• Energy Research
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Abstract The need to have access to accurate short term forecasts is essential in order to anticipate the energy production from intermittent renewable sources, notably wind energy. For hourly and sub-hourly forecasts, benchmarks are based on statistical approaches such as time series based methods or neural networks, which are always tested against persistence. Here we discuss the performances of downscaling approaches using information from Numerical Weather Prediction (NWP) models, rarely used at those time scales, and compare them with the statistical approaches for the wind speed forecasting at hub height. The aim is to determine the added value of Model Output Statistics for sub-hourly forecasts of wind speed, compared to the classical time series based methods. Two downscaling approaches are tested: one using explanatory variables from NWP model outputs only and another which additionally includes local wind speed measurements. Results of both approaches and of the classical time series based methods, tested against persistence on a specific wind farm, are considered. For both hourly and sub-hourly forecasts, adding explanatory variables derived from observations in the downscaling models gives higher improvements over persistence than the benchmark methods and than the downscaling models using only the NWP model outputs.

The problem of sizing an electricity storage for a 5000 inhabitants island supplied by both marine renewables (offshore wind and waves) and the mainland grid is addressed by a case study based on a full year resource and consumption data. Generators, transmission lines and battery storage are accounted for through basic simplified models while the focus is put on electricity import/export budget. Self-sufficiency does not seem a reasonable goal to pursue, but partial autonomy provided by renewable sources and a medium size storage would probably be profitable to the island community.

This paper addresses the simulation of a partially transparency, ventilated PV facade integrated into the envelope of an energy efficient building. Such an arrangement exploits the heat transfer between cavity air, the PV facade and the primary wall of the building for the purpose of PV cooling in summer (with natural convection) and heat recovery in winter (mechanical ventilation). A simplified physical model of the system is proposed for the summer operating configuration, which is more challenging from a numerical perspective. The model describes the active envelop in terms of a simplified geometry, and includes parameters such as density of PV cells, relative coverage of degree of transparency/opaque surfaces, and the ratio of height/width of the double-skin. For a given set of meteorological conditions, the surface and air temperatures, mass flow rate and PV power output are obtained by solving a system of thermal and aerodynamic balance equations. Validation of the model was undertaken using experimental data from a full scale prototype system installed in Toulouse, France as part of the RESSOURCES project (ANR-PREBAT2007). Coupling of the system to a simulated building was achieved with the aid of TRNSYS, and this combined system was evaluated in terms of heating and cooling needs for a range of French climates. It was found that the cooling needs are marginally higher for all locations considered, whereas the impact of the facade on the heating needs is weak as these needs are already low for these all locations.

Abstract Usually, wind sites are equipped with fast-running Horizontal Axis Wind Turbines of the airscrew type, which has a high efficiency. In this article, the argument is put forward that the choice of a wind turbine must not be based only on high efficiency. We propose a comparative criterion adapted to the comparison of a horizontal axis wind turbine with a vertical axis wind turbine: the L–σ criterion. This criterion consists in comparing wind turbines which intercept the same front width of wind, by allocating them a same reference value of the maximal mechanical stress on the blades or the paddles. On the basis of this criterion, a quantitative comparison points to a clear advantage of the Savonius rotors, because of their lower angular velocity, and provides some elements for the improvement of their rotor.

Abstract The purpose of this work is two-fold. First, to introduce a comparison between steady state and dynamic test methods for two different collectors. Second, to design a solar water heating system to satisfy both hot water and space heating demands for a multi-family house in Alexandria, Egypt.

The thermal regulation which is under preparation for Tunisian buildings is based on the thermal comfort level achieved in the building by natural means, since residential buildings are rarely heated or cooled. This paper investigates the appropriate building solutions in order to reach the highest possible comfort level, while keeping the overrun cost of construction at a reduced level.

Abstract We investigate the turbulent dynamics of the coupled atmosphere-ocean-sediment system around a wind turbine. To this end, a coupled two-dimensional idealized numerical model of the ocean and sediment layers, forced by an idealized offshore wind turbine wake is used. The turbine wake impacts the ocean surface and for strong wind and water layer thickness higher than 20 m, large scale eddies of the size comparable to the wake thickness are generated, leading to a turbulent dynamics in the ocean. The turbulence in the ocean is controlled by the shallow wake parameter S. The turbulent ocean dynamics is numerically integrated using time dependent simulations at fine horizontal resolution (1 m). From these simulations, eddy coefficients parametrizating the turbulent fluxes are proposed to be used in larger-scale (RANS) models. The ocean dynamics and the parameter values depend mainly on S. The ocean dynamics is laminar (S > 7.10−2), has a localized (7.10−2