M. M. Fernández

Predictions of the maximum energy extracted from salinity exchange inside porous electrodes

Capacitive energy extraction based on double layer expansion (CDLE) is the name of a new method devised for extracting energy from the exchange of fresh and salty water in porous electrodes. It is based on the change of the capacitance of electrical double layers (EDLs) at the electrode/solution interface when the concentration of the bulk electrolyte solution is modified. The use of porous electrodes provides huge amounts of surface area, but given the typically small pore size, the curvature of the interface and EDL overlap should affect the final result. This is the first aspect dealt with in this contribution: we envisage the electrode as a swarm of spherical particles, and from the knowledge of their EDL structure, we evaluate the stored charge, the differential capacitance and the extracted energy per CDLE cycle. In all cases, different pore radii and particle sizes and possible EDL overlap are taken into account. The second aspect is the consideration of finite ion size instead of the usual point-like ion model: given the size of the pores and the relatively high potentials that can be applied to the electrode, excluded volume effects can have a significant role. We find an extremely strong effect: the double layer capacitance is maximum for a certain value of the surface potential. This is a consequence of the limited ionic concentration at the particle-solution interface imposed by the finite size of ions, and leads to the presence of two potential ranges: for low electric potentials the capacitance increases with the ionic strength, while for large potentials we find the opposite trend. The consequences of these facts on the possibility of net energy extraction from porous electrodes, upon changing the solution in contact with them, are evaluated.

Multi-ionic effects on energy production based on double layer expansion by salinity exchange.

It has been recently shown that the free energy change upon salinity mixing in river mouths can be harvested taking advantage of the fact that the capacitance of charged solid/liquid interfaces (electrical double layers, EDLs) depends strongly on the ionic composition of the liquid medium. This has led to a new generation of techniques called Capmix technologies, one of them (CDLE or capacitive energy extraction based on DL expansion) based precisely on such dependence. Despite the solution composition playing a crucial role on the whole process, most of the research carried out so far has mainly focused on pure sodium chloride solutions. However, the effect of other species usually present in river and seawaters should be considered both theoretically and experimentally in order to succeed in optimizing a future device. In this paper, we analyse solutions of a more realistic composition from two points of view. Firstly, we find both experimentally and theoretically that the presence of ions other than sodium and chloride, even at low concentrations, may lead to a lower energy extraction in the process. Secondly, we experimentally consider the possible effects of other materials usually dispersed in natural water (mineral particles, microbes, shells, pollutants) by checking their accumulation in the carbon films used, after being exposed for a long period to natural sea water during CDLE cycles.

Effect of Solution Composition on the Energy Production by Capacitive Mixing in Membrane-Electrode Assembly

In this work, we consider the extent to which the presence of multivalent ions in solution modifies the equilibrium and dynamics of the energy production in a capacitive cell built with ion-exchange membranes in contact with high surface area electrodes. The cell potential in open circuit (OCV) is controlled by the difference between both membrane potentials, simulated as constant volume charge regions. A theoretical model is elaborated for steady state OCV, first in the case of monovalent solutions, as a reference. This is compared to the results in multi-ionic systems, containing divalent cations in concentrations similar to those in real seawater. It is found that the OCV is reduced by about 25% (as compared to the results in pure NaCl solutions) due to the presence of the divalent ions, even in low concentrations. Interestingly, this can be related to the “uphill” transport of such ions against their concentration gradients. On the contrary, their effect on the dynamics of the cell potential is negligible in the case of highly charged membranes. The comparison between model predictions and experimental results shows a very satisfactory agreement, and gives clues for the practical application of these recently introduced energy production methods.

Ca AND P BIOAVAILABILITY OF PROCESSED LENTILS AS AFFECTED BY DIETARY FIBER AND PHYTIC ACID CONTENT

The influence of germination for 6 days and dry-heating at 120°C for 15 min on the content of calcium, phosphorus, phytic acid and dietary fiber in lentils was studied. Germination lowered total phosphorus (P) and phytic acid content whereas dry-heating did not modify total P but slightly decreased phytic acid. Both treatments increased cellulose (CL) and lignin (LN) content and reduced hemicellulose (HMC) of lentils. The apparent digestibility coefficient (ADC) and balance of calcium (Ca) improved in animals fed dry-heated and germinated lentils which was related with the decrease in HMC. Phytic acid intake was significantly reduced by dry-heating and germination, the lowest P intake and P absorption observed in animals fed germinated lentils led to a null P balance. The results obtained in muscle and femur by comparing control (casein) and experimental diets indicated that muscle is more sensitive tissue than bone at lower Ca and P retention.

Fitness, fatness and cardiovascular profile in South Spanish and North Moroccan women

INTRODUCTION We studied the differences on physical fitness, fatness and cardiovascular profile in Spanish and Moroccan women. MATERIAL AND METHODS The study comprised 63 and 58 women aged 45-65 years from South of Spain and North of Morocco, respectively. We assessed fitness and body composition using standard procedures. We also assessed resting heart rate (RHR), blood pressure, fasting glucose, total cholesterol, LDL-cholesterol, HDL-cholesterol and triglycerides. RESULTS AND DISCUSSION Moroccan women had a better performance in the main health-related physical fitness components, i.e. higher levels of cardiorespiratory fitness (P=0.01) and (lower-body) muscular strength (P<0.001). Diastolic blood pressure (P=0.004), RHR and total cholesterol (both P=0.04) were lower in Moroccan women. No differences were observed in the prevalence of metabolic syndrome. CONCLUSIONS The women from Morocco had a healthier fitness and cardiovascular profile than the women from Spain. Further research on physical fitness and other health indicators in understudied populations is needed.

New energy sources: Blue energy study in Central America

Blue energy or salinity difference energy takes advantage of the free energy released in a mixture of two solutions with different salinity concentrations as it happens continuously in river mouths. Among the large number of available techniques that aim to harness blue energy, capmix (or capacitive mixing) methods allow to directly extract electrical energy without the need of any electromechanical converter such as turbines or heat engines. The main goal of this article is to analyze the potential of blue energy by capmix methods in Central America. So far, blue energy studies have been principally carried out in countries from the global North. Therefore, we describe experimental results with real sea and river waters from the Gulf of Fonseca, an area of special interest due to its hydrographic richness, which is situated among Honduras, El Salvador, and Nicaragua. An electrochemical cell, which consists of a pair of activated carbon electrodes coated with cationic and anionic polyelectrolyte layers, r...

Geometrical properties of materials for energy production by salinity exchange

Environmental context Oceans and seas have the potential to play a significant role in providing renewable and clean energy. In particular, salinity difference energy aims to extract the enormous amount of energy that is released when fresh water rivers flow into the oceans. Capmix methods are focused on this challenge by using capacitive carbon electrodes whose optimisation will certainly help in developing salinity difference energy. Abstract One of the most powerful marine renewable resources is salinity difference energy, also termed blue energy. Numerous techniques have been investigated to harvest this energy but, recently, the capmix proposal has increased in importance due to its easy implementation and use of low cost materials, very often activated carbon. Two methods based on this principle are tested in this work, namely CDLE (energy production by double layer expansion in bare electrodes) and SE (the electrodes are made ‘soft’ by polyelectrolyte coating). The characteristics of the carbon materials play a central role in capmix energy production. In this work, we focus on understanding the required pore structure that might be demanded from carbon samples. The balance between micro- and mesopores, the wettability of the material and its electrical resistance are explored by using hierarchical carbons, and their combination with graphene oxide and carbon nanotubes. It is found that the CDLE technique requires a large fraction of mesopores for easy solution exchange, while SE performance improves with a large amount of micropores. The addition of carbon nanotubes to the activated carbon reduces the capmix cycle duration, increasing the extracted power. In the case of electrodes containing graphene the internal resistance decreases, but the hydrophobicity of graphene oxide works against the improvement in energy extraction.