The research was published in the journal Joule. While the energy used to power air conditioners has a clear impact on greenhouse gas emissions, the effect of removing moisture from the air has so far escaped intense study. The researchers showed that controlling humidity is responsible for about half of energy-related emissions, while the other half is due to controlling temperature.
“This is a challenging problem that people haven’t solved since air conditioners became common more than half a century ago,” said Jason Woods, senior research engineer at NREL and co-author of the new study. His co-authors from NREL are Nelson James, Eric Cozubal and Eric Bonema. Associates at Xerox PARC, an R&D company working on ways to more efficiently remove moisture from the air, are Kristin Brief, Liz Voeller, and Jesse Rivest.
The growing need for air cooling is both a cause and effect of climate change, the researchers point out.
Even small amounts of moisture in the air can make people feel uncomfortable and even cause damage to buildings in the form of mold and mildew. In addition, controlling indoor humidity through commercially available air conditioning technologies impacts the environment in three ways:
1. They consume 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.
3. The manufacturing and distribution of these systems also releases greenhouse gases.
The researchers calculated that air conditioning is responsible for the equivalent of 1,950 million tons of carbon dioxide released annually, or 3.94 percent of global greenhouse gas emissions. Of that figure, 531 million tons comes from the energy expended to control the temperature and 599 million tons from removing moisture.
The balance of 1,950 million tons of carbon dioxide comes from global-warming-causing refrigerant leaks and emissions during the manufacturing and transportation of air-conditioning equipment. Managing humidity with air conditioners contributes more to climate change than controlling temperature. The problem is expected to worsen as consumers in more countries – particularly in India, China and Indonesia – increasingly install many more air conditioners.
“That’s a good and a bad thing,” Woods said.
“It’s good that more people can benefit from better comfort, but it also means that a lot more energy is used, and carbon emissions increase,” he said.
To calculate emissions to manage both temperature and humidity, the researchers divided the globe into a fine grid measuring 1 degree latitude and 1 degree longitude. Within each grid cell, the following characteristics were considered, population, GDP, estimated air conditioner ownership per capita, carbon intensity of the grid and hourly weather. They ran about 27,000 simulations around the world for representative commercial and residential buildings.
Climate change is affecting ambient temperature and humidity around the world, making it warmer and more humid. As part of the study, the researchers considered the impact of a changing climate on air conditioner energy use by 2050. For example, the study forecasts a 14 percent increase in air conditioner energy use in the hottest climate (Chennai, India) and 41 percent in the mildest (Milan, Italy) by 2050. An increase in global humidity is projected to have a larger impact on emissions than an increase in global temperature.
“We’ve made existing, age-old technology as efficient as possible,” Woods said.
“To achieve a transformative change inefficiency, we need to look at different approaches without the limitations of the existing one,” he said.
Existing vapor compression technology has been adapted to cool our buildings using a “vapor compression cycle”. This cycle uses harmful refrigerants to cool the air so low that its moisture content is reduced, often causing the air to become too cold and energy wasted.
Improvements in the vapor compression cycle are reaching practical and theoretical limits, thus pointing to the need to leapfrog into an entirely new way to cool and dehumidify buildings. New technologies that split this cooling and humidity control problem into two processes have shown the potential to improve efficiency by 40 percent or more.
Once such a technological 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 several partners such as Emerson and Blue Frontier.
The researchers report that the use of liquid desiccants fundamentally changes the way moisture is controlled and has a theoretical efficiency limit that exceeds 10 times that of the vapor compression cycle alone.
A hypothetical technique – only half of this new limit – would reduce cooling-energy emissions by 42 percent in 2050, the equivalent of avoiding 2,460 million tons of carbon dioxide annually. (ANI)
First published:March 16, 2022 at 1:51 pm