Experimental and Numerical Study of a Mobile Reversible Air Conditioning Heat Pump System Charge Imbalance when Switching Modes Lili Feng 1 (lfeng8@illinois.edu) Pega Hrnjak 1, (pega@illinois.edu) 1 Air Conditioning and Refrigeration Center MechSE Department, University of Illinois Urbana Champaign Creative Thermal Solutions
Outline Background of heat pump for electric vehicles Refrigerant charge imbalance when switching modes Charge determination for A/C and HP Charge retention in components measurement Charge imbalance modeling and minimization strategies Conclusion July 11 14, 16 Purdue Conferences /16
Background on reversible AC/HP systems for EV PTC heating drains battery quickly and reduces drive range largely Reversible AC/HP system largely reduces battery drainage for EV cabin heating compared to PTC heater A/C components available for heating function List of current EV models using heat pump: Nissan Leaf, Renault Zoe, KIA Soul, BMW i3, VW e Golf, and growing Many different reversible system configurations has been proposed, varying from direct refrigerant to air to indirect refrigerant to coolant toair, and mixed configurations July 11 14, 16 Purdue Conferences 3/16
Configuration based on Nissan Leaf system July 11 14, 16 Purdue Conferences 4/16
Components of the experimentally studied system 587 mm 49 mm 311 mm 9 tubes 197 mm 4 passes 15/14/18/11 19 tubes Evaporator Outdoor heat exchanger 189 mm 151 mm slabs tubes EEV July 11 14, 16 Purdue Conferences Compressor Accumulator Inner Condenser 5/16
Charge imbalance when switching modes The system needs more charge for A/C mode than for HP mode When reheat is needed in A/C mode, system needs even more charge Effect of imbalance: large accumulator size, liquid refrigerant entering compressor suction, oil return A/C mode HP mode July 11 14, 16 Purdue Conferences 6/16
Charge imbalance effects Liquid storage reservoir size Using accumulator for excess liquid storage affects oil return to compressor e.g.: minimum allowed suction quality (x suc ) 95% oil circulation ratio (OCR) 5% excess refrigerant liquid stored in accumulator (m ref,liq,accu ) 1 g Assuming oil and liquid refrigerant are well mixed m oil, accu m ref, liq, accu (1 x OCR ) (1 OCR) oil mass in accumulator 15.3 g Over charge system with oil Use internal heat exchanger Store excess liquid in high side Reduce charge imbalance suc July 11 14, 16 Purdue Conferences 7/16
Charge imbalance breakdown in components HP mode V[cm 3 ] 5 Internal volume HP ref. mass A/C ref. mass 15 1 1 8 6 4 m[g] A/C mode 5 Comp. Lines HEX's Accu. 15 1 HP internal volume A/C internal volume HP ref. mass A/C ref. mass 15 1 6 5 HP ref. mass A/C ref. mass 3 5 V [cm3] 9 6 9 6 m [g] V [cm3] 4 3 15 1 m[g] 3 3 1 5 Dis. Liq. ph Suc. line July 11 14, 16 Purdue Conferences I.D. cond. O.D. HX Evap. 8/16
Liquid line size minimization and balancing HP mode A/C mode HP liquid line in A/C mode: discharge line Balance between charge and COP A/C liquid line in HP mode: Before 3 way valve suction line Minimize length by placing 3 way valve close to outdoor heat exchanger outlet After 3 way valve bypassed Minimize diameter for not having flash evaporation July 11 14, 16 Purdue Conferences 9/16
Microchannel heat exchanger: finite volume/ε NTU method Break into elements along refrigerant flow List of empirical correlations Subject Correlation Assumptions: 1. Steady state flow and heat transfer;. Uniform distribution among parallel tubes; 3. Pressure drop in headers neglected; Single phase htc Gnielinski 1976 Single phase frictional pressure drop Churchill 1977 Two phase evaporation htc Kandlikar 14 Two phase condensation htc Cavallini et al 6 Condensing superheated zone htc Kondou and Hrnjak 1 Two phase frictional pressure drop Friedel 1976 (Evap) Cavallini et al 9 (Cond) Air side htc Chang and Wang 1996 Air side pressure drop Chang et al 1994 July 11 14, 16 Purdue Conferences 1 /16
Internal volume [cm 3 ] Microchannel heat exchanger: charge retention modeling Internal volume: headers take a large portion of the internal volume, and can retain the majority of refrigerant charge Microchannel tube charge retention: 35 3 5 15 1 Not very sensitive to selection of void fraction correlation? May be affected by maldistribution? Header charge retention: 5 Mass flux/flow regime Gravity Protrusion Headers MC tubes 58 61 315 8 18 173 I.D. Cond O.D. HX Evap July 11 14, 16 Purdue Conferences 11 /16
Compressor, accumulator: simplified model by fitting with data Compressor: Calculate shaft work, refrigerant mass flow rate, and discharge state from suction state, discharge pressure, and compressor speed Volumetric efficiency 1/ n poly Pdis volm 1 cv 1 Psuc Isentropic efficiency isen Pdis.37.577 P Q volm Heat loss from shell loss suc a( Tdis Tamb) b( Tsuc Tamb) Accumulator: m ref Calculate suction quality or superheat based on refrigerant charge amount left in the accumulator, hll, liq Cd, pinhole Apinhole P f hll P P P P P dyn fri res dyn G f Utube L D 1 C d, res g G P Utube g G Utube g hydr P P hyd Cd, res f P gh f ll res.94 3.51 6 Re D July 11 14, 16 Purdue Conferences 1 /16
System heating performance modeling results Good prediction of Q and HPF trends with varying working conditions ~1% over prediction of Q, primarily due to unaccounted maldistribution in outdoor heat exchanger (details can be found in paper) W[kW], Q[kW], HPF 4 3 1 3 4 5 6 7 8 9 Indoor air flow rate [kg/min] N cp =146[rpm], T outdoor = C, v ohex =4 m/s, T indoor =1 C Q_heating_meas Q_heating_mdl W_shaft_meas W_shaft_mdl HPF_meas HPF_mdl 1 15 Compressor speed [RPM] T outdoor = C, v ohex =4 m/s, m indoor =6 kg/min, T indoor =1 C 4 3 1 1 3 4 5 Outdoor air face velocity [m/s] N cp =146[rpm], T outdoor = C, m indoor =6 kg/min, T indoor =1 C W[kW], Q[kW], HPF W[kW], Q[kW], HPF 5 4 3 1 July 11 14, 16 Purdue Conferences 13 /16
Refrigerant mass [g] Refrigerant charge retention modeling results I 5 15 1 5 Model ran at the same operating conditions as experiments HP: T id = 1 [C], T od = 1 [C], mfr id = 6 [kg/min], Vel cai = 4 [m/s], SC = 6 [C], m charge = 181 [g] A/C: T id = 4 [C], T od = 4 [C], Vel eai =.5 [m/s], Vel cai = 3.7 [m/s], SC = 8.4 [C], SH = 18. [C] Indoor condenser: under prediction of charge in both HP and A/C Single phase inlet and outlet header at bottom, liquid can get trapped in inlet header Small intermediate header (~14 cm 3 ) Inlet and outlet at the same side, channels far away from inlet/outlet may get clogged by liquid refrigerant oil mixture HP measured A/C measured 96 7 7 79 45 7 13 18 I.D Cond O.D. HX Evap July 11 14, 16 Purdue Conferences 7 modeled in MC tubes 8 71 75 14 /16
Refrigerant mass [g] Refrigerant charge retention modeling results II 5 15 1 5 HP measured A/C measured 96 7 7 79 45 7 13 18 I.D Cond O.D. HX Evap July 11 14, 16 Purdue Conferences 7 modeled in MC tubes 8 71 75 Outdoor heat exchanger 38 g (7%) underestimation of imbalance: Header retention model accuracy channels clogged by liquid refrigerant oil mixture In A/C mode as a condenser, most of the charge resides in nd pass and outlet header Reverse flow direction in A/C mode to reduce nd pass and outlet header size Use the extra charge in HP mode, i.e., for flash V [cm3], m [g] gas bypass 1 1 8 6 4 41 internal volume HP mode 6 A/C mode 71 41 1 11 14 8 6 17 1 19 68 69 6 I.H. 1 1st pass O.H. 1 I.H. nd pass O.H. 15 /16 5 9
Summary and Conclusion A mobile AC/HP system based on Nissan Leaf configuration was studied System charge imbalance when switching ~3 g in total, ~15 g caused by inappropriate liquid line sizing, ~14 g caused by outdoor heat exchanger changing role Liquid line sizing Liquid line may work as discharge line or suction line in another mode, line size should be constrained by acceptable COP/HPF and Q reduction Minimize liquid line size for acceptable subcooling decrease Outdoor heat exchanger caused ~14 g of imbalance when switching modes As a condenser, most of charge resides in the nd pass and outlet header, reversing refrigerant flow direction can help to reduce charge in A/C mode Charge retention difference can be used for flash gas bypass in HP mode, to send only liquid to outdoor heat exchanger for better heat transfer performance July 11 14, 16 Purdue Conferences 16/16
Lili Feng lfeng8@illinois.edu July 11 14, 16 Purdue Conferences