Objetivos

The principle behind thin film heating is the same as for traditional electrical heating elements. However, these types of heating elements differ a lot in how the generated heat is subsequently transported. Initially, only flat sheets of thin film coated glass can be used for these are the only ones available commercially and it requires a lot of process optimisation to coat non-planar surfaces. This is also the reason why it is not possible to create differences in layer thickness or apply masks. Electrodes have to be applied in order to draw an electrical current through the thin film. Essential for proper functioning is a very low electrical resistance in the electrodes relative to the electrical resistance in the thin film. Also the electrical resistance has to be homogenous throughout the entire surface to ensure an even temperature. There are various ways of applying the electrodes depending on the geometry and desired thermal behaviour, but for now a rectangular

configuration will be used. To demonstrate the feasibility, facilitate prototyping and gain knowledge the various ways for creating the electrodes investigated. All in all this has led to roughly five methods: electroless deposition, electrolysis, metal spraying, applying a conductive adhesive through screen printing and applying a conductive foil. Subsequent experiments will have to point out what method works best.

Glass remains the best substrate in this particular case although other materials are pos- sible. This is because glass is transparent, hygienic, can withstand the CVD process and has the desired thermal and mechanical behaviour. Concerning the type of glass, tem- pered borosilicate glass is the most suited. There are many ways of treating the glass both mechanically and optically. The coating process called chemical vapour deposition (CVD) requires the substrate to be heated between 400°C and 450°C and, although the figures suggest differently, the tempered glass can still be used as a substrate for this coating process. Once the glass is tempered it cannot be reworked anymore. Hence, actions like polishing edges and drilling holes must be carried out prior to the tempering process. Laminating the glass is safer and also contributes to the desired thermal behaviour, but whether this is truly applicable at the desired temperature ranges is yet to be found out. To safely protect the tray from overheating a bimetallic overheat protector is required. The easiest way to make the temperature adjustable is by implementing a triac and po- tentiometer. A safer yet more expensive solution is by implementing a transformer and regulate the temperature by switch mode power supply (SMSPS). It is easy to implement a rough automatic temperature control but very difficult to make an accurate one because that requires a temperature sensor in the middle of the heated surface i.e. compromis- ing the aesthetic qualities. However, for such a simple device, an accurate temperature is not necessary and a simple thermistor at the edge suffices. Subsequently this thermistor can also be used to create a heating indicator. A short circuit protection can be made in an ordinary fashion i.e. implementing a simple fuse in the power supply. However, it is a different story with the automatic fault circuit interrupter. Most appliances don’t require such a feature but this one does because of the relatively vulnerable heating surface. If the surface breaks there will be shock hazard and to prevent this the sudden increase in electrical resistance can be used to trigger a switch that turns of the entire system auto- matically. To get insight in the power consumption, efficiency and required temperature range some preliminary calculations have been made. It turned out that the tray will have an efficiency of only roughly 17% if it is made in the fashion suggested. Luckily, there are some serious improvements like applying heat shields and lowering conductivity locally which could lower the energy consumption, required for keeping an aluminium pan at 60°C, from 303 Watts to 138 Watts. Also the material can be optimized to facilitate other thermal and electrical behaviour which will be important for other possible concept di- rections. From the product comparison it follows that the temperature range lies between 64 °C and 113 °C on average. This is the initial target value, since so little is still known about the exact thermal behaviour. Further research involving a worst case scenario will point out what is the optimal temperature range. All in all it seems that the thin film heated glass is very well capable of delivering sufficient power. This analysis has shown that, especially with further optimisations, this is a very feasible way of heating foods.

OVerall guideli

H6 Overall guideline and total list

of demands and wishes

To have a clear basis for the design all of the foregoing is condensed into a list of de- mands and wishes. This list is supplemented by a textual guideline.