Radiative cooling has emerged as a promising passive cooling strategy for buildings, leveraging the ability to emit heat in the form of long-wavelength infrared radiation (LWIR) to the cold sky. While this approach can significantly reduce cooling energy consumption during hot summer months, it inadvertently introduces a “heating penalty” during the winter season. The undesired cooling effect in cold weather exacerbates the heating demand, leading to increased energy consumption. Addressing this heating penalty is crucial for the widespread adoption of radiative cooling in buildings.

Despite the promising potential of radiative cooling for energy-efficient thermal regulation in buildings, several challenges remain to be addressed. One major hurdle is the “heating penalty” issue, where the undesired cooling effect during winter months exacerbates heating demands and energy consumption. Developing materials and systems that can adapt to seasonal changes and provide optimal performance year-round is crucial for widespread adoption. Additionally, the integration of radiative cooling solutions into existing building envelopes poses technical and aesthetic challenges. Ensuring compatibility with standard construction practices, materials, and building codes while maintaining the desired thermal properties and visual appearance is essential for successful implementation.

Looking ahead, the future of radiative cooling in buildings is expected to witness significant advancements. Research efforts will likely focus on developing novel materials with tunable optical properties, such as thermochromic, electrochromic, and mechanically reconfigurable structures, to enable dynamic control over radiative heat exchange. The integration of smart sensors and control systems will allow for real-time optimization of thermal performance based on weather conditions and occupant preferences. Furthermore, the synergistic combination of radiative cooling with other passive and active building technologies, such as green roofs, phase change materials, and advanced insulation, could lead to even greater energy savings and improved indoor thermal comfort. As the demand for sustainable and energy-efficient buildings continues to grow, radiative cooling is poised to play an increasingly important role in shaping the future of building thermal management.

Researchers at Columbia University have developed an innovative solution to overcome the heating penalty issue: the dynamic FinWall. This scalable wall design features rotatable fins with high emissivity surfaces on top and low emissivity surfaces on the bottom, enabling tunable angular emissivity for all-season thermal regulation.

Concept illustration of the FinWall design

 

The key to the FinWall’s success lies in its ability to adapt to different weather conditions. During the summer, the fins are oriented to exhibit high emissivity when facing the cold sky and low emissivity when facing the hot ground. This angular asymmetric emissivity ensures simultaneous radiative cooling to the sky and reflection of thermal radiation from the ground, resulting in optimal cooling performance.

In the winter mode, the fins are rotated to a specific angle, presenting a low emissivity appearance to the outside. By minimizing radiative heat exchange with the cold surroundings, the FinWall effectively mitigates the heating penalty associated with conventional radiative cooling walls.

Experimental demonstration of dynamic FinWalls

 

Field tests conducted in Queens, New York, demonstrated the FinWall’s impressive thermal regulation capabilities. Compared to a conventional high emissivity wall, the FinWall achieved a 2.0°C temperature elevation under cold weather conditions and a 3.1°C temperature drop under hot weather conditions. These temperature differences translate to substantial power savings:

37 W/m^2 less heating in winter and 53 W/m^2 less cooling in summer.

To assess the energy impact of FinWalls at a building scale, the researchers conducted EnergyPlus simulations on a mid-rise apartment building across different U.S. climate zones. The results were promising, with dynamic FinWalls saving an average of 24% annual HVAC energy compared to high emissivity walls and 10% compared to low emissivity walls.

Energy impact of FinWalls

 

The dynamic FinWall design presents a simple and effective solution to the heating penalty challenge, enabling all-season thermal regulation for building envelopes. The scalability and ease of integration with conventional wall surfaces make FinWalls an attractive option for both new construction and retrofitting existing buildings. As we strive towards more energy-efficient and sustainable built environments, solutions like the dynamic FinWall will play a crucial role in maximizing the benefits of radiative cooling while minimizing the winter heating penalty.

Studies on radiative cooling, such as the FinWall design presented in this blog post, are of great importance for advancing sustainable building design and mitigating the environmental impact of the built environment. Buildings account for a significant portion of global energy consumption and greenhouse gas emissions, with space cooling and heating being major contributors. By harnessing the principles of radiative cooling, buildings can passively regulate their temperature, reducing reliance on energy-intensive mechanical systems like air conditioners and heaters. This not only leads to substantial energy savings but also helps in reducing the carbon footprint associated with building operations.

Moreover, radiative cooling studies contribute to the development of innovative materials and technologies that can be readily integrated into building envelopes. These advancements not only improve the thermal performance of buildings but also enhance occupant comfort and well-being. By creating thermally adaptive and responsive building skins, radiative cooling solutions can help maintain a pleasant indoor environment across different seasons and climatic conditions. Furthermore, the knowledge gained from these studies can inform building design guidelines and standards, promoting the adoption of passive cooling strategies in both new construction and retrofit projects. As the world faces the urgent need to combat climate change and transition towards a more sustainable future, radiative cooling research plays a vital role in shaping the next generation of energy-efficient and environmentally friendly buildings.

Citation:
[1] https://www.sciencedirect.com/science/article/pii/S2666386424001838