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” 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 addressing barriers to the design, construction, and operation of net-zero energy buildings have been provided by the Department of Energy, the DOE National Laboratories, universities, and the private sector. 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 in order 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, a number of 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. Additionally, it was postulated that the use of 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 and less costly photovoltaic system to achieve net-zero. During the second demonstration year, these 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. This increase in surplus energy was largely due to decreased energy consumption from the measures taken with the heating and ventilation systems. 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 begin to focus on evaluating different technologies for particular subsystems.
NIST is in the process of making the data from the NZERTF available to the public. 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. NIST will put the data collected from the first year of data collection on a publicly available portal. To support that data release, a document will be developed that describes the data, and sample calculations will be provided to show possibilities for using the data.
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 an air-source heat pump with conventional ducting were implemented. An alternative approach using a Small-Duct High-Velocity (SDHV) system, has since been commissioned; NIST will evaluate the SDHV system while comparing its performance to the heat pump configuration used during the second demonstration year. This comparison will be accomplished by switching between the two systems at regular intervals (e.g, every day or every 3rd day, etc.). In addition to measuring the energy consumption of the two competing heat pump systems, the thermal comfort provided when each system is active will also be 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 has been installed in an upstairs bedroom of the NZERTF. This heat pump comparison study will be conducted over the course of a year to assess seasonal impacts on performance.
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, questions often arise related to the impact of a heat pump water heater on the home’s space conditioning load. To better quantify this impact, NIST instrumented the NZERTF’s heat pump water heater to measure the heat transferred between its evaporator, piping, and storage tank and the (basement) surrounding air. A study will be conducted to quantify the extra space heating and cooling requirements brought on by the heat pump water heater, both when operated with the solar thermal preheat system and without it. The two system configurations (i.e., with and without solar) will be alternated on a weekly basis. A variety of occupant hot water usage schedules will be imposed to more fully evaluate the range of impacts associated with using a heat pump water heater.
An additional study that is being conducted relates to the distribution of hot water from the water heater to end fixtures. NIST is measuring the amount of energy lost in this distribution system under a variety of usage patterns. NIST will report the energy transferred to the conditioned space and the extra water volume removed during temperature-sensitive draws. Options for minimizing these effects will be identified, implemented, and then evaluated. The tradeoffs associated with different home hot water distribution systems will be documented.
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. This shortcoming will complicate any evaluations of alternative ventilation strategies that could prove to be more effective or more energy efficient. To overcome this shortcoming, NIST will install an automated tracer gas system that can obtain the air change rates more easily and regularly than has been done in the past. Additionally, this system will allow for evaluation of air movement within the testbed. The data collected will help to improve computer models of ventilation systems in energy efficient homes.
To extend the results gathered from the NZERTF, an effort is underway to model the home under different climates or construction practices. One topic to be investigated is the effect of the foundation. The NZERTF was built with a basement, but there are a large number of homes that do not have basements. To extend the results to those homes, NIST will develop and model a home that is comparable to the NZERTF but which has a slab foundation. Heat transfer to and from the ground can have a significant impact on energy performance, so this computer model will help the team apply lessons learned from operation of the NZERTF to a wider range of homes.
Currently, the focus of the NZERTF has been on energy performance and indoor air quality. A key aspect of sustainable buildings that has not been studied is water use. To begin to explore how the NZERTF could be used as a testbed for efficient water technologies, a study will be conducted to benchmark the water use within the NZERTF to that of other homes. An effort will then be undertaken to design ways in which the facility could be used as a testbed for water technologies that could be used in homes. Such technologies could include those to ensure water quality, to reduce the amount of water used, or to reuse water.
 National Institute of Standards and Technology, "Measurement Science Roadmap for Net-Zero Energy Buildings: Workshop Summary Report", March 2010.
 National Science and Technology Council, "Net-Zero Energy, High Performance Green Buildings" October 2008.