HPC 2017 Performance simulation and exergy analysis on multi-stage compression high temperature heat pumps with R1234ze(Z) refrigerant

Transcription

1 Performance simulation and exergy analysis on multi-stage compression high temperature heat pumps with R1234ze(Z) refrigerant Bin Hu, Di Wu, R.Z. Wang Institute of Refrigeration and Cryogenics Shanghai Jiao Tong University

2 Outlines Introduction Multi-stage compression heat pump systems System simulation and exergy analysis Results and discussion Conclusions

3 Introduction 250 C Low-grade thermal energy 30 C 3

4 Introduction Compression heat pumps, absorption and adsorption heat pumps and chemical heat pumps are promising technologies for low grade heat recovery. Compression heat pumps has been widely applied in process industries such as lumber drying, food and beverage production, dyeing process, district heating and crude oil heating. Usually, industrial process and applications require heat pumps with higher output temperature. High-temperature heat pumps (HTHP) offer a most practical solution to this problem. 4

5 Introduction The HTHP recover heat from waste water and produce hightemperature hot water. Chamoun et al. developed a dynamic model for an industrial heat pump using water as refrigerant and the reported temperature range was around C. However, in order to achieve a high output temperature, the compression ratio of single-stage heat pump is very high, which lowers the compression efficiency and degrades the heating capacity and system COP. As a result, a multi-stage compression heat pump system is required to overcome those problems. 5

6 Introduction Compared to single-stage compression HPs, the temperature lift of multi-stage compression HPs is bigger and the compression efficiency is higher, so it is possible to achieve higher COP. Although there are some investigations on multi-stage compression heat pump systems, it is still lack of studies on waste heat recovery industrial heat pump with high temperature lift, especially with low GWP refrigerants. 6

7 Outlines Introduction Multi-stage compression heat pump systems

8 Two stage compression system 8

9 Two stage compression system Process 10 1: The liquid refrigerant in the evaporator absorbs heat from the waste water and vaporizes. Process 1 2: The super-heated refrigerant vapor is compressed to an intermediate temperature and intermediate pressure gas by the lower-stage compressor. Process 2(8) 3 4: The discharge gas from the lower-stage compression is mixed with the intermediate pressure vapor refrigerant from the flash tank. The mixed vapor enters the upper-stage compressor for the second-stage compression. Process 4 5: The high temperature and pressure refrigerant gas from upper-stage compression flows into the condenser where it exchanges heat with the water from the water supply system and becomes liquid refrigerant. Process 5 6: The saturated liquid refrigerant is further cooled down in the subccoler. Process 6 7: The high pressure refrigerant from the subcooler is throttled by the upperstage expansion valve and becomes a liquid-gas mixture of intermediate pressure. Process 7 8(9): The refrigerant mixture is then separated into liquid phase and vapor phase in the flash tank with intermediate pressure. Process 9 10: The liquid refrigerant is further throttled by the lower-stage expansion valve and becomes a liquid-gas mixture of low pressure and low temperature. Finally, the low pressure refrigerant mixture flows back to the evaporator where it absorbs heat from waste water and vaporizes for next cycle. 9

10 Three stage compression system 10

11 Three stage compression system Three-stage compression heat pump system : Ø the refrigerant from evaporator is compressed in the first-stage and then mixed with the vapor refrigerant of the first-stage pressure. The mixed vapor enters the compressor for the second-stage compression. Ø After mixing with the vapor refrigerant of the second-stage pressure, the mixed refrigerant is further compressed in the third-stage. Ø And then the refrigerant is cooling down in the condenser and flows through the thirdstage expansion valve. Ø The refrigerant is separated into liquid phase and vapor phase in the flash tank П. The vapor refrigerant of second-stage pressure is mixed with the discharge gas from the secondstage compression. Ø The liquid refrigerant is further cooling down in the subccoler and enters the secondstage expansion valve. The refrigerant is separated again in the flash tank І. Ø The vapor refrigerant of first-stage pressure is mixed with the discharge gas from the first-stage compression. Ø The liquid refrigerant enters the first-stage expansion valve and then is heated by the waste heat in the evaporator. 11

12 Outlines Introduction Multi-stage compression heat pump systems System simulation and exergy analysis

13 System simulation and exergy analysis Assumptions Degree of subcooling: 20 o C; Degree of superheating: 5 o C; Isentropic efficiency is calculated as a function of the pressure ratio; Condensing temperature is 5 o C higher than hot water temperature(120 o C); Evaporating temperature is 10 o C lower than the waste heat source temperature; The initial intermediate pressures are selected to result in equal pressure ratios across the compression stages to minimize the compressor power; Vapor refrigerant is injected to the next compression process, and the injection amount is controlled to maintain the discharge temperature after compression. Start Given values: T ww, T c, ΔT e, SH, SC, η is, m ww Guess values: P in,i Calculate: m in,vapor,i, m total T d -T set,d <0.005 Multi-stage heat pump: W, Q, COP End Yes No 13

14 System simulation and exergy analysis According to the second law of thermodynamics, exergy analysis equations are show as following: E + E + E = E + E + I heat, in mass, in work heat, out mass, out rr For the compression process: Compressor work, Electrical power, So, exergy loss, = ( h h0) T0( s s0) W = m( h h ) c out in W = W el c m e I = m( ) + W = m[( h h ) T ( s s )] + W comp in out el in out 0 in out el 14

15 System simulation and exergy analysis For condensation process: I = m ( ) + m ( ) cond R1234ze R1234 ze, in R1234 ze, out hw hw, in hw, out For evaporation process: I = m ( ) + m ( ) evap R1234ze R1234 ze, in R1234 ze, out ww ww, in ww, out For subcooling process: I = m ( ) + m ( ) sub R1234ze R1234 ze, in R1234 ze, out hw hw, in hw, out For expansion process: I = m ( ) = m T ( s s ) exp R1234ze in out R1234ze 0 out in Total exergy destruction I = I + I + I + I + I total comp cond evap exp sub 15

16 System simulation and exergy analysis Exergy efficiency is defined as the ratio of the total exergy increasement to the total power input of multi-stage compression heat pump. = ( ) E E W x hw, out ww, in el The exergy output: E = m ( ) = m [( h h ) T ( s s )] hw, out hw hw, out hw, in hw hw, out hw, in 0 hw, out hw, in Total exergy input: E = m ( ) = m [( h h ) T ( s s )] ww, in ww ww, in ww, out ww ww, in ww, out 0 ww, in ww, out 16

17 Outlines Introduction Multi-stage Compression Heat Pump Systems System simulation and exergy analysis Results and discussion

18 Results and discussion Fig. 4 Variation of total power consumption The power consumption decrease from 180 kw to 70.5 kw for single-stage compression system. For the two-stage and three-stage compression system, the total power consumption decrease from 170 kw to 68.4 kw and from 163 kw to 65.7 kw. Fig. 5 Variation of system COP Compared with single-stage compression system, the COP improvements of twostage compression system are 12.2% for 50 o C waste heat source temperature. The COP improvements of three-stage compression system are 19.8% for 50 o C waste heat source temperature. 18

19 Results and discussion Table 1 Simulation results of R1234ze(Z) heat pumps. Items Unit Single-stage Two-stage three-stage W com kw Q con kw Q eva kw COP m total kg s m in,1 kg s m in,2 kg s P suc Bar P in,1 Bar P in,2 Bar P dis Bar This table also shows the mass flow rate and pressure of each stage in the compression process. The reduced pressure ratio results in compressor work reduction, and finally results in COP improvement. 19

20 Results and discussion For single-stage compression system, exergy efficiencies decreased 18.3% For two-stage compression system, exergy efficiencies decreased 19.2%. For three-stage compression system, exergy efficiencies decreased 20.2%. Fig. 6 Variation of exergy efficiency with waste heat source temperature The exergy destruction of compression and expansion process became less and less serious. The total exergy destruction of the system is decreased while the exergy loss of condensation and evaporation process is a constant because of the fixed heat transfer approach temperature. 20

21 Results and discussion compression processes condensation processes expansion processes evaporation processes 21

22 Results and discussion The exergy destruction ratio of compression and condensation processes take up a large proportion. While the expansion and evaporation processes account for 13% and 12%, respectively. The simulation method is able to reflect the system performance and predict the exergy destruction for different heat pump process. 22

23 Outlines Introduction Multi-stage compression heat pump systems System simulation and exergy analysis Results and discussion Conclusions

24 Conclusions As the waste heat source temperature increased from 50 C to 90 C, system COP increases from 3.5 to 6.98 for twostage compression system and from 3.74 to 7.14 for threestage compression system, respectively. As the stage number increased for the same waste heat recovered, multi-stage compression heat pump has less power consumption. When the three-stage compression heat pump is applied, the COP improvement is 16.4% under 60 C waste heat source temperature conditions.

25 Conclusions The improvements of exergy efficiency are 6.9% and 11.8% for two-stage and three-stage compression systems when compared with single-stage compression system. With the waste heat source temperature increasing, the exergy destruction ratio of compression and expansion processes decreased, while that of condensation and evaporation processes increased. For the same operating conditions, three-stage compression heat pump has the minimum exergy destruction of compression and expansion process.

26 Thank you!