Numerical simulation of thermal mass enhanced envelopes for office buildings in subtropical climate zones

Current developments of building lightweight and energy efficient sandwich constructions are driven by sophisticated solutions that are lighter, of larger area, thinner, safer and smarter. This has led to more envelope manufacturers resorting to new sandwich laminates based on fiber reinforced polymer (FRP) and a phase change material is introduced into building of envelope structures in order to enhance thermal mass.

Professor Ben Y.B. Chan and Andrey A. Chernousov from The Hong Kong University of Science and Technology extended earlier studies on efficiency effects of a substantial thermal mass of a phase change material (PCM) layer surrounded by two layers with changeable thermal resistance and thermal mass which were separately determined for distinct envelope orientations to the sun track within the hot season in Hong Kong. Their work which is now published in journal, Energy and Buildings adapted the optimized cases for a typical office building with both windowless and glazed building envelopes in order to carry out more authentic modelling and estimate phase change material advantages.

Various studies on effect of phase change material on building envelope has been conducted with examples of phase change material presence at the interior, exterior or inside the envelope. First, it was suggested that phase change material thermal mass does not play a significant role in lightweight building envelopes with relatively high thermal resistance which has led to the studying of large-scale and compact system reinforced with fiber reinforced polymer is promising. The encased smart layer is equivalent to a PCM thermal mass operating between two layers of certain thermal resistance and thermal mass in this situation. This situation was touched in case where the appropriate PCM position for day-and-night cooling was shifted outwards from the interior because the PCM requires charging which can only occur due to outdoor fluctuations in air temperature, irradiation level, wind speed etc. This approach however has challenges due to non-affordability of 24h cooling for smart office building in subtropical megacities, provocation of room warming during hot summer nights, absence of night ventilation leading to heat energy accumulation over hot days in internal phase change material layer and uncertainty about relatively thick phase change material layers fencing an office space.

For the Implementation of this study, windowless envelope consisted of a polymeric foam core laminated by exterior and interior composites layers. Exterior layer was composed of aluminum facing 3mm thick and reinforced by 2mm of fiber reinforced polymer and interior composite is a combination of 2mm of FRP and a finished layer of 12mm gypsum plaster. Commercially, available polyisocyanurate foam (PIR) and organic PCM-RT were chosen for the core (t_C= 40mm) and smart phase change material layer (t_PCM= 5, 10 and 20mm) respectively. The phase change material was separated from the exterior by relative thermal resistance R_rel and the relative thermal mass C_rel. Layer thickness (t), thermal conductivity k and specific heat capacity c was also measured.

Thermal simulation of building envelope was achieved by EnergyPlus (ver 7.2) software where a semi-implicit Crank-Nicolson scheme (2nd order) was chosen as the conductive finite difference scheme which is based on Adams-Moulton solution approach while test validation was achieved in variable conditions in month of July by temperature change comparison in different layers of a large scale building envelope prototype (0.9×0.9m², t_PCM=2.5mm).

Heat flux q in optimization of phase change material was studied by total energy transferred into a modeled office Q_on through the envelope which was numerically computed for hottest month in Hong Kong. Authentic building modeling was conducted by optimized phase change material placement of various thickness and EnergyPlus parameters for the thermal modelling was also implemented.

Results on optimization of phase change material in the building envelope showed maximum reduction in heat energy Q_on transferred into the modeled office by 7.3, 8.7, 15 and 14% for west, east, north and south oriented office respectively. It was also observed that phase change layer presence obviously modifies the hourly air temperature increases when it depends on orientation to sun. As thicker the phase change material layer was used, the faster and higher the temperature increases within the off-time period.

Results also showed that phase change material type as a layer in the envelope has to be less isolated from indoor air media. Phase change material was also seen to balance heat flux profile much better if envelopes are under moderate heat loads.

Integrated parameter of the liquid phase fraction when oriented to west and south showed that phase change material provides minor charging or discharging only in the envelope although efficient phase change material work stimulates a higher alteration of V_liq as thicker phase change material better resists to the night ventilation conditions and holds more discharged phase change material makes the indoor temperature worse.

Customary office aeration at night and on Sundays without air conditioning was found to have negative consequences such as overheating which was attributed to the day by day heat energy accumulation during hot periods in summer and insufficient cooling to discharge the relatively thick phase change material layers through envelope interior (fiber reinforced polymer and gypsum plate).

Overheating effect was minimized in realistic office building model containing the envelope window assembly while phase change material layers effectively balances the diurnal cooling power. Additional thermal mass does not provide further improvement even though it embodies the envelope with a slightly higher thermal resistance.

This study highlighted that phase change material layer has to be less isolated on the interior to provide rapid thermal exchange and control of the office temperature.

Acknowledgement: This research is supported by UCRUSAL as part of the funded research on the development  of large-scale pre-insulated fiber reinforced aluminum envelopes and cantilevered roof systems.