Open Access
On the energy potential of daytime radiative cooling for urban heat island mitigation
(Elsevier, 2020) Carlosena Remírez, Laura; Ruiz-Pardo, Álvaro; Feng, Jie; Irulegi, Olatz; Hernández-Minguillón, Rufino J.; Santamouris, Mattheos; Ingeniería; Ingeniaritza
The objective of this paper is to present the potential of daytime radiative cooling materials as a strategy to mitigate the Urban Heat Island effect. To evaluate the cooling potential of daytime radiative cooling materials, 15 theoretical materials and seven existing materials were simulated: two radiative cooling materials, a coolmaterial, two white paints, a thermochromic paint and a construction material. The novelty of this study is that it shows that the optimal spectral characteristics of radiative cooling materials depending on the climate conditions and the type of application. A sensitivity analysis was performed to evaluate the impact of each wavelength emissivity on the ability to achieve sub-ambient radiative cooling. The sensitivity analysis comprised a total of 90 theoretical materials with 15 different wavelength combinations and 6 emissivity values. The heat transfer model, which includes conduction, convection, and radiation, was developed using a spectrally-selective sky model. Two conditions were considered: a very conductive surface and a highly insulated one. All the materials were simulated in two cities that suffer from the Urban Heat Island effect—Phoenix and Sydney. The mean surface temperature reduction achieved was 5.30 ◦C in Phoenix and 4.21 ◦C in Sydney. The results presented suggest that the type of application (active or passive) is a determinant factor in the design of radiative cooling materials. Modifying the spectra of the materials led to a substantial change in the cooling potential. A material that performs well in a dry climate as a passive solution could perform poorly as an active solution.
Open Access
Experimental development and testing of low-cost scalable radiative cooling materials for building applications
(Elsevier, 2021) Carlosena Remírez, Laura; Andueza Unanua, Ángel María; Torres, Luis; Irulegi, Olatz; Hernández-Minguillón, Rufino J.; Sevilla Moróder, Joaquín; Santamouris, Mattheos; Ingeniaritza; Ingeniaritza Elektrikoa, Elektronikoaren eta Telekomunikazio Ingeniaritzaren; Institute of Smart Cities - ISC; Ingeniería; Ingeniería Eléctrica, Electrónica y de Comunicación; Gobierno de Navarra / Nafarroako Gobernua
Urban overheating has a serious impact on building energy consumption. Daytime radiative cooling materials are an interesting passive solution for refrigeration. However, their costs and complex manufacturing hinder their current application. In this study, a series of scalable and lowcost daytime radiative cooling (DTRC) materials were designed, fabricated, and tested in a moderate climate (Cfb-Köppen-Geiger classification) and compared to aluminum and Vikuiti. The methodology was: i) material selection and design, (ii) optimization, (iii) fabrication, (iv) characterization, and (v) testing. The materials were fabricated using different substrates, aluminum and Vikuiti, and two kinds of formulations for the emissive layers based on silica-derived polymer polymethylsilsesquioxane (PMSQ) with embedded silica nanoparticles. The resulting aluminum DTRC materials had a mean solar reflectivity of 0.7 and 0.34 emissivity in the atmospheric window, the samples with Vikuiti had 0.97 and 0.89, respectively. During the experiment, the samples were exposed to different ambient conditions without a convection barrier and were contained in an extruded polystyrene board to eliminate conduction. The samples reached 7.32 °C and 9.13 °C maximum surface temperature reduction (below ambient) during the day and night, respectively. The samples with the commercial substrate achieved a mean reduction of 3.72 °C below ambient temperature. Although the aluminum samples did not achieve subambient cooling throughout the entire day, the emissive layer reduced the sample's surface temperature by an average of 1.7 °C. The PMSQ radiative cooling materials show great potential for future building applications. Suitability under different climates and experimental settings should be done to test broad applicability.
Open Access
Monitoring the thermal potential of low-cost radiative cooling materials under static and dynamic conditions of exposure
(Morlacchi Editore University Press, 2023) Chiatti, Chiara; Kousis, Ioannis; Fabiani, Claudia; Carlosena Remírez, Laura; Pisello, Anna Laura; Ingeniería; Ingeniaritza
Reflecting the radiation of the sun while emitting thermal radiation to cold outer space has proven to be an effective solution against urban overheating. The latter severely impact the energy consumption of buildings, outdoor pollution levels, and heat-related morbidity and mortality, which is why recent research has focused on new advanced mitigation technologies to be implemented in cities. Passive radiative cooling (PRC) has the potential to provide a temperature lower than ambient without any energy consumption. While conventional cooling prototypes reject heat to the air, PRCs reject heat to the outer atmosphere emitting radiation mainly in the 8-13 ¿m range, i.e., the so-called atmospheric window. This work investigates the thermal behavior of different radiative cooling materials under various exposure conditions to examine their effective cooling potential. The basic structure of the samples comprehends a highly reflective substrate (aluminum or Vikuiti) and a silica-derived emissive layer. After a preliminary characterization under controlled environmental settings, the samples were exposed outdoors, and their superficial temperature was monitored during the central hours of the day. Comparisons among samples and a benchmark aluminum reference layer were made, also considering the weather data collected during the days of exposure. Although the samples did not reach sub-ambient temperatures during the monitoring, the emissive layer significantly reduced the surface temperature. Furthermore, the effect of a tunable intermediate layer placed between the substrate and the emissive element was demonstrated to positively impact the thermal performance of the sample, thanks to its capability of changing the emissivity spectrum with temperature.

Carlosena Remírez, Laura

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