Space-Conditioning Options for High-Performance Buildings and Resilient Electrical Grids

Summary

Space-conditioning equipment accounts for 32 % of electricity use in homes[1] and 17 % in commercial buildings[2], costing $49 Billion and $21 Billion[3] USD annually. Advanced air-conditioning and heat-pump system adoption would lower electricity consumption and reduce these costs, and would support grid resilience in a market where overall demand is growing due to artificial intelligence and data centers [4]. Moreover, space-conditioning equipment must maintain comfort and accommodate reduced building loads and changing load profiles brought about by concurrent increased insulation, lower infiltration, and modern appliances with smart controls. This project focuses on evaluating space-conditioning options for high-performance residences, using controlled temperature and humidity chambers and a full-scale residential test facility (i.e., NIST RTF) for experimental studies. Heating and cooling loads in the RTF will be recorded and documented in detail and serve as the benchmark for comparing alternative space-conditioning equipment configurations. Through data analysis and simulations, the project will explore space-conditioning options for different climatic locations. 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 targeting high-performance homes with different grid-stabilizing options. Lastly, the RTF will be used to compare small-duct high-velocity (SDHV) ductwork with conventional larger ductwork, based on energy use, first and operating costs, and comfort.

Description

Measurement of the air velocity profile exiting a section of the rooftop air-conditioning unit's heat exchanger

Objective
Develop and deploy the measurement science to support the implementation of cost-effective and comfortable space-conditioning options for high-performance buildings that reduce stress on the grid.

Technical Idea
One approach to reducing electrical demand for HVAC is construction of high-performance homes or upgrading existing homes, which may require new solutions for space-conditioning equipment. High-performance homes have tighter envelopes and better insulation, and therefore are less sensitive to the external environment and more sensitive to internal loads. Thus, cooling systems must be able to cope with higher moisture removal demands (higher latent loads). An additional consideration for tighter homes is the integration of outdoor air mechanical ventilation systems, which provide improved ventilation and indoor air quality as compared to uncontrolled ventilation. The measured cooling and heating loads in the RTF will be used to explore various technologies in search of the most effective and economical options for high-performance homes. The focus of the study is on improvements that promote grid stability by reducing energy use and peak demand and will include simulation-based extrapolations of the RTF results to performance in different U.S. climatic regions. This will be achieved by validating and then using a building simulation model, the Transient Systems Simulation (TRNSYS) platform, and advancing simulation tools for designing high-performance heat pump systems. Special consideration will be given to exploring and understanding the performance of ground-source heat pump systems. This includes studying a ground-source integrated heat pump (GSIHP), a single appliance that can provide all the heating, cooling, and domestic hot water (DHW) for a home. These systems have potential to significantly reduce energy consumption and peak demand, thereby stabilizing the grid. The performance of the GSIHP will be analyzed in detail and compared with competing technologies. Lastly, the RTF will be used to compare small-duct high-velocity (SDHV) ductwork with conventional larger ducts by quantifying energy use, first and operating costs, and comfort. The SDHV ducts are less expensive to install but have an associated energy penalty from the added fan power requirements, and potentially more (or less) temperature stratification in the conditioned spaces.

Research Plan
To broaden the impact and guide the direction of the experimental research conducted on the RTF, all experiments will be augmented by detailed building simulation (using TRNSYS). The model has been and is being validated using the RTF's annual performance data, and will be used to evaluate the energy and economic merits of different space-conditioning technologies in varied U.S. climates. One application this year will be to study the seasonal performance of the GSIHP in a variety of US climates. The 12-month test data for the GSIHP in the RTF will be used to further validate and refine the TRNSYS model. Other future efforts include using the TRNSYS model to perform seasonal energy simulations for the carbon dioxide GSHP.

A high-performance ground-source integrated heat pump (GSIHP) will be studied. The system provides both space conditioning (SC) and water heating (WH) all in a single, efficient appliance. A test rig for evaluating ground-source systems was set up in the Large Environmental Refrigeration Chamber, and a GSIHP using a hydrofluorocarbon refrigerant (R-410A) was procured and instrumented. In FY25 and FY26, the system performance will be measured under controlled conditions in the environmental chamber. Following the laboratory tests, the GSHP/HPWH will be installed in the RTF where its performance will be recorded for 12 months in order to capture seasonal variation in performance. In FY26 and FY27, these measurements will be analyzed to establish performance merits of a GSIHP versus that of the air-source heat pump and HPWH used at the RTF during the earlier studies. Further, the measurements will be compared to performance estimates from the applicable industry-consensus standard, ASHRAE 2065, to provide guidance to improve the standard.

A carbon-dioxide (CO2) based GSHP will be studied. Application of CO2 as the refrigerant in a GSHP may allow for holding the thermodynamic cycle below or near the CO2 critical temperature, which should result in higher performance. Previously, a prototype water-to-air air conditioner using CO2 as a refrigerant was installed and instrumented in an environmental chamber. Shake-down tests and standard cooling performance tests were performed, and additional performance data were collected at extended operating conditions. In FY26, these data will be used to estimate the seasonal energy consumption based on the measured RTF cooling loads. The estimation will be performed using an empirical model that interpolates the data collected in the environmental chamber, and this model will be integrated with the larger TRNSYS RTF model. Further, a physics-based model of the CO2 system was developed and validated with the laboratory measurements. In FY26, the model will be used to evaluate equipment configurations and estimate heating performance and explore optimization of system superheat and subcooling. This optimization will be validated using experimental measurements of the CO2 system performance with parametrically varied superheat and subcooling.

Lastly, a comparative study will be performed for an air-source heat pump (ASHP) using “conventional” ventilation using large rectangular ducts vs. “small-duct high-velocity” (SDHV) ventilation, in the RTF. The systems will be compared for their energy use and comfort. The RTF was previously used to test the heating and cooling performance of (1) a conventional, two-stage ASHP with conventional ductwork, and (2) a Small Duct High Velocity (SDHV) variable-speed ASHP. Once this comparison testing was complete, the conventional, two-stage ASHP was connected to the existing SDHV ductwork to examine the energy-use and/or comfort penalty of a more restrictive ductwork on a conventional system. Initial examination of the data show that the indoor air handler energy consumption increased due to the more restrictive duct, yet the heat pump was still able to satisfy the heating and cooling demand. The SDHV ductwork was used in FY2025 to complete collection of cooling and heating data with the final goal of publishing the results in FY2026.

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References

[1] https://www.eia.gov/consumption/residential/data/2015/ - Table CE4.1: End-use consumption by fuel in the U.S. - totals

[2] https://www.eia.gov/consumption/commercial/data/2012/ - Table E5: Electricity consumption (kWh) by end use

[3] https://www.eia.gov/consumption/commercial/data/2012 - Table C13: Total electricity consumption and expenditures

[4] ANSI/ASHRAE Standard 206-2013 (R2017), https://webstore.ansi.org/Standards/ASHRAE/

Major Accomplishments

• Completed Report/NIST-contribution to IEA Annex 40, on performance of a heat pump in a net-zero energy home
• Released EVAP-COND software tool, version 4.0

Associated Product(s)

EVAP-COND Simulation Models for Finned-Tube Heat Exchangers