Solar energy conversion | EPFL Graph Search

Solar energy conversion describes technologies devoted to the transformation of solar energy to other (useful) forms of energy, including electricity, fuel, and heat. It covers light-harvesting technologies including traditional semiconductor photovoltaic devices (PVs), emerging photovoltaics, solar fuel generation via electrolysis, artificial photosynthesis, and related forms of photocatalysis directed at the generation of energy rich molecules. Fundamental electro-optical aspects in several emerging solar energy conversion technologies for generation of both electricity (photovoltaics) and solar fuels constitute an active area of current research. History Solar cells started in 1876 with William Grylls Adams along with an undergraduate student of his. A French scientist, by the name of Edmond Becquerel, first discovered the photovoltaic effect in the summer of 1839. He theorized that certain elements on the periodic table, such as silicon, reacted to the exposure of sun

Probing interactions of molecules and pigments on titanium dioxide surfaces for photovoltaic cells, using QCM-D

Hauke Arne Harms

Photovoltaic cells offer a sustainable way of generating electricity from sunlight, and they will be of growing economical importance for decades to come. Hybrid organic-inorganic solar cells, such as dye sensitized solar cells or perovskite solar cells, are an area of active research, and fundamental scientific advances can lead to impressive improvements of device efficiency. In this thesis, I present experimental studies on some of the fundamental properties of molecules and pigments on titanium dioxide surfaces for solar energy conversion. My main instrument is a quartz crystal microbalance with dissipation technique (QCM-D), and I develop special methods of measurement, which enable the characterization of mass uptake in mesoporous systems. In dye sensitized solar cells (DSC), dye molecules are adsorbed onto a mesoporous titanium dioxide (TiO2) photoanode. The dye molecule absorbs light, subsequently injects an electron into the conducting TiO2, and is regenerated from a hole transport material. Furthermore, the molecular film of dye molecules blocks recombination between electrons in the TiO2 conduction band and the oxidized form of the redox mediator in the hole transport material. In the first chapter, I study the adsorption of the ruthenium-based dye Z907 by QCM-D on flat TiO2 as a model system with sub-monolayer resolution. An adsorption isotherm is measured for Z907, which leads to a formal adsorption equilibrium constant of 5.1e6 M-1 and a number density of 0.76 molecules per nm2. The coadsorbate chenodeoxycholic acid (cheno) enhances the recombination-blocking capability of the adsorbed molecular layer and it prevents aggregation of dye molecules. In chapter two, I develop a method to quantify the ratio between cheno and dye molecules when coadsorbed on the TiO2 using a combination of QCM-D and subsequent fluorescence spectroscopy. I apply this method to study coadsorption with triphenylamine-, porphyrin-, ulllazine- and diketopyrrolopyrrole-based dyes. The role of cheno depends on its specific interaction with the dye molecule. In chapter three, I present a microscopic study on the molecular layer of the amphiphilic dye Z907 adsorbed on TiO2. In collaboration with two groups at EPFL, we present in-situ atomic force microscopy images with sub-nanometer resolution in a functionally relevant liquid environment. Our results reveal changes in the conformation and the lateral arrangement of the dye molecules, depending on their average packing density on the surface, and these results are confirmed by molecular dynamics simulations. Chapter four aims to understand the QCM-D signals on mesoporous TiO2 films attached to the QCM-D sensor. Their high accessible surface area makes mesoporous films interesting for sensing applications. For films of up to 200 nm thickness, I find a regular inertial loading, which allows for an archimedic measure of molecules adsorbed inside the mesoporous matrix. For thicker films, I observe a particular behaviour that is explained by the resonant coupling of the quartz crystal driving frequency to an eigenmode of the TiO2 film. In chapter five, I contribute to the fundamental understanding of the perovskite conversion reaction between PbI2 and CH3NH3PbI3 perovskite. This material is central to the latest, record-breaking perovskite solar cells. I provide evidence for that the back-conversion is reversible by release of methylammonium iodide from the perovskite film into selective solvents.

EPFL2014

Promotion de l'utilisation rationnelle de l'énergie dans l'industrie par PINCHLENI

Daniel Favrat, Pierre Krummenacher, François Maréchal, Adam Molyneaux

A computer system for the promotion of the rational use of energy in the industry has been developed. It combines the PINCHLENI software developed by the LENI (Laboratory of industrial Energy Systems) of the Swiss Federal Institute of Technology (Lausanne) and the web based platform EXSYS developed initially by the LASSC (Laboratory for the Analysis and the Synthesis of Chemical Systems) at the University of Liège (B) with the support of the European commission. It represents an efficient platform for solving process integration problems.? ? A web site allowing free download of the tools has been developed. It includes a users data base, a FAQ and a bug reporting section that will allow a more efficient and easier maintenance of the software tools.? ? The Windows version of PINCHLENI has been stabilized and a communication interface with the EXSYS platform has been developed. This system is an efficient system for computing the process integration including the optimal integration of energy conversion technologies like combustion, gas turbines, steam network, Rankine cycles, heat pumps and refrigeration cycles. This system is particularly well suited for solving site scale problems (as demonstrated in an industrial site application solved during this project).? ? A courseware (in English language) helps learning the utilisation of the software. It is composed of online manuals and teaching materials for the theoretical backgrounds, a FAQ list with a search engine as well as a hypertext tutorial document illsutrating the application of the two computer platforms for solving a small example.? ? Results of this research project will be found on the LENI web site: (http://leniwww.epfl.ch/).

2002

Multi-objective design and optimization of district energy systems including polygeneration energy conversion technologies

Céline Weber

In the present context of finding ways to decrease CO2 emissions linked with human activity, district energy systems including polygeneration energy conversion technologies are likely to play a major role. District energy systems meet the heating, hot water, cooling and electricity requirements of a district. Because they meet several types of energy requirements, and for more than one single building, district energy systems represent good opportunities to implement polygeneration energy conversion technologies. Polygeneration energy conversion technologies indeed provide different energy services simultaneously, helping to decrease the CO2 intensity compared to energy conversion technologies that meet only one energy service. Moreover, when providing energy to a whole district, polygeneration energy conversion technologies can take advantage of the various load profiles of the buildings by compensating the fluctuations and having therefore a smoother operation. A district energy system comprises essentially two parts: the plant with the polygeneration energy conversion technologies, and the distribution networks (heating and cooling). When designing the energy system for a district, one has therefore to define which type of polygeneration energy conversion technologies are best suited for the district, as well as which building are worse connecting to the system and which buildings shouldn't be connected (for instance if they are located too far away from the other buildings or if they have too small requirements to justify a connection from the plant). Moreover the operation strategy needs to be defined. In the present thesis, a method is developed that helps designing and optimizing district energy systems, from the structuring of the information available for the district (energy consumption profiles, location of the buildings, available energy sources, possible layouts for the pipes,...), over the thermo-economic modelling of the energy conversion technologies, the design of the network and the simulation of its operation strategy, and finally the evaluation of the results in terms of CO2 emissions and costs. The design and optimization of the district energy system is a multi-objective Mixed Integer Non Linear Programming problem. To solve this problem, a decomposition strategy including a master and a slave problem was developed. The master optimization problem takes care of the energy conversion technologies, whereas the slave optimization problem optimizes the network part. The two sub-problems are solved iteratively and result in the definition of a Pareto optimal curve that gives the trade-offs between the emissions and the costs for various configurations satisfying the requirements of the district. A configuration is characterized by given types and sizes of energy conversion technologies, their location in the district, the network layout, as well as the operation strategy of the technologies. Due to the time dependent energy consumption profiles and the geographical location of the buildings and plant, the method developed combines two well known types of problems, namely the multi-period optimization problems and the network problems. The method developed allows to take into account various constraints such as limited availability of energy sources, forbidden connections between buildings (for instance if a large river separates these two buildings), or else space limitations in underground technical channels. The capabilities of the method are demonstrated by means of a test case, as well as a real case in the Canton of Geneva. The results show the importance of considering all the energy services together (and not separately). Energy systems including a gas engine or a gas turbine combined cycle, together with heat pumps, indeed help decreasing both the emissions and the costs compared to the actual configurations. In the Geneva case study for instance, emissions can be decreased by up to 45%, with a simultaneous costs reduction of 24%. However, the method only deals with water networks, while in some cases space limitations and safety issues make the use of water impossible. A new type of district energy system based on CO2 as energy transfer medium (instead of water), is therefore developed in order to take such issues into account. This new system, that led to the submission of a patent, meets all the different types of energy requirements with only two pipes (instead of three or four like in conventional water based system), and uses the latent heat of CO2 as driving force, instead of the specific heat.

EPFL2008