# Energy Modeling for Buildings ## Table of Contents - [[#Overview]] - [[#Purpose and Applications]] - [[#Simulation Tools]] - [[#EnergyPlus]] - [[#IES Virtual Environment]] - [[#DesignBuilder]] - [[#Passive House Planning Package PHPP]] - [[#Other Tools]] - [[#Modeling Stages Aligned with Design]] - [[#Model Inputs and Data Requirements]] - [[#Weather Files and Climate Data]] - [[#ASHRAE 90.1 Appendix G Baseline Method]] - [[#Parametric Analysis]] - [[#Model Calibration]] - [[#Common Pitfalls and Quality Assurance]] - [[#Practical Notes for Architects]] - [[#References and Standards]] --- ## Overview Energy modelling is the computational simulation of building energy performance to predict annual energy consumption, peak loads, and the effectiveness of design strategies. It is an essential tool for achieving energy code compliance, green building certification, and performance targets such as [[Net Zero Energy Buildings]]. Energy models range from simple steady-state calculations to sophisticated hourly dynamic simulations that account for thermal interactions, occupancy patterns, weather variations, and HVAC system behaviour. --- ## Purpose and Applications Energy modelling serves multiple purposes across a project: - **Design optimisation**: Evaluating the energy impact of massing, orientation, envelope, and systems choices. - **Code compliance**: Demonstrating compliance with energy codes (e.g., [[ASHRAE 90.1 Energy Standard]], Part L of the Building Regulations). - **Green building certification**: Generating energy performance credits for [[LEED Certification System]], [[BREEAM Rating System]], and other systems. - **Passive House verification**: Supporting [[Passive House Standard]] certification through PHPP calculations. - **Life cycle cost analysis**: Comparing long-term cost implications of alternative strategies. - **Carbon assessment**: Quantifying operational carbon emissions for [[Operational vs Embodied Carbon]] analysis. - **Sizing mechanical systems**: Informing HVAC equipment selection through peak load calculations. --- ## Simulation Tools ### EnergyPlus - Developed by the US Department of Energy (DOE). - Free, open-source, whole-building dynamic simulation engine. - Sub-hourly timestep capability with detailed zone and system models. - Text-based input/output; typically used through graphical interfaces (DesignBuilder, OpenStudio). - Gold standard for research and compliance modelling in North America. - Extensive component libraries and validation documentation (ASHRAE Standard 140 / BESTEST). ### IES Virtual Environment - Commercial integrated building performance suite (IES VE). - Dynamic thermal simulation (Apache engine), daylighting (RadianceIES), CFD, and HVAC system modelling. - Strong user interface with integrated geometry modelling. - Widely used in the UK and Europe for Part L compliance and BREEAM assessments. - CIBSE-approved for UK National Calculation Methodology. ### DesignBuilder - Commercial graphical interface for the EnergyPlus simulation engine. - Accessible 3D modelling environment with drag-and-drop construction assemblies. - Integrated daylighting (Radiance), CFD, and HVAC templates. - Supports ASHRAE 90.1 Appendix G automated baseline generation. - Good balance between usability and simulation rigour. ### Passive House Planning Package PHPP - Spreadsheet-based steady-state energy balance tool developed by the Passivhaus Institut. - Monthly energy balance method derived from EN 13790. - Specifically calibrated for [[Passive House Standard]] verification. - Inputs: geometry (via designPH plugin), U-values, thermal bridges, ventilation rates, internal gains, shading factors. - Less suited for complex HVAC systems or cooling-dominated climates (though improving). ### Other Tools | Tool | Developer | Typical Use | |---------------|----------------|------------------------------------------| | OpenStudio | NREL / DOE | EnergyPlus front-end, parametric studies | | Sefaira | Trimble | Early-stage design feedback (SketchUp) | | TRNSYS | U. Wisconsin | Research-grade modular simulation | | Tas | EDSL | UK compliance and overheating analysis | | SBEM | BRE | UK simplified energy calculation method | | eQUEST | DOE / JJH | Quick energy analysis (DOE-2 engine) | --- ## Modeling Stages Aligned with Design Energy modelling should be integrated throughout the design process, not applied as an afterthought: | Design Stage (RIBA) | Model Type | Purpose | |---------------------|--------------------------------|------------------------------------------------| | Stage 1 (Preparation)| Benchmarking | Establish energy targets and budget | | Stage 2 (Concept) | Massing model | Test orientation, form, window-to-wall ratio | | Stage 3 (Spatial) | Envelope parametric model | Optimise insulation, glazing, shading | | Stage 4 (Technical) | Full systems model | Detailed HVAC, lighting controls, renewables | | Stage 5 (Construction)| As-built model update | Reflect specification changes | | Stage 7 (Use) | Calibrated model | Post-occupancy comparison and diagnostics | Early-stage models need not be precise. Approximate models informing design decisions at Stage 2 deliver greater value than precise models produced too late at Stage 4. --- ## Model Inputs and Data Requirements A comprehensive energy model requires the following categories of input: ### Geometry - Building footprint, floor-to-floor heights, zone layout. - Window areas, orientations, and positions. - Shading from overhangs, fins, neighbouring buildings, and topography. ### Envelope Properties - U-values for walls, roofs, floors, and glazing assemblies. - Solar heat gain coefficient (SHGC) and visible light transmittance (VLT) of glazing. - Thermal mass (heat capacity and density of construction materials). - Airtightness rate (infiltration at operating pressure). ### Internal Loads - Occupancy density (m²/person) and schedules. - Lighting power density (W/m²) and control strategy. - Equipment/plug load density (W/m²) and schedules. - Process loads (kitchens, servers, specialist equipment). ### HVAC Systems - System type (VAV, VRF, chilled beams, radiant, DOAS, etc.). - Equipment efficiencies (COP, EER, IEER, AFUE). - Distribution losses, fan power, pump power. - Control strategies (setpoints, setbacks, economiser modes). ### Domestic Hot Water - Demand (litres/person/day), generation efficiency, distribution losses. ### Schedules - Hourly profiles for occupancy, lighting, equipment, thermostat setpoints, and ventilation. --- ## Weather Files and Climate Data Simulation accuracy depends on representative weather data: - **TMY (Typical Meteorological Year)**: Statistical composite of typical weather conditions. Variants include TMY3 (USA), IWEC2 (international), CIBSE TRY (UK). - **DSY (Design Summer Year)**: Used for overheating risk assessment (CIBSE methodology). DSY-1, DSY-2, DSY-3 represent moderate to extreme summers. - **Future weather files**: Morphed files (e.g., CIBSE 2050s, Meteonorm projections) for climate change resilience testing. - **AMY (Actual Meteorological Year)**: Recorded data from a specific year, used for model calibration against monitored energy data. Weather files are typically in EPW (EnergyPlus Weather) format, containing hourly data for dry-bulb temperature, humidity, solar radiation, wind speed and direction, and atmospheric pressure. --- ## ASHRAE 90.1 Appendix G Baseline Method The Performance Rating Method (PRM) is the standard approach for LEED energy performance credits: 1. **Proposed model**: Represents the actual building design as specified. 2. **Baseline model**: An equivalent building meeting minimum ASHRAE 90.1 prescriptive requirements, with specific rules: - Baseline envelope properties per climate zone tables. - Baseline HVAC system type determined by building type and size (e.g., System 7: VAV with reheat for large offices). - Baseline lighting power density per space-by-space or building area method. - Four baseline orientations rotated 90° (averaged to remove orientation bias). 3. **Percentage improvement**: Calculated as: ``` % Improvement = (Baseline Cost - Proposed Cost) / Baseline Cost × 100 ``` Where costs are based on virtual energy rates specified in the standard. LEED v4.1 requires minimum 5% improvement (new construction) with up to 50% for maximum points. --- ## Parametric Analysis Parametric studies systematically vary design parameters to identify optimal combinations: - **Single-variable sweeps**: Vary one parameter (e.g., wall U-value from 0.10 to 0.40 W/(m²K)) while holding all others constant. - **Multi-variable optimisation**: Vary multiple parameters simultaneously using tools such as OpenStudio PAT, jEPlus, or GenOpt. - **Sensitivity analysis**: Rank parameters by their impact on total energy to focus design effort on the most significant variables. Typical parameters for parametric study: - Wall, roof, and floor insulation levels - Glazing type and window-to-wall ratio by orientation - Shading device depth and type - HVAC system type and efficiency - Lighting power density and control strategy - Airtightness rate --- ## Model Calibration Calibration aligns model predictions with measured energy data from an occupied building: - **ASHRAE Guideline 14 criteria**: - Monthly: NMBE ≤ ±5%, CV(RMSE) ≤ 15% - Hourly: NMBE ≤ ±10%, CV(RMSE) ≤ 30% - **IPMVP (International Performance Measurement and Verification Protocol)** provides a framework for measurement boundaries. - Calibration adjusts uncertain inputs (schedules, plug loads, infiltration) within reasonable ranges. - Calibrated models are used for measurement and verification (M&V) of energy conservation measures. --- ## Common Pitfalls and Quality Assurance - **Unrealistic schedules**: Using ASHRAE 90.1 default schedules when actual occupancy patterns differ significantly. - **Ignoring infiltration**: Under- or overestimating air leakage rates. - **Simplified HVAC**: Using idealised system models that do not capture part-load efficiency. - **Thermal bridging omission**: Failing to account for repeating and non-repeating thermal bridges in envelope U-values. - **Solar shading errors**: Incorrect modelling of external obstructions and self-shading. - **Single weather year bias**: Not testing sensitivity to weather file selection. Quality assurance protocols include peer review, ASHRAE 140 (BESTEST) validation, and comparison with benchmarks (CIBSE TM54, ASHRAE AEDG). --- ## Practical Notes for Architects - Commission energy modelling at RIBA Stage 2 to influence design, not at Stage 4 to justify it. - Provide accurate geometry and specifications to the energy modeller; errors in input produce errors in output. - Request parametric studies rather than single-design-point results to understand the sensitivity of options. - Use early-stage tools (Sefaira, Climate Consultant, PHPP) for rapid feedback during design workshops. - Ensure the energy modeller is accredited (ASHRAE BEMP, CIBSE LCEA, or PHI Certified Designer) for the relevant methodology. - Review model assumptions critically: occupancy schedules, plug load densities, and control strategies have enormous influence on results. - Plan for post-occupancy monitoring to verify model predictions and close the performance gap. --- ## References and Standards - ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential - ASHRAE Standard 140: Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs - ASHRAE Guideline 14: Measurement of Energy, Demand, and Water Savings - CIBSE TM54: Evaluating Operational Energy Performance of Buildings at the Design Stage - EN 13790: Energy Performance of Buildings — Calculation of Energy Use - [[Net Zero Energy Buildings]] - [[Passive House Standard]] - [[ASHRAE 90.1 Energy Standard]] - [[LEED Certification System]] --- #sustainability #energymodeling #simulation #ASHRAE #performance