Category Archives: EuroSun2008-2

Solar industrial process heat plants in operation

Data gathered in the framework of the IEA Task 33/IV include comprehensive information about the geographical distribution of the solar thermal plants, the industrial sectors addressed, the specific processes, the process temperatures, the solar thermal collectors technologies, the capacity installed, the type of back-up systems and some economics. This survey include the majority of the worldwide built examples with few exceptions such as China and Japan. At the present time data collection comprises 19 countries. Plants in operation in Austria, Greece, Spain, Germany, Italy and the USA represent about 75% of the total installed capacity reported.


Currently about 90 operating solar thermal plants for process heat are reported worldwide, with a total capacity of about 25 MWth (35,000 m2). The size of SHIP plants varies from small (around 10 kWth) to large scale installations over 800 kWth.

The majority of the SHIP plants operate in the sectors of food, wine and beverages, car washing facilities, metal treatments, textile and the chemical industries. Examples in food processing sectors (especially dairies) are particularly numerous: i. e. 20 plants corresponding to 23% of the sample. Wineries account for 4 of the 8 examples reported within the beverages sector, showing a large potential for future applications especially bottle washing and cooling of wine cellars.


Figure 2. Solar industrial process heat plant for a brewery in Austria (Source: AEE Intec)

Solar car washing facilities are concentrated in Germany and Austria (8 examples), while dairies in Greece and in Italy (6 examples). 10 solar facade integrated systems are in operation in Austria for space heating of factory buildings, while in Spain the most recent solar thermal applications are mainly in the food (e. g. olives, meat and fish processing) and transport equipment sectors (e. g. washing facilities for lorries and containers).

Solar heat is mainly used at 20-90 °C for process hot water, preheating of boiler feed-water and space heating (and cooling). Therefore standard selective flat plate collectors (FPC), working in the temperature range of 30-90 °С, result to be the most installed.

Cost figures, available for about 50% of the plants analysed, range from 450 to 1,100 €/kWth with few exceptions and some differences at national level. These costs refer to plants built before 2006, while cost figures for more recent plants are not available. In Austria and Spain, the investment cost (plant size < 350 kWth) is in the range between 470 and 700 €/kWth, while costs collected for Germany and Italy in average are higher. Costs for Greek plants are lower because of some targeted marketing strategies adopted in the 90ies by the solar thermal companies. Most of the reported plants benefited of public contributions between 30% and 50% of the total cost.

Solar application for drying woodchip in Scotland

A. Clemente, T. Grassie, D. Henderson and J. Kubie

Napier University, 190 Colinton Road, Edinburgh, EH10 5DT Scotland, UK


A novel solar dryer for drying woodchip has been developed in Scotland. In this paper, designs and performance of both solar collector and dryer have been presented separately. Woodchip drying performance has been analysed for a range of temperatures (10°C to 51°C) and flow rates (70m3/h to 280m3/h). Page model has been used for modelling the drying curves as a function of temperatures and drying velocities.

The thermal solar system considered consisted of a solar collector based on the transpired plate type and a small 10We PV panel unit employed to run a 5We fan. The performance of the system is presented in terms of air flow rate and temperature increments as a function of irradiance levels.

Keywords: Solar air heating, solar dryer, woodchip

1. Introduction

Woodfuel is a clean energy resource that reduces the dependency on imports of fossil fuels and contributes to the reduction of CO2 emissions that cause climate change. The main production of woodfuel in Scotland comes from forestry and timber industries. Changes in the energy policy and high production of forestry mass give a significant role to the wood fuels in the Scottish heat power market. [1]. Researchers have predicted that wood fuel production will be equivalent to 4.5 TWh or 11 % of the heat demand in Scotland [2].

Woodchip for burning is a bulky fuel characterized by the size and the shape of the chip and its heating value, highly dependant on the moisture content, MC. The percentage of water in a fresh cut Sitka Spruce wood sample can be up to 65 % MC on wet basis. Thus removing water from the woodchip is a necessary step in the wood fuel chain supply in order to improve the quality of the product: reduce storage and haulage costs and enhance the burning performance [3]. Drying wood requires time and energy. As an alternative to natural drying or fuel heated dryers, solar thermal systems can be used as a cheap and sustainable method to reduce the drying times, suitable for small scale producers [4].

Scotland is located at high latitudes (between 50°N and 60°N) and it has a moderate maritime climate. Despite the low average temperatures, there is a long period of daylight during the spring and summer time that makes solar energy an important power resource for preheating air applications. Previous works on solar ventilation have been accomplished in Scotland as a solar slate system by Odeh [5] and solar heater for pebble bed stores by Grassie [6].

A novel solar dryer has been designed in order to assess the capacity of drying woodchip using exclusively solar energy. The solar thermal system consists of a solar collector that increases the

temperature of air that has been delivered by a fan connected to a PV-panel. This warm air passes through the wet wood chip located on a tray.

The design and operation of the present system is considered in respect of drying woodchip in a small scale. Although woodchip is commonly dried in high volume rates in forestry factories, the decentralization of wood fuel production in Scotland leads to its use in medium and small size installations where users look for minimizing production costs [2] Woodchip usually is stored outdoors drying in natural air ventilation. So a solar thermal system can be used as a backup to reduce the drying times for a small woodchip production.

For effective woodchip drying it is necessary to supply the maximum flow rate at higher temperatures. The solar thermal system when run at high flow rates yields lower flow temperatures and vice-verse. The optimum system design and operation are a compromise between the performance of the woodchip dryer and the solar collector.

The solar dryer tests were taken in the workshops of Napier University in Edinburgh. The project consisted of two independents parts that were studied separately: the dryer and solar collector. The dryer was designed and built on basis of the outlet flow from a solar collector described in the paper. After the study of the woodchip drying performance, the dryer was connected to the solar collector for the study of the solar collector and solar dryer.

Experimental Investigations On Solar Driven Desalination Systems Using Membrane Distillation

J. Koschikowski*, M. Wieghaus*, M. Rommel*,

Vicente Subiela Ortin**, Baltasar Penate Suarez**, Juana Rosa Betancort Rodriguez**

* Fraunhofer Institute for Solar Energy Systems ISE
Heidenhofstr.2,79110 Freiburg, Germany
Tel +49-761-4588-5294
Fax +49-761-4588-9000
email ioako@ise. fhg. de


Playa de Pozo Izquierdo, s/n
35119 — Santa Lucia, Las Palmas
Tel: +34 928 727511 Fax: +34 928 727517
email: baltasarp@itccanarias. org


In many places world wide drinkable water is already a scarce good and its lack will rise dramatically in the future. Missing energy sources and no grid connections complicates the use of standard desalination techniques in these places. Fraunhofer ISE develops solar thermally driven compact desalination systems based on membrane distillation (MD) for capacity range between 100 and 500 l/day and larger systems for the capacity range up to 10m3/day. All systems can be operated energy self sufficient and almost maintenance free. Membrane distillation is a technique which is operated with thermal energy but also uses a membrane for the separation of pure water from the concentrated solution. The physical basics of transport processes in MD are described. Experimental investigations demonstrate that MD keeps important advantages for the operation in solar driven stand alone desalination systems. Altogether eight fully solar driven pilot plants were installed in 5 different countries. Measurements and experimental investigations of these demonstration units are provided.

Keywords: stand alone, desalination, solar thermal, membrane distillation

1. Introduction

In many places world wide drinkable water is already a scarce good and its lack will rise dramatically in the future. Today, sea and brackish water desalination plants are well developed in industrial scales to provide big cities with fresh water. Small villages or settlements in rural remote areas without infrastructure do not profit from these techniques. The technical complexity of the large plants is very high and can not easily be scaled down to very small systems and water demands. Furthermore, the lack of energy sources as well as a missing connection to the grid complicates the use of standard desalination techniques in these places. In arid and semi arid regions the lack of drinkable water often corresponds with a high solar insulation. This speaks for the use of solar energy as the driving force for water treatment systems. Especially in remote rural areas with low infrastructure and no grid connection, stand alone operating systems for the desalination of brackish or sea water are suitable to provide small settlements with clean potable water.

Within the scope of two projects subsidised by the European Union Fraunhofer ISE developed solar driven compact desalination systems for capacity range between 100 and 500 l/day and Two-


Loop-Systems for capacities up to 10m3/day. All systems are supplied by solar energy only. The energy for the desalination process is provided by solar thermal collectors and the auxiliary equipment as pumps and valves are powered by PV. The main advantage of the compact system is on the very low technical complexity enabling long term maintenance free operation periods. The Two-Loop-System is constructed for low maintenance operation as well, but has a higher technical complexity. A modular design of all systems is important in order to adapt them to a wide range of user profiles.

Membrane distillation is a technology which is operated with thermal energy but also uses a membrane for the separation of pure water from salty water. Apart from some experimental systems the MD-technology is currently not used for desalination, but with respect to the implementation in solar driven stand alone desalination systems it holds important advantages.

Eight fully solar driven pilot plants (2 Two-Loop-Systems and 6 Compact Systems) were installed in 5 different countries. Comprehensive measurements and experimental investigations were carried out on these pilot units for more than 3 years demonstrating on the one hand that membrane distillation is a very suitable technology but discovered on the other hand also significant potentials for improvements.

Easy retrofitting of process heat plants by solar boilers and direct heat input

Rolf Meissner*, Stefan Abrecht

Paradigma Energie — und Umwelttechnik GmbH & Co. KG, department large-scale thermal-solar systems,

Ettlinger Str. 30, 76307 Karlsbad, Germany

Corresponding author: r. meissner@paradigma. de

The results of nearly 20 years of Paradigma research, development as well as the practical experience with more than 30.000 already realized solar systems according to the AquaSystem principle give enough evidence that CPC-ETC technology with pure water as heat transfer fluid is probably the most promising way to realize large-scale solar-thermal systems (LSS) and demanding solar-thermal systems at all. High temperature collectors increase the efficiency factor, allow an all-season application and are the key for effective heat storage.

Keywords: process heat, CPC, heat transfer fluid water, large-scale solar-thermal systems

Materials and method

A chamber covering an area of 46.67 m2 with a North-South oriented central axis was used as the infrastructure. The East and West surrounding walls were 0.15 thick and 1m high — built with bricks and coated inside and outside with plaster — on which a glass with an aluminium frame is placed. The South wall, without glass, was built in the same way on a concrete foundation. The North wall is composed of a panel for the cooling system in the lower part and a glass surface in the upper part. The chamber structure also has:

• A low transparency cover made of semi-translucid glass fiber, under which a LTD long thermal life plastic cover was placed, with UV treatment to improve the chamber airtightness and to obtain an adequate cooling.

• Three iron box-like beds about 0.20m high, with an expanded metal grid base mounted on

0. 80m high legs. Inside the beds, a stony layer (0.05 m) and a pearlite layer (0.05 m) were

consecutively deposited. In two of the beds, used for the walnut production, a polyethylene pipe system of 0.0127 m in diameter was placed on the stony layer, separated at 0.10m one from the other, where water circulates at 22°C to heat the root zone. The pipes were covered with pearlite till the upper edge of the bed. One of the beds for the walnuts was prepared for micro grafting, so that apart from the basal heating system already described, an aerial polyethylene pipe system was added where water circulates at temperatures oscillating between 28°C and 31°C embracing the cicatrization area of the graft. On the third bed, prepared for carob rooting, a 0.80m wide and 0.60m high transparent plastic tunnel was built. It was set on an iron sheet and galvanized wire frame. The base heating of the carob stems was accomplished by the heat of the water in the pipes at 32°C, temperature above that required by the walnut plants.

• Mist system which allows the artificial creation of mist in the room keeping the relative humidity high at the level of the beds.

• Evaporating cooling system to diminish the temperature in the chamber. A straw savings panel was added to the North wall to maintain hydrocooling. The panel is 3.80 m wide, 1.00 m high and 0.20 m thick. Water is distributed from the upper part to maintain the straw humidity and the surplus water is taken by a channel for recirculation. Air is distributed by means of a 1.5 kW extractor fan placed on the opposite side (South).

Description of the sun — gas system

The system is made up of (Fig. 1):

• Two flat water collectors measuring 0.80 m x 2.96 m each.

• A hot water tank for house heating with a 110 l capacity, 5,75 W energy consumption, 235 l/h recuperation and a work capacity of 3,4 kg/cm2

• A 100 l water tank

• A 1/2 HP bomb for water circulation through the beds from the collectors to the hot water tank.

• Two 1” electrovalves and a 24 V-AC tension

• Two retention valves.

• “Thermostat” type sensor with a 30 — 90 °C range.

• A flow meter.

Sensors location

Three sensors were located inside the chamber at 2.00 m, 6.00 m y 8.00 m of the front NET wall along the central horizontal axis, at 1.50 m above the central bed. Sensors were put at the water inflow and outflow of each collector, and a thermostat type sensor was inside them. In the hot water tank, sensors were placed at the water inflow and outflow, and one inside so as to have the ignition reference. To measure the water volume entering the bed, a flow meter was installed. Radiation was measured inside and outside the chamber. In Fig. 2, the distribution circuit of the sensors and the water through the collectors, hot water tank and benches are shown.

To monitor the variables of the inner and outer environment, a computer equipped with Keithely 1600 and Pclab 812 acquisition cards was used. Sensors were used to measure air temperature type LM (semiconductor); Vaisala capacitive tips for humidity, Kipp & Zonen radiometers and LICOR 200SA pyranometers for solar inner and outer radiation, and a LICOR analogic lux-meter for the ilumination level.

image256 image257

Fig. 1. View of the propagation chamber and the sun — Fig.2. Diagram of the water distribution circuit gas system

Vegetal Material:

Walnut: young plants — six monts old — of native walnut were selected. They were obtained from seeds cultivated in greenhouses, and cv. Sunland walnut grafts 0.025 a 0.030 m long collected and kept in a cold chamber at 5°C.

Carob: stems from plants of the Central Valley of Catamarca were used for this work. They were 2 to 3 and 8 to 10 years old. Stems were taken from the basal part of the branch and were from 0.25 to 0.30 m in length and from 0.0025 to 0.0030 m in diameter. On each stem, 4 to 5 buds and leaves cut in halves were left to avoid greater perspiration. At the base of the stem, a bark scraping was made to favour the contact of the surface with the hormone solution.

First simulation results of a solar tower plant

Подпись: Fig. 5: Results of the solar concentrated power Pin, the power Prec and the gas temperature at the boiler inlet

Various specialized computer codes exist which are used for layout calculations and performance prediction of such solar power plant. For the solar field layout the software code WinDelsol is used, which allows the evaluation of the optimized heliostat arrangement and calculates the flux distribution on the receiver. Such results are used as input for the developed model under the MATLAB/Simulink environment. With the use of measured weather data (ambient temperature, direct normal irradiation) a realistic estimation of the annual plant performance is possible for a chosen location.

First simulation results with the model for the plant in Juelich for one day for the output power of the receiver Prec and the gas temperature at the boiler inlet are presented in Fig. 5. The chosen day was a clear summer day in Juelich with high solar concentrated radiation reaching the absorber (Fig. 5). The

temperature after the receiver rises at the beginning of the day and reaches the operation value. A control of the mass flow is been considered for a solar only operation without any storage.

5. Conclusion

Calculations of solar tower plants can be done with numerical procedures. The first results of the simulation analysis show that the created model library is a solid basis for the description of the components of the power block. The created component library will be developed further in order to describe the hybrid operation of the plant in Juelich. A future aim is to simulate the annual energy production of the solar tower plant with a gas turbine or a burner for different sizes and sides.


[1] Schwarzboezl P.: A TRNSYS Model Library for Solar Thermal Electric Components (STEC). Reference Manual. Release 3.0, November 2006.; available at: http://sel. me. wisc. edu/trnsys/trnlib/stec/stec. htm

[2] B. Hoffschmidt, G. Dibowski, M: Beuter, V. Fernandez, F. Tellez, et al.: TEST RESULTS OF A 3 MW SOLAR OPEN VOLUMETRIC RECEIVER. ISES Solar World Congress 2003, Goteburg, 14.-19. 06.2003, ISES, (2003)

[3] B. Hoffschmidt, P. Schwarzbozl, G. Koll, F. V. Quero: Design of the PS10 Solar Tower Power Plant. ISES Solar World Congress 2003, Goteburg, 14.-19. 06.2003, ISES, (2003)

[4] P. Schwarzboezl, R. Buck, C. Sugarmen, A. Ring, J. M. Crespo, P. Altwegg, J. Enrile Solar gas turbine systems: Design, cost and perspectives Solar Energy 80 (2006) 1231-1240

[5] K. Hennecke, P. Schwarzbozl, S. Alexopoulos, J. Gottsche, B. Hoffschmidt, M. Beuter, G. Koll, T. Hartz: SOLAR POWER TOWER JULICH The first test and demonstration plant for open volumetric receiver technology in Germany, Proceedings of the 14th Biennial CSP SolarPACES Symposium, Las Vegas, Nevada, 4-7 March 2008

[6] MATLAB/Simulink Manual, http://www. mathworks. com

[7] K. Hennecke, P. Schwarzbozl, B. Hoffschmidt, J. Gottsche, G. Koll, M. Beuter, T. Hartz (2007): The solar power tower Julich a solar thermal power plant for test and demonstration of air receiver. In: Goswami, Yogi; Zhao, Yuwen [Hrsg.]: 2007 ISES Solar World Congress, Beijing, Springer Verlag, S. 1749 — 1753, ISES Solar World Congress, Beijing, China, 2007-09-18 — 2007-09-21

A New Methodology for Optimum Design of Solar Power Plants. with Parabolic Trough Collectors

L. Gonzalez1*, E. Rojas1 and E. Zarza2

1 Ciemat — PSA, Avda. Complutense, 22, 28040 Madrid, Spain
3 Ciemat — PSA, Carretera de Senes, s/n, 04200 Tabernas, Almeria, Spain
lourdes. gonzalez@ciemat. es


In the last years, it has reappeared the interest that there was in the Eighties of last century in solar power plants with parabolic-trough collectors. These plants allow generating electricity like in conventional power plants, but replacing the primary energy source by the Sun, a renewable energy source. Although the scene has changed from the conditions in the Eighties, now another favourable conditions exist that promote the construction of new systems, both in the United States and in Europe. In particular, in Spain a change in the legislation for electricity generation has encouraged the implementation of renewable energies among which the solar thermal energy is. As the solar field supposes one of the main investments when erecting a thermosolar power plant, it is necessary to have an effective tool for the determination of its dimensions. This paper presents a new methodology for designing optimum parabolic trough solar fields by finding the best value around a first estimation. This first estimation of the solar field is obtained from a simplified calculation of the performance of a parabolic power plant at the site of interest, assuming a fixed solar direct irradiance at the solstice when the Sun gets its maximum declination (summer solstice for northern hemisphere and winter solstice for southern hemisphere). The performance of the power plant along a real year (i. e. using beam solar irradiation and ambient temperature data from a typical meteorological year at that site) and varying the solar field size around the first estimation is analyzed to obtain the optimum solar field. Keywords: thermoelectric, parabolic troughs, solar field, design, optimization, dumping of energy

1. Introduction

The Kyoto Protocol has set the guidelines to be followed to reduce the changes caused by greenhouse gases. This requires promoting energy savings, but above all the use of new sources of cleaner energy (i. e. renewable energies) is necessary. Solar thermal energy is a good example. One of the more promising fields of application for solar thermal energy is electricity generation by means of solar thermal power plants, which have the potential to provide, at least, 5% of global energy demand in 2040, [1].

Solar power plant technology with parabolic trough collectors is one of the more mature technologies at present, [2], with 340 MWe connected to the electricity grid in Southern California (SEGS plants) and 64 MWe in Nevada (Nevada Solar One plant). In 2004, the Spanish Royal Decree 4361 and its reviewed version in 2007 (Royal Decree 661/2007) launched the major Spanish power market players to be among the first 500MW. Most of them are promoting parabolic trough plants with up to 50MW nominal power, like Andasol-1 and 2 in Andalusia or

the Iberdrola (Spanish electric utility company) and IDAE (Spanish Institute for Energy Savings a Diversification) thermosolar power plant in Puertollano, Ciudad Real.

A solar thermal power plant has the same components as a conventional power plant with the exception of the steam boiler, which is replaced by a solar system. This system is mainly composed by the solar field, the heat exchanger and in some cases by a thermal storage. The solar field is formed by a number of parallel rows of parabolic trough collectors connected in series.

The working fluid (thermal oil) circulates through the absorber pipes from the entrance to the exit of each row.

As the solar field supposes one of the main investments when erecting a thermosolar power plant, it is necessary to have an effective tool for the determination of its dimensions. In order to optimize the design of the solar field, the reported simulation tools [3-4] work with an economic criterion as the single figure of merit. It means that every time the specific economical situation to apply changes, the optimization has to be run again from the very beginning — or to leave the design like it was-. Considering the example of Spain, where the premium payment for the electricity produced by solar thermal has been reviewed 3 times in 5 years, an optimization tool based only on economical aspects may waste of lot of design engineers’ time. The optimization methodology presented in this paper establishes an energy-related criterion prior the economic one, limiting to a small amount of new simulations to run if the economical framework changes. The energy-related criterion is aimed at minimizing the waste of energy in summer while maximizing the annual electrical energy production. As the specific feature of the methodology proposed is this energetic optimization, this paper presents the energy-related optimization and not the economical optimization that would follow.

The optimization methodology includes two parts:

* A Pre-design of the solar field. With a simplified calculation process, a first estimation of the solar field size is obtained.

* Optimization itself of the solar field size. The number of collectors per row is kept as in the pre­design, but the number of rows in the solar field is optimized. The number of sizes or, in other words, the different numbers of rows, to simulate is limited by assuming that the waste of energy in summer (called “dumping of energy”) is below 3% of the annual production. The number of simulations is, then reduced to just 3 or 4 cases or sizes. Every simulation gives the thermo­electrical performance of the solar power plant along a year using typical meteorological data at that site. The simulation is based on a simplified physical model of a parabolic trough power plant. A brief example is also presented in the last section to illustrate this methodology.

Potential for future applications

In spite of the current small contribution to the worldwide installed solar thermal power, the potential for providing heat to industrial application is really relevant.

Looking at this new promising market, several studies on the potential for solar industrial process heat have been recently performed in different countries such as Austria, Spain, Portugal, Italy and Netherlands. The main results of the potential studies performed in several countries all over the world are summarised with the key outcomes categorized by:

industrial heat demand by temperature range;

most suitable industry branches and processes for solar thermal use;

potential of application for solar thermal technologies in industry for several countries and at the European level.

Drying woodchip characteristics

1.1. Dryer setup

The woodchip drying process has been assessed experimentally in the lab. The range of flow rates and heat inputs covers the values expected from the solar thermal system, thus the dryer works at low drying velocities and low temperatures.


The dryer consisted of the supply unit and drying chamber where the woodchip was located. The supply unit consisted of the apparatus and instrumentation necessary to deliver the air flow and control its quality. The fan used was a PASPT 10W 12DC that blows a volume of air up to 280m3/h. The air temperature was regulated by an electrical resistor that provided heat up to 1200W.

The drying chamber was an isolated wooden box that contained the tray where the woodchip was held. The volume of the chamber was defined by the height, 65 cm and the area of the tray, a square area of 51 cm side. At the bottom of one of the sides, there was a square aperture (28 cm x 28 cm) that adapted the flow input getting into the box. Once the air crosses the tray of woodchip, the air was exhausted through a cardboard chimney located on the top of the drying chamber. The tray that holds the woodchip was made of aluminium sheet on the sides and a plastic mesh (3 mm square holes) at the base that allows the air pass through.

The woodchip treated for the study was shredded wood coming from Sitka Spruce trees grown in the Scottish forests. The woodchip employed had a normal distribution of sizes and shapes. The product was characterized by the length of the blade used to chip the wood; hence 2 cm was the characteristic dimension of the woodchips. The average of the initial moisture content in the sample, MC0, for all the tests was 53% MC.

Principle of Membrane Distillation

The principle setup of membrane distillation (MD) is based on a hydrophobic, microporous membrane as shown in figure 1. Due to the high surface tension of the polymeric membrane materials, liquid water is retained from entering the pores while molecular water in the vapor phase can pass through.

A hydrophobic membrane is characterized by the fact that the surrounding liquid water can not enter its pores (capillary depression). This effect depends on the relative intensity of cohesiveness between the liquid molecules and the adhesive power between liquid molecules and the membrane material. These forces are responsible for the contact angle © = 180 — © between the liquid surface and the membrane wall. In case of a non wet-able membrane, the contact angle is © > 90° and a convex meniscus as shown on the right hand side of figure 1 is formed. The hydrostatic pressure of the water columns on both sides of the membrane must remain less than the wetting pressure of the membrane in order to restrict liquid water from passing the pores.

Temperature and vapour pressure
profile across the membrane

Evaporator Condenser

channel channel

Fig 1: Principle of direct contact membrane distillation (DCMD)

Membranes for MD usually have a pore diameter of 0.1- 0.4 pm and are made from PTFE, PVDF or PP polymers. The driving force in a MD process is the vapor pressure difference across both

membrane interfaces. For direct contact membrane distillation (DCMD) , this vapor pressure difference arises due to a temperature gradient between a hot feed stream in the evaporator channel and a cooled permeate stream in the permeate channel.

In MD, it is assumed that mass transfer is based on convection and diffusion of water vapor through the microporous membrane. Finally, summarizing the constants of different equations describing the convection and diffusion process in a single transport coefficient C leads to the simplified equation: Nw = C ■ dp

Usually C is determined by experimental investigations and is in the order of 3■ 10-7 to 4■Ю6kg/m2sPa depending on membrane material and geometry.

As can be seen in the graph of figure 2, the pressure difference across the membrane dp is calculated for the temperature differences between the hot and cold membrane interface according

figure 1 AT10 = T1 — T0 from the gradient of vapor pressure curve dP at average temperature, dT

T = 0.5 ■ (T1 + T0). Respectively the mass transfer can be calculated according the following

Подпись: equation:Nw = СІР (71- T.)


Подпись: dp

The gradient can be calculated using the Clausius-Clapeyron equation with sufficient


Подпись: accuracy if AT10 < 10K :dp Ahv

dT RT2

Where Ahv is the latent heat needed for evaporation.


Fig 2: Pressure difference across the membrane dp calculated for the temperature differences dT between the

hot and cold membrane interface

There are three steps of heat transfer in MD. The first is the heat transfer from the hot bulk stream in the evaporator channel to the membrane interface. It is calculated as a function of the heat transfer coefficient a1 and the temperature difference between the bulk stream and membrane

interface Th — Tx. The second step is the heat transfer through the membrane. It consists of three different mechanisms: first is the heat transfer of the latent heat of vaporization Ahv which is transported with the vapor flux Nw through the membrane. The second mechanism is the heat flux through membrane material and the third is the heat conduction through water vapor and air in the membrane pores. Heat conduction through the membrane is adversarial because this fraction of energy can not be utilized for evaporation and must be considered as heat loss.

The third step is the heat transfer from the cold membrane interface to the cold bulk stream in the condenser channel.

Today MD is not used in large scale desalination plants. There are, however, several advantages which make this a preferred technology for small plants especially in remote applications where low temperature waste heat or a solar thermal heat supply is available, facilitating the energy self­sufficient operation of the unit.

The main advantages of MD are:

• The operating temperature of the MD process is in the range of 60 to 80 °C. This is a temperature range where solar thermal flat plate collectors have a sufficient efficiency or waste heat from co­generation plants is available.

• The membranes used in MD are resistant against fouling and scaling.

• Chemical pre-treatment of the water supply is not necessary.

• Intermittent operation of the module is possible without heat storage.

Подпись: salinity ofThe system efficiency and the produced water quality are almost independent from the the raw water.