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Energy saving and desalination of water
DESALINATION ELSEVIER
Desalination
108 (1996) 43-50
Energy saving and desalination of water Agustin Aragh Mesa, Claudio Miguez G6mez, and Rarndn Urcelay Azpitarte* Water Technologies Department, Institute for the Diversification and Saving of Energy (IDAE), Paseo de la Castellana 95, planta 21, 28046 Madrid, Spain. Tel.: +34-l-5568415; Fax.: +34-l-5.551389 Received
29 July 1996; accepted
10 August 1996
Abstract
The IDAE is a public entity of the Ministry of Industry and Energy which belongs to the Department of Energy and Mineral Resources. Its objective is to improve energy efficiency in the use of energy in the different consumer sectors and to promote production and consumption of renewable energies. The work of the IDAE in the different consumer sectors is derived from action planned in the Energy Saving and Efficiency Plan (ESEP) of the National Energy Plan 91/2000 and from strategies identified in the New Promotion of Spain’s Energy Saving Policy, which includes thinking ahead to promote activities so that consumer sectors will be able to avoid having to take costly corrective measures - where feasible - in the future. The activities to promote energy efficiency are therefore of particular interest in the new desalination sector which is already working on a large scale in Spain. Keywords: Energy efficiency; Cost optimization
1. Introduction Water has always been a source of energy for man, and it continues to be so to produce electricity in power stations. Today we use energy to change salt water into drinking water, by desalinating the water, a process that started way back in history. The greatest desalinator in the world is Nature itself which uses the free energy of the sun in its Natural Water Cycle, as documented by Aristotle. Later, philosophers observed Presented at the Second Annual Meetin of the Euro ean Desalination Society (EDS) on Desa K.mation an dpthe Environment, Genoa, Italy, October 20-23, 1996. *Corresponding
author.
001 l-9164/97/$17.00 Copyright PZZ SO01 l-9164(97)00007-6
e 1997 Published
how Mediterranean sailors obtained fresh water from seawater. Alexander of Aphrodisia and, subsequently, Saint Basil, the Archbishop of Cesarea, both described the process as follows: The sailors put seawater in a pot on a fast boil and hang sponges over the pot. They squeeze out the sponges and obtain fresh water. The reference which dates back furthest, three thousand five hundred years ago, can be read in the book of Exodus, chapter XV, verses 22 to 25: So Moses brought the children of Israel from the Red Sea and they went out into the wilderness of Shur; and they went three days in the wilderness, and found no water; And when they came to Marah, they could not drink of the waters of Marah for they were bitter:
by Elsevier Science B.V. All rights reserved.
44
A.A. Mesa et al. /Desalination
Therefore the name of it was called Marah. And the people murmured against Moses, saying, “What shall we drink?“. And he cried unto the Lord; and the Lord shewed him a tree, which when he had cast into the waters, the waters were made sweet. When the steam engine was invented, patents were brought out to desalinate seawater using steam. However, the first industrial plant was built in 1872 in Las Salinas, near Antofagasta, in Chile, using solar distillation. It was a glass-covered wooden structure and processed 22.5 m3/d until 1908. Since then, there has been such rapid development that according to IDA (International Desalination Association), at the beginning of the nineties, there was a total 16 million m3/d capacity in 100 m3/d plants installed throughout the world. Spain has 2.4% of these plants, representing the tenth largest installed capacity. 67% of this worldwide capacity is used for producing drinking water, 20% is for industrial use and 13% is for agricultural use and others. Disregarding technologies which are at experimental stages at present, such as desalination by freezing, and others which are difficult to apply at an industrial level, such as electrodialysis and solar distillation using the greenhouse effect, there are only three technologies which are apt for commercial use today: Reverse osmosis (RO) Distillation (MSF, MED) Vacuum distillation, a variant of the second technology (VVC). RO is based on the property of some special membranes which, under the pressure of the seawater (70 kg/cm2), allow part of the water through (30 to 40%) without salt (fresh water) and block out the rest (70 to 60%) which has a higher salt content (brine) (Fig. 1). The energy used by this method is approximately 5.6 kWNm3 of desalinated water when seawater is treated (30,000 to 40,000 ppm) and 1.4 to 1.7 kWNm3 when saline groundwater is treated (2,000 to 3,000 ppm). Electricity is the only form of energy
108 (1996) 43-50
Engine + auxiliary equqxn. = 5.6 kWh/m3
Fig. 1. Reverse
osmosis
technology.
used in this method and it is employed to move the turbopumps and auxiliary equipment. The high degree of energy efficiency of this technology is gained from the recovery of the brine water of the pressure (approximately 35-40% at present). The distillation technologies use the same basis as the mediaeval still. The modern version uses a multi-stage process, shown in Fig. 2, in such a way that initial evaporation occurs (heat is absorbed), and when this vapour condenses (heat is given off) the heat from this condensation is used to evaporate the following stage, and so on. The following energy is consumed: approximately 1.3 kWh electricity and 48.5 kWh heat, for each m3 of desalinated water (3% electricity and 97% heat). The VVC (vacuum vapour compression) uses the same basis as the MED, but a vacuum
A.A. Mesa et al. / Desalination
45
108 (1996) 43-SO
Heat rejection
Effects of heat recovery
condenser Non conden-
Hig h pres sure vapor (for TVC plant)
c w Kemrn condensation pump
Fig. 2. Diagram
of MED
Intermediary FO‘ J ,.>,rn” 1LV.A Y....LY
-
inhibitor
Feed pump
and TVC processes
Evaporator-condenser
?I
Seaw ‘ater
Non condensable
Fig. 3. Diagram
of the WC
process
gases
(vacuum
LI
vapour
compression).
is formed in the machine (Fig. 3). More electricity is required to form the vacuum, but the total energy consumption is reduced because less heat is required to evaporate the water. Approximately 8.5 kWh electricity and 13.2 kWh heat is required for each m3 of desalinated water (40% electricity and 60% heat).
2. Energy effkiency in desalination Energy costs invariably represent 50 to 75% of real operating costs, regardless of the technology used and the design of the seawater desalinator. The focal point in desalination is therefore its energy consumption. Since technologies available do
46
A.A. Mesa et al. /Desalination
not offer many possibilities of reducing energy consumption, reduction of the costs of desalinated water must therefore be sought through an extensive use of the imagination and application of engineering know-how in order to achieve an efficient supply of the energy necessary. The reduction of these costs must therefore be focused on energy efficiency. Optimization of costs does not therefore only effect technologies available. This is why the overall design of the plant is of such importance, including the type of energy supply that it requires. This is also why the initial design and engineering work is of such importance too, for it is this work that identifies the suitability of one particular desalinator model amongst the different models which could be used on a certain site. If this were not so, the focus would be placed solely on the choice between the different desalination technologies, which is just one aspect of the whole plant. It cannot be expected that costs will be optimized by the suppliers of the equipment; it would not be reasonable to demand this, and, in fact, this has never occurred. The IDAE has a wide and rich experience in this field, both in cogeneration installations, buildings and renewable forms of energy. In fact, the IDAE has carried out studies on desalination and made in-depth and detailed analyses of the alternatives in the conception and design of desalination plants. This was pioneer work in Spain, despite the fact that Spain houses 2.4% of the installations of the world capacity for desalination of seawater. Cost reduction must therefore be sought in the technical optimization of the energy supply of electricity and heat. Cogeneration offers the only solution, i.e., the simultaneous supply (and consumption) of electricity and heat in the same equipment. To gain a better understanding of this subject, it should be remembered that thermoelectric generation always produces surplus heat in addition to electricity. This heat cannot be used and it evaporates in the atmosphere. This phenomenon can be
108 (1996) 43-50
observed in all thermoelectric plants, There is a difference of approximately 3 1.5% in electric efficiency between the energy a fuel contains and the electricity that reaches the consumer. If this efficiency is measured as the electricity leaves the power plant, it could be as much as 35% in the electricity system in Spain today. However, if the heat which is produced at the same time as the electricity was also used, as occurs in cogeneration installations, electric efficiency could reach 85%. There are three clear possibilities in the application of cogeneration in water desalination: - A desalinator can be designed with a small electricity generator for self-supply, in which the electricity is used in RO and the heat is used in a MED. This is the design recommended by IDAE, and is called PAME (Autonomous Maximum Efficiency Plant). A variant of this desalinator is possible, maintaining the same maximum efficiency, but it does not have a selfsufficient electricity supply. The variant is called PAME 25 (the reasons for this name will be explained later). - Cogeneration can be applied in an electric power station, using the heat which evaporates into the atmosphere in a MED desalinator. This would be the case of a Dual Plant. Examples of this plant can be found in operation abroad. _ RO can be applied, using the heat to heat the water before it is processed in the membranes.
Figs. 4 and 6 show autonomous plants, i.e., plants which do not have any connection with an electricity network. They are therefore able to operate in isolation. Figs. 4a and 6a refer to the same plants, but connected to an electricity supply, which enables them to export approximately 25% of electricity produced, provided the electricity power installed is equal to or less than 25 MVA, in accordance with Royal Decree 1327/95 of 28 July.
,n 108 (1996) 43-50
41
+ Brinei,M7
Self-consumed Fuel 21.9
Fuel 23.1 M
m3/h
ksal. way 40.000 m-/d)
rsteml ‘~ A Seawater 23°C 3% improv. in efficiency
T L’“.“*“d+\4z0C
“30% extra power for start-up. Energy efficiency: 85%
Fig. 4. Autonomous (PAME).
*30% extra power for start-up.
Hoat
Maximum
Energy efficiency: 85%
Energy
Efficiency
Plant
Fig.
6. RO autonomous
plant
Exported electricity
Brme
Self-consumed Fuel 33.28
i Brine 1.667 m3/h Desal. water (40.000 m3/d
Seawater 23°C 3% improv. t in efficiency
1-+$j~5,~oC Energy efficiency: 85%
Energy efficiency: 85%
Fig. 4a. Maximum
Energy Efficiency
Plant (PAME 25).
Fig. 6a. RO plant with cogeneration.
Electricitv to oubhc network 98i
(186.2 MWh)
onsumedDistillation
,
&g)’ 1,?7! m3/h
TL._“_d y&r
VIYUI‘C:
Energy
effvziency:
85%
Heat
Fig. 5. Dual plant with combined
Seawater
(45,OOC‘I” /d) ;15 Hm /y)
cycle.
Fig. 5 shows a dual plant with combined cycle, capable of producing 45,000 m3/d of drinking water. Tables l-3 show the electricity con-
sumption and cost of desalinated water for each proposed alternative. Table 2 shows these figures in terms of operating costs and Table 3 in terms of total costs. Costs unrelated to energy consumption have been calculated from information available in publications on this subject, and from consulting manufacturers and suppliers. These costs vary greatly on the market, and a warning should therefore be given that there is no reliable information or statistics for these costs.
3. Conclusions In light of the data given in Tables 1-3, the following conclusions can be made.
48
A.A. Mesa et al. /Desalination
Table 1 Energy consumption
per m3 of desalinated
Technology
water kWh final energy Elect. Heat
RO seawater MED seawater WC seawater PAME seawater Combined cycle dual plant seawater RO autonomous plant seawater RO autonomous plant saline water
Table 2 Operating
108 (1996) 43-50
5.6 1.3 8.5
Efficiency of supply system Elect. (%) Heat (%)
0 31.5 48.5 31.5 13.2 31.5 Overall efficiency 85% 190 MW combined cycle 12 MW self-generation 2.3 MW, idem for saline water
85 85 85
Primary kWh
energy
17.8 61.2 42.5 13.2 11.7 14.2 3.6
costs (in PTA/m3) RO with network supply (%)
RO with cogeneration (1) (%)
PAME (1) (%)
Combined (%)
O&M, and chemicals Energy
20 (26.3) 56 (73.7)
23 (45) 28 (55)
22 (48) 24 (52)
13 (31.7) 28 (68.3)
Total
76 (100)
51 (100)
46 (100)
41 (100)
cycle (1)
(1) The energy costs are calculated with natural gas at 2.1 PTA/therm. If gas-oil at 2.7 PTA/therm is used, costs increase by 8 PTA/m3 for RO and 7 PTA/m3 for PAME. If fuel-oil at 1.8 PTA/therm is used, costs decrease by 4 PTA/m3 for RO and 3 PTA/m3 for PAME. External electricity supply (from the network) is calculated at 10.04 PTA/kWh (rate 3.1) both for RO with network supply and for the electricity consumed by the dual plant.
Table 3 Total costs (in PTA/m3) RO with cogeneration (%)
PAME (%)
20 (18) 56 (51) 34 (31)
23 (25) 28 (31) 40 (44)
22 (25) 24 (27) 43 (48)
13 (15.1) 28 (32.5) 45 (52.4)
110 (100) 4,000
91 (100) 4,770
89 (100) 5,150
86 (100) 5,310
RO with network supply (%) O&M, and chemicals Energy Amortization Total Investment
in millions
Note: Amortizations
of PTA
calculated
cycle (%)
over 20 years with 10% discount rate.
3.1. General conclusions With regard
Combined
to product
costs,
energy
efficiency and reduction in release of COz, the best way of desalinating water is the combined method which produces drinking water and
A.A. Mesa et al. /Desalination
the energy required for the process, i.e. using cogeneration. The simple reason for this is that the heat which is wasted in the electricity production in power stations is used in this case; the energy efficiency is therefore much more effective and as a result there is a saving in the energy supply required for the desalination process.
-
3.2. Energy efficiency
-
Cogeneration plants are much more efficient from an energy saving viewpoint than those which do not produce their own energy. The PAME plant saves almost 26% of the energy required by the most efficient technology which uses an electricity supply from the public network. This logically goes side by side with a reduction in CO2 release per m3 of desalinated water.
-
3.3. Cost of desalinated water The data given in Tables 2 and 3 are valid for comparing technologies, regardless of the non-energy costs, since any variation in such costs would affect all the options to a similar degree. These costs vary according to the type of fuel used in cogeneration: the cheapest is fuel-oil, the most expensive is gas-oil and the intermediary is natural gas, which was the fuel used for the calculations in Tables 2 and 3. Operating costs are clearly lowest in the solution offered by the combined cycle Dual Plant, closely followed by the PAME. There is a big difference between these options and the RO option which uses electricity from the public network, It can be observed that the majority of investment costs are recovered in an average of less than six years, with a 12% discount rate, which makes the investment highly profitable. This means that in the case of the dual plant, the electricity consumption of the MED is logically paid for at the network price (10.04 PTA/kWh). 3.4. Choice of system - The choice of a system is not a matter of
-
108 (1996) 43-50
49
competition between the different technologies, but is based on a suitable plant design. It should be warned that more information is required on heating water for RO treatment in order to find out the consequences of membrane ageing and the possible resulting increased costs in replacement membranes. For the dual plant solution, aspects such as energy planning should be taken into account, since this affects electricity production and is controlled by the LOSEN, with the possibility of operating profitably in the independent system. The PAME offers the best solution in terms of industrial operativeness for removable plants installed on the ground or on barges. All aspects of these plants can be designed and built in modules. The exportation of 25% of the electricity production does not significantly increase the cost of the investment, and may decrease the cost of desalinated water by 5 to 10 PTA/m3. The design can be improved in this case too, by using the heat from the condenser and from the heat rejected from the MED to heat the water in the RO. With regard to the transformation of an existing electricity power station into a Dual Plant, it should be warned that each station will require an individual solution, taking into account its design, turbines and service regime. However, it should be noted that a transformation will only be profitable if the following two conditions are met: that the plant is operated on a base, and that it produces the maximum amount of desalinated water that is technically possible. Only under these conditions is it possible to produce desalinated water at a price that is competitive with the abovementioned cogeneration solutions.
Power stations which are transformed into dual plants are capable of supplying much greater volumes of desalinated water than plants which are in operation at present, since between 400 and 500 m3/d, depending on the
50
A.A. Mesa et al. /Desalination
type of plant, can be profitably obtained per megawatt of power of the transformed station. This means, for example, that the Group 4 Waste Dump (283 Mw) could produce between 100,000 and 140,000 m3/d, depending on the design adopted for the transformation, with associated operating costs of approximately 65 PTA/m3 of desalinated water. However, this approach would obviously require joint regulation and planning of water and electricity, and considering the consequences of the system, power stations used would therefore have to be those which are currently held in reserve, as is the case of many thermal power stations on the Mediterranean coast. Finally, the above points can be summarized in a basic conclusion: Each case should be studied and analyzed individually in order to design a plant with the greatest energy efficiency to ensure that the desalinated water is produced at the minimum
108 (1996) 43-50
cost technically possible. Preliminary engineering work is therefore clearly required, which is totally different from the work of the suppliers of the equipment and assembly work, at least in comparison with such work that is carried out at present. This highly recommended initial optimization work, i.e., a preliminary project through which offers for contracting out building work can be received, provides an opportunity which is justified in itself, for if the drought plan installed a fresh water production capacity of 500,000 m3/d, with efficient desalinators instead of those normally used up until now, savings of the equivalent of just over 65,000 tonnes of oil per year (Tep) would be achieved. This is the amount of energy consumed each year in houses in an Andalusian town of 250,000 inhabitants. This would result in a reduction of 200,000 tonnes per year of greenhouse gas CO2 released into the atmosphere.
Desalination
108 (1996) 43-50
Energy saving and desalination of water Agustin Aragh Mesa, Claudio Miguez G6mez, and Rarndn Urcelay Azpitarte* Water Technologies Department, Institute for the Diversification and Saving of Energy (IDAE), Paseo de la Castellana 95, planta 21, 28046 Madrid, Spain. Tel.: +34-l-5568415; Fax.: +34-l-5.551389 Received
29 July 1996; accepted
10 August 1996
Abstract
The IDAE is a public entity of the Ministry of Industry and Energy which belongs to the Department of Energy and Mineral Resources. Its objective is to improve energy efficiency in the use of energy in the different consumer sectors and to promote production and consumption of renewable energies. The work of the IDAE in the different consumer sectors is derived from action planned in the Energy Saving and Efficiency Plan (ESEP) of the National Energy Plan 91/2000 and from strategies identified in the New Promotion of Spain’s Energy Saving Policy, which includes thinking ahead to promote activities so that consumer sectors will be able to avoid having to take costly corrective measures - where feasible - in the future. The activities to promote energy efficiency are therefore of particular interest in the new desalination sector which is already working on a large scale in Spain. Keywords: Energy efficiency; Cost optimization
1. Introduction Water has always been a source of energy for man, and it continues to be so to produce electricity in power stations. Today we use energy to change salt water into drinking water, by desalinating the water, a process that started way back in history. The greatest desalinator in the world is Nature itself which uses the free energy of the sun in its Natural Water Cycle, as documented by Aristotle. Later, philosophers observed Presented at the Second Annual Meetin of the Euro ean Desalination Society (EDS) on Desa K.mation an dpthe Environment, Genoa, Italy, October 20-23, 1996. *Corresponding
author.
001 l-9164/97/$17.00 Copyright PZZ SO01 l-9164(97)00007-6
e 1997 Published
how Mediterranean sailors obtained fresh water from seawater. Alexander of Aphrodisia and, subsequently, Saint Basil, the Archbishop of Cesarea, both described the process as follows: The sailors put seawater in a pot on a fast boil and hang sponges over the pot. They squeeze out the sponges and obtain fresh water. The reference which dates back furthest, three thousand five hundred years ago, can be read in the book of Exodus, chapter XV, verses 22 to 25: So Moses brought the children of Israel from the Red Sea and they went out into the wilderness of Shur; and they went three days in the wilderness, and found no water; And when they came to Marah, they could not drink of the waters of Marah for they were bitter:
by Elsevier Science B.V. All rights reserved.
44
A.A. Mesa et al. /Desalination
Therefore the name of it was called Marah. And the people murmured against Moses, saying, “What shall we drink?“. And he cried unto the Lord; and the Lord shewed him a tree, which when he had cast into the waters, the waters were made sweet. When the steam engine was invented, patents were brought out to desalinate seawater using steam. However, the first industrial plant was built in 1872 in Las Salinas, near Antofagasta, in Chile, using solar distillation. It was a glass-covered wooden structure and processed 22.5 m3/d until 1908. Since then, there has been such rapid development that according to IDA (International Desalination Association), at the beginning of the nineties, there was a total 16 million m3/d capacity in 100 m3/d plants installed throughout the world. Spain has 2.4% of these plants, representing the tenth largest installed capacity. 67% of this worldwide capacity is used for producing drinking water, 20% is for industrial use and 13% is for agricultural use and others. Disregarding technologies which are at experimental stages at present, such as desalination by freezing, and others which are difficult to apply at an industrial level, such as electrodialysis and solar distillation using the greenhouse effect, there are only three technologies which are apt for commercial use today: Reverse osmosis (RO) Distillation (MSF, MED) Vacuum distillation, a variant of the second technology (VVC). RO is based on the property of some special membranes which, under the pressure of the seawater (70 kg/cm2), allow part of the water through (30 to 40%) without salt (fresh water) and block out the rest (70 to 60%) which has a higher salt content (brine) (Fig. 1). The energy used by this method is approximately 5.6 kWNm3 of desalinated water when seawater is treated (30,000 to 40,000 ppm) and 1.4 to 1.7 kWNm3 when saline groundwater is treated (2,000 to 3,000 ppm). Electricity is the only form of energy
108 (1996) 43-50
Engine + auxiliary equqxn. = 5.6 kWh/m3
Fig. 1. Reverse
osmosis
technology.
used in this method and it is employed to move the turbopumps and auxiliary equipment. The high degree of energy efficiency of this technology is gained from the recovery of the brine water of the pressure (approximately 35-40% at present). The distillation technologies use the same basis as the mediaeval still. The modern version uses a multi-stage process, shown in Fig. 2, in such a way that initial evaporation occurs (heat is absorbed), and when this vapour condenses (heat is given off) the heat from this condensation is used to evaporate the following stage, and so on. The following energy is consumed: approximately 1.3 kWh electricity and 48.5 kWh heat, for each m3 of desalinated water (3% electricity and 97% heat). The VVC (vacuum vapour compression) uses the same basis as the MED, but a vacuum
A.A. Mesa et al. / Desalination
45
108 (1996) 43-SO
Heat rejection
Effects of heat recovery
condenser Non conden-
Hig h pres sure vapor (for TVC plant)
c w Kemrn condensation pump
Fig. 2. Diagram
of MED
Intermediary FO‘ J ,.>,rn” 1LV.A Y....LY
-
inhibitor
Feed pump
and TVC processes
Evaporator-condenser
?I
Seaw ‘ater
Non condensable
Fig. 3. Diagram
of the WC
process
gases
(vacuum
LI
vapour
compression).
is formed in the machine (Fig. 3). More electricity is required to form the vacuum, but the total energy consumption is reduced because less heat is required to evaporate the water. Approximately 8.5 kWh electricity and 13.2 kWh heat is required for each m3 of desalinated water (40% electricity and 60% heat).
2. Energy effkiency in desalination Energy costs invariably represent 50 to 75% of real operating costs, regardless of the technology used and the design of the seawater desalinator. The focal point in desalination is therefore its energy consumption. Since technologies available do
46
A.A. Mesa et al. /Desalination
not offer many possibilities of reducing energy consumption, reduction of the costs of desalinated water must therefore be sought through an extensive use of the imagination and application of engineering know-how in order to achieve an efficient supply of the energy necessary. The reduction of these costs must therefore be focused on energy efficiency. Optimization of costs does not therefore only effect technologies available. This is why the overall design of the plant is of such importance, including the type of energy supply that it requires. This is also why the initial design and engineering work is of such importance too, for it is this work that identifies the suitability of one particular desalinator model amongst the different models which could be used on a certain site. If this were not so, the focus would be placed solely on the choice between the different desalination technologies, which is just one aspect of the whole plant. It cannot be expected that costs will be optimized by the suppliers of the equipment; it would not be reasonable to demand this, and, in fact, this has never occurred. The IDAE has a wide and rich experience in this field, both in cogeneration installations, buildings and renewable forms of energy. In fact, the IDAE has carried out studies on desalination and made in-depth and detailed analyses of the alternatives in the conception and design of desalination plants. This was pioneer work in Spain, despite the fact that Spain houses 2.4% of the installations of the world capacity for desalination of seawater. Cost reduction must therefore be sought in the technical optimization of the energy supply of electricity and heat. Cogeneration offers the only solution, i.e., the simultaneous supply (and consumption) of electricity and heat in the same equipment. To gain a better understanding of this subject, it should be remembered that thermoelectric generation always produces surplus heat in addition to electricity. This heat cannot be used and it evaporates in the atmosphere. This phenomenon can be
108 (1996) 43-50
observed in all thermoelectric plants, There is a difference of approximately 3 1.5% in electric efficiency between the energy a fuel contains and the electricity that reaches the consumer. If this efficiency is measured as the electricity leaves the power plant, it could be as much as 35% in the electricity system in Spain today. However, if the heat which is produced at the same time as the electricity was also used, as occurs in cogeneration installations, electric efficiency could reach 85%. There are three clear possibilities in the application of cogeneration in water desalination: - A desalinator can be designed with a small electricity generator for self-supply, in which the electricity is used in RO and the heat is used in a MED. This is the design recommended by IDAE, and is called PAME (Autonomous Maximum Efficiency Plant). A variant of this desalinator is possible, maintaining the same maximum efficiency, but it does not have a selfsufficient electricity supply. The variant is called PAME 25 (the reasons for this name will be explained later). - Cogeneration can be applied in an electric power station, using the heat which evaporates into the atmosphere in a MED desalinator. This would be the case of a Dual Plant. Examples of this plant can be found in operation abroad. _ RO can be applied, using the heat to heat the water before it is processed in the membranes.
Figs. 4 and 6 show autonomous plants, i.e., plants which do not have any connection with an electricity network. They are therefore able to operate in isolation. Figs. 4a and 6a refer to the same plants, but connected to an electricity supply, which enables them to export approximately 25% of electricity produced, provided the electricity power installed is equal to or less than 25 MVA, in accordance with Royal Decree 1327/95 of 28 July.
,n 108 (1996) 43-50
41
+ Brinei,M7
Self-consumed Fuel 21.9
Fuel 23.1 M
m3/h
ksal. way 40.000 m-/d)
rsteml ‘~ A Seawater 23°C 3% improv. in efficiency
T L’“.“*“d+\4z0C
“30% extra power for start-up. Energy efficiency: 85%
Fig. 4. Autonomous (PAME).
*30% extra power for start-up.
Hoat
Maximum
Energy efficiency: 85%
Energy
Efficiency
Plant
Fig.
6. RO autonomous
plant
Exported electricity
Brme
Self-consumed Fuel 33.28
i Brine 1.667 m3/h Desal. water (40.000 m3/d
Seawater 23°C 3% improv. t in efficiency
1-+$j~5,~oC Energy efficiency: 85%
Energy efficiency: 85%
Fig. 4a. Maximum
Energy Efficiency
Plant (PAME 25).
Fig. 6a. RO plant with cogeneration.
Electricitv to oubhc network 98i
(186.2 MWh)
onsumedDistillation
,
&g)’ 1,?7! m3/h
TL._“_d y&r
VIYUI‘C:
Energy
effvziency:
85%
Heat
Fig. 5. Dual plant with combined
Seawater
(45,OOC‘I” /d) ;15 Hm /y)
cycle.
Fig. 5 shows a dual plant with combined cycle, capable of producing 45,000 m3/d of drinking water. Tables l-3 show the electricity con-
sumption and cost of desalinated water for each proposed alternative. Table 2 shows these figures in terms of operating costs and Table 3 in terms of total costs. Costs unrelated to energy consumption have been calculated from information available in publications on this subject, and from consulting manufacturers and suppliers. These costs vary greatly on the market, and a warning should therefore be given that there is no reliable information or statistics for these costs.
3. Conclusions In light of the data given in Tables 1-3, the following conclusions can be made.
48
A.A. Mesa et al. /Desalination
Table 1 Energy consumption
per m3 of desalinated
Technology
water kWh final energy Elect. Heat
RO seawater MED seawater WC seawater PAME seawater Combined cycle dual plant seawater RO autonomous plant seawater RO autonomous plant saline water
Table 2 Operating
108 (1996) 43-50
5.6 1.3 8.5
Efficiency of supply system Elect. (%) Heat (%)
0 31.5 48.5 31.5 13.2 31.5 Overall efficiency 85% 190 MW combined cycle 12 MW self-generation 2.3 MW, idem for saline water
85 85 85
Primary kWh
energy
17.8 61.2 42.5 13.2 11.7 14.2 3.6
costs (in PTA/m3) RO with network supply (%)
RO with cogeneration (1) (%)
PAME (1) (%)
Combined (%)
O&M, and chemicals Energy
20 (26.3) 56 (73.7)
23 (45) 28 (55)
22 (48) 24 (52)
13 (31.7) 28 (68.3)
Total
76 (100)
51 (100)
46 (100)
41 (100)
cycle (1)
(1) The energy costs are calculated with natural gas at 2.1 PTA/therm. If gas-oil at 2.7 PTA/therm is used, costs increase by 8 PTA/m3 for RO and 7 PTA/m3 for PAME. If fuel-oil at 1.8 PTA/therm is used, costs decrease by 4 PTA/m3 for RO and 3 PTA/m3 for PAME. External electricity supply (from the network) is calculated at 10.04 PTA/kWh (rate 3.1) both for RO with network supply and for the electricity consumed by the dual plant.
Table 3 Total costs (in PTA/m3) RO with cogeneration (%)
PAME (%)
20 (18) 56 (51) 34 (31)
23 (25) 28 (31) 40 (44)
22 (25) 24 (27) 43 (48)
13 (15.1) 28 (32.5) 45 (52.4)
110 (100) 4,000
91 (100) 4,770
89 (100) 5,150
86 (100) 5,310
RO with network supply (%) O&M, and chemicals Energy Amortization Total Investment
in millions
Note: Amortizations
of PTA
calculated
cycle (%)
over 20 years with 10% discount rate.
3.1. General conclusions With regard
Combined
to product
costs,
energy
efficiency and reduction in release of COz, the best way of desalinating water is the combined method which produces drinking water and
A.A. Mesa et al. /Desalination
the energy required for the process, i.e. using cogeneration. The simple reason for this is that the heat which is wasted in the electricity production in power stations is used in this case; the energy efficiency is therefore much more effective and as a result there is a saving in the energy supply required for the desalination process.
-
3.2. Energy efficiency
-
Cogeneration plants are much more efficient from an energy saving viewpoint than those which do not produce their own energy. The PAME plant saves almost 26% of the energy required by the most efficient technology which uses an electricity supply from the public network. This logically goes side by side with a reduction in CO2 release per m3 of desalinated water.
-
3.3. Cost of desalinated water The data given in Tables 2 and 3 are valid for comparing technologies, regardless of the non-energy costs, since any variation in such costs would affect all the options to a similar degree. These costs vary according to the type of fuel used in cogeneration: the cheapest is fuel-oil, the most expensive is gas-oil and the intermediary is natural gas, which was the fuel used for the calculations in Tables 2 and 3. Operating costs are clearly lowest in the solution offered by the combined cycle Dual Plant, closely followed by the PAME. There is a big difference between these options and the RO option which uses electricity from the public network, It can be observed that the majority of investment costs are recovered in an average of less than six years, with a 12% discount rate, which makes the investment highly profitable. This means that in the case of the dual plant, the electricity consumption of the MED is logically paid for at the network price (10.04 PTA/kWh). 3.4. Choice of system - The choice of a system is not a matter of
-
108 (1996) 43-50
49
competition between the different technologies, but is based on a suitable plant design. It should be warned that more information is required on heating water for RO treatment in order to find out the consequences of membrane ageing and the possible resulting increased costs in replacement membranes. For the dual plant solution, aspects such as energy planning should be taken into account, since this affects electricity production and is controlled by the LOSEN, with the possibility of operating profitably in the independent system. The PAME offers the best solution in terms of industrial operativeness for removable plants installed on the ground or on barges. All aspects of these plants can be designed and built in modules. The exportation of 25% of the electricity production does not significantly increase the cost of the investment, and may decrease the cost of desalinated water by 5 to 10 PTA/m3. The design can be improved in this case too, by using the heat from the condenser and from the heat rejected from the MED to heat the water in the RO. With regard to the transformation of an existing electricity power station into a Dual Plant, it should be warned that each station will require an individual solution, taking into account its design, turbines and service regime. However, it should be noted that a transformation will only be profitable if the following two conditions are met: that the plant is operated on a base, and that it produces the maximum amount of desalinated water that is technically possible. Only under these conditions is it possible to produce desalinated water at a price that is competitive with the abovementioned cogeneration solutions.
Power stations which are transformed into dual plants are capable of supplying much greater volumes of desalinated water than plants which are in operation at present, since between 400 and 500 m3/d, depending on the
50
A.A. Mesa et al. /Desalination
type of plant, can be profitably obtained per megawatt of power of the transformed station. This means, for example, that the Group 4 Waste Dump (283 Mw) could produce between 100,000 and 140,000 m3/d, depending on the design adopted for the transformation, with associated operating costs of approximately 65 PTA/m3 of desalinated water. However, this approach would obviously require joint regulation and planning of water and electricity, and considering the consequences of the system, power stations used would therefore have to be those which are currently held in reserve, as is the case of many thermal power stations on the Mediterranean coast. Finally, the above points can be summarized in a basic conclusion: Each case should be studied and analyzed individually in order to design a plant with the greatest energy efficiency to ensure that the desalinated water is produced at the minimum
108 (1996) 43-50
cost technically possible. Preliminary engineering work is therefore clearly required, which is totally different from the work of the suppliers of the equipment and assembly work, at least in comparison with such work that is carried out at present. This highly recommended initial optimization work, i.e., a preliminary project through which offers for contracting out building work can be received, provides an opportunity which is justified in itself, for if the drought plan installed a fresh water production capacity of 500,000 m3/d, with efficient desalinators instead of those normally used up until now, savings of the equivalent of just over 65,000 tonnes of oil per year (Tep) would be achieved. This is the amount of energy consumed each year in houses in an Andalusian town of 250,000 inhabitants. This would result in a reduction of 200,000 tonnes per year of greenhouse gas CO2 released into the atmosphere.