Radiant Cooling Research Articles

Multifunctional Wood Composite Aerogel with Integrated Radiant Cooling and Fog–Water Harvesting for All‐Day Building Energy Conservation

AbstractPassive radiative cooling, as a cooling technique with no energy input, can continuously radiate heat into the supercooled universe. However, the continuous cooling effect tends to cause the problem of nighttime overcooling. Moreover, non‐renewable radiative cooling materials and energy‐intensive processing methods lead to increased carbon emissions and resource consumption. Therefore, there is an urgent need to develop a renewable and environmentally friendly self‐adaption radiative cooling thermal management material. In this paper, a high‐performance self‐adaption thermal management wood composite aerogel material is designed and prepared by in situ growth of multi‐scale silicon dioxide on wood. The constructed passive radiative cooling material has a sub‐ambient cooling effect of up to 13.5 °C and 20.2 °C during daytime in winter and summer, respectively. Meanwhile, it has a certain thermal insulation performance (2.0 °C above ambient) due to low thermal conductivity (0.063370 ± 0.000329 W m−1 k−1) at night in winter. In addition, the material is also suitable for fog–water harvesting (fog–water harvesting rate of 59.27 ± 0.76 mg min−1) due to its hydrophobicity. This work can significantly promote the practical application of passive radiative cooling materials.

Experimental studies on the operating characteristics of air-layer-integrated radiant cooling units

Radiant cooling is widely acknowledged as an energy-efficient approach to indoor thermal environment regulation. It offers an improved environment in terms of thermal comfort, compared to traditional air-cooling methods, but suffers from condensation risk and insufficient cooling power in hot and humid climates. To overcome these limitations, an Air-Layer-Integrated Radiant Cooling Unit (AIRCU) was developed, which seals a layer of dry air between an infrared transparent membrane and the radiant cooling surface. This innovative approach allows the use of lower radiant cooling temperatures to increase cooling power while concurrently maintaining the membrane at a higher temperature to minimize the risk of condensation. In our previous studies, the heat transfer model of AIRCUs was established, and the feasibility of the concept was verified. However, there is a lack of research on the operating characteristics of AIRCUs. In this paper, both experimental and numerical studies are conducted to investigate the operating characteristics of the AIRCUs under diverse cooling temperatures, chilled water flow rates, and cooling loads. Results indicated that the thermal environment created by the AIRCUs was uniform, and it can be characterized by uncooled surface temperature and radiant cooling surface temperature. Under different load conditions, the chilled water supply temperature was suggested as the control variable to adjust the thermal environment. To satisfy the thermal comfort, the operative temperature should be controlled within 23.6 °C ∼26.4°C. This study provides valuable guidance on operating variable settings and environmental control of the AIRCU and promotes its application.

A comprehensive study on the impacts of thermal comfort on occupants’ thermophysiology and cognitive performances in a radiant cooling environment using physiological measurements

Radiant cooling (RC) systems are increasingly implemented in public buildings to enhance human thermal comfort and reduce building energy consumption. The quality of the indoor thermal environment significantly impacts occupants’ learning and work performance. This investigation explores indoor occupants’ dynamic physiological regulation, thermal comfort, and cognitive performance under radiant whole-wall cooling (RWWC) systems. Twenty-eight participants underwent a summer exposure test task at varying ambient temperatures, yielding four collected data types: indoor physical parameters, physiological parameters (including skin temperature (ST), heart rate (HR)/heart rate variability (HRV), and electroencephalography (EEG)), performance index (PI), and thermal perception. Results indicate that the RWWC system notably improves thermal perception and comfort, with 93 % of participants reporting comfort at a target temperature of 24℃. Air temperature (AT) exerted the most significant effect on changes in ST on the right upper arm (p < 0.01) and left hand back (p < 0.01) at 30℃ compared to the 22℃. The difference in whole-body mean skin temperature (MST) is 2.68℃. The HR decline was more pronounced in higher temperatures, and the HRV representativeness index approached ‘1′ at 24℃. PI was affected by temperature and task type, displaying a non-linear correlation with AT (R2 = 0.6056) and an inverted “U” shaped curve with TSV (R2 = 0.9523). Thermal perception correlates with MST, HRV, and EEG. The study carries implications for the reasonable design of comfortable and healthy RC environments.

Research on the Optimization of Solar Energy Utilization Based on Sky Radiant Cooling

Research on the Optimization of Solar Energy Utilization Based on Sky Radiant Cooling

Performance Assessment of an Integrated Low-Approach Low-Temperature Open Cooling Tower with Radiant Cooling and Displacement Ventilation for Space Conditioning in Temperate Climates

Cooling towers, by producing chilled water and by integration with radiant and displacement cooling systems, offer a possible alternative method for space conditioning of office buildings in temperate climates. This present study examines the operational feasibility of a cooling tower in conjunction with a radiant and displacement ventilation cooling system for office conditioning in four temperate climates. The climates are: cool and semi-humid (Birmingham, UK), cool and dry (Helsinki, FI), warm and humid (Paris, FR) and warm and dry (Prague, CZ). The system is capable of producing chilled water between 14 and 20 °C, with low approach tower temperatures (1–3 K). A mathematical model of the cooling tower system was developed and integrated with an office building energy simulation model. Using the integrated simulation model, assessment was carried out based on ASHRAE design day specifications, as well as a complete cooling seasonal analysis. Moreover, the performance of the system is benchmarked against a variable-air-volume cooling system. Energy savings for the system when benchmarked against a variable-air-volume air conditioning system, where the chiller COP (coefficient of performance) varies from 2.75 to 6.5, were 62% to 37% in Paris, 56% to 30% in Prague, 52% to 28% in Helsinki and 45% to 13% in Birmingham, respectively.

Experimental Study on Heat Transfer Characteristics of Radiant Cooling and Heating

Experimental Study on Heat Transfer Characteristics of Radiant Cooling and Heating

Subjective evaluation of thermal comfort of tropically acclimatized subjects in air conditioned, naturally ventilated and radiant cooling environments

The thermal sensation of people is subjective. It depends on the external climatic condition and thermal history of a person. Thermal comfort standards suggest a narrow temperature range to assert an environment as comfortable or otherwise. It is important to evaluate people’s thermal perception in different climatic regions and set comfort limits based on their thermal perception. This study evaluates the thermal perception of subjects in hot and humid climatic regions at higher indoor temperatures and air velocities. Three different thermal environments chosen for the study are air conditioned (AC), naturally ventilated (NV), and radiant cooling (RC) thermal environments. The study shows that more than 80% of subjects voted as comfortable in AC and RC thermal environments for the measured Top and va range of 25.3 – 28.4℃/0.1 – 0.3m/s and 25.9 – 29.1℃/0.1 – 0.9m/s respectively. The findings gathered from the study in NV environment indicate that 68% of subjects opt for the use of table fans over ceiling fans. The subjects' free control of air movement improves their thermal sensation over no control of air movement, indicating that they prefer table fans and free control of air movement. In AC, NV, and RC thermal environments, humidity and air quality remain uncontrolled. In AC and RC thermal environments, less than 20% of subjects voted unacceptable for the humidity sensation and Perceived air quality (PAQ) scales in nearly all the studied experimental conditions. In NV thermal environment, the percentage of subjects voting dissatisfied on the humidity sensation scale and PAQ scale is 30-52% and 26-37%, respectively. This outcome shows that a thermally comfortable environment with air movement improves subjects' humidity sensation and PAQ even when humidity and air quality are unaltered. The study points out that air velocity played a vital role in influencing the thermal sensation, humidity sensation and perceived air quality of subjects in the three studied thermal environments. Further the study shows that with free control of air movement will significantly aid in rising the comfort temperature band of tropically acclimatized subjects. Practical application The study shows the variation in limits of thermal comfort indices such as operative temperature and air velocity for tropically acclimatized subjects in three thermal environments. The same group of subjects participated in the subject study conducted in air conditioned, naturally ventilated and radiant cooling thermal environments. The outcomes indicate that subjects are comfortable at higher indoor temperature and air velocity. Further the outcomes showed that the humidity and air quality sensation of subjects is influenced by the subjects’ thermal sensation and air velocity.

Design and Analysis of Radiant Cooling System

Design and Analysis of Radiant Cooling System

Thermal Environment Analysis and Optimization for Large Space Buildings with Radiant Cooling Floors: A Case Study of Xianyang International Airport

Radiant cooling floors combined with ventilation systems have been widely applied in large space buildings. However, there has been a lack of research on system control strategies for their adaptation to weather changes. This study aimed to find control strategies for radiant cooling floors combined with displacement ventilation systems used in large space buildings in order to achieve energy conservation and environmental improvement. Supply air temperature and cooling surface temperature were determined to be the control variables. It was found that cooling capacity of the combined system and the comfort index, PMV (predicted mean vote), were linear in relation to the supply air temperature and cooling surface temperature. The linear equations regarding cooling capacity and PMV were established separately using environment data, and then the optimal region was determined. A case study on Terminal 3 of Xi’an Xianyang International Airport was conducted. The thermal environment was investigated through on-site measurements, questionnaires, and numerical simulations with CFD (computational fluid dynamics). It was found that supply air temperature and cooling surface temperature had a significant impact on PMV, and less impact on the cooling capacity. Therefore, it was determined that the supply air temperature should be altered first when the indoor temperature exceeds the upper limit, and then the cooling surface temperature should be changed if the indoor environment continues to overheat with the supply air temperature set to 18 °C. Thus, the supply air temperature was kept at 18 °C, and the floor surface temperature was set to be 22 °C on a high-temperature day. The average PMV was 0.87, and the cooling capacity of the combined system was 200 W/(m2·K), according to the CFD simulation. In addition, the surface heat transfer coefficient of the cooling floor was found to be 10.26 W/(m2·K). This research provides important references for the design and operational management of radiant cooling floors in large space buildings.

Effects of Plenum Load on Cooling Capacity Design of Ceiling Radiant Cooling Panels

Effects of Plenum Load on Cooling Capacity Design of Ceiling Radiant Cooling Panels

Energy saving potential in radiant cooling system by utilizing air-conditioning condensate: A strategy for green building rating

The applicability of recovered air-conditioning condensate in Ceiling Mounted Radiant Cooling Unit (CMRCU) for achieving thermal comfort in a cabin is studied through field assessment, exergy analysis, simulation, and experiments. The field assessment study in a 35 kW cooling capacity air-conditioned building reveals the condensate generation of 8.2 L/h at 15 ± 1 °C which can be effectively utilized as a cooling source in CMRCU. In addition, an exergy analysis is carried out to understand the effect of cooling by CMRCU using recovered condensate. Condensate-assisted CMRCU was shown to be beneficial in enhancing the system performance, as evidenced by a 31 % reduction in exergy consumption which contributed to a 5 % increase in exergy efficiency. Further, the cabin thermal comfort was examined using TRNSYS simulation at supply water temperatures of 14 °C and 16 °C and the results are compared with standards of EN ISO 7730:2005. Simulation studies confirmed the performance of CMRCU to attain thermal comfort with minimized ceiling condensation is at 16 °C, and the same is considered for the experiment. The experiment and predicted cabin temperature are in good agreement with a maximum variation of 5 %. Moreover, the study was extended with a thermal energy storage system utilizing polyethylene glycol 600 as the energy storage material, resulting in a 20-minute prolongation of thermal comfort through stored latent heat. Energy and economic analysis reveal the effective use of condensate resulting in 35 % of energy savings with a payback period of 6.2 years, fulfilling GRIHA Criterion-14, with offers to earn a maximum of 8 credits for green building certification.

A Review of Studies on Heat Transfer in Buildings with Radiant Cooling Systems

Due to their benefits in interior thermal comfort, energy saving, and noise reduction, radiant cooling systems have received wide attention. Radiant cooling systems can be viewed as a part of buildings’ maintenance structure and a component of cooling systems, depending on their construction. This article reviews studies on heat exchange in rooms utilizing radiant cooling systems, including research on conduction in radiant system structures, system cooling loads, cooling capacity, heat transfer coefficients of cooling surfaces, buildings’ thermal performance, and radiant system control strategy, with the goal of maximizing the benefits of energy conservation. Few studies have examined how radiant cooling systems interact with the indoor environment; instead, earlier research has focused on the thermal performance of radiant cooling systems themselves. Although several investigations have noted variations between the operating dynamics of radiant systems and conventional air conditioning systems, the cause has not yet been identified and quantified. According to heat transfer theory, the authors suggest that additional research on the performance of radiant systems should consider the thermal properties of inactive surfaces and that buildings’ thermal inertia should be used to coordinate radiant system operation.

Numerical and Parametric Study on Open-Type Ceiling Radiant Cooling Panel with Curved and Segmented Structure

A suspended open-type ceiling radiant cooling panel (CRCP) has been proposed recently. The main challenge is improving its cooling performance to overcome limitations for extensive use. Therefore, this study aims to optimize the design of CRCPs with curved and segmented structure to enhance heat transfer. A three-dimensional CFD model was developed to investigate the cooling capacity and heat transfer coefficient of the CRCPs installed inside a single enclosed room. Panel structure was determined based on four dependent parameters: the panel curvature width (L, m), the panel curvature radius (r, m), the void distance (d, m) between each panel or panel segment, and the panel coverage area (Ac, m2). The panel surface area (As, m2) and the ratio of panel curvature width to radius (L/r) were also examined. A total of 35 designs were compared under 7 different cooling load conditions, and 245 cases were carried out. The results show that the nominal cooling capacity and heat transfer coefficient rise with increasing curvature radius and decreasing curvature width. The void distance plays the most crucial role in influencing cooling performance. It is possible to simultaneously improve cooling performance, achieve uniform temperature distribution, and reduce the number of panels through structure optimization.

All-air vs. Radiant Cooling Systems: Analysis of Design and Operation Factors that Impact Building Cooling Loads (ASHRAE RP 1729)

ABSTRACT When compared to all-air systems, radiant cooling systems can improve chiller efficiency by using a higher refrigerant evaporation temperature. However, the details on differences in heat transfer and building cooling load between all-air and radiant systems are not well defined. This study uses experimental data and numerical modeling to examine the dynamics of cooling load and overall heat transfer for both radiant panel and all-air systems under various operating conditions. This study investigates the effect of: (a) heat gains (transmitted solar, internal radiative, convective), (b) set-point temperature control (air vs operative), (c) building thermal mass, (d) air circulation and surface convection, (e) panel surface area, (f) radiant system configuration (ceiling vs floor panels), and (g) intermittent system operation on the space cooling load of radiant cooling panels. Results show that for the same set-point air temperature radiant systems have slightly larger peak load than an all-air system regardless of dominant heat gain type. When considering set-point, control by operative temperature provides better performance for radiant systems. When air mixing and convection are enhanced, radiant panels allow for operational cost savings for space cooling as well as shifting building peak load to off-peak hours.

A Numerical Study on the Exergy Performance of a Hybrid Radiant Cooling System in an Office Building: Comparative Case Study and Analysis

This research investigated the exergy enhancement performance of a hybrid radiant cooling system adapting to a hot and humid summer conditions through comparative case studies and analyses. This study suggested three cooling systems: a general all-air system (AAS), a conventional radiant cooling system (CRCS), and a hybrid radiant cooling system (HRCS). As a case study, an office building with cooling systems was examined in the summer season in four different cities: Beijing, Shanghai, Chengdu, and Guangzhou, China. This study utilized the building energy performance simulation program to analyze the cooling loads of office space in a building with numerical approaches. The comparison analysis using the four different weather datasets showed simple and rational exergy efficiency and the overall impact ratio. According to the results, the ambient conditions, i.e., the surrounding temperature and the humidity ratio, significantly impacted the cooling systems’ exergy efficiency ratio. On the basis of the calculated energetic and exergetic performance, the HRCS had a higher exergy efficiency and a higher overall impact ratio. The HRCS system released an additional 20–30% of cooling output, and it could adapt well in extreme hot and humid weather conditions compared to the AAS and the CRCS system. The overall cooling impact ratio of the HRCS with an airbox convector was approximately 185% higher than that of the AAS and 8.5% higher than that of the CRCS. This study can provide the design references for the hybrid radiant cooling system and other cooling systems in hot and humid summer conditions.

A four-step method based on sequential solution of relevant model equations to predict condensation risk for a radiant cooling system

The problem of surface condensation constraints widespread application of radiant cooling (RC) system. Although, a dehumidification system is reported as a solution, the viability of using dehumidification for different climates is not conclusive. This study is conducted to determine the appropriation of dehumidification process for RC systems in different climates. An office building with an embedded RC system is modelled and simulated in EnergyPlus v9.6. Eighteen Asian cities are chosen based on ASHRAE's moisture zone classification for the simulation. Initially, the attainment of thermal comfort is assessed followed by an assessment of condensation. The results show that the system maintains required thermal comfort without an occurrence of condensation in eight cities with both dry and humid climates. It is noted that the condensation of an area does not depend on dry or humid zone of the area. Conversely, it is found that the outdoor humidity ratio is a major contributor towards the occurrence of condensation. A novel method is developed and validated that involves solving relevant model equations in a specific sequence to predict condensation risk based on outdoor humidity ratio. Results show that if the value of condensation risk is up to 20%, an RC system would provide thermal comfort without a dehumidification system and surface condensation. The proposed method will enable the designers and engineers to decide on provision of dehumidification for an RC system without going through rigorous simulation or experimental work.

Performance Improvement of The Radiant Cooling Panel Using Pressure-Vacuum Swing Adsorption with A Solid Desiccant Dehumidification

A growing environmental Impact of global warming and rapidly increasing cost of the electrical energy cause researchers in the field of refrigeration and air conditioning to develop the sustainable cooling technologies. The aim of this study was to investigate and design a radiant cooling panel with solid desiccant dehumidifier which was suitable for tropical climate to achieve thermal comfort. The influence of an interior ventilation fan to thermal comfort assessment was also examined. The designed system with CFD (Computation fluid dynamics) simulation used ANSYS software to compare with the measured data. The total area of the ceiling and wall radiant cooling panels were 16.83 m2 and 16.32 m2 with 26.28 L/min of supplied water by the cooling tower. The solid desiccant dehumidifier was the pressure-vacuum swing adsorption technology with containing 20 kg of silica-gel. The flow rate of supply air was approximately 90 kg/h. The results indicated that the PMV (Predicted mean voted) could be improving with the uses of cooling panel with dehumidifier during the day. The lower temperature and humidity ratio can improve thermal comfort. The patterns of temperature, the relative humidity and the PMV from CFD simulation were in the same agreement with the experimental results. In the case of the ventilation fan installed in the radiant cooling panel with dehumidifier, the average value of predicted mean vote was -0.2. Consequently, the PPD (Predicted percentage dissatisfied) was reduced to 6.2%. The daily electric power consumption was 5.94 kWh which was lower than the conventional air conditioning system about 16.27%.

Performance Analysis of a Geothermal Radiant Cooling System Supported by Dehumidification

Space cooling demand is increasing globally due to climate change. Cooling has also been linked to all 17 sustainable development goals of the United Nations. Adequate cooling improves productivity and thermal comfort and can also prevent health risks. Meanwhile, policy initiatives such as the European Union’s Green Deal require participants to cut greenhouse gas emissions and reduce energy use. Therefore, novel cooling systems that are capable of efficiently producing high levels of thermal comfort are needed. Radiant cooling systems provide a design capable of fulfilling these goals, but their application in hot and humid climates is limited due to the risk of condensation. In this study, we compare the performances of radiant cooling systems with and without dehumidification. The studied systems are supplied by geothermal energy. The study is conducted using building energy models of a small office building belonging to a three-building school complex located in Sant Cugat near Barcelona in Spain. The studied location has a Mediterranean climate. The simulations are conducted using IDA Indoor Climate and Energy 4.8 simulation software. The results show that the radiant cooling system with dehumidification (RCD) produces considerably improved thermal comfort conditions, with maximum predicted mean vote (PMV) reached during the cooling season being 0.4 (neutral) and the maximum PMV reached by the radiant cooling system without dehumidification (RC) being 1.2 (slightly warm). However, the improved thermal comfort comes at the cost of reduced energy and exergy efficiency. The RCD system uses 2.2 times as much energy and 5.3 times as much exergy as the RC system. A sensitivity analysis is also conducted to assess the influence of selected input parameters on the simulation output. The results suggest that maximising dehumidification temperature and minimising ventilation flow rate can improve the energy and exergy efficiency of the RCD system while having a minor effect on thermal comfort.

Numerical Prediction of Mean Radiant Temperature in Radiant Cooling Indoor Environments

Numerical Prediction of Mean Radiant Temperature in Radiant Cooling Indoor Environments

Analysis of Indoor Thermal and Humidity Environment of Radiant Cooling Coupled with Attached Jet System

The radiant cooling combined with attached jet air supply system is a temperature and humidity independent control system. Radiant cooling panels are used to bear the indoor sensible heat load, and the attached jet air supply is used to bear the indoor humidity load. Numerical simulation method is used to study the changes of indoor relative humidity and dehumidification efficiency of radiant cooling panels installed on ceiling and wall, as well as the advantages and disadvantages of all three systems taking into consideration the radiant cooling coupled with wall attached jet air supply with deflectors in terms of comfort. The results show that the attached jet has good adhesion to the cooling panel and will not leads to condensation; the dehumidification efficiency of wall attached jet is relatively poor; at the height of 0.1m, the thermal comfort of wall radiant cooling system is the best; at the height of 0.7m, the wall radiant cooling system with deflector is the best; in the working range of 1.1 ∼ 2.4m, the wall radiant cooling system is better than the ceiling radiant cooling system with or without deflector.