College undergraduates, high school students and middle school students.
After completing this unit, users will be able to:
- Describe (in general) how electric power is generated
- Identify the major energy resources used in electricity power production and their pros and cons relative to GHG emissions
- Apply emission factors to estimate the total GHG emissions for different energy resources used for electricity
- Relate regional differences in electricity mix to their associated GHG emissions and regional energy resources
- Manipulate data in a spreadsheet to produce graphs
- Analyze pie and bar graphs to interpret differences in electric generation and GHG emissions
- Gain familiarity with the lexicon of electricity generation vocabulary
The electric power industry contributed 40% of the total U.S. GHG emissions in 2009. This is attributed to the large quantities of fossil fuel combusted for electricity generation, which is the primary source of CO2 emissions in the United States. Understanding how electricity is made and some of the reasons for this significant contribution will help our citizens appreciate the types of changes that we need to make to reduce our emissions to mitigate climate changes.
Key Concepts and Vocabulary
Greenhouse gas: A greenhouse gas (GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, and nitrous oxide.
Electric energy: Electrical energy (electricity) refers to the flow of power or the flow of charges along a conductor. Electrical energy is a secondary source of energy, which means that we obtain electrical energy through the conversion of other forms of energy. The primary sources energy include coal, nuclear energy, natural gas, oil, the sun, wind, etc. The primary sources from which we create electrical energy are classified as non-renewable or renewable forms of energy. Electrical energy however is neither non-renewable or renewable.
Emissions factor: An emission factor is the relationship between the amount of pollution produced and the amount of raw material processed or number of product units produced. For example: GHG emissions per kWh electricity generated.
Steam turbine: A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. It is one of the critical processes in most electricity power plants to transfer the heat energy from fossil fuel combustion into mechanical energy required to move the magnets and copper coils that comprise the electric generator.
Energy efficiency: Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat. Increasing energy efficiency essentially enables you to do more with less energy input.
Lifecycle emissions: A product or processes’ lifecycle includes all of the steps from extracting raw materials to make the product, manufacturing of the product and all of its component parts, the product use and ultimate disposal or other end-of life fate. Lifecycle emissions include emissions (pollutants) for all of these processes summed together. We often associate emissions with just one step (e.g., automobile tail pipe emissions) rather than also considering the emissions for making the fuel and automobile (etc.) as well.
Electricity generation contributes nearly 40% of the total GHG emission generated in the United States and thus deserves significant attention and understanding if our Nation sets policies for reducing GHG emissions to help mitigate global climate change. Coal combustion, which accounts for 45-50% of our national electric energy generation, contributes over 81% of the GHG emissions from electric power generation. 
There is a variety of reasons for the high CO2 emissions from coal. Most importantly, a greater fraction of the mass of coal is comprised of carbon atoms. Thus, when it is combusted, there is more carbon per mass of fuel that can be oxidized to CO2. Another contributing factor is the efficiency of some of our older coal power plants. With efficiencies on the order of <30-40%, that means only about one-third of the energy value of coal is converted to electricity that leaves the power plant. Another 10% of that is lost through the electricity transmission and distribution system before it gets to your home. Inefficiencies in the electricity generation process are primarily attributed to unrecoverable heat energy from the combustion process and friction.
The figure below illustrates the primary steps in the production and use of electricity from coal resources. Each of the major processes in this sequence is termed a “stage” in the overall lifecycle process of creating and using electricity from coal. The primary steps include mining and processing coal to prepare it for combustion, combustion of coal to make steam, passing the steam through a turbine to rotate it (and convert the heat energy of the steam into mechanical energy), and finally the rotation of the generator, which is comprised of copper coils and magnets, to induce an electrical current.
GHG emissions are typically expressed as emission factors in units of mass of GHGs emitted per unit of electric energy produced (e.g., g CO2 eq./kWh or metric ton (Mg) CO2 eq./MWh). The figure below presents some emission factors for electricity generation. These include both those related to the actual fuel combustion (stack emissions) as well as other emissions associated with equipment and facility construction and operation, and fuel mining, processing and transportation. These other emissions stem from other stages in the lifecycle process of energy resource extraction through conversion to electricity. A couple of key points can be made about these values:
- Coal emissions are much higher than any other energy resource used for electricity, including other fossil fuels.
- There are a range of values for each of the primary sources due to different technologies used for combustion and electricity generation, different characteristics of fuels (e.g., lignite versus anthracite coal), and differences in the assumptions made to estimate these emission factors.
- There are GHG emissions even for renewable energy resources, although most are orders of magnitude lower than for the fossil fuels.
- forms of energy we use and how important they are to our lives (electricity, petroleum fuel for transportation, fuel for heating, embodied energy in goods, etc.);
- major sources of energy in the U.S.;
- the importance of electricity in terms of our Nation’s total greenhouse gas emissions;
- not all energy resources are equivalent – making electricity out of coal versus nuclear power (for example) has very different consequences related to GHG emissions; and,
- making electricity from fossil fuel is a complex series of steps, the sum of which is defined as the “lifecycle.” Most – but not all - of the GHG emissions from fossil fuel power plants is related to the fuel combustion.
- some analysis of the relative high or low emission factors for the region your school is in (or the students’ home towns) and the implications for those differences
- Should more be done to find alternative energy resources to make electricity from?
- How can increased energy efficiency at the end-user help?
- What types of new power generation technologies should be implemented to reduce GHG emissions?
- The central purpose of scientific inquiry is to develop explanations of natural phenomena in a continuing, creative process.
- The observations made while testing proposed explanations, when analyzed using conventional and invented methods, provide new insights into phenomena.
- Students use measurement in both metric and English measure to provide a major link between the abstractions of mathematics and the real world in order to describe and compare objects and data.
- Human Decisions and activities have had a profound impact on the physical and living environment.
- Models are simplified representations of objects, structures, or systems used in analysis, explanation, interpretation, or design.
Electricity generation in the United States is broken into 26 different regions. For example, most of New York State is in the upstate region (NYUP), but New York City (NYCW) and Long Island (NYLI) have their own regions. Within each of these regions, the total electricity generation and total GHG emissions are averaged to provide regional emission factors for the overall mix of electricity. This is often also done at the state level.
Table 1 provides values for regional emission factors that are averaged over all types of electricity generation. Emission factors for CO2 vary by more than a factor of three among the regions. New York State, for example, is well below the national average. These emission factors were compiled by the U.S. EPA’s eGrid program. They only include combustion related (stack) emissions, not total lifecycle emissions.
Table 1: 2007 GHG Annual Electricity Output Emission Rates by Region
(see map above)
Carbon dioxide (kg CO2/MWh)
Nitrous oxide (kg N2O/MWh)
This project module includes three in class activities and a homework assignment. The different activities are geared towards different age levels. Collectively, the set of activities can be used to explain electricity generation systems, their efficiency and differences in greenhouse gas generation.
Activity appropriate for:
Energy efficiency activity
Lifecycle electricity generation poster
Review of U.S. EPA Power Profiler results
Homework – application of electricity mix and emission factors to determine state average
* perhaps best without the prepared cut-outs the younger students get
Anticipatory Set A brainstorm about how we use energy and then specifically electric energy will get students engaged in thinking about energy overall and its split between electrical and other forms. A powerpoint lecture is available to introduce some of the key concepts:
Procedure This unit has three primary activities and an associated homework assignment. The selection of which activities are completed depends mostly on the age and learning level of the students. The first two activities are best completed by groups (3-4) students. The Review of the Power Profiler tool can be done individually or in pairs at a computer. An introductory powerpoint lecture is available to introduce aspects of the materials. Suggested timeline:
Day 1: Introduction and energy efficiency activity
Days 2-3: Create and briefly present the lifecycle posters
Closure The students should have mastered all of the learning objectives through the series of activities included here. A general discussion could include:
This unit can be used to connect science, technology and environmental science topics. There are opportunities for extensions in any of these areas. For example – more on the combustion chemical reactions for a physical science class, electricity generation technologies and alternatives in a technology class, or consequences (climate change, acid rain, etc.) for an environmental science class.
The following New York State Science Standards are supported by this module:
STANDARD 1—Analysis, Inquiry, and Design
Students will use mathematical analysis, scientific inquiry, and engineering design, as appropriate, to pose questions, seek answers, and develop solutions.
STANDARD 3- Mathematics
Students will understand mathematics and become mathematically confident by communicating and reasoning mathematically, by applying mathematics in real world settings, and by solving problems through the integrated study of number systems, geometry, algebra, data analysis, probability and trigonometry.
STANDARD 4- Science
Students will understand and apply scientific concepts, principles, and theories pertaining to the physical setting and living environment and recognize the historical development of ideas in science.
STANDARD 6—Interconnectedness: Common Themes
Students will understand the relationships and common themes that connect mathematics, science, and technology and apply the themes to these and other areas of learning
The following National Common Core Mathematics Standards are supported by this module: (http://www.corestandards.org/the-standards/mathematics )
Students will be able to:
G-MG.3. Apply geometric methods to solve design problems Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.
N-Q.2. Define appropriate quantities for the purpose of descriptive modeling.
G-MG.3. Apply geometric methods to solve design problems
S-ID.1. Represent data with plots on the real number line (dot plots, histograms, and box plots).
The lifecycle posters and Power Profiler worksheet can be collected and graded as part of the assessment. The college-level homework assignment also provides a means of assessing if the students understand the concepts and met the learning objectives.
Electricity, efficiency and emissions powerpoint lecture
Cut-outs for electricity generation lifecycle poster
Student worksheet for Power Profiler activity
Homework assignment - State emission factors for electricity (geared for college, perhaps also HS students)
 U.S. DOE EIA, What are greenhouse gases and how much are emitted by the United States? http://www.eia.gov/energy_in_brief/greenhouse_gas.cfm (accessed 6/2/2011)
 USEPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009, April 2011. USEPA #430-R-11-005 (http://www.epa.gov/climatechange/emissions/usinventoryreport.html )
 U.S. DOE EIA, Annual Energy Outlook, 2011: Emissions from Energy Use. April 2011, Report Number: DOE/EIA-0383(2011) Table 30 http://www.eia.gov/forecasts/aeo/topic_emissions.cfm
 U.S. DOE EIA, Emissions of Greenhouse Gases in the U. S. March 31, Report Number: DOE/EIA-0573(2009) http://www.eia.gov/environment/emissions/ghg_report/ghg_overview.cfm (accessed 6/2/2011)
 Spadaro, J.V., L. Langlois, and B. Hamilton, 2000. Greenhouse gas emissions of electricity generation chains – Assessing the difference. IAEA Bulletin, 422: 19-24. http://www.iaea.org/Publications/Magazines/Bulletin/Bull422/article4.pdf
 U.S. EPA eGRID2010 Version 1.1, http://www.epa.gov/cleanenergy/documents/egridzips/eGRID2010V1_1_year07_GHGOutputrates.pdf (accessed 6/2/2011)
Materials for Activities:
Lecture Support Materials:
Links to External Resources: