       
Overview
Using recent, publicly available government and non-profit supported data, our goal is to provide customers with an explanation of the environmental impact of the renewable energy available through PaloAltoGreen.
PaloAltoGreen customers purchase renewable energy in kilowatt-hours (kWh) of electricity. The methodologies used to calculate PaloAltoGreen's environmental benefits convert kWh into pounds of avoided carbon dioxide (CO2) emissions. This conversion allows us to translate abstract measurements (like kWh or pounds of CO2) into more tangible environmental equivalencies such as cars or passenger vehicles removed from the road, acres of trees storing carbon for one year, and garbage recycled as opposed to being landfilled.
Carbon Dioxide (CO2) Emissions Avoided
The first step in calculating environmental benefits is to convert the reduction of kilowatt-hours into avoided units of CO2. The CO2 reduction value of PaloAltoGreen renewable energy is calculated by multiplying the number of kilowatt-hours (kWh) a customer purchases by 1.218. This is the average number of pounds of CO2 prevented from entering the atmosphere for each kWh not used in the Western Electricity Coordinating Council (WECC) region (EPA, 2008). The WECC is the North American Electric Reliability Corporation (NERC region) from which PaloAltoGreen sources its renewable energy.
Example: the average Palo Alto household uses 650 kWh per month. 650 x 1.218 = 791.7 pounds of CO2 avoided, or 9,500 pounds annually.
Cars or Passengers Vehicles Removed from the Road
Once we know the pounds of avoided CO2 emissions for the average Palo Alto household, we can convert this number into the equivalent number of cars or passenger vehicles that would need to be removed from the road to have the same impact on CO2 emissions.
Passenger vehicles are defined as 2-axle 4-tire vehicles, including passenger cars, vans, pickup trucks, and sport/utility vehicles. In 2007, the weighted average of fuel economy of cars and light trucks combined was 20.4 miles per gallon (FHWA 2008). The average vehicle miles traveled in 2007 was 11,720 miles per year. In 2007, the ratio of CO2 emissions to total emissions (including carbon dioxide, methane, and nitrous oxide, all expressed as CO2 equivalents) for passenger vehicles was 0.977 (EPA 2009). The amount of CO2 emitted per gallon of motor gasoline burned is 8.89*10-3 metric tons.
To determine annual greenhouse gas emissions per passenger vehicle, the following methodology is used: average vehicle miles traveled is divided by average gas mileage to determine gallons of gasoline consumed per vehicle per year. The number of gallons of gasoline consumed is multiplied by CO2 per gallon of gasoline to determine CO2 emitted per vehicle per year. CO2 emissions are divided by the ratio of CO2 emissions to total vehicle greenhouse gas emissions to account for vehicle methane and nitrous oxide emissions. When simplified, the result is 5.23 metric tons CO2 equivalent per vehicle per year. (1 metric ton = 2,204.6 pounds) Note: due to rounding, performing the calculations given above may not return the exact results shown.
Example: the average Palo Alto household in PaloAltoGreen avoids 9,500 pounds of CO2 annually, or 4.3 metric tons. 4.3/5.23 = 0.82 cars removed from the road for one year, or 1 car removed for 10 months.
Acres of Trees/Pine or Fir Forest Storing Carbon for One Year
We can also use the pounds of avoided CO2 for the average Palo Alto household to compare with the equivalent number of acres of trees storing carbon for one year.
Growing forests store carbon. Through the process of photosynthesis, trees remove CO2 from the atmosphere and store it as cellulose, lignin, and other compounds. The rate of accumulation is equal to growth minus removals (i.e., harvest for the production of paper and wood) minus decomposition. In most US forests, growth exceeds removals and decomposition, so there has been an overall increase in the amount of carbon stored nationally.
The estimate of the annual average rate of carbon accumulation is based on two studies, one on Douglas fir in the Pacific Northwest (Nabuurs and Mohren, 1995), and the other on slash pine in Florida (Shan et al. (2001)). These two studies represent commercially important species from different regions and with different rotation periods (i.e., time between planting and harvesting). The calculation addresses only above-ground carbon; although carbon accumulates in roots, leaf litter, and soils, these below-ground carbon pools are not included.
Calculation for Slash Pine
The calculation uses the Gain Loss method, as outlined in the 2006 IPCC Guidelines, in order to estimate carbon stored annually per hectare in the slash pine plantation system described in the Shan et al. paper mentioned above. The general equation for this method is shown here. Carbon losses due to harvested wood products, firewood foraging, and other sources of wood removal are assumed to be zero.
Since this paper measured growth in a plantation of trees harvested at age 17, the value is for relatively young trees that grow more rapidly than older trees. The paper included several options in terms of management. The value used in the calculations below is the "control" - meaning that there was no fertilization (which had a big impact on growth) and no trimming of the understory for these trees. The calculation below uses the IPCC assumption that the carbon fraction is 47 percent of dry biomass.
The final result is 3.052 metric tons of carbon per hectacre per year * 0.4048 hectares per acre = 1.24 metric tons of carbon per acre per year.
Calculation for Douglas Fir
This calculation is based on results found in a 1995 paper by Nabuurs et al. The paper uses a model to calculate the amount of carbon sequestered in plots of various tree types across the world. The model uses turnover rates in order to calculate carbon stored in forests over time during different types of logging intervals. Parameters included in the model include basic wood density, allocation of net primary production, turnover rates of tree organs, resident times of litter and humus, current volume increment, and allocation of harvested wood. The parameters are specific for each of the six sites chosen for the study. Within each site, three areas of fertility and production are measured, although the study uses sample data from the "moderate" site during the discussion and results sections. The numbers presented below are also from the "moderate" site.
Since the paper is concerned with carbon sequestered in forests undergoing selective logging, the designers (U.S. EPA) of this calculation had to choose at what point during the harvesting cycle to measure the carbon sequestered. They decided to use the total carbon stock stored (including biomass and forest products, not including soil carbon) after 100 years of accumulation. The model in this paper assumes that the carbon fraction is 50 percent.
When simplified, the result is 3.27 metric tons of carbon per hectacre per year * 0.4048 hectares per acre = 1.32 metric tons of carbon per acre per year. One reason why this value is higher than the slash pine plantation number is because the Douglas fir trees had 100 years to accumulate biomass - including more years at a relatively rapid maturity rate than the slash-pine trees.
The average of these two values (slash pine and Douglas fir) is 1.28 metric tons of carbon per acre per year, which corresponds to 4.69 metric tons of CO2 per acre of pine or fir forests.
Example: the average Palo Alto household in PaloAltoGreen avoids 9,500 pounds of CO2 annually, or 4.3 metric tons. (1 metric ton = 2,204.6 pounds) 4.3/4.69 = 0.92 acres of trees storing carbon for a year.
Sources:
- Nabuurs, G.J., and G.M.J. Mohren. 1995. Modeling analysis of potential carbon sequestration in selected forest types. Canadian Journal of Forest Research 25(7):1157-1172.
- Shan, J.P., L.A. Morris, and R.L. Hendrick. 2001. The effects of management on soil and plant carbon sequestration in slash pine plantations. Journal of Applied Ecology 38(5):932-941.
- IPCC 2006, 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Prepared by the National Greenhouse Gas Inventories Programme, Eggleston H.S., Buendia L., Miwa K., Ngara T. and Tanabe K. (eds). Published: IGES, Japan. Volume 4. Available at http://www.ipcc-nggip.iges.or.jp/public/2006gl/index.html
- EPA Calculations and References
Garbage Recycled and Not Landfilled
To determine how much CO2 is prevented from entering the atmosphere by recycling waste rather than landfilling it, we used greenhouse gas emission factors from EPA's WAste Reduction Model (WARM) (EPA 2006). The emissions from landfilling all waste were compared to emissions from waste landfilled without mixed recyclables (e.g., paper, metals, and plastics). For every 2,000 pounds of waste recycled, the result was an emission reduction of 0.79 metric tons of carbon equivalent. (1 metric ton = 2,204.6 pounds.) To convert 0.79 into metric tons of carbon dioxide equivalent, it was multiplied by 44/12, the ratio in weight of carbon dioxide to carbon. When simplified, the result is 2.90 metric tons of carbon dioxide equivalent per 2,000 pounds of waste recycled instead of landfilled.
Example: the average Palo Alto household in PaloAltoGreen avoids 9,500 pounds of CO2 annually, or 4.3 metric tons. 4.3/2.90 = 1.49 metric tons (or 3,276 pounds) of waste recycled instead of landfilled.
Carbon Footprint Calculator
For users who only know the cost of their average electricity bill, the PaloAltoGreen Carbon Footprint Calculator calculates kilowatt hour (kWh) consumption by dividing the dollar amount by the average price per kWh for the average Palo Alto resident. The average residential price per kWh is $0.117433.
Example: A customer's average electricity bill is $75.00. 75/0.117433 = 638.66 kWh per month.
|
