Measuring Performance of Net-Zero Energy Homes Project

Uncertainty in the realized performance of green buildings is a barrier to increased adoption and investment in energy efficienct and environmentally friendly technologies.  NIST’s new Net-Zero Energy Residential Test Facility will provide a platform for comprehensive, accurate measurements that will be used to assess the in-situ performance of a net-zero energy home.  By its completion, this project will outfit the test facility with a full suite of high-quality monitoring equipment, measure its energy use, evaluate integrated performance metrics for net-zero energy homes, and publish guidelines for designing, operating, and monitoring net-zero energy homes.

Objective:  Develop the measurement science necessary to assess the performance of net-zero energy homes through the evaluation of performance monitoring techniques, assessment of integrated performance metrics, validation of building energy and ventilation models, and development of design, operation, and monitoring guidance by 2014.

What is the new technical idea?  Uncertainty in the realized performance of green buildings is a barrier to increased adoption and investment in energy and environmental savings technologies.^[1] Regarding net-zero energy buildings specifically, NIST’s “Measurement Science Roadmap for Net-Zero Energy Buildings”^[2]  and the National Science and Technology Council^[3] have emphasized the need for improved monitoring techniques, metrics, and models that would assess the energy performance of net-zero energy buildings (NZEB). Furthermore, a recent workshop conducted by NIST to develop a framework for guidelines to achieve net-zero energy homes (NZEHs) identified needs for guidance on, among other things, measuring the system performance of NZEHs, selection of appropriate heating and cooling systems, and collection and analysis of home energy data. Understanding how to measure the performance of NZEB will enable stakeholders to resolve differences between design intent and actual performance, enable comparisons of NZEB with conventional buildings, and allow governing bodies to enact measurable high-performance goals.

The construction of NIST’s Net-Zero Energy Residential Test Facility (NZERTF) will provide a platform to investigate effective monitoring techniques through the collection of comprehensive, highly accurate performance measurements of the energy flows within the house. Detailed performance measurements throughout the home and for each subsystem will enable the validation of whole-home building energy and ventilation models. These measurements will also be used to analyze potential performance metrics, including those representing whole-house performance and those integrating energy, economic, and environmental factors. The analysis will allow for the estimation of the benefits and costs of net zero energy construction relative to a comparable Maryland code compliant home. Finally, guidelines will be published describing the most effective design, operation, and monitoring of net-zero energy homes. The facility will be operated for one year in its as-built state to verify net-zero operation. After this initial one-year phase is completed, the facility will be converted to a testbed in which technologies and operational approaches will be tested to determine their effects on the performance of a NZEH. Considering the importance of system interactions, an experimental management plan will be devised to make best use of the facility to address the most critical research questions related to building energy use while ensuring that concurrent experiments do not have adverse effects on each other.

What is the research plan?  Comprehensive, accurate monitoring of the NZERTF must include conditions in and around the home, energy consumed by the home’s internal loads, thermal energy provided by the space conditioning and water heating systems, and contributions of the photovoltaic modules. A weather station will record the outdoor temperature, humidity, wind, and solar radiation conditions. Sensors will measure the ambient temperature, radiative temperature, and relative humidity in each room, which will also be used to calculate occupant comfort. The concentration of volatile organic compounds (VOCs), as a measure of indoor air quality, will be determined periodically. The electrical loads for the NZERTF will also be monitored carefully, including the electrical power of each lighting circuit, individual appliance loads, and general plug loads.  The performance of the space and hot water conditioning systems will be measured in sufficient detail to allow full characterization of their performance independently. These systems include the forced-air space conditioning equipment, ground-source heat pump, solar water heaters, heat pump water heater, heat recovery ventilator, whole-home dehumidifier, and radiant floor heating system. Performance of the photovoltaic modules will be measured before and after the DC/AC inverter.

A key driver of energy use in residential buildings is occupant behavior.  To fully account for such variability, the facility will be equipped with the means to actuate a variety of end-uses to simulate the effect of occupant behavior on the overall energy balance.  For example, the plumbing system will be equipped with valves to start and stop flow to end uses. These valves will be mated to sensors to determine flow from the fixture. This measurement and control system will be developed so that an arbitrary pattern of hot and cold water usage can be implemented to accurately estimate the energy consumption in the test facility. Similar activities will occur with appliances and in simulating the effect of the heat and moisture load from occupants. The basis for these usage patterns will be derived from the literature on building science, particularly from benchmark values given by DOE’s Building America program. In FY13, the focus will be on implementing a quality control plan for the data and thoroughly evaluating the uncertainty in the measurements taken throughout the house. The completion of a full year’s worth of data will enable analysis of all systems and reporting on the lessons learned in monitoring a net-zero energy home. A key part of the reporting will include the development of appropriate metrics that incorporate energy, environmental, and economic factors. Concurrent with the monitoring, the team will conduct energy and ventilation simulations using software to determine if the actual performance is meeting the predicted performance. If the results do not match the predictions, the causes for the discrepancies will be investigated and reported to clarify shortcomings of the models.

The enormous amount of data that will be generated will necessitate a formal data modeling and storage system. These data will be stored in a standard format based on taxonomy that has been proposed by DOE for reporting data from commercial buildings. This effort will clarify whether this taxonomy is appropriate for residential buildings and will lead to a guideline for reporting residential performance data.

Following the completion of a one-year phase to show net-zero energy performance, the facility will be used as a testbed for investigation of a variety of technologies that will enable the building community to achieve net-zero operation. In FY13, a number of tasks will be undertaken to enable such a testbed. First, baseline performance for the facility will be characterized and control tests will be developed to identify changes in performance with time. Next, estimates of time necessary to conduct tests of various systems that result in statistically significant findings will be catalogued. Finally, a plan will be developed to ensure that tests of systems in the future can be conducted efficiently and without undue interference with other aspects of the home.

Ultimately, a guideline for designing, operating, and monitoring of net-zero energy homes will document the experience gained in the NZERTF and an analysis of the effectiveness of the individual technologies and monitoring systems. The NIST workshop that developed a framework for future guidelines for NZEHs identified the need to develop a clearinghouse for currently available design guidelines. The DOE Building Technologies Program is developing such a clearinghouse, and NIST will add guidance from its design to this clearinghouse. To address the needs for guidance for measuring system performance in NZE homes and for collection and analysis of home energy data, NIST will draft a guideline document on the monitoring approach. The third guideline document that will be written will describe operational issues that are necessary to achieve net-zero energy homes. The report describing the framework for designing a net-zero home with the NZERTF as a case study will be completed in FY13. The guidelines on monitoring and operating NZEHs will be completed in FY14.

 

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[1] USGBC Research Committee, “A National Green Building Research Agenda” November 2007.

[2] National Institute of Standards and Technology, “Measurement Science Roadmap for Net-Zero Energy Buildings:  Workshop Summary Report”, March 2010.

[3] National Science and Technology Council, “Net-Zero Energy, High Performance Green Buildings” October 2008.

 

Recent Results:

Outcomes:

• Completed design of Net-Zero Energy Residential Test Facility in coordination with architect of record. (2011)
• Developed whole-building energy simulation model to predict energy performance of facility. (2012)

Outputs:

• Conducted workshop entitled “Strategies to Achieve Net-Zero Energy Homes: A Framework for Future Guidelines” (2011)
• Completed design and acquisition of monitoring system to measure key performance aspects of facility.  (2012)

Standards and Codes:

Current minimum efficiency building codes do not come close to mandating net-zero energy operation; the results of this project will instead yield experimental data that will assist in the development of aspirational design standards that approach net-zero energy homes through efficiency improvements and stipulations that buildings be built to accommodate renewable energy systems. To enable the development of such standards, the guidelines produced in this project will clarify issues that must be addressed when specifying a net-zero energy home that may not be as critical with code-compliant homes due to the increased airtightness, changes in heating and cooling demands, and heightened sensitivity to occupant behavior. The data obtained will help clarify the appropriateness of using currently-used energy simulation software to model the performance of homes in standards development as the requirements approach net-zero operation. Another key area where this project will impact standards is in translation of standard test methods for equipment in a laboratory setting to performance in actual buildings. NIST has a long history of developing test methods for building equipment, but net-zero energy homes place new demands on that equipment which must be reflected in test methods.