Calculations Used in Carbon Footprint Calculator
Written By: Jeremy Caves
jcaves@rice.edu
Car Emissions:
Car emissions are calculated by dividing the miles driven by the fuel efficiency of
the vehicle. This number is then multiplied by the CO2 emissions coefficient, 19.36 lbs
CO2/gallon1 of gasoline, and then divided by 2204.6 lbs/metric ton to obtain the tonnes of
CO2 emitted from car travel (Equation 1).
Equation 1
 MilesDrive n
 lbsCO2  


 FuelEfficiency  19.36 gallon    2204.6  CO2 Emissions (tonnes)



Airplane Emissions:
The total number of passenger revenue miles sold by U.S. airlines,
809,728,364,758 miles2, is divided by the total gallons of jet fuel purchased by United
States based carriers, 19,285,575,892 gallons3. Both of these numbers include domestic
and international travel by U.S. airlines in 2006. The result of this calculation is the fuel
efficiency per passenger, 41.986 passenger miles/gallon, and is averaged over all flights
operated by U.S. carriers. Because airplanes use significantly more fuel during takeoff,
this number is an overestimation for short flights, but an underestimation of fuel
efficiency for long flights.
The total number of miles flown is divided by the above fuel efficiency per
passenger and multiplied by 20.88 lbs CO2/gallon4, the CO2 emissions coefficient for jet
fuel. This number is then multiplied by 2.7, the Radiative Forcing Index (RFI), which is
1
Office of Policy and International Affairs. Technical Guidelines: Voluntary Reporting of Greenhouse
Gases (1605(b)) Program. United States Department of Energy; March 2006.
2
TranStats. “Revenue Passenger Miles by Unique Carrier for 2006.” Bureau of Transportation Statistics;
2006.
3
“Airline Fuel Cost and Consumption (1977-2007).” Bureau of Transportation Statistics; 2007.
4
Office of Policy and International Affairs. Technical Guidelines: Voluntary Reporting of Greenhouse
Gases (1605(b)) Program. United States Department of Energy; March 2006.
a measure of the global warming impact from greenhouse gases other than carbon
dioxide5. Airplanes emit both nitrous oxides (NOx) and water vapor (contrails) high in
the troposphere, both of which are greenhouse gases. Thus, the RFI accounts for the
global warming potential of airplane emissions due to gases other than CO2. Finally, this
number, in pounds of CO2, is divided by 2204.6 lbs/metric ton to obtain CO2 emissions in
tonnes (Equation 2).
Equation 2




 lbsCO 2 


MilesFlown
  2.7   2204.6  CO2 Emissions (tonnes)
 20.88

 gallon 
 41.986 passengerm iles 





gallon




Electricity:
Several numbers were important in developing the calculations of CO2 emissions
due to electricity use; these are detailed in Table 2.
Rice Central Plant Heat Rate
The calculation of the Central Plant’s heat rate is not straightforward and depends
largely upon which combination of boilers, generators, and chillers are in use. According
to Doug Wells, the Central Plant has a heat rate of 13.3MMBtu/Mwh. However, this
measurement excludes steam production, which is produced by the generators as a byproduct of electricity production. To determine how much natural gas is used to produce
steam, one of the boilers is operated to determine the ratio of natural gas input to steam
output. The quantity of natural gas used to produce an equivalent amount of steam as
during generator operation is then “credited” to the generator. However, the generators
run at a lower efficiency than the boilers (in terms of steam production), and this must be
accounted for when “crediting” the generators. In addition, each of the boilers operates at
a different efficiency, and the Solar and Ruston generators vary by more than 15% in
“Executive Summary.” Aviation and the Global Atmosphere. Intergovernmental Panel on Climate
Change; 1999.
5
their efficiency, complicating these calculations6. Ultimately, accounting for steam
production results in a 3-5 MMBtu/Mwh decrease in the Central Plant’s heat rate. Thus,
we chose a fairly conservative value, 10 MMBtu/Mwh, as the Central Plant’s heat rate for
use in our calculator. We relied heavily on communications with Mark Gardner and
usage of his “Plant Lineup Scenario 1” model, produced in the early part of this decade,
for this information.
Rice Central Plant CO2 Emissions Coefficient
The Central Plant emits 27411.7188 tonnes of CO2 annually7. This number is
derived from computer models and simulations of generator operations which predict the
total amount of CO2 released for a given amount of natural gas. Given that Rice
purchased 494,327MMBtus last year8, the Central Plant emits 122.25 lbs CO2/MMBtu,
and, with a heat rate of 10 MMBtu/Mwh, the CO2 emissions coefficient is 1.22 lbs
CO2/Kwh (Equation 3)
Equation 3
lbsCO 2
MMBtu
Kwh
 27411.7188tonnesCO2  2204.6 
 1000
 1.22

  10
494327 MMBtu
Mwh
Mwh
Kwh


Calculation of Reliant Energy’s CO2 Emissions Coefficient
Reliant Energy uses a fuel mix of 42% lignite, 43% natural gas, 9% nuclear, 1%
renewable, and 5% other production means to produce the electricity that Rice
purchases9,10. This electricity is slightly dirtier than the Texas average; where 100 is an
indexed value corresponding to the average CO2 emissions for electricity production in
Texas, Reliant emits CO2 at a 109 value11. The average CO2 emissions coefficient for
6
Mark Gardner, FE&P Energy Manager (personal communication)
Doug Wells, Director of Central Plant (personal communication)
8
Ibid.
9
Eric Valentine, Energy Manager, FE&P (personal communication)
10
“Electricity Facts: Reliant Energy Residential Services—12 Month OneRate Secure Plan.”
Powertochoose.org. February 2007.
11
Ibid.
7
Texas electric power, averaged over the years 1998-2000, is 1.46 lbs CO2/Kwh12.
Increasing the state average by 9%, to model Reliant’s slightly dirtier energy production,
yields a CO2 emissions coefficient of 1.59 lbs CO2/Kwh (Equation 4). We used 1.6 lbs
CO2/Kwh in our calculator to match the calculations of Dr. K. Zygourakis.
Equation 4
1.46
lbsCO 2  109 
lbsCO 2

  1.59
Kwh  100 
Kwh
where 109/100 is a ratio of Reliant’s CO2 emissions to the Texas state average for
electricity production (indexed to 100).
CO2 Emissions Due to Appliance Use
Table 3 lists constants used for appliance electricity consumption. Equation 5
was used to calculate the CO2 emissions from each appliance.
Equation 5

 Kwh  
 Electricit yConsumption
 

 day    365days  1.486 lbsCO2   2204.6  CO Emissions (tonnes)


2


UseSuite / Individual 
Kwh 







In Equation 5, “electricity consumption” is the daily consumption by the given
appliance, “use(suite/individual)” is the number of people using that appliance (1 for an
individual; 2, 3, 4, or 5 for suite use), and “1.486 lbs CO2/Kwh” is the weighted carbon
emissions coefficient for Rice electricity (assuming 30% is generated at the Central Plant
and 70% is purchased from Reliant). Note that, for blow-dryers, electricity consumption
was measured on a weekly basis; that is, instead of scaling up use by 365 days, use was
scaled up by 52 weeks, given that many women only blow-dry their hair several times per
week and not on a daily basis.
12
Energy Information Administration. Updated State-level Greenhouse Gas Emission Coefficients for
Electricity Generation 1998-2000. United States Department of Energy. April 2002.
CO2 Emissions Due to Air-Conditioning
We assumed that 30% of Rice’s electricity consumption was used for airconditioning and that, of this fraction, 20% was used to air-condition the colleges. These
assumptions were supplied to us by Richard Johnson, who himself assumed that all of the
Central Plant’s electricity production was being used for air-conditioning, amounting to
approximately 30% of Rice’s electricity consumption. As a result, per-capita electricity
consumption among on-campus students is 2,854Kwh (Equation 6).
Equation 6
99905Mwh  0.3  0.2  1000
2100students
Kwh
Mwh  2854 Kwh
student
Facilities, Engineering, and Planning (FE&P) uses 76º as the baseline temperature
for air-conditioning; any extra degree of cooling increases electricity use by 4%.
Conversations with Mark Gardner, FE&P Energy Manager, and Doug Wells, Director of
the Central Plant, indicated that these two figures are standard “rules-of-thumb” and were
obtained from engineering textbooks or websites. For our calculator, we assumed that, if
a student set his or her thermostat at 76º, he or she would be using 2,854 Kwh for airconditioning. For every extra degree of cooling, the calculator increases the electricity
consumption by 4%; similarly, for every degree higher than 76º to which the thermostat
is set (such as 78º or 80º), the calculator decreases the electricity consumption by 4%.
For instance, a student who sets his thermostat at 70º would use 3,539Kwh of electricity
for air-conditioning (Equation 7).
Equation 7
2854 Kwh 
76  thermostatsetting   4 2854 Kwh  Electricit yConsumption( Kwh)
100
where “thermostat setting” is the temperature at which the thermostat is set and
“electricity consumption” is the total electricity used, by that student, to keep his or her
room at that thermostat setting.
Finally, the total CO2 emissions due to air-conditioning is calculated using
Equation 8.
Equation 8
lbsCO 2

Electricit yConsumption( Kwh)  1.486

Kwh


  2204.6  CO 2 Emissions (tonnes)


CO2 Emissions Due to Steam Use
The colleges used 34.771 million pounds of steam during 200613. In our
calculator, we converted pounds of steam into kilowatt-hours, finding that each student
uses 1,655.76Kwh of steam (Equation 9). This is an unnecessary conversion which
requires using the heat rate, an uncertain figure, and future versions of the calculator
should only convert heat rate into equivalent values of Btus.
Equation 9
lbs
therms
MMBtu
Kwh
Kwh
 10
 10
 1000
 2100students  1655.76
therm
MMBtu
Mwh
Mwh
student
where 100lbs steam/therm and 10therms/MMBtu are standard conversions14.
34771000lbs  100
This number is subsequently converted to CO2 emissions using Equation 10.
Equation 10
1655.76Kwh  1.486
lbsCO2
 2204.6  1.116tonnesCO2
Kwh
For every extra degree of heating, an additional 4% energy is needed; however,
steam is used for both hot water and heating, and we lacked data which broke down what
13
Mark Gardner, FE&P Energy Manager (personal communication)
“Rules of Thumb.” A.B. Young Companies. http://www.abyoung.com/thumb.html. A website hosted
by A.B. Young Companies, a heating and cooling engineering firm, listing standard conversions for steam
and chilled water.
14
percentage of steam was destined for which end-use. Thus, we kept this figure constant
for every student.
CO2 Emissions Due to Commons/Servery Electricity Consumption
This calculation is quite crude due to a lack of data. The colleges use
10,335.795Mwh of electricity (approximately 10% of the Rice’s electricity
consumption)15; however, even if every student used every appliance we measured (listed
in Table 3) every day of the year, these appliances would only account for
1,717.727Mwh of college electricity consumption (Equation 11). Clearly, a large amount
of electricity is being used by servery equipment, common areas, and possibly ceiling
lights in dorm rooms (which we did not measure). Since appliance electricity
consumption is included in the total college electricity consumption, it is necessary to
subtract appliance electricity consumption from total college electricity consumption in
order to approximately gauge the total amount of electricity consumed due to
commons/serveries (see Equation 12). To do this, we assumed that every student used
every appliance we measured every single day of the year and subtracted this total from
the college electricity total to obtain an estimate of commons/servery electricity use.
While this assumption is almost certainly incorrect, it does avoid counting appliance
electricity use twice.
Equation 11
0.177  0.21  0.095  0.81  0.15  0.731  0.068 Kwh  365days  2100students  1717727 Kwh
day
where the above, decimal figures are drawn from Table ???.
Equation 12
10335.795Mwh  1717.727Mwh  8618.039Mwh
Thus, common areas and serveries draw 8,618.039Mwh of electricity, which,
divided among 2100 students is 4,103Kwh/student. This figure is then used to determine
15
Mark Gardner, FE&P Energy Manager (personal communication)
the CO2 emissions due to common areas and servery electricity consumption (Equation
13).
Equation 13
4103Kwh  1.486
lbsCO2
 2204.6  2.766tonnesCO2
Kwh
Green Energy CO2 Emissions:
At the very end, the calculator determines how much CO2 one would emit if Rice
purchased green energy. It assumes all domestic energy use (electricity, heating, and
cooling) would be produced via green energy. This total is calculated using Equation 14.
Equation 14
TotalCO2 Emissions (tonnes)  DomesticEnergyCO2 Emissions (tonnes)  GreenEnergyCO2 Emissions
where “Total CO2 Emissions” equals the sum of air travel, car travel, and domestic
energy CO2 emissions and “Domestic Energy CO2 Emissions” equals the sum of
appliance, cooling, steam use, and common areas/servery CO2 emissions.