the new generation of scalable urban HEat isLand mitigatIOn by means of adaptive photoluminescent radiative cooling Skins

Project Information

HELIOS

Grant agreement ID: 101041255

DOI

10.3030/101041255

EC signature date 6 April 2022

Start date 1 May 2022

End date 30 April 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 498 125,00

EU contribution

€ 1 498 125,00

Coordinated by

UNIVERSITA DEGLI STUDI DI PERUGIA
Italy

Periodic Reporting for period 1 - HELIOS (the new generation of scalable urban HEat isLand mitigatIOn by means of adaptive photoluminescent radiative cooling Skins)

Reporting period: 2022-05-01 to 2024-10-31

Summary of the context and overall objectives of the project

Urban environments are increasingly grappling with the combined impacts of human activity and solar radiation, which elevate surface temperatures and intensify the well-established Urban Heat Island (UHI) effect. This phenomenon not only amplifies the demand for cooling in buildings, but also deepens energy poverty, heightens health risks, and leads to broader socio-economic consequences. These challenges are particularly urgent as the majority of the global population is expected to reside in urban areas by 2100, with cities already responsible for approximately 80% of global CO2eq emissions.
HELIOS proposes a transformative approach to future urban infrastructure, targeting the urgent need for sustainable cooling alternatives to conventional air conditioning. Groundbreaking strides have been made with materials featuring high reflectivity and emissivity, as well as with emerging technologies like luminescent skins. However, the most promising innovations are expected to arise from smart systems that enable passive daytime radiative cooling, even under direct sunlight.
Passive radiative cooling offers tremendous potential, with experimental results showing temperature reductions exceeding 10K below ambient conditions by dissipating heat through the atmospheric window (AW). These radiative cooling technologies selectively control the incident radiation across the spectrum, achieving unprecedented cooling efficiencies. HELIOS seeks to advance these solutions by improving their adaptability to diverse climate conditions, exploring the role of photoluminescence, and scaling them up from material applications to urban infrastructure.
Despite promising results under specific conditions, significant challenges persist, particularly in terms of replicability, adaptability to varying climates, and integration of aesthetic factors like color, which can diminish cooling performance. HELIOS addresses these obstacles through the development of comprehensive multiscale analytical models, spanning building to urban contexts, while also contributing to the creation of standardized experimental protocols. These efforts aim to facilitate the broad, global deployment of passive radiative cooling technologies, revolutionizing urban sustainability.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The HELIOS project operates through four interconnected pillars, each addressing key aspects of radiative cooling (RC) technology and its urban application. These pillars work together to develop and optimize RC systems in a comprehensive manner.

Pillar 1 focuses on creating dynamic RC systems that modulate emissivity within the atmospheric window (AW). This involves a systematic design framework, including material selection for longwave emitters and reflective layers, and the integration of colored or adaptive elements. A recent perspective paper emphasizes the need for comparative studies of selective versus broadband RCs and the development of standardized experimental protocols.

Pillar 2 explores the integration of photoluminescence (PL) into the HELIOS RC prototype to enhance both aesthetic and energy efficiency. By combining fluorescence and phosphorescence, this pillar aims to boost energy transfer efficiency. Ongoing efforts focus on optimizing lead-free halide perovskite-based PL layers for urban use, offering aesthetic flexibility with different colors and surface textures. This work complements Pillar 1 by adding a functional and visual dimension to RC materials.

Pillar 3 focuses on material characterization and modeling through the creation of a validated experimental protocol. This protocol, which has been tested on existing RC prototypes, supports the numerical assessment of temperature-responsive radiative properties for integration into urban models.

Pillar 4 integrates PL into the Princeton Urban Canopy Model (PUCM) using experimental data and a new numerical phosphorescence description. Results show that red PL coatings can lower surface temperatures by up to 2.62°C and delay peak temperatures by five hours. The pillar also evaluates the impact of broadband and selective RCs on urban environments through a multilayer Urban Canopy Model (UCM) coupled with the Weather Research and Forecasting (WRF) model.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The HELIOS project has made pivotal strides in advancing radiative cooling (RC) technologies tailored for urban environments. A key achievement has been the development of advanced testing and characterization techniques, including the creation of a state-of-the-art climatic chamber. This facility simulates real-world conditions, providing a standardized platform for precisely evaluating and comparing RC materials, filling a critical gap in the field.
On the theoretical front, HELIOS has employed Density Functional Theory (DFT) to design RC materials with optimized properties, accelerating innovation by predicting material performance before physical prototyping. This computational approach has streamlined the development process, leading to more efficient material solutions.
In parallel, HELIOS has explored the integration of phosphorescent and fluorescent materials to enhance the functionality and aesthetic appeal of RCs. This novel combination has shown promise in creating dynamic, energy-efficient cooling systems that not only reduce building cooling loads but also improve urban lighting. By tuning the emission spectra of these materials, HELIOS offers tailored applications suited to various urban settings.
HELIOS has also examined the role of phosphorescent coatings in combating urban overheating. By incorporating these properties into the Princeton Urban Canopy Model (PUCM), the project has quantified their potential to mitigate Urban Heat Islands (UHIs). The results highlight the critical importance of optimizing material properties to maximize cooling effects in urban landscapes.
At a larger scale, HELIOS has integrated the Weather Research and Forecasting (WRF) model with a multilayer Urban Canopy Model (UCM) to assess RC performance across entire cities. This approach enables a comprehensive evaluation of how different RC materials affect urban microclimates, taking into account variables like building orientation, density, and material characteristics. Findings indicate that RC technologies can significantly reduce surface and air temperatures, improving both pedestrian comfort and urban livability.
In summary, the HELIOS project has significantly advanced RC technology through an integrated approach that combines experimental validation, computational modeling, and large-scale urban analysis.

HELIOS project logo

HELIOS motivation and breakthrough