ASHRAE Journal - April 2021 - 36

COLUMN ENGINEER'S NOTEBOOK

heat rejection, which in turn raises
FIGURE 2 Simplified causal loop diagram showing the positive feedback loops between outdoor temperature and
outdoor air temperature. (Closed local air-conditioning use.
feedback Loop A is seen on the left
side of Figure 2.) Even more compliFossil Fuel
Electricity
cated is the chronic globalized positive
Use
AC Use
feedback loop, which pumps emisUtility
sions into the atmosphere, thereby
Carbon
Heat
exacerbating the global warming
Loop B
Rejection
trend. (Closed global feedback Loop B
Refrigerant
Solar Heat
is seen on the right side of Figure 2.)
Leaks
The causal loop diagram in Figure 2
shows the interplay of happenings.
Atmosphere
Loop A
A " + " sign on the arrow means that
CO2e
Heat Island
Extreme
when the state at the arrow's source
Events
node (e.g., heat island, AC use) goes
Outdoor
Temperature
up, the action at the end of the arrow
Global
also goes up. Causal loop diagrams
Warming
are useful for visualizing positive/selfreinforcing loops versus negative/selfbalancing loops. They are also parFIGURE 3 Causal loop diagram modified to demonstrate how five classes of intervention can help reduce global
ticularly useful at identifying where
warming while still allowing air-conditioning use to support adaptation to warming environments anticipated this
changes could be sought to break or
century.
slow down the cycle. On our HVAC
Fossil
Technology Intervention
feedback loop, many are working in
Electricity
parallel to target strategic intervenFuel
AC Use
Use
tions at the nodes (Figure 3).
Utility
Fuel Source Intervention
* Design Interventions: ArchiCarbon
Heat
tects, landscape designers and
Renewables
Rejection
municipal agencies apply cool roofs,
Refrigerant
Solar Heat
street shade, absorptive ground
Diversion Intervention
Leaks
Refrigerant Intervention
cover and reflective road surfaces to
Design Interventions
Atmosphere
Heat
mitigate the solar inputs to the " heat
CCS
island " node, allowing condenser
e
CO
2
Extreme
Events
heat to be more quickly dissipated.
Island
Outdoor
* Diversion Interventions: To
Temperature
Global
attack the purple atmospheric
carbon and global warming nodes,
Warming
researchers explore carbon capture
and sequestration (CCS) technolopact of the green refrigerant leaks node by changing the
gies that pull carbon directly out of the air, with an
chemistry of the refrigerant itself.
audacious team proposing to use normal air movement
* Fuel Source Intervention: Certain states and muthrough HVAC equipment to create " crowd oil " from the
16
nicipalities have legislated a conversion to 100% renewcaptured carbon dioxide.
* Refrigerant Interventions: ASHRAE itself has a
able energy by 2050 to remove the yellow node's utility
long-standing relationship with the U.N. Environmental carbon content in the emissions loop.
Program to pursue low global warming potential (GWP)
* Technology Interventions: Finally, we come to the
refrigerants that fundamentally reduce the adverse imorange node of " Electricity Use " and the need for deep
36

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ASHRAE Journal - April 2021

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