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Introduction:
The Earth’s temperature has been on a steady rise, with 2021 marking the sixth-warmest year on record. The consequences of this warming trend are evident in regional and seasonal extremes, reduced snow cover and sea ice, intensified rainfall, and shifts in habitat ranges. One significant contributor to the escalating global temperature is the growing demand for air conditioning, which is projected to triple by 2050. As the world grapples with the environmental and energy implications of this increased cooling need, a revolutionary solution emerges — passive cooling systems. This article explores the urgency of addressing the rising cooling demand, the challenges posed by conventional air conditioning, and the potential transformative impact of passive cooling systems.
The Escalating Demand for Cooling:
The rise in global temperatures has led to an increased reliance on air conditioning for maintaining comfortable indoor environments. Paradoxically, the more we use air conditioning, the warmer it gets, creating a cycle of escalating demand. According to the International Energy Agency, the global stock of air conditioners is expected to reach 5.6 billion by 2050, necessitating new electricity capacity equivalent to the combined electricity capacity of the United States, the EU, and Japan today. This surge in demand comes with significant economic and environmental costs, making it imperative to explore alternative, sustainable cooling solutions.
Challenges and Environmental Implications:
Conventional air conditioning systems not only contribute to the rising demand for electricity but also present efficiency disparities across regions. For instance, ACs in Japan and the European Union are typically 25% more efficient than those in the United States and China. Bridging this efficiency gap through mandatory energy performance standards could potentially halve the energy growth from AC demand.
Additionally, supplying power to meet the escalating demand entails substantial costs and environmental implications. In this context, the urgent need for enhancing cooling efficiency and exploring sustainable alternatives becomes evident.
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 Systems:
A Sustainable Solution: Passive cooling systems emerge as a sustainable and efficient alternative to traditional air conditioning. These systems leverage free, renewable energy sources such as the sun and wind to provide cooling, ventilation, and lighting.
Unlike conventional ACs, passive cooling systems eliminate the need for mechanical or electrical devices, offering a cost-effective, secure, and environmentally friendly solution. Passive cooling aligns with the principles of low-energy architecture and vernacular architecture, emphasizing adaptation to local environmental conditions and natural elements.
Key Principles and Actions of Passive Cooling:
Passive cooling systems operate based on four fundamental actions: storing cold mass or air within the building envelope, avoiding direct external solar radiation heat gain, removing gained heat from interior or exterior sources, and slowing heat transfer from the external climate through the building envelope.
Passive cooling is an approach that focuses on providing thermal comfort by controlling heat gains and heat dissipation without involving mechanical or electrical devices. These actions collectively contribute to maintaining thermal comfort by controlling heat gains and dissipation without relying on energy-intensive mechanisms. Understanding the sources of heat gains is essential for implementing the appropriate passive cooling strategies.
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.
The integration of passive systems in the architectural design process requires many considerations on all levels of design stages. 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.
The four passive cooling actions include the following:
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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.
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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.
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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.
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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.
Innovations in Passive Cooling:
MIT researchers have developed an innovative passive cooling system that holds promise for off-grid locations and areas with limited access to reliable electricity.
As the world seeks sustainable alternatives, MIT researchers have developed an innovative passive cooling system that combines radiative cooling, evaporative cooling, and thermal insulation. This system, resembling a solar panel, can provide up to 19 degrees Fahrenheit of cooling from the ambient temperature. Notably, it offers a promising solution for off-grid locations and areas with unreliable access to electric power. By utilizing passive cooling, this system aims to preserve food crops and supplement conventional air conditioning without requiring a significant amount of power or water.
The system combines radiative cooling, evaporative cooling, and thermal insulation, resembling existing solar panels. Capable of providing up to 19 degrees Fahrenheit (9.3 degrees Celsius) of cooling from the ambient temperature, the system aims to extend the safe storage time for food crops. In locations with existing air conditioning systems, the technology could reduce the load on these systems, potentially improving overall energy efficiency. The system’s three-layered design includes an aerogel, hydrogel, and a mirror-like layer, offering a sustainable cooling solution.
The top layer, consisting of aerogel, is highly insulating and allows water vapor and infrared radiation to pass through. Evaporation of water from the layer below contributes to cooling, while infrared radiation releases heat into space. Below the aerogel, a layer of hydrogel serves as a water source for evaporative cooling. A mirror-like layer reflects incoming sunlight, reducing solar heating of the device. The system was tested effectively on the rooftop of an MIT building, achieving 9.3 degrees Celsius of cooling. Challenges include the cost of producing aerogel and ongoing research to enhance cost-effectiveness for mass production. Despite these challenges, the system’s integration of passive cooling technologies presents a novel approach with potential applications in food preservation and beyond.
Chinese scientists invent new passive cooling of buildings
A group of Chinese scientists found a new type of ceramic materials to realize passive cooling of buildings and save energy consumed by air conditioners, according to the recent issue of the journal Science.
The electricity used by cooling systems accounts for about 10 percent of global electricity consumption. Therefore, developing new cooling technologies for saving energy and promoting carbon neutrality is crucial.
Radiative sky cooling is a passive cooling technology that emits heat to outer space through the atmospheric window from 8 to 13 micrometers. If the radiated heat is greater than the solar energy absorbed, then daytime radiative cooling is achieved without any energy input.
The research team, led by Prof. Zhao Dongliang from the School of Energy and Environment at China’s Southeast University, analyzed the optical characteristics of different materials in the solar and infrared bands based on their crystal structure.
They put forward that ceramics mainly composed of alumina and silica with certain structural designs could achieve radiative sky cooling of buildings by a high sunlight reflection rate and high emissions of infrared radiation. Moreover, such ceramics are durable, thermostable, and water-resistant.
The study also introduced examples of such ceramic radiative cooling materials. After using such materials on the building roof, the energy consumption of air conditioners inside the building can be reduced by 26.8 percent compared with buildings using ordinary white paint.
The study said further research will be carried out to realize the massive production of such materials at a low cost.
Conclusion:
The imperative to address the escalating demand for cooling, driven by global warming, demands innovative and sustainable solutions. Passive cooling systems represent a transformative approach, harnessing natural elements to provide thermal comfort without adding strain to power grids or exacerbating environmental concerns. As the world grapples with the challenges of climate change, the adoption of passive cooling stands as a beacon of hope, offering a path toward a more sustainable and resilient future.
References and Resources also include:
https://news.mit.edu/2022/passive-cooling-off-grid-0920
https://english.news.cn/20231121/9932e90c6cc445f1bfe1194c60496dea/c.html
