The 20 year global warming potential of methane (natural gas) is 86. this means that one gram
of methane will trap the equivalent amount of heat in the atmosphere as 86 grams of co2 over a
twenty year timeframe. we will explore the importance of this fact for switching from natural
gas to an electric heat pump in this problem.
(1) the efficiency of a modern natural gas furnace in the home is 95% (i. e. 95 percent of
the energy that goes into the furnace as natural gas comes out as useable heat in the
home). using this information as well as the energy content for natural gas given (i. e.
one cubic foot of natural gas contains 1,031 btus of energy), find the number of cubic
feet of natural gas that would be necessary to heat a typical single family home in the
northeast that requires 71 million btu for space heating.
(2) for an air source heat pump with a seasonally averaged cop of 2.0, calculate the
number of kwh’s of electricity required to provide the 71 million btus required for
space heating. (hint: there are 3,412 btu per kwh.)
(3) for each cubic foot of natural gas that is burned, 0.0547 kilograms of co2 are
released. for each kwh of electricity in new york there are 0.246 kilograms of co2
released from direct combustion of fossil fuels. using your answers to part (1) and (2),
find the total amount of co2 that would be emitted by the natural gas furnace and the
heat pump powered by electricity from the grid.
(4) now calculate what the indirect emissions from methane leaks are for these two
systems. for natural gas, assume that one cubic foot of natural gas weighs 0.0200
kilograms and that the leakage rate from both upstream and downstream emissions is
3.80 percent representing the mixture of shale and conventional natural gas in 2013. for
the electricity sector add 0.215 kilograms of co2-equivalent for each kwh to account for
the fraction of electricity in the state coming from natural gas.
(5) based on your answers to part (3) and (4), how does the inclusion of methane leaks
affect the comparison of the direct use of natural gas and the use of an air source heat
pump powered by grid electricity.