Paper Conference

Proceedings of eSim 2016: 9th Conference of IBPSA-Canada


Practical building energy simulation applications

Ralph Culham

Abstract: The author provides a retrospective viewpoint of his over 30 year experience in applying building energy simulation methods to actual projects. The paper begins with ASHRAE Simplified Energy Analysis using the Modified Bin Method through DOS based Trane Trace 600, custom Excel Cogen spreadsheet analysis, DOE-2.2 building description language (BLD), eQUEST and finishing with EnergyPlus. The author highlights some of the trials and tribulations of his experience in energy modelling as well as the successful outcomes as it applied to specific projects. Further, the author identifies beneficial skills and knowledge beyond knowing the intricacies of the software programs. The author concludes with suggestions on strategies to transfer the knowledge gained by experienced building simulation modellers. BACKGROUND EARLY DESIGN, ANALYTICAL AND PROGRAMMING SKILLS The background to my migration to building energy simulation goes back four decades. Prior to graduation, I designed custom built homes on a drafting board to earn income for university. This developed my building plan reading skill significantly. During the early part of my engineering career, I designed a pneumatic start system for a gas turbine driven emergency plant. This developed my understanding of pneumatic systems, often a significant energy load in plants. Other early design work included HVAC systems for commercial, industrial, residential and mid-rise multi-residential buildings, wherein only one corridor makeup air system was required. Furthermore, I designed retrofit steam heating plants for long-term care facilities and retrofit hydronic heating systems for institutional buildings which included arenas where process heating was required for the Zamboni. Not only was I engaged in the design phase of construction documents, but I also performed on-site inspection of the installations. My analytical skills were developed as a sales application engineer wherein I analyzed the predicted natural gas well head production to optimize the selection of gas turbine driven natural gas compressors over a 25 year project life-time frame. Programming skills in Microsoft QuickBASIC were developed when I wrote a custom program to access the noise level at a critical receptor of a cogeneration package with multiple noise sources. Other efforts included on-site performance testing of gas turbines in the North West Territories and witness testing of industrial gas turbines in test cells and industrial fan testing, both in Ontario. This was an opportunity to validate on-site performance with manufacturer’s tabulated data. THE BEGINNING OF BUILDING ENERGY SIMULATION My beginning in building energy simulation began in the 1980’s when a colleague approached me with a project opportunity that he did not have the time to take on because of other commitments. An energy management company was installing building automation systems (BAS) in high-rise condominium buildings in and around the areas of Toronto, Ontario. They needed the annual baseline natural gas and electrical energy consumption and the potential energy savings and cost savings attributed to the installation of the building automation system. The purpose, the why of this project was to justify the expenditure of the automation control system. Notice this why – a single dollar amount of annual savings so that the energy management company could show the value of the system to the condominium developer. The condominium developer then could use this feature as an additional benefit to prospective buyers to reduce the annual condominium fees as compared to other buildings in the market place. ANALYSIS TOOLS Initial Computing Platform At that time I was still using a computer with an 8-bit Zilog Z80 microprocessor, 64 kilobytes or RAM, and two 5 ¼ -inch double-density floppy disk drives. The computer ran on the Digital Research CP/M operating system, short for Control Program for Microcomputers. It was equipped with several software packages including a word processor and a spread sheet program. I needed a method to solve the problem within the constraints of my computing system. Through research of the ASHRAE catalog, I ordered the report “Simplified Energy Analysis Using the Modified Bin Method” published in 1983. Simplified Energy Analysis Using the Modified Bin Method The modified bin method recognizes that the building and zone loads consist of time dependent loads (solar and scheduled loads) and temperature dependent loads (conduction and infiltration). To compute the energy consumption, two or more computational periods are selected, normally representing the occupied period and the unoccupied period. For each period, the time dependent loads are averaged and added to the conduction loads such that the load is characterized as a function of outside air temperature for the calculated period. The modified bin method utilizes bin weather data. Bin weather data is the number of hours in a specified time period, such as a year or a month that the dry bulb temperature is within a bin, wherein the bin may be 5 degrees F. The distribution is further divided into 3 periods of the day (01-08 hrs), (09-16 hrs), and (17-24 hrs). In expressing building loads as a function of outdoor temperature, the following assumptions are made: 1. All exterior loads can be expressed as a linear function of outdoor temperature. 2. On a daily basis, the interior loads can be averaged over the “system on” or “system off” time periods. The procedure begins with a load estimation for the occupied and unoccupied operating modes. Although the calculations are similar to the design type calculations, they are averaged internal and external loads, and are also performed at intermediate temperatures. The load calculations are performed at four temperature bins, judged to be significant for the given building and location. The bins are identified by their mid-point temperatures (e.g. the 89.5-94.5 F bin is identified as the 92 F bin), and represent the following four temperature bins: • Peak Cooling, Tpc • Intermediate Cooling, Tic • Intermediate Heating, Tih • Peak Heating, Tph An alternate approach to developing the loads is to develop an equation of the load profile. This results in a system of linear equations of the form: Y = mx +b, where Y represents the load, m is the slope, x is outside temperature, and b is the intercept. Using the methods described in the report, I developed a multi-page spread sheet for performing the analysis wherein each page represented a month of the year. Limitations on the Simplified Energy Analysis Method The procedure is based on time averaging techniques and as such has limited capability in accurately dealing with highly dependent problems. The major premise is that the net time dependent energy rate, added to or removed from the space during a given computational period, is equivalent to the average energy rate added or removed from the space times the duration of the computational period. The weakness is this premise is the approximation used in developing the average rates of energy gains or losses from the space. The thermal capacitance of the space will induce a time lag before the thermal load to the space actually becomes a load on the HVAC system. Thus, the load computed by averaging many not become the actual load on the HVAC systems. Furthermore, variation of space temperatures characteristic of any control system, which causes heat storage and release, is not accurately represented. This issue may also be argued in hourly simulation programs since the space temperature variation may occur within the hourly time step. Large space temperature variations associated with dead band controls and night set back and set up conditions may show significant variation with hourly simulators.⁽¹⁾ Simplified Energy Analysis Method Background The development of the Simplified Energy Analysis Using the Modified Bin report, was based on many years of effort by the members of the subcommittee for simplified energy analysis of the TC 4.7 Energy Calculations Committee, and was the result of research sponsored by ASHRAE. The report provides an excellent background source for an understanding of load calculations, air system component analysis, air system simulation, and plant component simulation. It also includes program code for analysis of a variety of systems. The report is still available in paperback format on the ASHRAE store web site. The following is an exert from Chapter I –
Pages: 570 - 579