A method of measuring sky temperature

For radiation heat exchange the sky is assumed to be a blackbody at some equivalent temperature known as the sky temperature. When correlating measured collector performance with meteorological data it is desirable to have complete information on all the meterological variables, one of which in the sky temperature.A new system for measuring sky temperature based on a heat flow meter has been constructed and calibrated. The heat flow meter is mounted in a constant velocity air stream to minimise variable convective heat transfer effects caused by the wind. The exposed surface of the heat flow meter, which contains a thermocouple embedded in the metal substrate for measuring surface temperature, is coated with a white paint having a low solar absorptance and a high thermal emittance. The lower polished metal surface of the heat flow meter faces a radiation shield.This paper presents the analysis on which the design was based together with methods of calibration and results of measurements made with the device. The results are encouraging and indicate that there is a need for further work to determine the absolute performance of similar devices over a wide range of conditions.     

New formulae for the equivalent night sky emissivity

The knowledge of the equivalent sky emissivity is important in computations of radiation cooling systems, as well as in the evaluation of the thermal losses in solar collectors. A new empirical formula has been developed which permits the determination of the equivalent nocturnal sky emissivity as a function of the site's altitude, the ambient temperature, the relative humidity and the degree of cloudiness, of mist or of haze present at the time. The formula has been found to agree well with the equivalent nocturnal sky temperature values measured at various Venezuelan sites and other places. Comparison is made with formulae from other investigators. A modification of Brunt's. Elsasser's and Angström's formulae for clear nocturnal sky is also suggested.     

About the equivalent radiative temperature for clear skies

The cold sky radiation constitutes a heat sink mainly used for passive cooling systems. The usefulness of this resource is commonly expressed in terms of the equivalent sky temperature T_c, whose accurate determination still remains as an interesting problem. Increasing the number and the diversity of the observations is necessary. An inexpensive pyrradiometer, made for that purpose, is described here. In addition, analysis of five years of observations with the Carpentras pyrradiometer (National Meteorology) lead us to propose a formulation for T_c which points out the influence of the boundary layer.     

Amplified radiative cooling via optimised combinations of aperture geometry and spectral emittance profiles of surfaces and the atmosphere

Net thermal radiation cooling, from surfaces at sub-ambient temperatures, to the night sky is amplified if the aperture to the sky is partially blocked with heat mirrors. The temperature at which radiation loss stagnates (the effective sky temperature) falls continuously as the aperture closes and is derived in terms of the aperture size and the spectral properties and temperatures of the atmosphere and of the emitting surface. Cooling surfaces must have high absorptance between 7.9 μm and 13 μm where the atmosphere is most transparent. The best response for the remainder of the Planck radiation spectrum surprisingly switches between two spectral extremes at a temperature which falls as the aperture gets smaller. A perfect absorber is best above this switch, while surfaces which reflect all of this radiation are best below it. A simple formula is presented for the cross-over temperature as a function of aperture size. With known material properties plus representative non-radiative heat gains a high emittance surface is generally better except when heat mirrors are not used. A known high emittance roof paint at 10° C below ambient, under a 45° aperture lined with shiny aluminium, can achieve a net output power near 135 W m⁻² under a clear sky.     

Practical procedure for predicting non-uniform temperature on the exposed face of arch dams

Temperature distribution on the exposed face (especially the downstream face) of arch dams is non-uniform, to a certain extent, due to the variation in solar radiation striking the surface. However, the temperature at the same elevation across the dam axis is generally assumed to be uniform in the design specifications for arch dams because there is no accepted procedure for defining the non-uniform temperature field. In this paper, a practical model for predicting the non-uniform temperature of the exposed face is presented by considering both solar radiation and shading effects. The ASHRAE clear sky model was adopted to calculate the solar radiation, and a ray-tracing algorithm was used to analyze the shade of surrounding terrain and the structure per se. The real-life case study presented in this study shows that the proposed model was effective in simulating and accessing reasonable thermal distribution. Subsequently, the case study also reveals that the non-uniform temperature had a significant effect on surface thermal stress and crack propagation.     

Building cluster and shading in urban canyon for hot dry climate: Part 1: Air and surface temperature measurements

Under low latitude conditions, minimization of solar radiation within the urban environment may often be a desirable criterion in urban design. The dominance of the direct component of the global solar irradiance under clear high sun conditions requires that the street solar access must be small. It is well known that the size and proportion of open spaces has a great influence on the urban microclimateThis paper is directed towards finding the interaction between urban canyon geometry and incident solar radiation. The effect of building height and street width on the shading of the street surfaces and ground for different orientations have been examined and evaluated. It is aimed to explore the extent to which these parameters affect the temperature in the street. This work is based on air and surface temperature measurements taken in different urban street canyons in EL-Oued City (hot and arid climate), Algeria.In general, the results show that there are less air temperature variations compared to the surface temperature which really depends on the street geometry and sky view factor. In other words, there is a big correlation between the street geometry, sky view factor and surface temperatures.     

POTSOL: Model to predict extraterrestrial and clear sky solar radiation

A model was developed to predict potential and clear sky solar radiation for any latitude. The model (POTSOL) uses the fundamental geometric relationships between the earth and sun to predict the theoretical solar radiation outside the earth's atmosphere, clear sky solar radiation received at the earth's surface after accounting for atmospheric interference, and clear sky solar radiation on a panel with any tilt angle between 0° and 90° from the horizontal. The only model input parameters are latitude (PHI), clearness number (CN), and panel tilt angle (PT). The model was verified using weather data obtained from the National Climatic Center, Asheville, North Carolina for Ely, Nevada.     

Estimation of dimensionless temperature distributions in spherical products during hydrocooling

Experiments and computations were performed for temperature distributions within a single spherical product cooled in water flow. Experimental center, half radius and surface temperature distributions of the individual tomatoes during cooling at a water temperature of 1°C and a flow velocity of 0.05 m/s were measured. The heat transfer coefficients determined by Dincer model for a single product were used in the computations. Comparisons between experimental results and theoretical predictions were observed in a very good agreement form.     

Field investigation of night radiation cooling under tropical climate

This paper discusses the feasibility of cooling by using night radiation under Thailand’s hot and humid climate. Four types of roof radiators were made by using common construction materials. They were examined under three sky conditions: clear, cloudy and rainy, respectively. Investigation was done, mainly, based on the temperature of different surfaces of a roof radiator. The experimental results showed that the depression of different surface temperatures is in the range of 1–6°C below ambient temperature under clear and cloudy skies. Under rainy skies, the temperature of different surfaces of roof radiators and ambient air was fairly close. Apart from sky conditions, the factors which affect the night radiation cooling are the thermal emissivity of materials and water condensation on the radiator surface area. Finally, this first study indicated that cooling by using night radiation is feasible mainly during tropical winter season.     

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.     

Solar radiation modelling in the urban context

This paper describes alternative methods for predicting surface irradiance in the urban context. In this the focus is on means of accounting for the effects of nearby obstructions on reducing direct sky radiation and on contributing reflected radiation. The first two methods involve abstracting the urban skyline into an effective canyon using isotropic and anisotropic tilted surface irradiance models. The third predicts the irradiance contribution from two hemispheres which are discretised into patches––given the radiance of the sky and dominant obstructions (if these exist) and associated view factors––so that we have a new simplified radiosity algorithm (SRA). Results from the three methods (isotropic canyon (IC), anisotropic canyon (AC) and simplified radiosity algorithm (SRA)) are compared with a ‘truth model' under the following circumstances: (i) unobstructed sky, (ii) sky obstructed by black surfaces, (iii) sky obstructed by grey diffusely reflecting surfaces. Results show conclusively that the SRA offers superior accuracy at comparable speed to the canyon models. The SRA also compares well with a ray tracing program, it can handle urban scenes of arbitrary geometric complexity and is readily amenable for inclusion into standard computer programs that require surface irradiance as an input.     

The thermal radiance of clear skies

Measurements of the longwave radiance of the sky were made during the summer of 1979 at Tucson, Arizona; Gaithersburg, Maryland; and St. Louis, Missouri. The global longwave radiation (wavelengths greater than 3 μm) was monitored with a pyrgeometer and the distribution of this radiation in several spectral bands at five different zenith angles was monitored with a spectral radiometer. This paper presents results for the global sky radiation during clear sky conditions. The spectral radiometer was used to calibrate the pyrgeometer and to detect the presence of clouds. The results can most appropriately be summarized in terms of the correlation between the global sky emissivity _sky and surface dewpoint temperature T_dp(°C). The global sky emissivity is defined as the ratio of sky radiance to σT_a⁴, where T_a is the absolute air temperature near the ground, and σ is the Stefan-Boltzmann constant. Based on 2945 night-time measurements in all three cities we find _sky=0.741 +0.0062T_dp with a standard error of estimate of 0.031. A similar relationship with almost identical coefficients holds during daylight hours.     

Evaluating the solar resource: a review of problems resulting from temporal, spatial and angular variations

Accurate assessment of the availability of solar energy at the Earth's surface is hampered by difficulties associated with the variability of solar radiation in space and time and with the angular position in the sky hemisphere. This paper assesses each of these problem areas by highlighting the difficulties and the attempts to offset them. Major emphasis is placed on extrapolation and interpolation procedures, the sensitivity of inclined surfaces to variations in the solar input, numerical models for calculating the irradiance of inclined surfaces and the time scales for which these computations are valid.Finally, the reader is urged to consider the significance of these difficulties in light of both radiation measurement errors and the sensitivity of the application to imprecise determinations of the radiation environment.     

Sky temperature modelisation and applications in building simulation

The sky temperature is an important parameter for simulation codes in building studies. A preliminary campaign of validation of a simulation software CODYRUN has demonstrated the misinterpretation of the radiative exchanges of long waves between the building and its environment. A bibliographical research has then led to the use models using dry air temperature to estimate sky temperature. However, these models has not been completely satisfactory as far as night clear sky are concerned. In this case, sky temperature remains overestimated. A research of a non linear model has been undertaken, leading to the use of neural networks with satisfactory results. Sky temperature is then calculated and reinjected into the simulation code. Comparison between simulated temperature and measures has turned to be acceptable.     

利用我国气象卫星遥感资料估算黄河源区净辐射辐照度的研究

以黄河源区为研究区域,利用我国静止气象卫星(FY-2D)及极轨气象卫星(FY-3A)资料,估算黄河源区一天中不同时刻的净辐射辐照度的空间分布。在晴天条件下,利用计算的向下太阳辐射照度,结合卫星遥感资料估算的地表反照率,得出地表太阳辐射收支。利用卫星遥感估算地表发射率和地表温度资料,结合地面观测站的气象观测资料,得到大气向下的长波辐射和地面向上的长波辐射值。在阴天条件下,借助FY-2D云顶的反照率资料,根据太阳辐射在大气中的衰减过程,推求透射比系数,进而得出阴天条件下地表净短波辐射照度。     

Méthode Approchée d'Estimation de I'lrradiation Solaire Journalière de Surfaces Réceptrices Orientées et Inclinées

La méthode proposée d'estimation approachée des irradiations journaliéres du rayonnement solaire total (direct, diffus et réfléchi) d'une surface réceptrice quelconque prend en considération, comme valeurs de base, les expositions énergétiques calculées pour des conditions de ciel serein, ainsi que les relations de corrélation liant les rayonnements globaux mesurés sur une surface horizontale à ces mêmes rayonnements par ciel serein et à la fraction d'ensoleillement. The proposed method for an approximate estimation of daily sums of irradiations of total solar radiation (direct, diffuse and reflected) on any oriented and inclined surface takes into consideration, as basic values, the corresponding irradiations calculated for clear sky conditions, together with the correlation relations between the global radiation measured on an horizontal surface and the same global radiation by clear sky and the ratio of duration of sunshine.     

Infrared sky radiation in Geneva

Continuous measurements of sky radiation have been performed in Geneva for more than two years. We present here some results, and we show that the radiative earth-sky deficit is correlated to the clearness of the day as evaluated from the amount of solar radiation reaching the ground. The corresponding model allows to determine the sky temperature within 2–3°C, with respect to ambient temperature, diffuse and global solar radiation. These results can be of help when computing energy balances involving sky radiation, especially in buildings and solar energy fields.     

Cloud classification in a mediterranean location using radiation data and sky images

Knowledge regarding the solar radiation reaching the earth’s surface and its geographical distribution is very important for the use of solar energy as a resource to produce electricity. Therefore, a proper assessment of available solar resource is particularly important to determine the placement and operation of solar thermal power plants. To perform this analysis correctly, it is necessary to determine the main factors influencing the radiation reaching the earth’s surface, such as the earth’s geometry, terrain, and atmospheric attenuation by gases, particles and clouds. Among these factors, it is important to emphasise the role of clouds as the main attenuating factor of radiation. Information about the amount and type of clouds present in the sky is therefore necessary to analyse both their attenuation levels and the prevalence of different sky conditions. Cloud cover is characterised according to attenuation levels, using the beam transmittance (k_b, ratio of direct radiation incident on the surface to the extraterrestrial solar radiation) and hemispherical sky images. An analysis of the frequency and duration of each type of cloud cover blocking the sun’s disk is also performed. Results show prevailing sky situations that make the studied area very suitable for the use of solar energy systems.     

Experimental testing of models for the estimation of hourly solar radiation on vertical surfaces at Arcavacata di Rende

More than 55,000 data of hourly solar radiation on a horizontal surface and on vertical surfaces exposed to the south, west, north and east, measured at Arcavacata di Rende (CS), were compared with hourly radiation data calculated by various calculation models.Erbs, Reindl et al. and Skartveit et al. correlations for the split of hourly global radiation in the diffuse and beam components were used together with the isotropic sky model and three anisotropic sky models.The agreement between experimental and calculated data is generally good.     

An investigation into sky temperature estimation,its variation,and significance in heat transfer calculations of solar cookers

The determination of sky temperature assumes great importance in engineering applications such as radiative cooling of buildings. Many studies that involve a radiative exchange with the sky employ different reported models of sky temperature interchangeably. However, until now, hardly any systematic study has been done to quantify the errors/variations that might be encountered in calculating this radiative exchange employing these different correlations. In the current paper, first, a thorough analysis has been presented on the sky temperature correlations and a possible range of variation in sky temperature based on the estimation of sky emissivity is computed. Both diurnal‐nocturnal variation in sky temperature and seasonal disparities in sky temperature have been reported. Next, the case of a box‐type solar cooker has been taken up for investigation with respect to the possible influence of the sky temperature estimation in predicting its performance parameter, first‐figure‐of‐merit on a daily, seasonal, and climatic basis. Our observations show an enormous difference in sky temperature depending upon the expressions of emissivity from which it is derived. The variability of sky temperature has a nominal influence on the prediction of first‐figure‐of‐merit, although a marked discrepancy is observed across the seasons at the same location.