Adaptation
Innovative Trombe wall The new Trombe wall that we propose is based on innovative materials like phase change material for latent heat storage and aerogel for thermal insulation, is shapeoptimised for best energy performance, is created with advanced rapid prototyping techniques like (robotic) FDM printing, enables daylight to enter the space and is adaptable to changing environmental or use conditions. This ensures that our envisioned Trombe wall not only works in winter to capture heat from the sun but also in summer to capture heat from internal heat sources thereby acting as a cooling device (fig 1). Another important aspect of the design we are developing is that we want the performance of the system to be part of its identity. Phase change materials The main ingredient of the Trombe wall that stores the heat from the sun is phase change material (pcm). A pcm is a material that can store a large amount of heat during the change from solid to liquid state. Inversely, it will release this heat during the change from liquid to solid state. As such it can increase the thermal inertia of a building. Water is one of the most well-known pcms. However, its melting/solidification temperature (0oC) is too low for practical applications in buildings. Pcms that are useful in buildings come in different types: paraffins, salt hydrates and eutectics. The pcm we are using in this
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project is a salt-hydrate with a phase change temperature of around 25oC. The advantage of salt-hydrates is that they are non-flammable and may come in transparent form when liquid. Surface and cross-section optimisation The heat transfer to and from the pcm occurs via (short wave) solar radiation, via (long wave) infrared radiation and via convection. This means that the ideal surface of the Trombe wall considers all these three modes of heat transfer. During experiments we observed that melting the pcm by solar radiation is a relatively fast process. However, solidifying the pcm via convective heat transfer takes very long. During the project therefore several different surface textures are being investigated both with simulations and with physical experiments. Figure 3 shows an example of a textured surface with increased surface area to enable faster convective heat transfer. Not only the surface needs to be optimised, also the internal cross-section needs to be fine-tuned. The temperature distribution inside the pcm is one aspect to consider, creating places of translucency and places to look through the wall are another. Simulations have shown that if a pocket of pcm is too high, a large temperature gradient arises. In a pcm pocket of 3 cm deep and 20 cm high the bottom may still be around 25oC (still solid)
Figure 1: winter and summer mode of the Trombe wall.
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