Integration: Net-zero energy design
Energy reduction is the most important design criteria for a NZEB. The architectural design of the building (orientation, insulation, building envelope, passive solar strategies) will account for much of the energy savings. Engineers must design and specify building systems that use the least amount of energy possible while providing the required amount of space conditioning and power for the building operations and occupants.
An energy reduction target should be set, and this will drive design decisions. Keep in mind that all the energy needed by the building must be supplied with renewable energy. Conventional payback analysis for efficiency measures must be avoided. Rather, efficiency opportunities must be compared to the cost of generating the equivalent amount of renewable energy. It is almost always easier and less expensive to save energy than to produce it, so efficiency measures that may not be financially viable on a typical building may prove to be effective on a NZEB.
Building energy modeling is a key part of the design process. It is used to assess building energy use and the interactive effects of energy efficiency measures. A baseline building design should be established early and modeled. As the building and system designs are refined, the energy model should be updated so the design team can determine whether the energy reduction goals are on track. Some building design applications contain a built-in energy modeling component. Often specialized energy modeling software is used, such as eQUEST (commercial buildings), BEOpt (residential buildings), or one of the many building energy software tools listed on the U.S. Dept. of Energy’s Energy Efficiency & Renewable Energy website.
Energy use intensity (EUI, measured in kBtu/sq ft/yr) is the metric used to evaluate the energy performance of buildings. The U.S. Energy Information Administration publishes the Commercial Buildings Energy Consumption Survey (CBECS), which is a national sample survey that collects information on the stock of U.S. commercial buildings, their energy-related building characteristics, and their energy consumption and expenditures. The most recently published survey contains data from 2003, and work is underway to compile data for 2012.
The 2003 national average EUI of all U.S. commercial buildings is 93 kBtu/sq ft/yr. In the New Buildings Institute research report “Getting to Zero 2012 Status Update: A First Look at the Costs and Features of Zero Energy Commercial Buildings,” the EUI of the zero-energy buildings considered in the study was 9 to 35 kBtu/sq ft/yr. The National Renewable Energy Laboratory (NREL) considers “highly energy efficient” buildings as those using 60% to 70% less than CBECS and refers to a target in the 25 to 30 kBtu/sq ft/yr range as a practical maximum for most zero-energy building applications. Achieving energy reductions of this magnitude is a big challenge, but as demonstrated by example projects across the country, it is achievable.
ASHRAE Vision 2020 established a baseline for buildings referencing ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings representing the “turn of the millennium.” Energy reduction targets from the baseline are phased in over time until 2030 when all buildings should be designed for net-zero energy. ASHRAE Standard 189.1: Standard for the Design of High-Performance, Green Buildings Except Low-Rise Residential Buildings, will be updated over time as the energy reduction targets increase.
Some of the most useful design resources are the Advanced Energy Design Guides (AEDG) published by ASHRAE and developed in collaboration with the AIA, the IES, the USGBC, and the U.S. Department of Energy (DOE). The original series of guides have an energy savings target of 30% over ASHRAE Standard 90.1-1999, while the most recent series of guides have an energy savings target of 50% over ASHRAE Standard 90.1-2004. Each guide addresses a specific building type.
The guides contain prescriptive energy-saving recommendations for each of the eight U.S. climate zones. They also contain a chapter on how to implement recommendations. By following the recommendations, advanced levels of energy savings can be achieved.
It is beneficial to know which areas to target for energy use reduction. The areas of highest energy use hold the greatest potential for reduction in terms of the overall total. For example, if water heating accounts for 2% of overall energy use, a 50% reduction is not nearly as significant as a 20% reduction in lighting, which accounts for 25% of overall energy use. Figure 1 shows the energy use intensity (EUI) breakdown for typical commercial office buildings. Lighting, heating, and cooling account for the majority of energy use, and these are the areas typically targeted for efficiency savings. Figure 2 shows the EUI for an example NZEB, NREL’s Research Support Facility in Golden, Colo. As lighting, heating, and cooling are optimized for efficiency, other categories constitute a more significant portion of the total energy use. In this case, the data center is the most significant energy use. For office building types and perhaps others, computing should be an area of focus for engineers.