This paper presents models/strategies for optimum performance of solar collector in closed loop systems. These models aim to maximize the obtained energy by thermal conversion of solar energy. The mass flow rate of the fluid from the primary circuit of the system is the control parameter. The semi empirical models and optimal control methods are in brief presented. The volume of the storage tank is important and the ratio Vs/Ac between this volume and area of the collectors is a key factor in appropriate sizing of the DHW system. Therefore, the paper establishes a relationship between this ratio and the mass flow rate of the fluid in the collector This paper also analyses the variation of the energetic performance (useful heat flux transferred to the storage tank, heat flux transferred to the water, water temperature in the storage tank) with the volume of the storage tank. Analysis was performed on an extensive set of meteorological data from Timisoara, Romania, with instantaneous data (measured at 15 seconds) for summer days, from July 2009, with different relative sunshine values, σ. Important differences have been observed between days with different stability levels - days more or less stable.
The paper is focused on the solar irradiance estimation in clear-sky conditions and an aerosol-loaded atmosphere. Two parametric models developed by our group and three empirical models are tested. The estimates of the parametric models are based on three atmospheric parameters (ozone, nitrogen dioxide and water vapor column content) and the aerosol properties quantified by means of several specific parameters (Ångström turbidity coefficient, single scattering albedo, asymmetry factor). The empirical models contain no inputs for aerosol properties. Data collected from 10 stations were used to test the models. The inputs for the parametric models were retrieved from Aerosol Robotic Network - AERONET. Global and diffuse solar irradiance data at high-quality standards were retrieved from the Baseline Surface Radiation Network BSRN. A comparative analysis of the models’ accuracy in estimating clear-sky solar irradiance is discussed from the perspective of aerosol proprieties.
A precise estimation of the electrical energy produced daily by photovoltaic (PV) systems is important both for PV owners and for electrical grid operators. It can be achieved if the received solar irradiance can be accurately estimated during any type of daily solar profile (clear, cloudy, mixed sky), not only average solar profile for larger periods of time, e.g. one month or season, as used in PV system design. The paper firstly describes an existing mathematical model, based on the Meliss approach, which uses mean monthly coefficients for estimating average direct and diffuse solar irradiance. This model is satisfactory for monthly/annual intervals but is not useful for daily estimations. Therefore in the second part of the paper an algorithm which allows to generate daily variations of the model’s coefficients for clear and cloudy sky conditions is proposed. The improved model with variable coefficients was tested during several representative days and can be used for estimating the effect which different meteorological conditions as fog/dew/frost have on the quantity and quality of the solar irradiance received by a PV convertor.
Ioana Tismanar, Maria Covei, Cristina Bogatu and Anca Duta
Photocatalytic degradation of organic pollutants from wastewater using titanium dioxide is recognized as an efficient process. To be feasible, this process needs to be solar- or visible light -activated, but, due to its wide band gap energy, titanium dioxide can only be activated by ultraviolet radiation. In this paper, thin films of titanium dioxide were deposited by Spray Pyrolysis Deposition using organo-metallic (titanium-iso-propoxide) and inorganic (titanium chloride) precursors, as a first step in optimizing the deposition process of titanium dioxide - carbon composite thin films, active in the visible spectral range. The thin films were characterized to outline the differences when using these two precursor solutions, when deposited on Fluorine doped Tin Oxide-glass, regular glass and microscopic glass, in terms of crystallinity (by using X-ray diffraction), elemental composition (using Energy Dispersive X-ray spectrometry), surface morphology (Scanning Electronic Microscopy and Atomic Force Microscopy). The photocatalytic activity of the titanium dioxide thin films was investigated based on the removal efficiencies of methylene blue from a synthetically prepared wastewater. The samples deposited using the inorganic precursor show higher roughness, and this proves to be the most important factor that influences the phocatalytic processes. After 8 hours of ultraviolet irradiation, methylene blue removal efficiencies up to 36% were observed; the highest removal efficiency was registered using the thin titanium dioxide film obtained using the titanium chloride precursor, deposited on fluorine doped tin oxide glass substrate, due to the titanium dioxide-tin dioxide semiconductor tandem formed at the interface, that limits the electron-hole recombination, thus increasing the photoctalytic performance of the substrate.
This paper proposes a new simple model to forecast daily global solar irradiation one day ahead using the Takagi-Sugeno fuzzy methods. The model is based on solar radiation data measured in Timisoara, Romania. The daily clearness index represents the direct variable used by the fuzzy algorithm. The model forecasts the clearness index at the moment of time t on basis of two previous values measured at time t-1 and t-2. An assessment of the model accuracy is performed.
Marius Paulescu, Nicoleta Stefu, Ciprian Dughir, Robert Blaga, Andreea Sabadus, Eugenia Paulescu and Sorin Bojin
Forecasting the solar energy production is a key issue in the large-scale integration of the photovoltaic plants into the existing electricity grid. This paper reports on the research progress in forecasting the solar energy production at the West University of Timisoara, Romania. Firstly, the experimental facilities commissioned on the Solar Platform for testing the forecasting models are briefly described. Secondly, a new tool for the online forecasting of the solar energy production is introduced. Preliminary tests show that the implemented procedure is a successful trade-off between simplicity and accuracy.
Florin Drăgan, Ørnulf Nordseth, Laurențiu Fara, Constantin Dumitru, Dan Crăciunescu, Vlad Muscurel and Paul Sterian
An investigation of silicon-based tandem solar cells incorporating Al-doped ZnO (AZO) and Cu2O metal oxides, via two of the most efficient methods of optical modeling, specifically ray tracing and transfer matrix algorithms, was performed. The simulations were conducted based on specialized software, namely Silvaco Atlas and MATLAB, as well as on OPAL2 simulation platform. The optical analysis involved the calculation of the spectral curves for reflectance, absorptance and transmittance for different thicknesses of the thin film layers constituting the cell. It was established the optimum thickness of the AZO layer based on the minimum reflectance and maximum transmittance. Moreover, several materials were investigated in order to determine the optimum buffer layer for the tandem solar cell, based on optical modeling. The optical parameters of the ZnO/Cu2O top subcell were optimized, in order to achieve the highest conversion efficiency of such heterojunction solar cell.
Regional climate models (RCMs) are used in a wide range of climate applications as they can provide high resolution (up to 10 to 20 km or less) and multi-decadal simulations of the climate system describing climate feedback mechanisms acting at the regional scale. However due to different forcing data and physics parametrisations regional climate models might produce different results. This study aims to achieve a state-of-the-art knowledge of bias-corrected surface solar radiation projections coming from 11 EURO-CORDEX regional climate models. First a comparison against 63 GEBA observations is elaborated indicating a general overestimation of surface solar radiation (SSR) in the RCMs by 6.12 W/m2 (4.4%). Next changes in surface radiation between the period of 2031-2060 and 1971-2000 are presented on annual and seasonal time scale. The model projections indicate robust increase in SSR mainly in the western part of the Mediterranean region, while the northern part of the continent is characterised by decreases in SSR till the middle of this century. The study emphasis the need of an overall validation of different climate models before introducing them in impact studies in order to have an overview regarding the uncertainties.
The movement of water on the land surface, within channels and through the soil is dependent on some hydrological factors. For surface flow, the velocity of flow increases with the bottom gradient of the channel and the flow depth, but when roughness increases, it decreases. For a given flow depth, the velocity decreases as the channel height increases. The construction of sub-surface drainage helps to remove excess soil water that can cause flood. To model overland flow, a kinematic-wave approach is applied so that flood prediction could be made.