Glass-free lightweight PV building elements: solutions to minimize weight and maximize durability

In densely populated countries, little free land is available for the deployment of photovoltaics (PV) in field installations. In addition, 40% of the world's demand for electricity is related to buildings. These facts provide a strong argument for the accelerated development of building-integrated PV (BIPV), as it enables electricity production with a minimal impact on free land. However, BIPV elements currently on the market show some limitations, which includes their relatively high prices, as most products are custom-made and produced in small volumes. Moreover, the relatively high weight (15-20 kg/m2) of these products may preclude their use in the potentially promising market of renovations of older buildings, for which excessive load could be a serious constraint.

With the aim of limiting the module weight while preserving excellent mechanical stability and durability, we demonstrate that, with careful material selection and a proper adaptation of the manufacturing processes, we are able to manufacture lightweight modules with a weight of 6 kg/m2. The weight reduction is achieved by replacing the backsheet, conventionally glass or polymer, with a rigid composite sandwich structure and by replacing the front cover glass with a thin polymer foil. In our approach, the conventional liquid epoxy used to glue skins and core is substituted with a thermoplastic adhesive foil (polyolefin) that ensures a good adhesion and stress transfer between the sandwich components. By associating this adhesive with a high-thermal-conductivity honeycomb core (aluminum), we can employ conventional lamination processes used in the PV industry to manufacture the backsheet and the full solar module stack simultaneously during a single process. These modules show an excellent resistance and stability when subjected to mechanical, climatic and electrical stress tests used in the solar industry. In fact, these solutions show a reduced power loss (lower than -5%), no major visual changes and preserve a good insulation resistance. These results demonstrate the potential of our design to successfully comply with the relevant PV qualification standards (e.g. IEC 61215).

We further demonstrate that our lightweight modules can successfully pass hail tests without cracked cells by tuning the mechanical properties of the frontsheet and backsheet simultaneously. The design of the lightweight PV modules is successfully scaled up to a sixteen-cell 0.81 × 0.81 m2 module.

Furthermore, the BIPV elements need to comply not only with PV standards but also with construction and safety standards, e.g. fire resistance. Preliminary results indicate that the proposed design in this thesis may successfully pass the relevant fire tests used for building elements (CEN/TS 1187). These first results also give indications on how to optimize the product further.

Finally, to reach a commercial market, some challenges must still be addressed: we must scale up to standard module sizes (1.6 m2), and we must develop an ideal mounting solution for a good integration of the modules into the building.

Modeling and Simulation of Energy Efficient Milling Process Plans for Prismatic Parts

Nenad Nesic

The milling process has been continuously optimized from many aspects: forces, velocities, stability, quality. Numerous papers have been published that report results in the domain of toolpath optimization with criteria such as raising the quality of the machined part, obtaining a stable machining process and maximizing the material removal rate. However, only recently researchers have started investigating the ecological impact of machining processes. With the constantly increasing prices of electrical energy and all environmental problems caused by the production and waste of the energy, it has become indispensable to look for ways to optimize machining processes from the energy consumption aspect, too. It is in this domain, then, that the work described in this dissertation contributes – to add a new criterion for the sustainable operation of machine tools: reducing the energy consumption during the use phase of the machine tool. The total energy spent in a machining process is the sum of power spent by all machine tool subsystems multiplied by the time that they are working. Therefore, the operating strategies for increasing energy efficiency for the high-speed machining processes must be oriented towards the reduction of the total machining time (productive and unproductive) and reduction of instantaneous consumed power in the machining process or, preferably, both. There are many causes for milling process time and energy inefficiency. The major sources of potential time reductions include: (i) the issues related to the geometry and topology of the toolpath („air-time”, overlapping segments, gouge and uncut regions, orientation of the toolpath), (ii) the issues related to the kinematics of the feed drives (the feed velocity/acceleration profile). The possible domains for power consumption reduction are related to the forces that exist in the machining process (cutting, inertial, gravitational and friction forces). The power invested to overcome the cutting force is necessary for the machining process but its consumption should be optimized through constant cutting engagement and feedrate scheduling strategies. The other forces are responsible for the pure mechanical power loss and they should be minimized. The optimization variables for inertial and gravitational forces include the feed motion profile and moving mass configuration, while friction losses are load and/or velocity dependent. In addition, there are some power losses in the electric components of machine tool drives (electrical power losses), whose dependency on velocity and mechanical load can also be modeled. Despite many research efforts to analyze and model the phenomena causing inefficiency of the machining process, they are usually not taken into account during milling process planning. In fact, rare are the attempts to use this knowledge to optimize the milling process with more than one of the above mentioned criteria. The reason for that lies in the fact that increasing the instantaneous values of tool engagement and feedrate in order to minimize total machining time also increases the consumed power and, indirectly, deteriorates the tool and/or destabilizes the cutting process. The existence of conflicting criteria in the objective functions imposes the use of evolutionary algorithms for multiobjective optimization. The process of experimental validation of such an optimization model requires costly and complex external hardware setup and it is also time consuming. Hence, there is an emerging need for the development of an accurate model, which would be used for the simulation of the milling process and testing the competing optimization strategies. The ultimate goal of contemporary manufacturing science and engineering is the development of a holistic virtual machine tool system. The motivation of this PhD research is to contribute to this objective by developing the “virtual milling process for prismatic parts (produced by 2.5D milling)”. This category of objects is selected, because the majority of CNC milling tasks can be performed using 2.5D milling. A very large number of mechanical parts are prismatic (their geometry consists exclusively of features that represent 2D contours extruded in a perpendicular direction). Parts that are even more complex are usually produced from a blank by a 2.5D roughing and 3D–5D finishing. Therefore, development of an accurate model of a 2.5D milling process represents a significant contribution to manufacturing engineering. Starting with above given problem description, we have defined the following research objectives: 1. Model power and energy consumption of a 2.5D milling machine as a function of the toolpath geometry (curvature and continuity), cutting engagement, real feedrate profiles and spindle rotational speed, taking into account moving mass configuration, all power losses (mechanical and electrical) in feed-axis and spindle drives and electromechanical constraints of servo drives; 2. Design an integrated software environment for simulation of the developed models; 3. Validate the model experimentally; 4. Demonstrate the potential of its use for multi-objective process optimization. Cutting engagement calculation is performed by using two competing methodologies, in order to make a comparison of their performances. The first one, the pixel based method, is based on discrete image processing, while the other, the exact method, is based on Boolean operations and vector geometry. The tests performed in more than 20 feature/toolpath combinations demonstrate that the exact method is more precise whereas the pixel-based method is computationally much faster. The machine tool operator commands a constant feedrate value. During the process, based on his/her subjective judgment, he/she can manually tune it to keep the process in the desired boundaries. One of the objectives of this research is to predict the real feedrate values so that they can be integrated in the part program before the process start. The feasible profiles of feed velocities for each toolpath segment are analyzed by answering the following questions: can the commanded feedrate be achieved and with what sequence of acceleration and deceleration phases, how much the tool has to slow down in corners and how does the feedrate profile degenerate in segments where the commanded feedrate cannot be reached? In order to determine all forces and torques, a machine tool model is developed. The following subsystems are modeled as rigid bodies (without vibrations): feed motor shaft, transmission, lead screw/nut pair, lead screw bearings, guides (sliding and rolling design), table with the workpiece and electrospindle (rotor, stator and bearings). All kinetic variables (linear and angular speeds; cutting, inertial and friction forces and torques) are then calculated in the joints of this multi-body system. The modeling of all speeds, forces and torques that occur in the machine tool system during the machining process, allows for calculation of various indicators of process time and energy efficiency: material removal rate, total machining time, useful power (invested in cutting), power loss due to the friction in the joints, power losses in electrical motors, electrical energy consumption of feed and spindle drives. In order to assess the accuracy and the reliability of the developed model two sets of experiments have been performed. The model validation is performed on several subsystem levels and the simulation results show a very good compliance with the experimental results. The first experimental setup consists of the force dynamometer, to measure cutting force components in linear axis directions, two laser optical position sensors, to measure the real feedrate and the power sensor, to measure the total power consumption of the machine tool electrical motors. The machining process was performed on the 6-axis machining center C.B. Ferrari A152 and the data acquisition platform was developed in LabVIEW 2010. The second experimental setup consists of the torque dynamometer, to measure the cutting torque directly on the spindle, the built-in current sensor that measures the motor current proportional to the electrical power consumed by the spindle and, again, the power sensor, to measure the total power consumption of the machine tool electrical motors. This time, MIKRON HPM 600U, a 5-axis milling machine equipped with the controller iTNC530, was used for the experimentation. The developed model is ready for practical implementation. Several examples have been prepared to demonstrate its capabilities in the domains of milling process simulation, sensitivity analysis and prospective optimization strategies. These examples include the prediction of various kinetostatic variables for given toolpaths, choice of the optimum machining strategy (minimization of machining time, moving mass and energy consumption) and process optimization by feedrate scheduling (reduction of cutting force fluctuation). As a conclusion, the major contribution of this PhD research is the development of a comprehensive rigid-body dynamics model of a machine tool system, aimed for the generation, simulation and optimization of 21⁄2D milling process plans. The model features several novelties compared to existing approaches: (i) it takes into account the machine tool rigid body dynamics and real tool kinematics in toolpath planning, (ii) it predicts all mechanical and electrical power losses in the machine tool system (iii) it allows the modeling of machine tool system electromechanical constraints. The model was experimentally validated on two real machine tools in different workshops and its capabilities for the simulation of different toolpath patterns and optimization of process parameters were successfully demonstrated using several examples.

EPFL2012

Polymer - Carbon Nanostructures Composites

Maryam Majidian

Synthesis and characterization of polymer-based composites containing carbon nanostructures is the focus of this dissertation. The main polymer which has been used for this research is SU8. The motivation of the work is to overcome the drawbacks of SU8 e.g. (high electrical and thermal resistance, brittleness) by addition of the carbon-based nanofillers. In this thesis, three classes of carbon nanostructures have been used as composite fillers: 2 dimentional graphene, 1 dimentional carbon nanotubes and 0 dimentional onion like carbons. The idea behind this material selection is to study the influence of dimensionality of the fillers on the electrical transport properties of the composites. In addition, we have performed comprehensive characterizations of graphene composites and addressed some questions about carbon nanotubes and nano onions composites. SU8 graphene composites were prepared using solution mixing method. The microstructure analysis of the composites showed a homogeneous dispersion of the graphene flakes. The study of electrical properties of the composite as function of the filler loading exhibited superior electrical conductivity compared to other graphene-base polymer composites. The study of the viscoelastic behavior of the composites showed that the rheological percolation is very close to zero, which we attribute to the polymer chain restriction due to high aspect ratio graphene fillers. The mechanical properties were evaluated with nanoindentation technique. 67 percent enhancement for Youngâs modulus and 75 percent enhancement for hardness were acquired. The possibility of the linkage between the filler and the matrix was investigated by spectroscopy techniques including Photoluminescence, Raman and Fourier Transform Infrared spectroscopy. Our findings suggest that covalent bonds are formed between SU8 and the functional groups on the surface of the graphene flakes. SU8-CNT composites were synthesized using both randomly dispersed and well-aligned tubes. For composites with randomly dispersed CNTs, the effect of nanotube length and polydispersity was investigated with experimental approach, for the first time. We have shown that the conductivity in such composites is proportional to mean length of CNTs, Ln, rather than weighted average length, Lw, which is predicted by theory. For case of aligned CNTs, we have measured the thermal conductivity, kappa, parallel and perpendicular to the orientation of the tubes, which exhibited an anisotropy close to 20 . The study was motivated by thermal management applications. From the same composite, lamellas with thickness ranged 20-100 nm were prepared using ultramicrotomy technique, for proton channeling applications. The successful sample preparation and pioneering channeling experiments give an encouraging outlook for future investigations in this field. Composites containing Poly methyl methacrylate as matrix and onion-like carbon as fillers were prepared for transport studies. The temperature and pressure dependence of the conductivity were measured. Due to the complexities associated to non homogeneous structure of these composites, we do not have a unified model to describe the dependence of conductivity upon concentration, temperature and pressure, and this question has remained open.

EPFL2016

Thermo-mechanical stability of lightweight glass-free photovoltaic modules based on a composite substrate

Christophe Ballif, Valentin Chapuis, Hengyu Li, Ana Cristina Oliveira Martins, Laure-Emmanuelle Perret Aebi, Alessandro Francesco Aldo Virtuani

Lightweight PV modules are attractive for building-integrated photovoltaic (BIPV) applications, especially for renovated buildings, where the additional load bearing capacity is limited. This work focuses on the development of a lightweight, glass-free photovoltaic (PV) module (6 kg/m(2)) composed of a composite sandwich back structure and a polymeric front layer. Sandwich structures are usually manufactured with a vacuum bag process and thermosetting liquid glues (e.g. epoxy resin). However, due to the long manufacturing process ( > 24 h), liquid adhesives are not compatible with conventional solar industry processes. This work presents the development of a robust glass-free PV module based on a composite sandwich architecture manufactured with a simple process. To simplify the production, the standard thermoset epoxy is substituted by different PV encapsulant foils (EVA, ionomer, polyolefin). The results show that a particular formulation of polyolefin is the ideal adhesive to produce a stable backsheet structure. The use of this polymer with a high thermal conductive core (aluminum honeycomb) allows a reduction of processing time from 24 h to 30 min. The mechanical properties of the composite sandwich structure showed an excellent stability under thermal cycling and damp heat with only 1% and 3% loss in bending stiffness, respectively. Two-cell lightweight PV modules manufactured with this backsheet show good electrical performance after thermal cycling and damp-heat tests, for which, respectively, an output power loss of only 3% and 2% is observed. This configuration is up scaled to a sixteen-cell module for which a power loss of only 3% is measured after damp-heat.