Net-Zero Energy Residential Test Facility Research Articles

Ventilation in a Residential Building Brings Outdoor NOx Indoors with Limited Implications for VOC Oxidation from NO3 Radicals.

Energy-efficient residential building standards require the use of mechanical ventilation systems that replace indoor air with outdoor air. Transient outdoor pollution events can be transported indoors via the mechanical ventilation system and other outdoor air entry pathways and impact indoor air chemistry. In the spring of 2022, we observed elevated levels of NOx (NO + NO2) that originated outdoors, entering the National Institute of Standards and Technology (NIST) Net-Zero Energy Residential Test Facility through the mechanical ventilation system. Using measurements of NOx, ozone (O3), and volatile organic compounds (VOCs), we modeled the effect of the outdoor-to-indoor ventilation of NOx pollution on the production of nitrate radical (NO3), a potentially important indoor oxidant. We evaluated how VOC oxidation chemistry was affected by NO3 during NOx pollution events compared to background conditions. We found that nitric oxide (NO) pollution introduced indoors titrated O3 and inhibited the modeled production of NO3. NO ventilated indoors also likely ceased most gas-phase VOC oxidation chemistry during plume events. Only through the artificial introduction of O3 to the ventilation duct during a NOx pollution event (i.e., when O3 and NO2 concentrations were high relative to typical conditions) were we able to measure NO3-initiated VOC oxidation products, indicating that NO3 was impacting VOC oxidation chemistry.

Evaluation of Multi-Functional Variable Refrigerant Flow System With Thermal Energy Storage and Photovoltaic-Based Distributed System for Net-Zero Energy Home Design

Abstract Efficient heating and cooling systems and renewable energy sources are crucial for effectively designing net-zero energy homes (NZEHs). The study proposes using a multi-functional variable refrigerant flow system with hydraulic heat recovery (MFVRF-H2R) to reduce heating, ventilating, and air-conditioning (HVAC) and hot water energy usage, offering a practical approach to enable NZEH solutions. Photovoltaic (PV)-based on-site power generation is utilized to achieve zero energy performance in residential buildings. A building energy simulation study is conducted to assess the effectiveness of the combined systems in various climate conditions. To develop the simulation model, the US National Institute of Standards and Technology (NIST)’s net-zero energy residential test facility is used as the benchmark for NZEH baseline models. The MFVRF-H2R system is incorporated into the NZEH baseline to propose a more-energy-efficient design with heat recovery technology. eQUEST and post-processing calculations are used to simulate NZEH performance, comparing whole-building energy end-use and PV capacity for the baseline and alternative models with MFVRF-H2R. Results demonstrate that the proposed variable refrigerant flow (VRF)-based NZEH design can provide potential energy savings of up to 32% for cooling energy under various climate zones. Moreover, the NZEH design with the proposed MFVRF-H2R can achieve up to a 90% reduction in domestic hot water usage compared to an NZEH design without VRF heat recovery technology. The study suggests that the MFVRF-H2R system can provide practical and realistic solutions for making HVAC energy-efficient by minimizing thermal waste and reusing it for other thermal parts of the building, such as hot water applications. Consequently, this study highlights the effectiveness of the MFVRF-H2R system in designing NZEHs while considering heat recovery and renewable energy technologies.

Evaluating indoor air quality and energy impacts of ventilation in a net-zero energy house using a coupled model

The National Institute of Standards and Technology constructed the Net-Zero Energy Residential Test Facility to support the development and adoption of cost-effective net-zero energy designs and technologies. In support of indoor air quality goals, contaminant source control approaches were implemented that minimized the use of products containing urea-formaldehyde resin and utilized products with relatively low volatile organic compound emissions. Indoor and outdoor concentrations of formaldehyde and acetaldehyde were approximately measured monthly for 15 months. Independent emission measurements of formaldehyde were made in a small chamber system to determine the emission rates from samples of the wood flooring, plywood, and wood cabinetry taken from the house. Blower door tests were performed to determine the leakage area of the exterior envelope, the interior floors, and transfer grilles between floors. Real-time formaldehyde concentration and energy measurements were used to verify the indoor concentrations and energy predictions of a coupled CONTAM-EnergyPlus model of the house. The verified model was then used to evaluate the impacts of different outdoor air ventilation rates on indoor concentrations and energy. The current work demonstrates the need for consideration of source control options during product selection and the provision of mechanical ventilation, especially in homes with relatively airtight envelopes.

Detailed energy model of the National Institute of Standards and Technology Net-Zero Energy Residential Test Facility: Development, modification, and validation

The National Institute of Standards and Technology Net-Zero Energy Residential Test Facility is a highly instrumented, highly configurable, single-family, net-zero energy house occupied by a virtual family of four. A detailed transient model of the Net-Zero Energy Residential Test Facility and the accompanying mechanical equipment was created using information available before construction; the model incorporated building geometric details and construction material properties, as well as manufacturers’ specifications for HVAC, water heating, solar photovoltaic and other equipment. This model represents the typical design paradigm, where actual building performance and detailed equipment operation are not known. This original model underpredicted the measured annual energy consumption by 13.8%. The measured data were used to understand and correct the sources of error at the component level; modifications to the heating, ventilation, and air-conditioning system, interior thermal capacitance, and domestic hot water system improved the energy consumption prediction to within 1.6% of measured data. The differences between the original and modified models are useful for understanding the sources, magnitudes, and possible corrections to errors in energy models for high-efficiency residences. The modified model will be used in future studies of alternative energy system configurations and control strategies, contributing to cost-effective and optimum design of net-zero energy houses in America.

Small Changes Yield Large Results at NIST's Net-Zero Energy Residential Test Facility.

The Net-Zero Energy Residential Test Facility (NZERTF) was designed to be approximately 60 % more energy efficient than homes meeting the 2012 International Energy Conservation Code (IECC) requirements. The thermal envelope minimizes heat loss/gain through the use of advanced framing and enhanced insulation. A continuous air/moisture barrier resulted in an air exchange rate of 0.6 air changes per hour at 50 Pa. The home incorporates a vast array of extensively monitored renewable and energy efficient technologies including an air-to-air heat pump system with a dedicated dehumidification cycle; a ducted heat-recovery ventilation system; a whole house dehumidifier; a photovoltaic system; and a solar domestic hot water system. During its first year of operation the NZERTF produced an energy surplus of 1023 kWh. Based on observations during the first year, changes were made to determine if further improvements in energy performance could be obtained. The changes consisted of installing a thermostat that incorporated control logic to minimize the use of auxiliary heat, using a whole house dehumidifier in lieu of the heat pump's dedicated dehumidification cycle, and reducing the ventilation rate to a value that met but did not exceed code requirements. During the second year of operation the NZERTF produced an energy surplus of 2241 kWh. This paper describes the facility, compares the performance data for the two years, and quantifies the energy impact of the weather conditions and operational changes.

A self-learning algorithm for estimating solar heat gain and temperature changes in a single-Family residence

To develop effective control optimization strategies for managing residential electricity consumption in a smart grid environment, predictive algorithms are needed that are simple to implement, minimize custom configuration, and provide sufficient accuracy to enable meaningful control decisions. This paper presents a self‐learning algorithm for predicting indoor temperature in a single-family residence using a first-order lumped capacitance (1R1C) model that can be used to evaluate the consequences of energy saving or load shifting strategies on thermal comfort. The algorithm is formulated in such a way that key design details such as window size/configuration, thermal insulation, and airtightness (all major factors that affect heat loss and solar heat gain) are combined into effective parameters that can be learned from observation. This approach eliminates the need for a custom configuration for each residence.The model was validated using experimental data from the National Institute of Standards and Technology (NIST) Net-Zero Energy Residential Test Facility. It was found that with a simple temperature decay test to determine a thermal time constant and a seven-day sliding window of training data to account for seasonal variations in other parameters, the algorithm can reliably predict indoor temperatures for a 24h period using a solar irradiance forecast, an outdoor air temperature forecast, and heat pump output. The average root mean square temperature prediction error was found to be 2.2%.

Performance Data from the NIST Net-Zero Energy Residential Test Facility.

Data were collected over two separate year-long test periods at the Net-Zero Energy Residential Test Facility, alaboratory designed to evaluate a variety of technologies and operational strategies that lead to energy efficient houses with comfortable and healthful indoor environments. In a net-zero energy building, all energy consumption over the course of a year is offset by on-site renewable energy production; this facility attempts to meet that goal through use of a photovoltaic array installed on the roof. Data are presented for one-year test periods over which the research team examined whether the facility would reach net-zero status. In both years, the house was operated in an all-electric configuration, with slight modifications made in the second year related to control schemes and equipment selection. A virtual family of four was simulated to carry out the operations that would typically occur in a home (e.g., appliance usage, lighting usage, hot water usage). Data are being released for the second year of operation at the time of publication of this document, with an expectation that data from the first year will be released at a later date.

Reducing the impacts of weather variability on long-term building energy performance by adopting energy-efficient measures and systems: a case study

This case study uses whole-building simulation software to investigate the ability of alternative sets of energy conservation measures to limit the sensitivity in energy use and heating, ventilation, and air-conditioning (HVAC) peak electricity demands by residential buildings due to weather variability. Four alternative building designs are considered: a net-zero energy design based on the National Institute of Standards and Technology (NIST) net-zero energy residential test facility (NZERTF), a comparable, yet less efficient design built according to the 2015 International Energy Conservation Code (IECC), and two low-energy designs at varying levels of energy-efficiency. The findings from this study reveal which measures best ensure robust annual energy performance and occupant comfort by low- and net-zero energy homes given weather variability, as well as how much resulting energy performances vary across the sets of conservation measures considered in this work.

Predicting energy performance of a net-zero energy building: A statistical approach

Performance-based building requirements have become more prevalent because it gives freedom in building design while still maintaining or exceeding the energy performance required by prescriptive-based requirements. In order to determine if building designs reach target energy efficiency improvements, it is necessary to estimate the energy performance of a building using predictive models and different weather conditions. Physics-based whole building energy simulation modeling is the most common approach. However, these physics-based models include underlying assumptions and require significant amounts of information in order to specify the input parameter values. An alternative approach to test the performance of a building is to develop a statistically derived predictive regression model using post-occupancy data that can accurately predict energy consumption and production based on a few common weather-based factors, thus requiring less information than simulation models. A regression model based on measured data should be able to predict energy performance of a building for a given day as long as the weather conditions are similar to those during the data collection time frame. This article uses data from the National Institute of Standards and Technology (NIST) Net-Zero Energy Residential Test Facility (NZERTF) to develop and validate a regression model to predict the energy performance of the NZERTF using two weather variables aggregated to the daily level, applies the model to estimate the energy performance of hypothetical NZERTFs located in different cities in the Mixed-Humid Climate Zone, and compares these estimates to the results from already existing EnergyPlus whole building energy simulations. This regression model exhibits agreement with EnergyPlus predictive trends in energy production and net consumption, but differs greatly in energy consumption. The model can be used as a framework for alternative and more complex models based on the experimental data collected from the NZERTF.

Energy use consequences of ventilating a net-zero energy house

A Net-Zero Energy Residential Test Facility (NZERTF) has been constructed at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, to demonstrate that a home similar in size, aesthetics, and amenities to those in the surrounding communities can achieve net-zero energy use over the course of a year while meeting the average electricity and water use needs of a family of four in the United States. The facility incorporates renewable energy and energy efficient technologies, including an air-to-air heat pump system, a solar photovoltaic system, a solar thermal domestic hot water system, and a heat recovery ventilation system sized to meet American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) Standard 62.2-2010 ventilation requirements. The largest energy end use within the home was space conditioning, which included heat loss through the building envelope, ventilation air supplied by the heat recovery ventilator (HRV), and internal loads. While HRVs are often described as being able to save energy when compared to ventilating without heat recovery, there have been no studies using a full year of measured data that determine the thermal load and energy impacts of HRV-based ventilation on the central heating and cooling system. Over the course of a year, continuous operation of the HRV at the NZERTF resulted in an annual savings of 7% in heat pump energy use compared with the hypothetical case of ventilating without heat recovery. The heat pump electrical use varied from an increase of 5% in the cooling months to 36% savings in the heating months compared with ventilation without heat recovery. The increase in the cooling months occurred when the outdoor temperature was lower than the indoor temperature, during which the availability of an economizer mode would have been beneficial. Nevertheless, the fan energy required to operate the selected HRV at the NZERTF paid for itself in the heat pump energy saved compared with ventilation without heat recovery.

Long term air quality monitoring in a net-zero energy residence designed with low emitting interior products

The National Institute of Standards and Technology (NIST) constructed a Net Zero Energy Residential Test Facility (NZERTF) to support the development and adoption of cost-effective Net Zero Energy (NZE) designs and technologies. One key design objective was to provide for occupant health and comfort through adequate ventilation and reduced indoor contaminant sources. To improve source control, guidelines were implemented to utilize products with relatively low volatile organic compound (VOC) emissions. Indoor and outdoor concentrations of formaldehyde and 30 other VOCs were measured approximately monthly during the first year of house operation. Indoor temperature and ventilation conditions were relatively constant over the study. Indoor minus outdoor (I–O) concentrations of many VOCs varied with outdoor temperature. Correlation analyses of I–O concentrations versus inverse outdoor temperature (1/K) suggested that some building envelope components were an indoor source of aldehydes (but not formaldehyde) and several other VOCs. Floor area specific emission rates were calculated and compared to values from several prior studies of conventional new houses. The average formaldehyde emission factor of 6.7 μg h−1 m−2 in this study was lower than literature values (29 μg h−1 m−2 to 45 μg h−1 m−2) indicating formaldehyde source control approaches were effective. VOC measurements at other indoor conditions demonstrated that eliminating mechanical ventilation contributed more to an increase in indoor VOC concentrations than an 8 °C increase in the indoor temperature.

Net-zero and beyond! Design and performance of NIST's net-zero energy residential test facility

A Net-Zero Energy Residential Test Facility has been constructed at the National Institute of Standards and Technology to demonstrate that a home similar in size, aesthetics, and amenities to those in the surrounding communities can achieve net-zero while meeting the needs of a four member family. The home incorporates a vast array of renewable energy and energy efficient technologies, a subset of which was used during the first year of operation, including an air-to-air heat pump system, a solar photovoltaic system, a solar thermal hot water system, and a heat recovery ventilation system (HRV). The solar photovoltaic system generated 13,523kWh of energy, exceeding the home's annual energy consumption by 484kWh during the 12-month test interval. The solar thermal hot water system provided 54% of the energy required to meet the hot water load. Use of the heat recovery ventilator, used to provide ventilation air to the home, resulted in 1965kWh of energy consumption, 514kWh to power the HRV and 1451 of energy used by the heat pump system to meet the additional sensible and thermal loads. This paper describes the facility and presents performance data for the first year of operation.

Energy performance sensitivity of a net-zero energy home to design and use specifications

The purpose of this report is to test the sensitivity of the energy performance of the National Institute of Standards and Technology (NIST)'s Net Zero Energy Residential Test Facility (NZERTF) design to variation in the assumed specifications of occupant behaviour and building design characteristics using whole building energy simulations. The analysis includes a total of 128 EnergyPlus (E+) simulations (DOE 2013) considering two levels for each of the seven factors that could impact the energy performance of the NZERTF: building design, air leakage, occupant behaviour, weather, building orientation, and heating and cooling setpoint temperatures. The results for each combination of the seven factors are analysed to determine the magnitude and significance of changing these factors, and the interaction effects between the factors. There is particular focus on the building design results to determine if the energy performance of the NZERTF is more or less stable than that of typical residential construction in Maryland.