Solar barometric distillation for seawater desalting Part II: Analyses of one-stage and two-stage distillation technologies

The report concerns design aspects for the recently proposed solar barometric distillation technology for seawater desalting (SW–SBD) with an underground barometric layout. Two types of SW–SBD desalting plants are analysed. The first plant utilizes a single-stage distillation process with one distillation heat exchanger (operative condensation temperature 50°C). The second plant utilizes a two-stage distillation process with two distillation heat exchangers connected in series (operative condensation temperatures 40 and 60°C). Vacuum solar collectors of simple design and construction, utilizing glass or suitable glass/polymer blends as transparent material, are proposed for the SW–SBD plants. The present analyses suggest that SW–SBD desalting technology may have a promising technico-economic potential. Field research on SW–SBD prototype plants is necessary to bring SW–SBD desalting technology to its full technological development.     

On the reduction of desalting energy and its cost in Kuwait

All seawater desalting processes, multi-stage flash (MSF), multi-effect boiling (MEB), mechanical vapor compression (MVC) and seawater reverse osmosis (SWRO) consume significant amounts of energy. The recent increase of fuel oil cost raises the cost of energy consumed for desalting water and the final water cost, and creates more interest in using more energy efficient desalting systems.

The most used desalting systems by distillation (MSF and MEB) are usually combined with power plants in what is called co-generation power desalting plants, CPDP. Fuel is supplied to the CPDP to produce both desalted water D and power W, and the fuel cost is shared between D and W. Exergy analysis and equivalent work are among the methods used to determine the fuel energy charged to each product. When desalting systems, such as SWRO and MVC, are not combined with a power plant, the fuel energy can be directly determined from its electrical power consumption.

In this paper, the fuel energy cost charged to desalting seawater in the presently used CPDP in Kuwait is calculated based on exergy analysis. The MSF, known by its high energy consumption, is the only desalting method used in Kuwait. The MSF units consume 258 kJ/kg thermal energy by steam supplied to the brine heater BH, 16 kJ/kg by steam supplied to steam ejectors, and 4 kWh/m3 mechanical energy for pumping. These MSF units are operated either by:

(1) Steam extracted from extraction/condensing steam turbines EC/ST as in as in Doha West, Azzour, and Sabbiya CPDP. This practice is used in most Gulf area.

(2) Steam supplied directly from boilers as occurred in single purpose desalting plants as Al Shuwaikh plant; or in winter time when no steam turbines are in operation in the CPDP to supply steam to the desalting units.

The CPDP have limited water to power production ratio. While they can cope with the increase of power demand, it cannot satisfy the water demand, which is increasing with higher pace than the power demand.

The case of steam CPDP used in Kuwait is presented in this paper as a reference plant to evaluate the amount of fuel energy consumed to desalt water in MJ/m3, its cost in $/m3. The resulted high fuel cost calls for some modifications in the reference CPDP to lower the energy cost, and to increase its water to power ratio. The modifications include the use of an auxiliary back-pressure steam turbine ABPST supplied with the steam presently extracted to the MSF units. The power output of the ABPST operates MVC or SWRO desalting units; while the ABPST discharged steam operates LT-MEB desalting unit. The desalting fuel energy costs when applying these modifications are also calculated by the exergy analysis and compared with that present situation.

It is also suggested to increase desalted water output by using separate SWRO desalting units operated by the existing power plants of typical η_c = 0.388, or by new combined gas/steam turbines power cycle GT/ST-CC of typical η_c = 0.54 under construction. The SWRO with energy recovery is assumed to consume typical 5.2 kWh/m³ electric energy.     

Solar stills made with tubes for sea water desalting

The report concerns basic technological features of simple solar stills utilizing tubes for sea water desalting. The evaporation section comprises horizontal transparent thin-walled plastic or glass tubes, of ~0.10–0.25 m inner diameter, half-filled with sea water which absorbs solar radiation. The condensation section is physically separated from the evaporation section, in a shaded space below it, and comprises horizontal plastic or metal tubes of ~0.01 m inner diameter. The wall thickness of condenser plastic tubes is rather small, ~50 μm.

Water vapour released by solar radiation in evaporator tubes flows into condenser tubes to be condensed into produced fresh water by delivering condensation latent heat to atmospheric air. Heat transfer by air convection may be helped by surface winds, often available in coastal areas. Enhanced fresh water productivity is expected with respect to conventional solar stills in which sea water evaporation and water vapour condensation occur in one confined space. Technological features of the proposed solar stills are analysed in some detail and specific experimental work is suggested on prototype solar stills in view of clarifying relevant aspects concerning transparent and opaque construction materials, assembling procedures, and the role of the various operative parameters vis-à-vis energy efficiency and fresh water productivity.     

Desalting seawater and brackish water: a cost update

This report is a second update of costs, originally presented in ORNL/TM-5070, which gave cost estimates for desalting seawater and brackish water based upon first quarter 1975 costs. The first update was based upon 1977 costs. The specific input to the report includes two earlier U.S. Department of Energy reports, recent work sponsored by the U.S. Office of Water Research & Technology, and new equipment and operating cost input from major equipment suppliers and users.

Cost estimates are given for desalting seawater by distillation and reverse osmosis, and for desalting brackish water by reverse osmosis and electrodialysis. The report includes the cost of generating steam and electrical energy on site using coal-fired boilers as well as oil-fired boilers, and dual purpose nuclear/electric seawater distillation plants. The energy costs for both reverse osmosis and electrodialysis are based upon the availability of electricity at a fixed rate. Cost data were computed as a function of plant size, and include both capital costs and construction costs which are considered as typical. These assumptions are used to develop the reference cases of total water cost. The manner of presentation is such, however, that the costs can be easily adjusted to reflect local conditions.     

膜蒸馏技术研究及应用进展

膜蒸馏作为一种新型分离技术,具有操作温度低、设备简单、脱盐率高等特点,在海水淡化、苦咸水脱盐、果汁浓缩等过程具有良好的应用前景。本文简述了膜蒸馏的工作原理、特点和膜材料的制备方法,指出当前膜材料的研究方向。综述了直接接触式、气隙式、真空式和气扫式4种基本膜蒸馏形式和几种改进的膜蒸馏形式的传热传质原理、研究现状和发展方向。重点介绍了可再生能源以及工业低温余热驱动膜蒸馏的技术特点、研究现状和应用,包括太阳能光伏/光热驱动膜蒸馏技术、太阳能热泵耦合驱动膜蒸馏技术、太阳池膜蒸馏技术、地热能梯级利用驱动膜蒸馏技术和低温余热驱动膜蒸馏技术等,并指出其发展方向。最后,探讨了膜蒸馏技术亟待研究和解决的问题,为该技术的进一步发展提供参考。     

Evaluation of an ion exchange system regenerated with seawater for the increase of product recovery of reverse osmosis brackish water plant

An experimental ion exchange unit was operated on the Red Sea shore. Softening of the reject containing 1,400 ppm calcium from a nearby RO brackish water plant was performed using seawater as regenerant. The ex- perimental results reported show that the calcium concentration in the softened reject was reduced to a level which enables further RO desalting of the reject at 50% product recovery. A preliminary economic evaluation of 4,000m³/day RO plant indicates that desalting of softened reject would be more economically advantageous than the continued operation of the existing thermal seawater desalting plants and should precede the commercial RO seawater desalting at this location.     

Recent advances in reverse osmosis and electrodialysis membrane desalting technology

The potential impact of recent developments in both reverse osmosis and electrodialysis membrane desalting technology are summarized.Particular emphasis is given to the status of advanced technology reverse osmosis membranes with chlorine resistance having single pass seawater desalination capability. Membranes capable of using low operating pressures for brackish water desalting are also reviewed.Results obtained with large prototype reverse osmosis modules and their potential effect on lowering plant capital costs are presented.Possible elimination of acid and use of ultrafiltration as the predominant pretreatment step in seawater desalination plants are also described.Recent developments in the high temperature electrodialysis program for seawater desalting and in the use of newly developed anion membranes for brackish water desalination are reviewed.Finally, the effect of recent budget cut-backs imposed on the office of Water Research and Technology (OWRT) and potential impacts on future membrane desalination R&D activity are discussed.     

Energy advantages of reverse osmosis in seawater desalination

The very rapid increase in energy costs during the past three years is causing a change in the preferred process technology for seawater desalination. The phase changes, evaporation, and condensation, required in the distillation processes make them more energy intensive than the ambient temperature liquid separation that occurs in the reverse osmosis (RO) process. This paper describes the RO process and how to calculate its energy consultation.The RO process requires only 5–7 KWh/m³ of product water compared to 15–16 KWh/m³ required by the most efficient distillation process. The productivity of a large dual purpose electricity/RO water plant is compared to the productivity of a commercially purchased state-of-the-art dual purpose electric/distillation water plant that is currently under construction. The RO potable water productivity is about 2X the distillate flow at the same fuel rate     

Comparison of solar thermal technologies for applications in seawater desalination

This paper deals with a global analysis of the use of solar energy in seawater distillation under Spanish climatic conditions. Static solar technologies as well as one-axis sun tracking were compared. Different temperature ranges of the thermal energy supply required for a desalination process were considered. At each temperature range, suitable solar collectors were compared in some aspects as: (1) fresh water production from a given desalination plant; (2) attainable fresh water production if a heat pump is coupled to the solar desalination system; (3) area of solar collector required for equivalent energy production. Results showed that direct steam generation (DSG) parabolic troughs are a promising technology for solar-assisted seawater desalination.     

Comparative economics of current and advanced desalting systems

A comparative investigation of the economics of desalting based on current and projected technology has been made. Current operating cost of various plant types operating in Israel are reported. These costs range from less than $.4/m³ for membrane plants desalting brackish water to more than three times as much for thermal plants desalting seawater. For new systems, two plant sizes were evaluated: 4,000 m³/day plants applying current technology and 100,000 m³/day plants applying projected technology. The water costs obtained for the various plant types and applied economic parameters, especially energy prices, range between $.2/m³ and $.6/m³ for brackish water desalting and from $.5/m³ to $2.4/m³ for seawater desalting.     

Coupling of nuclear heating reactor with desalination process

A seawater desalination plant using a nuclear heating reactor (NHR) coupled with the multi-effect distillation (MED) process was developed by the Institute of Nuclear Energy Technology, Tsinghua University, China. The seawater desalination plant was designed to supply potable water demand to some coastal location or island where both fresh water and energy sources are severely lacking. The NHR design possesses intrinsic and passive safety features, which was demonstrated by the NHR-5 experiences. The intermediate circuit and steam circuit were designed as the safety barriers between the NHR and MED desalination systems. With the power range of the heating reactor within 10 to 200 MWt, the desalination plant could provide 8,000 to 160,000 m³/d of potable water of appropriate quality. The design concept and parameters, safety features, and comparative investigation of coupling schemes are presented in the paper.     

Status of reverse osmosis desalination technology

Reverse osmosis (RO), a relatively new technology, is gradually becoming an established and economical method for demineralization of saline waters. Over 50 commercial plants ranging in size from 50,000 gpd to 2 million gpd (2 mgd) are producing fresh quality water for municipal and industrial uses from brackish water sources. The U.S. Congress has authorized construction of a 100 mgd plant in Yuma, Arizona to demineralize otherwise unusable high salinity irrigation return flows as part of the Colorado River Salinity Control. Engineering design and operation details together with cost information on some commercial plants and the planned 100 mgd plant will be presented.A review of the plant operation data indicates that is imperative for the plant owners and equipment suppliers to place due emphasis on providing adequate feed water pretreatment facilities and trained plant operation personnel to ensure trouble-free operation and to achieve furthur economy in desalting costs.Significant advances have been made in the development of RO process for sea water desalination. Soaring energy costs are providing incentive for plant owners to prefer RO plants (up to 100,000 gpd) over vapor compression distillation hardware. Results of the Federal Government Desalting R & D Programs clearly indicate that RO desalting costs will be at least 20–30% lower than distillation.     

Towards sustainable seawater desalting in the Gulf area

Gulf countries experienced rapid growth in the last four decades from oil production and its price increase. Natural water resources are very limited to meet this growth, and as result, desalted seawater in Kuwait became the main source of potable water, about 93% in 2002. The electric power and desalted water, produced in co-generation power desalting plants (CPDP), consumptions are continuously increasing, almost doubled every 10 years, due to population and standard of living increases. This led to the consumption of huge amounts of fuel, draining the country main fuel (and income) resource, and negatively affecting the environment. One tenth of Kuwait’s oil production was consumed by the CPDP in 2003. If the trend of almost doubling the consumption every 10 years prevails, the total oil production may not be sufficient to desalt seawater for people to drink, and to produce power to run space air conditioning units (a necessity for Kuwaiti harsh weather). It is essential therefore to look for energy efficient ways to produce power and desalted water so as to save the nation’s income of these non-renewable fuel resources, to save the environment and indeed life itself in Kuwait, and this is the objective of this paper. It reviews the presently used desalting methods and their energy demand, and the correctness of fuel allocation formulas for CPDP, to determine the most efficient methods to apply and the less efficient ones to avoid. Fourteen desalting cases are analyzed by using the current practice, with and without combination with power generation plants (using steam or gas or combined gas/steam turbines cycles). The specific fuel energy consumed and the emitted CO₂, SO_x, and NO_x per m³ desalted water were calculated for each case. The results show that operating thermally driven desalting systems by steam directly supplied from fuel-fired boilers is the most inefficient practice, and should be avoided. The use of the gas/steam turbine combined cycle, which is also the most efficient powergeneration cycle, to drive seawater reverse osmosis (SWRO) desalination plants is the most efficient combination. Also, all conservation measures in utilization of both water and power should be applied. Reclamation of waste water, at least for non-potable water needs must be promoted, because it consumes less energy and at cost much lower than those of desalting seawater.     

Professor Joseph W. McCutchan, 1918-1982

The paper compares the energy requirements of single and dual purpose MSF distillation with seawater reverse osmosis plant. Energy consumptions are given both as heat and power consumptions for distillation and as power consumption for R.O. To enable a true comparison to be made these inputs are referred back to the heat inputs from fuel needed at the boiler plant or appropriate thermal power plant.Energy recovery is also considered for reverse osmosis and it is shown that the energy input can be expected to decrease by some 35% for a typical example.Although the prime energy input needed for reverse osmosis is shown to be substantially lower than for dual purpose distillation, the overall costs taking account Of capital charges, energy, replacements and other operating costs, are found to be in a band width of about 5% for plants in the range of 5000 to 15000 m³/day. Reverse osmosis plant water costs are significantly less than distillation if membrane life increases from 3 years to 5 years, particularly with small plant capacities.     

Seawater integrated desalination plant without brine discharge and powered by renewable energy systems

Environmental effects are one of the main concerns of massive desalination facilities. To reach the objective of no brine discharge the salt from seawater must be completely separated and obtained as a secondary and valuable product. If no CO₂ emission increase is desired, the power source must be a combination of renewable energy systems (RES). This paper presents an analysis of an integrated desalination scheme consisting of two sequential systems: a multi-effect distillation (MED) plant and a mechanical vapour compression (MVC) system based on evaporator equipment. The energy is obtained by several wind turbines (WT) and a thermal solar collector (TSC) field. Separation of salt and water is achieved in a coupled multi-effect distillation-mechanical vapour compression (MED+MVC) two step process. The MED stage is driven by thermal solar collectors, whereas the energy consuming mechanical compression of the vapour (MVC) is fuelled by wind-powered turbines. Interestingly, the final products of this process are dry salt and fresh water. Such a system has been designed and dimensioned for a throughput of 100 m³/h of desalted water A preliminary study of the investment, amortization and exploitation costs of a combined MED+MVC+WT+TSC installation with these dimensions has been done. The price of desalted water, after considering the profits due to the sale of salt and electricity has been estimated at 0.59 ?/m³. If the initial investment has a 35% subsidy, a final price of 0.41 ?/m³ could be ensured, which is near the price associated to conventional energy sources. An outline of the solar collector system and the technical requirements of the wind turbines in needed to meet the energy demand of the MED+MVC system are also included.     

MEDA - a new thermal concept for desalting plants

The MEDA project comprises a highly efficient, self-contained, VTFE-VC-MSF sea water distillation plant with a capacity of 5000 tones of fresh water per day at a fuel-use performance ratio (i.e., a performance ratio which includes the electrical and steam consumption of all auxiliaries associated, with the desalination plant) of 11.8 kg/1000 kJ heat content of fuel.The main aim of the MEDA project is to demonstrate advanced desalting technology with a plant of sufficient capacity in various parts of the world under- varying seawater and climatic conditions. For this purpose the desalting plant is on a barge which can be towed to different locations where a demonstration is desired or requested.Advanced features installed on the barge are: continuous ion exchange pretreatment for removal of calcium and regeneration by brine-blowdown, high heat-transfer rates due to enhanced tubing and foamy up-flow, spirally-indented horizontal tubes in the MSF section, surfactant recovery, turbine driven steam compressor with topping effects, and reverse osmosis system with polishing unit for providing boliler make-up for plant start-up. The calcium removal by ion-exchange permits plant operation with a maximum brine temperature of 150°C. Product is redistilled in the MSF cooler to produce high purity distillate for desuperheater injection water and boiler make-up.R and D work is currently underway with a view to demonstrate reliability of operation, to verify projected performance and to evaluate corrosion behaviour of the various materials used in the plant.     

Test results from a solar boiler for saline water distillation

In this paper the performance of a low temperature, 50–70 C, solar boiler is presented. The boiler consisted of a double-glazed flat-plate solar collector, a flash chamber, a water cooled surface condenser, a circulating pump, and a temperature-controlled preheater to simulate constant temperature thermal storage. Data were obtained on this boiler from October 1980 to May 1981. During this period, the instantaneous total incident solar radiation at noon, as measured by a pyranometer in the plane of the collector ranged from 0.7 to 1.0 kWm^-2 and the diurnal total incident solar radiation ranged from 13 to 29 MJ m^-2day^-1. At these conditions the instantaneous efficiency of the collector ranged from 30 to 55 percent while the diurnal efficiency ranged from 20 to 44 percent.The results show that the performance of the solar collector was the dominant factor in determining the performance of this boiler. They also show that low temperature steam, 50 to 70 C, from such boilers can be produced to drive multieffect distillation systems using high performance evaporators for desalting saline water.