An NMF Based Model Library for Building Climate and Energy Simulation

During the academic year 95-96 an NMF based model library has been developed by the IDA group. The library contains detailed and simplified zone models, full finite difference and reduced RC-network wall models, different types of solar radiation calculation models, state-of-the-art window models, local climate control models and some supporting Fortran subroutines. Among the most interesting models are the detailed convective radiative zone with full non-linear radiative and convective heat transfer using a general view factor calculation algorithm and the RC-wall model. These models are described in some detail in the paper. The full library has been comparatively validated against the BRIS program. In its most basic version, the library is more or less a BRIS re-implementation in IDA. The execution times are compared for applications developed in a modular simulation environment vs stand-alone programs. The relative development times are estimated as well. INTRODUCTION Simulation is an important tool for building research and design. A building, together with its installations, is a complicated system, and accurate predictions of its behavior require dynamic simulation. Simplified tools have abounded for a long time and have been made more attractive with attractive user interfaces. More detailed tools, capable of accurate predictions, have also been available for several decades, but their use has been hampered by several factors: accurate predictions need large amounts of input data, thus, effective user interfaces require much more development effort; developed codes have been monolithic and difficult to adapt to changing and growing user needs. Modular simulation environments (MSE:s) developed during the last decade have the potential to change this picture. Interfaces using familiar drag-and-drop techniques are available. The maintenance problem has been alleviated by the modular approach combined with effective handling of model development, using e.g. the Neutral Model Format (NMF). THE NEUTRAL MODEL FORMAT This format for description of building simulation models was first presented at the BS'89 in Vancouver. It is a program independent language for modelling of dynamical systems using differential-algebraic equations. NMF serves both as clear model documentation for human readers and as direct input for automatic machine translation [Sahlin 1996a ]. Some key aspects of NMF are discussed when model examples are presented below. ASHRAE has supported NMF and paid for development of translators from NMF to TRNSYS and HVACSIM+ [Grozman 1996]. Translators have also been developed for SPARK [Nataf 1995], IDA [Shapovalov 1997, Kolsaker 1994], ESACAP [Pelletret 1994], and MS1 [Lorenz 1994]. THE IDA SIMULATION ENVIRONMENT A modular simulation environment, IDA, has been developed by the Division of Building Services Engineering, KTH, and the Swedish Institute of Applied Mathematics (ITM) [Sahlin 1996b]. IDA is now being made available to the Swedish building industry via a consortium of some thirty companies, building designers, contractors, property owners, and equipment manufacturers. The consortium develops building design applications based on the IDA software during the period 1995-98. In support of this effort, an NMF-based model library has been developed at KTH during the academic year 1995-96. SCOPE OF MODEL LIBRARY One aim of the consortium effort is to provide easy-touse applications for early design estimates. Another aim is that applications should be based on detailed physical models, so that complicated problems, involving interaction between building and systems, can be studied accurately. These diverging interests can be served by detailed model libraries, which can be used by interfaces with different focus and degree of detail. The library presented in this paper was developed as a first major step towards a complete and detailed model library for studies of room climate and energy consumption. It contains all models required for detailed climate studies. The library is used in a tool for early estimates of local cooling requirements; a tool with a very simple user interface, where some twenty parameters are given (see Figure 1). Remaining input parameters are defined by suitable default values, describing typical simulation objects in various categories, office, school, hospital, etc. CONTENT OF MODEL LIBRARY The library contains models for the following components zone, simplified and detailed walls, RC-models and full finite difference models state-of-the-art window models solar radiation on windows and facades, several radiation models local climate control. Descriptions of the RC-wall model and of the detailed convective radiative zone model with full non-linear radiative and convective heat transfer are given below. RC-WALL MODEL The most common way to model a wall is to discretize it in one dimension into a number of equidistant layers (nodes) by using some finite difference method. The number of nodes is selected as a compromise between execution time and required accuracy of the results. The same accuracy can be reached with fewer nodes with an RC-network, if the thermal resistances, the heat capacities and the construction of the RC-network are chosen properly [Akander 1996]. In the project, a three-node RC model has been used, see Figure 2. A least-square optimization procedure determines the parameter values that minimize the deviations between the model and the analytical solution, observed for harmonic oscillations with periods of 1, 3, 6, 12, 24, 48 and 96 hours. The calculation of optimal RC parameters is based on a physical description of the wall, given in the PARAMETERS section of the NMF model. It is performed before a simulation starts, as described in the PARAMETER_ PROCESSING section, which is executed prior to the actual simulation.