Computer Modeling - New & Existing Buildings

Why create an energy model for a new building?

Early, in the conceptual design of a building, the creation of an energy model enables the design team to evaluate the energy impact of the following types of options.

Architectural Features

  • Orientation of the building
  • Shape of the building – long and narrow versus boxy configuration. Long and narrow buildings can sometimes take advantage of natural ventilation from daily wind patterns depending upon building location
  • Types of window glazing: double glazed, triple glazed, optimum level of reflectivity, low emissivity
  • % of wall that is glass
  • Skylights
  • External shading devices - overhangs, vertical side fins
  • Internal shading devices, manual and automatically deployed
  • Material composition of walls, roofs and floors. The greater the mass of the building the more significant the temperature drift. Heat gain does not appear instantaneously in the building.
  • Wall, roof, door and floor U-values.
  • Glazing U-Values and shading coefficient

Mechanical Systems

  • Type of HVAC systems – there are almost 40 to choose from including hybrid system types
  • Types of fan equipment
  • Types of pump equipment
  • Types of direct expansion cooling equipment – air cooled versus water cooled
  • Types of chillers using chilled water for cooling – air cooled versus water cooled
  • Types of cooling towers
  • Free cooling with airside economizers
  • Free cooling with waterside economizers
  • Heat recovery wheels, sensible and total energy recovery
  • Water to water heat pumps
  • Air to air heat pumps
  • Geothermal heat pumps
  • Self contained package units
  • Equipment full load and part load efficiency for all of the above types of equipment

Electrical Systems

  • Many types of energy efficient lighting to choose from
  • Power distribution density – plug loads
  • Day lighting strategies

Where does one begin to create an energy model of a new building? Click on the following eQUEST Tutorial hot link.

Once a base building has been input, options can be evaluated relatively quickly. An estimate of construction costs and simulated energy savings will inform the design team whether or not the feature in question should be included in the design.

Why create an energy model for an existing building?

An energy model of an existing building can analyze the impact of energy conservation measures (ECMs) that are too difficult and time consuming to calculate manually with spreadsheets. A base model is first created to represent the architectural, mechanical and energy consuming systems of the building.

Three years of electricity and gas utility bills are analyzed using a spreadsheet. Anomalies in energy use are identified and adjusted to create a realistic base year consumption profile. Once the energy model has been created, it must be debugged to ensure that the systems are being simulated correctly. This can only be accomplished with an understanding of how HVAC systems and their control systems operate in the real world. The modeler must be adept with the use of the energy modeling program.

The DOE-2 simulation program can fill a room with computer output if the modeler requested every output possible for 8,760 hours of operation. Inputs and outputs must be "sanity checked" and triangulated ensuring a realistic output. The artist visualizes the picture before painting; the energy modeler must comprehend each type of option simulated.

The model must have its electricity, electrical demand and fuel use calibrated to the utility bills. The energy use and demand should calibrate monthly and annually for an accurate model. Reasonable expectations are that the model should calibrate within 95% of actual energy use monthly and annually. An experienced modeler aims for these optimum results based on his extensive engineering design and field smarts.

Once the model is calibrated, each ECM is simulated against the base model. The number of ECMs modeled is limited only by the modeler’s experience with energy engineering, HVAC design and DDC control systems. After the ECMs have been modeled and their installed costs, energy costs and simple paybacks have been analyzed, the ECMs with reasonable simple paybacks are identified. These ECMs are input into one final “run” to eliminate double accounting of energy savings due to synergism between measures.

A concept and budget report documents the systems input, assumptions made, measures evaluated, findings and recommendations. The executive summary presents a business case such that a budget can be allocated including engineering design fees.

As soon as the budget is approved, the project proceeds through concept design, design development and construction drawings. A fundamental part of the design process includes the preparation of a Measurement and Verification Plan. Smart electricity and gas meters are installed on the main services to the building. Depending on panel location and layout in the existing building, smart electrical sub meters are strategically located for 120 volt lighting, 347 volt lighting, plug loads, constant HVAC electrical loads and Variable Speed HVAC electrical loads where practical.

Flow meters for chilled water, heating water, domestic hot water are installed with matched input and output temperature sensors. The smart M&V software converts the flow rates and temperature differences into BTU/Hr instantaneous demands and pulsed energy accumulations. Installation of the M&V system enables performance tracking of ECMs in real time. Deviations from weather-corrected performance are flagged and alarms generated on the M&V system, enabling problems with system operation to be identified and repaired in a timely manner.

The project is tendered, a contract awarded and construction process begins. The CES Group provides testing and balancing, construction phase commissioning through project completion and turnover to the Owner. Owner training is provided, mechanical Operation and Maintenance manuals and a System manual are created for the Building Operators.