The role of aerosol models of the SMARTS code in predicting the spectral direct-beam irradiance in an urban area

Measurements of the ground-level spectral distribution of the direct-beam solar irradiance between 300 and 1000 nm were made in Athens, Greece, in May 1995. Results obtained using simple model for the atmospheric radiative transfer of sunshine (SMARTS) (version 2.9.2) parametric model for the urban atmosphere of Athens are analyzed and compared to the ground-level experimental spectral solar irradiance measurements obtained by the passive pyrheliometric scanner (PPS) in three discrete bands, UV (320–400 nm), VIS (400–700 nm) and NIR (700–1000 nm). The study uses two different input parameters for the aerosol characterization: the aerosol optical depth at 500 nm, t_α0.5, and the Ångström turbidity coefficient, β. The results clearly show that the nine aerosol models implemented in the SMARTS code lead to quite different predictions of the direct-beam spectrum, strongly depended on the input parameter. In all cases the inadequacies between the measured and the modeled direct-beam spectra are lower and higher when the urban and maritime aerosol models are used, respectively.     

A simple, solar spectral model for direct-normal and diffuse horizontal irradiance

A spectral model for cloudless days that uses simple mathematical expressions and tabulated look-up tables to generate direct-normal and diffuse horizontal irradiance is presented. The model is based on modifications to previously published simple models and comparisons with rigorous radiative transfer codes. This model is expected to be more accurate than previous simple models and applicable to a broader range of atmospheric conditions. The primary significance of this model is its simplicity, which allows it to be used on small desk-top computers. The spectrum produced by this model is limited to 0.3–4.0 μm wavelength with an approximate resolution of 10 nm.     

A method for correction of cosine errors in measurements of spectral UV irradiance

One of the sources contributing to the overall uncertainty of spectral UV radiation measurements is the cosine error of the spectroradiometer. It leads to measurement errors that depend on atmospheric conditions and on solar zenith angle, and thus time of the day and season. Though the foreoptics of modern instruments are designed such as to minimize cosine errors, there remain deviations from the ideal cosine response. We have worked out a method to further reduce that remaining cosine error in global spectral irradiance. This method was applied to spectra of global UV radiation taken with a Brewer spectroradiometer. The only additional input data needed to apply the method of cosine correction to spectral irradiance data are concurrent broad-band UV-B radiation measurements of diffuse and global radiation recorded with filter UV instruments, which are used to estimate the optical thickness referred to global UV radiation for the time when the spectral scan is taken. The method takes account of the variable conditions of cloudiness and turbidity. In the case of measurements taken with Brewer instrument No. 30, the cosine corrected global UV-B radiation was higher than the measured irradiance by 9–20%, and even its daily totals turned out to be higher than the uncorrected radiation by 13–18%. An estimate of the uncertainty of ±4 to ±8% was derived from a theoretical approach as well as from model calculations using a radiative transfer model.     

Interdisciplinary applications of a versatile spectral solar irradiance model: A review

A detailed review of different applications that have already been investigated with SMARTS, a versatile spectral solar irradiance model, is proposed here. This review provides examples of applications in many different disciplines, for which recent developments are discussed. Three main types of applications are considered, depending on their spectral range. Purely spectral applications encompass the determination of atmospheric constituents, the performance testing of spectroradiometers, and the improvement and validation of reference spectra for the rating of photovoltaic or glazing systems, or for new standards development in the field of weathering and material degradation. Narrow-band applications include the determination of different UV fluxes and of the UV index, and the prediction of illuminance on any horizontal or tilted surface, of the luminous efficacy of direct, diffuse or global radiation, of the photosynthetically active radiation, and of the irradiance transmitted by different bandpass filters. Finally, some specific broadband applications are reviewed: mesoscale predictions of radiation fluxes, evaluation of circumsolar effects in pyrheliometers, performance assessment of broadband radiation models, and turbidity determination from broadband irradiance data.     

A comparative study of SPCTRAL2 and SMARTS2 parameterised models based on spectral irradiance measurements at Valencia, Spain

Results obtained using the parametric models SPCTRAL2 and SMARTS2 for the urban area of Valencia, Spain, have been analysed and compared with experimental measurements at ground level obtained with two Li-cor 1800 spectroradiometers with a 6 nm resolution. The study used two different input parameters in both models for the aerosol characterisation: the aerosol optical thickness at 0.5 μm, τ_aλ(0.5), and the Angstrom turbidity coefficient β. The results obtained show that both algorithms reproduce quite correctly the spectral irradiance experimental values when an urban aerosol model parameterised by the τ_aλ(0.5) value is considered. In all the cases the deviations are lower when SMARTS2 code is used.     

An improved computer code for the simulation of solar heating systems

A generalised solar energy simulation code, which represents a significant improvement over fixed step iterative simulation codes, is presented. The key features of the code are the simultaneous solution of all differential equations arising from closed loops in the system, and the technique for advancing the simulation from “control function” to “control function”. Computer time is minimised by the use of sparse matrix techniques together with automatic time step control generated from an error bound estimate. The application of the improved code to the simulation of solar energy systems is demonstrated on two example problems. In the first problem, emphasis is placed on predicting the temperatures of the fluids within the system and the controller actions, while in the second problem the emphasis is on predicting long-term system performance. The simulation results of both these problems demonstrate that the improved code is suitable for both detailed system control and long-term system performance studies.     

A comparison of luminous efficacy models with illuminance and irradiance measurements

Several luminous efficacy models have been tested against simultaneous illuminance and irradiance measurements in Helsinki, Finland (60°11′N, 24°50′E). When compared with the measured values, the Perez luminous efficacy model had the lowest relative root mean square difference (RMSD) of 6.7%. All tested models had an RMSD within 2% of the constant model which used the measured yearly average of 110 lm/W as the constant. For all models, the error was smaller for high solar altitudes but greater for the low altitudes. The monthly average luminous efficacy was fairly constant between April and October but it was clearly lower during the winter months when the sun is very low.     

Dependence of the spectral surface irradiance on aerosol properties and surface reflectivity

A reduction in global surface irradiance occurs with increasing aerosol loadings when the aerosols are absorbing. For scattering aerosols, a reduction is pronounced for isotropic scattering (characteristic of small particles) but reduction is not as significant for scattering with a high anisotropy of a large forward peak (characteristic of large particles). This distinction between isotropic and anisotropic scattering becomes small or null over highly reflecting terrain; and actually for reflectivities higher than 0.5 and solar elevation angles close to the zenith, the global irradiance can be slightly higher for isotropic scattering than in the case of an anisotropy of a forward peak. Under such conditions, which can be encountered in reflective infrared bands over dense vegetation or over sandy deserts (close to noon, in low latitudes) the surface irradiance becomes nearly independent of the aerosol optical thickness.The above conclusions are derived based on analytical treatment of simplified single scattering approximation to the radiative transfer through a turbid atmosphere, which is valid only for a low or a moderate optical thickness.Computations of the spectral irradiance using the explicit expression are presented in the form of tables and graphs, for two anisotropy parameters of aerosols and three surface reflectivities.     

An analytical comparison of four approaches to modelling the daily variability of solar irradiance using meteorological records

Temporal solar variability significantly affects the integration of solar power systems into the grid. It is thus essential to predict temporal solar variability, particularly given the increasing popularity of solar power generation globally. In this paper, the daily variability of solar irradiance at four sites across Australia is quantified using observed time series of global horizontal irradiance for 2003–2012. It is shown that the daily variability strongly depends on sky clearness with generally low values under a clear or overcast condition and high values under an intermittent cloudiness condition. Various statistical techniques are adopted to model the daily variability using meteorological variables selected from the ERA-Interim reanalysis as predictors. The nonlinear regression technique (i.e. random forest) is demonstrated to perform the best while the performance of the simple analog method is only slightly worse. Among the four sites, Alice Springs has the lowest daily variability index on average and Rockhampton has the highest daily variability index on average. The modelling results of the four sites produced by random forest have a correlation coefficient of above 0.7 and a median relative error around 40%. While the approach of statistical downscaling from a large spatial domain has been applied for other problems, it is shown in this study that it generally suffices to use only the predictors at a single near point for the problem of solar variability. The relative importance of the involved meteorological variables and the effects of clearness on the modelling of the daily variability are also explored.     

Application of the computer code TOUGH2 to the simulation of supercritical conditions in geothermal systems

At the high pressures and temperatures found in deep geothermal systems, supercritical conditions can occur. Current numerical geothermal simulators are either not capable of modelling these conditions, or can do so only at significantly reduced computation speed. This paper describes modifications to the TOUGH2 simulator to extend its applicability. It employs the updated IAPWS-97 thermodynamic formulation, and uses density and temperature as primary thermodynamic variables under supercritical conditions. Results from test problems are in agreement with results produced by other simulators, giving confidence that the simulator can be used for modelling deep geothermal reservoirs.     

A preliminary assessment of a detailed two stream short-wave narrow-band model using spectral radiation measurements

A data bank of measurements of global, direct and diffuse solar spectral irradiances at ground level for clear skies has been compiled for Valencia (Spain) dating back to December 1992. The measurements were made with a commercial Li-cor 1800 spectroradiometer with a range of 300–1100 nm and a spectral resolution of 6 nm. A preliminary comparative assessment has been carried out between the experimental data and model data. The chosen model was a detailed narrow-band model (208 spectral intervals from 0.2 to 4 μm) developed at the “Laboratoire d'Optique Atmosphèrique (LOA)” of the University of Lille (France). This plane-parallel multilayer model uses a two-flux method to solve the radiative transfer equation and an exponential sum-fitting procedure to solve the absorption–scattering problem for finite spectral intervals. For this first comparative assessment we focused our attention on the capability of the LOA model to predict irradiance data (direct, global and diffuse) using four values of visibility (40, 23, 14 and 5 km) for two aerosol models (maritime and continental) in the boundary layer. These first results show the low sensitivity of global irradiance to different turbidity conditions. Conversely, the spectral direct and diffuse irradiances were highly influenced by the chosen aerosol model taking into account the visibility values. The spectral distribution of predicted global and direct irradiances are in relatively good agreement with the observed values. The diffuse data show larger discrepancies, which are in part due to the nature of the measurement process itself. However, the observed differences can be partially explained by taking into account the associated errors of the measured data, the elapsed time between the measured spectra and the prediction power of the LOA model.     

A comparative study of “in-out” and “out-in” hydrogen reaction alternatives for metal hydride beds using RET 1 computer code

The paper presents the results of a comparative study of two alternatives for progress for hydrogen reaction in metal hydride hydrogen storage beds; namely “In-Out” and “Out-In” through the application of a space-time dependent computer code “RET 1” for the solution of the associated mass and heat transfer problem. The reference model for the metal hydride storage bed is considered of cylindrical shape and composed of a multiple of a cylindrical cell, that either consists of a coaxial fluid tube surrounded by the granular iron-titanium metal hydride; called “In-Out” cell; or consists of a coaxial rod of the granular iron-titanium metal hydride surrounded by an annular fluid tube; called “Out-In” cell. The results of the study lead to the conclusion that for the same operating and physical conditions, the rate of hydrogen reaction in the “Out-In” cell is many times higher than that in the “In-Out” one.     

Design of a sun tracker for the automatic measurement of spectral irradiance and construction of an irradiance database in the 330–1100 nm range

An automatic global and direct solar spectral irradiance system has been designed based on two LICOR spectroradiometers equipped with fibre optics and remote cosine sensors. To measure direct irradiance a sun tracker based on step motors has been developed. The whole system is autonomous and works continuously. From the measurements provided by this system a spectral irradiance database in the 330–1100 nm range has been created. This database contains normal direct and global horizontal irradiances as well as diffuse irradiance on a horizontal plane, together with total atmospheric optical thickness and aerosol optical depth.     

Chemical looping gasification for hydrogen production: A comparison of two unique processes simulated using ASPEN Plus

The research compares the simulations of two chemical looping gasification (CLG) types using the ASPEN Plus simulation software for the production of H₂. The simulated biomass type was poultry litter (PL). The first CLG type used in situ CO₂ capture utilizing a CaO sorbent, coupled with steam utilization for tar reforming, allowing for the production of a CO₂-rich stream for sequestration. Near-total sorbent recovery and recycle was achieved via the CO₂ desorption process. The second type utilized iron-based oxygen carriers in reduction–oxidation cycles to achieve 99.8% Fe₃O₄ carrier recovery and higher syngas yields. Temperature and pressure sensitivity analyses were conducted on the main reactors to determine optimal operating conditions. The optimal temperatures ranged from 500 to 1250 °C depending on the simulation and reactor type. Atmospheric pressure proved optimal in all cases except for the reducer and oxidizer in the iron-based CLG type, which operated at high pressure. This CLG simulation generated the most syngas in absolute terms (2.54 versus 0.79 kmol/kmol PL), while the CO₂ capture simulation generated much more H₂-rich syngas (92.45 mol-% compared to 62.94 mol-% H₂).     

A computer study for estimation of seasonal energy consumption and selection of air-conditioner in six Saudi Arabian cities

A computer model was developed to simulate the building cooling load and the seasonal energy consumption of standard residential sized central air-conditioning systems. The model was first validated by comparing the predicted cooling energy consumption against the metered energy of a 100 m² residence. The model predicts within 10% of the metered value. The validated model was next used to compute the cooling load and seasonal energy consumption for similar 100 m² residences in six cities located in different climatic zones of Saudi Arabia. Simulations showed that the cities of Dhahran and Riyadh required a 17.6 kW system to satisfy the cooling load through the year, whereas the defined residences in the cities of Taif, Hail, Jeddah and Gizan required a 14.1 kW system. Our analysis showed that the selected residence in Gizan would consume 23,100 kWh annually, while a comparable residence located in Dhahran, having a more severe summer, required only 21,500 kWh. This difference may be explained by analysis of the weather data which revealed that Gizan required year-round cooling, whereas Dhahran needed cooling for only 283 days during the year. Investigation showed that, by selecting the next smaller capacity air-conditioning unit for each location than required to satisfy the load for 100% of the time, the annual power consumption may be reduced, on an average, by 10%, with about 7% of the hours during the cooling season when the air-conditioning load may not be satisfied.     

Impact of spectral irradiance distribution and temperature on the outdoor performance of amorphous Si photovoltaic modules

Performance of photovoltaic (PV) modules is evaluated under the standard test condition, which rarely meets actual outdoor conditions. Environmental conditions greatly affect the output energy of PV modules. The impact of environmental factors, especially solar spectrum distribution and module temperature, on the outdoor performance of amorphous Si (a-Si) and multicrystalline Si (mc-Si) PV modules is characterized. The results show that the output energy of a-Si modules mainly depends on spectrum distribution and is higher under blue-rich spectrum. In contrast, the output energy of mc-Si module is sensitive to module temperature but not to spectrum distribution.     

A comparison of two energy efficient motors

Limited testing provides evidence that a particular energy efficient motor (EEM) appears to have a unique susceptibility to reduced negative sequence impedance. The effect of the reduction in negative sequence impedance is to allow a higher negative sequence current flow under conditions of voltage unbalance or negative sequence harmonic distortion in the bus voltage. This higher current flow results in additional heating of the rotor and 120 Hz vibration, and may combine with other conditions to cause premature failure. In addition, energy efficient motors operate at slightly higher speeds, and may sometimes cause the driven loads to require more horsepower from the motor causing further overheating. Existing industry guidance for the application of three phase motors in poor power quality environments may be inadequate because this guidance does not consider the combined effect of such conditions as voltage unbalance, harmonic distortion and overvoltage. A specific application of an EEM is studied in this paper; the motor may be failing due to a combination of overvoltage, voltage unbalance, harmonic distortion and loading the motor to full rated load, while the motor has a service factor of 1.0     

A computer program for the elastostatics of a toroidal shell using the differential quadrature method

This paper presents a general analysis of the elastostatic problem of a sectorial toroidal shell. The solution to the problem is based on the Mushtari–Vlasov–Donnell shell theory and is developed using the two-dimensional quadrature method. A computer program incorporating the theory is attached, and an example of its application is given for the case of a fluid loading. Numerical results are determined and these are validated by a comparison with results obtained from a finite element solution.     

A comparison of two methods of estimating insolation over the tropical pacific ocean using cloudiness from satellite observations

Two methods of estimating insolation over the tropical Pacific using cloud cover from satellite observations are compared with each other and with average monthly ration data from four Pacific island stations. The two methods yield comparable values for latitudes within 15° of the equator but diverge at higher latitudes. Both methods overestimate insolation in the summer and underestimate insolation in the winter for the two stations above 15° latitude. Results support reports that tropical Pacific cloud types have a high mean transparency.