1EXPERIMENTAL RESULTS OF AQUASOL PROJECT: DEVELOPMENT OF AN ADVANCED HYBRID SOLAR-GAS MULTI-EFFECT DISTILLATION SYSTEM Diego Alarcn, Julin Blanco, Ana Lozano, Sixto Malato, Manuel I. Maldonado, Pilar Fernndez Environmental Applications of Solar Energy and Characterization of Solar Radiation CIEMAT-Plataforma Solar de Almera. P.O. Box 22, E-04200 Tabernas, Almera, SPAIN. Tel.: +34 950387960, Fax: +34 950365015, E-mail: diego.alarcon@psa.es Abstract Main objective of R&D AQUASOL Project has been the development of a zero-discharge, improved cost and energy-efficient seawater desalination technology based on the multi-effect distillation (MED) process. During the course of the demonstration phase, an advanced hybrid solar/gasdesalinationsystemhasbeenimplementedatthePlataformaSolardeAlmera installations, in order to evaluate under real atmospheric conditions its reliability and energy efficiency. This paper shows the first results obtained of the MED unit performance working with a 500-m2 CPC solar collector field and the new double-effect absorption (H2O-LiBr) heat pump, which has allowed a very important increase in the overall thermal efficiency of the process. Keywords: solar desalination, multi-effect distillation, absorption heat pump ------------------------------------ Introduction Intheyear2003Mankindconsumed10723Mtoe(milliontonsoilequivalent),oftotal primary energy supply (TPES), [IEA, 2003] and, the same year, it was scheduled an scenario of growth of 0.7% in the oil production until 2030 and then start to declining [1], realizing thattheoileraasdominantenergyfactorwillbeoverinthemediumterm.Possible alternative primary energy sources are also very problematic: nuclear energy in addition to the strongpopularcontestinmanypartsoftheWorld,hasalsolimitedfissionableuranium reserved(inthelongterm)andthesecurityconcern(potentialfabricationofweapons)of many countries and coal has the problem of very high CO2 emissions and its repercussions over climate change issue [2]. If this was the prospect in 2003, today many people would agree that this forecast is even darker with the main conclusion that there is no solution to any sustainable energy future without a strong participation of the renewables in general and the solar energy in particular, due to its highest potential among all existing renewable energies [3]. If the energy prospect is worrisome, much worst is the problematic related to water shortage. Water is essential to all life and today more than 1 billion people lack access to safe drinking 2water, being unsafe water and poor sanitation the cause of 80 percent of all diseases in the developing world, with more than 5 million deaths annually. Groundwater supplies about one third of the worlds population with water tables falling, in some cases, by 1 to 3 meters a year.Ifthepresenttrendcontinues,twooutofthreepeopleonEarthwillliveinwater stressed areas by 2025 [4]. The consequences of this analysis are very serious as the water problem can not be effectively addressed without considering the energy implications and the growth of human population: large additional amount of water will be needed within a few decades which are not only unavailable from the existing renewable resources but also the energy to produce it will not be easily available.Desalination is, many times, not only an interesting option but the only feasible and practical option as about 40 percent of world population lives in a 70 km strip from sea border [5]. In 2003, World installed desalination capacity was 37.75 hm3/day [6], being 64 percent of them fromseawater,with10350plantshavingacapacityhigherthan100m3/dia.Today,total productionofdesalinatedwatercouldcoverthenecessitiesofapopulationofabout100 million people [7]. First desalination country is Saudi Arabia, followed by Arab Emirates, United States of America and Spain. Market studies (Global Water Intelligence) showed estimated investments of more than 30 billionUS$innewdesalinationplantsworldwideintheperiod2005-2015,70percentof which would be of seawater. In the Mediterranean area, the estimated figure is 9.6 billion US$ (90 percent seawater). It is clear that the majority of these areas enjoy the availability of a large amount of solar energy and, if the energy situation is also considered, the solar option to drive desalination processes seems not only fully logical but, in the medium range, absolutely necessary. Therefore, it is clear that scientific and technological developments will be needed; this paper presents one of these developments. The AQUASOL project TheAQUASOLproject(EnhancedZeroDischargeSeawaterDesalinationusingHybrid Solar Technology), was the continuation of a large previous research of solar desalination carried out at Plataforma Solar de Almera [8, 9], widely described in many previous papers [10].Theproject(2002-2006)wasfinancedbytheEuropeanCommissionandwas successfully carried out with the collaboration of the following partners: Spain: CIEMAT, INABENSA, CAJ AMAR, Comunidad de Regantes Cuatro Vegas Portugal: INETI, AO SOL Energias Renovveis Greece: Hellenic Saltworks, National Technical University of Athens France: WEIR Entropie The project was focused in the technological development of specific technological aspects thatareexpectedtosignificantlyimprovethepresenttechno-economicefficiencyofsolar MEDsystemsandtherefore,reducethecostofwaterproduction.Tothisobjectivethe following main subsystem were modified (MED plant) or designed and erected (all others): 3A multi-effect distillation plant (14 effects, 3 m3/h nominal distillate production) A stationary CPC (Compound Parabolic Concentrator) solar collector field A thermal storage system based on water (total volume: 24 m3) A double-effect (LiBr-H2O) absorption heat pump A smoke-tube gas boiler An advanced solar dryer for final treatment of the brine (not installed at PSA) Figure 1. Conceptual layout and main subsystems of AQUASOL plant Figure 1 shows the configuration and interconnection of these main subsystems. The overall systemwasdesignedtomakefeasiblethefollowingthreedesalinationoperatingmodes depending on where the desalination unit energy supply comes from: Solar-onlymode:energytothefirstdistillationeffectcomesexclusivelyfromthermal energy from the solar collector field. Fossil-only mode: the double-effect absorption heat pump (DEAHP) supplies all of the heat required by the distillation plant. Hybridmode:theenergycomesfromboththeheatpumpandthesolarfield.Two different operating philosophies were here initially considered: -Theheatpumpworkscontinuously24hoursadaywitha30%minimum contribution. -Start-up and shutdown of the pump when requested, depending on the availability of the solar resource. Solar-only experiments Solar energy is collected by the solar field constituted by 252 stationary solar collectors (CPC Ao Sol 1.12x) with a total surface of 498.96 m2 arranged in four rows of 63 collectors each; 4Collectors inclination is 35 degrees and are arranged in reverse feeding mode to achieve equal distribution of total flow rate (14.97 m3/h, nominal) into the 4 rows (3.74 m3/h, each) without further regulation. Technical specifications of the collector field were previously described [11].Experimental procedure was the following: once a defined threshold (of about 300 W/m2) of solar radiation was available, the solar field pump was connected and water from the bottom of secondary water tank was recirculated throughout the solar field heating the piping and collector water volume. This scheme is maintained until the outlet temperature from solar fieldis8degreeshigherthantheoneatthebottomofsecondarytank(inletsolarfield temperature). At this moment the water is transferred to the upper part of the primary tank (see Fig. 1). Solar field main pump is stopped when the outlet temperature is only 2 to 4 degrees higher than the inlet one. During normal operation the flow rate through the solar collector field is continuously modified (using a frequency variator to drive the pump) to the achievement of a T (Toutput Tinput) of about 10 C to efficiency maximization. 01002003004005006007008009007:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00Local Time (Almera, Spain)Power (kW)0100200300400500600700800900Global Solar Radiation (W/m2)Solar power incident over collector field (35 inclination)Power delivered by sol ar collector field to the thermal storage systemSolar global radiation measuredover horizontal plane (W/m2) Figure 2. Power delivered by solar collectors (test date: 08.03.2006) Figure 2 shows the available solar radiation and the instantaneous power delivered by the solar collector field to the thermal storage tanks, constituted by two interconnected 12 m3 capacity tanks, filled with 10 m3 of water each. This volume of 20 m3 provides the overall system the needed time to switch from the solar-only mode to the fossil-only mode (30 min. estimated to reach full nominal conditions in the heat pump, working at partial load in the hybrid mode configuration) in case of a sudden change in solar irradiation conditions. The reasons to use two tanks, instead of just one, are, firstly, to increase the yearly contribution of solar energy to the system due to the physical separation between hot and cold water at solar modestart-upand,secondly,toguarantyacertaintemperaturestratificationnecessaryto avoid the heat pump water inlet temperature to exceed the permissible range (60 70 C). ConsideringthewholeexperimentaldayshowedintheFigure2(8thMarch2006),a48 5percent of overall efficiency of the solar field collector (energy provided to thermal storage tanks divided by the solar energy available) was calculated. Figure 3 shows the evolution of middle temperature within the two thermal storage tanks. 5055606570758085909510012:00 12:30 13:00 13:30 14:00 14:30 15:00Local Time (Almera, Spain)Temperature (C)020040060080010001200Solar Radiation (W/m)Global Solar Radiation over tilted surface (35)Solar field outlet temperatureSolar field inlet temperature Figure 3. Solar field outlet and inlet temperature evolution. Test date: 08.03.2006 Finally, Figure 4 shows the distillate production of the MED plant and the global thermal energy consumed. Distillate production achieved was higher than the nominal figure of 3 m3/h (for PSA MED plant), with an average thermal energy input of 69.17 kWhth/m3. This figure correspond with a constant inlet temperature in the first cell of MED plant of 71.5 C; if this temperatureisreducedto68.5C,theaverageenergyneededis67.8kWhth/m3(testof 09.03.2006). Standard thermal energy needed in conventional MED plants is in the range of 80 kWhth/m3 [12]. Electric power consumption is not considered in this analysis. 05010015020025030035040013:25 13:35 13:45 13:55 14:05 14:15 14:25 14:35 14:45 14:55 15:05 15:15 15:25Local Time (Almera, Spain)Power (kW)00.511.522.533.54Distillate production (m3/h)Distillate production (average value =3.23 m3/h)MED Power consumption (average value =223.61 kW) Test Date: 08/03/2006Consumed thermal energy / distillate production ratio: 69.17 kWh/m3 Figure 4. Solar-only mode. Distillate production and global thermal energy consumed by AQUASOL plant 6Fossil-only experiments Mainreasontoincludeafossilfuelsubsystemtoprovidetheenergyneededtodrivethe desalination process is the reduction of the amortization figure to minimize the existing gap between solar and conventional costs of different seawater desalination processes. The hybrid configuration makes possible the 24-hour operation without big and costly thermal storage systems and, therefore, significantly reducing the weight of the capital cost in the final cost of the distilled water.Another reason is the possibility, throughout the inclusion of the double effect absorption heat pump,torecovertheenergyusuallylostinthelasteffectoftheMEDplant,increasing significantly the overall efficiency of the plant. Finally, the hybrid concept makes possible the combined system avoiding the problems of operation if the solar energy is not available. The designing and construction of the heat pump with the possibility of working at partial load enable the combined operation of both systems, always optimising the performance of the overall system. Up to yet this hybrid mode has not been tested, so no operational results can be presented here. The concept of the double effect heat pump has been already described [9]. It increases the energyefficiencyofthedistillationprocessbymakinguseofthe35Csaturatedsteam produced in the last MED plant effect, which would otherwise involve the loss of the energy (about 100 kW) in the evacuation of the cooling fluid used for its condensation, as the cold focus. As the heat pump requires steam at 180 C a propane gas-fired system with a C-class smoke tube boiler was installed to provide this input energy (Fig. 5).The gas to be burnt is stored in a 2,450-liter tank providing an estimated autonomy of 143 hours at full load. Figure 5. Double effect absorption heat pump (left) and smoke tube boiler (right) designed and installed at AQUASOL plant When the heat pump is working, observed distillate production is lower than the achieved with the solar-only mode, due to the lower inlet temperature to the MED (about 63 C). The net thermal energy consumed by the plant was 37.4 kWhth/m3, figure which would be 62.4 kWhth/m3 (average values) if the energy from the 14th effect were not recovered by the heat pump (Figure 6). 705010015020025030035015:00 15:30 16:00 16:30 17:00 17:30Local Time (Almera, Spain)Power (kW)00.511.522.53Distillate (m3/h)Distillate production (Average value =2.39 m3/h)MED Input Power (Average value =149.25 kW)DEAHP Input Power (Average value =89.39 kW)Test date: 10.04.2006 Figure 6. Fossil-only mode. Distillate production and global thermal energy consumed by AQUASOL plant System efficiencies The most relevant indicator of the overall system efficiency is the Performance Ratio (PR) of theplant,whichindicatesthekilogramsofdistilledwaterproducedbyeach2326kJ of thermal energy given to the first cell of MED plant (thermal energy required to evaporate 1 kg of water at standard reference conditions of temperature and pressure). Figure 7 shows the PR obtained in the solar-only mode.02468101212:00 12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00Local Time (Almera, Spain)Performance RatioTest date: 08.03.2006Performance Ratio average value (from13:00 until 15:46 hours): 9.29 kJ /kg Figure 7. Performance Ratio of the AQUASOL plant in the solar-only mode If the same analysis is carried out in the fossil-only mode, the data showed in Figure 8 are obtained, where the PR of the system if the heat pump did not exist is also shown. 8051015202513:00 14:00 15:00 16:00 17:00Local Time (Almera, Spain)Performance ratioWith DEAHP (Average value =17.41 kJ /kg)Without DEAHP (Average value =10.03 kJ /kg)Test date: 10.04.2006 Figure 6. Performance Ratio of the AQUASOL plant in the fossil-only mode Another interesting study related with the efficiency of the system is the analysis of the losses of the thermal storage tanks. This can be made using the following formula: FIELD SOLARMEDthQU U Q2 1 + += Where QMED is the energy delivered to the first cell of MED plant, U1 is the variation of internal energy of the primary tank, U2is the variation of internal energy of the secondary tankandQSOLARFIELDistheenergycollectedbytheCPCsolarcollectorfield.Whenthis calculation is made to the overall experiment of 8th of March 2006 (solar-only mode), a figure of 3.5 percent of heat losses is obtained. When a similar approach is made to estimate the thermal losses during the night, figures between 5 and 7 percent are achieved.Discussion and conclusions This paper shows the first experimental results obtained with the AQUASOL plant working in the solar-only and gas-only modes. Plant specific consumption of thermal energy, in the solar-onlymode,isintherangeof60to70kWhpercubicmeterofdistillateproduced;these figures are reduced to 35 - 40 kWh/m3 when the heat pump is working (fossil-only mode). One interesting observed indication is that this figure seems to decrease as decrease the inlet temperaturetothefirstcellofMEDplant.Thisfact,iffullyconfirmed,wouldbevery interesting to coupling a MED plant with low temperature solar collectors, as the used CPCs, because the efficiency and operation possibilities of the overall system could significantly be increased.Tothisend,additionalexperimentswillbemadetoidentifythebestworking conditions of the plant. The thermal storage system is very efficient with reduced losses either in normal operation or during the night; in addition, this subsystem permits a simple and very flexible operation of 9theMEDplanteitherwiththesolarcollectorfieldorwiththegasandthedoubleeffect absorption heat pump. The hybrid mode (combined solar and gas energy inputs) has not been tested yet. This evaluation will be published in future papers. Acknowledgements TheauthorswishtothanktheEuropeanCommission(DGXIIResearch)foritsfinancial assistancewithintheEnergy,EnvironmentandSustainableDevelopmentProgramme (AQUASOL Project; Contract Nr. EVK1-CT2001-00102). References [1]InternationalEnergyAgency.(2003).Energyto2050,Scenariosforasustainable future. ISBN 92-64-01904-9. [2]Petit, J .R.; J ouzel, J . et al. (1999). Climate and atmospheric history of the past 420.000 years from the Vostok ice core in Antartica, Nature 399 (3J une), pp 429-436. [3]Swenson, R. (2005). The production peaks in petroleum and natural gas: information, misinformation,awareness,andimplications.ProceedingsoftheISES2005Solar World Congress, August 6-12, 2005, Orlando, FL, USA. [4]KofiAnnan,(2000).WeThePeoples.MillenniumReportoftheSecretary-General, United Nations. [5]El-Dessouky H.T. and Ettouney H.M. (2002) Fundamentals of Salt Water Desalination. Ed. Elsevier, Amsterdam. [6]Wangnick K. (2004) 2004 IDA Worldwide Desalting Plants Inventory. Report No. 18. Wangnick Consulting, Gnarrenburg, Germany. [7]Izaguirre Etxeberria, J .K. (2004). Gestin de Recursos Alternativos. smosis inversa: Desalacin de agua. Tecnologa del Agua, special number, pp. 4-11. Barcelona. [8]ZarzaE.(1991)SolarThermalDesalinationProject.FirstPhaseResults&Second Phase Description. Ed. Ciemat, Madrid[9]Zarza E. (1994) Solar Thermal Desalination Project. Phase II Results & Final Project Report. Ed. Ciemat, Madrid. [10]Zarza E. and Blanco M (1996). Advanced M.E.D. solar desalination plant: seven years ofexperienceatthePlataformaSolardeAlmera.Proceedingsofthe Mediterranean Conf. on Renewable Energy Sources for Water Production. EURORED Network. ISBN-960-90557-0-2; pp. 45-49. Santorini (Greece). 10-12 June, 1996 [11]Alarcn et al. (2005) Design and setup of a hybrid solar seawater desalination system: the AQUASOL Project. Proceedings of the ISES 2005 Solar World Congress, August 6-12, 2005, Orlando, FL, USA. [12]Ophir A. and Lokiec F. (2004) Review of MED fundamentals and costing. Proceedings oftheInternationalConferenceonDesalinationCosting,December6-8,2004, Lemesos, Cyprus. .