Passive Daytime Radiative Cooling

Passive Daytime Radiative Cooling (PDRC) is an innovative thermal management technique that enables surfaces to lower their temperature below the ambient air temperature under direct sunlight, without the need for external energy input. It functions by reflecting most of the incoming solar radiation while simultaneously emitting heat through the atmospheric transparency window into outer space. This phenomenon has gained substantial attention in recent years as a sustainable strategy for reducing energy consumption in buildings, vehicles, and infrastructure.

Physical Principles and Mechanism

PDRC operates based on two fundamental optical and thermal principles: high solar reflectance and high thermal emittance in the mid-infrared spectrum.

• Solar Reflectance: To minimise heat gain, a PDRC surface must reflect the majority of sunlight, particularly across the visible and near-infrared wavelengths (0.3–2.5 µm).
• Thermal Emittance: The surface simultaneously emits infrared radiation strongly in the atmospheric transparency window (8–13 µm), where the Earth’s atmosphere allows thermal radiation to escape into space.

When these two properties are balanced effectively, the net radiative heat loss exceeds the absorbed solar energy, enabling cooling even during peak daylight. This distinguishes PDRC from conventional radiative cooling, which typically functions only at night.

Historical Development and Research Progress

The concept of radiative cooling dates back several decades, but achieving daytime radiative cooling was long considered challenging due to strong solar irradiance. The breakthrough occurred in the early 2010s, when researchers developed spectrally selective materials capable of reflecting sunlight efficiently while maintaining high mid-infrared emissivity.
In 2014, scientists at Stanford University demonstrated a multilayer photonic structure composed of silicon dioxide and hafnium dioxide on a silver substrate, achieving surface temperatures about 5°C below ambient under direct sunlight. This experiment validated PDRC as a feasible passive cooling method. Since then, substantial progress has been made in developing cost-effective, scalable materials suitable for real-world applications.

Materials and Design Approaches

Various materials and structures have been engineered to enhance PDRC performance. These include:

• Photonic Multilayer Coatings: Engineered layers designed to achieve near-unity solar reflectance and high infrared emissivity.
• Polymer-Based Films: Low-cost alternatives such as polyethylene (PE) or polytetrafluoroethylene (PTFE) films embedded with reflective pigments like titanium dioxide (TiO₂).
• Porous and Nanostructured Materials: Micro/nano-scale roughness enhances multiple scattering of sunlight, improving overall reflectivity.
• Coloured and Functional Surfaces: Research has extended PDRC to coloured coatings that maintain aesthetic appeal while retaining cooling performance, using selective absorption filters.

These materials are often optimised using computational models to tailor spectral properties across solar and thermal bands.

Applications and Benefits

PDRC technology has broad potential across multiple sectors, particularly in energy-efficient building design and urban heat management.
1. Buildings and Urban Infrastructure:

• Roof and façade coatings using PDRC materials can reduce indoor temperatures by several degrees Celsius, significantly lowering air-conditioning demand.
• Reflective pavements and urban surfaces contribute to mitigating the urban heat island (UHI) effect.

2. Renewable Energy Systems:

• Cooling photovoltaic (PV) panels using PDRC coatings enhances power conversion efficiency, as PV cells operate less efficiently at high temperatures.

3. Textiles and Vehicles:

• Incorporation of PDRC fabrics into clothing and vehicle paints improves thermal comfort and reduces the need for active cooling systems.

4. Agricultural and Industrial Uses:

• Greenhouse films and storage facilities benefit from temperature regulation without electricity, preserving crops and reducing spoilage.

Advantages and Environmental Implications

The key advantages of PDRC include:

• Zero energy consumption, making it highly sustainable.
• Reduced greenhouse gas emissions through decreased reliance on air-conditioning systems.
• Enhanced energy efficiency and cost savings over time.
• Long-term stability and durability under various climatic conditions when advanced coatings are used.

Environmentally, PDRC aligns with the goals of net-zero energy buildings and climate-resilient urban planning, contributing to overall carbon neutrality.

Limitations and Technical Challenges

Despite its promise, PDRC technology faces several challenges before large-scale adoption can be achieved:

• Durability and weathering resistance: Long-term exposure to UV radiation, humidity, and pollutants can degrade coatings.
• Manufacturing costs: High-performance photonic structures often require complex fabrication processes.
• Aesthetic constraints: Highly reflective white surfaces may not be desirable in all architectural contexts, though coloured variants are under development.
• Geographical dependence: Performance may vary with humidity, atmospheric aerosols, and local climatic conditions affecting infrared emissivity.

Addressing these issues is central to transitioning PDRC from laboratory prototypes to commercial applications.

Recent Innovations and Future Prospects

Recent advances have focused on scalable fabrication techniques, such as spray coating, roll-to-roll processing, and self-assembly. Novel materials, including ceramic-based composites and biodegradable polymers, are being explored to improve environmental compatibility.
Moreover, integration with smart systems and adaptive materials—for instance, thermochromic or electrochromic coatings that adjust reflectivity dynamically—could enable hybrid cooling solutions combining passive and active regulation.
In urban environments, PDRC is increasingly being considered alongside green roofs and reflective pavements as part of integrated heat management strategies. International projects and government-supported pilot installations in countries such as the United States, China, and India are testing large-scale implementations of PDRC surfaces.