Space-Conditioning Options for Energy-Efficient Buildings

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 systems must reduce these costs by operating at higher efficiency. Moreover, these systems must accommodate reduced building loads and changing load profiles brought about by concurrent increased insulation, lower infiltration, and more efficient appliances with smart controls.

(Tasks 1-4, 6): The project focuses on evaluating space-conditioning options for energy-efficient residences, using environmental chambers and a test house for experimental studies.  Heating and cooling loads in the house 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 energy-efficient homes. Finally, the test house will be used to compare small-duct high-velocity (SDHV) ductwork with conventional larger ducts, to examine the energy-use and/or comfort penalty of a more restrictive but lower cost SDHV system.

(Task 5): This task number is reserved for future use. 

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 efficient and cost-effective space-conditioning options for energy-efficient buildings.

Technical Idea
(Tasks 1-4,6): The unique aspects of high-efficiency homes require 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. Thus, 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 in buildings with tighter envelopes. The measured cooling and heating loads in the test house will be used to explore various technologies in search of the most effective and economical options for energy-efficient homes. The focus of the study is on enhancing the efficiency of the equipment in the test house. However, a broader impact of the study will be attained by extrapolating the test house results to different weather zones and longer timeframes using a building simulation model developed on the Transient Systems Simulation (TRNSYS) platform. Special consideration will be given to exploring and understanding the performance of ground-source heat pump systems.  The project will also advance simulation tools for designing high-efficiency heat pump systems. Lastly, the test house will be used to compare small-duct high-velocity (SDHV) ductwork with conventional larger ducts, on the basis of energy use and comfort. The SDHV ducts are less expensive but more restrictive, so there is an associated energy penalty from the added fan power. Further, the higher air velocity in the SDHV may have associated changes in thermal comfort, which will be characterized by air temperature stratification within the house and within one of the bedrooms (using a grid of air temperature sensors).

Research Plan
Task 1 uses a TRNSYS building simulation model to broaden the impact and guide the direction of the experimental research conducted on the test house. The model was validated using the test house annual performance data (July 2013 – June 2014). Subsequently, the TRNSYS building simulation model was used to evaluate the energy and economic merits of different space-conditioning technologies to provide guidance for test house options beyond those used in the initial baseline tests. The compared technologies included: an air-source heat pump vs. a ground-source heat pump; a heat recovery ventilator vs. an enthalpy recovery ventilator; and a heat pump water heater vs. a solar-assisted electric water heater.  These options were evaluated for 15 cities in the U.S. that represented all the climate zones in the contiguous states. In FY2024, this model was used to simulate seasonal performance of the combined-appliance ground-source heat pump and heat-pump water heater (GSHP/WH) described in Task 2, for the Gaithersburg, MD climate. In FY 2025 the seasonal performance of the GSHP/WH will be simulated for a variety of US climates. Further, the TRNSYS model will be used to perform seasonal energy simulations for the carbon dioxide ground-source heat pump described in Task 3.

Task 2 studies the performance of a high-efficiency GSHP/WH, which provides both space conditioning (SC) and water heating (WH).  A test rig for evaluating ground-source systems was set up in the Large Environmental Refrigeration Chamber, and a GSHP/WH using a hydrofluorocarbon refrigerant (R-410A) was procured and instrumented. In FY2024 and FY2025, 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 test house where its performance will be recorded for 12 months. In FY2025 and FY2026, these measurements will be analyzed to establish performance merits of a GSHP/WH versus that of the air-source heat pump and HPWH used at the test house during the earlier studies. Further, the measurements will be compared to performance estimates from the applicable standard, ASHRAE 206[1].

Task 3 entails a study of a carbon dioxide (CO₂) GSHP. 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 FY2025, these data will be used to estimate the seasonal energy consumption based on the measured test house 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 test house model (from Task 1).  Further, a physics-based model of the CO₂ system was developed and validated with the laboratory measurements. In FY2025, 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.

Task 4 will develop and implement novel optimization methods in the EVAP-COND tool for designing air-to-refrigerant heat exchangers (evaporators and condensers). Version 4.0, released in FY2016, included the predefined “hair pin” pattern option, which improved the manufacturability of generated designs. Following that release, the robustness of EVAP and COND simulators was upgraded to accommodate high-glide zeotropic blends. The follow-on work expanded the utility of the EVAP-COND package to industry in its transition to new-generation refrigerants (Version 5.0, FY2020). These efforts included increasing the modeling capabilities to new fluids (single-component and blends) and synchronization of the refrigerant thermophysical properties representation with the latest version of NIST refrigerant property database (REFPROP). In FY 2024 a detailed model of an air-source air conditioner ACSIM was completed. In FY2025, EVAP-COND will be expanded to include cooling and heating heat exchangers using liquids (e.g., water).

Task 6 is a comparative study of air-source heat pump (ASHP) energy and comfort performance with “conventional” ventilation using large rectangular ducts, and “small-duct high-velocity” (SDHV) ventilation, in the test house. The test house 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 is being used in FY2024 to complete collection of cooling and heating data with the final goal of publishing the results in FY2025.

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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