Composition optimisation of working fluids for Organic Rankine Cycles and Kalina cycles

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Energy xxx (2013) 1e13

Contents lists available at SciVerse ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Composition optimisation of working ﬂuids for Organic Rankine Cycles and Kalina cycles Rachel Anne Victor a, Jin-Kuk Kim b, *, Robin Smith a a b

Centre for Process Integration, School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, UK Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 January 2013 Received in revised form 15 March 2013 Accepted 26 March 2013 Available online xxx

An optimisation model for the composition of mixed working ﬂuids for (Organic Rankine Cycles) ORCs and Kalina cycles has been developed. The temperatures investigated were 100 Ce250 C for the heat source and 30 C for the heat sink. The optimisation method of the composition was carried out with Simulated Annealing technique with the objective function of maximising the thermal efﬁciency of the cycle, based on 1 MW of heat source. The results show that the pure component organic ﬂuids are more energy-efﬁcient than mixed organic ﬂuids. The selection of organic working ﬂuids was studied for achieving maximum cycle efﬁciency at a given operating temperature. The composition of the Kalina cycle was also optimised and it was found that for a maximum temperature of 250 C, the minimum ammonia concentration in the ammoniaewater mixture was 73.8% mol fraction. A novel consideration of employing alcoholewater mixtures in the cycle was also investigated and the most efﬁcient mixture at 250 C was methanol and water mixture when compared to the Kalina cycle and steam Rankine Cycle. The study showed overall that the optimal choice of working ﬂuids for a particular type of cycle would depend on the operating temperature and pressure. The developed design method can be useful for engineers to evaluate the performance of the cycles considering a wide range of working ﬂuids, and determines the optimal choice of working ﬂuids and operating conditions of the cycle. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Organic Rankine Cycles Kalina cycles Mixed ﬂuids Optimisation

1. Introduction Sustainable power generation has become increasingly important due to the accelerated consumption of fuels over the recent years. The excessive use of fossil fuels not only depletes these natural resources at an exponential rate, but also contributes to serious environmental problems, such as global warming. It is necessary to identify alternative sources of energy that could be utilised for energy generation with minimal harmful effects to the environment. Current environmental concerns have led to interest in the recovery and utilisation of low-grade heat sources. Typical examples of low-grade heat sources are geothermal energy, biomass energy, solar heat and waste heat from industrial processes such as ﬂue gas and exhaust gases. The interest in these energy sources requires special attention because the temperatures at which these sources are available are relatively low (ranging from 60 C to 200 C). This

* Corresponding author. Tel.: þ82 2 2220 2331. E-mail address: [email protected] (J.-K. Kim).

results in low efﬁciency for power extraction, which makes power generation from such sources uneconomic. The Rankine cycle was initially identiﬁed as having the highest efﬁciency system when it was compared to other dynamic energy conversion systems [1]. Steam, the working ﬂuid for the conventional Rankine cycle, offered advantages such as low cost, nonﬂammability and non-toxicity [2]. In the case of low temperature heat sources, however, steam as a working ﬂuid operates at a relatively low efﬁciency. Alternatives to the Rankine cycle to increase the efﬁciency for low temperature applications are the (Organic Rankine Cycle) ORC and the Kalina cycle. The main challenge with the ORC is the choice of appropriate working ﬂuids. Aside from the working ﬂuid, a signiﬁcant difference between the conventional Rankine cycle and the ORC is the device employed for the expansion of the ﬂuid to generate power. The conventional Rankine cycle could utilise a steam turbine from the expansion of the pressurised steam, but the ORC would require a specialised expander device to cater for the properties of the organic working ﬂuid. The Kalina Cycle, introduced in the early 1980s, is a thermodynamic power cycle using a binary mixture of ammonia and water as the working ﬂuid. The binary properties of the mixture produce a

0360-5442/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.03.069

Please cite this article in press as: Victor RA, et al., Composition optimisation of working ﬂuids for Organic Rankine Cycles and Kalina cycles, Energy (2013), http://dx.doi.org/10.1016/j.energy.2013.03.069

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R.A. Victor et al. / Energy xxx (2013) 1e13

non-isothermal evaporation and condensation in the cycle which contributes to the higher efﬁciency as compared to pure working ﬂuids [3]. However, the main drawback identiﬁed in the Kalina cycle is the increase in the number of components required for the cycle. The objective of this paper is to investigate the performance of mixed working ﬂuids for the ORC and Kalina cycle for low temperature applications. The heat source and heat sink for this study were assumed at a constant temperature, therefore giving a ﬂat temperature proﬁle. For the ORC, various organic components were considered in this work. The temperature range investigated was between 100 and 250 C for both ORCs and Kalina cycles. The selection and/or composition optimisation of working ﬂuids for both ORCs and Kalina cycles was carried out with an automated design procedure, which had been implemented on commercial software, CRYO-intÒ(version 1.4) [4]. CRYO-intÒ is a commercial simulation and optimisation software package for the design of refrigeration cycles developed by Process Integration Limited. The optimal results were then further analysed with Aspen HYSYSÒ (version 2006.5) [5] for sensitivity analysis. 2. Literature review 2.1. Organic Rankine Cycle (ORC) The use of organic ﬂuids in a Rankine cycle system for low temperature applications, as shown in Fig. 1, has been known for a number of years. When the selection of the appropriate organic ﬂuids in the ORC is to be made, key issues to be considered are the thermodynamic and environmental performance of the cycle, as well as health and safety criteria.

Fig. 1. A simple closed Rankine Cycle.

Badr et al. [6] surveyed 68 possible organic working ﬂuids for the application of low-grade heat recovery for low power output applications. Three performance indicators were applied to evaluate the performance of working ﬂuids: heat transfer coefﬁcients of the working ﬂuids in the evaporator and condenser, feed pump power requirement, and the overall system energy efﬁciency. The working ﬂuid with the highest heat transfer coefﬁcient was implied to be the more favourable choice, because a high heat transfer coefﬁcient suggests a smaller heat transfer area which consequently reduces the capital cost of the heat exchangers. The feed pump power requirement for the working ﬂuids was related to the pressure differential across the pump, and the power required for pumping was less than 10% of the expander output. It was further suggested that high molecular weight compounds have less internal leakage rate in expanders, which results in a higher expander isentropic efﬁciency. These results produced were then compared to available experimental data, which showed a good agreement with their hypothesis. Devotta and Holland [7] compared the cycle efﬁciency of 24 working ﬂuids using four performance indicators for the selection of the appropriate working ﬂuid for the Rankine cycle, including the temperature at the evaporator, the temperature difference between the evaporator and condenser, the pressure ratio of the evaporator and condenser, and the thermal efﬁciency of the cycle. Thermodynamic properties of the working ﬂuids were obtained from published data, which were used to evaluate the cycle for the evaporator temperature of 80 C, 100 C, 120 C, 160 C and 200 C. 31 organic working ﬂuids were investigated for the application of a maximum working temperature of 100 C [8]. Different types of ORC cycles were identiﬁed for the purpose of matching the thermodynamic properties of the working ﬂuids to the requirements of the process. These cycles were classed based on two factors: the shape of the saturated vapour line in the temperature (T) e entropy (s) diagram and the state of the working ﬂuid from the evaporator to the turbine outlet. The two shapes of saturated vapour line discussed were the negative- and positive-sloped curve. The identiﬁcation of these shapes is important in order to choose the operating conditions for the different types of ﬂuids. For the expansion with negative-sloped saturated vapour line, the working ﬂuid enters the turbine inlet as a saturated vapour and leaves the turbine in the two phase region. It is important to note that for a conventional turbine expander, a certain liquid fraction tolerance can be accepted. Liquid formation in turbines can cause mechanical damage and erratic operations which are undesirable. The working ﬂuid entering the turbine inlet as a saturated vapour for the positive-sloped case leaves the turbine as a superheated vapour. Another case is a supercritical pressure cycle where the working ﬂuid is compressed up to supercritical condition before entering the turbine, and leaves in the two phase region. Another conﬁguration which could be applied for the supercritical pressure cycle is where the working ﬂuid leaves the turbine as a superheated vapour. The notation for 2a and 4a represents the points at which a recuperator is used in the cycle, shown in Fig. 1(b). Saleh et al. [8] also analysed the performance of the working ﬂuid with respect to the heat transfer between the heat source and working ﬂuid. It has been identiﬁed that the choice of working ﬂuid should take into account how the heat source is utilised in the cycle. Results showed that for subcritical cycles, achieving superheated vapour at the turbine inlet always gives a higher thermal efﬁciency. Coupling the recuperator with the superheated cycle was demonstrated to produce a more signiﬁcant increase in the thermal efﬁciency. Although the use of recuperators were addressed to be beneﬁcial for Rankine Cycles in most literature, Dai et al. [9] pointed out

Please cite this article in press as: Victor RA, et al., Composition optimisation of working ﬂuids for Organic Rankine Cycles and Kalina cycles, Energy (2013), http://dx.doi.org/10.1016/j.energy.2013.03.069

R.A. Victor et al. / Energy xxx (2013) 1e13

that the use of the recuperator is dependent on the type of cycle considered. It may be beneﬁcial to implement the heat exchange through the recuperator when the temperature of the working ﬂuid leaving the expander is signiﬁcantly higher than that of ﬂuid leaving the condenser. Lai et al. [10] further investigated alkanes, aromates and linear siloxanes as working ﬂuids for the maximum working temperatures of 250 C and 300 C, based on the same methodology by Saleh et al. [8]. Ranking points were used to analyse the performance of the working ﬂuids based on the minimum or maximum values of the thermal efﬁciency, volumetric ﬂowrate of inlet stream to the turbine, and the volumetric ﬂowrate of the outlet stream from the turbine. The analysis of the heat transfer from the heat source, however, involved minimising the heat capacity ﬂowrate for a given net work of 1 MW. The heat capacity ﬂowrate was then incorporated into the ranking point equation to analyse the performance of the working ﬂuid. Papadopoulos et al. [11] investigated the selection of optimal working ﬂuids for ORCs using (computer aided molecular design) CAMD and process optimisation techniques. For a simple closed Rankine Cycle, 15 properties based on thermodynamic, environmental, safety and process related properties were used as performance measures in the CAMD to generate suitable components for the cycle. The components generated were then employed in the process optimisation technique in which (Simulated Annealing) SA methods were used to minimise the cost of heat exchanger area. It was suggested that it would be reasonable to avoid any components with a signiﬁcantly higher maximum operating pressure and with a signiﬁcantly higher mass ﬂowrate than 10 kg/h. This assumption limits the choice of working ﬂuids, and the method employed is only limited to pure component ﬂuids, not for multicomponent mixture. The merits of employing mixed working ﬂuids were highlighted by Angelino and Colonna di Paliano [12]. A simple closed Rankine Cycle was considered in this study, with a recuperator where applied. The analysis was classiﬁed based on the cycle conﬁgurations as saturated cycle, superheated cycle and supercritical cycle, and the type of molecules as simple molecules and complex molecules. Their analysis could be summarised as mixed composition ﬂuids provide better heat proﬁle matching with the heat and cooling source. This is due to the non-isothermal evaporation and condensation of the ﬂuid which could reduce the mean temperature difference between the source and working ﬂuid in heat exchangers. However, it should be kept in mind that reducing the mean temperature difference implies that a higher heat exchanger area is required for a given heat ﬂow, suggesting a trade-off between operating and capital costs. 2.2. Kalina cycle The layout of a Kalina cycle is signiﬁcantly different from a Rankine cycle, due to the complexity of the design. In general, a Kalina cycle requires additional equipment involving a separator and intermediate heat exchangers. Ibrahim and Kovach [13] investigated the ammoniaewater mixture as working ﬂuids. The cycle conﬁguration is similar to the Rankine cycle with the addition of a separator downstream of the turbine. This conﬁguration allowed the manipulation of the ammoniaewater mixture, in which water is added to the ammoniarich stream before the condenser. The mixing of these streams allows a liquid mixture at the outlet of the condenser, as well as resulting in a condensing operating pressure above atmospheric pressure. An iterative energy balance procedure around the separator was used when manipulating the operating conditions (temperature, pressure and mass ﬂowrate) of the separator. Results

3

from this study showed that the mass fraction of ammonia-rich stream and the operating temperature and pressure of the separator affect the thermal efﬁciency of the cycle, and decreasing the separator pressure for a ﬁxed temperature and ammoniaewater composition increases the thermal efﬁciency of the cycle. The Kalina-type cycle studied was 10e20% more efﬁcient than a Rankine cycle under similar conditions, but no parameters or results of the Rankine cycle were provided for the comparison between the two cycles. Consideration of the turbine inlet conditions (composition and temperature) and the separator temperature as key parameters were further emphasised through an optimisation study done by Nag and Gupta [14]. Thermodynamic analysis was used to assess the optimal design for the cycle using ammoniaewater mixtures. This study also investigated the exergy loss in the system due to thermodynamic irreversibility for each component. Gibbs free energy equations for mixtures were used to evaluate the thermodynamic properties of the working ﬂuid. The second law cycle efﬁciency was then deﬁned as the ratio of the exergy output and the exergy input, taking into account the exergy losses in the components. The results suggested that increasing the temperature of the separator while decreasing the ammonia concentration at the inlet of the separator could increase the cycle efﬁciency. However, variation of the cycle efﬁciency for a given separator temperature with increasing ammonia concentration (0.55e0.80 mass fraction) varies at a maximum of 2%. An interesting fact that was highlighted in the exergy analysis is the different effects produced by the concentration of ammonia for different components. Increasing the ammonia concentration at the turbine inlet reduces the losses in the turbine, but increases the losses in the evaporator. The authors suggested that this is because low ammonia concentrations produce a better matching for the temperature proﬁle between the heat source and the working ﬂuid and therefore reduces the thermodynamic irreversibility of the system. The evaporator also exhibited the greatest losses as compared to the other components (turbine, pump and absorber) by nearly 30%. Ogriseck [15] investigated ﬁve Kalina cycle cases with varying ammoniaewater compositions, cooling water temperatures and condenser pressures based on an existing (combined heat and power) CHP plant. The layout of the Kalina cycle process investigated is similar to a Rankine cycle with the addition of two recuperators and a separator located upstream of the turbine expander. It was pointed out that the mixing ratio of the ammoniaewater mixture varies during the evaporation. This is due to the fact that ammonia has a lower boiling temperature than water and therefore evaporates more than water. The separator here is placed before the turbine expander so as to separate the ammonia-rich steam from the liquid phase mixture. This paper used the net electrical efﬁciency as the performance measure for the case studies. One important feature that was mentioned is the signiﬁcance of the condenser pressure to the efﬁciency of the cycle. Aside from utilising a lower temperature cooling utility, the pressure of the condenser could be reduced by decreasing the concentration of the ammonia in the mixture. 2.3. Comparison between ORC and Kalina cycle A study of the thermodynamic comparison between the Kalina and ORC cycle was carried out for the heat recovery from diesel engines [16]. The working ﬂuid for the ORC used was (hexamethyldisiloxane) MM. The thermodynamic properties of both MM and ammoniaewater mixture were calculated using commercial software. The Kalina cycle layout evaluated in this study is similar to the layout proposed by Ibrahim and Kovach [13] in which the separator is located downstream of the turbine followed by a series

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R.A. Victor et al. / Energy xxx (2013) 1e13

of heat exchangers and pumps. For the sake of simplicity, Bombarda et al. [16] assumed no pressure drops within the heat exchangers. This assumption suggests that the thermal efﬁciency of the cycle would be overestimated. The optimisation of the ammoniaewater mixture for maximum net work output required a very high operating pressure (over 100 bar). This suggested high capital and operating cost, and would not be reasonable especially for smallscaled plants. The ORC cycle considered for comparison in this study is a simple, closed cycle with a recuperator (Fig. 1b). The same parameters applied for the evaluation of the Kalina cycle were applied in the ORC cycle so as to perform a reasonable comparison. However, the condensing pressure applied in the ORC cycle for this working ﬂuid is 0.12 bar, below atmospheric pressure. As mentioned previously, not only is it inadvisable to operate below atmospheric pressure, but the condensing pressure also has a signiﬁcant effect on the thermal efﬁciency. Keeping all other variables constant, lowering the condensing pressure will increase the thermal efﬁciency of the cycle. The condensing pressure of 0.12 bar clearly suggests that the thermal efﬁciency of the ORC cycle will be signiﬁcantly higher than when operating at a more reasonable pressure (above atmospheric). Results from this study showed that the performance of the ORC cycle and the Kalina cycle at 100 bar to be competitive to one another. Reducing the pressure of the Kalina cycle to 50 bar showed a signiﬁcant decrease in the performance of the cycle. Zamﬁrescu and Dincer [17] proposed the use of a (Trileteral Flash Cycle) TFC employing the ammoniaewater mixture as working ﬂuids. The TFC, similar to the Rankine cycle, ﬂashes the working ﬂuid through a scroll expander into the two phase region and then fully condenses the working ﬂuid. It should be mentioned that unlike a turbine expander, a scroll expander has the advantage of operating with wet steam [18]. The TFC ammoniaewater cycle was ﬁrst analysed to maximise cycle efﬁciency, based on varying temperature difference at the condenser and varying turbine efﬁciency. Results showed that both parameters produced insigniﬁcant differences in the cycle efﬁciency for varying ammonia concentration. The study then proceeded to compare the ammoniaewater TFC cycle with four organic ﬂuids for the ORC as well as a Kalinatype cycle (Rankine cycle employing ammoniaewater mixture as a working ﬂuid). All cases applied a turbine inlet temperature of 150 C and the corresponding pressure of 6 bar. The four organic working ﬂuids used in this study were R-141b, R-123, R-245ca and R-21. The results from this comparison showed that all the organic ﬂuids achieved higher cycle efﬁciencies as compared with the ammoniaewater mixture. However, the exergy analysis suggested that the TFC cycle produced the highest exergy efﬁciency, more than double of the next highest exergy efﬁciency. This is due to the proﬁle matching obtained by the TFC conﬁguration as a single component cycle being less efﬁcient, due to its inability to match the heat proﬁle, especially at the condensing side. 2.4. Recent advances and summary Recent studies on ORCs have been extended to address complex cycle design which accommodates discontinuous waste heat recovery and considers the inﬂuence of outlet temperature of heat source on the cycle efﬁciency [19]. Industrial application of ORCs has been highlighted in the reﬁning process in which different working ﬂuids for using a process stream from 140 C to 45 C were tested and ranked in terms of maximum achievable power output, efﬁciency of the cycle, amount of the heat recovered, heat exchanger area, CO2 emissions and capital cost [20]. The utilisation of low grade heat in ORCs for steel industry has been investigated for improving energy efﬁciency through the environmental and techno-economic evaluation of a

case study using waste heat available from the stacks of a coke oven used in steel-making process [21]. Systematic investigation for using low grade heat in process industries has been made with the aid of process integration concept, which evaluates different technologies, including ORC, and identiﬁes the most appropriate way for industrial waste heat recovery [22]. The integrated application of the Kalina cycle in the context of energy generation systems has been reported. Peng et al.’s work [23] proposed an energy conversion system which combines the Kalina cycle with the intercooled gas turbine cycle in the solar thermal power system. The application of Kalina cycle for the coal ﬁred steam power plant has been investigated, which examines the possibility of using low-temperature heat of ﬂue gases for generating power with the aid of process simulation and optimisation techniques [24]. A review of studies on the Kalina cycle has been documented in the recent work, which addresses exergy and energy analysis for the Kalina cycle, explains different Kalina systems and properties of working ﬂuid. Also, engineering aspects in the design and operation of Kalina cycle has been discussed in the context of stability, environmental impacts, safety and corrosion [25]. A review of the previous work showed that the use of ORC and Kalina cycle has been proven to be beneﬁcial, especially for low temperature applications. Optimisation studies on the cycle outlined the signiﬁcant effects associated with choices of cycle parameters on the cycle efﬁciencies, for example, conﬁguration of cycles, use of recuperators and operating conditions on the condensing side. Although the importance of the choice of working ﬂuids to improve the performance of the cycle has been addressed, little work has been done to the computer-aided screening of optimal working ﬂuids, especially for mixed-composition ﬂuids. Also, the current paper aims to build a systematic design method which can effectively evaluate a wide range of possible working ﬂuids in Kalina and ORC cycles, and enables choice of the most appropriate working ﬂuids with their composition in a holistic manner. Therefore, the developed modelling and simulation framework can provide a practical and reliable decision-support tool for achieving energy-efﬁcient ORC and Kalina cycle in practice. Another major contribution from this work includes the detailed and thorough consideration of alcoholewater mixture in the optimisation study, leading to high energy efﬁciency in the energy generation. 3. Base case: modelling of a Rankine Cycle Although it is evident from many publications that the conventional steam Rankine Cycle is highly inefﬁcient for low temperature applications, this section intends to model a steam Rankine Cycle as a base case for the purpose of producing a reasonable comparison with the ORC and Kalina cycles. The cycle analysed in this study was the simple, closed Rankine cycle operating at subcritical conditions. The four main components considered in this cycle were the evaporator, turbine, condenser and pump as shown in Fig. 1. With the given parameters for this study as shown in Table 1, the objective of the case studies is to optimise the performance of the cycle for 1 MW of heat supplied at the evaporator for the temperature range from 100 C to 250 C. The temperatures of the heat source and heat sink were assumed to be constant. For the sake of simplicity, the study considered only above atmospheric pressure operations hence a minimum condensing pressure of 1 bar, which also avoids potential problems with the ingress of air into the cycle. Cost and safety concerns have been raised in previous publications regarding the maximum allowable pressure for a system. A maximum pressure of 50 bar was chosen so

Please cite this article in press as: Victor RA, et al., Composition optimisation of working ﬂuids for Organic Rankine Cycles and Kalina cycles, Energy (2013), http://dx.doi.org/10.1016/j.energy.2013.03.069

R.A. Victor et al. / Energy xxx (2013) 1e13

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Table 1 Cycle design parameters considered. Turbine isentropic efﬁciency, hturb Pump isentropic efﬁciency, hpump Minimum condensing pressure, P3, min Maximum turbine inlet pressure, P1, max Minimum temperature difference, DTmin Cooling water outlet temperature Condenser outlet temperature, T3 Pressure drop in heat exchangers

0.80 0.65 1 bar 50 bar 10 C 20 C 30 C 1 bar

as not to signiﬁcantly constrict any optimisation opportunities within the cycle. The mechanical efﬁciency for the pump and the generator efﬁciency for the turbine have been assumed to be unity, which would be a fair assumption as literature values suggested these efﬁciencies to be approximately 98% [15]. Thermodynamic calculations of the working ﬂuids were performed using Aspen HYSYSÒ (version 2006.5) simulator [5]. The following equations describe the thermodynamic modelling of each component under steady-state conditions. The amount of heat supplied by the heat source to the working ﬂuid in the evaporator, Qin, was expressed as shown in Equation (1) and the amount of heat rejection from the working ﬂuid to the environment, Qout, was expressed as shown in Equation (2).

_ 1 h4 Þ Q in ¼ mðh

(1)

_ 2 h3 Þ Q out ¼ mðh

(2)

Expressions for Qin and Qout would be deﬁned as shown in Equations (3) and (4) when applying a recuperator to the cycle.

_ 1 h4a Þ Q in ¼ mðh

(3)

_ 2a h3 Þ Q out ¼ mðh

(4)

The mechanical work generated by the expander, Wturb, and the mechanical work required by the pump, Wpump, were expressed as shown in Equations (5) and (6).

_ 2 h1 Þhturb Wturb ¼ mðh Wpump ¼

_ 4 h3 Þ mðh

hpump

(5) (6)

The net work generated by the cycle, Wnet, was therefore deﬁned as the difference between Wturb and Wturb:

Wnet ¼ Wturb Wpump

(7)

The ﬁrst and second law cycle efﬁciencies were applied to measure the performance of the cycle. The ﬁrst law cycle efﬁciency was deﬁned as the ratio of net work generated to the heat source supplied to the system (Equation (8)). However, hi alone does not measure the efﬁciency with regards to the true potential of the cycle [26]. The second law cycle efﬁciency, hii, was then applied, with relation to the Carnot cycle efﬁciency, hCC, which represents the maximum thermodynamic efﬁciency of the cycle. A better representation of the cycle efﬁciency could be achieved by calculating hii, which represents the effectiveness of the cycle with regards to the maximum energy that could have been extracted from the system in Equation (9).

hi ¼

Wnet Q in

(8)

hii ¼

hi hCC

(9)

Fig. 2. Simulation results: steam Rankine Cycle.

The steam Rankine Cycle shown in Fig. 1(a) was modelled in Aspen HYSYSÒ [5] employing PengeRobinson (Equation of State) EOS for the thermodynamic calculation. The conditions for a temperature of 30 C at 1 bar were speciﬁed for the outlet of the condenser. Due to the pressure drop speciﬁcation of 1 bar throughout the heat exchangers, the minimum working temperature of the steam Rankine Cycle modelled was 130 C at 2.7 bar (saturated vapour condition). The results of the base case simulation are shown in Fig. 2. The results shown in Table 2 were presented for turbine inlet temperature of 150 C, 200 C and 250 C, will be used later on for comparison with the other cycles. The results showed a decrease in efﬁciency as the working temperature decreases. Detailed results of the simulation for this base case can be found in Appendix A. For the same turbine inlet temperature, applying superheated vapour conditions would reduce the pressure of the working ﬂuid and hence the work extracted from the turbine. To demonstrate this, an analysis was performed for 250 C with a 10 C superheated conditions. The pressure at which steam is available for this condition was calculated to be 33 bar. Results showed that hi of the cycle decreased as compared to the saturated vapour condition of the same temperature. The hii of the cycle also decreased because the maximum available energy of the cycle remains constant. Therefore, only saturated vapour conditions at the turbine inlet were considered in the study, to maximise efﬁciency opportunities. It should also be noted that the base case modelled is not an optimum steam Rankine Cycle. An optimum cycle would employ the normal boiling temperature at the outlet of the condenser at which the liquid would be compressed from a higher temperature, reducing the work required by the pump. The increase in temperature at the condenser would also require a higher working ﬂuid ﬂowrate. 4. Selection of optimal working ﬂuids for ORC 4.1. Performance of pure working ﬂuids A wide range of organic compounds have been investigated. The majority are alkanes, cycloalkanes, aromates, siloxanes, alcohols, Table 2 Simulation results of a steam Rankine Cycle. Vapour condition

T1 ( C)

P1 (bar)

hi (%)

hCC (%)

hii (%)

Saturated vapour Saturated vapour Saturated vapour Superheated Vapour

150 200 250 250

4.8 15.4 39.1 33.0

4.65 10.68 15.01 14.29

87.50 90.47 92.30 92.30

5.31 11.80 16.26 15.48

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Table 3 Commercially-viable HCFCs identiﬁed by Montreal Protocol 2009.

Table 5 Additional organic compounds considered in the current study.

Group

Substance

ODP

GWP

Substance

Chemical formula

Group

CHFCl2 CHCl2CF3 CHFClCF3 CH3CFCl2 CH3CF2Cl

HCFC-21 HCFC-123 HCFC-124 HCFC-141b HCFC-142b

0.040 0.020 0.022 0.110 0.065

210 120 620 700 2400

i-Pentane n-Heptane n-Octane 22-Mbutane 33-Mpentane 2-Mheptane i-Butene 1-Pentene 1-Hexene 1-Heptene 1-Octene Propadiene Cyclopropane Cyclobutane Cyclohexane Cycloheptane Cyclooctane Cyclobutene Cyclopropene Ethanol 2-Propanol 1- Butanol

C5H12 C7H16 C8H18 C6H14 C7H16 C8H18 C4H8 C5H10 C6H12 C7H14 C8H16 C3H4 C3H6 C4H8 C6H12 C7H14 C8H16 C4H6 C5H8 C2H6O C3H8O C4H10O

Alkanes Alkanes Alkanes Alkanes Alkanes Alkanes Alkenes Alkenes Alkenes Alkenes Alkenes Allenes Cycloalkanes Cycloalkanes Cycloalkanes Cycloalkanes Cycloalkanes Cycloalkenes Cycloalkenes Alcohols Alcohols Alcohols

(chloroﬂuorocarbons) CFCs and (hydrochloroﬂuorocarbons) HCFCs. A number of HCFCs which have been identiﬁed by the Montreal Protocol as the most commercially-viable substances (Table 3) were considered in this work [27]. Eliminating CFC compounds, potential organic working ﬂuids which have been investigated from various previous studies are summarised in the list seen in Table 4. Results from these studies would not provide a reasonable comparison due to the fact that each study applied different cycle parameters. Therefore, the performance of these ﬂuids was analysed again based on the parameters used in this study. Expanding on organic ﬂuids studied in previous works, a further 22 organic ﬂuids were considered in this study, listed in Table 5. Additional substances from the same group with similar chemical formula were chosen to search potential working ﬂuids. The performances of the 40 organic working ﬂuids listed in Tables 3e5 were evaluated as pure component working ﬂuids in Aspen HYSYSÒ. The cycle conﬁguration for this analysis is the steam Rankine Cycle shown in Fig. 1(a) which is based on conventional simple cycle widely used in the production of power from heat [11,28]. The same cycle parameters given in Table 1 are used. The operating conditions for the outlet of the condenser would be speciﬁed for 30 C at 1 bar. The EOS applied in the simulations for the hydrocarbons (alkanes, alkenes, allene, cycloalkanes, cycloalkenes, aromatics, HCFCs and organoﬂourines) was PengeRobinson, while the EOS applied for the analysis of the alcohols was NRTL model. The results shown in Figs. 3e6 are the performance of various working ﬂuids for saturated vapour cycle conﬁgurations over the temperature range of 100 C and 250 C. The general trend observed from different chemical groups was that the higher molecular weight compounds produced a lower efﬁciency. However, the range at which these compounds could operate extended to the higher temperature more than that of the lower molecular weight compounds. This might be due to the higher critical temperature and higher normal boiling point. The critical pressure, however, decreased as the molecular weight increase, subsequently reducing the entropy difference between the two pressures. Therefore, the efﬁciency of the cycle was reduced.

The results showed that for the temperature range of 100 C and 150 C, C4 and C5 hydrocarbons produced the highest efﬁciencies within its group. The performance of refrigerants at this temperature was also similar to the hydrocarbons. For the temperature range of 150 Ce200 C, the ﬂuids with high energy efﬁciency were the refrigerants and methanol. At 250 C, only benzene exhibited an efﬁciency of higher than 10%. Compared to the performance of steam at this pressure, it could be concluded that the use of pure component organic ﬂuids was more beneﬁcial than steam for up to the temperature of approximately 200 C. 4.2. Composition optimisation of ORC working ﬂuids CRYO-intÒ is a commercial simulation and optimisation software package that can carry out the design and the optimisation of mixed working ﬂuids for refrigeration cycles [4]. The refrigeration cycle is essentially the reverse process of a power cycle, in which power input for the compressors is required, instead of extracting power from the system. The software was modiﬁed to incorporate power cycle components, like the turbine and pump to accommodate for the optimisation of mixed composition for power cycle working ﬂuids. CRYO-intÒ has two optimisation methods for both the refrigeration and power cycles: (Simulated Annealing) SA and (Non-Linear Programming) NLP [4]. To simulate a process, a ﬂowsheet for the cycle has to be generated followed by a set of calculation sequence.

Table 4 Potential organic working ﬂuids identiﬁed from previous work. Substance

Chemical formula

Group

n-Butane/R600a i-Butane/R600 n-Pentane n-Hexane 22-Mpropane Cyclopentane Benzene Toluene o-Xylene p-Xylene Diﬂuoroethane/R152a Tetraﬂuoroethane/R134a Methanol

C4H10 C4H11 C5H12 C6H14 C5H12 C5H10 C6H6 C7H8 C8H10 C8H10 C2H4F2 C2H2F4 CH4O

Alkanes Alkanes Alkanes Alkanes Alkanes Cycloalkanes Aromatics Aromatics Aromatics Aromatics Organoﬂourine Organoﬂourine Alcohol

Fig. 3. Performance of pure component working ﬂuids e alkanes.

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Fig. 4. Performance of pure component working ﬂuids e alkenes, allene and aromatics. Fig. 6. Performance of pure component working ﬂuids e HCFCs, organoﬂourines and alcohols.

The default optimisation objective is to minimise shaftpower. However, for the case of a power cycle, the speciﬁed objective is to maximise efﬁciency, which has to be incorporated into the calculation sequence. It should be noted that the modelling and optimisation framework can readily accommodate capital cost of the cycle in the objective function for carrying out more rigorous economic trade-off between operating and capital costs. However, optimisation study carried out is to focus on maximising energy efﬁciency, which is a reliable and sound performance indicator for comparing the cycle in a common design basis. Inclusion of capital costing may not be able to provide realistic design guidelines, due to considerable uncertainty in the capital costing parameters. A summary of the optimisation method is shown in Fig. 7. Although built-in physical properties calculation methods in CRYO-intÒ could be used for the simulation, CRYO-intÒ allows the use of the physical property library which is available in Aspen HYSYSÒ [4]. This allowed more ﬂexibility with the choice of physical property calculations, especially for the optimisation of mixed component ﬂuids where binary interaction parameters were required. The Rankine cycle ﬂowsheet generated in CRYO-intÒ is shown in Fig. 8(a) [4]. One of the limitations in the CRYO-intÒ software is the pressure drop speciﬁcation within heat exchangers. Letdown valves have been incorporated into the ﬂowsheet to accommodate for the loss of net work, due to pressure drops from the condenser and evaporator. A summary of the calculation sequence implemented for the composition optimisation of the working ﬂuids is shown in Fig. 8(b). A difﬁculty identiﬁed for optimising the mixed composition was to specify a minimum condensing pressure for the working ﬂuid at 30 C. If the desired pressure and temperature were speciﬁed at the condenser, there was the probability that the optimiser would set the working ﬂuid to be in the vapour phase.

The calculation of the pump would therefore lead to a negative shaftwork, misleading the objective function into maximising the negative shaftwork to maximise hi. To overcome this, the objective function was expressed as Equation (10). This expression, although not the most elegant method, would not only avoid the resulting condensing pressure below atmospheric pressure, but also kept the condensing pressure to a minimum where possible.

objective function ¼ hi

P3 1 P32

(10)

The optimisation method employed in the study was the SA (Simulated Annealing) method. This method is an effective method for obtaining the minimum of an objective function for highly nonlinear problem, although computational time, as compared to conventional deterministic NLP (nonlinear programming) methods, is much longer [29]. The optimisation was ﬁrst performed to select a most energyefﬁcient pure ﬂuid to be used for the turbine inlet temperatures of 100 C, 150 C, 200 C and 250 C and the results obtained are shown in Table 6. Different turbine inlet temperatures were then applied for the selected compound with the Aspen HYSYSÒ simulator [5], and the sensitivity of turbine inlet temperature on the cycle efﬁciency was obtained, as depicted in Fig. 9. The initial assumption of the project was that a mixed composition ﬂuid would result in a better performance than pure working ﬂuids. However, a closer look at the interactions between the compounds proved that there is no signiﬁcant beneﬁt for using mixed components for achieving a better performance, due to insigniﬁcant interactions between the compounds, especially for hydrocarbonehydrocarbon mixtures. When two compounds with different efﬁciencies were mixed, the resulting ﬂuid with mixed composition would be an average of the two. An example to illustrate this is shown with a mixture of n-Butane and n-Hexane. Two sets of mixtures for n-Butane and n-Hexane were investigated with 50e50 mol fraction and a 70e30 mol fraction for n-Butane and nHexane respectively. The cycle efﬁciencies of these mixtures are shown in Fig. 10. As expected, the efﬁciency of the resulting mixture lies between the efﬁciencies of the pure components. As the mol fraction of n-Butane in the mixture was increased, the trend of the efﬁciency of the mixture increasingly follows the trend for nButane. 5. Composition optimisation of a Kalina cycle

Fig. 5. Performance of pure component working ﬂuids e cycloalkanes and cycloalkenes.

The conﬁguration of a Kalina cycle could vary from the location of the separator (before or after the turbine) to the number of

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R.A. Victor et al. / Energy xxx (2013) 1e13

Fig. 7. Optimisation method in CRYO-int simulation software.

recuperators employed in the cycle. For the purpose of optimising the composition of the ammoniaewater mixture, the Kalina cycle in this work was scaled down to the most fundamental level, replicating the conﬁguration of a simple closed Rankine Cycle [11,28]. A saturated vapour conﬁguration was applied to the ammoniaewater mixture, hence eliminating the need for a separator. However, when the maximum pressure restriction of 50 bar was applied on the cycle, a two-phase mixture might form at the outlet of the evaporator, depending on the operating pressure. A separator was then placed before the turbine in this conﬁguration (Fig. 11). The rest of the cycle remained the same (i.e. two heat exchangers, a pump and a turbine). Water is a highly polar compound due to the hydrogen bonding, causing signiﬁcant interactions in the binary mixture, unlike hydrocarbonehydrocarbon mixtures which have small interactions. Built-in interaction parameters for PengeRobinson EOS for the ammoniaewater mixture are available on the simulation software Aspen HYSYSÒ [5], which was used to calculate the thermodynamic properties. The performance of ammonia as a pure ﬂuid was ﬁrst investigated and compared to the performance of steam. For a saturated vapour cycle conﬁguration, ammonia showed a signiﬁcantly high efﬁciency, even when comparing to the performance of organic ﬂuids, at a low temperature (Fig. 12). This was due to the low normal boiling point of the component. As compared to the normal boiling point of water, the extent to which ammonia could be applied for low temperature applications is promising. However, due to the low critical temperature of ammonia (i.e. 134 C), the saturated vapour cycle conﬁguration, with the cycle parameters set in this work, was only effective up to 130 C. As the concentration of the water increased in the ammoniae water mixture, the normal boiling point of the mixture increased. The computational time for the composition optimisation could be reduced by specifying the upper limit of the water composition to avoid operating below atmospheric pressure at the condenser side. The maximum water concentration for the ammoniaewater binary mixture is identiﬁed with the calculation sequence as given in Fig. 13.

Table 6 Selection of optimal pure compounds and its cycle efﬁciency.

Fig. 8. Simulation and optimisation framework for Rankine Cycles.

T1 ( C)

100

150

200

250

Compound selected

i-Butane

n-Butane

HCFC-141b

Benzene

hi (%) hii (%)

10.71 13.09

14.25 16.29

15.15 16.74

12.83 13.9

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Fig. 9. Sensitivity of turbine inlet temperature for selected working ﬂuids.

Fig. 11. Flowsheet of a Kalina cycle with a separator.

A temperature range from 25 C to 40 C was investigated for this analysis. Table 7 shows that as the condensing temperature increases, the maximum concentration of water in the mixture increases. This study considered a condensing temperature of 30 C, therefore the resulting maximum water mol fraction in the ammoniaewater mixture was 0.7549. A higher water fraction in the mixture would force the operation below atmospheric pressure. This value was used as the upper limit concentration for the optimisation of the ammoniaewater mixture. The Kalina cycle in CRYO-intÒ was modelled as a simple closed Rankine Cycle, similar to the ORC cases. The same cycle parameters were also applied for this case for consistency. The same calculation sequence for the ORC was used, but only with the two components: ammonia and water. The objective function for this calculation sequence was to maximise the ﬁrst law cycle efﬁciency hi. The expression related to pressure, as shown for the optimisation of the organic ﬂuids, was not necessary, due to the water concentration speciﬁcation. The optimisation of the ammoniaewater composition was performed for turbine inlet temperatures of 100 C, 150 C, 200 C and 250 C. Table 8 shows the optimal ammoniaewater composition in mol fractions generated from optimisation. These optimal results were then used in Aspen HYSYSÒ for performing sensitivity analysis as shown in Fig. 14. The results showed that for a saturated vapour cycle, as the turbine inlet temperature increased, the efﬁciency of the cycle could be improved by increasing the concentration of water in the mixture. Another sensitivity analysis was performed to

investigate whether a lower ammonia concentration of the mixture would produce a higher efﬁciency. The different concentrations of ammonia were investigated, and the results in Fig. 15 showed that reducing the concentration of ammonia further than 73.8% would not produce a cycle with better efﬁciency when the upper limit for the stream temperature of turbine inlet is 250 C. Although the efﬁciency of the cycle with mixed composition was lower than that with optimal composition, the performances were still higher than that of using steam alone. The optimal pressures for the ammoniaewater mixture produced for the saturated vapour conﬁguration were shown to be over the maximum pressure constraint. For that reason, another set of optimisations was performed while applying the maximum pressure constraint of 50 bar. Table 9 shows the results from the composition optimisation at 50 bar. Although steam presented a higher efﬁciency than an ammoniaewater mixture at 250 C, steam was a liquid (water) at 250 C at 50 bar. Allowing a reduced pressure of 39.1 bar showed that steam was a better ﬂuid than the ammoniaewater mixture at 250 C. The results of the sensitivity analysis for these results are shown in Fig. 16. Due to the low critical temperature of ammonia, the working range of ammonia for a saturated vapour conﬁguration is limited. However, compressing the working ﬂuid to 50 bar increased the working temperature range to the superheated vapour region, which allowed the ammonia to be used at higher temperatures beyond 130 C. On the other hand, the ammoniaewater mixture was limited to be operated at a lower pressure. Depending on the temperature and pressure, the mixture would exist as a two-phase mixture. This would result in not only an increase in the required

Fig. 10. Cycle efﬁciency of mixed organic ﬂuids.

Fig. 12. Performance of ammonia and water as pure components.

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R.A. Victor et al. / Energy xxx (2013) 1e13

Fig. 14. Sensitivity analysis of optimal ammoniaewater composition.

Fig. 13. Summary of maximum water concentration calculation sequence.

ﬂowrate, but a reduced efﬁciency due to fractions of the mixture that would not be expanded through the turbine. 6. Investigation of novel alcoholewater mixtures The fact that the Kalina cycle could yield high efﬁciencies despite being a mixed composition ﬂuid, initiated the research into organic-water mixtures. The majority of the organic ﬂuids investigated in this study are relatively insoluble in water, especially the alkanes and cycloalkanes [30]. Among the organic compounds which were investigated in this work, the alcohol group was the most soluble group with water. The alcohol compounds considered for the optimisation of alcoholewater compositions were methanol, ethanol, 2-propanol and 1-butanol. These components and water were investigated in the optimisation framework for identifying the optimal composition at the temperature of 250 C. This temperature was chosen on the basis that the ammoniaewater mixture yielded the highest efﬁciency at this temperature.

Table 7 Maximum water concentration in ammoniaewater binary mixture to avoid operating below atmospheric pressure. Condensing temperature, T3 ( C) Mole fraction Ammonia Water

25

30

35

40

0.2711 0.7289

0.2451 0.7549

0.2210 0.7790

0.1984 0.8016

Table 8 Optimal ammoniaewater composition. Turbine inlet temperature, T1 ( C) Pressure (bar) Mole fraction Ammonia Water Efﬁciency hi (%) hii (%)

100 62.5 1 0 13.16 16.08

Fig. 15. Sensitivity analysis of ammonia concentration.

Table 10 shows the results obtained from the optimisation of the alcoholewater composition. A sensitivity analysis for this composition was made to compare between the performance of the alcohol as a pure ﬂuid and the performance of steam. The operating pressure of the mixture, however, was above the maximum operating pressure of 50 bar for the saturated vapour cycle conﬁguration. An analysis on the performance of the mixture with a limiting pressure of 50 bar was carried out. As seen in Fig. 17, the efﬁciency of the mixture was higher for the temperature range of 220 Ce 250 C, exceeding the efﬁciency of the steam Rankine Cycle. Even with at 50 bar, the mixture is shown to have a higher performance for the range of 230 Ce250 C. The performance of the ﬂuid below that temperature would result in a two-phase mixture at the turbine inlet. Another sensitivity analysis was performed to consider impact of using different alcohols. The optimisation resulted in steam as the better performing working ﬂuid. An ethanol mol fraction of

Table 9 Optimal ammoniaewater composition with the pressure constraint of 50 bar. 150

200

250

107.6 0.949 0.051 15.87 18.14

129.7 0.886 0.114 16.85 18.62

178.1 0.738 0.262 17.66 19.13

Turbine inlet temperature, T1 ( C) Pressure (bar) Mole fraction Ammonia Water Efﬁciency hi (%) hii (%)

100 50 1 0 10.52 12.86

150 50 0.935 0.065 11.71 13.38

200 50 1 0 12.44 13.75

250 50 1 0 13.20 14.30

250 39.1 0 1 15.01 16.26

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Fig. 18. Analysis of ethanolewater mixture. Fig. 16. Sensitivity analysis for ammoniaewater mixture with pressure constraint.

Table 10 Optimal methanolewater mixture. Composition T1 ( C) P1 (bar) Efﬁciency

Methanol Water

hi (%) hii (%)

0.73488 0.26512 250 77.2 16.74 18.14

250 50.0 15.97 17.3

obtained. For the 100 Ce130 C, ammonia as a pure component had the highest efﬁciency followed by the n-Butane for the range of 130 C and 150 C. The ammoniaewater mixtures, at the different compositions, seemed to be the best performing ﬂuid at higher temperatures up to 250 C. However, it should be kept in mind that these performances required a high operating pressure, up to 180 bar. The performance achieved by ammonia also required an operating pressure above the maximum pressure which was set for the study. Applying the maximum pressure constraint, a different set of optimal working ﬂuid choices was obtained as shown in Fig. 20. The

60.2% in the mixture showed a higher efﬁciency of 12.32%. It could be seen from the trend in Fig. 18 that the mixture can provide a better cycle performance for the range of 170 Ce230 C. Grouping the results together, a pure methanol working ﬂuid was the best ﬂuid up to 220 C followed by the methanolewater mixture for up to 250 C. 7. Overall comparison The analysis of steam, organic working ﬂuids, ammoniaewater mixture and alcoholewater mixture presented different optimum working ﬂuids and compositions at different temperature intervals. These results were combined together in this section to investigate the optimum choice of working ﬂuid as a whole. The working ﬂuids which appeared to be the most appropriate choice for each of the cases are ﬁrst presented in Fig. 19 without any operating pressure constraints. The results showed that the optimal components were very competitive to one another, with a difference of thermal efﬁciencies as close as 1%. Also Fig. 19 clearly demonstrates that inappropriate choice of working ﬂuids and associated operating conditions results in a considerable reduction in energy efﬁciency about 5e12%, compared to the best efﬁciencies

Fig. 17. Sensitivity analysis of methanolewater mixture.

Fig. 19. Overall comparison for optimal working ﬂuids without pressure constraint.

Fig. 20. Overall comparison for optimal working ﬂuids with maximum pressure constraint.

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R.A. Victor et al. / Energy xxx (2013) 1e13

hydrocarbons and the refrigerant presented as the more efﬁcient working ﬂuids for the ranges of 100 Ce200 C, because of the characteristics of the components to produce high efﬁciency at lower operating pressures. Between 200 C and 230 C, methanol showed the highest efﬁciency among the working ﬂuids followed by a methanolewater mixture to up to 250 C. The results of the methanolewater mixture were shown to perform better than the Kalina cycle and steam-based cycle even with limiting the operating pressure.

8. Conclusions and future work This research showed that mixed-composition ﬂuids might only be beneﬁcial if there are strong interactions between the components, like the ammoniaewater mixture and the alcoholewater mixture. Due to weak interactions between the organic hydrocarbons, the pure component ﬂuids have been shown to produce higher efﬁciencies in a cycle. The composition of ammonia and water mixture should be optimised to achieve the optimal performance. Different compositions were obtained at different temperatures and pressures. The alcoholewater mixtures have showed potential in increasing the efﬁciency of a cycle, especially at temperatures of 220 Ce250 C. The optimisation framework developed in this study has been proven to produce reliable results for the optimisation of pure components and multi-component ﬂuids. Without the aid of the automated design procedure and optimisation solver, manually investigating different compositions for an optimum performance would have been too lengthy. The developed design framework allows users to evaluate impacts of using different working ﬂuid in the cycle in conﬁdence, and to fully appreciate the beneﬁt gained from simultaneous decision on the choice of working ﬂuid and optimal operating conditions. From Figs. 19 and 20, it was clearly illustrated that considerable loss in energy efﬁciency around 5e12%, can be avoided with the careful consideration in the choice of working ﬂuid and operating conditions. The results presented in this paper, however, were limited to a single cycle conﬁguration, namely, the saturated vapour cycle. Alternative cycle conﬁgurations to increase the efﬁciency of the cycle could include the use of superheated vapour at the turbine inlet, supercritical pressure cycles and implementing the use of a recuperator or even multiple recuperators in the cycle. The results presented in this paper were also limited to a constant temperature proﬁle for the heat source and heat sink. For a sloping temperature proﬁle, the organic mixture may be more beneﬁcial than the pure components for the exploitation of the temperature proﬁle. The difference between the sloping temperature proﬁle and the constant temperature proﬁle would suggest that the optimisation of the mixtures (organic, ammoniaewater or alcoholewater) would have been different. The results presented for novel alcoholewater mixtures showed considerable potential in increasing the efﬁciency of a power cycle.

Acknowledgement Financial support from European Union’s FP7 Programme EFENIS project (No. 296003) is gratefully acknowledged. This research was supported by the International Research & Development Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 20110031290).

Appendix A. Rankine Cycle results Table A1 Details of Rankine Cycle Simulation Results. Mass Wnet T1 P1 T3 P3 Molar ﬂow (kW) ( C) (bar) ( C) (bar) ﬂow (kmole/s) (kg/s)

hi

hii

(%)

(%)

Condensing duty (kW)

130 140 150 160 170 180 190 200 210 220 230 240 250

1.63 3.19 4.65 6.02 7.30 8.50 9.62 10.68 11.66 12.59 13.45 14.26 15.01

1.90 3.68 5.32 6.82 8.21 9.50 10.69 11.80 12.83 13.79 14.68 15.50 16.26

984 968 953 940 927 915 904 893 883 874 865 857 850

2.7 3.6 4.8 6.2 7.9 10.0 12.5 15.4 18.9 23.0 27.6 33.0 39.1

30 30 30 30 30 30 30 30 30 30 30 30 30

1 1 1 1 1 1 1 1 1 1 1 1 1

0.0211 0.0210 0.0208 0.0207 0.0206 0.0205 0.0204 0.0203 0.0203 0.0202 0.0201 0.0201 0.0201

0.38 0.38 0.38 0.37 0.37 0.37 0.37 0.37 0.37 0.36 0.36 0.36 0.36

16.30 31.92 46.52 60.20 73.00 85.00 96.24 106.78 116.64 125.88 134.52 142.59 150.11

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Nomenclature _ heat capacity ﬂowrate [kW/ C] C: h: enthalpy [kJ/kg]

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_ mass ﬂowrate [kg/s] m: P: pressure [bar] Q_ : heat ﬂow [kW] s: entropy [kJ/K] T: temperature [ C] _ volumetric ﬂowrate [m3/s] V: W: work [kW] Greek symbols h: efﬁciency [e] Abbreviations CAMD: computer-aided molecular design CFC: chloroﬂuorocarbon CHP: combined heat and power EOS: equation of state GWP: global warming potential HCFC: hydrochloroﬂuorocarbon MM: hexamethyldisiloxane NLP: non-linear programming ODP: ozone depleting potential ORC: Organic Rankine Cycle SA: simulated annealing TFC: trilateral ﬂash cycle

Please cite this article in press as: Victor RA, et al., Composition optimisation of working ﬂuids for Organic Rankine Cycles and Kalina cycles, Energy (2013), http://dx.doi.org/10.1016/j.energy.2013.03.069