Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
This post is actually in answer to the comment asking for a description of how DOE-2 works.
DOE-2 actually consists of two independent programs - doebdl, the input processor, and doesim, the simulation engine.
doebdl reads a user input file, which could be as short as a dozen lines or as long as 10,000 lines, and creates from that two binary building description files containing all the information needed by doesim. doebdl takes virtually no time at all to run, but minimizes the need for doesim to digest the input information.
doesim originally consisted of four modules - LOADS, SYSTEM, PLANT, ECONOMICS - that are run sequentially, although I'm told that in DOE-2.2 SYSTEM and PLANT have been combined. LOADS simulates the heat gains and losses into each space for an entire run-period (year) at a user-specified reference temperature. The simulation is then repeated in SYSTEM, which derives the true temperature in each space, taking into account the heat input or extraction provided by the HVAC system, as well as the effect of outside ventilation air. If there is no HVAC, SYSTEM solves for the space floating temperature. If there is HVAC, SYSTEM simulates the actions of the HVAC and returns the heat input or extraction (up to the system capacity, natch), energy consumption, and final space temperature. For buildings with a central plant, the simulation is repeated once more with PLANT (DOE-2.1E only), which simulates the actions of the big stuff to meet the SYSTEM demand for heating/cooling by fluid type. ECONOMICS allow users to input utility rates and charges,and derive energy costs.
In comparison to EnergyPlus, DOE-2 is more loosely coupled. However, that doesn't mean there's no feedback between the three modules. For example, when SYSTEM solves for the zone temperature, the space loads are recalculated using the zone Weighting Factor, and the conductive loads adjusted by the Zone Conductance. As implied earlier, the effect of undersized HVAC on zone conditions is taken into account. However, up through DOE-2.1E there was no feedback from PLANT to SYSTEM, which was solved in DOE-2.2 by merging the two modules.
DOE-2 was designed in the late 1970's, when computers were much less powerful (I remember a 5-zone building taking 40 minutes on an IBM 370 that would now take 5 seconds on a PC), so computational efficiency, i.e., speed, was imperative. This applied not only to the overall solution technique, but throughout the program design. For example, (1) I/O is handled largely in binary, with the weather data read in two-week chunks, and output reports converted to text once at the end, (2) shading uses a very fast algorithm and done only for one day of each month. I was asked to do some timing comparisons in a 2013 study and found DOE-2.2 to be from 22 to 81 times faster than EnergyPlus (see www.whiteboxtechnologies.com/download_AutoDesk_EP_feasib_rpt.htm , p. 98).
Of course, there's always the danger of losing accuracy. However, I'm still waiting to see how bad is the problem, whether it's irreparable or just identifying problem areas to avoid or make improvements.
| | 2 | No.2 Revision |
This post is actually in answer to the comment asking for a description of how DOE-2 works.
DOE-2 actually consists of two independent programs - doebdl, the input processor, and doesim, the simulation engine.
doebdl reads a user input file, which could be as short as a dozen lines or as long as 10,000 lines, and creates from that two binary building description files containing all the information needed by doesim. doebdl takes virtually no time at all to run, but minimizes the need for doesim to digest the input information.
doesim originally consisted of four modules - LOADS, SYSTEM, PLANT, ECONOMICS - that are run sequentially, although I'm told that in DOE-2.2 SYSTEM and PLANT have been combined. LOADS simulates the heat gains and losses into each space for an entire run-period (year) at a user-specified reference temperature. The simulation is then repeated in SYSTEM, which derives the true temperature in each space, taking into account the heat input or extraction provided by the HVAC system, as well as the effect of outside ventilation air. If there is no HVAC, SYSTEM solves for the space floating temperature. If there is HVAC, SYSTEM simulates the actions of the HVAC and returns the heat input or extraction (up to the system capacity, natch), energy consumption, and final space temperature. For buildings with a central plant, the simulation is repeated once more with PLANT (DOE-2.1E only), which simulates the actions of the big stuff to meet the SYSTEM demand for heating/cooling by fluid type. ECONOMICS allow users to input utility rates and charges,and derive energy costs.
In comparison to EnergyPlus, DOE-2 is more loosely coupled. However, that doesn't mean there's no feedback between the three modules. For example, when SYSTEM solves for the zone temperature, the space loads are recalculated using the zone Weighting Factor, and the conductive loads adjusted by the Zone Conductance. As implied earlier, the effect of undersized HVAC on zone conditions is taken into account. However, up through DOE-2.1E there was no feedback from PLANT to SYSTEM, which was solved in DOE-2.2 by merging the two modules.
DOE-2 was designed in the late 1970's, when computers were much less powerful (I remember a 5-zone building taking 40 minutes on an IBM 370 that would now take 5 seconds on a PC), so computational efficiency, i.e., speed, was imperative. This applied not only to the overall solution technique, but throughout the program design. For example, (1) I/O is handled largely in binary, with the weather data read in two-week chunks, and output reports converted to text once at the end, (2) shading uses a very fast algorithm and done only for one day of each month. I was asked to do some timing comparisons in a 2013 study and found DOE-2.2 to be from 22 to 81 times faster than EnergyPlus (see www.whiteboxtechnologies.com/download_AutoDesk_EP_feasib_rpt.htm here, p. 98).98 (120/152 absolute)).
Of course, there's always the danger of losing accuracy. However, I'm still waiting to see how bad is the problem, whether it's irreparable or just identifying problem areas to avoid or make improvements.
