impact_model.rb

Copyright © 2010 Brighter Planet. See LICENSE for details. Contact Brighter Planet for dual-license arrangements.

 
require 'leap'
require 'timeframe'
require 'date'
require 'weighted_average'
require 'builder'

ElectricityUse carbon model

This model is used by Brighter Planet’s carbon emission web service to estimate the greenhouse gas emissions of electricity use.

Timeframe and date

The model estimates the emissions that occur during a particular timeframe. To do this it needs to know the date on which the electricity use occurred. For example, if the timeframe is January 2010, a electricity use that occurred on January 5, 2010 will have emissions but a electricity use that occurred on February 1, 2010 will not.

Calculations

The final estimate is the result of the calculations detailed below. These calculations are performed in reverse order, starting with the last calculation listed and finishing with the emission calculation. Each calculation is named according to the value it returns.

Methods

To accomodate varying client input, each calculation may have one or more methods. These are listed under each calculation in order from most to least preferred. Each method is named according to the values it requires. If any of these values is not available the method will be ignored. If all the methods for a calculation are ignored, the calculation will not return a value. “Default” methods do not require any values, and so a calculation with a default method will always return a value.

Standard compliance

Each method lists any established calculation standards with which it complies. When compliance with a standard is requested, all methods that do not comply with that standard are ignored. This means that any values a particular method requires will have been calculated using a compliant method, because those are the only methods available. If any value did not have a compliant method in its calculation then it would be undefined, and the current method would have been ignored.

Collaboration

Contributions to this carbon model are actively encouraged and warmly welcomed. This library includes a comprehensive test suite to ensure that your changes do not cause regressions. All changes should include test coverage for new functionality. Please see sniff, our emitter testing framework, for more information.

 
module BrighterPlanet
  module ElectricityUse
    module ImpactModel
      def self.included(base)
        base.decide :impact, :with => :characteristics do

Emission calculation

Returns the emission estimate (kg CO2e). This is the emission produced by generating the electricity used during the timeframe, including transmission and distribution losses.

 
          committee :carbon do

Emission from date, emission factor, loss factor, and energy

 
            quorum 'from date, emission factor, loss factor and energy', :needs => [:date, :emission_factor, :loss_factor, :energy] do |characteristics, timeframe|
              date = characteristics[:date].is_a?(Date) ?
                characteristics[:date] :
                Date.parse(characteristics[:date].to_s)
  • Checks whether the electricity was used during the timeframe
 
              if timeframe.include? date
  • Divides energy (kWh) by (1 – loss factor) to give total electricity used, including transmission and distribution losses (kWh)
  • Multiplies by emission factor (kg CO2e / kWh) to give emission (kg CO2e)
 
                characteristics[:energy] / (1 - characteristics[:loss_factor]) * characteristics[:emission_factor]
              else
  • If the electricity was not used during the timeframe, emission is zero
 
                0
              end
            end
          end

Emission factor calculation

Returns the grid average emission factor of the area where the electricity was used (kg CO2 / kWh).

 
          committee :emission_factor do

Emission factor from eGRID subregion

 
            quorum 'from eGRID subregion', :needs => :egrid_subregion do |characteristics|

Looks up the emission factor of the eGRID subregion.

 
              characteristics[:egrid_subregion].electricity_emission_factor
            end
          end

Loss factor calculation

Returns the average transmission and distribution loss factor for the area where the electricity was used.

 
          committee :loss_factor do

Loss factor from eGRID region

 
            quorum 'from eGRID region', :needs => :egrid_region do |characteristics|

Looks up the loss factor of the eGRID region.

 
              characteristics[:egrid_region].loss_factor
            end
          end

eGRID region calculation

Returns the eGRID region where the electricity was used.

 
          committee :egrid_region do

eGRID region from eGRID subregion

 
            quorum 'from eGRID subregion', :needs => :egrid_subregion do |characteristics|

Looks up the eGRID region in which the eGRID subregion is located.

 
              characteristics[:egrid_subregion].egrid_region
            end
          end

eGRID subregion calculation

Returns the eGRID subregion where the electricity was used.

 
          committee :egrid_subregion do

eGRID subregion from zip code

 
            quorum 'from zip code', :needs => :zip_code do |characteristics|

Looks up the eGRID subregion in which the zip code is located.

 
              characteristics[:zip_code].egrid_subregion
            end

eGRID subregion from default

 
            quorum 'default' do

Uses the average of all eGRID subregion, weighted by electricity generation.

 
              EgridSubregion.find_by_abbreviation 'US'
            end
          end

Energy calculation

Returns the electricity used (kWh).

 
          committee :energy do

Energy use from client input

Uses the client-input energy (kWh).

Energy from default

 
            quorum 'default' do

Uses the 2008 U.S. annual household average electricity use (kWh).

 
              11_040
            end
          end

Date calculation

Returns the date on which the electricity was used.

 
          committee :date do

Date from client input

Uses the client-input date.

Date from timeframe

 
            quorum 'from timeframe' do |characteristics, timeframe|

Assumes the electricity was used on the first day of the timeframe.

 
                timeframe.from
            end
          end

Zip code calculation

Returns the zip code where the electricity was used.

Zip code from client input

Uses the client-input zip code.

Timeframe calculation

Returns the timeframe during which to calculate emissions.

Timeframe from client input

Uses the client-input timeframe.

 
        end
      end
    end
  end
end