Page 130 - Zero Net Energy Case Study Buildings-Volume 1

ZERO NET ENERGY CASE STUDY BUILDINGS, VOL. 1 OBSERVATIONS
Observations
In the six ZNE buildings discussed in this monograph, some common issues and conclusions about ZNE building design and performance are apparent. All buildings shared common process elements—in each case the design teams engaged in:
• Target setting
• Designing to the target
• Building to the design
• Monitoring, diagnosis and correction
At this point there are, to be sure, imperfections in this developing area of building design and construction; experience, feedback and new building technologies are needed to achieve solu- tions scalable to widespread practice. Several observations based on these case study buildings are examined below together with resulting conclusions about near-future industry development.
Target Setting: Motivation and Purpose for Pursuing a ZNE Design
The back stories of the case study projects all indicate that the choice to pursue a ZNE design resulted primarily from a client’s commitment to such an environmental goal, often promoted by a highly motivated A/E team. This was always accompanied by the confidence of the client in the ability of the design team to deliver a ZNE-performing building within acceptable budget constraints. With the project vision established, the entire team (client and A/E team) worked to- gether at the beginning of the process to establish the energy-use and energy production targets that would guide all design decisions along the way.
As the case studies show, these buildings were able to achieve ZNE per- formance because the targets were clear and deliberate from the begin- ning, a prerequisite for successfully reaching the envisioned goal.
Designing to the Target: Building Modeling Versus Actual Performance
Once a performance target of ZNE is established, the A/E team uses whole building energy modeling as a method of determining approximate performance based on many assumptions about building schedule and occupancy, internal plug loads and a set of weather data points for a statistically-determined representative year. Inevitably, initial modeling efforts are approxima- tions owing to the large number of variables. However, as noted in the paper by Brown et. al.1, with new ZNE policy goals as well as new disclosure requirements in some cities and states, there is an increasing expectation that building energy modeling results should more accurately align with actual measured building energy use. In addition, the size of the energy production system—in most cases by far the most costly component of “getting to zero”—is often deter- mined from the results of the energy modeling of the building. It is therefore imperative that modeling software improves in “predictive” capability, especially with regard to the new building technologies being employed for low energy performance.
Modeling what the actual performance of a building will be after it is built and occupied is both an art and a science. Being able to compare modeling results from the design phase with actual measured performance when the building is occupied will provide designers with a better sense of this art. In the Packard Foundation Headquarters building and several other ZNE buildings, for example, the design engineers noted that energy models typically underestimated heating en- ergy use. The explanation offered is that ideal construction tolerances are assumed in the energy modeling software, which are not normally attained in the field. Similarly, lighting energy use by high performance systems with daylighting and occupancy controls is often overestimated, as in the case of the Stevens Library. The modeling inaccuracy for lighting seems to be the result of the complexity of the interactions among the lighting system, the available daylight, and the
1 K. Brown, A. Daly, J. Elliott, C. Higgins, J. Granderson, “Hitting the Whole Target: Setting and Achieving Goals for Deep Efficiency Buildings”, Proceedings of the 2010 ACEEE Summer Study, Panel 3 Paper 734
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Zero Net Energy Case Study Buildings: Volume 1