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. 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. Nanolubricants for chillers have been identified as a promising technique for reducing energy consumption in buildings. 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. 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 and re-calibrated. In FY12, the apparatus was used to test R134a and two low-GWP test refrigerants. Three more low-GWP refrigerants will be tested during the FY13 effort. For each refrigerant, roughly eighty discrete operating points will be recorded to characterize the steady-state heat flux versus mass flow rate. The low-GWP candidate refrigerants have been selected with guidance from chemical manufacturers. The new heat transfer measurements for low-GWP refrigerants will broaden the fluid data base, which will be used to extend an existing NIST heat transfer correlation.
Laboratory characterization of low-GWP refrigerants will take place 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 sink. In FY12, the MB-HP was instrumented and calibrated, and benchmark tests with R134a were made. In FY13, the rig will be used to evaluate the cycle performance of three new low-GWP refrigerants and mixtures.
The modeling element of this project supports two NIST Standard Reference Data programs, CYCLE_D and REFLEAK. FY11 saw updates of both programs with new versions released in FY12. 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 development work on this software started in FY12. The beta version of this model will be made available for outside testing with its release scheduled by the end of FY13. It will serve as a novel tool for evaluating and rating the performance of refrigerants operating in the subcritical vapor-compression cycle.
On October 29-30, 2012, NIST will host the ASHRAE/NIST Refrigerants Conference: Moving Towards Sustainability. The goal of the conference is to discuss cooling/refrigeration technology options in the context of the impending phase down of high-GWP refrigerants, and to inform manufacturers, consultants, researchers, and government officials about the very latest trends and directions going forward.
In FY11, NIST completed procurement and installation of a chiller for research purposes. In FY12, a system to measure the evaporator refrigerant\lubricant composition was 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 were completed without nanoparticles, and chiller baseline tests without nanoparticles. In FY13, tests with nanolubricant will be performed to demonstrate the impact of the nanolubricant on the chiller performance.
 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
 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.
 Measurement Science Roadmap for Net-Zero Energy Buildings, Workshop Summary Report, NIST Technical Note 1660, March 2010
 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; 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-ΔT/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.
Researchers use novel fluids with nanoparticles to improve the efficiency of chillers that cool large buildings.
Start Date:October 1, 2011
Lead Organizational Unit:el
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