Experimental investigation for total equivalent temperature difference (TETD) values of building walls and flat roofs

An experimental investigation for determining total equivalent temperature difference (TETD) values of building walls and flat roofs was performed. The TETD values are functions of the time lag, decrement factor and sol–air temperature. The time lag and decrement factor depend on the highest and lowest temperatures at the inner and outer surfaces of the walls or roofs, and the time periods involved in reaching these temperatures. Also, the sol–air temperature depends on essentially solar radiation and outside air temperature. For these reasons, two testing rooms each consisting of four walls and one flat roof, an air conditioner, thermocouples, data logger and a computer were constructed to measure all required temperatures. Inside and outside air temperatures, and surface temperatures of each wall and roof layers were measured in each minute and saved on the computer over a period of 24 h in the summer season of Gaziantep, Turkey. Data for the hourly solar radiation on the walls were computed using hourly measured solar radiation data on horizontal surface. The TETD values for eight different walls and two different roofs commonly used in Turkey were computed by using the measured temperatures and solar radiation flux. The TETD values for the walls and the roofs were also obtained for selected cities in Turkey by utilizing their outside air temperature and solar radiation inputs.     

Comparison of experimental and theoretical results for the transient heat flow through multilayer walls and flat roofs

This study deals with comparison of experimental and theoretical results of transient temperature variations in multilayered building walls and flat roofs, and heat flow through the building structures. Experimental and theoretical models are presented to find the transient temperature variations in these structures and heat flow through these elements, which depends on inside surface and room air temperatures. Instantaneous inside and outside air temperatures, and surface temperatures of each wall and roof layers are measured by using the experimental model consisted of two rooms, cooling units, measuring devices and computers. A computer program based on the theoretical model is developed to perform numerical calculations. Hourly temperature variations of the nodal points are computed numerically over a period of 24 h by using the hourly measured ambient air temperatures and solar radiation flux on a horizontal surface for the city of Gaziantep (37.1°N), Turkey, and also by using thermophysical properties of the structures. Results obtained from the experimental and theoretical models are compared with each other, and validation of the theoretical model is verified in this paper. Computations for various multilayer building walls of briquette, brick, blokbims, and autoclaved aerated concrete (AAC), which are commonly used in Turkey are repeated for finding heat gain through these structures, and results are compared to determine suitable wall material. It is observed that AAC and blokbims are more suitable wall materials than briquette and brick due to heat flow through these elements.     

The determination of hourly insolation on an inclined plane using a diffuse irradiance model based on hourly measured global horizontal insolation

Using only measured hourly values of global insolation on a horizontal surface, a method has been developed for computing the corresponding hourly values of insolation on a surface inclined at any angle and oriented in any direction. The method uses a solar radiation model in which the diffuse component is calculated from global horizontal radiation using three different relationships; the appropriate equation is selected according to the value of the ratio of measured hourly global insolation to hourly global insolation computed for clear sky conditions. The method has been checked using measured hourly values in Melbourne over a 5-yr period of insolation on both a horizontal surface and a plane inclined at 38° to the horizontal facing north. The differences between the computed hourly values and the measured hourly values are found to be approximately normally distributed about zero with a standard deviation of 0.16 MJ m⁻². This method is particularly useful for predicting the heat output of inclined solar flat plate collectors when only measured global horizontal insolation is available, which is often the case. Good agreement was found between the predicted output of a typical collector using measured 38° insolation and the computed hourly values using this method. Since the method has been checked only against Melbourne data it should be applied elsewhere with caution, but it is believed to have general application.     

A RECTANGULAR SOLAR POND MODEL INCORPORATING EMPIRICAL FUNCTIONS FOR AIR AND SOIL TEMPERATURES

A novel theoretical model, capable of giving the temporal temperature variation at any point inside or outside a non-insulated rectangular solar pond at any time, is presented. Incorporating the finite difference approach, the model makes use of one- and two-dimensional heat balances written on discrete regions in the brine and in the soil adjacent to the pond. These simultaneous equations are solved for the local temperatures, using a computer program. Values of hourly averaged air temperature and daily averaged soil temperature for the site were used as input parameters, and empirical functions for the time-dependence of these variables were incorporated into the theoretical model. It was found necessary to use this level of detail of the meteorological data for reliable predictions on the solar ponds. The model results are compared with measured results on an actual solar pond built in Cukurova, Turkey. The modelled and experimental temperature profiles are found to be in a very good agreement. The results indicate that the thickness of the salt gradient region of a solar pond should not be less than 1.3 m. Heat losses form the pond side-walls was found not to effect the performance of solar ponds when the surface area is greater than 100 m².     

A proposal for “correction values” for winter outdoor design temperatures

This study aims to find a correlation between winter outdoor design temperature (WDT) and mass of the building envelope. The daily variations of the inside surface temperatures and heat fluxes of the walls under various climatic conditions and different wall constructions have been calculated by a computer program based on the response factor technique, which uses variable outside air temperature and solar radiation and constant inside air temperature values as input climatic data. The analysis of the relation between mass of the walls and inside surface heat fluxes resulted with the correction values for winter design temperature (WDTCV) depending on the mass of the wall and on the direction of facades for different climatic zones.     

On the selection of shape and orientation of a greenhouse for composite climates

Five most commonly used single-span shapes of greenhouses such as even-span, uneven-span, vinery, modified arch and quonset type have been selected for comparison in a composite type of climate. The length, width and height (at the centre) are kept the same for all the shapes. The comparison is based on total solar radiation input (beam, diffused and ground reflected) to each shape through each wall, inclined surfaces and roofs. Total solar radiation is theoretically computed for each shape in east–west and north–south orientations and compared for each month of the year. The computed values for each shape are then introduced in a transient thermal model developed to compute the hourly inside greenhouse air temperature for the selected day of the year in each month. It is observed that the variation in greenhouse shape can cause up to 3.5–5.5 °C change in the inside air temperature during different hours of the day. Experimental validation of the model is carried out using the measured inside air temperature data (for a typical summer day) for an even-span greenhouse (in which capsicum is grown) at Ludhiana (31°N and 77°E), Punjab, India. The predicted and measured values are in close agreement. The results show that even-span or quonset shape in east–west orientation is the most suitable shape for year-round agricultural operations.     

On the selection of shape and orientation of a greenhouse: Thermal modeling and experimental validation

In this study, five most commonly used single span shapes of greenhouses viz. even-span, uneven-span, vinery, modified arch and quonset type have been selected for comparison. The length, width and height (at the center) are kept same for all the selected shapes. A mathematical model for computing transmitted total solar radiation (beam, diffused and ground reflected) at each hour, for each month and at any latitude for the selected geometry greenhouses (through each wall, inclined surfaces and roofs) is developed for both east-west and north-south orientation. Computed transmitted solar radiation is then introduced in a transient thermal model developed to compute hourly inside air temperature for each shape and orientation. Experimental validation of both the models is carried out for the measured total solar radiation and inside air temperature for an east-west orientation, even-span greenhouse (for a typical day in summer) at Ludhiana (31°N and 77°E) Punjab, India. During the experimentation, capsicum crop is grown inside the greenhouse. The predicted and measured values are in close agreement. Results show that uneven-span shape greenhouse receives the maximum and quonset shape receives the minimum solar radiation during each month of the year at all latitudes. East-west orientation is the best suited for year round greenhouse applications at all latitudes as this orientation receives greater total radiation in winter and less in summer except near the equator. Results also show that inside air temperature rise depends upon the shape of the greenhouse and this variation from uneven-span shape to quonset shape is 4.6 °C (maximum) and 3.5 °C (daily average) at 31°N latitude.     

Estimation of hourly solar radiation for India

The ASHRAE constants predict high values of the hourly beam radiation and very low values of the hourly diffuse radiation when used to predict radiation at Indian locations. Hence a procedure has been developed for the estimation of direct, diffuse and global hourly solar radiation on a horizontal surface for any location in India. To calculate hourly solar radiation, an exponential curve, similar to the one used by ASHRAE, was fitted to the measured solar radiation data of six cities from different regions of India. The statistical analysis was carried out for the data computed using ASHRAE constants and the set of constants obtained for India using the measured data of four different Indian cities selected randomly. Three statistical indicators were used to compare the accuracy of the developed procedure. The results show that ASHRAE constants are not suitable to estimate hourly solar radiation in India. Hourly solar radiation estimated by constants obtained for India are fairly comparable with measured data. The mean percentage error with Indian constants for these four Indian cities was found as low as 2.27, −6.29 and −6.09% for hourly beam, diffuse and global radiation, respectively.     

Techniques for the precise estimation of hourly values of global, diffuse and direct solar radiation

The method usually used to compute solar radiation, when no measured data are available, is the well-known regression technique relating mean daily totals of global and diffuse solar radiation with the mean duration of sunshine. Using this method and taking into account the first order multiple reflections between the ground and the atmosphere, regression parameters were obtained from the monthly mean values of daily totals of global solar radiation and sunshine at a network of 16 stations in India. Daily values of global and diffuse solar radiation were then computed for 121 stations, where sunshine data are available for periods of 6–28 yr, using interpolated values of the regression parameters. Where no sunshine data were available, global and diffuse solar radiation were computed from cloud observations, using the inverse relationship between sunshine and cloudiness. Further, using the empirical relationship between daily totals and the corresponding hourly values of global and diffuse solar radiation, two sets of curves were prepared valid for the whole country, using which mean hourly values of global and diffuse radiation could be deduced from the corresponding daily totals, with a high degree of accuracy. The paper discusses the validity of the techniques used for computing daily and hourly values of global and diffuse solar radiation from sunshine and cloud amounts at an extended network of 145 stations in India and stresses the fact that such techniques are successful, only if accurate data on both radiation and sunshine are available at a widely distributed network of stations for a minimum period from at least 5 to 6 yr, using carefully calibrated and well-maintained instruments of the required quality. Theoretical models have also been used to compute clear sky noon values of global, diffuse and direct solar radiation from the solar constant, allowing for attenuation by atmospheric constituents such as ozone, water vapour, dust and aerosols. Using a simple model, calculations of global and diffuse solar radiation on clear days were made for 145 stations from values of the solar constant and measured values of ozone, water vapour and atmospheric turbidity. A method of extending the technique to overcast skies and partly clouded skies is discussed. The values of the mean annual transmission factor for global solar radiation under cloud-free conditions using the two methods show excellent agreement and establishes the soundness of the regression technique on one hand and the reliability of the theoretical model used for computing clear sky radiation, on the other.     

Comparisons of the hourly and daily global irradiances of Turkey on non-horizontal surfaces

Global irradiances incident on a horizontal surface are estimated from the bright sunshine hour measurements over Turkey. Global radiation and its components on daily and hourly bases are computed for the surfaces with any inclination and orientation. Some demonstration results are presented for the six climatologically different regions. Numerous hourly and daily computer output tables of global direct and diffuse radiations are obtained for 50 locations, which have been used to prepare a solar radiation handbook of Turkey.     

Investigation of thermal performance of-double pass-flat and v-corrugated plate solar air heaters

In this paper, the double pass flat and v-corrugated plate solar air heaters are investigated theoretically and experimentally. Analytical models for the air heater with flat and v-corrugated plates are presented. Numerical calculations have been performed under Tanta (latitude, 30^° 47^′ N) prevailing weather conditions. The theoretical predictions indicated that the agreement with the measured performance is fairly good. Comparisons between the measured outlet temperatures of flowing air, output power and overall heat losses of the flat and v-corrugated plate solar air heaters are also presented. The effect of mass flow rates of air on pressure drop, thermal and thermo hydraulic efficiencies of the flat and v-corrugated plate solar air heaters are also investigated. The results showed that the double pass v-corrugated plate solar air heater is 11-14% more efficient compared to the double pass flat plate solar air heater. It is also indicated that the peak values of the thermo hydraulic efficiencies of the flat and v-corrugated plate solar air heaters are obtained when the mass flow rate of the flowing air is 0.02 kg/s.     

Estimates of the Linke turbidity factor over Zimbabwe using ground-measured clear-sky global solar radiation and sunshine records based on a modified ESRA clear-sky model approach

This paper describes a procedure that can be used to calculate values for Linke atmospheric turbidity factors at air mass 2 (TL2) over Zimbabwe. Ground measured daily global solar radiation on clear days over 3years is used to evaluate TL2 for those stations that measure global radiation. The evaluation makes use of the clear-sky model of the European Solar Radiation Atlas (ESRA) combined with a diffuse transmittance model developed in the study. For those stations that do not measure global radiation but have sunshine duration records, global radiation values are generated through Angstrom type regression coefficients between the clearness index and relative sunshine duration. The TL2 values that are generated from the ESRA model are higher than those obtained from the study model (root mean square error (RMSE) up to 1.0 Turbidity Units). When compared to TL2 values from this study, the worldwide database SoDA, give significantly higher TL2 values (RMSE up to 2.2 Turbidity Units), underlining the value addition obtained in using measurement derived values in place of SoDA values.The values obtained from the study may be used to yield better estimates of clear-sky solar radiation for Zimbabwe. A better estimate of the clear-sky solar radiation will in turn improve the accuracy of the global radiation estimates from satellite based methods.     

建筑墙体热、湿及空气耦合传递

为了研究建筑墙体的热质耦合传递规律，该文考虑了太阳辐射的影响，建立了在第三类边界条件下建筑墙体非稳态热、湿及空气渗透耦合传递的物理及数学模型，开发了相应的数值模拟计算软件。给出了墙体热、湿及空气耦合传递时各部位模拟计算温度随时间的变化曲线及不同使用年限湿容量沿墙体厚度的分布规律，并将墙体的模拟计算结果与现场实测热流及温度进行了对比分析。     

Effect of using a reflecting sheet in an air gap on the thermal performance of hollow walls and roofs

In this paper, the effect of placing a reflecting sheet in an air gap on the thermal performance of single hollow, double hollow and insulated hollow walls and roofs is studied. One face of the wall or roof is exposed to solar radiation and ambient air and the other is in contact with room air at constant temperature. These results are also compared with the predictions made in the case when a water film is maintained on the roof or wall. Numerical calculations are for a typical hot day (the 26th of May, 1979) in New Delhi. It is seen that the use of a reflecting sheet in single hollow and double hollow walls and roofs is more economical—and gives a better performance—than the water film system.     

Hourly vs daily method of computing insolation on inclined surfaces

Insolation on south-facing inclined planes has been computed using hourly values of total and diffuse radiation, obtained from experimental data. Such a computation procedure is then compared with the widely used method by Liu and Jordan for obtaining daily insolation on surfaces tilted toward the equator. Very small differences are noted between the results obtained by the two methods. These differences are mainly due to three factors; (a) Liu and Jordan's formulation uses a theoretical day-length while the hourly method uses day-length as incicated by the radiation data; (b) hourly method takes into account the asymmetries of total and diffuse radiation around solar noon while the daily method implicitly assumes symmetry of the same; (c) the daily method assumes uniform atmospheric transmissivity to beam radiation throughout the day. On the other hand, the hourly method assumes constant atmospheric transmissivity for one hour only.     

Thermal modelling of asymmetric overlap roof greenhouse with experimental validation

Global solar radiation availability model and thermal model for newly designed asymmetric overlap roof shape (AORS) greenhouse are presented and experimentally validated. Instantaneous solar radiation flux is utilised in a dynamic thermal model to ascertain the hourly plant and inside air temperature. The AORS is also compared with the previously developed two best greenhouse shapes. An experimental validation of both the models is carried out for the measured instantaneous solar radiation, plant and inside air temperature for a typical day in summer at Ludhiana (31°N and 77°E), Punjab, India. During the experimentation, a tomato crop was grown inside the greenhouse. From the results, it can be inferred that an east–west orientation AORS greenhouse should be preferred due to a lesser solar radiation capture in summer months. The predicted plant and air temperatures are in good agreement with the measured data having a root mean square error of 4.69 and 3.7, respectively.     

Properties and performance of advanced reflective paints to reduce the cooling loads in buildings and mitigate the heat island effect in urban areas

High reflective coatings and paints spread on the roof and walls can be very useful to reduce the cooling loads in buildings to ensure thermal comfort in the built environment. The solar reflectance of construction and cooling materials was measured with a spectrophotometer. A surface temperature monitoring campaign compared the thermal profiles of typical Italian construction materials with an innovative sustainable white paint, obtained with a special mixture of milk and vinegar of very high solar reflectance. Two building-integration cool-roof campaigns were run in the experimental building, Casa Intelligente of ENEA, in which indoor and outdoor air temperature and roof surface temperatures were monitored. This campaign, run in the summer of 2005 and 2006, allowed us to verify the influence of cool roofs to mitigate indoor air temperatures and to compare the behaviour of different cool-roof technologies.     

Modeling of the optimum tilt of a solar chimney for maximum air flow

The aim of this work is to develop a mathematical model to determine the tilt that maximizes natural air flow inside a solar chimney using daily solar irradiance data on a horizontal plane at a site. The model starts by calculating the hourly solar irradiation components (direct, diffuse, ground-reflected) absorbed by the solar chimney of varying tilt and height for a given time (day of the year, hour) and place (latitude). In doing so it computes the transmittance and absorbance of the glazing for the various solar irradiation components and for various tilts. The model predicts the temperature and velocity of the air inside the chimney as well as the temperatures of the glazing and the black painted absorber. Comparisons of the model predictions with CFD calculations delineate the usefulness of the model. In addition, there is a good agreement between theoretical predictions and experiments performed with a 1 m long solar chimney at different tilt positions.     

Solar and thermal energy gain through windows in Jordan

Solar gain and thermal energy transfer through windows is studied for three different sites in Jordan using the TRNSYS computer program. Solar and thermal energy is calculated using the monthly average daily data for the above-mentioned three regions. Calculation of hourly radiation on a vertical plane is presented, and also the method of determination of the amount of radiation transmitted through the glazing layers is given. The effect of window orientation on the total solar gain is analysed. It is found that for all directions, solar gain is season-dependent, and this dependency varies from one direction to another. Calculations are carried out for two cases of glazing location: case 1, glazing flush with the outside of the wall; and case 2, glazing recessed by 15 cm from the outside wall, which represents a window with overhang and sidewalls. The number of glazing layers is taken as 1, 2 and 3 to observe the effect on solar gain as well as on the thermal energy exchange between the inside and outside of the building. During the calculations, the temperature of the inside is fixed at 22°C for the entire year. The results are tabulated to serve as a database for solar and thermal energy in Jordan.     

Test reference year generation from meteorological and simulated solar radiation data

In this paper, a new method for generating test reference year (TRY) from the measured meteorological variables is proposed. Hourly recorded data of air temperature, relative humidity and wind velocity for two stations, Valladolid and Madrid (Spain) were selected to develop the method and a TRY was obtained. Monthly average solar radiation values were calculated taking into account the temperature and solar radiation correlations. Four different methodologies were used to evaluate hourly global solar radiation from hourly weather data of temperature and, as a consequence, four different TRYs with common data sets of temperature, relative humidity and wind velocity were generated for Valladolid and Madrid (Spain) stations. In order to evaluate the four different methodologies, TRYs data were compared with long-term measured data series using statistical estimators such as average, standard deviation, root mean square error (rmse) and mean bias error (mbe). Festa and Ratto and the TAG model, from Aguiar and Collares-Pereira, respectively, turned out to be the best methods for generating hourly solar irradiation data. The best performance was shown by the TRY0 year which was based on the solar radiation models mentioned above. The results show that the best reference year for each site varies with the season and the characteristics of the station.