Novel Working Fluids for High-Efficiency HVAC&R Equipment Project

Concerns over global warming and ozone depletion will limit or phase out several refrigerants currently used in commercial and residential cooling and heating equipment. Consequently, 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. This project will also seek cost-neutral efficiency improvements of chillers through application of nanolubricants - lubricants with dispersed nano-size particles. Nanolubricants have been demonstrated to substantially improve refrigerant pool-boiling heat transfer and therefore can potentially be a cost-effective technology for improving the efficiency of chillers.

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; and to demonstrate improved energy efficiency of chillers through application of nanolubricants, by 2013.

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 goal of these studies is to evaluate refrigerant's performance in the theoretical thermodynamic cycle, in a heat exchanger, and in an optimized system, the last being the ultimate measure of refrigerant's merit because it considers all of the refrigerant's thermophysical and transfer properties.  The output of this project will include a new, cycle simulation-based methodology that will determine refrigerant performance parameters, capacity and coefficient of performance, while accounting for the refrigerant's thermodynamic and transfer properties.  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. These laboratory measurements will augment the cycle simulation-based analysis involving all thermophysical properties. The new methodology will provide more realistic predictions of low-GWP refrigerant performance than conventional theoretical models based on refrigerant thermodynamic properties alone. 

Reducing energy consumption by cost-effective means is essential for buildings to achieve net-zero.^[2] Nanolubricants for chillers have been identified as a promising technique for reducing energy consumption in buildings.^[3]  Accordingly, this project will also exploit the boiling heat transfer enhancing characteristics of nanolubricants to improve the efficiency of a chiller.  EL measurements have shown substantial improvements in pool-boiling heat transfer when a conventional lubricant was replaced by a lubricant containing nanoparticles^[4].  This project will identify the best nanofluids for chillers and will demonstrate the impact of a nanolubricant on efficiency in a complete chiller system test.

What is the research plan? The local convective-boiling heat-transfer coefficient of candidate R134a replacements inside a micro-fin tube will be determined.  In FY11, the test apparatus was repaired, re-calibrated and used to conduct check out tests with R134a.  For FY12, two low-GWP refrigerants will be tested. For each refrigerant, roughly eighty discrete operating points will be recorded to characterize the steady-state heat flux versus mass flow rate.  These tests will be followed by a second low-GWP candidate refrigerant, which will be selected with guidance from chemical manufacturers.  The intent is to develop a simple, accurate correlation by modifying an existing NIST correlation. 

Laboratory characterization of low-GWP refrigerants will take place in a mini-breadboard heat pump (MB-HP). In FY2011, this laboratory apparatus was completely rebuilt and equipped with specialized, variable area heat exchangers, variable-speed compressor, and two water chillers to control the temperatures of the heat source and sink.  In FY2012, the MB-HP will be fully instrumented and calibrated. Benchmark tests on the new MB-HP rig using R134a will begin once the measurement instrumentation, data acquisition, and calibration steps are complete.  The rig will be used to evaluate the cycle performance of new low-GWP refrigerants and mixtures after the benchmark tests are complete.

The modeling element of this project supports two NIST Standard Reference Data programs, CYCLE_D and REFLEAK. FY2011 saw new releases of both programs in response to user's feedback. Also, the simulation methodology and design specifications were formulated for a new generation cycle model, Cycle-ΔT/UA, which will account for both thermodynamic and transport properties of refrigerants. This program will be applied for determining relative merits of low-GWP refrigerants evaluated within this project.  The beta version of this model will be completed in FY2012 and available for outside testing with its release scheduled for FY2013. It will serve as a novel tool for evaluating and rating the performance of refrigerants operating in the subcritical vapor-compression cycle.

In FY11, NIST completed procurement and installation of a chiller for research purposes.  In FY12, a system to measure the evaporator refrigerant\lubricant composition will be developed and calibrated in the laboratory and then it will be installed in the chiller evaporator.  In addition, shakedown tests of the chiller, the lubricant composition measurement system, the data acquisition, and the data reduction procedure will be completed without nanoparticles.  Then chiller baseline tests without nanoparticles will be completed.  In the first half of FY13, tests with nanolubricant will be performed to demonstrate the impact of the nanolubricant on the chiller performance.

[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] Griffith, B., Long, N., Torcellini, P., Judkoff, R., Crawley, D., Ryan, J., 2007. Assessment of the Technical Potential for Achieving  Net Zero-Energy Buildings in the Commercial Sector; NREL Report No. TP-550-41957.

[3]Measurement Science Roadmap for Net-Zero Energy Buildings, Workshop Summary Report, NIST Technical Note 1660, March 2010

[4]Kedzierski, M. A, and Han, D. H., 2006, "Influence of Additives and Concentration on R123/Paraffinic Mineral Oil Pool Boiling Heat Transfer," NISTIR 7336, U.S. Department of Commerce, Washington, D.C., Appendix A

Standards and Codes:  ASHRAE Standard 34; the new version of 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 modeling element of the project 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.

Recent Results: 

Output: Convective-boiling test apparatus repaired and re-calibrated

Output: MB-HP apparatus rebuilt

Output: Brown, J.S., Domanski, P.A., Lemmon, E., 2011. Standard Reference Database 49, CYCLE_D: NIST Vapor Compression Cycle Design Program, Version 5.0, August 2011.

Output: Didion, D.A., Kim, M.S., Domanski, P.A. Standard Reference Database 73, REFLEAK: NIST Leak/Recharge Simulation Program for Refrigerant Mixtures, Version 4.0, August 2011.

Output:Kedzierski, M. A., 2011, Effect of Al₂O₃ Nanolubricant on a Passively Enhanced R134a Pool Boiling Surface, submitted to I. J. Trans. Phenomena.

Output:Kedzierski, M. A., 2011, Viscosity and Density of Al2O3 Nanolubricant, Proceedings of the 23rd IIR International Congress of Refrigeration, ICR718, Prague, Czech Republic.

Output:In total, outputs from FY11 and FY10 include: three (5) peer reviewed journal publications; two (2) NIST Technical Notes; five (5) Seminar presentations, two (2) conference papers, and two NIST SRD databases.

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

Impact: 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 Committee to evaluate received submittals.

Impact: Manufacturers, consultants, and academics use Standard Reference Database 49, CYCLE_D,  to evaluate different refrigerant options including the CO₂ transcritical cycle.