Investigation on effects of piping on heating performance of multi-split variable refrigerant flow system

Transcription

1 12 th IEA Heat Pump Conference, 15 th -18 th May, 2017, Rotterdam Investigation on effects of piping on heating performance of multi-split variable refrigerant flow system Ziai Li Department of Building Science, Tsinghua University, Beijing May 16,

2 2 Contents Background Motivation and method Results and discussion Conclusions and outlook

3 Multi-split variable refrigerant flow (VRF) system Outdoor units Refrigerant pipe ü Space saving ü Fewer transportation loss ü Efficient part-load operation ü Individual control of indoor units ü Easy maintenance and management Widely applied in east Asian and European countries Sales of VRF units shared larger than 40% of China market of central air-conditioning products in 2016 Indoor units Office Hospital Hotel Villa Types: Air-cooled heat pump type, heat recovery type, water-cooled type, gas engine heat pump type, etc. Configuration: ODU up to 70kW, IDU number up to 60, outdoor-to-indoor pipeline length up to 149m, height difference up to 46m 3

4 Issue about piping of VRF system in application Performance degradation caused by piping (L and H) in application Field cooling performance of multi-split VRF systems in one office building in Beijing, 2005 (Note: applying centralized control method) 4.0 Refrigerant pipe L 1 H L pipe =L 1 +L 2 L 2 System COP First floor Fifth floor Seventh floor Fourth floor Seventh floor: average COP =3.32 Fifth floor: average COP=3.11 First floor: average COP=2.92 0% 20% 40% 60% 80% 100% Part-load ratio Concern on piping effect on VRF system performance Data resource: Department of building science, Tsinghua University. Reports on public building energy saving diagnosis in the summer of

5 Issue about piping of VRF system in application Performance degradation caused by piping (L and H) in application 3 3 ΔP EEV0 ΔP LP ΔP EEV1~N 2 2 ΔP GP ΔP ΔP EEV1~N LP ΔP EEV ΔP GP R410A, cooling heating mode Cooling mode: L pipe à P suc and Tsh suc à ηv cp, mr cp, Qe and COP Heating mode: L pipe à T cond à Qc and COP Cooling/Heating mode: H à Insufficiency of ΔP of indoor EEVs or overpressure at the entry of indoor EEVs 5

6 Research status of piping effect of VRF system Reference Approach Control method Condition Highlights W. Shi, et al (2007) K. Zheng, et al (2007) Z. Guo (2008) X. Wang (2010) D. Zhou, et al (2011) Simulation, R22 Field test, R22 Experiment, R410A Simulation, R410A Simulation, R410A Y. Pan et Simulation, al. (2012) R22 Z. Li, et al (2016) Simulation, R410A Constant outlet superheat of evaporators Not mentioned Not mentioned Constant suction pressure and continuous operation of indoor units Constant suction superheat and continuous operation of indoor units Constant outlet superheat of evaporators Constant outlet superheat of evaporators Rated cooling/heating condition (given fixed compressor speed) Field cooling/heating condition Variable cooling conditions (given different active indoor units) Rated cooling condition (given certain cooling capacity) Rated cooling condition (given certain cooling capacity) Rated cooling condition (given fixed compressor speed) Rated cooling condition (given fixed compressor speed) The cooling and heating COP decrease 25% and 11% respectively at pipe length of 100m The cooling capacity degradation is mainly caused insufficiency of ΔP for indoor EEV caused by pressure drop along liquid pipe while the heating capacity degradation is mainly caused by the heat leakage to environment along gas pipe The effect of pipe length on performance under part-load cooling conditions is smaller than that under full load condition The cooling COP decreases as pipe length increases without bypass loop while it is almost not affected by pipe length when suction superheat is controlled by bypass loop The effect of pipe length on performance under part-load cooling conditions is less than that under full load condition The highest COP can be obtained when indoor units are located equally from the outdoor unit The cooling capacity reduces by 14% and the COP reduces by 15% as the main pipe length increases from 10 m to 190 m 6

7 7 Research status of piping effect of VRF system Brief summary The available simulation work focuses on pipe length effect on the cooling performance of VRF system given fixed compressor speed. There is lack of study on the piping effect on its heating performance given various heating load The control method of the simulated systems is either indefinite or unpopular currently The simulation assumed a constant subcooling degree of condenser exit instead of using refrigerant mass charge conservation under part-load conditions The experiment and field test study need more detailed information of control method of the tested system to analyse the piping effect under various operation conditions This study works on the effect of pipe length and height difference on VRF system performance under part-load heating conditions with the current heating control method based on simulation

8 8 Contents Background Motivation and method Results and discussion Conclusions and outlook

9 Current heating control method of VRF systems Indoor unit Outdoor unit Actuator Control Target / Set value Indoor fan User control Rated air volume EEV1~N Compressor speed Automatic Automatic Target air temperature T ai =20 & indoor HEX exit subcooling degree ΔT sc 1 Constant discharge pressure set value T sat (P dis, cp set )=54 Outdoor fan Automatic Rated air volume EEV0 Automatic Compressor suction superheat set value ΔT sh, suc, set EEV0 Secondary liquid pipe EEV1 Outdoor heat exchanger Main liquid pipe Indoor heat exchanger #1 EEV2 Four-way valve Compressor Main gas pipe Indoor heat exchanger #2 EEVN Gas-liquid separator Outdoor unit Secondary gas pipe Indoor unit Indoor heat exchanger #N 9

10 Methodology Establishment of steady-state heating model of VRF system Validation of refrigerant pipe model Component design of a VRF system Simulation on heating performance using constant discharge pressure control method (variable pipe length and height difference) Model simplification Regardless of the effect of lubricating oil Inlet refrigerant of gas-liquid separator is superheated and no refrigerant is accumulated No frosting occurred out of the outdoor heat exchanger 10

11 Model development Variable speed compressor: Efficiency model [1] m = nv h / v cp th v i,cp W = m ( h - h )/ f cp cp o,cp i,cp loss ho,cp s -hi,cp ho,cp = hi,cp + hs h = f ( P, P ) h s = f2( Pi,cp Po,cp v 1 i,cp o,cp, ) Fin-and-tube evaporator, condenser: One-dimensional distributed-parameter model [2] EEV: Correlation for the variable area expansion device m = C A() z 2 r ( P -P ) hi,eev eev D i,eev i,eev o,eev = h CD = ri,eev / ro,eev o,eev f Pressure drop along refrigerant pipeline : Two-phase flow empirical model 2 dpf f J Single phase[3]: -( ) = dl 2D r Two-phase [4]: ( ) [1] R. Koury, L. Machado, K. Ismail, Numerical simulation of a variable speed refrigeration system, International Journal of Refrigeration 24 (2001) [2] Shao S, Li X, Shi W, et al. A universal simulation model of air-cooled condenser consisting of plate-fin-tube[c]//asme 2003 Heat Transfer Summer Conference. American Society of Mechanical Engineers, 2003: [3] S. W. Churchill, Friction-factor equation spans all fluid-flow regimes. Chemical Engineering 84: i 1/ f = 8é 8/ Re + K - ù ë û é e ù K = ( ) - ê2.457 ln(( ) ) ú Re ë Re Di û dp dl f 2D 2 f L0 -( ) = i J r fl0 = 8c Re -k 2 2 (2-q)/2 (2-q)/2 (2-q) L0 = 1 + ( Y -1)[ bx (1- x) + x ] Potential pressure drop: dp r - = - - L f z G ( ) rl g {1 a(1 )} dz rl 2 L

12 Validation of refrigerant pipe model Indoor unit Compressor T Evaporator P P T T Gas-liquid separator Gas pipe P P T EEV Liquid pipe 5 HP variable-speed compressor Indoor unit was located 30 m higher 100-meter-long liquid pipe and a 100-meterlong gas pipe Wrapped with 18 mm insulation Refrigerant mass flow rate is calculated based the suction and discharge refrigerant state of the compressor by employing the compressor efficiency model P T P T Condenser Outdoor unit Experimental data centre room air-conditioner for validation of refrigerant pipe model 12

13 13 Validation of refrigerant pipe model Ambient temperature ( o C) Compressor frequency (Hz) Measurement Simulation Deviation of simulation Suction temperature ( o C) Suction pressure (MPa) Discharge pressure (MPa) Gas pipe pressure drop (MPa) Liquid pipe pressure drop (MPa) Refrigerant mass flow rate (kg/h) Gas pipe pressure drop (MPa) Liquid pipe pressure drop (MPa) Gas pipe pressure drop Liquid pipe pressure drop % 0.8% 16.2% -12.0% -21.4% -1.3% -20.6% -19.2% -26.4% -3.0% 1.4% 8.6% 7.4% 10.6% 14.6% 13.2% 13.4% 14.6%

14 Solving procedure for VRF system heating simulation Start Inputting system configuration, operating condition, Mass_charge, ΔTsh o,e,set, P o,cp,set, Q c,1 ~Q c,n Assuming f cp, P i,cp, P o,mlp Calculating m cp, W cp, T o,cp by compressor model Calculating P i,e, h i,e, x eev0 by evaporator model and EEV model Assuming m c, j, h o,slp, j Calculating P i,slp,j, h i,slp,j, P o,sgp,j, h o,sgp,j by liquid refrigerant pipe and gas refrigerant pipe models Calculating P i,mlp, h i,mlp, P o,mgp, h o,mgp by liquid refrigerant pipe and gas refrigerant pipe models, j=1 Adjusting m c, j Calculating P o,c, j, h o,c, j, x eevj by condenser model and EEV model No e Q <e Q '? j=n? Yes No j=j+1 Adjusting f cp Adjusting P i,cp No No Yes e m <e m '? Yes e h <e h '? Yes Adjusting P o,mlp No e mass <e mass '? [5] Li Z, Wang B, Li X, et al., Simulation on effects of subcooler on cooling performance of multi-split variable refrigerant Yes flow systems with different lengths of refrigerant pipeline, Energy and Buildings, 126 (2016) End 14

15 15 Contents Background Motivation and method Results and discussion Conclusions and outlook

16 Configuration of VRF system for simulation Compressor Rated nominal cooling performance (pipe length=10m) Indoor HEX Outdoor HEX Displacement, V th Refrigerant 85 cm 3 /rev R410A Quantity 1 Rotation speed Rated rotation speed Refrigerant charge (circulating) Rated cooling capacity 20~100 rps 60 rps 3.0 kg 28 kw Rated suction & discharge pressure T sat,suc =5.2, T sat,dis =47.6 Rated cooling capacity 7.0 kw Quantity 4 Rated heat exchange capacity 35.4 kw Quantity 1 EEV Nominal diameter of EEV0/EEV1~N 6.0 mm/ 2.4 mm Main pipe External diameter Do of main liquid/ gas pipe Addition of refrigerant charge 12.7mm(liquid), 25.4mm(gas) 0.11 kg/m 16

17 17 Results and discussion Compressor frequency (Hz) Effect of main pipe length (T ai =20, T ao =7, RH ao =50%, T sat (P dis, cp set )=54 ) Compressor frequency Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW Inlet pressure of condensers Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW Pressure (MPa) COP (W/W) COP Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW Suction saturation temperature of compressor Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW Temperature ( o C) Length of main gas pipe and main liquid pipe (m) Length of main gas pipe and main liquid pipe (m) As L MGP and L MLP increased from 5 m to 165 m, Compressor frequency increases by 3~5 Hz to output the same heating capacity Te reduces by about 0.5 o C P in of condensers reduces by MPa (about 1 o C of Tc drop) at Qc=26kW COP comp decreases from 3.28 to 3.11 at Qc=26 kw

18 18 Results and discussion Compressor frequency (Hz) Effect of outdoor-to-indoor height difference (T ai =20, T ao =7, RH ao =50%, T sat (P dis, cp set )=54 ) Compressor frequency Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW Pressure difference of EEV0 Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW Pressure difference(mpa) COP (W/W) COP Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW Outlet subcooling degree of condensers Qc=26.0kW Qc=23.6kW Qc=21.2kW Qc=18.8kW Subcooling degree ( o C) Height difference between the outdoor unit and indoor units (m) Height difference between the outdoor unit and indoor units (m) Compressor frequency and COP comp show little change (tiny decrease of COP comp caused by length increase) while ΔTsc of condensers increase slightly Available ΔP for EEV0 decreased markedly (decreased from 2.25 MPa to 1.55 MPa at Qc=26 kw), affecting adjustability and system stability

19 19 Contents Background Motivation and method Results and discussion Conclusions and outlook

20 20 Conclusions and outlook Conclusions A model has been established for heating operation of multi-split VRF system applicable to analysis of piping effect and control method According to simulation of a designed VRF system under the constant discharge pressure control method: (1) Lengthening of horizontal pipe will reduce COP apparently in heating mode even under part-load conditions (2) Height difference in range from -50 m to 50 m had low effect on COP but it would affect the available pressure drop of EEVs Outlook Application of the developed model to study the effect of piping under more operation conditions and different control methods, aiming at a comprehensive evaluation on the application of multi-split VRF systems in buildings

21 THANKS FOR YOUR ATTENTION!