- Email: [email protected]
Design of an autonomous low-temperature solar Rankine cycle system for reverse osmosis desalination
Desalination 183 (2005) 73–80
Design of an autonomous low-temperature solar Rankine cycle system for reverse osmosis desalination D. Manolakosa*, G. Papadakisa, Essam Sh. Mohameda, S. Kyritsisa, K. Bouzianasb a
Department of Agricultural Engineering, Farm Structures Laboratory, Agricultural University of Athens, 75, Iera Odos Street 11855 Athens, Greece Tel. þ30 (210) 5294033; Fax þ30 (210) 5294023; email: [email protected] b Hellas Energy K. Bouzianas P. Moschovitis & Co, 10, Saint George Square 11257 Athens, Greece Received 28 January 2005; accepted 21 February 2005
Abstract The present paper regards the design outline, of a low temperature solar organic Rankine cycle system for reverse osmosis (RO) desalination. The thermal processing taking place is described briefly below: Thermal energy produced from the solar collectors array evaporates the working fluid (HFC-134a) in the evaporator surface, changing the fluid state from sub-liquid to super heated vapour. The super-heated vapour is then driven to the expanders where the generated mechanical work produced by the processing drives the high pressure pump of the RO unit, circulation pumps of the Rankine cycle (HFC-134a, cooling water pump), and the circulator of the collectors. The saturated vapour at the expanders’ outlet is directed to the condenser and condensates. HFC-134a condensation is necessary in the Rankine process. On the condenser’s surface, seawater is pre-heated and directed to the seawater reservoir. Seawater pre-heating is applied to increase the fresh water recovery ratio. The saturated liquid at the condenser outlet is then pressurised by the HFC-134a pump. Specific innovations of the system are low temperature thermal sources can be exploited efficiently for fresh water production; solar energy is used indirectly and does not heat seawater; the RO unit is driven by directly mechanical work produced from the process; the system condenser acts as sea water pre-heater and this serves a double purpose; (1) increase of feed water temperature implies higher fresh water production, (2) decrease of temperature of ‘‘low temperature reservoir’’ of Rankine cycle implies higher cycle efficiencies. Such a system can be an alternative to PV-RO systems, while low temperature energy sources like thermal wastes may be used for RO desalination. Based on the design results, a prototype installation is going to be realised. Keywords: Rankine cycle system; Reverse osmosis; Desalination
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.02.044
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D. Manolakos et al. / Desalination 183 (2005) 73–80
1. Technical description of the system The low temperature solar organic rankine cycle (LTSORC) system for RO desalination consists of the following sub-systems and components (Fig. 1): 1) High efficiency vacuum tube solar collectors’ array 2) Circulator 3) Evaporator 4) Condenser 5) Expanders 6) HFC-134a pump 7) RO unit 8) Insulated seawater reservoir 9) Fresh water reservoir 10) RO energy recovery system The system operation is described briefly below.
Fig. 1. Schematic representation of the system.
Thermal energy produced by the solar array (1) preheats and evaporates the working fluid (HFC-134a) in the evaporator surface (3). The water temperature at collector inlet is about 70 C and the outlet temperature about 77 C. The super-heated vapour is driven to the expanders (5) where the generated mechanical work drives the RO high pressure pump high pressure pump, cooling water pump, HFC-134a pump (6) and circulating pump (2). The saturated vapour at the expanders’ outlet is directed to the condenser (4). On the condenser surface, seawater is preheated and directed to the seawater reservoir (8). Seawater pre-heating is applied to increase the fresh water recovery ratio in a value not exceeding the temperature limit set by the membranes manufacturer. The
D. Manolakos et al. / Desalination 183 (2005) 73–80
seawater tank is insulated. The saturated liquid at the condenser outlet is pressurised by the pump (6). An energy recovery system (10) is coupled to the RO unit thus declining the energy consumption to 2.5 kWh/m3 product. Figs. 1 and 2 illustrate the thermodynamic analysis of the states of the above described thermodynamic process while Table 1 summarises the properties of cycle states. The cycle consists of the following processes: 1!2: Isentropic expansion (expander) 2!3: Isobaric heat rejection (condenser) 3!4: Isentropic compression (HFC-134a pump) 4!1: Isobaric heat supply (evaporator) 2. Assessment of system advantages
Significant advantages of using the proposed solar Rankine system for RO desalination are:
Fig. 2. Molier chart of Rankine cycle (working fluid states).
75
Use of market available components (expanders, heat exchangers, pumps) widely applied in heating and cooling industry available at low cost implies lower investment cost compared to other solar thermal technologies. Ideal exploitation of low temperature energy sources. This concerns the efficient utilisation of thermal wastes of industries, diesel generators and geothermal energy fields, as well as thermal energy coming from biomass or municipal solid wastes. Mechanical work is driven directly to RO pumps so there is a direct efficiency gain from no transformation to electrical power. It can be easily standardised due to most of the system components are market available.
76
D. Manolakos et al. / Desalination 183 (2005) 73–80
Table 1 States of Rankine cycle State S1 Super-heated vapour, evaporator outlet, expander inlet S2 Saturated vapour, expander outlet condenser inlet S3 Saturated liquid, condenser outlet S4 Sub-cooled liquid, pre-heater inlet S40 Saturated liquid, evaporator inlet
T ( C) 75.8
P (kPa)
H (kJ/kg)
S (kJ/kg K)
2200
435.7
1.7138
35
887.91
417.5
1.7138
35
887.91
249.2
1.1676
2200
248.0
1.1676
2200
307.8
1.3433
35 71.7
T, temperature; P, pressure; H, enthalpy; S, entropy.
Rankine cycle approaches the efficiency of Carnot cycle meaning maximum efficiency. Continuous and safe operation at low temperatures. The working fluid (HFC-134a) is not corrosive and environmentally friendly. Low maintenance cost since except the fact that most of the components are market available, they are characterised by long life time (e.g. expander >150,000 h) Due to low O & M cost, simplicity in construction and reliable operation it fits perfectly for applications in isolated, not grid connected areas. Compared to PV-RO desalination system this system prevails in the following: Water storage is used instead of batteries (electrical energy storage) It is environmentally friendly since no batteries are used (problems of old batteries disposal). The absence of batteries implies less maintenance (usually 5 years batteries’ replacement). This is critical for installations in isolated areas where the maintenance is the key factor for long life-time.
Since no electricity is produced, the system is safer for the end users. No qualified staff is needed for O & M. The fresh water cost is expected to be at competitive level. Compared to thermal systems operating in the same temperature range the proposed one is characterised by a much higher efficiency and much less product water cost. Variable pressure working conditions in RO system, implies higher energy availability and higher efficiencies in the whole system, making it more flexible to the variability conditions of RES (solar).
Several applications of solar Rankine cycle are referred in the literature. Most of them concern the cycle application for electricity generation or heat production. Ying You et al. [1] proved that re-heat generative Rankine cycle is suitable for parabolic solar collectors. B.J. Huang et al [2] investigated an integral type solar assisted heat pump consisted of a Rankine refrigeration cycle. V.M. Nguyen et al. [3] developed a small-scale solar Rankine system designed to operate at low temperatures for electricity generation with
77
D. Manolakos et al. / Desalination 183 (2005) 73–80
4.3% efficiency. M. Aghamohammadi et al. [4] analysed the performance of a solar Rankine cycle system for water pumping applied in Iran. The international state-of-the art shows that scientific efforts of solar Rankine cycle have been so far concentrated to heat or electricity generation. No RTD efforts for seawater desalination are mentioned. The application of this thermodynamic cycle for RO desalination represents a significant step forward beyond state of the art. 3. System design The system design concerns, as a first step, the definition of the size and technical specifications of the technologies involved in the system that is the solar collectors and their circulator, the heat exchangers (pre-heater, evaporator, condenser), the expanders, the HFC-134a pump, as well as the RO unit with the energy recovery system. The prototype system is going to be installed at a location near Athens, so the required meteorological data to be used for the design can be those of Athens without significant error. Well known softwares such as TRNSYS simulation software and other commercially available, provided by manufacturers of systems’ components were applied. A brief description of the design procedure is provided below. 3.1. Solar radiation simulator For the simulation of solar insolation in a titled surface the Type 16: Solar radiation processor component of TRNSYS 15.0 simulation programme is used. Several meteorological data enter as inputs, derived from the site of concern, such as radiation on horizontal surface, ground reflectance, slope of surface etc. The outputs supply a set of solar geometry and total, diffuse and beam radiation on both horizontal and inclined surface. Since the
prototype system is to install near Athens, meteorological data of Athens are used. 3.2. Solar collectors array The SOLAMAX direct flow evacuated solar energy collector manufactured by Thermomax Ltd. has been considered. For the assessment of solar array behaviour Type 71: Evacuated tube solar collector of TRNSYS 15.0 is applied. In Table 2 the basic data of the collectors array are presented. 3.3. Solar collectors’ circulation pump The circulation pump has to overcome the total pressure drop of the system caused by the different components at the given flow rate VS. As the flow rate VS is already determined (see 3.2) the pressure drop ps in [Pa] of the system has to be calculated. The system pressure drop ps equals the sum of all single pressure drops of components in the installation connected in series (not parallel). Mainly these are: Table 2 Characteristics of solar collector’s array Manufacturer
Thermomax Ltd.
Type Number of tubes/collector Overall approx. dimensions, m Capacity, l Number of collectors Number of collectors connected in series Slope, Efficiency coefficients (aperture area) a0, a1,W/m2C, a2W/m2C2 Maximum rate of energy gain, kW Maximum flow rate, l/min Inlet temperature, C Outlet temperature, C
SOLAMAX 20 1.4151.998 4 80 2 40 0.769, 1.61, 0.0032 2100.1 196.8 70 77.3
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D. Manolakos et al. / Desalination 183 (2005) 73–80
Solar collector, Pipe work and Heat exchanger.
ps ¼
3.4. Heat exchangers
npc þ pp þ phe 9807
ð1Þ
with ps is the system pressure drop [m], n the number of 20 tube collectors [–], pc the pressure drop of 20 tube collectors [Pa], pp the pressure drop of pipe work [Pa], phe the pressure drop of heat exchanger [Pa], 9807 [Pa] to [m]. The pressure drop pc is provided by the collector’s manufacturer. Heat exchanger’s manufacturers also provide the pressure drop phe. Concerning the calculation of pipe work pressure drop, this can be done applied the well known formulas below. Pipe work pressure drop is expressed as: pp ¼ 9807ðplin þ ploc þ pkin Þ
ð2Þ
or lVs2 X Vs2 þ k d2g 2g k 2 V þ s 2g
plin þ ploc þ pkin ¼
ð3Þ where pp Pressure drop of pipe work [Pa], plin the Linear pressure drop [m], ploc the Local pressure drop [m], pkin the Kinetic energy losses [m], the Friction factor calculated from Colebrook equation, l the Pipe length [m], d the Diameter of pipe [m], 9807 [m] to [Pa]. The above formulas provide the total pressure drop in the collectors’ circuit and since flow rate is already known, the selection of circulation pump can be easily done. Table 3 presents the circulation pump characteristics:
Preheater and evaporator: For selecting the suitable preheater and evaporator, selection software provided by CIAT heat exchangers manufacturer is used. Table 4 presents the basic technical characteristics of the preheater and evaporator. Condenser: CIAT software is also applicable for condenser’s selection. The selected condenser has the following characteristics (Table 5). 3.5. HFC-134a pump The work of the HFC-134a pump is expressed as here below: _ 3 ðp4 p3 Þ Wp ¼ nis nm mv
ð4Þ
with, Wp: Pump work [W] m: HFC-134a flow rate [kg/sec] p3, p4: Pressure at states 3, 4 respectively [Pa] v3: Specific volume [m3/kg] nis: Isentropic efficiency [–] nm: Mechanical efficiency of the pump [–] (Table 6) 3.6. Expanders Four expanders are used for the operation of HP pump of RO system, HFC-134a circulation pump, cooling water pump and collectors’ circulator. The total power to be distributed to the above mentioned consumptions is 7 kW. Table 3 Collectors circulator characteristics Flow rate l/min Head, m Efficiency, % Nominal power, kW Rotation speed, rpm
200 18 75 1.85 2850
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D. Manolakos et al. / Desalination 183 (2005) 73–80 Table 4 Preheater and evaporator characteristics
Manufacturer Model Dimensions L w h, mm Duty, kW Fluids, hot/cold Pressure drop, hot/cold, mmWG Type Flow rate hot/cold, m3/h
Preheater
Evaporator
CIAT EXL-1440 129 265 528 40 Water/HFC-134a 237/2140 Counter flow 11
CIAT EXL-1440 129 265 528 65 Water/HFC-134a 237/2140 Counter flow 11
Table 5 Condenser characteristics Manufacturer
CIAT
Model Dimensions L w h, mm Duty, kW
FKM 273 20 2U 2165 279 415 100 Tube side Sea water 25/33 — — 399 11 m3/h
Fluid Inlet/outlet temperature, C Discharge temperature, C Condensing temperature, C Pressure drop, mmWG Flow rate
Shell side HFC-134a — 75 34.8 — 1700 kg/h
Table 6 HFC-134a pump characteristics Type
Vane
Flow rate, kg/h Suction Pressure, bar Discharge pressure, bar Power, kW Efficiency, %
3.7. RO-desalination recovery system
1920 9 22 0.9 80
plant
with
energy
For the analysis of the RO system the ROSA 6.0 software supplied by FilmTecTM Company is applied. The system details are illustrated in the Tables 7 and 8 below. 4. Results and discussion The organic solar Rankine system for RO desalination can be successfully applied for
fresh water production from both sea and brackish water. Particularly, compared with PV-RO systems, is more environmentally friendly, with less O&M needs and simpler. The Table 9 below summarises the energy and mass results in a yearly base. The results show that a system efficiency of about 7% is achieved. The efficiency is directly related to the selected temperature level of operation. The higher the temperature difference between high and low temperature reservoir, the higher the efficiency. But the scope of research is to examine the behaviour of Rankine system at temperatures in the range of 70–80 C, so as to extract tangible results about the exploitation of low temperature thermal sources. Although this efficiency does not seem attractive at first sight, it is comparable to the efficiency of a PV system. Additionally,
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D. Manolakos et al. / Desalination 183 (2005) 73–80
Table 7 RO configuration Number of stages
1
Element model Number of pressure vessels Number of elements Recovery, % Water temperature, C Water classification
SW30HR-320 1 3 20 25 Seawater (open intake) SDI<5 Clark pump 8.36
Energy recovery system Specific energy (without energy recovery), kWh/m3 Specific energy (with energy recovery), kWh/m3 HP pump power, kW
2.50 2.50
Table 8 RO flows, pressures and TDS
Feed Concentrate Permeate
Flow (m3/h)
Pressure (bar)
TDS (mg/l)
5.00 4.00 1.00
47.82 47.34 0.00
42,710 53,325 252
Table 9 Basic energy and mass results Collectors’ energy gain, MWh/y HFC-134a pump, MWh/y Energy for condensation, MWh/y Energy for preheating, MWh/y Energy for evaporation, MWh/y Energy from expansion, MWh/y System efficiency, % Energy for desalination, MWh/y Fresh water produced, m3/y Specific energy consumption, kWh/m3
101.3 0.89 90.83 35.5 65.8 7.10 7 2.53 1012 2.5
it is the maximum practical technology to exploit low temperature thermal sources (in the range of 70–80 C). Taking into account the above mentioned advantages, the system becomes an
attractive alternative technology for both exploitation of low temperature thermal sources and generation of fresh water from solar energy. Some future aspects of R&D are Examination of different organic fluids that may improve system performance. Identification of different operation points of the ORC, as a function of working fluid temperature, so as to define the optimum efficiency. For that it should be taken into consideration the behaviour of expanders and solar collectors’ efficiency under different operation temperatures. Both the above parameters are directly related each other, since selection of different operation temperature implies possibly the need of using different working fluid for a better performance. The fresh water cost with solar collectors configuration is calculated at 12.5E/m3 while with energy saving applications is 0.5E/m3. Acknowledgements The system will be developed under the project COOP-CT-2003–507997, partially financed by European Commission. References [1] Y. You and E.J. Hu, A medium-temperature solar thermal power system and its efficiency optimisation. Appl. Thermal Eng., 22 (2002) 357–364. [2] B.J. Huang and J.P. Chyng, Performance characteristics of integral type solar-assisted heat pump. Solar Energy, 71(6) (2001) 402–414. [3] V.M. Nguyen, P.S. Doherty and S.B. Riffat, Development of a prototype low-temperature Rankine cycle electricity generation system. Appl. Thermal Eng., 21 (2001) 169–181. [4] M. Aghamohammadi et al. Performance of a solar water pump in southern part of Iran.
Design of an autonomous low-temperature solar Rankine cycle system for reverse osmosis desalination D. Manolakosa*, G. Papadakisa, Essam Sh. Mohameda, S. Kyritsisa, K. Bouzianasb a
Department of Agricultural Engineering, Farm Structures Laboratory, Agricultural University of Athens, 75, Iera Odos Street 11855 Athens, Greece Tel. þ30 (210) 5294033; Fax þ30 (210) 5294023; email: [email protected] b Hellas Energy K. Bouzianas P. Moschovitis & Co, 10, Saint George Square 11257 Athens, Greece Received 28 January 2005; accepted 21 February 2005
Abstract The present paper regards the design outline, of a low temperature solar organic Rankine cycle system for reverse osmosis (RO) desalination. The thermal processing taking place is described briefly below: Thermal energy produced from the solar collectors array evaporates the working fluid (HFC-134a) in the evaporator surface, changing the fluid state from sub-liquid to super heated vapour. The super-heated vapour is then driven to the expanders where the generated mechanical work produced by the processing drives the high pressure pump of the RO unit, circulation pumps of the Rankine cycle (HFC-134a, cooling water pump), and the circulator of the collectors. The saturated vapour at the expanders’ outlet is directed to the condenser and condensates. HFC-134a condensation is necessary in the Rankine process. On the condenser’s surface, seawater is pre-heated and directed to the seawater reservoir. Seawater pre-heating is applied to increase the fresh water recovery ratio. The saturated liquid at the condenser outlet is then pressurised by the HFC-134a pump. Specific innovations of the system are low temperature thermal sources can be exploited efficiently for fresh water production; solar energy is used indirectly and does not heat seawater; the RO unit is driven by directly mechanical work produced from the process; the system condenser acts as sea water pre-heater and this serves a double purpose; (1) increase of feed water temperature implies higher fresh water production, (2) decrease of temperature of ‘‘low temperature reservoir’’ of Rankine cycle implies higher cycle efficiencies. Such a system can be an alternative to PV-RO systems, while low temperature energy sources like thermal wastes may be used for RO desalination. Based on the design results, a prototype installation is going to be realised. Keywords: Rankine cycle system; Reverse osmosis; Desalination
*Corresponding author. Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.02.044
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D. Manolakos et al. / Desalination 183 (2005) 73–80
1. Technical description of the system The low temperature solar organic rankine cycle (LTSORC) system for RO desalination consists of the following sub-systems and components (Fig. 1): 1) High efficiency vacuum tube solar collectors’ array 2) Circulator 3) Evaporator 4) Condenser 5) Expanders 6) HFC-134a pump 7) RO unit 8) Insulated seawater reservoir 9) Fresh water reservoir 10) RO energy recovery system The system operation is described briefly below.
Fig. 1. Schematic representation of the system.
Thermal energy produced by the solar array (1) preheats and evaporates the working fluid (HFC-134a) in the evaporator surface (3). The water temperature at collector inlet is about 70 C and the outlet temperature about 77 C. The super-heated vapour is driven to the expanders (5) where the generated mechanical work drives the RO high pressure pump high pressure pump, cooling water pump, HFC-134a pump (6) and circulating pump (2). The saturated vapour at the expanders’ outlet is directed to the condenser (4). On the condenser surface, seawater is preheated and directed to the seawater reservoir (8). Seawater pre-heating is applied to increase the fresh water recovery ratio in a value not exceeding the temperature limit set by the membranes manufacturer. The
D. Manolakos et al. / Desalination 183 (2005) 73–80
seawater tank is insulated. The saturated liquid at the condenser outlet is pressurised by the pump (6). An energy recovery system (10) is coupled to the RO unit thus declining the energy consumption to 2.5 kWh/m3 product. Figs. 1 and 2 illustrate the thermodynamic analysis of the states of the above described thermodynamic process while Table 1 summarises the properties of cycle states. The cycle consists of the following processes: 1!2: Isentropic expansion (expander) 2!3: Isobaric heat rejection (condenser) 3!4: Isentropic compression (HFC-134a pump) 4!1: Isobaric heat supply (evaporator) 2. Assessment of system advantages
Significant advantages of using the proposed solar Rankine system for RO desalination are:
Fig. 2. Molier chart of Rankine cycle (working fluid states).
75
Use of market available components (expanders, heat exchangers, pumps) widely applied in heating and cooling industry available at low cost implies lower investment cost compared to other solar thermal technologies. Ideal exploitation of low temperature energy sources. This concerns the efficient utilisation of thermal wastes of industries, diesel generators and geothermal energy fields, as well as thermal energy coming from biomass or municipal solid wastes. Mechanical work is driven directly to RO pumps so there is a direct efficiency gain from no transformation to electrical power. It can be easily standardised due to most of the system components are market available.
76
D. Manolakos et al. / Desalination 183 (2005) 73–80
Table 1 States of Rankine cycle State S1 Super-heated vapour, evaporator outlet, expander inlet S2 Saturated vapour, expander outlet condenser inlet S3 Saturated liquid, condenser outlet S4 Sub-cooled liquid, pre-heater inlet S40 Saturated liquid, evaporator inlet
T ( C) 75.8
P (kPa)
H (kJ/kg)
S (kJ/kg K)
2200
435.7
1.7138
35
887.91
417.5
1.7138
35
887.91
249.2
1.1676
2200
248.0
1.1676
2200
307.8
1.3433
35 71.7
T, temperature; P, pressure; H, enthalpy; S, entropy.
Rankine cycle approaches the efficiency of Carnot cycle meaning maximum efficiency. Continuous and safe operation at low temperatures. The working fluid (HFC-134a) is not corrosive and environmentally friendly. Low maintenance cost since except the fact that most of the components are market available, they are characterised by long life time (e.g. expander >150,000 h) Due to low O & M cost, simplicity in construction and reliable operation it fits perfectly for applications in isolated, not grid connected areas. Compared to PV-RO desalination system this system prevails in the following: Water storage is used instead of batteries (electrical energy storage) It is environmentally friendly since no batteries are used (problems of old batteries disposal). The absence of batteries implies less maintenance (usually 5 years batteries’ replacement). This is critical for installations in isolated areas where the maintenance is the key factor for long life-time.
Since no electricity is produced, the system is safer for the end users. No qualified staff is needed for O & M. The fresh water cost is expected to be at competitive level. Compared to thermal systems operating in the same temperature range the proposed one is characterised by a much higher efficiency and much less product water cost. Variable pressure working conditions in RO system, implies higher energy availability and higher efficiencies in the whole system, making it more flexible to the variability conditions of RES (solar).
Several applications of solar Rankine cycle are referred in the literature. Most of them concern the cycle application for electricity generation or heat production. Ying You et al. [1] proved that re-heat generative Rankine cycle is suitable for parabolic solar collectors. B.J. Huang et al [2] investigated an integral type solar assisted heat pump consisted of a Rankine refrigeration cycle. V.M. Nguyen et al. [3] developed a small-scale solar Rankine system designed to operate at low temperatures for electricity generation with
77
D. Manolakos et al. / Desalination 183 (2005) 73–80
4.3% efficiency. M. Aghamohammadi et al. [4] analysed the performance of a solar Rankine cycle system for water pumping applied in Iran. The international state-of-the art shows that scientific efforts of solar Rankine cycle have been so far concentrated to heat or electricity generation. No RTD efforts for seawater desalination are mentioned. The application of this thermodynamic cycle for RO desalination represents a significant step forward beyond state of the art. 3. System design The system design concerns, as a first step, the definition of the size and technical specifications of the technologies involved in the system that is the solar collectors and their circulator, the heat exchangers (pre-heater, evaporator, condenser), the expanders, the HFC-134a pump, as well as the RO unit with the energy recovery system. The prototype system is going to be installed at a location near Athens, so the required meteorological data to be used for the design can be those of Athens without significant error. Well known softwares such as TRNSYS simulation software and other commercially available, provided by manufacturers of systems’ components were applied. A brief description of the design procedure is provided below. 3.1. Solar radiation simulator For the simulation of solar insolation in a titled surface the Type 16: Solar radiation processor component of TRNSYS 15.0 simulation programme is used. Several meteorological data enter as inputs, derived from the site of concern, such as radiation on horizontal surface, ground reflectance, slope of surface etc. The outputs supply a set of solar geometry and total, diffuse and beam radiation on both horizontal and inclined surface. Since the
prototype system is to install near Athens, meteorological data of Athens are used. 3.2. Solar collectors array The SOLAMAX direct flow evacuated solar energy collector manufactured by Thermomax Ltd. has been considered. For the assessment of solar array behaviour Type 71: Evacuated tube solar collector of TRNSYS 15.0 is applied. In Table 2 the basic data of the collectors array are presented. 3.3. Solar collectors’ circulation pump The circulation pump has to overcome the total pressure drop of the system caused by the different components at the given flow rate VS. As the flow rate VS is already determined (see 3.2) the pressure drop ps in [Pa] of the system has to be calculated. The system pressure drop ps equals the sum of all single pressure drops of components in the installation connected in series (not parallel). Mainly these are: Table 2 Characteristics of solar collector’s array Manufacturer
Thermomax Ltd.
Type Number of tubes/collector Overall approx. dimensions, m Capacity, l Number of collectors Number of collectors connected in series Slope, Efficiency coefficients (aperture area) a0, a1,W/m2C, a2W/m2C2 Maximum rate of energy gain, kW Maximum flow rate, l/min Inlet temperature, C Outlet temperature, C
SOLAMAX 20 1.4151.998 4 80 2 40 0.769, 1.61, 0.0032 2100.1 196.8 70 77.3
78
D. Manolakos et al. / Desalination 183 (2005) 73–80
Solar collector, Pipe work and Heat exchanger.
ps ¼
3.4. Heat exchangers
npc þ pp þ phe 9807
ð1Þ
with ps is the system pressure drop [m], n the number of 20 tube collectors [–], pc the pressure drop of 20 tube collectors [Pa], pp the pressure drop of pipe work [Pa], phe the pressure drop of heat exchanger [Pa], 9807 [Pa] to [m]. The pressure drop pc is provided by the collector’s manufacturer. Heat exchanger’s manufacturers also provide the pressure drop phe. Concerning the calculation of pipe work pressure drop, this can be done applied the well known formulas below. Pipe work pressure drop is expressed as: pp ¼ 9807ðplin þ ploc þ pkin Þ
ð2Þ
or lVs2 X Vs2 þ k d2g 2g k 2 V þ s 2g
plin þ ploc þ pkin ¼
ð3Þ where pp Pressure drop of pipe work [Pa], plin the Linear pressure drop [m], ploc the Local pressure drop [m], pkin the Kinetic energy losses [m], the Friction factor calculated from Colebrook equation, l the Pipe length [m], d the Diameter of pipe [m], 9807 [m] to [Pa]. The above formulas provide the total pressure drop in the collectors’ circuit and since flow rate is already known, the selection of circulation pump can be easily done. Table 3 presents the circulation pump characteristics:
Preheater and evaporator: For selecting the suitable preheater and evaporator, selection software provided by CIAT heat exchangers manufacturer is used. Table 4 presents the basic technical characteristics of the preheater and evaporator. Condenser: CIAT software is also applicable for condenser’s selection. The selected condenser has the following characteristics (Table 5). 3.5. HFC-134a pump The work of the HFC-134a pump is expressed as here below: _ 3 ðp4 p3 Þ Wp ¼ nis nm mv
ð4Þ
with, Wp: Pump work [W] m: HFC-134a flow rate [kg/sec] p3, p4: Pressure at states 3, 4 respectively [Pa] v3: Specific volume [m3/kg] nis: Isentropic efficiency [–] nm: Mechanical efficiency of the pump [–] (Table 6) 3.6. Expanders Four expanders are used for the operation of HP pump of RO system, HFC-134a circulation pump, cooling water pump and collectors’ circulator. The total power to be distributed to the above mentioned consumptions is 7 kW. Table 3 Collectors circulator characteristics Flow rate l/min Head, m Efficiency, % Nominal power, kW Rotation speed, rpm
200 18 75 1.85 2850
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D. Manolakos et al. / Desalination 183 (2005) 73–80 Table 4 Preheater and evaporator characteristics
Manufacturer Model Dimensions L w h, mm Duty, kW Fluids, hot/cold Pressure drop, hot/cold, mmWG Type Flow rate hot/cold, m3/h
Preheater
Evaporator
CIAT EXL-1440 129 265 528 40 Water/HFC-134a 237/2140 Counter flow 11
CIAT EXL-1440 129 265 528 65 Water/HFC-134a 237/2140 Counter flow 11
Table 5 Condenser characteristics Manufacturer
CIAT
Model Dimensions L w h, mm Duty, kW
FKM 273 20 2U 2165 279 415 100 Tube side Sea water 25/33 — — 399 11 m3/h
Fluid Inlet/outlet temperature, C Discharge temperature, C Condensing temperature, C Pressure drop, mmWG Flow rate
Shell side HFC-134a — 75 34.8 — 1700 kg/h
Table 6 HFC-134a pump characteristics Type
Vane
Flow rate, kg/h Suction Pressure, bar Discharge pressure, bar Power, kW Efficiency, %
3.7. RO-desalination recovery system
1920 9 22 0.9 80
plant
with
energy
For the analysis of the RO system the ROSA 6.0 software supplied by FilmTecTM Company is applied. The system details are illustrated in the Tables 7 and 8 below. 4. Results and discussion The organic solar Rankine system for RO desalination can be successfully applied for
fresh water production from both sea and brackish water. Particularly, compared with PV-RO systems, is more environmentally friendly, with less O&M needs and simpler. The Table 9 below summarises the energy and mass results in a yearly base. The results show that a system efficiency of about 7% is achieved. The efficiency is directly related to the selected temperature level of operation. The higher the temperature difference between high and low temperature reservoir, the higher the efficiency. But the scope of research is to examine the behaviour of Rankine system at temperatures in the range of 70–80 C, so as to extract tangible results about the exploitation of low temperature thermal sources. Although this efficiency does not seem attractive at first sight, it is comparable to the efficiency of a PV system. Additionally,
80
D. Manolakos et al. / Desalination 183 (2005) 73–80
Table 7 RO configuration Number of stages
1
Element model Number of pressure vessels Number of elements Recovery, % Water temperature, C Water classification
SW30HR-320 1 3 20 25 Seawater (open intake) SDI<5 Clark pump 8.36
Energy recovery system Specific energy (without energy recovery), kWh/m3 Specific energy (with energy recovery), kWh/m3 HP pump power, kW
2.50 2.50
Table 8 RO flows, pressures and TDS
Feed Concentrate Permeate
Flow (m3/h)
Pressure (bar)
TDS (mg/l)
5.00 4.00 1.00
47.82 47.34 0.00
42,710 53,325 252
Table 9 Basic energy and mass results Collectors’ energy gain, MWh/y HFC-134a pump, MWh/y Energy for condensation, MWh/y Energy for preheating, MWh/y Energy for evaporation, MWh/y Energy from expansion, MWh/y System efficiency, % Energy for desalination, MWh/y Fresh water produced, m3/y Specific energy consumption, kWh/m3
101.3 0.89 90.83 35.5 65.8 7.10 7 2.53 1012 2.5
it is the maximum practical technology to exploit low temperature thermal sources (in the range of 70–80 C). Taking into account the above mentioned advantages, the system becomes an
attractive alternative technology for both exploitation of low temperature thermal sources and generation of fresh water from solar energy. Some future aspects of R&D are Examination of different organic fluids that may improve system performance. Identification of different operation points of the ORC, as a function of working fluid temperature, so as to define the optimum efficiency. For that it should be taken into consideration the behaviour of expanders and solar collectors’ efficiency under different operation temperatures. Both the above parameters are directly related each other, since selection of different operation temperature implies possibly the need of using different working fluid for a better performance. The fresh water cost with solar collectors configuration is calculated at 12.5E/m3 while with energy saving applications is 0.5E/m3. Acknowledgements The system will be developed under the project COOP-CT-2003–507997, partially financed by European Commission. References [1] Y. You and E.J. Hu, A medium-temperature solar thermal power system and its efficiency optimisation. Appl. Thermal Eng., 22 (2002) 357–364. [2] B.J. Huang and J.P. Chyng, Performance characteristics of integral type solar-assisted heat pump. Solar Energy, 71(6) (2001) 402–414. [3] V.M. Nguyen, P.S. Doherty and S.B. Riffat, Development of a prototype low-temperature Rankine cycle electricity generation system. Appl. Thermal Eng., 21 (2001) 169–181. [4] M. Aghamohammadi et al. Performance of a solar water pump in southern part of Iran.