A national laboratory of the U.S. Department of Energy Office of Energy Efficiency & Renewable Energy National Renewable Energy Laboratory Innovation for Our Energy Future A Methodology for Validating Technical Report NREL/TP-550-42059 Building Energy Analysis April 2008 Simulations R. Judkoff, D. Wortman, B. O’Doherty, and J. Burch National Renewable Energy Laboratory This work was performed during the early 1980s and the report was written in 1983 but not published. It was, however, distributed informally to some experts in the field and was referenced as SERI/TR-254-1508. It was recently prepared for publication, but the information has not been updated. NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 A Methodology for Validating Technical Report NREL/TP-550-42059 Building Energy Analysis April 2008 Simulations R. Judkoff, D. Wortman, B. O’Doherty, and J. Burch National Renewable Energy Laboratory Prepared under Task No. 54004000 National Renewable Energy Laboratory 1617 Cole Boulevard, Golden, Colorado 80401-3393 303-275-3000 • www.nrel.gov Operated for the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy by Midwest Research Institute • Battelle Contract No. DE-AC36-99-GO10337 NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at http://www.osti.gov/bridge Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone: 865.576.8401 fax: 865.576.5728 email: mailto:[email protected] Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone: 800.553.6847 fax: 703.605.6900 email: [email protected] online ordering: http://www.ntis.gov/ordering.htm Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste Foreword This is a publication of work that was almost completed in August 1983. Final publication was never completed at that time because of funding issues. There was, however, a limited distribution of the final draft to leading experts in the field, and the report has been referenced in a number of documents nationally and internationally. Since that time great strides have been made in computer hardware. It is now possible for a building design practitioner to run a full-blown simulation of building energy performance on a laptop computer, and there are literally hundreds of such computer programs throughout the world. Thus, there is renewed interest in the theory of how to validate building energy simulation programs. We have therefore cleaned up the few cosmetic edits that remained in the previous final draft and formally published it as NREL/TP-550-42059 (originally SERI/TR-254- 1508). Although the simulation programs referred to in this report have long since been replaced by many subsequent versions of software, the underlying theory of how to validate, diagnose, and design good validation experiments has remained substantially unchanged since we first proposed this methodology. i Executive Summary Objective To develop a validation methodology for building energy analysis simulations (BEAS), collect high-quality, unambiguous empirical data for validation, and apply the validation methodology to the DOE-2.1, BLAST-2MRT, BLAST-3.0, DEROB-3, DEROB-4, and SUNCAT 2.4 computer programs. Discussion This report covers background information, literature survey, validation methodology, comparative studies, analytical verification, empirical validation, comparative evaluation of codes, and conclusions. Section 1.0 establishes the historical context in which the Solar Energy Research Institute (SERI) studies evolved. The history of computerized building energy analysis is traced and the case is made that earlier methods do not contain algorithms that can accurately determine all heat flow quantities, especially for natural heating and cooling applications. These programs, though versatile for conventional buildings, are highly questionable for analyzing innovative design options. Newer state-of-the-art programs, such as DOE-2.I, BLAST-3.0, DEROB-4, and SUNCAT-2.4, have not yet been sufficiently validated over a wide enough range of parameters to be used with confidence. Researchers, representatives of the building industry, and several government-sponsored planning groups have expressed the need for a systematic approach to the validation issue. Section 2.0 reviews a sampling of the literature on the validation of building energy analysis simulations, which shows that previous validation studies left four areas needing further investigation: • Validation with empirical data from full-scale buildings: In previous studies there generally have not been sufficient data to understand observed differences between calculated and empirical results. Little effort has been directed toward performing follow-up experiments and reducing ambiguities in the data. Furthermore, data are lacking for buildings using natural environmental control systems. • Validation with empirical data from test cells: Many validation studies have been done using single-zone test cells; however, few investigators have confidence in the extrapolation from single- to multi-zone predictions. Nevertheless, there are few or no data from multi-zone test cells or multi-zone unoccupied buildings. • Analytical verification and code-to-code comparisons: Most comparative software studies have been done on conventional buildings. Studies on buildings that use natural environmental control systems are lacking. Additionally, previous comparative validation studies have not exploited the combined analytical and comparative approach. • Validation methodology: Although validation studies have been performed, no systematic validation methodology has been developed. ii Section 3.0 presents the SERI methodological approach to validation. All differences between measured and calculated results are attributed to four external and three internal error types in building energy analysis techniques. We define various levels of validation according to the degree of control exercised over these error sources. Additionally, we identify four types of extrapolations inherent in validation studies. The relative strengths and weaknesses of empirical, analytical, and comparative validation techniques are shown with respect to these extrapolations, the control of error sources, and the simulation process. Finally, we present a systematic approach to validation that includes these three validation techniques. When properly applied, the methodology assists in establishing the parametric range within which a building energy analysis simulation can be used with confidence. Section 4.0 discusses the two comparative studies we conducted at SERI. In the first study, we compared the DOE-2.l, BLAST-2MRT, DEROB-3, and SUNCAT-2.4 programs by modeling a simple high- and low-mass direct gain building with Madison, Wisconsin, TMY (typical meteorological year) data. One of the programs yielded annual results significantly different from the others. We subsequently uncovered a flaw in the thermal solution algorithm that required the code author to extensively rewrite this program. The other three programs compared closely for annual results but differed somewhat in hourly temperature profiles. In the second comparative study, DOE-2.l.BLAST-3.0, DEROB-4, and SUNCAT-2.4 were again compared except that Albuquerque, New Mexico, TMY data were also used. In this case we observed a large scatter in the annual results from all four programs. The large differences indicated the need for further investigation to determine which, if any, of these programs was correct. Section 5.0 describes the analytical verification studies we conducted to determine if the differences between the building energy analysis simulations shown in the second comparative study could be attributed to the faulty numerical solution of specific heat transfer mechanisms. A set of analytical solutions was derived for simple cases that could also be modeled by the computer programs. Any differences between the analytical solution and the results obtained with the simulation indicate a mistake in the numerical solution algorithm of the code for that specific combination of heat transfer mechanisms, given the boundary conditions. The major test types were: • Temperature decay • Steady-state overall conductivity • Infiltration • Glazing assembly conductivity • Glazing assembly transmissivity • Mass charging by radiation. iii Except for DEROB-3, we found