Passive cooling to improve thermal performance and reduce energy consumption of residential buildings

Earth’s temperature has risen by 0.14° Fahrenheit (0.08° Celsius) per decade since 1880, but the rate of warming since 1981 is more than twice that: 0.32° F (0.18° C) per decade. 2021 was the sixth-warmest year on record based on NOAA’s temperature data. That extra heat is driving regional and seasonal temperature extremes, reducing snow cover and sea ice, intensifying heavy rainfall, and changing habitat ranges for plants and animals—expanding some and shrinking others.

The rise in cooling demand will be particularly important in the hotter regions of the world.

The warmer it gets, the more we use air conditioning. The more we use air conditioning, the warmer it gets.

The growing use of air conditioners in homes and offices around the world will be one of the top drivers of global electricity demand over the next three decades, according to new analysis by the International Energy Agency that stresses the urgent need for policy action to improve cooling efficiency.

The global stock of air conditioners in buildings will grow to 5.6 billion by 2050, up from 1.6 billion today – which amounts to 10 new ACs sold every second for the next 30 years, according to the report. Global energy demand from air conditioners is expected to triple by 2050, requiring new electricity capacity the equivalent to the combined electricity capacity of the United States, the EU and Japan today.

Supplying power to these ACs comes with large costs and environmental implications. One crucial factor is that the efficiency of these new ACs can vary widely. For example, ACs sold in Japan and the European Union are typically 25% more efficient than those sold in the United States and China. Efficiency improvements could cut the energy growth from AC demand in half through mandatory energy performance standards.

Making cooling more efficient would also yield multiple benefits, making it more affordable, more secure, and more sustainable, and saving as much as USD 2.9 trillion in investment, fuel and operating costs.

Passive cooling uses free, renewable sources of energy such as the sun and wind to provide cooling, ventilation and lighting needs for a household. This additionally removes the need to use mechanical cooling.

Applying passive cooling means reducing differenecs between outdoor and indoor temperatures, improving indoor air quality and making the building both a better and more comfortable environment to live or work in. It can also reduce levels of energy use and environmental impacts such as greenhouse gas emissions.

Passive cooling systems were the main design driver in low energy architecture and vernacular architecture especially in hot climate regions. Buildings were designed and built to adapt to local environmental conditions and to use natural elements to provide occupants with the required thermal comfort around the year.  Interest in passive design for either heating or cooling has grown recently – particularly in the last decade – as a part of a movement towards sustainable architecture.

The integration of passive systems in the architectural design process requires many considerations on all levels of design stages. Passive cooling is an approach that focuses on providing thermal comfort by controlling heat gains and heat dissipation without involving mechanical or electrical devices. The performance quality of this approach depends totally on the interaction of the building’s design and devices with the surrounding environmental factors, such as sun rays, ambient air temperature, wind, and humidity, to achieve energy balance for occupants.

Heat gain sources include internal and external sources. The internal heat gains are produced from human activities, artificial lights, equipment, and appliances used by the occupants, while the external heat gains result from the interaction of the building with the outdoor environment. Heat gain or loss has four forms: first, heat gains caused by solar radiation passing through opaque envelope materials and heating the interior spaces with the greenhouse effect, second, heat gain caused by direct sun rays transmitted through windows and transparent surfaces into the interior spaces, third, heat gains caused by conduction between the building envelope and the surrounding environment, and, fourth, heat gains through convection caused by air infiltration and ventilation exchange between the outdoor and indoor environment.

Understanding the sources of heat gains that affect thermal comfort in the building is essential for deciding the type of actions to be taken to avoid as much heat gains as possible, to slow the heating process to remove the uncontrolled gained heat, or to store cold air or elements.

The four passive cooling actions include the following:

1. Storing of cold mass or air within building envelope. This action is defined by keeping cold air or mass away from direct heat gains to provide spaces with cold air or cool down the air before entering the interior spaces like courtyards, basements, earth spaces, and thermal masses.

2. Avoidance of direct external solar radiation heat gain. This action is conducted by applying design considerations and devices in the building. Avoidance could be applied by using shading windows and glazed areas, using landscape, designing of self-shading forms, and considering colors and reflectivity of external surfaces.

3. Removal of gained heat from the interior or exterior sources. This action is required to remove portion of undesirable heat that could not be avoided or slowed. The action can be performed through controlled ventilation, by using wind towers, earth tunnels, and windows to support ventilation requirements.

4. Slowing heat transfer from the external climate through the building envelope. This action is conducted by using techniques like efficient insulation and double glazing window units.

Passive cooling system could benefit off-grid locations

As the world gets warmer, the use of power-hungry air conditioning systems is projected to increase significantly, putting a strain on existing power grids and bypassing many locations with little or no reliable electric power. Now, an innovative system developed at MIT offers a way to use passive cooling to preserve food crops and supplement conventional air conditioners in buildings, with no need for power and only a small need for water.

The system, which combines radiative cooling, evaporative cooling, and thermal insulation in a slim package that could resemble existing solar panels, can provide up to about 19 degrees Fahrenheit (9.3 degrees Celsius) of cooling from the ambient temperature, enough to permit safe food storage for about 40 percent longer under very humid conditions. It could triple the safe storage time under dryer conditions.

The findings are reported today in the journal Cell Reports Physical Science, in a paper by MIT postdoc Zhengmao Lu, Arny Leroy PhD ’, professors Jeffrey Grossman and Evelyn Wang, and two others. While more research is needed in order to bring down the cost of one key component of the system, the researchers say that eventually such a system could play a significant role in meeting the cooling needs of many parts of the world where a lack of electricity or water limits the use of conventional cooling systems.

“This technology combines some of the good features of previous technologies such as evaporative cooling and radiative cooling,” Lu says. By using this combination, he says, “we show that you can achieve significant food life extension, even in areas where you have high humidity,” which limits the capabilities of conventional evaporative or radiative cooling systems.

In places that do have existing air conditioning systems in buildings, the new system could be used to significantly reduce the load on these systems by sending cool water to the hottest part of the system, the condenser. “By lowering the condenser temperature, you can effectively increase the air conditioner efficiency, so that way you can potentially save energy,” Lu says.

The system consists of three layers of material, which together provide cooling as water and heat pass through the device. In practice, the device could resemble a conventional solar panel, but instead of putting out electricity, it would directly provide cooling, for example by acting as the roof of a food storage container. Or, it could be used to send chilled water through pipes to cool parts of an existing air conditioning system and improve its efficiency. The only maintenance required is adding water for the evaporation, but the consumption is so low that this need only be done about once every four days in the hottest, driest areas, and only once a month in wetter areas.

The top layer is an aerogel, a material consisting mostly of air enclosed in the cavities of a sponge-like structure made of polyethylene. The material is highly insulating but freely allows both water vapor and infrared radiation to pass through. The evaporation of water (rising up from the layer below) provides some of the cooling power, while the infrared radiation, taking advantage of the extreme transparency of Earth’s atmosphere at those wavelengths, radiates some of the heat straight up through the air and into space — unlike air conditioners, which spew hot air into the immediate surrounding environment.

Below the aerogel is a layer of hydrogel — another sponge-like material, but one whose pore spaces filled with water rather than air. It’s similar to material currently used commercially for products such as cooling pads or wound dressings. This provides the water source for evaporative cooling, as water vapor forms at its surface and the vapor passes up right through the aerogel layer and out to the environment.

Below that, a mirror-like layer reflects any incoming sunlight that has reached it, sending it back up through the device rather than letting it heat up the materials and thus reducing their thermal load. And the top layer of aerogel, being a good insulator, is also highly solar-reflecting, limiting the amount of solar heating of the device, even under strong direct sunlight.

“The novelty here is really just bringing together the radiative cooling feature, the evaporative cooling feature, and also the thermal insulation feature all together in one architecture,” Lu explains. The system was tested, using a small version, just 4 inches across, on the rooftop of a building at MIT, proving its effectiveness even during suboptimal weather conditions, Lu says, and achieving 9.3 C of cooling (18.7 F).

“The challenge previously was that evaporative materials often do not deal with solar absorption well,” Lu says. “With these other materials, usually when they’re under the sun, they get heated, so they are unable to get to high cooling power at the ambient temperature.”

The aerogel material’s properties are a key to the system’s overall efficiency, but that material at present is expensive to produce, as it requires special equipment for critical point drying (CPD) to remove solvents slowly from the delicate porous structure without damaging it. The key characteristic that needs to be controlled to provide the desired characteristics is the size of the pores in the aerogel, which is made by mixing the polyethylene material with solvents, allowing it to set like a bowl of Jell-O, and then getting the solvents out of it. The research team is currently exploring ways of either making this drying process more inexpensive, such as by using freeze-drying, or finding alternative materials that can provide the same insulating function at lower cost, such as membranes separated by an air gap.

While the other materials used in the system are readily available and relatively inexpensive, Lu says, “the aerogel is the only material that’s a product from the lab that requires further development in terms of mass production.” And it’s impossible to predict how long that development might take before this system can be made practical for widespread use, he says.

This work “represents a very interesting and novel system integration approach of passive cooling technologies,” says Xiulin Ruan, a professor of mechanical engineering at Purdue University, who was not associated with this research. Ruan adds, “By combining evaporative cooling, radiative cooling, and insulation, it has a better cooling performance and can be effective in a wider range of climates than evaporative cooling or radiative cooling alone. The work could attract significant practical applications, such as in food preservation, if the system can be made at reasonable cost.”

The research team included Lenan Zhang of MIT’s Department of Mechanical Engineering and Jatin Patil of the Department of Materials Science and Engineering.

References and Resources also include:

https://news.mit.edu/2022/passive-cooling-off-grid-0920

https://www.iea.org/news/air-conditioning-use-emerges-as-one-of-the-key-drivers-of-global-electricity-demand-growth

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