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AVL qpunkt Deutschland GmbH
Public
Efficiency of Single-Stage & 2-
Stage Flash Tank Heat Pump
Systems for Electric VehiclesBen Banney
| | 21 September 2018 | 2Public
Contents
1.Simulation Goals
2.Systems simulated / Methodology used
3.Results
4.Conclusions
5.Further Work
| | 21 September 2018 | 3Public
1.Simulation Goals / 2.Methodology
GOAL: To examine the potential for 2-Stage Heat Pump Systems in electric vehicles.
SYSTEMS:
• Single Stage Heat Pump System with and without Internal Heat Exchanger (IHX)
• 2-Stage Heat Pump System with phase separating tank; with and without IHX
METHODOLOGY:
1. Comparison of the above systems for varying heating power requirements
• Boundary Conditions: 20lpm/60°C coolant inlet temperature (heater circuit); 0.5kg/s / -10°C ambient air inlet temperature to the Evaporator
2. Examination of the effect of higher DTs (heater circuit -> ambient temperature) on COP for 1- and 2-Stage Systems -> Assumption of higher coolant inlet temperatures: 70 and 80°C in addition to the reference case of 60°C.
| | 21 September 2018 | 4Public
2.Methodology:1-Stage Heat Pump System Boundary Conditions and Control
Coolant 20lpm60°C
Air 0.5kg/s-10°C
Compressor170cc: GT-SUITE Library
Indirect Condenser
(optional)Internal Heat Exchanger - Master
(optional)Internal Heat Exchanger - Slave
Evaporator(Condenser in evaporating mode)
Compressor speed controlled for heating power
Control of superheat (2K) after Evaporator via XV
| | 21 September 2018 | 5Public
2.Methodology:2-Stage Heat Pump System Boundary Conditions and Control
Coolant 20lpm60°C
Air 0.5kg/s-10°C
Compressor 1170cc: GT-SUITE Library
Indirect Condenser
(optional)Internal Heat Exchanger - Master
(optional)Internal Heat Exchanger - Slave
Evaporator(Condenser in evaporating mode)
Vap.
Liquid
Compressor 2170cc: GT-SUITE Library
Compressor speeds controlled for heating power-same speed for both compressors
Control of intermediate pressure level via XV such that:PR_comp_1= PR_comp_2(PR = Pressure Ratio)
Control of superheat (2K) after Evaporator via XV
| | 21 September 2018 | 6Public
2. Methodology: Components
Evaporator: modified condenser from a GT-SUITE Template: Tube and Fin 380x530x16
- Modified to for an expanding (evaporating) flow from inlet to outlet and for reduced refrigerant pressure losses
Internal Heat Exchanger (IHX): – Shell and Tube Heat Exchanger
Ø 21/16mm Shell / Tube
Indirect Condenser (iCond):Plate Heat Exchanger, 50 plates: 300 x 80mm
Compressor: 170cc: GT-SUITE Library. 2-Stage cases use this same compressor for high and low pressure duties.
| | 21 September 2018 | 7Public
3.Results: COP/Compressor Speed Single-Stage Systems with & w/out IHX
Heating powers >4,5kW not possible since the max. compressor speed (4000rpm) is reached
Note: COPs based on Fluid Powers -> motor/frictional losses for the compressor are not taken account of!
| | 21 September 2018 | 8Public
3.Results: COP/Compressor Speed Single-Stage Systems with & w/out IHX
Heating powers <4,5kW not possible since the min. compressor speed (850rpm) is reached
Note: COPs based on Fluid Powers -> motor/frictional losses for the compressor are not taken account of!
| | 21 September 2018 | 9Public
3. Results: 2 St. Systems: Why is the COP higher w/out IHX?
Operating point for compressor_1 almost identical-> with or without IHXThis is true for the reduced mass flow, but due to the changed temperatures, the actual mass flow is approx. 10% less with IHX.
Without IHX
With IHX
X
X
| | 21 September 2018 | 10Public
3. Results: 2 St. Systems: Why is the COP higher without IHX?
Dhcomp1=36.95 kJ/kg Dhcomp1=43.87 kJ/kg
ሶ𝑚refrig=67.73 kg/h
ሶ𝑚refrig=60.96 kg/h
Lower refrigerant mass flow (low pressure circuit) and higher Dhcompressor1 for the case with IHX lead to a lower heat take up and higher compressor power with IHX.
QEvaporator=2.382 kW
QEvaporator=2.161 kW
PComp1=0.695 kW
PComp1=0.742 kW
Example for Heating Power 5.5kW
| | 21 September 2018 | 11Public
3. Results: 1 St. Systems: Why is the COP higher with IHX for 1 stage?
COP=1.52
COP=1.65
ሶ𝑚refrig=84.22 kg/h
ሶ𝑚refrig=66.78 kg/h
Despite a lower refrigerant mass flow with IHX the higher DhEvaporator due to Subcooling in the IHX leads to a higher heat take up and higher COP.
QEvaporator=1.186 kW
QEvaporator=1.380 kW
PComp1=2.291 kW
PComp1=2.117 kW
Example for Heating Power 3.5kW
| | 21 September 2018 | 12Public
3.Results: COP/Compressor Speed Single- and 2-Stage Systems w & w/out IHX
Note: COPs based on Fluid Powers -> motor/frictional losses for the compressor are not taken account of!
| | 21 September 2018 | 13Public
3.Results: COP=f(Coolant Inlet Temp.) Single- and 2-Stage Systems
Note: COPs based on Fluid Powers -> motor/frictional losses for the compressor are not taken account of!
| | 21 September 2018 | 14Public
4. Conclusions
• For a single stage system:
• COPs of approx. 1.5 to 1.75 (based on compressor fluid power) for heating powers of 3 – 4kW are simulated.
• An IHX adds an additional 10% in COP.
• For a two stage system:
• For the heating powers tested* higher COPs are possible than with a single stage system, even for the lowest system DTs assumed here (60°C heating circuit; -10°C ambient)
• In contrast to the single stage system, use of the assumed IHX leads to a COP reduction and not an increase.
• For higher temperature differences (heat source to heat sink – here 80 and 90K were tested additionally) 2-Stage systems are less susceptible to a reduction in COP than Single-Stage systems.
*There is a trend for 1 St. systems of increasing COPs with decreasing heating powers so the optimum would have to be found here in order to show that 2-Stage compression results in a higher max. COP for this hardware and these temperatures.
| | 21 September 2018 | 15Public
5. Further Work
It would be useful:
• for a given heating power requirement to find out whether a 2-Stage system with this hardware can deliver a better COP (for the 60°C heating circuit coolant and -10°C ambient temperature condition). This would also require finding an optimum compressor size for both systems.
• to test whether the optimum intermediate pressure results in the same pressure ratio for both stages (equal pressure ratios are used in this work) or whether alternative pressure ratios might be helpful.
• to test how independent the system performance is of refrigerant charge, and whether the charge used is adequate for other operating conditions.
• to find the minimum ambient temperature at which the heat pump operation is possible/practical.
www.avl.com
Thank YouKontakt
AVL qpunkt Deutschland GmbH
Niederlassung Ingolstadt
Straußenlettenstr. 15
D-85053 Ingolstadt
Mobil: +49 151 6160 6457
E-Mail: [email protected]
Web: www.qpunkt.at
Ben Banney
System-Simulation
Mobil: +49 151 4261 4541
E-Mail: [email protected]
Steffen Lerch
Abteilungsleitung Simulation