Whole Building Energy Modeling and Measurements Project

Residential buildings use approximately 22% of the total energy consumed in the U.S. The objective of this project is to increase the energy efficiency of the residential building sector by reducing the uncertainties in tools and techniques that are currently used to make energy predictions during three phases of a building’s lifetime: construction/design, regular operation, and retrofit.

Objective:  To improve estimates of residential whole-building energy consumption by 2014 through characterization of uncertainties of tools used in modeling whole buildings in support of national energy efficiency standards, providing energy feedback to building occupants, and selecting retrofit options to improve the performance of existing buildings.

What is the new technical idea?  Providing measures for residential energy use on a whole building scale is challenging because of the interacting components and systems that come into play. As these building components and systems increase in efficiency, a holistic assessment of energy consumption is necessary to target effective ways to better energy performance in homes. In this project, NIST will focus on improving the measurement science utilized by standards developers, designers, occupants, and retrofit professionals to assess the energy performance of a residence by quantifying the uncertainties associated with these assessments.

Whole building energy simulations often inform the development of next-generation building codes for energy efficiency by using the analyses to understand the implications of proposed code changes. NIST will work toward determining the sensitivity of residential building energy simulations to various parameters, such as construction and occupant factors, to make the energy savings predictions of the standards to which the modeled buildings will be compliant more complete. Once the building is in operation, real-time energy feedback to occupants is needed to continue to meet and exceed the efficiency goals described in the construction standards; it is estimated that up to a 20% energy savings can be achieved through behavioral changes that result from appropriate feedback. NIST has been evaluating commercially-available residential energy monitoring systems (REMS) and will develop a metric and a test method to quantify measurement accuracy and communication performance so commercial products can meet the needs of building occupants. Finally, to fully address energy efficiency of residential buildings, it is imperative to identify ways to improve the performance of existing buildings. Energy auditing is a process by which retrofits needed to improve the energy performance of a building are determined. NIST will enable quantification of the uncertainty in audit recommendations by formalizing a procedure to implement a round-robin program so that building professionals and homeowners have greater confidence in energy efficiency improvements resulting from retrofits.

Quantifying the uncertainties associated with these modeling tools, sensor systems, and guidelines will provide more confidence to building designers, operators, homeowners, and standards developers to move the nation toward net-zero energy, high performance buildings.

What is the research plan?  To decrease uncertainty in predictions of energy efficiency improvements of proposed code provisions, NIST will assess the sensitivity of computer models used to evaluate those proposals. Assessments of prescriptive standards and codes such as ASHRAE 90.2 and the International Energy Conservation Code performed by the residential building energy codes community typically utilize single building types to represent the nation’s residential building stock. In FY13, NIST will create EnergyPlus models of multiple prototype buildings using data from the Census Bureau’s American Housing Survey and the DOE Residential Energy Consumption Survey to expand the representation of the U.S. building stock. NIST will then perform a sensitivity study to quantify the uncertainty and variability in modeling results when factors such as location, occupant behavior, construction type, and square footage are varied. These factors can all be examined by changing inputs into the software, with occupant behavior being examined through changes in hot water use, thermostat setpoints, appliance usage, and plug load usage that are documented in the building science literature.

To assess the uncertainty in energy use estimates from continuous feedback systems, NIST will utilize a testbed to develop metrics and test methods to assess such systems. A hardware test platform for electricity sensors that also possesses the functionality to assess the software included to retrieve sensor data was designed in FY12 to assess the accuracy of commercially available residential energy monitoring systems (REMS). The platform makes use of individual energy end-uses (e.g., small and large appliances, and plug loads) to simulate what would be used in a home. In FY13, NIST will expand the test platform to include the measurement of simultaneous household loads and multiple sensors to further represent typical home energy use. Current investigations have shown that certain appliances have variable power states, a trend emerging with appliances incorporating more complex electronics. The results of testing end-uses on the platform will allow NIST to categorize various household electrical loads and determine which REMS are the most appropriate for these categories. FY13 tasks will culminate in the draft and evaluation of a test method to assess the performance of residential electricity monitoring systems, with the aid of a load generator to simulate the sum of household loads and the development of suitable metric(s) of comparison.

To characterize the uncertainty in projected energy efficiency improvements arising from residential retrofits, NIST will investigate the variability in the predictions from energy audits of existing buildings. In FY12, with the assistance of a grantee, a qualitative “round robin” study was performed with multiple auditors conducting energy audits on the same homes using short-term evaluative tools such as blower doors, infrared thermography, and flow measuring devices. Results showed widely varying retrofit recommendations and estimated savings from those recommendations. In FY13, NIST will formalize the round-robin procedure to identify key areas of variability in a statistically significant manner and will work with the retrofit community to assess this framework, starting by implementing a round robin during a leading home performance conference. This effort will result in a process that will help reduce variability in energy audit results and increase confidence in predictions of energy savings. NIST will work with standards and credentialing residential energy retrofit organizations to incorporate lessons learned from the studies into new standards. The first step in this endeavor will be to create a map of gaps and overlaps in the standards landscape. NIST will team with a contractor or grantee to assess the residential auditing standards and evaluate the round robin framework.

Recent Results:

Outputs:

• Publication submitted to Home Energy Magazine entitled “Past, Present and Future Directions in Single-Family Energy Auditing and Retrofits.” (2012)
• Presentation at 2012 RESNET Building Performance Conference on results of energy audit round robin.
• Three peer-reviewed journal articles and 6 conference papers describing use of wireless sensor networks in buildings. (2010 – 2012)

Outcome:

• Phase II SBIR grantee developed commercial residential energy monitoring system. (2012)

Standards and Codes:

Results of sensitivity analyses of residential buildings models will be shared with ASHRAE Standards Committee 90.2 and a proposed ASHRAE committee on high performance, green residential buildings.  (Boyd/Kneifel/Healy)  Results of energy audit analysis will lead to modifications of standards and guidelines developed by organizations such as the Residential Energy Services Network (RESNET) and the Building Performance Institute (BPI). (Healy) Test method for residential energy monitoring systems and results of its evaluation will be disseminated to organization deemed most appropriate.  Candidate organizations include ASHRAE, ASTM International, and IEEE (Ullah/Guo)