Resultado de la inspección del Procedimiento de toma de rehenes

The multizone simulation tool used in this project was EnergyPlus (EnergyPlus 2012a). EnergyPlus is an energy analysis and thermal load simulation program, based upon the BLAST (Building Loads Analysis and System Thermodynamics) and DOE-2 programs which were developed in the 1970s and 1980s as energy analysis tools for designers to size HVAC equipment, improve energy performance etc. (EnergyPlus 2012b). By using a user defined description of a building’s geometry and construction, along with a description of any services and mechanical systems, EnergyPlus can calculate energy consumptions for heating and cooling, internal temperatures, ventilation flow rates and a number of other values.

92 5.2. EnergyPlus

There are a number of different multi-zone building ventilation modelling tools available. The capabilities of a number of tool were investigated before EnergyPlus was selected as being the most suitable. The following explains the main reasons for this selection and also highlights some of the shortcomings of EnergyPlus.

Advantages of EnergyPlus:

• As EnergyPlus’s primary function is to serve as a whole building energy analysis tool, it carries out thermal and energy calculations alongside the ventilation calcula- tions. Energy usage and the affect of thermal loads are often important elements in building ventilation studies, although this functionality is not included in all tools. For example CONTAM (Walton & Dols 2013), which is a popular multizone venti- lation prediction tool, does not include this functionality but requires coupling with a thermal calculation engine such as TRNSYS (TRNSYS 2013).

• The EnergyPlus software is opensource. This is advantageous for a number of rea- sons, firstly it means that the software and all of its corresponding documentation is free but more importantly it is possible to develop the software and change function- ality if this is required. As well as the ability to make changes to the software, the opensource nature of EnergyPlus also makes linking it to other tools a much easier process. This could be beneficial to this project, as it would allow EnergyPlus to be linked to other tools which have the ability to simulate more advanced control methods (Wetter 2011).

• The input files (.idf) are in ASCII text which can be created and read by the user (although this can be a time intensive process). The advantage of using a standard text format for input files is that it allows for third-party interfaces which can be tailored to specific applications using EnergyPlus as the simulation engine. This has resulted in a number of front-ends for which can be used to speed up the creation of EnergyPlus input files or to carry out specific calculations without having to deal with the complexities of creating .idf input files.

Disadvantages:

• Energyplus lacks a graphical user interface (GUI). Although this does not affect the results generated directly, it does make it easier for user errors to occur as well as increasing the time required to generate input files. This disadvantage has been largely mitigated by using DesignBuilder (DesignBuilder 2011) to create the building geometry and other aspects of the input file, before making finer adjustments using EnergyPlus’s IDF Editor. DesignBuilder is a building simulation tool which runs on the EnergyPlus calculation engine. It’s graphical interface makes creating building geometry, setting up simulations and obtaining results a much quicker and easier task than using EnergyPlus. However, DesignBuilder is much less flexible, both in terms of setting up more complex simulations and exporting all of the results which may be of interest. For this reason DesignBuilder will be used to save time

Chapter 5. Simulation Model 93

by generating a basic input file with the building geometry, which is then exported and fine-tuned within EnergyPlus.

• As is the case with all multizone models; situations where there is a non-uniform air temperature distribution cannot be accurately modelled. This can occur in large spaces such as atria and in displacement ventilation scenarios. To overcome this some multizone programs such as CONTAM have recently incorporated computational fluid dynamics (CFD) into the software (Walton & Dols 2013). This allows the user to specify zones in the model to be calculated using CFD methods, using the adjacent well-mixed zones as boundary conditions. The results are more accurate than those from a standalone multizone model without the computational time or resources increasing too drastically (as would be the case if CFD was used for all zone in a building). However, it is possible to link EnergyPlus to a separate CFD software (couplings of EnergyPlus and different CFD softwares has previously been achieved, for example Zhai et al. (2002)).

In this study no atria or high spaces with flow dominated by buoyancy are being modelling. Therefore one of the key disadvantages of EnergyPlus is negated. EnergyPlus has also been utilised in a number of studies investigating MPC of building systems. Being used as either the predictive model or as the plant model upon which control schemes are tested (Neto & Fiorelli 2008, Ruano et al. 2006).

5.2.1 EnergyPlus Modelling Overview

The EnergyPlus software can essentially be considered as a collection of a large number of individual modules. These modules are called upon, depending on the type of calculation being carried out, and collectively calculate energy consumption and other information. This is achieved by simulating the building including any plant systems when they are exposed to environmental and operating conditions. The core of the simulation is a model of the building based upon the fundamental heat balance principles (EnergyPlus 2012a). As naturally ventilated buildings are the topic of this investigation, most of the calculations will be carried out by two of the three main blocks within the Integrated Solution Manager. Specifically the Surface Heat Balance Manager and the Air Heat Balance Manager. The Building Systems Simulation Manager will still be used in the calculation, to simulate any heating systems and other equipment such as lighting, but its primary function of simulating mechanical plant and air-conditioning will not be utilised. The critical module for natural ventilation calculations is the AirflowNetwork model. The thermal response of the building is also an important factor in this investigation. This is calculated using the Conductive Transfer Function (CTF) calculation module. This is briefly described in Appendix A.4. A detailed explanation of all the individual elements within the program is given in the extensive EnergyPlus documentation (EnergyPlus 2012a).

EnergyPlus calculates results using an integrated simulation. This means that all three major element, building, systems, and plant, are solved simultaneously. This is in contrast

94 5.2. EnergyPlus

to other programs such as BLAST or DOE-2, where the building zones, air handling systems, and other plant equipment are simulated sequentially with no feedback from one another (EnergyPlus 2012a). The starting point for the sequential simulation is the calculation of the zone conditions, using a a zone heat balance, this then updates the zone conditions and determines any heating or cooling loads at all time steps. This information is then passed on to the air handling simulation, which determines the systems response; but this response does not affect zone conditions.

When utilising EnergyPlus for building energy simulation there are two ways of dealing with air exchange rates for natural ventilation, scheduled values can be used or values can be calculated at each time step. Scheduled ventilation rates can be specified by the user based upon typical values or from manual calculation however while this may be acceptable for some applications, such as during an initial design stage, calculated values are required in this study. The ventilation flow rates are calculated using the EnergyPlus Airflow Network.

The Airflow Network carries out calculations at each timestep. The flow rates are driven by pressure differences, due to temperature and wind. To begin the calculation the node pressures are determined using a linear approximation which relates airflow to pressure drop:

˙

mi = Ciρ(∆Pi/µ) (5.1)

where ˙

mi = Air mass flow rate at the ith linkage (kg/s)

Ci= Air mass flow coefficient (m3)

∆Pi = Pressure difference across the ith linkage (Pa)

µ = Air viscosity (Pa·s)

A linkage model connects two nodes, an inlet and an outlet, these two nodes are linked by a linkage component. This could be a window, an air vent, a crack etc. It is the linkage component which gives the relationship between airflow and pressure. Bernouli’s equation is used to calculate the pressure difference:

∆P =  Pn+ ρV_n2 2  −  Pm+ ρV_m2 2  + ρg(zn− zm) (5.2) where

∆P = Total pressure difference between nodes n and m (Pa) Pn, Pm= Entry and exit static pressures (Pa)

Chapter 5. Simulation Model 95

ρ = Air density (kg/m3₎

g = Acceleration due to gravity (m/s2) zn, zm= Entry and exit elevations (m)

In a typical simulation there will be several nodes representing internal zones and the external conditions. These nodes can be connected by a range of linkage models to cre- ate a network. A more detailed description of the simulation procedure utilised by the AirflowNetwork can be found in Appendix A.