Steady State Modeling of Advanced Vapor Compression Systems with Multiple Air and Refrigerant Loops

Transcription

1 Steady State Modeling of Advanced Vapor Compression Systems with Multiple Air and Refrigerant Loops Mohamed Beshr, Vikrant Aute, Reinhard Radermacher Center for Environmental Energy Engineering Department of Mechanical Engineering University of Maryland, College Park, MD 20742

2 Contents Motivation Literature Review Objectives Summary and Conclusions 2

3 Motivation HVACR systems are necessary to make buildings inhabitable and for food preservation Majority of HVACR systems are based on vapor compression technology HVAC accounts for* 40% of primary energy use in residential buildings 33% of primary energy use in commercial buildings Continuous need for Improving the efficiency of vapor compression systems Reducing their cost and environmental impact *U.S. Department of Energy, "Energy Efficiency Trends in Residential and Commercial Buildings",

4 Motivation (cont d) Steady state simulation tools are the key to Maximizing performance Minimizing cost Minimizing environmental impact Making the first prototype 95% right Reliable simulation tool * Robustness Speed Accuracy Scalability *G. L. Ding, "Recent Developments in Simulation Techniques for Vapor-Compression Refrigeration Systems," International Journal of Refrigeration, vol. 18, pp ,

5 Vapor Compression (VC) System Solver Condenser VC System Simulation Tool Expansion Device System Solver Non-linear Equation Solver Compressor 5 Evaporator

6 VC System Solver Medium temperature display cases Compressor racks Cascade HX 6 Low temperature display cases 6

7 Literature Simulations studies consist of transient and steady state modeling 60+ significant publications about steady state vapor compression system solvers 10+ vapor compression steady state tools 7

8 Model Formulation Options Global scheme Hard coding the equations of the component models within the system solver Improves the solver robustness Inflexible Component-based scheme Decoupling the system solver from the component models System solver requires no specific details about the component model equations or data Easily handle arbitrary system configurations Lower solver robustness 8

9 Solution Techniques Early Literature Methodology Successive Substitution Davis & Scott, 1976; Hiller & Glicksman, 1976 State variable is solved before moving on to the next variable along refrigerant flow direction Simultaneous Solution Parise, 1986 All the unknown variables are solved simultaneously Number of unknowns Low High (more than 2x) Computational Speed Fast Robustness High Low Flexibility Low - Usually designed for fixed system configuration Slow - High computational cost High 9

10 New Comprehensive Solver Component-based High flexibility Scalable Fast and robust 10

11 Existing VC System Simulation Tools Detailed Component Models Componentbased Optimization Arbitrary System Configurations Multiple Fluids Paths NIST CYCLE_D x x x x x CEEE TCM x x x x x SysMo Ltd SmoWeb x x x x x FKW KMKreis x x x x x Comments Thermodynamic models DTU CoolPack x x x x ACMODEL x x x x IMST-ART x x x x Emerson SDS x x x ORNL HPDM x x x runs CEEE VapCyc x x Used by more than 40 companies New Solver 11

12 Objectives Develop a comprehensive component-based vapor compression system steady state solver that Handles arbitrary system configurations Evaluates the performance of any newly proposed system configuration Reduces the design and optimization time Handles multiple fluids paths Multi-circuit heat exchangers Multiple secondary fluids Air paths Allows better control over convergence criteria Test and validate the solver 12

13 COMPREHENSIVE VAPOR COMPRESSION SYSTEM SOLVER 13

14 Unknown Variables Condenser P 5 h 5 P 4 h 4 P 3 h 3 P 2 h 2 Expansion device Compressor Outlet pressure Evaporator P 6 h 6 P 7 h 7 P 1 h 1 Inlet pressure and enthalpy Outlet pressure m ref 14 unknown variables [P 1 P 2 P 3 P 4 P 5 P 6 P 7 h 1 h 2 h 3 h 4 h 5 h 6 h 7 ] Successive substitution 4 variables [P1 P2 P6 h1] 14

15 Solver Outline Start Determine junctions, unknowns, and residuals on the fly Identify additional inputs required User Inputs Determine the unknowns Formulate the residual equations No Inlet port state known Yes Run component Run compressor(s) Check next component in the component list No No End of component list Yes All port states known Yes Calculate residuals Update state variables No Solver converged Yes Results Analysis 15

16 Arbitrary Cycle Solver 16

17 Multiple Fluid Solver Start R-404A User Inputs Determine the unknowns Formulate the residual equations No Inlet port state known Yes Run component Run compressor(s) No End of component list Yes Check next component in the component list No All port states known Yes CO 2 Calculate residuals Update state variables No Solver converged Yes Results Analysis 17

18 Residential Air Source Heat Pump System Power Consumption (kw) - Cooling Capacity (kw) - Cooling 4-5% 12-5% Numerical % Numerical % Experimental Experimental 18

19 Vapor Injection Heat Pump System with a Flash Tank Air inlet Vapor line Liquid line Condenser 5 Power consumption (kw) -10% 12 Capacity (kw) -10% Evaporator Numerical % Numerical % Distributor Vapor Line Experimental Experimental Xu, X., Investigation of Vapor Injection Heat Pump System with a Flash Tank Utilizing R410A and Low- GWP Refrigerant R32, Ph.D. Dissertation, University of Maryland, 2012 Liquid Line Air inlet 19

20 Supermarket System Water/Glycol Mix High-Pressure CO 2 Gas Cooler Medium-Pressure CO 2 Low-Pressure CO Nominal capacity of 8.1 kw CO 2 Loop Gas cooler: plate fin HX Suction HX: tube in tube HX Evaporators: plate HX Subcooler HX: plate HX R-134a Loop Condenser: plate fin HX Subcooler HX: plate HX The laboratory portion of this effort was performed by the Electric Power Research Institute with the support of Southern California Edison 20 Subcooler 10 Throttle Valve 4 Subcooler HX Flash Tank EEV Manuscript presented at the 12th IIR Gustav Lorentzen Natural Working Fluids Conference, EEV 5 MT Evap. 7 LT Evap Liquid/Suction HX Gas Cooler/ MT Suct. HX LT Compressor MT Compressor Variable-speed with a nominal capacity of 17.6 kw Single-speed with a nominal capacity of 6.8 kw

21 Supermarket System Capacity (kw) Numerical % -3% Experimental +3% -3% +3% -3% 21

22 Summary and Conclusions Creating a new generation of vapor compression system steady state solvers that can handle arbitrary system configurations and multiple fluids paths Verified the solver Validated the solver Residential ASHP system (5%) Vapor injection heat pump system (10%) Two-stage CO 2 supermarket refrigeration system with mechanical subcooler (3%) 22

23 23 Thank You