1. Numerical Analysis of the Impact of Thermal Inertia from the Furniture / Indoor Content and Phase Change Materials on the Building Energy Flexibility
Hicham Johra, Per Kvols Heiselberg, Jérôme Le Dréau
Introduction
Furnishing elements and indoor items have complicated shapes and are made of various materials. Therefore, many numerical models for building energy simulation assume empty rooms. However, this simplification could be problematic for dynamic thermal simulation of light-structure buildings. For example, some researchers pointed out that indoor content can delay the temperature increase in a room by up to 7 hours.
ted in a single equivalent planar element with explicit geometric representation and location in the thermal zone.
Detailed explicit furniture: Detailed explicit modelling of each indoor item with realistic geometry, location in the room and thermal properties.
Impact on building energy flexibility
The energy flexibility of a building is often defined as the ability to control its energy use in function of the grid requirement. Here, the heating systems of the residential house study cases are controlled with a strategy for heat storage in the indoor environment by the mean of indoor temperature set point modulation according to electricity spot price signal. The energy flexibility index is calculated with the relative amount of energy use shifted from high and medium price periods to low price periods.
Characterization of the indoor content thermal mass
No study or clear guidance concerning typical values for the furnishing / indoor content parameters in buildings could be found. A simple survey has been performed on residential and single office buildings in Denmark to determine the amount of indoor content per m² of floor area. The physical characteristics of representative material categories of the indoor items are suggested. With this information, the effective thermal mass of the indoor content has been calculated for different types of room.
Temperature set point modulation with price control signal.
Variation of yearly heating use profile with heat storage strategy.
House insulation level plays a dominant role in the improvement of the building energy flexibility. However, activated thermal mass, regardless of its nature and location, can also significantly improve the building heat storage capacity and therefore its energy flexibility potential up to a certain point. Under-floor heating system presents better performance regarding energy flexibility compared to convective radiators.
Results show that the impact of indoor content / furniture thermal mass is not negligible for light-structure buildings. Finally, phase change materials integrated in wallboards or furniture elements can appreciably increase light-structure buildings energy flexibility.
Indoor content mass in buildings.
Daily effective heat capacity of indoor content in buildings.
Properties of the representative indoor content material categories.
Modelling of the indoor content thermal mass
Different modelling methods of the indoor content for energy and indoor climate simulations are reviewed hereafter:
First order RC network: xR1C; all effective thermal mass (indoor content, air, structure) is aggregated in a single thermal node.
Higher order RC network: xR2C, xR3C, xRyC; different kinds of thermal mass are aggregated in different capacitance nodes.
Distinct indoor content thermal mass capacitance: Indoor content / furniture elements are aggregated in a dedicated capacitance node.
Equivalent virtual sphere: Internal mass bodies aggregated in a single virtual sphere with equivalent size and thermal properties.
Equivalent virtual planar element: Indoor content / furniture aggregated in an equivalent virtual planar element without explicit geometry or placement in the room.
Equivalent geometric planar element: Indoor content aggrega-
Effect of building effective thermal mass on energy flexibility
Influence of indoor content thermal mass on energy flexibility.
Conclusion
This numerical study showed that the empty-room assumption is not valid for the simulation of light-structure buildings with dynamic heat storage control. Different technics can be used to integrate the indoor content in building energy models with different degrees of complexity. Finally, even if insulation level plays the main role for building energy flexibility, significant improvement can be made by increasing the effective thermal mass with, for example, phase change materials integrated in wallboards or furniture elements.
Contact Information
Hicham JOHRA – PhD Student – Aalborg University, Denmark
Prof. Per Kvols HEISELBERG – Aalborg University, Denmark
Assoc. Prof. Jérôme LE DRÉAU - La Rochelle University, France
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