Objective: Develop the measurement science necessary to evaluate various technological approaches to achieve net-zero energy residential 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.
What is the new technical idea? NIST’s “Measurement Science Roadmap for Net-Zero Energy Buildings” 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 addressing barriers to the design, construction, and operation of net-zero energy buildings have been provided by the building industry. Using the NZERTF as a test bed, these measurement science needs will be addressed and performance metrics for emerging technologies developed. Methods to optimize the overall performance of the facility will also be explored. Interactions between various subsystems will be quantified to capture their impact and 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 complemented by computer simulations to identify the most promising approaches and economic analysis to determine the most cost-effective approaches.
During two separate demonstration years, the NZERTF achieved net-zero operation while meeting the demands of a typical family of four and being subjected to extremely harsh winters. During the first demonstration year, issues were identified that were expected to 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 outdoor air in strict accordance with, rather than exceeding, ASHRAE Standard 62.2 requirements. During the second demonstration year, changes resulted in an increase in the amount of surplus energy from 4 % of energy consumed in the first year to 18 % of energy consumed during the second year. In depth analyses of each system have been carried out for the two demonstration years, and results have been reported in a number of publications. Additionally, various computer models have been developed to extend the lessons learned by estimating performance in different climates or under different usage scenarios. Computer models have also been developed to help assess how alternative configurations would have performed in the NZERTF. For example, a range of water heating systems have been analyzed to determine which one would result in the lowest energy consumption. Indoor air quality has been measured and modeled to examine the impact of energy efficient ventilation techniques on the indoor environment. In upcoming years, the team will focus more on evaluating different technologies for particular subsystems.
Several methods to heat and cool the house were built into the facility when it was originally constructed. During the two demonstration years, two variations of a two-speed air-source heat pump with conventional ducting were implemented. During FY17, an alternative approach using a Small-Duct High-Velocity (SDHV) system was commissioned and then studied while operating over the cooling, heating, and swing seasons. In addition to measuring the energy consumption of the heat pump systems, the thermal comfort provided was also quantified. To expand the team’s means for evaluating thermal comfort, a measurement system consisting of a reconfigurable, 3-dimensional array of temperature, humidity, and air flow sensors was installed within a NZERTF upstairs bedroom. This same thermal comfort measurement system along with other sensors throughout the house will continue to be used in upcoming years. A focus in the upcoming year will be on the challenge in achieving uniform thermal conditions in efficient homes because of the relatively low demands on the heating and cooling system. In FY18, researchers will explore the energy and comfort tradeoffs from using different strategies for providing conditioned air to the house. The NZERTF contains fully-variable electronic dampers in each supply duct that allow researchers to control airflow to each room. Strategies will be selected that maximize comfort, attempt to minimize energy consumption, and allow comparisons between perimeter versus central supply options. The results will contribute to improving duct design and to quantifying the merits of different zoning approaches for net-zero energy homes.
The domestic hot water system consists of a solar preheat tank that feeds into a heat pump water heater. A computational study confirmed that this approach is the most energy efficient; however, recent advances in heat pump water heater technology may change how future homes will provide hot water. One such technology advancement is a heat pump water heater that uses CO2 as a refrigerant and that places its evaporating unit outdoors as opposed to indoors. This positioning will impact the overall energy consumption by the house, as the evaporating unit essentially serves as an air conditioner if placed indoors. The CO2 unit also promises to perform with much higher efficiencies compared to heat pump water heaters with more conventional refrigerants. NIST will install such a unit in the NZERTF in FY18 after completing laboratory characterization in FY17. Tests will be run over four seasons to capture data on field performance and enable comparison of that performance to results from laboratory test methods. In future years, the team intends to explore water heating systems that are combined with geothermal heat pumps. To advance towards that goal, a laboratory test apparatus will be constructed in FY18 to evaluate such systems. Those results can be used in comparing to field performance in the NZERTF that will be captured at a later date.
An important aspect of any high performance home is indoor air quality. To assist in promoting good indoor air quality, ventilating the home is a crucial step. Preliminary computer models have been developed of the ventilation rates and projected contaminant levels, but there is uncertainty in the rate of outdoor air change of the facility and the effectiveness of ventilation in removing contaminants. This shortcoming will complicate any evaluations of alternative ventilation strategies that could prove to be more effective or more energy efficient. To assist in determining outdoor air infiltration, NIST will work with a team to validate and refine a protocol for measuring infiltration in a high-performance home. To help assess the performance of ventilation system, researchers will also design CO2 injection systems that will mimic the impact of occupants. In conjunction with a newly installed tracer gas system, this capability will allow for in-depth characterization of ventilation effectiveness.
To extend the results gathered from the NZERTF, an effort is underway to model the home under different climates, construction practices, and configurations. Of particular interest is the upper limit to energy efficiency that can be attained with alternative technology (termed a “MaxTech” arrangement). Work will be conducted to model the performance of high-efficiency space conditioning and water heating technologies to determine a realistic lower bound on the energy consumption of the NZERTF.
While Net-Zero Energy homes typically account for energy on a yearly basis, the timing of the power draw from or power supply to the grid is becoming very important in terms of electric grid performance. The project team will work with the NIST Smart Grid Program to begin to explore how the NZERTF can be used to assess metrics for residential building performance as it relates to grid management. To start, the electrical performance over the first two years of operation will be fully characterized to baseline the key metrics of electrical performance prior to future studies of integration of the home with the Smart Grid. Plans will be developed to emulate utility-initiated demand-side management. To stay updated regarding trends in usage, the team will reassess electrical assumptions in operating the home, particularly how plug load consumption has changed since the facility was commissioned in 2012.
NIST has and will continue to make available to the public data from the NZERTF. Joint work with the Department of Commerce Data Team has identified the user needs for the data and a path forward to make best use of the data. The data releases are complemented with documentation that describes the data and provides sample calculations.