Performance analysis of an organic Rankine cycle for a reverse osmosis desalination system using zeotropic mixtures

Performance analysis of an organic Rankine cycle for a reverse osmosis desalination system using zeotropic mixtures

Desalination 381 (2016) 38–46 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Performance an...

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Desalination 381 (2016) 38–46

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Performance analysis of an organic Rankine cycle for a reverse osmosis desalination system using zeotropic mixtures Donghan Geng ⁎, Yuhong Du, Ruiliang Yang Tianjin Polytechnic University, Tianjin 300387, China

H I G H L I G H T S • An ORC for RO desalination system with zeotropic mixture is proposed. • The effects of seawater temperature increase on the cycle performance are analyzed. • The best composition of mixtures and seawater temperature increase are identified.

a r t i c l e

i n f o

Article history: Received 6 July 2015 Received in revised form 25 November 2015 Accepted 26 November 2015 Available online xxxx Keywords: Organic Rankine cycle Reverse osmosis Zeotropic mixture Temperature rise

a b s t r a c t The use of low-grade thermal energy to power desalination processes by coupling an organic Rankine cycle (ORC) with seawater reverse osmosis (RO) is a promising technology to reduce the cost and environmental impact associated with the use of fossil fuel sources. A low-enthalpy geothermal ORC for a RO desalination system with zeotropic mixtures is proposed. Zeotropic mixtures can improve the thermodynamic performance of ORC systems owing to their excellent temperature glide characteristics during evaporation and condensing processes. A case study with butane/pentane (R600/R601) and butane/isopentane (R600/R601a) is investigated, aiming to analyze the effect of seawater temperature increase on the cycle performance. In the temperature range investigated, the power profit first increases rapidly then decreases. For the mixture R600/R601 at a mole fraction of 0.9/0.1, the maximum power profit value of 29.3 kW occurred with the temperature rise of 26 K, and for the mixture R600/R601a at a mole fraction of 0.9/0.1, the maximum power profit value of 30.9 kW occurred with the temperature rise of 27 K. The results show that, it is necessary to consider the effects of seawater temperature increase in both the ORC and RO simultaneously to design the ORC-RO system. © 2015 Elsevier B.V. All rights reserved.

1. Introduction There are three dominant seawater desalination technologies, multieffect distillation, multi-stage flash and reverse osmosis (RO). Of these, RO has gradually gained more popularity, mainly because of the recent progress in the membrane industry and development of high-efficiency energy recovery equipments [1]. In RO process, energy prices remain a drawback in terms of desalination economics [2]. To reduce the cost and environmental impacts associated with the use of fossil fuel sources, one possible solution is to adopt renewable sources [3]. Of the available renewable sources, the use of low-temperature thermal energy, including waste heat, geothermal heat and solar sources, for organic Rankine cycle (ORC) coupled

Abbreviations: ORC, organic Rankine cycle; RO, reverse osmosis; HPP, high pressure pump; TDS, total dissolved solids; GWP, global warming potential; ODP, ozone depression potential; NDP, net driving pressure. ⁎ Corresponding author. E-mail address: fi[email protected] (D. Geng).

http://dx.doi.org/10.1016/j.desal.2015.11.026 0011-9164/© 2015 Elsevier B.V. All rights reserved.

with reverse osmosis desalination has received an increasing amount of interest. Manolakos et al. [4] pioneered theoretical and experimental research in this field. They presented a system that the high pressure pump (HPP) of RO was directly driven by the expander of the ORC sub-system, and the results proved that their concept was technically feasible. Later, they proposed a cascade ORC cycle for RO desalination consisting of two cycles of different temperatures, and proved that the proposed two-stage ORC significantly improved the efficiency and reduced the cost of the previously-developed single-stage lowtemperature ORC for RO desalination [5]. However, Delgado-Torres et al. [6] argued that the efficiency of a single ORC with R245fa was higher than that of a two-stage system within the same temperature conditions. Nafey et al. [7] made a detailed investigation of a combined ORC-RO desalination system with different types of solar collectors, based on energy, exergy, and economic analyses. Moreover, they also did a thermo-economic analysis of different energy recovery units and concluded that the pressure exchanger configuration was more economical than either stand alone or Pelton wheel turbine configurations

D. Geng et al. / Desalination 381 (2016) 38–46

Nomenclature m m p Q s W x A TCF FF ST SE NE Pnet Π Pf ΔPfs ΔTE ΔTC

specific enthalpy (kJ/kg) mass flow rate (kg/s) pressure (MPa) heat transfer rate (kW) specific entropy (kJ/(kg·K))h power (kW) mole fraction membrane's permeability to water (m3/(m2·s·bar)) temperature correction factor membrane fouling factor total membrane's effective surface area (m2) membrane area of each element (m2) number of membrane elements net driving pressure (MPa) osmotic pressure in the concentrate flow (MPa) feed pressure at the inlet of the vessel (MPa) average pressure losses between the feed and concentrate flows (MPa) evaporator pinch temperature (K) condenser pinch temperature (K)

Greek symbols η efficiency Subscripts 1–6 point corresponding to Fig. 2 exp. expander con condenser p pump eva evaporator th thermal efficiency net net work output pro power profit in inlet out outlet f working fluid h heat source sw seawater s isentropic

[8]. Li et al. [9] proposed a supercritical organic Rankine cycle for a seawater reverse osmosis system with two types of heat sources. In addition,they also presented a co-generation system to produce electricity and freshwater using a solar supercritical ORC coupled with a desalination unit. This system can reduce the negative impact of intermittent solar energy without using thermal energy storage by converting solar energy to desalinated water [10]. In the design of the ORC cycle, one crucial issue is the selection of working fluids for the given temperature. Previous studies regarded more about pure working fluids; however, zeotropic mixtures have received significant attention recently, because of their excellent temperature glide characteristics during evaporation and condensation processes, which lead to a better temperature match with heat source/sink. This allows for the system irreversibility to be minimized and the cycle efficiency to be increased. Angelino et al. [11] made a comparison of n-pentane and mixture nbutane/n-hexane for a low-temperature ORC system. Their results showed that the mixture yielded 6.8% more electricity than the pure working fluid. Wang et al. [12–13] made a theoretical and experimental analysis of a zeotropic mixture of R245fa/R152a. The results showed that the zeotropic mixture had higher collector and thermal efficiencies than the pure R245fa. Chen et al. [14] proposed a supercritical ORC

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system with zeotropic mixtures. The comparison of R134a/R32 (0.7/ 0.3) and R134a suggested that the cycle efficiency with R134a/R32 increased by 10–30%. Heberle [15] investigated the exergetic efficiency of a subcritical cycle with isobutene/isopentane and R227ea/R245fa. The case study indicated that exergetic efficiency increased by 4.3% to 15%. Chys et al. [16] presented a mixture selection method and optimized mixture concentrations to realize the maximum ORC thermal efficiency for the low-temperature heat source, which increased from 10.85% to 11.57% with zeotropic mixture of isopentane/cyclohexane. Liu et al. [17] presented a method to determine the optimal ORC condensation pressure using the zeotropic mixtures and investigated the effects of the condensation temperature glide of the zeotropic mixture on the ORC thermodynamic performance. Lecompte et al. [18] examined the cycle performance of a subcritical ORC with zeotropic mixtures, their study suggested that the second law efficiency with mixtures can be improved by 7.1% to 14.2% and the exergy loss of condenser can be reduced with 3% to 6%. Radulovic et al. [19] conducted a parametric optimization of a supercritical ORC with six mixtures. Their results indicated that the thermal and exergetic efficiencies of R143a/R124 were higher than those for R143a/RC318 in the range of the parameters investigated. Guo et al. [20] analyzed the effects of three types of working fluids, a mixture that matched the heat source, a mixture that matched the heat sink and a pure working fluid on the performance of ORC system. The result suggested that the mixture that matched the heat sink achieved the highest thermal efficiency. Zhao et al. [21] presented a thermodynamic model mainly including Jacob number and the ratio of evaporation and condensation temperatures, to forecast the thermal efficiency, work output and exergetic efficiency of the ORC cycle with zeotropic working fluids. Mavrou et al. [22] investigated the performance of working fluid mixtures including conventional working fluids and mixtures designed by the method proposed by Papadopoulos [23]. The result suggested that the mixture of neopentane/2-Fluoromethoxy2-methylpropane (70%/30%) exhibited the best performance. It can be observed that the Rankine cycle performance with zeotropic mixtures have been investigated in terms of the thermal efficiency, the exergetic efficiency and net work output as a function of different process parameters, such as the mixture type, mixture composition, temperature of the heat source, etc. However, unlike the sole ORC system, the coolant in the ORC-RO system is seawater, and its temperature rise also directly affects the RO performance. Consequently, both the temperature match of the seawater with the mixtures and the effect of the seawater temperature rise on the RO performance should be considered simultaneously for the design of the ORC-RO system. However, there have been no reports on this subject till now. Therefore, the cycle performance of an ORC-RO system with zeotropic working fluids will be analyzed in the current study, with emphasis on the effects of the seawater temperature rise. 2. Cycle 2.1. System description The ORC-RO system analyzed in this study is shown in Fig. 1, which is composed of an ORC engine and a desalination unit. A heat source of low-temperature geothermal water and heat sink of raw water are used as the external inputs for the proposed system. A basic Rankine cycle is adopted, including a feed pump, an evaporator, an expander, and a condenser. The working fluid undergoes the following processes: pressurized by the pump, the working fluid is heated by the heat source and vaporized in the evaporator, and the generated high pressure vapor expands through the expander and generates power for the RO system, then, the low pressure vapor at the outlet of the expander is cooled by the feed seawater in the condenser, and pumped back to the evaporator, completing the cycle. In the desalination sub-system, feed seawater is used to condense the working fluid, which consequently preheats the seawater. The shaft

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D. Geng et al. / Desalination 381 (2016) 38–46

Fig. 1. Schematic of ORC-RO system.

In this section, detailed models and performance parameters for the main components are presented.

where mf is the mass flow rate of the working fluid, h2s is the working fluid enthalpy after the isentropic expansion process, h1 and h2 are the enthalpies of the working fluid in the inlet and outlet of the expander, respectively. For a non-superheated subcritical cycle, the turbine inlet temperature is equal to the dew point temperature for zeotropic mixtures at the expander inlet pressure (also the pressure of point 6).

2.2.1. Thermodynamic modeling of ORC unit The thermodynamic processes for an ORC system with a zeotropic mixture are illustrated in the T-S diagram (Fig. 2). The corresponding analysis of the Rankine engine under steady state conditions is carried out based on mass and energy balance. Furthermore, heat loss and pressure drops are not considered. The basic equations for each component are described as follows.

2.2.1.2. Condenser (corresponding to process 2–4 in Fig. 2). This is an isobaric heat rejection process, where 2–3 refers to a precooling process (the mixture is cooled to saturated-vapor state), and 3–4 refers to a phase change process (the mixture is cooled from saturated-vapor state to saturated-liquid state). Point 3 refers to the saturated-vapor state under the condensation pressure. The total transferred energy Qcon is expressed as:

power of the expander drives the high pressure pump (HPP), which raises the pressure of preheated water and forces it through the membrane. 2.2. Mathematical models

2.2.1.1. Expander (corresponding to process 1–2 in Fig. 2). In Fig. 2, 1–2s is the isentropic expansion process under ideal conditions, while 1–2 is the actual expansion process. The isentropic efficiency of expander ηeva is defined as: ηeva ¼

h1 −h2 : h1 −h2s

ð1Þ

The work output of expander is calculated as: W exp ¼ m f ðh1 −h2 Þ

ð2Þ

 f   Q con ¼ mf h f ;2 −h f ;4 ¼ msw hsw;out −hsw;in

ð3Þ

where msw is the mass flow rate of the feed seawater, hf,2 and hf,4 are the enthalpies of the working fluid in the inlet and outlet of the condenser, respectively, hsw,in and hsw,out are the enthalpies of the feed seawater in the inlet and outlet of the condenser, respectively. For a subcritical cycle with a zeotropic working fluid, the location of the pinch point can exist at the dew point of the mixture in the condenser while the temperature glide of the mixture is lower than the temperature increase of the heat sink, otherwise at the bubble point of the mixture [17]; in this case with the seawater heat sink, the location of the pinch point exists at the dew point (illustrated in Fig. 2). ΔT C ¼ T f ;3 −T SW;3 :

ð4Þ

2.2.1.3. Pump (corresponding to process 4–5 in Fig. 2). In Fig. 2, process 4–5 s is the isentropic pressurization process under the ideal condition, while process 4–5 is the actual pressurization. The isentropic efficiency of pump ηp is defined as: ηp ¼

h5s −h4 : h5 −h4

ð5Þ

The power applied by a pump with an isentropic efficiency Wp can be determined as:  W p ¼ m f h5 −h4 Þ

Fig. 2. Temperature-entropy diagram of ORC with zeotropic working fluid.

ð6Þ

where h5s is the working fluid enthalpy after the isentropic pressurization process, h4 and h5 are the enthalpies of the working fluid in the inlet and outlet of the pump, respectively. For a subcritical cycle with saturated liquid at the pump inlet, the pump inlet temperature is equal to the bubble point temperature for zeotropic mixtures at the pump inlet pressure.

D. Geng et al. / Desalination 381 (2016) 38–46

2.2.1.4. Evaporator (corresponding to process 5–1 in Fig. 2). This is an isobaric heat absorption process where 5–6 refers to a preheating process (the mixture is heated to saturated-liquid state), and 6–1 refers to a phase change process (the mixture is heated from saturated-liquid state to saturated-vapor state). Point 6 refers to the saturated-liquid state under the evaporation pressure. The total transferred energy Qeva is expressed as:     Q eva ¼ m f h f ;1 −h f ;5 ¼ mh hh;in −hh;out

ð7Þ

where mh is the mass flow rate of the heat source, hf,1 and hf,5 are the enthalpies of working fluid in the inlet and outlet of the evaporator, respectively, hh,in and hh,out are the enthalpies of the heat source in the inlet and outlet of the evaporator, respectively. For the subcritical cycle in this study, the pinch point exists at the incipient boiling point of the working fluid, since the heat source is geothermal water (in Fig. 2). ΔT E ¼ T h;6 −T f ;6 :

ð8Þ

2.2.1.5. Performance indicators. The net work output Wnet, the thermal efficiency ηth and the relative difference of thermal efficiency Sη are defined as follows: W net ¼net W exp −W p

ð9Þ

ηth ¼ W net =Q eva

ð10Þ

Sη ¼ dηth jdT rise;sw :

ð11Þ

2.2.2. Mathematic modeling of RO unit The mathematical model of RO unit was presented by Crossley [24] and developed by DOW Chemical Company [25]. The permeating water flow rate is a function of the permeability factor for water molecules A, the total membrane's effective surface area ST, the temperature correction factor TCF, the membrane fouling factor FF, and the net driving pressure Pnet. Q p ¼ A  ST  TCF  FF  P net :

ð12Þ

ð13Þ

where SE is the element area, NE is number of membrane elements. The temperature correction factor TCF is given as:    8 1 1 > > ; T sw ≥25 C − < exp 2640  298 273 þ T sw  TCF ¼ 1 1 > > : exp 3020 ; T sw b25 C − 298 273 þ T sw

ð14Þ

where Tsw is the seawater temperature. The net driving pressure Pnet is a function of the feed pressure at the inlet of the vessel Pf, the osmotic pressure in the concentrate flow Π, and the average pressure losses between the feed and concentrate flows ΔPfs, which can be expressed as: 1 P net ¼ P f − ΔP fs −ΔΠ: 2

ð15Þ

The power consumption of RO unit Wneed can be calculated as: W need ¼ P f msw;in =ηHPP :

3. Case study 3.1. Technical details Based on the above system model, steady-state simulations are carried out on MATLAB2010b platform, connecting to REFPROP 9.0, which can provide the thermodynamic properties of the working fluids. The validity of the designed RO system is verified with ROSA9.0 software. The simulated system is designed for a given water demand of 78 m3 per day with maximum salt concentration of 500 ppm of water product. SW30HRLE-400i RO membranes are used, and a single-stage RO system using 1 pressure vessel and 7 elements in the vessel connected are considered for this system. The operating parameters and boundary conditions of the ORC and RO modules are listed in Table 1 and Table 2, respectively. 3.2. The selection of working fluid Because there was no investigation about the effects of the seawater temperature increase in the condenser in previous studies, working fluids are selected based on their properties and operating conditions in the current case [26]. Isentropic and dry fluids are preferred for non-superheated subcritical cycles, because wet fluids may drop into the two-phase region, leading to liquid droplet impingent in the turbine blades. Considering the environmental aspects, working fluids with a relatively high global warming potential (GWP) and ozone depletion potential (ODP), are excluded from consideration. Based on the analysis outlined above, R600, R601 and R6001a are selected as the components for the synthesis of the zeotropic mixture. Table 3 shows a list of the considered working fluids. As for the determination of the composition of individual constituents, just as suggested by Heberle et al. [14] and Zhao et al. [27], component separation phenomenon should be taken into consideration, because the temperature difference in the phase-change process causes the difference of the circulating composition and charge composition, leading to lower performance of ORC system. In this paper, mole fractions that allow for a temperature glide of less than 15 °C are error allowed, and the effect of the composition shift can be neglected. The temperature-entropy diagrams of R600/R601 (at a mole fraction of 0.1/0.9, 0.5/0.5, and 0.9/0.1) and R600/R601a (at a mole fraction of 0.1/0.9, 0.5/0.5, and 0.9/0.1) are shown in Fig. 3. 4. Results and discussion

The total membrane's effective surface areaSTis given as: ST ¼ NE  SE

41

ð16Þ

4.1. Condensation temperature glides of zeotropic mixtures for various mole fractions The condensation temperature glide of the mixtures varies with the component mole fractions. The mole fractions in this study are varied from 0.1 to 0.9, Fig. 4 depicts the condensation temperature glides of two types of zeotropic mixtures for various mole fractions. It can be observed that for the mixture R600/R601, the maximum temperature glide with a value of 10.56 °C occurs at the a mole fraction of 0.5/0.5, Table 1 Operating parameters for the geothermal ORC. Parameter

Value

Geothermal water flow rate, mh (kg/s) Geothermal water inlet temperature, Th,in (°C) Evaporating temperature, TE (°C) Evaporator pinch temperature, ΔTE (K) Condenser pinch temperature, ΔTC (K) Inlet flow rate of seawater water, msw (kg/s) Inlet temperature of seawater, Tsw,in (°C) Isentropic efficiency of the expander, ηexp (%) Isentropic efficiency of feed pump, ηp (%)

1 150 Variable 10 5 3 5 85 75

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Table 2 Operating parameters for the RO unit. Parameter

Value

Inlet seawater TDS (mg/l) Water classification Seawater recovery (%) Element type Effective area of each membrane, SE (m2) Maximum operation pressure, Pmax (MPa) Average flux (L/m2 h) Stage Pressure vessels per stage Elements per pressure vessel Seawater flow to the system, msw (kg/s) Fouling factor, FF (%) High pressure pump efficiency, ηHPP (%)

35,000 Seawater(Well/MF)SDI b 3 30 SW30HRLE-400i 37 8.3 15 1 1 7 3 70 80

while for the mixture R600/R601a, it occurs at a mole fraction of 0.4/0.6 with a value of 6.52 °C, both of which meet the requirement of less than 15 °C discussed earlier. Because the evaporating pressure of each mixture is larger than the corresponding condensation pressure, the temperature glide during condensation process is definitely larger than that in the evaporating process; therefore, it can guarantee that the temperature glides during the evaporation and condensation process do not exceed 15 °C. 4.2. The effect of the seawater temperature increase on thermal efficiency Fig. 5 indicates the influence of the temperature rise of seawater in the condenser on the thermal efficiency of the ORC for various mixtures investigated. It can be seen that the cycle thermal efficiency monotonously decreases with an increase in the temperature rise of the seawater for all of the mixtures. It can be found in Fig. 5(a) that for the mixture R600/R601 with a concentration of 0.1/0.9, the corresponding thermal efficiency decreases from 16.2% to 9.8%, as the temperature rise of the seawater varies between 6 K and 28 K. For the mixtures of different mole fractions, the thermal efficiency increases slightly with the decrease of mole fraction of R600. The difference between the best and worst mixture is approximately 1% for the same seawater temperature increase. From Fig. 5(b) the same tendency can be found. When using R600/R601a with a concentration of 0.1/0.9, the corresponding thermal efficiency decreases from 16.3% to 10.1%, as the temperature rise of the seawater varies between 6 K and 28 K. Among all the mixtures investigated, there exists a maximum value of 16.9% with a temperature rise of 7 K when using R600/R601a with a concentration of 0.9/0.1, while the minimum value of 9.8% occurs with a temperature rise of 28 K, when using R600/R601 with a concentration of 0.1/0.9. To have a detailed assessment, the relative difference in the thermal efficiency to the temperature rise is calculated to investigate the reduction rate of the thermal efficiency with the increase in the seawater temperature rise. Because all of the mixtures of different mole fraction have the same tendency, only the relative difference of the thermal efficiency of mixtures at a mole fraction of 0.9/0.1 is shown in Fig. 6, where it can be observed that the relative difference of the thermal efficiency shows a monotone decreasing trend in the given range. Take R600/R601 as an

Fig. 3. Temperature-entropy diagrams of different zeotropic mixtures.

example (Fig. 6(a)), the relative difference monotonously decreases from − 1 × 10− 3 to − 5 × 10−3 as the temperature rise of seawater varies between 7 k and 28 K. For an increase of 1 K in the temperature, an absolute reduction from 0.1 to 0.5 percentage points in the thermal efficiency can be calculated, which suggests that the reduction amplitude of the thermal efficiency rises gradually with the increase in the temperature rise of the seawater. 4.3. The effect of the seawater temperature increase on net work output Fig. 7 shows the influence of the temperature rise of the seawater on the net work output with different zeotropic mixtures. It can be observed that the net work output first increases with an increase in the temperature rise of seawater, and then decreases with further seawater temperature increases for all of the mixtures investigated. When using R600/R601 with a concentration of 0.5/0.5 as the working fluid, the corresponding work output first increases from 15.7 kW to 44.1 kW, as the temperature rise of the seawater varies between 6 K and 26 K, and then starts to decrease until it reaches 43.3 kW. The maximum value occurs with the seawater temperature rise of 26 K. Similarly, in Fig. 7 (b), it

Table 3 Physical properties of the selected working fluid. Working fluid

Butane (R600)

Pentane (R601)

Isopentane (R601a)

Critical temperature (°C) Critical pressure (MPa) ASHRAE 34 safety GWP 100 years ODP

151.98 3.79 A3 20 0

196.55 3.86 A3 20 0

187.2 3.70 A3 20 0

Fig. 4. Condensation temperature glide vs mole fraction of the first component R600.

D. Geng et al. / Desalination 381 (2016) 38–46

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Fig. 5. Effect of temperature rise of seawater on thermal efficiency for various mixtures.

can be found that the maximum value of 46.1 kW occurs with a seawater temperature rise of 26 K for mixture R600/R601a with a concentration of 0.5/0.5. Among all of the mixtures investigated, the best performing working fluid is R600/R601 at a concentration of 0.9/0.1, with corresponding maximal work output of 49 kW with a temperature rise of 27 K. Fig. 8 shows the influence of the composition of the mixture on the net work output with different temperature rises of the seawater. It can be observed that the net work output increases with the increase of mole fraction of R600. Additionally, the influence is gradually obvious with the increase in the temperature rise of the seawater. It can be found in Fig. 8(a) that as the composition of R600 varies from 0.1 to 0.9, the corresponding net work output increases from 17 kW to 18.4 kW with a temperature rise of 7 K; while the net work output increases from 41.1 kW to 47.3 kW with a temperature rise of 25 K. From Fig. 8(b) the same tendency can be found for the mixture R600/ R601a. The net work output increases from 17.2 kW to 18.6 kW with a temperature rise of 7 K; while the net work output increases from 43 kW to 48.7 kW with a temperature rise of 25 K.

4.4. Comparison of the power output and the desalination power consumption In previous studies [9], solar thermal energy and bioenergy are usually treated as recirculating sources, while the waste heat and geothermal source analyzed in this study are considered to be once through sources. For the former, the thermal efficiency is usually used as the indicator to evaluate the system performance, while for the latter, it is more proper to take the net work output as performance indicator. As such, in this section, we will make a comparison between the net work output of the ORC and power consumption of the RO unit, to determine the optimal working fluid and relevant parameters. Firstly, based on the analysis above, the mixture R600/R601 at a mole fraction of 0.9/0.1 and the mixture R600/R601a at a mole fraction of 0.9/0.1 are selected to be the working fluids in this section.

Concerning the operation of the RO unit, previous studies [28–29] suggested that the permeated total dissolved solids (TDS) of the RO increased as the feed seawater temperature rose. However, Sassi [29] noted that, for low and medium seawater concentrations (15,000– 35,000 ppm), the water quality constraint was easily realized in this range of feed concentrations for all temperatures. Feed flow with a TDS level is set at 35,000 ppm, and the computed temperature is restricted to 35 °C in the current study, because within this range, the requirements of both the water product and membrane can be met simultaneously. The effect of the seawater temperature on permeate TDS will not be discussed. This paper focuses on the required power of an RO unit with varying feed temperatures, which depends on the feed pressure. The applied feed pressure of an RO system is the pressure required to overcome the resistance of two independent pressures, including net driving pressure (NDP) and the osmotic pressure. As Franks investigated [30], the NDP and osmotic pressure are inversely influenced by the feed water temperature, and the effect of temperature on these two pressures varied with different parameters such as the range of temperature change, feed TDS and membrane characteristics. The magnitude of energy savings associated with increasing temperature is calculated under the operational conditions in the current case. To make a detailed analysis of the relationship between the temperature rise of seawater in the condenser and the systematic performance, a comparison of the power output of ORC and the desalination consumption power Wpro, defined as Wnet − Wneed is studied, as shown in Fig. 9. When Wpro b 0, it suggests that the net work output of ORC cannot provide the total power of RO, and other forms of energy such as electricity are needed; when Wpro N 0, it suggests that a portion of the output work can drive the HPP, while the excess can be for other uses, such as for conversion into electricity or driving the feed pump. In brief, the larger the power profit Wpro is, the better the system becomes. It can be observed from Fig. 9 that for temperature in the investigated range, the power profit first increases rapidly, then the rising rate decreases gradually, and finally the power profit starts to decrease. For the mixture R600/R601 at a mole fraction of 0.9/0.1, the power profit first

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Fig. 7. Effect of temperature rise of seawater on net work output for various mixtures. Fig. 6. Relative difference of thermal efficiency vs temperature rise of seawater.

increases from −5.5 kW to 29.3 kW. From the analysis above, this is because the power output of ORC increases while the consumption power of the RO decreases in this temperature range. Then, the power output of ORC decreases, and the consumption power of RO decreases slightly simultaneously; therefore, the combined action leads to the decreasing of power profit. The stagnation point exits at 29.3 kW of power profit and 26 K of the temperature rise. For the mixture R600/R601a at a mole fraction of 0.9/0.1, the maximum power profit value of 30.9 kW occurs with the seawater temperature rise of 27 K. 5. Conclusions A low-enthalpy geothermal ORC for a RO desalination system with zeotropic mixtures is proposed, based on which, the thermal efficiency, work output and overall performance of the whole system as a function of temperature rise of seawater with working fluids of R600/R601 and R600/R601a are investigated. The main results can be extracted as follows: The ORC cycle thermal efficiency monotonously decreases with an increase in the temperature rise of seawater. There exists a maximum

value of 16.9% with a temperature rise of 7 K when using R600/R601a with a concentration of 0.9/0.1. For all of the mixtures investigated, the net work output first increases and then decreases with an increase in the seawater temperature rise. The best performing working fluid is R600/R601 at a mole fraction of 0.9/0.1 with corresponding maximal output work of 49 kW, with the seawater temperature rise of 27 K. In the temperature range investigated, the power profit first increases rapidly then decreases. For the mixture R600/R601, the maximum power profit value of 29.3 kW occurs with a temperature rise of 26 K, and for the mixture R600/R601a, the maximum power profit value of 30.9 kW occurs with a temperature rise of 27 K. The above results show clearly how the temperature change of the seawater influences the ORC and RO systems, which suggests that the best condition for the ORC is not certainly optimal for the whole system. The temperature increase of seawater in the condenser is a nonnegligible process parameter, and it is necessary to consider the effects of the seawater temperature increase on both the ORC and RO simultaneously in the design and analysis of the ORC-RO system. However, these values differ depending on the detailed operational conditions, and the emphasis in the current study lies on the design strategy itself. More comprehensive analysis can be carried out for

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ORC-RO systems with various parameters such as the heat source temperature, feed concentration, and membrane permeability. Additionally, an experimental study will be needed in further work. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51205288) and Tianjin Natural Science Foundation (No. 13JCYBJC15900). References

Fig. 8. Net work output vs mole fraction of the first component R600.

Fig. 9. Power profit depending on temperature rise of seawater.

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