Diffuse Radiation Research Articles

Typical and extreme weather datasets for studying the resilience of buildings to climate change and heatwaves

We present unprecedented datasets of current and future projected weather files for building simulations in 15 major cities distributed across 10 climate zones worldwide. The datasets include ambient air temperature, relative humidity, atmospheric pressure, direct and diffuse solar irradiance, and wind speed at hourly resolution, which are essential climate elements needed to undertake building simulations. The datasets contain typical and extreme weather years in the EnergyPlus weather file (EPW) format and multiyear projections in comma-separated value (CSV) format for three periods: historical (2001–2020), future mid-term (2041–2060), and future long-term (2081–2100). The datasets were generated from projections of one regional climate model, which were bias-corrected using multiyear observational data for each city. The methodology used makes the datasets among the first to incorporate complex changes in the future climate for the frequency, duration, and magnitude of extreme temperatures. These datasets, created within the IEA EBC Annex 80 “Resilient Cooling for Buildings”, are ready to be used for different types of building adaptation and resilience studies to climate change and heatwaves.

Assessing the Effectiveness of Reflective and Diffusive Polyethylene Films as Greenhouse Covers in Arid Environments

The application of diffusive and reflective polyethylene (PE) films as greenhouse coverings in arid climates presents an opportunity to improve the microclimate of the greenhouse and achieve consistent light distribution within the crop canopy. Nevertheless, there is still a lack of understanding regarding the properties of these covers and their impact on the microclimate and the growth parameters of crops. This study aimed to assess the impact of different covers on the diffusion of beam radiation during transmission, microclimatic parameters, and growth parameters of cucumbers in each of the greenhouses they covered. In the study, three PE covers were evaluated: a reflective cover (RC), a diffusive film (DC), and a locally produced cover (LPC) as the control treatment. The covers were installed on three identical, single-span, evaporatively cooled greenhouses named GH1/LPC, GH2/RC, and GH3/DC, which were utilized for cultivating cucumber crops. The results indicated that the diffusive nature of the tested films increased the ratio of diffuse to global solar radiation (D/G) from 0.22 outside the greenhouses to 0.49, 0.42, and 0.41 inside GH1/LPC, GH3/DC, and GH2/RC, respectively. Similarly, the ratio of diffuse to direct beam radiation (D/B) showed an increase, with values of 0.95, 0.70, and 0.68 inside GH1/LPC, GH3/DC, and GH2/RC, respectively, compared to the outside value of 0.28. The DC used in GH3 showed a favorable microclimate by reducing the air temperature and improving the relative humidity. Accordingly, the vegetative growth of the cucumbers was significantly improved in GH3/DC, reflected in increases in their biomass, followed by GH2/RC and GH1/LPC. The highest crop yield (p ≤ 0.05) of 12.3 kg/m2 was achieved in GH3/DC, followed by 10.2 kg/m2 in GH2/RC and 10.1 kg/m2 in GH1/LPC. Interestingly, the LPC not only stood out as a low-cost option but also displayed excellent diffusive–radiative properties, and demonstrated reasonable growth development and productivity for the cucumber crops. Consequently, the LPC emerges as a practical and cost-effective greenhouse covering material for crop production in arid climates.

Spatiotemporal diffusion metamaterials: Theories and applications

Diffusion metamaterials with artificial spatial structures have significant potential in controlling energy and mass transfer. Those static structures may lead to functionality and tunability constraints, impeding the application scope of diffusion metamaterials. Dynamic structures, adding the temporal dimension, have recently provided a new possibility for electric charge and heat diffusion regulation. This perspective introduces the fundamental theories and practical constructions of spatiotemporal diffusion metamaterials for achieving nonreciprocal, topological, or tunable properties. Compared with traditional static design, spatiotemporal modulation is promising to manipulate diffusion processes dynamically, with applications of real-time thermal coding and programming. Existing spatiotemporal diffusion explorations are primarily at macroscopic systems, and we may envision extending these results to microscale and other physical domains like thermal radiation and mass diffusion shortly.

Integrating daylight with solar thermal modeling: A dynamic calculation method for complex fenestration systems

This study developed a mathematical model named the DL-E model, which links annual dynamic daylight and solar heat gain calculations for Complex Fenestration Systems (CFSs). The DL-E model utilizes the Klems Bidirectional Scattering Distribution Function (BSDF) to characterize CFSs and can consider the shading effect of obstacles, as well as solar radiation reflected from obstacles and the ground. The DL-E model uses the Perez all-weather model to calculate transmitted solar radiation contributed by diffuse solar radiation, rather than the simplified sky model used in EnergyPlus. Results calculated with the DL-E were compared with those computed with EnergyPlus, and it was found that for most of the results, the both methods yielded closely aligned outcomes. However, for the diffuse transmitted solar radiations, reflected solar radiation from the ground in the case with a vertical obstacle, and the reflected solar radiations from the obstacle, the values calculated with DL-E showed significant differences compared to those calculated with EnergyPlus due to the use of the simplified sky model in EnergyPlus. In contrast, the values calculated with the DL-E model which used the Perez all weather sky model displayed very small difference compared to those calculated with Radiance, which means the values derived with EnergyPlus may produce significant errors. Additionally, a new workflow has been developed for calculating building load and energy for CFSs to avoid the manual conversion of the BSDF files.

RHDLPP: A multigroup radiation hydrodynamics code for laser-produced plasmas

In this paper, we introduce the RHDLPP, a flux-limited multigroup radiation hydrodynamics numerical code designed for simulating laser-produced plasmas in diverse environments. The code bifurcates into two packages: RHDLPP-LTP for low-temperature plasmas generated by moderate-intensity nanosecond lasers, and RHDLPP-HTP for high-temperature, high-density plasmas formed by high-intensity laser pulses. The core radiation hydrodynamic equations are resolved in the Eulerian frame, employing an operator-split method. This method decomposes the solution into two substeps: first, the explicit resolution of the hyperbolic subsystems integrating radiation and fluid dynamics; second, the implicit treatment of the parabolic part comprising stiff radiation diffusion, heat conduction, and energy exchange. Laser propagation and energy deposition are modeled through a hybrid approach, combining geometrical-optics ray-tracing in sub-critical plasma regions with a one-dimensional solution of the Helmholtz wave equation in super-critical areas. The thermodynamic states are ascertained using an equation of state, based on either the real gas approximation or the quotidian equation of state (QEOS). For ionization calculations, the code employs a steady-state collisional-radiation (CR) model using the screened-hydrogenic approximation. Additionally, RHDLPP includes RHDLPP-SpeIma3D, a three-dimensional spectral simulation post-processing module, for generating both temporally-spatially resolved and time-integrated spectra and imaging, facilitating direct comparisons with experimental data. The paper showcases a series of verification tests to establish the code's accuracy and efficiency, followed by application cases, including simulations of laser-produced aluminium (Al) plasmas, pre-pulse-induced target deformation of tin (Sn) microdroplets relevant to extreme ultraviolet lithography light sources, and varied imaging and spectroscopic simulations. These simulations highlight RHDLPP's effectiveness and applicability in fields such as laser-induced breakdown spectroscopy, extreme ultraviolet lithography sources, and high-energy-density physics.

Artificial ground reflector size and position effects on energy yield and economics of single‐axis‐tracked bifacial photovoltaics

AbstractArtificial ground reflectors improve bifacial energy yield by increasing both front and rear‐incident irradiance. Studies have demonstrated an increase in energy yield due to the addition of artificial reflectors; however, they have not addressed the effect of varying reflector dimensions and placement on system performance and the impact of these parameters on the reflectors' financial viability. We studied the effect of high albedo (70% reflective) artificial reflectors on single‐axis‐tracked bifacial photovoltaic systems through ray‐trace modeling and field measurements. In the field, we tested a range of reflector configurations by varying reflector size and placement and demonstrated that reflectors increased daily energy yield up to 6.2% relative to natural albedo for PERC modules. To confirm the accuracy of our model, we compared modeled and measured power and found a root mean square error (RMSE) of 5.4% on an hourly basis. We modeled a typical meteorological year in Golden, Colorado, to demonstrate the effects of artificial reflectors under a wide range of operating conditions. Seventy percent reflective material can increase total incident irradiance by 1.9%–8.6% and total energy yield by 0.9%–4.5% annually after clipping is considered with a DC–AC ratio of 1.2. Clipping has a significant effect on reflector impact and must be included when assessing reflector viability because it reduces reflector energy gain. We calculated a maximum viable cost for these improvements of up to $2.50–4.60/m2, including both material and installation, in Golden. We expanded our analysis to cover a latitude range of 32–48°N and demonstrated that higher‐latitude installations with lower energy yield and higher diffuse irradiance content can support higher reflector costs. In both modeling and field tests, and for all locations, the ideal placement of the reflectors was found to be directly underneath the module due to the optimized rear irradiance increase.

The combined effect of diffuse radiation and leaf wetness on functional traits and transpiration efficiency on a cloud forest species.

Cloud forests are unique biomes that thrive in foggy environments for a substantial part of the season. Fog in cloud forests plays two critical roles: it reduces incoming radiation and creates a humid environment, leading to the wetting of the canopy. This paper aims to investigate the combined effect of both radiation and wetness on Myrica faya Wilbur-a cloud forest species present in subtropical regions-both directly in plants and through simulations. Experiments consisted of a controlled environment with two levels of radiation and leaf wetness: low radiation/wet conditions, and high radiation/no-wetness; and three treatments: continuous low radiation and wetness, continuous high radiation and no wetness and alternate high low radiation and alternate wetness. The results revealed that a combination of low radiation and leaf wetness significantly improves leaf stomata conductance and increases the specific leaf area (SLA). Changes in SLA were driven by leaf size changes. However, the minimum leaf conductance (gmin) did not respond to any of the treatments. The simulations focused on exploring the impact of radiation and canopy wetness on transpiration efficiency (TE), i.e. the ratio between photosynthesis (An) and transpiration (Tc). The simulations demonstrated that TE increased exponentially as the canopy was gradually wetted, regardless of the radiation environment. This increase in TE results from Tc approaching zero while An maintains positive values. Overall, this study provides an integrated understanding of how fog alters M. faya functioning and, potentially, other cloud forest tree species.

Visualization of Solar Radiation Using Three-Dimensional Computer Graphics Technologies

Currently, the prediction, calculation and rationing of solar radiation is a serious problem that has a wide impact on the spheres of human activity. Creating and maintaining a comfortable living environment, as well as solving the world's energy problems, are the fundamental criteria that determine its relevance. This article discusses the possibility of using three-dimensional graphics technologies to calculate insolation. The paper considers an algorithm for calculating the four main components of the visualization of global solar radiation: direct beam radiation, diffuse radiation, reflected beam radiation and reflected diffuse radiation. The freely distributed open source software Blender 3D is used as a tool for working with computer graphics. In the course of the study, calculations based on the principle of bidirectional path tracing, which is used in the visualization algorithms of the Cycles graphics engine, are presented. The use of the technology of transferring complex data to the raster image format allows you to create a separate texture map for each component of the simulated lighting. Based on the obtained texture maps, it is possible to calculate the global solar radiation for all created or imported 3D models in the three-dimensional space of Blender 3D.

Moving beyond the Aerosol Climatology of WRF-Solar: A Case Study over the North China Plain

Abstract Numerical weather prediction (NWP), when accessible, is a crucial input to short-term solar power forecasting. WRF-Solar, the first NWP model specifically designed for solar energy applications, has shown promising predictive capability. Nevertheless, few attempts have been made to investigate its performance under high aerosol loading, which attenuates incoming radiation significantly. The North China Plain is a polluted region due to industrialization, which constitutes a proper testbed for such investigation. In this paper, aerosol direct radiative effect (DRE) on three surface shortwave radiation components (i.e., global, beam, and diffuse) during five heavy pollution episodes is studied within the WRF-Solar framework. Results show that WRF-Solar overestimates instantaneous beam radiation up to 795.3 W m−2 when the aerosol DRE is not considered. Although such overestimation can be partially offset by an underestimation of the diffuse radiation of about 194.5 W m−2, the overestimation of the global radiation still reaches 160.2 W m−2. This undesirable bias can be reduced when WRF-Solar is powered by Copernicus Atmosphere Monitoring Service (CAMS) aerosol forecasts, which then translates to accuracy improvements in photovoltaic (PV) power forecasts. This work also compares the forecast performance of the CAMS-powered WRF-Solar with that of the European Centre for Medium-Range Weather Forecasts model. Under high aerosol loading conditions, the irradiance forecast accuracy generated by WRF-Solar increased by 53.2% and the PV power forecast accuracy increased by 6.8%. Significance Statement Numerical weather prediction (NWP) is the “go-to” approach for achieving high-performance day-ahead solar power forecasting. Integrating time-varying aerosol forecasts into NWP models effectively captures aerosol direct radiation effects, thereby enhancing the accuracy of solar irradiance forecasts in heavily polluted regions. This work not only quantifies the aerosol effects on global, beam, and diffuse irradiance but also reveals the physical mechanisms of irradiance-to-power conversion by constructing a model chain. Using the North China Plain as a testbed, the performance of WRF-Solar on solar power forecasting on five severe pollution days is analyzed. This version of WRF-Solar can outperform the European Centre for Medium-Range Weather Forecasts model, confirming the need for generating high spatial–temporal NWP.

Detecting clear‐sky periods from photovoltaic power measurements

AbstractA method for detecting clear‐sky periods from photovoltaic (PV) power measurements is presented and validated. It uses five tests dealing with parameters characterizing the connections between the measured PV power and the corresponding clear‐sky power. To estimate clear‐sky PV power, a PV model has been designed using as inputs downwelling shortwave irradiance and its direct and diffuse components received at ground level under clear‐sky conditions as well as reflectivity of the Earth's surface and extraterrestrial irradiance, altogether provided by the McClear service. In addition to McClear products, the PV model requires wind speed and temperature as inputs taken from ECMWF twentieth century reanalysis ERA5 products. The performance of the proposed method has been assessed and validated by visual inspection and compared to two well‐known algorithms identifying clear‐sky periods with broadband global and diffuse irradiance measurements on a horizontal surface. The assessment was carried out at two stations located in Finland offering collocated 1‐min PV power and broadband irradiance measurements. Overall, total agreement ranges between 84% and 97% (depending on the season) in discriminating clear‐sky and cloudy periods with respect to the two well‐known algorithms serving as reference. The disagreement fluctuating between 6% and 15%, depending on the season, primarily occurs while the PV module temperature is adequately high and/or when the sun is close to the horizon with many more interactions between the radiation, the atmosphere and the ground surface.

A Generalized Selectively Relaxed Matrix Splitting Preconditioning Strategy for Three-Dimensional Flux-Limited Multi-Group Radiation Diffusion Equations

A Generalized Selectively Relaxed Matrix Splitting Preconditioning Strategy for Three-Dimensional Flux-Limited Multi-Group Radiation Diffusion Equations

EVALUATION OF NEW DEVELOPED MODELS FOR ESTIMATION OF DIFFUSE SOLAR RADIATION OVER IKEJA, LAGOS STATE, NIGERIA

This study investigates the estimation of diffuse solar radiation in Ikeja, Nigeria (6.58°N, 3.33°E, 39.40m above sea level). Employing a 22-year dataset (2001-2022) of monthly average climatic data from NASA website, nineteen new models were developed to predict diffuse solar radiation. These models incorporated various meteorological parameters including global solar radiation, wind speed, temperature, pressure, and relative humidity. A categorized approach was used, with models falling into five groups: modified Page, Liu and Jordan; clearness index with one additional variable; two-variable; three-variable; and a four-variable model. Evaluation using five statistical tests tools of Mean Bias Error (MBE), Root Mean Square Error (RMSE), Mean Percentage Error (MPE), t-test, and Coefficient of determination (R²) identified Equation 28h (the model relating diffuse radiation to clearness index and temperature) as the most suitable model out of all the models in each of the five categories for estimation of diffuse solar radiation for Ikeja. Hence, emphasizing the importance of considering both atmospheric clarity and temperature for accurate diffuse solar radiation estimation in this coastal location.

FORECASTING THE CONSUMPTION OF ELECTRICAL ENERGY ON THE CEB ELECTRICAL NETWORK IN LOME IN TOGO BY APPROACHES: SIMPLE LINEAR REGRESSION, RECURRENT NEURAL NETWORKS AND GENETIC ALGORITHMS

Forecasting the electrical power to be consumed requires planning production on several levels. In this work we used data from the Electric Community of Benin. Temperature, relative humidity, wind speed, normal direct irradiance, precipitation and diffuse radiation are the meteorological variables that made it possible to analyze the forecasts. The objective is to do learning with genetic algorithms, LSTM recurrent neural networks and simple linear regression after a characterization and a correlation study and then to submit the results to performance evaluation criteria. The results of the characterization made it possible to understand that certain variables are significant and influence the consumption of electrical energy. The study of the correlation gives 94% between direct normal irradiance and diffuse irradiance. Both give with the temperature 67% for one and 68% for the other. Regarding modeling, the results are bad with genetic algorithms if we take into account the correlation coefficient (R² = 28.84%), good with simple linear regression (R² = 69.08%) and very interesting for networks of recurrent neurons where we find: MAE = 0.11 MSE = 0.02 MAPE = 18.50% RMSE = 13.09% RRMSE = 18.25% and R² = 96.11%. Given these results, we deduce that short- and long-term memory recurrent neural networks (LSTM) are very well suited to predicting the electrical power consumed on the CEB electrical network.

A CERES-based dataset of hourly DNI, DHI and global tilted irradiance (GTI) on equatorward tilted surfaces: Derivation and comparison with the ground-based BSRN data

The NASA CERES SYN1deg(Ed4.1) satellite-based products include hourly global horizontal irradiance (GHI), diffuse horizontal irradiance (DHI) and direct horizontal irradiance (DirHI) as well as hourly solar zenith angle (SZA), and the direct normal irradiance (DNI) can thus be calculated by dividing the DirHI by cos(SZA). The spatial resolution of the dataset is 1° latitude by 1° longitude, and the time span is from March 2000 to near present (October 2023 as of this writing). While the GHI of the dataset agrees well with the ground-based Baseline Surface Radiation Network (BSRN) data, the DHI and DirHI, and thus DNI, show appreciable biases against the BSRN data. Nevertheless, the uncertainty of the DHI and DNI, as exhibited in the standard deviations of their differences from the BSRN data, are smaller than results from the DirIndex model, a global-to-beam model, applied to the CERES data, albeit the model is one of the two best out of 140 models according to a study. This means that the original CERES hourly DNI, though biased, renders the spatiotemporal variability better than does the DirIndex model. For this reason, we decided to perform a bias correction on the hourly DHI and DNI. With the corrected hourly DHI and DNI, we are able to use the isotropic model to calculate the global tilted irradiance (GTI) and the global tracker irradiance (GTrI) on an hourly basis. The BSRN DHI and DNI, on the other hand, are available on near-instantaneous time scales (1-, 2-, 3, or 5-minute intervals) and thus are more conducive to the isotropic model, and for the first time, we use the BSRN data to validate the GTI and GTrI, as opposed to GHI and DHI that have previously been used for validation purpose. For comparison and validation, we also use the monthly-mean-based LJCR method, an independent method as used by RETScreen®, which requires monthly mean GHI and DHI, to directly calculate the monthly mean DNI, GTI and, to extend the model, GTrI, and the results agree well with the BSRN data, which proves the LJCR method itself and corroborates the corrected DHI and DNI as well as the derived GTI and GTrI. The monthly mean DHI and DNI from the Whitlock Method, also monthly-mean-based, are evaluated as well, and we find that minor modification can dramatically improve the method.

Optical performance investigation for spatially separated non-imaging concentrator with congruent plane concentrating surface

The thermal deformation on the concentrating surface of traditional CPC (Compound Parabolic Concentrator, CPC) is effectively addressed by SCSA-CPC (Separation of Concentrating Surface and Absorber CPC, SCSA-CPC). However, the energy flux density distribution on the absorber of SCSA-CPC is uneven and the concentrating surface is still a curved paraboloid structure, which hinders practical engineering applications and lacks significant economic benefits. In the present research, five kinds of SS-CPCs (Spatially Separated CPC, SS-CPC) with different numbers of congruent plane concentrating surfaces, namely SS-CPC2, SS-CPC3, SS-CPC4, SS-CPC5 and SS-CPC6 were constructed based on non-imaging optics principles. The optical performances of the SS-CPCs were investigated and compared to SCSA-CPC with the same specification. It is found that SS-CPC allows for an increased acceptance angle of up to 41° while effectively improving the uniformity of energy flux density distribution on the absorber. The results also showed that the annual beam radiation collection time can be extended by 43.11 % and SS-CPC has a particular advantage in collecting diffuse radiation, indicating its high adaptability to weather changes. Moreover, due to the congruent planar structure of the concentrating surface, SS-CPC holds promising prospects for practical engineering applications.

New Models for Estimation of Diffuse Solar Radiation for Warri, Nigeria

This study utilized a 22-year dataset (2001-2022) comprising monthly average daily global solar radiation, diffuse solar radiation, relative humidity, atmospheric pressure, wind speed, and mean temperature for Warri (Latitude 5.52 oN, Longitude 5.73 oE and 6.10 m above sea level). Nineteen models, categorized into modified Page, Liu, and Jordan models; clearness index and one variable models; two variable models; three variable models; and four variable models, were developed and statistically tested using the following validation indices of Mean Bias Error (MBE), Root Mean Square Error (RMSE), Mean Percentage Error (MPE), t-test, and coefficient of determination (R2). Among the models, the linear page model from the modified Page, Liu, and Jordan category, the model that relates the diffuse solar radiation with clearness index and mean temperature for clearness index and one variable models, the model that relates wind speed and atmospheric pressure for two variable models, and the model that relates wind speed, atmospheric pressure and relative humidity for three variable models were found more appropriate. Upon ranking the best-performing models from each category, the model that relates that diffuse solar radiation with clearness index and mean temperature performed best as the most suitable model for estimating diffuse solar radiation in Warri.

Study on the Engineering Application of LNG Accident Consequence Model in the Design of Terminal Sump

In order to provide a theoretical basis for the design of the LNG terminal sump, this paper proposes a suitable mathematical model and solution method by analyzing the physical process of evaporation diffusion and thermal radiation of the sump. Combined with the engineering practice, a lot of calculations have been carried out on the vapor diffusion concentration and pool fire thermal radiation intensity of the terminal sump, and the correlations between the vapor concentration and thermal radiation intensity and their influencing factors have been quantitatively calculated. Based on the analysis of the calculation results, it is concluded that the vapor concentration and thermal radiation value at the fire control room of the wharf are strongly related to the equivalent diameter of the sump and the distance from the fire control room to the sump. Among them, the distance from the fire control room to the sump is more closely related to the vapor concentration, and the equivalent diameter of the sump is more closely related to the thermal radiation; Keeping 19.2~33.8meters between the fire control room and the sump can basically ensure that the vapor concentration at the fire control room does not exceed the standard; In order to ensure that the thermal radiation at the fire control room does not exceed the standard, the distance between the fire control room and the sump should not be less than 15meters, and the equivalent diameter of the sump should not be greater than 7meters. The research methods and conclusions proposed in this paper can provide useful reference for engineering designers, and also build a bridge for mutual reference and application for theoretical research and engineering practice.

Shedding light on the increased carbon uptake by a boreal forest under diffuse solar radiation across multiple scales.

Solar radiation is scattered by cloud cover, aerosols and other particles in the atmosphere, all of which are affected by global changes. Furthermore, the diffuse fraction of solar radiation is increased by more frequent forest fires and likewise would be if climate interventions such as stratospheric aerosol injection were adopted. Forest ecosystem studies predict that an increase in diffuse radiation would result in higher productivity, but ecophysiological data are required to identify the processes responsible within the forest canopy. In our study, the response of a boreal forest to direct, diffuse and heterogeneous solar radiation conditions was examined during the daytime in the growing season to determine how carbon uptake is affected by radiation conditions at different scales. A 10-year data set of ecosystem, shoot and forest floor vegetation carbon and water-flux data was examined. Ecosystem-level carbon assimilation was higher under diffuse radiation conditions in comparison with direct radiation conditions at equivalent total photosynthetically active radiation (PAR). This was driven by both an increase in shoot and forest floor vegetation photosynthetic rate. Most notably, ecosystem-scale productivity was strongly related to the absolute amount of diffuse PAR, since it integrates both changes in total PAR and diffuse fraction. This finding provides a gateway to explore the processes by which absolute diffuse PAR enhances productivity, and the long-term persistence of this effect under scenarios of higher global diffuse radiation.

Evaluating the cloud effect on solar irradiation by three-dimensional cloud information

A detailed analysis of the effect of clouds on irradiance yields insights into the interaction between clouds and radiation and the precise forecasting of short-term solar radiation. Existing research has mainly focused on the impact of the cloud fraction (CF), i.e., the area distribution of clouds, on radiation. However, three-dimensional cloud information, such as the cloud thickness, also significantly affects solar radiation. This study proposes an innovative index based on the luminance of clouds in all-sky images (ASIs) to characterize the three-dimensional information of clouds, namely, CT, which is usually omitted in the binarization of ASIs. By using two-year synchronous ground irradiance and ASI observation data, the relationship between CT and the cloud radiative effect (CRE) is studied for different types of clouds under two situations, the sun being obscured and unobscured by clouds. The comparison with the commonly used CF indicates that CT has the same or higher negative correlation with the CRE of global horizontal irradiance. Moreover, the correlation between CT and CF with the CREs of direct normal irradiance (DNI) and diffuse irradiance (DFI) is explored in detail. The results indicate that CT has a more negative correlation with the CRE of DNI than that of CF when the sun is obscured, particularly for altocumulus clouds. A detailed discussion indicates that cloud radiative enhancement effects mainly occur under cumulus clouds with a minimum distance between the clouds and the sun (CSMD) of less than three solar radii, but cirrus clouds and altocumulus clouds can also induce cloud radiative enhancement effects.

Understanding the climatology and long-term trends in solar radiation using ground based in-situ observations in India

Understanding the variations of solar power potential over the country is essential for the optimum utilisation of solar energy in power generation, which demands accurate information of solar radiation and its variations. In the present study, we investigate the climatology and trends of global radiation (GR), diffuse radiation (DR), bright sunshine hours (BHS) and technical potential of solar power (Solar Photovoltaic potential; SPV potential) using in-situ data procured from India Meteorological Department for the period 1985-2019. GR is high (low) over the northwest and inland areas of peninsular (extreme north and northeast) India, whereas DR is high (low) over the coastal stations (extreme northern parts of the country). BHS is more (less) over northwest (north, northeast and southern peninsular) India. The country has SPV potential in the range of 1800-3400 Wm-2 with substantial regional variations. High (low) SPV potential is observed in the northwest regions (north, northeast and southern peninsular India). The GR and BHS (DR) have (has) a significant decreasing (increasing) trend in most parts of the country. However, the rate of decreasing (increasing) of GR (DR) has been weakened (strengthened) in the recent decade. The technical potential of solar power has a significant decreasing trend in most of the selected stations which is alarming. It necessitates the wide use of solar panels with better efficiency to meet the energy requirements from solar resources.