Phase changing solar shading

On the integration of phase change materials to improve the solar shading efficiency

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Energy consumption around the world increases by the day. The primary energy usage has risen with approximately 49% and the CO2 emissions with 43% during only the last two decades. The building sector in the Netherlands contributes for approximately 36% to this energy consumption and office buildings are one of the larger consumers (Rijksdienst voor Ondernemend Nederland, 2018). Even
though sustainability measures have been established, a contradicting trend occurs where building envelopes become more transparent and lightweight. This results in a lack of thermal mass and so increases the sensitivity to external influences. Solar shading applications are often applied to control the influences of the sun. This research project explores how the implementation of phase
change materials (PCM’s) could reduce this sensitivity. PCM’s present a suitable replacement for thermal mass, since PCM’s are known for their high storage densities in small volumes. How the combination of solar shading and PCM will affect the energy consumption of an office, is researched through its influence on the thermal and visual comfort of occupants. If both comforts are improved and stabilized by this application, less energy is needed to maintain this comfortable environment. This is defined in the main research question: “What is the most optimal PCM solar shading system which contributes to a stabilized indoor climate and a reduction of energy consumption in an office setting for both winter- and summertime in the Netherlands?”.
The key element within this question is the PCM solar shading system which exists out of two aspects; the PCM and the solar shading system. Both aspects include different parameters which determine whether a certain typology is suitable for this particular combination within an office setting. The influence of a sun shading system on the indoor climate is mainly determine by its thermal and visual performance. Whereas the thermal performance is determined by the
difference between external and internal applications, the visual performance depends on the flexibility of the system. In general, an external blind system would achieve the highest performance on both levels. However, if a PCM is combined with a shading system, an internal application would be more beneficial (Fiorito, 2012). Which PCM is most suitable has been established based on safety factors and appurtenant performances. A salt-hydrate is selected in this case, based on its nonflammable and non-toxic character, high latent heat storage and high thermal conductivity. The first two characteristics ensure a safe application and the latter two increase both the storage density
and heat transfer, and thus the performance.
Both PCM and shading related parameters are simulated to establish their individual and combined impact on the thermal and visual environment. The PCM related parameters concern the melting temperature and the layer thickness. These parameters are only simulated based on their thermal influence in Comsol Mutliphysics. Comsol is a simulation platform which is used to optimize physic-based designs through numerical simulations. The melting temperature showed to have one of the most influential effects. Since a too low temperature causes for overheating of the material and a too high temperature for a lack of activation, a specific value is required to provide any (positive) effect on the indoor environment. The tightly integrated relation to the surrounding conditions results in two different temperatures for summer and winter with 27 and 23 oC, respectively. The layer thickness on the other hand mainly influences the heat transfer. A thickness of 2 cm resulted in a relatively stable temperature flow throughout multiple days, whilst still being able to release heat in
both summer and winter conditions.
Shading related parameters are the position, depth, shape and encapsulation material of the blind. These parameters represent various aspects of a common blind system which can be altered to increase mainly the visual performance. However, these parameters might influence the amount, distribution and heat transfer of the PCM, which leads to a different thermal performance as well.
Visual performance related simulations are executed in Grasshopper, a graphical algorithm editor which uses plug-ins to apply performance simulations. The first parameter which is simulated in Grasshopper determines the influence of horizontally and vertically positioned blinds on the visual comfort. These simulations showed that vertical blinds perform well with low angled sun (winter
conditions) and horizontal blinds with high angled sun (summer conditions). However, horizontal blinds present the highest overall performance when different angles of the blinds towards the sun are considered. The visual results are used as a guideline due to limitations in the thermal models and assumed small influences.
The blind depth has a significant impact on the thermal performance. An increase in depth namely results in an increased PCM volume, unless the thickness is adjusted to maintain a constant volume. Additionally, it should be considered that an increased depth is linear proportional to an increased opening between the blinds. When these considerations are taken into account, a blind of 7x2 cm performs best. The thermal effects of different blind shapes, have a less
significant impact. A rule of thumb describes that smooth surfaces increase heat transfer and rough surfaces create obstructing air pockets. Visual simulations support the preference for smooth surfaces as a crescent shaped blind allows for the highest amount of comfortable daylight. All visual results are based on
a transparent encapsulation material to visually show the PCM. Polycarbonate has been selected in this case based on its transparent, tough and non-flammable character.
All parameter results are suitable for both winter and summer conditions, except for the melting temperature. When separate melting temperatures are applied, both thermal and visual enhancements can be achieved under different conditions. During summer an average temperature reduction of 1,5 oC, stabilized temperature fluctuations and 10% increase on visual comfort are
achieved. Smaller improvements are achieved during winter with a 1 oC increase and 5% visual comfort enhancement. Ultimately, when these influences are
converted, the general energy consumption of an office can be reduced by approximately 6,9%.
The results regarding the energy reduction are only achieved when the PCM is able to completely regenerate during the night. Hence, the effect of positioning, rotation and additional ventilation are studied to estimate how it could stimulate the regeneration process. In general, a horizontally positioned PCM blind will experience difficulties to initiate the solidification process compared to a vertical positioned element. However, once the blind is rotated, the horizontal blind will
provide an evenly distributed solid PCM layer. Rotation cause for the solidified particles to find more points of attachment, and thus the possibility to advance over the entire surface area. Additional ventilation will contribute during the initial phases, since it accelerates the solidification. To what extend these actions are necessary strongly depends on the surrounding conditions.
Both the evaluated parameters and regeneration process come together into one concept proposal with an appurtenant behavioural strategy. To determine how one system could function for both summer and winter conditions, three different system set-ups have been analyzed. Conclusively, a compromise regarding the melting temperature was required in order to obtain a practical application. Since an opportune melting temperature is a key component of the PCM blind system, a thermochromic layer is added to the blind surface. Similar to the PCM, the thermochromic layer will change its properties around a specific temperature. The material will change in tint and gain the ability to block solar radiation. Hence, excessive radiation is prevented from affecting the PCM. Through this combination one system would be able to contribute to a stabilized indoor climate whilst reducing the energy consumption.