COOLing for Electricity Production: Battery-free Technology

Project Information

COOLed

Grant agreement ID: 101087974

DOI

10.3030/101087974

EC signature date 10 March 2023

Start date 1 June 2023

End date 31 May 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 000 000,00

EU contribution

€ 2 000 000,00

Coordinated by

AGENCIA ESTATAL CONSEJO SUPERIOR DE INVESTIGACIONES CIENTIFICAS
Spain

Periodic Reporting for period 1 - COOLed (COOLing for Electricity Production: Battery-free Technology)

Reporting period: 2023-06-01 to 2025-11-30

Summary of the context and overall objectives of the project

As urban populations continue to increase, smart cities have become essential for improving the quality of life and urban services. In this sense, Internet of Things (IoT) devices, which collect and process data in real time, are essential. These devices consume very little power -between 1 µW and 1 mW- but need to operate continuously, often in remote or off-grid areas. Currently, batteries are their primary power source, which is not ideal; batteries have limited lifespans, can harm the environment, and require regular replacement or recharging. A fundamental question arises: Can we power IoT devices without batteries, anytime and anywhere?
Traditional renewable sources like solar, wind or tidal energy are not always available and they still depend on batteries for storage. This project introduces a disruptive, battery-free solution that works 24/7, independent of ambient conditions. The idea is to combine two cutting-edge technologies: radiative cooling, which uses the cold of outer space to passively lower temperature, and thermoelectric generators (TEGs), which turn temperature differences into electricity. The project aims to develop a self-powered system that integrates:
· A radiative cooler made of 3D polymer metamaterials that can reflect sunlight and emit heat efficiently -even during the day. Unlike conventional coolers that work mostly at night, COOLed’s approach uses specially designed polymer structures that don’t need mirrors or reflective coatings, making them more versatile and effective.
· A TEG based on 2D transition metal selenides. To solve the problem of improving the efficiency of thermoelectric materials, COOLed will explore 2D selenides, offering a high performance with abundant, non-toxic materials, and manufactured using scalable, low-cost methods.
Together, these elements will form a lightweight, scalable, and eco-friendly energy source that can deliver 1 W/m² -sufficient to power IoT devices in urban or remote environments.
This is a multidisciplinary, cutting-edge initiative in materials science and energy harvesting. It explores new 3D and 2D material platforms, opens up new possibilities in how we power devices, and supports the green transition. From reducing electricity use and heat waste to supporting European sustainability goals, COOLed could help reshape how cities function in a more energy-responsible future.

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

We carried out a study on 3D-AAO fabrication focused on tuning different morphologies of the nanostructures to enhance optical performance. For the first time, these morphologies were explored, aiming to optimize light scattering and solar reflectance, thereby improving radiative cooling efficiency.
3D polymer metamaterials showed high infrared emissivity (up to 92 %) and moderate solar reflectance (~85 %), enabling theoretical cooling powers up to ~198 W/m² during the day and ~144 W/m² at night.
We performed a complementary research line focused on cellulose-derived nanostructured materials, selected for their sustainability, low cost, and favorable optical properties. Three chemically modified cellulose derivatives -nitrocellulose (NC), mixed cellulose ester (MCE), and cellulose acetate (CA)- were fabricated and characterized. MCE emerged as the most promising due to its optimal balance of porosity, hydrophilicity, and surface area (11.91 m²/g). Optical measurements showed high solar reflectance (~97%) and infrared emissivity (~96%), resulting in strong radiative cooling potential.
Optical and thermal performance of these cellulose-derived nanostructured materials were validated via outdoor testing on the rooftop of the institute, confirming their effective cooling performance under real-world conditions.
A thermal simulation model was also developed to predict device performance across different climates. The model is globally applicable by adapting key parameters and has been validated with experimental data.
In line with the thermoelectric generator (TEG) development, thin films of metal selenides were synthesized via electrodeposition and evaluated for TEG applications. Films achieved record power factors (the highest one: 7655 µW/m·K² and a zT of 2.89 at room temperature). Prototype TEGs were fabricated and outperformed existing devices in the 5–55 K range.
These multidisciplinary efforts have resulted in significant advancements in nanostructured materials for thermal management and energy harvesting applications.

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)

-We have developed innovative nanostructured polymer materials. These structures allow precise control over how materials interact with heat and light, making them excellent candidates for passive radiative cooling (PCR). Their potential to passively dissipate heat -without any energy input- could significantly reduce energy consumption in buildings and smart urban environments, contributing to greener, sustainable cities.
-Our focus on polymers -not only for their thermal performance but also for their abundance, flexibility, and low environmental impact- positions our research within a circular and sustainable vision of material development. These materials are compatible with scalable manufacturing and offer an alternative to more polluting or costly technologies.
-We have performed advanced simulations to better understand and optimize heat flow within our devices. This modelling work is critical to ensuring that the materials and architectures we have developed translate into real-world performance.
-Through the electrodeposition of metal selenides, we have achieved record-high thermoelectric performance at room temperature, on flexible polymer substrates. This opens new opportunities for low-temperature, energy-harvesting devices that are compatible with flexible and lightweight electronics.
-Although integration is still under development, the parallel advancement of both high-emissivity 3D polymer structures and efficient 2D thermoelectric materials is laying the groundwork for a completely new kind of energy-harvesting device. The synergy between passive cooling and thermoelectric generation holds the promise of creating autonomous, battery-free systems that can function continuously, even in remote or off-grid environments.
-Worth noting that all the growth processes are perfectly implemented to the industry with large areas.

FUTURE KEYS:
-We have evaluated the outdoor RC performance of cellulose-derivative materials. The ongoing measurements for other polymers are necessary to strengthen their potential as candidates for PCR.
-In the electrodeposition of metal selenides, the main keys are to optimize the configuration of the multilayers to achieve the best possible thermoelectric performance, and to electrodeposit thicker multilayers in order to be able to completely detach the films from the gold substrate and transfer them to non-conductive substrates -allowing the measurement of their thermoelectric properties.

Cooling performance in outdoor experiments of different networks of cellulose-derivative materials