ASHRAE Journal - September 2021 - 30

COLUMN ENGINEER'S NOTEBOOK
By reducing " operational carbon, "
good MEP design can eliminate
Scope 1 emissions and prepare
systems to take advantage of everreducing
Scope 2 emissions linked
to electricity's increasing use of
renewable energy in the future. The
mechanical engineer's decarbonization
work does not end there,
as there is lingering responsibility
for Scope 3 emissions in at least
two ways, water use and embodied
carbon.
FIGURE 3 Tracking carbon reduction by emissions scope for low carbon design. (Source: Arup)
1 Eliminate on-site fossil fuel use. [Scope 1]
2 Better than code energy efficient performance to reduce demands of " all-electric " /heat pump building with green
power purchase agreement. [Scope 2]
3 Reduce water demand with graywater and storm water reuse, and low-flow fixtures. [Scope 3]
4 Reduce primary structure and MEP embodied energy by 20% minimum target. [Scope 3]
5 Minimize interior finishes and target other embodied carbon reductions. [Scope 3]
6 Harvest on-site solar PV and battery storage systems to
reduce grid demands. [Scope 2]
7 Embodied carbon for structure and MEP systems is sunk in
year one. [Scope 3]
Accounting for the carbon emissions
of wastewater is explicitly
required by the definition of Scope
3. Of equal importance is the
acknowledgement of the carbon
intensity of potable water. These values
will be unique to the local water
supplier, considering the energy for
water treatment as well as pumping. Awareness of utility-scale
trends around the water-carbon-energy nexus
will be useful for this aspect of Scope 3 management.
Water efficiency is thus a proactive climate action. A
collection of tools and Scope 3 calculation methods for
water management can be found in a guidance document
from the River Network.12 Water use for cooling
towers and their backwash will require carbon accounting
not typically present in current energy modeling. In
drier locations, despite higher energy use, air-source
heat pumps and dry heat rejection may become preferable
when the whole life carbon impact of energy and
water sources is evaluated. Where cost-effective and
not detrimental to the surrounding ecosystem, heat
rejection into geothermal fields, naturally occurring
water bodies or underground flows may be beneficial
to explore to reduce evaporation losses on water-cooled
equipment.
Scope 3
Scope 1
Scope 2
4
5
3
2
1
Baseline
2
Day 1
6
Year 1
May 2020 ASHRAE Journal column " Counting Carbons
and Circular Diets " for more detailed information on
this topic.13 Attention to material quantities, recycled
content of metals, use of refurbished equipment, and
local manufacturing will rise in importance for Scope 3
emissions.
As shown in the simple carbon reduction visualization
The second Scope 3 concern is embodied carbon of
the mechanical systems, which covers the carbon emitted
in getting materials to and then installed on-site.
This includes the indirect emissions associated with
energy use and the direct emissions of a variety of
global-warming chemicals associated with raw material
extraction, manufacturing, transportation, installation,
and disposal at the end of usefulness. Please refer to the
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S E PTEM B E R 2021
of Figure 3, whole life carbon analysis for a new building
design tracks against the business-as-usual baseline
to justify decisions, just as we did with traditional
energy-based life-cycle cost analysis. Unlike traditional
paradigms in which energy efficiency often paid for first
cost increases over a 25 to 30 years, low carbon solutions
are not necessarily lower in energy costs or first costs.
Building-wide energy efficiency and resource use reductions
are the ultimate key to moving toward net zero carbon
buildings in the absence of a common carbon valuation
or compliance-based regulations requiring building
decarbonization.
Decarbonization of Electricity
U.S. President Biden's recent commitment to a " carbon
pollution-free power sector by 2035 " 14 highlights
how some of the first government-level climate change
commitments have been focused on electrical grid
decarbonization. This will be a substantial effort given
7
Metric Tons of Carbon

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ASHRAE Journal - September 2021

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