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Scientists show
large impact of controlling humidity on greenhouse gas emissions
BY
National Renewable
Energy Laboratory 03/15/22
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Credit: Pixabay/CC0 Public Domain
Greenhouse gas emissions from air
conditioners are expected to climb as economic growth drives efforts
to control both temperature and humidity, according to an analysis by
scientists from the National Renewable Energy Laboratory and Xerox
PARC.
The research, which explores the
environmental impact of controlling
humidity, appears in the journal Joule as "Humidity's
impact on
greenhouse gas emissions from air conditioning." While the energy
used to power air conditioners has clear implications on greenhouse
gas emissions, the impact from removing moisture from the air has
escaped in-depth study until now. The researchers showed that
controlling humidity is responsible for roughly half of the
energy-related emissions, with the other half due to controlling
temperature.
"It's a challenging problem that people
haven't solved since air conditioners became commonplaces more than a
half-century ago," said Jason Woods, an NREL senior research engineer
and co-author of the new study. His co-authors from NREL are Nelson
James, Eric Kozubal, and Eric Bonnema. The collaborators from Xerox
PARC, an R&D company working on ways to remove humidity more
efficiently from the air, are Kristin Brief, Liz Voeller, and Jessy
Rivest.
The researchers pointed out the
increasing need to cool the air is both a cause and an effect of
climate change.
Even a small amount of moisture in the
air can cause people to feel uncomfortable and even damage buildings
in the form of mold and mildew. Furthermore, controlling indoor
humidity through commercially available air conditioning technologies
impacts the environment in three ways: 1) They consume a considerable
amount of electricity, 2) they use and leak CFC-based refrigerants
with global warming potential that is 2,000 times as potent as
carbon dioxide, and 3) the manufacturing and delivery of these
systems also release greenhouse gasses.
The researchers calculated air
conditioning is responsible for the equivalent of 1,950 million tons
of carbon dioxide released annually, or 3.94% of
global greenhouse gas emissions. Of that figure, 531 million tons
comes from energy expended to control the temperature and 599 million
tons from removing humidity. The balance of the 1,950 million tons of
the carbon dioxide come from leakage of global-warming-causing
refrigerants and from emissions during the manufacturing and transport
of the air conditioning equipment. Managing humidity with air
conditioners contributes more to climate change than controlling
temperature does. The problem is expected to worsen as consumers in
more countries—particularly in India, China, and Indonesia—rapidly
install many more
air conditioners.
"It's a good and a bad thing," Woods
said. "It's good that more people can benefit from improved comfort,
but it also means a lot more energy is used, and carbon emissions are
increased."
To calculate the emissions to manage both
temperature and humidity, the researchers divided the globe into a
fine grid measuring 1 degree of latitude by 1 degree of longitude.
Within each grid cell, the following characteristics were considered:
population,
gross domestic product, estimated air conditioner ownership per
capita, carbon intensity of the grid, and hourly weather. They ran
nearly 27,000 simulations across the globe for representative
commercial and residential buildings.
Climate change is affecting ambient
temperatures and humidity around the globe, making it warmer and more
humid. As part of the study, the researchers considered the impact of
the changing climate on air conditioner energy use by 2050. For
example, the study projects air conditioner energy use to increase by
14% in the hottest climate studied (Chennai, India) and by 41% in the
mildest (Milan, Italy) by 2050. The increase in global humidity is
projected to have a larger impact on emissions than the increase in
global temperatures.
"We've already made the existing,
century-old technology nearly as efficient as possible," Woods said.
"To get a transformational change in efficiency, we need to look at
different approaches without the limitations of the existing one."
Existing vapor compression technology is
optimized to cool our buildings using a "vapor compression cycle."
This cycle uses harmful refrigerants to cool air down low enough to
wring out its moisture, often over-cooling the air and wasting energy.
Improving the vapor compression cycle is reaching practical and
theoretical limits, thus pointing to a need to leap-frog to an
entirely new way to cool and dehumidify buildings. New technologies
that split this cooling and humidity control problem into two
processes show potential to improve efficiency by 40% or more. Once
such technology space is the use of liquid desiccant-based cooling
cycles such as the many liquid desiccant air conditioning technologies
that NREL is currently developing with many partners, such as Emerson
and Blue Frontier.
The researchers point out that the use of
liquid desiccants fundamentally changes the way humidity is controlled
and has theoretical efficiency limit that is 10 times higher than the
vapor compression cycle alone. A hypothetical technology—at only half
this new limit—would reduce cooling-energy emissions by 42% in 2050,
with the equivalent of avoiding 2,460 million tons of carbon dioxide
annually.
More information:
Jason Woods et al, Humidity's impact on greenhouse gas emissions
from air conditioning, Joule (2022).
DOI: 10.1016/j.joule.2022.02.013
Journal information:
Joule
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