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Τεκμήριο Corrigendum to “Modeling of the optimum tilt of a solar chimney for maximum air flow”(2012-02) Sakonidou, E. P.; Karapantsios, T. D.; Balouktsis, A. I.; Chassapis, D.There is an error in Eqs. (45) and (46) describing the average gas velocity in a solar chimney. These equations are used to describe the air velocity as a function of the chimney geometry and of the average density and, therefore, temperature of the air inside the chimney. In these equations sin(s) (s: slope of the chimney with respect to the horizontal), should not be raised to the second power. The term sin(s) originates from transforming the vertical chimney equations to the inclined case. In doing this, one should either keep the vertical Cartesian axes system and so replace the length of the chimney, L, with the product L∗ sin(s), or rotate the axes by s and then replace the acceleration of gravity by g∗ sin(s). Apparently, it is naive to do both. This error affected the estimated average gas velocity presented in Figs. 7a, 8a, 9a, 12a and 13. The CFD predictions shown in the same plots are irrelevant to our code, i.e., are not affected by the error. Inspection of the written numerical code indicates that the error also did not affect the predictions of the Air temperature, Glazing temperature and Absorber temperature shown in the figures. The authors give below a short account of the significance of this error to the main conclusions of the article. Regarding Figs. 8 and 9, the correct optimum tilt for maximum air flow is ∼50–60o (∼65–70o in the paper) which is again in fair agreement with the CFD prediction of ∼60o. Fig. 12a displays optimum tilt of the chimney and maximum air velocity values versus the months of a year. The correct values of the optimum tilt are lower by ∼5–15o than those presented in the paper and the maximum air velocity values are higher by at most ∼10% (for summer months) than those presented in the paper. However, the qualitative trend of both the maximum air velocity and optimum tilt versus the months of a year remains the same. Fig. 13 discusses whether it is worth it to incline a chimney to its maximum air velocity tilt compared to its operation at a vertical position (tilt = 90o). The highest deviation in air velocity between the two cases is expected to occur in summer time where the new calculations indicate a deviation of ∼18% instead of the mentioned in the paper ∼10%. For winter time, the deviation between the inclined and vertical cases is expected to be insignificant. All in all, the closing statements of the article that for winter time the gain is not so large “against concerns about the stability of the construction” and for summer time “decisions must be made more carefully” are still valid.Τεκμήριο A five-level symmetrically defined selective harmonic elimination PWM strategy: analysis and experimental validation(2008-01) Agelidis, V. G.; Balouktsis, A. I.; Dahidah, Mohamed S. A.A five-level symmetrically defined multilevel selective harmonic elimination pulsewidth modulation (MSHE-PWM) strategy is reported in this paper. It is mathematically expressed using Fourier-based equations on a line-to-neutral basis. An equal number of switching transitions when compared against the well-known multicarrier phase-shifted sinusoidal PWM (MPS-SPWM) technique is investigated. For this paper, it is assumed that the four triangular carriers of the MPS-SPWM method have nine per unit frequency resulting in seventeen switching transitions for every quarter period. For the proposed MSHE-PWM method, this allows control of sixteen harmonics and the fundamental. It is confirmed that the proposed MSHE-PWM offers significantly higher converter bandwidth in the standard range of the modulation indices. Moreover, when the bandwidth is reduced to be equal with the one offered with the MPS-PWM, the modulation index can be increased resulting in a higher gain and at a reduced switching frequency overall. Selected solutions for the switching transitions are presented and verified experimentally in order to confirm the effectiveness of the proposed technique.Τεκμήριο Modeling of the optimum tilt of a solar chimney for maximum air flow(2008-01) Sakonidou, E. P.; Karapantsios, T. D.; Balouktsis, A. I.; Chassapis, D.The aim of this work is to develop a mathematical model to determine the tilt that maximizes natural air flow inside a solar chimney using daily solar irradiance data on a horizontal plane at a site. The model starts by calculating the hourly solar irradiation components (direct, diffuse, ground-reflected) absorbed by the solar chimney of varying tilt and height for a given time (day of the year, hour) and place (latitude). In doing so it computes the transmittance and absorbance of the glazing for the various solar irradiation components and for various tilts. The model predicts the temperature and velocity of the air inside the chimney as well as the temperatures of the glazing and the black painted absorber. Comparisons of the model predictions with CFD calculations delineate the usefulness of the model. In addition, there is a good agreement between theoretical predictions and experiments performed with a 1 m long solar chimney at different tilt positions.Τεκμήριο On attaining the multiple solutions of selective harmonic elimination PWM three-level waveforms through function minimization(2008-03) Agelidis, V. G.; Balouktsis, A. I.; Cossar, C.Selective harmonic elimination pulsewidth modulation techniques are some of the control methods used in voltage/current source converters. However, challenges such as the task of finding all the multiple sets of solutions of the switching angles for a given problem may be difficult to deal with. In this paper, a direct minimization of the nonlinear transcendental trigonometric Fourier functions in combination with a random search is discussed. The unipolar (three-level) waveform is used to illustrate the proposed method confirming its ability to find multiple sets of solutions, including a case where 51 angles are sought for single- and three-phase applications. A simple harmonic distortion factor is studied for each set of solutions to assess their performance against the noneliminated harmonics. The results presented both at theoretical and experimental level are in close agreement and confirm the robustness of the proposed approach.