In order to improve the quality of aggregate energy savings estimates, a building-specific Retrofit Energy Savings Estimation Model (RESEM) was developed by the Lawrence Berkeley National Laboratory for the DOE Institutional Conservation Program (ICP). RESEM is a user-friendly tool which allows ICP staff to reliably determine the energy savings directly caused by ICP-supported retrofit measures implemented in a building. For maximum accuracy and validity, energy savings are calculated from a combination of computer simulation and actual utility data. This tool has the technical capabilities to describe, simulate, and evaluate detailed building energy usage and savings for a variety of non-residential building types.

The RESEM interface has been designed to lead the unfamiliar user through the entire energy savings analysis process. However, RESEM allows the experienced user substantial freedom in accessing the simulation powers of this tool. RESEM runs on any IBM-PC or 100% compatible with 640 Kbytes of RAM. The program is written in Microsoft C and executes in conjunction with RAM-resident screen handling utilities.

Considerable development effort has been invested in the RESEM interface. Although text-based, RESEM uses pull-down menus and pop-up data entry screens and dialog boxes. These features assist the user in visually maintaining the context of detailed work within the overall savings analysis sequence. Descriptive hint lines appear onscreen to guide the user in selecting menu options and entering technical data. Default values are automatically entered into most data fields prior to display for editing. Pop-up windows dynamically list allowable choices for appropriate data fields.

Energy savings analysis begins with the development of a base (pre-retrofit) building description. RESEM will automatically generate a complete building description for primary, secondary, and university schools, and for hospitals. For these building types, the only required user input consists of: a building name, year of construction, total floor area, and building type. The complete building description is generated from this minimal input by applying a built-in set of production rules, for the given building type, to a modifiable external library of standard building elements. The generated building description includes all details regarding building geometry, envelope construction elements, HVAC system, lighting, equipment, occupancy, and usage schedules. All aspects of this building description can be accessed for review and modification. This building description will be used to perform an energy simulation of the pre-retrofit building.

RESEM is complex and sophisticated enough in its energy modeling to explicitly reflect the influence of a wide range of design, operation, and weather parameters. Energy simulation algorithms are based on the ASHRAE modified bin method, modified to use monthly bins and to simulate complete HVAC systems and plant equipment performance at each bin condition. Although based on existing public domain methods, all code has been designed and rewritten to provide maximum speed consistent with required accuracy. A twelve month simulation of a five thermal zone building takes less than 45 seconds on an IBM-PC/XT with an 8087 math coprocessor. The same simulation takes less than 5 seconds on an 80386 based machine. This extremely fast simulation engine encourages multiple iterations exploring the energy use consequences of modifications to the building description. A completed simulation can display building energy consumption for each fuel type and end use.

The next stage of the analysis sequence involves modifying the base building description so that its simulated energy use matches actual utility data collected for the building. RESEM allows the user to enter utility data for arbitrary consumption periods for each fuel type used in the building. An energy simulation is then performed using bin data (processed from NOAA weather data) for the time period(s) corresponding to the utility data. RESEM uses a unique approach for automatically modifying the building description to match the entered utility data. To perform this automatic reconciliation, the user need only identify the type of modification desired. This means selecting from a list of 25 key building parameters to include in the automatic reconciliation process. Once the parameters of interest have been selected, RESEM will determine the optimum amount of modification for each parameter and automatically make this modification.

Following reconciliation of the base building, the next stage of the analysis sequence involves describing the energy conservation retrofits which have been implemented in the base building. For many of the specific types of retrofits recognized in the ICP program (e.g., replacing the heating plant burner), RESEM menu options will lead the user to minimal input screens and then automatically make the required building description modifications. For more unique or complex retrofit measures, the user can access detailed input screens containing all building description parameters. RESEM also allows additional modifications to be described which may affect post-retrofit utility data, but which should not be considered in the calculation of energy savings. All post-retrofit modifications are then adjusted in a manner similar to the base building reconciliation so that the post-retrofit building simulation matches user entered post-retrofit utility data.

Based on all the above information, RESEM then calculates energy savings using monthly ratios between simulated performance for the utility data time periods and for long-term average weather. This approach bases savings estimates directly on the metered utility data. Savings estimates are reported for the overall retrofit building as well as the implemented ECM modifications in both natural units (e.g., kilowatt-hours) and in energy use index units of BTUs per square foot per year.