- Email: [email protected]
An energy-efficient submarine desalination plant
DESALINATION ELSEVIER
Desalination 122 (1999) 171-176 www.elsevier.com/Iocate/desal
An energy-efficient submarine desalination plant D. Colombo, M. de Gedoni, M. Reali* ENEL - Ricerca Polo Idraulico e Strutturale, V. P ozzobonelli 6, 20162 Milano, Italy Tel. +39 (02) 7224 3600; Fax +39 (02) 7224-3530; eraail: [email protected]
Received 3 December 1998; accepted February 28, 1999
Abstract
The socio-economical development of many regions lacking a sufficient supply of fresh water (FW) may at times be planned with the help of desalination technologies for sea and brackish water (SW and BW). Since these technologies require large energy fluxes in the form of heat or electricity and produce sizable salt fluxes in the form of brines (B), relevant environmental problems have to be duly taken into account. One of the most energy-efficient industrial desalination technologies, reverse osmosis (RO) requires -3-10 kWh of electric energy per cubic meter of FW produced from SW. At ENEL Research Division (Polo Idraulico e Strutturale, Milano), this desalination technology has been made the object of system design analyses which have brought out three novel, highly efficient schemes (submarine, underground, and ground-based) for FW production from SW. The present report concerns the submarine RO scheme RODSS (reverse osmosis deep sea system) which has been designed in collaboration among ENELResearch (Milano), ETA (Firenze), JRC (Ispra), and WIP (Muenchen), under the auspices of the European Commission. RODSS has its RO desalinating units sited offshore at a suitable operative depth and achieves a remarkable energy efficiency through the free exploitation of the hydrostatic pressure ofSW. RODSS will be described in sufficient detail to demonstrate that it requires much less electric energy than any conventional (ground-based) desalination plant of comparable FW production capacity. The discussion will clarify the main operative and maintenance features of the proposed submarine desalination technology which seems to have a promising technicoeconomical potential also vis-/l-vis advantageous couplings with renewable energy sources. Keywords: Energy efficiency; Seawater desalination; Submarine and underground reverse osmosis
1. Introduction Water represents a strategic resource for global sustainable development aimed at human welfare *Corresponding author.
while saving the environment. As a consequence, desalination technologies, ever more frequently assumed in socio-economical development plans, represent a technico-industrial sector in great expansion.
0011-9164/99/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S O 0 1 1 - 9 1 6 4 ( 9 9 ) 0 0 0 3 8 - 7
172
D. Colombo et al. / Desalination 122 (1999.) 171-176
ENEL Research Division, Polo Idraulico e Strutturale, Milano, is engaged in the development of innovative energy-efficient seawater (SW) and brackish water (BW) desalination technologies for the production of desalinated water (DW) for either civil or industrial use. Among the most advanced industrial desalination technologies, reverse osmosis (RO) is based on the use of semipermeable membranes which, assembled in special modules and flushed with high pressure salt water fluxes, withhold salts and let desalinated water permeate through them. The pumping of salt water at a high pressure (50-80 bar), sufficiently larger than the operative osmotic pressure, determines the main electric energy requirement in this desalination technology. This pumping represents a critical factor since the SW flow rate is much larger (3-4 times) than the produced DW flow rate. In optimized conventional plants equipped with special energyrecovery pump-turbines, the basic energy requirement is some 5 kw/m 3(while plants without energy recovery require some 10 kw/m3). ENEL Project SUROS (seawater ultimate reverse osmosis systems) addresses the strategic topic of developing innovative highly energyefficient RO desalination plants for providing drinking/irrigation water to areas lacking a sufficient supply of fresh water (FW). Three innovative RO desalination schemes have been investigated, two for SW desalination (submarine plant RODSS and underground plant EUROS), and one for either SW or BW desalination (ground-based plant EROS). The overall aim is to develop cost-competitive desalination technologies with a broad application potential in Europe and beyond (for isolated villages in coastal areas, etc.). The following discussion will concern only the RODSS system which will be analyzed below in Section 2 with regard to its basic design layout, and in Sections 3 and 4, respectively, with regard to its remarkable energy efficiency and to construction, operation, and maintenance issues.
Finally Section 5 presents general conclusive comments.
2. RODSS design layout The potential exploitation of the hydrostatic pressure of seawater at a sufficient operative depth was considered by several investigators in the 1960s in view of increasing the energy efficiency of the then developing RO-SW industrial desalination technology (see, e.g., [1-6] and related references). The technological risk features inherent in a conceptual submarine desalination system are presently considered not critical in view of the outstanding developments which have meanwhile occurred in submarine and offshore technologies on the one hand, and in the manufacturing and maintenance technologies of RO membrane modules on the other. The diagrammatic sketch of Fig. 1 presents the basic layout of RODSS, a large production capacity submarine RO plant, designed at ENEL Polo Idraulico e Strutturale (Milano, Italy), in collaboration with ETA (Firenze, Italy), JRC (Ispra, Italy), and WIP (Muenchen, Germany). Hydrostatic pressure acts as the driving pressure for the RO process: DW produced at about atmospheric pressure and collected in a submarine tank at a depth of-500 m is pumped to the sea surface while the resulting brine (B) is disposed of into the sea. RODSS has a large production capacity, 20,000 m3/d, to achieve economies of scale and is composed by several (four) submarine RO units sited at -500m below sea level. Each RO unit produces -5,000 m3/d of DW. An extra (fifth) RO unit, similar to the four RO units working in the sea depth, is kept assembled and stored in the RODSS shore warehouse in view of substitution and maintenance operations. The main RODSS components are: RO desalination units, filter boxes, SW feed pump, connection cable for delivering the electric power
D. Colombo et al. / Desalination 122 (1999) 171-176
--
F i g . 1. R O D S S
--
173
ine >
simplified scheme.
from the coast to the immersed plant, DW pump, DW sealine, SW feed conduit, and B discharge conduit. For RODSS, most of the required electric power is for pumping produced DW from the sea depth up to the shore while relatively little electric power is required for circulation pumping of pressurized SW and B: RODSS reduced high pressure pumping is the basis of its great energy efficiency analyzed below in Section 4. RODSS also exploits the advantageous RO characteristics of deep SW by eliminating the inlet SW pre-treatment assembly, which constitutes a considerable cost factor in conventional surfacebased RO plants. While innovative, the RODSS technology utilizes well known and commercially available components and techniques. Since submarine and offshore technologies are mature ones, no critical technological risks are envisaged. Clearly, the specific features of the deep-sea environment have to be duly taken into account, e.g., RODSS desalination units (RO modules, tanks, and pumps) must be positioned sufficiently above the sea floor so as to avoid contamination from sea floor
sediments. Also, the RODSS design avoids, as much as possible, costly submarine operations: its RO units can be easily hoisted up to the sea surface via buoyancy devices so that all required operations can be made after essentially conventional procedures.
3. Energy efficiency The energy efficiency of the RODSS scheme, in terms of required electric energy per cubic meter of produced DW, will be quantitatively evaluated after one set of assumed plant parameters without optimization options. The computed energy efficiency of any desalination plant is to be compared with the minimum mechanical energy, -2.5 MJ or N0.7 kWh, required to produce 1 m 3 of fresh water from seawater [7]. For quantitative reference data from an operating RO plant, we consider the Ghar Lapsi RO-SW desalination plant in Malta which produces -2x10 4 m3/d of DW with a specific electric energy consumption of-6.12 kWh/m 3 [8]: operation of the RO process may conservatively be
174
D. Colombo et aL /Desalination 122 (1999) 171-176
assumed to require - 5 . 0 kWh/m 3 (about seven times the minimum theoretical consumption). Let us now assume a RODSS plant o f - 2 x l 0 4 m3/d D W production capacity and having its RO units at a depth h - - 5 0 0 m. In order to evaluate its energy efficiency, we have to compute its required overall pumping power WT which is a sum o f two contributions: the D W pumping power WD and the S W - B pumping power WsB. For our computation, the following expressions are required: (I) =* (I) S = (I) D + (I) B
(1)
r = (I)D/(I) S
(2)
Ax B = ( 1 - 0 -1 A~ s
(3)
r = 0.25 =* • s = 0.926 (m3/s); ~B = 0.694 (m3/s); Pa = 1.035 × 103 (kg/m3); Ax B = 3.37x 10 6 Pa (= 33.3 atm); ps = 5.12x 106pa (= 50.5 atm); Ap = P s - A~B = 1-75x106 Pa (= 17.3 atm); Flow rates in single RO unit: ~ D = 0 . 0 5 7 8 7 (m3/s); ~ s = 0.2315 (m3/s); ~ B =
0.1735 (m3/s);
3.2. Computation o f D W pumping power for single RO unit
g = 9.8 m s -2
Ap = Ps - AXB
(4)
u = 4 X -I • D -2
(5)
v = 1.0 × 10 -6 (m2/s) for water at 20°C
D W is pumped from the RO unit in the sea depth up to the shore through one conduit: D D = 0.20 m; UD = 1.84 m/s; L D -= 2000 m;
Re = u D/v
(6)
f = 0.012 for turbulent water flow
Ah o =-20.7 m; h D = h + Ah D - 520.7 m; =, W D -= 0.369 M W
Ah = ( 2 g O ) - ~ f L u ~
(7)
hD = h + AhD
(8)
hsB = Ah s + Ah B + AhBM
(9)
rl = 0 . 8
3.3. Computation o f S W - B pumping power f o r single RO unit SW is pumped from the sea into RO modules through one conduit:
W= ~ P g h
(10)
D s = 0.40 m; u s = 1.84 m/s; L s ~ 10 m;
BIT = WD+ WSB
(11)
Ah s - 0.05 m; h s = Ah s ~ 0.05 m;
WSB = Ws + WB
(12)
=* Ws is negligible (-= 145 W)
3.1. Submarine RODSS plant design data D W production capacity: 2x104 (m3/d) OD= 0.2315 (m3/s); h = 500 m; A~ s = 2.53x106 Pa (=24.9 arm); Ps = 1.025x 10 a (kg/m3);
B is pumped from the RO modules through one conduit to be disposed o f into the sea: D B = 0.40 m; UB = 1.38 m/s; L B -= 100 m; Ah B - 0.3 m; AhBM -= 10 m; hB = AhB + AhBM -= 10.3 m;
D. Colombo et al. / Desalination 122 (1999) 171-176 =' WB ~ 0.0224 MW =* WSB= Ws + WB ~ 0.0224 MW
3. 4. Computation o f total pumping power for single RO unit WT = Wo + WSB= 0.3914 MW The computed total pumping power WTimplies that for the production of 1 m 3 of DW from SW the RO process in our assumed submarine RODSS desalination plant requires -6.763 MJ or -1.88 kWh of electric energy which is -0.37 times the energy required (-5.0 kWh) in the Malta Ghar Lapsi plant. Since the cost of energy represents a major portion of the DW cost from RO-SW desalination plants, the RODSS remarkable energy efficiency clearly represents a rather promising economic asset.
4. General R O D S S features
4.1. Operational aspects Operation of RODSS would parallel that of conventional (ground-based) RO-SW desalination plants without risk of mechanical failures for membrane modules as may occur in surface plants due to high system vibrations induced by the main SW delivery pumps. Such failures would not be expected in RODSS RO units since the SW flowing through them would be pumped by conventional centrifugal low-head circulation pumps. The flow of SW through the membrane modules should also be more steady and more easily regulated according to set operational parameters. On the other hand, the low temperature of deep SW would reduce membrane permeation flux so that RODSS would require more RO modules than a conventional plant of equivalent DW production capacity. Specific
175
experimentation, design analyses, and productivity projections are required to check this important operational aspect so as to reduce investment costs as much as possible. As for maintenance and substitution procedures, no critical problems are envisaged since RODSS is specifically designed to have these required operations performed at its shore warehouse. An important operative advantage, as far as pretreatment issues are concerned, is expected for RODSS because deep SW is rather pure so that microbiological fouling of membranes should not occur. As for the ecological viewpoint, it is to be noticed that the rather fastidious if not dangerous noise of conventional RO--SW desalination plants is not to be found in submarine RODSS since all operating pumps are placed into the sea depth. B discharge problems do not clearly present any critical issue for RODSS since B is disposed of in the sea depth where it can be easily diluted by surrounding SW. As for potentially exploitable energy sources, the great RODSS energy efficiency suggests a favorable coupling with renewable energy sources, in particular photovoitaic and wind power plants, in islands and isolated sites not linked to any electric power grid.
4.2. Economic and social aspects A preliminary technico-economic analysis has indicated a rather promising economic potential for RODSS and that implementation of this remarkably energy-efficient desalination technology may usefully affect the overall development of many selected areas (arid islands and coasts) lacking an adequate supply of FW [9]. Implementation of RODSS technology may thus be considered within a transnational context for coastal areas in all continents with a sufficiently deep sea floor close to the shore.
D. C o l o m b o et al. / Desalination 122 (1999) 1 7 1 - 1 7 6
176
5.
Conclusions
The proposed novel submarine RODSS desalination plant presents advantageous features from the energy, operational, and ecological viewpoints when compared with conventional R O - S W desalination plants of equivalent fresh water production capacity. Implementation of RODSS desalination technology also would seem to promise rather useful couplings with renewable energy sources in selected sites, particularly in isolated areas and small islands without an electric power grid. While no critical technological issues are envisaged, for a thorough clarification of the technico-economical features of the novel desalination scheme, a specific experimental pilot project, including construction and field testing of a small production capacity RODSS prototype, is to be carried out. The forthcoming phase of the European Innovation Project for RODSS will concern the overall important experimental verification of the RODSS concept [9].
6.
Symbols
D f
--
g h Ah
--
AhBM
--
Z
m
p
m
6p
--
r
m
Re
m
Conduit inner diameter, m Friction coefficient Gravitational acceleration, m s -2 Plant depth or overall hydrostatic head, m Distributed head losses in conduit, m Concentrated head losses in RO modules, m Conduit length, m Hydrostatic pressure, Pa Driving pressure for RO process, Pa Recovery coefficient Reynolds number for conduit flow
U
--
W
--
Average flow velocity in conduit, m s -1
--
Pumping power, w Total pumping power, w
Greek rl
--
V
--
P ¢b
---
Ar~
--
Pumping efficiency coefficient Kinematic viscosity, m 2 s-1 Density, kg m -3 Flow rate, m 3 s-1 Osmotic pressure difference between salt and fresh water, Pa
Subscripts
B D F S
-----
Brine Desalinated water Fresh water Seawater
References
[1] EuropeanProject Prodesal, Towards the large-scale development of decentralised water desalination, Contract RENA-CT94-0018, 1994. [2] M. Reali, M. de Gerloni and A. Sampaolo, Desalination, 109 (1997) 269. [3] B.C. Drude, Desalination, 2 (1967) 325. [4] E.F. Miller, Chemical Engineering, 1968. [5] B.C.Drude and E. Klapp, in: Proc., 4th Int. Symp. on Fresh Water from the Sea, Heidelberg, 4 (1973) 125. [6] P. Glueckstem, Desalination, 40 (1982) 143. [7] K.S.Spiegler, Salt Water Purification, Plenum Press, New York, 1977. [8] DuPont, date from brochure on the 2×104 m3/d RO-SW desalination plant, Ghar Lapsi, Malta, 1991. [9] EuropeanProject, INNOVATION Programme, CEC Contract No. IN 20519 D, 1997-98.
Desalination 122 (1999) 171-176 www.elsevier.com/Iocate/desal
An energy-efficient submarine desalination plant D. Colombo, M. de Gedoni, M. Reali* ENEL - Ricerca Polo Idraulico e Strutturale, V. P ozzobonelli 6, 20162 Milano, Italy Tel. +39 (02) 7224 3600; Fax +39 (02) 7224-3530; eraail: [email protected]
Received 3 December 1998; accepted February 28, 1999
Abstract
The socio-economical development of many regions lacking a sufficient supply of fresh water (FW) may at times be planned with the help of desalination technologies for sea and brackish water (SW and BW). Since these technologies require large energy fluxes in the form of heat or electricity and produce sizable salt fluxes in the form of brines (B), relevant environmental problems have to be duly taken into account. One of the most energy-efficient industrial desalination technologies, reverse osmosis (RO) requires -3-10 kWh of electric energy per cubic meter of FW produced from SW. At ENEL Research Division (Polo Idraulico e Strutturale, Milano), this desalination technology has been made the object of system design analyses which have brought out three novel, highly efficient schemes (submarine, underground, and ground-based) for FW production from SW. The present report concerns the submarine RO scheme RODSS (reverse osmosis deep sea system) which has been designed in collaboration among ENELResearch (Milano), ETA (Firenze), JRC (Ispra), and WIP (Muenchen), under the auspices of the European Commission. RODSS has its RO desalinating units sited offshore at a suitable operative depth and achieves a remarkable energy efficiency through the free exploitation of the hydrostatic pressure ofSW. RODSS will be described in sufficient detail to demonstrate that it requires much less electric energy than any conventional (ground-based) desalination plant of comparable FW production capacity. The discussion will clarify the main operative and maintenance features of the proposed submarine desalination technology which seems to have a promising technicoeconomical potential also vis-/l-vis advantageous couplings with renewable energy sources. Keywords: Energy efficiency; Seawater desalination; Submarine and underground reverse osmosis
1. Introduction Water represents a strategic resource for global sustainable development aimed at human welfare *Corresponding author.
while saving the environment. As a consequence, desalination technologies, ever more frequently assumed in socio-economical development plans, represent a technico-industrial sector in great expansion.
0011-9164/99/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S O 0 1 1 - 9 1 6 4 ( 9 9 ) 0 0 0 3 8 - 7
172
D. Colombo et al. / Desalination 122 (1999.) 171-176
ENEL Research Division, Polo Idraulico e Strutturale, Milano, is engaged in the development of innovative energy-efficient seawater (SW) and brackish water (BW) desalination technologies for the production of desalinated water (DW) for either civil or industrial use. Among the most advanced industrial desalination technologies, reverse osmosis (RO) is based on the use of semipermeable membranes which, assembled in special modules and flushed with high pressure salt water fluxes, withhold salts and let desalinated water permeate through them. The pumping of salt water at a high pressure (50-80 bar), sufficiently larger than the operative osmotic pressure, determines the main electric energy requirement in this desalination technology. This pumping represents a critical factor since the SW flow rate is much larger (3-4 times) than the produced DW flow rate. In optimized conventional plants equipped with special energyrecovery pump-turbines, the basic energy requirement is some 5 kw/m 3(while plants without energy recovery require some 10 kw/m3). ENEL Project SUROS (seawater ultimate reverse osmosis systems) addresses the strategic topic of developing innovative highly energyefficient RO desalination plants for providing drinking/irrigation water to areas lacking a sufficient supply of fresh water (FW). Three innovative RO desalination schemes have been investigated, two for SW desalination (submarine plant RODSS and underground plant EUROS), and one for either SW or BW desalination (ground-based plant EROS). The overall aim is to develop cost-competitive desalination technologies with a broad application potential in Europe and beyond (for isolated villages in coastal areas, etc.). The following discussion will concern only the RODSS system which will be analyzed below in Section 2 with regard to its basic design layout, and in Sections 3 and 4, respectively, with regard to its remarkable energy efficiency and to construction, operation, and maintenance issues.
Finally Section 5 presents general conclusive comments.
2. RODSS design layout The potential exploitation of the hydrostatic pressure of seawater at a sufficient operative depth was considered by several investigators in the 1960s in view of increasing the energy efficiency of the then developing RO-SW industrial desalination technology (see, e.g., [1-6] and related references). The technological risk features inherent in a conceptual submarine desalination system are presently considered not critical in view of the outstanding developments which have meanwhile occurred in submarine and offshore technologies on the one hand, and in the manufacturing and maintenance technologies of RO membrane modules on the other. The diagrammatic sketch of Fig. 1 presents the basic layout of RODSS, a large production capacity submarine RO plant, designed at ENEL Polo Idraulico e Strutturale (Milano, Italy), in collaboration with ETA (Firenze, Italy), JRC (Ispra, Italy), and WIP (Muenchen, Germany). Hydrostatic pressure acts as the driving pressure for the RO process: DW produced at about atmospheric pressure and collected in a submarine tank at a depth of-500 m is pumped to the sea surface while the resulting brine (B) is disposed of into the sea. RODSS has a large production capacity, 20,000 m3/d, to achieve economies of scale and is composed by several (four) submarine RO units sited at -500m below sea level. Each RO unit produces -5,000 m3/d of DW. An extra (fifth) RO unit, similar to the four RO units working in the sea depth, is kept assembled and stored in the RODSS shore warehouse in view of substitution and maintenance operations. The main RODSS components are: RO desalination units, filter boxes, SW feed pump, connection cable for delivering the electric power
D. Colombo et al. / Desalination 122 (1999) 171-176
--
F i g . 1. R O D S S
--
173
ine >
simplified scheme.
from the coast to the immersed plant, DW pump, DW sealine, SW feed conduit, and B discharge conduit. For RODSS, most of the required electric power is for pumping produced DW from the sea depth up to the shore while relatively little electric power is required for circulation pumping of pressurized SW and B: RODSS reduced high pressure pumping is the basis of its great energy efficiency analyzed below in Section 4. RODSS also exploits the advantageous RO characteristics of deep SW by eliminating the inlet SW pre-treatment assembly, which constitutes a considerable cost factor in conventional surfacebased RO plants. While innovative, the RODSS technology utilizes well known and commercially available components and techniques. Since submarine and offshore technologies are mature ones, no critical technological risks are envisaged. Clearly, the specific features of the deep-sea environment have to be duly taken into account, e.g., RODSS desalination units (RO modules, tanks, and pumps) must be positioned sufficiently above the sea floor so as to avoid contamination from sea floor
sediments. Also, the RODSS design avoids, as much as possible, costly submarine operations: its RO units can be easily hoisted up to the sea surface via buoyancy devices so that all required operations can be made after essentially conventional procedures.
3. Energy efficiency The energy efficiency of the RODSS scheme, in terms of required electric energy per cubic meter of produced DW, will be quantitatively evaluated after one set of assumed plant parameters without optimization options. The computed energy efficiency of any desalination plant is to be compared with the minimum mechanical energy, -2.5 MJ or N0.7 kWh, required to produce 1 m 3 of fresh water from seawater [7]. For quantitative reference data from an operating RO plant, we consider the Ghar Lapsi RO-SW desalination plant in Malta which produces -2x10 4 m3/d of DW with a specific electric energy consumption of-6.12 kWh/m 3 [8]: operation of the RO process may conservatively be
174
D. Colombo et aL /Desalination 122 (1999) 171-176
assumed to require - 5 . 0 kWh/m 3 (about seven times the minimum theoretical consumption). Let us now assume a RODSS plant o f - 2 x l 0 4 m3/d D W production capacity and having its RO units at a depth h - - 5 0 0 m. In order to evaluate its energy efficiency, we have to compute its required overall pumping power WT which is a sum o f two contributions: the D W pumping power WD and the S W - B pumping power WsB. For our computation, the following expressions are required: (I) =* (I) S = (I) D + (I) B
(1)
r = (I)D/(I) S
(2)
Ax B = ( 1 - 0 -1 A~ s
(3)
r = 0.25 =* • s = 0.926 (m3/s); ~B = 0.694 (m3/s); Pa = 1.035 × 103 (kg/m3); Ax B = 3.37x 10 6 Pa (= 33.3 atm); ps = 5.12x 106pa (= 50.5 atm); Ap = P s - A~B = 1-75x106 Pa (= 17.3 atm); Flow rates in single RO unit: ~ D = 0 . 0 5 7 8 7 (m3/s); ~ s = 0.2315 (m3/s); ~ B =
0.1735 (m3/s);
3.2. Computation o f D W pumping power for single RO unit
g = 9.8 m s -2
Ap = Ps - AXB
(4)
u = 4 X -I • D -2
(5)
v = 1.0 × 10 -6 (m2/s) for water at 20°C
D W is pumped from the RO unit in the sea depth up to the shore through one conduit: D D = 0.20 m; UD = 1.84 m/s; L D -= 2000 m;
Re = u D/v
(6)
f = 0.012 for turbulent water flow
Ah o =-20.7 m; h D = h + Ah D - 520.7 m; =, W D -= 0.369 M W
Ah = ( 2 g O ) - ~ f L u ~
(7)
hD = h + AhD
(8)
hsB = Ah s + Ah B + AhBM
(9)
rl = 0 . 8
3.3. Computation o f S W - B pumping power f o r single RO unit SW is pumped from the sea into RO modules through one conduit:
W= ~ P g h
(10)
D s = 0.40 m; u s = 1.84 m/s; L s ~ 10 m;
BIT = WD+ WSB
(11)
Ah s - 0.05 m; h s = Ah s ~ 0.05 m;
WSB = Ws + WB
(12)
=* Ws is negligible (-= 145 W)
3.1. Submarine RODSS plant design data D W production capacity: 2x104 (m3/d) OD= 0.2315 (m3/s); h = 500 m; A~ s = 2.53x106 Pa (=24.9 arm); Ps = 1.025x 10 a (kg/m3);
B is pumped from the RO modules through one conduit to be disposed o f into the sea: D B = 0.40 m; UB = 1.38 m/s; L B -= 100 m; Ah B - 0.3 m; AhBM -= 10 m; hB = AhB + AhBM -= 10.3 m;
D. Colombo et al. / Desalination 122 (1999) 171-176 =' WB ~ 0.0224 MW =* WSB= Ws + WB ~ 0.0224 MW
3. 4. Computation o f total pumping power for single RO unit WT = Wo + WSB= 0.3914 MW The computed total pumping power WTimplies that for the production of 1 m 3 of DW from SW the RO process in our assumed submarine RODSS desalination plant requires -6.763 MJ or -1.88 kWh of electric energy which is -0.37 times the energy required (-5.0 kWh) in the Malta Ghar Lapsi plant. Since the cost of energy represents a major portion of the DW cost from RO-SW desalination plants, the RODSS remarkable energy efficiency clearly represents a rather promising economic asset.
4. General R O D S S features
4.1. Operational aspects Operation of RODSS would parallel that of conventional (ground-based) RO-SW desalination plants without risk of mechanical failures for membrane modules as may occur in surface plants due to high system vibrations induced by the main SW delivery pumps. Such failures would not be expected in RODSS RO units since the SW flowing through them would be pumped by conventional centrifugal low-head circulation pumps. The flow of SW through the membrane modules should also be more steady and more easily regulated according to set operational parameters. On the other hand, the low temperature of deep SW would reduce membrane permeation flux so that RODSS would require more RO modules than a conventional plant of equivalent DW production capacity. Specific
175
experimentation, design analyses, and productivity projections are required to check this important operational aspect so as to reduce investment costs as much as possible. As for maintenance and substitution procedures, no critical problems are envisaged since RODSS is specifically designed to have these required operations performed at its shore warehouse. An important operative advantage, as far as pretreatment issues are concerned, is expected for RODSS because deep SW is rather pure so that microbiological fouling of membranes should not occur. As for the ecological viewpoint, it is to be noticed that the rather fastidious if not dangerous noise of conventional RO--SW desalination plants is not to be found in submarine RODSS since all operating pumps are placed into the sea depth. B discharge problems do not clearly present any critical issue for RODSS since B is disposed of in the sea depth where it can be easily diluted by surrounding SW. As for potentially exploitable energy sources, the great RODSS energy efficiency suggests a favorable coupling with renewable energy sources, in particular photovoitaic and wind power plants, in islands and isolated sites not linked to any electric power grid.
4.2. Economic and social aspects A preliminary technico-economic analysis has indicated a rather promising economic potential for RODSS and that implementation of this remarkably energy-efficient desalination technology may usefully affect the overall development of many selected areas (arid islands and coasts) lacking an adequate supply of FW [9]. Implementation of RODSS technology may thus be considered within a transnational context for coastal areas in all continents with a sufficiently deep sea floor close to the shore.
D. C o l o m b o et al. / Desalination 122 (1999) 1 7 1 - 1 7 6
176
5.
Conclusions
The proposed novel submarine RODSS desalination plant presents advantageous features from the energy, operational, and ecological viewpoints when compared with conventional R O - S W desalination plants of equivalent fresh water production capacity. Implementation of RODSS desalination technology also would seem to promise rather useful couplings with renewable energy sources in selected sites, particularly in isolated areas and small islands without an electric power grid. While no critical technological issues are envisaged, for a thorough clarification of the technico-economical features of the novel desalination scheme, a specific experimental pilot project, including construction and field testing of a small production capacity RODSS prototype, is to be carried out. The forthcoming phase of the European Innovation Project for RODSS will concern the overall important experimental verification of the RODSS concept [9].
6.
Symbols
D f
--
g h Ah
--
AhBM
--
Z
m
p
m
6p
--
r
m
Re
m
Conduit inner diameter, m Friction coefficient Gravitational acceleration, m s -2 Plant depth or overall hydrostatic head, m Distributed head losses in conduit, m Concentrated head losses in RO modules, m Conduit length, m Hydrostatic pressure, Pa Driving pressure for RO process, Pa Recovery coefficient Reynolds number for conduit flow
U
--
W
--
Average flow velocity in conduit, m s -1
--
Pumping power, w Total pumping power, w
Greek rl
--
V
--
P ¢b
---
Ar~
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Pumping efficiency coefficient Kinematic viscosity, m 2 s-1 Density, kg m -3 Flow rate, m 3 s-1 Osmotic pressure difference between salt and fresh water, Pa
Subscripts
B D F S
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Brine Desalinated water Fresh water Seawater
References
[1] EuropeanProject Prodesal, Towards the large-scale development of decentralised water desalination, Contract RENA-CT94-0018, 1994. [2] M. Reali, M. de Gerloni and A. Sampaolo, Desalination, 109 (1997) 269. [3] B.C. Drude, Desalination, 2 (1967) 325. [4] E.F. Miller, Chemical Engineering, 1968. [5] B.C.Drude and E. Klapp, in: Proc., 4th Int. Symp. on Fresh Water from the Sea, Heidelberg, 4 (1973) 125. [6] P. Glueckstem, Desalination, 40 (1982) 143. [7] K.S.Spiegler, Salt Water Purification, Plenum Press, New York, 1977. [8] DuPont, date from brochure on the 2×104 m3/d RO-SW desalination plant, Ghar Lapsi, Malta, 1991. [9] EuropeanProject, INNOVATION Programme, CEC Contract No. IN 20519 D, 1997-98.