This paper presents a review of windows filled of water-based liquids which absorbs solar radiation and will be referred to herein as water-based windows (WBWs). These liquids have ...spectrally-selective and high thermal capacity properties for optical and thermal performance improvement. A WBW relates to windows that features transparent, semi-transparent, colorless or colored visual performance. Its profile consists of liquid layers inside clear or solar control multilayer glazing or tubular structures. A detailed analysis of WBW design differences and performance is carried out and studies were classified according to common characteristics regarding two main categories: WBW without and with fluid exchange. Optical and thermal methodologies are evaluated, highlighting the limitations and accomplishments for future researches. A water-filled WBW can transmit more light compared to the same glazing when empty. It can also perform substantially better than many commercially available glazing products, in terms of either promoting solar heat gain or reducing cooling demand. There was a predominance of cases with better heating performance for WBWs without fluid exchange. On the other hand, most cases of WBWs with fluid exchange where better for cooling performance if the circulating fluid was not preheated. Also, water heated in WBWs can be applied in the building facilities.
The transitioning from overall steady-state thermal conditions indoors to non-steady and non-uniform conditions encompasses the airflow field generated by means of air conditioning and ventilation ...systems, devices for increased air movement and even cross-ventilation in naturally ventilated buildings. The indoor airflow field is susceptible to large variations with respect to time and space, which makes its characterisation and evaluation in real occupancy buildings a complex task. A literature review focusing on dynamic and non-uniform airflows applied for human thermal comfort in buildings was conducted with the aim of documenting recent findings on three main topics: airflow characterisation, thermal comfort evaluation and thermal comfort prediction. Over 150 articles from the past two decades of research carried out in climate chambers, experimental rooms, laboratories and real occupancy buildings were reviewed. The main findings indicate that the more dynamic and unpredictable airflows tested in studies with subjects had better preference, sensation and comfort ratings when compared to constant and sinusoidal patterns. Field validation is required to (1) verify the applicability of simulated natural airflows in real occupancy buildings and (2) optimize delivered mean air speeds – up to 0.9 m/s – and their respective time spans – from 10 seconds to 3 minutes – in intermittent airflow patterns, to avoid the risk of draughts and lack of air motion. Temporal variations in air velocity, whose standard deviation was up to 0.5 m/s in studies with sources of increased airflow, should be addressed in detail with respect to the corresponding subjective feedback. Furthermore, there is a gap to be filled regarding the incorporation of temporal variations into the prediction models to be proposed and validated.
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•The more dynamic and unpredictable airflows were better rated for thermal comfort.•Optimization of air speeds and their respective time spans is needed in future works.•Applicability of simulated natural airflow in real buildings needs to be checked.•Temporal variations of air velocity should be inputted in comfort prediction models.
•We compare mechanical properties of lightweight concrete with vermiculite and EPS.•Lower amount of lightweight aggregate and air-entraining provides higher strength.•Vermiculite lightweight concrete ...has lower thermal conductivity than with EPS.•EPS lightweight concrete has higher strength and is lighter than with vermiculite.
This study aimed to compare mechanical and thermal properties of lightweight aggregate concrete with two kinds of lightweight aggregates, vermiculite and Expanded Polystyrene (EPS) and using air-entraining agent and superplasticizer admixture. For better reliability, a statistical analysis of the results compressive strength and density was used. The factors of the 22 full factorial design were: amount of lightweight aggregate (55% and 65%) and quantity of air-entraining agent (0.5% and 1.0%). The results showed that the addition of air-entraining agent left the lightweight concretes even lighter, but less resistant. EPS lightweight concrete has higher strength and is lighter than with vermiculite. Vermiculite lightweight concrete had lower thermal conductivity than with EPS. The better lightweight aggregate content was 55%.
•Thermal performance of four reduced-scale roofs analysed experimentally.•Heat flows, temperature and thermal resistance were verified during the tests.•Thermal resistance for an eight-month period ...based on upward and downward heat flows.•White roof can half the thermal gain and allows thermal losses at night for buildings in hot climates.
A better understanding of the thermal performance of construction systems may help engineers and architects to select the most suitable materials for the solutions that meet their objectives. In warm-climate countries like Brazil, the thermal performance of roofs is of particular importance. Thus, this study presents an evaluation of the thermal performance of roofs built in reduced scale on an experimental test rig, in Florianópolis, capital of the state of Santa Catarina, southern Brazil. Four roofs were evaluated, all with fibre cement tiles and a polyvinyl chloride (PCV) ceiling: the first was used as a reference, the second had the tiles painted white, the third had a radiant barrier installed and the fourth had conventional thermal insulation based on expanded polystyrene (EPS). All roofs were instrumented with temperature and heat flow sensors and data was collected over a period of eight months. The results show that all of the test roofs had a lower thermal gain than the reference roof, with reductions of 55% for the roof with white tiles, 67% with a radiant barrier and 70% with conventional thermal insulation. The thermal resistance for downward heat flow of the roof with conventional thermal insulation was 1.29m2K/W and that of the roof with the radiant barrier was 1.20 m2K/W, with a difference between the two of only 7%. When comparing the thermal resistance obtained experimentally with values obtained with the international standard ISO 6946 and with the Brazilian standard NBR 15220-2 it was found that the radiant barrier roof presented the greatest differences, of 41% and 30%, respectively. This demonstrates that it is important to review the calculation procedure, especially when a radiant barrier is used for thermal insulation.
This paper presents a study of the optical performance of window systems with embedded water layers, here called water-based windows (WBWs). Spectrophotometer measurements were performed on four WBWs ...prototypes, in addition to a single and double-glazed window. A numerical model was also developed to calculate the optical performance of multilayer WBWs, which was validated by laboratory measurement results. Furthermore, a parametric performance assessment of multilayer WBWs is caried out for windows using only clear glazing and layers of water and air. The results corroborate previous studies that the use of water in glazing ensures higher visible light transmittance (Tvis) values while it reduces solar transmittance (Tsol) rates compared to the same glazing without water. Increasing the thickness of a water layer in a WBW does not interfere in Tvis values, but decreases direct solar gain. Increasing the number of “glass-water” interfaces on a WBW has little impact on its optical performance. On the other hand, more interfaces of “glass-air” increases the glazing reflectance and reduces both Tsol and Tvis values. Therefore, different combinations of air, water and glass layers in a WBW can result in very different optical performance results. Last but not least, the optical performance of WBWs when varying the angle of solar incidence (aoi) showed the same pattern compared to single or multi-glazing with air.
•Combing water layers and air gaps increases the spectral selectivity of glazing.•An optical numerical model for simulation of glazing with water is presented.•Multilayer windows with water increase light transmission.
•Modeling of transient evaporative cooling using idealized and real weather data.•Parametric study inferring on the impact of the environmental conditions on the effectiveness of evaporative ...cooling.•World wide TMY analysis aiming to identify the best climates where evaporative cooling is more effective.
The current manuscript applies a previously developed and validated evaporative cooling theoretical model to a wide range of scenarios aiming to infer the actual effectiveness of evaporative cooling on building roofs as a way to reduce the excessive thermal load. The presented analysis is divided in three main sections, one of parametric nature, which considers standard or idealized variation for wind speed, solar irradiation, ambient temperature and relative humidity, one using real weather data and the third one that based on TMY data from several locations around the world, which are employed to evaluate the applicability of evaporative cooling for different climates. The parametric analyses indicate that air relative humidity and temperature variations present, arguably, the strongest influence on the performance of the evaporative cooling compared to other roof heat gain reduction methods. The global analysis indicates that arid climates such as BSk and BWh, from the Köppen classification, are more likely to display encouraging results with respect to the evaporative cooling whereas in humid equatorial climates the benefits are modest.
•A self-pumping, self-optimizing passive latent heat cooling mechanism for roofs.•A multi-variable theoretical modeling of porous medium coatings for heat shielding.•Experimental validation using ...real transient outdoor irradiation, temperature, humidity and wind speed data.•Discussion of the heat transfer mechanisms affecting the effectiveness cooling technique investigated.
This study aims to introduce and experimentally validate a model to predict the potential of evaporative cooling as a way to mitigate unwanted heat gain in buildings’ roofs. The experimental setup, which emulates a roof on a small-scale, was developed to independently measure the instantaneous heat flux permeating through two equal porous medium samples under the same environmental conditions, with the only difference that one was completely dry and other was fully wet, thus creating a reference heat gain value to which the wet, evaporative sample could be compared. The numerical model solves an energy balance within the porous sample and a surface energy balance at its upper surface while considering the environment’s temperature, humidity, wind speed and solar irradiation as boundary conditions. The experimental and numerical model have their results compared for two different conditions: (i) steady-state and indoors without solar irradiation and (ii) transient outdoors under real weather. The direct comparison showed that the theoretical model developed is capable of predicting, with good accuracy, the heat flux flowing through the porous medium for most tests and that the wet porous medium can, for certain conditions, outperform its dry counter version, despite having higher thermal conductivity and absorptance due to its wetness.
•An experimental rig to evaluate the thermal performance of roofs in the field is presented.•Eight roofs with trapezoidal shape, built in reduced scale, were simultaneously evaluated.•Heat flows, ...temperature and thermal resistance were verified during the tests.•Measured values were compared with results obtained through computer simulations.•There was good agreement between the results obtained with the rig and through computer simulations.
Reducing the thermal gain through the roof is one of the best approaches to decreasing energy consumption and to providing thermal comfort to the users of residential or commercial single-story buildings. In this context, this paper describes an experimental test rig for use in the field, which was employed to simultaneously evaluate the thermal performance of eight roofs in reduced scale in the city of Florianópolis, Santa Catarina State, southern Brazil. The experimental test rig is comprised of a temperature-controlled environment over which the roofs were constructed. All roofs have a trapezoidal air cavity with the same dimensions, in order to carry out the calibration of the test rig. In the calibration tests, the surface temperatures, the heat flow in the roofs and the thermal resistance for the upward and downward directions of heat flow were investigated. Furthermore, computer simulations and measurements of a roof with real dimensions were also carried out in order to verify whether the experimental test rig could provide results compatible with those obtained in a real situation. The calibration results show good agreement between the results obtained for the eight roofs, with a maximum variation of 8.7% in the thermal resistance values. On comparing the values for the thermal resistance obtained with the experimental test rig and the computer simulation, there were differences of 11.7% for the upward direction and 26.1% for the downward direction of heat flow. In relation to the comparison of the results obtained between the experimental test rig and the measurement values for the roof with real dimensions, the differences were 5.6% for the upward and 34.3% for the downward heat flow.
•LEED Certification and Solar Reflectance Index. Checklist of optical and thermal properties of pigmented glazed roof tiles.•A simplified procedure that shows the relationship between CIELab ...parameters of roof tiles and Solar Reflectance Index is evaluated.•The Kubelka-Munk model in solar spectral range is used for to understand the optical behavior of pigmented roof tiles and to suggest improvement in the solar reflectance performance.
Urbanization and the resulting concentration of people in cities has led to the emergence of Urban Heat Islands (UHI), which is associated with several problems. One strategy for UHI mitigation is the use of cool coatings for pavements and buildings, for instance, on roof tiles. Ceramic roof tiles are widely used in Brazil and in the rest of South America, but they have not been adequately studied regarding their thermal emittance and solar radiation reflectance. These two properties can be used to determine whether or not a roof tile coating can be classed as “cool”, according to the Leadership in Energy and Environmental Design (LEED) certification. In this paper, the solar reflectance, thermal emittance and Solar Reflectance Index (SRI) index were determined for “Portuguese” and “American” colored glazed roof tiles, mostly produced in Brazil. Nineteen glazed roof tiles (from four different producers) with different colors were studied according the LEED certification. The thermal emittance was determined using a portable emissometer and the reflectance spectral curves were measured using a UV–Vis-NIR spectrophotometer. All measurements were performed according to the ASTM standards. The colorimetric coordinates in the visible range (for the D65 illuminant) were determined to evaluate the relationship between the luminous reflectance factor (Y) and the SRI. The optical behavior of the colored glazes for the full range of the solar spectrum was determined using the Kubelka-Munk (K-M) model and the relationship between the absorption (K) and scattering (S) coefficients. The results show that most of the colored glazed tiles did not meet the 2013 LEED requirement, that is, SRI ≥ 39 for roof tiles with a steep slope. A polynomial relationship between the Y factor and the SRI was found (R2 value > 0.95), therefore the tile colors that would probably not qualify for SRI certification could be estimated. The K-M analysis showed that for some glazes there was a high absorption of radiation in the near infrared region, suggesting that the pigments used for these glazes do not exhibit cool properties. Therefore, cool pigments with higher reflectance should be used to improve the performance of the glazes with a low SRI.
