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Space-Conditioning Options for Energy-Efficient Buildings

Summary

Space-conditioning equipment accounts for 32 % of electricity use in homes  and 17 % in commercial buildings , annually costing $49 Billion and $21 Billion  USD respectively.  For this reason, reaching the goal of net-zero energy buildings requires high-efficiency air-conditioning and heat-pump systems.  These systems must accommodate the shifting building load paradigm brought about by concurrent advances in insulation, lower infiltration, and more efficient appliances with smart controls. 

The project focuses on evaluating space-conditioning options for energy-efficient residences, where environmental chambers and the NIST Net-Zero Energy Residential Test Facility (NZERTF) serve as test beds 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 space-conditioning-equipment 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.

Description

Measurement of the air velocity profile exiting a section of the rooftop air-conditioning unit's heat exchanger
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 development and implementation of the most efficient and cost-effective space-conditioning options for energy-efficient buildings.

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.  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 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 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 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 Transient Systems Simulation (TRNSYS) platform.  Special consideration will be given to exploring and understanding the performance of geothermal heat pump systems.  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.  The model was validated using the NZERTF 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 NZERTF 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 the upcoming years, the model will be used to simulate seasonal performance of the carbon dioxide geothermal heat pump described in Task 3.

Task 2 includes studying the performance of a high-efficiency combined-appliance ground-source heat-pump and heat-pump water heater (GSHP/HPWH), 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/HPWH using an HFC refrigerant (R410A) was procured and instrumented.  The future effort entails measuring the system performance under controlled conditions in the environmental chamber.  The system performance will be characterized while operating as a GSHP only and as a GSHP/HPWH.  Following the laboratory tests, the GSHP/HPWH will be installed in the NZERTF, where its performance will be recorded under cooling and heating operation.  The system will first be tested as a GSHP only for 12 months, and then as a GSHP/HPWH for another 12 months. These measurements will be analyzed to establish performance merits of a GSHP/HPWH  versus that of the air-source heat pump and HPWH used at the NZERTF during the earlier annual studies. 

Task 3 entails a study of a carbon dioxide (CO2) geothermal air conditioner; the use of CO2 as the working fluid is a significant novelty. 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. Additional performance data were collected at extended operating conditions, these data will be used to estimate the seasonal energy consumption based on the measured NZERTF cooling loads. The estimation will be performed using an empirical model that interpolates the data collected in the environmental chamber; this model will be integrated with the larger TRNSYS NZERTF model (from Task 1).  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.  The performance of the CO2 system will be compared to the system employing a conventional refrigerant (R410A) from Task 2.  Additionally, a physics-based model of the CO2 system will be developed and validated with the laboratory measurements.  The model will be used in the future to evaluate equipment configurations and guide the experimental tests.

Task 4 involves the development and implementation of 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 objective of further work is to expand the utility of the EVAP-COND package to industry in its transition to new-generation refrigerants. These efforts include increasing the modeling capabilities to new fluids (single-component and blends), synchronization of the refrigerant thermophysical properties representation with the latest version of NIST refrigerant property database (REFPROP), and developing the capability to simulate large heat exchangers (larger number of tubes and tubes depth rows) above the current size limit based on typical residential applications. Further development of a detailed model of an air-source air conditioner (ACSIM) is also undertaken to research FDD methods and low-GWP refrigerants.

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
Created October 28, 2011, Updated October 17, 2019