Although the integral quantities of atmospheric turbulence are conveniently measured using sonic anemometers, obtaining relevant finescale variables such as the kinetic energy dissipation using conventional hot-film/wire techniques remains a challenge because of two main difficulties. The first difficulty is the mean wind variability, which causes violation of the requirement that mean winds have a specific alignment with the hot-film/wire probe. To circumvent this problem, a combination of collocated sonic and hot-film anemometers, with the former measuring mean winds and aligning the latter in the appropriate wind direction via an automated platform, is successfully designed and implemented. The second difficulty is the necessity of frequent and onerous calibrations akin to hot-film anemometry that lead to logistical difficulties during outdoor (field) measurements. This is addressed by employing sonic measurements to calibrate the hot films in the same combination, with the output (velocity) to input (voltage) transfer function for the hot film derived using a neural network (NN) model. The NN is trained using low-pass-filtered hot-film and sonic data taken in situ. This new hot-film calibration procedure is compared with the standard calibration method based on an external calibrator. It is inferred that the sonic-based NN method offers great potential as an alternative to laborious standard calibration techniques, particularly in the laboratory and in stable atmospheric boundary layer settings. The NN approximation technique is found to be superior to the conventionally used polynomial fitting methods when used in conjunction with unevenly spaced calibration velocity data generated by sonic anemometers.Corresponding author address: E. Kit, School of Mechanical Engineering, Tel-Aviv University, Ramat Aviv, Tel Aviv 69978,
This paper investigates the challenges in developing a domestic scale solar thermal electricity system using a Stirling engine. All the system components, the parabolic troughs, the heat transfer fluid and the Stirling engine are individually analyzed and described. The analysis includes a market survey and performance assessments of the various components. In this paper a mathematical analysis for a gamma type Stirling engine is carried out. The mathematical analysis demonstrates how such a low/medium temperature engine can be developed. The design is based on an atmospheric air as the working fluid. The market survey reveals that low/medium temperature Stirling engines are not currently readily available. The few that are mentioned in literature are still in their development stage. The size of the presently available parabolic troughs is too large for domestic rooftop installations but can be easily installed on industrial rooftops. On the other hand a range of heat transfer fluids suitable for domestic Stirling engine systems are quite readily available from the present market. All these factors present a real challenge for developing a domestic Stirling engine system for generating enough energy to make it feasible to feed into the electricity grid. Some comments are further given in the conclusions' section of the paper in which the wider implications of the study are discussed in relation with photovoltaic systems.
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