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Measuring Performance of Net-Zero Energy Homes Project


Develop the measurement science necessary to evaluate various technological approaches to achieve net-zero energy buildings through the development of innovative monitoring techniques, methods of test, and performance metrics while concurrently providing high quality experimental data for the validation and improvement of building energy and indoor air quality models.


A Net-Zero Energy Residential Test Facility (NZERTF) has been constructed and extensively instrumented. The facility incorporates a high-performance envelope, a dedicated fresh air ventilation system, three distinct ground-source heat exchangers, a radiant basement floor heating system, variable capacity photovoltaic and thermal solar systems, and three distinct means of distributing conditioned air throughout the house. A smart meter and various communication systems within the house enable research to capture the benefits of smart appliances and energy storage technologies. The sensible, latent, and electrical plug loads generated by a typical four-member family are replicated using computer controlled devices located throughout the facility. During its first year of operation, the NZERTF has demonstrated that a home similar in size, aesthetics, and amenities to those in the surrounding communities can generate as much energy as it consumes on an annual basis (http://www.nist.gov/el/nzertf). During the second year of operation, optimization of the controls and ventilation system further improved the energy performance. The NZERTF provides a platform for comprehensive, accurate measurements to explore various designs, technologies, and control strategies to achieve net-zero and an exemplary test bed for energy efficient, renewable, energy storage, and smart-grid technologies. The insight gained, lessons learned, and data generated from this project will result in enhanced public awareness of net-zero energy buildings and measurement science that accurately captures the performance of net-zero energy buildings, renewable energy systems, and energy efficient technologies. The data generated from this project will validate/improve building energy and indoor air quality models that will be used to design the next generation of single-family housing.

What is the new technical idea?

NIST's "Measurement Science Roadmap for Net-Zero Energy Buildings" and the National Science and Technology Council have emphasized the need for improved monitoring techniques, metrics, and models to assess the energy performance of net-zero energy buildings (NZEB). Additional measurement science needs representing barriers to the design/construction/operation of net-zero energy buildings have been provided by the Department of Energy, the National Laboratories, universities, and the private sector. Using the NZERTF as a test bed, the identified measurement science needs will be addressed and performance metrics for emerging technologies developed. Methods to optimize the overall performance of the facility will be explored. Interactions between various subsystems will be quantified in order to capture their impact and their influence on the methods of test used for individual subsystems. Development of the measurement science required to understand how net-zero energy buildings can be designed/constructed in the most effective manner will result in greater numbers of low energy homes being constructed.

What is the research plan?

The NZERTF will be operated using various technologies and control strategies to determine various means to achieve net-zero. The experimental work will be complimented by computer simulations to identify the most promising approaches and economic analysis to determine the most cost-effective approaches.

In FY14, the NZERTF achieved net-zero while meeting the demands of a typical family of four and while being subjected to an extremely harsh winter. During the net-zero demonstration, a number of issues were identified, that if addressed, should yield significantly improved performance. These improvements included better integration/sizing of the auxiliary resistance heat unit, improved controls to better anticipate conditions within the NZERTF, and the introduction of fresh air in strict accordance with, rather than exceeding ASHRAE Standard 62.2 requirements. It was also observed that using a ducted dehumidification system, in lieu of relying on the heat pump's dedicated dehumidification mode may significantly decrease the energy required to provide dehumidification. The result of these changes/improvements could result in a smaller, less costly, photovoltaic system needed to achieve net-zero. In FY15, these changes/improvements were implemented and the affected subsystems are being closely monitored to assess their impact on energy consumption and indoor air quality. For the six months following these changes the NZERTF has performed at a higher level and reduced the energy deficit that was observed over the same time interval by approximately 50 percent. Using the coupled CONTAM/TRNSYS model the impact of various operational strategies on concentrations of various pollutants was computed.

During FY14 it was observed that significant heat losses occurred from the plumbing associated with the water heating systems and distribution lines within the facility. These thermal losses increased the space-cooling load and decreased the space-heating load. During FY15 the magnitude of these loses is being  quantified through the use of improved instrumentation and data monitoring techniques.  Simulations are being performed in FY15 to assess alternative methods of providing domestic hot water, such as an expanded photovoltaic system in lieu of a solar hot water system in combination with a photovoltaic system. Using FY14 data, the predicted performance of the photovoltaic systems is being compared to the actual performance. Improvements in the instrumentation used to measure the photovoltaic performance, the water distribution system,  and solar hot water system have been completed in FY15.  

In FY15, the entire data collection/reduction procedures was improved. Upgrades to the current software to accommodate the previously described hardware changes have been made. A database has been completed that gives NIST researches complete access to all collected data in a timely and organized manner. Data sets from the NZERTF are being used for model validation and to better understand equipment performance. It is expected that NIST can leverage the efforts of outside researchers to magnify the impact of the data collected during operation of the facility. An internal roadmap will be developed to organize future research activities using the NZERTF facility for maximum impact.

In FY 16 the second year full-year of operation will be completed, permitting an in-depth assessment of the changes made to improve the energy performance of the NZERTF. A high-velocity air distribution heat pump system will be installed. The operation of this system will be alternated on a daily basis with a centrally ducted heat pump system to assess the advantages/disadvantages of the two system on energy efficiency, thermal comfort, and ability to effectively remove moisture from the home. The heat pump water heater, currently installed in the NZERTF,  will be instrumented and interactions between the heat pump water heater and space conditioning load of the NZERTF will be quantified. Tests will be conducted to determine the thermal impact of the water distribution system on the NZERTF's space conditioning load for various water usage profiles. Short term test will be conducted to quantify material emissions with temperature and relative humidity and to measure the thermal time constant of the NZERTF.  

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

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

Major Accomplishments:

Impact of Standards and Tools:

  • Net-energy successfully demonstrated for an entire year; data set generated to quantify the performance of the house and all of its systems.