This article was originally published in the September/October 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online September/October 1998
Putting the Byte into Your Analysis Toolkit
by Bion Howard
When it comes to energy efficiency analysis, you have specific needs. Chances are, there's a software package out there that fits your needs. But if you're new to the world of energy software, it's hard to know where to start.
There are more than 125 EDS tools available to evaluate the energy-related aspects of buildings, and others are in development. Some tools are best for executing a basic load calculation; others, for doing year-long simulations with custom climate data. Some are best at the beginning of the design process. Others are useful for coming up with detailed performance reports on a finished building. Choosing the correct EDS is like choosing any other tool-you figure out which is the right one for the job. In the end, it is better to select a few EDS tools that meet your specific needs than to try to use many different ones, because it takes considerable time to become proficient with most of them.Tool Types There are three major types of EDS tool that you can use.
Correlation-based tools analyze data statistically and simplify it into a mathematical representation (a curve), which is then correlated (curve-fitted) with similar shapes from other data. Correlation data are often corrected with sensitivity study information to take into account systematic variations on usual situations. Since correlation tools are based on curve-fits to results from a more detailed simulation program, they can be no more accurate than the source program and are limited to the combinations and ranges of input used in the development of the correlation. They are less flexible than true simulation tools. WinGuide and PEAR are examples of correlation-based tools.
Seasonal-based tools are usually set up to minimize the time it takes to perform an analysis, by using only portions of the full year in order to make reasonably reliable calculations when run time is a concern. This approach can produce somewhat more accurate results on an individual building than a correlation-based tool can provide, but it still sacrifices some detail and cannot analyze whole-year weather data. Seasonal tools are highly simplified and cannot model dynamic effects very well. A well known example is REM/Design, from Architectural Energy Corporation.
Simulation tools model the building down to each hour (or less) of the year. In principle, they are the most accurate. They typically use either thermal networks (model interactions between individual components based on heat and mass transfer) or response factors that are generated to describe the characteristics of building materials and systems, and interactions. Once a building model is created, the software uses hourly weather data to calculate building loads, fuel use of the mechanical equipment, and indoor conditions.
The user has considerable control over the level of detail in a simulation model. But he or she must have more expertise in building science and mechanical engineering to select correct input values, to check his or her work, and to analyze the detailed performance reports that these tools produce. Simulation tools include DOE-2 and Energy-10 (see Energy-10, HE Sept/Oct '97, p. 37), though the two use quite different approaches to simulating building performance.
Within these three categories of energy design tools, there are many individual programs that you can turn to, depending on the task at hand.Early-Design Tools Early in the schematic phase, when a building may exist only on the back of an envelope, I use either a correlation-based software program, WinGuide; a seasonal-based tool, REM/Design; or a simulation-based tool, Energy-10. These software packages allow me to quickly verify my initial recommendations. For example, sometimes I think a roof might need a large overhang to avoid air conditioning penalties. All three of these EDS tools can rapidly recalculate different scenarios, and can show whether adding the overhang would cut too much off solar gain during the heating season. Such results are helpful for creating reports for builders in the early phase of design.
This compact and useful program is MS-WindowsÐbased and is a significantly revised version of the passive solar oriented BuilderGuide program developed initially by the National Renewables Energy Laboratory (NREL) for the Passive Solar Industries Council (PSIC). It is placed within a Visual Basic shell and has some very simple but useful tools to edit the materials and building propoerty values. There is also a rotating function so a designer or builder can examine the impact on overall energy performance of changes in the orientation of a home design. The graphical bar-charting function allows for quick comparisons between energy scenarios on the same building, and shows whether or not the building meets the Model Energy Code (International Energy Conservation Code-see p. 7).
REM/Design is an example of a tool whose engine is based on seasonal performance. Such programs often include a significant amount of default data, which can be accepted or, in selected cases, can be opened for editing by the user. REM/Design was recently updated for use in concert with the 1998 Criteria of the Energy Efficient Building Association (EEBA).
Energy-10 was developed by NREL with Lawrence Berkeley National Laboratory (LBNL) and others. It is marketed by the PSIC. Energy-10 includes a very useful manual, and comes with a training course called Designing Low Energy Buildings. Energy-10's user-friendly training and support make it stand out from other EDS tools.
One key advantage of Energy-10 is the Autobuild function in which the program erects a default rudimentary model. You then can edit the model to reflect the actual building, which is much quicker than creating a model from scratch. I also appreciate Energy-10's ability to run optimizations-which let me consider competing efficiency measures on the basis of their costs versus their benefits-and to suggest that I go get a cup of coffee while the results are being compiled.
I think Energy-10 is the best-developed simulation-based tool and the easiest one to use. But despite the recent release of version 1.2, there is still room for improvement. Specifically, I would like more control over inputting and modifications to the data sets and more emphasis on residential buildings. The program was originally written for small commercial applications. There are some minor bugs-such as problems getting correct units for energy factors to be furnished under some conditions-but NREL claims that these are likely to be taken care of in the next version. In my mind, these problems are pretty minor.
Consider Building Types
These programs are suited to different types of building. REM/Design, for example, is best suited to production housing, while WinGuide is intended specifically for passive solar designs. If the project is passive solar and will have thermal mass added, I use either WinGuide or Energy-10. If home efficiency is emphasized, I would be more likely to use REM/Design. It can also consider typical sun-tempered passive solar design.Fastest Spreadsheet in the West Sometimes I'm asked to do a real quick-and-dirty study. My client may say, Help me tweak this house so it meets the International Energy Conservation Code (IECC) or, I'm getting hassled by my building official and can't make my calculations work. For this sort of simple work, I use an Excel spreadsheet based on ACCA's Manual J, or possibly Program for Energy Analysis of Residences (PEAR) or MEC-Check, which is short for Model Energy Code-Check.
At my company, we developed-first in Lotus 1-2-3 for DOS, and later in Microsoft Excel-worksheets based on ACCA's Manual J. This permits us to quickly plug in many details about a building that can be taken off plans. Then we can calculate how much heated or cooled air might be required in a given room, and how large a home's space conditioning system would be. Many code jurisdictions require or recommend that a Manual J calculation be done to size home heating and air-conditioning systems. Manual J is also cited in the IECC.
There is also a computer program called Right-J that does Manual J calculations, but we don't currently use it, as we are happy with our Excel spreadsheet. Wright Associates has issued a new Windows-based version of Right-J that is packaged with other computerized versions of ACCA technical manuals, such as Right-D, which designs duct systems according to Manual D. HVAC contractors who do a lot of equipment-sizing work for home builders and retail strip malls might benefit from checking out these programs.
For simple calculations on new home designs, I still like the 10-year-old PEAR, which was developed by LBNL. This program is correlation based and uses a data set produced from thousands of individual DOE-2 runs performed on a set of typical houses with rather basic energy features. The user has only moderate control, selecting from a preset features listing, and most users have no control over the data that underpin the program.
Although it is DOS based and sort of clunky by today's standards, PEAR is handy for a quick run-through of envelope and basic mechanical system options. Its menu options are simple; it can compare different option packages with a base-case house; and it does a quick and simple economics analysis to boot.
MEC-Check, developed by Pacific Northwest National Laboratory, is helpful for quickly checking a home's compliance with the IECC. Like PEAR, it is a DOS program and may be difficult to set up on Windows-based PCs. It is best run in full-screen mode. It can be downloaded with documentation via the Internet (see Software Resources).
Home Energy Saver
Developed by LBNL, the Home Energy Saver is available on-line as a Web-based analysis tool. It provides information and energy analysis for the whole house by letting the user build a virtual home to estimate energy savings.Detailed Analysis For more detailed energy analyses of homes, I use typically use REM-Design or Energy-10. But sometimes, I turn to one of the following programs.
Before I started using Energy-10 for residential building studies, I typically used a simulation-based program called Micro-PAS. It was developed by Enercomp, based on the earlier CALPAS code by that the Berkeley Solar Group wrote for the California Energy Commission. One advantage of Micro-PAS is that it is a thermal network program. Thermal network programs are inherently flexible; they can model a wide variety of simulations. In this kind of model (one that requires some skill and good data on material properties), you can erect in the computer nearly any sort of building-even complex heat storage systems.
If I am called on to evaluate a passive system with a rock bed or air-core block bed system, I go straight to Micro-PAS. I have used the program in the past to do energy studies of buildings with these systems, and the buildings performed well when constructed. I saved the files from those buildings, and now I can simply update some variables to analyze a new building. Making a habit of saving old input files is useful, particularly in structured programs like Micro-PAS and Energy-10. It takes a lot less time to update old project files than to type in a new project from scratch.
The National Energy Audit Tool (NEAT) was originally distributed to weatherization agencies, utilities, and community action programs. It is a DOS-based program for personal computers, and though learning to use it can be somewhat difficult, it can produce useful results-including an economic assessment of the savings-to- investment ratios of selected energy efficiency measures.
In NEAT, a user inputs data on the building and its heating and cooling systems. The program responds with a prioritized list of cost-effective energy efficiency measures. The selected measures are customized for individual homes. An upcoming version will also be able to analyze utility bill histories. According to Oak Ridge National Laboratory, as of October 1995 NEAT was being used by about 500 local weatherization agencies in 30 states to make retrofitting decisions on low-income, single-family dwellings.
Multifamily buildings, such as garden apartments and condos, may have multiple zones and complex HVAC systems more like the ones that commercial structures employ. We usually use the detailed DOE-2 program to tackle such projects. A thorough site study gathers input data, sometimes over a three-day period. We use these data, along with building plans and as-built updates to plans, to create a building description for use by DOE-2. We run the simulation several times to check the inputs for reasonableness before we explore efficiency options. Sometimes we perform an initial analysis with Energy-10. If it is a big enough project, then once Energy-10 has recommended the best measures, we run DOE-2 as well. This is the one case in which we will use two EDS tools to confirm one another's results. It can be important to run multiple programs in expensive projects, where more than $250,000 may be available for improvements (see Simulations Save Money).Green Building The integration of green building attributes into EDS packages has a long way to go. Building codes and incentive programs in places like Austin, Texas, and Boulder, Colorado, are going beyond energy, into green building. These codes and programs require a minimum level of green attributes, including water conservation, recycled building materials, energy efficiency, and waste reduction.
Like energy codes, the green building programs allow trade-offs between different measures. However, there are no widespread standards for environmental attributes. There are standards for energy and water use, but these are only two parts of the equation. This poses a problem for EDS packages-consumers want to know how green a building is, but different software packages have different ways of calculating environmental effectiveness.
Professor Jan Krieder, of the University of Colorado, has examined software assessment tools with his graduate students. When he and his students ran one home design through various applications, they received reliable and consistent results for energy efficiency, but the green building scores were inconsistent. When run on the same house design in several states, such as Texas and Colorado, where green building programs exist, the different tools did not equally indicate that code compliance was met.Special Programs If you or your clients participate in programs such as Energy Star Homes, Edison Electric Institute E-Seal, GoodCents Environmental, or certification by insulation manufacturers, their requirements might affect which EDS you choose.
Some programs require that specific proprietary EDS tools must be employed in order to meet certain objectives and requirements. In other programs there is considerable leeway for selecting among competing tools.
Some utility and government incentive programs rely on analyses of individual buildings, either in plan review or as a result of field assessments such as energy audits, home energy ratings, and residential environmental performance assessments. These require numbers calculated from an EDS. As a practicing Energy Star Ally, for example, I must use computer tools such as REM/Design and DOE-2 to demonstrate that residential buildings are at least 30% better than the IECC.
Both the Residential Energy Services Network (RESNET) and the Home Energy Ratings System Council (HERS-Council) have created accreditation standards for energy performance assessment systems and support personnel. If you are consulting for builders who want to qualify for home energy ratings, you should get software that will conform to the RESNET or HERS-Council standards. Both groups require that accredited software be tested with NREL's HERS-BESTEST procedure. Contact either HERS Council or RESNET to find out whether they recommend an EDS that would be most suitable for residential energy efficiency performance assessments.Know Your Needs Keep in mind the following details when you shop for an EDS tool:
Bion Howard is a building environmental science and technology consultant with Howard Associates in Harwood, Maryland. He is a member of the Energy Efficient Building Association (EEBA).
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