Assessment of high temperature organic Rankine cycle engine for polygeneration with MED desalination: A preliminary approach

Energy Conversion and Management 53 (2012) 108–117

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Assessment of high temperature organic Rankine cycle engine for polygeneration with MED desalination: A preliminary approach Daniel Maraver a,⇑, Javier Uche a,b, Javier Royo b a b

CIRCE – Centre of Research of Energy Resources and Consumptions, Zaragoza, Spain Department of Mechanical Engineering, University of Zaragoza, Zaragoza, Spain

a r t i c l e

i n f o

Article history: Received 12 November 2010 Received in revised form 16 August 2011 Accepted 16 August 2011 Available online 29 September 2011 Keywords: Polygeneration Organic Rankine cycle (ORC) Multi-effect distillation (MED) Biomass

a b s t r a c t The objective of this paper is to assess a proposed simultaneous energy and water generation system through a main combination of technologies: organic Rankine cycle (ORC) for heat and power generation, multi-effect distillation (MED) water desalination and a cold generation thermally activated technology (TAT). It is also a primary objective the performance evaluation of the ORC subsystem applied to the use of high temperature renewable energy sources to activate the proposed configuration. This study is divided into three main sections. First of all, the energy feasibility of the proposed configuration is analysed through the Fuel Energy Saving Ratio (FESR) in order to determine the optimal distribution of the heat generated by the prime mover, i.e. the ORC, obtaining that the highest savings correspond to the complete use of heat for domestic hot water (DHW) or heating, which limits the amount of heat used for the activation of MED and TAT subsystems only up to 40%, when the polygeneration system is compared with a high performance stand-alone one. In addition, the ORC subsystem was modelled using the Aspen Plus process simulator, while heat to MED unit was simulated as an input of the model. This part of the paper analyzes a comprehensive list of candidate working fluids for the ORC polygeneration application, and a selection was made of the most interesting according to the preliminary conclusions extracted from the first section of this paper: fluorobenzene and octamethyltrisiloxane could be the most suitable organic fluids for the proposed system. The third main section of this paper deals with the liability of the proposed polygeneration configuration activated by biomass thermal oil boilers through a techno-economic evaluation, revealing that approximate payback periods between 4 and 20 years might be obtained for biomass prices and MED engine specific investment cost in the range of 0–200 €/t and 0–15,000 €/m3/day, respectively. The main conclusions extracted from the paper results reveal that the amount of heat generated in the ORC condenser used for desalination severely limits the primary energy savings in comparison to conventional systems, as well as the electric efficiency obtained with every single fluid. In consequence, the working fluids that might be used in order to obtain a valid FESR and an assumable economic performance for investors are limited. The importance of the proposed configuration activated by biomass combustion lies in the well-known environmental advantages of this renewable resource, plus the energy advantages from the efficiency point of view (despite the lower efficiency of the subsystems in comparison with internal combustion engines or compression chillers) proving that it is possible to achieve small scale water desalination and electricity, cold and heat generation with energy savings in comparison with conventional systems. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction and objectives Plants where electricity is generated in combination with desalted water have been widely studied by previous authors [1– 16]. Thermal seawater desalination with both multi-stage flash (MSF) and multi-effect distillation (MED) is an energy-intensive ⇑ Corresponding author. Tel.: +34 976 762 582; fax: +34 976 732 078. E-mail addresses: [email protected] (D. Maraver), [email protected] (J. Uche), [email protected] (J. Royo). 0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2011.08.013

process, and cogeneration is the most suitable technology to reduce this drawback. The most common system is to use steam turbines to desalt seawater in MSF with steam bleedings [2,3,7,10], although gas turbines (in open or combined cycle) or organic Rankine cycles have been also proposed in combination with reverse osmosis (RO) [1,3,11–13]. Other authors have studied the use of internal combustion engines for hybrid desalination plants coupled with MED and RO [17]. With the aim of increasing overall system efficiency and reducing distillation cost, several researchers have proposed the concept

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109

Nomenclature QH QC We SC Vdes

thermal energy, heating (kW) thermal energy, cooling (kW) electricity (kW) specific consumption of the desalination technology (kW h/m3) desalted water (m3)

Greek letters g energy efficiency k heat fraction Subscripts act activation C cooling chpe prime mover, electric chph prime mover, thermal D desalination e electricity th thermal Ufix heating or DHW Abbreviations AR annual revenues (€)

of polygeneration [14,18]. One approach to these types of combined systems might combine a power plant, a desalination system to supply fresh water, an absorption chiller for cooling requirements, and other equipment to provide hot sanitary water supply. One common characteristic in combined power and desalination is the use of low grade heat or waste heat from one top cycle. In particular, TVC and MED are the most efficient systems when low pressure and temperature steam does not directly comes from a boiler or a turbine extraction: MED has the ability of using this heat in the most efficient (water per heat used) thermal desalination process currently in use [17], reducing to a minimum the energy requirements of these installations. Combined water desalination and power generation with biomass resources is not in general a promising alternative since organic residues are not normally available in arid regions and growing of biomass requires more fresh water than it could generate in a desalination plant [8]. Even though, this issue concerns to the development of large scale plants and some Bioenergy resources, while other type of resources such as algae and energy crops, and smaller scale plants are a promising alternative for biomass desalination [19,20]. The objective of this paper is to comprehensively assess an integrated configuration, based on renewable sources (biomass combustion), to produce desalted water and generate power, heat and cold in order to have better understanding of its potential use and to evaluate the steady-state operating conditions which lead to energy savings in comparison with conventional systems.

2. Description of the proposed polygeneration system The proposed polygeneration system consists of three main subsystems: the ORC engine, MED plant and TAT (LiBr–H2O absorption chiller due to its higher market applicability and availability and better performance with low temperature heat sources). The schematic configuration is represented in Fig. 1. The configuration proposed is based on a small scale ORC engine, which produces electricity and low grade heat, i.e., is acting as the polygeneration system’s prime mover. This heat is used directly for heating purposes or DHW, to

AS CCHP CHP COP DHW FESR HX IC ICE IR LBSE LHV LT MED MSF NPV ORC PP RES RO TAT TVC

annual savings (€) combined cooling heating and power combined heating and power coefficient of performance (dimensionless) domestic hot water Fuel Energy Saving Ratio (dimensionless) heat exchanger investment costs (€) internal combustion engine interest rate (dimensionless) lithium bromide–water simple effect low heating value (MJ/kg) life time (years) multi-effect distillation multi-stage flash net present value (€) organic Rankine cycle payback period (years) renewable energy source reverse osmosis thermally activated technology thermal vapour compression

Fig. 1. Scheme for energy generation and water distillation in the proposed configuration.

generate cooling energy in an absorption chiller and to produce desalted water in a MED unit. 3. Energy analysis of the proposed polygeneration plant Many evaluation criteria have been formulated to quantify the benefits achieved by use of CHP, CCHP and polygeneration plants over traditional ones. Some well-known parameters are the Energy Utilization Factor (EUF), the Primary Energy Savings (PES), the Artificial Thermal Efficiency (ATE), the Fuel Energy Saving Ratio (FESR), the Exergy Efficiency (w) or the Greenhouse Gases Emission Reduction Potential (DGHG). Each of those parameters considers a particular aspect of energy flows; moreover, they might lead to different conclusions. Although it is possible to express a preference for the FESR and the Exergy Efficiency, which better take into consideration the different values of energy flows, it must be stressed that all these parameters might be incomplete, in some occasions, when selecting the best plant among different configurations. All these criteria are usually calculated on a large time period, but they do not contemplate the management of a polygeneration plant. Nevertheless, the aim of this paper is not to obtain the optimal plant configuration but to assess a proposed system based on a RES and to evaluate the steady-state operating conditions which lead to energy savings in comparison with conventional systems. In order to assess a theoretical simplified case, a polygeneration scheme based on ORC as a prime mover, absorption cooling and MED desalination is proposed in comparison with an equivalent

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Fig. 2. Schematic comparison between the polygeneration system assessed and the selected conventional alternative. Adapted from [25].

stand-alone system (Fig. 2). Table 1 reports the representative parameters for the analyzed subsystems, including the reference parameters defined by the Directive 2004/8/EC [21] for combined electricity and heat production. On the other hand, due to the lack of a reference system for cooling and water desalination, some new reference values were proposed by the authors. Previous authors [22] have defined the Fuel Energy Saving Ratio, parameter which regulates several national policies for the national support to energy efficient plants [23], for example Italy [24], as the quantity of the saved energy represented by the difference between two terms: the fuel consumed in a stand-alone system and the one in a polygeneration scheme. According to this definition and the scheme represented in Fig. 2, the FESR parameter is calculated with the expression in:

FESR ¼ 1  f1=½gchpe =RefEg þ kC  COPTAT =COPref  gchph =RefEg þ kD  SCRO;ref =SCMED  gchph =RefEg þ ð1  kC  kD Þ  gchph =RefHgg

ð1Þ

where gchpe, gchph, COPTAT and SCMED are the characteristic parameters of the proposed plant and RefEg, RefHg, COPref and SCRO,ref represent the reference system, as depicted in Table 1, while kC and kD are the heat fractions assigned to TAT and MED activation. After evaluating the FESR for the proposed systems (polygeneration vs. high efficiency stand-alone), it is important to stress that total available heat has to be shared between its direct use for heating and TAT and MED activation, so kD and kC cannot be assigned arbitrarily [25]. Therefore, it is necessary to define an equation with an independent variable, setting first the amount of residual heat that can be used for heating or DHW (kUfix). This amount is defined by Eq. (2). Subsequently, for a set value of kUfix the fractions of heat assigned to cold generation and seawater desalination are defined by Eqs. (3) and (4). Negative values of kD and kC lack physical sense, thus, a limit is needed in order to generalize the value of the equations (Eqs. (5) and (6)).

kUfix ¼ 1  ðkD þ kC Þ

ð2Þ

kD ¼ ð1  kUfix Þ  kC

ð3Þ

kC ¼ ð1  kUfix Þ  kD

ð4Þ

0 6 kC 6 ð1  kUfix Þ

ð5Þ

0 6 kD 6 ð1  kUfix Þ

ð6Þ

Once defined the previous mathematical expressions, the fuel savings were evaluated and represented in Fig. 3, which shows the FESR surface for different kUfix values. In the four shown cases, it is noticeable that the positive FESR values were in the range between 0% and 20%, for the different combinations of kD and kC. However, the FESR could even had negative values due to the higher rate of thermal energy destined to the MED and TAT subsystems. The maximum FESR value was obtained when the MED plant nor the TAT were not in operation. Fig. 3a and b, which represent heat fractions destined for heating or DHW of 20% (kUfix = 0.2) and 40% (kUfix = 0.4) respectively, show a few points where the FESR surface has positive values. In consequence, it can be stressed that an approximate feasible limit value for the heat fraction used for heating or DHW should be a minimum of 60% (Fig. 3c). Once the discrete preliminary analysis of the FESR was made, a sensitivity analysis (Fig. 4) is also required in order to:  Assess the behaviour of the system when varying the main parameters of every subsystem (COPTAT, gchph, gchpe, kD and SCMED).  Determine the minimum required electric efficiency of the prime mover to obtain a positive FESR. A 10% for the heat fractions corresponding to the MED (kD) and TAT (kC) activation was selected here. From the comparison (Fig. 4a), it is seen that the factors decrease their influence in the sequence gchph, gchpe, COPTAT, SCMED and kD. For instance, with a 10% rise of gchpe and gchph, the increases of FESR are, respectively, 30% and 60%. According to this sensitivity assessment (Fig. 4b), the electric efficiency value required to obtain a minimum FESR is 12.66%, which limits the number of potential working fluids that might be used in the power cycle, as shown in the next section. 4. Modelling of the ORC subsystem The ORC system was modelled with Aspen Plus [26]. Required inputs for the MED generator where obtained from a six-stage model developed by the authors according to the one carried out by Al-Juwayhel et al. [27]. The main objective of this section is to

Table 1 Input parameters used in the polygeneration configuration and the equivalent system proposed. Subsystem

Parameter

Value

Unit

Source

ORC

Electric efficiency (gchpe) Thermal efficiency (gchph) Electric efficiency (RefEg) Specific consumption (SCMED) Specific consumption (SCRO,ref) Coefficient of operation (COPTAT) Coefficient of operation (COPref) Thermal efficiency (RefHg)b

18.0 80.0 52.5 50.0 4.0 0.7 3.0 90.0

% % % kW ht/m3 kW he/m3 – – %

[32] [32] [21] [25] [25] [33] [34] [21]

Reference power planta MED RO Absorption chiller Reference chillerb Reference boilerc a b c

Combined cycle activated by gas. Small scale compression chiller. Gas boiler.

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Fig. 3. Theoretical FESR surface for the polygeneration and the stand-alone systems proposed: (a) kUfix = 0.2; (b) kUfix = 0.4; (c) kUfix = 0.6; (d) kUfix = 0.8.

Fig. 4. Sensitivity analysis of the influence factors: (a) FESR variation (%) due to influence factor variation (%); (b) FESR variation due to different electric efficiency values of the prime mover.

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make a first selection of working fluids including the operating pressure at the inlet of the turbine and the condenser. 4.1. Model description High temperature ORC was modelled along with the heat exchange at the condenser and the corresponding activation stages of the desalination and cold generation subsystems, i.e., different heat exchangers representing the heat consumer, the brine heater of the MED plant and the generator of the absorption chiller. The efficiency of the ORC was determined following Eq. (7).

gORC ¼ W net =Q ORC ¼ ðW turbine  W pump Þ=Q ORC

ð7Þ

Input parameters used in the model were given in Table 2, where stream numbers and nomenclature were referred to that of Fig. 5. To obtain the optimum operating pressures and a first selection of fluids, the ORC was modelled as a saturated cycle, and the thermodynamic properties of all the fluids were calculated by means of the Peng–Robinson (organic fluids) and the NRTL (condensing fluid) equation of state, according to Bruno et al. [4]. 4.2. Preliminary selection of working fluids In ORC applications, the choice of working fluid is critical since the fluid must have not only thermo physical properties that match the application but also adequate chemical stability at the desired working temperature. Aspen Plus has a comprehensive library of fluids that can be used for power generation in Rankine cycles. Initially, the working fluids considered can be linear, branched and

cyclic hydrocarbons, refrigerants and siloxanes. Table 3 shows an extensive list of working fluids for high and low-temperature ORC, with some of their relevant thermo physical properties and the sources that had previously analysed its behaviour in power cycles. This table might be used by other authors as a state of the art of working fluids for power generation with ORC. Mixtures of fluids have not been explored at the present stage of work. Although some siloxane mixtures have been previously studied, the benefits do not translate into better net power output [28]. 4.3. Simulation results From the initial list of potential working fluids presented in Table 3, refrigerants containing chlorine were discarded because of their ban in relation to the ozone depletion problem. Fluids were also discarded which demonstrated an isentropic or a negative slope of the saturated vapour line in the temperature–entropy diagram in order to reduce the need for superheating. An additional constraint was the critical temperature, so a limit of 180 °C was established by the authors, below which fluids should not be considered for the analysed polygeneration configuration for two main reasons:  This configuration has been proposed to be activated by high temperature renewable sources (within low characteristic temperatures of the organic Rankine cycles in comparison to conventional ones), such as biomass combustion or even concentrated solar energy.

Table 2 Input parameters used in the polygeneration model. Subsystem

Equipment

Parameter

Value

Source

ORC

Absorption chiller Heating/DHW

Generator –

Isentropic efficiency Quality outlet (str. 3) Outlet temp. (str. 4) Efficiency Quality outlet (str. 6) Quality outlet Vapour temperature Vapour flow Water temperature Water temperature

0.8 1 (saturated vapour) 5 (°C) sub cooling 0.8 0 (saturated liquid) 1 (saturated vapour) 70 (°C) 0.2092 (kg/s) 90 °C 90–70 °C

[4]

MED

Turbine Regenerator Condenser Pump Preheater Evaporator Generator

Fig. 5. An integrated polygeneration plant with ORC engine modelled with Aspen Plus.

[27] [33] –

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D. Maraver et al. / Energy Conversion and Management 53 (2012) 108–117 Table 3 Preliminary selection of working fluids. Component name

Alt. name

Pc (bar)

Tc (°C)

Source

Methane (CH4) Carbon-tetrafluoride Hexafluoroethane Trifluoromethane Carbon dioxide Ethane Fluoromethane Pentafluoroethane 1,1,1-Trifluoroethane Octafluoropropane Difluoromethane Chloropentafluoroethane Pentafluorodimethylether Propylene/propene Propane Chlorodifluoromethane 1,1,1,2-Tetrafluoroethane 1,1,1,2,3,3,3-Heptafluoropropane 1,1,1,2,2-Pentafluoropropane Dichlorodifluoromethane 1,1-Difluoroethane Decafluorobutane Octafluorocyclobutane 1,1,2,2-Tetrafluoroethane 2-Chloro-1,1,1,2-tetrafluoroethane Trifluoroiodomethane (CF3I) 1,1,1,3,3,3-Hexafluoropropane Cyclopropane Dimethylether Propyne (methylacetylene) Ammonia Pentafluoromethoxyethane Isobutane 1-Chloro-1,1-difluoroethane 1,1,1,2,3,3-Hexafluoropropane Methyl-chloride Isobutylene/Isobutene (or 2-methylpropene) Dichlorotetrafluoroethane Dodecafluoropentane Bis-difluoromethyl-ether Perfluoro-N-pentane (C5F12) N-butane 1,1,1,3,3-Pentafluoropropane 1,1,2-Trifluoroethane 1,1,1,2,2,3,3,4-Octafluorobutane Neopentane (2,2-dimethyl-propane) Heptafluoropropyl-methyl-ether 2-Difluoromethoxy-1,1,1-trifluoroethane Dichlorofluoromethane 1,1,2,2,3-Pentafluoropropane 1,1,1,2,2,3,4,5,5,5-Decafluoropentane 1,1-Dichloro-2,2,2-trifluoroethane Ethyl-amine 1,1,1,3,3-Pentafluorobutane Isopentane (2-methyl-butane) 1,2-Dichloro-1,2,2-trifluoroethane Diethyl-ether N-pentane Trichlorofluoromethane 1,1-Dichloro-1-fluoroethane Dibromodifluoromethane Trichlorotrifluoroethane Methyl-formate 1,2-Dibromotetrafluoroethane 2,2-Dimethylbutane Isohexane 2,3-Dimethylbutane N-hexane Methanol Ethanol Hexafluorobenzene Hexamethyldisiloxane Fluorobenzene N-heptane

– R-14 R-116a R-23a R-744a R-170 R-41a R-125 R-143a R-218 R-32 R-115 RE125 R-1270 R-290 R-22 R-134a R-227ea R-245 R-12 R-152a R-3-1-10 R-C318 R-134 R-124 – R-236fa R-C270 RE170 – R-717 RE245mc R-600a R-142b R-236ea R-40 – R-114 FC-4-1-12 RE134 PF-5050 R-600 R-245fa R-143 R-338mccq – RE347mcc RE245 R-21 R-245ca R-43-10mee R-123 R-631 R-365mfc R-601a R-123a R-610 R-601 R-11 R-141b R-12b2 R-113 R-611 R-114b2 – – – – – – – MM – –

46.0 36.8 30.5 48.3 73.8 48.7 59.0 36.3 37.6 26.8 57.4 30.8 33.6 45.3 41.8 49.9 40.6 28.7 31.4 39.5 44.5 23.2 27.8 46.4 36.2 39.1 31.9 54.8 53.4 56.3 113.3 28.9 36.4 40.6 34.1 66.2 39.7 32.4 20.5 42.3 20.2 37.9 36.1 45.2 27.2 31.6 24.8 34.2 51.8 38.9 22.9 36.6 56.2 32.7 33.7 37.4 36.4 33.6 43.7 42.1 40.7 33.8 60 33.9 30.8 30.4 31.3 30.6 81.0 40.6 32.8 19.1 35.4 27.3

83 46 20 26 31 32 44 66 73 73 78 79 81 91 96 96 101 101 107 111 112 113 114 119 122 122 124 124 127 129 132 134 135 137 139 143 144 145 147 147 149 152 153 157 159 160 165 170 178 174 179 183 183 187 187 188 193 196 197 204 205 213 214 215 216 225 227 235 240 241 244 245 258 267

[35] [28] [36] [28,36,37] [36] [35,36,38] [36] [36,37,39,40] [36,37,39] [4,36,39] [36,37,39,41] [37,42] [39] [36,38,39] [28,35,36,38–41,43–46] [36,37,40] [4,36,37,39–41,43–45,47–53] [4,36–39,43,44,47,51,54] [4] [37,41,42,48] [36,37,39–43,45,51,55,56] [4,36] [4,36,38,39,41,51] [4] [36,37,40] [39] [38,39,47,51] [36,37,39] [39] [57] [35,36,41,43,46,48,58–61] [39] [4,35,36,38–41,43,45,54,58,59,62–65] [36,37,40] [4,36,37,39,59,64] [40] [51,57] [37,41,42] [36] [4,37,39] [39,61,64] [4,28,35–37,39,41,43,45,52,59,63,64] [36–39,43–45,47,49,51,52,54–57,62,64] [37] [39] [39,43,63,66] [39] [4,37,39] [36,37] [36,39,43,45,59,62,64,65] [67] [36,37,40–42,44,45,50,59,61,64,65,67–70] [4,71] [50,51,64] [4,37,39,43,46,51,54,57,60,62–64,66,68,72] [37] [4] [4,28,36,37,41,43–45,50,61,63,64,66–68] [37,42,48,59,68] [36,37,41,50,59,64] [4] [37,38,41,42,45,48,53,57,59,64,65,67,69] [4,71] [4] [67] [51,64] [67] [4,28,35,39,45,60,64,67] [41] [41,50] [67] [4,6,28,58,60,63,67,73,74] [67] [4,35,46,50,67] (continued on next page)

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D. Maraver et al. / Energy Conversion and Management 53 (2012) 108–117

Table 3 (continued) Component name

Alt. name

Pc (bar)

Tc (°C)

Source

Cyclohexane Benzene (C6H6) Octamethyltrisiloxane N-octane Octamethylcyclotetrasiloxane Toluene (C7H8) N-nonane (C9H20) Decamethyltetrasiloxane Dibromomethane p-Xylene Ethylbenzene N-decane Decamethylcyclopentasiloxane Dodecamethylpentasiloxane N-propylbenzene N-undecane Dodecamethylcyclohexasiloxane Water Tetradecamethylhexasiloxane N-dodecane (C12H26) N-butylbenzene Tribromomethane

– – MDM – D4 – – MD2M R-30b2 – – – D5 MD3M – – D6 R-718 MD4M – – R-20b3

40.7 48.8 14.4 25 13.1 41.3 22.7 12.2 71.7 34.8 36.1 21.0 11.6 9.3 32 19.7 9.5 220.6 7.9 17.9 28.9 60.9

280 289 291 296 312 319 321 326 338 342 344 345 346 354 365 366 371 374 379 382 388 423

[4,41,51,57,67] [36,42,46,48,60,64,66,68,69] [4,28,67,72–75] [4,35,45,67] [6,45,60,67,74] [4,6,36,42,45,57,60,63,64,66,68,69,72] [35,45] [4,28,67,73,75] [4] [42,68,69] [4,72] [35] [6,74] [28,73,75] [4,72] [35] [75] [35,36,41,46,48,51,58,59,70] [6,28,73] [35,45] [4,72] [4]

simulation results are presented in Table 4, which represents the system performance of selected working fluids using the parameters shown in Table 2 at the points of Fig. 5; kUfix and kD parameters were 0.8 and 0.1 respectively. The last two columns of this table represent the specific ORC power per unit of mass of working fluid. Highest ORC efficiencies were obtained with high critical temperature fluids. However, condensing pressure is low and consequently their density at the turbine outlet after expansion is also quite low, as shown in Table 4. In other words, these fluids would

 The integration with space heating and MED and TAT technologies limits the low temperature and pressure of the organic Rankine cycle hence its characteristic electric efficiency and, in consequence, the global FESR of the entire polygeneration system (as seen in previous section of this paper). A process simulation was developed with the selected fluids in order to determine the best top and down pressures for the ORC using a saturated cycle configuration and for each case. The Table 4 Simulation results of the selected working fluids with a saturated cycle configuration. #

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Working fluid

Isopentane (2-methyl-butane) Diethyl-ether N-pentane Dibromodifluoromethane 1,2-Dibromotetrafluoroethane 2,2-Dimethylbutane Isohexane 2,3-Dimethylbutane N-hexane Hexafluorobenzene Hexamethyldisiloxane Fluorobenzene N-heptane Cyclohexane Benzene (C6H6) Octamethyltrisiloxane N-octane Octamethylcyclotetrasiloxane Toluene (C7H8) N-nonane (C9H20) Decamethyltetrasiloxane p-Xylene Ethylbenzene N-decane Decamethylcyclopentasiloxane Dodecamethylpentasiloxane N-propylbenzene N-undecane Dodecamethylcyclohexasiloxane Tetradecamethylhexasiloxane N-dodecane (C12H26) N-butylbenzene Tribromomethane

1

2

3

4

5

6

T (°C)

p (bar)

T (°C)

p (bar)

T (°C)

T (°C)

T (°C)

T (°C)

184.98 191.65 195.10 191.22 205.97 213.68 221.66 223.76 231.49 241.38 241.38 266.11 265.87 279.90 287.30 289.29 292.86 311.64 319.66 318.34 324.71 340.30 341.34 340.72 342.22 351.81 359.58 363.72 367.95 379.69 384.92 384.63 411.89

33.00 35.00 33.00 34.00 30.00 30.00 29.00 30.00 29.00 32.00 18.00 35.00 27.00 40.00 48.00 14.00 24.00 13.00 40.00 22.00 12.00 34.00 35.00 20.00 11.00 9.00 30.00 19.00 9.00 8.00 18.00 28.00 55.00

120.71 118.79 123.43 97.52 128.96 153.36 157.96 160.07 161.24 155.67 187.35 173.06 185.60 180.85 153.29 232.53 215.75 254.98 193.99 239.45 269.98 222.11 225.09 269.26 288.68 300.41 253.84 295.32 316.79 329.59 310.16 292.37 121.32

7.00 7.00 6.00 7.00 4.00 5.00 4.00 4.00 3.00 2.00 1.00 2.00 1.00 2.00 2.00 0.20 0.50 0.10 1.00 0.20 0.05 0.40 0.40 0.20 0.02 0.01 0.20 0.20 0.005 0.005 0.08 0.30 0.30

97.88 103.82 100.70 96.12 96.33 110.61 111.40 109.46 108.10 103.84 100.14 109.13 97.97 106.21 104.29 99.17 102.19 101.43 111.68 98.21 101.09 106.87 104.52 119.16 93.46 93.76 104.92 137.19 91.79 102.51 129.78 139.19 104.28

92.88 98.82 95.71 91.12 91.33 105.61 106.40 104.49 103.10 97.84 95.14 104.13 92.97 101.21 99.29 94.17 97.19 96.43 106.68 93.21 96.09 101.87 99.52 114.16 88.46 88.76 99.92 132.19 86.79 97.51 124.78 134.19 99.28

95.29 101.19 98.05 93.91 93.39 107.69 108.31 106.40 104.89 99.36 96.26 105.87 94.33 103.37 101.79 94.94 98.26 97.05 108.52 94.06 96.69 103.25 100.95 114.92 88.94 89.11 101.03 132.90 87.13 97.81 125.36 135.27 102.14

113.12 112.44 115.60 94.95 118.27 142.27 145.46 147.25 147.89 139.77 168.81 154.81 165.73 163.52 138.89 209.40 192.38 230.34 173.41 213.06 244.42 196.03 199.11 242.63 262.42 273.98 224.98 268.88 290.01 303.75 281.84 264.32 114.31

q2

gORC (%)

mORC (kg/s)

wturb (kJ/kg)

10.44 10.09 10.97 11.44 13.49 12.32 13.19 13.78 14.70 16.03 17.54 18.79 19.67 19.40 19.27 22.49 22.10 24.26 21.80 25.10 25.42 24.86 25.35 24.43 28.18 28.94 27.40 24.02 30.53 29.78 26.46 25.14 25.78

0.20477 0.19654 0.19098 0.62843 0.63285 0.21845 0.20115 0.20480 0.18783 0.35180 0.29449 0.18759 0.18140 0.17176 0.15604 0.33372 0.18182 0.37663 0.16152 0.18168 0.35687 0.16350 0.16565 0.18811 0.40153 0.38946 0.17082 0.19633 0.42413 0.42349 0.19449 0.18009 0.39815

40.17 40.05 44.87 14.02 16.46 42.24 50.91 52.50 60.66 35.14 46.42 78.55 86.57 90.83 98.96 54.45 98.36 52.85 109.72 114.98 59.18 126.63 128.35 107.02 60.13 64.08 137.15 100.25 63.37 61.27 114.24 116.35 54.83

(kg/m3) 17.84 18.47 14.96 55.82 33.94 13.48 10.48 10.39 7.64 10.90 4.37 5.36 2.69 4.62 4.57 1.13 1.42 0.68 2.42 0.61 0.34 1.04 1.03 0.63 0.16 0.08 0.55 0.67 0.05 0.05 0.28 0.86 2.33

D. Maraver et al. / Energy Conversion and Management 53 (2012) 108–117

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Table 5 Input summary for the economic evaluation of the proposed polygeneration system. Inputs

Units

Value

Source

Investment costs Absorption chiller Biomass boiler MED plant ORC module

€/kWc €/kWth €/(m3  day) €/kWe

600 200 1000 3500

[25] [76] [25] [4]

Financial parameters Lifetime Interest rate

Years %

20 10

[77] [77]

Fig. 7. Payback period (solid lines) and NPV (dash lines) versus specific investment of MED and ORC.

5. Techno-economical assessment

Fig. 6. Payback period (solid lines) and NPV (dash lines) versus primary energy cost and desalted water reference cost.

require large-volume equipment in their operation [4], which is the case of siloxanes. These fluids are mainly used in commercial cogeneration applications, which is the case proposed here, where the condensing pressure is considerably high, thus making it advantageous to generate heat in the condensing process at the expense of lower cycle efficiency. Different groups of fluids can be recognized in this table:  Fluids with a net efficiency close to 30% with low pressures under atmospheric one, such as the siloxane fluids D6 and MD4M.  Fluids with atmospheric pressure at the condensing section and with an efficiency of around 20%, such as n-heptane and cyclohexane.  Finally, there is a group of fluids with lower critical temperatures (below isohexane) working at high condensing pressures, with high density at the turbine outlet but showing net efficiency values lower than 13%, which are not suitable for the proposed polygeneration configuration since a positive FESR was not targeted. Fluorobenzene and octamethyltrisiloxane offer good cycle efficiencies as well as acceptable densities and specific power capacities compared with other efficient fluids.

A techno-economic evaluation was also performed to illustrate the liability of the ORC engine (with octamethyltrisiloxane as working fluid), integrated with a MED unit and a LBSE absorption chiller, activated by a conventional biomass thermal oil boiler. Economic efficiency is crucial for the application of CHP, CCHP and polygeneration systems. Many factors, such as investment and operating costs and the fuel price were obviously taken into account in the assessment. Main assumptions of the analysis were constant demands, high operation periods (8000 h/year) and a distribution of heat between heating, MED and LBSE of 80%, 10% and 10% respectively. The economic parameters calculated and discussed in this paper were the payback period (PP) and the net present value (NPV), which are defined in:

PP ¼ IC=AS

NPV ¼ IC þ

ð8Þ LT X ½AR=ð1  IRÞi 

ð9Þ

i¼1

According to their economic performance, the choice of polygeneration system or stand-alone one is influenced by three groups of factors:  Prices of raw materials and services.  Capital investment and operating cost.  Technical properties of technologies, such as efficiencies and coefficients of performance. All these factors influence this techno-economic assessment as well, in particular, the annual revenues, operating and investment costs. The annual savings are defined as the difference between the annual revenues and the operating costs. The annual revenues parameter takes into account two different aspects:

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D. Maraver et al. / Energy Conversion and Management 53 (2012) 108–117

 The savings from desalted water that does not have to be supplied by other producers, taking into account that this water would also be produced by MED technology (2–8 €/m3 [29]).  The electricity sale incomes under a cogeneration scheme, according to the Spanish legislation [30]. Expected CO2 bonus for the proposed polygeneration configuration system will raise the annual revenues, but they were not considered in the present paper. The operating cost is referred to the primary energy consumption of the proposed system, i.e. the amount of biomass required to produce high temperature heat through a standard boiler (90% of thermal efficiency) [31], with an average LHV of 15 MJ/kg. The investment cost is calculated with different specific costs (presented in Table 5) where it is also stressed the interest rate and the lifetime period considered in the present economic assessment. Results of the techno-economic assessment are presented in two different sensitivity analyses (Figs. 6 and 7). In the first place, Fig. 6 shows the economic indices variation with the change of biomass price and desalted water reference cost. Secondly, Fig. 7 presents the behaviour of PP and NPV depending on the investment costs of the MED unit and the ORC engine, which regarding the proposed configuration are the two subsystems with lower market penetration at small scale. It is relevant to stress that the variation of the MED unit investment cost has higher influence on the PP and the NPV than the one of biomass cost. For example, the maximum variation of the PP regarding biomass price is approximately 8 years while in the case of the MED unit investment cost it raises up to 20 years for the considered ranges (0–200 €/t and 0–15,000 €/ m3/day respectively). In resume, payback periods between 4 and 20 years might be obtained when developing this type of polygeneration configuration. Positive net present values might also be obtained when considering high desalted water reference costs (4–8 €/m3), without dependency from the biomass cost. Meanwhile, positive net present values might only be obtained when the specific investment of the MED unit is below 7000 €/m3/day.

6. Conclusions This paper mainly deals with the preliminary evaluation and analysis of a proposed polygeneration system integrating a small scale ORC engine, a MED desalination plant and a LBSE absorption chiller. Special emphasis was made on the ORC subsystem regarding the choice of the adequate working fluid. This configuration is considered of innovative nature, according to the extensive bibliography review and state of the art carried out by the authors. The innovative aspects of this work consist on the ORC and MED integration which has not been extensively studied in previous works in comparison with other polygeneration configurations, such as ORC–RO or ICE–MED integration, systems suitable for its combination with RES such as biomass combustion. This arrangement is specially indicated to provide energy and fresh water in isolated areas, when RO is not adequate to treat raw water and enough biomass exists, for example in islands with access to new developed sources of biomass (algae or new types of residual biomass or energy crops). Through the study, conclusions can be draw as follows: (1) The Fuel Energy Saving Ratio (FESR) was assessed in order to determine the optimal distribution of the heat generated by the ORC, obtaining the highest savings with larger amounts of heat destined for heating or DHW. This aspect limits the amount of heat used for the activation of MED and TAT

subsystems to 40% when analyzing the polygeneration system proposed in comparison to a high performance standalone one. (2) A comprehensive list of working fluids has been analyzed to optimize the performance of organic Rankine cycle engines for thermal desalination. The fluids reviewed were chosen based on the criterion of having a positive slope of the vapour saturation line in the temperature–entropy diagram, not containing chlorine in their chemical composition and having a high critical temperature due to the desired final application (biomass combustion). Fluorobenzene and octamethyltrisiloxane are the most suitable organic fluids for the proposed system, attending to high electric efficiency and acceptable values of pressure and specific volume at the turbine outlet. (3) Economic evaluation shows that the proposed polygeneration system activated by biomass boilers has a high initial cost but this investment can be recovered in a period of time comprised between 4 and 20 years depending on the biomass resources cost. Main result of this paper set the favourable techno-economic conditions for future market development of energy integration based on biomass boiler, ORC, LBSE and MED, which is a promising alternative upon favourable conditions, as well as the steady-state operating conditions which lead to energy savings in comparison with conventional systems. Acknowledgements This work is financially supported by the Ministerio de Ciencia e Innovación of Spain, BIO3 Project, Ref. ENE2008-03194/ALT. The authors would also like to thank Carlos Rubio. References [1] El-Nashar AM. Optimal design of a cogeneration plant for power and desalination taking equipment reliability into consideration. Desalination 2008;229:21–32. [2] Fath H, Al-Khaldi F, Abu-Sharkh B. Numerical simulation and analysis of a patented desalination and power co-generation cycle. Desalination 2004;169:89–100. [3] Darwish MA, A1 Najem N. Co-generation power desalting plants: new outlook with gas turbines. Desalination 2004;161:1–12. [4] Bruno JC, López-Villada J, Letelier E, Romera S, Coronas A. Modelling and optimisation of solar organic Rankine cycle engines for reverse osmosis desalination. Appl Therm Eng 2008;28:2212–26. [5] Alarcón-Padilla D-C, García-Rodríguez L. Application of absorption heat pumps to multi-effect distillation: a case study of solar desalination. Desalination 2007;212:294–302. [6] Delgado-Torres AM, García-Rodríguez L. Preliminary assessment of solar organic Rankine cycles for driving a desalination system. Desalination 2007;216:252–75. [7] Kamal I. Integration of seawater desalination with power generation. Desalination 2005;180:217–29. [8] Eltawil MA, Zhengming Z, Yuan L. A review of renewable energy technologies integrated with desalination systems. Renew Sustain Energy Rev 2009;13:2245–62. [9] Yari M, Mahmoudi SMS. Utilization of waste heat from GT-MHR for power generation in organic Rankine cycles. Appl Therm Eng 2010;30:366–75. [10] Bouzayani N, Galanis N, Orfi J. Comparative study of power and water cogeneration systems. Desalination 2007;205:243–53. [11] Cardona E, Piacentino A. Optimal design of cogeneration plants for seawater desalination. Desalination 2004;166:411–26. [12] Almulla A, Hamad A, Gadalla M. Integrating hybrid systems with existing thermal desalination plants. Desalination 2005;174:171–92. [13] Mussati SF, Aguirre PA, Scenna NJ. Optimization of alternative structures of integrated power and desalination plants. Desalination 2005;182:123–9. [14] Rubio C, Uche J, Dejo N. Optimization of desalted water production in a poligeneration scheme for the tourist sector. Desalination 2008;223:464–75. [15] Mathioulakis E, Belessiotis V, Delyannis E. Desalination by using alternative energy: review and state-of-the-art. Desalination 2007;203:346–65. [16] Cardona E, Piacentino A, Marchese F. Performance evaluation of CHP hybrid seawater desalination plants. Desalination 2007;205:1–14.

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