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Seawater desalination plant by reverse osmosis powered with photovoltaic solar energy. A supply option for remote areas

T. Espino, B. Peñate#, D. Henríquez, J. Betancort, G. Piernavieja

(1) Energy, Water and Bioengineering Division, Instituto Tecnológico de Canarias, S.A.

(2) Playa de Pozo Izquierdo s/n –35119 (Santa Lucía) Gran Canaria, Spain Phone +34 928 727503; Fax +34 928 727517; emails: baltasarp@itccanarias.org; gpiernavieja@itccanarias.org

Abstract This paper presents the results of Instituto Tecnológico de Canarias (ITC) project called DESSOL-Desalination with Photovoltaic Solar Energy. This project is the optimization of the one first version of the system, which was development in close collaboration with Aachen University of Applied Sciences (Julich, Germany) in 1998 [1]. The DESSOL project proposes a solution for the water scarcity in isolated areas that the conventional electric grid does not reach. We make use of photovoltaic solar energy (PV) to power a pilot sea water desalination system based on reverse osmosis (RO) with a mean production capacity of 3 m³/day (7 operating hours annual average) with no electricity of a thermal origin, but with certain solar radiation conditions. This experience has shown the technical and economic viability of these systems, which can be efficiently extrapolated to other water productions or qualities. Keywords: Reverse osmosis desalination; Photovoltaic system; Batteries. 1. Introduction In the last decade, desalination, especially by reverse osmosis (RO), has become one of the principal safe sources of supply of potable water, and even water for agricultural use, in Spain and other countries with limited natural water resources. It is in the Mediterranean, Africa, the Middle East, and son on, where the potable water supply is a high priority problem. The water resources in these areas are limited and even diminishing, since water becomes saline due to sea water filtrations and there is extensive irrigation. All this leads to the need to identify new sources of supply such as the desalination of brackish or sea water. However, the installation of desalting plants leads to total dependence on power, meaning that the water-energy combination becomes of the utmost importance, to a point where in many places it is true that “if there is no power, there is no water”. This is where renewable energies play a fundamental role as a new source of electricity to help to obtain the water that is so vital for human use (European Water Charter. Strasbourg, 1968). The autonomous photovoltaic solar-reverse osmosis (PV-RO) system with power storage and control system presented here is capable of satisfying the water demand of an area isolated from the electric grid (50–75 inhabitants) with a scarcity of potable water. The optimised system [1] has a production capacity of 400 L/h (at 60 bar) from sea water, operating an average of 8 hours per day in the summer and 6 in the winter. PV-RO systems have been devised which are only simulations based on a design[2-4] or which have been designed and tested with a manual plant in continuous operation [5,6]. Our real installation approaches the model from the automation perspective, using a control programme to manage and optimise the hours of solar radiation available.

# Corresponding author.

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2. Location of the desalination plant The autonomous PV-RO system tested is in the ITC facilities on the Pozo Izquierdo beach, in the municipality of Santa Lucía on the island of Gran Canaria (Las Palmas-Spain). The ITC is a publicly-owned company of the Government of the Canary Islands. Under its Technical Management is the Energy, Water and Bioengineering Division, which is responsible for fostering the industrial development of the Canary Islands in the field of renewable energies and water technologies, facilitating the participation of businesses and acting as a platform for R&D and innovation testing. The weather conditions at Pozo Izquierdo make it an area of excellent qualities for R&D concerning renewable energies. The following table (Table 1) shows some information on the location and its climate.

Table 1: Location and climate data (2001) referring to the ITC facilities at Pozo Izquierdo beach (Gran Canaria).

Orientation of the terrain South-west

Surface area available 109,000 m²

Distance from the sea <100 m

Maximum altitude above sea level 3 m

Mean annual solar radiation (on hz. surface) (*) 2,044 kWh/m²

Mean annual wind speed 7.8 m/s

Mean annual temperature 23.5ºC

Mean annual humidity 65 - 70%

Annual rainfall 105 mm (5 –10 days of rain/year)

(*) Compare with the 1,653 kWh/m² of Madrid [7] and the 1,718 kWh/m² of Almería [8].

Figure 1: Aerial views of the ITC facilities at Pozo Izquierdo

beach. Figure 1 shows a series of aerial views of the ITC facilities at Pozo Izquierdo. Figure 2 defines the specific technical areas of the project: sea water collection (1), the Desalination dome (2), the pond of produced water (3), the photovoltaic field (4) and the battery house (5).

Figure 2: Location of the principal components on the ITC facilities.

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3. Description of the PV-RO system The system basically consists of a sea water RO desalination plant operating and an isolated photovoltaic system (PV) with power storage based on batteries, producing the power required for the desalination process. Figure 3 is a diagram of the principal power and water lines of the PV-RO system tested.

Figure 3: Diagram of the PV-RO system1.

RO Desalination plant The sea water desalination plant uses RO technology and has a rated production at 60 bar of 10 m³/d (24 hours), producing potable water with a mean conductivity value of less than 1,000 µS/cm and specific rated consumption in the desalination process of 5.5 kWh/m³. It is a compact installation in an AISI 316 stainless steel frame, and has two parallel lines of 6 spiral -wound membranes each (2.5” x 40”) (Figure 4). All the high pressure water lines are in flexible rubber and the low pressure line is made out of plastic (PVC and PE-HD). The plant thus has optimal resistance in corrosive atmospheres and therefore requires minimal maintenance. Table 2 provides a list of the components and the principal characteristics of the lines comprising the system.

Figure 4: 10 m³/d sea water desalination plant.

1 Clean in place

RO lines 12 elements

(2.5” ∅)

Pressure: 3 bar Flow: 2,5 m³/h P: 1 kW

Pressure <70 bar Flow max: 1,5 m³/h P: 2,2 kW (60 bar)

Cartridge FiltersSea water

Brine flow

Product

Capacity: 10 m³/d Water quality<500 mg/L

Caudal max : 8 m3/h P : 0,75 kW

Water line

Electrical line

HIGH PRESSURE

PUMP

CIP1 PUMP

WELL PUMP

ELECTRICAL SUPPLY (PV)

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Table 2: Components of the sea water desalination plant.

Pre-treatment line High pressure line

Motor-driven feed valve Stainless steel ¾” ∅ Low pressure switch 1 bar cut-out pressure

Pre-filtration manometer 0-10 bar High pressure pump 900 L/h (2.2 kW)

Cartridge filter 25 µm Pump output manometer 0-100 bar

Cartridge filter 5 µm Membrane tubes 12 tubes (2 m. long)Max. pressure 1000 psi

Post-filtration manometer 0-10 bar RO membranes 12 SW-2540

Temperature sensor PT-100 High pressure switch 67 bar cut-out pressure

Tube output manometer 0-100 bar

Brine rejection line High pressure transmitter

Needle valve Stainless steel 904 L¾” ∅

Motor-driven by-pass valve

Stainless steelClosure time 150 s Product line

Brine rotameter Buoy200-1800 L/h Product rotameter Buoy

0-600 L/hConductivity and pH transmitter and sensor Compensating tank 60 L

Product water counter

CIP line Conductivity and pH transmitter and sensor

Clean tanks 2 x 200 L (each)

Motor-driven valve Stainless steel ¾” ∅Centrifugal impulse pump

700 L/h0.75 kW

One interesting new aspect of the plant is the design of the hydraulic system for the plant’s operating mode when the plant is shut down on a daily basis, to prevent the process brine from remaining in contact with the membranes all night. A flushing process according to the brine volume contained is of vital importance. Therefore, the centrifugal cleaning pump pumps around 300 L of product water (from the cleaning tanks) at a pressure of about 3-4 bar. Once this process is complete, the product water in the compensating tank acts by direct osmosis until the concentrations are the same. The tubes thus remain submerged in water with a low salinity value and are prevented from deteriorating. The plant’s electric installation includes an automatic mechanism which not only collects data from the system but also authorises the plant’s start-up, shut-down and control. The inclusion of this component has reduced the electric system to a minimum. Electrical-Supply - PV system The power generating system has a 4.8 kWp photovoltaic field consisting of sixty-four 75 Wp A-75 modules, power accumulation consists of twenty-four 385 Ah 2 V vessels, a 75 A regulator, a 4.5 kW inverter and a protection panel with which we separate and protect the panel lines, also including the battery output protection fuses. The following figure (Figure 6) shows a diagram of the photovoltaic system used in the project.

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Figure 5: Diagram of the 4.8 kWp photovoltaic system.

Basically, the photovoltaic field is connected to a bank of accumulators providing it with power (Figure 8). This process is controlled by a regulator, a device in charge of managing the charge process, preventing the batteries becoming over or undercharges with a view to lengthening their lives as much as possible. Each time the regulator cuts off the charge process, the power provided by the photovoltaic generator is lost. This factor is used to design the plant’s control programme, with which we can practically eliminate these charge cut-out periods, thus making the best possible use of the solar energy available.

Figure 6: Photovoltaic field and accumulators bank area. The accumulators fulfil two functions. On the one hand, they keep the input voltage to the inverter stable and, on the other, they make it possible to make use of the surplus radiation in the middle of the day, when more power is supplied by the photovoltaic system than is consumed by the plant. This surplus is later used to operate the system in the early and late hours of the day, when the process is inverted. This means that the charge regulator never cuts out the charge process, and there is 100% use of the solar energy available. The direct rated voltage of the photovoltaic system, which is 48 V, is transformed by the inverter into 220 V (AC) to adapt it to the plant’s operating voltage. Control system The system is controlled by an PLC mechanism and a control system developed in a Windows environment (Figure 9) which acts as an interface between the automatic mechanism and a computer in order to provide on-line data graphs. In turn, the control system stores the values of all the system’s variables once a minute in a data file, so that its evolution can later be analysed. The ITC control - programme is responsible at all times for controlling the plant’s operating status, which varies with the capacity remaining in the accumulation system (C), which in turn depends on the solar radiation available and the power consumed by the plant during operations. Measuring this parameter throughout the day, the control decides when the plant

PV field 64 modules (A-75)

Regulator 48 V/75 A

Inverter DC/AC

48 V/4,500W

Bank of accumulators 19 kWh

4.8 kWp

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starts up and shuts down (in other words, the operating period), adapting operating hours to the daily incident solar radiation. Thirteen operating status categories have been established for the plant, including on hold (Status 1), daily start-up (Status 2), daily shut-down (Status 5), flushing (Status 3), operating status (from Status 6 to 11), alarm (Status 4) and manual operating status (Status 12 and 13).

Figure 7: Main menu of the PV-RO system’s control Scada. The operating status categories are the daily transient phases in which the control programme sets the plant as soon as start-up status is detected, and they are responsible for deciding when the plant starts up and shuts down, depending on the battery capacity (C) and the time of day. Table 3 shows the value of each status category, depending on whether the order is to start up or shut down. The plant only starts up once a day and therefore there is only one shut-down per day, accompanied by a brine flushing process.

Table 3: RO plant operating status categories.

START-UP SHUT-DOWN

Status 6 C>377 Ah and 08:30 a.m. Status 6 18:00 or 19:30 p.m.

Status 7 C>310 Ah and 10:00 a.m. Status 7 C<248 Ah or 17:00 p.m.

Status 8 C>248 Ah and 10:30 a.m. Status 8 C<186 Ah or 17:00 p.m.

Status 9 No start-up decided Status 9 C<186 Ah or 16:00 p.m.

Status 10 186 ≤ C ≥ 248 Ah and 10:30 a.m. Status 10 C<248 Ah or 12:00 a.m.

Status 11 C>186 Ah and 12:00 p.m. Status 11 C<186 Ah or 14:00 p.m.

From an analysis of this table, we can see that the desalting plant will operate for a maximum of 11 hours (status 6 is maintained throughout the day) and a minimum of 2 hours (status 11 is maintained throughout the day). From the monthly data available, it has been seen that the plant operates for an average of 8 hours per day (3.2 m³/d) in the summer and 6 in the winter (2.4 m³/d). Since this is a discontinuous operation, the control programme, of which the following are significant aspects, is of priority importance:

The control adapts the plant’s operating time to the solar radiation available on the site. This has been possible thanks to the use, for the first time, of a battery monitor. This equipment is constantly measuring the power status of the battery and informing control. This factor means that this type of plant can be installed anywhere in the world, obtaining the maximum yield from the power system. The plant will produce more or less water, depending on the radiation available on the site in question. The maximum use of the solar radiation available is patent in the data collected. As we explained earlier, they reveal that the charge regulator has practically not been active, indicating that all the solar energy available is being used.

The installation of motor-driven valves in the feed system, in the wash line and the

needle valve by-pass, controlled by the control programme, means that start-ups, shut-downs and wash processes are fully automatic, so that the plant does not need to be manned.

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4. Discussion. Assessment of the system The most significant meteorological parameters in the study of isolated photovoltaic systems are the ambient temperature, which inversely affects the yield of the photovoltaic cells, and the solar radiation available. Logically, this study has been conducted considering the information for Pozo Izquierdo, where the project is located. The system’s production and performance, although it is a point of reference for its application anywhere in the world, is specific to this site, which is considered to be one of the best areas in the world for the use of renewable energies. We should also remember that in this installation, the inclination of the photovoltaic modules is altered four times a year (25º from the horizontal plane from February to April, 0º from May to July, 25º from August to October and 45º from November to January), thus obtaining a mean annual photovoltaic conversion rate of 92%. Solar radiation available The monthly mean daily radiation values on a horizontal surface at Pozo Izquierdo are shown on table 4. From these values, we can see that at Pozo Izquierdo there is a mean annual daily value of 5.6 kWh/m², equivalent to 2,044 kWh/m²/year

Table 4: Monthly mean solar radiation values at Pozo Izquierdo (2001).

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

KWh/m² day 3.5 4.6 5.5 6.6 7.6 8.2 7.8 7.2 5.8 4.5 3.3 2.9

Observing the data obtained from the system in this year’s operating period, we can see that the system’s mean production rate is around 23 kWh/d, with the installation’s global yield2 in the period at around 83%. Study of the plant’s operation To summarise our study of the data collected during the operating period, we will describe the most significant aspects and different operating days in different radiation conditions. The plant’s energy balance is obtained by analysing four basic parameters:

Plant production (L/h): litres of water produced per time unit. The system’s electric consumption (Wh): the power demanded by the plant to obtain a

given volume of product at a pre-established water quality (normally between 800 and 1000 µS/cm),

Quality of the water produced (µS/cm). Daily solar radiation (W).

Data analysis The data analysed is presented in graph form to see their evolution after several days of operation. Figure 10, representing five consecutive days and it shows the evolution of the incident solar radiation (W), which is nearly a parabola on the days with good solar radiation; over the graph (rectangular graph at the top), we can see the electric intensity consumed by the plant (A), the variable from which we obtain the RO plant’s operating hours and the flow rate produced. The other rectangular curve (bottom of the graph) represents the evolution of the instant conductivity value of the water produced (µS/cm).

2 The global yield of the installation includes the photovoltaic yield, the battery yield and the electrical line losses.

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Figure 8: Plant operation over several days.

Taking the most representative parameters of several days with high and low solar radiation, we obtain the following values:

Table 5: Summary of the operation on a day with high and low solar radiation.

Parameters High solar radiation day Low solar radiation day

Start-up time 8:30 a.m. 10:30 a.m.

Shut-down time 19:30 p.m. 17:00 p.m.

Initial battery capacity 378 Ah 308 Ah

Final battery capacity 367 Ah 277 Ah

Battery energy balance -11 Ah -31 Ah

Total power consumed (in direct current) 550 Ah 350 Ah

PV field production 25 kWh 12.5 kWh

Total plant operating time 11 hours 7 hours

Plant production during the day 4,165 litres 2,430 litres

Mean conductivity 880 µS/cm 662 µS/cm

From an analysis of this data, we reach the following conclusions:

On days with good radiation, the plant can operate continuously for 11 hours, with a very small battery discharge. The energy balance shows that the use of a larger accumulation system is not justified, since the plant practically directly consumes the power generated.

The control programme system adapts the possible operating hours to the solar radiation available. Whereas in the first case the plant operates for 11 hours, for days with low radiation we only obtain an average of 7 operating hours, and the battery is more discharged, since the little radiation available is not capable of recharging the battery to the initial values.

Following is a summary of the system’s performance after several days of very low radiation. Based on the data analysed we can evaluate that the batteries are on low charge, so the RO plant will operate the minimum possible hours until a day of good radiation, in which case the control reacts by making the plant operate for a few hours a day until the batteries charge level

0.00

1.25

2.50

3.75

5.00x10

3(W)

0.50

1.13

1.75

2.38

3.00x10

3(µS/cm)

0.00

0.37

0.75

1.12

1.50x10

1(A)

23/05/2002 22:29:2224/05/2002 22:29:2225/05/2002 22:29:2226/05/2002 22:29:2227/05/2002 22:29:22(GMT)Tiempo ---> 5 Días; 8 Horas

Potencia Campo FotovoltaicoE(>0):132880.08; E(<0):-1869.69Conductividad ProductoIntensidad de consumoE(>0):499.26; E(<0):0.00

PV field power (W) Product water conductivity (uS/cm)

Desalination Plant Load (A)

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is recovered. In these cases, the solar radiation is entirely used to return the batteries to their normal charge level. Table 6: Summary of the operation on a day of high radiation after several days of low radiation – battery charge recovery

(29/09/2002).

Start-up time 10:30 a.m.

Shut-down time 12:00 p.m.

Initial battery capacity 217 Ah

Final battery capacity 326 Ah

Battery energy balance +109 Ah

Total power consumed (in direct current) 105 Ah

PV field production 16.6 kWh

Total plant operating time 1.5 hours

Plant production during the day 570 litres

Mean conductivity 908 µS/cm

To date, and after monitoring the project for over 12 months, we have observed, in the worst case scenario, a discharge of 264 Ah. This has allowed us to optimise the battery capacity, establishing its value at 385Ah in C1003. The bank of batteries will be at most 68% discharged in this worst case scenario, and this will only occur a few times a year, which has a direct impact on lengthening the life of the battery. Night time operations have been tested, with the plant only operating at night and the system charging the batteries during the day. There are factors which make night time operations totally inadvisable, since there is no charge available to power a 24-hour operation. These factors are:

During the day, and with medium-high radiation, the voltage at the battery terminals remains around 50 - 52 V in relation to the system’s rated voltage of 48 V. When the plant operates at night, since there is no solar collection in the system, the battery voltage is around 46 - 47 V. This means that the amperes-hour (Ah) taken from the battery are greater, since it is operating at a lower voltage. The result of all this is that for the same quantity of water, we need more A per hour during the night than during the day. The data obtained reveals a consumption difference of between 30 and 40 Ah in eight operating hours.

The component which most benefits from daytime consumption is the battery. During the day, and only during the first and last hours of the plant operation, the battery suffers a very small discharge, since demand then exceeds generation. In the middle hours of the day, when generation is equal to or exceeds demand, the battery is practically not discharged at all. With a night time operation, the battery is subject to complete charge-discharge cycles, in the best of cases discharging 30 or 40% in each cycle. If the battery does not have time to recover during the day, this could reach a figure of 50% or more. This would logically represent greater wear and the battery will have a shorter life.

Preliminary economic feasibility of the process Having confirmed the technical viability of the DESSOL plant, and before considering its economic viability, we must remember that these systems are not designed to compete with conventional centralised desalting processes, but to satisfy the need for water in remote coastal or inland areas (with brackish water). These areas, largely located on the northern-sub-Saharan coast of Africa and in parts of South America and Asia, are typically far from water and electric distribution networks, and in places where road or sea transport greatly increases the cost of the resource. They have a road transport network for water using tankers or private transport with sales prices ranging from 0.01 – 0.04 €/L, or 13.30 – 19.00 €/m³ (data specific to Mauritania) [9]. 3 Parameter used to define the battery capacity at a discharge regime defined by the capacity divided by 100.

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Two hypothetical cases have been studied in which a system of these characteristics is installed (Table 7). The first (case 1) is a situation in which the system is a private business which has to obtain profits to justify sale of the resource. In case 2, the operation is not aimed at making a profit, and has partial external financing, providing that the system finances itself when in operation. In both cases, the minimum sales prices are lower than they actually are in these remote areas, and the water is available at the point of consumption, without depending on transport or availability in the closest towns.

Table 7: Summary of the economic viability study on the Dessol PV-RO system tested.

Study variables Case 1 Case 2

Years of use 15 15 Mean annual production 1,050 m³ 1,050 m³ Membrane replacement 2 years 2 years Battery replacement 5 years 5 years Annual operating and maintenance costs 5% investment 5% investment External contribution (grant) 0% 25% investment Internal ROI (return on investment) 8% 6% Minim sales price of the water produced (+) 9.62 €/m³ 7.51 €/m³

(*) Obtaining a mean net value of 0.00 € 5. Conclusions We have designed, tested and optimised a sea water RO desalination plant powered by photovoltaic solar energy produced in a 4.8 kWp PV field, in the ITC facilities at Pozo Izquierdo. The RO plant produces 400 L/h (operating at 60 bar) and is satisfactorily adapted to the discontinuous operating mode to which it is exposed, obtaining rated operating data within the design parameters in the year of experimentation. The system tested is presented as a technically and economically viable solution to the potable water supply in remote coastal or inland areas far from towns, with a mean daily solar radiation value. It is a feasible alternative not only to the transport of water to these locations, but also to the use of this technology for treating polluted water in large territories. The desalination plant has been equipped with a double brine flushing system for its daily shut-downs, which delays the ageing and loss of efficiency of the RO membranes when it is not operating. The PV-RO system has a fully automatic control system which manages the power generated and converts it on a daily basis into water, thus obtaining an average of 8 operating hours per day in the summer and 6 hours in the winter (7 hours annual average). The power storage capacity of the bank of batteries, 19 kWh, is fully optimised, making the installation technically and economically viable. One totally original aspect is measuring the variation in battery capacity in real time by means of a battery monitor with RS232 output, making full use of the solar radiation available and the generation of a large amount of useful information with a view to analysing battery variations over time. The night time use of the battery capacity is not recommended, since this wastes part of the power stored.

Acknowledgements The authors wish to thank to Dr. A. Neskakis and Dr. D. Herold for the cooperation provided in the development of the process.

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[7] E. Lorenzo, La Energía que producen los sistemas fotovoltaicos conectados a la red, Era Solar nº 107, 2002, page 24.

[8] Progensa, S.A., Solar radiation in Spain database, 2001.

[9] Parc National Du Banc D’Arguin, Mauritania, Rapport diagnostic de la situation d’eau et d’energie au niveau des villages et sites au PNBA, page 14, 2002