- Email: [email protected]
The SDAWES project: lessons learnt from an innovative project
DESALINATION ELSEVIER
Desalination 168 (2004) 39-47
ww .elsevier.com/locate/desal"
The SDAWES project: lessons learnt from an innovative project •
a*
r
Vicente Javier Sublela , Jos6 Antonio Cartab, Jaime Gonzalez
C
"Renewable Energies and Water Treatment Department, Technological Institute of the Canary Islands (ITC), Playa de Pozo Izquierdo s/n 35119, Santa Lucia Gran Canaria, Spain Tel. +34 (928) 72 75 03; Fax: +34 (928) 72 75 17; email: [email protected] bMeehanical Engineering Department, CElectronic Engineering & Automatics Department, Las Palmas de Gran Canaria University, Campus de Tafira s/n, 35017 Gran Canaria, Spain Received 16 February 2004; accepted 26 February 2004
Abstract
The Seawater Desalination by an Autonomous Wind Energy System(SDAWES) project was developed to produce a natural scarce resource (fresh water) by the use of a natural, renewable resource (wind energy). The basic concept consists in the connection of three kinds of desalination systems: reverse osmosis, vacuum vapour compression and reversible electrodialysis to a stand-alone wind energy system to produce fresh water from seawater on a significant scale (total nominal water production: 440 m3/d). The main objectives of the project were to identify the best desalination system for connection to a stand-alone wind farm and to assess the influence of the variations of wind energy on the operation of the desalination plants and on the quality of the water produced. This is a project where several lessons were learnt after two years' testing experience. This paper presents the main problems detected during that period and the experimented proposed solutions. Keywords: Stand-alone system; Wind energy; Reverse osmosis; Vapour compression; Electrodialysis
I. I n t r o d u c t i o n
The supply o f fresh water is one of the most important problems in the world today, specially in developing countries, as has been exposed by several desalination studies [ 1-4]. Energy supply *Corresponding author.
is probably the other most important global issue. Both resources are necessary for all of humanity since they are part o f all the economic and environmental analysis and have important social repercussions; for instance, water-related diseases cause the death o f 5 million people each year, and about 2.3 billion people suffer from diseases linked to dirty water [5].
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004.
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi;10.1016/j.desal.2004.06.167
40
V.J. Subiela et al. / Desalination 168 (2004) 39-47
One of the most recent, popular and hopeful solutions is seawater desalination by renewable energies; the Seawater Desalination by an Autonomous Wind Energy System (SDAWES) project has made a small contribution to this point: use a natural and abundant energy resource, wind, to produce a scarce resource, fresh water. Three desalination processes were tested: reverse osmosis (RO), vapour compression (VC) and reversible electrodialysis (ER). The project, which began in February 1996 and finished in May 2001 is located on the Gran Canaria Island (Canary Islands, Spain) where fresh water supply is an historical problem. The desalination industry is growing, and desalted water production has been increasing over the past years, which reached a total installed capacity of about 700,000 m3/d in 2002 (69% from seawater) [6]. The increase in population and the rainfall reduction have intensified the situation over the last years. On the other hand, the island has an important natural resource - - wind energy - which cannot be completely used to produce electricity. It is known that the connections of wind farms to small grids (the case of an island) is limited due to the instabilities produced by the oscillations of a variable power supply. Therefore, stand-alone desalination is an interesting way of using this remaining energy resource. The present paper focused on the lessons learnt to be taken into account ira new SDAWES project is be begin. As a curious comment, it is interesting to mention that, during a general electric failure that affected almost all the grid in the Gran Canaria Island (09.10.2000), one of the two wind turbines was the only one that continued operating as it was connected off-grid. The project has had certain international relevance as many foreigners have visited it. The most significant event was the preparation of an official proposal of a similar system to be installed in Tan-Tan (Morocco). This was a Canary
Islands Government initiative developed during the year 2000.
2. Basic description of the system A simple description of the system and its main components follows. Previous publications present a more detailed explanation with an extended description [7-9]. The basic elements of the tested system are the stand-alone wind energy system, the water production system and the control system. A basic description of each component follows. 2.1. Stand-alone wind energy system
This is composed by an off-grid wind farm with two 230 kW synchronous wind turbines, a 1,500 rpm flywheel coupled to a 100 kVA synchronous machine, an isolation transformer and a UPS of 7.5 kW. The electric connections among these elements and the loads can be seen in Fig. 1; the two electric circuits (supplied from the off-grid wind farm (solid line) and from the general grid (dotted line) can be clearly observed. 2.2. Hydraulic system
This part of the project is made up of the desalination plants, the piping circuits, the seawater pumping station and the fresh water tank. Fig. 2 illustrates the hydraulic connections of this system. The seawater is moved from a beach well to the desalination plants (only RO and VC). There are two pumping groups: one for the RO plants (2x13 kW pumps), and the other for the VC unit (2x9 kW pumps). Desalted water is collected in a pipe and conducted to a fresh water storage tank. As there is no natural brackish water source, the EDR plant was connected in a closed circuit. Artificial brackish water was prepared by mixing distilled water (from the VC unit) and seawater;
41
V.J. Subiela et al. / Desalination 168 (2004) 39-47
............... 77 T't:~
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Fig. 1. Basic electric system. t~evor~eOm~o~s ~ar',t~
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11
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42
KJ. Subiela et al. / Desalination 168 (2004) 39--47
this water was stored in two tanks to feed the plant and desalted water and brine were introduced, in turn, in the tank (see Fig. 2). Three desalination processes were tested: • Reverse osmosis (RO): eight identical units (25 m3/d each) operating at 60-70 bar, with a nominal specific consumption of 7.2 kWh/m 3. (Tested average values about 6.9 kWh/m 3 have been observed.) • Vapour compression (VC): a 50 m3/d unit operating with 20% of vacuum (0.2 bar), evaporation temperature of 62°C and average specific consumption 16 kWb./m 3. • Reversible electrodialysis (EDR): A nominal production of 190 m3/d with a specific consumption of 3.3 kWh/m 3. The flow range production is 35-100% and the feed water salinity varies from 2500 to 7500 #S/cm. 2.3. Control system
The control system is composed of the following elements: • a seven PLC network: a main PLC connected to the control PC and six secondary PLCs connected directly to each part of the system, • a control PC where the specific control software is installed, • an acquisition data PC where the data are stored and analyzed. The information goes in both directions through the network: signals of the system <-> PLCs net <-> control PC. Inside the control PC, two main activities take place simultaneously: the compiled data are recorded and displayed in several windows, and the control software checks the system and gives it, if necessary, appropriate instructions.
3. Description of the experience 3.1. Start-up and operation o f the system
The process of starting-up and operation of the stand-alone system can be essentially described
as follows: when the start-up signal is given, the system checks the wind speed and the software control decides if there is enough wind to start up the isolated system (minimum average of 6 m/s during 5 min or similar). Under these conditions, one of the wind turbines starts to accelerate the flywheel by a 22 kW start-up motor until it reaches 48 Hz; then the synchronous machine is activated to generate a three-phase grid of 400 V, which is detected as a reference by the wind turbine (WT). Then the WT introduces energy to the only connected load - - the flywheel - - until it reaches an upper speed limit of 52 Hz, standalone, grid ready. From that moment, the other WT and the loads can be connected to the isolated grid; the WTs change the blade angle to adapt the supplied power to the consumed power. If the wind speed decreases, the control system will detect the reduction of frequency and will order a reduction in the consumption by disconnecting plants or modifying the working point until reaching the nominal frequency (52 Hz); if the wind is very weak, all the loads will be stopped. Carta et al. described the process in detail [7,8]. 3.2. Developed activity
The general work plan was distributed in seven different large tasks according to the physical parts of the system (wind farm, RO plant, VC plant, EDR plant, control system) and the technical documentation to be generated (Technological Handbook and Industrial Package). Broadly speaking, the official European project plan (February 1996--July 2000) was as follows: The first 2 years were dedicated to analysis, design activities and contacts with suppliers and local administration. The installation and commissioning works were made during the third year; the tests, data analysis and corrections were carried out in the fourth year. The last 6 months were basically dedicated to drawing up the final reports. Later, complementary tests and
V.J. Subiela et al. / Desalination 168 (2004) 39--47 analysis were made over more than 1 year. Conceming the heart of the project (tests), it should be noted that the stand-alone wind farm was installed, commissioned, tested and operated with different loads and different wind conditions. It was checked the operation of the system was stable thanks to a double control: "pitch", under high wind conditions (generated power > consumed power), and "load", under low wind conditions (generated power < consumed power). In this way the isolated grid frequency was controlled between 48 and 52 Hz. 3.3. Description o f the learnt experience Different kinds of technical and practical problems were detected. This section describes the most relevant situations that happened during the project. 3.3.1 The electric system One of the most important inconveniences was the start-up when wind speed was close to the lower limit (average value of 6 m/s during 5 min). The total process lasted about 30 min; this situation can delay the process 1 or 2 h, or even more, depending on the day. Several minor failures in small components of the electric system came into view, the most common being located in the orientation blade motors of the wind turbines; this is presumed to be due to the continuous regulation to keep the nominal frequency (52 Hz) stable. On the other hand, during the first years of the project, the Enercon Company carried out many tests to improve the start-up process; they concluded it was very difficult to generate the isolated grid directly with the synchronous machine, so they solved the problem by installing an asynchronous motor mechanically connected to the flywheel to start it up. Two main difficulties appeared in the operation of the flywheel: • high mechanical friction, which consumed an
43
important amount of energy. It especially was affected when the wind power was down and loads had to be switched off; and overheating one of the bearings with the consequent fault and halting of the start-up process. After installing the complete system, two problems were found inside the flywheel building: high temperature due to the heat produced by the flywheel-synchronous machine component, and the presence of dust from the outside. The ceiling of this building was designed to be mobile in order to install the flywheel into it by a crane, so the highest part of the walls were open, and the wind brought sand, dust and humidity inside. 3.3.2. The hydraulic system 1. Pumps and pipes. A different set of mechanical problems appeared in the pumps: vibrations, heat, insufficient flow, and overload. Particular mechanical problems were found during off-grid operation since the frequency was slightly higher (52 Hz). A particular point should be mentioned: the pressure variations in the feed water pipe. Initially, a solenoid butterfly by-pass valve was installed in the RO circuit feed water pipe to get a constant pressure independent of the number of connected RO units. It seemed an easy control, as the pump operation curve was known and a pressure transmitter provided the instantaneous value. The experience demonstrated that it was not possible to reach a stable pressure with enough speed; only a short period of time was available since one RO unit starts up in 1 min. The water is taken from a beach well 35 m deep, located at 100 m from the coast; the advantage of this configuration was checked with the introduction of marine life, and consequent fouling has been avoided. 2. RO plant: Apart from the normal operation problems such as leakages, dirt and other less
44
v.J. Subiela et al. / Desalination 168 (2004) 39-47
frequent situations, specific difficulties appeared during the off-grid operation, caused especially by the step-loads connection. If the stand-alone grid went down due to lack of wind or other problems, the loads were progressively switched off. When this happens quickly, the remaining energy in the flywheel is not enough to supply power to wash the membranes of the connected RO units. However, each unit is provided with a fresh water tank on the top of the unit to feed fresh water into the membranes. It was found that the volume was not enough to complete the washing. The practical experience was complemented with a thorough theoretical analysis [10,11]. 3. VC unit: VC start-up was quite long (90 min) consuming 20 kW, and then the normal operation (30 kW in the compressor + discontinuous 10 kW in the electric heater). The on-grid tests demonstrated that the unit was not designed to operate under discontinuous power supply. After several weeks of start-and-stop operation, the plant stopped working altogether. The evaporation chamber was opened and a hard layer of scale was discovered. After acid cleaning, the plant was restarted, but as it was not operating 24 h/d, the problem appeared again. A full detailed report about the modelling, technical description and comparison analysis of the process can be found in the literature [12-14]. 4. EDR unit: The operation of the EDR unit required the preparation of the desired brackish water to feed by mixing seawater and freshwater in 2x5,000 1 tanks. It was not possible to reach a complete mix, and thus the feed water had variable salinity. However, the most serious problem with this plant was the generation of distortions in the electrical system in the form of current harmonics caused by the DC drivers (power converters for electricity supply to the membrane stacks) and the frequency converters in the pumps to control the plant flow rate [15]; this phenomenon was also detected in the VC plant, caused by the frequency converter of the
compressor. The most significant consequences were noise and heating in the RO high-pressure pumps. More technical information was presented by Veza et al. [15-17]. 3.3.3. In the control system
This has been probably the most critical point. On several occasions the control PC failed to stop the recording and control processes. It is thought that the simultaneous recording and control processes could cause isolated overloads in the computer. Part of the failures could be due to the operative system (WINDOWS 98). A very successful operation was observed in the PLCs as no failure was detected since the beginning of the project. An extensive description of the concept includes technical details [ 18].
4. Lessons learnt
The lessons learnt during the project can be divided into two categories: tested solutions and solutions to be applied in the future. 4.1. Tested solutions 4.1.1. In the electric system
The performed tests demonstrated the stability of the whole system. A thorough analysis with graphic information from real data was carried out by Carta et al. [7]. The flywheel was partially dismounted to determine the overheating of the rear bearing; it was substituted and correctly lubricated. The overheating in the flywheel building was solved by analyzing the movement of air inside the building and installing a fan on the ceiling. The intake of wet air was avoided by screwing a piece of impermeable canvas down the open part of the ceiling. 4.1.2. In the hydraulic system
The advantages and disadvantages of each desalination process operating under variable
V.J. Subiela et aL / Desalination 168 (2004) 39-47
45
Table I Relation of the main advantages and disadvantages of each desalination process in isolated system operation Desalination system
Advantages
Disadvantages
RO
• Fast start-up and stop • Absence of harmonics • Low specific energy consumption • Variable continuous power consumption
• •
VVC
EDR
• •
Variable continuous power consumption Fast starting-up and stop
Conditions were identified. It was concluded that the most suitable process was RO, mainly due to the fast start-up and the absence of an harmonics generation (see Table 1). After several months o f testing, a specific know-how was acquired which is based on a set o f recommendations (see Section 4.2.2). 4.1.3. In the control system After several checks, correct operation was found for all the software involved in the processes. A compatible version of each software was installed and the control software was simplified. 4.2. Proposed solutions (for future systems) 4.2.1. In the electric system The use of high variable speed flywheels has already been proposed as an interesting way o f improving the efficiency in the operation o f stand-alone grids. The R&D centres at CIEMAT and CEDEX have been researching this field [ 19]. On the other hand, a better mechanical and aerodynamic design should be done to increase energy efficiency. These subjects imply a specific engineering development, adapted to each system
Discontinuous power consumption Pressure control in the feed water circuit
• S l o w start-up • Scaling if discontinuous operation • Harmonic distortion • Specific stable temperature and pressure • Only for brackish water • High harmonic distortion (due to the conversions DC/AC/DC)
to be projected. The proposal of a hybrid system (diesel + wind) has already been defined [20]. 4.2.2. In the hydraulic system Different correction and improvement measures were proposed for each desalination process: 1. Reverse osmosis: The first proposal was to substitute the eight units by one bigger variable flow unit; this solution would need a specific frequency converter and variable flow seawater pumps. This option has already been installed in recent conventional RO desalination plants [21 ]. As previously mentioned, this device would produce harmonics that should be avoided by appropriate filters. On the other hand, a specific design of the plant should be developed, including strong components, high-quality materials, high-resistance membranes, and a suitable solution for selfwashing during breaks. If the pumps are still operating, allow the intake of seawater from the feed water (it is better to have seawater than brine in the membranes) or include a fresh water tank with the correct size. The sudden change of pressure in the membranes during breaks (from 60 to 1 bar in a very short time) is a strong load and could be avoided by installing a solenoid
46
V.J. Subiela et aL / Desalination 168 (2004) 39--47
needle valve to control the pressure at each moment. Finally, a reduction in power consumption by installing an energy recovery element (turbine, pressure exchanger) is recommended. 2. Vapour compression: A special design would be necessary in order to prepare the plant for interrupted operation. Some ideas should be studied: inclusion of a hot water storage system with solar thermal energy contribution to reduce start-up time, addition of a specific seawater washing of the evaporation plates to remove the rests of concentrated brine, improvement of the sealing system to preserve the vacuum for a longer time. 3. Reverse electrodialysis: A specific filter or filters should be designed to avoid the variety of harmonics generated in the plant. As this system has a limitation in the salinity of water, seawater could be used as feed water ifa previous RO unit was installed; a low-quality fresh water RO unit (old/used membranes) would be the ideal plant, as membrane life would be extended. The pumping system could also be significantly improved. Several pumps set could substitute for only one pump. The system should get maximum efficiency for each possible flow. Working with only one pump does not reach this result as different operation points (one for each flow) have to be used. The best control pressure in the feed water would be interesting; a special needle valve should be installed to guarantee the stability of the pressure.
The conclusion was that although it is practical to have the control software in the PC, so that modifications can be done quickly, the software must be installed in a stronger medium - the main PLC - - and prepare the PC only for illustrating online information and the recorded data. The new wind speed prediction models could be included in the control software to startup the stand-alone system at the most suitable moment.
4.3. In the control system
Acknowledgements
Intense efforts were carried out to solve the problems of this part: increase the RAM, substitution of the PC, modifications of the control software, among others. In spite of all these attempts for improving the operation of the PC, the failures were not totally eliminated.
This project was co-financed by the European Commission (Joule III Program, Contract No. JOR3-CT95-0077), it was co-ordinated by ITC, with the participation of other European partners: ULPGC (Spain), Der-Ciemat (Spain), Enercon (Germany), Crest (UK) and NeI (UK).
5. Conclusions One of the most recommended tasks to do after a long-term project is to look back at all the work done and analyze what was good and bad. An assessment activity has shown us which parts of the project should be repeated, modified or removed, and which additions should be included for future experience. The ideas presented are a set of recommendations to be taken into account before designing seawater desalination systems by autonomous wind energy systems. Concerning coordination and organizational aspects, it is important to note that a multidisciplinary project like this requires a complete technical team with different specialists. Contributions from suppliers are also indispensable, and specific and continuous contacts should be established with them in order to develop specific technology and technical components for these kinds of systems.
V.J. Subiela et al. / Desalination 168 (2004) 39-47
References [1] R. Clarke, Water: The International Crisis. MIT Press, 1993. [2] P. Gleick, Water in Crisis. Oxford University Press, New York, 1993. [3] R. Petrella, The Water Manifiesto:Arguments for a Word Water Contract. Zed Books, 2001. [4] V. Belessiotis and E. Delyannis, Water shortage and renewable energies (RE) desalination possible technological applications. Desalination, 139 (2001) 133-138. [5] Facts and figures: Water and Health. Official site of the International Year of Freshwater 2003, UNESCO, UN, http://www.wateryear2003.org/en/ ev. hp@URL_ID= 1600&URL_DO=DO_TOPIC& URL SECTION=201.html. [6] Fundaci6n Centro Canario del Agua, http://www. fcca.es/Docs/Listadesaladoras.xls. [7] J.A. Carta, J. Gonz~lez and V. Subiela, Solar Energy, 75 (2003) 153. [8] J.A. Carta, J. Gonzfilez andV. Subiela, Desalination, 161 (2004) 33-47. [9] I. Cruz, A. Gonz~lez, R. Calero, C. Rodriguez, J. Gonz~lez and J.A. Carta, SDAWES desalination plants connected to an autonomous wind energy system, Proc. European Union Wind Energy Conference, Goteborg, Sweden, 1996. [ 10] D.G. Infield and Z. Rahal, 'Computer modelling of a large-scale stand-alone wind-powered desalination plant, in: Wind Energy Conversion from Theory to Practice, R. Hunter, ed., Mechanical Engineering Publications, Edinburgh, 1997, pp. 129-134. [11] Z. Rahal, and D.G. Infield, Wind powered stand alone desalination, European Wind Energy Conference, IWEA, Dublin Castle, Ireland, 1997, pp. 802-806. [12] G Skivington, A review of vapour compression processes for the seawater desalination and wind energy systems (SDAWES) project, National Engineering Laboratory, Report No. 271/96, 1997.
47
[13] G.R. Skivington, Mathematical model of a vapour compression desalination plant for autonomous application of wind power, European Wind Energy Conference (EWEC'99), Nice, France, 1999. [14] M. McCourt and G.R. Skivington, The vacuum vapour compression plant, National Engineering Laboratory. Fourth Annual Progress Report for SDAWES, Contract JOR3-CT95-0077, 1999- 2000. [ 15] J.M. Veza, B. Pefiate and F. Castellano, Desalination, 141 (2001) 53. [16] J.M. Veza, B. Pefiate and F. Castellano, Electrodialysis desalination in an off-grid wind energy system, Proe Conf. Policies and Strategies for Desalination and Renewable Energies, Santorini, Greece, 2000. [17] J.M. Veza, B. Pefiate and F. Castellano, Desalination, 160 (2004) 211-221. [18] J. Gonz~lez, R. Vega, J.A. Carta, W. Janssen, R. Calero and J. Caballero, A control system design for an autonomous wind-park with different types of desalination plants in the Canary Islands, Proc. European Union Wind Energy Conference, Dublin, Ireland, 1997. [19] I3. Iglesias, L. Garcia-Tabarts, A. Agudo, I. Cruz and L. Arribas, Design and simulation of a standalone wind-diesel generator with a flywheel energy storage system to supply the required active and reactive power, Proc. 31 stAnnual Power Electronics Specialists Conference, Vol. 3., pp 1381-1386. [20] L.M. Arribas, I. Cruz, R. Calero, V. Subiela, J.A. Carta, A. Beekmann, D. Infield and M. McCourt, The SDAWES project: a hybrid system? Proc. European Union Wind Energy Conference, Kassel, Germany, 2000. [21] J. Sadhwani and F. Deniz, Behaviour of the asynchronous electric motors to reduction of energy consumption in reverse osmosis plants. Desalination, 157 (2003) 1-8.
Desalination 168 (2004) 39-47
ww .elsevier.com/locate/desal"
The SDAWES project: lessons learnt from an innovative project •
a*
r
Vicente Javier Sublela , Jos6 Antonio Cartab, Jaime Gonzalez
C
"Renewable Energies and Water Treatment Department, Technological Institute of the Canary Islands (ITC), Playa de Pozo Izquierdo s/n 35119, Santa Lucia Gran Canaria, Spain Tel. +34 (928) 72 75 03; Fax: +34 (928) 72 75 17; email: [email protected] bMeehanical Engineering Department, CElectronic Engineering & Automatics Department, Las Palmas de Gran Canaria University, Campus de Tafira s/n, 35017 Gran Canaria, Spain Received 16 February 2004; accepted 26 February 2004
Abstract
The Seawater Desalination by an Autonomous Wind Energy System(SDAWES) project was developed to produce a natural scarce resource (fresh water) by the use of a natural, renewable resource (wind energy). The basic concept consists in the connection of three kinds of desalination systems: reverse osmosis, vacuum vapour compression and reversible electrodialysis to a stand-alone wind energy system to produce fresh water from seawater on a significant scale (total nominal water production: 440 m3/d). The main objectives of the project were to identify the best desalination system for connection to a stand-alone wind farm and to assess the influence of the variations of wind energy on the operation of the desalination plants and on the quality of the water produced. This is a project where several lessons were learnt after two years' testing experience. This paper presents the main problems detected during that period and the experimented proposed solutions. Keywords: Stand-alone system; Wind energy; Reverse osmosis; Vapour compression; Electrodialysis
I. I n t r o d u c t i o n
The supply o f fresh water is one of the most important problems in the world today, specially in developing countries, as has been exposed by several desalination studies [ 1-4]. Energy supply *Corresponding author.
is probably the other most important global issue. Both resources are necessary for all of humanity since they are part o f all the economic and environmental analysis and have important social repercussions; for instance, water-related diseases cause the death o f 5 million people each year, and about 2.3 billion people suffer from diseases linked to dirty water [5].
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004.
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved doi;10.1016/j.desal.2004.06.167
40
V.J. Subiela et al. / Desalination 168 (2004) 39-47
One of the most recent, popular and hopeful solutions is seawater desalination by renewable energies; the Seawater Desalination by an Autonomous Wind Energy System (SDAWES) project has made a small contribution to this point: use a natural and abundant energy resource, wind, to produce a scarce resource, fresh water. Three desalination processes were tested: reverse osmosis (RO), vapour compression (VC) and reversible electrodialysis (ER). The project, which began in February 1996 and finished in May 2001 is located on the Gran Canaria Island (Canary Islands, Spain) where fresh water supply is an historical problem. The desalination industry is growing, and desalted water production has been increasing over the past years, which reached a total installed capacity of about 700,000 m3/d in 2002 (69% from seawater) [6]. The increase in population and the rainfall reduction have intensified the situation over the last years. On the other hand, the island has an important natural resource - - wind energy - which cannot be completely used to produce electricity. It is known that the connections of wind farms to small grids (the case of an island) is limited due to the instabilities produced by the oscillations of a variable power supply. Therefore, stand-alone desalination is an interesting way of using this remaining energy resource. The present paper focused on the lessons learnt to be taken into account ira new SDAWES project is be begin. As a curious comment, it is interesting to mention that, during a general electric failure that affected almost all the grid in the Gran Canaria Island (09.10.2000), one of the two wind turbines was the only one that continued operating as it was connected off-grid. The project has had certain international relevance as many foreigners have visited it. The most significant event was the preparation of an official proposal of a similar system to be installed in Tan-Tan (Morocco). This was a Canary
Islands Government initiative developed during the year 2000.
2. Basic description of the system A simple description of the system and its main components follows. Previous publications present a more detailed explanation with an extended description [7-9]. The basic elements of the tested system are the stand-alone wind energy system, the water production system and the control system. A basic description of each component follows. 2.1. Stand-alone wind energy system
This is composed by an off-grid wind farm with two 230 kW synchronous wind turbines, a 1,500 rpm flywheel coupled to a 100 kVA synchronous machine, an isolation transformer and a UPS of 7.5 kW. The electric connections among these elements and the loads can be seen in Fig. 1; the two electric circuits (supplied from the off-grid wind farm (solid line) and from the general grid (dotted line) can be clearly observed. 2.2. Hydraulic system
This part of the project is made up of the desalination plants, the piping circuits, the seawater pumping station and the fresh water tank. Fig. 2 illustrates the hydraulic connections of this system. The seawater is moved from a beach well to the desalination plants (only RO and VC). There are two pumping groups: one for the RO plants (2x13 kW pumps), and the other for the VC unit (2x9 kW pumps). Desalted water is collected in a pipe and conducted to a fresh water storage tank. As there is no natural brackish water source, the EDR plant was connected in a closed circuit. Artificial brackish water was prepared by mixing distilled water (from the VC unit) and seawater;
41
V.J. Subiela et al. / Desalination 168 (2004) 39-47
............... 77 T't:~
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Fig. 1. Basic electric system. t~evor~eOm~o~s ~ar',t~
Brae weE1
.4-
Brine Water ~
i
!
0, 0, seo~o'~ pum~
Pt0ducl ~ t ~
11
Brine Water
Fig. 2. Hydraulic system.
42
KJ. Subiela et al. / Desalination 168 (2004) 39--47
this water was stored in two tanks to feed the plant and desalted water and brine were introduced, in turn, in the tank (see Fig. 2). Three desalination processes were tested: • Reverse osmosis (RO): eight identical units (25 m3/d each) operating at 60-70 bar, with a nominal specific consumption of 7.2 kWh/m 3. (Tested average values about 6.9 kWh/m 3 have been observed.) • Vapour compression (VC): a 50 m3/d unit operating with 20% of vacuum (0.2 bar), evaporation temperature of 62°C and average specific consumption 16 kWb./m 3. • Reversible electrodialysis (EDR): A nominal production of 190 m3/d with a specific consumption of 3.3 kWh/m 3. The flow range production is 35-100% and the feed water salinity varies from 2500 to 7500 #S/cm. 2.3. Control system
The control system is composed of the following elements: • a seven PLC network: a main PLC connected to the control PC and six secondary PLCs connected directly to each part of the system, • a control PC where the specific control software is installed, • an acquisition data PC where the data are stored and analyzed. The information goes in both directions through the network: signals of the system <-> PLCs net <-> control PC. Inside the control PC, two main activities take place simultaneously: the compiled data are recorded and displayed in several windows, and the control software checks the system and gives it, if necessary, appropriate instructions.
3. Description of the experience 3.1. Start-up and operation o f the system
The process of starting-up and operation of the stand-alone system can be essentially described
as follows: when the start-up signal is given, the system checks the wind speed and the software control decides if there is enough wind to start up the isolated system (minimum average of 6 m/s during 5 min or similar). Under these conditions, one of the wind turbines starts to accelerate the flywheel by a 22 kW start-up motor until it reaches 48 Hz; then the synchronous machine is activated to generate a three-phase grid of 400 V, which is detected as a reference by the wind turbine (WT). Then the WT introduces energy to the only connected load - - the flywheel - - until it reaches an upper speed limit of 52 Hz, standalone, grid ready. From that moment, the other WT and the loads can be connected to the isolated grid; the WTs change the blade angle to adapt the supplied power to the consumed power. If the wind speed decreases, the control system will detect the reduction of frequency and will order a reduction in the consumption by disconnecting plants or modifying the working point until reaching the nominal frequency (52 Hz); if the wind is very weak, all the loads will be stopped. Carta et al. described the process in detail [7,8]. 3.2. Developed activity
The general work plan was distributed in seven different large tasks according to the physical parts of the system (wind farm, RO plant, VC plant, EDR plant, control system) and the technical documentation to be generated (Technological Handbook and Industrial Package). Broadly speaking, the official European project plan (February 1996--July 2000) was as follows: The first 2 years were dedicated to analysis, design activities and contacts with suppliers and local administration. The installation and commissioning works were made during the third year; the tests, data analysis and corrections were carried out in the fourth year. The last 6 months were basically dedicated to drawing up the final reports. Later, complementary tests and
V.J. Subiela et al. / Desalination 168 (2004) 39--47 analysis were made over more than 1 year. Conceming the heart of the project (tests), it should be noted that the stand-alone wind farm was installed, commissioned, tested and operated with different loads and different wind conditions. It was checked the operation of the system was stable thanks to a double control: "pitch", under high wind conditions (generated power > consumed power), and "load", under low wind conditions (generated power < consumed power). In this way the isolated grid frequency was controlled between 48 and 52 Hz. 3.3. Description o f the learnt experience Different kinds of technical and practical problems were detected. This section describes the most relevant situations that happened during the project. 3.3.1 The electric system One of the most important inconveniences was the start-up when wind speed was close to the lower limit (average value of 6 m/s during 5 min). The total process lasted about 30 min; this situation can delay the process 1 or 2 h, or even more, depending on the day. Several minor failures in small components of the electric system came into view, the most common being located in the orientation blade motors of the wind turbines; this is presumed to be due to the continuous regulation to keep the nominal frequency (52 Hz) stable. On the other hand, during the first years of the project, the Enercon Company carried out many tests to improve the start-up process; they concluded it was very difficult to generate the isolated grid directly with the synchronous machine, so they solved the problem by installing an asynchronous motor mechanically connected to the flywheel to start it up. Two main difficulties appeared in the operation of the flywheel: • high mechanical friction, which consumed an
43
important amount of energy. It especially was affected when the wind power was down and loads had to be switched off; and overheating one of the bearings with the consequent fault and halting of the start-up process. After installing the complete system, two problems were found inside the flywheel building: high temperature due to the heat produced by the flywheel-synchronous machine component, and the presence of dust from the outside. The ceiling of this building was designed to be mobile in order to install the flywheel into it by a crane, so the highest part of the walls were open, and the wind brought sand, dust and humidity inside. 3.3.2. The hydraulic system 1. Pumps and pipes. A different set of mechanical problems appeared in the pumps: vibrations, heat, insufficient flow, and overload. Particular mechanical problems were found during off-grid operation since the frequency was slightly higher (52 Hz). A particular point should be mentioned: the pressure variations in the feed water pipe. Initially, a solenoid butterfly by-pass valve was installed in the RO circuit feed water pipe to get a constant pressure independent of the number of connected RO units. It seemed an easy control, as the pump operation curve was known and a pressure transmitter provided the instantaneous value. The experience demonstrated that it was not possible to reach a stable pressure with enough speed; only a short period of time was available since one RO unit starts up in 1 min. The water is taken from a beach well 35 m deep, located at 100 m from the coast; the advantage of this configuration was checked with the introduction of marine life, and consequent fouling has been avoided. 2. RO plant: Apart from the normal operation problems such as leakages, dirt and other less
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v.J. Subiela et al. / Desalination 168 (2004) 39-47
frequent situations, specific difficulties appeared during the off-grid operation, caused especially by the step-loads connection. If the stand-alone grid went down due to lack of wind or other problems, the loads were progressively switched off. When this happens quickly, the remaining energy in the flywheel is not enough to supply power to wash the membranes of the connected RO units. However, each unit is provided with a fresh water tank on the top of the unit to feed fresh water into the membranes. It was found that the volume was not enough to complete the washing. The practical experience was complemented with a thorough theoretical analysis [10,11]. 3. VC unit: VC start-up was quite long (90 min) consuming 20 kW, and then the normal operation (30 kW in the compressor + discontinuous 10 kW in the electric heater). The on-grid tests demonstrated that the unit was not designed to operate under discontinuous power supply. After several weeks of start-and-stop operation, the plant stopped working altogether. The evaporation chamber was opened and a hard layer of scale was discovered. After acid cleaning, the plant was restarted, but as it was not operating 24 h/d, the problem appeared again. A full detailed report about the modelling, technical description and comparison analysis of the process can be found in the literature [12-14]. 4. EDR unit: The operation of the EDR unit required the preparation of the desired brackish water to feed by mixing seawater and freshwater in 2x5,000 1 tanks. It was not possible to reach a complete mix, and thus the feed water had variable salinity. However, the most serious problem with this plant was the generation of distortions in the electrical system in the form of current harmonics caused by the DC drivers (power converters for electricity supply to the membrane stacks) and the frequency converters in the pumps to control the plant flow rate [15]; this phenomenon was also detected in the VC plant, caused by the frequency converter of the
compressor. The most significant consequences were noise and heating in the RO high-pressure pumps. More technical information was presented by Veza et al. [15-17]. 3.3.3. In the control system
This has been probably the most critical point. On several occasions the control PC failed to stop the recording and control processes. It is thought that the simultaneous recording and control processes could cause isolated overloads in the computer. Part of the failures could be due to the operative system (WINDOWS 98). A very successful operation was observed in the PLCs as no failure was detected since the beginning of the project. An extensive description of the concept includes technical details [ 18].
4. Lessons learnt
The lessons learnt during the project can be divided into two categories: tested solutions and solutions to be applied in the future. 4.1. Tested solutions 4.1.1. In the electric system
The performed tests demonstrated the stability of the whole system. A thorough analysis with graphic information from real data was carried out by Carta et al. [7]. The flywheel was partially dismounted to determine the overheating of the rear bearing; it was substituted and correctly lubricated. The overheating in the flywheel building was solved by analyzing the movement of air inside the building and installing a fan on the ceiling. The intake of wet air was avoided by screwing a piece of impermeable canvas down the open part of the ceiling. 4.1.2. In the hydraulic system
The advantages and disadvantages of each desalination process operating under variable
V.J. Subiela et aL / Desalination 168 (2004) 39-47
45
Table I Relation of the main advantages and disadvantages of each desalination process in isolated system operation Desalination system
Advantages
Disadvantages
RO
• Fast start-up and stop • Absence of harmonics • Low specific energy consumption • Variable continuous power consumption
• •
VVC
EDR
• •
Variable continuous power consumption Fast starting-up and stop
Conditions were identified. It was concluded that the most suitable process was RO, mainly due to the fast start-up and the absence of an harmonics generation (see Table 1). After several months o f testing, a specific know-how was acquired which is based on a set o f recommendations (see Section 4.2.2). 4.1.3. In the control system After several checks, correct operation was found for all the software involved in the processes. A compatible version of each software was installed and the control software was simplified. 4.2. Proposed solutions (for future systems) 4.2.1. In the electric system The use of high variable speed flywheels has already been proposed as an interesting way o f improving the efficiency in the operation o f stand-alone grids. The R&D centres at CIEMAT and CEDEX have been researching this field [ 19]. On the other hand, a better mechanical and aerodynamic design should be done to increase energy efficiency. These subjects imply a specific engineering development, adapted to each system
Discontinuous power consumption Pressure control in the feed water circuit
• S l o w start-up • Scaling if discontinuous operation • Harmonic distortion • Specific stable temperature and pressure • Only for brackish water • High harmonic distortion (due to the conversions DC/AC/DC)
to be projected. The proposal of a hybrid system (diesel + wind) has already been defined [20]. 4.2.2. In the hydraulic system Different correction and improvement measures were proposed for each desalination process: 1. Reverse osmosis: The first proposal was to substitute the eight units by one bigger variable flow unit; this solution would need a specific frequency converter and variable flow seawater pumps. This option has already been installed in recent conventional RO desalination plants [21 ]. As previously mentioned, this device would produce harmonics that should be avoided by appropriate filters. On the other hand, a specific design of the plant should be developed, including strong components, high-quality materials, high-resistance membranes, and a suitable solution for selfwashing during breaks. If the pumps are still operating, allow the intake of seawater from the feed water (it is better to have seawater than brine in the membranes) or include a fresh water tank with the correct size. The sudden change of pressure in the membranes during breaks (from 60 to 1 bar in a very short time) is a strong load and could be avoided by installing a solenoid
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V.J. Subiela et aL / Desalination 168 (2004) 39--47
needle valve to control the pressure at each moment. Finally, a reduction in power consumption by installing an energy recovery element (turbine, pressure exchanger) is recommended. 2. Vapour compression: A special design would be necessary in order to prepare the plant for interrupted operation. Some ideas should be studied: inclusion of a hot water storage system with solar thermal energy contribution to reduce start-up time, addition of a specific seawater washing of the evaporation plates to remove the rests of concentrated brine, improvement of the sealing system to preserve the vacuum for a longer time. 3. Reverse electrodialysis: A specific filter or filters should be designed to avoid the variety of harmonics generated in the plant. As this system has a limitation in the salinity of water, seawater could be used as feed water ifa previous RO unit was installed; a low-quality fresh water RO unit (old/used membranes) would be the ideal plant, as membrane life would be extended. The pumping system could also be significantly improved. Several pumps set could substitute for only one pump. The system should get maximum efficiency for each possible flow. Working with only one pump does not reach this result as different operation points (one for each flow) have to be used. The best control pressure in the feed water would be interesting; a special needle valve should be installed to guarantee the stability of the pressure.
The conclusion was that although it is practical to have the control software in the PC, so that modifications can be done quickly, the software must be installed in a stronger medium - the main PLC - - and prepare the PC only for illustrating online information and the recorded data. The new wind speed prediction models could be included in the control software to startup the stand-alone system at the most suitable moment.
4.3. In the control system
Acknowledgements
Intense efforts were carried out to solve the problems of this part: increase the RAM, substitution of the PC, modifications of the control software, among others. In spite of all these attempts for improving the operation of the PC, the failures were not totally eliminated.
This project was co-financed by the European Commission (Joule III Program, Contract No. JOR3-CT95-0077), it was co-ordinated by ITC, with the participation of other European partners: ULPGC (Spain), Der-Ciemat (Spain), Enercon (Germany), Crest (UK) and NeI (UK).
5. Conclusions One of the most recommended tasks to do after a long-term project is to look back at all the work done and analyze what was good and bad. An assessment activity has shown us which parts of the project should be repeated, modified or removed, and which additions should be included for future experience. The ideas presented are a set of recommendations to be taken into account before designing seawater desalination systems by autonomous wind energy systems. Concerning coordination and organizational aspects, it is important to note that a multidisciplinary project like this requires a complete technical team with different specialists. Contributions from suppliers are also indispensable, and specific and continuous contacts should be established with them in order to develop specific technology and technical components for these kinds of systems.
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References [1] R. Clarke, Water: The International Crisis. MIT Press, 1993. [2] P. Gleick, Water in Crisis. Oxford University Press, New York, 1993. [3] R. Petrella, The Water Manifiesto:Arguments for a Word Water Contract. Zed Books, 2001. [4] V. Belessiotis and E. Delyannis, Water shortage and renewable energies (RE) desalination possible technological applications. Desalination, 139 (2001) 133-138. [5] Facts and figures: Water and Health. Official site of the International Year of Freshwater 2003, UNESCO, UN, http://www.wateryear2003.org/en/ ev. hp@URL_ID= 1600&URL_DO=DO_TOPIC& URL SECTION=201.html. [6] Fundaci6n Centro Canario del Agua, http://www. fcca.es/Docs/Listadesaladoras.xls. [7] J.A. Carta, J. Gonz~lez and V. Subiela, Solar Energy, 75 (2003) 153. [8] J.A. Carta, J. Gonzfilez andV. Subiela, Desalination, 161 (2004) 33-47. [9] I. Cruz, A. Gonz~lez, R. Calero, C. Rodriguez, J. Gonz~lez and J.A. Carta, SDAWES desalination plants connected to an autonomous wind energy system, Proc. European Union Wind Energy Conference, Goteborg, Sweden, 1996. [ 10] D.G. Infield and Z. Rahal, 'Computer modelling of a large-scale stand-alone wind-powered desalination plant, in: Wind Energy Conversion from Theory to Practice, R. Hunter, ed., Mechanical Engineering Publications, Edinburgh, 1997, pp. 129-134. [11] Z. Rahal, and D.G. Infield, Wind powered stand alone desalination, European Wind Energy Conference, IWEA, Dublin Castle, Ireland, 1997, pp. 802-806. [12] G Skivington, A review of vapour compression processes for the seawater desalination and wind energy systems (SDAWES) project, National Engineering Laboratory, Report No. 271/96, 1997.
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[13] G.R. Skivington, Mathematical model of a vapour compression desalination plant for autonomous application of wind power, European Wind Energy Conference (EWEC'99), Nice, France, 1999. [14] M. McCourt and G.R. Skivington, The vacuum vapour compression plant, National Engineering Laboratory. Fourth Annual Progress Report for SDAWES, Contract JOR3-CT95-0077, 1999- 2000. [ 15] J.M. Veza, B. Pefiate and F. Castellano, Desalination, 141 (2001) 53. [16] J.M. Veza, B. Pefiate and F. Castellano, Electrodialysis desalination in an off-grid wind energy system, Proe Conf. Policies and Strategies for Desalination and Renewable Energies, Santorini, Greece, 2000. [17] J.M. Veza, B. Pefiate and F. Castellano, Desalination, 160 (2004) 211-221. [18] J. Gonz~lez, R. Vega, J.A. Carta, W. Janssen, R. Calero and J. Caballero, A control system design for an autonomous wind-park with different types of desalination plants in the Canary Islands, Proc. European Union Wind Energy Conference, Dublin, Ireland, 1997. [19] I3. Iglesias, L. Garcia-Tabarts, A. Agudo, I. Cruz and L. Arribas, Design and simulation of a standalone wind-diesel generator with a flywheel energy storage system to supply the required active and reactive power, Proc. 31 stAnnual Power Electronics Specialists Conference, Vol. 3., pp 1381-1386. [20] L.M. Arribas, I. Cruz, R. Calero, V. Subiela, J.A. Carta, A. Beekmann, D. Infield and M. McCourt, The SDAWES project: a hybrid system? Proc. European Union Wind Energy Conference, Kassel, Germany, 2000. [21] J. Sadhwani and F. Deniz, Behaviour of the asynchronous electric motors to reduction of energy consumption in reverse osmosis plants. Desalination, 157 (2003) 1-8.