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 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 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) and 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. Previously, the model was validated using the NZERTF annual performance data (July 2013 – June 2014). Additionally, the TRNSYS building simulation model was 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 included: 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 the upcoming year, the analysis of these options will be extended to economic merits.
Task 2 initiates the project’s effort to study 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 will be set up in the Large Environmental Refrigeration Chamber, and a GSHP/HPWH using an HFC refrigerant (R410A) will be procured, instrumented, and tested under controlled conditions to obtain its baseline performance data. The system performance will be characterized while operating as a GSHP only, and as a GSHP/HPWH. In future years, 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 ground-source heat pump 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. Previously, a prototype water-to-air air conditioner using CO2 as a refrigerant was installed and instrumented in an environmental chamber. In the current year, shake-down tests and performance measurements of the CO2 air conditioner at standard rating conditions will be taken. In future years, an extensive test matrix will be developed and executed to allow determining the seasonal energy consumption based on the measured NZERTF cooling loads. 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. The performance of the CO2 will be compared to the system employing a conventional refrigerant (R410A) from Task 2.
Task 4 entails 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 FY2014, included the predefined “hair pin” pattern option, which improved the manufacturability of generated designs. Following that release, the model was expanded to allow simulations with low global-warming-potential refrigerant mixtures. The robustness of EVAP and COND simulators were updated to accommodate high-glide zeotropic mixtures. In preparation for release of Version 5 with the capability to simulate large heat exchangers with up to 10 depth rows (the current limit is five depth rows), the condenser simulator was upgraded to 10 depth rows. This upgrade will be implemented in the evaporator model in the upcoming year. We will also initiate the development of a new graphical user interface to facilitate the new modeling capabilities.