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Thermoeconomic analysis of configuration methods for modular Organic Rankine Cycle units in low-temperature applications
Energy Conversion and Management 127 (2016) 25–34
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Thermoeconomic analysis of configuration methods for modular Organic Rankine Cycle units in low-temperature applications Markus Preißinger ⇑, Sabrina Schatz, Anne Vogl, Andreas König-Haagen, Dieter Brüggemann Chair of Engineering Thermodynamics and Transport Processes, Center of Energy Technology, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 28 June 2016 Received in revised form 19 August 2016 Accepted 29 August 2016
Keyword: Organic Rankine Cycle modular Configuration approach Waste heat recovery Low-temperature Economic analysis
a b s t r a c t Organic Rankine Cycle (ORC) is a promising technology for the utilization of low-grade waste heat. However, due to tailor-made power plants for different heat source temperatures, specific investment costs are still too high to be profitable. This study compares two different methods to configure a modular ORC in the temperature range of 373–463 K. The first method assumes a simple adaption of the mass flow rate within the ORC (mass flow method). In the second method, simultaneous adaption of mass flow rate and working pressure (combined method) take place. The common purpose of both methods is the optimization of the net power output for heat source temperatures lower and higher than the reference plant. Analyses are carried out for common fluorinated refrigerants (R227ea and R236ea) as well as for isoalkanes (isobutane and isopentane). It is shown that within a wide range of temperatures the deviation in net power output between the simpler mass flow method and the more sophisticated combined method is below 10%. However, the deviation strongly depends on the location of the pinch point and on the choice of the working fluid. In general, it is shown that the mass flow method in combination with a working fluid, for which the pinch point is located at the beginning of the preheating, is thermodynamically favorable for plant manufacturers. Economic analyses subsequently compare both methods with respect to payback period and cash flow. Additional investment costs for the combined method are allowed to be up to 10% in order to reach higher profitability than units with mass flow method. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Organic Rankine Cycle (ORC) is a well proven technology for geothermal power generation and biomass fired power plants. In both applications, the ORC can be operated economically due to the special framework conditions. Geothermal power plants are tailor-made and separately designed for each specific source based on heat flow, well temperature and geological conditions. The ORC is merely an adjunct to the overall investment costs as drilling of the borehole is the most expensive part. Hence, tailor-made ORC units are economically feasible despite its expensive engineering and high investment costs. Furthermore, measures to increase the efficiency (like transcritical mode of operation or use of fluid mixtures) can be profitable. For an increase in investment costs of 20%, payback period can be still reduced by almost three years and even if implementation costs of these measures would almost double overall specific investment costs of the ORC, these measures can still be profitable [1].
⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Preißinger). http://dx.doi.org/10.1016/j.enconman.2016.08.092 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.
Compared to geothermal electricity production, payback periods of biomass fired ORC units are mainly influenced by the investment costs of the ORC unit and the price of biomass. Manufacturers have focused on modular concepts to reach low investment costs. Mostly, biomass combustion is coupled with a thermal oil circuit and subsequently coupled to the ORC. This simplifies the development of the ORC as it can be operated at the same boundary conditions concerning evaporation temperature and pressure. Just the size of the ORC has to be adapted on the capacity of the specific biomass furnace. Therefore, the mass flow rate of the working fluid and the size of the components change; however, the technical concept of the ORC remains unchanged. This reduces the investment costs as a serial-production is almost possible. Based on this approach, about 280 power plants in over 30 countries with an electric power of more than 400 MW have been sold. These units are based on just two concepts (with and without split cycle) and seven power classes (from 600 kW to 3000 kW) and have been studied intensively in the past [2–4]. For waste heat recovery from industrial processes, aspects from geothermal and biomass applications occur simultaneously: temperature and heat flow of industrial heat sources vary in a wide range (like geothermal sources), however, low investment costs
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Nomenclature Ar _ m N2 O2 P p Q_ R s T
argon mass flow rate (kg/s) nitrogen oxygen electric power (kW) pressure (bar) heat rate (kW) refrigerant specific entropy (kJ/kg K) temperature (K)
Abbreviations mole-% mole percent ORC Organic Rankine cycle R227ea 1,1,1,2,3,3,3 – heptafluoropropane
are as crucial as in biomass applications to reach payback times which are accepted by industrial decision makers. To account for this, thermoeconomic evaluation became popular within research and development. Quoilin et al. [5] investigate small scale ORCs in waste heat recovery application and analyze the correlation between the economic and thermodynamic optimum for different working fluids. Furthermore, an overview of different ORC applications is given including a market review with technical challenges and different cost figures for commercial modules [6]. Andreasen et al. [7] determine a general methodology for the optimization of ORC with special attention to the selected working fluid and maximum net power output. They found that mixed working fluids increase the cycle net power output while reducing its pressure levels. Oyewunmi and Markides [8] carry out a multi-objective cost-power optimization of a low-temperature ORC system by using different working-fluid mixtures. Compared to pure fluids the results for the mixtures show a thermodynamic improvement, however, accompanied with higher costs. Imran et al. [9] investigate the thermo-economic optimization of basic ORC as well as single and double stage regenerative ORC for waste heat recovery with the aim of maximum thermal efficiency and minimum specific investment costs. Comparing five working fluids, R245fa shows the best values and the basic ORC has low specific investment costs and thermal efficiency compared to regenerative ORC. Direct evaporation has also been widely tested to avoid the investment costs of an intermediate circuit for low temperature heat sources [10]. Next to the mentioned thermoeconomic optimization, many publications within literature deal with multi-objective optimization of ORCs. To name a few, Fergani et al. [11] focused on exergetic analysis of a specific application in the cement industry including exergoenvironmental evaluation. Hu et al. [12] investigated a heat source temperature of 90 °C in geothermal electricity generation, Read et al. [13] on pressurized water with 120 °C. A major drawback of such studies is the fact that they focus on the optimization of one specific heat source temperature. If different heat source temperatures are investigated [14], optimization is carried out for each single heat source temperature. In this study one reference plant is fixed and the optimization takes place for lower and higher heat source temperatures compared to the reference plant. Based on direct evaporating systems, it is the aim of this paper to investigate methods to decrease construction and, therefore, investment costs by using one and the same concept for different temperatures of the industrial waste heat. Two different ORC configuration methods (mass flow adaption and combined mass flow/working pressure adaption to the specific heat source conditions) are discussed for four common
R236ea t. €
1,1,1,2,3,3 – hexafluoropropane thousand €
Greek letters Dh specific enthalpy difference (kJ/kg) _ Dm mass flow rate difference (kg/s) DP net power output difference (kW) DT temperature difference (K) Subscripts net net power output opt optimal ORC Organic Rankine Cycle PP pinch point Pump pump Turb turbine
ORC working fluids (two synthetic and two natural refrigerants). Compared to a previous work [15], special care is given to an economic analysis giving information about payback period and cash flow for both methods. The study focuses on low temperature industrial waste heat (<463 K) and investigates the impact of different investment costs for both methods on the profitability of the overall system. 2. Fundamentals The ORC (Fig. 1) consists of pump, evaporator, turbine, recuperator and condenser. The pump (1) raises the working fluid to its higher pressure level. It is preheated in the recuperator (2) and vaporized in the evaporator (3) before it expands within the turbine (4). A generator, linked to the turbine, produces electricity. The medium is desuperheated in the recuperator (2) and liquefied in the condenser (5). The recuperator is optional and reduces the energy input in the evaporator thus improving thermal efficiency. Furthermore, it reduces the transferred heat flow in the condenser and, consequently, the required heat exchange area for the condenser and its fan power. Four commonly known working fluids are considered: hydrocarbons isobutane and isopentane as well as refrigerants R227ea and R236ea (T,s-diagrams are given in Fig. 2). R227ea is chosen instead of the also well-known fluid R134a as it is a dry working fluid. R236ea is chosen instead of R245fa due to its slightly lower critical pressure and temperature.
Fig. 1. Schematic design of the investigated ORC.
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Mainly, ORC systems or working fluids are compared with respect to thermal efficiency [16], exergetic efficiency [17] or exergy loss distribution [18]. However, in this study the net power output is used as crucial factor, as industrial waste heat is a free heat source and, therefore, one can focus on maximizing the net power output:
Pnet ¼ jPTurb j PPump
ð1Þ
The calculations are carried out in AspenPlus V7.3 [19] using the Peng-Robinson equation of state [20] which is used due to its applicability for different kinds of working fluids including refrigerants and alkanes [21–23]. 3. Methodology In the subsequent context, air (78 mole-% N2, 21 mole-% O2, 1 mole-% Ar) is applied as heat source (hot exhaust gas) and heat sink (air cooler) in the plant design of Fig. 1. The temperature of the heat source is varied for sensitivity purpose (Fig. 3). Lowtemperature waste heat in the range of 373–463 K is considered with a step size of 5 K. Note that a heat source temperature of 373 K could lead to water condensation and corrosion problems for real industrial waste heat (including deposits, water, etc.) [24]. To keep the results more general (e.g. for adaption on waste heat sources like hot water or thermal oil from oil cooled manufacturing machines), these aspects have not been addressed in this study. Further boundary conditions are listed in Table 1 whereas a constant isentropic efficiency for all four working fluids is used as a first assumption to show that the subsequent approach is useful for ORC manufacturer. A more sophisticated turbine model based on size parameter and rotational speed [25] or on dynamic losses [26,27] is beyond the scope of this study. The turbine efficiency of 80% can be reached for turbines even with a pressure ratio of up to 15 [28,29]. The generator efficiency is taken from literature and has also been validated in a test rig at our institute [28]. The pinch point in the evaporator is higher than in the other heat exchangers due to the low heat exchange coefficient of the gaseous side. The condenser inlet temperature is a typical mean value for climatic boundaries in Middle Europe. Let us now assume a manufacturer who wants to construct an ORC unit for a heat source temperature of 418 K and a mass flow rate of 5 kg/s based on the boundary conditions above. To do so, the ORC is simulated and the optimization of the net power output at a given minimal pinch temperature difference results in a specific evaporation pressure and mass flow rate within the ORC. To increase the number of sold units, the manufacturer wants to modify the unit for different heat source temperatures. Hence, the heat source temperature is varied between 373 K and 463 K at a constant mass flow rate of 5 kg/s.
Fig. 2. T,s-diagram of investigated fluids.
Fig. 3. Variation of the waste heat temperature from a reference temperature (solid line) to higher and lower values (dashed line) in steps of ±5 K (working fluid: isopentane).
Table 1 Boundary conditions. Parameter
Value
Reference mass flow rate of the heat source Reference temperature of the heat source Pinch temperature difference of the evaporator Pinch temperature difference of the internal recuperator Pinch temperature difference of the condenser Inlet temperature of the condenser Temperature difference in the condenser Isentropic efficiency of the pump Isentropic efficiency of the turbine Efficiency of the generator
5 kg/s 418 K 35 K 10 K 10 K 288 K 15 K 0.80 0.80 0.95
In some cases, the modification of the waste heat temperature leads to an unwanted violation of the pinch point in the evaporator. To avoid this, the manufacturer has two different opportunities: . variation of the mass flow rate of the working fluid at a constant evaporation pressure (mass flow method), . simultaneous variation of the mass flow rate and the evaporation pressure of the working fluid (combined method). The mass flow method has the advantage that the pressure ratio of the pump and the turbine remain constant. Therefore, the adjustment of these two components on different heat source temperatures is easier than in the combined method, for which the pressure ratio has to be changed additionally. Hence, method 1 is favorable concerning the construction and investment costs; method 2 has advantages concerning the achievable net power output. Compared to studies which include further optimization parameters like the degree of superheat [30,31], the methods in this paper with just two parameters are simpler which is in accordance to the aim of reduced constructional effort and, therefore, investment costs. Consequently, the first goal of this study is the quantification of the net power difference between the mass flow method and the combined method depending on the temperature of the heat source. It has to be noted that the two different methods have to be understood as configuration methods for manufacturers to serve different heat source temperatures and, therefore, broaden the number of applications and clients. The methods are not investigated as control strategies during the operation of the ORC. Studies on control strategies can be found elsewhere [32–35]. Based on the thermodynamic results, an economic investigation is carried out to reach the second goal, an evaluation of the profitability. The aim is to examine the differences in payback
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period and cash flow for all four fluids and the two different methods. The boundary conditions for the profitability analysis are listed in Table 2. Some of the parameters are chosen according to typical market conditions in Germany (electricity price, specific personnel costs, tax rate, inflation). Furthermore, it is assumed that waste heat is available all year (8000 operating hours) and that the whole ORC plant is financed by loan capital. The value for the specific investment costs are taken from an ORC in the cement industry [36,37] and are converted to the present value by an annual price increase for chemical plants of approximately 2.3% [38]. Consequently, the specific investment costs are assumed to be 3297 €/kW. As the condenser is air-cooled, fan power is assumed to be 1% of the rejected heat [39]. Personnel requirements are 130 h per year. This value is slightly lower than the range Obernberger et al. [40] have given for a biomass-fired demonstration unit (3–5 h per week). 4. Thermodynamic evaluation 4.1. Base case Firstly, the base case scenario with a waste heat temperature of 418 K and a mass flow rate of 5 kg/s is investigated. For each fluid the optimal pressure for a maximized net power output as well as the payback period and cash flow are summarized in Table 3. The lowest payback period occurs for isopentane and R236ea (9.65 a), the highest one for isobutane is 3.2% higher (9.96 a). Hence, all fluids are on a similar level. For the cash flow the difference between the highest (R227ea, 44.73 t. €) and lowest value (isobutane, 35.85 t. €) is far higher and amounts 24.8%. It can be concluded that for the reference case R227ea performs best concerning thermodynamic and economic aspects mainly due to its high optimal working pressure. Note that the absolute values of the payback period are influenced by the boundary conditions from Table 2 and may differ for other countries [43]. However, the relative difference between the working fluids and between the subsequent methods will remain constant. 4.2. Method 1: Mass flow method Next to the base case scenario, the temperature of the heat source is varied and, according to method 1, the mass flow rate of the ORC working fluid is adapted for each specific temperature to avoid a pinch point violation in the evaporator. Fig. 4 shows the correlation of the heat source temperature and the net power output. The curves of all fluids in Fig. 4 have an almost linear progression except the one of R227ea. This can be explained by thermodynamics. The net power output is calculated by:
Table 2 Boundary conditions of the economic analyses. Parameter
Value
Operational life Operating hours Electricity price Specific costs for cooling Specific costs for maintenance Specific costs for insurance Specific personnel costs Personnel hours Inflation electricity costs Inflation in general Tax rate Interest rate Loan capital Payback period Redemption rate
20 a 8000 h per year 12.88 €-cent [41] 1.0% of rejected heat [39] 2.0% of investment costs [39] 2.0% of investment costs [42] 40 €/h 130 h/a [40] 4.0% per year 2.0% per year 28.8% 4.0% 100% 20 a 5.0%
_ ORC ðjDhTurb j DhPump Þ Pnet ¼ jPTurb j PPump ¼ m
ð2Þ
The enthalpy differences within the turbine and the pump are determined by the pressure levels within the ORC. As the evaporation pressure and the condensing pressure are constant for the mass flow method, the net power output is just influenced by the mass flow rate adjustment in the ORC. As the mass flow rate of the ORC is linearly dependent on the transferred heat flow of the heat source and, consequently, on the heat source temperature, a linear progression occurs. For the working fluid R227ea, the curvature in Fig. 4 can be explained by a subdivision into three temperature ranges (Fig. 5). In the low temperature range (373–418 K), the mass flow rate increases by the constant specific mass flow difference _ DT = 0.082 kg/s K, which leads to an increase of the net power Dm/ of DP/DT = 0.688 kW/K. At higher temperatures (>448 K), the slope _ DT = 0.041 kg/s K and DP/DT = 0.351 kW/K, decreases to Dm/ respectively. In the transition zone (423–443 K), the mass flow rate of the working fluid does not increase by a constant factor. Therefore, the graph is slightly curved. The explanation can be found in the T, Q_ -diagram of the evaporator (see Fig. 6). It can be seen that within the temperature range between 373 K and about 418 K, the pinch point is located at the beginning of the evaporation (circled area in Fig. 6a). In the transition zone (423– 443 K) the pinch point shifts towards the preheating (circled area in Fig. 6b). At temperatures above 448 K, the pinch point is located at the beginning of the preheater and the slope of the curve in Fig. 5 is constant again. Note that the working fluids isobutane, isopentane and R236ea do not show this behavior in Fig. 4 but at temperatures above the considered range. For R236ea for example, the pinch point shifts at about 473 K based on the higher critical temperature compared to R227ea (see Fig. 2). 4.3. Method 2: Combined method In the combined method, the working pressure is adjusted additionally to the mass flow rate to optimize the cycle. Fig. 7 shows the maximum net power output of the combined method analogue to Fig. 4 of the mass flow method. Again, R227ea shows a different behavior than the other three fluids. In this case, the graph is s-shaped, which can be explained based on Eq. (2) and the additional degree of freedom within the system. Next to the mass flow rate of the ORC and the location of the pinch point, the pressure ratio within the pump and the turbine and, consequently, the enthalpy differences in Eq. (2) can change as well. The interaction of the three parameters and the net power output can be seen in Fig. 8. In zone 1 (T < 418 K), the mass flow rate and the pressure ratio increases which leads to a slightly left bended graph. In zone 2 (418 K < T < 443 K), the graph gets right bended. As the maximum pressure is already reached in zone 2, this behavior is due to the shift of the pinch point from the evaporation to the preheating. Lastly, in zone 3 (T > 443 K), the pinch point is located at the preheating and the graph becomes linear. This behavior is similar to the mass flow method (see Fig. 4). In general, the net power output depends more strongly on the evaporation pressure before the pinch point shift (therefore, at low heat source temperatures). After the shift, the mass flow change has more influence on the net power output. 4.4. Discussion Based on the results in the previous chapter, a comparison of the mass flow and the combined method, as well as an analysis of the base case is carried out. This leads to valuable information for plant manufacturer to point out whether or not the combined method should be preferred for configuration of a modular ORC
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M. Preißinger et al. / Energy Conversion and Management 127 (2016) 25–34 Table 3 Optimal working pressure at a heat source temperature of 418 K and a mass flow rate of 5 kg/s. Working fluid
Popt (kW)
popt (bar)
Payback period (a)
Cash flow (t. €)
Isobutane Isopentane R227ea R236ea
16.1 15.6 19.1 16.2
12.8 4.0 19.5 9.2
9.96 9.65 9.89 9.65
35.85 36.74 44.73 38.78
Fig. 4. Net power output over heat source temperature for mass flow method (dashed line: base case 418 K).
Fig. 5. Mass flow rate and net power output for R227ea.
design. Furthermore, the discussion serves as a guideline in which temperature range the mass flow method is sufficient. Regarding the base case, a relation between the performance of the ORC and thermo-physical properties can be found. For the
Fig. 7. Net power output of the combined method with a varying waste heat temperature (dashed line: base case 418 K).
fluids studied in this paper, Aljundi [44] ascertained that thermal efficiency depends on the critical temperature of the working fluid. The higher the critical temperature, the higher is the thermal efficiency. For the net power output, it is vice versa. The fluid with the minimum critical temperature leads to the maximum power. At the design point (Table 3 and Fig. 4) R227ea with a critical temperature of 374.8 K performs best, followed by isobutane (407.8 K) and R236ea (412.4 K), which have almost the same critical temperature. Isopentane with a significantly higher critical temperature of 460.4 K is least efficient. Regarding the mass flow method, three out of four graphs in Fig. 4 show a linear behavior. However, the analysis of the graph of R227ea shows that the linear behavior depends on the location of the pinch point. Therefore, it is essential for plant manufacturer to know whether the pinch point is located at the beginning of the preheating or of the evaporation. If the pinch point is located at the beginning of the preheating, the heat source temperature can be increased as much as needed; the net power output will always be proportional to the temperature increase. If the pinch point is located at the beginning of the evaporation, the curve will flatten at higher heat source temperatures due to the pinch point shift.
Fig. 6. T,Q_ -diagram for R227ea at a waste heat temperature of (a) 413 K and (b) 443 K.
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Fig. 8. Net power output, evaporation pressure and mass flow rate for R227ea (dashed line: base case 418 K).
Furthermore, if the pinch point is permanently located at the beginning of the preheating, an upscale of heat exchange equipment for modular ORC design can easily and, therefore, economically be achieved. If a pinch point shift occurs within the intended temperature range, possibly more sophisticated changes in the heat exchanger are necessary due to different flow characteristics. Regarding the combined method, a curved graph instead of a linear behavior occurs. However, R227ea is still the most efficient fluid for a heat source temperature lower than 443 K. This means that after the pinch point shift to the preheating, R227ea is not competitive anymore with the other fluids for which the working pressure of the ORC can still be increased.
To compare both design methods, Figs. 8 and 9 show the net power output of the two design methods for each working fluid. It is obvious that the net power output of the combined method is generally higher than the net power output of the mass flow method. However, it has to be kept in mind that the combined method also leads to a change in turbine characteristics (pressure and volume flow ratio). This leads to additional development and engineering costs of these components. Furthermore, a serial production is more difficult, thus increasing the construction cost and lowering the profitablity of the ORC. Furthermore, it can be observed that for R227ea (Fig. 9a) both methods behave similar above a temperature of 418 K. Due to the low critical temperature of R227ea, the maximum pressure within the ORC is already reached at a temperature of 418 K. Therefore, above 418 K, both methods are identical because only the mass flow rate can be adjusted to the heat source temperature. This can be an advantage for ORC manufacturer, as within a wide temperature range (418–463 K), the optimal ORC design point has a constant working pressure. However, it is not possible to construct ORC units with the mass flow method at heat source temperatures below 393 K as the pinch point criteria DTPP = 35 K is violated. This is a disadvantage compared to the combined method, for which the system could be constructed even at temperatures of 373 K due to lower working pressures. Note that a smaller value for the pinch point temperature difference in the evaporator would lead to different temperature at which the pinch point shift occurs. However, the pinch point shift will still occur so the results can be adapted qualitatively to other values of the pinch point temperature difference. Same is true for different mass flow rates of the heat source.
Fig. 9. Comparison of the net power output of both methods for (a) R227ea, (b) R236ea, (c) isobutane and (d) isopentane.
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M. Preißinger et al. / Energy Conversion and Management 127 (2016) 25–34 Table 4 Payback period and cash flow without considering cooling or personnel costs. Working fluid
Payback period without cooling (a)
Payback period without personnel costs (a)
Cash flow without cooling (t. €)
Cash flow without personnel costs (t. €)
Isobutane Isopentane R227ea R236ea
7.74 7.43 7.76 7.49
6.01 5.69 6.52 5.85
65.83 66.41 80.95 69.27
95.42 96.32 104.30 98.35
A similar behavior can also be observed for the working fluids R236ea (Fig. 9b), isobutane (Fig. 9c) and isopentane (Fig. 9d). Contrary to R227ea, the mass flow method and the combined method for these fluids differ within the whole temperature range as even at the highest heat source temperature the maximum working pressure is still not reached. The arising question is, in which temperature range plant manufacturers can still avoid the combined method with the more complicated pressure adjustment and, therefore, with higher investment costs. First tendencies for this economic question can be given by considering the relative deviation between both methods for each working fluid. If we assume in a first step that a deviation of 10% between the two methods is still economically attractive for a plant manufacturer, R227ea can be used from 408 K to 463 K. Any other fluid can be operated up to a temperature of 443 K. However, the lower temperature differs. The mass flow method is within 10% at a temperature of 398 K for isopentane, 403 K for R236ea and 408 K for isobutane, respectively. As the deviation between both methods is low, the following chapter discusses the economics of both methods in more detail. 5. Economic analysis First, the economic analysis is performed for the base case. Table 4 demonstrates the influence of cooling costs and personnel costs. By neglecting personnel costs, the cash flow averagely increases by a factor of 2.5 and the payback period decreases by a factor of 0.4 compared to the base case (see Table 2). Neglected cooling costs result in 1.3 times higher cash flow and a decrease of the payback period by a factor of 0.55. In Fig. 10 the relation between working hours, payback period and cash flow is illustrated for isobutane. Two cases concerning cooling costs are displayed: the base case with 1% fan power and the case with neglected cooling costs. The strong relation between the parameters emphasizes the need to take into account both personnel and cooling costs. The next step includes the comparison of the mass flow and the combined method with regard to payback period and cash flow.
Fig. 10. Payback period and cash flow of isobutane plotted against working hours with and without consideration of cooling costs.
Fig. 11 illustrates both parameters plotted against the heat source temperature for all analyzed fluids. Payback periods above a value of 20 years are neglected. For R227ea, both methods show nearly the same behavior above the reference temperature of 418 K as the net power output in Fig. 9a has similar values as well. For temperature lower than the reference temperature, the deviation between both methods is negligible. Hence, the combined method is not profitable for R227ea. For the three other fluids, the combined method has a higher cash flow and lower payback period for heat source temperatures above 418 K. For temperatures below, both methods are on a similar level even though the mass flow method shows slightly higher values. In Chapter 4.4, a minimum temperature was given for which the net power output of both methods deviates less than 10%. The same deviation can be found for the cash flow at the following temperatures: for R236ea and isopentane at 428 K, for isobutane at 433 K. The highest deviation occurs for the maximum temperature of 463 K. The cash flow of the mass flow method for R236ea amounts 171.23 t. €. Compared to this, the value for the combined method is around 64% higher with 280.97 t. €. A similar deviation of around 63% for the cash flow between the two methods results for the working fluid isobutane. Isopentane shows a deviation of 53%. For the payback periods the deviation is far lower. For the maximum heat source temperature, a drop of 5.2% for isobutane, 11.0% for isopentane and 9.2% for R236ea is calculated. The lowest payback period is found for the combined method and isopentane as a working fluid. So far, the specific investment costs of both methods had the same value (3297 €/kWel, see Chapter 3). In reality, the specific investment costs of the combined method are higher due to the need of the additional pressure adjustment from module to module compared to the mass flow method for which heat exchangers, turbine and pump can be constructed at a constant maximum pressure within the whole temperature range. To examine the influence of this aspect, the specific investment costs of the combined method are increased compared to the mass flow method. Fig. 12a and b shows the development of the payback period and cash flow exemplarily for isobutane with additional 10% (3626.7 €/kW) and 30% (4286.1 €/kW) investment costs for the combined method. The results for the mass flow method are taken from Fig. 11. As expected, the payback period generally increases and the cash flow decreases with increasing investment costs of the combined method. For 10% additional investment costs, the payback period of the combined method is higher in the whole temperature range. However, the combined method is still advantageous concerning cash flow for temperatures higher than 438 K. A difference of 10% between the cash flow values of the two methods is not reached before 448 K. Considering additional 30% investment costs, the mass flow method is favorable in the whole temperature range considering payback period and cash flow. The main conclusion for plant manufacturer based on the chosen boundary conditions and the working fluid isobutane is as follows: if a modular concept with the mass flow method has more than 10% lower investment costs than the more sophisticated combined method, lower net power output is overcompen-
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Fig. 11. Comparison of payback period and cash flow of both methods for (a) R227ea, (b) R236ea, (c) isobutane and (d) isopentane.
Fig. 12. Comparison of payback period and cash flow of isobutane and (a) additional 10% or (b) additional 30% investment costs of the combined method.
sated by lower specific investment costs within the investigated temperature range. Contrary, the combined method is profitable if specific investment costs of both methods are almost similar (deviation less than 10%). 6. Conclusion A guide line for the configuration of modular ORC units at low heat source temperatures is proposed and an economic analysis
is carried out. The evaluation includes a mass flow method (adjusting the mass flow of the ORC to the heat source temperature) and a combined method (adjusting the mass flow and the pressure of the working fluid). The results can be summarized as follows: Concerning the investigated configuration methods, working fluids for which the pinch point is located at the beginning of the preheating behave different than the ones at the beginning of the evaporation.
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ORCs based on the combined method yield higher net power output than the ones on the mass flow method. However, within a certain temperature range, the mass flow method can be used with less than 10% decrease of net power output. In order to achieve a simple configuration of modular ORC units, the mass flow method and a fluid for which the pinch point is located at the preheating is favorable. Including personnel and cooling costs for the ORC plant is crucial for an appropriate economic analysis. The mass flow method is economically favorable if additional specific investment costs for the more sophisticated combined method exceeds about 10%. In future research, the economic analysis is elaborated by including specific cost functions for the components of the ORC. Furthermore, appropriate correlations for the efficiency of the turbine will be implemented. Finally, we are up to further validate the assumptions for the ORC components within the test field Organic Rankine Cycle at our Center of Energy Technology. This allows for a holistic thermoeconomic evaluation of the two configuration methods.
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Thermoeconomic analysis of configuration methods for modular Organic Rankine Cycle units in low-temperature applications Markus Preißinger ⇑, Sabrina Schatz, Anne Vogl, Andreas König-Haagen, Dieter Brüggemann Chair of Engineering Thermodynamics and Transport Processes, Center of Energy Technology, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany
a r t i c l e
i n f o
Article history: Received 28 June 2016 Received in revised form 19 August 2016 Accepted 29 August 2016
Keyword: Organic Rankine Cycle modular Configuration approach Waste heat recovery Low-temperature Economic analysis
a b s t r a c t Organic Rankine Cycle (ORC) is a promising technology for the utilization of low-grade waste heat. However, due to tailor-made power plants for different heat source temperatures, specific investment costs are still too high to be profitable. This study compares two different methods to configure a modular ORC in the temperature range of 373–463 K. The first method assumes a simple adaption of the mass flow rate within the ORC (mass flow method). In the second method, simultaneous adaption of mass flow rate and working pressure (combined method) take place. The common purpose of both methods is the optimization of the net power output for heat source temperatures lower and higher than the reference plant. Analyses are carried out for common fluorinated refrigerants (R227ea and R236ea) as well as for isoalkanes (isobutane and isopentane). It is shown that within a wide range of temperatures the deviation in net power output between the simpler mass flow method and the more sophisticated combined method is below 10%. However, the deviation strongly depends on the location of the pinch point and on the choice of the working fluid. In general, it is shown that the mass flow method in combination with a working fluid, for which the pinch point is located at the beginning of the preheating, is thermodynamically favorable for plant manufacturers. Economic analyses subsequently compare both methods with respect to payback period and cash flow. Additional investment costs for the combined method are allowed to be up to 10% in order to reach higher profitability than units with mass flow method. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Organic Rankine Cycle (ORC) is a well proven technology for geothermal power generation and biomass fired power plants. In both applications, the ORC can be operated economically due to the special framework conditions. Geothermal power plants are tailor-made and separately designed for each specific source based on heat flow, well temperature and geological conditions. The ORC is merely an adjunct to the overall investment costs as drilling of the borehole is the most expensive part. Hence, tailor-made ORC units are economically feasible despite its expensive engineering and high investment costs. Furthermore, measures to increase the efficiency (like transcritical mode of operation or use of fluid mixtures) can be profitable. For an increase in investment costs of 20%, payback period can be still reduced by almost three years and even if implementation costs of these measures would almost double overall specific investment costs of the ORC, these measures can still be profitable [1].
⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Preißinger). http://dx.doi.org/10.1016/j.enconman.2016.08.092 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.
Compared to geothermal electricity production, payback periods of biomass fired ORC units are mainly influenced by the investment costs of the ORC unit and the price of biomass. Manufacturers have focused on modular concepts to reach low investment costs. Mostly, biomass combustion is coupled with a thermal oil circuit and subsequently coupled to the ORC. This simplifies the development of the ORC as it can be operated at the same boundary conditions concerning evaporation temperature and pressure. Just the size of the ORC has to be adapted on the capacity of the specific biomass furnace. Therefore, the mass flow rate of the working fluid and the size of the components change; however, the technical concept of the ORC remains unchanged. This reduces the investment costs as a serial-production is almost possible. Based on this approach, about 280 power plants in over 30 countries with an electric power of more than 400 MW have been sold. These units are based on just two concepts (with and without split cycle) and seven power classes (from 600 kW to 3000 kW) and have been studied intensively in the past [2–4]. For waste heat recovery from industrial processes, aspects from geothermal and biomass applications occur simultaneously: temperature and heat flow of industrial heat sources vary in a wide range (like geothermal sources), however, low investment costs
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Nomenclature Ar _ m N2 O2 P p Q_ R s T
argon mass flow rate (kg/s) nitrogen oxygen electric power (kW) pressure (bar) heat rate (kW) refrigerant specific entropy (kJ/kg K) temperature (K)
Abbreviations mole-% mole percent ORC Organic Rankine cycle R227ea 1,1,1,2,3,3,3 – heptafluoropropane
are as crucial as in biomass applications to reach payback times which are accepted by industrial decision makers. To account for this, thermoeconomic evaluation became popular within research and development. Quoilin et al. [5] investigate small scale ORCs in waste heat recovery application and analyze the correlation between the economic and thermodynamic optimum for different working fluids. Furthermore, an overview of different ORC applications is given including a market review with technical challenges and different cost figures for commercial modules [6]. Andreasen et al. [7] determine a general methodology for the optimization of ORC with special attention to the selected working fluid and maximum net power output. They found that mixed working fluids increase the cycle net power output while reducing its pressure levels. Oyewunmi and Markides [8] carry out a multi-objective cost-power optimization of a low-temperature ORC system by using different working-fluid mixtures. Compared to pure fluids the results for the mixtures show a thermodynamic improvement, however, accompanied with higher costs. Imran et al. [9] investigate the thermo-economic optimization of basic ORC as well as single and double stage regenerative ORC for waste heat recovery with the aim of maximum thermal efficiency and minimum specific investment costs. Comparing five working fluids, R245fa shows the best values and the basic ORC has low specific investment costs and thermal efficiency compared to regenerative ORC. Direct evaporation has also been widely tested to avoid the investment costs of an intermediate circuit for low temperature heat sources [10]. Next to the mentioned thermoeconomic optimization, many publications within literature deal with multi-objective optimization of ORCs. To name a few, Fergani et al. [11] focused on exergetic analysis of a specific application in the cement industry including exergoenvironmental evaluation. Hu et al. [12] investigated a heat source temperature of 90 °C in geothermal electricity generation, Read et al. [13] on pressurized water with 120 °C. A major drawback of such studies is the fact that they focus on the optimization of one specific heat source temperature. If different heat source temperatures are investigated [14], optimization is carried out for each single heat source temperature. In this study one reference plant is fixed and the optimization takes place for lower and higher heat source temperatures compared to the reference plant. Based on direct evaporating systems, it is the aim of this paper to investigate methods to decrease construction and, therefore, investment costs by using one and the same concept for different temperatures of the industrial waste heat. Two different ORC configuration methods (mass flow adaption and combined mass flow/working pressure adaption to the specific heat source conditions) are discussed for four common
R236ea t. €
1,1,1,2,3,3 – hexafluoropropane thousand €
Greek letters Dh specific enthalpy difference (kJ/kg) _ Dm mass flow rate difference (kg/s) DP net power output difference (kW) DT temperature difference (K) Subscripts net net power output opt optimal ORC Organic Rankine Cycle PP pinch point Pump pump Turb turbine
ORC working fluids (two synthetic and two natural refrigerants). Compared to a previous work [15], special care is given to an economic analysis giving information about payback period and cash flow for both methods. The study focuses on low temperature industrial waste heat (<463 K) and investigates the impact of different investment costs for both methods on the profitability of the overall system. 2. Fundamentals The ORC (Fig. 1) consists of pump, evaporator, turbine, recuperator and condenser. The pump (1) raises the working fluid to its higher pressure level. It is preheated in the recuperator (2) and vaporized in the evaporator (3) before it expands within the turbine (4). A generator, linked to the turbine, produces electricity. The medium is desuperheated in the recuperator (2) and liquefied in the condenser (5). The recuperator is optional and reduces the energy input in the evaporator thus improving thermal efficiency. Furthermore, it reduces the transferred heat flow in the condenser and, consequently, the required heat exchange area for the condenser and its fan power. Four commonly known working fluids are considered: hydrocarbons isobutane and isopentane as well as refrigerants R227ea and R236ea (T,s-diagrams are given in Fig. 2). R227ea is chosen instead of the also well-known fluid R134a as it is a dry working fluid. R236ea is chosen instead of R245fa due to its slightly lower critical pressure and temperature.
Fig. 1. Schematic design of the investigated ORC.
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Mainly, ORC systems or working fluids are compared with respect to thermal efficiency [16], exergetic efficiency [17] or exergy loss distribution [18]. However, in this study the net power output is used as crucial factor, as industrial waste heat is a free heat source and, therefore, one can focus on maximizing the net power output:
Pnet ¼ jPTurb j PPump
ð1Þ
The calculations are carried out in AspenPlus V7.3 [19] using the Peng-Robinson equation of state [20] which is used due to its applicability for different kinds of working fluids including refrigerants and alkanes [21–23]. 3. Methodology In the subsequent context, air (78 mole-% N2, 21 mole-% O2, 1 mole-% Ar) is applied as heat source (hot exhaust gas) and heat sink (air cooler) in the plant design of Fig. 1. The temperature of the heat source is varied for sensitivity purpose (Fig. 3). Lowtemperature waste heat in the range of 373–463 K is considered with a step size of 5 K. Note that a heat source temperature of 373 K could lead to water condensation and corrosion problems for real industrial waste heat (including deposits, water, etc.) [24]. To keep the results more general (e.g. for adaption on waste heat sources like hot water or thermal oil from oil cooled manufacturing machines), these aspects have not been addressed in this study. Further boundary conditions are listed in Table 1 whereas a constant isentropic efficiency for all four working fluids is used as a first assumption to show that the subsequent approach is useful for ORC manufacturer. A more sophisticated turbine model based on size parameter and rotational speed [25] or on dynamic losses [26,27] is beyond the scope of this study. The turbine efficiency of 80% can be reached for turbines even with a pressure ratio of up to 15 [28,29]. The generator efficiency is taken from literature and has also been validated in a test rig at our institute [28]. The pinch point in the evaporator is higher than in the other heat exchangers due to the low heat exchange coefficient of the gaseous side. The condenser inlet temperature is a typical mean value for climatic boundaries in Middle Europe. Let us now assume a manufacturer who wants to construct an ORC unit for a heat source temperature of 418 K and a mass flow rate of 5 kg/s based on the boundary conditions above. To do so, the ORC is simulated and the optimization of the net power output at a given minimal pinch temperature difference results in a specific evaporation pressure and mass flow rate within the ORC. To increase the number of sold units, the manufacturer wants to modify the unit for different heat source temperatures. Hence, the heat source temperature is varied between 373 K and 463 K at a constant mass flow rate of 5 kg/s.
Fig. 2. T,s-diagram of investigated fluids.
Fig. 3. Variation of the waste heat temperature from a reference temperature (solid line) to higher and lower values (dashed line) in steps of ±5 K (working fluid: isopentane).
Table 1 Boundary conditions. Parameter
Value
Reference mass flow rate of the heat source Reference temperature of the heat source Pinch temperature difference of the evaporator Pinch temperature difference of the internal recuperator Pinch temperature difference of the condenser Inlet temperature of the condenser Temperature difference in the condenser Isentropic efficiency of the pump Isentropic efficiency of the turbine Efficiency of the generator
5 kg/s 418 K 35 K 10 K 10 K 288 K 15 K 0.80 0.80 0.95
In some cases, the modification of the waste heat temperature leads to an unwanted violation of the pinch point in the evaporator. To avoid this, the manufacturer has two different opportunities: . variation of the mass flow rate of the working fluid at a constant evaporation pressure (mass flow method), . simultaneous variation of the mass flow rate and the evaporation pressure of the working fluid (combined method). The mass flow method has the advantage that the pressure ratio of the pump and the turbine remain constant. Therefore, the adjustment of these two components on different heat source temperatures is easier than in the combined method, for which the pressure ratio has to be changed additionally. Hence, method 1 is favorable concerning the construction and investment costs; method 2 has advantages concerning the achievable net power output. Compared to studies which include further optimization parameters like the degree of superheat [30,31], the methods in this paper with just two parameters are simpler which is in accordance to the aim of reduced constructional effort and, therefore, investment costs. Consequently, the first goal of this study is the quantification of the net power difference between the mass flow method and the combined method depending on the temperature of the heat source. It has to be noted that the two different methods have to be understood as configuration methods for manufacturers to serve different heat source temperatures and, therefore, broaden the number of applications and clients. The methods are not investigated as control strategies during the operation of the ORC. Studies on control strategies can be found elsewhere [32–35]. Based on the thermodynamic results, an economic investigation is carried out to reach the second goal, an evaluation of the profitability. The aim is to examine the differences in payback
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period and cash flow for all four fluids and the two different methods. The boundary conditions for the profitability analysis are listed in Table 2. Some of the parameters are chosen according to typical market conditions in Germany (electricity price, specific personnel costs, tax rate, inflation). Furthermore, it is assumed that waste heat is available all year (8000 operating hours) and that the whole ORC plant is financed by loan capital. The value for the specific investment costs are taken from an ORC in the cement industry [36,37] and are converted to the present value by an annual price increase for chemical plants of approximately 2.3% [38]. Consequently, the specific investment costs are assumed to be 3297 €/kW. As the condenser is air-cooled, fan power is assumed to be 1% of the rejected heat [39]. Personnel requirements are 130 h per year. This value is slightly lower than the range Obernberger et al. [40] have given for a biomass-fired demonstration unit (3–5 h per week). 4. Thermodynamic evaluation 4.1. Base case Firstly, the base case scenario with a waste heat temperature of 418 K and a mass flow rate of 5 kg/s is investigated. For each fluid the optimal pressure for a maximized net power output as well as the payback period and cash flow are summarized in Table 3. The lowest payback period occurs for isopentane and R236ea (9.65 a), the highest one for isobutane is 3.2% higher (9.96 a). Hence, all fluids are on a similar level. For the cash flow the difference between the highest (R227ea, 44.73 t. €) and lowest value (isobutane, 35.85 t. €) is far higher and amounts 24.8%. It can be concluded that for the reference case R227ea performs best concerning thermodynamic and economic aspects mainly due to its high optimal working pressure. Note that the absolute values of the payback period are influenced by the boundary conditions from Table 2 and may differ for other countries [43]. However, the relative difference between the working fluids and between the subsequent methods will remain constant. 4.2. Method 1: Mass flow method Next to the base case scenario, the temperature of the heat source is varied and, according to method 1, the mass flow rate of the ORC working fluid is adapted for each specific temperature to avoid a pinch point violation in the evaporator. Fig. 4 shows the correlation of the heat source temperature and the net power output. The curves of all fluids in Fig. 4 have an almost linear progression except the one of R227ea. This can be explained by thermodynamics. The net power output is calculated by:
Table 2 Boundary conditions of the economic analyses. Parameter
Value
Operational life Operating hours Electricity price Specific costs for cooling Specific costs for maintenance Specific costs for insurance Specific personnel costs Personnel hours Inflation electricity costs Inflation in general Tax rate Interest rate Loan capital Payback period Redemption rate
20 a 8000 h per year 12.88 €-cent [41] 1.0% of rejected heat [39] 2.0% of investment costs [39] 2.0% of investment costs [42] 40 €/h 130 h/a [40] 4.0% per year 2.0% per year 28.8% 4.0% 100% 20 a 5.0%
_ ORC ðjDhTurb j DhPump Þ Pnet ¼ jPTurb j PPump ¼ m
ð2Þ
The enthalpy differences within the turbine and the pump are determined by the pressure levels within the ORC. As the evaporation pressure and the condensing pressure are constant for the mass flow method, the net power output is just influenced by the mass flow rate adjustment in the ORC. As the mass flow rate of the ORC is linearly dependent on the transferred heat flow of the heat source and, consequently, on the heat source temperature, a linear progression occurs. For the working fluid R227ea, the curvature in Fig. 4 can be explained by a subdivision into three temperature ranges (Fig. 5). In the low temperature range (373–418 K), the mass flow rate increases by the constant specific mass flow difference _ DT = 0.082 kg/s K, which leads to an increase of the net power Dm/ of DP/DT = 0.688 kW/K. At higher temperatures (>448 K), the slope _ DT = 0.041 kg/s K and DP/DT = 0.351 kW/K, decreases to Dm/ respectively. In the transition zone (423–443 K), the mass flow rate of the working fluid does not increase by a constant factor. Therefore, the graph is slightly curved. The explanation can be found in the T, Q_ -diagram of the evaporator (see Fig. 6). It can be seen that within the temperature range between 373 K and about 418 K, the pinch point is located at the beginning of the evaporation (circled area in Fig. 6a). In the transition zone (423– 443 K) the pinch point shifts towards the preheating (circled area in Fig. 6b). At temperatures above 448 K, the pinch point is located at the beginning of the preheater and the slope of the curve in Fig. 5 is constant again. Note that the working fluids isobutane, isopentane and R236ea do not show this behavior in Fig. 4 but at temperatures above the considered range. For R236ea for example, the pinch point shifts at about 473 K based on the higher critical temperature compared to R227ea (see Fig. 2). 4.3. Method 2: Combined method In the combined method, the working pressure is adjusted additionally to the mass flow rate to optimize the cycle. Fig. 7 shows the maximum net power output of the combined method analogue to Fig. 4 of the mass flow method. Again, R227ea shows a different behavior than the other three fluids. In this case, the graph is s-shaped, which can be explained based on Eq. (2) and the additional degree of freedom within the system. Next to the mass flow rate of the ORC and the location of the pinch point, the pressure ratio within the pump and the turbine and, consequently, the enthalpy differences in Eq. (2) can change as well. The interaction of the three parameters and the net power output can be seen in Fig. 8. In zone 1 (T < 418 K), the mass flow rate and the pressure ratio increases which leads to a slightly left bended graph. In zone 2 (418 K < T < 443 K), the graph gets right bended. As the maximum pressure is already reached in zone 2, this behavior is due to the shift of the pinch point from the evaporation to the preheating. Lastly, in zone 3 (T > 443 K), the pinch point is located at the preheating and the graph becomes linear. This behavior is similar to the mass flow method (see Fig. 4). In general, the net power output depends more strongly on the evaporation pressure before the pinch point shift (therefore, at low heat source temperatures). After the shift, the mass flow change has more influence on the net power output. 4.4. Discussion Based on the results in the previous chapter, a comparison of the mass flow and the combined method, as well as an analysis of the base case is carried out. This leads to valuable information for plant manufacturer to point out whether or not the combined method should be preferred for configuration of a modular ORC
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M. Preißinger et al. / Energy Conversion and Management 127 (2016) 25–34 Table 3 Optimal working pressure at a heat source temperature of 418 K and a mass flow rate of 5 kg/s. Working fluid
Popt (kW)
popt (bar)
Payback period (a)
Cash flow (t. €)
Isobutane Isopentane R227ea R236ea
16.1 15.6 19.1 16.2
12.8 4.0 19.5 9.2
9.96 9.65 9.89 9.65
35.85 36.74 44.73 38.78
Fig. 4. Net power output over heat source temperature for mass flow method (dashed line: base case 418 K).
Fig. 5. Mass flow rate and net power output for R227ea.
design. Furthermore, the discussion serves as a guideline in which temperature range the mass flow method is sufficient. Regarding the base case, a relation between the performance of the ORC and thermo-physical properties can be found. For the
Fig. 7. Net power output of the combined method with a varying waste heat temperature (dashed line: base case 418 K).
fluids studied in this paper, Aljundi [44] ascertained that thermal efficiency depends on the critical temperature of the working fluid. The higher the critical temperature, the higher is the thermal efficiency. For the net power output, it is vice versa. The fluid with the minimum critical temperature leads to the maximum power. At the design point (Table 3 and Fig. 4) R227ea with a critical temperature of 374.8 K performs best, followed by isobutane (407.8 K) and R236ea (412.4 K), which have almost the same critical temperature. Isopentane with a significantly higher critical temperature of 460.4 K is least efficient. Regarding the mass flow method, three out of four graphs in Fig. 4 show a linear behavior. However, the analysis of the graph of R227ea shows that the linear behavior depends on the location of the pinch point. Therefore, it is essential for plant manufacturer to know whether the pinch point is located at the beginning of the preheating or of the evaporation. If the pinch point is located at the beginning of the preheating, the heat source temperature can be increased as much as needed; the net power output will always be proportional to the temperature increase. If the pinch point is located at the beginning of the evaporation, the curve will flatten at higher heat source temperatures due to the pinch point shift.
Fig. 6. T,Q_ -diagram for R227ea at a waste heat temperature of (a) 413 K and (b) 443 K.
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Fig. 8. Net power output, evaporation pressure and mass flow rate for R227ea (dashed line: base case 418 K).
Furthermore, if the pinch point is permanently located at the beginning of the preheating, an upscale of heat exchange equipment for modular ORC design can easily and, therefore, economically be achieved. If a pinch point shift occurs within the intended temperature range, possibly more sophisticated changes in the heat exchanger are necessary due to different flow characteristics. Regarding the combined method, a curved graph instead of a linear behavior occurs. However, R227ea is still the most efficient fluid for a heat source temperature lower than 443 K. This means that after the pinch point shift to the preheating, R227ea is not competitive anymore with the other fluids for which the working pressure of the ORC can still be increased.
To compare both design methods, Figs. 8 and 9 show the net power output of the two design methods for each working fluid. It is obvious that the net power output of the combined method is generally higher than the net power output of the mass flow method. However, it has to be kept in mind that the combined method also leads to a change in turbine characteristics (pressure and volume flow ratio). This leads to additional development and engineering costs of these components. Furthermore, a serial production is more difficult, thus increasing the construction cost and lowering the profitablity of the ORC. Furthermore, it can be observed that for R227ea (Fig. 9a) both methods behave similar above a temperature of 418 K. Due to the low critical temperature of R227ea, the maximum pressure within the ORC is already reached at a temperature of 418 K. Therefore, above 418 K, both methods are identical because only the mass flow rate can be adjusted to the heat source temperature. This can be an advantage for ORC manufacturer, as within a wide temperature range (418–463 K), the optimal ORC design point has a constant working pressure. However, it is not possible to construct ORC units with the mass flow method at heat source temperatures below 393 K as the pinch point criteria DTPP = 35 K is violated. This is a disadvantage compared to the combined method, for which the system could be constructed even at temperatures of 373 K due to lower working pressures. Note that a smaller value for the pinch point temperature difference in the evaporator would lead to different temperature at which the pinch point shift occurs. However, the pinch point shift will still occur so the results can be adapted qualitatively to other values of the pinch point temperature difference. Same is true for different mass flow rates of the heat source.
Fig. 9. Comparison of the net power output of both methods for (a) R227ea, (b) R236ea, (c) isobutane and (d) isopentane.
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M. Preißinger et al. / Energy Conversion and Management 127 (2016) 25–34 Table 4 Payback period and cash flow without considering cooling or personnel costs. Working fluid
Payback period without cooling (a)
Payback period without personnel costs (a)
Cash flow without cooling (t. €)
Cash flow without personnel costs (t. €)
Isobutane Isopentane R227ea R236ea
7.74 7.43 7.76 7.49
6.01 5.69 6.52 5.85
65.83 66.41 80.95 69.27
95.42 96.32 104.30 98.35
A similar behavior can also be observed for the working fluids R236ea (Fig. 9b), isobutane (Fig. 9c) and isopentane (Fig. 9d). Contrary to R227ea, the mass flow method and the combined method for these fluids differ within the whole temperature range as even at the highest heat source temperature the maximum working pressure is still not reached. The arising question is, in which temperature range plant manufacturers can still avoid the combined method with the more complicated pressure adjustment and, therefore, with higher investment costs. First tendencies for this economic question can be given by considering the relative deviation between both methods for each working fluid. If we assume in a first step that a deviation of 10% between the two methods is still economically attractive for a plant manufacturer, R227ea can be used from 408 K to 463 K. Any other fluid can be operated up to a temperature of 443 K. However, the lower temperature differs. The mass flow method is within 10% at a temperature of 398 K for isopentane, 403 K for R236ea and 408 K for isobutane, respectively. As the deviation between both methods is low, the following chapter discusses the economics of both methods in more detail. 5. Economic analysis First, the economic analysis is performed for the base case. Table 4 demonstrates the influence of cooling costs and personnel costs. By neglecting personnel costs, the cash flow averagely increases by a factor of 2.5 and the payback period decreases by a factor of 0.4 compared to the base case (see Table 2). Neglected cooling costs result in 1.3 times higher cash flow and a decrease of the payback period by a factor of 0.55. In Fig. 10 the relation between working hours, payback period and cash flow is illustrated for isobutane. Two cases concerning cooling costs are displayed: the base case with 1% fan power and the case with neglected cooling costs. The strong relation between the parameters emphasizes the need to take into account both personnel and cooling costs. The next step includes the comparison of the mass flow and the combined method with regard to payback period and cash flow.
Fig. 10. Payback period and cash flow of isobutane plotted against working hours with and without consideration of cooling costs.
Fig. 11 illustrates both parameters plotted against the heat source temperature for all analyzed fluids. Payback periods above a value of 20 years are neglected. For R227ea, both methods show nearly the same behavior above the reference temperature of 418 K as the net power output in Fig. 9a has similar values as well. For temperature lower than the reference temperature, the deviation between both methods is negligible. Hence, the combined method is not profitable for R227ea. For the three other fluids, the combined method has a higher cash flow and lower payback period for heat source temperatures above 418 K. For temperatures below, both methods are on a similar level even though the mass flow method shows slightly higher values. In Chapter 4.4, a minimum temperature was given for which the net power output of both methods deviates less than 10%. The same deviation can be found for the cash flow at the following temperatures: for R236ea and isopentane at 428 K, for isobutane at 433 K. The highest deviation occurs for the maximum temperature of 463 K. The cash flow of the mass flow method for R236ea amounts 171.23 t. €. Compared to this, the value for the combined method is around 64% higher with 280.97 t. €. A similar deviation of around 63% for the cash flow between the two methods results for the working fluid isobutane. Isopentane shows a deviation of 53%. For the payback periods the deviation is far lower. For the maximum heat source temperature, a drop of 5.2% for isobutane, 11.0% for isopentane and 9.2% for R236ea is calculated. The lowest payback period is found for the combined method and isopentane as a working fluid. So far, the specific investment costs of both methods had the same value (3297 €/kWel, see Chapter 3). In reality, the specific investment costs of the combined method are higher due to the need of the additional pressure adjustment from module to module compared to the mass flow method for which heat exchangers, turbine and pump can be constructed at a constant maximum pressure within the whole temperature range. To examine the influence of this aspect, the specific investment costs of the combined method are increased compared to the mass flow method. Fig. 12a and b shows the development of the payback period and cash flow exemplarily for isobutane with additional 10% (3626.7 €/kW) and 30% (4286.1 €/kW) investment costs for the combined method. The results for the mass flow method are taken from Fig. 11. As expected, the payback period generally increases and the cash flow decreases with increasing investment costs of the combined method. For 10% additional investment costs, the payback period of the combined method is higher in the whole temperature range. However, the combined method is still advantageous concerning cash flow for temperatures higher than 438 K. A difference of 10% between the cash flow values of the two methods is not reached before 448 K. Considering additional 30% investment costs, the mass flow method is favorable in the whole temperature range considering payback period and cash flow. The main conclusion for plant manufacturer based on the chosen boundary conditions and the working fluid isobutane is as follows: if a modular concept with the mass flow method has more than 10% lower investment costs than the more sophisticated combined method, lower net power output is overcompen-
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Fig. 11. Comparison of payback period and cash flow of both methods for (a) R227ea, (b) R236ea, (c) isobutane and (d) isopentane.
Fig. 12. Comparison of payback period and cash flow of isobutane and (a) additional 10% or (b) additional 30% investment costs of the combined method.
sated by lower specific investment costs within the investigated temperature range. Contrary, the combined method is profitable if specific investment costs of both methods are almost similar (deviation less than 10%). 6. Conclusion A guide line for the configuration of modular ORC units at low heat source temperatures is proposed and an economic analysis
is carried out. The evaluation includes a mass flow method (adjusting the mass flow of the ORC to the heat source temperature) and a combined method (adjusting the mass flow and the pressure of the working fluid). The results can be summarized as follows: Concerning the investigated configuration methods, working fluids for which the pinch point is located at the beginning of the preheating behave different than the ones at the beginning of the evaporation.
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ORCs based on the combined method yield higher net power output than the ones on the mass flow method. However, within a certain temperature range, the mass flow method can be used with less than 10% decrease of net power output. In order to achieve a simple configuration of modular ORC units, the mass flow method and a fluid for which the pinch point is located at the preheating is favorable. Including personnel and cooling costs for the ORC plant is crucial for an appropriate economic analysis. The mass flow method is economically favorable if additional specific investment costs for the more sophisticated combined method exceeds about 10%. In future research, the economic analysis is elaborated by including specific cost functions for the components of the ORC. Furthermore, appropriate correlations for the efficiency of the turbine will be implemented. Finally, we are up to further validate the assumptions for the ORC components within the test field Organic Rankine Cycle at our Center of Energy Technology. This allows for a holistic thermoeconomic evaluation of the two configuration methods.
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