Markku J. Lampinen

Analysis of Free Energy and Entropy Changes for Half‐Cell Reactions

A comprehensive study of different entropy scales used in chemical thermodynamics is presented, and a semi-absolute entropy scale is introduced, by which problems involving noncharged and charge species can be considered correctly and further, the heat generation of half-cell reactions can be calculated. In this context, the entropy of an electron in a metal is derived. The entropy changes for electrode reactions are calculated, and the heat distribution among the electrodes of the cell is solved

Thermodynamic optimizing of pressure-retarded osmosis power generation systems

A transport equation for a solution flow increasing due to osmosis inside a hollow cylindrical fibre is derived. The equation can be applied for either direct, pressure-retarded or reverse osmosis, when the membrane is highly selective. This transport equation is used to study theoretically the net power delivered, and the entropy generated by two different concepts of a pressure-retarded osmosis power production system. As a result, the system can be optimized either by maximizing the net power or maximizing the ratio (Ψ) between the net power and entropy generation. In both cases the optimal values of the initial hydrostatic pressure difference between the inner and the outer sides of the fibre, the initial velocity of the solution and the fibre length could be specified. However, in some cases these two methods of optimization result in remarkably different optimal values. The resulting net power, when Ψ was maximized, was found to drop to less than half the maximum net power. The local entropy generation was found always to result in a minimum value at a certain longitudinal position inside the fibre.

Preparation of Air Electrodes and Long Run Tests

In this study we have tested some alternative catalysts for oxygen reduction in an alkaline electrolyte (7 M KOH). The catalysts tested were: CoTMPP, LaNiO 3 , Pt (for comparison), and MnO 2 . The electrodes were manufactured by the rolling method. The best electrodes made in this way were tested in a long run test, testing times ranging from 425 to 660 h. The decay in potential during the 660 h run was 0.041 m V/h

Mathematical modeling of falling liquid film evaporation process

A mathematical model of evaporation process from a laminar falling liquid film on a vertical plate of constant temperature is presented. The model is developed with and without interfacial shear stress due to the vapor flow at the liquid film surface. The vapor pressure drop, vapor exit velocity and cooling rate are calculated for different liquid mass flow values. It is shown that lower liquid mass flow produces higher cooling rate. The results also show that the interfacial shear stress has a considerable negative effect on the cooling rate. It is proved that there exists an optimum distance between the plates, which gives the maximum volumetric cooling rate.

Thermodynamic analysis of thermoelectric generator

The aim of this research has been to investigate the role of the second law in thermoelectric phenomena. Considering the thermoelectric generator as a heat engine cycle process, the Kelvin relations for the thermoelectric circuit are derived from the energy balance and from the second law without using reversibility or equilibrium assumptions. A new formula has been derived for the thermal efficiency of the thermoelectric generator, also taking into account Thomson heat. An equation for maximum efficiency is derived and a representative numerical example of this equation is presented.

Preparation and measurement of air electrodes for alkaline fuel cells

Abstract The optimum structure and materials for the gas diffusion layer of air electrodes for alkaline fuel cells were studied. These electrodes contain carbon black, pretreated in different ways ( e.g. , boiling nitric acid, heat treatment in different atmospheres), and polytetrafluorethylene as a hydrophobic binder. The samples prepared were studied by different methods, e.g. , air permeability, electrical conductivity and porosity. Heat-treated cobalttetramethoxyphenylporphyrine was used as catalyst in the active layer.

Theory of Effective Heat-Absorbing and Heat-Emitting Temperatures in Entropy and Exergy Analysis with Applications to Flow Systems and Combustion Processes

Abstract The heat integral in the entropy equation is separated into heat-absorbing and heat-emitting parts. Defining the effective heat-absorbing and heat-emitting temperatures for the boundary of the system over which the heat flows, we derive a general formula for the real working power in terms of effective temperatures with corresponding entropy generation rates. The method gives an explicit solution to the classical problem of how to determine the correct temperature T_ in each case for the power loss = (temperature T_) × (entropy generation rate σ). For different types of heat exchangers, we derive mathematical presentations for the effective heat-absorbing and heat-emitting temperatures; we thus distinguish the entropy generation rates, or exergy losses, for each fluid flow separately. The mathematical formulae for combined-flow systems are illustrated by a district heating system. As an example of chemical processes, we analyze the entropy generation rate in a combustion chamber, and we solve the problem of the real power loss of isobaric combustion. We also show that an adiabatic reversible combustion of carbon in air chambers could be self-pressurized theoretically, even up to several hundred bar, without compressors – and that the efficiency of the gas turbine process could be the same as when fuel cells are used.

MODELING AND MEASUREMENTS OF INFRARED DRYERS FOR COATED PAPER

ABSTRACT We have developed models to determine the radiative heat transfer of infrared dryers. Using the models, the efficiencies of several IR constructions are calculated. The radiation properties of all parts of the IR dryer are measured with an IT-IR spectrometer using integrating sphere techniques. With these models and measured results, the total efficiencies of several dryer constructions are calculated for: an electric dryer with a pambolic mirror, an elecmc dryer with a flat ceramic mirror, and a gas-fired dryer. The effect of a back reflector on these dryers is also calculated. The efficiencies of all these dryers are compared to each other.

Thermodynamic analysis of the interaction of the xylem water and phloem sugar solution and its significance for the cohesion theory

The cohesion theory explains water transport in trees by the evaporation of water in the leaves (transpiration), which in turn generates the tension required for sap ascent, i.e. the flow of pure water from the soil through the root system and the non-living cells of the tree (xylem tracheids) up to the leaves. Only a small part of this water flow entering the leaves is used in photosynthesis to produce sugar solution, which is transported from the leaves through the living cells (phloem) to everywhere in the tree where it is needed and used. The phloem sieves are connected to the xylem tracheids by water transparent membranes, which means that the upflow of pure water and downflow of sugar solution interact with each other, causing the osmotic pressure in the sugar solution (Münch model). In this paper we analyse this interaction with a thermodynamic approach and we show that some open questions in the cohesion theory can then perhaps be better understood. For example, why under a quite high tension the water can flow in the xylem mostly without any notable cavitation, and how the suction force itself depends on the cavitation. Minimizing Gibbs energy of the system of xylem and phloem, we derive extended vapor pressure and osmotic pressure equations, which include gas bubbles in the xylem conduits as well as the cellulose-air-water interface term. With the aid of the vapor pressure equation derived here, we estimate the suction force that the cavitation controlled by the phloem sugar solution can generate at high moisture contents. We also estimate the suction force that the transpiration can generate by moisture gradient at low moisture contents. From the general osmotic pressure equation we derive an equation for calculating the degree of cavitation with different sugar solution concentrations and we show the conditions under which the cavitation in the xylem is totally avoided. Using recent field measurement results for a Scotch pine, the theory is demonstrated by showing its predictions for possible amounts of cavitation or embolism from morning hours to late afternoon.

Electrochemical evaluation of sintered metal hydride electrodes for electric vehicle applications

Powder-sintered technology for manufacturing thick metal hydride electrodes has been investigated. A slurry consisting of hydrogen storage alloy mixed with fine nickel powder and some additives was pasted onto porous metal substrate. The results with porous metal substrates were compared with those obtained with nickel foam, perforated nickel strips, perforated copper foil and copper mesh. The effect of sintering parameters on the electrochemical properties of the sintered metal hydride electrode has been characterized. The results show that activation and high-rate dischargeability of the sintered hydride electrode was strongly improved compared to that obtained with conventional pasting processes.