ASHRAE Journal - September 2021 - 15

TECHNICAL FEATURE
A.
On-Site Natural Gas
B.
80% " Resource-to-Room " Effi ciency
125 Units
Input Energy
Refi ning + Transportation
6 Units
Natural Gas-Based Grid
135% " Resource-to-Room " Effi ciency
74 Units
Input Energy
Refi ning + Transportation
4 Units
Natural Gas
Plant
Distribution Losses
1 Unit
Boiler
Combustion Losses
18 Units
100 Units Heating
Energy
203 lb CO2/MMBtu of Heat
Distribution Losses
2 Units
Ambient Energy
67 Units
Heat
Pump
100 Units Heating
Energy
158 lb CO2/MMBtu of Heat
Carbon Free
Figure 1 Calculations for grid carbon intensity include the following assumptions: natural gas emission value of 117 lb CO2/MMBtu,6 methane leakage rate of 2.3%,7 air-source heat pump COP of
3, natural gas boiler efficiency of 90% and a methane global warming potential of 84.8 Since methane has a large global warming potential relative to CO2, we included the assumption that methane
would be 84 times more potent than CO2 for global warming impacts. Due to the immediate concern for climate change, we used the 20-year methane global warming potential value.
building. While both embodied and operational carbon
emissions must be reduced in a decarbonization effort,
embodied carbon is a one-time carbon emission during
the construction process. Operational carbon, on the
other hand, is continuously emitted and signifi cantly
outweighs the embodied carbon over a 50-year lifetime
of a building. This article will only be addressing operational
carbon, which is where the responsibility of those
in ASHRAE lies.
Building HVAC systems, water heating and even cooking
will be transformed by the move to decarbonized
buildings.
Grid Carbon Intensity
The ease of implementing building decarbonization is
greatly dependent on the local electricity grid. A building
owner is able to make the decision to eliminate
on-site fossil fuel use and add photovoltaic (PV) panels
to their building, but has no control over the source of
energy at the utility level. Three potential scenarios exist
for decarbonizing depending on the grid.
Scenario 1: Carbon-Intense Grid
Many electric grids are still heavily reliant on fossil
fuels with over 62% of utility-scale electricity generation
originating from fossil fuels in the U.S.3 Over the
last decade improvements in power plant effi ciency and
the switch to more affordable natural gas instead of coal
have reduced carbon emissions of electricity. Switching
from a gas boiler to a heat pump, even somewhere like
Mississippi, which uses mostly natural gas electricity,
will still reduce emissions. For a grid that is 100% natural
gas generation with an effi ciency similar to the assumption
in Figure 1, heat pump heating systems (Figure 1b)
would result in a 22% reduction in carbon emissions
over on-site natural gas boilers (Figure 1a).
In areas that rely heavily on fossil fuel-based electricity,
building owners have the option of installing on-site
solar systems. Some states make it possible to directly
purchase renewable energy either from wholesale providers
or " community choice aggregation " (CCA)4 providers.
Other options for eliminating carbon emissions are
purchasing renewable energy credits or carbon credits to
offset emissions. Renewable energy certifi cates (RECs)5
are issued for renewable energy generated but not tied
to any specifi c location. By purchasing carbon credits,
an individual or company is essentially fi nancing carbon
reduction projects to offset their own emissions.
S E P T E M B E R 2 0 2 1 ashrae.o rg ASHRAE JOURNAL
15
Ambient Energy
67 Units
Heat
Pump
100 Units Heating
Energy
Distribution Losses
2 Units
Combustion Losses
35 Units
C.
Renewable-Based Carbon-Free Grid
285% " Resource-to-Room " Effi ciency
35 Units
Input Energy

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ASHRAE Journal - September 2021

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