Thermodynamic comparison and Dynamic Simulation of Direct and Indirect Solar ORC system with PCM storage Jahan Zeb Alvi, Muhammad Imran, Gang Pei*, Jing Li, Guangtao Gao, Junaid Alvi
Outline Introduction Operating and boundary conditions Thermodynamic modelling Validation of the Computational Model for PCM Climatic Data of Islamabad-Pakistan Results, Analysis and Discussions Conclusions
Introduction A thermodynamic comparison between a novel direct solar ORC system (DSOS) and indirect solar ORC system (ISOS) is carried out in this study. A phase change material (PCM) heat storage unit is integrated with both systems to ensure the stability of power generation. Water and R245fa are selected as a heat transfer fluids (HTFs) for ISOS and DSOS respectively. However, R245fa is used as working fluid for both systems. Weekly, monthly and annual dynamic simulations are carried out to compare the performance of both systems using hourly weather data of Islamabad, Pakistan. MATLAB programming environment is used to simulate both solar ORC systems associated with latent heat thermal storage and ORC unit under time-varying solar radiation conditions.
Direct Solar ORC system V 2 V 3 V 4 G Solar collectors V 1 P 1 PCM storage Condenser V 5
Indirect Solar ORC system V 2 V 3 V 4 PCM storage Expander G Solar collectors V 1 Evaporator P 2 P 1 Condenser V 5
Operating and boundary conditions The operation modes of storage system are divided into charging and discharging mode. The minimum threshold level of solar radiation system start up, is selected to be 4 W/m 2 otherwise system stops or undergoes to discharging process. The initial temperature of PCM is selected to be 373.15 K. This depicts that PCM is in solid phase at the beginning of simulation process. The PCM storage system is designed to work at melting point temperature of the PCM. It means that major part of energy is released or absorbed at melting point of PCM. HTF mass flow rate during charging mode is selected to be 3 kg/s and it increases with increment in collector outlet temperature. However, HTF bypass mass flow rate is kept constant at rate of.5 kg/s in both charging and discharging mode. Discharging limit of the storage tank is maintained to 37K which means that the system is allowed to discharge the storage in sensible heat region.
Thermodynamic modelling The ORC efficiency is defined by the ratio of the net power output to the heat supplied ORC W Q net ORC The overall electricity efficiency of the solar ORC is expressed by CF. inc sys ORC cl Increment in capacity factor of the systems is calculated by relative increment in working hours by use of PCM storage. Wh Wh w,pcm wo, pcm Wh w,pcm
Validation of the Computational Model of PCM storage Temperature ( o C) 6 55 5 45 4 35 3 Theoratical result 1 2 3 25 2 15 Experimental Result (Zivkovic) 1 2 3 4 5 6 Time (seconds)
Climatic Data of Islamabad- Pakistan Solar radiation received at collector surface (kwh/m 2 /day) Ambient Temperature ( o C) 8 35 7 3 6 25 5 2 4 3 15 2 1 1 5 1 2 3 4 5 6 7 8 9 1 11 12 Time (month)
Temperature( o C) Solar radiations (W/m 2 ) Hottest week of the year Hourly average daily variation in PCM and HTF temperature during charging mode for ISOS Solar radiations HTF(water) PCM 39 12 385 1 38 8 375 6 37 4 365 2 36 8 9 1 11 12 13 14 15 16 Time (hr)
Temperature ( o C) Solar radiation (W/m2) Hottest week of the year Hourly average daily variation in PCM and HTF temperature during charging mode for DSOS Solar radiations PCM HTF (R245fa) 388 12 1 383 8 378 6 4 373 2 368 9 1 11 12 13 14 15 Time (hr)
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 11 13 15 Temperature ( o C) Hottest week of the year Hourly average variation in temperature of PCM and HTF during discharging mode for ISOS PCM HTF (water) 375 37 365 36 355 35 345 34 335 33 Length of PCM storage tank (nodes)
Temperature ( o C) Hottest Week of Summer Hourly average variation in temperature of PCM and HTF during discharging mode for DSOS PCM HTF (R245fa) 38 37 36 35 34 33 32 31 3 1 4 7 1 13 16 19 22 25 28 31 34 37 4 43 46 49 52 55 58 61 64 67 7 73 76 79 82 85 88 91 94 97 1 13 16 Length of storage tank (nodes)
System Efficiency (% ) Net Power (kw) Hottest Week of Summer Overall system efficiencies and power output 14 12 1 8 6 4 2 DSOS pow ISOS pow DSOS eff ISOS eff 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 21 22 23 24 Time(hr) 16 14 12 1 8 6 4 2
System Efficiency (%) Net Power (kw) Coldest Week of winter Overall system efficiencies and power output ISOS pow DSOS pow ISOS eff DSOS eff 7 8 6 7 5 4 6 5 4 3 3 2 2 1 1 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 21 22 23 24 Time (hr)
Performance during every month of the year Net Power (kw/day) System efficiency (%) Overall system efficiencies and power output ISOS Power DSO Power ISOS eff DSOS eff 16 9 14 8 12 1 8 6 4 7 6 5 4 3 2 2 1 1 2 3 4 5 6 7 8 9 1 11 12 Time (months)
Performance during every month of the year Energy stored (MJ/day) Amount of energy stored during charging mode ISOS DSOS 5 45 4 35 3 25 2 15 1 5 1 2 3 4 5 6 7 8 9 1 11 12 Time (months)
Performance during every month of the year Power (kw/day) Power transferred to HTF during discharging mode ISOS DSOS 12 1 8 6 4 2 1 2 3 4 5 6 7 8 9 1 11 12 Time (months)
Performance during every month of the year Power (kw/day) Increment in capacity factor ISOS DSOS 12 1 8 6 4 2 1 2 3 4 5 6 7 8 9 1 11 12 Time (months)
Conclusions ISOS has shown 1.71% system efficiency and able to provide 34.2 kw/day power while DSOS has shown 4.5 times higher system efficiency and 2.8 times higher power on annual basis. Average annual amount of energy stored by PCM during charging phase for ISOS is 4.24 MW/day higher than DSOS. However, in comparison with ISOS, DSOS has delivered 33.8 kw/day more power to HTF during discharging phase of the PCM on annual basis. Maximum benefits of PCM storage are observed during the summer season compared to the winter season at selected operating conditions. Furthermore, average annual increment in capacity factor by using PCM storage are found to be 21.71% and 17% for DSOS and ISOS respectively