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Low-GWP Refrigerants for High-Efficiency HVAC&R Equipment Project

Summary:

Concerns over global warming and ozone depletion will limit or phase out several refrigerants currently used in commercial and residential cooling and heating equipment. The environmental criteria for the future refrigerants include zero ozone depletion potential (ODP), low global warming potential (GWP), and high efficiency. This project will benchmark the heat transfer properties of the leading replacement candidates and will measure and model their overall thermal performance in a vapor compression system.

Description:

Objective: To develop heat transfer and system performance information on low-GWP refrigerants, which will assist in the selection and implementation of the best replacements for high-GWP hydrofluorocarbon (HFC) refrigerants by 2016.

What is the new technical idea? Most of the currently used HFC refrigerants will be phased out or phased down.1 Consequently, candidate replacement fluids must be researched and evaluated. The scope of the evaluation includes two-phase heat transfer and cycle measurements that will support the development of a prediction model for a comprehensive and fair comparison of low-GWP refrigerants. For complete characterization of new fluids, fundamental heat transfer and pressure drop measurements will be taken in a convective-boiling heat transfer apparatus, and cycle performance will be measured in a laboratory heat pump apparatus. The prediction model will be a new cycle simulation-based methodology that will determine refrigerant performance parameters, volumetric capacity, and coefficient of performance while accounting for the refrigerant’s thermodynamic and transfer properties. The new methodology will provide more realistic predictions of low-GWP refrigerant performance than conventional theoretical models based on refrigerant thermodynamic properties alone.

What is the research plan? The research plan encompasses three tasks. Within Task 1, the local convective-boiling heat transfer coefficient of candidate replacements for high-GWP refrigerants inside a micro-fin tube will be determined. In FY12 and FY13, the apparatus was used to test R134a and three low-GWP test refrigerants. In FY13, test fluids were selected by theoretically evaluating the evaporative and condensing heat transfer performance of 40 potential low-GWP refrigerants as identified by the Air-Conditioning, Heating, and Refrigeration Institute’s (AHRI) Low-GWP Alternative Refrigerants Evaluation Program (AREP). Three more low-GWP refrigerants will be tested during the FY14 effort. For each refrigerant, roughly eighty discrete operating points will be recorded to characterize the heat flux and mass flow versus thermodynamic quality. The heat transfer measurements for low-GWP refrigerants will broaden the fluid database, which will be used to extend the validity of an existing NIST heat transfer correlation to low-GWP fluids and smaller diameter tubes.

NIST will be working closely with chemical manufacturers that have supplied the test refrigerants. Some of these test refrigerants may be mildly flammable having an ASHRAE 2L designation.2 Consequently, we will have to identify the required modifications to safely test the 2L refrigerants.

Task 2 consists of the laboratory characterization of low-GWP refrigerants in a mini-breadboard heat pump (MB-HP). In FY11, this laboratory apparatus was completely rebuilt and equipped with specialized, variable-area heat exchangers, a variable-speed compressor, and two water chillers to control the temperatures of the heat source and the heat sink. In FY12, the MB-HP was instrumented and calibrated. In FY13, shakedown-baseline tests with R134a were done followed by tests with the first low-GWP fluid. In FY14, the rig will be used to evaluate the cycle performance of three new low-GWP refrigerants and mixtures.

The modeling element of this project (Task 3) supports two NIST Standard Reference Databases (SRDs): CYCLE_D and REFLEAK. FY11 produced updates of both programs with new versions released in FY12. Also in FY12, the simulation methodology and design specifications were developed for a new generation cycle model, Cycle-?T/UA, which accounts for both thermodynamic and transport properties of refrigerants. Inclusion of transport phenomena within the heat exchangers enables the analysis and optimization of the fundamental trade-off between the pressure drop penalty and refrigerant heat transfer enhancement with increasing refrigerant mass flux. The FY13 effort produced the first version of this model, which is limited to the basic vapor compression cycle. During FY 2014, this model will be expanded to include three advanced cycle options offering improved efficiency by limiting throttling irreversibilities. The Cycle-?T/UA program will serve as a novel tool for evaluating and rating the performance of refrigerants operating in the subcritical vapor-compression cycle. It will be upgraded and will become a new NIST Standard Reference Database in FY16.

 


[1] 2009 Proposal by United States, Canada and Mexico to amend the Montreal Protocol http://ozone.unep.org/Meeting_Documents/mop/21mop/MOP-21-3-Add-1E.pdf

[2] ANSI/ASHRAE, 2010, “Standard 34-2007: Designation and Safety Classification of Refrigerants”

 

Major Accomplishments:

Outcomes:
  • Created new knowledge and understanding of the potential of nanolubricants for chillers, which has resulted in industry interest.

Impacts:

  • Standard Reference Database 73, REFLEAK, (http://www.nist.gov/srd/nist73.cfm) is used by industry to prepare their submittals to ASHRAE Standard Project Committee 34. It is also used by this ASHRAE SPC 34 to evaluate received submittals.
  • Manufacturers, consultants, and academics use Standard Reference Database 49, CYCLE_D, (http://www.nist.gov/srd/nist49.cfm) to evaluate different refrigerant options including new HFO refrigerants and the CO2 transcritical cycle. 

Standards and Codes:

ASHRAE Standard 34; REFLEAK is synchronized with the submittal requirements for a zeotropic mixture to ASHRAE Standard Project Committee 34; it simplifies preparing the submittals by industry and evaluating these submittals by the ASHRAE Committee.

The Cycle-?T/UA simulation model will formulate a new, advance practice of determining the relative performance merits of refrigerants, which may become a standard method for evaluating new working fluids.

The nanolubricant work is in the research stage; therefore the outcomes will not directly influence standards and codes in the immediate future. When the application of nanolubricants in chillers has been realized, industry would likely welcome a guideline or standard for evaluating nanolubricants, and this element of the project will provide the base knowledge for that standard.