Space-conditioning equipment accounts for 41 % of the Nation's primary energy consumption in buildings, and 15 % of the Nation's overall primary energy consumption. For this reason, reaching the goal of net-zero energy buildings requires high-efficiency air-conditioner and heat pump designs. These designs must accommodate the shifting building load paradigm brought about by better insulation, lower infiltration, and more efficient appliances with smart controls.
The project focuses on evaluating space-conditioning options for energy-efficient residences, where the NIST Net-Zero Energy Residential Test Facility (NZERTF) will serve as the test bed for experimental verification. Heating and cooling loads in the NZERTF will be recorded and documented in detail and serve as the benchmark for comparing alternative configurations. Through data analysis and simulations, the project will explore space-conditioning options for different climatic locations in search of optimal solutions for these localities. This project will also develop simulation models for vapor compression systems, the dominant technology for comfort space conditioning. These models will be used to evaluate different system configurations for energy-efficient homes.
What is the new technical idea?
The unique aspects of high-efficiency homes dictate new solutions for space conditioning equipment. Because of tighter envelopes and better insulation, high-efficiency homes are less sensitive to the external environment and more sensitive to internal loads. As a result, cooling systems must be able to cope with higher moisture removal demands (higher latent loads) as a percentage of the overall cooling demand. An additional consideration for heat pumps in new homes is the integration of outdoor air mechanical ventilation systems, which are required to maintain indoor air quality for buildings with tighter envelopes.
The overall concept of the project is to use the cooling and heating loads measured in the Net-Zero Energy Residential Test Facility (NZERTF) to explore various energy technologies in search of the most effective and economical options for energy-efficient homes. The main focus of the study is on enhancing the efficiency of the equipment in the NZERTF. However, a broader impact of the study will be attained by extrapolating the NZERTF results to different climatic regions and longer timeframes using a building simulation model developed on the TRNSYS platform. Special consideration will be given to exploring and understanding the performance of geothermal heat exchangers (GHXs). The project will also advance simulation tools for designing high-efficiency heat pump systems, and will involve some considerations for proper equipment commissioning since faulty equipment installation can negate efficiency benefits otherwise attainable with high-efficiency air conditioners and heat pumps.
What is the research plan?
Task 1 involves using a TRNSYS building simulation model to broaden the impact and guide the direction of the experimental research conducted on the NZERTF. In FY2015, the model was validated using the NZERTF annual performance data (July 2013 – June 2014). In FY2016, the TRNSYS building simulation model will be used to evaluate the energy merits of different space-conditioning technologies to provide guidance for NZERTF options beyond those used in the initial baseline tests. The considered technologies will include: air-source heat pump versus ground-source heat pump; heat recovery ventilator versus enthalpy recovery ventilator; and heat pump water heater versus solar-assisted electric water heater. In FY2017, the analysis of these options will be extended to economic merits.
Task 2 entails measurements on the NZERTF's three geothermal heat exchangers to develop their detailed characterization and understanding of their performance. Specifically, Thermal Response Tests will be conducted for individual legs of these heat exchangers to learn about the degree of performance uniformity (or disparity) between the individual legs and possible interactions between them affecting their performance.
Task 3 initiates the project's effort to study the performance of a geothermal air conditioner with the NZERTF. In FY2016, a prototype water-to-air heat pump using CO2 as a refrigerant will be installed and instrumented in an environmental chamber for performance characterization. Performance measurements will take place in FY2017. The use of CO2 is a significant novelty of the study. Application of CO2 in a geothermal air conditioner may allow for holding the thermodynamic cycle below the CO2 critical temperature, which should result in good performance.
Task 4 entails the development and implementation of novel optimization methods in the EVAP-COND tool for designing air-to-refrigerant heat exchangers. Version 4.0, released in FY2014, included the predefined "hair pin" pattern option, which improved the manufacturability of generated designs. In FY2015 the model was expanded to allow simulations with arbitrary low global-warming-potential refrigerant mixtures defined by the user. In FY2016, the simulation capabilities will be expanded to allow simulations of large heat exchangers with up to 10 depth rows (the current limit is five depth rows). This effort will include the development of a new graphical user interface to facilitate the new modeling capabilities.
Task 5 will produce an analysis of the impact of commissioning faults on performance (including indoor humidity control) of a residential split air conditioner installed in two types of residential houses in five U.S. climatic regions. This study will consider in detail the effect of different indoor fan's control options on the indoor humidity level throughout the whole cooling season.