SDAWES PROJECT (SEAWATER DESALINATION WITH AN AUTONOMOUS WIND ENERGY SYSTEM)

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1 SDAWES PROJECT (SEAWATER DESALINATION WITH AN AUTONOMOUS WIND ENERGY SYSTEM) R. Calero 1, V. Subiela 1, J.A. Carta 2, A. Beekmann 3, I. Cruz 4, D. Infield 5, M. McCourt 6 (1)CIEA-ITC,SA,(2)DIM-ULPGC,(3)ENERCON,(4)IER-CIEMAT,(5)CREST, (6)NEL (1) Cebrián, Las Palmas de G.C., Spain. Fx (34) (2) Campus de Tafira. Gran Canaria. Spain. Fx (34) (3) Dreekamp 5. D Aurich. Germany. Fx (49) (4) Avda. Complutense Madrid. Spain. Fx (34) (5) Loughborough University, Leicestershire, LE11 3TU, U.K. Fx (44) (6) East Kilbride, Glasgow, Scotland, G75 OQR, U.K. Fx (44) ABSTRACT: The SDAWES project pursues the use of a natural renewable resource: wind, to produce a natural scarce resource: water. It consists in connecting three kind of desalination systems: Reverse Osmosis (RO), Vacuum Vapour Compression (VVC) and Electrodyalysis Reversible (EDR) to an off grid wind farm to produce fresh water on a significant scale. The main objectives of the project are to identify the best desalination systems for connection to an off grid wind farm and to assess the influence of the variations of the wind energy on the behaviour of the desalination plants elements and on the quality of the produced water. This project is located in Gran Canaria island (Spain) and is co-financed by the European Commission, (Joule III Program); it began in February 96 and it will finish in July INTRODUCTION Fresh water supply is a historical problem in the archipelago of Canary Islands, and particularly in Gran Canaria. The increase in population and the rainfall reduction has intensified the situation in the last years. On the other hand, the island has an important natural resource: wind energy, which cannot be completely used to produce electricity due to the limit of connections to the weak general grid; therefore stand-alone desalination is an interesting way of using this remaining energetic resource. The University of Las Palmas has been working in this subject for several years, as it has been referenced in the previous publications regarding the SDAWES project, [1], [2]. The partners involved are Centro de Investigación en Energía y Agua (CIEA) from the company Instituto Tecnológico de Canarias, S.A. (ITC, SA) as co-ordinator of the project; three departments of University of Las Palmas of Gran Canaria (ULPGC): Department of Mechanical Engineering (DIM), Department of Electronics, Telematics and Automatics (DETA) and Department of Process Engineering (DIP); ENERCON, the supplier of the wind turbines; the research centre Instituto de Energías Renovables of Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (IER- CIEMAT), and two research centres from U.K.: Centre of Renewable Energy Systems Technology (CREST), and National Engineering Laboratory (NEL).

2 2 PROJECT OBJECTIVES Although a preliminary report of the project was already done [1], a more extended and updated explanation is described in this paper. The main objective of the project is to identify the best desalination systems for connection to a medium power off-grid wind farm. This objective is developed according to the following points: Design of a wind farm to be operated isolated from the grid Determination of the behaviour of each desalination system (RO, VVC, EDR) working under intermittent and variable load operation Design, installation and working of a RO system with several units, making possible the connection and disconnection of each unit as function of the instantaneous power. Determination of the life of the membranes working under intermittent operation Determination of the water production quality in function of the variations of the wind Assessment of the advantages and disadvantages of each desalination system working in the isolated system: determination of the optimal design of each plant. Adaptation of the VC and the EDR plants to work connected to an off grid wind farm: definition of the working conditions and limits. Design, installation and assessment of a control system to make possible the automatic working of the system. 3 BASIC METHOD OF APPROACH A general view of the installation can be seen in figure 1. The elements of the complete system are the following: Fig. 1: General view of the installations. (1) Pumping Station. (2) Product water tank. (3) Brackish water tanks. (4) Desalination dome. (5) Flywheel building. (6) Wind Turbines. (7) Feed water pipe circuit Wind Farm It is composed by two 230 kw wind turbines, a 1,500 rpm flywheel coupled to a 100 kva synchronous machine, an isolation transformer and a UPS of 7.5 kw. 3.2 Desalination Plants There are ten plants installed: Eight reverse osmosis (RO) units (25 m3/d each), with a specific consumption of 7.2 kwh/m 3 One vacuum vapour compression (VVC) plant unit (50m 3 /day),

3 working at 0.2 bar with a specific consumption of 16 kwh/m 3, and a variable speed compressor ( rpm). One unit of electrodialysiselectrodialysis reversible (EDR) with a production of 190 m 3 /d, with a specific consumption of 3.3 kwh/m 3, and a variable production: 35% - 100% (obtained by the variable feed flow and the variable stack voltage). 3.3 Water Circuits In the pumping station there are two seawater pumping groups one for the RO plants (2 x 13 kw), and the other for the VVC (2 x 9 kw). The water is taken from a well of 35 mts. deep, located at 100 mts. from the coast; this configuration avoids the introduction of marine life, and the consequent fouling. There are four pipe circuits: Two feed water pipes: one for RO plants, and other for the VC plant A product pipe, from the desalination area to the 200 m 3 product tank A brine pipe, from the desalination dome to a specific brine well. As there is no natural brackish water source, the EDR plant is connected in a closed circuit. An artificial brackish water was prepared by mixing distilled water and seawater; this water is stored in two tanks. These tanks feed the plant, and the outputs of the plant (desalted water and brine) are introduced in the tanks again. 3.4 Control System The control system is composed by the following elements: A seven PLC network: a main PLC, connected to the control PC and six secondary PLC s connected directly to each part of the system: one to the wind farm, two to the RO plants, one to the VC plant, one to the feed water pipe and pumps, and one into the EDR plant. A control PC, where the specific control software is installed An acquisition data PC, where the data is stored and analysed The information goes in both directions through the network: the secondary PLC s take the signals from the plants and send them to the main PLC and the main PLC send the orders from the control PC to the different parts of the system. 4 WORKING OF THE SYSTEM When the start-up signal is given, the system measures the wind speed and decides if there is enough wind to start up the isolated system (minimum average of 6 m/s during 5 minutes or similar). Under these conditions, one of the wind turbines starts to accelerate the flywheel 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 the upper speed limit of 52 Hz. From that moment the normal loads can be connected; the WT will 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 the frequency and request a reduction in the consumption by disconnecting

4 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. The system has two control modes: from the wind farm (in case of excess of wind) and from the loads control (in case of shortage of wind). A complementary explanation can be consulted in [2]. 5 PROGRESS SO FAR As a general assessment at this point of the project (more than four years since the beginning) it can be said that as a original R&D, several unexpected difficulties have appeared, which have forced the partners to create original solutions. It has meant, on the one hand, a cost in time and in money; and on the other hand, a very interesting learning experience. The different tasks developed according to the plan presented to European Commission is described as follows: 5.1 Task 1: Wind Farm The wind turbines were installed in Due to problems with local regulations, there was a delay in the commissioning of the machines (May 98). The rest of the elements were installed in July 99 (see the flywheel in figure 2); the off grid wind farm was started up in August 99. A team of the ULPGC achieved the automatic (from the control PC) starting up of the off grid wind farm in November 99. ENERCON has developed the elements Fig. 2 Flywheel and synchronous machine. design and the power control software, in co-ordination with the partners involved in this task (CIEA-ITC, SA and DETA-ULPGC). A technical team from ENERCON came to Gran Canaria to finish the installation of the isolated system and to start up the off grid wind farm; CIEA-ITC, SA co-ordinated all the necessary cabling installation works. Fig. 3. View of Reverse Osmosis units CREST and IER-CIEMAT have developed a short term simulation

5 considering the load variation and the wind speed data at Pozo Izquierdo. 5.2 Task 2: Reverse Osmosis Plants A view of this installation can be seen in figure 3. The eight 25 m 3 /d reverse osmosis (RO) plants were designed, constructed and ordered in Due that the civil works at Pozo Izquierdo were not finished, part of the time the plants were stored in an external building; the definitive installation at Pozo Izquierdo was carried out in One of the plants was started up in August 1998 in a private desalination plant; the rest of the plants were started up in April CIEA-ITC, SA, in collaboration with DIM-ULPGC elaborated the seawater feed system design; later both partners worked in the installation, in the tuning and the starting up of the plants. It was necessary to order a local private company to do these tasks. CREST has developed a complete model of the internal behaviour of the RO plant and the working of the plant connected to a variable power supply. 5.3 Task 3: Vapour Compression Plant A photo of this plant can be seen in figure 4. The 50 m 3 /d Vapour Compression (VC) plant was ordered in December 97. It was installed and started up in July 98. The supplier, in coordination with CIEA-ITC, SA, designed the plant in order to modify the standard design to adapt it to the project specifications. The plant works in vacuum (0.2 bar) and is provided with a variable speed compressor. The modelling and simulation of the plant has been developed by NEL. In co-ordination with CIEA-ITC, SA, a set of tests were performed in the installed plant (February 99) in order to collect data to validate the model. The plant was working normally until March 99, when scaling problems were detected. After several contacts with the supplier, they gave the instructions to solve the problems; the plant was cleaned up (November 99) and re-started up in January New difficulties appeared with new delays, and the plant began to work normally in May Fig 4. Vapour Compression plant 5.4 Task 4: Electrodialysis Plant A photo of this plant can be seen in figure 5.

6 Several simulations were done by DIP-ULPGC using the software of the supplier. As a result of this analysis, the design of the plant was decided. On the other hand, CREST has developed a specific model. The plant was ordered in December 97 and installed in August 98. A set of modifications was prepared in order to adapt the plant to the specifications of the system. As this desalination system works only with brackish water, and there is no natural source, several options were studied to feed the plant. For research and economical reasons, it was decided to mix seawater and distilled water (from the VC plant) in a tank and work in a closed circuit as a preliminary solution; other possibilities will be studied in the future. Fig. 5. Electrodialysis plant isolated system has been simulated. The plant was started up in April 99. Since then, several tests in grid connection have been performed; in this way, the different working points have been defined and the working in 5.5 Task 5: General Control System The control network composed by the PLC s and the PC s was designed at the beginning of the project (1996). An specific software has been programmed by DETA-ULPGC in order to control the loads connected to the off grid wind farm. The plants and the wind turbines can be switched on and off from the PC; the point of working in VC and EDR plants can be changed from the PC as well. The PLC s were acquired in December 98 and installed in April 99. The relevant cables (connection to the plants and connection among the PLC s for communication) were installed in June 99. After finishing and testing this installation, it was possible to start up the isolated system (wind farm, feed water pump and RO plants) from the PC control (December 99). 6 MAJOR FINDINGS The preliminary major findings are the following:

7 6.1 Checking the stability of the system The stability is possible due to the double control: from the wind farm: from the wind farm, by changing the blade angle in case of excess of wind; and from the control system, by reducing the power consumption in case of lack of wind. 6.2 Determination of the pressure control in the RO feed pipe Depending on the number of the connected RO plants, the flow changes and varies the pressure; several tests were performed to determine the control of the pressure. 6.3 Optimisation of the system (wind farm with RO) A simulation model has been used to identify the optimal installation of RO plants connected to an off grid wind farm. It has been decided to use only RO plants because it is the most suitable desalination system for seawater with the smallest specific consumption. Product Flow m³/year m3/d Plants Fig.6. Rejected energy and water production as function of the plants % Reyected Energy Specific Cost (ptas/m 3 ) Number of Plants Fig. 7. Specific cost as function of installed plants The figure 6 shows the rejected energy and the production of water depending on the number of RO plants installed; the production of water increases with the plants and the rejected energy decreases, because with less loads it is more difficult to adapt the consumption to the available power. In the figure 7 it can be seen the specific investment cost in relation with the number of plants, showing that there is an optimal number of plants to get a minimum cost. 7 POSSIBLE BREAKTHROUGHS Although a more complete analysis of the results will be done at the end of the project, some breakthroughs can be foreseen: 7.1 Operation of an off grid wind farm The starting up and operation of two medium power wind turbines working in parallel within an isolated system has been an original achievement of this project.

8 7.2 Determination of the optimal desalination system powered by wind energy It is one of the main objectives of the project. For the moment, preliminary aspects have been concluded about the advantages and disadvantages of each desalination system. (See table 1). Table 1. Relation of the main advantages and disadvantages of each desalination system in isolated system operation. Desalination Advantages Disadvantages system RO Fast starting-up and stop Discontinuous power consumption Difficult pressure control in the feed water circuit VVC Variable continuous power consumption Slow starting-up Scaling if discontinuous operation EDR Variable continuous power consumption Fast starting-up and stop Only for brackish water Harmonic distortion (due to the conversion AC/DC) 7.3 Determination of the modifications in the desalination systems in order to improve the working in an isolated wind grid The suppliers of the VVC and EDR plants prepared an specific design to include the possibility of a variable power consumption in order to achieve a better connection to the off grid wind farm; however, a more complete analysis should be done. The installed RO system does not include any modification, hence there are important possibilities to improve the system in future projects, for instance the substitution of several small plants by one sole big plant with a variable flow high pressure pump. 7.4 Assessment of the influence of several system parameters in the water quality This is a future breakthrough because several tests are necessary to conclude the variations in the water quality. 8 MAJOR OBSTACLES Many difficulties and obstacles have appeared along more than four years of working in the project. From a technical point of view the main problems have been the following: 8.1 Control program debugging It has been necessary to modify several times the original software to solve all the control problems that have appeared during the tests. 8.2 Malfunctions in electronic instruments There are many electronic instruments installed to take the signals (more than 130) which will be recorded in the acquisition data PC. Due to different reasons (wrong connections, low quality of the equipment, difficulties in the calibration) several failures have happened.

9 8.3 High harmonic distortion The EDR plant operates in DC, therefore it includes converters AC / DC. There are more converters in that unit (pumps) and in the VC unit (compressor). These elements have been causing harmonic distortion and excessive reactive power consumption (power factor less than 0.5 in EDR unit). A deeper analysis will be done to know all the aspects of this problem and the possible solutions. 9 APPLICATION PERSPECTIVES A first meeting was celebrated in October 1999 to deal with the future plans of the project; as there was no previous experience in this area, the main objective was to define a framework for future agreements about exploitation plans. As a result of that meeting, the present intentions of the partners for future applications can be described. 9.1 Informative actions As a result of all the obtained data, analysis and experiences, a great information can be popularised by means of conferences, publications and thesis. In fact, partners of the project have already published several papers, and there are more planned publications. 9.2 Social actions The lack or bad conditions of drinking water supply is an important problem in many parts of the world, mainly in developing countries, where 50% of people do not have access to fresh water and 80% of illnesses are related to bad quality of water [3]. This project offers an interesting possible solution in those places with wind energy resources and problems with fresh water supply: coastal (seawater) and continental (brackish water) areas isolated from the grid and in small islands (weak grid). On the other hand, the development of this kind of projects will contribute to the spawning of the local employment through renewable energy enterprises. 9.3 Environmental actions Desalination is a high power consumption process that normally needs electricity (about 5 kwh/m 3 in case of big size RO plants with energy recovery). At present, fossil or nuclear fuels generate most of electricity. The substitution of the conventional generation systems by a pollution free one as wind energy means important environmental benefits: reduction in CO 2 emissions (about 4 kg for each m 3 of produced water considering 5 kwh/m 3 [4]), reduction in acid rain gases (SO x, NO x ), elimination of nuclear accidents and nuclear wastes. 9.4 Industrial actions A technical and economical viability study is included as one of the main parts of the project. This is the basic document to prepare a future economical exploitation plan. Contacts with companies should be done to include all the conclusions of the project regarding the modifications in the design of desalination plants to improve the connection to wind energy generation system.

10 As a preliminary economical analysis, a simulation software has been programmed to know which is the optimal installation of desalination plants (only RO) connected to an off grid wind farm. The results showed it would be possible to produce water with a competitive cost (about 0.6 euros/m 3 ). 10 CONCLUSIONS AND FUTURE WORK SDAWES is a prototype project to develop in depth knowledge of the combination of desalination systems and wind energy. After obtaining the conclusions of this experience, an important specific knowledge will have been acquired in order to design more and more improved systems. The main lines are the following: 10.1 Advanced optimisation of the existing system Although a preliminary optimal system has been defined, a deeper analysis should be done to determine all the aspects and details of the installation Development of specific designs of desalination plants As it has been mentioned, an analysis should be done to assess the improvements in the desalination plants to obtain specific designs adapted to be connected to an off grid wind farm. The variation in wind speed means variation in the frequency; therefore one of the aspects of new plants design is the possibility of variable frequency operation Development of a larger system SDAWES is a project to produce water on a significant scale. Therefore, one of the future objectives is to design larger systems; wind turbine technology has already developed medium and high power machines (600 1,500 kw) Improvement in energy storage The flywheel is the storage energy element, in SDAWES it operates at 1,500 rpm. The design of higher speed flywheel will improve the energy storage and thus system operation. The future research requirements that can be emphasised are the available financial support, the participation of wind turbines and desalination plants manufacturers and the availability of a trained researchers team. 11 REFERENCES [1] Cruz, I. et al. Sewater Desalination Plants connected to an Autonomous Wind Energy System. Proceedings of European Union Wind Energy Conference, Goteborg, May [2] González, J. et al. A control system design for an autonomous wind park with different types of desalination plants in the Canary Islands. Proceedings of European Union Wind Energy Conference, Goteborg, May [3] European Commission, (DG XVII), Desalination Guide Using Renewable Energies, Greece, [4] Own calculations considering a conventional fuel oil power plant.