AHU Dynamic Modeling and Validation

Dynamic AHU simulation models are important tools for the development and evaluation of AFDD methods. Dynamic performance modeling of the AHU systems has been of interest for well over 20 years.

Various building heating, ventilating, and air conditioning simulation models have been developed during the past decade for different purposes (Reddy et al., 2006): 1) Simplified Spreadsheet Programs, such as BEST (Waltz, 2000); 2) Simplified System Simulation Method, such as SEAM and ASEAM (Knebel 1983 and ASEAM 1991); 3) Fixed Schematic Hourly Simulation Program, such as DOE-2 (Winkelmann et al., 1993), and BLAST (BSL, 1999); 4) Modular Variable Time-Step Simulation Program, such as TRNSYS (SEL, 2000), SPARK (SPARK, 2003), ESP (Clarke and McLean, 1998), Energy Plus (Crawley et al., 2004), ASHRAE Primary and Secondary Toolkits (Bourdouxhe et al,, 1998 and Brandemuehl, 1993); and 5) Specialized Simulation Program, such as HVACSIM+ (Park et al, 1985), GEMS (Shah, 2001), and other CFD programs (Broderick and Chen, 2001). Detailed building and HVAC simulation model reviews can also be found in Kusuda (1999 and 2001), Bourdouxhe et al. (1998), Shavit (1995), Ayres and Stamper (1995), and Yuill and Wray (1990). Based on available reviews, several simulation software can be used for dynamic AHU model development and are discussed in further detail below.

HVACSIM+ (Park et al, 1985) developed by National Institute of Standards and Technology (NIST) uses a unique hierarchical variable time step approach in which components are grouped into blocks and blocks into super-blocks. The actual breakdown of the system is left to the user. Each super-block is an independent subsystem, whose time evolution is independent of other super-blocks. The only exception is the building envelope, which uses a fixed, user-specified time step. The time step in a super-block is a variable, which is automatically and continuously adjusted by the program to maintain numerical stability. HVACSIM+ is especially appropriate for simulating secondary systems and

control strategies, and has been undergoing experimental validation and improvements for several years (Dexter et al., 1987).

TRNSYS (SEL, 2000) developed by the Solar Energy Laboratory, University of Wisconsin Madison, uses a component based methodology in which: 1) a building is decomposed into components, each of which is described by a FORTRAN subroutine, 2) the user assembles the arbitrary system by linking component inputs and outputs and by assigning component performance parameters, and 3) the program solves the resulting non- linear algebraic and differential equations to determine system response at each time step.

SPARK (SPARK, 2003), which is similar to a general differential/algebraic equation solver, is an object-oriented software system that can be used to simulate physical systems that are described by differential and algebraic equations. In SPARK, components and subsystems are modeled as objects that can be interconnected to specify the model of the entire system. Models are expressed as systems of interconnected objects, either created by the user or selected from a library. An HVAC tool kit library is supplied with SPARK. An on-going project (Xu and Haves, 2001) conducted by the Lawrence Berkeley National Laboratory extends the current SPARK HVAC library to include more equipment models, such as AHUs and chillers, as well as models related to control systems.

SIMBAD (SIMBAD, 2004) is a family of HVAC system toolboxes developed for the MATLAB/SIMULINK environment (MATLAB 2006). It provides 11 modules which simulate building and zones, production and storage devices such as boiler and storage tank, hydronic and air flow distribution, heat emission devices such as zone terminal units, control devices, and weather and occupancy profiles.

EnergyPlus (Crawley et al., 2004) is a building energy simulation program developed from BLAST and DOE-2. It includes many innovative simulation capabilities such as time steps of less than an hour, modular systems and plant integrated with heat balance-based zone simulation, multizone air flow, thermal comfort, and photovoltaic systems (EnergyPlus, 2005).

ASHRAE 825-RP (Norford and Haves, 1997) extended the ability of HVACSIM+ and TRNSYS in the following areas: 1) new models such as those for controller, sensor, and air flow path were developed; 2) component models of the building fabric and mechanical equipment were enhanced; 3) a real building (building E51), including the AHU system, was simulated and documented in detail to demonstrate the use of the models. This model is referred as the E51 model hereafter. Although the E51 model was not validated, it provided a good framework and model structure for other system model development.

2.6.2 AHU Model Validation

There are publications in the literature that discuss HVAC system dynamic model verification and validation such as those focus on a) component models (Clark et al., 1985, Zhou and Braun, 2007); b) primary systems (Henze et al. 1997 and Wang et al., 2000); and c) air conditioning process and its interaction with building zones (Brandemuehl et al., 1990 and Ahmed et al., 1998).

Two papers were found that specifically discussed AHU dynamic model validation: z Chen and Deng (2006) developed a dynamic simulation model for a direct expansion

VAV air conditioning system consisted of a VAV air distribution subsystem and a DX refrigeration plant. AHU model was part of the overall model. A test rig was developed for validating the model. However, the validation process only included comparing model outputs and experimental data under an open loop step change of compressor speed (one speed adjustment). No real weather conditions or internal loads was applied to the model.

z Nassif et al. (2008) developed a series of simplified component models for an AHU, a VAV terminal unit, building zone, and their control systems. Real operation data collected from the system control system were used to obtain model parameters. Model outputs were compared with system measurements. However, the component models were not connected to each other and it was unknown how well the entire system model would perform if all component models were connected.

The above literature review indicates that there is a lack of a comprehensive validation study for AHU dynamic models that compares the entire system model predictions with real operation data. In this study, three perspectives of the 1312 AHU model are to be validated:

1. Parameters: during the model development process, all parameters are obtained from either nominal design values or from manufacturer catalogs. Those values often do not reflect the true parameters for a real system. An important part of the validation process is to first "tune/calibrate" the parameters in the simulation model from system measurements;

2. Component models: component models used in HVACSIM+ may not be able to simulate the test facility AHU performance satisfactorily because HVACSIM+ component models generally represent new and ideal component behaviors; and 3. System performance: even after all component models perform agreeably, the

system performance may still not be satisfactory due to error propagation and numerical calculation stability.