- Email: [email protected]
Humidification–dehumidification desalination system driven by geothermal energy
Desalination 249 (2009) 602–608
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Humidification–dehumidification desalination system driven by geothermal energy A.M.I. Mohamed a,⁎, N.A.S. El-Minshawy b,1 a b
Mechanical Engineering Department, Faculty of Engineering, Suez Canal University, Egypt Mechanical Engineering Department, College of Engineering, Qassim University, B.O. Box 1299 Burydah, Saudi Arabia
a r t i c l e
i n f o
Article history: Accepted 22 December 2008 Available online 7 October 2009 Keywords: Desalination Renewable energy Geothermal energy Humidification–dehumidification Simulation
a b s t r a c t The work presented in this paper focuses on desalinating sea water system using a humidification– dehumidification process as it is supplied with water heated by geothermal energy as clean and renewable natural resources of energy. Computer simulation of the behavior under various working conditions of the desalination system was carried out to predict the variations of key output. Such variables include the ratio of sea water mass flow rate related to air mass flow rate, cooling water temperature difference across the condenser, geothermal source inlet temperatures to the heat exchanger and the amount of produced distilled water. To validate the computer program, a comparison between the experimental and theoretical results was conducted, and a good agreement had been obtained. The result showed that, the optimum value of the ratio between sea water mass flow rate to air mass flow rate was found to be in the range of 1.5 to 2.5. Improvement in the fresh water productivity at the optimum ratio of sea water mass flow rate to the air flow rate was observed by increasing both the geothermal source inlet temperature and the cooling water temperature difference across the condenser. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Earth is a water-rich planet, which is fortunate because water is key to man's progress. It is essential for agricultural and industrial growth and is required to support growing urban populations. Most of the countries in the Middle East are arid, and they are facing great challenges due to limited water resources and aridity. Desalination seems to be the most suitable solution. Desalination of sea water and brackish water has been given high priority as a source of water for domestic, industrial and agricultural applications in this region [1]. Standard desalination techniques such as multi-stage flash, multi-effect, vapor compression and reverse osmosis are expensive technologies especially when driven by conventional energy sources. Additionally, the geothermal energy is environmentally advantageous energy source which produces far less air pollution than fossil-fuel sources. The life of a geothermal resource may be prolonged by re-injecting the waste fluid which is the most common method of disposal. Geothermal energy as one of the renewable energy resources represents an alternative source of energy, which provide many advantages compared to conventional energy produced from fossil fuel [2]. Most desalination thermal process is performed at relatively low temperature. Consequently low enthalpy geothermal energy is
⁎ Corresponding author. Tel.: +20 101597112; fax: +20 663400936. E-mail addresses: [email protected] (A.M.I. Mohamed), [email protected] (N.A.S. El-Minshawy). 1 Tel.: +966 55 3856918; fax: +966 6 3802992. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.12.053
gradually emerging as successful renewable energy source of producing fresh water with great advantages [3,4]. Akpinara and Hepbasli [5] studied the exergetic performance of the geothermal heat pump systems installed in Turkey based on the actual operation data. They found that the geothermal thermal application is suitable for the developing countries as an available source of energy. This is due to their higher energy utilization efficiencies than those of both conventional heating and cooling systems. Manologloua et al. [6] presented the socio-economic impacts of a geothermal desalination plant on the island of Milos (Greece), which suffers from lack of water. Several comparative studies on the different renewable energy sources that are used for desalination of brackish and sea water are published [7–9]. Tzen and Morris [7] and Rodriguez [8] found that geothermal energy is suitable for different desalination processes at reasonable cost wherever a proper geothermal source is available. One of the main advantages is that no energy storage is required. Many workers have studied the humidification–dehumidification desalination system (HDDS) for different low temperature energy sources (solar, geothermal, PV systems, etc.). Bourouni et al. [10] presented and analyzed the operation and performance of different HDDS plants worldwide. They recommended that HDDS installations can be used for the low temperature part of classical distillers, this is to avoid effects of vacuum in which distillers have to function. A new sea water solar HDDS was described by Chafik [11]. Nafey et al. [12,13] investigated theoretically and experimentally the solar HDDS system. The results showed that the productivity of the unit is strongly influenced by the air flow rate, cooling water flow rate and total solar
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
energy incident through the day. Recently, Orfi et al. [14,15] proved theoretically that the daily production of fresh water by solar HDDS system depends on the ratio between the salt water and the air mass flow rates. More recently, Yamal and Solmus [16] designed a theoretical model to simulate the solar HDDS system based on the idea of closed water and open air cycles. They found that the system productivity increased by about 8%. Four configurations are analyzed for the air humidification–dehumidification water desalination system by Eettouney [17]. Al-Enezi et al. [18] experimentally evaluated the desalination process characteristics as a function of the flow rate of the water and air streams, the temperature of the water stream and the temperature of the cooling water. The previous studies of the HDDS focused on the use of solar energy as renewable source to drive the desalinating sea water system. This is because energy cost is one of the most important elements in determining the cost of water production from desalination plants. The present work concerns with the use of the geothermal energy, at low enthalpy, as a heat source for the HDDS. Also included are design guide lines for the proposed desalinating sea water system. 2. Geothermal desalination system The selection of the appropriate renewable energy/desalination technology depends on a number of factors. These include, plant size, feed water salinity, remoteness, availability of grid electricity, technical infrastructure and the type and potential of the local renewable energy resource. Among the several possible combinations of desalination and renewable energy technologies, some seem to be more promising in terms of techno-economic feasibility than others. However, their applicability strongly depends on the local availability of renewable energy sources and the quality of water to be desalinated. Geothermal sources were classified according to their reservoir enthalpy/temperatures as low (b100 °C), medium (100–150 °C) and high temperature (N150 °C). For water production, geothermal energy plays an important role as a source of energy for low enthalpy/temperature desalination systems. In the phase-change or thermal processes, the distillation of sea water is achieved by utilizing a thermal energy source, normally at temperature around 100 °C. Thus, for humidification–dehumidification sea water desalination system low temperature geothermal energy source is suitable. In the present work, it is assumed that the operating behavior of a desalination system driven by geothermal energy is in a steady state case. The most promising system equations, boundary conditions, and optimum working conditions are conducted via computer program. The computed results of the program had been validated and compared with the experimental results. The influences of some special arrangement features of the desalination unit from the thermodynamic, heat and mass transfer are significantly considered. 3. Experimental apparatus and measurement techniques The main objective of the experimental test facility is to validate the different simulated heat and mass transfer processes in the simulation model. The humidification–dehumidification desalination system with geothermal heat source test facility is shown in Fig. 1. The system operates at atmospheric pressure in which air is used as a carrier for vapor. The hot water is injected to the top of the humidifier tower equipped with packed bed to increase the contact surface and therefore improve the humidification rate. The air is firstly heated in an air-preheater and is then passed through the backed bed to meet the sprayed hot water and becomes moist or saturated at point (3). The air is discharged to the
603
humidification tower using an air blower and the air flow rate is controlled by a by-pass and control valves. Hot and saturated air flows toward the dehumidifier (condensation tower), then it condenses as it comes into contact with the cold condensation cooling water coils to remove the water moisture from the carrier air to obtain the distilled fresh water. Digital temperature/humidity measuring instrument (Testo 625, K-type temperature sensor) is used to measure each point of the air temperature and relative humidity at each point of the system. This is done with an accuracy of ± 2% RH and the air temperature measurement with an accuracy of ±0.5 °C. The temperature of the hot water, sprayed water, and cooling water are measured using Ktype thermocouples, with an accuracy of 0.1 °C. Each of air, hot water and sprayed heated water flow rates was measured using a glass Rotameter (KDG memory) fitted with each circuit, as shown in Fig. 1. The accuracy of the measured quantity of each is about 1% of the measured value. To estimate the uncertainty of the heat and mass transfer rate, the mathematical method of the root mean square was applied [19]. The uncertainty is determined by the square root of the sum of the squares of the uncertainties of the separated terms. Substituting the individual expected uncertainties of the measured quantities into the main equation yields to the uncertainty in the heat and mass transfer quantity, not exceed ±4.59%. A set of experiments were conducted with different operating conditions including water to air mass flow rates (varied from 1.5 to 2.5), different inlet temperatures to the heat exchanger from the heating source (varied from 60 to 90 °C). During the experiments, air temperature and humidity of the air were measured after the steady state conditions. The heating water temperatures were also measured for the whole system. 4. Theoretical model description The mathematical model was developed according to the energy and mass balance equations for each process in the cycle. It mainly includes heat exchanger energy balance, humidification process, dehumidification process and the system boundary conditions. The system is assumed to be adiabatic (no heat losses). The system is also working under the atmospheric pressure. It is assumed to be working under steady state conditions. The sea water properties are assumed to be equal to the fresh water properties. The psychometric conditions of the air had been calculated referred to ASHRAE standard properties [20]. The following equations below describe the mathematical formulation of the model. 4.1. Heat exchanger energy balance equation The heat exchanger type is shell and tube with counter flow between the geothermal hot water and the sea water. The energy equation of the heat exchanger is as follows; Q˙ g = Q˙ s = Q˙ HTgs :
ð1Þ
This equation yields to; m˙ g cp ðtgi −tgo Þ = m˙ s cp ðtso −tsi Þ = UAHT Δθm :
ð2Þ
4.2. Air preheater energy balance equation The air preheater is used to heat the inlet air to the humidifier, the energy balance leads to the next equation; Q˙ a = m˙ a cp ðta2 −ta1 Þ = UAHT Δθm :
ð3Þ
604
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
Fig. 1. Schematic description of the experimental test rig. a) Heat source, b) heat exchanger, c) water tank, d) air preheater, e) humidifier, f) dehumidifier, and g) cooling water.
where tci and tco are the inlet and outlet cooling temperatures in the dehumidifier (condenser) and ṁ d is the distillate mass flow rate exit from the dehumidifier. The quantity of the produced fresh water is given by;
4.3. Humidifier energy and mass balance equations The mass balance equation is described as; dm˙ h dx
3 2
= m˙ a
dωh 3 : dx 2
ð4Þ
By integration of the equation over the length of the humidifier, then m˙ h = m˙ a ðω3 −ω2 Þ = m˙ sp − m˙ b :
ð5Þ
The heat balance equation inside the humidifier is; Q˙ a = Q˙ sp −Q˙ b :
ð6Þ
Then; m˙ a ðha3 −ha2 Þ = m˙ sp cp tsp −m˙ b cp tb2
ð7Þ
where tb2 is the wet bulb temperature at point (2). 4.4. Dehumidification energy and mass balance equations The mass balance equation is described as; dm˙ d dx
4 3
= m˙ a
dωd 4 : dx 3
ð8Þ
By integration of the equation over the length of the dehumidifier, then m˙ d = m˙ a ðω3 −ω4 Þ:
ð9Þ
The heat balance equation inside the dehumidifier is; Q˙ a = Q˙ c = Q˙ HT :
ð10Þ
Then m˙ a ðha3 −ha4 Þ = m˙ c cp ðtco −tci Þ + m˙ d cp tb4 = UAHT Δθm
ð11Þ
m˙ w = m˙ d :
ð12Þ
5. Simulation model and its validation A computer simulation program based on the energy and mass balance equations mentioned above has been developed by means of VISUAL BASIC programming language. The simulation program is then used to investigate the different effects of the system working conditions and the temperature of the geothermal source on the productivity of the system. In this simulation program, energy and mass balance equations and boundary conditions are solved simultaneously using the analytical and iteration methods. In order to validate the computer simulation program results, a comparison between the experimental and the theoretical results had been applied. The next figures, Figs. 2–4, illustrate a comparison between experimental and theoretical results obtained from the computer simulation program. This is for each sprayed water temperature to the humidifier, dry air temperature exit from the humidifier, and the difference in the value of moisture content across the condenser in (kg fresh water/kg dry air). In this comparison the same boundary, design and physical conditions for both of experiments and computer runs are considered. The comparison has been done for different geothermal sources (from 60 to 90 °C), as well as different ratios of sea water mass flow rate enter the heat exchanger to the air mass flow rate that enters the preheater, which varied from 1 to 3. The comparison between the present theoretical and experimental results showed, in general, acceptable agreement. This includes, each of the trend and the quantity values of the results. The maximum difference between the theoretical and experimental results was found to be about 12%. This value was expected due to uncertainties in the measured quantities of the experiments and the simplifying assumptions in the theoretical solution in addition to the computational errors and the accuracy of calculations. Fig. 5 shows the variation of the production rates in (kg/h) for different geothermal source temperatures at different sea water mass
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
Fig. 2. Comparison between experimental and theoretical results for sprayed water temperature.
flow rates to air mass flow rate factor. For this case, the maximum value of the production rate was obtained at (ṁ s/ṁ a) equals 2 when geothermal temperature is above 80 °C. A comparison between the present experimental and theoretical values of the difference of air moisture content across condenser per unit area of the condenser against previous literature data of Orfi et al. [14] is presented in Fig. 6.
605
Fig. 5. Variation of the productivity from the system with different geothermal source temperatures at different sea water mass flow rates to air mass flow rate factor.
There is acceptable agreement between the results of the present work and Orfi's results. The deviation between the present work results and Orfi's is due to difference in some boundary conditions between both cases. However, the present system results show a good agreement with the data. 6. Results and discussion Various theoretical works have been carried out to assess the performance of utilizing the geothermal energy for the purpose of sea water desalination. In the present work the effect of the different working parameters on the productivity of the considered desalinating sea water system has been investigated. The design of the plant is shown in Fig. 7. The air is heated in an air-preheater using the geothermal energy source after passing the heat exchanger. The sea water at inlet to plant is used as a cooling water of the condenser. The considered working parameters are: • The ratio of sea water mass flow rate related to air mass flow rate (ṁ s/ṁ a) • Cooling water temperature difference across the condenser, DTcd • Geothermal source inlet temperatures to the heat exchanger.
Fig. 3. Comparison between experimental and theoretical results for dry air at humidifier exit.
A systematic calculation has been studied to assess the effect of the above parameters on the values of the fresh water productivity from the considered desalination system. The obtained results are then
Fig. 4. Comparison between experimental and theoretical results for difference of the air moisture content across condenser.
Fig. 6. Comparison of the present experimental and theoretical results against previous literature data of Orfi et al. [14].
606
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
Fig. 7. Geothermal desalination plant layout.
analyzed for each individual test in order to assess the optimum workable conditions. The relation between the different operating parameters was theoretically studied, including geothermal temperature inlet to the system feed sea water, air and cooling water flow rates. Fig. 8 illustrates the effect of geothermal water temperature and the ratio of sea water mass flow rate to air mass flow rate on the sprayed water temperature inlet to the humidifier. In addition, Fig. 9 illustrates the effect of the same two parameters (Tg and (ṁ s/ṁ a)) on the dry air temperature at humidifier exit. These figures show an increase in the sprayed water temperature and the dry air temperature at exit from the humidifier with the increase of the geothermal source temperature. In turn, decreasing the ratio of sea water mass flow to air mass
flow rate at constant geothermal source temperature leads to the decrease in each of the sprayed water temperature and the dry air temperature at humidifier exit. Accordingly, the fresh water productivity from the system is reduced. A systematic calculation has been carried out to assess the effect of the different parameters on the values of the fresh water productivity for the considered desalination system. The obtained results from the present work are then analyzed for each individual test, and then introduced in various groups for comparison in order to assess the optimum workable conditions. A comparison between the various groups and the outcomes is illustrated in Figs. 10–12). The effect of sea water flow rate related to the air mass flow rate on the air moisture content, across the condenser for various cooling
Fig. 8. Variation of sprayed water temperature with geothermal temperature source for different ratios of sea water mass flow rate to air mass flow rate.
Fig. 9. Variation of dry air temperature at humidifier exit with geothermal temperature source for different ratios of sea water mass flow rate to air mass flow rate.
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
607
Fig. 10. The effect of sea water flow rate related to the air mass flow rate on the air moisture content across the condenser for various (DTcd) at geothermal source temperature of 70 °C.
Fig. 12. The effect of sea water flow rate related to the air mass flow rate on the air moisture content across the condenser for various (DTcd) at geothermal source temperature of 90 °C.
water temperature differences in the condenser (DTcd) at geothermal source temperature of 70 °C, is illustrated in Fig. 8. It can be seen that the difference of the air moisture content across the inlet and outlet of the condenser increases firstly with the ratio of the mass flow rate of sea water to air mass flow rate till certain value of (ṁ s/ṁ a), then decline as the (ṁ s/ṁ a) increases. Furthermore, the effect of condenser dimensionless temperature on the difference of the air moisture content across the inlet and outlet of the condenser is also presented for geothermal source temperature of 70 °C. It can be noticed that as the condenser dimensionless temperature rise increases the considered desalination system productivity also increases. The same effect of the (ṁ s/ṁ a) and DTcd is repeated but for the geothermal source temperature of 80 °C and 90 °C, as shown in Figs. 11 and 12. As illustrated, an increase in the system productivity has been noticed at higher values of geothermal source temperature, especially at high values of DTcd.
to drive sea water desalination system to convert sea water to fresh water. The fresh water productivity increases by increasing the ratio of sea water mass flow rate to air mass flow rate up to a certain value then declines. The optimum value of the ratio between sea water mass flow rate to air mass flow rate was found to be in the range of 1.5 to 2.5. Increasing the geothermal source inlet temperature leads to a higher fresh water productivity at the optimum value of the ratio of sea water mass flow rate to the air flow rate. Increasing the cooling water temperature difference across the condenser causes a higher fresh water productivity at the optimum value of the ratio of sea water mass flow rate to the air flow rate.
7. Conclusions The present wok was carried out to evaluate the performance of sea water desalination system powered by the geothermal energy as a renewable energy that is available with a great level worldwide. The following can be concluded: • The utility of the low enthalpy geothermal energy that is available in the temperature range up to 100 °C, is found to be a good candidate
Fig. 11. The effect of sea water flow rate related to the air mass flow rate on the air moisture content across the condenser for various (DTcd) at geothermal source temperature of 80 °C.
•
• •
•
Symbols
U
Area, m2 Specific heat capacity, kJ/kg K Cooling water temperature difference across the condenser, °C Enthalpy, kJ/kg Mass flow rate, kg/s Temperature, °C Heat transfer rate, kW Heat transfer coefficient, kW/m2 K
Greek Δ Δθ ω
Difference Logarithmic mean temperature difference, °C Absolute humidity, g/kg
A cp DTcd h ṁ t Q̇
Subscripts a Air b Brine c Cooling water d Dehumidification g Geothermal h Humidifier HT Heat transferred i Inlet o Outlet s Sea water sp Sprayed water w Fresh water
608
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
References [1] W.A. Abdettahman, Urban water management in developing arid countries, Water Resources Development 16/1 (2000) 7–20. [2] M.K. Ghose, Environmentally sustainable supplies of energy with specific reference to geothermal energy, Energy Sources 26 (2004) 531–539. [3] T.J. Hammons, Geothermal power generation worldwide: global perspective, technology, field experience, research and development, Electric Power Components and Systems 32 (2004) 529–553. [4] O. Ozgener, G. Kocer, Geothermal heating applications, Energy Sources 26 (2004) 353–360. [5] E.K. Akpinara, A.A. Hepbasli, Comparative study on exergetic assessment of two ground-source (geothermal) heat pump systems for residential applications, Building and Environment 42/5 (2007) 2004–2013. [6] E. Manologloua, P. Tsartasb, A. Markou, Geothermal energy sources for water production—socio-economic effects and people's wishes on Milos island: a case study, Energy Policy 32 (2004) 623–633. [7] E. Tzen, R. Morris, Renewable energy sources for desalination, Solar Energy 75 (2003) 375–379. [8] L. Rodrhguez, Renewable energy applications in desalination: state of the art, Solar Energy 75 (2003) 381–393. [9] S. Kalogirou, Seawater desalination using renewable energy sources, Progress in Energy and Combustion Science 31 (2005) 242–281. [10] I. Bourouni, M. Chaibib, and L. Tadrist, Water desalination by humidification and dehumidification of air: state of the art, 37 (2001) 167–176.
[11] E. Chafik, A new seawater desalination process using solar energy, Desalination 153 (2002) 25–37. [12] A. Nafey, H. Fath, S. El-Helaby, A. Soliman, Solar desalination using humidification dehumidification processes: part I. a numerical investigation, Energy Conversion and Management 45 (2004) 1243–1261. [13] A. Nafey, H. Fath, S. El-Helaby, A. Soliman, Solar desalination using humidification– dehumidification processes: part II. an experimental investigation, Energy Conversion and Management 45 (2004) 1263–1277. [14] J. Orfi, M. Laplante, H. Marmouch, N. Galanis, B. Benhamou, Experimental and theoretical study of a humidification dehumidification water desalination system using solar energy, Desalination 168 (2004) 151–159. [15] J. Orfi, M. Laplante, Air humidification–dehumidification for a water desalination system using solar energy, Desalination 203 (2007) 471–481. [16] C. Yamal, I. Solmus, Theoretical investigation of a humidification dehumidification desalination system configured by a double-pass flat plate solar air heater, Desalination 205 (2007) 163–177. [17] H. Eettouney, Design and analysis of humidification dehumidification desalination process, Desalination 183 (2005) 341–352. [18] G. Al-Enezi, H. Ettouney, N. Fawzy, Low temperature humidification dehumidification desalination process, Energy Conversion and Management 47 (2006) 470–484. [19] J.P. Holman, Experimental Methods for Engineers, McGraw-Hill, New York, 1986. [20] F.E. Jakob, ASHRAE Fundamentals, CHAPTER 5 (1984).
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Humidification–dehumidification desalination system driven by geothermal energy A.M.I. Mohamed a,⁎, N.A.S. El-Minshawy b,1 a b
Mechanical Engineering Department, Faculty of Engineering, Suez Canal University, Egypt Mechanical Engineering Department, College of Engineering, Qassim University, B.O. Box 1299 Burydah, Saudi Arabia
a r t i c l e
i n f o
Article history: Accepted 22 December 2008 Available online 7 October 2009 Keywords: Desalination Renewable energy Geothermal energy Humidification–dehumidification Simulation
a b s t r a c t The work presented in this paper focuses on desalinating sea water system using a humidification– dehumidification process as it is supplied with water heated by geothermal energy as clean and renewable natural resources of energy. Computer simulation of the behavior under various working conditions of the desalination system was carried out to predict the variations of key output. Such variables include the ratio of sea water mass flow rate related to air mass flow rate, cooling water temperature difference across the condenser, geothermal source inlet temperatures to the heat exchanger and the amount of produced distilled water. To validate the computer program, a comparison between the experimental and theoretical results was conducted, and a good agreement had been obtained. The result showed that, the optimum value of the ratio between sea water mass flow rate to air mass flow rate was found to be in the range of 1.5 to 2.5. Improvement in the fresh water productivity at the optimum ratio of sea water mass flow rate to the air flow rate was observed by increasing both the geothermal source inlet temperature and the cooling water temperature difference across the condenser. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Earth is a water-rich planet, which is fortunate because water is key to man's progress. It is essential for agricultural and industrial growth and is required to support growing urban populations. Most of the countries in the Middle East are arid, and they are facing great challenges due to limited water resources and aridity. Desalination seems to be the most suitable solution. Desalination of sea water and brackish water has been given high priority as a source of water for domestic, industrial and agricultural applications in this region [1]. Standard desalination techniques such as multi-stage flash, multi-effect, vapor compression and reverse osmosis are expensive technologies especially when driven by conventional energy sources. Additionally, the geothermal energy is environmentally advantageous energy source which produces far less air pollution than fossil-fuel sources. The life of a geothermal resource may be prolonged by re-injecting the waste fluid which is the most common method of disposal. Geothermal energy as one of the renewable energy resources represents an alternative source of energy, which provide many advantages compared to conventional energy produced from fossil fuel [2]. Most desalination thermal process is performed at relatively low temperature. Consequently low enthalpy geothermal energy is
⁎ Corresponding author. Tel.: +20 101597112; fax: +20 663400936. E-mail addresses: [email protected] (A.M.I. Mohamed), [email protected] (N.A.S. El-Minshawy). 1 Tel.: +966 55 3856918; fax: +966 6 3802992. 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.12.053
gradually emerging as successful renewable energy source of producing fresh water with great advantages [3,4]. Akpinara and Hepbasli [5] studied the exergetic performance of the geothermal heat pump systems installed in Turkey based on the actual operation data. They found that the geothermal thermal application is suitable for the developing countries as an available source of energy. This is due to their higher energy utilization efficiencies than those of both conventional heating and cooling systems. Manologloua et al. [6] presented the socio-economic impacts of a geothermal desalination plant on the island of Milos (Greece), which suffers from lack of water. Several comparative studies on the different renewable energy sources that are used for desalination of brackish and sea water are published [7–9]. Tzen and Morris [7] and Rodriguez [8] found that geothermal energy is suitable for different desalination processes at reasonable cost wherever a proper geothermal source is available. One of the main advantages is that no energy storage is required. Many workers have studied the humidification–dehumidification desalination system (HDDS) for different low temperature energy sources (solar, geothermal, PV systems, etc.). Bourouni et al. [10] presented and analyzed the operation and performance of different HDDS plants worldwide. They recommended that HDDS installations can be used for the low temperature part of classical distillers, this is to avoid effects of vacuum in which distillers have to function. A new sea water solar HDDS was described by Chafik [11]. Nafey et al. [12,13] investigated theoretically and experimentally the solar HDDS system. The results showed that the productivity of the unit is strongly influenced by the air flow rate, cooling water flow rate and total solar
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
energy incident through the day. Recently, Orfi et al. [14,15] proved theoretically that the daily production of fresh water by solar HDDS system depends on the ratio between the salt water and the air mass flow rates. More recently, Yamal and Solmus [16] designed a theoretical model to simulate the solar HDDS system based on the idea of closed water and open air cycles. They found that the system productivity increased by about 8%. Four configurations are analyzed for the air humidification–dehumidification water desalination system by Eettouney [17]. Al-Enezi et al. [18] experimentally evaluated the desalination process characteristics as a function of the flow rate of the water and air streams, the temperature of the water stream and the temperature of the cooling water. The previous studies of the HDDS focused on the use of solar energy as renewable source to drive the desalinating sea water system. This is because energy cost is one of the most important elements in determining the cost of water production from desalination plants. The present work concerns with the use of the geothermal energy, at low enthalpy, as a heat source for the HDDS. Also included are design guide lines for the proposed desalinating sea water system. 2. Geothermal desalination system The selection of the appropriate renewable energy/desalination technology depends on a number of factors. These include, plant size, feed water salinity, remoteness, availability of grid electricity, technical infrastructure and the type and potential of the local renewable energy resource. Among the several possible combinations of desalination and renewable energy technologies, some seem to be more promising in terms of techno-economic feasibility than others. However, their applicability strongly depends on the local availability of renewable energy sources and the quality of water to be desalinated. Geothermal sources were classified according to their reservoir enthalpy/temperatures as low (b100 °C), medium (100–150 °C) and high temperature (N150 °C). For water production, geothermal energy plays an important role as a source of energy for low enthalpy/temperature desalination systems. In the phase-change or thermal processes, the distillation of sea water is achieved by utilizing a thermal energy source, normally at temperature around 100 °C. Thus, for humidification–dehumidification sea water desalination system low temperature geothermal energy source is suitable. In the present work, it is assumed that the operating behavior of a desalination system driven by geothermal energy is in a steady state case. The most promising system equations, boundary conditions, and optimum working conditions are conducted via computer program. The computed results of the program had been validated and compared with the experimental results. The influences of some special arrangement features of the desalination unit from the thermodynamic, heat and mass transfer are significantly considered. 3. Experimental apparatus and measurement techniques The main objective of the experimental test facility is to validate the different simulated heat and mass transfer processes in the simulation model. The humidification–dehumidification desalination system with geothermal heat source test facility is shown in Fig. 1. The system operates at atmospheric pressure in which air is used as a carrier for vapor. The hot water is injected to the top of the humidifier tower equipped with packed bed to increase the contact surface and therefore improve the humidification rate. The air is firstly heated in an air-preheater and is then passed through the backed bed to meet the sprayed hot water and becomes moist or saturated at point (3). The air is discharged to the
603
humidification tower using an air blower and the air flow rate is controlled by a by-pass and control valves. Hot and saturated air flows toward the dehumidifier (condensation tower), then it condenses as it comes into contact with the cold condensation cooling water coils to remove the water moisture from the carrier air to obtain the distilled fresh water. Digital temperature/humidity measuring instrument (Testo 625, K-type temperature sensor) is used to measure each point of the air temperature and relative humidity at each point of the system. This is done with an accuracy of ± 2% RH and the air temperature measurement with an accuracy of ±0.5 °C. The temperature of the hot water, sprayed water, and cooling water are measured using Ktype thermocouples, with an accuracy of 0.1 °C. Each of air, hot water and sprayed heated water flow rates was measured using a glass Rotameter (KDG memory) fitted with each circuit, as shown in Fig. 1. The accuracy of the measured quantity of each is about 1% of the measured value. To estimate the uncertainty of the heat and mass transfer rate, the mathematical method of the root mean square was applied [19]. The uncertainty is determined by the square root of the sum of the squares of the uncertainties of the separated terms. Substituting the individual expected uncertainties of the measured quantities into the main equation yields to the uncertainty in the heat and mass transfer quantity, not exceed ±4.59%. A set of experiments were conducted with different operating conditions including water to air mass flow rates (varied from 1.5 to 2.5), different inlet temperatures to the heat exchanger from the heating source (varied from 60 to 90 °C). During the experiments, air temperature and humidity of the air were measured after the steady state conditions. The heating water temperatures were also measured for the whole system. 4. Theoretical model description The mathematical model was developed according to the energy and mass balance equations for each process in the cycle. It mainly includes heat exchanger energy balance, humidification process, dehumidification process and the system boundary conditions. The system is assumed to be adiabatic (no heat losses). The system is also working under the atmospheric pressure. It is assumed to be working under steady state conditions. The sea water properties are assumed to be equal to the fresh water properties. The psychometric conditions of the air had been calculated referred to ASHRAE standard properties [20]. The following equations below describe the mathematical formulation of the model. 4.1. Heat exchanger energy balance equation The heat exchanger type is shell and tube with counter flow between the geothermal hot water and the sea water. The energy equation of the heat exchanger is as follows; Q˙ g = Q˙ s = Q˙ HTgs :
ð1Þ
This equation yields to; m˙ g cp ðtgi −tgo Þ = m˙ s cp ðtso −tsi Þ = UAHT Δθm :
ð2Þ
4.2. Air preheater energy balance equation The air preheater is used to heat the inlet air to the humidifier, the energy balance leads to the next equation; Q˙ a = m˙ a cp ðta2 −ta1 Þ = UAHT Δθm :
ð3Þ
604
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
Fig. 1. Schematic description of the experimental test rig. a) Heat source, b) heat exchanger, c) water tank, d) air preheater, e) humidifier, f) dehumidifier, and g) cooling water.
where tci and tco are the inlet and outlet cooling temperatures in the dehumidifier (condenser) and ṁ d is the distillate mass flow rate exit from the dehumidifier. The quantity of the produced fresh water is given by;
4.3. Humidifier energy and mass balance equations The mass balance equation is described as; dm˙ h dx
3 2
= m˙ a
dωh 3 : dx 2
ð4Þ
By integration of the equation over the length of the humidifier, then m˙ h = m˙ a ðω3 −ω2 Þ = m˙ sp − m˙ b :
ð5Þ
The heat balance equation inside the humidifier is; Q˙ a = Q˙ sp −Q˙ b :
ð6Þ
Then; m˙ a ðha3 −ha2 Þ = m˙ sp cp tsp −m˙ b cp tb2
ð7Þ
where tb2 is the wet bulb temperature at point (2). 4.4. Dehumidification energy and mass balance equations The mass balance equation is described as; dm˙ d dx
4 3
= m˙ a
dωd 4 : dx 3
ð8Þ
By integration of the equation over the length of the dehumidifier, then m˙ d = m˙ a ðω3 −ω4 Þ:
ð9Þ
The heat balance equation inside the dehumidifier is; Q˙ a = Q˙ c = Q˙ HT :
ð10Þ
Then m˙ a ðha3 −ha4 Þ = m˙ c cp ðtco −tci Þ + m˙ d cp tb4 = UAHT Δθm
ð11Þ
m˙ w = m˙ d :
ð12Þ
5. Simulation model and its validation A computer simulation program based on the energy and mass balance equations mentioned above has been developed by means of VISUAL BASIC programming language. The simulation program is then used to investigate the different effects of the system working conditions and the temperature of the geothermal source on the productivity of the system. In this simulation program, energy and mass balance equations and boundary conditions are solved simultaneously using the analytical and iteration methods. In order to validate the computer simulation program results, a comparison between the experimental and the theoretical results had been applied. The next figures, Figs. 2–4, illustrate a comparison between experimental and theoretical results obtained from the computer simulation program. This is for each sprayed water temperature to the humidifier, dry air temperature exit from the humidifier, and the difference in the value of moisture content across the condenser in (kg fresh water/kg dry air). In this comparison the same boundary, design and physical conditions for both of experiments and computer runs are considered. The comparison has been done for different geothermal sources (from 60 to 90 °C), as well as different ratios of sea water mass flow rate enter the heat exchanger to the air mass flow rate that enters the preheater, which varied from 1 to 3. The comparison between the present theoretical and experimental results showed, in general, acceptable agreement. This includes, each of the trend and the quantity values of the results. The maximum difference between the theoretical and experimental results was found to be about 12%. This value was expected due to uncertainties in the measured quantities of the experiments and the simplifying assumptions in the theoretical solution in addition to the computational errors and the accuracy of calculations. Fig. 5 shows the variation of the production rates in (kg/h) for different geothermal source temperatures at different sea water mass
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
Fig. 2. Comparison between experimental and theoretical results for sprayed water temperature.
flow rates to air mass flow rate factor. For this case, the maximum value of the production rate was obtained at (ṁ s/ṁ a) equals 2 when geothermal temperature is above 80 °C. A comparison between the present experimental and theoretical values of the difference of air moisture content across condenser per unit area of the condenser against previous literature data of Orfi et al. [14] is presented in Fig. 6.
605
Fig. 5. Variation of the productivity from the system with different geothermal source temperatures at different sea water mass flow rates to air mass flow rate factor.
There is acceptable agreement between the results of the present work and Orfi's results. The deviation between the present work results and Orfi's is due to difference in some boundary conditions between both cases. However, the present system results show a good agreement with the data. 6. Results and discussion Various theoretical works have been carried out to assess the performance of utilizing the geothermal energy for the purpose of sea water desalination. In the present work the effect of the different working parameters on the productivity of the considered desalinating sea water system has been investigated. The design of the plant is shown in Fig. 7. The air is heated in an air-preheater using the geothermal energy source after passing the heat exchanger. The sea water at inlet to plant is used as a cooling water of the condenser. The considered working parameters are: • The ratio of sea water mass flow rate related to air mass flow rate (ṁ s/ṁ a) • Cooling water temperature difference across the condenser, DTcd • Geothermal source inlet temperatures to the heat exchanger.
Fig. 3. Comparison between experimental and theoretical results for dry air at humidifier exit.
A systematic calculation has been studied to assess the effect of the above parameters on the values of the fresh water productivity from the considered desalination system. The obtained results are then
Fig. 4. Comparison between experimental and theoretical results for difference of the air moisture content across condenser.
Fig. 6. Comparison of the present experimental and theoretical results against previous literature data of Orfi et al. [14].
606
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
Fig. 7. Geothermal desalination plant layout.
analyzed for each individual test in order to assess the optimum workable conditions. The relation between the different operating parameters was theoretically studied, including geothermal temperature inlet to the system feed sea water, air and cooling water flow rates. Fig. 8 illustrates the effect of geothermal water temperature and the ratio of sea water mass flow rate to air mass flow rate on the sprayed water temperature inlet to the humidifier. In addition, Fig. 9 illustrates the effect of the same two parameters (Tg and (ṁ s/ṁ a)) on the dry air temperature at humidifier exit. These figures show an increase in the sprayed water temperature and the dry air temperature at exit from the humidifier with the increase of the geothermal source temperature. In turn, decreasing the ratio of sea water mass flow to air mass
flow rate at constant geothermal source temperature leads to the decrease in each of the sprayed water temperature and the dry air temperature at humidifier exit. Accordingly, the fresh water productivity from the system is reduced. A systematic calculation has been carried out to assess the effect of the different parameters on the values of the fresh water productivity for the considered desalination system. The obtained results from the present work are then analyzed for each individual test, and then introduced in various groups for comparison in order to assess the optimum workable conditions. A comparison between the various groups and the outcomes is illustrated in Figs. 10–12). The effect of sea water flow rate related to the air mass flow rate on the air moisture content, across the condenser for various cooling
Fig. 8. Variation of sprayed water temperature with geothermal temperature source for different ratios of sea water mass flow rate to air mass flow rate.
Fig. 9. Variation of dry air temperature at humidifier exit with geothermal temperature source for different ratios of sea water mass flow rate to air mass flow rate.
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
607
Fig. 10. The effect of sea water flow rate related to the air mass flow rate on the air moisture content across the condenser for various (DTcd) at geothermal source temperature of 70 °C.
Fig. 12. The effect of sea water flow rate related to the air mass flow rate on the air moisture content across the condenser for various (DTcd) at geothermal source temperature of 90 °C.
water temperature differences in the condenser (DTcd) at geothermal source temperature of 70 °C, is illustrated in Fig. 8. It can be seen that the difference of the air moisture content across the inlet and outlet of the condenser increases firstly with the ratio of the mass flow rate of sea water to air mass flow rate till certain value of (ṁ s/ṁ a), then decline as the (ṁ s/ṁ a) increases. Furthermore, the effect of condenser dimensionless temperature on the difference of the air moisture content across the inlet and outlet of the condenser is also presented for geothermal source temperature of 70 °C. It can be noticed that as the condenser dimensionless temperature rise increases the considered desalination system productivity also increases. The same effect of the (ṁ s/ṁ a) and DTcd is repeated but for the geothermal source temperature of 80 °C and 90 °C, as shown in Figs. 11 and 12. As illustrated, an increase in the system productivity has been noticed at higher values of geothermal source temperature, especially at high values of DTcd.
to drive sea water desalination system to convert sea water to fresh water. The fresh water productivity increases by increasing the ratio of sea water mass flow rate to air mass flow rate up to a certain value then declines. The optimum value of the ratio between sea water mass flow rate to air mass flow rate was found to be in the range of 1.5 to 2.5. Increasing the geothermal source inlet temperature leads to a higher fresh water productivity at the optimum value of the ratio of sea water mass flow rate to the air flow rate. Increasing the cooling water temperature difference across the condenser causes a higher fresh water productivity at the optimum value of the ratio of sea water mass flow rate to the air flow rate.
7. Conclusions The present wok was carried out to evaluate the performance of sea water desalination system powered by the geothermal energy as a renewable energy that is available with a great level worldwide. The following can be concluded: • The utility of the low enthalpy geothermal energy that is available in the temperature range up to 100 °C, is found to be a good candidate
Fig. 11. The effect of sea water flow rate related to the air mass flow rate on the air moisture content across the condenser for various (DTcd) at geothermal source temperature of 80 °C.
•
• •
•
Symbols
U
Area, m2 Specific heat capacity, kJ/kg K Cooling water temperature difference across the condenser, °C Enthalpy, kJ/kg Mass flow rate, kg/s Temperature, °C Heat transfer rate, kW Heat transfer coefficient, kW/m2 K
Greek Δ Δθ ω
Difference Logarithmic mean temperature difference, °C Absolute humidity, g/kg
A cp DTcd h ṁ t Q̇
Subscripts a Air b Brine c Cooling water d Dehumidification g Geothermal h Humidifier HT Heat transferred i Inlet o Outlet s Sea water sp Sprayed water w Fresh water
608
A.M.I. Mohamed, N.A.S. El-Minshawy / Desalination 249 (2009) 602–608
References [1] W.A. Abdettahman, Urban water management in developing arid countries, Water Resources Development 16/1 (2000) 7–20. [2] M.K. Ghose, Environmentally sustainable supplies of energy with specific reference to geothermal energy, Energy Sources 26 (2004) 531–539. [3] T.J. Hammons, Geothermal power generation worldwide: global perspective, technology, field experience, research and development, Electric Power Components and Systems 32 (2004) 529–553. [4] O. Ozgener, G. Kocer, Geothermal heating applications, Energy Sources 26 (2004) 353–360. [5] E.K. Akpinara, A.A. Hepbasli, Comparative study on exergetic assessment of two ground-source (geothermal) heat pump systems for residential applications, Building and Environment 42/5 (2007) 2004–2013. [6] E. Manologloua, P. Tsartasb, A. Markou, Geothermal energy sources for water production—socio-economic effects and people's wishes on Milos island: a case study, Energy Policy 32 (2004) 623–633. [7] E. Tzen, R. Morris, Renewable energy sources for desalination, Solar Energy 75 (2003) 375–379. [8] L. Rodrhguez, Renewable energy applications in desalination: state of the art, Solar Energy 75 (2003) 381–393. [9] S. Kalogirou, Seawater desalination using renewable energy sources, Progress in Energy and Combustion Science 31 (2005) 242–281. [10] I. Bourouni, M. Chaibib, and L. Tadrist, Water desalination by humidification and dehumidification of air: state of the art, 37 (2001) 167–176.
[11] E. Chafik, A new seawater desalination process using solar energy, Desalination 153 (2002) 25–37. [12] A. Nafey, H. Fath, S. El-Helaby, A. Soliman, Solar desalination using humidification dehumidification processes: part I. a numerical investigation, Energy Conversion and Management 45 (2004) 1243–1261. [13] A. Nafey, H. Fath, S. El-Helaby, A. Soliman, Solar desalination using humidification– dehumidification processes: part II. an experimental investigation, Energy Conversion and Management 45 (2004) 1263–1277. [14] J. Orfi, M. Laplante, H. Marmouch, N. Galanis, B. Benhamou, Experimental and theoretical study of a humidification dehumidification water desalination system using solar energy, Desalination 168 (2004) 151–159. [15] J. Orfi, M. Laplante, Air humidification–dehumidification for a water desalination system using solar energy, Desalination 203 (2007) 471–481. [16] C. Yamal, I. Solmus, Theoretical investigation of a humidification dehumidification desalination system configured by a double-pass flat plate solar air heater, Desalination 205 (2007) 163–177. [17] H. Eettouney, Design and analysis of humidification dehumidification desalination process, Desalination 183 (2005) 341–352. [18] G. Al-Enezi, H. Ettouney, N. Fawzy, Low temperature humidification dehumidification desalination process, Energy Conversion and Management 47 (2006) 470–484. [19] J.P. Holman, Experimental Methods for Engineers, McGraw-Hill, New York, 1986. [20] F.E. Jakob, ASHRAE Fundamentals, CHAPTER 5 (1984).