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.     

An RE consultant''s perspective on “Rio+10”

Ian Draisey from Dulas, a UK-based renewable energy consultancy, provides a perspective on the World Summit on Sustainable Development in Johannesburg. Are strong local community initiatives the keys to global sustainability?     

“You do not have to be a utility goliath to build a windfarm” or “David can do it!”

Many landowners believe that to build a windfarm on their land they have to find a developer or be approached by a large utility.This paper, gives examples both in Europe and the USA, of where the landowner has taken the initiative and developed wind energy on his own land to his maximum advantage.The problems and risks are addressed and evaluated. Various ways of financing projects are investigated and the rewards to the landowners highlighted.Land is an essential ingredient in any wind energy conversion development. The winds blowing over the land are rights of the landowner. This paper illustrates how the landowner can gain most benefits from those “windrights”.     

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.     

Process risk assessment — Using science to “do it right”

In today's process industry environment, it is becoming more and more important for companies to evaluate the risks associated with their plants. There are, however, many risk assessment methods that exist with varying degrees of complexity and cost. Choosing the right assessment method which will provide the information management needs to address the facility's risks is often the most difficult part in the whole assessment process. With this paper we are providing an overview of two assessment methods, namely HAZOP which is a qualitative method and FACILITY RISK REVIEW which uses different techniques of a qualitative and quantitative nature. These methods are similar to those currently being used in the risk assessment process at the facilities of the SASOL GROUP OF COMPANIES.     

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.     

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.     

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.     

“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.     

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.     

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.     

A “sleeper” awakes: tidal current power

Tidal currents are formed by the energy dissipated by the tides. These can be thalassic, estuarine or fluvial. There has been a growing interest to harness these currents and turn the energy into electrical power. The potential is considerable. The technology is in existence or adaptable. The sites are numerous, though as with most ocean energies, only a small fraction of the global potential could be converted. The paper provides a brief review of an area that has been scrutinized for more than quarter of a century.