WIND ENERGY BASED REVERSE OSMOSIS SYSTEM FOR DESALINATION OF BRACKISH WATER

1R.R.UTKEKAR, 2N.R.RAYKAR

1,2Sardar Patel College of Engineering, , E-mail: [email protected]

Abstract- The development of Renewable energy based desalination systems is important on islands and in the coastal areas. This paper presents design of such system proposed to be located at Devgad which is one of the potential sites vetted by National Institute of Wind Energy (NIWE) for wind energy project in the coastal region of . Most past studies of wind powered desalination used the wind power indirectly that is it was used to generate electricity first and then the electricity was used for desalination by RO process. In this work wind energy is used directly to raise the feed water pressure for RO desalination. It involves less energy conversion and thus higher efficiency. The brackish water desalination system presented here comprises of 10 kW wind turbine which converts wind energy directly into pressure energy. The system can produce freshwater at total dissolved solids (TDS) of less than 100 mg/l from brackish water up to 10,000 mg/l of total dissolved solids (TDS). The rated maximum capacity of plant is 31.26m3/day of clean water at average wind velocity 5.81m/s. The recovery rate of RO unit is about 40 to 50 percent. The pre-treated seawater passing through the Reverse Osmosis (RO) membrane is separated into clean water and brine. The brine from RO has a high pressure that has been reused to pressurize the feed water using pressure exchanger, which significantly reduces power consumption of the system. The design procedure developed in the present study may be adopted for systems working under similar conditions.

Keywords- Renewable energy, Desalination, Wind energy, Reverse osmosis, and Pressure exchanger.

I. INTRODUCTION pressure energy, a RO unit with energy recovery device (ERD) and a suitable pre-treatment technology In recent years, increase in population and to control scaling and fouling of RO membrane. The industrialisation has led to severe water shortage as calculations for actual capacity corresponding to real well as rise in cost of fossil fuels. Out of all water life site data are explained. available in earth, a vast majority (about 97%) is sea water and remaining little amount is the fresh water. Further nearly 68% of fresh water is frozen and 30% is underground with just about 0.3% being available as surface water of lakes, rivers, etc. [1]. The problem of water shortage can be solved by two ways, first by recycling the fresh water and second by doing desalination of seawater. Oceans are practically inexhaustible source of water, which however are of high salinity. The water shortage problem may be solved by sea water desalination. Which demands power for separation of salts which if extracted from fossil fuels can cause damage to the environment. Hence there is a need to develop environment- friendly and renewable energy driven sea water desalination system. Generally, Desalination Fig.1. Seawater desalination processes processes can be categorized into two major types: phase change thermal and single phase membrane II. MAIN COMPONENTS OF SYSTEM FOR processes. Fig.1 shows classification of desalination REVERSE OSMOSIS BASED SEA WATER processes. Among all the processes reverse osmosis DESALINATION process require relatively less amount of energy to desalinate sea water [2]. The coastal areas near sea Reverse osmosis is a well-established technology for present a high availability of wind power resources. the desalination of sea water [3].When concentrated Therefore a wind powered reverse osmosis system water is separated from pure water by a presents a promising choice for desalination based on semipermeable membrane, the pure water tends to renewable energy. diffuse through the membrane into concentrated In this study, design of wind energy driven RO plant 3 solution. This well-known natural process is termed of fresh water capacity 31.26 m /day at proposed as osmosis. The reverse osmosis phenomenon is not location of Devgad, Maharashtra is presented. The natural. By applying external pressure in excess of design consists of system for conversion of wind to natural osmotic pressure, water in concentrated side is

Proceedings of IEEEFORUM International Conference, 13th August, 2017, Pune, India 64 Wind Energy Based Reverse Osmosis System for Desalination of Brackish Water pushed to the pure water side through the membrane. The tubular pressure vessels are connected in parallel The osmotic pressure difference between seawater at every stage. The number of stages in RO system is and clean water can be calculated by defined as the number of pressure vessels in series ∆ = 0.078( − ) Eq.(1) from start to exit of feed water flow path. The number where∆ = osmotic pressure in bar, TDS is total of stages depends on system recovery rates, the dissolved solids in mg/l [4]. number of elements per vessel, and the feed water quality (Table 2). From this expression, the osmotic pressure of sea water at TDS concentration of 35000 mg/l is about 27 bars. Brackish water as a feed water for RO desalination process would require a smaller osmotic pressure. The osmotic pressure of brackish water at TDS concentration of 3000 mg/l is about 2.3 bars. The applied pressure to feed water should be greater than osmotic pressure. Generally for sea water desalination, applied pressure is taken as 55 bar when Table 2: Number of Stages of a brackish water system [5] TDS is 32000 ppm and for brackish water 15.5 bar when TDS is 2000 ppm [4]. Fig.2 shows simple 2.1 Energy Recovery Devices schematic depicting working of a RO system. Sea water desalination technology has one disadvantage. Large amount of energy is consumed during pumping of feed water. Once the desalination is complete, the remaining reject water which is at high pressure has to be eliminated as waste. This pressure energy can be reused and thus, the energy could be recycled. This idea led to the development of energy recovery devices(ERDs) that prevent the potential loss of energy in the sea water during reverse osmosis process. Pelton turbines, Francis

Fig.2. RO Desalination Process turbine, Pressure exchanger, Turbocharger are some of the major ERDs used in seawater desalination As the feed water enters the RO membrane under plant. The selection of ERD for desalination system pressure, the water molecules pass through the semi- depends on lot of factor. The main factor in the permeable membrane. The salts and other selection of ERD is the form of the recovered energy: contaminants are not allowed to pass and are (i) electrical energy or(ii) feed water pressure energy. discharged through the concentrate stream. There are Francis turbine, Pelton turbine converts pressure many types of membrane available in practice. These energy of brine into electrical energy. The pressure membranes are available as membrane modules. exchanger and turbocharger utilize the brine pressure Generally spiral wound modules are used in RO to increase feed water pressure energy. desalination process [5]. The standard industrial spiral-wound modules are 100 mm and 200 mm in Pressure exchanger is the most widely used energy diameter and 1000 mm long. RO plants require large recovery device in the world [6]. The pressure number of modules depending on the capacity of exchanger captures hydraulic energy from the high plant. The modules are placed inside the tubular pressure reject stream of seawater from reverse pressure vessels. Usually Standard six modules are osmosis process and transfers this energy to low placed in one tubular pressure vessel. The Eq.(2) pressure feed water. Because the pressure exchanger gives the number of modules required for a given itself consumes no electrical power, overall energy capacity of plant [5]. consumption is drastically reduced. The system with ()∗ No. of modules = Eq.(2) pressure exchanger requires a much smaller high ∗ ( ) pressure pump than what has historically been used. Where Gfd = design flux is an important factor and it The technology also separates the high pressure pump depends on quality of feed water. Table 1 gives the from the energy recovery device. These two factors range of Gfd applicable for different water sources combine to give plants much more flexibility in operations and can allow plants to reduce their power consumption by as much as 60 percent.

2.2 RO Pre-treatment and Post-treatment Systems Pre-treatment using mechanical and chemical means are necessary for RO system to prevent fouling, scaling, and degradation of RO membrane. Primary Table 1: Flux ranges for different source waters [5] objective of pre-treatment is to make feed water

Proceedings of IEEEFORUM International Conference, 13th August, 2017, Pune, India 65 Wind Energy Based Reverse Osmosis System for Desalination of Brackish Water compatible with membrane, which result in increase In case of traditional wind turbine, wind energy is the efficiency and life of the RO membrane. The type converted into electrical energy using electrical and extent of pre-treatment system will depend on the generator. This electrical energy then can be used to type of feed water, i.e., surface water or brackish drive the pump in RO system. In every stage of water or seawater. Table 3 gives the general guideline conversion from wind to electricity and electricity to for acceptable feed water quality for a typical RO pressure, there will be significant losses. Due to that system. disadvantage, windmills which transfer wind energy directly into pressure using mechanical transmission are gaining importance. Windmills have some limitation, windmills are preferred if tower height is low and pump capacity is low. To reduce the energy consumption of desalination plant it is important to design an energy efficient wind energy conversion system. A new concept for the design of wind turbine incorporates the idea of using fluids to transfer energy. Hydraulic drive systems often experience

Table 3: Guidelines for acceptable RO feed water quality [7] small losses compare to mechanical, electrical drive systems. Fig.3 shows the wind energy to pressure For most RO systems, Cartridge filter is a minimum energy conversion using hydraulic transmission. In pre-treatment component, even for cleanest ground such system, Main pump is directly coupled to water sources. The reason is that sometimes foulants/ windmill rotor through gear box. To avoid cavitation scalants are not in the source water but are coming in main pump, continuous water supply should be from other sources. Therefore cartridge filter should made available at the inlet of pump. One booster not be viewed as pre-treatment but as a last defence pump is provided at wind turbine base, which creates for protecting RO elements. the pressure required at suction of main pump. If chlorination is used to control microbiological Booster pump is driven by electric energy. The basic growth in the pre-treatment, overfeeding will cause requirement of such is that the sea water must be degradation of thin film composite RO elements. available in pre-treated water tank. Therefore an Activated carbon pre-treatment is used As the direction of wind changes wind turbine should for organic removal or dechlorination to protect RO have such mechanism that the wind turbine blades membrane from degradation. align with the wind direction. Generally active yaw To remove sand, clay, suspended solids and turbidity, mechanism is used. The active yaw systems are conventional treatment is used. Unlike traditional equipped with motor which is able to rotate the sand filters, multimedia water filter are composed of nacelle of wind turbine against the stationary tower three filtration media, ordered in decreasing porosity. based on automatic signals from wind direction Because of their multi-layer design, multimedia water sensors. filters are able to trap and retain a far larger number of particles than traditional sand filters. Trapping sediment and particulates throughout the entire depth of the filter bed, allows multimedia water filter to operate for much longer periods of time than conventional sand filters and also process of multimedia filtration produces high quality, filtered water at much faster flow rates than traditional sand filtration. Depending on the permeate water application and quality of permeate water post-treatment on water is necessary. The quality of treated water from RO is compared with drinking water standard as per the World health organisation (WHO) by doing water quality analysis. Fig.3. Wind energy to pressure conversion using hydraulic 2.3 Wind Energy to Pressure Energy Conversion transmission adopted from [8] System Desalination requires large amount of energy to The various components of modern active yaw separate the salt from seawater. In seawater RO systems vary depending on the design characteristics desalination system energy is required to drive the but all the active yaw systems include a means of high pressure pump, which can be easily obtained rotatable connection between nacelle and tower (yaw through wind energy abundantly available in coastal bearing), a means of active variation of the rotor region. orientation i.e. yaw drive, a means of restricting the rotation of the nacelle (yaw brake) and control system

Proceedings of IEEEFORUM International Conference, 13th August, 2017, Pune, India 66 Wind Energy Based Reverse Osmosis System for Desalination of Brackish Water which processes the signals from wind direction III. DESIGN OF WIND ENERGY DRIVEN sensors and gives proper commands to the actuating REVERSE OSMOSIS SYSTEM FOR mechanisms. The nacelle is mounted on a roller DESALINATION OF BRACKISH WATER bearing and the rotation is achieved using hydraulic motor. The yaw system with hydraulic drive has The section provides details of design of wind energy inherent benefits compare to electric drive such as: driven RO system for desalination of brackish water. high power to weight ratio and high reliability. The The calculations are done for site conditions hydraulic systems often also allow for the elimination corresponding to of Maharashtra. of yaw brake mechanism and their replacement with cut off valves. 3.1 Description of System The pump shaft is coupled to the windmill rotor shaft The Process flow diagram developed for wind driven at nacelle. When wind changes its direction nacelle RO plant is as shown in Fig.5. The brackish water is rotate, so pump also rotate, therefore a suitable taken from source using feed pump. This water is connection between stationary inlet and outlet pipe collected in settling tank which provides some degree and rotating pump is required. Swivel joint is a device of purification by settling of suspended particles at used to transfer fluid under pressure from stationary the bottom of tank. The water from settling tank is inlet to rotating outlet, preserving and isolating the then pretreated using multimedia filter (MMF), fluid connection. Fig.4 shows a typical two passage granular activated carbon filter (GACF) and collected swivel joint.For connecting inlet and outlet pipe to in pretreated water tank. The pretreated water is first the pump on rotating assembly requires two passage lifted at the level of windmill rotor by booster pump. swivel joint which is capable of sustaining the It is then pressurized using main pump which is maximum pressure in pipe and maximum rpm of directly driven by wind turbine. The pressurized feed rotation. water then passes through single pass RO unit through accumulator and cartridge filter. Accumulator is used to overcome flow variation due to wind fluctuation effect. The clean water from RO unit iscollected in clean water tank. The brine water which still has high pressure is used in pressure exchanger to pressurize the extra pretreated water directly from pretreatment tank. This provision saves the main pump’s work and further increases the plant capacity. The depressurized brine water from pressure exchanger is drained back to sea.

Fig.4. Two passage Swivel joint [9]

Fig.5. Process flow diagram of Wind RO desalination plant

The proposed system comprises of wind turbine of and its elevation is 40 m above the sea. Equating rated capacity 10 kW for handling of brackish water input power from wind turbine to pump and pump (TDS below 10000 mg/l) desalination. The mean power. annual wind power density at Devgad is 218 W/m2and the annual average wind velocity is 5.81 Pr * ηg = Eq.(4) m/s over the year 2015-16. This site is near to the sea ∗

Proceedings of IEEEFORUM International Conference, 13th August, 2017, Pune, India 67 Wind Energy Based Reverse Osmosis System for Desalination of Brackish Water 3 Where Pr = Rotor power of wind turbine (kW), ηg = Where ρ = density of air (1.23 kg/m ), A = Rotor 2 Efficiency of gearbox (80%), P = RO system pressure swept area (68m ), V = Wind velocity (m/s) and Cp = (30 bar), Qp = Main pump water flow (lph) and Rotor efficiency (0.48). calculation factor = 518 [9]. By substituting Eq. (8) in Eq. (4) main pump water From Eq. (4) the main pump water flow at average flow is 3 wind velocity (5.81 m/s) is 5117 lph. The RO unit is Qp (lph) = 15.33 * V Eq.(9) designed for 50 percent planned recovery rate (ratio of clean water flow to feed water flow). Therefore, Desalinated water flow for average recovery rate of from table 2, single stage RO unit is used for Ro system is 43% of feed water flow to RO (Q0) 3 3 desalination. Generally six membranes are fitted in Qclean(m /day) = 0.1594* V Eq.(10) one pressure vessel. The overall recovery rate of single stage RO unit is From Eq. (10) the rated maximum capacity of plant is ∗∑ ∗ 3 R = Eq.(5) 31.26 m /day at average wind velocity 5.81 m/s. The variation of wind speed affects the performance And Qi = (1-r)*Qi-1 of membrane and service life of component. Control where Q0 = Feed water flow to RO system is necessary to overcome the wind fluctuation n = No. of pressure vessels effect. The pressure stabilizer or accumulator is used i = Membrane number as control device. It is connected between the main Qi = Inlet flow to membrane pump and RO unit. It provides constant feed flow to r = Recovery of single membrane the RO unit. Fig.8 shows the calculated average main R = Overall recovery of RO system (single stage) pump flow vs average wind velocity. This data is The average recovery rate for single membrane is important in design of the control system. The assumed as 9 percent [10]. Therefore from Eq.(5) the average flow rate of main pump is 1438 lph (Fig.8). actual average overall recovery rate of single stage The time variation of desalinated water flow rate is RO unit is calculated as 43%. calculated and it is shown in Fig.9. This plot is used For pressure exchanger, feed water flow is considered to analyse the fluctuation in desalinated water flow equal to that of brine water flow for 50% recovery throughout the year. The annual average clean water rate of RO system. production rate of the system is 14.95m3/day. Qpx = 0.50* (Qp + Qpx) Eq.(6)

Where Qpx = feed flow to the pressure exchanger (lph) Total feed water to the RO system is increased from 5117 lph to 10234 lph by using pressure exchanger as energy recovery device. Thus, energy consumption of pump is reduced by 50%. Also the clean water production rate of plant is two times of clean water production rate of plant without energy recovery device.

3.2Performance Predictions for the System Fig.6 shows the month wise average wind speed for year 2016 at Devgad site which is measured at 10 m Fig.6. Average wind velocity for year 2016 at 10 m height from height from the ground.This data is extrapolated to ground at proposed site estimate the wind speed at a standard wind turbine height. The height of 10 kW wind turbine given by manufacturer is 20 m. The wind profile power law is used to find out wind speed at given height. The wind profile power law relationship is 1/7 u = ur (h/hr) Eq.(7)

Where u is wind velocity at h height and ur is wind velocity at hr height. Using this equation, average wind speed data at 10 m height is converted into 20 m height as shown in Fig.7. Rotor power of the wind turbine is ∗ ∗∗ Pr= Eq.(8) Fig.7. Average wind velocity for year 2016 at 20 m height from ground at proposed site

Proceedings of IEEEFORUM International Conference, 13th August, 2017, Pune, India 68 Wind Energy Based Reverse Osmosis System for Desalination of Brackish Water mg/l from brackish water containing up to 10000 mg/l of total dissolved solids (TDS). c) Using pressure exchanger as energy recovery device the clean water production rate of plant is two times of clean water production rate of plant without energy recovery device for 50 percent plant recovery.

ACKNOWLEDGEMENT

Authors would like to express their sincere thanks for the support and guidance received from Dr.

Fig.8. Average main pump water flow for year 2016 P.K.Ghosh, Institute of Chemical Technology, Mumbai, during the project.

REFERENCES

[1] Kalogirou, Soteris A. "Seawater desalination using renewable energy sources." Progress in energy and combustion science 31, no. 3 (2005): 242-281. [2] https://en.wikipedia.org/wiki/Desalination [3] puretecwater.com/downloads/basics-of-reverse-osmosis. [4] Liu, C. C. K., W. Xia, and J. W. Park. "A wind-driven reverse osmosis system for aquaculture wastewater reuse and nutrient recovery.“Desalination 202, no. 1 (2007): 24- 30. [5] http://www.filmtech.co.in/ [6] Guirguis, Mageed Jean. Energy recovery devices in seawater reverse osmosis desalination plants with emphasis on efficiency and economical analysis of Fig.9. Average desalinated water flow for year 2016 isobaric versus centrifugal devices. University of South Florida, 2011. CONCLUSIONS [7] https://www.amtaorg.com [8] Diepeveen, N. "Seawater-based hydraulics for offshore wind turbines." Delft University Wind Energy Research In this study, the basic design of wind energy based Institute (DUWIND) (2009). RO system for desalination of brackish water for [9] https://www.dsti.com/rotary-unions/standard coastal site of Maharashtra is developed. Some of the [10] www.lenntech.com [11] Miranda, Marcos S., and David Infield. "A wind-powered salient features of the system design are listed below seawater reverse-osmosis system without a) The system comprises of 10 kW capacity wind batteries." Desalination 153, no. 1 (2003): 9-16. turbine. Rated maximum clean water [12] Eltawil, Mohamed A., Zhao Zhengming, and Liqiang production capacity of system is Yuan. "A review of renewable energy technologies 3 integrated with desalination systems." Renewable and 31.26m /day.The predicted average output of Sustainable Energy Reviews 13, no. 9 (2009): 2245-2262. system based on wind data of year 2016 is [13] Lai, Wenyu, Qingfen Ma, Hui Lu, ShaojieWeng, Junqing 14.95 m3/day. Fan, and Haixuan Fang. "Effects of wind intermittence and b) The brackish water desalination system fluctuation on reverse osmosis desalination process and solution strategies." Desalination 395 (2016): 17-27. developed can produce drinkable freshwater at [14] https://www.mahaurja.com total dissolved solids (TDS) of less than 100 [15] http://www.indiawaterportal.org/articles/indian-standard- drinking-water-bis-specifications-10500-1991

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Proceedings of IEEEFORUM International Conference, 13th August, 2017, Pune, India 69