Humidity greatly impacts emissions from air conditioning

According to the IEA, air conditioning was responsible for about 1Gt of CO₂ emissions worldwide in 2020. ^[1] According to a recent study, this figure could reach 1.95 Gt if emissions from the manufacturing and transport of air conditioning equipment are included. ^[2] While the energy used to power air conditioners has clear implications on greenhouse gas emissions, the environmental impact of controlling humidity is less known. 

 

Yet, in a high-temperature and high-humidity environment, the dehumidification load is a large proportion of the total air-conditioning load. To increase the dehumidification capacity, traditional vapour compression refrigeration cycle air-conditioning systems are used to reduce the evaporation temperature, which in turn increases the system's energy consumption ratio. Simultaneously, to ensure the accuracy of temperature control in a room, the air must be reheated after cooling and dehumidification, leading to an unnecessary waste of energy. ^[3]

 

According to scientists from National Renewable Energy Laboratory (NREL) and from an R&D company, removing moisture from the air is responsible for about half of the energy-related CO₂ emissions from air conditioning, with the other half due to temperature control. ^[2] The researchers calculated that of the total global greenhouse emissions from air conditioning, 531 million tonnes come from energy expended to control temperature and 599 million tonnes from dehumidification.

 

The researchers have estimated that by 2050, the increase in global humidity will have a greater impact on emissions than the increase in global temperatures. Under high temperature and humidity conditions, effective dehumidification methods are of crucial importance to reduce the energy consumption of the air-conditioning system and improve the comfort of an air-conditioned room. ^[3] This is the case, for instance, with liquid desiccant-based cooling cycles, which allow cooling and humidity control to be split into two processes. According to the researchers, such technologies have the potential to improve efficiency by 40% or more. ^[2]

 

 

Sources

[1] IEA (2021), Cooling, IEA, Paris. https://www.iea.org/reports/cooling

[2] National Renewable Energy Laboratory (NREL). News Release: Scientists Show Large Impact of Controlling Humidity on Greenhouse Gas Emissions. https://www.nrel.gov/news/press/2022/nrel-shows-impact-of-controlling-humidity-on-greenhouse-gas-emissions.html

[3] Chen, L., Chen, S. H., Liu, L., & Zhang, B. (2018). Experimental investigation of precooling desiccant-wheel air-conditioning system in a high-temperature and high-humidity environment. International Journal of Refrigeration, 95, 83-92. https://iifiir.org/en/fridoc/experimental-investigation-of-precooling-desiccant-wheel-141500

 

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