Energy, Conventional and Advanced Exergy Analysis of Low Temperature Geothermal Binary-ashing Cycle Using Zeotropic Mixtures

: Due to deep utilization of geobrine and high net power output, binary flashing cycle (BFC) is 14 deemed to be the future geothermal energy power generation technology. The BFC using 15 R245/R600a zeotropic mixtures is presented in this paper. The thermodynamic model of the system 16 is built, and energy, conventional and advanced exergy analysis are carried out, to reveal the real 17 optimization potential. It is demonstrated that the optimal composition mass fraction of R245fa and 18 dryness of working fluid at the evaporator outlet ranges are 0.30~0.50 and 0.40~0.60, considering 19 the thermodynamic performance and the flammability of the mixtures, simultaneously. 20 Conventional exergy analysis indicates that the maximum exergy destruction occurs in condenser, 21 followed by expander, evaporator, flashing tank, preheater, high-pressure pump and low-pressure 22 pump. While the advanced exergy analysis reveals that the expander should be given the first 1 priority for optimization, followed by condenser and evaporator. The BFC has a large potential for 2 improvement due to higher avoidable exergy destruction, about 48.6% of the total system exergy 3 destruction can be reduced. And the interconnections among system components are not very strong, 4 owing to small exogenous exergy destructions. It also demonstrates the effectiveness of advanced 5 exergy analysis, and the approach can be extended to other energy conversion systems to maximize 6 the energy and exergy savings for sustainable development. 7

11 11 12 12 m X m X m X  (9) 3 where the subscript "FT" denotes the flashing tank. Note that the flashing temperature is assumed 4 to be the average of evaporation temperature and condensation temperature [16]. 5 The power output of the expander is given by 6 where ηexp represents the expander isentropic efficiency; the subscripts "exp" and "s" denote 8 expander and isentropic, respectively. 9 The mass balance of the expander is given by where t is the temperature, o C; cp denotes specific heat capacity, kJ/(kg•K); the subscript "cf" 15 represents cooling water. 16 The mass balance of the condenser is given by The power consumption of the low-pressure pump is given by 20 where ηLPP represents the low-pressure pump isentropic efficiency; the subscript "LPP" represents low-pressure pump. 1 The mass balance of the low-pressure pump is given by 2 54 mm  where the subscripts "hf" and "pre" represent the geofluid and preheater, respectively. 7 The mass balance of the preheater is given by The power consumption of the high-pressure pump is given by 11 where ηHPP represents the high-pressure pump isentropic efficiency; the subscript "HPP" represents 13 high-pressure pump. 14 The mass balance of the high-pressure pump is given by The energy balance of the evaporator is given by 18 The mass balance of the evaporator is given by 20 where ηth represents the thermal efficiency. 6

Conventional exergy analysis 7
The exergy balance of the BFC is given by 8 where T0 is ambient temperature, o C; e is specific exergy, J/kg; ED is exergy destruction, W. 10 The specific entropy is given by 11 where h0 is specific enthalpy under ambient state, J/kg; s0 is specific entropy under ambient state, 13 J/(kg·K). 14 The exergy destruction for k-th component is given by The exergetic efficiency of the BFC is given by 11

Advanced exergy analysis 13
The exergy can identify the system inefficient components, nevertheless the interactions among 14 components and the actual energy saving potential are not taken into account. The advanced exergy 15 analysis considers the detailed interaction among components and intends to improve the quality of 16 the results achieved from conventional exergy analysis. 17 ED,K AV is avoidable exergy destruction, which can be reduced by technical progress, W. 1 The unavoidable exergy destruction is given by 2 The exergy destruction of the k-th component can also be divided into endogenous and exogenous 4 parts. The endogenous exergy destruction is associated with the irreversibility of the k-th component 5 itself, while the exogenous exergy destruction is related to the irreversibility of other components. 6 where ED,K EN is endogenous exergy destruction, W; ED,K EX is exogenous exergy destruction, W. 8 In combination with the two splitting methods, the exergy destruction can be divided into four parts 9 as follows: 10 where ED,K AV,EN , ED,K AV,EX , ED,K UN where Cmin is the minimum inerting volume concentration, %; vmax is the maximum flame 5 propagation velocity of the flammable working fluid, m/s; Cst is stoichiometric concentration,%； 6 Φ is the suppression coefficient of flammable retardant refrigerant. 7 The suppression coefficient of flammable retardant refrigerant can be obtained by group 8 contribution method. The suppression coefficient of R245fa is 0.29. 9 The minimum inerting mass concentration of R245fa in R245fa/R600a mixtures can be calculated

Model verification 1
The comparison between the results of the developed model and the ones in the literature is 2 conducted to demonstrate its accuracy. The comparison under the identical operational conditions 3 and working fluid is presented in Fig. 2, where it can be observed that the numerical prediction is 4 close to the referenced data. As can be seen, only small differences exist between the results, indeed, 5 the largest relative error of thermal efficiency is lower than 1.30%. It indicates a good agreement 6 between the presented and referenced model.

Synergy optimization of mixtures composition mass fraction and dryness 11
The largest difference between ORC and BFC is the working fluid dryness at the evaporator 12 outlet. One of the key issues of geothermal power generation using zeotropic mixtures is the design of composition mass fraction. The input parameters for the synergy optimization of mixtures 1 composition mass fraction and dryness at the evaporator outlet are listed in Table 1 Fig.7. 6 For writing convenience, evaporator, expander, flashing tank, condenser, low-pressure pump, high-7 pressure pump and preheater are abbreviated as "Eva", "Exp", "FT", "Con", "LPP", "HPP" and 8 "Pre". As can be observed, the maximum exergy destruction is occurred in condenser (4.024 kW), 9 accounting for 26.45% of the total exergy destruction. It is indicated that the condenser should be

Advanced exergy analysis 1
On the basis of the results derived from conventional exergy analysis, advanced exergy analysis 2 is conducted to reveal the realistic improvement potential. For splitting exergy destruction into 3 unavoidable and avoidable exergy destruction, the unavoidable operating condition of each unit 4 need to be set. The main assumptions for real (actual operating conditions used for conventional 5 analysis), unavoidable (with extremely high efficiency), and theoretical (theoretical maximum 6 efficiency used to simulate the theoretical cycle) operating conditions of the components are listed 7 in Table 3. 8 Under the working conditions in Table 3, the state point parameters for real, unavoidable, and theoretical operating conditions are listed in Tables 4-6. As can be seen, the net power output of 1 theoretical operating condition is 92.8% and 7.08% higher than those of real and unavoidable 2 operating conditions. The thermal efficiency of theoretical operating condition is 68.63% and 6.32% 3 higher than those of real and unavoidable operating conditions. The exergy efficiency of theoretical 4 operating condition is 78.16% and 6.64% higher than those of real and unavoidable operating 5 conditions. 6  In comparison with conventional exergy analysis, advanced exergy analysis further details 1 the system exergy destruction and clarify the optimization direction. The 2 avoidable/unavoidable exergy destruction of each component is illustrated in Fig. 8. It can be 3 seen, the avoidable and unavoidable exergy destructions of the expander are 3.387 kW and 4 0.4878 kW. The avoidable exergy destruction of the expander is the largest among all the 5 components due to the occurrence of the expansion depressurization process, which is an intensely irreversible process. And the avoidable exergy destruction of the expander accounts 1 for 87.6% of the total exergy destruction of the expander. That is to say, most of the expander 2 exergy destruction can be avoidable. The avoidable and unavoidable exergy destructions of the unavoidable. All the flashing tank exergy destruction is unavoidable. To sum up, the expander 10 should draw the most attention, followed by condenser, evaporator, preheater, high-pressure 11 pump and low-pressure pump. If there is only unavoidable exergy destruction in the BFC 12 system, the total system exergy destruction can be reduced from 15.214 kW to 7.813 kW. While 13 results from the conventional exergy analysis indicates that the condenser should be firstly 14 optimized, followed by expander and evaporator, which is contradictory with that of the 15 advanced exergy analysis. The comparison of component improvement priority between 16 conventional and advanced exergy method presents a large difference due mainly to the 17 different criteria. Furthermore, conclusions can be drawn that the engineers should focus on 18 the avoidable exergy destruction rather than the total exergy destruction.

Fig. 8 Avoidable/unavoidable exergy destruction of each component 22
The exogenous/endogenous exergy destruction of each component is displayed in Fig. 7. As 1 can be seen, for any component of the BFC system, endogenous exergy destruction account for a 2 large proportion of the total exergy destruction. It indicates that the exergy destruction of the BFC 3 is mainly caused by the irreversibilities of the components themselves rather than their interactions. 4 Thus, it can be concluded that the interconnections between the system components are imcompact. 5 Therefore, the improvement of each component should be put in the first place when system 6 optimization is required. The interdependencies among system components can be positive or 7 negative, which could be caused by mass flow change or thermodynamic property variation of 8 working fluid through the specific component owing to the introduction of additional 9 irreversibilities. The exogenous exergy destructions of the low-pressure pump, expander and 10 condenser are all negative values, which is the result of differences in mass flow between 11 endogenous and real operating conditions. That is to say, the endogenous exergy destruction is 12 greater than the real exergy destruction. For the three components, performance improvement of 13 other components not only cannot reduce the exergy reduction, but also increase it. The increasing 14 in expander isentropic efficiency is the only useful measurement to reduce its exergy reduction. 15 16 Fig. 9 Exogenous/endogenous exergy destruction of each component 17

Conclusions 18
In present study, the BFC using R245fa/ R600a zeotropic mixtures is proposed. Taking 19 consideration of the thermodynamic performance and working fluid flammability, the optimal 20 component mass fraction and dryness at the evaporator outlet introduced by the ORC system analysis are conducted in sequence. The significant conclusions are summarized as follows: 1 (1) The recommended composition mass fraction of R245fa and dryness of working fluid at the 2 evaporator outlet ranges are 0.30~0.50 and 0.40~0.60, at which the BFC achieves the optimal 3 thermodynamic performance, and the flammability of the working fluid can be suppressed. 4 (2) It is indicated that the maximum exergy destruction is occurred in condenser, followed by 5 expander, evaporator, flashing tank, preheater, high-pressure pump and low-pressure pump by 6 conventional exergy analysis. The exergy destructions of preheater, high-pressure pump and low-7 pressure pump can be ignored. The condenser should be given the first priority. The exergy 8 destruction in the heat exchangers accounts for 52.40% of the total exergy destruction 9 (3) The optimization sequence of BFC components deduced from the conventional and advanced 10 methods is quite different. It is demonstrated that the priority should be given to the expander 11 because of its largest avoidable exergy destruction exergy destruction, followed by condenser and 12 evaporator. From the viewpoint of unavoidability, about 48.6% of total system exergy destruction 13 can be avoidable. And the interconnections among system components are not very strong, owing 14 to small exogenous exergy destructions. Takes into account the interrelationships between 15 components and the technical limitations of system components, the advanced exergy analysis could 16 diagnose the detailed interactions among components of the BFC system and facilitate an 17

Availability of data and materials 17
The datasets used and/or analysed during the current study are available from the corresponding 18 author on reasonable request.
The authors gratefully acknowledge the financial supports provided by the Natural Science 1