A note on the formation of “salt-finger” and “diffusive” interfaces in three-component systems

Thin density interfaces determine the fluxes of heat or solute through doubly-diffusive convection. Vertical transports are achieved by either “salt finger” convection or molecular diffusion. The influence of a third diffusing property upon the type of interface formed at an initial density discontinuity is explored here. There may be a strong dependence upon molecular diffusivities, and some interfaces are observed to have a complicated structure.     

Modelling and experimentation of the “Shell” dryer

The “Shell” dryer is an African solar food dryer using natural convection. Its design has been defined by users, in function of local working conditions and of its easy using. But it is necessary for the best behaviour of this dryer to optimize its air flow section design. We present here a theoretical study of a “rustic” process, through mathematical modelisation tools.     

A further study on the theory of “falling ponds”

The theory of “falling pond” is investigated and the stability requirements of such ponds are discussed. Based on the possible maximum temperature gradients, curves determining the regions of operation of stable falling solar ponds are presented. Also the effects of the thickness of the non-convecting layer and the bottom temperature on the stability of such ponds are discussed. Comparison between a MgCl₂ and NaCl falling solar pond is made and typical profiles for concentration are presented.     

Performance of the “shower” cooling tower in different climates

The paper summarizes experimental data from testing a new type of a passive cooling system developed by the author, the “Shower” tower, and compares its performance under three different climatic conditions: Los Angeles in California, USA, Riyadh (Saudi Arabia) and Yokohama (Japan).     

Thermal performance of the “Rohini”—An improved wood cookstove

A detailed thermal analysis of the “Rohini”, a new damperless, three-pot, L-shaped improved cooking stove, developed by the authors, is reported. The stove was designed for an optimum power output at a wood-burning rate of 1.0 kg/h, which is the average rating of the stove used in rural areas. The performance of this stove has been evaluated at different power outputs. The study shows that the thermal efficiency of the stove is not unduly affected by operation at burning rates lower than the design value but shows a fall at the higher burning rate of 1.5 kg/h. The rate of heat gained by the individual pots and the various heat losses, such as convection, radiation and flue gas losses, at different burning rates, are also discussed. This study shows that widespread use of this stove in India could save nearly 150 MT wood/yr.     

Mesoscale modeling of a “Dunkelflaute” event

In the near future, wind and solar generation are projected to play an increasingly important role in Europe's energy sector. With such fast‐growing renewable energy development, the presence of simultaneous calm wind and overcast conditions could cause significant shortfalls in production with potentially serious consequences for system operators. Such events are sometimes dubbed “Dunkelflaute” events and have occurred several times in recent history. The capabilities of contemporary mesoscale models to reliably simulate and/or forecast a Dunkelflaute event are not known in the literature. In this paper, a Dunkelflaute event near the coast of Belgium is simulated utilizing the Weather Research and Forecasting (WRF) model. Comprehensive validation using measured power production data and diverse sets of meteorological data (e.g., floating lidars, radiosondes, and weather stations) indicates the potential of WRF to reproduce and forecast the boundary layer evolution during the event. Extensive sensitivity experiments with respect to grid‐size, wind farm parameterization, and forcing datasets provide further insights on the reliability of the WRF model in capturing the Dunkelflaute event.     

Thermal evaluation of a “Winter House”

In this paper, the transient thermal model of a “Winter House” has been presented for a cold climatic region of Srinagar in India. In order to reduce the fluctuations in living room temperature, the effect of an isothermal mass has been incorporated in the thermal model. An analytical expression for room temperature has been derived to evaluate the performance of the “Winter House” by incorporating the effect of movable insulation during the night. Numerical calculations were done in terms of system as well as climatical parameters. Some isothermal mass, like water, has been considered to stabilize the room temperature throughout the day and right. For further heating, the effect of the roof as an air collector has also been taken into account.     

Thermal performance of “still on roof” system

This paper presents an analysis of periodic heat flux through a roof, on which a solar still has been placed. It is seen that on a typical hot day in Delhi, for high reflectivity of the bottom of the basin (on account of deposits or otherwise) the daily heat flux in the room gets reduced by 40% and the production of distilled water is 0.6 kg/m² day; the produced swings in the heat flux also get reduced in magnitude. For a typical cold day in Delhi, if a black dye is mixed with basin water the daily heat flux in the room increases by a factor of two and the yield of the distillate becomes 5.0 kg/m² day.     

Apparent “gettering” of the Staebler-Wronski effect in amorphous silicon solar cells

The stability behaviour of intrinsic amorphous silicon materials incorporated in a p⁺-i-n⁺ solar cell structure is considerably different from that observed by electrical characterization methods in individual thin films. This is due to the fundamental difference in Fermi-level position in a single layer compared to the situation occuring in devices. We have employed the differences in the re-equilibration behaviour that have been observed in various intrinsic materials when the Fermi-level is shifted towards the valence band edge, in order to design a cell with a new profiled i-layer which would possess an improved electric field distribution after light soaking compared to cells with a constant i-layer. The contribution of the interface region to the stabilized conversion efficiency is greatly improved, whereas the first 50 nm of the cell structure remain unchanged. Thus, it appears that the Staebler-Wronski effect is gettered away from the junction, much like the impurity gettering concept in crystalline solar cells.     

The “shell” dryer—modelling and experimentation

The “shell” dryer is an African solar food dryer which uses natural convection. Its design has been defined by users, in terms of local working conditions and ease of use. However, it is necessary to optimize the design of the dryer's air flow section for best performance. We present here a theoretical study of a simple process using mathematical modelling tools.     

Relationship between “clearness index” and “cloudiness index” at a tropical station (Ilorin, Nigeria)

Total (global) solar radiation, H, and diffuse solar radiation, H_d were measured at Ilorin (8° 30′N 4° 42′E) Nigeria. From these, the daily values of H/H₀ and H_d/H were computed, where H₀ is the extraterrestrial radiation. The relationship between the two ratios and their variation with the prevailing atmospheric conditions were examined.The ratios were found to be opposite in characteristics. H/H₀ has high values in clear atmospheres and low values in cloudy or turbid atmospheres; and vice-versa for H_d/H. The two ratios are negatively linearly related and with this fact, two simple mathematical models were obtained for estimating H_d/H in terms of H/H₀.     

“Active Flux” DTFC-SVM Sensorless Control of IPMSM

This paper proposes an implementation of a motion-sensorless control system in wide speed range based on “active flux” observer, and direct torque and flux control with space vector modulation (DTFC-SVM) for the interior permanent magnet synchronous motor (IPMSM), without signal injection. The concept of “active flux” (or “torque producing flux”) turns all the rotor salient-pole ac machines into fully nonsalient-pole ones. A new function for $L_{q}$ inductance depending on torque is introduced to model the magnetic saturation. Notable simplification in the rotor position and speed estimation is obtained, because the active flux position is identical with the rotor position. Extensive experimental results are presented to verify the principles and to demonstrate the effectiveness of the proposed sensorless control system. With the active flux observer, the IPMSM drive system operates from very low speed of 2 r/min at half full-load up to 1400 r/min. Higher speed is possible, in principle, with flux weakening.      

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 10¹⁶/cm³ could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 Å/s for microcrystalline silicon could be achieved.     

A “once-through” solar water heating system

In China, solar water heater is being popularized and most existing solar water heaters are the natural circulation system. Due to some shortcomings of the natural circulation system, a “once-through” system is proposed. In a once-through system, the storage tank can be placed below the collector, therefore, the load on the roof will be cut down significantly. This system also has the advantages of no mixing of hot and cold water, no reverse flow and being able to provide hot water earlier, etc. Both theoretical and experimental investigations have been conducted to compare the collector efficiences between the once-through and natural circulation systems and they coincide with each other very well. The once-through system performs worse in the morning but better in the afternoon than the natural circulation system and the difference of daily collector efficiency between these two systems is negligible.     

“Wet-type” solar cells with semiconductor electrodes

The photovoltages and photocurrents arising from “wet” solar cells of the type, semiconductor electrode¦aqueous solution of electrolyte¦platinum electrode, have been measured and discussed. Earlier work by the present authors and others on such “wet” solar cells using a semiconductor electrode is briefly and critically reviewed. The effect of dyes, either dissolved in the solution or coated on the electrode surface, was studied in detail including the effect of reducing agents, which turned out to act as electron suppliers. The photovoltaic effects in nonaqueous solutions were also studied. The photocurrent efficiencies of the dye-sensitized cells were improved by increasing the quantities of dyes absorbed on the surface and reached 19 per cent for monochromatic illumination at the wavelengths of the absorption peaks of the dyes and under sufficient anodic biases.