ENERGY-10, an award-winning PC-based design tool, helps architects and building designers quickly identify the most cost-effective, energy-saving measures for small commercial and residential buildings. ENERGY-10 can identify the best combination of energy-efficient strategies, including daylighting, passive solar heating, and high-efficiency mechanical systems. Using ENERGY-10 at a project's start takes less than an hour and can result in energy savings of 40%–70%, with little or no increase in construction cost.

ENERGY-10 is the software component of Designing Low-Energy Buildings with ENERGY-10, a collaborative project of the National Renewable Energy Laboratory’s Center for Buildings and Thermal Systems, the Sustainable Buildings Industry Council, Lawrence Berkeley National Laboratory, and the Berkeley Solar Group.

ENERGY-10 removes a major barrier to energy-efficient building design.

For years we have heard about buildings that use half the energy of conventional buildings, provide a superior indoor environment, and cost no more to build. These include offices, schools, mental health facilities, post offices, banks, churches, and houses. Detailed monitoring of several of these buildings confirms their excellent performance. Post-occupancy evaluations clearly indicate that occupants love these buildings. People often comment on the delightful nature of their bright, daylit interior. If these buildings are so great, why don't we see more of them? One answer is that they used to cost more to design. In addition, many architects lack the analysis tools required to accurately predict a building's comfort and performance before construction.

The solution to these problems is ENERGY-10, a computer program that takes the drudgery and uncertainty out of performance analysis by making it fast, easy, fun, and accurate. The program uses a process called simulation, which means calculating in advance how the building will operate hour by hour throughout an entire year, using weather data recorded near the building site. For years, NASA has used simulation routinely to plan all of its space missions. Now building designers can use similar techniques to plan their buildings. Architects who had waited years for this software hailed its development.

Energy-efficient buildings make extensive use of daylighting, bringing natural sunlight into the interior through high windows and special apertures called light shelves and roof monitors. These techniques were in common use a century ago. They eliminate glare because beam sunlight is diffused evenly throughout the room.

Daylighting allows building occupants (or, better yet, automatic controls) to dim or turn off heat-generating electric lights. This saves by reducing the amount of electricity used for both lighting and air conditioning. Savings that result from reducing the size of cooling equipment can more than pay for the modifications required. With good design, heating, cooling, and lighting costs can all be reduced. The ENERGY-10 program guides the designer to correct solutions, providing sound energy-efficient designs.

Ultimately, occupants are the major beneficiary of these buildings. Many occupants of low-energy buildings have already demonstrated lower rates of absenteeism, higher productivity, and more relaxed and contented feelings. The test scores of students in several North Carolina schools (figs 1–4) are 5%–21% higher than those of students in schools without natural light. Mental health patients have shown higher rates of recovery and less stress when treated in low-energy buildings. The economic effects of these ancillary benefits complement the savings from lower utility bills.

How Is This Software Different?
Computer simulations for buildings have been around for years. What is different about ENERGY-10? The answer lies in two of ENERGY-10's unique features: AutoBuild and Rank.

 

Fig. 3: Durant School


Fig. 4: Durant School
The hardest part of building energy analysis is describing the building to the computer. This is particularly problematic if the building has not yet been designed. But this is exactly when an energy analysis is most valuable. If the building is already designed, it is too late.

NREL has developed a way to define and evaluate a building before it's been designed. This procedure, called AutoBuild, is programmed into ENERGY-10 and can be done faster than it can be explained. The idea is to describe the building based on a few pieces of information that are known in the predesign phase of a project: location, total square footage, and use category (for example, office, warehouse, school, residence). Additionally, the number of stories and type of heating, ventilating, and air-conditioning (HVAC) system may be known. Starting from these five inputs, a plain rectangular (or shoe-box) building can be defined. This is called a reference case (or base case).

The next step is to perform an energy analysis of the reference case. The results will be enlightening, especially if you are new to energy analysis. Energy use and energy cost are determined, as well as how these are divided between heating, cooling, lights, fans, and other uses, such as plug loads. The results are quantitative and graphic. You can explore the output graphics to determine how big the peak loads are and when they occur. Most importantly, you gain insight.

The AutoBuild wizard goes one step further. It generates not just one building description, but two. The second building is derived from the reference case. It is the same rectangular shoe box but with a potpourri of energy-efficient strategies applied. These might be added insulation, energy-efficient lights, daylighting, passive solar heating, and extra mass for heat storage (the user gets to select the strategies). This second building, called the low-energy case, is also evaluated and the results are displayed alongside the reference case results. This shows the potential for savings from the selected strategies. Again, you can delve into the hourly graphs to see just why and when the savings are being made.

Another unique feature of ENERGY-10 is Rank, which automates the process of sequentially applying several energy-efficient strategies of potential interest, evaluating their consequences, and ranking the results. NREL strongly suggests that these steps be carried out at the beginning of each project to save time later on. The designer can enter the preliminary design phase knowing the energy strategies that are likely to be most important and effective.

As the design proceeds, the designer should modify the low-energy case building description (that AutoBuild generated) to represent the building that is actually being contemplated. ENERGY-10 is based on the principle that everything is defaulted and everything can be changed (including the defaults). This saves time in setup, yet provides great flexibility in defining the building. It also allows the user to evaluate the consequences of decisions as the design evolves. In the end, the architect can present the design to the client while feeling confident that he or she has designed a comfortable, energy-efficient building. The architect also will know that the future owner and occupants will appreciate the lower utility bills as well as the high quality of the indoor environment.

 
How It Works

ENERGY-10 is designed to complement the normal architectural design process, as shown in the diagram.

Predesign
In the predesign phase (sometimes called the programming phase), both reference case and low-energy case building descriptions are generated using AutoBuild, the automatic building-generation wizard. The wizard creates a basic shoe-box reference building with general characteristics required to satisfy the architectural program. (The user can modify this building description, if desired.) The low-energy case is also automatically generated at the same time by using the Apply feature. This applies a set of user-defined solar and energy-efficient strategies (EESs) to the reference case to create the low-energy case. These two buildings are then simulated and evaluated to identify (1) the energy issues of greatest importance and (2) the effectiveness of the selected EESs.

Next, the Rank feature is used to set priorities among the selected EESs. The designer is now well equipped to begin the building design, knowing which strategies should be incorporated. The designer presents these results to the client, and the two parties agree on energy-performance goals for the building.

Preliminary Design
In the preliminary design phase (sometimes called the schematic design phase), the user adjusts the original low-energy case building description to be consistent with the first building scheme proposed, simulates that building, and compares the results with the original two shoe-box designs and with the performance goals. This requires

manually computing the wall surface and other areas of the building and entering the values in appropriate places in the building-description dialog boxes. New walls, roofs, and floors can be created to describe multiple orientations or tilts. The evolving building description can be called the current design. This is a repetitive process (as shown in the diagram)—design, adjust the model, review, design, adjust the building description, evaluate, design, adjust the building description, review, etc. Each step represents a different design scheme. The Apply feature can be used to speed up the process of adjusting the model. The preliminary design is complete when the process has produced a design that satisfies the client.

Keeping a Record
The design process outlined here will result in a design progression such as the one shown here.

As the design evolves, the results for each scheme are saved using the Keep feature of ENERGY-10. ENERGY-10 can display the results of the initial cases and the various schemes as shown. Other results, such as annual costs or peak electric load, can also be graphed.

In the design development phase, the user further modifies the building description to be consistent with the final building design. ENERGY-10 results are compared with the original two reference buildings and with the performance goals established in the schematic design. Fine-tuning design can be done and design adjustments can be made at this point.

In the construction documents phase, the user develops specifications to ensure that each element of the building and each piece of equipment will meet the design parameters assumed in the energy evaluation.

Perspective
Design is a complex process, involving numerous considerations, many of which conflict. Energy performance is only one of a host of important issues facing the designer. A successful design integrates daylighting with passive solar heating and natural cooling to decrease heating, lighting, and cooling loads using conventional materials.

We strongly recommend you read Designing Low-Energy Buildings, an excellent general guide to the integration of energy-efficient strategies into the design process. This guide, along with the ENERGY-10 software, are the key materials in the Sustainable Buildings Industry Council's workshops on low-energy buildings.

 
Features


Automated Tasks
Graphic Output
Simulation Analysis

Automated Tasks
ENERGY-10 is fast and easy to use. That's because ENERGY-10 automates tedious design tasks. These include:

AutoBuild generates buildings in a snap. In the first step, provisional building data, such as wall heights and lengths and thermostat set points, are calculated based on four inputs you supply. You can edit these provisional values or not, as you choose. In the second step, the AutoBuild wizard generates two complete building descriptions, a reference case and a low-energy case. As the two building descriptions are being generated, hundreds of numbers are defaulted into the buildings' databases. However, you always have complete freedom to adjust any parameter.

 

Apply automatically incorporates a combination of energy-efficient strategies into your building description. Apply is a special feature of ENERGY-10 that provides a quick way for you to determine the combined effect of a group of strategies. Apply automatically makes global modifications in a building description. These modifications are made based on (1) your selection of any or all of the energy-efficient strategies (EESs), and (2) the characteristics you specify regarding each strategy. Both selections are made under the EE Strategies menu. Apply is used automatically as part of the AutoBuild process to create the low-energy case, starting with the reference case. But it can also be used at any point during the design process.

 


Daylighting retrofit: before and after
Rank automatically evaluates energy-efficient strategies individually. Rank is a special feature of ENERGY-10 that provides a quick way for you to determine the individual effect of several strategies. Rank automatically makes global modifications in a building description. These modifications are made based on (1) your selection of any or all of the EESs, and (2) the characteristics you specify regarding each strategy. Both selections are made under the EE Strategies menu.

Rank applies strategies one at a time, whereas Apply applies several strategies at once. Rank first simulates the unmodified building and then applies the first strategy selected, does a simulation, saves some results, and unapplies the strategy. It proceeds automatically through all selected strategies in the same way. At the conclusion, it displays the results, ranked according to annual energy use. The user can then select other ranking criteria. Rank results are saved as part of a variant. Rank leaves the original building unmodified.

ENERGY-10 runs under Windows. The Windows user interface is both intuitive and powerful. The building description is contained in dialog boxes. Building parameters are all defaulted during AutoBuild, but they can all be changed later. A powerful feature of Windows is that the output graphics can be copied to the Clipboard and subsequently pasted into another application, such as a word processor or presentation software.

Extensive on-line help is available. The ENERGY-10 user is supported by comprehensive, context-sensitive help screens. Many "how-to" sections detail common procedures. Extensive background descriptions are given. When possible, Help relies on graphical information. An example is the description taken from Help showing how a building's windows are modeled as center-of-glass and frame, relying on Lawrence Berkeley National Laboratory's Window-4 computer program results.

Graphic Output
ENERGY-10 offers a rich selection of graphic output options to make it easy and fast to assimilate the results of the billions of calculations that are made to execute an annual hour-by-hour simulation. These range from bar charts that show annual results to hour-by-hour line graphs showing heat flows, energy use, or daylighting savings. Examples are shown for a 6500-sq-ft bank building in Columbia, Missouri. The results illustrate the huge savings that can be obtained at little or no added cost. The HVAC capacities are reduced by 30%–40%, freeing up money that can be allocated to the other upgrades that make the downsizing possible.

These graphs enable you not only to quickly assess the bottom-line results, but to understand the reasons that the results behave as they do. This knowledge helps you develop the best strategies for your design. The graphs can be copied to the Windows Clipboard and then pasted into your report—in fact, that's what we did on this page.

 

This graph shows savings due to daylighting. Lighting in the reference case is 35 MWh. Nine MWh is saved by more efficient lights and ballasts; 13 MWh is saved by dimming the lights; leaving only 13 MWh required, a 63% reduction. And this does not include the savings due to reduced cooling, which is accounted for in the cooling calculations.

 

Another of the more than 20 view options shows typical days in each month, comparing the two buildings. Temperatures and heat flows are shown. This contains information about both the daily behavior and how this changes month to month.

For other graph options, see Simulation Analysis. These graphs show up better on the ENERGY-10 screen than in this web page, where they have been changed from their original vector representation to a raster (JPEG) representation, with some inherent loss of detail. The detail is retained when the image is pasted into a word-processor.

Simulation Analysis
ENERGY-10 gives you comprehensive evaluations of energy use with a technique called simulation, the most widely accepted method for doing building energy analysis. NASA uses simulation to plan spacecraft missions; you can use simulation to plan your building. Simulation predicts conditions in the building ACH day of the year, based on assumed occupancy characteristics.

Simulation results for Columbia, MO, indicating inside and outside temperatures, solar and internal gains, and output energy from the HVAC system for two days in April.
In ENERGY-10 this is done by calculating heat transfer from point to point within the building each hour throughout a simulated year. Weather data consists of site-specific, hour-by-hour values such as temperatures, solar radiation, humidity, and wind speed. Each month of data is from a different year, selected to be typical of the long-term average for that particular month.

The graphs generated by ENERGY-10 show typical simulation results. The advantage of simulation analysis is that it provides a reasonably realistic picture of how an actual building would perform, assuming that it is constructed and operated in accordance with your specifications and your suppositions regarding operational behavior, such as thermostat settings, HVAC control settings, and internal gains.

Many graphic output options are available. This graph shows a typical building's total energy use broken down by end use. This graph will look quite different after energy-efficient measures are implemented.

The simulation analysis in ENERGY-10 accounts for a detailed evaluation of solar gains through windows, heat flowing into and out of walls, thermal storage in all building materials, and HVAC performance. The simulation is performed for an entire year, summing hourly energy consumption into monthly totals, determining monthly costs, and then annual totals. It is a little like monitoring a building before it is constructed.

 

This graph shows the behavior of a daylighting system with continuous dimming. The lower black line is the required total lighting level. The yellow area indicates the artificial lighting required to reach the required total lighting level, with the lights dimmed to account for the available daylight. The blue area indicates the difference (plotted negative for clarity), which is the savings due to dimming the lights. The upper curves show the exterior illuminance. The middle day is clear, and the last day is almost totally cloudy except for a couple of hours in the afternoon. Note that the savings on these two days is comparable because the diffuse sun is particularly effective.
Daylighting in the building is simulated in a similar way, accounting for visible transmittance through windows, switching or dimming characteristics of the lighting controls, and reduction in heat energy generated by the lights, which, in turn, affects the building's thermal performance.

The daylighting analysis divides the building into five lighting zones and employs the split-flux method. The thermal analysis is done using a thermal-network mathematical modeling approach. HVAC analysis is quasi-steady state.

 

Resources

Bibliography
Weather Data
Workshops

 




Bibliography
Most of the publications listed here are available online in PDF format. (Learn more about PDFs.)

Balcomb, J. Douglas (1999). "Using ENERGY-10, to Design Low Energy Buildings". A summary report that describes ENERGY-10, and presents a detailed worked example carrying a particular building from pre-design through preliminary design. 20 pp. (PDF Format, 118KB)

Balcomb, J. Douglas (1999). "ENERGY-10, Slide Show". A graphic presentation of the ENERGY-10 program, adapted from a PowerPoint presentation. 21 slides. (PDF Format, 966KB)

Balcomb, J. Douglas; Beeler, George (1998). "Designing Low-Energy Buildings with ENERGY-10." Proc. Solar '98 (24th Passive Solar Conference), American Solar Energy Society, June 15-18, 1998, Albuquerque, NM. Shows ENERGY-10 as it would be used in the design of the Environmental Technology Center at the Sonoma State University. (PDF Format, 73KB)

Balcomb, J. D. (1998). "The Coming Revolution in Building Design." Proc. PLEA '98, Passive and Low-Energy Architecture, Lisbon, June 1-3, 1998. Lecture upon acceptance of the Passive and Low Energy Architecture Lifetime Achievement award. The paper is a general discussion of the huge benefits of low-energy building design, the critical role of design tools, and the impediments and solutions to wide-scale implementation. (PDF Format, 59KB)

Balcomb, J. Douglas (1998). "ENERGY-10, Designer Friendly Simulation for Smaller Buildings." Building Performance, Issue 1, Spring 1998, Journal of the Building Environmental Performance Analysis Club, UK. A general overview.

Balcomb, J. Douglas (1997). "ENERGY-10, A Design Tool for Low-Energy Buildings." Proc. Building Simulation '97, International Building Performance Simulation Association, Sept. 8-10, 1997, Prague, Czech Republic. A technical overview. (PDF Format, 81KB)

Balcomb, J. Douglas; Prowler, Donald (1997). "ENERGY-10, The Making of a Design Tool." Proc. Solar '97 (22nd Passive Solar Conference), American Solar Energy Society, April 25-30, 1997, Washington, D.C. Reviews the initial development of the Designing Low-Energy Buildings with ENERGY-10 package. (PDF Format, 58KB)

Balcomb, J. Douglas (1997). "ENERGY-10, A Design Tool Computer Program for Efficient Houses." Proc. 1996 EBBA Excellence in Building Conference, Energy Efficient Building Association, November 14-17, 1996, Minneapolis, MN. Shows how ENERGY-10 can be used in residential design. (PDF Format, 81KB)

Balcomb, J. Douglas; Crowder, R. Scott (1995). "ENERGY-10, A Design Tool Computer Program for Buildings." Proc. Solar '95 (20th Passive Solar Conference), American Solar Energy Society, July 15-20, 1995, Minneapolis, MN. The original paper published on ENERGY-10.

Judkoff, Ron; Neymark, Joel (1995). "International Energy Agency Building Energy Simulation Test (BESTEST) and Diagnostic Method." National Renewable Energy Laboratory. BESTEST is designed to help programmers develop reliable software to predict energy performance in buildings, and to assure building designers that a particular software package is accurate and appropriate for their usage.

Weather Data
ENERGY-10 simulations use hourly weather data for a particular location. The data are taken from the updated Typical Meteorological Year (TMY2) data set. There are 239 locations in the TMY2 set, and all have been converted to the binary format read by ENERGY-10.

The map shows these locations. Instructions are given to download any of these files. In the future, ENERGY-10 will be distributed with all these files on the CD-ROM and a weather data utility called WeatherMaker. This program has three functions: (1) converting weather data file formats (2) evaluating the data (see example at right) (3) adjusting the data to be more representative of a neighboring location. The program comes with monthly data for 3958 locations so that ENERGY-10 weather files can easily be made for any of these places. ENERGY-10 weather files can be made for locations outside the United States with the WeatherMaker program.

Workshops
The Sustainable Buildings Industry Council (SBIC) conducts two-day workshops on ENERGY-10. The text for the workshops is Designing Low-Energy Buildings, which provides design information on

energy-efficient buildings emphasizing the 16 strategies highlighted in ENERGY-10. These workshops are held in facilities where the participants alternate between lectures about the strategies and hands-on use of the ENERGY-10 program at computer terminals. Participants receive Designing Low-Energy Buildings and a copy of ENERGY-10. For a list of past workshops and a schedule of upcoming workshops, click here.