Simulation software of the hygrothermal environment of buildings based on detailed thermodynamic models
Ozaki, A., Watanabe, T. and Takase, S.
2004 eSim 2004
Ozaki, A., Watanabe, T. and Takase, S., (2004), "Simulation software of the hygrothermal environment of buildings based on detailed thermodynamic models", eSim 2004.
Abstract:
A Heat, Air and Moisture (HAM) simulation software program called THERB for has been developed for the purpose of estimating the hygrothermal environment within buildings. Authors have already developed THERB as a simulation software tool to estimate the thermal environment within residential buildings, however the new THERB program has been expanded to include complete HAM features including principles of moisture transfer within walls. Generally simulation software to predict temperature, humidity, heating and cooling load of building spaces does not take into account moisture transfer in wall assemblies. Humidity calculation in most software is simply affected by ventilation and focuses on just the building spaces. THERB for HAM was developed to simulate humidity conditions in both building spaces and wall assemblies in detail. THERB for HAM excels in the theories for describing actual building physics. Most of thermal theories on radiation and convection and ventilation utilize existing models already developed for THERB, however the water potential, which is defined as thermodynamic energy, is a newly adopted feature of THERB, which incorporates moisture transfer including moisture sorption and desorption of walls. Authors have also proposed such a moisture transfer model called P-model and developed simulation software to analyze moisture behaviour in wall assemblies. Thus THERB for HAM represents the combined simulation software capabilities of the original THERB software and the new features of the P-model, which can now predict the hygrothermal environment of the whole building (all spaces and wall assemblies of the building) taking into consideration the complex relationship between heat and moisture transfer and air flow. In this paper, the basic theories used in THERB for HAM are described. Accuracy of THERB for HAM is verified through the comparison of calculation and monitoring results of a residential building. Furthermore the difference of calculation results is clarified based whether or not moisture sorption and desorption of walls are taken into account. INTRODUCTION THERB for HAM is a dynamic simulation software which can estimate temperature, humidity, sensible temperature, and heating/cooling load for multiple zone buildings and wall assemblies. The heat and moisture transfer models used in THERB such as conduction, convection, radiation and ventilation (or air leakage) are based upon the detailed phenomena describing actual building physics [1], and can be applied to all forms of building design, structure or occupant schedules, etc. All the phenomena are calculated without simplification of the heat and moisture transfer principles of any building component or element. The paper explains prominent features of the models, and accuracy of T HERB for HAM is also described.
THEORETICAL FEATURE OF THERB
FOR HAM
The following outlines the algorithms for heat and moisture transfer used in THERB for HAM, which are derived from fundamental building physics principles. Conductive Heat Transfer: The finite difference method is applied to the model of one-dimensional transient thermal conduction of multi-layer walls. Regarding thermal conduction to the ground, the finite difference method of two or three dimensions is applied to the previous calculation of the ground temperature and then the results are used as the input excitation for conductive calculation of the earthen floor and bas ement walls.
Convective Heat Transfer: By default, the convective heat transfer coefficients are recalculated at every time step on all surfaces of the exterior, interior and cavities of buildings using dimensionless equations which are derived from either the profile method for boundary layer [2] (based on the energy equation, the momentum equation and the fluid friction) or defined from the exper imental findings according to natural or
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