A comparative performance analysis of stand-alone, off-grid solar-powered sodium hypochlorite generators

Sodium hypochlorite (NaClO) is a chemical commodity widely employed as a disinfection agent in water treatment applications. Its production commonly follows electrochemical routes in an undivided reactor. Powering the process with photovoltaic (PV) electricity holds the potential to install stand-alone, independent generators and reduce the NaClO production cost. This study reports the comparative assessment of autonomous, solar-powered sodium hypochlorite generators employing different photovoltaic (PV) technologies: silicon hetero-junction (SHJ) and multi-junction (MJ) solar cells. For Si hetero-junctions, the series connection of either four or five SHJ (4SHJ and 5SHJ, respectively) cells was implemented to obtain the reaction potential required. MJ cells were illuminated by a novel planar solar concentrator that guarantees solar tracking with minimal linear displacements. The three solar-hypochlorite generators were tested under real atmospheric conditions, demonstrating solar-to-chemical conversion efficiencies (SCE) of 9.8% for 4SHJ, 14.2% for 5SHJ and 25.1% for MJ solar cells, respectively. Simulations based on weather databases allowed us to assess efficiencies throughout the entire model year and resulted in specific sodium hypochlorite yearly production rates between 7.2-28 g(NaClO) cm(-2) (referred to the PV surface), depending on the considered PV technology, location, and deployment of electronics converters. The economic viability and competitiveness of solar hypochlorite generators have been investigated and compared with an analog disinfection system deploying ultraviolet lamps. Our study demonstrates the feasibility of off-grid, solar-hypochlorite generators, and points towards the implementation of SHJ solar cells as a reliable technology for stand-alone solar-chemical devices.

Contribution to the active generator principle for high power electric supply

Massimiliano Visintin

In recent years the use of solid state frequency converters is rapidly increasing in the industrial and power plants where electric machines are installed, since it allows variable speed operations for the electric system, thus becoming a key factor which is capable of increasing the overall efficiency, as well as the global plant flexibility. The benefits that can be reached when electric motors are under investigation can be so enticing, in particular from the perspective of the operating machine coupled to the motor, that drawbacks take a back seat and can be overcome. On the contrary, power electronic for variable speed is generally not used in power generation, since the gains brought out by the present technology are not sufficient to attract anyone's interest. There are some exceptions in the wind- and the hydro power plant businesses, where there are some cases of variable speed already introduced. However in the former applications the output power is limited to some MW, while in the latter the converter is sized only to a fraction of the whole generating power, since it feeds the machine's rotor.This dissertation investigates on a possible electronic converter for power generation in the 40 MW range and above, which is also attracting from both the cost's and the efficiency's point of view, as well as from the operation reliability perspective and thus may be proposed as a breakthrough in this field. Among the possible frequency converters, those ones have been selected which perform natural commutations only, i.e. those that employ thyristors as the semiconductor devices, since the efficiency, the robustness and the relatively high performances are actually the key success factors to be addressed from the beginning. Well known matrix converter topologies will be referred to, as well as new matrix converter arrangements will be brought out and analyzed: their behaviours will be targeted by the dissertation, in particular when different strategies are adopted for controlling the operations. Specifically comparisons will be carried out when the same converter topology is used, but driven by either the well-known cycloconverter strategy or the newly brought out "active generator" one. In addition several generator winding arrangements will be proposed and analyzed, with the aim of increasing the actual feasibility of the proposed solution. Finally some test results on a sized down experimental rig will be analyzed and compared to the simulation outcomes.

EPFL2006

Monte Carlo simulations of quantum dot solar concentrators: ray tracing based on fluorescence mapping

Charu Datta Galande, André Gabriel Kostro, Jean-Louis Scartezzini, Andreas Schueler

One promising application of semiconductor nanostructures in the field of photovoltaics might be quantum dot solar concentrators. Quantum dot containing nanocomposite thin films are synthesized at EPFL-LESO by a low cost sol-gel process. In order to study the potential of the novel planar photoluminescent concentrators, reliable computer simulations are needed. A computer code for ray tracing simulations of quantum dot solar concentrators has been developed at EPFL-LESO on the basis of Monte Carlo methods that are applied to polarization-dependent reflection/transmission at interfaces, photon absorption by the semiconductor nanocrystals and photoluminescent reemission. The software allows importing measured or theoretical absorption/reemission spectra describing the photoluminescent properties of the quantum dots. Hereby the properties of photoluminescent reemission are described by a set of emission spectra depending on the energy of the incoming photon, allowing to simulate the photoluminescent emission using the inverse function method. By our simulations, the importance of two main factors is revealed, an emission spectrum matched to the spectral efficiency curve of the photovoltaic cell, and a large Stokes shift, which is advantageous for the lateral energy transport. No significant energy losses are implied when the quantum dots are contained within a nanocomposite coating instead of being dispersed in the entire volume of the pane. Together with the knowledge on the optoelectronical properties of suitable photovoltaic cells, the simulations allow to predict the total efficiency of the envisaged concentrating PV systems, and to optimize photoluminescent emission frequencies, optical densities, and pane dimensions. Keywords: quantum dots, photoluminescence, fluorescent planar solar concentrators, photovoltaics, solar cells, Monte Carlo simulations, ray tracing, polarization

SPIE2008

Electrocatalysis induced by surface redox activities on conductive metal oxide electrodes

Stéphane Fierro

Iridium dioxide electrodes form part of the dimensionally stable anodes (DSA®) and this electrode material is widely used in many industrial processes namely water electrolysis, metal electro-winning, cathodic protection and electro-organic synthesis due to the high electrochemical activity and stability of this electrode material. IrO2-based electrodes can be prepared using different techniques but the most common is the thermal decomposition of H2IrCl6 precursor solution on an inert substrate like titanium. Within the water stability potential domain, the charging/discharging process is attributed to the slow diffusion of protons within the IrO2 coating together with the electrical double layer capacitance. In fact the valence state of the Ir surface atoms of the coating varies from +IV to +VI in the potential domain between the on-set potentials of H2 and O2 evolution. Concerning the oxygen evolution reaction, direct evidence was found that the IrO2 coating participates actively in the reaction using an electrolyte solution containing isotopically labeled H218O. In fact, measurements of the relative amounts of electrogenerated 16O2 and 18O16O have demonstrated that the hydroxyl radicals coming from water discharge interact strongly with IrO2 resulting in the formation of the higher oxide (IrO3) and the decomposition of that oxide produces oxygen. IrO3 is thus the intermediate involved in the OER on these electrodes. During the oxidation of organic compounds, direct evidence was found by marking the IrO2 electrode with 18O that the higher valence state oxide IrO3 participates effectively also in this process. In fact, the oxidation of a solution of formic acid on a marked IrO2 coating containing 18O has shown that C16O18O is evolved proving that the oxidation of organic compounds occurs on IrO3 with competing side-reaction of oxygen evolution. The low overpotential of the OER allows performing selective electro-oxidation of a wide variety of organic compounds. In fact, the competing side reaction of oxygen evolution 'buffers' the potential around values where the oxidation products are not further oxidized showing that the IrO2 electrode is particularly suited for electro-organic synthesis. However, the current efficiency of the oxidation process remains low due to the competing side reaction of oxygen evolution.

EPFL2010