Brian Barton
Figure 4-2 illustrates the environmental impacts of renewable electricity generation, highlighting the significant advantage of renewable energy technologies in reducing CO2 and other greenhouse gas (GHG) emissions compared to fossil-fuel-based electricity generation. This comparison underscores the lower life cycle GHG emissions expected from all forms of renewable electricity production, providing valuable insights into the emissions associated with different energy sources and their impact on the environment.
| Characteristics | Values |
|---|---|
| Pollutants | CO2e GHG emissions |
| Source | Conventional coal and natural gas plants |
| Alternative with lower emissions | Renewable electricity production |
| Example of renewable electricity production with low emissions | Closed-loop geothermal systems |
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What You'll Learn
GHG emissions from renewable electricity generation
Greenhouse gas (GHG) emissions from electricity generation are a significant contributor to global emissions. The commercial, residential, and industrial sectors' emissions increase when indirect emissions from electricity end-use are included, as these sectors consume large amounts of electricity, particularly for heating, ventilation, air conditioning, lighting, and appliances.
In 2022, the electric power sector was the second largest source of U.S. greenhouse gas emissions, accounting for 24-25% of the total. While coal use in the United States accounted for 55% of CO2 emissions from the electric power sector, it only represented 20% of electricity generation. On the other hand, natural gas use accounted for 39% of electricity generation, and petroleum use was less than 1%. The remaining generation came from non-fossil fuel sources, with nuclear power at 19% and renewable energy sources at 21%, including hydroelectricity, biomass, wind, and solar. Most of these non-fossil sources are non-emitting or have significantly lower emissions than fossil fuels.
The EU's electricity sector has also made progress in reducing GHG emissions, with a 59% decrease in GHG intensity compared to 1990 and a 20% decrease compared to 2022. This reduction is attributed to the decline in coal generation, the growth of solar and wind power, and the resumption of hydro and nuclear power to average generation levels. The highest rates of decarbonisation in electricity production over the 1990-2023 period were achieved in Luxembourg (90% decrease), Denmark (89%), Slovakia (82%), and Finland (82%).
To further reduce GHG emissions, additional policies and measures are needed to accelerate the deployment of renewable energy sources, incentivize energy conservation, and optimize the build-out of electricity infrastructure. Life Cycle Assessment (LCA) is a valuable tool for analyzing and comparing the environmental impacts of renewable and non-renewable energy systems, aiding in technology evaluation and selection. Renewable energy technologies (RETs) have lower life cycle GHG emissions than conventional fossil fuel sources, with offshore wind technologies having the lowest emissions among the studied RETs.
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CO2 emissions from electric power production
The burning of fossil fuels, such as coal, oil, and natural gas, to produce electricity results in the release of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and other greenhouse gases. In 2023, utility-scale electric power plants burning these fossil fuels accounted for about 99% of the associated CO2 emissions in the United States. The remaining 1% came from other fuels and gases derived from fossil fuels and certain types of geothermal power plants.
Despite the progress made in reducing CO2 emissions from electric power production, this sector still contributes significantly to overall emissions. Electricity generation accounts for over 40% of all energy-related emissions, despite only 20% of final energy consumption being in the form of electricity. Coal combustion is particularly carbon-intensive, and while coal use accounted for 55% of the electricity sector's CO2 emissions in the United States in 2022, it only represented 20% of electricity generated that year.
There are ongoing challenges and opportunities in reducing greenhouse gas emissions associated with electric power production, transmission, and distribution. One proposed method is carbon capture and storage (CCS), which involves capturing CO2 emissions from large power stations and injecting them underground. However, this technology is currently expensive and difficult to implement effectively.
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High-energy requirements of PV panel manufacturing
The manufacturing of PV panels is a complex process that involves multiple steps and requires a significant amount of energy. The energy requirements for PV panel manufacturing can vary depending on the specific technologies, processes, and materials used. Here is an overview of the high-energy requirements associated with PV panel production:
- Polysilicon Production: Polysilicon, a key component in PV panels, is produced using highly reactive gases obtained from metallurgical-grade silicon, hydrogen, and chlorine. The Siemens process, for instance, involves passing a silicon-hydrogen-chlorine compound gas over a heated silicon filament, requiring substantial energy input to achieve the necessary high temperatures.
- Thin Film PV Manufacturing: Thin film PV modules, commonly using cadmium telluride, are manufactured as a single unit. This process includes coating float glass with a transparent conductive layer, depositing the absorber material, laser scribing for cell patterns and interconnects, and lamination to ensure waterproofing. Each of these steps demands energy, particularly the high-temperature processes.
- Wafer Surface Texturing: PV panels undergo chemical texturing of the wafer surface to enhance light absorption. This process requires energy to power the chemical treatments and surface modifications.
- Assembly: PV panel assembly involves intricate processes such as tabbing and stringing, where copper ribbons connect adjacent cells. This stage of manufacturing consumes energy for soldering, quality control, and the assembly line machinery.
- Supply Chain and Transportation: The production and transportation of raw materials, components, and finished PV panels across the supply chain contribute significantly to energy consumption. Energy is expended in mining, refining, and delivering these materials to manufacturing sites and end markets.
- High-Temperature Processes: Certain steps in PV panel manufacturing, such as the production of metallurgical-grade silicon, require extremely high temperatures ranging from 1500°C to 2000°C. Achieving and maintaining these temperatures during production processes contribute to the high-energy requirements.
While PV panels have high-energy requirements during the manufacturing process, it is important to consider the long-term energy generation and environmental benefits they provide. PV panels are a form of renewable energy that can help reduce carbon emissions and contribute to a more sustainable energy landscape.
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Fossil fuel use in biomass combustion
Fossil fuel combustion and biomass burning are closely linked. As the world transitions to fossil-fuel-based energy, there is a reduced reliance on biomass burning for energy acquisition and livelihoods. This shift is reflected in the inverse relationship between fossil fuel and biomass burning emissions. From 1997 to 2010, global average annual carbon emissions from biomass burning were approximately one-third of fossil fuel emissions. Notably, the geography of combustion reveals that areas with high biomass burning emissions do not coincide with regions of high fossil fuel burning emissions.
Biomass, an essential fuel in many countries, particularly developing nations, is used for cooking and heating. It encompasses a range of materials, including wood and wood processing waste, agricultural crops and residues, biogenic materials in municipal solid waste, and animal manure. Direct combustion is the predominant method for converting biomass into usable energy. However, burning biomass releases carbon dioxide (CO2), a greenhouse gas.
Despite this, biomass is considered a carbon-neutral energy source because the source plants capture nearly as much CO2 through photosynthesis as is released during combustion. Additionally, biomass serves as an alternative energy source to fossil fuels. For instance, using wood, wood pellets, and charcoal for heating and cooking can reduce overall CO2 emissions compared to fossil fuel use. Nevertheless, wood smoke contains harmful pollutants such as carbon monoxide and particulate matter, which can be mitigated by employing modern wood-burning stoves and fireplace inserts.
The use of biomass also extends beyond direct combustion for energy production. Pyrolysis, for instance, is a process that involves heating organic materials in a low-oxygen environment to produce bio-oil, charcoal, renewable diesel, methane, and hydrogen. Hydrotreating further processes bio-oil to create renewable diesel, gasoline, and jet fuel. Gasification is another technique where controlled amounts of oxygen or steam are injected to generate synthesis gas (syngas), a fuel suitable for diesel engines, heating, and electricity generation.
Furthermore, biomass contributes to the production of transportation fuels, enhancing energy security and independence. Ethanol, derived from biomass fermentation, is used as a vehicle fuel. Biogas, also known as biomethane or renewable natural gas, is obtained from anaerobic digestion at sewage treatment plants and livestock operations. It has similar applications to fossil fuel natural gas when properly treated.
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CO2e emissions from geothermal systems
The environmental effects of geothermal energy depend on how it is used and converted into energy. Direct-use applications and geothermal heat pumps have minimal negative environmental effects and can even be beneficial by reducing the use of more harmful energy sources. Geothermal power plants do not burn fuel and so produce 97% less acid rain-causing sulfur compound emissions and 99% less carbon dioxide than fossil fuel plants of a similar size.
However, geothermal power plants do emit small amounts of sulfur and carbon dioxide and the extent of these emissions varies depending on the geological conditions and technology used. The wider use of geothermal systems has led to the exploitation of systems with relatively high greenhouse gas (GHG) concentrations in the reservoir fluid. Data from some sites in Italy and Turkey show that GHG emissions from these plants can be higher than 500 g/kWh and, in some cases, on par with or higher than coal-fired power plants.
Computer modelling is used to determine the increase in CO2 emissions resulting from the operation of a geothermal power plant and whether this will be offset in the future by a lowering of emissions after the plant is shut down. A study of the Ohaaki geothermal field in New Zealand showed that during the lifetime of the project, extra CO2 was emitted, but after the project was shut down, emissions reduced below their natural level. Over a long time period (300 years), there was no extra CO2 emitted as a result of the operation of the Ohaaki power plant.
Despite these emissions, geothermal energy generally produces fewer carbon emissions than fossil fuel plants. It is a predictable, constant, and reliable energy source that is unaffected by weather or seasonal variations.
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Frequently asked questions
Figure 4-2 shows that all forms of renewable electricity production are expected to have significantly lower life cycle GHG emissions than electricity production from conventional coal and natural gas plants.
In the United States, the electric power sector is the largest source of CO2 emissions, accounting for more than 2.3 billion metric tons in 2007, or over 40% of total energy-related emissions.
Renewable electricity generation from closed-loop geothermal systems has low GHG emissions. However, if the reservoir gases are vented into the atmosphere, CO2e GHG emissions can be high.
