Abstract
As the need for sustainable development increases, building simulation is becoming more crucial, and it is heading towards new challenges, dimensions, and concepts beyond the building envelope. Buildings are not isolated entities, which are not just responsible for about 35% of the total annual energy demand, but dynamically interact with their environment whose affected perimeter may be far wider than thought. In order to expand the building simulation perimeter to the entire impacted environment, all relevant variables must be factored in on a common base with a uniform metric. Among many attempts in this direction, exergy analysis establishes a uniform metric on all grounds and promises to make building simulation tools to more environmentally conscious.
INTRODUCTION
The objective of this entry is to highlight the importance of building simulation for analyzing and designing truly green and high performance buildings for a sustainable future. It also describes the need for new simulation tools that can cover a wider energy utilization and environment window, including the second law of thermodynamics, in a wide scope beyond the building envelope. More than ever, we need to know how efficient, stable, safe, functional, comfortable, and environmentally sustainable a building is under different indoor and outdoor conditions, functional and occupational requirements, and various equipment and system dynamics. Existing building simulation (BS) tools generally apply to a single building in a narrow window of the environment. These simulation and modeling tools allow us to design with methods to analyze, optimize, and control a building by calculating and compiling the crucial information regarding the overall performance of the building as accurately, precisely, and completely as possible, so that energy savings and a comfortable indoor environment are achieved in the building envelope with minimum possible cost. Only a few of the latest tools, however, include an environmental footprint analysis of a building, and these are rather limited.
NEED FOR BUILDING SIMULATION
People spend about two-thirds of their time indoors, with diverse and often conflicting needs relating to indoor air quality. According to several standards, including American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standard 90.1 and ASHRAE Standard 90.2,[1,2] the primary targets are energy efficiency and indoor air quality, which conflict. According to the U.S. Department of Energy (DOE) Energy Efficiency and Renewable Energy Office (EERE), there are 81 million buildings in the U.S., of which 75% were built before 1979 and need a substantial retrofit or replacement of their Heating, Ventilating, and Air-conditioning (HVAC) systems. Fifteen million more buildings are expected by 2010. As a result, residential and commercial buildings in the U.S. are responsible for about 39% of the annual U.S. primary energy consumption, more than 70% of the total electric power consumed,[3] and close to 40% of COo emissions (Fig. 1).[4] Buildings, which use natural gas for HVAC and domestic hot water, produce 20% of the total CO2 gas emissions, estimated to be responsible for 60% of the greenhouse effect.
Consequently, buildings are the focal point of the sustainability quadrilemma of energy, economy, environment, and people. In order to resolve this quadrilemma, new building and retrofit designs must establish an optimum balance among all elements of environment, economy, people’s needs, and rational use of energy resources. If it were possible to utilize the abundant, unused, low-exergy renewable and waste energy resources, all existing buildings could be heated and cooled. However, conventional HVAC systems cannot couple with low-exergy energy resources directly— because of their temperature incompatibility with low-exergy energy resources—unless HVAC equipment is oversized, or the resource temperature is conditioned. Both measures are cost-, energy-, and space-intensive, which greatly diminish the advantages of renewable and waste energy resources. This complexity requires developing new HVAC systems, with an outreach to the environment beyond the building envelope and a special emphasis on building-environment-energy source relations. Today’s green buildings may not be truly green unless the simulation window expands beyond the building envelope, with a clear understanding of exergy. Fig. 2 shows sample layers and scale of ideal building simulation windows for sustainability. Today there are almost 300 building simulation tools in 21 countries. However, in spite of this large availability and easy access, they do not yet address the overall picture; they are limited to the building envelope or its close vicinity.
Fig. 1 Carbon emissions from fossil fuels in different sectors.
ENERGY AND EXERGY ASPECTS OF BUILDINGS
“The absurdity of cutting butter with a chainsaw is immediately obvious to anyone.”[5] On the same token, a conventional HVAC system uses high-grade energy in low-grade space heating or cooling and degrades the original energy resource, most often fossil fuel. According to Dincer and Rosen,[6] “Many scientists and engineers suggest that the impact of energy-resource utilization on the environment is best addressed by considering exergy. The exergy of a quantity of energy or a substance is a measure of the usefulness or quality of the energy or substance or a measure of its potential to cause change. In other words, exergy of any flow or resource of energy is the potential of useful work that is available, and a conventional HVAC system wastes most of it.”[7] In fact, exergy is not only wasted but destroyed, because exergy flow is irreversible. Therefore, it is no surprise that the exergy efficiency of existing HVAC systems are less than 10%.[8] An earlier study showed that this efficiency is 5% on average for Swedish homes.[9] The shortcomings of the definition of energy efficiency are particularly apparent for tasks in which fossil fuels are used just for low-temperature heating. Since fossil fuels burn at very high flame temperatures—up to 2000 K[10]—the useful work potential (exergy) of fossil fuels is high. When fossil fuels are used for hot water heating, space heating, or even industrial steam production, most of the exergy is destroyed in these processes.[11] Indoor space heating furnaces have an estimated exergy efficiency of 6%, and heat pumps, when combined with conventional HVAC systems are not much better at 9%.[12] On the other hand, thermal efficiency of HVAC systems has almost reached a saturation point, well above 90% on average, except for thermal energy transport and distribution losses. Therefore, according to Annex 37,[13] the priority must now be shifted towards exergy efficiency in improving buildings, starting from their root causes. Fortunately, most of the root causes can be detected by developing next-generation building simulation codes that address the exergy efficiency at different scales—most importantly the HVAC system, which must be related to the environment and to the primary energy at its source, including the exergetic importance of thermal energy storage systems.[14'15] The rational exergy efficiency is the ratio of the minimum exergy required by a given HVAC load to the actually available exergy of the energy source used in satisfying that load.[16]
Fig. 2 Sample layers and scale of ideal building simulation windows.
Fig. 3 Heating, ventilating, and air-conditioning life cycle cost including the cost of exergy destruction.
The minimum amount of exergy required to satisfy a unit heating load for an indoor space at a dry-bulb air temperature Ta, in reference to the temperature of the environment Tg[17]:
If the reference temperature of the environment is the ground temperature of 278 K (5°C) in winter, and the indoor air design temperature is 291 K (18°C), the ideal emin for an HVAC system directly utilizing ground heat is 0.0447. For a conventional HVAC system, the available exergy of the used energy source relates to the resource temperature and Tg. For a natural gas-fired furnace with a flame temperature of 1273 K, the available unit exergy is:
The exergy efficiency using Eq. 1 is then 0.057 (6%). This means most of the exergy is destroyed. If the same indoor space could be directly heated by an energy source at 302 K (29°C), W could be much higher: 56%. These statements and equations are also valid for electric-based chiller or heat pump space cooling, if electric power is generated in a thermal plant. Fig. 3 illustrates the contrast between low-exergy HVAC systems and conventional HVAC systems as a function of operating fluid temperature.1-16-1 The contrast is a result of including the cost of exergy destruction in the life cycle cost (LCC) analysis. Generally, low-exergy HVAC systems are a hybrid of radiant and convective heat transfer equipment, which can operate at moderate fluid temperatures.[18]
ILLUSTRATIVE EXAMPLES
Fig. 4A-D illustrate the importance of a wider building simulation window for sustainability.
1. A Conventional HVAC System with a Central Power Plant. In Fig. 4A, according to a conventional building energy simulation tool covering only the building envelope, the building seems to be energy efficient and may comply with most of the energy codes. Once a wider simulation window is used, covering the plant, the transmission lines, and the environment, it becomes apparent that the exergy efficiency is less than 10%.
2. A Ground Source Heat Pump (GSHP) Building HVAC System with a Central Power Plant. According to the same small building simulation window, the ground source heat pump in Fig. 4B makes sense only from the building owner/ operator’s side, because of a high COP. From a wider window, the overall efficiency remains almost the same and the exergy efficiency is still below 10%. The small window does not reveal how low the exergy efficiency is and does not show the ways and means to increase the exergy efficiency.
3. A Decentralized Micro Combined Heat and Power (MCHP) Plant with a Conventional HVAC System. In Fig. 4C, a wider simulation window shows that the overall energy efficiency is improving substantially, because power transmission losses are eliminated. Because the decentralized plant utilizes the primary energy resource for electric power generation on location, and captures most of the waste heat to provide thermal energy to the building, exergy efficiency has also improved. The exergy efficiency may further increase if a low-exergy HVAC system is installed, because waste heat can be more effectively utilized in the building.
4. A Green Power (MCHP) System with Low-exergy HVAC System. In Fig. 4D, both the energy and exergy efficiencies are high. Yet, this solution cannot be visible in a narrow simulation window.
Fig. 4 (A) A centralized energy and power system with buildings using conventional heating, ventilating, and air-conditioning plant and equipment. (B) A centralized energy and power supply system with ground source heat pumps. (C) A decentralized micro combined heat and power system. (D) A decentralized, green energy system.
We can now conclude that whether it is thermal efficiency or exergy efficiency, or both, the simulation window must be far wider than the building envelope for sustainability. The building simulation window may only be reduced to building scale if decentralized energy systems with high efficiency green energy components are considered, as in Fig. 4D.[19]
MAJOR COMPONENTS OF BUILDING SIMULATION
Current theory of building energy simulation is based upon load and energy calculations developed by ASHRAE.[20] The selection of a simulation program for a given task depends on the project requirements, time, and cost of the analysis, experience of the user, and availability of suitable simulation tools and data.[20] Keeping in mind that future BS tools must include exergy analysis on a macro scale, we nevertheless currently need to select a BS tool best suited for a specific project. During the selection process, one needs to consider the following two requirements: (1) algorithms and data must be from reliable, well-established, published sources and (2) validation of the tool must be possible with available validation packages.[21] It is desirable to use a tool with an open structure or open end for future collaborative, modular versions. Expertise and training required must be compatible with the resources and background of the users. In addition, one must ask the following questions before the final decision:
• Who uses this tool? How many active users are there? Which institutions and countries use it?
• Is there a discussion group for this tool?
• What is the target audience? Does it suit your background, expertise, and position?
• What are the main features? Does it satisfy your needs?
• Is this tool compatible with other tools, programs, and databases? If yes, which programs are compatible? Can you bundle them?
• What are the required input and output data? How extensive are they, and what is their format and scope? Are there databases that can be readily used, or are databases already provided in the tool from recognized resources, and are they code approved?
• How many default inputs does the program tend to provide? Generally, more default inputs make it user-friendlier but substantially less accurate, depending on how the defaults are prepared and the assumptions involved.
• What is the computer platform, and which programming language is used?
• How much support can you get, and for how long can you get it? Can you get live support? Is the support free? How much and how detailed are educational materials, if provided?
• What are the costs, licensing options, terms, and conditions?
• Is a trial version available? What is the trial period and is it supported?
• Does the program check necessary code compliances? If yes, which codes?
• Will upgrades be available? What are the terms and conditions?
• What are the speed, capacity, and technical coverage of the program? Does it provide optimization and knowledge base tools?
The following basic components of building simulation must be covered:
• All sensible and latent loads
• Heat losses and gains from the envelope
• Internal gains and losses
• Electrical loads
• Water supply loads
• Waste management loads
• Ventilation loads for acceptable indoor air quality (IAQ)[22'23]
• Human comfort requirements and human behavior in controlled indoor environments
• Radiant asymmetry
• Mean radiant temperature
• Air velocity
• Wet-bulb and dry-bulb air temperature
• Relative humidity
• Mean radiant temperature
• Operative temperature
• AUST (Area Averaged Uncontrolled Surface Temperature)
• Convective and radiant heat transfer split
• Asymmetric thermal radiation
• Draft
• Vertical air temperature difference
• Warm or cold floors
• Hot ceiling
• Heat stress
• Comfort analysis [24'25]
• Duct losses[26]
• Domestic hot water demand modeling and supply[27]
• CBR (chemical, biological, and radiological) attack risk assessment and simulation[28]
• Fragility analysis (earthquakes, etc.)[29]
• Environmental ingress (like moisture penetration from foundations)
• Solar gains
• Shading
• Lighting simulation
INTEGRATED BUILDING DESIGN SYSTEM
Integration of simulation into the building design process can ensure that important data and information for each major design decision are provided in a timely fashion. By establishing design links and exchange between architecture and engineering, an integrated building design system (IBDS) can be developed.[30] Some researchers have taken the initiative to develop more efficient and flexible use of simulation tools. The COMBINE (Computer Models for the Building Industry in Europe) project[31] and the AEDOT (Advanced Energy Design and Operation Technologies) project in the U.S.[32] are typical examples. While design, simulation, operation, and control functions are becoming integrated, all equipment and systems must be bundled around a common protocol. Future integrated building simulation models must know exactly how each piece of equipment or sub-system behaves in the building through BACnet protocol,[33] and the equipment must be architectured accordingly.
AN OVERVIEW OF CURRENT BS TOOLS
According to G. Augenbroe,[34] “A broad range of simulation software applications has become available for a variety of building performance assessments over the last three decades …” The maturation of building simulation into a recognized and indispensable discipline for all professions—involved in the design, engineering, operation, and management of buildings—has now become the imminent challenge. Two key aspects dominate this evolution process: (1) attaining an increased level of quality assurance and (2) offering efficient integration of simulation expertise and tools in the overall building process. Major shortcomings of current simulation tools include:
• Input is usually lengthy and often cumbersome to prepare and compile.
• Parametric studies are not always possible.
• Outputs are generally too voluminous and need further analysis and interpretation.
• The learning curve may be too long and overly frustrating for the novice user.
• The user interface is generally overlooked and not user-friendly.
• The software is not flexible enough to test all possibilities.
• The software does not allow for the design and testing of new components. Off- the-shelf types of equipment must be used. This limitation reduces creative design opportunities.
• Most programs require long run times and a large memory. Personal Computer (PC) versions are available, but the compromises involved must be carefully weighed against the ease and simplicity of using them.
• Lifecycle analyses that include and recognize the exergy efficiency and the cost of exergy waste do not exist.
• Hybrid HVAC systems cannot easily be modeled.
• Optimization tools on a wide simulation window are not yet available.
Building simulation tools are available in the whole building level, equipment and component level, system level, retrofit level, and green building levels. Here only a very small number of available simulation tools are sampled. U.S. DOE EERE[35] provides a comprehensive and up-to-date listing and detailed information. Their Building Energy Software Tools directory lists almost 300 simulation programs categorized under: (1) whole-building analysis (load calculation, renewable energy, retrofit analysis, sustainability, and green buildings), (2) codes and standards, (3) materials, components, equipment, and systems, and (4) other applications.1-36-1 A short list of typical BS tools is provided.
1. APACHE. Thermal design (heating, cooling, and latent load calculations). Equipment sizing, codes and standards checks, dynamic building thermal performance analysis, systems and controls performance, and energy use.[37]
2. APACHE-HVAC. Flexible and versatile system HVAC and controls modeling. Integrated simulation of building and HVAC systems.[38]
3. BLAST. The zone models of BLAST (Building Loads Analysis and System Thermodynamics) are based on the heat balance method. It performs hourly simulations of buildings, air handling systems, and central plant equipment. The output may be coupled to the LCCID (Life Cycle Cost in Design).[39]
4. BSim2002. Comprises different programs like a graphic model editor, thermal and moisture building simulation tool, dynamic solar and shadow simulation, daylight calculation tool, and compliance checker. Computer-aided design import and building integrated Photovoltaic system.[40]
5. BEA. Building Energy Analyzer is a system-screening tool to evaluate a variety of commercially available HVAC and power generation options. Uses the DOE-2.1E computational engine, includes a life cycle cost analysis module, and handles complex utility rates structures.[41]
6. BUS+ + . New generation platform for building energy, ventilation, noise level, and indoor air quality simulations. A network assumption is adopted, and both steady state and dynamic simulations are possible.[42]
7. DOE-2. This is an hourly, whole-building energy analysis program that calculates energy performance and life cycle cost of operation. Can be used to analyze energy efficiency of given designs or efficiency of new technologies. Other uses include utility demand-side management and rebate programs, development and implementation of energy efficiency standards, and compliance certification. Training and expertise required.[43]
8. EE4 CODE. Used to determine the compliance of a building to Canada’s Model National Energy Code for Buildings (MNECB). EE4 CODE may also be used to perform noncompliance energy analyses and thus to predict the annual energy consumption of a building and to assess the impact of design changes, based on DOE-2.1E.[44]
9. EED. A program for borehole heat exchanger design in a ground source heat pump system (GSHP) and borehole thermal storage. In very large and complex tasks, EED allows for the retrieval of the approximate required size and layout before initiating analyses that are more detailed.[45]
10. EnergyPlus. This is a whole-building energy simulation program that builds on BLAST and DOE-2. Includes advanced simulation capabilities, including time steps of less than an hour, modular systems simulation modules that are integrated with a heat balance-based zone simulation, and input and output data structures tailored to facilitate third-party interface development. EnergyPlus Version 1.2.2 was released in April 2005. EnergyPlus and weather data for more than 900 locations worldwide can be downloaded at no cost from the EERE home page.[46]
11. EZDOE. This tool is an easier-to-use personal computer version of DOE-2. EZDOE calculates the hourly energy use of a building and its life-cycle cost of operation, given information on the building’s location, construction, operation, and heating and air conditioning system.[47]
12. Right Suite-Residential. All-in-one HVAC software performs residential loads calculations, duct sizing, energy analysis, equipment selection, cost comparison calculations, and geothermal loop design.[48]
13. TRACE 700. Follows the algorithms recommended by ASHRAE. Used for assessing the energy and economic impacts of building-related selections, such as architectural features, comfort-system design, HVAC equipment selections, operating schedules, and financial options.[49]
14. TRNSYS. TRNSYS (TRaNsient System Simulation Program) includes a graphical interface, a simulation engine, and a library of components that range from various building models to standard HVAC equipment, to renewable energy and emerging technologies. TRNSYS includes a method for creating new components that do not exist in the standard package.[50]
15. VisualDOE. A Windows interface to the DOE-2.1E energy simulation program. Users construct a model of the building’s geometry using standard block shapes, using a built-in drawing tool, or importing DXF files. Building systems are defined through a point-and-click interface. A library of constructions, fenestrations, systems, and operating schedules is included, and the user can add custom elements. Up to 99 alternatives can be defined.[51]
16. ESP-r. Developed by the Energy Simulation Research Unit, University of Strathclyde, this is a general simulation tool that can be used to address a broad range of thermal performance problems, most often used for buildings.[52]
17. HVAC Solution. Allows users to graphically design and specify HVAC equipment, picking objects like boilers, pumps, fan coils, and air handlers. Using drag-and-drop methods, an HVAC system can be built. Once the system is built, the tool automatically renders equipment schedules and export schematics.[53]
18. DUCTSIZE. Calculates optimal duct sizes using the static regain, equal friction, or constant velocity method. Data entry can be accomplished manually or taken graphically from either Drawing Board or AutoCAD. A library of fan data for noise calculations is built into the [54] program.
19. Hydronics Design Studio. Assists in analyzing the thermal and hydraulic performance of modern hydronic heating systems in residential and light commercial buildings. The professional version performs tasks like heating load analysis, series baseboard circuit analysis, piping heat loss estimating, expansion tank sizing, radiant circuit analysis, injection mixing simulation, buffer tank simulation, and fuel cost comparisons.[55]
20. FLOVENT. Calculates airflow, heat transfer, and contamination distribution for built environments. FLOVENT uses techniques of Computational Fluid Dynamics (CFD) packaged in a form that addresses the needs of mechanical engineers involved in the design and optimization of ventilation systems.[56]
21. ArchiPhysics-Solar. A passive solar energy system for free running buildings. Shows the adaptive comfort level, as well as indoor and outdoor air temperatures for given building geometry, location, and glazing.[57]
22. MOIST. This program predicts the combined transfer of heat and moisture in multi-layer building construction. Inputs hourly weather data and predicts the moisture content and temperature of the construction layers as a function of time of year. It can be used to develop guidelines and practices for controlling moisture in walls, flat roofs, and cathedral ceilings.[58]
23. Daylight. This program calculates the daylight factor distribution in a room. A user-friendly program by Archiphysics.[59]
24. BREEZE. This is a tool for estimating ventilation rates, developed by Building Research Establishment (BRE) in England.[60]
SUSTAINABLE DESIGN AND GREEN BUILDINGS
There are currently some desktop tools available for architects and engineers for sustainable design of buildings. However, these tools perform life cycle environmental impact assessment based solely upon building materials and related items.[61-63] The energy balance, exergy balance, and thermo-physical interactions of the building with the environment are not included.
1. ATHENA. This is an environmental impact estimator program developed by ATHENA Sustainable Materials Institute. The program has a large database of building materials and performs an environmental impact assessment.1-64-1
2. BEES. The BEES (Building for Environmental and Economic Sustainability) software enables the use of cost-effective, environmentally preferable building products. The tool is based on consensus standards and includes actual environmental and economic performance data for nearly 200 building products.[65]
3. RETScreen. Developed by the Natural Resources of Canada, the RETScreen International Clean Energy Project Analysis Software is a decision support tool that can be used worldwide to evaluate the energy production, life-cycle costs, and greenhouse gas emission reductions for various types of energy efficient and renewable energy technologies. Also includes product, cost, and weather databases, as well as a detailed online user manual.[66]
4. GBS. Seamlessly links architectural 3-D CAD building designs with energy analysis. Green Building Studio (GBS) enables architects to quickly calculate the operational and energy implications of early design decisions. GBS uses the DOE-2 simulation engine to calculate energy performance and generates geometrical input files.[67]
More information can be found at the Building Energy Simulation Tools (BEST) Web site,[68] which lists the available tools and their main features and provides Internet sites and references. The most up-to-date information about the energy design tools can be obtained from the International Building Performance Simulation Association.1-69-1
CONCLUSION
With the ever-increasing need for high performance buildings with truly green features, it is virtually inevitable that future building simulation tools will become an integral part of other macro-scale simulation tools for the environment, the energy sector, and the community. In achieving this goal, building simulation tools must be more compatible, open-structured, and open-ended for greater modularity, compatibility, and data exchange.
