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Seawater desalination costs cut through power plant co-location
featurearticle Historically, the two key barriers for the wider implementation of seawater desalination have been the costs of water production and the environmental impact of the plant concentrate. An innovative approach for improving the economics of seawater desalination and at the same time reducing its impact on the environment is the co-location of membrane desalination plants with existing coastal power generation stations. In the following article Nikolay Voutchkov describes in detail the co-location concept.
Seawater desalination costs cut through power plant co-location
T
he co-location approach includes the direct connection of the membrane desalination plant intake and discharge facilities to the discharge outfall of an adjacently located once-thru coastal power plant. This configuration allows the use of the power plant cooling water not only as source water for the seawater desalination plant, but also as blending water to reduce the salinity of the desalination plant concentrate prior to the discharge to the ocean. As shown on Figure 1, under typical operational conditions the seawater enters the power plant intake facilities and after screening is pumped through the power plant condensers to cool them, and thereby remove the waste heat generated during the electricity generation process. The cooling water discharged from the condensers is typically 5-15 °C warmer than the source ocean water and is usually conveyed to the ocean via a separate discharge outfall. Under the co-location concept, the intake of the seawater desalination plant is connected to the discharge canal of the power plant to collect a portion of the cooling water for desalination. After the desalination plant source seawater is pre-treated, it is processed in a reverse osmosis (RO) membrane desalination system, which produces two key streams – low salinity permeate, which after conditioning is conveyed for potable water supply, and concentrate, which has a salinity that is typically two times higher than the source seawater. Under the co-location configuration, the desalination plant concentrate is conveyed to the power plant discharge outfall downstream of the point of the desalination plant intake connection. Co-location with a power station on a large-scale was first used by Poseidon Resources for the Tampa Bay Seawater Desalination Project, and since then has been considered for numerous plants in the United States and worldwide. The intake and discharge of the Tampa Bay seawater desalination plant are connected directly to the cooling water discharge outfalls of the Tampa Electric (TECO) Big Bend power station (Figure 2). The TECO power station discharges an average of 5.3 billion m3 of cooling water per day. The desalination plant takes an average 167 000 m3/day of this cooling water to produce 95 000 m3/day of potable water. The desalination plant concentrate is discharged to the same TECO cooling water outfalls downstream from the point of the seawater desalination plant intake connection. In order for the co-location concept to be cost-effective and easy to implement, the power plant cooling water discharge flow has to be at least three to four times larger than the desalination plant capacity, and the power plant outfall length has to be
24 September 2004
adequate to avoid entraining and recirculation of concentrate into the desalination plant intake. Special consideration has to be given to what effect the power plant operations will have on the discharge water quality because this discharge provides the intake source water for the desalination plant. Some older power generation plants, which are equipped with copper or nickel condenser tubes that exhibit significant corrosion, may not be suitable for co-location. Significant elevation of copper, nickel or iron in the power plant discharge may render this discharge unsuitable to be used as source water for membrane seawater desalination plants. This is because these metals, if occuring in large quantity, may cause irreversible fouling of the membrane elements.
Sharing common intake Normally, coastal power plants with once-tru cooling use large volumes of seawater. Because the power plant intake seawater has to pass through the small diameter tubes (typically 20 mm or less) of the plant condensers to cool them, power plant discharge cooling water is already screened through bar racks and fine screens similar to these used at surface water intake desalination plants. Therefore, a desalination plant with intake that is connected to the discharge outfall of a power plant usually does not require the construction of a separate intake structure, intake pipeline and screening facilities (bar-racks and coarse screens). Since the cost of a new surface water intake for a desalination plant is typically between 5-20% of the total plant construction expenditure, power plant co-location yields significant construction cost savings. Sharing intake infrastructure also has environmental benefits because it avoids the need for new construction in the ocean and the seashore area near the desalination plant. The construction of a separate new open intake structure and pipeline for the desalination plant could cause significant disturbance of the benthic marine organisms on the ocean floor. The use of intake beach wells instead of open intakes would have similar negative environmental impact on the seashore marine organisms during the beach well construction. Another clear environmental benefit of the co-location of power and desalination plants is the reduced overall entrainment, impingement and entrapment of marine organisms compared to what would happen with the construction of two separate intake structures - one for the power plant and one for the desalination ISSN 0015-1882/04 © 2004 Elsevier Ltd. All rights reserved
featurearticle
Figure 1: Co-location of membrane desalination and power plants. plant. This benefit stems from the fact that total biomass of the impacted marine organisms is typically proportional to the volume of the intake seawater. By using the same intake seawater twice (first for cooling and then for desalination) the net intake inflow of seawater and marine organisms is minimized.
Sharing common discharge Under the co-location configuration, the power plant discharge serves both as an intake and discharge to the desalination plant. Four key benefits stem from this arrangement: 1. The construction of a separate desalination plant outfall structure is avoided, thereby decreasing the overall costs for seawater desalination. 2. The salinity of the desalination plant discharge is reduced as a result of the mixing and dilution of the membrane concentrate with the power plant discharge, which has ambient seawater salinity. 3. A portion of the discharge water is converted to potable water, so the total amount of the power plant thermal discharge is reduced, which in turn lessens the negative effect of the power plant thermal discharge on the aquatic environment. 4. The blending of the desalination plant and the power plant discharges results in accelerated dissipation of both the salinity and the thermal discharges. The cost of construction of a separate ocean outfall could be significant and its avoidance would result in a measurable reduction of plant construction expenditures. In addition, the length and configuration of the desalination plant concentrate discharge outfall are closely related to the discharge salinity. Normally, the lower the discharge salinity, a sorter outfall and less sophisticated discharge diffuser configuration are required to achieve environmentally safe concentrate discharge. The blending
Filtration+Separation
of the desalination plant concentrate with the lower salinity power plant cooling water often reduces the overall salinity of the ocean discharge within the range of the natural variability of the seawater at the end of the discharge pipe, thereby alleviating the need for complex and costly discharge diffuser structures. In addition, the power plant thermal discharge is lighter than the ambient ocean water because of its elevated temperature, and therefore it tends to float on the ocean surface. The heavier saline discharge from the desalination plant draws the lighter cooling water downwards, and thereby engages the entire depth of the ocean water column into the heat and salinity dissipation process. As a result, the time for dissipation of both discharges shortens significantly and the area of their impact is reduced.
Other co-location advantages One of the key additional benefits of co-location is the overall reduction of the desalination plant power demand and associated costs of water production as a result of the use of warmer source water. As mentioned previously, the source water of the RO plant is typically 5 °C to 15 °C higher than the temperature of the ambient ocean water. This is a significant benefit, especially for desalination plants with cold source seawater (such as ocean water in Northern Europe). This is because the RO membrane separation of seawater, which is on average 10 °C warmer, requires approximately 5-8% lower feed pressure, and therefore proportionally lower energy use and power costs for seawater desalination. Since the power costs are approximately 20-40% of the total costs for the production of desalinated water, the use of warmer source water could have a measurable beneficial effect on the overall water production costs. As a result of the co-location, the desalination plant unit power costs could be further decreased by avoiding the need for power grid transmission and the associated fees. Typically, the
September 2004 25
featurearticle
Figure 2: Co-location of Tampa Bay seawater desalination plant intake/discharge and TECO power plant discharge. power tariff (unit power cost) structure includes two components – fees for power production and for power grid transmission. Often, the power transmission grid portion of the tariff is 30% to 50 % of the total unit power costs. By connecting the desalination plant directly to the power plant electricity generation equipment, the grid transmission portion of the power fees could be substantially reduced or completely avoided, thereby further reducing the seawater desalination costs. Co-location of power and desalination plants may also have advantages for the power plant host. In addition to the benefit of having a new customer and generating revenue by leasing power plant property to locate the desalination plant, the power plant host also gains a user of a steady power demand and a high power load factor. This continuous high-quality power demand allows the power plant host to operate its power generation units at optimal regime, which in turn reduces the overall costs of power production.
Conclusions Co-location of desalination plants with large power generation stations may yield measurable improvements in the economics of seawater desalination and offer cost-reduction advantages as a result of the use of shared intake and discharge facilities, and reduced desalination power costs. In addition, co-location may provide environmental benefits for both the power station and the desalination plant because of the accelerated dissipation of the thermal and saline discharges, and the reduction of impact on the marine benthic and seashore habitats by avoiding the construction of new intake and outfall structures and pipelines in the sea.
Contact: Nikolay Voutchkov, senior vice president – Technical Services, Poseidon Resources Corp, 1055 Washington Boulevard, Stamford, CT 06901, USA. Tel: +1 203 327 7740; E-mail: [email protected]; Website: www.poseidon1.com
26 September 2004
ISSN 0015-1882/04 © 2004 Elsevier Ltd. All rights reserved
Seawater desalination costs cut through power plant co-location
T
he co-location approach includes the direct connection of the membrane desalination plant intake and discharge facilities to the discharge outfall of an adjacently located once-thru coastal power plant. This configuration allows the use of the power plant cooling water not only as source water for the seawater desalination plant, but also as blending water to reduce the salinity of the desalination plant concentrate prior to the discharge to the ocean. As shown on Figure 1, under typical operational conditions the seawater enters the power plant intake facilities and after screening is pumped through the power plant condensers to cool them, and thereby remove the waste heat generated during the electricity generation process. The cooling water discharged from the condensers is typically 5-15 °C warmer than the source ocean water and is usually conveyed to the ocean via a separate discharge outfall. Under the co-location concept, the intake of the seawater desalination plant is connected to the discharge canal of the power plant to collect a portion of the cooling water for desalination. After the desalination plant source seawater is pre-treated, it is processed in a reverse osmosis (RO) membrane desalination system, which produces two key streams – low salinity permeate, which after conditioning is conveyed for potable water supply, and concentrate, which has a salinity that is typically two times higher than the source seawater. Under the co-location configuration, the desalination plant concentrate is conveyed to the power plant discharge outfall downstream of the point of the desalination plant intake connection. Co-location with a power station on a large-scale was first used by Poseidon Resources for the Tampa Bay Seawater Desalination Project, and since then has been considered for numerous plants in the United States and worldwide. The intake and discharge of the Tampa Bay seawater desalination plant are connected directly to the cooling water discharge outfalls of the Tampa Electric (TECO) Big Bend power station (Figure 2). The TECO power station discharges an average of 5.3 billion m3 of cooling water per day. The desalination plant takes an average 167 000 m3/day of this cooling water to produce 95 000 m3/day of potable water. The desalination plant concentrate is discharged to the same TECO cooling water outfalls downstream from the point of the seawater desalination plant intake connection. In order for the co-location concept to be cost-effective and easy to implement, the power plant cooling water discharge flow has to be at least three to four times larger than the desalination plant capacity, and the power plant outfall length has to be
24 September 2004
adequate to avoid entraining and recirculation of concentrate into the desalination plant intake. Special consideration has to be given to what effect the power plant operations will have on the discharge water quality because this discharge provides the intake source water for the desalination plant. Some older power generation plants, which are equipped with copper or nickel condenser tubes that exhibit significant corrosion, may not be suitable for co-location. Significant elevation of copper, nickel or iron in the power plant discharge may render this discharge unsuitable to be used as source water for membrane seawater desalination plants. This is because these metals, if occuring in large quantity, may cause irreversible fouling of the membrane elements.
Sharing common intake Normally, coastal power plants with once-tru cooling use large volumes of seawater. Because the power plant intake seawater has to pass through the small diameter tubes (typically 20 mm or less) of the plant condensers to cool them, power plant discharge cooling water is already screened through bar racks and fine screens similar to these used at surface water intake desalination plants. Therefore, a desalination plant with intake that is connected to the discharge outfall of a power plant usually does not require the construction of a separate intake structure, intake pipeline and screening facilities (bar-racks and coarse screens). Since the cost of a new surface water intake for a desalination plant is typically between 5-20% of the total plant construction expenditure, power plant co-location yields significant construction cost savings. Sharing intake infrastructure also has environmental benefits because it avoids the need for new construction in the ocean and the seashore area near the desalination plant. The construction of a separate new open intake structure and pipeline for the desalination plant could cause significant disturbance of the benthic marine organisms on the ocean floor. The use of intake beach wells instead of open intakes would have similar negative environmental impact on the seashore marine organisms during the beach well construction. Another clear environmental benefit of the co-location of power and desalination plants is the reduced overall entrainment, impingement and entrapment of marine organisms compared to what would happen with the construction of two separate intake structures - one for the power plant and one for the desalination ISSN 0015-1882/04 © 2004 Elsevier Ltd. All rights reserved
featurearticle
Figure 1: Co-location of membrane desalination and power plants. plant. This benefit stems from the fact that total biomass of the impacted marine organisms is typically proportional to the volume of the intake seawater. By using the same intake seawater twice (first for cooling and then for desalination) the net intake inflow of seawater and marine organisms is minimized.
Sharing common discharge Under the co-location configuration, the power plant discharge serves both as an intake and discharge to the desalination plant. Four key benefits stem from this arrangement: 1. The construction of a separate desalination plant outfall structure is avoided, thereby decreasing the overall costs for seawater desalination. 2. The salinity of the desalination plant discharge is reduced as a result of the mixing and dilution of the membrane concentrate with the power plant discharge, which has ambient seawater salinity. 3. A portion of the discharge water is converted to potable water, so the total amount of the power plant thermal discharge is reduced, which in turn lessens the negative effect of the power plant thermal discharge on the aquatic environment. 4. The blending of the desalination plant and the power plant discharges results in accelerated dissipation of both the salinity and the thermal discharges. The cost of construction of a separate ocean outfall could be significant and its avoidance would result in a measurable reduction of plant construction expenditures. In addition, the length and configuration of the desalination plant concentrate discharge outfall are closely related to the discharge salinity. Normally, the lower the discharge salinity, a sorter outfall and less sophisticated discharge diffuser configuration are required to achieve environmentally safe concentrate discharge. The blending
Filtration+Separation
of the desalination plant concentrate with the lower salinity power plant cooling water often reduces the overall salinity of the ocean discharge within the range of the natural variability of the seawater at the end of the discharge pipe, thereby alleviating the need for complex and costly discharge diffuser structures. In addition, the power plant thermal discharge is lighter than the ambient ocean water because of its elevated temperature, and therefore it tends to float on the ocean surface. The heavier saline discharge from the desalination plant draws the lighter cooling water downwards, and thereby engages the entire depth of the ocean water column into the heat and salinity dissipation process. As a result, the time for dissipation of both discharges shortens significantly and the area of their impact is reduced.
Other co-location advantages One of the key additional benefits of co-location is the overall reduction of the desalination plant power demand and associated costs of water production as a result of the use of warmer source water. As mentioned previously, the source water of the RO plant is typically 5 °C to 15 °C higher than the temperature of the ambient ocean water. This is a significant benefit, especially for desalination plants with cold source seawater (such as ocean water in Northern Europe). This is because the RO membrane separation of seawater, which is on average 10 °C warmer, requires approximately 5-8% lower feed pressure, and therefore proportionally lower energy use and power costs for seawater desalination. Since the power costs are approximately 20-40% of the total costs for the production of desalinated water, the use of warmer source water could have a measurable beneficial effect on the overall water production costs. As a result of the co-location, the desalination plant unit power costs could be further decreased by avoiding the need for power grid transmission and the associated fees. Typically, the
September 2004 25
featurearticle
Figure 2: Co-location of Tampa Bay seawater desalination plant intake/discharge and TECO power plant discharge. power tariff (unit power cost) structure includes two components – fees for power production and for power grid transmission. Often, the power transmission grid portion of the tariff is 30% to 50 % of the total unit power costs. By connecting the desalination plant directly to the power plant electricity generation equipment, the grid transmission portion of the power fees could be substantially reduced or completely avoided, thereby further reducing the seawater desalination costs. Co-location of power and desalination plants may also have advantages for the power plant host. In addition to the benefit of having a new customer and generating revenue by leasing power plant property to locate the desalination plant, the power plant host also gains a user of a steady power demand and a high power load factor. This continuous high-quality power demand allows the power plant host to operate its power generation units at optimal regime, which in turn reduces the overall costs of power production.
Conclusions Co-location of desalination plants with large power generation stations may yield measurable improvements in the economics of seawater desalination and offer cost-reduction advantages as a result of the use of shared intake and discharge facilities, and reduced desalination power costs. In addition, co-location may provide environmental benefits for both the power station and the desalination plant because of the accelerated dissipation of the thermal and saline discharges, and the reduction of impact on the marine benthic and seashore habitats by avoiding the construction of new intake and outfall structures and pipelines in the sea.
Contact: Nikolay Voutchkov, senior vice president – Technical Services, Poseidon Resources Corp, 1055 Washington Boulevard, Stamford, CT 06901, USA. Tel: +1 203 327 7740; E-mail: [email protected]; Website: www.poseidon1.com
26 September 2004
ISSN 0015-1882/04 © 2004 Elsevier Ltd. All rights reserved