Solar distillation : the solar-driven case

This paper considers distillation systems where all the plant's thermal energy requirements are provided by solar energy. Fairly small capacity plants are considered operating in remote arid regions. Such systems consist of solar collectors and thermal storage coupled to a distillation plant.Each of these aspects is considered as well as the type of control strategy to be used. Using a dynamic computer simulation model, two solar-driven systems are considered: one using flat-plate collectors and a 76°C MEE desalination plant, the other using evacuated tubular collectors and a 95°C MEE plant. Various combinations of collector area and thermal storage volume are considered, and optimal values determined for each case. For the particular example chosen (lOOm³/day, located at 23$\text{1}\text{2}$°S) the flat-plate collector system was superior at current prices, giving an overall water cost of $5.82/m³ versus $7.34 for the second system. Both these costs are considerably less than water costs from conventional solar stills.     

Validating the performance simulation program “SOLDES” using data from an operating solar desalination plant

A computer program (named SOLDES) was developed to simulate the operation of solar desalination plants which utilize evacuated tube collectors, heat accumulators and multiple-effect distillation (MED) systems. The heat accumulator used is of the thermally stratified type using pure water as the storage fluid. The procedure was written in Fortran language and consists of a main program, 22 sub-programs, two system data files and four meteorological data files. The absorber area of the solar collector field can be varied between 500 m² and 20,000 m²; the storage capacity per unit collector area of the heat accumulator can vary between 0.05 and 1.00m³/m²; the capacity of the evaporator can be varied between 100 m³/d to 2000 m³/d. The heat collecting system uses a bypass circuit to allow the heat collecting fluid (pure water) to recirculate back to the solar collector field when the outlet temperature from the collector field is below a set-point. When the collector outlet temperature rises above the set-point, operation is switched over to the accumulator side. A solar-cell-type controller is used to start and stop the water circulating pump of the collector field. The operation of the MED evaporator is controlled by the state of charge of the heat accumulator by the use of set-point switches which allow the evaporator to start up when the accumulator water temperature is above a set-point and to shut down if the water temperature drops below the set point. In order to validate the SOLDES program, a comparison was made between the predicted results of the program and the actual measured data from a solar plant of similar design features to the simulation program. The selected plant was the one in actual operation in Abu Dhabi, UAE, which has almost identical design features as the simulation program and has been in operation since 1984. The data from the plant collected during 1985 were used to compare the simulation results for the months of January and June. These two months were found to be typical of a winter month (January) and of summer months (June). Except for days when a plant interruption took place, such as a power failure, the agreement between the measured and simulation data appears to be quite good.     

A vertical multiple-effect diffusion-type solar still coupled with a heat-pipe solar collector

We proposed a newly designed, compact multiple-effect diffusion-type solar still consisting of a heat-pipe solar collector and a number of vertical parallel partitions in contact with saline-soaked wicks. The solar collector and the vertical multiple-effect diffusion-type solar still can be folded or separated when it is carried, so that the still would be easy to carry and shipping cost would be very cheap. The solar energy absorbed on the solar collector is transported as latent heat of working fluid to the vertical multiple-effect diffusion-type still where the energy is recycled to increase the productivity of distillate. The performance of the proposed still is analyzed theoretically, and the still is predicted to produce 21.8 kg/m²d distilled water on a sunny autumn equinox day of 22.4 MJ/m²d solar radiation, and the productivity is greater than that of a vertical multiple-effect diffusion-type still coupled with bulky basin type still.     

An investigation of high operating temperatures in mechanical vapor-compression desalination

It is common practice to operate mechanical vapor-compression (MVC) seawater desalination systems at temperatures lower than 80°C. This study presents the detail engineering and economics of a MVC system operating at 172°C. The literature indicates that high overall heat transfer coefficients for the evaporator are possible at high temperatures with dropwise condensation on the steam side and pool boiling on the liquid side. Employing high operating temperature delivers the following advantages: (1) low compression work, (2) small latent heat transfer area, and (3) small compressor. These advantages potentially reduce operating costs and capital investment. The disadvantages follow: (1) pretreatment required to prevent fouling of heat exchangers, (2) careful selection of materials to prevent corrosion, and (3) larger sensible heat transfer area is required. A desalination plant is designed herein to produce 37,854 m³/d (10 mil gal/day), which is financed with a 5%, 30-y municipal bond. To maximize energy efficiency, combined-cycle cogeneration is employed. For the US ($5.00/GJ energy), the product water selling price is estimated to be $0.49/m³ ($1.86/thous gal). For the Middle East ($0.50/GJ energy) the product water selling price is estimated to be $0.38/m³ ($1.44/thous gal). These are attractive prices relative to competing technologies.     

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.     

Solar barometric distillation for seawater desalting part I: Basic layout and operational/technical features

The report presents an innovative solar barometric distillation technology for seawater desalting (SW-SBD) which has several attractive features such as: (1) efficient vacuum solar collectors of simple construction; (2) a barometric layout for quasi-steady operation at subatmospheric pressure; (3) a demister system of simple mechanical construction; and (4) an electronic control system regulating plant productivity parameters on available solar radiation flux. The proposed seawater desalting technology consists of single-effect distillation with water vapor produced and heated at subatmospheric pressure in a solar collector loop; the basic layout, operational features, and energy efficiency are presented and analysed in detail. Subsequent reports will provide technico-economical data from prototype desalting plants in view of industrial implementation. A promising specific electric energy consumption,∼2 kWm³ offresh water produced, was found by estimating pumping power requirements of major SW-SBD plant pumps. A sizable reduction in produced water cost with respect to an existing solar seawater desalting plant in Abu Dhabi is expected on the basis of a preliminary economic evaluation for a ∼100 m³/d plant prototype.     

Parametric study on a vertical multiple-effect diffusion-typesolar still coupled with a heat-pipe solar collector

A parametric investigation was theoretically performed for the vertical multiple-effect diffusion-type solar still, which consists of a number of vertical partitions in contact with saline-soaked wicks with narrow gaps between the partitions, coupled with a heat-pipe solar collector. The proposed still has some advantages: the still's size is compact, the still can produce distilled water without electricity, and the productivity is greater than that of conventional multiple-effect diffusion-type solar stills. We theoretically predicted the optimum angle of the solar collector on the spring and autumn equinox and the summer and winter solstice days, and also performed parametric investigations of the design and operation conditions; it was found that productivity increased with an increase in the number of partitions and the temperature of the saline water fed to the wicks, and with a decrease in the ratio of the solar collector area to each partition area, the thickness of the diffusion gaps between partitions, and the feeding rate of saline water to the wicks.     

Design and simulation of a novel solar photovoltaic system assisted single-slope solar still distillation unit

Water is an essential component of our lives. Conventional seawater desalination, based on fossil fuel energy, is primary in meeting freshwater demands. Thus, solar desalination still emerged as an alternative technology that employs environmentally friendly renewable energy. Here, we aim to design and simulate a novel hybrid solar photovoltaic (PV) system coupled with a single-slope solar still unit for freshwater production. Various design techniques were utilized to fine-tune the model towards producing 3–4.6 kg/m² · day of distillate water, thereby calculating the design aspects such as tank size, energy, and cost. The results revealed that a conventional solar desalination system had 22% lower efficiency than the proposed novel still distillation unit assisted with a solar PV system (connected to a heating element). The maximum efficiency of 45% has been recorded at the peak solar insolation due to the combination of the solar PV system. According to our design constraints, only a 3 m² basin area was required to achieve a productivity of ${\mathit{P}}_{\mathit{st}}$ = 1–5 kg/day. Design analysis showed that the total capital cost of a conventional still can be significantly reduced from 2600 to 1500 $/unit with PV system integration at the specified productivity and optimal solar radiation of ~17 MJ/m² · day at peak time (02.00 PM). This work paves the way towards maximizing solar energy utilization from PV integration with solar desalination to achieve high freshwater productivity in single-basin solar still systems.     

Investigation of Performance Improvement of the Evaporation Process in Raw Sugar Manufacturing by Increasing Heat Transfer Surfaces

A multiple-effect evaporator is used to evaporate water from sugar juice in a series of pressure vessels. Steam is used for evaporation in the first vessel, and vapor from all vessels, except the last one is used for evaporation in subsequent vessels. In order for evaporator surfaces to be used efficiently, juice entering the evaporator should be at the saturation temperature. Therefore, incoming juice with a low temperature must be heated in a juice heater. The heating medium in the juice heater is vapor bled from the evaporator. It is apparent that the multiple-effect evaporator and the juice heater interact through mass and energy balances. Previous investigations have focused on only the multiple-effect evaporator, and paid little attention to the juice heater. This paper presents the model of the interaction between the multiple-effect evaporator and the juice heater. The model is used to investigate how variations of surfaces in the evaporator and the juice heater affect the two performance parameters of the system, which are the amount of sugar juice processed by the system and the ratio of the amount of water evaporated from sugar juice in the evaporator to the amount of high-pressure steam required by the evaporator.     

新型水平环路热虹吸管启动性能

热管作为热超导元件已开始应用于中高温槽式太阳能热利用系统,但太阳辐射频繁变化的特点使得热管启动性能对热管集热性能影响很大。本文研制一套水平环路热管(HLTS),以导热姆为工质,搭建了热管启动性能测试平台,考察充液量、热通量及环境温度对启动性能影响。结果表明,工质流动方向为单向流,充液量对HLTS运行的影响较大,最佳充液量3.325 kg,当水平蒸发段工质充满度为100%时,蒸发段传热流型发生改变,管内形成脉动流;热通量增大时启动时间减少,启动温度为150℃不变;环境温度<9℃时,对热管启动有很大影响,冬季工质凝固,HLTS启动时间延长,影响热管集热。     

Optimum surface area distribution of quadruple-effect evaporator in sugar juice evaporation process

The evaporation process in raw sugar manufacturing consists of juice heater, multiple-effect evaporator, and crystallizer. Two performance parameters of the evaporation process are the rate of processed sugar juice and the steam economy, defined as the ratio between water content of sugar juice entering the process to the amount of steam required to run the process. If the total surface area of the multiple-effect evaporator is fixed, the performance parameters are affected by the distribution of the surface area among the effects of the evaporator. Although the optimization of evaporator surface area distribution has been investigated in previous studies, they have considered only the multiple-effect evaporator, and overlooked interactions between the three components of the evaporation process. The current investigation proposes a model of the evaporation process that takes into account interactions between the three components of the process through mass and energy balances. The model is used to find the optimum surface area distribution that maximizes the rate of sugar juice processed by the system and the optimum surface area distribution that maximizes the steam economy are different.     

采用不同集热器的太阳能有机朗肯-闪蒸循环性能分析

针对太阳能有机朗肯-闪蒸循环（SBFC）,本文基于EES（engineering equation solver）软件建立数学模型,以R245fa为循环工质,在集热器出口热水温度介于353.15～373.15K范围内,对平板集热器（FPC）和真空管集热器（ETC）驱动的SBFC系统热力性能以及投资成本进行了对比分析。选用的性能指标参数包括净输出功、热效率、第二定律效率、不可逆损失以及比初始投资。结果表明,随着集热器出口热水温度的增大,SBFC系统的热力性能显著升高,且采用ETC的系统比FPC系统在热力性能上更具优势,该优势随着热水温度的升高而持续增大。研究同时表明,在SBFC的投资成本中,太阳能集热器投资占比最大,且采用ETC的系统比初始投资远大于采用FPC的系统;此外,集热器出口热水温度的升高有利于降低SBFC系统的比初始投资。     

Design of a small solar-powered desalination system

A design is presented for a solar/thermal system configured to power a reverse osmosis (RO) desalination unit to produce 7000 gallons of fresh water in an eight hour period. A field of line-focus tracking solar collectors is used to heat a high pressure liquid-vapor water storage tank supplying two compound reciprocating steam engines, one direct-connected to the RO high-pressure pump and the other to an electric generator for auxiliary power. An auxiliary heating loop with an oil-fired boiler is also used to supply the steam engines.The system operates in either all-solar, all-oil, or mixed solar/oil modes. Primary operating mode is assumed to be a mixed solar/oil mode in which the oil-fired boiler is used only to prevent shutdown of the RO system during the course of a partly sunny day. In this mode, the RO system does not come on line in the morning until the solar collector field has brought the high-pressure storage tank to a point near maximum operating pressure. Thereafter, the oil-fired boiler comes on automatically whenever the storage tank is drawn down to a pressure near minimum full-power operation (due to inadequate or intermittent insolation) and remains on, supplying the steam engines, until the solar collectors have again brought the storage tank to the high-pressure cutoff.In the all-solar mode, the system continues to operate at reduced power as storage tank pressure drops below the point at which the oil-fired boiler would otherwise come on. A portion of the RO system is shut down to maintain pressure in the remainder.The all-oil mode is used whenever fresh water is required during non-sunny periods, or to increase fresh water production in sunny periods.     

Applicability of flashing desalination technique for small scale needs using a novel integrated system coupled with nanofluid-based solar collector

The present study introduces an attempt for the application of flash desalination technique for small scale needs. An integrated system uses a flashing desalination technique coupled with nano-fluid-based solar collector as a heat source has been made to investigate both the effect of different operating modes and that of the variation of functioning parameters and weather conditions on the fresh water production. The flashing unit is performed by similar construction design technique of commercial multi-stage flashing (MSF) plant. The thermal properties of working fluid in the solar collector have been improved by using different concentrated nano-particles. Cu nano-particle is used in the modeling to determine the proper nano-fluid volume fraction that gives higher fresh water productivity. An economic analysis was conducted, since it affects the final cost of produced water, to determine the cost of fresh water production. Although a system may be technically very efficient, it may not be economical. The effect of different feed water and inlet cooling water temperatures on the system performance was studied. The mathematical model is developed to calculate the productivity of the system under different operating conditions. The proposed system gives a reasonable production of fresh water up to 7.7 l/m²/day under the operation conditions. Based on the cost of energy in Egypt, the estimated cost of the generated potable water was 11.68 US$/m³. The efficiency of the system is measured by the gained output ratio (GOR) with day time. The gained output ratio (GOR) of the system reaches 1.058. The current study showed that the solar water heater collecting area is considered a significant factor for reducing the water production cost. Also, the produced water costs decrease with increasing the collecting area of the solar water heater. The volume fractions of nano-particle in solar collector working fluid have a significant impact on increasing the fresh water production and decreasing cost.     

新型太阳能槽式与平板式联合集热溶液双效再生系统

提出了一种新型太阳能槽式与平板式联合集热溶液双效再生模式,采用槽式集热管与气液分离器实现沸腾式溶液再生,并回收利用其蒸汽冷凝热作为平板式集热再生装置的一部分热源供给,实现非沸腾式溶液再生。然后建立了其稳态传热传质过程的简化数学模型,对采用氯化钙溶液的再生过程进行了实例计算。结果表明：太阳能利用系数为68.8%,与普通平板式集热再生器相比,再生效果明显改善。     

新型太阳能海水淡化技术研究进展

总结了微纳尺度下毛细驱动的局域热法太阳能海水淡化的相关机制,包括毛细驱动水分传输、太阳能热量传递机理。叙述了界面太阳能局域热法光蒸汽系统最近发展,并归纳了在1个太阳辐照下突破单级产水量理论极限的新型研究思路,指出了太阳能局域热法海水淡化的当前挑战,包含盐分结晶和蒸汽冷凝效率低等问题。认为界面太阳能局域热法海水淡化技术在效率和成本上都具有极大的优势,系统材料制备简单、结构紧凑、系统创新明显,特别适用于偏远山区和海岛地区电力缺乏的家庭生活淡水制备。随着研究者更多地转向保证蒸汽高效凝结和收集、水蒸汽潜热回收利用,预计这种新型海水淡化技术的未来应用范围将不断扩大。     

Solar distillation: the solar-assisted case

We consider here the case where a solar collector system, without storage, is linked to an otherwise stand-alone, fuel-driven desalination plant. The choices involved in selecting the type of solar collector and the design constrains on other components of the thermal source loop are outlined. A rationale is outlined for determining the most economic collector area for Such a situation and the proportion of the plant's energy requirement that can be provided by solar energy is discussed. Results for a specific collector/ desalination plant system, which include the effect of solar collector cost and fuel oil price on the economics, are presented. The decisions and constraints involved in selecting an optimum solar-assisted desalination system for a particular application are outlined.     

Experimental study of a multiple-effect humidification solar desalination technique

The object of this research is to experimentally investigate the principal operating parameters of a new desalination process working with an air multiple-effect humidification-dehumidification method. A test set-up was designed and constructed to carry out and optimize this technique. The main parts of the present set-up consist of a heat equipment device (heat exchanger), a spray humidifier and a dehumidifier system. This equipment was used to simulate the seawater desalination process experimentally with an eight-stage air solar collector heating-humidifying system. The outlet temperature of the air solar collector was correlated for use in the desalination process as a solar heating device. The operating conditions studied were: ratio of water to dry air mass flow rate through the system, humidifier inlet absolute humidity, dry air mass flow rate through the system and solar irradiation or humidifier inlet air temperature. The experimental results obtained were used to put stress and correlate the influence of the different operating conditions on the behavior of the eight-stage air heating-humidifying desalination process. The ratio of water to dry air mass flow rates was optimized, precisely 45%. The value of dry air mass flow rate through the system can be also varied with solar radiation in order to have a maximum of humidity content at the end of the system and though working in an adiabatic humidification process.     

Economic evaluation of desalination by small-scale autonomous solar-powered membrane distillation units

Solar-powered desalination is an attractive and viable method for the production of fresh water in remote arid areas. One of the most important factors determining desalination decisions is economics. This paper presents an economic assessment performed to estimate the expected water cost, which is the ultimate measure of the feasibility of the stand-alone system. Based on the calculations, the estimated cost of potable water produced by the compact unit is $15/m³, and $18/m³ for water produced by the large unit. Membrane lifetime and plant lifetime are key factors in determining the water production cost. The cost decreases with increasing the membrane and/or the plant lifetime.