Passive daytime radiative cooling (PDRC) offers a sustainable solution for reducing space cooling energy demand. However, achieving high cooling performance alongside scalability and durability remains a key challenge in PDRC. In this study, we propose a scalable and durable dual-layer radiative cooling paint (DRCP), composed of a bottom PDMS/TiO2 layer and a top PDMS/Al2O3 layer, fabricated using a spray-coating method. The particle size and layer thickness were determined via Monte Carlo simulations based on Mie scattering theory to maximize solar reflectance across the entire solar spectrum. The fabricated DRCP achieved a solar-weighted reflectance of 91.7% and an average emissivity of 95.9%, resulting in a peak subambient cooling temperature of-3.2 degrees C under 1060 W/m2 solar irradiance. Thermal durability was confirmed through 40 thermal cycles and a 30-day outdoor exposure test; over 99.7% of the initial solar-weighted reflectance was restored after water rinsing. EnergyPlus simulations demonstrated annual cooling energy savings of up to 44.6 GJ in hot desert climates. These findings highlight the potential of DRCP as a scalable, durable, and energy-efficient PDRC solution for real-world applications.

Due to its advantage in solar energy absorption, plasmonic nanofluid has been intensively studied to improve the thermal efficiency for solar energy harvest. On the basis of extensive studies of the optical properties of the nanofluids and their applications on direct absorption solar collectors (DASCs) in the temperature regime, researchers have started working on the application of nanofluids to direct absorption parabolic trough solar collectors (DAPTSCs) for mid-to high-temperature harvest. In this study, DAPTSCs with different concentric partitions have been proposed and studied to enhance their solar thermal conversion. The partition varies from one single tube to two and three concentric tubes, with each partition containing a nanofluid with a certain absorption coefficient. Furthermore, systematic optimization has been conducted for a DAPTSC with two nanofluid partitions. A deep neural network was used for building a surrogate model for the DAPTSC and the genetic algorithm was applied for optimization. The analysis showed that a low-temperature outer layer is necessary to avoid thermal losses. Trade-off needs to be made considering the structure complexity, the nanoparticle concentration, and the thermal efficiency of the DAPTSC.

The performance of an aluminum-air flow battery (AAB) unit cell is experimentally studied for application to a tri-generation system as a district heating resource of sensible heat storage, hydrogen production, and electric power generation. A layer-type unit cell is designed to protect against leakage of hydrogen gas during operation and to ensure electrolyte circulation in the AAB system. The electrolyte is made with NaOH pellets dissolved in purified water. A direct current (DC) loader is used to measure the electric power of the AAB unit cell, while a resistance temperature detector (RTD) is used to measure the electrolyte temperature at the unit cell inlet and outlet. The objective of the present work is to study the effect of operation parameters [i.e., electrolyte temperature (T-in), mass flow rate ((m) over dot(NaOH)), and molar concentration (X-NaOH)] on the performance of the tri-generation AAB system. The electric power of the AAB unit cell increases in the electrolyte temperature from 20 degree celsius to 50 degree celsius with increases in the NaOH molar concentration from 1 M to 4 M. The performance measurement of the AAB unit cell shows that the total surface power density of sensible heat, hydrogen, and electric power increased with increases in electrolyte temperature from 35 degree celsius to 55 degree celsius. The tri-generation system operates for 140 min under the condition of T-in = 40 degree celsius, (m) over dot(NaOH)=14.48 g/s, and X-NaOH=4 M.

A series of zeolite 13X with various cations was tested as a candidate for water-adsorption-based thermal storage. In the case of pristine commercial zeolite 13X pellet, >99.9 % of cation in the zeolite is confirmed to be Na+. Via conventional cation-exchange method, the Na+ could be almost completely exchanged to Li+, K+, Cs+, Mg2+, Ca2+, and Ba2+ to the thermodynamical levels. The improvement of adsorption thermal storage requires both the increases of water adsorption capacity and heat of desorption. Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) measurements were performed for the zeolites, showing that Li+-, and Mg2+-ion form zeolite samples exhibited 26 % and 29 % higher water adsorption capacities at room temperature under fully saturation condition than pristine sample, respectively, and that especially Mg2+-ion form shows 18 % higher heat of desorption than pristine Na+-form. Although Li+-ion form shows notably high water adsorption capacity, its desorption occurs mostly at low temperature (<388 K). The increase of thermal storage as exchanging the cation to Mg2+ and Li+ was fundamentally explained by density functional theory calculation. The cations induce the strong polarization of the adsorbed water molecules, which leads to strong interactions between water molecules and an increase in the amount of water stored.

The study presents an in-depth examination of a radiative cooling-based air-conditioning (RCAC) system, focusing on the impact of radiative cooling panel size and cold-water storage tank volume on energy efficiency. Using detailed annual simulations, the thermal behavior, efficiency of the air-conditioning cycle, and power consumption under different operating modes are evaluated. The performance of the RCAC system for a single-family house was compared with that of a conventional air source air-conditioning system (ASAC). Results indicate that the RCAC system can achieve up to a 30.7 % reduction in annual cooling energy consumption. The most efficient configuration, with a 10 m2 2 panel and a 4000 kg water tank, improved the system's coefficient of performance (COP) by 8.9-30.8 %. Additionally, the study highlights the importance of considering pumping power, as it can significantly impact overall system performance. These findings underscore the potential of the RCAC system to significantly reduce energy usage in cooling applications, providing a sustainable and energy-efficient alternative to traditional systems.

Radiative cooling (RC) technology has attracted a lot of attention due to its spontaneity and sustainability for cold energy generation. One of the primary objectives of enhancing RC performance is to prevent the absorption of incoming radiation. In particular, absorbing solar irradiance on an RC surface is known as a hazardous obstruction of the radiative cooling effect of the surface. This paper proposes a polymer-based RC film with dense aluminum oxide (Al2O3) microparticles to achieve high solar reflection. We have developed a fabrication recipe for densely dispersing Al2O3 particles in the polydimethylsiloxane (PDMS) medium to utilize a scattering effect as much as possible. As a result, the fabricated film contains 36.2% of Al2O3 by volume, and the resulting solar reflection is more than 94%. Additionally, both theoretical and experimental investigations are done to determine the cooling power and temperature of the fabricated film. The calculated cooling power of the proposed RC film is about 90 W/m2 under a standardized weather condition. Especially, an outdoor experiment is demonstrated to measure the RC film's cooling temperature. Under cool weather conditions, the resulting temperature is measured to be approximately 3.5∘C, which is 6.3∘C cooler than a conventional white-colored surface.

The production of heat and hydrogen gas by water splitting was experimentally investigated in a circulating flow reactor. Aluminum (Al) alloy pellets (i.e., AA6061; 10 mm long and 0.5 mm in diameter) and aluminum beverage can scraps (10 mm wide by 10 mm long and 0.24 mm thick) were used as the energy source, while sodium hydroxide (NaOH; 98 % purity) pellets were used to make aqueous alkali solutions. The temperature, mass flow rate, and molar concentration of NaOH solutions were varied in the range of 25 - 85 degree celsius, 4.4 - 21.4 gsol/s, and 1 - 4 MNaOH, respectively, to conduct a parametric study. The experimental results showed an increase in hydrogen (H2) production rate with increasing solution temperature and NaOH molar concentration, whereas the H2 evolution rate was not reduced significantly with increases in the amount of AA6061 pellets charging the reactor or the mass flow rate of the NaOH solution. From the one-cycle operation results, the average heat generation rate and hydrogen production rate for AA6061 pellets and aluminum beverage can scraps were 3.26 Whth/gAA6061, 2.06 Whth/gAlcan, 3.54 WhH2/gAA6061, and 2.72 WhH2/gAlcan, respectively. Generation of a combination of heat and power for next-generation district heating was deemed possible using the combination of common AA6061 pellets, Al beverage can scraps, and aqueous NaOH solutions in a circulating flow reactor.

This study aimed to investigate the feasibility of employing solar energy for seasonal thermal storage and utilization in horticulture facilities. The implementation of geothermal heat pump systems in large-scale horticulture farms has encountered challenges due to the substantial investment costs and operational stability concerns related to heat source temperatures. To address these limitations, this study proposes the novel concept of seasonal thermal storage and utilization. By incorporating a suitably sized storage tank, the capacity requirements of costly energy generation facilities like heat pumps can be significantly reduced, ensuring more consistent annual operation. Nevertheless, the installation of solar collectors for seasonal thermal storage may present additional challenges due to lack of space to accommodate large collectors, particularly in urban areas where the cost of land is high. This study considered two variations of seasonal thermal storage and utilization models, and conducted a comprehensive analysis of their operational characteristics and energy utilization. Through simulations and comparative analyses using an in-house developed simulation tool, the optimal design variables are determined, and the detailed operational characteristics are evaluated. The findings of this study furnish technical evidence supporting the viability of seasonal thermal storage and utilization, offering substantial benefits that can be applied in horticulture facilities.

Radiative cooling, which cools an object below its surrounding temperature without any energy consumption, is one of the most promising techniques for zero-energy systems. In principle, the radiative cooling technique reflects incident solar energy and emits its thermal radiation energy into outer space. To achieve maximized cooling performance, it is crucial to attain high spectral reflectance in the solar spectrum (0.3–2.5 μm) and high spectral emittance in the atmospheric window (8–13 μm). Despite the development of various radiative cooling techniques such as photonic crystals and metamaterials, applying the cooling technology in practical applications remains challenging due to its low flexibility and complicated manufacturing processes. Here, we develop a high-performance radiative cooling film using PDMS/TiO2 microparticles. Specifically, the design parameters such as microparticle diameter, microparticle volume fraction, and film thickness are considered through optical analysis. Additionally, we propose a novel fabrication process using low viscosity silicone oil for practical fabrication. The fabricated film accomplishes 67.1 W/m2 of cooling power, and we also analyze the cooling performance difference depending on the fabrication process based on the measurement and optical calculation results.

Sorption thermal energy storage (STES) systems utilizing zeolite 13X present a promising solution to pressing global energy challenges. In this study, we explore the influence of absolute humidity and flow rate on the heat release process within a STES system, with a focus on local and overall performance considering temperature profile, degree of adsorption reaction, and average thermal power. A numerical model has been developed to investigate the adsorption kinetics of zeolite 13X and water, which is validated through experiments on pressure drop and transient temperature changes. In this study, we introduce P(%), a novel factor providing a holistic perspective of the adsorption process throughout the reactor. Through the analysis of P(%), we elucidate the link between the adsorption reaction, local heat transfer characteristics, and average thermal power within the reactor. Our findings reveal that increasing absolute humidity and flow rate accelerates the adsorption reaction of zeolite, leading to reduced discharge time. Our findings indicate that the adsorption reaction rate significantly decreases when P(%) approaches 95%. It is noteworthy, however, that the specific threshold of P(%) can vary based on the adsorbent type or reactor design. Despite this, P(%) can be utilized as a factor to establish the criteria for optimal control of STES. This research provides a guideline for optimum operational control and reactor design of STES systems.

The potential of radiative cooling (RC) technology to passively and sustainably harvest cold energy has received a lot of interest. Since RC surface is installed outdoors for the long term and operates under time-varying environmental conditions, photonic structures constructing the RC surface need to be carefully designed based on proper theoretical modeling. However, a theoretical model for accumulated cooling performance that reflects environmental factors was scarcely developed for photonic structure design. This paper presents a more practical approach for estimating RC performance as an accumulated cold energy production (ACEP). ACEP stands for the time integration of cooling power, which is modeled to comprehensively account for a wide range of time-dependent environmental factors, including mostly neglected cloud coverage as well. The realistic performance of an RC surface at any location on Earth can be estimated without conducting any experiments. Utilizing the benefits of ACEP, photonic structures are separately optimized for 15 different locations with different climates. As a result, the universal optimum for all climates achieves 2 to 4 percent greater ACEP than the conventionally driven optimum. Based on the optimized results, we have developed a strategy for selecting radiative cooling structures based on regional climate conditions utilizing ACEP as a guiding metric. Furthermore, the impact of each environmental factor on RC performance is investigated by analyzing the ACEP components.

A passive radiative cooling system that can produce cold water at night and act as an active cooling channel during the daytime is proposed to enhance the photovoltaic efficiency of a conventional PV module. At night, the PV module acts as a radiative cooling surface on which cold energy can be harnessed by passing water underneath the PV module. We show that a thermosyphon effect can circulate water in a closed-loop at night without any power consumption. On the other hand, the stored cold water can be fed through the water channel and cool the PV module during the daytime. A quasi-steady-state one-dimensional simulation code has been developed to analyze the annual performance of the proposed radiative-cooling-assisted PV module system. In this study, we have taken into account two parameters: the water mass (or water storage tank size), which ranges from 100 to 700 kg, and the insulation covering the storage tank, which ranges from no insulation to perfect insulation. By taking the total amount of water as the main design parameter of the proposed system, we show that it is beneficial to install a tank to hold more than 300 kg. Moreover, no insulation on the storage tank is found to be desirable for further enhancing the photovoltaic efficiency. It is also revealed that the efficiency enhancement is closely related to the climate conditions, in particular, the diurnal temperature range. Thorough analysis indicates that the proposed system is appropriate for hot, dry locations where the diurnal temperature range is relatively large. For example, the proposed system can enhance the net power output by 6.4% as compared to a conventional PV module in Phoenix. We think that the proposed radiative cooling system will be a great way to make conventional PV modules work more efficiently.

For passive radiative cooling applications, all-day harvested cold energy is an essential performance indicator concerning its long-term and continuous operation. To this end, high mid-infrared emissivity of emitter surface is a key parameter for steadily boosting the net cooling power regardless of the time-varying environment. In this work, hexagonal-array grating-patterned PDMS film on a silver reflector is proposed for efficient 'all-day' radiative cooling. The optimized grating structure achieves the weighted emissivity of 0.985 in the atmospheric transparent window with the solar reflectivity of 0.933, leading to the predicted all-day average cooling power of 85 ~ 130 W/m(2). We also show from outdoor experiments that the proposed structure can produce 4.1 6.3 W/m(2) extra all-day cooling power compared to a flat PDMS emitter. Furthermore, the grating-patterned surface introduces self-cleaning feature that is desirable for long-term usage.

This study investigated thermochemical heat storage with zeolite 13X to provide an insight into the design and operation of a heat storage system for power-to-heat (P2H) applications. The heat storage system consists of a storage chamber with 21.2 liters of its capacity stacked by zeolite 13X. Experiments were conducted based on the variation of operating parameters, such as charging temperature, absolute humidity, and flow rate. The results show that the amount of available heat linearly increases with charging temperature; that is, its value at 220 °C is twice that at 100 °C. The maximum energy storage density is calculated as 0.56 GJ/m3. The average heat power varies in the range of 0.4–0.7 kW depending on the amount of supplied water. In addition, a linear correlation between the reacted water and discharged heat is provided. It was confirmed that the thermal storage efficiency was 60–70 %.

Daytime radiative cooling has recently drawn much attention due to its potential for use in next-generation cooling systems. So far, two indicators have been used to estimate radiative cooling performance: (i) cooling temperature of a radiative cooling surface at equilibrium condition; and (ii) instantaneous cooling power from a radiative cooling surface at ambient temperature. These quantities, however, deal with only a small thermal mass of a sample itself (i.e., they do not consider a real-world system to which a radiative cooling surface is applied) and cannot directly indicate the energy saving caused by the radiative cooling effect. Here, we propose a device that can directly measure daily averaged radiative cooling power as well as the resulting cooling energy reduction. To this end, two enclosures with different top covers (i.e., one with commercially available white paint and the other with radiative cooling paint) are prepared with several thermoelectric coolers attached through the side walls. Two different outdoor experiments are carried out; that is, one maintaining constant enclosure temperature and the other maintaining constant temperature difference between enclosure and ambient. The first test is designed for verifying the cooling energy saving of building, and the second test is for quantifying the cooling density from radiative cooling in an enclosure with a large thermal mass. By measuring the temperature and power consumption in the enclosures, the radiative cooling performance of two different enclosures can be thoroughly and quantitatively analyzed, which potentially can lead to the direct examination of cooling energy saving of buildings exploiting the daytime radiative cooling effects. With the proposed device, we show that the radiative cooling paint can produce the daily averaged radiative cooling of 10.9∼45.2 W/m2 and the corresponding cooling load of thermoelectric coolers can be reduced by 15.7∼50.0%. We anticipate this device to be a starting point for more realistic and sophisticated evaluation of radiative cooling performance.

We proposed a hybrid heating, ventilation, and air-conditioning (HVAC) system that integrates a solar thermal collector and a radiative cooling panel as a heat source and a heat sink, respectively. The annual performance of the proposed system was analyzed for a single-family house and compared with that of the conventional solar-assisted heat pump (SAHP) and radiative-cooling-assited heat pump (RAHP) sys-tems. We found that the seasonal coefficient of performance of the proposed system can be enhanced by 27.3% for cooling compared to the SAHP system in cold-dry climate and by 61.8% for heating compared to the RAHP system in hot-dry climate. The detailed analysis revealed that the sizes of the solar collector and the radiative cooling panel, and the total amount of water (i.e., working as a heat source or a heat sink for the proposed HVAC system) are the key parameters affecting the annual performance of the pro-posed HAHP system. By using an optimization technique, we showed that these parameters should be varied depending on climate zones for optimal operation. Interestingly, it was found that installing both solar collector and radiative cooing panel can be more efficient than the conventional systems regardless of climate conditions (i.e., cold-dry: Denver; hot-dry: Pheonix; and marine: Los Angeles). The sensitivity analysis of each parameter revealed that the annual power consumption is less sensitive to the tank water volume for cold-dry and hot-dry climates, but the tank volume becomes important for marine cli-mate. For considered three locations, the proposed system was found to be able to reduce the annual power consumption by about 3% ti 29% as compared to the conventional RAHP or SAHP system. (c) 2021 Elsevier B.V. All rights reserved.

Daytime radiative cooling has drawn much attention recently because a target surface can be passively maintained at sub-ambient temperature. In order to implement a daytime radiative cooling device (simply 'radiative cooler'), strong thermal emission should be focused in the mid-infrared regime (8-13 mu m), called 'atmospheric transparent window'. At the same time, absorption of the solar irradiation should be minimized. In the present study, for optimal performance of daytime radiative cooling, a mixed-integer genetic algorithm was employed to achieve maximal infrared emission as well as minimal solar absorption. The combination of total number of layer, materials, and thickness of each layer in the multilayered radiative cooler were determined through optimization. The optimized multilayer structure exhibited the spectrally-averaged (in the 8-13 mu m wavelength range) normal emissivity value of 0.96 and the solar-weighted absorptivity of 0.03. The corresponding net cooling power was found to be 101.0 W/m(2), and a sub-ambient cooling temperature of 11.2 degrees C (i.e., below ambient temperature) was predicted in daytime at air-mas 1.5 condition. Besides, the mechanism of enhanced emission in the infrared region and suppressed absorption in the solar spectrum were thoroughly investigated. We also derived the expected performance of the optimized radiative cooler for various conditions of the environmental parameters, such as convection heat transfer coefficient, ambient temperature, and precipitable water level. (C) 2020 Elsevier Ltd. All rights reserved.

In order to ensure high crop yield and good quality in greenhouse horticulture, the major environment control variables, such as temperature, humidity, and CO2 concentration, etc., need to be controlled properly, in order to reduce harmful effects on crop growth by minimizing the fluctuation of the thermal condition. Even though a hot water-based heating system is evidently superior to a hot air-based heating system, in terms of the thermally stable condition or energy saving, a hot air-based heating system has occupied the domestic market due to its economic efficiency from an initial investment cost saving. However, the intrinsic drawbacks of a hot air-based heating system, being more frequent variation of thermal variables and an inordinate disturbance on crops due to its convective heat delivery nature, are believed to be the main reasons for the insufficient crop yield and/or the quality deterioration. In addition, the current thermal environment monitoring system in a greenhouse, in which a sole sensor node usually covers a large part of cultivating area, seems to have a profound need of improvement in order to resolve those problems, in that the assumption of thermal uniform condition, which is adequate for a sole sensor node system, cannot be ensured in some cases. In this study, the qualitative concept of the new control variable—the degree of uniformity—is suggested as an indicator to seek ways of enhancing the crop yield and its quality based on the multiple sensor nodes system with a wireless sensor network. In contrast to a conventional monitoring system, for which a newly suggested concept of qualitative variable cannot be estimated at all, the multiple sensor nodes-based thermal monitoring system can provide more accurate and precise sensing, which enables the degree of uniformity to be checked in real-time and thus more precise control becomes possible as a consequence. From the analysis of the results of the experiment and simulation, it is found that the crops in plastic vinyl houses can be exposed to a serious level of non-uniform thermal condition. For instance, the temperature difference in the longitudinal and widthwise direction is 3.0 °C and 6.5 °C, respectively for the case of 75 × 8 m dimension greenhouse during a typical winter season, and it can be hypothesized that this level of non-uniformity might cause considerable damage to crop growth. In this paper, several variants of control systems, within the framework of the multiple sensor nodes system, is proposed to provide a more thermally-stable cultivating environment and the experimental verification is carried out for different scales of test greenhouses. The results showed that a simple change of heating mode (i.e., from a hot air- to a hot water-based heating system) can bring about a significant improvement for the non-uniformity of temperature (more or less 80%), and an additional countermeasure, with local heat flux control, can lead to a supplementary cut of non-uniformity up to 90%. Among the several variants of local heat flux control systems, the hydraulic proportional mass flow control valve system was proven to represent the best performance, and it can be hypothesized that the newly suggested qualitative variable—the degree of uniformity—with the multiple sensor nodes system can be a good alternative for seeking enhanced cultivating performance, being higher crop yield and better quality along with energy cost saving.

A high energy, efficient, harmonious, ecological greenhouse has been highlighted by advanced future agricultural technology recently. This greenhouse is essential for expanding the production cycle toward growth conditions through combined thermal environmental control. However, it has a negative effect on farming income via huge energy supply expenses. Because not only production income, but operating costs related to thermal load for thermal environment control is important in farming income, it needs studies such as a harmonious ecological greenhouse using natural ventilation control. This study is simulated for energy consumption and thermal environmental conditions in a three-layered greenhouse by natural ventilation using window opening. A virtual 3D model of a three-layered greenhouse was designed based on the real one in the Gangneung area. This 3D model was used to calculate a thermal environment state such as indoor temperature, relative humidity, and thermal load in the case of a window opening rate from 0 to 100%. There was also a heat exchange operated for heating or cooling controlled by various setting temperatures. The results show that the cooling load can be reduced by natural ventilation control in the summer season, and the heat exchange capacity for heating can also be simulated for growth conditions in the winter season.