TRNFlow - Airflow Simulation in Buildings Brochure

TRNFLOW TRANSSOLAR Energietechnik GmbH 1TRNFLOW: Integration of the airflow model COMISinto the multizone building model of TRNSYSM. Hiller 1, S. Holst 1 , T. Welfonder 1, A. Weber 2, M. Koschenz 2TRANSSOLAR Energietechnik GmbH1 Curiestraße 2, D- 70563 Stuttgart, tel.: +49 711 / 67976 – 0, fax: +49 711 / 67976-11holtine@transsolar.com, http://www.trnsys.de2 EMPA, Abteilung Energiesysteme/Haustechnik, CH-8600 DübendorfIntroductionIn the thermal building model TRNSYS Type 56 the air flows between the zones and from theoutside are defined by the user as input values. In natural ventilation systems these values de-pend on external wind pressures and the inside and outside temperatures. To account for thissituation, a coupling with an air flow model is absolutely necessary. TRNFLOW integrates themulti-zone air flow model COMIS into the type 56. An internal solver, optimized for this task,iterates in each time step between the two models until their solutions are consistent. The ex-isting user surface TRNBuild was extended in such a way that data for the air flow model can beentered. Thus a user-friendly handling of the coupling is assured. With TRNFLOW, a immensestep forward has been made for building simulation with TRNSYS.Fig. 1: Interaction of the thermal and the airflow modelCoupling of TRNSYS and COMIS in the PastIn order to achieve sustainable buildings new energy systems have been generatedusing natural effects to renew the air and lead away the heat. Examples are passivenight cooling, double facades, solar chimneys, inside courtyards and so on. In thesesystems the mutual impacts of thermal and air flow behavior are very distinctive. Thusfor numerical building simulation programs an integral approach is inevitable.TRNFLOW TRANSSOLAR Energietechnik GmbH 2Already in 1993 in the frame of the IEA project Annex 23 the EMPA has developed acoupling of the multizone air flow model COMIS with the thermal building and systemsimulation program TRNSYS which was presented at the TRNSYS Userday 1994. Theself-contained program COMIS was modified to TRNSYS Type 157 which can be linkedto the thermal building model Type 56 within the TRNSYS-Deck via in- and outputs. Theinput information of the air flow model are read in by Type 157 from the standardCOMIS Input File (CIF). The TRNSYS solver iterates the results of the two models untilthey match (Fig. 2)..DCK.BUI.BLDOutputMeteo.COF.CIF.SETStudioCOMISTRNBuild T 56TRNSYSMultizonebuildingInfiltrationVentilationCouplingT 157COMIS.TRNZoneOutdoorWindow andoperationAir FlowsTRNSYSSTUDIOPrinterReaderFig. 2: Former solution of the integration of COMIS as TYPE 157into TRNSYS Type 56This coupling has been successfully applied by several projects and simulation tasks.However the coupling wasn’t very user-friendly and required a laborious handling. Asthe mutual classification of the data is made by hand the inputs tend to be incorrect.This susceptibility is also increased by the redundancies of the two input files. Further-more it was proven that the TRNSYS solver is not always the perfect solution for such asystem. The solver possibly has to be supported by additional convergence promotingTypes what makes the handling again more difficult. Yet the need of an integral ap-proach concerning thermal building dynamics and natural air exchange is clearly neces-sary. Therefore with TRNFLOW an improved version including a deepened integrationof the two models is available now.Multizone Air Flow Model COMISMultizone air flow models idealize the building as a network of nodes and airflow links. Anode represents a room volume which a set of state variables can be assigned to.Cracks, window joints and openings, shafts as well as ventilation components like inletsand outlets, ducts and fans represent the links. Boundary conditions and thereby alsoinput factors are:• State variables of the air in the zones• Local wind pressuresThe pressure pZ is a free parameter in the node which is evaluated according to thecontinuity equation (mass flow balance in the node = 0). This results in nZ equationswhere nZ represents the number of zones.TRNFLOW TRANSSOLAR Energietechnik GmbH 3Fig. 3: Modeling of a building with zones and current outletsThe relation between mass flow rate m and pressure difference dp and thus the zonepressure pZ is not linear. Therefore an iterative process is used to solve the system ofequations. The mass flow rates per link and all dependent factors such as air exchangerates, air age etc. are calculated of the resulting zone pressures pZi. The calculation isstatic without an explicit consideration of the timestep. In principle calculating a condi-tion based on a new time is independent of the previous timestep.Coupling Concept of TRNFlowThe indoor temperatures are important boundary conditions of a multi zone air flowmodel which mostly can only be defined on the basis of the user`s guess. The indoor airtemperatures calculated by the thermal model strongly depend on the exchange of airbetween the zones as well as the outside. To link the two models and mutually use theresults is the obvious consequence. Thus the multi zone air flow model COMIS is com-pletely integrated into Type 56. This means that the exchange of data between thethermal and the air flow model is made internally and no longer by inputs and outputs.The proper classification of air flows (infiltration, ventilation, couplings) and tempera-tures to the air flow node resp. the thermal zones and the appropriate other model isautomatically carried out by the program.asdThermal modelTRNSYSTYPE56 - Weather data - Window or fan operation factors - Pollutant schedulesAirflow modelInfiltrationVentilationCoupling(Flowmatrix)Zone temperaturesPRINTERTYPEOUT COFInputs from other TYPES - Weather data - Heating - Cooling - etc.and humidityBLD TRN CIFDCKFig. 4: Coupling concept of TRNFlow: the air flow model is fully integrated in Type 56TRNFLOW TRANSSOLAR Energietechnik GmbH 4The input files of both models are kept in the existing formats (BUI, CIF) but can be cre-ated by the common user interface TRNBuild. Air flow model data depending on timelike wind velocity or window opening factors are defined as inputs or schedules. Outputslike air flows or zone pressures are declared as outputs by means of new NTYPES andcan be written into an output file using a printer type or processed otherwise. The stan-dard COMIS Output File (COF) is optionally also available.User Interface TRNBuild for TRNFlowThe functionality of the existing user interface TRNBuild has been enlarged. Beneathtraditional data of the thermal model also the required input information of the air flowmodel will be entered (see figure 5). This data is also stored in the BUI file and can beread in from it. Analog to the Standard Type 56 files (BUI,TRN, BLD) TRNBuild alsocreates a COMIS Input File (CIF) which is read by TRNFLOW. The CIF created this wayis - with few additions – completely equivalent to the format described in the COMIS 3.1User`s Guide [2].Fig. 5: User interface TRNBuild for TRNFLOWInternal Solver of TRNFlowDue to the integration of the air flow model into Type 56 the TRNSYS Solver can nolonger be used for the iteration process between the two models. I.e. the solutions haveto reach convergence using a solver integrated into Type 56. Depending on the bound-ary conditions the coupling of the two models forms a recursive flow of information withnegative feedback. E.g. at cold outdoor air an increasing indoor temperature imposesan increasing exchange of air. This counters another increase of the indoor temperatureso that a balance will appear (Fig. 6). If the negative feed back gets too strong there willbe the risk of getting an unstable system. A solver algorithm with a successive substitu-tion method (TRNSYS solver 0) then also becomes unstable. Therefore the new solverTRNFLOW TRANSSOLAR Energietechnik GmbH 5integrated into Type 56 automatically dampens individually every recursive flow of in-formation according to the iteration process so that an optimum of stability and conver-gence is achieved.? m=f( )?? m=f( )Fig. 6: Recursive flow of information between air flow and thermal modelDuct Network SystemBeneath the room nodes (thermal zones) the air flow model needs additional auxiliarynodes for the ventilation duct network as junctions of the individual parts of the ductsystem. In order to keep the thermal model small these nodes are not modeled asthermal zones in Type 56.The temperatures of the auxiliary nodes are directly calculated from the temperatures ofthe joining air flows. Thermal capacitance or other heat gains or losses of these nodesare not considered. In the thermal model air flows of the auxiliary nodes into the spacezones are interpreted as ventilations.To model an air heater or cooler, resp. a humidifier or dehumidifier, temperature as wellas humidity can be defined for an auxiliary node by means of a constant, input orschedule variable. The power necessary to reach this set point for the air can be ob-tained by an output.Figure 7 shows an example for the illustration of a duct network. The reference tem-perature of 20°C is defined for the auxiliary node AN6. For Room3 a ventilation of33kg/s with 17.9°C results. Room1 and Room2 have zero ventilation.18 °C 20°C15°CAN1 AN2 AN3AN4 AN520°C10kg/s15kg/s 8kg/sAN6ROOM1 ROOM2 ROOM3Fig. 7: Example duct network systemResulting temperature of aux. nodes:Aux. nodes Temperature [°C]AN1 18.8AN2 17.9AN3 17.9AN4 18.0AN5 15.0.TRNFLOW TRANSSOLAR Energietechnik GmbH 6Example: Double FacadeThe development of TRNFLOW was finished in March 2003. Since that date it has al-ready proved it’s strength in various projects like in the simulation of cross ventilation ofoffices, natural ventilation of double facades and atria, the pressure difference at doorsof high-rise buildings or pollutant concentration in a building. Figures 8 and 9 show amulti storey building with double facade as an example.TRNFLOW demonstrates its suitability for large building models by an example withmore than 60 thermal zones and lots of auxiliary nodes. No numerical or other difficul-ties have been obtained in those runs.Fig. 8: Air flow network model ofa multi story building with double facadeFig. 9: ONLINE Plot of the zone temperatures: floor 2, double facade 2, staircase andoutside temperature as well as the air flow referring to the room volume between floor2and the staircase and the number of internal iterationsTRNFLOW TRANSSOLAR Energietechnik GmbH 7Conclusions• TRNFLOW is a tool which allows an easy calculation of natural ventilation, passivenight cooling, double facades and exhaust air shafts quite fast.• No new input file structure or user interface has to be learned. Simply the knownuser-friendly TRNBuild has been expanded. Due to the full integration the requiredinput data is greatly reduced and less error-prone.• The newly-developed internal solver is featured with high stability. Due to the inte-grated automatical optimization of convergence the user is no longer bothered withnumerical questions.Further InformationIf you have further questions, please do not hesitate to contact your TRNSYS distribu-tor:TRANSSOLAR Energietechnik GmbHCuriestraße 2D-70563 StuttgartTel.: +49/711/ 679 76 - 0Fax: +49/ 711 / 67976 - 11e-mail: hotline@transsolar.comhttp://www.trnsys.deReferences[1] H. Feustel, B. Smith, V. Dorer, A. Haas A. Weber et al.: COMIS 3.1 – Programfor Modelling of Multizone Airflow and Pollutant Transport in Buildings, 2001EMPA Dübendorf, Switzerland[2] H. Feustel, B. Smith, V. Dorer, A. Haas A.Weber et al.: COMIS 3.1 – User'sGuide, 2001 EMPA Dübendorf, Switzerland[3] Klein, S.A. et al.. 2006. TRNSYS 16.1: a Transient System Simulation Program,SEL, University of Wisconsin, Madison, USA