Buildings account for almost 41% of the world’s energy consumption, which contributes to 30% of the annual greenhouse gas emissions . Trombe wall integrating phase change materials (PCM) is a particular passive solar technique that has shown great potentialities and can reduce effectively the building energy consumption.
During a cold day, this wall is heated due to the incident solar radiation, melting the PCM. While at night, when the outdoor temperature falls below the phase change temperature, the heat stored by the PCM is released warming the building. On the other hand, the integration of PCM in a transparent element of the building envelope enhances the ability of energy storage, since the PCM will be directly exposed to the solar radiation. This technology aims to smooth the indoor temperature and decrease the energy fluctuations, providing daylighting at the same time.
A new kind of translucent storage wall was studied in France (Figure 1) by the ANR INERTRANS (1) project and patented . A Ph.D. research work was presented at the issue of the project . A new study on this wall is presented here and a new acronym for this wall is proposed: TIM-PCM Trombe wall which stands for “Translucent Insulating Material – Phase Change Material Trombe wall”. Many features, which are not found in a conventional Trombe wall, are combined by such a wall: it provides heat gains from solar radiation, high thermal insulation, heat storage and release, natural daylighting and visual communication to the outside world.
It is composed, from outside to inside, of a glass pane having a thickness of 0.8 cm, a 4-cm thick bed of granular silica aerogel (transparent insulation material), and an eutectic of fatty acids as PCM filled in glass bricks of dimension 19cm 19cm 5cm. The Silica aerogel granulate bed provides super heat insulation, meaning that it has a thermal conductivity lower than that of still air (0.018 w/ (m K) for aerogel, 0.026 w/ (m K) for still air). This particular silica aerogel is chosen to meet the TIM–PCM wall transparency and insulation principles.
The wall also provides sound insulation and solar and light transmission. The chosen PCM has a comfortable phase change temperature with long-term stability. It provides solar absorption, energy storage, and restitution, in addition to light transmission. It absorbs the solar radiation when being in the solid state, thus increasing its temperature until the complete melting is achieved, and transmits solar radiation when being in liquid phase.
The thermal performance of the TIM-PCM wall is tested in a full-sized test cell located in Sophia Antipolis, Southern France, within the center for Processes, Renewable Energies and Energy Systems (PERSEE) of Mines ParisTech graduate school. In the winter season, particularly in sunny cold days, the PCM absorbs solar radiation, melts, and then releases the stored heat to the building at night by solidifying. While during the summer, an overheating problem is encountered mainly due to solar gains, the PCM remains in its liquid state and it is unable to release the stored heat at night.
Thus, to enhance the energy performance of the wall, numerical modeling of the heat transfer mechanisms in the wall materials is required and especially the melting with combined natural convection and radiation. Usually, the building models with integrated PCM ignore the convection effect in the liquid region, due to the complexity of CFD models and the required high computational time. It was proven that this assumption is not always adequate and convection effect must be considered in liquid PCM.
Most previous works studying PCM-enhanced transparent components have been mainly developed for the heating season, and there is a little quantification of their real advantages or disadvantages in terms of energy efficiency and indoor environmental comfort in the summer season. Thus, the present work focuses on the analysis of the summer performance of the innovative translucent TIM-PCM wall and on its effect on thermal comfort. The numerical model developed in this study aims to provide an easy tool to use and fast enough to be adopted as a design tool, to investigate the potentials and disadvantages of the TIM-PCM wall under different operative conditions and different climates and to propose solutions to optimize its performance in summer, without the need of performing extensive and expensive experimental analysis.
A one-dimensional numerical model describing the heat transfer mechanisms occurring in the PCM layer in combination with the other transparent wall layers is developed considering the effect of thermal bridges caused by the joints of the bricks. Mesh sensitivity analysis was carried out for the numerical model to make sure that the results are independent of the numerical domain.
A modified enthalpy method is adopted to solve the phase change problem. The natural convection occurring in the liquid PCM is accounted for using the enhanced thermal conductivity approach coupled with the scaling theory, and the absorbed shortwave radiation flux is also added into the energy equation as a source term using a simplified solution algorithm. The unsteady energy equation is written for each node and solved numerically. The developed TIM-PCM wall model computes the temperature field and the solar radiation transmitted to the test cell through the wall at each time step, these outputs are then linked to TRNSYS to simulate the energy performance of the whole building. Details of the numerical model are found in .
The numerical model is validated comparing the hourly profile of the measured data and simulated results of the internal surface temperature and the indoor test room air temperature for seven consecutive days in summer (figure 2) and winter seasons. The root mean square error (RMSE) and the percentage root mean square error (PRMSE) are found between 0.57 ⁰C – 1.43 ⁰C and 1.87% – 6.99% respectively, showing a good agreement between numerical model and experiments. It was proved that to be more realistic, natural convection in the liquid PCM should not be neglected when modeling phase change in the wall.
The validated model is then used to optimize the wall performance in the summer season. It was shown that shading devices can effectively reduce overheating while natural night ventilation decreases the indoor temperature without affecting the PCM performance due to the fact that the outdoor temperature is always higher than the phase change temperature. The use of a glass with selective solar reflection properties depending on the season (Prisma Solar glass) instead of the ordinary glazing is shown also to be a very effective way to overcome the overheating problem in summer while preserving the TIM-PCM advantages during winter.
The thermal comfort is studied in an office room integrating TIM-PCM wall in the south orientation with specific internal gains conditions and under different climates. Five different cities with different climates were chosen according to the Köppen–Geiger classification. Figure 3 shows the percentage of occupied time when overheating will possibly occur according to ASHRAE 55 adaptive comfort model for two different levels of satisfaction for the different climates in three months of the hot season.
It was shown that the thermal comfort and cycling of PCM in summertime depends on the climate conditions. In the hot-summer Mediterranean climate (Csa), Venetian blinds of slats rotated at 45 degrees combined with an overhang of 1 m projection can ensure thermal comfort with the TIM-PCM wall. In oceanic (Cfb) and warm-summer humid continental climates (Dfb), thermal comfort can be ensured by the sole use of an overhang of 1 m projection and Venetian blinds respectively. In subarctic climate (Dfc) the PCM achieves complete diurnal cycling and thermal comfort can be attained providing natural night ventilation.
In this work, the developed numerical model represents a good starting point for simulations on different configurations of the novel TIM-PCM wall and allows to investigate deeply its abilities and drawbacks under different operative conditions, orientations, geometries and different climates, typically during the hot season, without the need of performing the expensive experimental analysis. As future work, life cycle cost analysis, payback periods and optimization of TIM-PCM wall configuration under different climates conditions will be studied using the validated numerical model.
These findings are described in the article entitled Thermal behavior of a translucent superinsulated latent heat energy storage wall in summertime, recently published in the journal Applied Energy. This work was conducted by Farah Souayfane and Farouk Fardoun from the Lebanese University and Pascal Henry Biwole from MINES Paris Tech.
(1) INERTRANS is an acronym of Translucent Inertia, project funded by the French National Research Agency (ANR), PREBAT program 2007).
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- Souayfane F, Biwole PH, Fardoun F. Thermal behavior of a translucent superinsulated latent heat energy storage wall in summertime. Applied Energy. 2018 May 1; 217:390-408.