Advanced Solar Desalination for Tourism Industry in Cyprus

CONTENTS

1 Introduction
1.1 Water Consumption in the Tourism Industry

2 Market Analysis of Cyprus
2.1 Economy of Cyprus
2.1.1 Tourism Industry in Cyprus
2.1.2 Tourist Occupancy in Cypriot Hotels
2.2 Need for Desalination in Cyprus

3 Desalination
3.1 Classification of Desalination Technologies
3.1.1 Membrane Desalination
3.1.2 Thermal Desalination
3.2 Comparison of Performance of Desalination Technologies
3.2.1 Advantages of MED Desalination Process
3.2.2 Limitations
3.3 Existing Desalination Plants in Cyprus
3.4 Case Study: MED Desalination Technology
3.4.1 Existing Facilities

4 MED Desalination
4.1 Evaporator
4.2 Demister
4.3 Condenser and Flash Distiller
4.4 Brine Feed Configuration:
4.4.1 Forward Brine Feed Configuration
4.4.2 Backward Brine Feed Configuration
4.4.3 Parallel Brine Feed Configuration
4.5 Selection of Desalination Plant Capacity
4.5.1 Calculation of Desalination Capacity
4.6 Sensitivity Analysis of the Plant Capacity
4.7 Selection of No. of Effects for MED Plant
4.8 Pre and Post - Treatment
4.8.1 Pre- Treatment
4.8.2 Post Treatment

5 MED Desalination Plant Calculations
5.1 Conditions of Operation
5.1.1 Inlet Sea Water Salinity
5.1.2 Brine Rejection Salinity
5.1.3 Concentration Factor
5.1.4 Recovery Ratio
5.2 Development of Mathematical Model
5.2.1 Mathematical Model Flow Diagram
5.2.2 Latent Heat of Vapourisation
5.3 Calculation of Distillate Flow Rate
5.4 Calculation of Heat Transfer Areas
5.4.1 Selection of Heat Transfer Coefficient
5.4.2 Evaporator Area
5.4.3 Condenser Area
5.5 Temperature Distribution in MED Plants

6 MED Plant Calculations
6.1 Improvement in Efficiency
6.1.1 Thermo - vapour Compressor

7 Comparison of Performance
7.1 Gained Output Ratio
7.2 Specific Electricity Consumption
7.3 Specific Thermal Consumption
7.4 Comparison of Performance of MED and MED - TVC Plants

8 Operational Characteristics
8.1 Inlet Sea Water Preheating
8.1.1 Design of Sea Water Preheater
8.1.2 Selection of Heat Exchanger

9 Simulation of Desalination Plant
9.1 Selection of Sea Water Operation Temperature
9.2 Input Values for Desalination Plant
9.3 Output Modules
9.3.1 Flow Diagram
9.3.2 Expert Mode Calculations
9.4 Simulation Results

10 Final Design
10.1 Final Layout
10.2 Selection of Materials
10.2.1 Corrosion
10.2.2 Materials for Desalination Plant Components
10.2.3 Pump Materials
10.2.4 Piping

11 Energy Source for Desalination Plant
11.1 Current Studies in Cyprus
11.2 Solar Potential of Cyprus
11.2.1 Solar Potential of Bafra
11.2.2 Selection of the Data Source

12 Selection of Solar Technology
12.1 Comparison of Technologies
12.2 Parabolic Trough Collectors
12.2.1 Performance of the Parabolic Trough Collectors
12.2.2 Losses in the Parabolic Trough Collectors
12.2.3 Collector Efficiency
12.2.4 Collector Field Efficiency
12.2.5 Capacity Factor
12.2.6 Requirement of steam
12.2.7 Steam Generation Process
12.2.8 T-s Diagram for Solar PTC Plants

13 Solar Plant Calculations
13.1 Solar Multiple
13.2 Thermal Energy Storage
13.2.1 Sensible Heat Storage
13.2.2 Latent Heat Storage
13.2.3 Chemical Energy (Bond Energy) Storage
13.2.4 Comparison of Technology
13.2.5 Selection of Storage Medium
13.3 Thermal Energy Storage Calculations
13.3.1 Design of Storage Tank
13.4 Final Results

14 Summary

15 Bibliography

16 List of Tables

17 List of Tables

18 Appendix

ACKNOWLEDGEMENTS

First and foremost, I express my sincerest gratitude to FH Prof. Rudolf Kraft. It has been a privilege to have him as a supervisor for my thesis. I appreciate all his contributions in terms of time and suggestions.

I would like to thank Dr. Ahmet Lokurlu for his valuable guidance and the continuous encouragement during the internship and project work on this particular topic. I would also like to thank the HR Manager and colleagues at Soliterm GmbH for their support. Special thanks to International Atomic Energy Agency (IAEA) and SOLARGIS for providing me with the necessary data for my thesis work. I am deeply indebted to the International Office, Mag. Daniela Hochstöger, Dr. Robert Höller, Dr. Peter Zeller and Head of Studies, Dr. Micheal Steinbatz for providing me an opportunity as a Master Stu- dent in Sustainable Energy Systems to carry out research on such a topic.

Last but not the least, I would like to thank my friends, classmates, family and my mother without whom the work would not have been possible.

KURZFASSUNG

Trotz das Wissen von globale Trinkwasserverfügbarkeit, fast die Hälfte der Welt hat die Wasserknappheit besonders die Mittelmeer und Nordafrikanische Länder. Das Hauptziel dieser Masterarbeit ist eine Machbarkeitsanalyse der „Multiple Effect Distillation“ (MED), die thermische Meerwasserentsalzungstechnolgie.

Die Arbeit beschreibt eine Fallstudie eines Simulation für eine Entsalzugsanlage eines Hotelstandortes in Nord Zypern. Die Studie verfügt über Anlagendesign und Leistungsdaten, damit erreicht man niedrigen spezifischen thermischen und elektrischen Bedarf. Ein detailliertes mathematisches Modell wurde für die Entsalzungsanlage entwickelt, die auf den Stoff- und Energiebilanzen für jede Stufe in der MED - Entsalzungsanlage aufbaut. Ein verbessertes Modell für diese Entsalzungstechnologie wäre eine zusätzliche Einheit eines Thermo - Dampf - Kompressor. Mit dem verbesserten Modell wurde der GOR Werte verdoppelt und der spezifische thermische Wärmebedarf auf die Hälfte seines Wertes gesenkt.

Mittels einer Meerwasservorwärmungstechnologie, könnte diese Entsalzungsanlage ganzejährige betrieben werden. Damit kann die Soletemperatur auf 43 bis 48 °C abgehoben werden. Das Modell wird mit der Simulationssoftware DEEP bewertet. Mittelmeerländer haben ein großes Potenzial für erneuerbare Energien besonders in der Solarthermie. Mitteltemperaturkollektoren wie z.B. Parabolrinnenkollektoren erreichen die Temperatur von 180 °C und wären eine Lösung für Dampfversorgung dieser Entsalzungsanlage. Der Ersatz fossilen Brennstoffe durch solarthermische Energie ist eine saubere und nachhaltige Lösung und ein idealer Maßstab für die zukünftigen erneuerbaren Entsalzungsanlagen.

ABSTRACT

Despite the fact that global water is apparently abundant, almost half of the world faces the water scarcity especially in the Mediterranean and North African regions. The main purpose of this work is the assessment of Multiple Effect Distillation, a thermal desalination technique in water stressed countries in the Mediterranean region.

The thesis describes a case study for desalination plant solution of capacity 900 m³ /d for a hotel location in Cyprus. The study features plant design and performance data ensuring low specific electrical and thermal energy consumption. A detailed mathematical model is developed for the 8 - effect desalination plant which is based on the mass and energy balances for the streams flowing through each stage of the MED unit. The performance of the desalination can be predicted. An improved model for the desalination technology can be suggested by installing an additional unit called as thermo - vapour compressor. The Gained Output Ratio (GOR) and specific thermal energy consumption values of 13.6 and 40.1 kWhth/m³ respectively are obtained from the improved model.

The plant can be operated throughput the year by implementing an innovative technique of sea water preheating. This helps in achieving the required brine feed temperature of about 43 - 48 °C. The improved plant model is assessed by a simulation software, DEEP. The Mediterranean region has huge potential in renewable energy especially in solar thermal energy. A medium temperature parabolic trough collector operating at 180 °C will be used to supply the steam for desalination plant. With the fossil fuels powering most of the desalination plants, the use of solar thermal energy can be a clean and sustainable option and an ideal benchmark for the future renewable desalination plants.

Key Words:

Tourism Industry, Cyprus, MED - Desalination, Thermo - Vapour Compressor, Seawater Preheating, Parabolic Trough Collectors

1 INTRODUCTION

An issue of water scarcity is getting acute in the European (EU) countries especially in the Mediterranean regions with a burden of increasing population, affluence and the change in lifestyles. Water stress can be observed either due to the excess demand of water during a certain period or the poor quality of its use. The Water Stress Index (WSI) is one of the main indicators to quantify the water stress in in any region or country.

In 2003, OECD classified the water stress severity into three distinct groups namely moderate, medium high and high as shown in table 1.

Table 1: Classification of Water Stress Indices

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Amongst the EU countries, eastern Mediterranean regions like Cyprus ranks high in terms of percentage of water stress with a value of about 65 %. Figure 1 shows the water stress index value for different countries in Europe.

Figure 1: Water Stress Index in Europe¹

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Additionally, the World Resource Institute (WRI) suggests that Cyprus has one of the highest with a baseline water stress score of 5.00. The main reason is the rapidly expanding tourism industry in Cyprus².

1.1 WATER CONSUMPTION IN THE TOURISM INDUSTRY

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Figure 2: Share of Tourism on Domestic Water Use³

On an average, the global daily water consumption for a tourist is 222 l/d. Cyprus is the world’s second largest water consumer in terms of the tourism industry. The Guest to Hosts Ratio is 1:1, which in other cases should ideally not exceed 1:6. As seen in the figure 2, Cyprus ranks second amongst the countries with respect to the water consumption for international tourism due to its long coastlines and pleasant weather.

The sector-wise consumption of water for Cyprus can be seen in figure 3.

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Figure 3: Sector-wise Water Demand in Cyprus⁴

The international tourism is responsible for 17.4 % of the domestic water in Cyprus which corresponds to about 4.8 % of the total national water usage. Various tourist activities like golf and skiing add to the water use. The coastal locations generally dominate in terms of the water consumption. The consumption of water by golf courses varies considerably, depending on soils, climate and golf course size. In Cyprus a golf courses require 10,000 to 15,000 cubic meters per hectare per year⁵. Due to the recent approvals for large gardens and landscaping activities, the water demand is expected to rise in the near future.

Different data sources suggest the value for the per capita tourist water consumption in Cyprus. According to Gössling Report (2011), the per capita water consumption by tourists in Cyprus is about 0.400 m³ /d (400 litres/d)⁶. This would be the best estimate for the per capita water consumption value of the tourists in Cyprus.

2 MARKET ANALYSIS OF CYPRUS

Cyprus is a small island nation located in the east Mediterranean region and third most populous country in the Mediterranean. Previously it was controlled by the British Empire and was granted independence in 1960. The Cyprus is partitioned into two parts, the area controlled by the Cypriot Government covers 59 % of the total area, whereas the Northern Cyprus controlled by the Turkish Authority covers an area of 37 %. The remaining area passing through Nicosia is UN Buffer zone.

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Figure 4: Cyprus - Political Map

Cyprus experiences a subtropical Mediterranean climate. The rainfall occurs usually during the winter, whereas the summers in Cyprus are mostly dry. The average daily temperatures in the range of 7 to 35 °C. Most of the months in the year (generally May - October) have maximum ambient temperatures greater than 25 °C.

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Figure 5. Average Daily Ambient Temperatures (Source: METEONORM 7.1)

2.1 ECONOMY OF CYPRUS

The economy of Cyprus is mainly dependent on agriculture. Tourism, trade and the public sectors also play an important role in the economy. The tourism industry contributes about 6 % of its GDP.

2.1.1 TOURISM INDUSTRY IN CYPRUS

Cyprus is one of the popular destinations in summer for the tourists across the globe. Cyprus is divided into two separate parts namely Northern Cyprus (Turkish Occupied) and Republic of Cyprus (Greek Occupied). However the statistical data with respect to tourism, industry and other sectors is represented as a single country “Cyprus (CY)” in the EU as well as worldwide.

Tourism industry is one of the largest economic sectors in Cyprus. Cyprus ranks as the 40th most popular tourist destination in the world. Considering the current political scenario in Turkey, the total number of tourists are expected to increase in Cyprus. About 80% of the tourist arrivals in Cyprus are from the European countries. British tourists remain the most traditional tourists in Cyprus. Due to a strong legislation against casino business in the gulf countries, Cyprus is a popular destination with its casino resorts. The golf courses are also a major tourist attraction in Cyprus. Most of these golf courses are located in Paphos. The Northern Cypriot Government is also planning to build new golf courses in Famagusta.

Some of the popular destinations in Cyprus are:

− Ayia Napa
− Larnaca
− Limassol
− Paphos
− Nicosia
− Famagusta
− Kyrenia

2.1.2 TOURIST OCCUPANCY IN CYPRIOT HOTELS

According to the 2015 CYSTAT data for tourism, Cyprus had an annual arrival of 2.6 million tourists⁷. The period between July and September has the maximum inflow of the tourists. According to the 2015 EUROSTAT data, figure 6 shows the percentage of occupancy for the tourists in month of September⁸

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Figure 6: Net Bed Occupancy by Tourists in the Hotels in EU, September, 2015⁸

All the hotels irrespective of their locations (either in the Republic of Cyprus or Northern Cyprus) are clubbed together for data analysis in the EU 28 as Cyprus (CY). Therefore the net bed occupancy levels can be applicable to all the resorts and hotels lying in both the regions. Knoema data in figure 7 provides an exact value of the monthly bed occupancy level of the tourists. Cyprus has the maximum demand in terms of bed occupancy mainly due to its casinos, golf courses and water sports.

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Figure 7: Monthly Net Bed Occupancy by Tourists in Cyprus (Source: Knoema)⁹

2.2 NEED FOR DESALINATION IN CYPRUS

Due to the gradual decrease in rainfall over a period of time, the ground water has been over exploited in Cyprus. These existing conditions have a constant saline water intrusion along with quality deterioration in coastal aquifers and depletion of inland aquifers. Sea water intrusion in the aquifers has lowered the quality of fresh water in various areas near Limassol and Larnaca.

Cyprus has certain problems with respect to water stress. One major problem in Cyprus is that of over-pumping of water through illegal boreholes which is a common activity practised in several countries in the Mediterranean region. This activity has a serious effect on the quality of groundwater. With regards to this issue Helmer (1997) stated that in Cyprus, due to the numerous illegal boreholes and uncontrolled water withdrawals, salt water intrusion has become a serious problem. The underground water in certain areas such as Larnaca are so salty that this water is not suitable to irrigate the salt - tolerant crops. Although the use of illegal bore holes is widely used by the agricultural sector, they are also used by several other industries such as the tourism industry. Such extraction is forcing water authorities in several islands to find out the possible alternative solutions for the production of fresh water such as desalination¹⁰.

Owing to these concerns regarding the water scarcity and the tourist occupancy level along with a 648 km long coastline, sea water desalination is one of the prospective solutions in Cyprus.

3 DESALINATION

The desalination process consists of production of distilled water from salt water sources such as sea water or brackish water. As desalination is an endothermic process, it requires a considerable amount of energy¹¹. Figure 8 shows the basic principle on which any desalination technology works.

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Figure 8: Basic Principle of Desalination Technology

Desalination has been developed in the 4th Century itself. Potable water was produced by boiling the sea water and absorbing the steam by sponge. Since then, sea water desalination technology has become quite popular. There has been a phenomenal increase in use of this technology since last six decades. The total global installed desalination capacity was 67 million m³ /d in 2012. The latest estimation by June, 2015 indicates 18426 plants with a worldwide cumulative capacity of about 86.8 million m³ /d. Figure 9 shows the desalination technologies by different feed water categories.

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Figure 9: Available Worldwide Desalination Capacity based on Feed Water Sources¹²

About 150 counties worldwide have installed desalination plants. The gulf countries share majority of these installations. Figure 10 shows the country - wise share of the installed desalination plant capacity.

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Figure 10: Country - wise Installed Desalination Plant Capacity¹³

3.1 CLASSIFICATION OF DESALINATION TECHNOLOGIES

The general classification of desalination technologies is according to three criteria:

− Product extracted from sea water
− Type of separation process
− Type of energy used

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Figure 11: Classification of Desalination Technologies¹⁴

3.1.1 MEMBRANE DESALINATION

Membranes or filters play an important role in separating the salts from the saline water. Technologies like Electro dialysis (ED) and Reverse Osmosis (RO) use this principle for the production of fresh water. Although this desalination technology was used to obtain fresh water from brackish water, various techniques have also been developed for distillation of seawater. Identifying the need to remove contaminants that affect the longterm filter operation, several filtration techniques like micro - filtration, nano - filtration and ultra - filtration have been developed.

3.1.2 THERMAL DESALINATION

Thermal distillation process involves heating the saline water to produce water vapour. This vapour in turn is condensed to obtain distilled water. Multistage Flash Distillation (MSF) was one of the widely used desalination technique. However due to the increasing problems of scaling and corrosion, a new technology Multiple Effect Distillation (MED) was developed. Other technologies existing in low capacities are Vapour Compression (VC) methods. The global share of the technologies can be seen in figure 12.

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Figure 12: Available Worldwide Desalination Capacity based on Technologies¹²

Nevertheless, RO technology continues to lead the pack in terms of global desalination installed capacity. In spite of its domination, the thermal distillation technologies such as MED, MSF and VC distillation are rapidly expanding and are anticipated to play an important role in the future.

3.2 COMPARISON OF PERFORMANCE OF DESALINATION TECHNOLOGIES

Depending on the energy source, the desalination technologies have their process efficiency characterized on the basis of either specific thermal or specific electrical energy consumption (or both). Due to the lower performance characteristics of MSF and MVC technologies, MED and RO type technologies are used as reference calculations as shown in table 2.

Table 2: Comparison of Performance of Desalination Technologies¹⁵

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The RO plants have certain limitations:

− The sea water requires a lengthy pre-treatment process before entering the membrane.
− The water quality obtained has a Total Dissolved Solids (TDS) level up to 500 parts per million (ppm) which requires a demanding post - treatment process.
− The requirement of higher amount of electrical power for the pumps.

On the contrary, the MED Plant produces fresh water with a TDS level up to 10 ppm. The MED Plants are more reliable, easy to maintain and do not require an extensive pre-treatment or post-treatment.

3.2.1 ADVANTAGES OF MED DESALINATION PROCESS

There are certain merits associated with the use of MED Plants namely:

- Lower electricity consumption
- Possibility of powering by waste heat from the power plants
- High quality distillate (TDS level ~10 ppm)
- Less complicated pre - and post - treatment

3.2.2 LIMITATIONS

In comparison to the membrane technologies, MED technology requires an additional source of energy demand in the form of heat energy.

3.3 EXISTING DESALINATION PLANTS IN CYPRUS

Since 1960, the Government of Cyprus has emphasized on production of good quality water. The average annual rainfall has significantly decreased. From the year 1991-2010, the average annual rainfall decrease by 15 % compared to the rainfall from 1961-1990. These conditions have forced Cypriot Government to implement alternative fresh water production techniques like sea water desalination technology¹⁶.

As per 2015, Cyprus has installed seven main desalination plants with a total capacity of about 2.72 million m³ per day. Other than Garyllis, the only major brackish water desalination plant, all other desalination plants have sea water as its primary source. All the desalination plants use RO technology for production of fresh water. The desalination plants are either of the Build, Own, Operate and Transfer (BOOT) or Build, Operate and Transfer (BOT) type.

The first desalination plant was installed in the year 1997 at Dhekelia. Over the years desalination plants were installed at various location like Limassol, Larnaca and Paphos. Majority of these desalination plants lie in the Republic of Cyprus. The water supply in the Northern Cypriot regions takes place through various pipelines from the Republic of Cyprus and Turkey.

Figure 13 shows the major desalination plants installed in Cyprus.

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Figure 13: Desalination Plant Locations in Cyprus

Most of the desalination plants are located in the southern part of Cyprus. Table 3 provides the data of these existing desalination plants.

Table 3: Desalination Plants in Cyprus¹⁶

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Currently several desalination plants installed in Cyprus use RO technology. Therefore MED Desalination solution can be an appropriate technology for its performance assessment in the water stressed regions like Cyprus. Due to the concentration of the desalination plants mostly in the Republic of Cyprus, a water stressed location in the Northern Cypriot region is selected for the case study.

3.4 CASE STUDY: MED DESALINATION TECHNOLOGY

The tourism industry can be considered as a typical application for the assessment of desalination plant performance. A case study is carried out for implementation of such MED desalination plant technology for a five star hotel, Kaya Artemis Resort and Casino located in Bafra, Northern Cyprus.

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Figure 14: Kaya Artemis Resort and Casino

Located in the district of Famagusta, Kaya Artemis Resort and Casino is a five star resort spread over an area of 165,000 m². The hotel and casino covers a space of about 65000 m² with a total capacity of 2500 beds. It has several lavish restaurants and swimming pools with one of them heated by using a conventional heater during winter period. It also has several amenities like fitness, spa, water sports, and entertainment. However the main attraction is its casino. As casinos are not allowed in the resorts in the middle - east, the Kaya Artemis Resort and Casino is an important location. Therefore the occupancy level of the tourists is rather high.

3.4.1 EXISTING FACILITIES

The Kaya Artemis Resort and Casino has an existing Certuss boiler for the generation of steam in various applications such as laundries and space heating. It also has two machine rooms with chillers for supplying the necessary cooling demand for its hotel and casino.

Figure hast been removed for publication.

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Figure 15: Existing Facilities in Kaya Artemis Resort

3.4.1.1 SPECIFICATIONS OF THE SERVICES

3.4.1.1.1 CHILLERS

− Machine Room 1: Compression Chillers (2 units) Capacity: 864 kW and 462 kW

− Machine Room 2: Compression Chillers (2 units) Capacity: 1050 kW and 1050 kW

3.4.1.1.2 BOILER: CERTUSS STEAM BOILER

− Capacity: 2000 kg/hr

4 MED DESALINATION

MED Desalination is a thermal desalination process which consumes heat and electricity for the production of distilled water. MED Desalination is one of the improved desalination techniques replacing the traditional thermal desalination technologies like MSF. The steam cycle (Rankine Cycle) enables lesser power losses compared to MSF. The MED Desalination uses inexpensive materials, but has excellent scaling control and operates at a top brine temperature of 70 °C. In principle, MED plants can be configured for high temperature or low temperature operation.

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Figure 16: Multiple Effect Desalination¹⁷

In the MED technology, the feed water is sprayed or distributed onto the surface of the evaporator surface in a thin film so as to enable its evaporation as shown in figure 16. The evaporation of the feed water takes place in different chambers (effects), hence the name Multiple Effect Distillation. Some of the studies refer to MED desalination as Multiple Effect Evaporation (MEE). The inlet feed water from saline sources such as seas or lakes is heated by the steam from the boiler or the power cycle. It produces the water vapour which heats the feed water in the succeeding effect. The pressure in each effect (chamber) is less than the preceding effect .The process continues up to the given no. of effects. Vapour produced in the last effect is condensed in a condenser.

Condenser helps in ejecting the excess heat energy from the plant. Additionally it uses this ejected energy for heating the incoming sea water. This can help in achieving an adequate temperature for injecting the feed water into each evaporator effect.

Figure 17 shows the basic layout of the MED plant. It includes n effects and n-1 flash distiller boxes. The effects are numbered from 1 to n. The vapour moves from left to right (considering the direction of flow).

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Figure 17: MED Desalination Layout

The inlet sea water always moves in the direction perpendicular to flow of steam in the evaporator effects. Steam is introduced into the tube in the first effect, whereas, the feed water is sprayed on the shell side. The brine spray forms a thin falling film on the succeeding rows within the evaporator. As a result the brine temperature increases beyond its saturation temperature and evaporates to form a vapour in the effect. This vapour is used to heat the 2nd effect, which later condenses on the tube side after giving away its latent heat of vaporization. Each effect produces the vapour which in turn heats the feed water in the succeeding effect. This process continues till n effects. Each effect has an evaporator, brine spray nozzles, brine pool and space for the vapour. Condenser is located at the end of last evaporator effect.

The maximum vapour temperature in the first effect are also called as Top Brine Temperature (TBT). The temperature and pressure decreases gradually in each effect due to the boiling point elevation (BPE), non-equilibrium allowance and frictional losses in the demister and condensation. Therefore the amount of vapour formed in each effect is less than the amount formed in the previous effect.

The main considerations for the desalination plant is the design of evaporator, demister and condenser and the selection of brine feed configuration.

4.1 EVAPORATOR

Thermal desalination involves generation of fresh water vapour from the sea water or brackish water. The incoming steam from the succeeding effect condenses on one side of the evaporator, whereas distilled vapour is produced on the other side. Evaporator and condenser are the most important components in the MED desalination plant. Several configurations are carefully studied in order to select the appropriate configuration of tube bundles and design of the evaporator.

The tube bundles should not be exposed to the brine. This can avoid the problems related to scaling and corrosion. Thus the capacity of heat exchanger surface with anti-scaling materials can be reduced. MED plants have several available configurations for the heat exchanger like vertical tube falling film, vertical tube climbing film, horizontal tube falling film and plate heat exchanger¹⁸.

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Figure 18: Two Effect Submerged Tube Evaporator¹⁸

Submerged tube evaporators are used generally for household purposes. Earlier they were also used in certain industrial desalination plants. However due to rapid fouling and scaling of the outside surface of the tubes, they are no longer used. It also requires a lengthy and expensive procedures for cleaning the tube bundles.

Figure 19 shows the plate type evaporators. The steam condenses on one side of the plate whereas the water evaporates on the other sides. The plates are manufactured using materials such as metal, plastic or polymers. These evaporators have high heat transfer coefficient, smaller space requirements and lower fouling resistances. Yet these evaporators are only available on an experimental scale.

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Figure 19: Plate Type Heat Exchanger¹⁸

The drawbacks of fouling and difficulty in cleaning the tube bundles is minimised by using falling film configuration. There are two types of configurations available namely vertical and horizontal falling film evaporators.

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Figure 20: Vertical Film Evaporator¹⁸

Due to the difficultly in maintaining the film formation, dry patches may occur giving rise to scaling and uneven tube expansion. Thus the horizontal falling film configuration as shown in figure 21 is the most preferred technology.

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Figure 21: Horizontal Film Evaporator¹⁸

The horizontal film evaporators have the following characteristics: − High wetting rates

− High heat transfer coefficients
− Better monitoring of scaling or fouling.
− Efficient water distribution over the heat exchanger surfaces

The horizontal falling film configuration is used in most of the MED and MSF plants due to their characteristics to handle seawater scaling.

4.2 DEMISTER

The demister is a component installed in each evaporator effect which helps to avoid the brine droplets mixing with the generated vapour or the distillate. Several types of designs for demisters are available such as mesh type, vane pack or other structures. Mesh type is one of the commonly used demister designs.

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Figure 22: Wire Mesh Demister¹⁹

As shown in the figure 22, the mesh demister consists of several layers bound together in order to retain liquid droplets entrained by the water vapour. Demisters are placed horizontally such that the collected entrained vapour droplets on the wire mesh merge into larger droplets. Wire mesh demisters are capable of producing distillate with a salinity as low as 0 - 5 ppm. Performance of demister depends on various factors such as vapour velocity, wire diameter, thickness of the mat and the construction materials.

4.3 CONDENSER AND FLASH DISTILLER

Last stage in the desalination plant is the condenser. It is mainly used for releasing the heat of vapour produced in the last evaporation effect. Condenser utilises this energy and heats the inlet sea water up to an adequate brine feed temperature. The distillate from each effect might possess a fraction of vapour which can damage the piping and the pump during its circulation. Therefore the distillate is allowed to pass through a separator which helps to obtain pure liquid distillate. The vapour formed in flash distiller is used as a steam input for the removal of non - condensable gases formed in the evaporator effects.

Other than these important components, it is important to remove the non - condensable gases from the system. The vents are provided for each evaporator and condenser effects for better purging and removal of non-condensable gases. Lesser effective removal of non-condensable gases causes more losses of heat inside the desalination plant. The vent for last effect (condenser) is connected to the vacuum producing equipment to compress the non-condensable gases. Usually a steam jet ejector is used to operate the gas separator which finally ejects these gases into the atmosphere.

4.4 BRINE FEED CONFIGURATION

The configuration for feeding in the sea water for desalination needs to be designed in such a way that it would be less complicated and energy extensive. This allows better vaporisation of brine and performance. Depending on the direction of the brine feed flow with respect to the vapour flow different configuration of brine feed are available namely Conventional, Backward and Parallel²⁰.

4.4.1 FORWARD BRINE FEED CONFIGURATION

The brine feed is directly fed into the first effect. After the vapour formation, remaining brine is circulated in the succeeding effect. The circulation continues until the last effect, where the leftover brine is discharged into the environment. The forward feed configuration as shown in figure 23, is not used on an industrial scale in the desalination plants. However the sugar and textile industries use this technology.

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Figure 23: Forward Brine Feed Configuration²⁰

4.4.2 BACKWARD BRINE FEED CONFIGURATION

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Figure 24: Backward Brine Feed Configuration²⁰

As shown in figure 24, in the backward feed configuration, the sea water first enters in the last effect which has lowest temperature and pressure. Due to the increase in pressure and temperature across the effects, brine pumping units are required between each effect. Thus this configuration requires higher pumping power and maintenance cost. In addition, the brine with higher concentration has higher temperature, which implies that the temperature concentration profile is beyond the soluble limits of Calcium Sulphate (CaSO4).

4.4.3 PARALLEL BRINE FEED CONFIGURATION

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Figure 25: Parallel Brine Feed Configuration²⁰

Unlike the conventional brine feed configuration, the parallel brine feed configuration as shown in figure 25 distributes the brine feed in such a way that brine from each effect is collected individually and then discharged together into the environment. Better value of overall heat transfer coefficient is achieved due to an equal distribution of brine feed in each effect.

The parallel feed configuration possesses certain benefits: − Simpler piping design

− Reduction in pressure drops for the pipes due to reduced no. of bends
− Lower pump power requirement for brine circulation
− Effective heating of brine in each effect due to reduced feed flow.

The selection amongst the three configurations also depends on the variation in the salt solubility. Salt solubility is a function of the top brine temperature and the maximum brine concentration. Higher brine temperature or salinity values leads to scale formation. Thus the pumping energy demand is higher and heat transfer efficiency will be reduced, thereby yielding a lower product flow.

Figure 26 and 27 shows the salt solubility for various brine feed configurations.

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Figure 26: CaSO4 Solubility for Forward/Backward Feed Configuration²⁰

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Figure 27: CaSO4 Solubility for Parallel Feed Configuration²⁰

From the figure 26, for the forward and backward feed configurations, the highest concentration brine is subjected to highest temperature. Thus the concentration profile crosses the solubility limit for CaSO4. This makes the makes the backward brine feed configuration inappropriate. The parallel feed configuration has several advantages compared to forward feed as mentioned in its benefits. For reducing the frequency of chemical cleaning, the operational temperature of the brine is limited to 70 °C. Therefore considering the solubility limits of CaSO4 and the merits, the parallel brine feed configuration is chosen for the desalination plant.

4.5 SELECTION OF DESALINATION PLANT CAPACITY

The two main parameters to be estimated for the selection of the desalination plant capacity are per capita water consumption by the tourists and net bed occupancy.

− Per Capita Water Consumption for Tourists (Wcapita)

As per section 1, the daily per capita water consumption by the tourists in Cyprus is 0.400 m³ (400 litres).

− Net Bed Occupancy Rate {O.Rne1)

As per the occupancy rates in Figure 7 in the topic Tourist Occupancy in Cypriot Hotels, the month of August has the highest bed occupancy level of 90 %. This value is used for calculation of the plant capacity.

4.5.1 CALCULATION OF DESALINATION CAPACITY

The Desalination plant capacity is calculated using following steps: Step 1: The maximum bed capacity of the hotel.

Step 2: The maximum net bed occupancy rate (As per the Occupancy Level in Figure 7). Final Step: Total Desalination Plant Capacity

For the given bed capacity ܤ୫ୟ୶, the desalination plant capacity can be calculated as:

For the given values the water consumption comes out to be 900 m³ /d.

4.6 SENSITIVITY ANALYSIS OF THE PLANT CAPACITY

Desalination plant capacity depends on two main factors namely, per capita water consumption, and the net bed occupancy by the tourists. The change in any of these parameters affects the capacity of the desalination plant.

The per capita water demand is high during the summer seasons for water sports, showers and restaurants. The tourist occupancy level is not significantly changed. However the trends show an increase of about 1.5 % in the peak occupancy level each year. Due to the introduction of grey water techniques, the demand can be reduced in near future. For an existing demand, the per capita consumption of water is assumed as 0.4 m³ /d.

Therefore the selection of capacity value of 900 m³ /d is justified.

4.7 SELECTION OF NO. OF EFFECTS FOR MED PLANT

An iterative method is used to find out the appropriate no. of effects for the MED Plant. The latent heat at the top brine temperature is calculated. This indicates that the latent heat of vaporisation is used for the first effect for calculation of quantity of steam. The calculations have been carried for several effects from 7 to 10. An average pressure drop of 0.03 bar and a temperature drop of 2.5 °C is considered between each effects.

Table 4 shows the distillate flow in the first effect, required steam flow rate for heating the feed water in the first effect and estimated brine rejection temperature.

Table 4: Selection of No. of Effects

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Following factors are considered for the final selection of no. of effects: − Specific Heat Consumption (kWhth/m³ )

− Total heat exchanger surface area for desalination plant − Estimated brine rejection temperature
− Gained Output Ratio (GOR) value

The selection of the no. of effects mainly depends on the percentage savings made in the specific heat consumption. Figure 28 shows the correlation of the specific heat consumption and the no. of stages (considering condenser along with the evaporator effects) for the MED desalination plant.

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Figure 28: Specific Heat Consumption of MED Desalination Plant²¹

From figure 28 it is clear that the higher no. of effects have lower specific heat consumption. Along with the significant energy savings and higher GOR value, the investment costs are also a major constraint. Increasing the no. of stages also increases the investment costs. The investment costs are linearly proportional to the no. of stages, whereas, the specific energy consumption has a hyperbolic co-relation with respect to the no. of stages.

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Figure 29: Specific Thermal Energy and Investment Cost per year v/s No. of Stages²¹

Therefore the no. of effects are selected considering the energy savings, investment costs as well as other specific requirements. In this case of a proposed plant for the Cypriot resort, sea water preheating is also a necessary aspect to be considered for selection of no. of effects which will be discussed later in section 8.1. For the plant, the brine rejection temperature after the 8th effect is reduced to such an extent that it is difficult to use the overall brine rejection temperature for preheating the inlet sea water.

Considering the required heat exchanger area, the estimated brine temperature and the amount of steam required, the MED desalination plant design with 8 evaporator effects and a condenser is the most suitable one.

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