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THE RESEARCH OF DIRECT HEATING SOLAR SEAWATER DESALINATION SYSTEMJuyuan JiangTianjin University of Technology Tianjin 300191，ChinaRunxi SuTianjin Institute of Seawater Desalination and Multipurpose Utilization, State Oceanic Administration,Tianjin 300193, ChinaZhibin XuHimin Solar Energy Group Shandong 253090, ChinaMingxian CuiTianjin University of Technology, Tianjin 300191，ChinaABSTRACTThe new seawater desalination technology of direct heating seawater by solar energy is put forward in this article. The solar energy collector is made of newly developed PPR mill micron material, which makes it corrosion-resistant and hardly deform less than 135 ℃, so as to guarantee its sealing. This new desalination technology combines flashing with MED, which makes it possible for seawater to be directly heated by solar energy. It also fits solar energy as an unsteady heat source. Besides, the operating conditions which avoid exchanger deposition are presented. Compared with traditional technology, it can save an exchanger, avoid temperature drop of heat exchanger, raise heat efficiency, and increase the production ratio by more than 10%.1. INTRODUTIONBecause of population increasing, water extension and environment depravation, the fresh water resources run short. The seawater desalination has already become one of the major development burgeoning industries in coastal region. Currently, the day supplying water using seawater desalination is 32,400,000 m3 in the world, the cooling water for year is already over 60 million cubic meters and there is 125 nations adopted seawater desalination device [1]. In recent years，China intensified the development and application study of the seawater desalination, the day supplying water of the seawater desalination in 2005 was about 150,000 m3 [2]. Seawater desalination by solar energy is growing recognition because of using the reborn green energy, the develop countries devoted a large amount of manpower, materials and capital to research. Much experience and result of solar seawater desalination were obtained in our country, the technique level measures up to advanced world standards since the China's first demonstration device of solar seawater desalination is launched by Institute of energy, Guangzhou, Academia Sinica in the 1980s [3].2. DIRECT HEATING SOLAR SEAWATER DESALINA-TION SYSTEMSo far the method of indirect heating seawater by solar energy is adopted in solar seawater distillation desalination system both in multi state flash (MSF) and in low temperature multi effect distillation (LT – MED) [3,4,5].The freshwater yield M d (kg/h) of MSF is calculated as follows:,,()p fd f h b NfgcM M t tγ=−i（1）Where: c p,f is specific heat of seawater(kJ/（kg℃）)；M f is8 OTHER INDIRECT SOLAR TECHNOLOGIES AND APPLICATIONS2539seawater flow rate(kg/h)；t h is the highest temperature ofbrine(℃)；t b,N is the lowest temperature of brine( ℃)；γfg is latent heat of evaporation(kJ/kg).Seen from calculation (1), under the constant seawater flow rate, the fresh water yield increased with the raising of the difference in temperature between first state and the end state. Generally, because the end state temperature is conditioned by cooling seawater temperature and is already a fixed value, the method of raising first state temperature is adopted to raise fresh water yield. The output temperature of a solar energy collector system without variation in collector form has a certain maximum temperature. Our purpose is that temperature t h of the sea water coming into distil room is near to or is equal to temperature t i as possible. The system showed in figure 1, Seawater directly flew into the solar energy collector, then output to first state flash chamber. Do it this way, heat exchange loss is avoided, the first state’s evaporation temperature is raise, and the freshwater yield is decreased because of t h =t i .When the heat loss among evaporators and variety of evaporation latent heat are neglected and fresh water yield in each effect is same, the total yield of fresh water M （kg/h ）of LT-MED is calculated by)()()(3,2101t t t t t INT t t ac r t t mc M N b i h h pf i p Δ−Δ−Δ−×−+−= （2） Where: m is the flow of heating water in solar energy collector(kg/h)；a is the discharge coefficient of first effect ；Δt 1 is the rise in boiling point because of hydrostatic pressure and a increase in the seawater concentration(℃) ；Δt 2 is the loss temperature caused by indirect exchanger(℃)；Δt 3 is the average difference in temperature of each effect(℃)；C P1 is the average specific heat of cyclic water in solar energy collector (kJ /kg. ℃)；C Pf is the average specific heat of seawater in first effect (kJ/kg. ℃).Seen from formula (2), the fresh water yield of LT-MED also relate to the difference in temperature between the firsteffect and the last effect when the difference in each effecttemperature is fixed. If such of the craft is changed to directheating solar desalination, Δ t 2 becomes zero, first effect’stemperature is raised obviously, all available of differencein temperature is extended, and the number of effects is increased on the premise of certain difference in each effecttemperature.Fig. 1: Process chart of direct heating solar seawaterdesalination system. A new craft of flash combining with low temperature multi-effects is developed. In this equip, the first state is flash, the rest of the sates are low temperature multi-effects. Flash is conjunct to direct heating solar seawater system, the unsteady condition of solar energy is solved by low temperature multi-effects. The total energy efficiency is gotten an increasing. Compare with traditional indirect heating seawater, middle heat exchanger is cut off in direct heating system, the flash temperature in first state is raised 3-5 , ℃performance ratio is raised 10% above (have relation with total number of effects).3. THE KEY TECHNIQUE RESEARCH OF THE SEAWATER DESALINATION SYSTEM SCALE INHIBITION 3.1 The Experiment Research of Header Tank ofAdoption Nanometer MaterialThe traditional solar energy seawater distillation instancesadopting indirect heating mainly because the seawater isavoided getting into solar energy collector directly that leadto corrosion problem and the vacuum tube knot dirtProceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement2540problem and the whole solar energy collector closeness problem etc. Our research team carried on a research to these key problems.The solar energy collector system mainly includes vacuum tube and header tank. The stainless steel header tank is taken corrosion severity by seawater. The general engineering PPR material takes bad temperature characteristic and appears to transform after using a period of time. So it can't meet a requirement. Nanometer PPR header tank bearing seawater corrosion and having good temperature characteristic is developed. The table 1 belowshows the major technological index.（a ）plastic header tank （b ）nanometer plastic header tankFig. 2: The photograph of header tank interface.After repeated experiments, nanometer PPR header tank was proved meeting the requirements of solar seawater desalination. This header tank don't transform and aging with long hours. Experimental results contrast with the common PPR can be seen from figure 2. From figure 2(a) we can see the conjunction making of general PPR material transforms obviously. Nanometer PPR header tank making almost don't transform under the same indoor temperature and the conjunction keeps original closeness. These characteristics ensure the normal operation of the solar energy collector operated at a pressure. The use effect is showed as figure 2(b)3.2 The Research of Anti-scaleThe problem of anti-scale exists in solar energy collector system of seawater desalination and heat exchange parts of distillation plants. We studied and solved this problem from two aspects. On the one hand, reasonable operating temperature is defined to scale inhibition in the operation. This text will mainly elaborate this point. On the other hand scale inhibitor of little contaminate is developed. (The research of scale inhibitor will be communicated in anotherpaper)TBALE 1: TECHNICAL CHARACTERISTICS OF NAN- OMETER PPR HEADER TANKTest conditionTest contentTemperature℃Pressure MPaTime h Test resultsthermal stability110 1.9 8000unbreak ，non-bleed vibrationgeometric recovery ratio 135 11.0%hydraulic tes t 95 3.5 1000 unbreak ，non-bleed agingre s i stance toultraviolet336 maintenanc e ratio of tension intens ity is 92.5%The main composition of scale is a material that consists of calcium [7]: CaCO 3, CaSO 4, Mg (OH) 2. The separation condition of these three salts has differentiations. The curve of three salts separated out is showed in figure 3[8].CaS046543210150140130120110100908070605040302010CaS04 12X 1x 2（temperature ℃）salinity factorFig. 3: The curve of three salts separated out.From three curves in the diagram, CaSO 4·2H 2O just separated out at great density of seawater and will not become dirt generally. The density and temperature condition of CaSO 4 ·12H 2O is high, such as when the salinity is 7% the temperature of separation is 120 . In ℃low temperature multi-effect craft the separation of anhydrous calcium sulfate is mainly guarded against. From the diagram we can see that anhydrous calcium sulfate has8 OTHER INDIRECT SOLAR TECHNOLOGIES AND APPLICATIONS 2541a lowest curve, when the salinity is 7% the temperature ofseparation is 120 and in the density (the salinity is 3.5%)℃of standard seawater the temperature of separation is 115 ℃. Giving consideration to every aspect, we defined that the highest operation temperature is 70 in direct℃heating solar seawater desalination process, such as X1 point in diagram. X2 point is in the end of curve. X2 showed that the limit density of seawater is less than or equal to 7%. The line between X1 and X2 is the operating curve of salinity and temperature. This line is completely under the curve of three salts separated out. Scale in heat exchanger part is avoided as far as possible from an operated aspect. Under thus craft condition, there was no significant scale found on exchanger surface though long time experiment.4. CONCLUSION1) Direct heating solar seawater desalination method raised thermal efficiency and increased freshwater yield;2) The header tank in solar energy collector system made of recently develop nanometer PPR material not only defends seawater corrosion, but also has good temperature variation characteristic;3) The craft of low operating temperature solved scale problem in the process of distillation;4) Flash is combined with low temperature multi-effect. This is the suitable craft for direct heating solar seawater desalination to adapt to solar energy unstable heat supply. 5. REFERENCES(1) Leon Awerbuch, “Desalination in the World—GeneralOverview Integration of Power Water and Security”, Desalination and Water Reuse International Forum, Tianjin, China, 2004：47-62.(2) Congkai Gao, “The history and prospect of desalinationtechnological development in china”, Desalination and Water Reuse International Forum, Tianjin, China, 2004：5-7.(3) J ianming Zhao, “the Seawater Utilization”, TianjinScience and Technology Press，1994.(4) H.Nishikawa, T.Tsuchiya, Y.Narasaki, “Triple effectevacuated solar still system for getting fresh water from seawater”, APPLIED THERMAL ENGINEERING，1998（18）：1067-1075.(5)Julián Blanco,Diego Alarcón, Bernardo Sánchez., “TECHNICAL COMPARISON OF DIFFERENT SOLAR-ASSISTED HEAT SUPPLY SYSTEMS FORA MULTI-EFFECT SEAWATER DISTILLATIONUNIT”, ISES Solar World Congress, Sweden， 2003：14-19.(6)Hongfei Zheng，Kaiyan He，Ziqian Chen, “SolarDesalinization Technology”, Beijing Institute of Technology Press，2005.(7) ZHOU Li-min, JIN Jie-yun, LIU Zhi-rong, “ControllingCalcium Sulfate Scalein Process of Multi-stage Distillation Desalination of SeaWater”, Sea-lake Salt and Chemical Industry ，2005，34（4）：6-25.(8) BORIE, “OPERATING TEMPERATURE ANDSCALING IN THE M.E.EV APORATORS SOME ENLIGHTENMENTS”, Desalination and Water Reuse International Forum, Tianjin, China, 2004：95-106.。
