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A. Franco (2011)
Power Production from a Moderate Temperature Geothermal Resource with Regenerative Organic Rankine CyclesEnergy for Sustainable Development, 15
F. Heberle, M. Preißinger, D. Brüggemann (2012)
Zeotropic mixtures as working fluids in Organic Rankine Cycles for low-enthalpy geothermal resourcesRenewable Energy, 37
R. Dipippo (2005)
Chapter 7 – Dry-Steam Power Plants
M. Preißinger, F. Heberle, D. Brüggemann (2012)
Thermodynamic analysis of double‐stage biomass fired Organic Rankine Cycle for micro‐cogenerationInternational Journal of Energy Research, 36
R. Rayegan, Y. Tao (2011)
A procedure to select working fluids for Solar Organic Rankine Cycles (ORCs)Renewable Energy, 36
G. Kosmadakis, D. Manolakos, S. Kyritsis, G. Papadakis (2010)
Design of a two stage Organic Rankine Cycle system for reverse osmosis desalination supplied from a steady thermal sourceDesalination, 250
S. Karellas, A. Schuster (2008)
Supercritical Fluid Parameters in Organic Rankine Cycle ApplicationsInternational Journal of Thermodynamics, 11
D. Peng, D. Robinson (1976)
A New Two-Constant Equation of StateIndustrial & Engineering Chemistry Fundamentals, 15
M. Chys, M. Broek, Bruno Vanslambrouck, M. Paepe (2012)
Potential of zeotropic mixtures as working fluids in organic Rankine cyclesEnergy, 44
D. Tempesti, G. Manfrida, D. Fiaschi (2012)
Thermodynamic analysis of two micro CHP systems operating with geothermal and solar energyApplied Energy, 97
N. Lai, M. Wendland, J. Fischer (2007)
Working fluids for high-temperature organic Rankine cyclesEnergy, 32
H. Hettiarachchi, Mihajlo Golubovic, W. Worek, Y. Ikegami (2007)
Optimum design criteria for an Organic Rankine cycle using low-temperature geothermal heat sourcesEnergy, 32
A. Borsukiewicz-Gozdur, W. Nowak (2007)
Comparative analysis of natural and synthetic refrigerants in application to low temperature Clausius–Rankine cycleEnergy, 32
Exploitation of geothermal sources based on advanced Organic Rankine Cycle (ORC) is studied with respect to energetic, plant-specific and economic aspects. Three natural and five synthetic refrigerants are investigated as pure fluids within a sub- and transcritical ORC. Furthermore, a zeotropic mixture of R227ea/R245fa is analysed under subcritical conditions. It is shown that transcritical ORC is a promising way to optimize geothermal power plants. The gross power can be increased by .15% compared with standard subcritical processes. Economic analysis indicates that transcritical ORC as well as zeotropic mixtures lead to significantly lower payback periods and, even when taking into account higher specific investment costs and higher mean cash flow. Keywords: organic Rankine cycle; transcritical; fluid mixtures; geothermal; economic evaluation *Corresponding author. zet@uni-bayreuth.de Received 18 January 2013; revised 12 March 2013; accepted 20 March 2013 ......... ................. ................ ................. ................. ................ ................. ................. . ............... ................. ................. within the heat exchange equipment, a transcritical mode of 1 INTRODUCTION operation can be used. Saleh et al.[9] concluded that for the working fluid R134a, a rise in thermal efficiency of 17% can be In 2010, geothermal applications within Germany accounted reached; Schuster et al.[10] specified potential for 8% increase for ,0.01% of the overall power generation. In contrast, the in exergetic efficiency. Both studies are mainly based on federal government aims for 20% power generation out of re- thermodynamic analysis and consider no special applications. newable energy sources. As geothermal power plants can In the present study, eight different fluids out of two chemical provide base load power—compared with other sources like classes are investigated for the usage in geothermal power wind and solar energy—the technology shows high potential plants. We focus on the potential of transcritical mode of oper- for renewable power generation. To use brine temperatures up ation and the usage of zeotropic mixtures. Next to the thermo- to 1908C, binary power plants like the Organic Rankine Cycle dynamic analysis, the processes are evaluated with respect to (ORC) or the Kalina Cycle are predominantly chosen and, plant-specific and economic aspects. therefore, many publications are available in the literature [1]. Hettiarachchi [2] focused on low-temperature geothermal heat sources from energetic and exergetic point of view. Franco [3] emphasized on regenerative ORC to limit the reinjection tem- 2 METHODOLOGY perature of the brine. Furthermore, three main ways of improv- ing exergetic efficiency as well as power output of a standard Figure 1 shows a scheme of a standard ORC with internal re- subcritical ORC are reported in the literature. Firstly, multi- cuperator for geothermal applications. In our study, pure stage processes were analysed by several research groups [4 – 6]. fluorinated refrigerants R227ea, R236ea, R236fa, R245fa and Secondly, due to non-isothermal phase-change, the usage of RC318 as well as hydrocarbons isobutane, isobutene and iso- fluid mixtures is considered. Heberle et al.[7] reported an in- pentane are considered in the sub- and transcritical modes of crease in second law efficiency of 8% for the zeotropic mixture operation. The geothermal temperature is varied between 100 isobutane/isopentane compared with the pure working fluid and 1908C in steps of 10 K. Firstly, the processes are compared isobutane at a brine temperature of 1208C. Borsukiewicz- with regard to exergetic and thermal efficiency as well as gross Gozdur and Nowak [8] compared natural and synthetic refrig- power output and heat exchange capacity. To compare the erants for temperatures up to 1158C including the zeotropic usage of fluid mixtures with the sub- and transcritical modes of mixture propane/ethane. Finally, to avoid pinch-point restrictions operation, the zeotropic mixture R227ea/R245fa is investigated International Journal of Low-Carbon Technologies 2013, 8, i62 – i68 # The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com doi:10.1093/ijlct/ctt021 Advance Access Publication 2 May 2013 i62 Advanced Organic Rankine Cycle for geothermal application Figure 1. A scheme of a geothermal ORC with internal recuperator. Table 1. Boundary conditions for sub- and supercritical modes additionally within the economic analysis, as it showed high of operation efficiency in a previous analysis [7]. Exergetic efficiency is calculated as net power output Variable Symbol Value divided by total exergy flow rate of the brine: Mass flow brine m ˙ 65 kg/s brine Temperature range brine T 100 – 1908C brine jP þ P j jP þ P j P T P T h ¼ ¼ : ð1Þ Pressure range subcritical ORC p p(508C) –p(s ) ORC,sub max ex m _ ½ðh h Þ T ðs s Þ brine he;in 0 0 he;in 0 Pressure range transcritical ORC p 1.01 – 1.30 p ORC,trans crit Temperature difference at pinch point DT 5K pp P and P are the power of the pump and the turbine, respect- Inlet temperature of the cooling water T 158C P T cool Temperature difference of the cooling water DT 5K ively. The exergy of the brine is calculated using the enthalpy cool Turbine isentropic efficiency h 80% i,T and entropy of the heater inlet (index he,in) and of the dead Pump isentropic efficiency h 75% i,P state (index 0 with T ¼ 158C). m ˙ is the mass flow rate of 0 brine the brine. The brine is assumed to be pure water. Thermal efficiency is defined as net power output divided by heat flux of heater, using index he,out for the outlet of the heater: jP þ P j jP þ P j P T P T h ¼ ¼ : ð2Þ th m _ ðh h Þ Q ORC he;in he;out he The overall heat exchange capacity is calculated by the sum of the specific heat exchange capacities of each heat exchanger (heater, internal recuperator and condenser): jQj UA ¼ ð3Þ LMTD where Q is the heat flux within the heat exchanger; LMTD, the Figure 2. s -boundary condition for sub- and transcritical ORC. max logarithmic mean temperature difference. The processes are simulated in AspenPlus [11] using the 3 RESULTS Peng – Robinson equation of state [12] with the Boston – Matthias modification for supercritical pressures. The bound- Results are presented in four subchapters. Firstly, exergetic effi- ary conditions are listed in Table 1. According to Figure 2, the ciency is analysed at two different brine temperatures (150 and maximum pressure for the subcritical process is limited by the 1908C) typically for geothermal sources in Europe. Secondly, requirement of dry expansion (no droplet formation within exergetic and thermal efficiency, gross power output and heat the turbine). Therefore, the maximum entropy of the dew line exchange capacity are exemplarily investigated at a brine tem- is calculated for each fluid and the corresponding temperature perature of 1508C. Thirdly, results for the sub- and transcritical T for subcritical as well as the minimum turbine inlet max,sub modes of operation are presented at different brine temperatures. temperature T for transcritical ORC is educed, respect- min,trans Lastly, we analysed both modes in comparison with zeotropic ively [13]. Superheating the working fluid in the range between mixtures with regard to economic aspects in a case study for T and T was not considered due to a required in- max,sub min,trans the Upper Rift Valley located between Frankfurt and Basel. crease in heat exchange area [10]. International Journal of Low-Carbon Technologies 2013, 8, i62 – i68 i63 M. Preißinger et al. 3.1 Exergetic analysis at brine temperatures 3.2 Thermodynamic investigation at a brine of 150 and 19088888C temperature of 15088888C Figure 3 shows the exergetic efficiency over the working pres- For typical conditions in the Upper Rift Valley, a thermodynam- sure for the sub- and transcritical modes of operation at two ic analysis is carried out in Figure 4. For the sake of clarity, just different brine temperatures. Three groups of fluids can be two group 1 fluids (R236fa and isobutene), one group 2 fluid identified: (1) Group 1 fluids show a bell-shaped curve for (isopentane) and one group 3 (RC318) fluid are presented. It exergetic efficiency due to pinch-point restrictions and relative- can be seen that though exergetic efficiency (a) is lowest for iso- ly high critical temperatures. For these fluids, exergetic effi- pentane, thermal efficiency (b) is highest. However, for plant ciency rises at low working pressures as the enthalpy difference manufacturers and operators, the attainable net power output of in the turbine increases. Having reached a maximum, the exer- an available source is crucial due to economic aspects. getic efficiency drops as the mass flow rate has to be decreased Therefore, thermal efficiency is not an appropriate parameter to maintain the required minimum temperature difference in for comparison purposes as it can be misleading. As for geother- the heater. Therefore, less heat can be coupled to the ORC, mal power plants, it is possible to purchase the electric power which leads to lower efficiencies. For the same reason, the exer- for the pump cheaper than power infeed to the grid is refunded getic efficiency decreases at supercritical pressures. (2) Group 2 by legislation; gross power output is even more crucial than fluids also show a bell-shaped curve for subcritical pressures; exergetic efficiency. Figure 4c shows an increase in gross power operation under supercritical conditions is not possible output of 12% for a transcritical process with RC318 compared anymore due to pinch-point restrictions. (3) Group 3 fluids with 4.4% in exergetic efficiency. For R236fa, exergetic efficiency are characterized by a steady increase in exergetic efficiency for actually decreases by 1.6%, whereas an increase in gross power rising the working pressures as the critical temperature is low output of 3% can still be achieved for transcritical mode of op- enough to avoid pinch-point restrictions. eration. According to investment costs, one has to consider that As can be seen for a brine temperature of 1508C, three for RC318 the increase in power output is coupled with an in- fluids are of group 1 (R236ea, R236fa, isobutane), three fluids crease in heat exchange capacity UA of 6%. For R236fa, the UA of group 2 (R245fa, isobutene, isopentane) and two fluids of value is decreased by 7%; however, gross power output increases group 3 (RC318, R227ea). Therefore, exergetic efficiency can at the same time as mentioned afore. This leads to the conclu- just be increased for two working fluids in transcritical mode sion that a thermo-economic investigation is necessary to decide of operation. For all other fluids, subcritical conditions lead to on or against transcritical mode of operation. higher efficiencies. However, at a brine temperature of 1908C just one fluid (isopentane) reaches the maximum efficiency 3.3 Thermodynamic analysis at different brine under subcritical conditions, and all other fluids perform best at supercritical pressures. Furthermore, comparison of Figure 3a temperatures and b leads to the conclusion that exergetic efficiency decreases In addition to the results for specific temperatures (1508C/ for fluids with low critical temperature (group 3 fluids, R227ea 1908C), Table 2 summarizes maximum exergetic efficiency and and RC318) at higher brine temperatures, whereas for group 1 corresponding fluid for the whole investigated temperature and group 2 fluids, the exergetic efficiency increases. range. We can conclude from Table 2 that the brine temperature Figure 3. Exergetic efficiency for a brine temperature of 1508C(a) and 1908C(b). i64 International Journal of Low-Carbon Technologies 2013, 8, i62 – i68 Advanced Organic Rankine Cycle for geothermal application Figure 4. Exergetic efficiency (a), thermal efficiency (b), gross power (c) and heat exchange capacity (d) over ORC working pressure for selected fluids (brine temperature: 1508C). Table 2. Exergetic efficiency for sub- and transcritical ORC at different conditions. Therefore, at a brine temperature of 1408C, effi- brine temperatures ciency increase is lower than at 130 and 1508C. In Table 3, results for gross power output are shown. It is evident that rela- Subcritical Transcritical tive deviation between sub- and supercritical ORC is higher T (8C) max. h (%) fluid max. h (%) fluid Dh (%) brine ex ex ex than for exergetic efficiency (Table 2). Similar to exergetic effi- ciency, maximum improvements are reached at a temperature 100 32.5 R227ea n.a. n.a. n.a. 110 36.2 R227ea n.a. n.a. n.a. range of 130 to 1608C and exceed a value of 15%. We notice 120 41.3 R227ea 38.7 R227ea 26.3 again that at some temperatures, two different fluids perform 130 44.1 R227ea 46.8 R227ea 6.2 best under sub- and supercritical conditions, respectively. 140 47.3 RC318 48.8 R227ea 3.0 150 48.3 RC318 50.4 RC318 4.4 160 49.0 R236fa 51.7 R236fa 5.6 3.4 Economic case study at brine temperature 170 52.6 R236ea 53.4 R236fa 1.4 180 54.4 R236ea 54.4 R236ea 0.0 of 15088888C 190 54.0 R236ea 55.2 R236ea 2.2 Based on the economic boundary conditions given by Janczik et al.[14] (Table 4), the case of the Upper Rift Valley at a brine has to be .1208C to improve exergetic efficiency by a transcriti- temperature of 1508C is economically evaluated. Firstly, sub- cal ORC. Within the temperature range of 130 to 1608C, an in- and transcritical ORC are simulated with RC318 as working crease of up to 6.2% is possible, whereas at higher temperatures fluid. Table 5 summarizes gross power, electric power of the the effect is less distinctive. At brine temperatures of 140 and pump, payback period and mean cash flow. Based on similar 1708C, maximum efficiency under subcritical conditions is specific investment costs of 1250 E/kW for the ORC, the el reached with a different working fluid than under transcritical payback period can be reduced by 10% using a transcritical International Journal of Low-Carbon Technologies 2013, 8, i62 – i68 i65 M. Preißinger et al. Table 3. Gross power output for sub- and transcritical ORC at different brine temperatures Subcritical Transcritical T max. P fluid max. P fluid DP brine gross gross gross (%) (MW) (MW) (%) 100 1.0 R227ea n.a. n.a. n.a. 110 1.4 R227ea n.a. n.a. n.a. 120 2.0 R227ea 2.0 R227ea 21.0 130 2.5 R227ea 2.8 R227ea 13.4 140 3.1 RC318 3.6 R227ea 15.4 150 3.6 RC318 4.0 RC318 11.9 160 4.1 R236fa 4.6 RC318 12.8 170 5.0 R236ea 5.4 R236fa 7.7 180 5.8 R236ea 6.1 isobutane 6.0 190 6.4 R236ea 6.9 isobutane 8.1 Figure 5. Gross power output of the mixture R227ea/R245fa over mole fraction of R227ea for a brine temperature of 1508C. Table 4. Boundary conditions for economic analysis Table 6. Payback period and mean cash flow for the investigated base Parameter Value case with pure working fluid R227ea and fluid mixture R227ea/R245fa Operating time 8000 h/a Subcritical Transcritical Fluid mixture Compensation period/life time 20 a Compensation 0.25 E/kWh Investment costs (E/kW ) 1250 1250 1250 el Drilling costs 22.29 MME Electric gross power (kW) 3193.0 4029.2 3608.0 Thermal water circuit 1.77 MME Electric power of the pump (kW) 382.3 821.3 357.6 Electricity costs 0.1 E/kWh Payback period (a) 12.69 9.40 10.15 Interest rate 6.5% Mean cash flow (MME) 1.98 2.61 2.43 Tax rate 28.2% the heat exchange equipment and has been analysed in further Table 5. Payback period and mean cash flow for the investigated base detail in continuative studies [7, 15] case with working fluid RC318 Firstly, Table 6 compares the payback period and the mean cash flow based on constant investment costs similar to Subcritical Transcritical Transcritical Table 5. We can conclude that for the working fluid R227ea, Investment costs (E/kW ) 1250 1250 1500 el the payback period can be reduced by .25% for transcritical Electric gross power (kW) 3610.5 4039.5 4039.5 mode of operation. Using fluid mixture R227ea/R245fa, the Electric power of the pump (kW) 388.7 685.0 685.0 decrease in payback period is around 20%. Furthermore, the Payback period (a) 10.21 9.09 9.57 mean cash flow is significantly higher for transcritical mode of Mean cash flow (MME) 2.41 2.69 2.67 operation and fluid mixtures. However, one should assume higher investment costs for ORC. If we assume 20% overhead for the specific investment transcritical mode of operation due to a higher working pres- costs of a transcritical ORC (=1500 E/kW ) due to the higher sure and increased UA-values for the heat exchange equipment. el pressure level and UA value, the payback period for the super- Similar effects occur when using fluid mixtures, as it is known critical ORC is calculated to 9.6 years. This is still 5% lower from the literature that due to lower heat exchange coefficients, compared with a conventional subcritical ORC. Additionally, more voluminous heat exchangers are needed [7]. Therefore, the mean cash flow is hardly influenced by increased invest- Figures 6 and 7 show the payback period and the mean cash ment costs as can be seen in Table 3. flow for variable investments costs. For each optimization strat- In a next step, the sub- and transcritical modes of operation egy, three values for the investment costs are given. Firstly, the are compared with the usage of zeotropic mixtures as working investment costs remained constant at 1250 E/kW as shown el fluid. Therefore, based on the preliminary work of Heberle in Table 4. Secondly, the investment costs for the transcritical et al.[7], Figure 5 shows the gross power output of the mixture ORC and the ORC with fluid mixtures are increased by 20% R227ea/R245fa. For a brine temperature of 1508C, a peak (=1500 E/kW ). Lastly, we calculated the value of the invest- el occurs at a mole fraction of 80% R227ea and 20% R245fa. The ment costs, which leads to the same payback period as for the behaviour of the gross power output with respect to the mole base case using R227ea under subcritical conditions (i.e. 12.7 fraction is caused by the non-isothermal phase-change within years, Table 6). i66 International Journal of Low-Carbon Technologies 2013, 8, i62 – i68 Advanced Organic Rankine Cycle for geothermal application still be reached even if one assumes investment costs about twice as high as for the base case. The reason for this can be seen in the high ratio between drilling costs and costs for the power plant within geothermal applications which diminish the effect of the costs for the power plant. Additionally, even for an equal payback period of 12.7 years, the mean cash flow can be increased from 1.98 MME (subcritical, 1250 E/kW ) el to 2.32 MME (zeotropic mixture, 2418 E/kW ) or even 2.43 el MME (transcritical, 2721 E/kW ) as displayed in Figure 7. el Therefore, the decision on investment should always be based on both parameters, the payback period and the mean cash flow. Lastly, in addition to thermodynamic and economic aspects, one has to mention that refrigerants have a global warming po- tential which accounts for 3360 (R227ea), 1350 (R236ea), 9659 (R236fa), 1020 (R245fa) and 10 090 (RC318) [16]. Due to the fact that within Europe upcoming regulations may ban working fluids with a GWP of . 2500 by 2017, this should be kept in mind for a proper fluid selection for future geothermal power Figure 6. Payback period with respect to investment costs. plants. 4 CONCLUSION Sub- and transcritical ORC were investigated for geothermal application with respect to energetic and plant-specific aspects and compared with zeotropic mixtures from an economic point of view. The main conclusions can be summarized as follows: † Gross power output can be increased by 15% and exergetic efficiency by 6% for a transcritical ORC within the investi- gated temperature range. † However, subcritical ORC performs best at brine tempera- tures of ,1308C. † For the Upper Rhine Graben at a brine temperature of 1508C, the payback period can be reduced from 10.2 to 9.1 years using RC318 as a working fluid. † For geothermal power plants with high drilling costs, tran- Figure 7. Mean cash flow with respect to investment costs. scritical ORC and zeotropic mixtures can reach equal payback times like subcritical ORC even at investment costs In general, transcritical mode of operation performs slightly twice as high as assumed for subcritical ORC. Additionally, better than an ORC with zeotropic mixtures. For example, the the mean cash flow can be increased by .8%. payback period of a transcritical ORC with specific investment costs of 1500 E/kW still has a payback period of ,10 years, In the future, we want to extend our investigations for transcri- el and an ORC using fluid mixtures is slightly .10 years assum- tical mode of operation and zeotropic mixtures with an exer- ing the same investment costs. However, for both the optimiza- goeconomic approach and experimental measurements of heat tion strategies, the payback period of the subcritical ORC can transfer coefficients. International Journal of Low-Carbon Technologies 2013, 8, i62 – i68 i67 M. 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Elsevier. 2006 Oxford [a.o.]. gwps.html. i68 International Journal of Low-Carbon Technologies 2013, 8, i62 – i68
International Journal of Low-Carbon Technologies – Oxford University Press
Published: Jul 2, 2013
Keywords: organic Rankine cycle transcritical fluid mixtures geothermal economic evaluation
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