Concept of Restaurants Optimised for Solar Cooking

Overview & Synopsis

Le Présage is the first restaurant concept in Europe whose cooking system is powered by solar thermal energy. Having confirmed the feasibility of the concept with its "guinguette solaire", the company now wants to optimise the cooking system and integrate it into an innovative building with high environmental quality. The cooking system comprises three elements: a Scheffler reflector that collects and concentrates sunlight; a secondary reflector that reflects the concentrated flux upwards; and a cast-iron plate that acts as a solar absorber on the underside and as a cooking plate on the top. The Scheffler reflector is a parabolic dish that is flexible, enabling it to concentrate solar radiation at a fixed point throughout the year, without requiring the reflector to rotate around this point. As a result, the focal point can be positioned inside a building in which a kitchen can be installed. The aim of this doctoral research project is to understand and evaluate the performance of this promising concentrating solar cooker concept. In the first chapter of this thesis, the overall performance of the solar cooker is evaluated in order to obtain an initial estimate that can serve as a benchmark for comparison with future improvements. First, the different ways of modelling and evaluating solar cooker performance are explored. The specific characteristic time constant, the opto-thermal ratio and the corrected standard power are selected from other performance indicators and expressed using an energy balance based on a lumped-element model. Next, the test bench developed, similar to the cooker used by the Le Présage restaurant, is described. Experiments were carried out using a protocol similar to that used by ASABE, and regression analyses of temperature, efficiency and normalised power are detailed. The opto-thermal ratio of the cooker varies from 0.06 m2 · K · W−1 to 0.22 m2 · K · W−1 , while the corrected standard power reaches up to 416 W. In the second chapter, the Scheffler reflector is studied in more detail. The objectives are to evaluate its energy performance and to obtain information about the shape of the heat flux it delivers. Theoretical and numerical studies in the literature have produced highly variable results, and experimental data is currently lacking. A second test bench and another experimental protocol were therefore developed. Using infrared thermography and inverse techniques, detailed maps of heat flux densities on a vertical absorber were obtained from measurements carried out on different days between March and July. The average energy efficiency is estimated at 68 %. The heat flux distributions provided are then successfully fitted to a two-dimensional Gaussian model. The model parameters are used to calculate, among other things, the mean major and minor diameters of the ellipse containing 99.7 % of the heat flux, i.e. 47 cm and 37 cm respectively. The surface concentration ratio and the heat flux concentration ratio are evaluated at 36 and 24 respectively. The issue of repeatability of measurements and seasonal/daily variations is also addressed. Overall, this work provides a detailed data set on the performance of the solar cooker studied using two innovative methodologies. These results provide fundamental information for identifying improvements to the system, for modelling the reflector and for optimising secondary optical systems and the absorber.