Controlled atmosphere pv concentrator

The purpose of this project was to develop a PV concentration module concept that consists on enclosing in a box the most sensible components of PV concentration modules (PV and reflectors). At this way, it is possible to eliminate degradation problems related to this type of systems in reflectors and PV module encapsulating organic elements (EVA, silicone, etc...) as browning of the encapsulating material). Elimination of encapsulating elements let to use nearly standard Si-cells in the prototypes of the CAC concentrator system, so reducing the costs of the whole system. The CAC PV module concept avoids the use of fast degradation encapsulating elements and protects the most sensible elements (cells and reflectors) by enclosing them in a non-aggressive ambient. This ambient conditions are achieved through dehydration of the air inside the CAC box.

A matrix for aluminium extrusion has been built for the heat-sink element of the CAC module. The material used for the heat-sink is Al97Mg3. After different tests and designs, in the final heat-sink design, 12 additional aluminium sheets with 0.65 mm thickness were attached in the back of the heat-sink. The aluminium sheets were attached using screws ans grease KP30 for the interface between aluminium pieces. This grease improved 51.9% the thermal transfer between the aluminium pieces according to the tests realised. The KP 30 Thermal Grease is a ceramic filled single-component silicone with high thermal conductivity. The uncured heat conducting paste does not dry out. Special storage is not necessary and can therefore be stored under normal climates for up to 12 months. Any appearance of settling of the filling material must be intensively mixed before KP 30 is used. The whole heat-sink design provided the appropriate thermal resistance for the concentration module, where its performance at different wind speeds has also been tested and resistance calculated.

The dehydrator has been designed to extract the internal humidity of the system CAC. The operation is the following one: the humid air that comes from the CAC, crosses a sensor of humidity and it is taken to the entrance of the compressor through the electro-valve and of a filter of air. The air that leaves the compressor is taken to a homogenizator of pressure with the purpose of transforming the intermittent flow into continuous flow, it crosses a first filter of salts endowed with element heater, next it crosses a second filter of salts, where it finishes retaining the humidity and raisin through a valve of on-pressure and of the electro-valve to the CAC. When the salts have absorbed a lot of humidity, a process of regeneration of the same ones begins.This process is carried out activating the two electro valves to commute the circuits of air and feeding the electro heator of the first filter of salts. The operation takes place in the following way: The compressor takes ambient air (through a filter) of the interior of the box. This air crosses the two filters of salt, but in this case, the salts of the first filter are hot, so the humidity of the salts is transferred to the flow of air that also crosses the second filter heating and absorbing the humidity. The electro-valve throws the air out to the exterior by means of a long tube to avoid the feedback to the entrance of air of the box.

This task has been carried out using a variety of resources. These included solar libraries, web search engines, solar industry catalogues and directories, personal contacts, glass and mirror industry sources, discussion groups, and a wealth of other investigative resources. The election of glass has been carried out considering the transmittance of the glass for the PV cell spectral response. The best results have been obtained for the Saint Gobain glass SGG DIAMANT float-glass with a transmittance greater than 91% for 4-5 mm thickness (ideal for the CAC prototype). Different Anti Reflective (AR) coatings have been tested for finding the best results. The best AR coating found improves 2.5% the transmittance of the glass window for the Si spectral response. The AR coating has also shown stable results for the IEC 61215 standard. Different reflective materials have been selected according to its properties. High reflectance and flexible films are ideal for this purpose. The best material found is the 3M SA-85 solar film. The SA-85 is aluminium vapour coated on 2-mil polyester film with an outdoor weatering acrylic coating over the aluminium. Unfortunately, 3M revealed that 3M would no longer be manufacturing this material and once the current supply in the field runs out, none would be available in the future.

Three different aluminium matrixes have been designed under a continuous feedback communication scheme. The first one is ideated for the tray and heat-sink element which composes an unique piece.The other two are for the CAC module frame. The module is also provided by a passive heat sink element that evacuates the remainder heat to the out side. So PV cells thermal and electric isolation problems are solved reducing the insolationg film sited between the backside of the cells and the heat sink. Although this situation reduces the thermal problem, the electrical is solved isolating the heat sink from the surroundings.

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