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Abstract In this work we review and recalibrate existing models, and present a novel comprehensive model for estimation of the downward atmospheric longwave (LW) radiation for clear and cloudy sky conditions. LW radiation is an essential component of thermal balances in the atmosphere, playing also a substantial role in the design and operation of solar power plants. Unlike solar irradiance, LW irradiance is not measured routinely by meteorological or solar irradiance sensor networks. In most cases, it must be calculated indirectly from meteorological variables using simple parametric models. Under clear skies, fifteen parametric models for calculating LW irradiance are compared and recalibrated. All models achieve higher accuracy after grid search recalibration, and we show that many of the previously proposed LW models collapse into only a few different families of models. A recalibrated Brunt-family model is recommended for future use due to its simplicity and high accuracy (rRMSE = 4.37%). To account for the difference in nighttime and daytime clear-sky emissivities, nighttime and daytime Brunt-type models are proposed. Under all sky conditions, the information of clouds is represented by cloud cover fraction (CF) or cloud modification factor (CMF, available only during daytime). Three parametric models proposed in the bibliography are compared and calibrated, and a new model is proposed to account for the alternation of vertical atmosphere profile by clouds. The proposed all-sky model has 3.8–31.8% lower RMSEs than the other three recalibrated models. If GHI irradiance measurements are available, using CMF as a parameter yields 7.5% lower RMSEs than using CF. For different applications that require LW information during daytime and/or nighttime, coefficients of the proposed models are corrected for diurnal and nocturnal use.

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 ϵsky and surface dewpoint temperature Tdp(°C). The global sky emissivity is defined as the ratio of sky radiance to σTa4, where Ta 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 ϵsky=0.741 +0.0062Tdp with a standard error of estimate of 0.031. A similar relationship with almost identical coefficients holds during daylight hours.

Abstract The interaction between air pollutants and solar radiation is discussed in terms of three examples: measurements of the direct solar beam to determine atmospheric turbidity; the role of solar radiation in initiating photochemical smog processes; and urban-rural differences of incident solar energy. Turbidity measurements from several non-urban stations, particularly in the south-central and south-eastern U.S., show a slight increase during the past 10–15 yr, with most of the change occurring during summer. Measurements at certain urban stations indicate a decreasing turbidity trend. Coincident measurements of incident solar radiation at urban and rural sites are presented for St. Louis and Los Angeles. The urban-rural differences of incident total and ultraviolet (UV) solar energy ranged from near zero to more than 50 per cent. The solar depletion was much larger over Los Angeles than St. Louis. The impact of solar radiation on photochemical smog over Los Angeles is shown be discussing the sensitivity of ozone concentrations to variations of UV radiation. The results of a numerical photochemical diffusion model indicate that the seasonal decrease of UV energy from July to November has a substantial effect on ozone levels, especially over the western part of the Los Angeles Basin.

Abstract This paper focuses on theoretical and experimental analysis used to establish the limiting heat flux for passively cooled thermoelectric generators (TEG). 2 commercially available TEG’s further referred as type A and type B with different allowable hot side temperatures (150 °C and 250 °C respectively) were investigated in this research. The thermal resistance of TEG was experimentally verified against the manufacturer’s specifications and used for theoretical analysis in this paper. A theoretical model is presented to determine the maximum theoretical heat flux capacity of both the TEG’s. The conventional methods are used for cooling of TEG’s and actual limiting heat flux is experimentally established for various cold end cooling configurations namely bare plate, finned block and heat pipe with finned condenser. Experiments were performed on an indoor setup and outdoor setup to validate the results from the theoretical model. The outdoor test setup consist of a fresnel lens solar concentrator with manual two axis solar tracking system for varying the heat flux, whereas the indoor setup uses electric heating elements to vary the heat flux and a low speed wind tunnel blows the ambient air past the device to simulate the outdoor breezes. It was observed that bare plate cooling can achieve a maximum heat flux of 18,125 W/m2 for type A and 31,195 W/m2 for type B at ambient wind speed of 5 m/s while maintaining respective allowable temperature over the hot side of TEG’s. Fin geometry was optimised for the finned block cooling by using the fin length and fin gap optimisation model presented in this paper. It was observed that an optimum finned block cooling arrangement can reach a maximum heat flux of 26,067 W/m2 for type A and 52,251 W/m2 for type B TEG at ambient wind speed of 5 m/s of ambient wind speed. The heat pipe with finned condenser used for cooling can reach 40,375 W/m2 for type A TEG and 76,781 W/m2 for type B TEG.

Abstract As solar radiation and the weather appear to be affected by identical periods of variation, it is therefore likely that weather changes are produced by solar variation, and can be predicted when the periods are known. Forecasts for precipitation and temperature at St. Louis and Peoria are compared graphically with actual weather data, based on 5-month smoothed running means and extending from 1854 to 1939, as evidence of the author's thesis.

Abstract While pavements are currently a significant contributor to the urban heat island effect, modified pavements could play a role in cooling cities. Specifically, the radiative and thermal properties of pavements can be manipulated to alter the timing and magnitude of sensible heat storage and release into the urban airshed. The present study developed and applied a one-dimensional heat transfer model to Phoenix AZ as a case study to explore the potential for modified pavements in a hot desert climate. We explored solar reflective (0.35 albedo) and traditional (0.10 albedo) asphalt pavements. Pavement thermal conductivity and heat storage capacity values were varied across a reasonable range based on existing products. The results from this study show that daytime peak surface temperature of highly conductive (2.4 WK-1m−1) and thermally massive pavements (4.0 MJK-1m−3) is 17.5 °C cooler than less conductive (0.4 WK-1m−1) and thermally light (1.0 MJK-1m−3) pavements. However, at night, the more thermally massive and conductive pavements were as much as 10.8° C warmer than the less conductive and less massive counterpart. Therefore, when the focus is on reducing excess urban heat during the day, it is better to use pavement materials with higher conductivity and thermal storage. However, when the focus is on reducing the nocturnal heat island, it is wise to use lower conductivity and lower thermal storage pavements. Regardless of thermal properties, the use of reflective pavement surfaces will mitigate both the daytime and nocturnal urban heat island effect. We find up to a 10° C reduction in peak daytime surface temperature and a modest reduction of nocturnal temperatures (by up to 1.0° C) when pavement albedo is increased by 0.25. However, reflective pavements result in a higher influx of net radiation for pedestrians walking on these surfaces during the daytime. So, reflective pavement projects should be evaluated for potential adverse impacts on thermal comfort. Nevertheless, depending upon the time of day and the overall solar reflectivity of pedestrian clothing and skin, we argue that the benefits of reduced surface and air temperatures associated with highly reflective paving likely outweigh the adverse effects for pedestrians. However, for regions where daytime thermal comfort is of primary concern, pavements with higher thermal conductivity and thermal storage may perform better than reflective pavements.

Abstract A study on a novel photo-bio screen (PBS) used as a shading system in a real building is presented. The green microalgal culture (Scenedesmus obliquus) of the PBS allows a screening of the direct sunlight and a production of biomass containing bioactive compounds. The PBS was tested in a kindergarten classroom at Saint Marcel (Aosta Valley, north-west of Italy) and monitored for 3 weeks (June-July 2016). The visible transmittance Tv of PBS was determined through in situ illuminance measurements, while the daylight amount in the room and the energy demand for lighting EDl were calculated through Diva-for-Rhino simulations (using the median measured Tv as input). The analysis was split in two phases: (i) the real room (with south-facing windows and external obstructions); (ii) the same room without obstructions, analyzed parametrically by changing the site (Turin, Ostersund, Athens, and Abu Dhabi) and the orientation (south, west, north, and east). For both phases, the results for PBSs were compared to what obtained applying a traditional venetian blind VB of comparable light transmission to the window. From the monitoring campaign, the Tv of the PBS was found to have a quite high variation as a function of the dynamic boundary conditions, so an median value of 0.75 was identified as the reference Tv. From simulations, it was found that the daylight amount and the EDl for PBS and the VB were comparable, with slightly better results for the PBS in Turin and Athens and slightly better results for the VB in Ostersund and Abu Dhabi.

Abstract The Kyoto Protocol to the Climate Change Convention sets out legally binding emission targets and timetables for developed countries. In order to ease compliance, it allows countries to achieve their emission targets through the ‘Kyoto Mechanisms’. These mechanisms comprise International Emissions Trading (ET), Joint Implementation (JI), and a Clean Development Mechanism (CDM). This paper analyses the capacity of the proposed mechanisms of the Kyoto Protocol to promote investment in renewable energy technologies (RETs). Analysis of abatement costing studies indicates that the increasing use of renewable energy tends to be a higher cost option compared with other greenhouse gas (GHG) abatement technologies. This finding, however, does not make RETs unattractive for GHG mitigation as such because, apart from their vast technical potential to reduce GHG emissions, RETs have great capacity to contribute to other aspects of sustainable development. The extent of investment into renewable energy induced by the Kyoto mechanisms will depend on whether the rules and guidelines that are to be developed in the coming years will explicitly support renewables. The Kyoto mechanisms could be instrumental in leading to significant investment into these resources if rules are defined appropriately.

Abstract Solar irradiance nowcasts can be derived with sky images from all sky imagers (ASI) by detecting and analyzing transient clouds, which are the main contributor of intra-hour solar irradiance variability. The accuracy of ASI based solar irradiance nowcasting systems depends on various processing steps. Two vital steps are the cloud height detection and cloud tracking. This task is challenging, due to the atmospheric conditions that are often complex, including various cloud layers moving in different directions simultaneously. This challenge is addressed by detecting and tracking individual clouds. For this, we developed two distinct ASI nowcasting approaches with four or two cameras and a third hybridized approach. These three systems create individual 3-D cloud models with unique attributes including height, position, size, optical properties and motion. This enables us to describe complex multi-layer conditions. In this paper, derived cloud height and motion vectors are compared with a reference ceilometer (height) and shadow camera system (motion) over a 30 day validation period. The validation data set includes a wide range of cloud heights, cloud motion patterns and atmospheric conditions. Furthermore, limitations of ASI based nowcasting systems due to image resolution and image perspective constrains are discussed. The most promising system is found to be the hybridized approach. This approach uses four ASIs and a voxel carving based cloud modeling combined with a cloud segmentation independent stereoscopic cloud height and tracking detection. We observed for this approach an overall mean absolute error of 648 m for the height, 1.3 m/s for the cloud speed and 16.2° for the motion direction.