Vassilis Gekas

Environmental impacts from the solar energy technologies

Abstract Solar energy systems (photovoltaics, solar thermal, solar power) provide significant environmental benefits in comparison to the conventional energy sources, thus contributing, to the sustainable development of human activities. Sometimes however, their wide scale deployment has to face potential negative environmental implications. These potential problems seem to be a strong barrier for a further dissemination of these systems in some consumers. To cope with these problems this paper presents an overview of an Environmental Impact Assessment. We assess the potential environmental intrusions in order to ameliorate them with new technological innovations and good practices in the future power systems. The analysis provides the potential burdens to the environment, which include—during the construction, the installation and the demolition phases, as well as especially in the case of the central solar technologies—noise and visual intrusion, greenhouse gas emissions, water and soil pollution, energy consumption, labour accidents, impact on archaeological sites or on sensitive ecosystems, negative and positive socio-economic effects.

Mass transfer in the membrane concentration polarization layer under turbulent cross flow : I. Critical literature review and adaptation of existing sherwood correlations to membrane operations

In its first part, this work contains: (a) a critical review on the mass transfer correlations under turbulent duct flow, as they appeared in the literature ( 1934-1984); (b) a discussion on the factors influencing mass transfer during membrane operations (reverse osmosis and ultrafiltration), like porosity and roughness of the membrane wall and change of viscosity and diffusion coefficient due to the strong concentration gradient; (c) a proposal for a modified mass transfer equation, including the term (Sc/Scw)0.11 where Scw is the Schmidt wall number with Dw, and vw both functions of the concentration at the membrane wall (Cw), as well as a correlation factor accounting for the effect of roughness and permeate flux. The second part of this paper will contain an application of the modified Sh equation in the characterization of UF membranes.

Apparent mass diffusivities in fruit and vegetable tissues undergoing osmotic processing

Abstract Apparent mass (water and solute) diffusivities were measured during osmotic processing of model fruit (apple) and vegetable (potato) tissues. Among process parameters, temperature had the largest positive effect on moisture ( D w ) and soluble solids ( D ss ) diffusivity. Increased concentration gave increased moisture and decreased solute diffusivities. The dehydration efficiency index ( D w D ss ratio) increased with concentration but decreased or remained constant with temperature. Using the right size of osmotic solute it was possible to maintain satisfactory moisture diffusivities with nearly zero net solute uptake. Freeze/ thaw-induced tissue damage caused a dramatic decrease in dehydration efficiency. Both moisture and solute diffusivities followed Arrhenius kinetics with activation energies ( E a ) ranging between 21.2 and 29.7 kJ mol −1 K −1 . Cell level explanations on response differences are attempted.

Apple shrinkage upon drying

Abstract In this paper results from a study of the shrinkage of Mutsu apple slices during drying in a connective oven are presented. The results are expressed as relationships between volume and thickness reduction as well as between volume reduction and water content. No differences were observed with respect to slice size. Volume change was correlated to thickness change through an exponent of 1.50±0.24 for all samples measured. Volume change was linearly correlated with water content with r = 0.95.

Osmotic dehydration of apples —effects of agitation and raw material characteristics

Abstract The effects of agitation and structural differences on osmotic dehydration were investigated. Osmotic dehydration was performed in an agitated vessel at 20 °C using a 50% sucrose solution as the osmotic medium. The impellers Reynolds number was used for agitation quantification. Samples were separated into inner and outer apple parenchymatic tissue, the intercellular space interconnectivity and aspect ratio (length to width ratio) being higher in inner than outer tissue. Structural differentiation revealed a strong effect on process responses. Solid gain (kg/kg i.m.) was higher in inner than in outer apple parenchymatic tissue independently of agitation level. Water loss (kg/kg i.m.) was lower in inner than outer apple parenchyma at the same Reynolds number. Water loss was higher in the turbulent flow region than in the laminar flow region. Thereby, external mass transfer limitations were verified for our experimental conditions. Solid gain did not show significant differences between laminar and turbulent flow regions. The data indicate that free convection is the mechanism used by the solution in pore penetration, although lack of understanding of this phenomenon at the cell level prevented conclusions from being drawn. Attempts to explain experimental variations revealed indications suggesting the influence of initial bulk density and initial water content on water loss and solid gain.

Contact angles of ultrafiltration membranes and their possible correlation to membrane performance

The captive bubble method was used to describe the wetting characteristics of a number of commercial ultrafiltration (UF) membranes. The membranes belonged to one of two homologous (same material, different cut-off) series made by the same manufacturer. One series was made of polysulphone and the other one of cellulose triacetate. The porosimetric characteristics of the membranes have also been measured. The combined data were used to explain the fouling behaviour of the membranes upon ultrafiltration of solutions containing dextran, whey protein concentrate and silicate sols. The cellulose triacetate series is characterized by lower receding contact angle and smaller contact angle hysteresis and shows better flux behaviour (permeate flux during UF and pure water flux recovery at the end of UF) than the polysulphone series. Within the same series the mean permeability pore size shows a better correlation with membrane flux behaviour than with contact angle hysteresis.

Artificial membranes as carriers for the immobilization of biocatalysts

Abstract Artificial membranes, due to their interesting properties (high surface area per unit volume, possibility to combine separation with chemical reaction etc.) have interested biotechnologists in their attempt to find carriers for the immobilization of biocatalysts. Various systems and modes of operation have been used with the biocatalyst either in soluble or in insolubilized form. Progress in the above fields over the past decade is summarized and discussed.

Determination of diffusion coefficients in volume-changing systems—Application in the case of potato drying

Abstract This work was based on a modified Crank approach in order to determine the diffusion coefficient in systems where volume changes occur during dyring. The modified model was applied to the case of potato drying, for both raw and blanched potatoes, at two temperatures: 60 and 80°C. The degree of volume change (shrinkage) was different for raw and blanched potatoes, but only small differences have been observed for the diffusion coefficient of reference, which was evaluated at 2·25 (± 0·13) × 10 −10 m 2 /s for raw potatoes and 2·48 (± 0·28) × 10 −10 m 2 /s for blanched potatoes.

Analysis of the heat transfer coefficient during potato frying

Abstract The objective of this work was to study the dependence of the heat transfer coefficient (h) on the water loss rate of potato during frying. An indirect method was used where a metal piece with the same geometry of the potato pieces was placed on top of various potato samples at different frying times, and its temperature was recorded for 20–30 s. Another method consisted of direct recording of the temperature within a potato slice, close to the surface. Water loss rate was estimated by image analysis of bubbles. After immersion in hot oil, the potato temperature increases and water starts vapourising, leaving the surface in the form of bubbles that flow through the oil. The water loss rate increases until complete drying of the potato surface and then decreases till the end of frying. The h value showed the same behaviour increasing up by two times in relation to the values measured in the absence of bubbling, with maximum values depending on the oil temperature and potato geometry (443–750 W m−2 K−1). The percentage of heat transferred to the potato that is used for water evaporation showed an increase with time up to complete surface drying.

Osmotic dehydration of apples. Shrinkage phenomena and the significance of initial structure on mass transfer rates

The existence of two well-defined structures in the parenchymatic tissue of apples (the flesh) gave us the opportunity to study the significance of the initial structure on mass transfer rates of two apple varieties, Kim (Sweden) and Granny Smith (Argentina) when subjected to osmotic dehydration. Our results verified the importance of the initial structure for osmotic processing responses. Knowledge of the properties of the tissue and the solution penetration into the intercellular space, permitting us to explain the kinetics found. Shrinkage properties such as volume changes, bulk density, particle density and porosity, have been studied macroscopically for both structures, inner (close to core) and outer (close to skin) and presented as a function of water content in a manner similar to air drying practice. A comparison with shrinkage properties observed in air drying is attempted.