1、96 2009 ASHRAEABSTRACTThis paper provides an overview of the recent develop-ments of Heat, Air and Moisture modeling of Whole Buildings, which were carried out within a collaborative project of the International Energy Agency. The project has strived to advance the possibilities to calculate the int
2、egrated phenom-ena of heat, air and moisture flows while including the impor-tant interactions that take place in buildings between the various building materials, components, and room air, and how those conditions are influenced by occupants and HVAC systems. Principles and some applications of dif
3、ferent levels of modeling are presented: simplified modeling of moisture buff-ering, whole building coupled models as well as more detailed contributions for airflow modeling, including CFD models.INTRODUCTIONHumidity levels in building components and in indoor air depend on vapor transfers, on ener
4、gy fluxes and resulting temperature levels and on air flows. The interactions between these phenomena are essential for the whole building (WB) Heat Air and Moisture (HAM) response. Indeed, the levels of relative humidity in the indoor air are strongly dependent not only on the moisture transfers be
5、tween the air and the construction and moisture sources, but also on the correct analysis of airflows and of temperature levels, dependent upon proper energy balances.When the impact of moisture on whole building energy response is considered, the first and essential step is to correctly represent t
6、he moisture balance, including vapor absorption and desorption from hygroscopic surfaces. In some practical applications, when only an estimate of the indoor climate is of interest, this can be done using simplified models for moisture buffering, such as Effective Moisture Penetration Depth (EMPD) m
7、odel. However when the moisture level in constructions is of interest, the investigations require use of coupled heat and mass transfer models to describe the complex physics in walls. When a detailed field of moisture in the air or in the constructions is needed, Computational Fluid Dynamics (CFD)
8、can help to get a precise prediction of the conditions. All these approaches, from EMPD to CFD, are complementary, and are of interest in HAM simulations of buildings.Whole Building Simulation Tools at PresentDuring the past few decades there has been quite some development and professional use of t
9、ools that describe some of the processes and building elements, which should be considered when whole building heat, air and moisture trans-port are analyzed. These tools have different capabilities with regard to the level of detail represented.Most of the simulation tools which are able to represe
10、nt the whole building, including the mass and energy flows in a building and in simple heating and ventilating systems in dynamic conditions, are mainly coming from energy perfor-mance simulation field. A list of such tools, which have been developed for more than two decades now, can be seen in htt
11、p:/www.eere.energy.gov/buildings/tools_directory/. Some of them can calculate the moisture level in the indoor air and can account for moisture storage in hygroscopic materials. Mois-ture storage is modeled either using simple lumped models (see for instance Kerestecioglu et al., 1989) or using a de
12、tailed From EMPD to CFDOverview of Different Approaches for Heat Air and Moisture Modeling in IEA Annex 41Monika Woloszyn, PhD Carsten Rode, PhD Angela Sasic Kalagasidis, PhDMember ASHRAEArnold Janssens, PhD Michel De Paepe, PhDMember ASHRAE Associate Member ASHRAEMonika Woloszyn is an associate pro
13、fessor in Civil Engineering (Thermal Sciences Centre) at University Lyon 1, Villeurbanne, France. Carsten Rode is an associate professor in Civil Engineering at the Technical University of Denmark, Kgs. Lyngby, Denmark. Angela Sasic Kalagasidis is an assistant professor at Department of Building Tec
14、hnology at Chalmers University of Technology, Gothenburg, Sweden. Arnold Janssens is an associate professor in building physics at the Department of Architecture and Urban planning, faculty of engineering, and Michel De Paepe is an associate professor in thermodynamics and heat transfer at the Depar
15、tment of Flow, Heat and Combustion Mechan-ics, faculty of engineering, Ghent University in Gent, Belgium.LO-09-008 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. A
16、dditional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.ASHRAE Transactions 97description of the heat and mass transfer phenomena in the building envelope, which also allows the moisture level in building element
17、s to be assessed (Rode and Grau, 2003, Holm et al., 2003).Detailed simulation tools, where multi-dimensional fields of state variables can be assessed were mainly devel-oped for air flow simulation tools at the room level (CFD codes, e.g. Teodosiu et al. 2003) or for building envelopes, as for insta
18、nce around thermal bridges (see for instance Knzel et al., 2000, Funk and Grunewald, 2002). Some of CFD tools deal with airborne moisture transport, and even take the impact of moisture on the airflow into account. They also represent the heat transfer in the air and in the envelope. However in gene
19、ral, these tools do not take moisture flow between air and porous materials into account. The multidi-mensional tools to assess heat, air and moisture flows in enve-lope constructions have in general fairly good procedures to represent the outdoor environmental exposures, e.g. from weather data file
20、s, but the indoor environment often has to be assumed and specified by the user. Hence, whole building models should take into account location and orientation of the building (climatic zone), vari-ous heating, ventilating and air conditioning systems, air infil-tration or exfiltration, user behavio
21、r (number of people, activities, moisture and heat production, window ventilation, etc.) and type of room (bathroom, living room, office, etc.), see Figure 1. However, it should also be realized that the assembly of building elements themselves constitute one of the most important factors to determi
22、ne the indoor climate, and thus there is a mutual link between the envelope and room condi-tions.Typical Tasks for Whole Building HAM Simulation ToolsThe reason why so many levels of detail of simulation tools have been developed can be easily explained by the number of different questions that the
23、simulation tools are supposed to answer. For instance:How to optimize the global energy performance of a building, without ruining the indoor air quality (IAQ)?Are the building envelope elements at risk regarding moisture damage?In some cases these questions can be answered using energy performance
24、simulation tools, whereas in other situations, a detailed assessment of HAM flows in one room, or in complex assemblies of building envelopes is needed.Scope of the Subtask 1, IEA Annex 41Modeling of different physical aspects of buildings (Heat, Air and Moisture) has been a key element of Annex 41,
25、 involv-ing most of the participants. Annex 41 of the International Energy Agencys (IEA) Energy Conservation in Buildings and Community Systems program (ECBCS) was a four-year cooperative project on “Whole Building Heat, Air and Mois-ture Response” (2004-2007). The project sought to deepen the knowl
26、edge about the integrated heat, air and moisture trans-port processes when the whole building is considered. Alto-gether researchers from some 39 institutions representing 19 different countries participated in the project. The objective of one of the subtasks: Subtask 1Modelling and Common Exer-cis
27、es was to encourage the development and testing of new models that:integrate several physical aspects of buildings (Heat, Air and Moisture),operate on various levels of buildings: from porous mate-rials, over composite building constructions to whole buildings with their furnishing, systems and user
28、s,consider indoor as well as outdoor climatic conditions, andadopt 1-, 2- and 3-dimensional aspects, or combinations, as appropriate.The purpose of Subtask 1 has been to advance develop-ment in modeling the integral heat, air and moisture transfer processes that take place in “whole buildings”. It i
29、s believed that a full understanding of these processes for the whole building is absolutely crucial for future energy optimization of buildings, as this cannot take place without a coherent and complete description of all hygrothermal processes.This paper presents an overview of different simulatio
30、n tools and approaches that have been discussed and improved within the project.Figure 1 Building with some examples of hygrothermal loads.98 ASHRAE TransactionsNUMERICAL MODELS FOR HAM SIMULATION IN BUILDINGSModeling EquationsHeat and mass transfers in building can be described by energy and mass c
31、onservation equations. Energy balances consider the flows of heat by conduction, convection and radi-ation. Mass balances for water vapor include moisture flows by vapor diffusion, convection and liquid transport, and mass balances for dry air comprise air flows driven by natural, exter-nal or mecha
32、nical forces. In coupled heat, air and moisture analyses, the interactions between different phenomena, as shown in Figure 2, must be considered.There are two main methods to write conservation equa-tions. The theoretical laws are in general expressed in differ-ential form, applied to a representati
33、ve elementary volume. In numerical modeling practice however, the size of the control volume is always finite, even if it can vary in extremely large proportions (from mm3to several m3for buildings). This second method is employed in the following. The volume element can be a part of an air zone in
34、a building, or a part of a building material. Energy conservation equation. In a very general control volume it can be written as the flow rate of thermal and mechanical energy that enters the volume in, W (Btu/h) minus the flow rate of thermal and mechanical energy that leaves the control volume ou
35、t, W (Btu/h) plus the rate of ther-mal and mechanical energy generated in control volume , W (Btu/h) that must equal the rate of increase in the energy stored in the control volume Estored, J (Btu) during time t, s (h):(1)In most building applications, kinetic and potential energy contributions can
36、be neglected. Variation of energy of the control volume is then proportional to temperature varia-tions T, K (R), density , kg/m3(lbm/ft3) and specific heat c, J/(kg m3) Btu/(lbm F) of the material composing the control volume V, m3(ft3).(2)Air mass conservation equation. Air flows in buildings invo
37、lve significant transfer of energy and moisture. They can be assessed by analyzing air mass and momentum conserva-tion equations. A general expression of mass balance, also called the “continuity equation” applied to a control volume can be written as:(3)where mairis the mass of air, kg (lbm), airis
38、 density of air, kg/m3 (lbm/ft3), Q is the volumetric air flow rate, m3/s (ft3/h).For most building application, the left hand side of the equation can be practically eliminated, i.e. the mass of air in a volume can be regarded as constant, and there is an instan-taneous equilibrium between the mass
39、 of air flowing into and out of a volume. The conservation equation has the same form for control volumes situated in both the air zone or in the envelope.The momentum conservation equation, also called the equation of motion (derived from Newtons second law) assumes that the change in momentum of a
40、 fluid cell is the sum of all forces applied on this fluid cell. The summation of forces includes pressure forces, shear forces due to the viscosity and other forces, such as gravity.Moisture Conservation Equation. It should include both gaseous and liquid forms, e.g. by looking to vapor sources, tr
41、ansport by the air, diffusion and adsorption in solids. Water in its solid form (ice) may also appear in buildings, caus-ing mechanical damages.The equation for conservation of mass m, kg (lbm) of water within a volume element can be written in a general form as:(4)mmoisture= mvapor+ mliquid(5)where
42、 G is the total moisture flow through the surfaces of the volume element, kg/s (lbm/h), and is moisture produc-tion rate, kg/s (lbm/h).mliquid in the second term of Equation (5) refers to the moisture in any condensed or adsorbed phase. For moisture in a porous material, the mass of water vapor in t
43、he pores is most likely to be negligible compared to the mass in the adsorbed or condensed phases. The opposite is the case for cavities in Figure 2 Main interactions between heat and mass flows.EgenerateddEstoreddt- inout Egenerated+=dEstoreddt- cVdTdt-=dmairdt- air in,Qinair out, Qout=dmmoisturedt
44、- Gvapordiffusion in,Gliquid in,Gconvection in,+=Gvapordiffusion out,Gliquid out,Gconvection out,+()msource+msourceASHRAE Transactions 99building assemblies and spaces in buildings, where the mass of vapor will most likely dominate over the mass of condensed moisture. Nevertheless, the mass of humid
45、ity in the air of a room is rather small compared to the mass of moisture that is stored in furnishing and building constructions.Interfaces between Control Volumes. In addition to the conservation equations formulated for each control volume, the numerical model is complemented by additional equati
46、ons expressing the conditions at interfaces. In general these condi-tions translate the continuity of physical potentials (e.g. temperature, partial pressures) and heat and mass flows over the boundaries. In coupled HAM modeling, the interface equations include interactions between heat and mass tra
47、nsfers.Numerical ResolutionAll balance and interface equations are implemented in simulation tools and solved using some numerical methods for space and time discretization. Possible numerical methods are:Finite Difference Methods (FDM)/Finite Control Vol-ume methods (FCV),Finite Element Method (FEM
48、)Response Factor/Transfer Function method.Spatial discretization requires dividing the whole build-ing into small computational cells. While some important processes take place in narrow layers of air or building mate-rial, other processes in the same building do not require as finely discretized an
49、alyses. It is important to consider both heat and mass transfers with the most appropriate accuracy and computational efficiency when the hygrothermal condi-tions in rooms and building assemblies are to be predicted. The computational procedures may therefore need to work on different levels of spatial resolution. The choice of the size of control volumes, referred further as granularity, is then very important.Granularity or Spatial Discretization of an Air Volume. The term gra
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