Thermodynamic and thermo-economic analysis of dual-pressure and single pressure evaporation organic Rankine cycles
Introduction
The main energy consumption fields of the world are industry [1] and building [2]. Due to the emission regulations and the increasing of energy prices, waste heat recovery [3] and application of renewable energy [4] have received more and more attention. Organic Rankine cycle (ORC) has been considered as an effective and promising method for translating low-media temperature heat to power [5]. Therefore, it has been widely used in the waste heat recovery [6], [7]. However, the thermodynamic performance of the ORC is limited by the cycle structure. Lecompte et al. [8] pointed out that there is a mismatch between the working fluid isothermal phase change process and the heat source temperature linear drop process in the subcritical ORC, which means a large heat transfer difference and increases the exergy loss in the evaporator.
In order to improve the temperature match in the evaporator, some scholars introduced the modified ORC, such as the recuperated cycles [9]. Maraver et al. [10] investigated the ORC performance with and without a recuperator. The result showed that the recuperated ORC could improve the thermal efficiency, but reduce the utilization rate of heat source. Therefore it could not increase the net power output.
Furthermore, using zeotropic mixtures as working fluids is also an approach to improve the temperature match between heat source and working fluid, because the zeotropic mixtures have temperature glides during the phase change process [11], [12]. Nevertheless, the temperature match between the zeotropic mixtures and heat source is still not satisfactory, especially when the temperature drop of heat source is much larger than the temperature glide of the zerotropic mixtures in the evaporation process, as shown in Ref. [13]. Liu et al. [14] also found that ORC system using zeotropic mixtures showed better cycle performance when the temperature increase of cooling water was nearly equal to condensation temperature glide; however, there is still large exergy loss in the evaporator.
In the transcritical ORC (TORC), the working fluid temperature continues to rise by absorbing heat at the pressure above the critical pressure in the evaporator [15], [16]. That will improve the temperature match in the evaporator. Therefore, the average heat transfer temperature difference will be decreased, and the cycle performance would be improved, compared with the subcritical ORC [17], [18]. Besides the performance advantages, TORC also have some drawbacks. Li et al. [19] investigated the performances of the subcritical and transcritical ORCs using R1234ze (E) as working fluid. They found that the TORC had better thermal efficiency; however its heat absorption capacity decreased compared to the subcritical ORC. Maraver et al. [10] researched performances of subcritical and transcritical ORC systems. It was found that the highest second law efficiency was achieved by transcritical ORC using R134a when the geothermal heat source temperature was 170 °C. However, Maraver et al. [10] also emphasized that there were also some drawbacks for the TORC, such as high pressure and high expansion ratios.
Besides, dual-loop ORC, also called cascade ORC, was also proposed for heat recovery. Shu et al. [20] investigated the dual-loop ORC to recovery both the flue gas waste heat and cylinder cooling water waste heat. The proposed cycle achieved a power output higher than that of the Steam Rankin cycle. Yang et al. [21] recently analyzed the thermodynamic and economic performance of dual-loop ORC system for recovering the engine waste heat. They found that the dual-loop ORC system could provide a maximum net power output (23.62 kW) and a minimum EPC (0.41 $/kW h) at the rated condition. The thermal efficiency of the system is between 8.97% and 10.19% over the whole operating range. Indeed, dual-loop ORC was more suitable for two or more heat sources heat recovery, as also shown in Ref. [22]. However, this kind of cycle still had high exergy destruction in the evaporator, especially in the high-stage pressure, on account of the isothermal phase change of the working fluid.
Different from the dual-loop ORC, the DORC (dual-pressure ORC) system splits the pure working fluid into two evaporation process with different pressure values, which could reduce the heat transfer temperature difference in the evaporator. The advantages of the DORC have been presented by some researches. Franco et al. [23] evaluated the system performance improvement potential of the DORC system. They found that the DORC using isobutene or n-pentane as working fluid could increase the exergy efficiency by 15–21% compared to the SORC in the recovery of geothermal water at 160 °C. Peris et al. [24] investigated system performances of the 6 ORC layouts using 10 working fluids for recovering the jacket water waste heat from the engines. The results showed that the dual-pressure layout using SES36 as working fluids achieved the maximum efficiency of 7.15%. Stijepovic et al. [25] investigated the potential of multi-pressure ORC for recovering the waste heat of a 320 °C heat source using the exergy composite curves approach. The results indicated that, because of the rise of the heat capacity absorbed from the heat source, the power output of the multi-pressure ORC was higher than that of the SORC. It was also found that the third-pressure ORC improved a marginal system performance compared with the DORC system. Guzovic et al. [26] evaluated the feasibility of the DORC for recovering a 175 °C geofluid. The authors showed that the DORC system increased the exergy efficiency from 52% to 65%, and brought an obvious increase in the net power output from 5270 to 6371 kW. Li et al. [27] investigated and compared the system performances of SORC and DORC (two-stage ORC) for recovering the geothermal heat source of 100 °C. The results showed that the DORC improved the net power output from 622.7 to 669.7 kW compared to the SORC. And the irreversible loss in the evaporator decreased from 360.8 to 295.2 kW. Thierry et al. [28] analyzed the performance of the DORC using mixture as working fluid for recovering waste heat of 90–110 °C. The results indicated that the DORC improved efficiency from 6.85% to 7.70% compared to the SORC when heat source temperature was 90 °C. Li et al. [29] investigated and evaluated system performances of series two-stage ORC (STORC), parallel two-stage ORC (PTORC) and single pressure ORC (SORC) using 90–120 °C geothermal water as heat source. The results showed that PTORC and STORC could reduce exergy loss in the evaporator and improve the net power output. Moreover, it also found that STORC showed best system performance, and could be widely used in engineering. Shokati et al. [30] compared the SORC, DORC, dual-loop ORC and Kalina cycle from the viewpoints of energy, exergy and exergoeconomic. They found that the net power output of the DORC was maximum, 15.22% higher than the SORC. Manente et al. [31] proposed and compared the SORC and DORC using geothermal heat source of 100–200 °C. The results indicated whether the thermodynamic performance of the DORC was better than that of SORC depended on the working fluid and heat source temperature. The increment of the power output of the DORC decreased, and at last disappeared as the temperature of heat source increased. Sadeghi et al. [32] analyzed and compared the performances of the simple ORC, PTPRC and STORC using zeotropic working fluids for heat recovery from geothermal water. The minimum turbine size parameter (TSP) and maximum net power output were chosen as the optimized goals. It was observed that STORC yielded the maximum net power and R407A was the most suitable working fluid candidate. Li et al. [33] studied the applicable range of the DORC and investigated the influences of the working fluid critical temperature on the suitable heat source temperature range. Results showed that the SORC and DORC systems had different heat source temperature applicable range, which was closely relevant to the working fluid critical properties. In addition, they found that there was a linear relationship between applicable range of heat source temperature and the critical temperature of the working fluid. Li et al. [34] investigated and evaluated the thermal economic performance of the separate and induction turbine layouts, and analyzed the effects of high-stage and low-stage pressure on the thermal economic performance. They found that induction layout produced more net power, and the maximum decrement of the specific investment cost was 34.2%.
However, the existing researches on DORC mainly concentrated on the system parameters optimization or the performance comparisons with other ORC layouts, and few published researches focus on the DORC thermo-economic performance. In fact, the DORC system is more complex because of the two evaporation process, two turbines and pumps. Although the thermodynamic performance of the DORC is better than that of the SORC, components investment of the DORC also increases simultaneously. Therefore, it is essential to investigate the applicability of the DORC from the aspect of the thermo-economic.
In this paper, the thermodynamic and the economic performances of DORC and SORC are investigated and compared. The thermodynamic and economic models of the SORC and DORC systems are built by the Matlab software. The influences of the high-stage and low-stage pressures on SORC and DORC systems parameters (Outlet temperature of heat source, working fluid mass flow rate, thermal efficiency, net power output, investment cost and electricity production cost) are investigated. On the basic of this, the optimization processes are carried out to compare the thermodynamic and economic performances of DORC and SORC systems. Then the feasibility and advantage of the DORC system is discussed in view of thermodynamic and economic performances. In addition, the reasons behind the advantages of DORC system are also investigated.
Section snippets
Methodology
This section introduces the SORC and DORC systems using isobutane as the working fluid for the following thermodynamic and thermo-economic comparisons. Moreover, boundary conditions, assumptions and optimization process are also presented for the analysis of the cycle performances.
Model verification
The calculation results of this paper were compared with the data presented by Manente et al. [31] for SORC and DORC. The same input parameters and boundary conditions are shown in Table 5. The output parameters and comparison results of the SORC and DORC at heat source temperature of 100 °C and 150 °C are presented in Tables 6 and 7 respectively. It can be seen that the maximum relative error is 3.56%, which is the mass flow rate of the working fluid in the SORC, followed by 3.42%, which is
SORC
Fig. 4 presents the influences of the heat source inlet temperature (Ths) and evaporation pressure (Pe) on the variation of the net output power (Wnet) of the SORC. It is obvious that, when Ths is 100–160 °C, Wnet firstly increases and then decreases as Pe increases. When Ths is higher than 160 °C (180 and 200 °C), Wnet increases with the rise of Ths.
The reason is as follows: When Ths is between 100 and 160 °C, the position of the pinch point temperature difference (ΔTpp) in the evaporator
Conclusions
The thermodynamic and thermo-economic performances of SORC and DORC using isobutane as working fluids were investigated and evaluated. The effects of the two-stage pressures and the heat source temperature on the system performances are also analyzed. Main conclusions obtained are listed as follows:
- (a)
Thermodynamic performance of DORC using isobutane as working fluid is better than that of SORC as the heat source temperature is lower than 177.2 °C; DORC system no longer has performance advantage
Acknowledgement
This work has been supported by the Natural Science Foundation of Shandong Province (Nos. ZR2014EEP026 and ZR2016AP07) and the Scientific Research Foundation of Ludong University (No. 27860301).
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