The electric power system relies on fossil fuels for electricity generation. The implementation of grid projects can have a significant effect on carbon and criteria pollutant emissions by changing the demand profile of a system. The Grid Project Impact Quantification (GridPIQ) tool leverages fossil fuel generation and emissions data from the U.S. Environmental Protection Agency (EPA) to estimate emissions impacts for a given grid project based on context. This empowers stakeholders to consider another potential benefit stream for grid projects, to articulate project impacts, and to quickly screen alternatives.
About AVERT
The AVERT (AVoided Emissions and geneRation Tool) is a tool developed by the EPA with a primary objective of enabling end users to estimate the emissions benefits from energy efficiency and renewable energy policies and programs. AVERT allows users to estimate changes in emissions of carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen oxide (NOx), and particulate matter less than 2.5 micrometers in diameter (PM2.5) based on changes in load shape. CO2 is a greenhouse gas, SO2 and NOx are criteria pollutants with national air quality standards, and PM2.5 consists of fine inhalable particles which can cause serious health problems and haze. More information can be found at the EPA's website.
Fundamentally, the AVERT statistically aggregates fossil fuel generation data from the EPA's publicly available Air Markets Program Data (AMPD) tool to estimate typical emissions output at various system power levels. The AVERT splits the United States into ten different regions, and emissions rates are determined for each year (2007-2015) for each region. To determine emissions rates, each year is further split into ozone season (May-September) and non-ozone season (October-April), a delineation that is driven by differing emissions requirements. Note that electrical generating units smaller than 25 MW do not report to AMPD and are thus excluded. For more information about AVERT, please visit their website.
GridPIQ Methodology
GridPIQ uses emissions rates aggregated from the AVERT to estimate grid project emissions (both hourly and aggregate) via fossil-fuel correction, scaling, and interpolation/extrapolation. Care is taken to properly map input demand profile time ranges to the proper subset of AVERT data (which is aggregated by region, year and ozone vs non-ozone season).
Line Losses
The project's location within the grid will dictate which losses are affected. Depending on whether the load is increased or decreased following a project implementation, the transmission and distribution line losses will either increase or decrease accordingly. A project located at the point of consumption will impact both these losses whereas a project located at the bulk generation level will impact neither. A project at the substation will impact the transmission but not the distribution losses.
For more information on line losses, see this page.
Fossil Fuel Load Correction
Fossil fuel generation is classified as the generation of electrical energy using fossil fuels such as use coal, natural gas, oil, and other minor resources. To quantify certain impacts, including emissions and fossil fuel costs, an understanding of how much load is served by fossil fuel generation is necessary. Therefore, if the load profile being used for a grid project represents total generation or total demand, a mechanism is required for approximating the corresponding fossil fuel generation.
For more information on fossil fuel correction, see this page.
Scaling
The AVERT aggregates the country into ten large regions, which do not match the size of a typical grid project. GridPIQ provides multiple scaling methods to get around this. The graphic user interface (GUI) currently only allows ‘peak’ scaling, while the application program interface (API) allows “peak” or no scaling (“none”). In the “peak” case, a different scaling factor is used for each given year of data – pre-project and post-project power values are scaled to AVERT region size for interpolation/extrapolation of emissions, and the results are scaled back afterwards.
In the “peak” scaling case, the maximum demand value for each year of the user-provided demand profile is determined (after fossil fuel correction). Note that these maximums are determined for the provided profile before grid project implementation. Similarly, the yearly maximum values for the selected AVERT region are obtained from the AVERT data-set. For each year, the AVERT maximum value is divided by the corresponding pre-project profile maximum value to create yearly scaling factors. Each hourly pre-project and post-project power value is then multiplied by its corresponding (based on year) scaling factor to be used for interpolation/extrapolation in the AVERT data. Resulting hourly emissions are scaled back to project size by using the reciprocal of the factors used to scale to AVERT region size. The peak scaling methodology assumes that the fuel mixture and dispatch order of the project’s electricity supply is similar to that of the larger AVERT region. An additional inherent assumption in the ‘peak’ scaling method is that project represents an area that meets its own demand with its own supply (think balancing area).
In the “none” scaling case, project impacts are assumed to affect the marginal generation of the larger AVERT region. The difference between pre-project and post-project power values (after line loss adjustment and fossil fuel correction) are subtracted from the selected AVERT region’s hourly fossil fuel generation. The resulting emissions from the AVERT region and the AVERT region minus project differences can then be compared to assess project emissions impacts.
Interpolation and Extrapolation
The AVERT data provide emissions information for a discrete range of regional fossil fuel generation power levels. To approximate emissions for power values that do not exactly fall on one of these power levels, simple linear interpolation and extrapolation is leveraged. Emissions for power values below the minimum value provided by AVERT are estimated by extrapolating toward zero, assuming that emissions are zero when generation is zero.
A fundamental assumption in this procedure is that input hourly power values are average for the hour.
GridPIQ informs the user how many hourly emissions values were estimated by interpolation rather than extrapolation. The given AVERT data contains extrapolated power/emission levels, and these are accounted for in the provided counts.