A. Louche

DESIGN OF HYBRID-PHOTOVOLTAIC POWER GENERATOR, WITH OPTIMIZATION OF ENERGY MANAGEMENT

Abstract A methodology is developed for calculating the correct size of a photovoltaic (PV)-hybrid system and for optimizing its management. The power for the hybrid system comes from PV panels and an engine-generator – that is, a gasoline or diesel engine driving an electrical generator. The combined system is a stand-alone or autonomous system, in the sense that no third energy source is brought in to meet the load. Two parameters were used to characterize the role of the engine-generator: denoted SDM and SAR, they are, respectively, the battery charge threshold at which it is started up, and the storage capacity threshold at which it is stopped, both expressed as a percentage of the nominal battery storage capacity. The methodology developed is applied to designing a PV-hybrid system operating in Corsica, as a case study. Various sizing configurations were simulated, and the optimal configuration that meets the autonomy constraint (no loss of load) was determined, by minimizing of the energy cost. The influence of the battery storage capacity on the solar contribution is also studied. The smallest energy cost per kWh was obtained for a system characterized by an SDM=30% and an SAR=70%. A study on the effects of component lifetimes on the economics of PV-hybrid and PV stand-alone systems has shown that battery size can be reduced by a factor of two in PV-hybrid systems, as compared to PV stand-alone systems.

Correlations for direct normal and global horizontal irradiation on a French Mediterranean site

Abstract The purpose of this study is to establish some correlations on direct normal and global horizontal irradiation for a mediterranean site. The correlations have been developed for monthly mean values, daily and hourly values of irradiation data. We have used linear or polynomial regressions between energetical ratio and sunshine duration. For the monthly means and the daily values, the ratio I I M fits very well the sunshine duration while, for hourly values, we express I h I o,h as a function of H h H o,h . In all cases, these correlations are, for this mediterranean site, in good accordance with experimental data. Nevertheless, a study for other locations has to be developed.

Dew water collector for potable water in Ajaccio (Corsica Island, France)

We report on the development of an inexpensive radiative condenser for collecting atmospheric vapor. Based on the experience gained using a small working model in Grenoble (France), a prototype of 10×3 m2 was established in Ajaccio (Corsica, France). The condensing surface is a rectangular foil made of TiO2 and BaSO4 microspheres embedded in polyethylene and has an angle of 30° with respect to horizontal. The hollow part of the device, thermally isolated, faces the direction of the dominant nocturnal wind. Dew measurements were correlated with meteorological data and compared to dew condensed on a horizontal polymethylmethacrylate (PMMA, Plexiglas) reference plate. The plate served as a reference standard unit and was located nearby. Between July 22, 2000 and November 11, 2001 (478 days), there were 145 dew days for the reference plate (30%), but 214 dew days for the condenser (45%). This yield corresponds to 767 l (3.6 l, on average, per dew day). The maximum yield in the period was 11.4 l/day. Dew mass can be fitted to a simple model that predicts dew production from simple meteorological data (temperature, humidity, wind velocity, cloud cover). Chemical analyses of the water collected from the plate were performed from October 16, 1999 to July 16, 2000 and from the condenser, from July 17, 2000 to March 17, 2001. The following parameters were investigated: suspended solids, pH, concentration of SO42−, Cl−, K+, Ca2+ ions. Only Cl− and SO42− ions were sometimes found significant. Wind direction analyses revealed that Cl− is due to the sea spray and SO42− to the combustion of fuel by an electrical plant located in the Ajaccio Gulf. Except for a weak acidity (average pH≈6) and high concentration of suspended solids, dew water fits the requirements for potable water in France with reference to the above ions.

Determination of Ångström's turbidity coefficient from direct total solar irradiance measurements

Abstract A method of determining Angstroms turbidity coefficient from the measured total (broadband) direct normal solar irradiance is described here. This direct normal irradiance can be expressed in terms of the individual transmittances of the various atmospheric attenuators, such as ozone layer thickness, precipitable water vapor thickness, and α and β of the Angstroms turbidity formula. From the resulting parameterization equation, an explicit expression for β is obtained. By assigning a fixed value to α, the value of β is determined. Use of this method is demonstrated with data from Ajaccio (France), a mediterranean coastal station.

Forecasting and simulating wind speed in Corsica by using an autoregressive model

Alternative approaches for generating wind speed time series are discussed. The method utilized involves the use of an autoregressive process model. The model has been applied to three Mediterranean sites in Corsica and has been used to generate 3-hourly synthetic time series for these considered sites. The synthetic time series have been examined to determine their ability to preserve the statistical properties of the Corsican wind speed time series. In this context, using the main statistical characteristics of the wind speed (mean, variance, probability distribution, autocorrelation function), the data simulated are compared to experimental ones in order to check whether the wind speed behavior was correctly reproduced over the studied periods. The purpose is to create a data generator in order to construct a reference year for wind systems simulation in Corsica.

An analysis of linke turbidity factor

Abstract Standard values of the optical thickness of the clean dry atmosphere are based on very old values of the extraterrestrial solar spectral irradiance and atmospheric transmittance. Based on the modern values of the extraterrestrial solar spectrum and its attenuation by the atmospheric components, a new value of the optical thickness of the clean dry atmosphere has been proposed. This optical thickness results in values slightly higher than the older values. Hence the new procedure will result in lower values of the Linke turbidity factor. A relationship between the old and the new values is presented.

PV-hybrid power systems sizing incorporating battery storage: an analysis via simulation calculations

A methodology is developed to determinate the optimal size of PV-hybrid subsystems and to optimize the stand-alone system management. Two particular parameters characterize the back-up engine generator: SDM and SAR, respectively the starting and the stopping thresholds calculated as a part of the nominal storage capacity. Using simulation calculations, the optimal configuration leading to the autonomy constraint, is determined on the basis of a minimization of the kWh cost. The operating back-up generator strategy study has shown that the smallest kWh cost is obtained for a system characterized by a SDM=30% and SAR=70% of the nominal battery capacity.

Autonomous photovoltaic systems: Influences of some parameters on the sizing: Simulation timestep, input and output power profile

In this paper, an optimized sizing method of an autonomous photovoltaic system is applied to a case study located in Corsica. Using a global systemic approach based on solar radiation data and load profile, the photovoltaic system operation behaviour is first simulated. The system autonomy constraint leads to an infinity of system configurations which can be reduced by additional physico-technical constraints. Adding, as a parameter, the kilowatt-hour cost and its minimization constraint, the best system configuration is determined. In this study of the influence of the temporal repartition of the load and of the simulation time step, we determine the important energy surplus produced by the optimized autonomous photovoltaic system.

Modelling and performance of a copolymer solar water heating collector

Abstract The performance of a solar flat-plate thermal collector wholly manufactured in a copolymer material is studied. The influence of different parameters of the system such as the insulation thickness, the flow rate and the fluid layer thickness is analysed. Thermal performance, productivity and efficiency of such a solar system is presented for a Mediterranean site. Yearly mean efficiencies are about 56.5% in no wind and about 49.0% for a wind speed of 5 m s−1. The use of polymer materials reduces the collector weight by 50% in comparison with a traditional metal collector, this renders easier installation.

Autonomous hybrid photovoltaic power plant using a back-up generator: A case study in a Mediterranean island

Using a stand-alone photovoltaic array as a unique source of supply for a local electric demand generally induces a significant excess in energy production. A solution to the problem appears through the use of a hybrid system, which gives a good way to improve the energy balance together with minimum investment. A sizing method starting from meteorological conditions in Corsica was extended to a twin-source system: solar/fuel electric generator with each source contribution being optimized. The results obtained from such an approach show that a 75% solar rate is an optimal value to obtain an energy balance fitted to the local demand when compared to other systems for decentralized electricity generation. The methodology was tested and validated, through an existing hybrid PV system in Corsica (44 kW). This paper does not present a general sizing method for the hybrid system, but is just a case study.