• Energy Research

Owing to excellent solar reflectivity and sky window emissivity, disordered heterogenous materials, including filler-abundant matrices, paints, and coatings, as well as foam-like, fiber-stacked and composite porous structures, form a major class for efficient passive radiative cooling. Contrary to well-established empirical understanding, this work offers a generalized analytical overview of their macroscopic thermo-optical properties from the microscopic electromagnetic perspective of Maxwell-Garnett effective medium theory. With the family of micro-porous poly(vinylidene-fluoride)/poly(methyl-methacrylate) blends as a representative example, procedures for tailoring mid-infrared spectral emissivity via effective permittivity are outlined. Theoretical framework and design scheme are validated by finite difference time domain simulation and Fourier transform infrared spectrometry. It is shown that poly(vinylidene-fluoride) and poly(methyl-methacrylate) form a pair of complementary constitutive materials for near unity thermal emission through the atmospheric window. Optimized binary polymeric blend, prepared by spray-coating method, features a window emissivity of 98% and realizes nocturnal radiative cooling with a temperature reduction of 6.8 °C and a cooling power of 94 W/m2 in an outdoor field investigation. It can serve as a promising bifunctional material for simultaneous radiative heat dissipation and capacitive energy storage, which meets the demand for nocturnal, radiative cooling aided thermoelectricity generation and storage potential.

Abstract Thermochromic smart windows and radiative coolers are two passive cooling technologies, whose adoption as windows and roofs, respectively, is feasible for building energy-saving. However, to the authors' knowledge, the investigation of annual energy performance incorporating both techniques is scarce at the time of writing. Therefore, a passive hybrid system involving both technologies is proposed in this study. A perovskite thermochromic smart window and three different radiative coolers were chosen based on their superior performance. The energy performance of the passive hybrid system in a prototypical medium-sized office building was simulated using EnergyPlus and the results were rigorously analyzed. Both thermochromic smart window and radiative cooler could reduce total energy consumptions by up to 10.6% and 23.0%, respectively, regardless of building's year of completion, while the synergic system saved up to 32.0%. Among the chosen cities of various climates, thermochromic smart windows and radiative coolers perform better in cities where cooling demand dominates. The west- and east-facing thermochromic smart windows could mitigate more energy usage in contrast to the other orientations. If this passive hybrid system can be offered at a reasonable cost, the technology is likely to be a viable energy-efficient option for buildings.

During the past few decades, the growing demand for air conditioning has caused a significant increase in demand for primary energy resources. Adsorption cooling system is one of the technologies which could be powered by renewable energy. This study aims to improve the performance of a solar-powered adsorption chiller by applying a novel composite adsorbent, a mixture of activated carbon, silica gel and calcium chloride. Modeling is established to investigate the cooling performance of a composite adsorbent based adsorption chiller driven by flat-type solar collectors with three different configurations of glaze: (1) single glazed cover; (2) double glazed cover and (3) transparent insulation material (TIM) cover. The simulation results show that the coefficient of performance (COP) and the specific cooling power (SCP) of the adsorption chiller depend hugely on the solar collector temperature. It is found that a double glazed cover shows the best cooling performance and 30 m2 is the most optimized solar collector area. Two to three hours of pre-heating time is required to initiate the desorption process of the adsorber in a day of operation. This newly developed silica activated carbon/CaCl2 composite material as adsorbent used in the adsorption chiller could achieve a high mean COPsc of 0.48. Its satisfactory performance suggests that this novel composite material has a potential to be used in the adsorption chiller system even if it is powered by unstable solar energy.

Radiative sky cooling, which utilizes outer space as the cooling source for heat dissipation, is attracting global interests for its ability to cool passively without using any energy. Considering the swift advancements in the production of cost-effective radiative sky cooling materials on a large scale, it is of practical significance to incorporate radiative sky cooling technology into buildings to improve energy efficiency, necessitating amendment of the corresponding regulations. With this objective, the present research particularly focuses on the impact of radiative sky cooling-based super-cool roof strategies on the code of practice for Overall Thermal Transfer Value (OTTV), which is a prescriptive building regulation introduced by the Hong Kong government for building design, aimed at providing correction factors for local typical roofs integrated with advanced radiative sky cooling materials. To achieve the purpose, a new model for predicting roof heat transfer based on radiative sky cooling was established. Scaled-down experiments were conducted to fully validate the developed model with sufficient accuracy. The findings indicate that the implementation of super-cool roof strategies through radiative cooling-based techniques can substantially decrease roof heat gains, with reductions ranging from 73.9 % to 90.7 %, yield considerable energy savings, averaging between 3.5 and 45.0 kWh/m2, as well as significant cost savings on cooling expenses, averaging from 5.8 to 74.2 HKD/m2. Moreover, the adoption of such strategies has the potential to reduce carbon emissions by 6.3 to 81.2 kg/m2 during the cooling season. These outcomes vary depending on factors like the coefficient of performance (COP), radiative cooling materials, and specific roof types. Correlations between roof heat gains and OTTVs are established with a high accuracy of fit, achieving a coefficient of determination as high as 0.978. The average correction factors of five cutting-edge radiative cooling materials range from −0.00563 to −0.01655 ...

Abstract Cost reduction and enhanced cooling performance are strongly demanded for daytime passive radiative cooling due to its attractive cooling strategy that does not require any energy input. Its potential application varies widely from air conditioning systems for buildings, photovoltaic cells, electronic device cooling and automobiles. However, recently proposed daytime passive radiative coolers are based on photonic structures which are high in cost. A relatively cheap metal oxide material, TiO2, which lowers the cost but is highly emissive in the mid-infrared range has been used, also improving the cooling performance of the photonic daytime passive radiative cooler. An optimized TiO2–SiO2 alternating multi-layered photonic daytime radiative cooler with average emissivity of 0.84 within 8–13 μm while reflecting 94% of incident solar energy is developed. Its net cooling power is estimated to be 136.3 W/m2 at ambient air temperature of 27 °C which shows an improvement of 90 W/m2 compared to that of the HfO2-SiO2 photonic radiative cooler. Last, a field test has been conducted in Hong Kong's subtropical climate (i.e. relative humidity = 60–70%) to investigate its feasibility, and with the help of solar shading, successfully demonstrated temperature reduction of 7.2 °C with a net cooling power of 14.3 W/m2 under direct sunlight.

Abstract In the passive radiative cooling process, a sky-facing surface emitting thermal radiation through the bandwidth coincident with the atmospheric window highly transparent to the radiation within 8–13 μm can preserve the temperature below ambient spontaneously. The cold surface can act as a fundamental building block for energy conversion, in which thermo-radiative energy conversion can be the simplest form and realized by a functionalized fluid-wall heat transfer interface. Energy conversion efficiency denotes the ratio of enthalpy converted by the working fluid to the cooling effect harvestable from the sky. In parallel with fluid cooling capacity, they are discussed by thermal and energy responses of a cooling system subjected to a perturbation in fluid flow, and demonstrated by measurement on a wafer-sized system acted by an equivalent heat current. According to interfacial heat transfer characteristics, cooling performance can be classified into inhibition, transition and saturation regimes, where the saturated performance is the most outstanding. However, fluid cooling and energy conversion capacities are always inversely correlated, where the reduction in fluid temperature decreases with increasing flow rate, but efficiency increases with increasing flow rate. Experimental results, in line with the theoretical prediction, show that 12.4 μL/s of water can be chilled by −4.1 °C at an overall efficiency of 14%, but 116 μL/s of water can be weakly chilled by −1.5 °C at an elevated efficiency of 49%. The dilemma in energy efficient collection of cooling fluid is an innate physical mechanism restricted by Newton's law of cooling and the 1st law of thermodynamics.

Abstract In this study, a compact dual adsorber adsorption cooling system (ACS) prototype was built using the zeolite 13X/CaCl2 composite adsorbent with water as the adsorbate. The adsorbers were constructed by directly coating the composite adsorbent on parallel flow finned heat exchangers to enhance the heat and mass transfer performance. The compactness of the ACS is of great concern for use in buildings, where space is always limited. Through a better adsorber design, the specific cooling power (SCP) is largely improved from 106 W/kg to 377 W/kg (256% improvement) under the same desorption temperature, 85 °C, and chilled water inlet temperature, 14 °C, even though the cooling water temperature is increased from 22 °C to 28 °C. Besides, four different operation sequences, namely basic cycle, mass recovery cycle, pre-heating & pre-cooling cycle, and mass recovery with pre-heating & pre-cooling cycle, were studied to optimize the system performance. It is found that performing the pre-heating & pre-cooling cycle can further increase the SCP to 401 W/kg. This promising result shows that the ACS has potential to be installed in buildings to achieve the goals of heating/cooling energy saving.

Reducing the energy consumed by space cooling and providing outstanding thermally insulated windows are essential requirements for a smart green building. In this study, a new building design concept for envelope/façade is proposed and demonstrated, providing a solution for thermal management in buildings. This new design for the envelope/façade of buildings comprises two major passive and energy-free technologies, a daytime radiative cooler and a thermochromic smart window. The PDMS-silica-silver daytime passive radiative cooler can provide a cooling effect to the indoor environment without energy input, meanwhile the thermochromic smart window using PNIPAm hydrogel can passively modulate the solar irradiance entering buildings through windows. Hence, the energy needed for indoor heating, cooling and lighting can be reduced significantly. To evaluate the energy-saving performance, model houses are built, one of which is assembled using the proposed building design. The orientation effect on the indoor air temperature of the model houses is investigated. Under intensive solar irradiance, a maximum reduction of 4.8 °C of the indoor air temperature is achieved in the model house constructed using the proposed building design. An energy saving of about 17% of air-conditioning systems in buildings constructed with this new design is expected during daytime operation.