Passive cooling and the future of HVAC

In a rapidly heating world, researchers have looked into the future of cooling technologies and found that air conditioning must take a back seat.

While air conditioning undoubtedly saves lives, its growing use is straining the electricity grid, increasing greenhouse gas emissions and making cities hotter in the process.

UNSW Sydney Professor Mat Santamouris AM has co-authored a review of passive cooling technologies that indicates keeping buildings cool without relying solely on air conditioning will be critical for adapting to climate change.

Published in Nature Reviews Clean Technology, the review examines in detail the latest advances in passive cooling technologies, listing both benefits and drawbacks. It explores emerging materials for radiative, evaporative and combined radiative/evaporative cooling, as well as solar control systems and intelligent ventilation technologies that can help buildings shed heat without consuming electricity.

Santamouris, an expert in innovative heat mitigation technologies and strategies for cities, said passive cooling should no longer be viewed as a niche architectural feature, but as essential infrastructure for a warming world.

“Air conditioning saves lives and will remain essential during extreme heat,” he said.

“But we cannot air-condition our way out of climate change. If every building depends entirely on mechanical cooling, we create enormous pressure on electricity systems while adding even more heat to our cities.”

A voracious demand for cooling

The review charts a rapid global growth in cooling demand, with electricity consumption for cooling accounting for almost 10% of total electricity use. Ten new air conditioners are sold every second and by 2050, the number of residential air-conditioning units is projected to increase to almost 5.6 billion worldwide.

At the same time, billions of people living in hot climates still lack access to affordable cooling.

Stark findings

The review gives examples of grids being overloaded during heatwaves thanks to continuous AC use — causing blackouts, transformer failures and unstable supply. Such events are becoming more common: in the US, grid failures increased by 151% between 2015–2016 and 2020–2021, while heat-related outages rose by 60% in 2024 compared to the prior decade.

In Asia, extreme heat in 2024–2025 triggered major outages in Laos, the Philippines, Bangladesh and Pakistan, leading to fatalities and health system disruptions. Meanwhile, in Europe, 2025 heatwaves reportedly drove a 14% surge in electricity use in France, Germany, Italy and Spain, doubling daily power prices and stressing utilities.

Avoiding reliance on AC

The researchers have presented passive cooling technologies as a way to reduce energy demand while making buildings safer and more comfortable, particularly for vulnerable communities.

“The best cooling strategy is to stop unwanted heat entering buildings in the first place,” Santamouris said. “Shading, reflective materials, smarter ventilation and new cooling materials can dramatically reduce indoor temperatures before an air conditioner even needs to switch on.”

Rather than replacing air conditioning, Santamouris and co-author Dr Konstantina Vasilakopoulou from RMIT argue passive cooling should become the first layer of defence, with mechanical systems providing additional cooling only when required.

According to the review, integrating passive cooling strategies with efficient building design could reduce cooling demand by as much as 80% in hot climates while lowering peak electricity demand and improving resilience during power outages.

The review examined passive cooling technologies that are already in use, including reflective cooling materials that release heat directly into the atmosphere and hybrid cooling systems that combine multiple passive approaches.

It also evaluated emerging technologies such as super-cool materials, combined radiative/evaporative coatings, sophisticated external shading systems and personalised ventilation.

Cool benefits

As well as reducing energy use, the researchers said passive cooling can make cities healthier and more resilient as extreme heat events become more frequent.

Keeping buildings and neighbourhoods cooler can reduce the risk of heat-related illness, ease pressure on electricity networks during heatwaves and improve comfort for people who cannot afford to run air conditioners. Passive cooling measures can also help buildings remain safer during power outages, when mechanical cooling systems are unavailable.

An example tested in Australia is ‘cool roofs’ — roof systems designed to reflect a large fraction of the solar radiation that hits them, efficiently releasing absorbed heat back to the atmosphere. According to modelling cited in the review, use of cool roofs in air-conditioned buildings would provide estimated energy savings of 10–60% in Darwin. And in non-air-conditioned housing, cool roofs can lower peak indoor temperatures by up to 10°C, again based on a scenario considering buildings in Australia.

A whole-system approach

Santamouris said the greatest benefits will come from combining passive cooling with efficient air conditioning, rather than treating them as competing approaches.

“There is no single solution to keeping cities cool. We need a whole-system approach that starts with climate-responsive building design, shading and better materials, then uses the most efficient cooling technologies only when they are really needed.”

The review calls for stronger building standards and planning policies that encourage climate-responsive design, alongside investment in technologies that reduce heat entering buildings and lessen demand on electricity infrastructure as cities continue to warm.

‘Passive cooling for the built environment’ can be read at doi.org/10.1038/s44359-026-00177-y.

Image credit: iStock.com/jamesteohart