P. Wibulswas

Performance analysis of multiple effect vertical still with a flat plate solar collector

The paper presents an analytical model for predicting performance of a multiple effect vertical still having a flat plate solar collector as a heat generator. The solar radiation, the ambient temperature and the wind speed are used as the input and the output are the temperature-time history of the evaporating and the condensing surfaces in the distillation unit and the distilled water production. The results agree quite well with the experiment. Simulation of long term performance of the system shows that as the ratio of the evaporating surface area to the solar collector area, Aevap/Acoll increases, the distilled water output also increases but when the ratio is over 5 the productivity output increases slightly. When the last condensing plate is covered with a wetted cloth, the system performance is better than when the surface is cooled by ambient air.

Transient simulation of vertical solar still

Abstract This paper presents a transient analysis of a vertical solar still having two sides of absorbing surfaces covered with glass. A modification of a theory developed by Spalding ( Convective Mass Transfer , 1963) has been used to estimate mass transfer in the still. The predicted results agree very well with experimental data.

Thermogravimetric analysis of Thai lignite—I. pyrolysis kinetics

Abstract Nonisothermal thermogravimetric (TG) data have been used with simple first-order and multi-reaction models to determine the pyrolysis kinetics of Thai lignite at slow heating rates (5–50 K/min). Under the conditions studied, the heating rate showed little effect on the TG curve and ultimate yield of volatiles, but significantly affected the volatile evolution rate. Simple first-order kinetics seem to be inadequate to characterize the weight loss during pyrolysis, and the derived activation energies are too low for organic decomposition. However, good agreement between predicted results and TG data were obtained when using multi-reaction models, particularly the distributed activation energy and volatile product models.

Prediction of mass transfer rates in solar stills

We present a modification of a mass transfer theory developed by Spalding to predict mass transfer rates in solar stills of different inclinations. In previous applications to solar stills, the authors considered the mass fractions at the evaporating surface and the bulk state to evaluate the driving force for mass transfer. In this study, the mass fraction at the condensing surface on the cover is employed instead of the bulk mass fraction on the moist air in the still, since the former can be more accurately determined. Predicted results from the modified theory and results from relation of Dunkle1 are compared with experimental values. The results of the modified theory agree very well with the experimental results, whereas Dunkles relation is accurate only in horizontal stills and stills with small inclinations.

Thermogravimetric analysis of Thai lignite. II, Char combustion kinetics

Abstract In this paper, nonisothermal thermogravimetric (TG) data have been used with the integral method to estimate the combustion kinetics of Thai lignite char obtained by pyrolysing the parent lignite at about 1173 K in LPG flame. The results show that the reaction is affected by pore diffusion when a high heating rate is used. The derived activation energy is about 21.31 kcal/gmol and 12.60 kcal/gmol for chemical kinetics and pore diffusion control, respectively. A small discrepancy is observed when these results are compared with those obtained from the literature. The difference may be due to the parent lignite and pyrolysis method used when preparing the sample.

Pyrolysis and volatile combustion of a single large lignite particle

A model for the evolution and combustion of volatiles from a single large lignite particle in a hot oxidizing gas stream is proposed. The analysis is divided into preignition and postignition stages. The transfer of heat both to and through the particle, as well as multireaction pyrolysis kinetics, are considered in order to find the overall devolatilization rate. The model also includes allowance for the effects of volatile species on the flame temperature. Predictions of transient weight, ignition and extinction time delay for a flame supported by the volatiles are in good agreement with the experiments.

Modeling of a charcoal cook stove

A model for simulating a charcoal cook stove is proposed. We develop mass and energy balances and evaluate these in terms of variations of temperature in the combustion zone with time during combustion. The model is used to predict the combustion rate of charcoal. For semi-empirical use, a correlation has been established between air velocity and temperature. The pressure drop across the charcoal bed was found to be negligibly small. The model yields results in good agreement with experimental values. The charcoal-combustion rate is an exponential function of time.

Upgrading heat by a reversed absorption heat pump

Analysis of a reversed absorption heat pumps performance for upgrading low temperature waste-heat from industrial processes and solar heat from a flat-plate solar collector has been carried out. In the case of upgrading waste-heat, the ratio of output to input heat decreases as the temperature at which the output heat is needed increases. For water heating, the performance of the conventional solar water heating system is better than that of the heat pump system because in the latter case, heat is rejected from the condenser.