How to use building energy modeling tools
The building design industry is quickly becoming reliant on energy models as a means of proving total building energy performance. Here we will briefly discuss the primary uses of energy models, including topics of concern when approaching energy modeling for innovative or unconventional systems, and a preview of the new Dept. of Energy (DOE) Simergy interface for its energy modeling engine, EnergyPlus.
The Building Technologies Program of DOE’s Energy Efficiency and Renewable Energy arm has compiled a list of 408 building modeling software tools available for “evaluating energy efficiency, renewable energy, and sustainability in buildings.” With so many types of software to use, designers need to understand the original purpose of creating an energy model. Primary reasons include the following:
- Early phase comparison of design alternatives
- Submission of documents to municipal or state reviewing authorities for energy code compliance
- Submission of documents to the U.S. Green Building Council’s LEED process to justify points under the Energy and Atmosphere Credit 1
- Discussions with owners, utilities, or ESCOs (energy service companies) wishing to achieve a guaranteed energy savings to justify increased first-cost investments or incentives to apply energy-saving measures.
Early-phase analysis of design alternatives
At the start of a project it is often quite useful to build a simplified whole or single-zone building energy model of simplified geometry in order to compare design alternatives for the purposes of making well-informed decisions within an integrated design team. These models are usually used to test façade material performance or HVAC system alternates. The two pieces of freeware most often used for this process are the EQuest Schematic Design Wizard and, specific to façade performance, COMFEN. Both of these packages were produced by the DOE. In addition, there are a number of other commercially available packages that provide energy modeling capabilities, often as an extension to load analysis programs or building information modeling (BIM) software (see link to sidebar below).
One example of using energy modeling was to explore the integration of an ice-storage system into a community college campus chilled water loop in order to reduce energy costs within the context of a heavily penalized demand rate structure. The software package was capable of reviewing the charging and discharging cycles of the ice as compared to the anticipated energy use by the technology-intensive classrooms, resulting in the optimization of the number of tanks versus the overall size of the chiller plant. This analysis was able to prove the lifecycle cost benefit of incorporating the more complex system, even taking glycol costs and increased maintenance costs into account.
Energy modeling for energy code compliance
The first thing to understand when considering whether an energy model is required for code compliance is the status of the state energy code for the project site. The DOE hosts a website summarizing the details of state energy code adoption for commercial and residential projects. Particularly useful are maps that show which states allow the use of COMcheck and REScheck (see Figures 1 and 2).
Both pieces of software are available as either downloads or online software programs that allow the user to demonstrate direct compliance with a number of model energy codes (International Energy Conservation Codes versions 2000 to 2009 and ASHRAE 90.1 versions 2001 to 2007, and state or county-specific modifications thereof). The software has been developed by the DOE through Pacific Northwest National Labs. The software is free, and easy to use if the design is strictly compliant with the prescriptive approaches in the relevant codes and able to accommodate the preprogrammed “typical” input for occupancy types. While it is capable of internal calculations associated with the building envelope trade-off method and the energy cost budget method, its output cannot be used for comparative analysis of energy use data, and the software is written only to show compliance success or failure.
Looking at the whole picture
It is often necessary to revert to a whole building energy analysis software package when simplified energy code compliance is not an available path due to the jurisdiction, the complexity of the systems selected, or the need to trade off energy use between different design disciplines. For many years in the United States, these more sophisticated packages have primarily been based on a common freeware calculation engine produced by the DOE called DOE-2. EQuest is the most current public freeware interface for DOE-2. EQuest includes some preprogrammed defaults when one uses the “wizard” for schematic design, and the more detailed interface allows the user to dig much deeper into the program to manipulate input files to represent a wide variety of systems.
The primary drawback to the use of whole building energy modeling for code compliance is that it often requires the production of at least two energy models: one for the proposed building, and one for a “baseline” building of the same shape as the proposed one but in compliance with the strict prescriptive aspects of the code. An automated baseline building generation subroutine is built into the commercially available EnergyPro interface for both the preapproved algorithms of the California Energy Commission requirements for its special state-specific energy code as well as an ASHRAE 90.1 comparison, but this is a notable exception to the other packages. The onus is always on the professional engineer to ensure that both models adequately represent the building components in a way that would allow parallel direct comparison for energy performance characteristics.
Complex systems are always quite difficult to model in the more prescriptive approaches found within many manufacturer-based software packages. For instance, few packages adequately represent partially transmissive shading devices parallel to the window surface, as one might find with perforated metal shading scrims. Another example of system complexity is that of accommodating innovative HVAC systems. While some of the commercially available software packages now include modules to incorporate active chilled beams, for many years a “workaround” approach of zero-energy fan coil units mimicking the energy use of the beams was the only input method available. The risk exposure for engineers forced to use “workarounds” is quite substantial when submitting code compliance forms, especially in an industry-wide trend toward accountability for energy use.
It should be noted that ASHRAE has released its building rating system, called Building Energy Quotient, or bEQ. It is similar in theory to systems used for public energy-use reporting in certain European countries, and a few U.S. states already have laws on the books requiring building energy-use reporting in the near future. It is likely that this trend in the industry will see upcoming contracts that require the design team to bear some financial risk or professional liability with regard to actual energy use, as evidenced by the General Services Administration retaining 0.5% of design team fees until building energy performance is achieved (as reported in Engineering News Record, May 14, 2012).
Energy modeling for LEED
Energy modeling to meet the requirements of the U.S. GBC LEED for New Construction 2009 Energy and Atmosphere Prerequisite 2 and Credit 1 will generally need to show compliance with some form of a whole building energy modeling approach. For greater detail, the rating system should be downloaded for free. It should be noted that there are three options for showing compliance with these energy use credits, all of which are based on whole building modeling:
1. Whole building energy simulation LEED explicitly in compliance with the procedures described in Appendix G of ASHRAE 90.1-2007.
2. Prescriptive compliance with the recommendations of the ASHRAE-published 30% Advanced Energy Design Guides (AEDGs) for a limited number of small-building occupancy types (with drafts of LEED 2012 referring to the recently published 50% AEDGs instead). These guides can be downloaded for free.
3. Compliance with the prescriptive measures of the Advanced Building Core Performance Guide by the New Buildings Institute.
The latter two options are prescriptive guides generated out of extensive iterative energy modeling, with the resulting recommendations representing the “best practice” across all regions of the United States for systematically achieving the stated percentage savings for small buildings. These approaches are appropriate as alternates to project-specific energy modeling for buildings small enough to comply with the prescriptive guidelines.
For those who wish to pursue the whole building energy modeling as per ASHRAE 90.1-2007 Appendix G, it is required that five energy models are produced and managed throughout the process. These are particular items to note about the simulation procedure that often trip up the first time modeler.
1. Appendix G describes a cost-basis of comparison, which uses utility rates common to both the baseline and proposed building models. Absolute energy savings is not necessarily the ultimate goal of the Appendix G process.
2. Appendix G requires the gross geometry of the baseline and proposed buildings to be identical. To take into account an intelligent placement of the building, the process forces the baseline building to be rotated to the four ordinal directions (and to have unique HVAC sizing in each direction) in order to generate an average “baseline” energy use intensity.
3. Appendix G requires that the baseline building matches the skylight and window percentage and distribution by directional orientation of the proposed building up to a 40% window-wall ratio and a 5% skylight-roof ratio, but applies the prescriptive code-required thermal performance characteristics for glazing and opaque wall construction.
4. Appendix G requires that unmet load hours for both baseline and proposed models be less than 300 each and for the proposed model to have no more than 50 unmet hours above the baseline.
The LEED review process for submitted energy models is extensive, so it is incumbent on the professional engineer to thoroughly document the inputs and to review the hourly results reports for any inconsistencies in equipment behavior or reporting by system output type prior to preparing the final package of information. The engineer should cross-check compliance with ASHRAE 62.1 or other state or municipality-mandated ventilation requirements under turn-down conditions. Similarly, it is important to identify any unique modeling techniques upfront in the documentation and provide a narrative description of the thought process and assumptions by which the data was entered to represent a nonstandard system. This is particularly important as there is greater emphasis on comparing modeled performance to actual measured energy performance in upcoming versions of LEED.
Modeling for investment opportunities
The final area in which energy modeling is most often used is for determining the return on investment from the energy savings that are likely to be accrued from additional first cost outlays. This can be applied to both selecting equipment for new buildings and retro-commissioning older buildings.
A recent example includes a design-build team’s win of a large-scale chilled water central plant at an airport, which was heavily contingent on proving that the increased cost of the combination of solar chillers and a large chilled water storage tank would ultimately provide substantial energy savings over the 25-year life of the equipment. This was particularly crucial, as many public and institutional owners struggle to get increased operations budgets, even with the introduction of new installations and the ever-rising cost of energy. Thus having some sort of anticipated cap on future energy use has great benefit as a design criterion from the start of the project.
On the other end of the equipment life span is the case of a large-scale data center in which energy use had dramatically increased due to the densification of heat load in the move to blade servers at the site. After a recent energy audit, the engineers created a substantial number of energy models to explore economizer methods, increased setpoint temperatures, and the effects of improved efficiency as compared to existing equipment. Again, when payback periods for project energy savings meet or beat the general return on investment criteria for commercial owners, it is possible to justify deep energy saving retrofits on the basis of future savings. Many institutional and large-scale commercial portfolio holders have created revolving investment funds and a long-term staggered energy improvement policy that recoup energy savings from early “quick win” energy improvements to fund later installation of more substantial energy conservation measures.
Both examples demonstrate that detailed energy modeling at a deep level of complexity requires substantial reliance on the building energy modeling professional (BEMP), a subdivision of our usual HVAC engineering now officially identified by ASHRAE as an independent skill set worthy of certification. But the DOE is working hard to improve software to make energy modeling a run-of-the-mill skill, taught in engineering departments and architecture schools. Given that 40% of the country’s energy is used in our building stock, putting intuitive but mathematically robust energy modeling software into the hands of practitioners is a key component of our nation’s drive toward both net-zero new buildings and dramatically retrofitted existing buildings, and the improved energy security that would come with them.
It is clear from the analytical work associated with the recent ASHRAE 50% Advanced Energy Design Guides that pursuing building designs with greater than 50% savings as compared to current codes will require whole building energy modeling. The California Energy Code update cycles continue to recommend further mandatory energy savings (~30% more reduction in the newly announced 2013 code) on the drive toward code-required net-zero residential and commercial buildings by 2020 and 2030. If historical patterns hold, the ASHRAE 90.1 development cycle will follow in a similar manner. Soon, deep familiarity with whole building energy modeling techniques will be crucial to keeping up with just designing code-compliant buildings within the United States.
Erin McConahey is principal in mechanical engineering with Arup. She has worked internationally and now leads multidisciplinary design teams on a wide variety of project types. McConahey is a member of the Consulting-Specifying Engineer editorial advisory board and a Career Smart Engineers Conference presenter.