Air conditioners

It’s a wet heat: Study shows humidity’s impact on AC emissions

You’ve probably heard this phrase many times: It’s not the heat, it’s the humidity. The adage appears to apply not just to human comfort but to greenhouse gas emissions as well.

The amount of energy used to power air conditioners has been well documented. New research from the National Renewable Energy Laboratory in Colorado examines the less-studied environmental impact of removing humidity from the air. 

The researchers found that about half of energy-related emissions from air conditioning are caused by controlling the air moisture — not temperature. 

“It’s a challenging problem that people haven’t solved since air conditioners became commonplace more than a half-century ago,” Jason Woods, an NREL senior research engineer, said in a news release.

And while climate change causes an increasing desire to cool the air, the researchers say air conditioning is also contributing to worsening climate change.

Increasing AC use

The use of air conditioners to improve humans’ comfort levels has been popular in the United States for decades, and it is quickly spreading across the world. For example, more air conditioners are being installed in countries such as China, India, and Indonesia. 

“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,” Woods said.

As climate change intensifies, the potential grows for greater air conditioner use — and consequently, more emissions. The research team projects that by 2050, air conditioner energy use will increase 14% in the hottest climate studied (Chennai, India) and by 41% in the mildest climate (Milan, Italy).

Effects of humidity and AC

Humidity increases are expected to have an even greater impact on emissions than temperature increases. That’s because even a little air moisture can cause discomfort. It can also damage buildings, such as by fostering mold and mildew growth.

The solution for decades has been to control humidity with commercially available air conditioning technologies. But they impact the environment in several ways:

  • Large amounts of electricity consumption
  • CFC-based refrigerant leaks
  • Greenhouse gas emissions during air conditioner manufacturing and delivery

The study

The researchers divided the globe into a grid to measure localized emissions from managing temperature and humidity. They examined factors including population, gross domestic product, estimated air conditioner ownership per capita, carbon intensity of the grid, and hourly weather. They performed approximately 27,000 simulations for commercial and residential buildings.

They determined that air conditioning is responsible for 3.94% of global GHG emissions annually, or about 1,950 million tons of CO2. About 599 million tons is caused by removing humidity while 531 million tons is from controlling temperature. The remaining 1,950 million tons are from refrigerant leaks (CFCs are 2,000 times as potent as carbon dioxide) and air conditioner manufacturing and transportation.

Working on an additional problem

Besides an increase in the global use of AC, the researchers identified another barrier: the technology. 

The current vapor compression technology in air conditioners uses harmful refrigerants to cool air and remove moisture, plus it wastes a lot of energy. But improvements to this technology are reaching their limits.

“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.”

Scientists say a step-change advancement is needed to identify a new way to heat and cool buildings. Part of this could come from splitting the temperature and humidity control functions on ACs; the researchers believe that could improve energy efficiency by 40%. 

NREL is currently working with commercial partners, including St. Louis-based Emerson, to develop these new cooling technologies. One system under investigation uses liquid desiccants, which absorb humidity from the air and exhibit increased energy efficiency compared with vapor compression cycle technologies. Developers predict a hypothetical liquid desiccant cooling technology has the potential to reduce cooling-energy emissions by 42% in 2050.