Feasibility Study of Brackish Water Desalination in the Egyptian Deserts and Rural Regions Using PV Systems

Transcription

1 Feasibility Study of Brackish Water Desalination in the Egyptian Deserts and Rural Regions Using PV Systems G.E. Ahmad, *J. Schmid National Research Centre, Solar Energy Department P.O. Box 12622, El-Tahrir St., Dokki, Cairo, Egypt *Institute Für Solare Energieversorgungstechnik, Kassel, Germany Abstract Fresh water is the most important source for life on the earth. In the Egyptian deserts and rural areas, there is a shortage in fresh water in spite of the presence of large sources of brackish water. Solar energy is abundant in these remote areas of Egypt, where the amount of sunshine hours is around 3500h per year. This paper introduces a feasibility study of water desalination in these areas using Photovoltaic energy as the primary source of energy. The availability of water resources and solar energy in these areas has been investigated. Also, a design of a PV-powered small-scale reverse osmoses water desalination system is carried out and economically estimated. It is found that the cost of producing 1m 3 of fresh water using the small PV-powered RO water desalination systems is 3.73 $. This cost is based on using a small system that is operating during the daylight only. If the system size and the daily period of operation are increased, the price of producing fresh water will be decreased in these regions. Also, it is important to mention that using renewable energy sources in feeding different systems in these rural areas with their energy demands will maintain their environment clean and healthy for people life. 1. INTRODUCTION Water desalination is one of the most important factors that can help in developing the remote areas and the desert. Water desalination is the method by which brackish water can be changed to fresh water suitable for drinking and irrigation and other uses of people and animals. Water desalination can be carried out by different techniques that lies under two categories (Kalogirou Soteris, 1996), Gocht et al (1998); 1- Thermal processes 2- Membrane processes The thermal processes can be subdivided into the following processes: a- Multistage flash evaporation b- Multiple effect boiling c- Vapour compression d- Freezing e- Solar distillation While the membrane processes are subdivided into: a- Reverse osmosis b- Electrodialysis The water desalination processes require significant quantities of energy to achieve the salt separation and to get the fresh water. The amount and shape of the energy required differs according to the used technique in water desalination. In Egypt, the rapid industrial growth and population increase in rural areas has resulted in a large escalation of demand for fresh water. According to some studies, the demand for potable water in Egypt is estimated to be 12.9 x10 9 m 3 /year in year 2025; i.e. it is expected to be about 3.5 times the present demand of 3.7x 10 9 m 3 /year (Mogahed & Mekhemer, 1991), (Abdelrassoul 1998) So, we study the feasibility of using PV powered RO water ISES 2001 Solar World Congress 1031

2 desalination plants in the Egyptian rural regions that suffer from a sharp shortage of fresh water, while they have a large amounts of underground brackish water and solar energy. 2. WHY COUPLING REVERSE OSMOSIS UNITS AND PV GENERATOR Water desalination by the technique of reverse osmosis is proved to be the lowest energy consuming technique according to many studies. It consumes nearly around half of the energy needed for thermal process, Voros et al (1998). Also; the modularity of reverse osmosis units, their simplicity of operation, their compact sizes and lower environmental impacts give them the priority to be used for water desalination in remote areas. Water desalination by reverse osmosis units removes not only inorganic ions, but also organic matters, viruses and bacteria. On the other hand, PV generators are direct, simple, maintenance free, quiet, clean, renewable and economic power sources in rural areas. Due to all the above mentioned reasons we are studying the use of PV generators to feed water desalination reverse osmosis units in the Egyptian remote areas that are still far away from the local electrical grid. 3. WATER RESOURCES AND SOLAR ENERGY AVAILABILITY To study the feasibility of carrying out a project like brackish water desalination in remote areas powered by PV generator, it is very important to ensure the availability and specifications of brackish water resources and solar energy in these regions. From this point of view, tables 1 and 2 introduce the specifications of the water resources and solar energy in some remote areas of Egypt (Mohamad, 1987), (Ahmad & Mohamad 2000). Table 1: Sample of wells in rural regions of Egypt and their water specifications (Mohamad, 1987) Site Water Depth (m) Water specifications Salinity (PPM) Sidr Dahab Nuibaa North coast East Owaynat Table2: The average daily solar energy on horizontal and tilted planes in kwh/m2/day, mean ambient temperature and average sun shine hours for a complete year for Cairo city in Egypt (Ahmad & Mohamad 2000) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec H hori H tilted T a PSSH From table 2; the average daily solar energy in a complete year on a tilted surface by an angle equal to latitude angle (β=φ) can be estimated to be about 6 kwh/m 2 /day. This corresponds to 6 hours of peak sunshine intensity of 1000 W/m LOAD ENERGY REQUIREMENTS FOR WATER DESALINATION In this study, it is intended to design a small PV-powered reverse osmosis water desalination system. It will be installed in Solar Energy Dept., National Research Centre, Cairo, to study the feasibility of coupling reverse osmosis units with PV energy systems in the Egyptian environment. It will be used also for demonstration purposes. ISES 2001 Solar World Congress 1032

3 It is expected that the system will operate only during the daytime (i.e. during sunshine hours) to avoid the problems of operation with large battery storage that will be required in the case of night operation. The water to be desalinated by the system under consideration is under ground brackish water of salinity up to 2000 PPM. It is required to get an average daily production of 1 m 3 of fresh water. According to table 2, the average peak sunshine hours are estimated to be 6 hours/day. It is found that the suitable available RO units in the markets for this production rate have the following specifications: Feed water salinity 2000 PPM Fresh water production ( m 3 /hr) Maximum recovery rate (50%) Motor power (0.5 HP) Auxiliary loads Booster pump + control (0.3 HP) AC voltage (220V, 50 Hz) Thus the total power required by RO unit is 0.8 HP (i.e. 597 W) The load Power = 597 W Assuming that the system will operate for 7 hours daily Load energy requirements = 597 * 7 = 4179 Wh/day = kwh/day 5. SIZING THE COMPONENTS OF PV GENERATOR 5.1 PV Array It is clear from the previous section that the energy requirements by the RO unit will be kwh/day, but the inverter losses, the battery losses and the PV array thermal losses have to be taken into consideration. Assuming the inverter losses to be about 10%, the battery losses are 15% and the PV array thermal losses in the Egyptian environment 15%. Thus, the peak power of the PV array can be determined as follows (Ahmad & Mohamad 2000); Peak power of PV array = E L / (PSSH * F th * η b * η inv ) (1) The average peak sun shine hours in Cairo, PSSH = 6 hrs, The PV array thermal factor in Cairo, F th = 0.85 Peak power of PV array = / (6 * 0.85* 0.85* 0.9) = 1.07 kw The size of the PV array = 1.07 kw. If PV modules of the mono-crystalline silicon type are selected, each of 50 W as a peak power (I SC =3.27 A, V OC =21.7 V at STC), then 22 modules will be needed. Considering that the DC voltage required for the inverter is 24 V then the PV array will consist of 11 parallel strings each of two series PV modules. 5.2 Battery Storage and Control Due to the rapidly variations in PV power in cloudy days; this can cause a problem for RO plants in particular, as the mechanical parts of the osmosis modules suffer the dynamic strains of the pressure changes (European Network, 1996). So small battery storage is very important option for this system to stabilise the energy input to the RO unit and compensate the effect of solar energy variations. From this point of view, the battery storage is selected to be able to operate the system for one day (7hrs). Assuming a battery maximum depth of discharge of 50 %, then the required maximum battery capacity will be calculated as follows: ISES 2001 Solar World Congress 1033

4 Battery capacity (kwh) = E L / (DOD * η b * η inv ) (2) / (0.5*0.85*0.9) = kwh Considering that 24 V voltage output is required from the battery bank The battery bank capacity in AH = * 1000/24 = 455 AH A battery bank consisting of 12 batteries each of 500 AH, 2V can be used. The suitable battery charge controller for this system is expected to be able to handle the short circuit current of the PV array. Thus it is selected with the specification of 24V, 40A. 5.3 The Inverter The brackish water desalination RO units available in the markets are normally of the AC type single phase or three phase. There are also some DC units but their prices are higher and need more maintenance. So it is preferred to use that of the AC, single-phase type. So the inverter is needed to change the DC output power of the PV array to AC power. According to the system design, the inverter is required to handle the peak power of the PV array and its input and output voltages are suitable for the operation of the battery and RO unit. The selected inverter has the specifications of 24V / 220V single phase 1200 W. Fig.1 shows a block diagram of the designed PV-powered brackish water desalination RO system. Feed Brackish Water PV Array Battery Charge Controller Battery Storage DC/AC Inverter RO Unit Brine Water Fresh Water Fig. 1 Block diagram of the Photovoltaic powered RO desalination plant 6. THE SYSTEM PERFORMANCE As stated in previous sections, the RO unit is expected to operate about 7 hrs/day. The energy required will be supplied by a PV generator and a battery bank that can be used during low intensity periods. In fact, the generated energy from the PV generator will vary during different months of the year according to the variations in the environmental conditions. These variations in the generated energy will affect the daily amount of produced water. The average daily-generated energy can be computed during different months depending on the information of; peak power of PV generator, average daily peak sunshine hours, power conditioning efficiency, and the PV array thermal factor as presented in equation (3). E generated = P peak * PSSH * F th * η b * η inv (3) Using the above equation, a comparison is made between the average daily-generated energy during different months and that required by the load to operate the system for 7 hrs daily as shown in fig.2. ISES 2001 Solar World Congress 1034

5 6 5 Daily required energy by RO Average daily generated energy Energy (kwh/day Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Time (months) Fig.2 The average daily generated energy versus that required by the RO unit at different months. It is found from fig.2 that the energy generated during summer months are expected to be more than that required by the RO unit by about 20%. This surplus energy can be used to get more water during summer. On the other hand, the average daily energy produced during winter months is expected to be less than that required by the load by about 12%. Also, the amount of energy generation during spring and autumn months are expected to be enough to operate the unit according to the design. It is clear that the need for water increases during summer, while it decreases during winter months. This fact is very helpful in feeding RO units by PV generators that gives more energy during summer months. Fig.3 introduces the expected water production during different months. The variations in water production agree with the variations of energy generation of fig. 2 and both of them are related to the daily period of sunshine hours during different months. Average daily water production m Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Time (months) Fig.3 The expected water production during different months To verify the system operation according to the design, the maximum output power of one unit of PV modules specified in the design is measured during a complete day in March at different solar intensity levels. Fig. 4 shows the variations in the PV module output power during that day. The generated energy from the module according to measurements during that day is found to be Wh/day. This means that the output energy of 22 modules will be kwh/day. If the batteries and inverter efficiencies are taken into consideration, the energy available for the load at that day will be 4 kwh/day. This amount of energy represents about 96 % from the energy required by the load and this result agrees with the curves of figures 2,3 for that time of the year. ISES 2001 Solar World Congress 1035

6 Solar Intensity (W/m2) Solar Intensity PV module output Power Module Maximum Power (W) Time (hrs) Fig.4 Instantaneous variations of PV module maximum output power and solar intensity through a complete day of 23/3/ ECONOMICAL OVERVIEW To study the economics of obtaining potable water from under ground brackish water with salinity up to 2000 ppm, it is necessary to estimate the initial cost and the operating cost of the system according to the present prices. Table 3 introduces the prices of the system components and also the operating and maintenance costs. It is important to take into consideration that the expected life time of the system is estimated to be 20 years, while that of the batteries and membranes of the RO unit is expected to be 5 years. This means that batteries and RO membrane will be replaced three times after 5,10, and 15 years. The prices of replacement are considered the same as the first time prices. It is shown in table 3 that the life cycle cost of the PV-powered small brackish water desalination RO system is $. The cost of desalinating 1 m 3 of brackish water using this system can be calculated as follows: If it is assumed that the system will operate 7-hrs daily (on the average estimation), then 1m 3 will be delivered daily. Assuming also that the system will operate only 300 days/year. The expected amount of water desalinated in 20 years = 6000 m 3 Thus the cost of desalinating 1 m 3 = 22365/6000 = 3.73 $ Increasing the size of the system and daily period of operation where the relation between the prices and the size is not linear can decrease this cost. Also, increasing the daily production will increase the cost of operation and maintenance only while the initial cost will not be affected greatly and this accordingly will decrease the cost. ISES 2001 Solar World Congress 1036

7 Table 3: The PV-powered brackish water desalination system cost according to the prices of year 2000 from the Internet The Item The Cost ($ ) The PV array (1100 W) (6 $/W) 6600 The battery bank (11 kwh) (120 $/kwh) 1320 The battery charge controller (0.5 $/W P ) 550 The inverter (1 $/W P ) 1200 The reverse osmosis unit The auxiliaries The installation (10% from initial cost) 1207 The membrane replacement three times (100 $/membrane) The maintenance and operation (2% annually from initial cost) The battery replacement three times (1320$ * 3) 3960 Total life cycle cost $ 8. CONCLUSIONS The study confirmed the importance and suitability of using the technique of Photovoltaic powered RO desalination plants in remote areas of the Egyptian desert. The spread of these systems can contribute in providing these regions with potable water in suitable prices (3.73 $/m 3 ) that can be decreased by using larger systems than that described in this study. Also the technique agree with Egyptian plan that tends to depend on renewable energy sources in developing remote areas. NOMENCLATURE E generated E L F th H hori H tilted The average daily-generated energy (Wh/day) The average daily load energy (kwh/day) PV array thermal factor The average daily solar energy incident on horizontal plane in (kwh/m 2 /day). The average daily solar energy incident on tilted plane by an angle equal to latitude angles (30 ) in (kwh/m 2 /day). P peak Peak power of PV generator (W) PPM Part per million PSSH The average Peak sun shine hours (corresponding to solar intensity 1000 W/m 2 ). T a The mean ambient temperature in C. η b Battery efficiency Inverter efficiency η inv REFERENCES Kalogirou Soteris (1996), Mediterranean Conference On Renewable Energy Sources For Water Production, Santorini, Greece, June 10-12, 1996, pp Gocht W., Sommerfeld, Rautenbach R., Melin Th., Eilers L., Neskakis A., Herold D., Horstmann V., Kabariti M., and Muhaidat A. (1998). Renewable Energy, 14 (1-4), Mogahed, M.M. and Mekhemer, S.S. (1991), Desalination in the Egyptian Context: IAEA First Regional Meeting: Nuclear Desalination As A Source Of Low Cost Potable Water, Cairo, Egypt, Abdelrassoul Roshdy A. (1998). Renewable Energy, 14 (1-4), Voros N.G., Kiranoudis C.T. and Maroulis Z.B. (1998). Desalination, 115, Mohamad M.A. (1987), Utilisation of Solar Cells in Developing Remote Areas, Academy of Scientific Research and Technology Report No. (3),, Egypt, Dec Ahmad G.E., Mohamad M.A. (2000). Energy Conversion and Management, 41(12), European Network to integrate renewable energy into water production. (1996), 2 nd Interim Report, January ISES 2001 Solar World Congress 1037

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