Wind power a reliable source for desalination

Wind power a reliable source for desalination

Desalination, 67 (1987) 559-564 Else&r Science Publishers B.V., Amsterdam-Printed 559 in The Netherlands WIND POWER A RELIABLE SOURCE FOR DESALINAT...

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Desalination, 67 (1987) 559-564 Else&r Science Publishers B.V., Amsterdam-Printed

559

in The Netherlands

WIND POWER A RELIABLE SOURCE FOR DESALINATION R. McBride, James Howden & Co. Ltd., Glasgow, Scotland R. Monis, Scottish Development Agency, Glasgow, Scotland. W. Hanbury, University of Glasgow, Scotland.

Abstract Recent developments in wind turbine technology mean that wind power can now be regarded as a reliable and cost-effective power source for many areas of the world. This paper reviews experience to date on the development of wind turbines and in using wind power in coniunction with RO slants for oroduction of notable water. The maior nractical uroblem of co&ling a windpowei sour& to fn RO system lies in limiting the startuplshutdown frequency of the RO plant. The economics of the combined. unit are briefly explored. Finally, the paper looks at areas-of the world where wind powered desalination could be applied.

Wind turbines or as they used to be called - windmills - were one of the first sources of power to be harnessed by mankind to supplement his own efforts. The earliest wind turbines that are recorded were located in Persia at around 200 BC [l] and were used to grind corn. These were fairly primitive vertical axis machines. The use of this type of wind turbine spread throughout the Islamic world. With the development of a geared transmission system the more familiar horizontal axis machine was developed to produce the furled sail windmill which is still in use today in many countries around the Mediterranean. The design of this machine has remained largely unchanged over the centuries. In an interesting example of Technology Transfer, the Crusaders, in the 13th Century, returning from the Holy Land introduced the wind turbine to Europe. These wind turbines were also used for arindine corn but bv the 14th Centurv the Dutch had taken the lead in this technology. Th;y dev$oped the slat&d blade design which is now synonymous with Holland and were using it in conjunctton with the Archimedes screw to drain marshes and lakes in the Rhine Delta. While there have obviously been developments in this machine over the centuries these were relatively minor in comparison to the changes that were to come. The Industrial Revolution and the coming of steam largely displaced wind power as a useful energy source except in specific applications such as the pumping-of water for drainage or supply and in the generation of small amounts of electricity in isolated areas such as farms. In the United States it is estimated that around 6 million small multi-bladed machines of 1 HP or less have been built for these purposes and that around 150,000 of these are still in operation. Within the last 12 years and in particular over the last 5 years, spectacular developments in wind turbine technology have taken place. This has been brought about by two important but independent factors. Firstly the oil crisis of 1973 which escalated the price of oil and made us aware for perhaps the fust time that energy particularly in the convenient form of oil was a finite resource not totally within our own control. Secondly, and perhaps in the long term mom importantly, we were becoming more aware that man was polluting the entire world’s atmosphere by burning increasing quantities of fossil fuels. In looking at alternative sources of energy, wind energy was readily identified as being available in virtually limitless quanuttes and was non polluting. As a consequence of this considerable sums of money have been spent in developing efficient reliable wind turbines ca able of generating useful amounts of electricity. This is manifested today in the windfarms tKat have been built in the United States where since 1982 wind turbines capable of generating around 2000 Me awatts have been installed. These have largely been built on three locations in California where t%e wind regime is articularly favourable. Figure 1 shows the number of wind turbines erected in California and & err generating capacity. The

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average size of these wind turbines is amund IN&W and is increasing. ln 1985, James Howden a windfarm of 75 300kW machines in California (Fig. 2).

ofGlasgowerected

Growth of wind turbines in California 16ooo

1so0

14000

1400

12000

1200

10000

1000

8ooO

-D

No. of Turbines

-+-

Mwcapacity

800

600

4cmo

400

2ooo

200

0 1981

0 1982

1983

1984

1985

1986

Machines up to 3 Megawatts have been built in the USA, Sweden and most recently in the U.K. (Fig. 3). These very large machines have all been built with government funds for research purposes but we are now in.a situation where commercially reIiabIe wind turbines with outputs of up to 1 Megawatt are avalable and where machines with outputs of up to 5OOKW are being ?roduc.ed, on a large. enough scale to considerably reduce the unit price. Almost aII of the major mdusmahsed counmes have major programmes for the development of wind power in hand.

Figure3. #iometerdiameter rotorfor the Wind Energy Gmp’s 3Mw Ttiine on Ckkney.

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Figure 2. The Howden Wind Turbines at Altmont.

California

Wind turbines have in the past acquired a reputation for unreliability, largely due to the failures of large research machines. More recently, series production of small and medium sized machines has enabled manufacturers to offer lower-cost machines that can demonstrate high reliability in service. It has been estimated that in California, where the majority of the worlds wind farms are located, availability of most machines have been in excess of 90%. Several groups of machines have demonstrated availabilities of 98 to 99%. One of the problems of utilising wind power in process applications is the variable nature of the resource. While the wind is relatively predictable it is seldom constant and there will be periods when there will be none at all. The storage of wind energy in the form of electrical power is really only practical when small amounts are involved. Storage batteries are very expensive. To run a process of any magnitude on stored electrical energy is therefore not a practical proposition. However if the product of the process can be stored inexpensively then it may be practical to oversize the process equipment to allow for downtime. Water is just such a medium in that it can be stored for long periods of time without deterioration and the storage vessels are relatively cheap. A further requirement is that the process is relatively insensitive to start-up- and shut-down. In Desalination Technology there are two processes which have this characteristic. These are Reverse Osmosis and Electrodialysis. There is therefore a natural match between these two processes and wind power. Of the two processes R.O. is the more common and some work has been done in establishing the viability of wind powered R.O. Hence while these remarks address R.O. they are in the main applicable to ED for brackish applications.

Experience on wind powered RO The idea of using wind power to drive a reverse osmosis plant is not new. Lawland [3] investigated the economics of wind driven desalination systems in the mid 1960’s. The Brace Institute [4], in Canada, ran a small reverse osmosis unit in the early 1970’s in which the flow rates were oscillated in order to simulate a wind power input to the plant, although no development of their system seems to have taken place since. The Canadian design was for a direct mechanical drive of the high pressure pumps by the wind turbine. More recently, however, small experimental, genuinely wind-driven, units have been run on the French island of Planier and also on Suderoog, a small island off the North Sea coast of Germany. Both of these involved electrical power generation by wind turbines and use of this power to drive reverse osmosis plants whenever the wind power was sufficient.

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Some theoretical or design studies have also been undertaken - most notably those by Husseiny et al in the U.S. in 1980 [7] and by Femn in Holland in 1985 [8]. The Practical Experience. a). The German Experience. The most significant experience is that provided by the joint MAN/GKSS project in which a 4.8 cum/day GKSS seawater reverse osmosis plant was hooked up to an Aeroman IV11 aero enerator, producing a rated output of 11 kW, designed and built by MAN [6]. The plant was run for a one year period from August 82 to July 83. The MAN aerogenerator had an automatic pitch controller which allowed the power frequency to be stablised at 50 Hertz, and thus allowed the use of standard electrical components in the reverse osmosis plant. The reverse osmosis plant apparently showed no adverse effects due to the continual start-up and shut-down cycles that it inevitably had to go through due to the intermittent nature of windpowered operation. The RO plant, although designed to use two membrane modules in series on the high pressure brine line, was operated during this test with only one module installed. This virtually halved the potential recovery of the lant and doubled the potential specific energy consumption. The specific energy consumption ac l!ieved m the test was quoted at 36.3 kWhr / cu.m. The average production rate of the plant when o erating was 2.64 cum/day. The RO plant utilisation factor was 45%. Since plant utilisation Pactors of 80 to 95% are nearer the norm for conventionally powered plant, it would indicate that wind driven plants would need to be significantly oversized and would suffer a resulting capital cost penalty compared to conventionally driven plant GKSS were contacted to see if there was any further information to that given in reference 16. On the question of how and when they allowed the RO plant to start up, they said that they had arrived at a criterion of 20 minutes continuous wind speed above the cut in velocity before allowing the RO plant to start up. This avoided excessive start-up/shut-down cycling of the water plant. Once this criterion had been instituted there had been no trouble with the plant - it had operated perfectly satisfactorily. b). The French Experience. Experiments with a 4 kW Aerowatt 4100 IT7 wind generator connected to a 0.5 m3/hr seawater reverse osmosis plant operating at 25% recovery were carried out by CEA - Cen Cadarache [5] during 1982 and 1983 on the island of Planier and subsequently at Faraman. The RO plant employed a pelton turbine for energy recovery, retrieving 1.2 kW from the outgoing brine flow. Again the major problem reported in these experiments was the frequent start up shut down cycles particularly when the wind speed was in the region of the cut in s eed. When operating at a wind speed in excess of 7 m/s the combination gave an output of 0.5 m J /br yielding an energy consumption of 7.8 kW/m3. The frequent cycling problems experienced at Planier and Faraman led to experiments in which battery energy storage systems were included in a simulated wind powered system at Cadarache in 1984. Economic studies based on the results of these experiments favoured a system which employed DC motors and battery storage and indicated that water costs comparable with conventional desalination systems might be expected. Technical Aspects The practical experience on wind powered RO systems has been with relatively small capacity systems. Since the time of these experiments significant developments have taken place, most notably the availability of larger, production, wind turbines which can provide wind generated electrical power on a larger and more economic scale. It now remains to combine these ‘new generation’ wind turbines together with larger RO plants to form viable self contained water plants. From a study of the literature and discussion with some of the people that have had experience of the small scale plant there is no indication that them should be any particular problem in hooking up a standard seawater reverse osmosis plant with a larger wind turbine generator using

563 a conventional electrical drive. The only matters that would require some careful design would be the relative sizes of the wind turbine and the RO plant and the cut-in and cut-out criteria for the RO plant to avoid excessive startup shutdown cycles. The optimisation of such a plant would certainly require detailed study and simulation under the actual conditions in which the plant is to serve. The electrical transmission route is probably the easiest to follow. Although some direct drive system may hold out a slightly greater efficiency, it would certainly require much more development work, particularly in the control aspects. The most sensible route forward would appear to be acquisition of a standard RO plant to run under test conditions connected to a standard wind turbine generator. Suitable modifications to either may then be made to enhance the matching of the combination. It is unlikely that energy storage would prove economical in these larger systems, although energy recovery for seawater plants would almost certainly be so. Economic Factors The average intalled capital costs for Californian wind turbines have decreased from over $3,OOO/kW,in 1981, to $1,25O/kW in 1986 [A] and the estimated O&M costs are of the order of lc/kWh [B]. The average installed costs of seawater RO plants are in the order of from $1,000 to $1,500 per cubic meter per day capacity. Using these specific costs together with load factors for both the wind turbine and the RO ~~~otw;p_propriateto their combined operation a simplistic water cost estimate may be made as Wind Turbine &stings Specific Capital Cost $/kW AmortisatiOn rate % Load factor %

fiWh 1.250 ::

CL&d charge

6.49

Power cost

::LG

SWRO Castings Specific Capital cost $/m3/day 1,250 Specific Energy Consumption kWh/m3 8.0 Load factor % 60

$h3

Capital Chemicals Staff Membrane replacement Power @ 7.49 c/kWh spares Water Costs

0.86 0.08 0.16 0.17 Kz $1194 lm3

Prospects The prospects for Wind Powered Reverse Osmosis plants would appear to be good enough to merit serious further investigation. There are no serious technical difficulties in matching up an electricity generating wind turbine with an electrically driven reverse osmosis plant. The economics of running the reverse osmosis plant at a low load factor would have to be assessed for each particular case. The sort of performance figures that one might expect to get from such a combination desalinating seawater on a site with a reasonable wind regime might look as follows:Wind Turbine:

Rated output actual average

60kW 20 kW

RO Plant:

Rated output actual average

80 m3/d 48 m3/d

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The actual ratio of wind turbine size to ROplant sire that might be used in any instance should result from an optimisation making use of data that will be. site specific for both the wind turbine and for the RO plant. The most promising potential market for wind powered RO is in present or potential future island tourist developments in places such as the Canaries, the West Indies, the Mediterranean islands, the Pacific Islands etc. Generally, if wind powered electricity generation is an economic proposition in any of these places and water is scarce (which it usually is), then wind powered reverse osmosis should also be economic. The economics of a combination of a wind turbine with an RO plant is helped by the fact that water is a storable commodity. The RO plant should be able to utilise the wind turbine efficiently, although the converse will not generally be true in that the RO plant will be operating at a low load factor (typically 60 to 70%). Now that significantly larger and more reliable wind turbines have become available, wind powered desalination is poised to make the breakthrough into commercial applications. The next step should be a full scale demonstration plant. Islands with water and fuel shortages and with a good wind regime should consider this possibility seriously.

REFERENCES Eldridge, “Wind Machines”, Van Nostrand. Milborrow, D. International Notes, WINDirections, VI (1987) No. 4,21-25. T.A. Lawland, The Economics of Wind Powered Desalination Systems, Brace Institute Technical Report T-36, June 1967. R Alward, E.R. Lising and T.A. Lawland, The Effect of a Variable Power Input on the Performance of a Reverse Osmosis De&nation Unit, 4th Int. Symp. on Fresh Water from the Sea, Vol. 4,25-34, 1973. Libert, J.J. and Maurel, A., Desalination and Renewable Energies - A Few Recent Developments, Desalination, 39 (198 1) Petersen, Cl., Fries, S., Kaiba, K. and Knunz, D.., A Wind-Powered Water Desalination Plant for a Small Island Communtty at the German Coast of the North Sea - Design and Working Experience, Proc. 3rd Int. Conf., Inverness, Scotland, U.K. (1983) 173-180 Unione, A., McClymont, A., Dix, T. and Husseiny, A., Wind Powered Reverse Osmosis for Self-Contamed Village Applications, A Design fltid~30Proc. 7th Int. Symp. on Fresh Water from the Sea, Vol. 2 (1980) Feron, P., The Use of Windpower in Autonomous Reverse Osmosis Seawater Desalination, Wind Engineering Vo1.9,No.3,1985