Effective Sky Temperature Research Articles

A novel approach based on equivalent sky radiative temperature for quick computation of radiative cooling in building energy simulation

Passive radiative cooling materials (RCM) can cool surfaces by radiating heat to outer space via long-wave radiation. In simulation application, co-simulation of the thermal process between RCMs and building structures is generally required; however, the high computing time cost and low integration level have hindered the integrated whole-building simulation in architectural design. Traditional building energy simulations utilize the effective sky temperature to numerically approximate the effect of sky radiation. However, given the limitations of effective sky temperature concept, it is more suitable to evaluate diffusive gray surfaces while not fully capturing the characteristics of radiative heat transfer for spectrum-selective surfaces. In this study, an innovative equivalent-sky-radiative temperature was introduced as a new equivalent parameter to accurately simulate the thermal performance of RCMs in buildings. Embedding this new parameter into whole-building energy simulation tools such as EnergyPlus and DeST is expected to significantly improve the calculation accuracy and reduce the computational cost. Case studies in this work have shown an average error of 5.47 W/m2. The generation of the new parameter for 50 cities worldwide also demonstrated the technical feasibility of applying the equivalent-sky-radiative temperatures to evaluate the thermal performance and energy savings of real-world RCMs in buildings.

Sub-ambient radiative cooling and its application in buildings

Radiative cooling can effectively reduce energy consumption for building applications. As a passive cooling technology, a radiative cooling system radiates heat into space via infrared radiation whenever the effective sky temperature is colder than the body surface. Although radiative cooling has been proposed for many years, its application is limited to nighttime operation due to the constraints of the materials and diurnal radiative cooling technology. The radiative cooling surfaces recently developed, which can produce approximately 100 W/m2 average daily cooling power, are perfectly applicable for employment in a passive cooling system during the day. This paper reviews the development of radiative cooling techniques and their application in buildings. The existing models for the heat balance of radiative cooling systems are introduced, and the contributions of solar radiation, forced convection, and atmospheric conditions are also discussed in detail. Recent advancements in diurnal cooling approaches and associated radiative cooling surfaces are outlined, and the application prospects are analyzed, accounting for the energy saving potential. In addition, several feasible radiative cooling systems are proposed in this study.

Monitoring the Effective Ambient and Sky Temperature Based on Infrared Sensor for Advanced Thermal Calculations

Building performance simulations and advanced thermal analysis are becoming the basis for a well-established practice of a building sector. This approach requires many input data which are typically not available on site. Apart from already well-practiced climate variable quantities, such as ambient temperature, solar radiation and parameters of wind, more complex data are needed for advanced building thermal analysis. One of those is based on longwave radiation level. A pyrgeometer is a device that measures longwave radiation part of whole thermal radiation phenomena. This can be determined based on sky or effective ambient temperature monitoring. Secondly, both variables might be approximated by infrared sensors as an applicable option in the calculation of longwave radiation heat exchange between the external surface and the ambient building environment. The paper presents data obtained both by pyrgeometer and infrared sensor corresponding to their mutual comparison to demonstrate its application when longwave radiation exchange needs to be calculated or analyzed in advanced. Integrating of the infrared sensor with aim to monitor the effective ambient and/or sky temperature enables its applicability as an alternative, integrated and less cost consuming method towards the monitoring by commercial pyrgeometer.

A comparative study of the thermal performance of primary mirror at the four typical sites

We propose a heat dissipation model to assess whether the material and thickness of the primary mirror of a telescope design exceed the upper constraint of the requirement in terms of the four typical sites: Dome A of Antarctica, Ali of Tibet, Mauna Kea of Hawaii and Chajnantor of Chile. It accounts for thermal conduction, heat convection, and radiation heat transfer whose effective sky temperature depends on the ambient air temperature, vapor pressure, and relative humidity. In addition, the values of the heat convection coefficient, sharply decreasing due to the low atmospheric pressure, are considered. We apply the model to analyze the thermal performance of a 2.5 m primary mirror of a wide field survey telescope (WFST). Under the conditions of the four typical sites in the world, a comparative study of WFST for two thicknesses of the primary mirror is presented via steady thermal analysis and transient thermal analysis using the proposed model. The results reveal that all RMS errors on the mirror surface of primary mirrors are smaller than 5 nm, which fulfill the performance requirements of WFST, and demonstrate that the proposed model is useful for evaluating the heat dissipation of the primary mirror on those sites.

Effect of clouds and dust storms on the sky radiation exchange for buildings located in hot–dry climates

This article evaluates the impact of effective sky temperatures on building radiation exchange under clear, cloudy, and dusty conditions for extremely hot and dry climates. In part, a dusty sky temperature model has been introduced as a function of atmospheric aerosol optical depth. The sky radiative exchange was evaluated using a one-dimensional transient heat transfer model with numerical calculations performed using the fully implicit finite-difference method. The newly available ASHRAE 2013 clear sky model was evaluated and implemented to calculate the hourly incident solar radiation for a horizontal roof under the extremely hot–dry climate conditions of Riyadh, Saudi Arabia. Results showed that in clear sky conditions, sky longwave radiation contributes to a reduction of the total heat gain. A daily mean clear sky cooling around 2645 and 2385 W-hr/m2 was estimated for July and January, respectively. In contrast, cloud and dust covers increase effective sky temperature and diminish the role of sky radiative cooling. Depending on severity, the mean contributed sky cooling heat exchange was found to range between 436 and 1636 W-hr/m2 for dust storm and scattered cloudy sky conditions, respectively. Similarly, the ASHRAE 2013 clear sky model and the sky temperature models were shown for four other extremely hot–dry global sites.

The effective sky temperature: an enigmatic concept

To properly simulate the condensation process and design a water recovery system from condensation of atmospheric water vapour on a surface maintained below the dew point by radiative heat loss from the surface to night sky, an accurate estimation of the effective sky temperature is required. To estimate the effective night sky temperature, an experimental system consisting of a series of metal plates embedded on a heat transfer panel, a weather station and a control system was used. The results obtained from theoretical analyses and preliminary experiments are presented and discussed. A special emphasis is given to a mathematical solution of the system of equations that need to be solved to obtain the effective sky temperature. It is shown that although the system of equations works well for the direct heat transfer problem, there is a serious difficulty to solve the inverse heat transfer problem to retrieve the desired parameters.

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.

Calibrating pyrgeometers outdoors independent from the reference value of the atmospheric longwave irradiance

In this article, we describe a method for the calibration of thermopile pyrgeometers in the absence of a reference for measurement of atmospheric longwave irradiance. This is referred to as the incoming longwave irradiance in this article. The method is based on an indoor calibration using a low-temperature blackbody source to obtain the calibration coefficients that determine the pyrgeometer's radiation characteristics. From these coefficients the outgoing irradiance of the pyrgeometer can be calculated. The pyrgeometer is then installed outdoors on an aluminum plate that is connected to a circulating temperature bath. By adjusting the temperature bath to the approximate value of the effective sky temperature, the pyrgeometer's body temperature is lowered changing the pyrgeometer's thermopile output. If the incoming longwave irradiance is stable, the slope of the outgoing irradiance versus the pyrgeometer's thermopile output is the outdoor net irradiance responsivity (RS net), independent of the absolute value of the atmospheric longwave irradiance. The indoor calibration coefficients and the outdoor RS net are then used in the pyrgeometer equation to calculate the incoming longwave irradiance. To evaluate this method, the calculated irradiance using the derived coefficients was compared to the irradiance measured using a pyrgeometer with direct traceability to the World Infrared Standard Group (WISG). This is maintained at the Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Switzerland. Based on results from four pyrgeometers calibrations, this method suggests measurement agreement with the WISG to within ±3 W/m 2 for all sky conditions.

Influence of site dependent effects on the assessment of the radiative cooling potential

The assessment the radiative cooling potential requires the knowledge of some site dependent boundary conditions affecting to the performance of the respective technological systems. These boundary conditions are mainly related to the existence of certain climatological solicitations that will determine the feasibility or not for each application. This paper presents a contribution to a more accurate assessment of the radiative cooling potential based on the analysis of the influence in the estimation of effective sky temperature and effective sky temperature depression of local or transient effects associated to the existence of a non standard vertical atmospheric structure. These eventual deviations are usually not considered in the conventional estimation models, based exclusively on surface information, and the suitability of the proposed approach is established by comparison with measured data.

Enhanced solar still performance using a radiative cooling system

A basin type solar still is integrated with a packed bed storage tank which is used as an external condenser for the still. The packed bed condenser is cooled during the night using a radiative cooling panel by circulating pure water into the packed bed condenser and the radiative cooling panel. At the end of the cooling process, the packed bed tank attains low temperature, which is very close to the effective sky temperature. At the begining of daylight, the vapor produced by the solar still is sucked naturally by the packed bed condenser and condenses within it. A mathematical model describing the behavior of the modified still is proposed. The effects of different designs, climate and operating parameters on the still performance are investigated.

A THEORETICAL AND EXPERIMENTAL INVESTIGATION OF A RADIATIVE COOLING SYSTEM

A radiative cooling system is built in Irbid-Jordan and utilized to cool and store fluids by direct radiation to the low effective sky temperature. The radiative cooling system is built to verify the validity of a mathematical model proposed to describe the system dynamic thermal behavior. The constructed radiative system has the ability to create a 15°C reduction in the mean temperature of a 120 l capacity storage tank using 0.6 m 2 radiative cooling panel over one night and under climate conditions that vary within the range of 28–55% for relative humidity and 16–27°C for ambient temperature, which are the average spring conditions of Irbid-Jordan (Latitude=32.5°N, Longitude=35.5°E). This implies that the system can emit 13 MJ/(m 2.night) to the sky. It is found that the theoretical and the experimental predictions of the proposed model are in a qualitative good agreement.

Nocturnal radiation measurements in Nigeria

Experimental work was carried out to determine the net flux of the long-wave radiation from the earth's surface to the atmosphere for clear nights in Nsukka, Nigeria. Values of the effective sky temperature were also obtained. The experimental radiator consisted of a flat mild steel plate coated with high emissivity black enamel paint. Data were obtained for the months of February through May. This seasonal period is free from the harmattan dust haze which characterizes part of the dry season and is free from the rain-bearing clouds of the wet season. A comparison of the measured downward flux of the atmospheric radiation was made with the data generated from the empirical formulas of some investigators using standard surface observations. Generally, the theoretical models yielded values that are lower than the observed results. However, the analytical values derived from Idso and Jackson's empirical model gave the best agreement with the experimental data. Agreement was within 0–5 per cent. The effective sky temperatures obtained are on the average 12.2°C less than the surface air temperatures. This was in close agreement with the result of Exell for the tropical climate of Thailand. The overall results presented are suitable for use in Nigeria and other regions with similar tropical or near-equatorial climate.

A “thermometer” for effective sky temperature

The measurement of sky emissivity or effective sky temperature is essential toward assessing the potential for radiative cooling. Such measurements have been made with radiometers[l,2] for a variety of locations throughout the United States. On summer nights, for example, desert skies can have effective sky temperatures as much as 35K below ambient dry bulb temperature[3], which attests to the impressive potential for radiative cooling. It is the purpose of this paper to propose a simple experimental technique for the determination of effective sky temperature-a method that requires a relatively unsophisticated and inexpensive apparatus. This method is based on elementary theoretical considerations of heat exchange between a radiator and the nocturnal sky. The nighttime heat exchange between a radiator at temperature T and its surroundings can be expressed by

Effective sky temperatures for several Martian atmospheric models

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Performance correlation of horizontal plastic solar water heaters

Statistical and experimental methods are used to determine the optical efficiency and the heat lost by radiation and convection in a horizontal plastic solar hot water heater. The heater consisted of a water filled polyethylene bag with a clear top and black bottom. It was insulated on the sides and bottom by styrofoam and covered on the top with one to three layers of clear Tedlar. The effective sun angle, effective sky temperature, ambient temperature and the number of covers are included as variables. The upward heat losses were higher than those predicted by the heat transfer coefficients of Hottel and Woertz.

Absorption and emission in the 8-millimeter region by ozone in the upper atmosphere

Abstract : The absorption of solar radiation and the emission from the atmosphere (effective sky temperature) were measured over the ranges of the millimeter region corresponding to rotational lines of ozone centered at 30,056 Mhz, 36,025 Mhz, and 37,830 Mhz. The measurements were made possible by the special development of a frequency-switching radiometer and a gain-compensation technique. The contributions of ozone to atmospheric absorption and emission are evaluated from the solution of the radiative transfer equation on the basis of a line-width function of temperature and pressure and the ARDC 1959 Standard Atmosphere; the contributions of the water vapor and the oxygen are determined from the theories of Barrett and Chung for H2O and Meeks for O2. The intensity of the 37,830-Mhz line calculated by Gora was verified in a laboratory study. The width of this line was also found to coincide with the value 2340 Mhz at standard atmospheric conditions as derived by Walshaw from the 9.6-micron ozone band. Both emission and absorption of the atmospheric ozone lines were detected, with the line strengths in reasonable agreement with the model calculation. (Author)