ECOS

PROCEEDINGS OF ECOS 2017 - THE 30TH INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS JULY 2-JULY 6, 2017, SAN DIEGO, CALIFORNIA, USA

Thermo-economic comparison of single and dual loop ORC for industrial waste heat recovery Lorenzo Toccia,b, Claudia Toroa a

Department of Mechanical and Aeropsace Engineering “Sapienza” University of Rome, Via Eudossiana 18, Rome, Italy [email protected], [email protected] b

Entropea Labs limited, 2a, Greenwood road, E81AB London, UK e-mail: [email protected]

Abstract: Waste heat recovery (WHR) is widely recognized as a promising technology to improve the degree of sustainability in thermal processes: it improves fuel utilization, producing an eco-friendly electrical power from a material flow that would otherwise be discharged into the environment. Industrial interest in Organic Rankine Cycle (ORC) technology is growing since it allows for the production of “free” electricity out of wasted thermal energy. The optimization of the system architecture is a key aspect in the minimization of the investment cost. Several investigations available in literature demonstrate that the specific cost of a competitive ORC for industrial applications should not exceed 2000 €/kW. Different optimization techniques have been proposed to properly select the ORC design parameters. Among those, non-linear programming algorithms can be conveniently applied to this specific field. This paper compares two different ORC architectures. The first one consists in a regenerated ORC with a non-flammable working fluid, the second one being a dual ORC cycle in which two ORCs operating on different working fluids are stacked on top of each other. The objective is to understand to what extent the latter configuration offer better performance. The thermodynamic model of the ORC system developed using the software MATLAB® is presented. Using nonlinear programming techniques of the open source software NOMAD, the cycle operative parameters have been optimized to maximize the power exploitation. Hence, a thermo-economic analysis is carried out to investigate over the possible improvements of the configurations proposed. This study underlines the need for an optimization approach in which the thermodynamic parameters are selected with the objective to lower the ORC system specific cost.

Keywords: ORC, non-linear programming, optimization, energy efficiency, dual loop ORC.

1. Introduction The adoption of an Organic Rankine Cycle (ORC) to exploit useful power from thermal sources of low and medium temperature is widely considered among the most prominent heat-recovery technologies. Bianchi [1] highlighted that the potential of the ORC technology in the exploitation of heat source at medium temperature such as the ICE exhaust gas. The ORC power plant receives as an input the industrial waste thermal energy which is discharged to the environment and converts it into electricity by means of a thermodynamic cycle. When designing a waste heat recovery system, the objective is twofold. Firstly, the efficiency of the plant needs to be maximized to reduce the specific cost of the system. Furthermore, the temperature of the hot gas released to the environment has to be minimized, to maximize the energy recovered by the system. Arguably the efficiency of the ORC plants depend on the selection of the working fluid and of the thermodynamic parameters. Hung [2] performed a thermodynamic analysis that covers different fluids such as benzene, p-xylene, ammonia, toluene, R 11 and R 12 and identified R 113 and R 123 as the best performing ones. Yu and Shu [3] studied a bottoming ORC applied to a 300 hp internal


combustion engine (ICE), considering both exhaust gas and jacket water as heat sources; their results show an increase in thermal efficiency up to 6.1 % and a recovered electric power of 15.5 kW. He [4], after an analysis of LNG engine waste heat recovery, concludes that R 236fa outperforms its competitors in terms of thermal efficiency. Tian [5] claims it is possible to reach up to 16 % cycle efficiency using R 141b as an operating fluid in WHR system for engine applications. In waste heat recovery applications, the efficiency of the cycle and the amount of energy recovered from the heat source need to be maximized, to reduce the specific cost of the plant. Braimakis [6] claims that the maximization of the exergy efficiency ensures to reach the aforementioned objectives. Li [7] performed an exergy analysis of ORC for low grade heat source, stating that regeneration improves the system performance. He also demonstrated that the evaporator is the system component with the highest exergy destruction. Generally, companies that develop waste heat recovery systems try to minimize the specific cost of the plant to offer a competitive pay-back period (PBP) to the end user. To this end, different architectures have been proposed to improve the performance of the ORC system, which in turns, reduces the specific cost of the plant. Among those, dual loop configuration received attention recently. Zhang [8] studied a dual loop bottoming Organic Rankine Cycle applied to a 4 cylinders ICE; the high temperature (HT) loop recovers heat using R 245fa as operative fluid, while the low temperature (LT) loop uses R 134a. The cycle performance has been calculated for several engine operative conditions, the conclusions being that the waste energy recovery (WER) system allows for an efficiency increases of about 14-16% in the peak effective thermal efficiency region and 38-43% at part load and in the high speed region. These authors [9] studied a dual loop configuration for marine applications. Water extracts the thermal energy from the heat source at high temperature while R 245 fa and R 600 have been compared to exploit the thermal energy of the heat source at low temperature. The efficiency of the dual loop cycle resulted 9% higher than that of the single loop ORC. This work compares the single and the dual loop configurations of ORC plants. The thermodynamic model of the ORC system has been implemented and the power output of the system maximized using non-linear programming techniques. Toluene and R 1233zdE have been selected to recover the thermal energy from the exhaust gas at high temperature and at low temperature respectively. The single loop and the dual loop architectures have been compared in the exploitation of industrial heat at different temperatures. The results show that the dual loop system improves the power production of up to 31.9%. In addition, the adoption of the dual loop configuration allows to achieve a system specific cost as low as 1250 €/kW.

2. Description of the configurations analysed In this paper two different plant configurations have been simulated. Specifically, single loop and dual loop ORC system have been considered to exploit the thermal energy discharged by industrial processes. Figure 1 and Figure 2 show the plant layout respectively for the single loop and the dual loop configurations.


Figure 1 Single loop ORC

Figure 2 Dual loop ORC

Toluene and R 1233zdE have been considered in this study. Firstly, Toluene and R 1233zdE have been simulated in the single loop architecture to evaluate the power output they can achieve. Then, the dual loop ORC has been investigated to evaluate whether the dual loop configuration results in an increase in the performance with respect to the single loop ORC. Notice that in the dual loop


configuration, Toluene has been considered to recover the thermal energy of the exhaust gas at high temperature whereas R 1233zdE has been considered to recover the thermal energy left in the exhaust gas at the outlet of the Gas-Toluene evaporator. In fact researchers proved that Toluene performs well in the recovery of thermal energy at high temperature [10] while R 1233zdE offers good performance when the temperature of the heat source is low [11]. Table 1 Thermodynamic properties of the working fluids

Name Toluene R 1233zdE

Critical temperature [K] 591.75 439.6

Critical pressure [bar] 41.26 36.236

3. Mathematical model Figure 3 displays the thermodynamic transformations of an ORC on the Temperature- Entropy diagram.

Figure 3 ORC cycle

Table 2 shows a description of the transformations that take place in each one of the components of the ORC. Table 2 ORC thermodynamic transformations

Transformation 1-2 2-5 5–6 6 – 6a 6a - 1

Component Pump Evaporator Turbine Regenerator Condenser

Description Fluid pressurized by the pump Fluid vaporized Expansion process Heat recovery to pre-heat the pressurized fluid Fluid condensation

The thermodynamic model and the optimization algorithm used are presented in subsections 3.1 and 3.2.


3.1. Thermodynamic model of the ORC The thermodynamic model of the ORC has been derived using the software MATLAB®. The model is based on mass and energy balance equations applied at each component of the system. Equations (1) – (8) are representative of the model implemented.

MASS CONSERVATION EQUATIONS Heat source Working fluid

𝑚̇ℎ𝑠 = 𝑐𝑜𝑛𝑠𝑡 𝑚̇𝑤𝑓 = 𝑐𝑜𝑛𝑠𝑡

(1) (2)

Cooling fluid

𝑚̇𝑐𝑓 = 𝑐𝑜𝑛𝑠𝑡

(3)

𝑚̇ℎ𝑠 ∗ (ℎ5,ℎ𝑠 − ℎ2𝑎,ℎ𝑠 ) = 𝑚̇𝑤𝑓 ∗ (ℎ5 − ℎ2𝑎 )

(4)

𝑃𝑡𝑢𝑟𝑏𝑖𝑛𝑒 = 𝜂𝑡𝑢𝑟𝑏𝑖𝑛𝑒 ∗ 𝑚̇𝑤𝑓 ∗ (ℎ5 − ℎ6 )

(5)

𝑚̇𝑤𝑓 ∗ (ℎ6𝑎,𝑤𝑓 − ℎ1,𝑤𝑓 ) = 𝑚̇𝑐𝑓 ∗ (ℎ6𝑎,𝑐𝑓 − ℎ1,𝑐𝑓 )

(6)

𝑚̇𝑤𝑓 ∗ (ℎ6 − ℎ6𝑎 ) = 𝑚̇𝑤𝑓 ∗ (ℎ2𝑎 − ℎ2 ) 𝑚̇𝑤𝑓 ∗ (ℎ2 − ℎ1 ) 𝑃𝑝𝑢𝑚𝑝 = 𝜂𝑝𝑢𝑚𝑝

(7)

ENERGY BALANCE EQUATIONS Evaporator Turbine Condenser Regenerator Pump

(8)

Where 𝑚̇ is the fluid mass flow rate, ℎ is the enthalpy, 𝑃 is the power and 𝜂 is the efficiency. The subscripts refer to the thermodynamic points depicted in Figure 3. Notice that the model refers to the single loop ORC. The dual loop ORC is based on the duplication of the set of equations.

3.2. The optimization algorithm In this study, a derivative free optimization algorithm has been used. Such an algorithm performs well when solving optimization problems in which the objective function and the constraints are highly non-linear, such as thermodynamic problems. Optimization algorithms are configured to reach the optimal value of the objective function computing the least possible number of iterations. Notice that derivative free algorithms are weak from an optimization stand point, in that they use little pieces of information to solve the problem. This leads to the convergence to an optimal value which results less accurate with respect to algorithms that use derivatives. In this work, a directional direct search method has been used to perform the calculations, which is implemented in the optimization software NOMAD, an open source software of the Opti toolbox. Table 3 shows the main steps of the optimization algorithm.


Table 3 Optimization algorithm procedure

Directional direct search algorithm Initialization A point 𝑥0 is defined form which the algorithm starts the optimization process. Search step The algorithm evaluates the objective function in a finite number of points, looking for a new point 𝑥 such that 𝑓(𝑥) ≤ 𝑓(𝑥0 ). If such a point is reached, the algorithm skips the pool step. Pool step The algorithm chooses a positive spanning set 𝐷𝑘 and it computes the set of pull points 𝑃𝑘 = {(𝑥𝑘 + 𝛼𝑘 𝑑𝑘 ) ∶ 𝑑𝑘 ∈ 𝐷}. The objective function 𝑓 is evaluated in the set 𝑃𝑘 , following a pre-defined order. If a point (𝑥𝑘 + 𝛼𝑘 𝑑𝑘 ) ∶ 𝑓(𝑥𝑘 + 𝛼𝑘 𝑑𝑘 ) ≤ 𝑓(𝑥𝑘 ), the algorithm sets 𝑥𝑘 = 𝑥𝑘 + 𝛼𝑘 𝑑𝑘 and the evaluation of the function in the set of pull points is stopped. Step size update If the iteration is successful, the algorithm maintain or increase the step size. Otherwise, the step size is diminished. The procedure starts over again from the search step.

3.3. The optimization model The objective of this study was the maximization of the power output of the ORC system, for both the single loop and the dual loop ORC. Equations (9) – (20) report the objective function and the constraints of the optimization problem in the single loop case. OBJECTIVE FUNCTION 𝑃𝑡𝑢𝑟𝑏𝑖𝑛𝑒 − 𝑃𝑝𝑢𝑚𝑝

(9)

𝑇1 − 𝑇1,𝑐𝑓 ≥ 𝛥𝑇𝑚𝑖𝑛

(10)

𝑇7 − 𝑇7,𝑐𝑓 ≥ 𝛥𝑇𝑚𝑖𝑛

(11)

𝑇6𝑎 − 𝑇6𝑎,𝑐𝑓 ≥ 𝛥𝑇𝑚𝑖𝑛

(12)

𝑇2𝑎,ℎ𝑠 − 𝑇2𝑎 ≥ 𝛥𝑇𝑚𝑖𝑛 𝑇3,ℎ𝑠 − 𝑇3 ≥ 𝛥𝑇𝑚𝑖𝑛 𝑇5,ℎ𝑠 − 𝑇5 ≥ 𝛥𝑇𝑚𝑖𝑛 𝑇6 − 𝑇2𝑎 ≥ 𝛥𝑇𝑚𝑖𝑛 𝑇6𝑎 − 𝑇2 ≥ 𝛥𝑇𝑚𝑖𝑛 𝑇5 ≤ 𝑇𝑑𝑒𝑐𝑜𝑚𝑝

(13) (14) (15) (16) (17) (18)

𝑇10 ≤ 𝑇3 ℎ2𝑎 ≤ ℎ3

(19) (20)

CONSTRAINTS

Where 𝛥𝑇𝑚𝑖𝑛 is the minimum temperature difference allowed in the heat transfer process, which is set to 10 𝐾 and 𝑇𝑑𝑒𝑐𝑜𝑚𝑝 is the decomposition temperature of the fluid. The subscripts are in agreement with the thermodynamic points displayed in Figure 3. The decision variables are listed in Table 4.


Table 4 Decision variables of the optimization problem

Decision variables Working fluid mass flow rate Bottom pressure of the ORC cycle Top pressure of the ORC cycle Super-heating rate Degree of regeneration

𝑚̇𝑤𝑓 𝑝𝑏𝑜𝑡𝑡𝑜𝑚 = 𝑝2 𝑝𝑡𝑜𝑝 = 𝑝1 𝛥𝑇𝑠ℎ = 𝑇5 − 𝑇4 ℎ6 − ℎ9 𝑅= ∈ [0 − 1] ℎ6 − ℎ7

Notice that the optimization problem of the dual loop ORC presents as the objective function the sum of the power produced by each single loop ORC while the constraint set and the set of the decision variable are duplicated.

4. Results The objective of section 4 is twofold. First, the results of the thermodynamic optimization of the plant layouts under investigation is presented. Then, based on the thermodynamic optimization, a thermo-economic analysis is presented, which highlights the power output of the plant for which the dual loop configuration presents a lower specific cost for the energy produced.

4.1. Thermodynamic optimization results Figure 4 shows the comparison of the specific power output with respect to the mass flow rate of exhaust gas of the single loop configuration (see Figure 1) for several temperature of the gas, when considering Toluene and R 1233 zdE as the working fluids.

Specific power output [kW/kg/s]

60

Single loop ORC power output

50 40 30 20 10 0 150 C

200 C Toluene

250 C

300 C

350 C

R 1233 zdE

Figure 4 Comparison of Toluene and R 1233 zdE in the single loop configuration


Notice that Toluene performs better than R 1233 zdE when the temperature of the exhaust gas is high (i.e. over 300 °C) while R 1233 zdE performs better than Toluene in any other case. As it has already been demonstrated in the past, there exist a correlation between the critical temperature of the working fluid (see Table 1) and the temperature of the heat source. Figure 5 shows the comparison between the single loop ORC using Toluene as the working fluid and the dual loop configuration.


70

Toluene single loop vs dual loop ORC

60 50 40 30 20 10 0 150 C

200 C Single loop

250 C

300 C

350 C

Dual loop

Figure 5 Comparison of the Toluene single loop ORC and the dual loop ORC

The dual loop configuration ensures an increase in the specific power output of the system for any given temperature of the exhaust gas. However, Figure 5 highlights that the percentage increase in the power output varies at different temperature of the heat source. Figure 6 shows the comparison of the single loop ORC when R 1233 zdE is used as the working fluid and the dual loop ORC system.


70

R 1233 zdE single loop vs dual loop ORC

60 50 40 30 20 10 0 150 C

200 C Single loop

250 C

300 C

350 C

Dual loop

Figure 6 Comparison of the R 1233 zdE single loop ORC and the dual loop ORC


It is interesting to notice that the performance of the single loop ORC using R 1233 zdE as the working fluid is similar to that of the dual loop configuration, when the temperature of the exhaust gas is less than 300 °C (see Figure 6). When the temperature of the exhaust gas is above 300 °C, the performance of the dual loop ORC is better than those of the single loop system. It can be concluded that dual loop configuration does not improve the performance of the system with respect to the single loop configuration using R 1233 zdE as the working fluid in the exploitation of the thermal energy of the exhaust gas in case their temperature is below 300 °C. When the temperature of the heat source is above 300 °C, the dual loop configuration allows to increase the production of power with respect to the single loop configuration of up to 31.9%. The obtained optimal values of the decision variables are presented in Table 5 for each heat source temperature and working fluids. Table 5 Optimization procedure main results (mHS=1kg/s)

Type

Working fluid

THS,in

Single Loop Single Loop Single Loop Single Loop Single Loop Single Loop Single Loop Single Loop Single Loop Single Loop Dual Loop Dual Loop Dual Loop Dual Loop Dual Loop Dual Loop Dual Loop Dual Loop Dual Loop Dual Loop

Toluene Toluene Toluene Toluene Toluene R1233zdE R1233zdE R1233zdE R1233zdE R1233zdE Toluene R1233zdE Toluene R1233zdE Toluene R1233zdE Toluene R1233zdE Toluene R1233zdE

150 200 250 300 350 150 200 250 300 350 150

[°C]

200 250 300 350

𝑚̇𝑤𝑓 [kg/s]

𝑝2 [kPa]

𝑝1 [kPa]

𝛥𝑇𝑠ℎ [°C]

𝑅 [%]

P [kW]

𝑇𝐻𝑆,𝑜𝑢𝑡 [°C]

0.107 0.1818 0.2624 0.3465 0.414 0.3313 0.5222 0.7216 0.7388 0.8819 0.000766 0.3299 0.001 0.5293 0.0642 0.7191 0.1092 0.8476 0.1509 0.95

20 20 20 20 20 192 197 203 203 208 20 192 20 197 20 203 20 207 20 210

77 161 319 617 400 855 2298 3500 3500 3500 160 843 487 2010 1066 3500 1915 3500 3432 3500

0 0 0 0 0 0 0 40 98 100 12 1 12 1 19 8 29 21 36 35

0 0 0 0 0 0 0 0 0 0 14 9 85 11 87 46 87 73 94 80

3.19 8.68 17.19 29.37 54.32 6.29 15.11 30.18 38.355 45.9 0.038 6.45 0.0815 15.88 6.98 26.32 14.58 34.2 23.9 41.5

103 115 122 112 77 80 85 50 50 51 150 80 199 83 218 57 242 50 269 51

Table 5 shows that the optimization of the dual loop configuration leads to negligible values for the Toluene mass flow rate when the temperature of the heat source is below 250 °C. This implies that a single loop loop configuration using R 1233 zdE as the working fluid would be more appropriate when the temperature of the heat source is low. As opposed to the low temperature case, when the temperature of the heat source increases above 300 °C, the dual loop configuration ensures the production of a power output that is greater with respect to the single loop configurations analysed. The reason being that a better usage of the thermal energy content of the heat source can be obtained when the temperature of the heat source is above 300 C. These authors [9] proposed a dual loop ORC system considering Water and R 245fa as the working fluids. The results obtained in this work show that the dual loop configuration using Toluene and R 1233 zdE as the working fluids results in an increase of the recovered power with respect to the one proposed in [9]. In the rest of the paper, the thermo-economic comparison of the configurations analysed will be carried on, to investigate if the dual loop configuration becomes more cost effective than the single loop configuration in the power range 300 – 1000 kW.


4.2. Thermo-economic comparison of optimized ORC plants Thermo-economic analysis combines the exergy analysis of a process with the evaluation of all the costs related to this process evaluated with a techno-economic analysis [12]. The evaluation of the cost of the product of an energy conversion system is performed with a technoeconomic analysis. The correct cost evaluation is achievable when all the fixed and variable costs linked to the process are determined. Several economic approaches can be used to estimate the major costs involved in a plant. In this paper the total revenue requirement (TRR) method is adopted [13]. Once the purchase cost of the components is evaluated, this method allows to calculate the cost rate Żk associated to each component. The cost correlations based on the exponential scaling law taken from Turton et al. [14] have been used to evaluate the cost of the components. The goal of the thermoeconomic analysis is the calculation of the costs associated with all the streams included in the process, above all the cost of the final product (electricity). The cost balance of each component lead to the cost balance for the overall system, in which the fuel cost entering the system (equal to zero for ORC using wasted heat) and the total investment and O&M cost rate of the plant are balanced by the cost of the final products of the process. Figure 7 and Figure 8 show the comparison of the obtained cost of electricity [€/kWh] for the Toluene, R1233 zdE and dual loop ORC plants as a function of the hot gases temperature for 300 kW and 1000 kW. Notice that the plant has been considered to operate for 70 % of the time for 25 years with an interest rate of 5 %. The size affects the global costs through an inverse proportion: increase of the size leads to a decrease of the costs, furthermore temperatures might have an impact, as between 200 °C – 250 °C dual loop shows to be the most convenient choice while it becomes Single loop Toluene for higher temperature.

Cost of electricity (P=300kW) Dual

Toluene

R1233zdE

0.04 0.035

[€/kWh]

0.03 0.025 0.02 0.015 0.01 0.005 0 150

200 250 300 Temperature of the heat source [°C]

350

Figure 7 Comparison of the cost of electricity [€/kWh] for the Toluene, R1233 zdE and dual loop ORC plants as a function of the hot gases temperature (Power output =300 kW).


Cost of electricity (P=1000 kW) Dual

Toluene

R1233 zdE

0.04 0.035

[€/kWh]

0.03 0.025 0.02 0.015 0.01 0.005 0 150

200 250 300 Temperature of the heat souce [°C]

350

Figure 8 Comparison of the cost of electricity [€/kWh] for the Toluene, R1233 zdE and dual loop ORC plants as a function of the hot gases temperature (Power output =1000 kW).

Figure 9 and Figure 10 show the cost of electricity, expressed in €/kWh, of the different configurations proposed when the temperature of the heat source is respectively 200 °C and 300 °C . It is interesting to notice that, when the temperature of the heat source is 200°C, the dual loop system results as the most cost-effective plant scheme in terms of electricity produced for the considered range of 3001000kW, whilst when the temperature of the heat source is higher, the single loop using Toluene as the working fluid offers lower specific cost.

Cost of electricity (THG=200°C) Dual

Toluene

R1233 zdE

0.024

[€/kWh]

0.022 0.02 0.018 0.016 0.014 0.012 0.01 0

200

400

600 P [kW]

800

1000

1200

Figure 9 Comparison of the cost of electricity [€/kWh] for the Toluene, R1233 zdE and dual loop ORC plants as a function of the output power (Hot gases temperature =200 °C).


Cost of electricity (THG=300°C) Dual

Toluene

R1233 zdE

0.024

[€/kWh]

0.022 0.02 0.018 0.016 0.014 0.012 0.01 0

200

400

600 P [kW]

800

1000

1200

Figure 10 Comparison of the cost of electricity [€/kWh] for the Toluene, R1233 zdE and dual loop ORC plants as a function of the output power (Hot gases temperature =300 °C).

Figure 11 shows the comparison of the specific investment cost [€/kW] of the different system architectures proposed.

a)

Specific cost of the plant (R 1233 zdE) 2600 2400

[€/kW]

2200 THG=150°C 2000

THG=200°C

1800

THG=250°C

1600

THG=300°C

1400

THG=350°C

1200 300

400

500

600

700

P [kW]

800

900

1000


b)

Specific cost of the plant (Toluene)

[€/kW]

2600 2400

THG=150°C

2200

THG=200°C

2000

THG=250°C

1800

THG=300°C

1600

THG=350°C

1400 1200 300

400

500

600

700

800

900

1000

P [kW]

c)

Specific cost of the plant (Dual cycle) 2600 2400

[€/kW]

2200 2000

THG=200°C

1800

THG= 250°C THG=300°C

1600

THG=350°C

1400 1200 300

400

500

600

700

800

900

1000

P[kW]

Figure 11 Comparison of the specific investment cost [€/kW] for the R1233 zdE (a), Toluene (b) and dual loop (c) ORC plants as a function of the output power and hot gases temperature.

It is possible to notice that the single loop configuration using Toluene as the working fluid offers the lowest specific investment cost among the different options analysed (see Figure 11b). In particular, the lowest specific investment cost can be reach considering a temperature of the exhaust gasses of 300 °C. This system shows indeed the lower specific investment cost due to a better thermodynamic


heat exchange matching between hot gas and working fluid which directly allows to reduce the heat exchanger design and manufacturing costs. Nevertheless, the actual advantage of an ORC system has to be evaluated considering the mass flow rate of the exhaust gasses, which provides a direct measure of the size of the engine where the hot gases themselves come from. Figure 12 shows the comparison among the different plant schemes analysed considering both the specific investment cost and the specific mass flow rate of the exhaust gas.

Figure 12 Comparison of the specific investment cost [€/kW] and specific hot gas mass flow rate [kg/d/kW] for the Toluene, R1233 zdE and dual loop ORC plants as a function of gases temperature (Output power =1000kW).

As shown in Figure 12, the Single Loop configuration using Toluene as the working fluid presents higher values of mHG /kW ratios with respect to the other configurations. The Dual Loop and the single loop configuration using R 1233 zdE as the working fluid have similar tendencies up to T=300°C. When the temperature of the heat source exceeds 300 °C, the dual loop system becomes more performing in that is requires less mass flow rate of the heat source for the production of the same amount of power. For what concern the specific investment cost, Figure 12 shows that the Dual Loop configuration results to be more convenient than the single loop configurations analysed when the temperature of the heat source is equal to 200 °C.


5. Conclusions In this paper, a thermo-economic comparison of single loop and dual loop ORC schemes has been performed. The objective was the investigation of whether the dual loop system is convenient over the single loop systems, both from a thermodynamic and a thermo-economic point of view. The results show that from a thermodynamic perspective the dual loop configuration allows to increase the amount of power produced per unit mass flow rate of the heat source. This advantage increases as the temperature of the heat source increases. Specifically, an improvement in the power production of up to 31.9% has been obtained using the dual loop configuration proposed. From a thermo-economic perspective, the dual loop configuration results more cost effective than the single loop configurations considered when the temperature of the heat source is equal to 200°C, where the dual loop configuration achieves the specific cost of 1250 €/kW in the production of 1000 kW electric power.

References [1] Bianchi M, De Pascale A. Bottoming cycles for electric energy generation: parametric investigation of available and innovative solutions for the exploitation of low and medium temperature heat sources. Applied Energy. 2011;88:1500-9. [2] Hung T-C. Waste heat recovery of organic Rankine cycle using dry fluids. Energy Conversion and Management. 2001;42:539-53. [3] Yu G, Shu G, Tian H, Wei H, Liu L. Simulation and thermodynamic analysis of a bottoming Organic Rankine Cycle (ORC) of diesel engine (DE). Energy. 2013;51:281-90. [4] He S, Chang H, Zhang X, Shu S, Duan C. Working fluid selection for an Organic Rankine Cycle utilizing high and low temperature energy of an LNG engine. Applied Thermal Engineering. 2015;90:579-89. [5] Tian H, Shu G, Wei H, Liang X, Liu L. Fluids and parameters optimization for the organic Rankine cycles (ORCs) used in exhaust heat recovery of Internal Combustion Engine (ICE). Energy. 2012;47:125-36. [6] Braimakis K, Preißinger M, Brüggemann D, Karellas S, Panopoulos K. Low grade waste heat recovery with subcritical and supercritical Organic Rankine Cycle based on natural refrigerants and their binary mixtures. Energy. 2015;88:80-92. [7] Li G. Organic Rankine cycle performance evaluation and thermoeconomic assessment with various applications part I: Energy and exergy performance evaluation. Renewable and Sustainable Energy Reviews. 2016;53:477-99. [8] Zhang H, Wang E, Fan B. A performance analysis of a novel system of a dual loop bottoming organic Rankine cycle (ORC) with a light-duty diesel engine. Applied Energy. 2013;102:1504-13. [9] Sciubba E, Tocci L, Toro C. Thermodynamic analysis of a Rankine dual loop waste thermal energy recovery system. Energy Conversion and Management. 2016;122:109-18. [10] Franchetti B PA, Pesmazoglou I, Sciubba E, Tocci L. Thermodynamic and technical criteria for the optimal selection of the working fluid in a mini - ORC. ECOS 2016 conference. 2016. [11] Molés F, Navarro-Esbrí J, Peris B, Mota-Babiloni A, Barragán-Cervera Á, Kontomaris KK. Low GWP alternatives to HFC-245fa in Organic Rankine Cycles for low temperature heat recovery: HCFO-1233zd-E and HFO-1336mzz-Z. Applied Thermal Engineering. 2014;71:204-12. [12] El-Sayed YM. The Thermoeconomics of Energy Conversions: Elsevier; 2003. [13] Bejan A, Tsatsaronis G, Moran MJ. Thermal Design and Optimization. New York: Wiley; 1996. [14] Lecompte S, Lemmens S, Huisseune H, van den Broek M, De Paepe M. Multi-Objective Thermo-Economic Optimization Strategy for ORCs Applied to Subcritical and Transcritical Cycles for Waste Heat Recovery. Energies. 2015;8:2714.