- Email: [email protected]
Potential application of a flash-type barometric desalination plant powered by waste heat from electric-power stations in Cyprus
Applied Energy 83 (2006) 1089–1100
APPLIED ENERGY www.elsevier.com/locate/apenergy
Potential application of a flash-type barometric desalination plant powered by waste heat from electric-power stations in Cyprus G.G. Maidment *, I.W. Eames, M. Psaltas, A. Lalzad Department of Engineering Systems, Faculty of Engineering, Science and the Built Environment, London South Bank University, 103 Borough Road, London SE1 0AA, UK Available online 25 January 2006
Abstract This paper describes and evaluates the results of a study into the problems of freshwater production and shortages on the island of Cyprus. The use of a novel barometric flash-type desalinator, driven by otherwise waste-heat from the island’s power-stations, is proposed as a means of increasing freshwater supplies. Mathematical models are described and used to investigate the thermodynamic performance and economic viability of the proposed system. Although water and electricity supply data for the island of Cyprus were used for the purposes of this investigation, the overall findings are thought have a wider applicability. Ó 2005 Published by Elsevier Ltd.
1. Introduction As with most other Mediterranean islands, the island of Cyprus increasingly suffers from a shortage of drinking water, particularly when the population increases many fold during the tourist season. This paper describes the current situation in Cyprus and investigates the potential use of waste heat from electric-power stations in the production of drinking water from seawater using a novel barometric flash-type desalination device, which is currently under development at London South Bank University, London. Whilst
*
Corresponding author. Tel.: +44 207 815 7676; fax: +44 207 815 7699. E-mail address: [email protected] (G.G. Maidment).
0306-2619/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.apenergy.2005.11.006
1090
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
Nomenclature A a Cp h hfg i m n pv Q_ C T U Vd x y e k
heat-transfer area (m2) annual income from sales of water (Cy£) specific heat-capacity at constant pressure (kJ/kg K) enthalpy (kJ/kg) latent enthalpy of vapourisation discounted interest rate mass flow (kg/s) time over which the return on investment is calculated (years) present value heat rate to desalinator condenser coil (kW) temperature (K or °C) overall heat-transfer coefficient (kW/m2 K) annual production rate of freshwater (m3/annum) dryness fraction specific yield heat-exchange effectiveness m_ c m_ D
Sufficies c seawater flow to desalinator condenser D heated seawater flow to desalination chamber d desalinated water flow s saturation condition in desalination chamber
this paper concerns itself primarily with the situation in Cyprus, the results described herein could be applied to the situations in many other countries. 2. Freshwater supplies in Cyprus Cyprus is one of the largest islands in the Mediterranean Sea, with a resident population of approximately 800,000 people, which is increasing annually at a rate of 1%. The island also has about 2.5 million visitors each year, which significantly impacts on drinking-water supplies. At this time, the total daily water consumption is Cyprus is approximately 700,000 m3 of which domestic water-use represents 25% [1]. Today, of the total water consumption in Cyprus, 76% is used by agriculture whilst the remainder is required for domestic use and industry, which includes tourism. Cyprus currently suffers from, an inadequate and unreliable rainfall and, as a result, a lack of freshwater, particularly in the summer months. Water shortages can occur even when the island receives its average annual rainfall, which at this time is 500 mm. This is because rainfall tends to be concentrated during the winter months [2]. Furthermore, climate change is affecting water supplies. Over the last century, there has been a regular reduction in annual rainfall by approximately 1 mm each year, as shown in Fig. 1. This too has impacted on the availability of freshwater supplies, which have reduced by around
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1091
Fig. 1. Time-averaged annual rainfall in Cyprus between 1901 and 2002.
25% since the 1970s [2]. Cyprus currently faces an increasing shortfall in water supplies, which has led to an over exploitation of groundwater. This has resulted in saline water intrusion and consequent quality deterioration in the coastal aquifers and depletion of inland aquifers. In 2001, annual groundwater extraction was estimated to exceed natural recharge by 15.3 M m3 [3]. Improvements in water-management techniques have helped to reduce the shortfall. During the colonial period, a freshwater water management programme led to the construction of reservoirs capable of storing up to 300 million cubic metres of freshwater [4]. Also, in the early 20th century, emphasis was given to the development of groundwater systems for the provision of drinking water, because these provided a low-cost supplies, which were of good quality and required minimal infrastructure investment. By 1930, the exploitation of the Famagusta and Morphou aquifers had began with thousands of boreholes being drilled. This continued up to 1960, but over-pumping in areas like Famagusta, Morphou and Akrotiri resulted in the depletion of these key aquifers. More recently, the Water Development Department of the Ministry of Agriculture, Natural Resources and Environment has begun to utilised desalination systems in conjunction with dams to store freshwater. 3. Cyprus and desalination With effective desalination systems, the seawater surrounding Cyprus could provide an almost limitless source of freshwater. Desalination of seawater was first introduced in Cyprus on a large-scale in 1997, with the opening of a 20,000 m3/day reverse-osmosis plant at Dhekelia, which was soon expanded to 40,000 m3/day. The plant operates on a buildown-operate-transfer basis and the desalinated water is sold to the Government, at source, at a varying unit price around Cy£0.62/m3 [4]. A new reverse-osmosis seawater desalina-
1092
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
tion plant, of 51,667 m3/day nominal capacity, is presently being constructed next to Larnaca Airport [5]. 4. Cogeneration of drinking water and electricity The use of otherwise wasted low-grade heat from power stations to drive desalination plants is environmentally attractive and may provide cost benefits. The Electricity Authority of Cyprus (EAC) was established in 1952. It is an independent non-profit making semi-government corporation responsible for the generation, transmission and distribution of electricity on the island [6]. At this time, EAC operates three fossil-fuelled power stations, at Moni, Dhekelia and Vasilikos [6]. These are situated along the southern coastline of the island and together produce 800 MWe from steam-turbine powered generators. The condensers of these units are cooled by seawater. EAC also operates gas-turbine powered generators at Moni and Vasilikos producing an additional 188 MWe. With a continuing growth in electricity demand, a further 300 MWe increase in generator capacity is planned for 2006 from fossil-fuel fired stations [5]. By 2006, therefore, the amount of waste heat available to produce desalinated seawater will be of the order of 1.8 GW. Fig. 2 shows a schematic representation of a 60 MWe steam-turbine powered electricity generator used in Dhekelia. The design-point data for the power cycle are listed in Table 1. Fig. 3 shows the same electric-power generation system combined with a barometric flashtype desalination plant. The water-electricity cogeneration system illustrated in Fig. 3 is used here to assess the potential for producing freshwater from seawater using low-grade waste-heat. An advantage of this system is that its addition it will not affect the power output or the thermal efficiency of the steam-turbine power plant. A low-temperature barometric flash desalination system, illustrated in Fig. 3, was first proposed by Panayiotou and Eames [7] and described by Lalzad et al. [8] with further theoretical work being described by Eames et al. [9]. This system is at present the subject of an experimental study.
Fig. 2. The 60 MWe power plant at Dhekelia (design-point data as in Table 1).
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1093
Table 1 Showing the conditions of the process steam at design-point operation – refer to Fig. 2 State point (Fig. 2 only)
Flow (kg/s)
P (bar)
T (°C)
h (kJ/kg)
1 2 3 4 5 6 7 8 9 10 11
64.44 11.33 3.83 2.646 3.52 3.761 40.05 47.787 47.787 64.44 64.44
87 24.8 10.08 3.73 1.673 0.455 0.049 0.049 3.73 108.9 97.5b
510 223.5 180.2 141.1 114.7 79.0 32.7(sat) 32.7(sat) 34.6 143.0 220.7
3414.4 3120.7 2918.7 2766.0a 2613.2 2468.3 2202 136.94 141.39 610.5 949.2
a b
Estimated. Estimated assuming equal pressure loss through feed-heaters and boiler.
Fig. 3. The cogeneration system with proposed barometric desalinator.
5. Operation of the proposed desalination system The main feature of the desalination system shown in Fig. 3 is that evaporation of seawater can occur at saturation pressures of a few thousand Pascals and therefore, at normal ambient temperatures. The plant consists of an air-tight barometric chamber, positioned so the free surface of the condensate or seawater held-up in the chamber is at least 10.3 m above the water level in both the drinking-water reservoir and return-seawater pipe as shown in Fig. 3. Seawater enters the system via a pump from where the flow is divided into three parts. One part is directed to a condenser in the barometric chamber, a second part is heated at it flows through the power-plant condenser and the remaining part (which
1094
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
can be much smaller than the first two) is directed through the primary nozzle of an ejector, as shown in Fig. 3. The purpose of the ejector is to create suction in the drainpipe (h) to assist with the transfer of seawater via the return pipe. As the heated seawater leaves the power-plant condenser, part of it is directed via a pipe, (a) in Fig. 3, to the top of the barometric chamber. Because of the low pressure in the chamber, little pump-work is required to achieve this lift. The heated seawater enters the barometric chamber through a spray nozzle, which maintains the required pressure difference between the flow in pipe (a) and chamber (c), within the barometric chamber. As the heated water passes through the spray nozzle (d) it expands and cools due to thermodynamic throttling. This causes a proportion of the seawater to evaporate in a ‘flash’ process to form low-temperature steam. The proportion of seawater converted to steam will equal the dryness fraction (or quality) of the vapour within the chamber. The vapour produced within the inner-compartment (c) of the barometric chamber flows into the outercompartment, where it is cooled and condensed by the condenser coil (e). The condensate produced then flows down under gravity to the bottom of the outer-compartment, where it collects and flows, again under gravity, down drainpipe (f) into the drinking-water storage vessel as shown in Fig. 3. Meanwhile, concentrated seawater collects at the bottom of the inner-compartment, from where it flows, under gravity and assisted by ejector (i), through drainpipe (h) and into the seawater return pipe and from here, back into the sea. 6. Theoretical analysis of proposed plant Although the results of this analysis are based upon a individual power plant at Dhekelia, the theoretical approach can be applied generally for the system shown in Fig. 3. Fig. 4 illustrates the theoretical model used in this study. For simplicity, it has been assumed that the thermodynamic properties of seawater equal those of pure-water and that the walls of the barometric chamber are adiabatic. Also, the effects of non-condensable gases released from seawater when heated or expanded are neglected in this case. Referring to Fig. 4, the specific yield of desalinated water for this system y¼
ðh3 hf ÞT s m_ d ¼x¼ hfg T s m_ D
ðkilogram of freshwater per kilogram of feed-waterÞ;
where x is the dryness fraction or quality of vapour in the barometric vessel. As the temperatures considered here, (0–100 °C), it can assumed that the enthalpy of water, h = CpT and, therefore, y¼
C p ðT 3 T s Þ ; hfg T s
ð1Þ
where hfg 2500.8 2.35Ts (kJ, kg1) (for 0 °C 6 Ts 6 100 °C). Applying the steady-flow energy equation to the condenser cooling coil (e), in Fig. 3, the heat rate is given by Q_ c ¼ m_ c C p eðT s T 1 Þ.
ð2Þ
Also, referring to Fig. 4, an energy balance on the barometric chamber yields, Q_ c ¼ m_ D C p ðT 3 T s Þ.
ð3Þ
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1095
Fig. 4. Theoretical model of the proposed desalination system.
Equating Eqs. (2) and (3), cancelling like terms and solving for Ts gives Ts ¼
T 3 þ keT 1 ; 1 þ ke
ð4Þ
where k ¼ mm__ Dc . Fig. 5 shows the variation in saturation temperature, Ts, with ke for a heated seawater entry temperature, T3 of 30 °C, and a range of coastal seawater inlet-temperature, T1. ke can be increased by either increasing the condense coil effectiveness (e) or the ratio flow between the coolant and heated seawater, as defined above. The effectiveness of the condenser is given by UA e ¼ 1 exp . m_ c C p
Fig. 5. Variation in saturation temperature in the desalination vessel with ke (see Eq. (4)).
1096
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
Fig. 6. Variation in specific desalinated water yield as a function of power-plant cooling-water inlet-temperature and ke, for a power-plant condenser cooling-water discharge temperature of 30 °C.
The results in Fig. 5 show that for a given seawater inlet-temperature, (T1), as the value of ke increases, the saturation temperature (Ts) and pressure both decrease within the barometric chamber (see Fig. 6). Substituting from Eq. (4) into (1) gives the specific yield of barometric desalination process, Cp ke m_ d ¼y¼ ð5Þ ðT 3 T 1 Þ. hfg 1 þ ke m_ D These results show that higher yields can be expected when the coastal seawater temperature is low. However, this might be misleading as T3 is a function of T1 and the heat-rejec-
Fig. 7. Variation in annual desalinated-water production rate with ke (100% utilisation).
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1097
tion rate of the power plant, which in the case of the system under consideration, is 84 MW. With this in mind, and on substituting relevant conversion factors, Eq. (5) gives V_ d in 3 m /annum as V_ d ¼ 31; 536 y m_ D .
ð6Þ
Eq. (6) describes the variation in yield as a function of ke. Fig. 7 shows the variation in annual yield as a function of ke. In this case, it was assumed that all the seawater flowing through the power-plant condenser, ðm_ E Þ entered the barometric chamber for desalination, i.e., m_ D ¼ m_ E . For a given value of ke, this gives the maximum annual yield for the heat-rejection rate of the power-plant, 84 MW, with a seawater temperature outlettemperature of 30 °C from the power-plant condenser and a coastal seawater temperature of 15 °C. 7. Economic analysis of the case-study The ke factor is important because of its influence on both operating costs and capital costs: k is directly proportional to the flow of seawater through the condenser in the barometric chamber and consequently is directly proportional to the pumping cost; e is a function of both seawater flow through the condenser in the barometric chamber and the heatexchange area of the condenser, which is related to capital cost. Therefore, the greater the value of ke, the greater either the pumping cost or capital cost or both. These costs must be offset against income from the sale of freshwater. Based on a price of Cy£0.43 per cubic metre, Fig. 8 shows the potential annual income from the sale of freshwater produced by waste heat from one power plant, assuming a 50% utilisation. In terms of return on investment, it is usual to discount the income over time. If the freshwater were sold to the Government, as it is now, and the payments were made annu-
Fig. 8. Variation in annual income from desalinated water sales with ke, for 50% utilisation.
1098
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
ally in equal amounts (a) them the present-value (pv), in Cy£ units, of this uniform series of incomes with interest paid (compounded) annually is [10] n ð1 þ iÞ 1 pv ¼ a . ð7Þ n ið1 þ iÞ Fig. 9 shows the variation in gross present value sales, based on Eq. (7), for a range of saleprices and interest rates. A 5-year term was chosen in this case as the nominal payback period. Fig. 10, therefore, gives an indication of the capital and discounted operating expenditure that might be covered by the sale of freshwater in that period. Clearly, the return on investment is a function of those parameters listed in Fig. 10. If, instead of selling it to the government, the freshwater produced was sold directly the public then the income stream could be considered to be continuous. In this case, the present-value, (pv), in Cy£ units, of sales, discounted at an interest rate, i, over the life cycle of the project, n-years, is given by 1 ein pv ¼ a . ð8Þ i Eq. (8) was derived from equations provided by Stoecker [10]. Fig. 10 shows the variation in gross present value of income from sales, from Eq. (8), for the same conditions as in Fig. 10. Comparing Figs. 9 and 10 it can be seen that selling direct to the public could provide a small increase in gross present value over a 5-year term and therefore a greater allowance for capital and operating cost investment. For an inter-
gross present value of sales discounted over 5 years (thousands of CY£)
1000 900 800
Uniform series of annual incomes compounded annually i = 6% i = 5% i = 4% i = 3% i = 2%
700 600
T 1 = 15 o C T 3 = 30 o C mD/m E = 1 =1 Q E = 84MW Utilization = 50%
500 400 300 200 0.2
0.3
0.4
0.5
0.6
0.7
desalinated water sale price (CY£) Fig. 9. Variation in present-value of uniform series of annual incomes from the sale of desalinated water over 5 years, compounded annually, (Eq. (7)).
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1099
Fig. 10. Variation in present-value of uniform series of annual incomes from the sale of desalinated water over 5 years, compounded continuously (Eq. (8)).
est rate of 5% over a 5-year term sales to the public would produce an approximate 2% increase in gross present value. The results of the economic analysis are strongly a function of coastal seawater temperatures, flows and heat-rejection rates by the power plant. The financial viability of a waterelectricity cogeneration system, as proposed here, would require further data for the annual variations with time of seawater temperatures and power-plant heat-rejection rates. Based on a total heat-rejection rate of 1.8 GW for all EAC power stations, the total maximum annual yield of freshwater for a ke value of unity would be approximately 11.25 million cubic metres per annum. This figure is based on the data given in Fig. 7 multiplied by the total heat rejected at this time by all EAC power stations (1.8 GW) and divided by the heat rejected by the plant at Dhekelia used in this case study (i.e., 0.084 GW). 8. Conclusions Based on a total heat-rejection rate of 1.8 GW for all electric-power stations in Cyprus, as discussed earlier, the total maximum annual yield of freshwater for a ke value of unity would be approximately 11.25 million cubic metres per annum. This figure is based on the data given in Fig. 7 multiplied by the total heat rejected by all EAC power stations (1.8 GW) and divided by the heat rejected by the plant at Dhekelia used in this case study (i.e., 0.084 GW).
1100
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
The proposed barometric flash-type desalination plant described here is simple to manufacture, and provides an efficient employment of otherwise waste heat. Further research is now underway to demonstrate the concept and optimise to design parameters. Acknowledgement The financial support of the Cyprus Council for Assisting Refugee Academics (CARA) is gratefully acknowledged. References [1] Savvides L, Do¨rflinger G, Alexandrou K. The assessment of water demand of Cyprus, The ‘‘Re-assessment of the water resources and demand of the island of Cyprus’’, vol. II: ISBN Vol. II 9963-1-7005-6; 2001. Accessed by: http://www.emwiscy.org/PDF/Synthesis.pdf. [2] Rossel F. Hydrometeorological study examining changes in recorded precipitation: The ‘‘Re-assessment of the water resources and demand of the island of Cyprus’’, vol. I; 2001. [3] Georgiou A. Assessment of groundwater resources of cyprus volume I – water resources. The ‘‘Reassessment of the water resources and demand of the island of Cyprus, vol. I: ISBN Vol. I 9963-1-7004-8; 2002. Accessed by: http://www.emwis-cy.org/PDF/Synthesis.pdf. [4] Klohn W. Synthesis report, The ‘‘Re-assessment of the water resources and demand of the island of Cyprus, vol. I: ISBN Vol. I 9963-1-7004-8; 2002. Accessed by: http://www.emwis-cy.org/PDF/Synthesis.pdf. [5] Psaltas M. An investigation of cogeneration systems using heat to produce water and electricity in Cyprus. An undergraduate project report, London South Bank University; 2004. [6] Parteous A. Desalination technology: development and practice. London, New York: Applied Science Publishers; 1983. p. 1–30. [7] Panayiotou P, Eames IW. An experimental investigation into water purification through an environmentally powered desalination device. A paper presented at the meeting of the Chartered Institute of Water Engineering Management, Sheffield Hallam University, 26th April; 1995. [8] Lalzad AK, Eames IW, Panayiotou P, Maidment G, Karayiannis TG. Low-pressure solar distillation-plant. In: Proceedings of the 2nd international heat powered cycles conference, HPC’01, Paris; 2001. p. 51–4. [9] Eames IW, Karayannis TG, Panayiotuo P. The results of a theoretical study of a novel barometric desalination plant. In: Proceedings of eurotherm seminar No. 72, thermodynamics, heat and mass transfer of refrigeration machines and heat pumps, Valencia, Spain, 31 March–2 April; 2003. p. 417–22. [10] Stoecker WF. Design of thermal systems. 3rd ed. New York: McGraw-Hill; 1989, ISBN 0-07-061620-5 [Chapter 3].
APPLIED ENERGY www.elsevier.com/locate/apenergy
Potential application of a flash-type barometric desalination plant powered by waste heat from electric-power stations in Cyprus G.G. Maidment *, I.W. Eames, M. Psaltas, A. Lalzad Department of Engineering Systems, Faculty of Engineering, Science and the Built Environment, London South Bank University, 103 Borough Road, London SE1 0AA, UK Available online 25 January 2006
Abstract This paper describes and evaluates the results of a study into the problems of freshwater production and shortages on the island of Cyprus. The use of a novel barometric flash-type desalinator, driven by otherwise waste-heat from the island’s power-stations, is proposed as a means of increasing freshwater supplies. Mathematical models are described and used to investigate the thermodynamic performance and economic viability of the proposed system. Although water and electricity supply data for the island of Cyprus were used for the purposes of this investigation, the overall findings are thought have a wider applicability. Ó 2005 Published by Elsevier Ltd.
1. Introduction As with most other Mediterranean islands, the island of Cyprus increasingly suffers from a shortage of drinking water, particularly when the population increases many fold during the tourist season. This paper describes the current situation in Cyprus and investigates the potential use of waste heat from electric-power stations in the production of drinking water from seawater using a novel barometric flash-type desalination device, which is currently under development at London South Bank University, London. Whilst
*
Corresponding author. Tel.: +44 207 815 7676; fax: +44 207 815 7699. E-mail address: [email protected] (G.G. Maidment).
0306-2619/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.apenergy.2005.11.006
1090
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
Nomenclature A a Cp h hfg i m n pv Q_ C T U Vd x y e k
heat-transfer area (m2) annual income from sales of water (Cy£) specific heat-capacity at constant pressure (kJ/kg K) enthalpy (kJ/kg) latent enthalpy of vapourisation discounted interest rate mass flow (kg/s) time over which the return on investment is calculated (years) present value heat rate to desalinator condenser coil (kW) temperature (K or °C) overall heat-transfer coefficient (kW/m2 K) annual production rate of freshwater (m3/annum) dryness fraction specific yield heat-exchange effectiveness m_ c m_ D
Sufficies c seawater flow to desalinator condenser D heated seawater flow to desalination chamber d desalinated water flow s saturation condition in desalination chamber
this paper concerns itself primarily with the situation in Cyprus, the results described herein could be applied to the situations in many other countries. 2. Freshwater supplies in Cyprus Cyprus is one of the largest islands in the Mediterranean Sea, with a resident population of approximately 800,000 people, which is increasing annually at a rate of 1%. The island also has about 2.5 million visitors each year, which significantly impacts on drinking-water supplies. At this time, the total daily water consumption is Cyprus is approximately 700,000 m3 of which domestic water-use represents 25% [1]. Today, of the total water consumption in Cyprus, 76% is used by agriculture whilst the remainder is required for domestic use and industry, which includes tourism. Cyprus currently suffers from, an inadequate and unreliable rainfall and, as a result, a lack of freshwater, particularly in the summer months. Water shortages can occur even when the island receives its average annual rainfall, which at this time is 500 mm. This is because rainfall tends to be concentrated during the winter months [2]. Furthermore, climate change is affecting water supplies. Over the last century, there has been a regular reduction in annual rainfall by approximately 1 mm each year, as shown in Fig. 1. This too has impacted on the availability of freshwater supplies, which have reduced by around
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1091
Fig. 1. Time-averaged annual rainfall in Cyprus between 1901 and 2002.
25% since the 1970s [2]. Cyprus currently faces an increasing shortfall in water supplies, which has led to an over exploitation of groundwater. This has resulted in saline water intrusion and consequent quality deterioration in the coastal aquifers and depletion of inland aquifers. In 2001, annual groundwater extraction was estimated to exceed natural recharge by 15.3 M m3 [3]. Improvements in water-management techniques have helped to reduce the shortfall. During the colonial period, a freshwater water management programme led to the construction of reservoirs capable of storing up to 300 million cubic metres of freshwater [4]. Also, in the early 20th century, emphasis was given to the development of groundwater systems for the provision of drinking water, because these provided a low-cost supplies, which were of good quality and required minimal infrastructure investment. By 1930, the exploitation of the Famagusta and Morphou aquifers had began with thousands of boreholes being drilled. This continued up to 1960, but over-pumping in areas like Famagusta, Morphou and Akrotiri resulted in the depletion of these key aquifers. More recently, the Water Development Department of the Ministry of Agriculture, Natural Resources and Environment has begun to utilised desalination systems in conjunction with dams to store freshwater. 3. Cyprus and desalination With effective desalination systems, the seawater surrounding Cyprus could provide an almost limitless source of freshwater. Desalination of seawater was first introduced in Cyprus on a large-scale in 1997, with the opening of a 20,000 m3/day reverse-osmosis plant at Dhekelia, which was soon expanded to 40,000 m3/day. The plant operates on a buildown-operate-transfer basis and the desalinated water is sold to the Government, at source, at a varying unit price around Cy£0.62/m3 [4]. A new reverse-osmosis seawater desalina-
1092
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
tion plant, of 51,667 m3/day nominal capacity, is presently being constructed next to Larnaca Airport [5]. 4. Cogeneration of drinking water and electricity The use of otherwise wasted low-grade heat from power stations to drive desalination plants is environmentally attractive and may provide cost benefits. The Electricity Authority of Cyprus (EAC) was established in 1952. It is an independent non-profit making semi-government corporation responsible for the generation, transmission and distribution of electricity on the island [6]. At this time, EAC operates three fossil-fuelled power stations, at Moni, Dhekelia and Vasilikos [6]. These are situated along the southern coastline of the island and together produce 800 MWe from steam-turbine powered generators. The condensers of these units are cooled by seawater. EAC also operates gas-turbine powered generators at Moni and Vasilikos producing an additional 188 MWe. With a continuing growth in electricity demand, a further 300 MWe increase in generator capacity is planned for 2006 from fossil-fuel fired stations [5]. By 2006, therefore, the amount of waste heat available to produce desalinated seawater will be of the order of 1.8 GW. Fig. 2 shows a schematic representation of a 60 MWe steam-turbine powered electricity generator used in Dhekelia. The design-point data for the power cycle are listed in Table 1. Fig. 3 shows the same electric-power generation system combined with a barometric flashtype desalination plant. The water-electricity cogeneration system illustrated in Fig. 3 is used here to assess the potential for producing freshwater from seawater using low-grade waste-heat. An advantage of this system is that its addition it will not affect the power output or the thermal efficiency of the steam-turbine power plant. A low-temperature barometric flash desalination system, illustrated in Fig. 3, was first proposed by Panayiotou and Eames [7] and described by Lalzad et al. [8] with further theoretical work being described by Eames et al. [9]. This system is at present the subject of an experimental study.
Fig. 2. The 60 MWe power plant at Dhekelia (design-point data as in Table 1).
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1093
Table 1 Showing the conditions of the process steam at design-point operation – refer to Fig. 2 State point (Fig. 2 only)
Flow (kg/s)
P (bar)
T (°C)
h (kJ/kg)
1 2 3 4 5 6 7 8 9 10 11
64.44 11.33 3.83 2.646 3.52 3.761 40.05 47.787 47.787 64.44 64.44
87 24.8 10.08 3.73 1.673 0.455 0.049 0.049 3.73 108.9 97.5b
510 223.5 180.2 141.1 114.7 79.0 32.7(sat) 32.7(sat) 34.6 143.0 220.7
3414.4 3120.7 2918.7 2766.0a 2613.2 2468.3 2202 136.94 141.39 610.5 949.2
a b
Estimated. Estimated assuming equal pressure loss through feed-heaters and boiler.
Fig. 3. The cogeneration system with proposed barometric desalinator.
5. Operation of the proposed desalination system The main feature of the desalination system shown in Fig. 3 is that evaporation of seawater can occur at saturation pressures of a few thousand Pascals and therefore, at normal ambient temperatures. The plant consists of an air-tight barometric chamber, positioned so the free surface of the condensate or seawater held-up in the chamber is at least 10.3 m above the water level in both the drinking-water reservoir and return-seawater pipe as shown in Fig. 3. Seawater enters the system via a pump from where the flow is divided into three parts. One part is directed to a condenser in the barometric chamber, a second part is heated at it flows through the power-plant condenser and the remaining part (which
1094
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
can be much smaller than the first two) is directed through the primary nozzle of an ejector, as shown in Fig. 3. The purpose of the ejector is to create suction in the drainpipe (h) to assist with the transfer of seawater via the return pipe. As the heated seawater leaves the power-plant condenser, part of it is directed via a pipe, (a) in Fig. 3, to the top of the barometric chamber. Because of the low pressure in the chamber, little pump-work is required to achieve this lift. The heated seawater enters the barometric chamber through a spray nozzle, which maintains the required pressure difference between the flow in pipe (a) and chamber (c), within the barometric chamber. As the heated water passes through the spray nozzle (d) it expands and cools due to thermodynamic throttling. This causes a proportion of the seawater to evaporate in a ‘flash’ process to form low-temperature steam. The proportion of seawater converted to steam will equal the dryness fraction (or quality) of the vapour within the chamber. The vapour produced within the inner-compartment (c) of the barometric chamber flows into the outercompartment, where it is cooled and condensed by the condenser coil (e). The condensate produced then flows down under gravity to the bottom of the outer-compartment, where it collects and flows, again under gravity, down drainpipe (f) into the drinking-water storage vessel as shown in Fig. 3. Meanwhile, concentrated seawater collects at the bottom of the inner-compartment, from where it flows, under gravity and assisted by ejector (i), through drainpipe (h) and into the seawater return pipe and from here, back into the sea. 6. Theoretical analysis of proposed plant Although the results of this analysis are based upon a individual power plant at Dhekelia, the theoretical approach can be applied generally for the system shown in Fig. 3. Fig. 4 illustrates the theoretical model used in this study. For simplicity, it has been assumed that the thermodynamic properties of seawater equal those of pure-water and that the walls of the barometric chamber are adiabatic. Also, the effects of non-condensable gases released from seawater when heated or expanded are neglected in this case. Referring to Fig. 4, the specific yield of desalinated water for this system y¼
ðh3 hf ÞT s m_ d ¼x¼ hfg T s m_ D
ðkilogram of freshwater per kilogram of feed-waterÞ;
where x is the dryness fraction or quality of vapour in the barometric vessel. As the temperatures considered here, (0–100 °C), it can assumed that the enthalpy of water, h = CpT and, therefore, y¼
C p ðT 3 T s Þ ; hfg T s
ð1Þ
where hfg 2500.8 2.35Ts (kJ, kg1) (for 0 °C 6 Ts 6 100 °C). Applying the steady-flow energy equation to the condenser cooling coil (e), in Fig. 3, the heat rate is given by Q_ c ¼ m_ c C p eðT s T 1 Þ.
ð2Þ
Also, referring to Fig. 4, an energy balance on the barometric chamber yields, Q_ c ¼ m_ D C p ðT 3 T s Þ.
ð3Þ
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1095
Fig. 4. Theoretical model of the proposed desalination system.
Equating Eqs. (2) and (3), cancelling like terms and solving for Ts gives Ts ¼
T 3 þ keT 1 ; 1 þ ke
ð4Þ
where k ¼ mm__ Dc . Fig. 5 shows the variation in saturation temperature, Ts, with ke for a heated seawater entry temperature, T3 of 30 °C, and a range of coastal seawater inlet-temperature, T1. ke can be increased by either increasing the condense coil effectiveness (e) or the ratio flow between the coolant and heated seawater, as defined above. The effectiveness of the condenser is given by UA e ¼ 1 exp . m_ c C p
Fig. 5. Variation in saturation temperature in the desalination vessel with ke (see Eq. (4)).
1096
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
Fig. 6. Variation in specific desalinated water yield as a function of power-plant cooling-water inlet-temperature and ke, for a power-plant condenser cooling-water discharge temperature of 30 °C.
The results in Fig. 5 show that for a given seawater inlet-temperature, (T1), as the value of ke increases, the saturation temperature (Ts) and pressure both decrease within the barometric chamber (see Fig. 6). Substituting from Eq. (4) into (1) gives the specific yield of barometric desalination process, Cp ke m_ d ¼y¼ ð5Þ ðT 3 T 1 Þ. hfg 1 þ ke m_ D These results show that higher yields can be expected when the coastal seawater temperature is low. However, this might be misleading as T3 is a function of T1 and the heat-rejec-
Fig. 7. Variation in annual desalinated-water production rate with ke (100% utilisation).
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1097
tion rate of the power plant, which in the case of the system under consideration, is 84 MW. With this in mind, and on substituting relevant conversion factors, Eq. (5) gives V_ d in 3 m /annum as V_ d ¼ 31; 536 y m_ D .
ð6Þ
Eq. (6) describes the variation in yield as a function of ke. Fig. 7 shows the variation in annual yield as a function of ke. In this case, it was assumed that all the seawater flowing through the power-plant condenser, ðm_ E Þ entered the barometric chamber for desalination, i.e., m_ D ¼ m_ E . For a given value of ke, this gives the maximum annual yield for the heat-rejection rate of the power-plant, 84 MW, with a seawater temperature outlettemperature of 30 °C from the power-plant condenser and a coastal seawater temperature of 15 °C. 7. Economic analysis of the case-study The ke factor is important because of its influence on both operating costs and capital costs: k is directly proportional to the flow of seawater through the condenser in the barometric chamber and consequently is directly proportional to the pumping cost; e is a function of both seawater flow through the condenser in the barometric chamber and the heatexchange area of the condenser, which is related to capital cost. Therefore, the greater the value of ke, the greater either the pumping cost or capital cost or both. These costs must be offset against income from the sale of freshwater. Based on a price of Cy£0.43 per cubic metre, Fig. 8 shows the potential annual income from the sale of freshwater produced by waste heat from one power plant, assuming a 50% utilisation. In terms of return on investment, it is usual to discount the income over time. If the freshwater were sold to the Government, as it is now, and the payments were made annu-
Fig. 8. Variation in annual income from desalinated water sales with ke, for 50% utilisation.
1098
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
ally in equal amounts (a) them the present-value (pv), in Cy£ units, of this uniform series of incomes with interest paid (compounded) annually is [10] n ð1 þ iÞ 1 pv ¼ a . ð7Þ n ið1 þ iÞ Fig. 9 shows the variation in gross present value sales, based on Eq. (7), for a range of saleprices and interest rates. A 5-year term was chosen in this case as the nominal payback period. Fig. 10, therefore, gives an indication of the capital and discounted operating expenditure that might be covered by the sale of freshwater in that period. Clearly, the return on investment is a function of those parameters listed in Fig. 10. If, instead of selling it to the government, the freshwater produced was sold directly the public then the income stream could be considered to be continuous. In this case, the present-value, (pv), in Cy£ units, of sales, discounted at an interest rate, i, over the life cycle of the project, n-years, is given by 1 ein pv ¼ a . ð8Þ i Eq. (8) was derived from equations provided by Stoecker [10]. Fig. 10 shows the variation in gross present value of income from sales, from Eq. (8), for the same conditions as in Fig. 10. Comparing Figs. 9 and 10 it can be seen that selling direct to the public could provide a small increase in gross present value over a 5-year term and therefore a greater allowance for capital and operating cost investment. For an inter-
gross present value of sales discounted over 5 years (thousands of CY£)
1000 900 800
Uniform series of annual incomes compounded annually i = 6% i = 5% i = 4% i = 3% i = 2%
700 600
T 1 = 15 o C T 3 = 30 o C mD/m E = 1 =1 Q E = 84MW Utilization = 50%
500 400 300 200 0.2
0.3
0.4
0.5
0.6
0.7
desalinated water sale price (CY£) Fig. 9. Variation in present-value of uniform series of annual incomes from the sale of desalinated water over 5 years, compounded annually, (Eq. (7)).
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
1099
Fig. 10. Variation in present-value of uniform series of annual incomes from the sale of desalinated water over 5 years, compounded continuously (Eq. (8)).
est rate of 5% over a 5-year term sales to the public would produce an approximate 2% increase in gross present value. The results of the economic analysis are strongly a function of coastal seawater temperatures, flows and heat-rejection rates by the power plant. The financial viability of a waterelectricity cogeneration system, as proposed here, would require further data for the annual variations with time of seawater temperatures and power-plant heat-rejection rates. Based on a total heat-rejection rate of 1.8 GW for all EAC power stations, the total maximum annual yield of freshwater for a ke value of unity would be approximately 11.25 million cubic metres per annum. This figure is based on the data given in Fig. 7 multiplied by the total heat rejected at this time by all EAC power stations (1.8 GW) and divided by the heat rejected by the plant at Dhekelia used in this case study (i.e., 0.084 GW). 8. Conclusions Based on a total heat-rejection rate of 1.8 GW for all electric-power stations in Cyprus, as discussed earlier, the total maximum annual yield of freshwater for a ke value of unity would be approximately 11.25 million cubic metres per annum. This figure is based on the data given in Fig. 7 multiplied by the total heat rejected by all EAC power stations (1.8 GW) and divided by the heat rejected by the plant at Dhekelia used in this case study (i.e., 0.084 GW).
1100
G.G. Maidment et al. / Applied Energy 83 (2006) 1089–1100
The proposed barometric flash-type desalination plant described here is simple to manufacture, and provides an efficient employment of otherwise waste heat. Further research is now underway to demonstrate the concept and optimise to design parameters. Acknowledgement The financial support of the Cyprus Council for Assisting Refugee Academics (CARA) is gratefully acknowledged. References [1] Savvides L, Do¨rflinger G, Alexandrou K. The assessment of water demand of Cyprus, The ‘‘Re-assessment of the water resources and demand of the island of Cyprus’’, vol. II: ISBN Vol. II 9963-1-7005-6; 2001. Accessed by: http://www.emwiscy.org/PDF/Synthesis.pdf. [2] Rossel F. Hydrometeorological study examining changes in recorded precipitation: The ‘‘Re-assessment of the water resources and demand of the island of Cyprus’’, vol. I; 2001. [3] Georgiou A. Assessment of groundwater resources of cyprus volume I – water resources. The ‘‘Reassessment of the water resources and demand of the island of Cyprus, vol. I: ISBN Vol. I 9963-1-7004-8; 2002. Accessed by: http://www.emwis-cy.org/PDF/Synthesis.pdf. [4] Klohn W. Synthesis report, The ‘‘Re-assessment of the water resources and demand of the island of Cyprus, vol. I: ISBN Vol. I 9963-1-7004-8; 2002. Accessed by: http://www.emwis-cy.org/PDF/Synthesis.pdf. [5] Psaltas M. An investigation of cogeneration systems using heat to produce water and electricity in Cyprus. An undergraduate project report, London South Bank University; 2004. [6] Parteous A. Desalination technology: development and practice. London, New York: Applied Science Publishers; 1983. p. 1–30. [7] Panayiotou P, Eames IW. An experimental investigation into water purification through an environmentally powered desalination device. A paper presented at the meeting of the Chartered Institute of Water Engineering Management, Sheffield Hallam University, 26th April; 1995. [8] Lalzad AK, Eames IW, Panayiotou P, Maidment G, Karayiannis TG. Low-pressure solar distillation-plant. In: Proceedings of the 2nd international heat powered cycles conference, HPC’01, Paris; 2001. p. 51–4. [9] Eames IW, Karayannis TG, Panayiotuo P. The results of a theoretical study of a novel barometric desalination plant. In: Proceedings of eurotherm seminar No. 72, thermodynamics, heat and mass transfer of refrigeration machines and heat pumps, Valencia, Spain, 31 March–2 April; 2003. p. 417–22. [10] Stoecker WF. Design of thermal systems. 3rd ed. New York: McGraw-Hill; 1989, ISBN 0-07-061620-5 [Chapter 3].