW or more. Initial costs remain a barrier, but life cycle costs could actually be cheaper than for gas-fired peakers.

Other storage technologies include supercapacitors—also known as ultracapacitors—used as a complement to batteries. Adiabatic compressed air and liquid metal batteries are in an earlier stage of development than lead-acid, lithium-ion, or flow batteries. Pumped hydro and compressed air are considered mature technologies. Pumped hydro is currently the most effective technology for large-scale operations, but severe limitations on the availability of new sites may prevent significant expansion in this sector.

Flow batteries, already at a high level of technology readiness, use abundant materials and do not pose significant hazards to health, safety, or the environment. Their large economies of scale and the possibility of a virtually infinite number of charge-discharge cycles make them an attractive target for development. However, flow batteries are not yet cost-competitive, and lithium-ion batteries enjoy a strong edge in the market. Improvements in cost, membrane lifetimes (or dispensing with the membranes altogether), and power and energy density could improve this technology's market share.

Today’s flow batteries are not energy-dense enough for use in passenger vehicles, but they could be used for ships, trains, and large trucks. Flow batteries, together with power-to-gas, could account for approximately 17% of the global market for energy storage following the integration of wind and solar power into utility grids in the next decade, but lithium-ion batteries will continue to dominate, claiming almost half of this market. However, global supplies of lithium, which already affect markets for lithium-ion batteries, could become a limiting factor.

Going Net Zero

Net-zero buildings generate at least as much energy as they use annually. By 2016, the number of net-zero buildings in the U.S. and Canada had topped the 4,000 mark—an increase of more than 20% over 2015. The U.S. has set a 2030 goal for all new federal buildings to meet one of four types of net-zero qualifications, and one 2018 EEI survey indicated that more than 60% of U.S.-based organizations are "extremely or very likely" to join this trend over the next decade.

The American Geophysical Union (AGU) recently completed a basement-to-rooftop overhaul of its 62,000-square-foot headquarters building in Washington, D.C., striving for Net Zero Energy Building Certification. Chris McEntee, AGU executive director and CEO, notes that multiple stakeholders looked at more than 50 strategies before selecting about half of them for implementation, based on the principles of reduction, reclamation, absorption, and generation. “The real challenge is the integration of multiple technologies,” she says.

The project saved materials manufacturing and disposal costs by renovating the existing building and recycling or reusing signage, bricks and banisters. Porcelain fixtures were crushed to make flooring materials. The rainwater cistern captures and reuses water, and hydroponic remediation “green walls” filter the indoor air. Light sensors and smart temperature controls ensure efficient energy usage.

The existing forced-air cooling system was replaced with a hydronic radiant cooling system coupled with a dedicated outdoor air system. This change significantly reduced the building’s energy consumption and improved the thermal comfort of the workspace, according to Roger E. Frechette, PE, managing principal for Interface Engineering, which took on the renovation. Because the radiant cooling system is passive and contains few moving parts, he anticipates the new HVAC system will require less maintenance and have a longer service life than the one it replaced. Radiant cooling has been used in Germany since the 1970s, according to Frechette, but this technology is much newer in the U.S.

Solar PV panels on the roof provide direct current to LED lights, computers, monitors, and other devices, reducing the power loss typically realized when transforming electric power from DC to AC. The building’s electrical system automatically switches to AC city power (with a centralized DC power convertor) when demand exceeds supply.

A municipal sewer heat exchanger, the first of its kind in the U.S., provides a geothermal source for heating and cooling the building and contributes to the building’s high-performance HVAC system. AGU signed a covenant with municipal utility DC Water that allows for the diversion of a small amount of wastewater from a nearby municipal storm and sanitary sewer line to a settling tank. This water is then routed through a heat exchange system in the building's basement before returning to the sewer. The system’s “free cooling” mode significantly reduces the annual energy consumption of the building and eliminates the need for a rooftop cooling tower and the potable water supply that such a tower would require.

Image credit Beth Bagley, AGU

Thermal energy

Although geothermal energy provides only a small part of the world’s energy, it has a small surface footprint, and it produces power continuously (bed load) with efficiencies approaching those for hydrocarbon-fired power plants, while emitting little to no CO2. Engineered and enhanced geothermal systems are currently at an early to intermediate stage of technology maturity. Levelized costs in 2018 for geothermal energy generation are at $71–$111 per MWh, compared with $41–$74 for gas combined cycle or $60–$143 for coal.

Dry, in-field geothermal wells, currently in an intermediate stage of development, are promising targets for margin stimulation. Hot dry rock technology, currently in an early developmental stage, could heat subsurface reservoirs of water or supercritical CO2. Both methods require cost-effective deep drilling technology and the ability to create and manage subsurface fluid reservoirs.

Low-temperature solar-thermal energy, primarily of interest to small-scale consumers, is used worldwide for heating water and in industrial processes such as drying or food processing. Nations facing higher prices for fossil fuels are more likely to use solar thermal water and space heaters. China is currently the largest market for solar-thermal, but significant progress is expected in low-temperature solar thermal power and hybrid solar-thermal and PV systems for home heating and water heating in North America. Incentives to build and adopt these systems, system cost, and ease of installation could increase commercial and residential use.

Some custom-designed combined heat and power installations are in operation at large facilities with significant demand for both electrical power and thermal energy. Small-scale, mass-produced and modular thermal appliances remain a promising but undeveloped area.

Change Checklist

Switching to clean, renewable energy sources requires optimizing a combination of several factors:

  • Motivation: Organizational mission, cost trends, regulatory requirements
  • Project scale: Single-building, office park, public utility grid
  • Financial resources: Capital investment, tax rebates, grant money
  • Materials availability: Existing infrastructure, access to components and replacement parts
  • Knowledge availability: Designers, vendors, builders, maintenance personnel
  • Regulatory infrastructure: Uncharted legal/regulatory territory, historic preservation and environmental considerations
  • Technology maturity
  • Capital costs
  • Operating costs
  • Intermittency: Backup sources, storage

Smart systems and efficiency measures

“Going green” can mean finding ways to use less energy. On the small scale, using LEDs and window films and shades can reduce electricity and air conditioning costs. The emerging Internet of Things, which could eventually get a boost from 5G wireless technology, enables homeowners and facility managers to switch appliances and equipment on and off remotely, and motion sensor systems can turn off lights and adjust climate controls in unoccupied rooms. Building-scale sensor networks can alert operators to investigate a system that is using an unusually large amount of energy.

Advanced high-voltage direct current (HVDC) electrical systems, currently at an intermediate stage of development, can reduce the need for energy-inefficient individual alternating current power adapters in office buildings that use large numbers of computers and other DC devices. Some HVDC demonstration projects are already in operation (see “Going Net Zero”).

Small distributed sources provide power to individual buildings and homes, but they also supply electricity to public power grids during peak generation times (see “Untapped Potential”). If small power sources are to provide more than 15% of the total grid load, new tools and methods, currently in an intermediate stage of development, will be required in order to ensure sustainability, system reliability, and cost-effectiveness. Flexible systems, capable of handling variable inputs from many of intermittent power sources not synchronized to user demand, will require energy and distribution management systems that include sensors at every stage. This could impose significant capital costs as public utilities upgrade their capabilities.

More intentional design and retrofitting of power plants could also reduce power distribution losses, but state regulators often do not consider energy distribution in their efficiency goals. Grid modernization could enable time-of-use pricing, better load leveling and demand response, and a greater incorporation of variable and distributed power generation sources. However, issues related to cybersecurity and mitigating the impacts of natural disasters, as well as the challenges of integrating new and legacy technologies, must be addressed.

Nuclear energy

Nuclear power currently provides over 10% of the world’s electricity, and it presents an opportunity for an increasingly urbanized globe. It can effectively be deployed on a large scale, and it’s one of the more reliable and predictable energy sources powering today’s grid. Fuel is a low proportion of the costs of nuclear power, leading to price stability. The fuel is on-site, so there is no need for continuous delivery. Its power is available on demand, with quick ramp-up, and provides low carbon emissions. This makes it particularly reliable in all environments, regardless of peak load times.

In the 2018 edition of the International Energy Agency’s World Energy Outlook (WEO), the agency predicted a 1,121 terawatt-hour increase in nuclear generation between 2016 and 2040. This would require an increase in capacity of about 100 GW.

Utility-scale battery storage capacity in the U.S. is expected to grow from its current total of less than 10 GW to 40 GW in 2050, driven in part by growth in the solar and wind power sectors.

Other clean energy options

Between 2019 and 2027, the compound annual growth rate for fuel cells is projected to top 18%, driven by demand from the transportation, power generation, data center, and automotive industries. Fuel cell power plants currently provide backup power at some small facilities including hospitals, and they can be part of a renewable energy strategy for larger operations, such as data centers. At the end of 2016, the U.S. had 56 large-scale fuel cell generating units, each greater than 1 MW, most of which came online since 2013. Fuel cells collectively provided 810,000 MWh of electricity in 2016, representing 0.02% of total U.S. electricity generation. As of 2018, the levelized cost of fuel cell energy generation was $103–$152 per MWh, compared with $41–$74 for gas combined cycle or $60–$143 for coal.

Hydroelectric power remains a mainstay renewable electricity source in the U.S. It was virtually the only such source in 1950, providing more than 100 billion kWh, and this amount had tripled by 2018. As of 2014, large conventional U.S. hydropower generating plants (at least 30 MW of capacity) had an installed capacity of almost 80 GW. The technical ability to double this capacity exists already, but regulatory and financial constraints will likely hinder reaching this target before 2035.

Other clean energy options are in less mature phases. Wave, tidal, and ocean thermal technologies are mainly in the demonstration phase. Ocean current technologies are still in the proof-of-concept phase. Carbon capture and storage, while not yet cost-competitive, could reduce greenhouse gas emissions from fossil fuel-based power plants.

Using carbon-containing fuels efficiently can be a step toward zero carbon emissions: Advanced natural gas power generation technologies could provide greater than 50% thermal efficiency, for example. Another carbon fuel option, biomass, spans technology readiness levels from the experimental to the mature. Progress is expected in the technological development and commercial availability of leaching and torrefaction, which are currently at the plant demonstration stage.

Biomass electricity generation in the U.S. topped the 50 billion kWh mark in the early 1990s, but it has not grown past 64 billion kWh since then. By comparison, solar power in the U.S. grew from 0.37 billion kWh in 1990 to 25 billion kWh in 2015, reaching almost 67 billion kWh in 2018. Wood pellets manufactured in the U.S. and Canada are exported to Europe as a replacement for coal, but wood burning remains controversial because of concerns about deforestation and CO2 emissions.

Dozens of technologies are already on the market for converting a building, campus, or regional utility grid to clean energy, and dozens more are in various stages of development. Implementing these technologies and integrating them into a diversified energy ecosystem presents many challenges for engineers to solve, but the financial and environmental payoffs are steadily increasing.

Untapped Potential

Clean energy technologies with the greatest breakthrough potential include offshore wind, energy storage, advanced nuclear reactors, and hydrogen. However, power generation technology is not the biggest barrier to adopting low-carbon energy generation. Most of the technologies are ready for widespread deployment, but significant work is needed on system integration to match supply with demand and markets.

Next-generation grids likely will use a combination of renewable energy sources to meet the demand for more energy and greater renewables penetration. This creates a need to integrate energy generation technologies with systems for power transmission and load leveling, smart systems for efficient energy use, and large-scale carbon management. Clean energy innovation ecosystems could integrate federal agencies such as national labs with regional and local initiatives, the private sector, universities, and philanthropies. These collaborations are needed to address the size and complexity of existing systems, the vital role these systems play in the economy, and public expectations of safety and reliability.

Even wind and solar, the most established renewable energy technologies, require expanded research and development to increase penetration and decrease costs. Components, materials, and entire systems must evolve. Solar PV efforts must focus on reducing costs and using smart systems to integrate power from multiple small PV sources into public utility grids. Onshore wind energy generation is well established, but resource planning and integration with utility grids require more innovation.

Offshore wind energy offers significant potential for innovation in foundation designs and installation processes. Design and management of offshore wind farms, preparing for hurricanes and surface icing, and better power transmission to onshore customers are other areas calling for innovation. Floating offshore wind turbines could open up new areas for wind farm installation. An overall trend toward larger turbines poses a range of materials and engineering challenges.

Advanced hydrothermal and geothermal technologies (e.g., hot rock resources) will require long-term, sustained research and development to optimize and demonstrate designs, reduce costs, and bring concepts to market. Rock fracture and fracture propagation mechanisms are particular areas of interest.

Hydropower, a mature technology, has room for improvement in reliability, efficiency, safety, and reduction of costs and environmental impact. Ocean power is still in its infancy, requiring design development for key components and subsystems, as well as simplified installation procedures. Areas to watch include ocean thermal energy conversion, salinity gradient power and ocean current technology.

Written by Nancy McGuire

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