- Email: [email protected]
Laboratory demonstration of a photovoltaic-powered seawater reverse-osmosis system without batteries
Desalination 183 (2005) 105–111
Laboratory demonstration of a photovoltaic-powered seawater reverse-osmosis system without batteries Murray Thomson*, David Infield Centre for Renewable Energy Systems Technology (CREST), Loughborough University, LE11 3TU, UK Tel. þ44 (1509) 228144; Home: þ44 (1509) 213902; Fax: þ44 (1509) 610031; email: [email protected] Received 25 February 2005; accepted 10 March 2005
Abstract A prototype photovoltaic-powered reverse-osmosis system has been constructed at CREST, Loughborough, UK. The rate of production of fresh water varies throughout the day according to the available solar power, and thus, the system operates without need of batteries. The system is designed to operate from seawater, and a Clark pump brine-stream energy recovery mechanism is coupled with a variable recovery ratio technique to achieve a specific energy consumption of less than 4 kWh/m3 over a wide range of operation. Measurements showing the variable operation over a two-day period are presented and discussed. Keywords: Solar; Photovoltaic; PV; Seawater; Reverse osmosis; Energy recovery; Renewable energy
1. Introduction The use of renewable energy for desalination is a very attractive proposition, addressing both environmental concerns and the more general need for long-term sustainability. More specifically, solar photovoltaic-powered reverse osmosis (PV-RO) is considered one of the more promising technology combinations [1–3], particularly for small-scale systems where other technologies are less competitive. Indeed, PV-RO systems operating from
brackish water are already commercially available [4] and further developments are ongoing [5]. Operation from seawater is more challenging from an energy perspective, and early demonstrations tended to require large PV arrays, making them commercially unattractive. Developments have been made however, and ongoing demonstrations of seawater PVRO are installed in Europe [6,7] and by Kunczynski in Mexico [8], the latter being particularly impressive in terms of total water desalinated and energy efficiency.
*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.2005.03.031
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M. Thomson, D. Infield / Desalination 183 (2005) 105–111
Energy efficiency in seawater RO systems is critically dependent on the brine-stream energy-recovery device. In medium and large-scale systems, Pelton wheels are widely employed and various alternative devices [9–11] now compete to further improve system efficiencies. Unfortunately, none of these devices are well-suited to small-scale RO systems, and many small-scale systems are built without any energy-recovery; this keeps capital costs down but has a heavy penalty in ongoing energy costs. When the energy is to be provided by PV, the energy costs are unavoidably capitalized and brinestream energy-recovery becomes a high priority. Keefer recognised this in 1985 and designed and demonstrated a pump-integrated energy-recovery mechanism within a seawater PV-RO system [12]; high manufacturing costs inhibited further development, but a similar concept is now used for brackish PV-RO [4]. In 1996, Dulas Limited demonstrated use of a Danfoss axial-piston hydraulic motor for energy recovery in seawater RO [13]. CREST adopted the Dulas test rig in 2000, but then designed a system [14,15] based on the Clark pump from Spectra Watermakers [16], which offered significantly higher efficiency. Meanwhile, Kunczynski was putting similar devices to the test in a real application [8]. CREST and Duals took the view that the PV-RO system should be designed without batteries. This was based on experience with batteries in other PV systems in hot remote locations and with minimal maintenance. Without batteries, the RO part of the system can only operate when the sun is shining. The thinking is that it is much easier to store the fresh water in a tank than it is to store the equivalent energy in batteries. The developers of the brackish PV-RO systems mentioned above took a similar view, and their systems do operate without batteries. The opposing view is that RO systems are best
run at constant flow, 24 hours a day, and this is the norm for almost all medium and large-scale RO systems. Obviously, it makes good use of the capital invested in the RO hardware, but is also provides consistently high-quality product water and may reduce fouling potential within the RO membrane elements. The ongoing demonstrations of seawater PV-RO mentioned above [6–8] all employ batteries. The laboratory testing presented in this paper does not. 2. System under test The batteryless PV-RO test rig shown in Fig. 1 was assembled at CREST in 2003. The operation of the Clark pump and the two motorised pumps was described previously [14,17]. In summary: the Moineau (progressing cavity) pump raises the feed water to a medium-pressure (6–11 bar). The Clark pump raises this to high-pressure (40–70 bar) by virtue of the energy it recovers from the concentrate. The flow ratio of the Clark pump is fixed by design, which normally gives a 10% water recovery ratio at the membranes. However, the plunger pump injects an additional high-pressure feed, which increases the water recovery ratio to any desired value. Both pump motors are equipped with variable-frequency inverters. These are controlled, firstly, to provide maximum-power-point tracking (MPPT), ensuring that the total power drawn from the PV array tracks the maximum available as the irradiance varies throughout the day. Secondly, the controller adjusts the relative speeds of the two pumps in order to optimise the water recovery ratio and so maximise the flow of product water. The performance of such a system had been predicted [14,15], based largely on computer simulations. The test rig was assembled in order to verify these predictions, and more generally, to demonstrate operation with variable flow, pressure and recovery ratio.
M. Thomson, D. Infield / Desalination 183 (2005) 105–111 Photovoltaic Array
107
Reverse-Osmosis Modules Inverter
Inverter
Controller
Induction Motor
Induction Motor
Plunger Pump
Filter Moineau Pump
Clark Pump
“Seawater” Intake
Concentrate Discharge
Fresh water product
Tank
Fig. 1. Test rig configuration.
For the purpose of the demonstration, the controller shown in Fig. 1 was implemented in LabView software on a personal computer. The same computer and software also logged measurements made by numerous sensors throughout the system (not shown in Fig. 1). The feed water used for demonstration was straight NaCl solution, nominally at 32,800 mg/L, which is isosmotic with ASTM standard seawater. The product and concentrate streams were fed directly back into the feed tank, thus maintaining the feed concentration within plus and minus 500 mg/L. A computer-controlled heat exchanger (not shown in Fig. 1) was used to maintain the temperature of the feed water very close to 25 C.
The PV array used for testing comprised eighteen BP Solar Saturn BP585F monocrystalline silicon modules, having a total rating of 1.53 kWpeak, This is only 64% of the design value 2.4 kWpeak that was used in the performance predictions [15]. Further modules were available but could not be connected because the variable-frequency inverters were industrial units with a DC voltage limit of 400 V. Alternative PV modules with different power-to-voltage ratios are readily available and could be used in future to provide an array power much closer to the design value. The array used for testing has a fixed orientation (no solar-trajectory tracking).
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M. Thomson, D. Infield / Desalination 183 (2005) 105–111
The plunger pump and its motor were not quite optimum sizes, having been adopted from an earlier test rig. Thus, they required a toothed-belt coupling and had lower efficiency than optimum. Testing took place in Loughborough, UK.
3. Initial results and discussion 3.1. Product water flow The overall operation of the batteryless PV-RO is illustrated in Fig. 2: the product water flow varies in direct response to the available sunlight. At dawn, the PV array provides voltage to the inverters (Fig. 1). The controller then starts the Moineau pump and increases its speed to match the available power from the PV. As the sun
rises further, the controller starts the plunger pump, which increases the water recovery ratio and hence the product flow. There were many passing clouds during the two-day test, and, in the absence of any batteries, the arrival of each cloud caused an immediate reduction or halt in product flow. The pressures throughout the system also reduced but, were not lost completely (because the pumps are positive displacement). As soon as the cloud had passed, the pumps would again accelerate and production was restored. The overall effect of the clouds is clearly apparent in Fig. 2. The total product water recorded over the two-day test was 2.93 m3. The system is intended to produce more than twice this amount, but bear in mind that this testing was performed in the UK, with many passing
1000
Irradiance (W/m2)
Irradiance Product Flow
500
200
0
0 06:00
10:00
14:00
18:00
22:00
02:00
06:00
10:00
Time (hh:mm)
Fig. 2. Measured irradiance and product water flow June 9th and 10th 2003.
14:00
18:00
Product Flow (L/h)
400
109
M. Thomson, D. Infield / Desalination 183 (2005) 105–111
3.2. Energy consumption In order to see the performance of the RO rig itself, Fig. 3 shows the product flow against DC power coming from the PV array, measured at the input to the inverters. Water production starts at just 100 W and then increases more or less proportionally to the DC power. Operation over such a wide range of input power allows the system to make almost complete use of the variable power available from the PV array, without need of batteries. The points in Fig. 4. represent the same measured data as shown in Fig. 3, but this time presented as specific energy consumption in kWh/m3. The line is the predicted characteristic [Fig. 7. of reference 14 but with the spread caused by temperature removed]. The slightly-higher energy consumption shown in the measured results is 400 350
Product Flow (L/h)
300 250 200 150 100 50 0
0
200
400
600
800
1000
1200
1400
1600
DC Power (W)
Fig. 3. Product flow versus DC power (averaged at 1-minute intervals, covering the whole of the two-day test).
6
Specific Energy (kWh/m3)
clouds, and that the PV array was only 64% of the design value. Nearer to the equator and with a full-size array, the system may still be expected to produce around 3 m3/d as predicted [15].
Measured Predicted
5
4
3
2
1
0
0
200
400
600
800
1000
1200
1400
1600
DC Power (W)
Fig. 4. Specific energy consumption (kWh/m3).
due, at least in part, to the reduced efficiency offered by the particular plunger pump and motor used in testing, as mentioned earlier. For perspective, bear in mind that a smallscale seawater RO system, without energy recovery, would typically consume two to three times more energy. 3.3. Product water quality Energy efficiency was the primary objective throughout the design of the batteryless PV-RO system. For this reason, the system has a very generous membrane area relative to the product flow. Unfortunately, this also leads to the concentration of the product water being increased. Furthermore, the intermittent operation of the system also causes a significant rise in product water concentration, particularly following a break in production: a passing cloud may stop the flow of product water, but the (unwanted) salt passage through the membrane continues. Fortunately, the periods of high concentration tend to coincide with periods of low flow, but even taking this into account, the average concentration of the product water over the two-day test was over
110
M. Thomson, D. Infield / Desalination 183 (2005) 105–111
1100 mg/L. To improve matters, a diverter valve could be fitted in the product line and automated to reject product water exceeding a chosen concentration threshold. For example, discarding less than 1% of the product volume would bring the average concentration below 800 mg/L. Operation with fewer clouds would also improve the product water quality significantly. Lastly, the required quality of the product water should perhaps be considered in the context of the current World Health Organization ‘‘Guidelines for drinking-water quality, third edition’’ [18]. In particular, chapter 10 states: ‘‘The palatability of water with a TDS level of less than 600 mg/litre is generally considered to be good; drinking-water becomes significantly and increasingly unpalatable at TDS levels greater than about 1000 mg/litre. The presence of high levels of TDS may also be objectionable to consumers, owing to excessive scaling in water pipes, heaters, boilers and household appliances. No health-based guideline value for TDS has been proposed.’’ 4. Conclusions The operation of a seawater PV-RO system without batteries has been demonstrated. The use of a Clark pump energy recovery mechanism, coupled with variable flow and variable water recovery ratio, provides the necessary energy efficiency over a wide operational range. The concentration of the product water was unacceptably high during the tests presented here, but could perhaps be made acceptable without recourse to batteries. However, the case against using batteries has not been proven: demonstrating that the system works does not prove that it is the most economic approach overall. Also, the reliability of the system remains unproven; for example, the longevity of the membranes under
intermittent, variable and low-flow conditions is unknown. Clearly, reliability is a critical factor for any desalination system intended to provide drinking water in remote locations, and it is recommended that reliability be treated as a priority alongside energy-efficiency in the future development of PV-RO. A more detailed presentation of the system described in this paper is given in the author’s thesis [19]. References [1] CRES, Desalination Guide Using Renewable Energies, THERMIE – DG XVII, European Commission Report. 1998, CRES, Greece. [2] R. Oldach, Matching renewable energy with desalination plants. 2001, IT Power Ltd. Sponsored by MEDRC: The Middle East Desalination Research Center: Muscat, Sultanate of Oman. [3] D. Assimacopoulos, R. Morris and A. Zervos, Water, Water Everywhere, Desalination Powered by Renewable Energy Sources. REFOCUS, Elsevier, 2001 [4] Solarflow – Water Purification – Solar Powered Reverse Osmosis, http://www.sesltd.com.au/ html/waterpure.htm, accessed 19 February 2005. [5] B.S. Richards, C. Remy and A.I. Scha¨fer, Sustainable Drinking Water Production From Brackish Sources Using Photovoltaics, in 19th EPSEC. 2004. [6] E. Tzen, D. Theofilloyianakos, M. Sigalas and K. Karamanis, Design and development of a hybrid autonomous system for seawater desalination. Desalination, 166 (2004) 267–274. [7] T. Espino, B. Penate, G. Piemavieja, D. Heroid, and A. Neskakis, Optimised desalination of seawater by a PV powered reverse osmosis plant for a decentralised coastal water supply. Desalination, 156 (2003) 349–350. [8] Y. Kunczynski, Development and optimization of 1000–5000 GPD solar power SWRO, in IDA World Congress on Desalination and Water Reuse. 2003: Bahamas [9] DWEER (Dual Work Exchanger Energy Recovery), http://www.calder.ch/htm/dweer.htm, accessed 21 February 2005.
M. Thomson, D. Infield / Desalination 183 (2005) 105–111 [10] ERI (Energy Recovery Inc.) Pressure Exchanger, http://www.energyrecovery.com, accessed 21 February 2005. [11] FEDCO Hydraulic Pressure Booster, http:// www.fluidequipmentdev.com/hpb.htm, accessed 21 February 2005. [12] B.G. Keefer, R.D. Hembree and F.C. Schrack, Optimized matching of solar photovoltaic power with reverse osmosis desalination. Desalination, 54 (1985) 89–103. [13] J. Gwillim, Village Scale Photovoltaic Powered Reverse Osmosis, ODA TRD Report No. R6245. 1996, Dulas Limited: Machynlleth, Wales, UK. [14] M. Thomson, M.S. Miranda and D. Infield, A small-scale seawater reverse-osmosis system with excellent energy efficiency over a wide operating range. Desalination, 153 (2003) 229–236. [15] M. Thomson and D. Infield, A photovoltaicpowered seawater reverse-osmosis system without batteries. Desalination, 153 (2003) 1–8.
111
[16] Spectra Clark Pump Technology Overview, http://www.spectrawatermakers.com/technology, accessed 23 February 2005. [17] M. Thomson and D. Infield, A ReverseOsmosis System for the Desalination of Seawater Powered by Photovoltaics Without Batteries, in Proceedings of Renewable Energy Sources for Islands, Tourism and Water Desalination Conference. 2003, EREC (European Renewable Energy Council): Crete, Greece. 551–557. [18] WHO Guidelines for Drinking-Water Quality, 3rd ed., http://www.who.int/water_sanitation_ health/dwq/gdwq3/en/, accessed 25 February 2005. [19] M. Thomson, Reverse-osmosis desalination of seawater powered by photovoltaics without batteries. PhD Thesis, University of Loughborough, UK, 2004. Also available at: http://wwwstaff.lboro.ac.uk/elmt/Thesis.htm
Laboratory demonstration of a photovoltaic-powered seawater reverse-osmosis system without batteries Murray Thomson*, David Infield Centre for Renewable Energy Systems Technology (CREST), Loughborough University, LE11 3TU, UK Tel. þ44 (1509) 228144; Home: þ44 (1509) 213902; Fax: þ44 (1509) 610031; email: [email protected] Received 25 February 2005; accepted 10 March 2005
Abstract A prototype photovoltaic-powered reverse-osmosis system has been constructed at CREST, Loughborough, UK. The rate of production of fresh water varies throughout the day according to the available solar power, and thus, the system operates without need of batteries. The system is designed to operate from seawater, and a Clark pump brine-stream energy recovery mechanism is coupled with a variable recovery ratio technique to achieve a specific energy consumption of less than 4 kWh/m3 over a wide range of operation. Measurements showing the variable operation over a two-day period are presented and discussed. Keywords: Solar; Photovoltaic; PV; Seawater; Reverse osmosis; Energy recovery; Renewable energy
1. Introduction The use of renewable energy for desalination is a very attractive proposition, addressing both environmental concerns and the more general need for long-term sustainability. More specifically, solar photovoltaic-powered reverse osmosis (PV-RO) is considered one of the more promising technology combinations [1–3], particularly for small-scale systems where other technologies are less competitive. Indeed, PV-RO systems operating from
brackish water are already commercially available [4] and further developments are ongoing [5]. Operation from seawater is more challenging from an energy perspective, and early demonstrations tended to require large PV arrays, making them commercially unattractive. Developments have been made however, and ongoing demonstrations of seawater PVRO are installed in Europe [6,7] and by Kunczynski in Mexico [8], the latter being particularly impressive in terms of total water desalinated and energy efficiency.
*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.2005.03.031
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M. Thomson, D. Infield / Desalination 183 (2005) 105–111
Energy efficiency in seawater RO systems is critically dependent on the brine-stream energy-recovery device. In medium and large-scale systems, Pelton wheels are widely employed and various alternative devices [9–11] now compete to further improve system efficiencies. Unfortunately, none of these devices are well-suited to small-scale RO systems, and many small-scale systems are built without any energy-recovery; this keeps capital costs down but has a heavy penalty in ongoing energy costs. When the energy is to be provided by PV, the energy costs are unavoidably capitalized and brinestream energy-recovery becomes a high priority. Keefer recognised this in 1985 and designed and demonstrated a pump-integrated energy-recovery mechanism within a seawater PV-RO system [12]; high manufacturing costs inhibited further development, but a similar concept is now used for brackish PV-RO [4]. In 1996, Dulas Limited demonstrated use of a Danfoss axial-piston hydraulic motor for energy recovery in seawater RO [13]. CREST adopted the Dulas test rig in 2000, but then designed a system [14,15] based on the Clark pump from Spectra Watermakers [16], which offered significantly higher efficiency. Meanwhile, Kunczynski was putting similar devices to the test in a real application [8]. CREST and Duals took the view that the PV-RO system should be designed without batteries. This was based on experience with batteries in other PV systems in hot remote locations and with minimal maintenance. Without batteries, the RO part of the system can only operate when the sun is shining. The thinking is that it is much easier to store the fresh water in a tank than it is to store the equivalent energy in batteries. The developers of the brackish PV-RO systems mentioned above took a similar view, and their systems do operate without batteries. The opposing view is that RO systems are best
run at constant flow, 24 hours a day, and this is the norm for almost all medium and large-scale RO systems. Obviously, it makes good use of the capital invested in the RO hardware, but is also provides consistently high-quality product water and may reduce fouling potential within the RO membrane elements. The ongoing demonstrations of seawater PV-RO mentioned above [6–8] all employ batteries. The laboratory testing presented in this paper does not. 2. System under test The batteryless PV-RO test rig shown in Fig. 1 was assembled at CREST in 2003. The operation of the Clark pump and the two motorised pumps was described previously [14,17]. In summary: the Moineau (progressing cavity) pump raises the feed water to a medium-pressure (6–11 bar). The Clark pump raises this to high-pressure (40–70 bar) by virtue of the energy it recovers from the concentrate. The flow ratio of the Clark pump is fixed by design, which normally gives a 10% water recovery ratio at the membranes. However, the plunger pump injects an additional high-pressure feed, which increases the water recovery ratio to any desired value. Both pump motors are equipped with variable-frequency inverters. These are controlled, firstly, to provide maximum-power-point tracking (MPPT), ensuring that the total power drawn from the PV array tracks the maximum available as the irradiance varies throughout the day. Secondly, the controller adjusts the relative speeds of the two pumps in order to optimise the water recovery ratio and so maximise the flow of product water. The performance of such a system had been predicted [14,15], based largely on computer simulations. The test rig was assembled in order to verify these predictions, and more generally, to demonstrate operation with variable flow, pressure and recovery ratio.
M. Thomson, D. Infield / Desalination 183 (2005) 105–111 Photovoltaic Array
107
Reverse-Osmosis Modules Inverter
Inverter
Controller
Induction Motor
Induction Motor
Plunger Pump
Filter Moineau Pump
Clark Pump
“Seawater” Intake
Concentrate Discharge
Fresh water product
Tank
Fig. 1. Test rig configuration.
For the purpose of the demonstration, the controller shown in Fig. 1 was implemented in LabView software on a personal computer. The same computer and software also logged measurements made by numerous sensors throughout the system (not shown in Fig. 1). The feed water used for demonstration was straight NaCl solution, nominally at 32,800 mg/L, which is isosmotic with ASTM standard seawater. The product and concentrate streams were fed directly back into the feed tank, thus maintaining the feed concentration within plus and minus 500 mg/L. A computer-controlled heat exchanger (not shown in Fig. 1) was used to maintain the temperature of the feed water very close to 25 C.
The PV array used for testing comprised eighteen BP Solar Saturn BP585F monocrystalline silicon modules, having a total rating of 1.53 kWpeak, This is only 64% of the design value 2.4 kWpeak that was used in the performance predictions [15]. Further modules were available but could not be connected because the variable-frequency inverters were industrial units with a DC voltage limit of 400 V. Alternative PV modules with different power-to-voltage ratios are readily available and could be used in future to provide an array power much closer to the design value. The array used for testing has a fixed orientation (no solar-trajectory tracking).
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M. Thomson, D. Infield / Desalination 183 (2005) 105–111
The plunger pump and its motor were not quite optimum sizes, having been adopted from an earlier test rig. Thus, they required a toothed-belt coupling and had lower efficiency than optimum. Testing took place in Loughborough, UK.
3. Initial results and discussion 3.1. Product water flow The overall operation of the batteryless PV-RO is illustrated in Fig. 2: the product water flow varies in direct response to the available sunlight. At dawn, the PV array provides voltage to the inverters (Fig. 1). The controller then starts the Moineau pump and increases its speed to match the available power from the PV. As the sun
rises further, the controller starts the plunger pump, which increases the water recovery ratio and hence the product flow. There were many passing clouds during the two-day test, and, in the absence of any batteries, the arrival of each cloud caused an immediate reduction or halt in product flow. The pressures throughout the system also reduced but, were not lost completely (because the pumps are positive displacement). As soon as the cloud had passed, the pumps would again accelerate and production was restored. The overall effect of the clouds is clearly apparent in Fig. 2. The total product water recorded over the two-day test was 2.93 m3. The system is intended to produce more than twice this amount, but bear in mind that this testing was performed in the UK, with many passing
1000
Irradiance (W/m2)
Irradiance Product Flow
500
200
0
0 06:00
10:00
14:00
18:00
22:00
02:00
06:00
10:00
Time (hh:mm)
Fig. 2. Measured irradiance and product water flow June 9th and 10th 2003.
14:00
18:00
Product Flow (L/h)
400
109
M. Thomson, D. Infield / Desalination 183 (2005) 105–111
3.2. Energy consumption In order to see the performance of the RO rig itself, Fig. 3 shows the product flow against DC power coming from the PV array, measured at the input to the inverters. Water production starts at just 100 W and then increases more or less proportionally to the DC power. Operation over such a wide range of input power allows the system to make almost complete use of the variable power available from the PV array, without need of batteries. The points in Fig. 4. represent the same measured data as shown in Fig. 3, but this time presented as specific energy consumption in kWh/m3. The line is the predicted characteristic [Fig. 7. of reference 14 but with the spread caused by temperature removed]. The slightly-higher energy consumption shown in the measured results is 400 350
Product Flow (L/h)
300 250 200 150 100 50 0
0
200
400
600
800
1000
1200
1400
1600
DC Power (W)
Fig. 3. Product flow versus DC power (averaged at 1-minute intervals, covering the whole of the two-day test).
6
Specific Energy (kWh/m3)
clouds, and that the PV array was only 64% of the design value. Nearer to the equator and with a full-size array, the system may still be expected to produce around 3 m3/d as predicted [15].
Measured Predicted
5
4
3
2
1
0
0
200
400
600
800
1000
1200
1400
1600
DC Power (W)
Fig. 4. Specific energy consumption (kWh/m3).
due, at least in part, to the reduced efficiency offered by the particular plunger pump and motor used in testing, as mentioned earlier. For perspective, bear in mind that a smallscale seawater RO system, without energy recovery, would typically consume two to three times more energy. 3.3. Product water quality Energy efficiency was the primary objective throughout the design of the batteryless PV-RO system. For this reason, the system has a very generous membrane area relative to the product flow. Unfortunately, this also leads to the concentration of the product water being increased. Furthermore, the intermittent operation of the system also causes a significant rise in product water concentration, particularly following a break in production: a passing cloud may stop the flow of product water, but the (unwanted) salt passage through the membrane continues. Fortunately, the periods of high concentration tend to coincide with periods of low flow, but even taking this into account, the average concentration of the product water over the two-day test was over
110
M. Thomson, D. Infield / Desalination 183 (2005) 105–111
1100 mg/L. To improve matters, a diverter valve could be fitted in the product line and automated to reject product water exceeding a chosen concentration threshold. For example, discarding less than 1% of the product volume would bring the average concentration below 800 mg/L. Operation with fewer clouds would also improve the product water quality significantly. Lastly, the required quality of the product water should perhaps be considered in the context of the current World Health Organization ‘‘Guidelines for drinking-water quality, third edition’’ [18]. In particular, chapter 10 states: ‘‘The palatability of water with a TDS level of less than 600 mg/litre is generally considered to be good; drinking-water becomes significantly and increasingly unpalatable at TDS levels greater than about 1000 mg/litre. The presence of high levels of TDS may also be objectionable to consumers, owing to excessive scaling in water pipes, heaters, boilers and household appliances. No health-based guideline value for TDS has been proposed.’’ 4. Conclusions The operation of a seawater PV-RO system without batteries has been demonstrated. The use of a Clark pump energy recovery mechanism, coupled with variable flow and variable water recovery ratio, provides the necessary energy efficiency over a wide operational range. The concentration of the product water was unacceptably high during the tests presented here, but could perhaps be made acceptable without recourse to batteries. However, the case against using batteries has not been proven: demonstrating that the system works does not prove that it is the most economic approach overall. Also, the reliability of the system remains unproven; for example, the longevity of the membranes under
intermittent, variable and low-flow conditions is unknown. Clearly, reliability is a critical factor for any desalination system intended to provide drinking water in remote locations, and it is recommended that reliability be treated as a priority alongside energy-efficiency in the future development of PV-RO. A more detailed presentation of the system described in this paper is given in the author’s thesis [19]. References [1] CRES, Desalination Guide Using Renewable Energies, THERMIE – DG XVII, European Commission Report. 1998, CRES, Greece. [2] R. Oldach, Matching renewable energy with desalination plants. 2001, IT Power Ltd. Sponsored by MEDRC: The Middle East Desalination Research Center: Muscat, Sultanate of Oman. [3] D. Assimacopoulos, R. Morris and A. Zervos, Water, Water Everywhere, Desalination Powered by Renewable Energy Sources. REFOCUS, Elsevier, 2001 [4] Solarflow – Water Purification – Solar Powered Reverse Osmosis, http://www.sesltd.com.au/ html/waterpure.htm, accessed 19 February 2005. [5] B.S. Richards, C. Remy and A.I. Scha¨fer, Sustainable Drinking Water Production From Brackish Sources Using Photovoltaics, in 19th EPSEC. 2004. [6] E. Tzen, D. Theofilloyianakos, M. Sigalas and K. Karamanis, Design and development of a hybrid autonomous system for seawater desalination. Desalination, 166 (2004) 267–274. [7] T. Espino, B. Penate, G. Piemavieja, D. Heroid, and A. Neskakis, Optimised desalination of seawater by a PV powered reverse osmosis plant for a decentralised coastal water supply. Desalination, 156 (2003) 349–350. [8] Y. Kunczynski, Development and optimization of 1000–5000 GPD solar power SWRO, in IDA World Congress on Desalination and Water Reuse. 2003: Bahamas [9] DWEER (Dual Work Exchanger Energy Recovery), http://www.calder.ch/htm/dweer.htm, accessed 21 February 2005.
M. Thomson, D. Infield / Desalination 183 (2005) 105–111 [10] ERI (Energy Recovery Inc.) Pressure Exchanger, http://www.energyrecovery.com, accessed 21 February 2005. [11] FEDCO Hydraulic Pressure Booster, http:// www.fluidequipmentdev.com/hpb.htm, accessed 21 February 2005. [12] B.G. Keefer, R.D. Hembree and F.C. Schrack, Optimized matching of solar photovoltaic power with reverse osmosis desalination. Desalination, 54 (1985) 89–103. [13] J. Gwillim, Village Scale Photovoltaic Powered Reverse Osmosis, ODA TRD Report No. R6245. 1996, Dulas Limited: Machynlleth, Wales, UK. [14] M. Thomson, M.S. Miranda and D. Infield, A small-scale seawater reverse-osmosis system with excellent energy efficiency over a wide operating range. Desalination, 153 (2003) 229–236. [15] M. Thomson and D. Infield, A photovoltaicpowered seawater reverse-osmosis system without batteries. Desalination, 153 (2003) 1–8.
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