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 and environmental savings 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 a guideline for designing, operating, and monitoring net-zero energy home

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, and validation of building energy and ventilation models 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).  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.  Finally, guidelines will be published describing the most effective measurement and analysis techniques for net-zero energy homes. 

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 and relative humidity in each room.  Additional sensors for air velocity, radiant temperature, floor temperature, and the vertical temperature gradient will measure 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.  Each measurement system will be assembled, calibrated, and tested in a mock-up by May 2012 to allow for a quick installation and commissioning in the house once construction is complete.  Before the start of the one-year net-zero energy proving period, the performance of each system within the house, including the monitoring system, will be verified.  

Such comprehensive monitoring will generate enormous amounts of data that must be organized to allow quick analysis of the home's operational state and detailed investigations of the performance of the home, its subsystems, and even individual elements.  An open, standard data model will be implemented that facilitates access to NZERTF performance data, including a publicly available website.  

In future years, NZERTF performance data will be used to evaluate the accuracy and data-requirements of whole-home performance metrics that integrate energy, economic, and environmental factors.  The detailed performance of each system within the NZERTF will be compared to predictions by various building energy and ventilation models to identify possible limitations.   Finally, 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. 

[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.