• Energy Research

This study assesses existing literature on radiative cooling through bibliometric and keyword analyses, shedding light on both quantitative and qualitative aspects of the subject. The research adheres to a systematic methodology, encompassing query formulation, data extraction, data curation, and analysis, accompanied by author interpretations and discussions. The evaluation encompasses the scrutiny of radiative cooling patents and scientific publications. For patents, trends, and geographic distribution are analyzed, while for scientific publications, a comprehensive overview of subtopics, subject areas, top journals of publication, distinctive trends, geographical distribution, affiliations, document types, and the central focus of previous studies are examined. From the results, the main questions on research diversity, dimensions, dominance, methodological approach, evolution, trends, and commercial relevance among others are discussed. From this investigation, it was found out that, although research on radiative cooling dates to the 1880 s, it is in the last decade when substantial growth was experienced across multiple disciplines. China and the United States of America emerged as the top contributors in this research domain. This publication is part of the grant PID2021-126643OB-I00, funded by MCIN/AEI/10.13039/ 501100011033/ and by “ERDF A way of making Europe”. This publication is also part of the grant TED2021- 131446B-I00, funded by MCIN/ AEI/10.13039/501100011033/ and by the “European Union NextGenerationEU/PRTR” and of the grant PDC2022-133215-I00, funded by MCIN/ AEI/10.13039/ 501100011033/. The authors would like to thank Generalitat de Catalunya for the project awarded to their research group (2021SGR 01370).

Most of the storage systems available on the market use water as storage medium. Enhancing the storage performance is necessary to increase the performance of most systems. The stratification phenomenon is employed to improve the efficiency of storage tanks. Heat at an intermediate temperature, not high enough to heat up the top layer, can still be used to heat the lower, colder layers. There are a lot of parameters to study the stratification in a water tank such as the Mix Number and the Richardson Number among others. The idea studied here was to use these stratification parameters to compare two tanks with the same dimensions during charging and discharging processes. One of them is a traditional water tank and the other is a PCM-water (a water tank with a Phase Change Material). A PCM is good because it has high energy density if there is a small temperature change, since then the latent heat is much larger than the sensible heat. On the other hand, the temperature change in the top layer of a hot water store with stratification is usually small as it is held as close as possible at or above the temperature for usage. In the system studied the Phase Change Material is placed at the top of the tank, therefore the advantages of the stratification still remain. The aim of this work is to demonstrate that the use of PCM in the upper part of a water tank holds or improves the benefit of the stratification phenomenon.

The effect of using external vertical fins in Phase Change Materials (PCM) modules to improve the natural convection coefficient in water is studied in this paper. The use of PCM in a water tank working with a solar system is able to store a lot of energy, but it is necessary to transfer this energy to the water during demand. Optimization of the natural convection to the water is of great importance for the heat transfer. External fins increase the heat transfer surface and the heat transfer coefficient changes. Experimental work was carried out to determine natural convection heat transfer coefficients for PCM modules with two different external vertical fin geometries. Results were presented as a temperature variation over time of the PCM and the water, and the heat transfer coefficient as a function of the temperature difference for different fins. These results were used to validate a simulation code to determine the behaviour of the system using finned PCM modules. The results prove the technical potential of external fins for heat storage systems using PCM. The experimental results were used to determine different correlations for Nusselt number as a function of different dimensionless numbers.

Abstract An exergy analysis, which only considers the unavoidable exergy destruction, is conducted for single, double, triple and half effect Water–Lithium bromide absorption cycles. Thus, the obtained performances represent the maximum achievable performance under the given operation conditions. The coefficient of performance (COP), the exergetic efficiencies and the exergy destruction rates are determined and the effect of the heat source temperature is evaluated. As expected, the COP increases significantly from double lift to triple effect cycles. The exergetic efficiency varies less among the different configurations. In all cycles the effect of the heat source temperature on the exergy destruction rates is similar for the same type of components, while the quantitative contributions depend on cycle type and flow configuration. Largest exergy destruction occurs in the absorbers and generators, especially at higher heat source temperatures.

At present, much interest is being shown in absorption refrigeration cycles driven by low temperature heat sources, such as solar energy or low-grade waste-heat. Double-lift absorption cycles working with ammonia-water have been recommended for refrigeration applications which require cold at 0°C and which are activated by waste heat between 70 and 100°C. This paper discusses the potential of the organic fluid mixtures trifluoroethanol (TFE)-tetraethylenglycol dimethylether (TEGDME or E181) and methanol-TEGDME as working pairs in series flow and vapour exchange double-lift absorption cycles. The ammonia-water mixture was used for comparison purposes. The results show that the performances of these cycles improve significantly when they have the above mentioned organic fluid mixtures as working pairs. For example, the coefficient of performance of the vapour exchange cycle working with TFE-TEGDME is 15% higher than with ammonia-water. In this study, we used a modular software package, which we developed for the thermodynamic properties and cycles simulation of absorption systems.

Climate change is becoming more important day after day. The main actor to decarbonize the economy is the building stock, especially in the energy used for Domestic Hot Water (DHW), heating and cooling. The use of renewable energy sources to cover space conditioning and DHW demands is growing every year. While solar thermal energy can cover building heating and DHW demands, there is no technology with such potential and development for space cooling. In this paper, a new concept of combining radiative cooling and solar thermal collection, the Radiative Collector and Emitter (RCE), through the idea of an adaptive cover, which uses different material properties for each functionality, is for the first time experimentally tested and proved. The RCE relies on an adaptive cover that uses different material properties for each functionality: high spectral transmittance in the solar radiation band and very low spectral transmittance in the infrared band during solar collection mode, and high spectral transmittance in the atmospheric window wavelength during radiative cooling mode. Experiments were performed during the summer period in Lleida (Dry Mediterranean Continental climate). The concept was proved, demonstrating the potential of the RCE to heat up water during daylight hours and to cool down water during the night. Daily/nightly average efficiencies up to 49% and 32% were achieved for solar collection and radiative cooling, respectively.

The potential of the fluid mixture water-(LiBr + Lil+ LiNO3 +LiCl) (5:1:1:2 molar) is studied for air-cooled absorption air-conditioning systems. This multicomponent system shows a considerably higher solubility than that of water-LiBr and is also less corrosive. It is, therefore, one of the potential alternatives to replace water-LiBr in future air-cooled absorption chillers. A comparative study based on thermodynamic simulation of the single-and double-effect cycles with water-LiBr and the new fluid mixture is first reported. Once the operating conditions were established in terms of temperature and concentration in the generator and the absorber, the absorption process of a falling film flowing on the inner surface of a vertical tube at typical air cooling thermal operating conditions of the absorber was modelled in order to compare the absorption rates of the multicomponent salt mixture with those of water-LiBr. This study was completed by an experimental characterization of the absorption process in a vertical falling film tube. The results show that the multicomponent salt solution is more suitable than water-LiBr if the temperatures in the generator and absorber/condenser are low and high, respectively. Therefore this new working fluid can be recommended for air-cooled absorption air-conditioning systems driven by low temperature heat sources.