Thermoeconomic analysis of a seawater reverse osmosis plant

Transcription

1 Desalination 181 (2005) Thermoeconomic analysis of a seawater reverse osmosis plant Vicente Romero-Ternero a *, Lourdes García-Rodríguez a, Carlos Gómez-Camacho b a Dpto. Física Fundamental y Experimental, Electrónica y Sistemas, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, La Laguna (Tenerife), Canary Islands, Spain Tel. +34 (922) ; Fax: +34 (922) ; vromero@ull.es. b Dpto. Ingeniería Energética y Mecánica de Fluidos, Universidad de Sevilla, E.S.I., Camino de los Descubrimientos s/n, Sevilla, Spain Received 9 September 2004; accepted 11 February 2005 Abstract Thermoeconomy is a useful and powerful tool that combines thermodynamics and economics. It can evaluate how irreversibility and costs of any process affect the exergoeconomic cost of the product flows. The thermoeconomic analysis of a seawater reverse osmosis desalination plant with a 21,000 m 3 /d nominal capacity located in Tenerife (Canary Islands, Spain) is given. This analysis extends the exergy analysis performed in a previous paper where further details about features of desalination facility, flow diagram, equipment purposes and flows of the process are widely provided. The main result indicates that economics predominates over the thermodynamics aspect; thus the influence of the operational parameters on the unit cost of the final product is significantly limited. Reverse osmosis skid is the most influential equipment on both the thermodynamic and economic aspects. As well, pretreatment has a large influence on the unit cost of the final product, essentially due to O&M costs. The unit cost of external consumption and the annual real discount rate are the most influential parameters on the sensitivity analysis of the final product and the high-pressure pump efficiency the most important of the operational ones; conversely, membrane replacement is the least important among the parameters analysed. Keywords: Thermoeconomic analysis; Reverse osmosis plant 1. Introduction This paper deals with a thermoeconomic analysis of the Santa Cruz de Tenerife desalination plant (a seawater reverse osmosis (SWRO) plant located at Santa Cruz de Tenerife, Tenerife, *Corresponding author. Canary Islands, Spain). For further information about this analysis see Romero-Ternero [1]. Some of latest references regarding thermoeconomy and desalination processes have been reported in García-Rodríguez et al. [2 4], Uche et al. [5,6] and El-Sayed [7,8]. Thermoeconomy is based on the thermodynamic potential exergy, which takes into account /05/$ See front matter 2005 Elsevier B.V. All rights reserved

2 44 V. Romero-Ternero et al. / Desalination 181 (2005) the energy as well as its potential use (quality). Thermoeconomic analysis provides valuable information about the influence of the efficiency and cost of equipment on the efficiency of the global plant and the cost of the product. Therefore, this thermoeconomic analysis can identify where the major chances of improvements are in the production process. In particular, the methodology of Valero and Lozano [9] has been considered for this thermoeconomics analysis. Previously, a comprehensive exergy analysis of the Santa Cruz de Tenerife desalination plant was performed by Romero-Ternero et al. [10] where the main features, diagram flow, equipment purposes and flows of desalination facility, as well as a fuel product losses definition for analysis were given in great detail. Thus, both the exergetic and the thermoeconomics analysis provide an important overview of the performance of the Santa Cruz de Tenerife desalination plant. 2. Thermoeconomic analysis The effects of irreversibility and fixed costs on the exergoeconomic cost of the products were evaluated. Details about fixed costs, thermoeconomics fundamentals and economic settings are given for the Santa Cruz desalination facility. Fixed costs represent the sum of all those costs not included as exergy terms in the flow chart (given by Fig. 1 in this case). Thus, fixed costs consist of two main groups of costs: investment and O&M. For the Santa Cruz de Tenerife desalination facility, O&M costs include proper operational and maintenance, spare parts, membrane replacement and auxiliary consumption expenditures. Auxiliary consumption represents all external consumption not included in the flow chart: chemical dosing, membrane cleaning, regulation and control, blowdown pumping, lighting and other minor consummables. Fig. 1. Flow chart for the analysis of the Santa Cruz de Tenerife desalination plant.

3 V. Romero-Ternero et al. / Desalination 181 (2005) Thermoeconomics fundamentals The thermoeconomic analysis is based on the following items: C For any flow, economic cost per unit of exergy (unit exergoeconomic cost) is assigned. Exergoeconomic cost is equal to the unit exergoeconomic cost multiplied by the exergy rate. C For any flow from the environment, external valuation of the unit exergoeconomic cost is performed. For our system, this involves seawater (free) and external consumption. C For any flow without later usefulness (losses), zero unit exergoeconomic cost is assigned. This involves blowdown. C For any equipment, fuel product balance of exergoeconomic cost is performed. In this balance, the sum of the exergoeconomic costs of the products (outlet useful flows) is equal to the sum of the exergoeconomic costs of the fuel components (exergy flows which contribute to generate the products) and the fixed costs: Product cost (A P ) = fuel cost (A F ) + fixed costs (Z) Economic setting For economic calculations, a 20-year lifetime and a 5% real discount rate are considered. This discounting reflects the time value of money once the effect of inflation is removed (a real discount rate can be approximated by subtracting expected or actual inflation from a nominal interest rate). Discounting is performed with respect to a basis year, namely the previous year to production onset. It is assumed that building of the desalination plant is done in this basis year. Taxes, amortisation and salvage value are not taken into account. Discounting affects all the terms of the fuel product balance. As a result, the unit exergoeconomic cost of the product (c P ) is given by Eq. (1). It is determined by the unit costs (c F ) and exergy rates (Ex F ) of the fuel components, the rate of discounted fixed costs (X) and the exergy rate of the product (Ex P ). The rate of discounted fixed costs (X) takes into account discounted fixed costs during the lifetime of the desalination facility (Z n ), availability (A) and discount factor (t r ) Investment costs (1) According to official figures [11], the total investment cost of the SWRO plant is about 24.6 million. In accordance with specific costs of this type of plant [12,13], it is supposed that 25% of the investment costs are due to fresh water distribution (additional, significant piping and civil works were necessary to interconnect the desalination facility and the municipal storage tanks). The remaining 75% distribution (see [12 16]) is shown in Fig. 2. Civil works, piping and valves, instrumentation and control, electrical equipment and other represents 40% of the total investment costs without fresh water distribution (30% if the Fig. 2. Distribution of the total investment cost of the Santa Cruz de Tenerife desalination plant (without distribution).

4 46 V. Romero-Ternero et al. / Desalination 181 (2005) Fig. 3. Investment cost (10 3 ) of the Santa Cruz de Tenerife desalination plant by equipment. 1 seawater pumping, 2 pretreatment, 3 high-pressure pump, 4 regulation valve, 5 reverse osmosis skid, 6 Pelton turbine, 7 posttreatment, 8 product pumping, 9 distribution. latter is included). These costs do not belong to any of the equipment of the flow chart of Fig. 1, and thus they will be shared among them, with two exceptions: the regulation valve, which has a negligible investment cost, and distribution. Thus, these shared costs contribute 4.3% per equipment. Besides that, half of the civil works is assigned to seawater pumping (70%) and posttreatment (30%) equipment. Finally, shared, own and total investment costs for all equipment are shown in Fig. 3. It can be seen that reverse osmosis skid (19%) and distribution are the main contributions to the total investment costs. Seawater pumping, pretreatment and high-pressure pump investment costs are about a 12% each. The Pelton turbine is 7.5%, posttreatment and pumping product are about 6% each, representing the smaller contributions (and this is the only equipment in which the contribution of the shared investment costs is larger than the contribution of their own investment costs). Distribution of investment costs among mechanical equipment is based on its consumption ratio (5:2:1 for high-pressure pump, Pelton turbine, seawater and product pumping) [10] and cost data [17,18]. Therefore, 55% of the mechanical equipment investment cost is assigned to the high-pressure pump, 22.5% to the Pelton turbine and the remaining 22.5% to seawater and product pumping (half for each) O&M costs The costs needed for operation and maintenance (O&M) were analysed. Membrane replacement costs are also included in O&M costs, since an annual membrane replacement rate is assumed. Assumptions for O&M costs are shown in Table 1, itemized by equipment. With respect to shared costs, the percentages of investment costs that have been considered for the calculation are: Equipment: 20.25% (piping and valves, instrumentation and control and electrical equipment); C Building: 7.5% (half of civil works); C Insurance: 57.8% (all the investments costs except for civil works, distribution and other ). The calculations for labour and insurance represent about two-thirds of the O&M shared costs; the remaining one-third corresponds to equipment and auxiliary consumption; building has a small contribution. Mechanical equipment O&M affects seawater and product pumping, the high-pressure pump and the Pelton turbine (respectively, 1, 8, 3 and 6). The percentage applied to mechanical equipment O&M costs varies in the literature from 2% [19] to 4% [12]. Specifically, it has been assumed the latter, which represents a more expensive, preventive maintenance but consequently a better performance.

5 V. Romero-Ternero et al. / Desalination 181 (2005) Fig. 4. Discounted O&M costs (10 3 ) of the Santa Cruz de Tenerife desalination plant over a lifetime (20 years with a real annual discount rate of 5%) itemized by equipment. Table 1 O&M cost assumptions (in ) itemized by equipment for the Santa Cruz de Tenerife desalination plant 1 Seawater pumping Mechanical equipments O&M: 4% of investment [12] Intake O&M: 1 % of investment 2 Pretreatment Chemicals: 3 c/m 3 a [19] Cartridge filters replacement: 0.8 c/m 3 [19] 3 High-pressure pump Mechanical equipment O&M: 4% of investment [12] 5 Reverse osmosis skid Annual replacement rate: 8% [12,15,16,19], 780/membrane b [19] O&M: 1% of investment 6 Pelton turbine Mechanical equipment O&M: 4% of investment [12] 7 Posttreatment Chemicals: 0.7 c/m 3 [19] 8 Product pumping Mechanical equipment O&M: 4% of investment [12] Shared costs Equipment O&M: 2% of investment Building O&M: 1% of investment Labour: 25,000/worker/year [19] 11 workers [12,19,20] Insurance: 2% of total equipment investment c Auxiliary consumption: 0.5 kwh/m 3 [20] at 6 c/kwh a Pretreatment was designed with a specific cost of 5.5 c/m 3 ; however, the high quality of seawater made a more soft, inexpensive actual pretreatment possible. b In acceptable accordance with the literature [17,21,22]. c Without civil works, distribution and other costs. Calculations establish total annual O&M costs close to 1,557,000 (6.3% of the total investment), 58% due to shared costs. The discounted O&M costs rise approximately to 19,400,000 over the desalination facility s lifetime. The distribution by equipment is shown in Fig. 4. Pretreatment and reverse osmosis are the main expensive equipment (and the only ones where the contribution of their own O&M costs are higher than shared O&M costs). Pretreatment exhibits the highest O&M cost (27%) and reverse osmosis skid contributes 18% of the total. The remaining equipment is about 9 14%. The regulation valve has negligible O&M costs. Distribution O&M costs were not considered a responsibility of the desalination facility.

6 48 V. Romero-Ternero et al. / Desalination 181 (2005) Fig. 5. Discounted fixed costs (10 3 ) of the Santa Cruz de Tenerife desalination plant over a lifetime (20 years with annual real discount rate of 5%), itemized by equipment Fixed costs Fixed costs involve both investment and O&M. The sum of the discounted total fixed costs rises approximately to 44,000,000 over the desalination facility s lifetime. Distribution by equipment is shown in Fig. 5. Seawater pumping (1), high-pressure pump (3) and reverse osmosis skid (5) have higher investment than O&M costs. Pretreatment and reverse osmosis skid are the most important contributions, 18% each. The contribution of the previous stages, namely seawater pumping and pretreatment, to the total fixed costs rise to 29%; the core stages, i.e., high-pressure pump, reverse osmosis skid and Pelton turbine, contribute 41%; and the final stages 30% (about half, due to distribution). Shared fixed costs represent 43% of the total, 17% investment and 26% O&M costs. Therefore, shared fixed costs have a contribution of 6% per piece of equipment, which represents one-third of the reverse osmosis skid or pretreatment fixed costs. The distribution of own fixed costs by equipment is shown in Table 2, with a contribution of investment and O&M costs of 25% and 18%, respectively. Consequently, investment and O&M costs are balanced if distribution is not considered: 49 51% (56 44% if distribution is included). Finally, the rate of discounted fixed costs for equipment is presented in Fig. 6 (with a total of Table 2 Contribution of the own fixed costs of equipment to the total fixed costs of the Santa Cruz de Tenerife desalination plant Equipment Investment (%) O&M (%) 1: Seawater pumping : Pretreatment : High pressure pump : Reverse osmosis skid : Pelton turbine : Posttreatment : Product pumping Total c/s). These values are needed to calculate the discounted exergoeconomic cost of the products generated by the equipment [see Eq. (1)]: 90% availability of the desalination plant, 5% annual real discount rate and 20-year lifetime are assumed Exergoeconomic cost of the flows Once fixed costs have been calculated, Eq. (1) provides the discounted unit exergoeconomic cost or concise unit cost (c/mj). For any stream, the rate of discounted exergoeconomic cost (c/s), or brief cost, is given by the product of its unit exergoeconomic cost and its exergy rate (kw).

7 V. Romero-Ternero et al. / Desalination 181 (2005) Fig. 6. Rate of discounted fixed costs (c /s) of the Santa Cruz de Tenerife desalination plant (90% availability, 5% discount rate and 20-year lifetime) itemized by equipment. Fig. 7. Exergoeconomic cost (c /s) of the flows of the Santa Cruz de Tenerife desalination plant. 10 seawater, 21 pumped seawater, 32 feed (pretreated seawater), 43 high-pressure feed, 54 high-pressure feed to skid, 65 high-pressure blowdown, 06 blowdown, 75 product, 87 posttreated product, 98 pumped product, 09 final product, W10 seawater pumping; W30 external consumption of high-pressure pump, W36 Pelton turbine recovery, W80 product pumping. For equipment, this cost reflects the influence of irreversibility (related to the thermo-dynamic performance of the process inside the equipment) and fixed costs in the generation of the products (useful outlet flows). The cost of the flows is shown in Fig. 7. Seawater (10) has zero cost (no process is needed to generate it). Blowdown (06) has zero cost as well because it is a useless outlet of the global process. The unit cost of external consumption is imposed by the market price (1.67 c/mj or 6 c/kwh). The highest cost is for high-pressure feed flow (43), with a value of 19.6 c/s (high-pressure feed to skid (54) has the same value because the fixed costs of the regulation valve are null). This result is justified since the high-pressure pump has the highest consumption of fuel (4362 kw), and this consumption has a mean unit cost of 4.12 c/mj, which is approximately two and a half times more expensive than the external one. Moreover, about half of the cost of the high-pressure feed flow is due to energy recovery. As a consequence, if energy recovery would be removed, the mean unit cost of fuel would decrease to 2.53 c/mj and the cost of the high-pressure feed flow to 12.7 c/s. However, energy recovery is a suitable process since it increases the performance of the global process and reduces the cost of the final product. Finally, costs by stages (previous, core and final stages) are shown in Fig. 8.

8 50 V. Romero-Ternero et al. / Desalination 181 (2005) Fig. 8. Exergoeconomic cost diagram of the Santa Cruz de Tenerife desalination plant by stages. Fig. 9. Unit exergoeconomic cost (c /MJ) of the flows of the Santa Cruz de Tenerife desalination plant. As shown, the fuel product increase is widely dominated by fixed costs in the previous and final stages. In the core stages, the cost of the product is penalized by the blowdown with a 5% contribution (see Appendix B), given that the potential use of its chemical exergy rate with respect to seawater is wasted. The unit cost of any flow is presented in Fig. 9 (see appendix A). First of all, a remarkable increase of the unit cost of the products involving the previous stages is shown. This increase is based on the significant influence of the fixed costs (29% of the total) but the small exergy gain associated with the previous stages (mainly pretreatment). Consequently, the equipment represents the highest fuel product increase of the unit costs (with reverse osmosis skid) and moreover the unit cost of feed (14.8 c/mj) is the most expensive one. As expected, the fuel product increase of the unit costs of the previous stages is largely dominated by fixed costs (higher than 90%). With regard to core stages, the influence of fixed costs and thermodynamic performance are practically balanced (about 50% each for whole core stages). The mean unit cost of fuel (the sum of the cost divided by the sum of the exergy rate for feed and high-pressure pump external consumption) is 3.07 c/mj. Thus, the fuel product increase of the unit cost due to core stages is 6.31 c/mj, the difference with respect to the unit cost of product (9.38 c/mj).

9 V. Romero-Ternero et al. / Desalination 181 (2005) In core stages, a considerable reduction in unit costs between the high-pressure feed (43) and feed (32) is disclosed. This reduction is due to the higher increase of exergy yields by the highpressure pump with respect to the exergy destruction of the process. The mean unit cost of the fuel of the high-pressure pump (feed, external consumption and energy recovery) is 4.12 c/mj. For the high-pressure pump, the influence of the fixed costs on the fuel product increase of the unit cost (0.70 c/mj) reaches 57%. The rise of the unit cost of the high-pressure feed to skid (54) is exclusively due to the exergy destruction in the regulation valve, since negligible fixed costs are considered for this equipment. For the reverse osmosis skid, the high-pressure feed to skid (54) and high-pressure blowdown (65) are the same unit cost (inlet and outlet components of a fuel must have the same exergoeconomic unit cost), and consequently, inefficiencies and fixed costs of the equipment are exclusively loaded on the product (75). Fixed costs contribute 35% to the fuel product increase of the unit cost (4.39 c/mj); therefore, thermodynamic performance dominates (exergy destruction is 54% and losses 11%). With reference to the Pelton turbine, the unit cost of energy recovery is 5.80 c/mj. This is about 3.5 times higher than the unit cost of external consumption. This increase is due to the inefficiencies and fixed costs of the high-pressure pump (seed of the hydraulic energy of highpressure blowdown) and the Pelton turbine, also loaded on a stream with a lower exergy rate. As pointed out previously, this result does not mean a more expensive final product. In this way, a Pelton turbine rate of discounted fixed costs/ recovery exergy rate ratio lower than the unit cost of external consumption is requisite to improve the unit cost of the final product using energy recovery. Consequently, for the desalination system analysed, energy recovery is economically suitable for a unit cost of external consumption higher than 2.4 c/kwh (far enough from the actual 6 c/kwh). With reference to final stages, the mean unit cost of the fuel is 7.87 c/mj, and the fuel product increase is 4.22 c/mj. Fixed costs dominate with a fraction close to 60% (70% without distribution). The pumped product (98) presents a lower unit cost than the previous one because the cost associated with pump operation is lower than the generated exergy rate increase. The unit cost of the final product is 12.1 c/mj and the global fuel product increase is 10.4 c/mj. A summary of unit costs by stages is shown in Fig. 10. As observed, the product cost is penalized by blowdown (see also Fig. 8). The unit cost of blowdown is determined in accordance with the balance of the desalination plant as a whole. Finally, the cost per cubic meter (c/m 3 ) for any mass flow is shown in Fig. 11. It can be seen that there are some significant differences with respect to unit exergoeconomic costs. In contrast with the unit exergoeconomic cost, feed (32) is not the more expensive flow because the high unit exergoeconomic cost is compensated for by low specific exergy. Similarly, since the high-pressure feed flow (43) has a high specific exergy, its cost is substantially increased. Another significant increase takes place between high-pressure feed to skid (54) and product (75), the latter with a cost of 58.5 c/m 3. The cost of the final product (09) is 76.7 c/m 3 (10% previous, 66% core and 27% final stages) Final considerations In the analysis performed, the major influence of the reverse osmosis skid equipment was disclosed. This equipment, with the highest exergy destruction [10], presents the highest fixed costs as well (see Figs. 5 and 6). Influence of exergy destruction (D) and fixed costs (Z) by equipment on the unit cost increase

10 52 V. Romero-Ternero et al. / Desalination 181 (2005) Fig. 10. Exergoeconomic unit cost diagram by stages of the Santa Cruz de Tenerife desalination plant. Fig. 11. Cost (c /m 3 ) of the mass flows of the Santa Cruz de Tenerife desalination plant. Table 3 Influence of exergy destruction (D) and fixed costs (Z) by equipment on the exergoeconomic unit cost increase of pumped (98) and final (09) product with respect to the exergoeconomic unit cost of global fuel (external consumption) Equipment Pumped product Final product D (%) Z (%) D + Z D (%) Z (%) D + Z 1: Seawater pumping : Pretreatment : High pressure pump : Regulation valve : Reverse osmosis skid : Pelton turbine : Posttreatment : Product pumping : Distribution Total of pumped (98) and final (09) product with respect to unit cost of global fuel (external consumption) is shown in Table 3. As shown, there is a wide influence of fixed costs, close to 80%. This is an essential result since prospective improvements in thermodynamic performance are clearly restricted by the economics of the process.

11 V. Romero-Ternero et al. / Desalination 181 (2005) Reverse osmosis skid leads with about onequarter of the total increase. About half is attributed to four items (pretreatment, high-pressure pump, Pelton turbine and distribution), with an individual contribution in the range of 12 15%. The remaining one-quarter is due to seawater and product pumping, posttreatment and the regulation valve. Similar results are obtained for the pumped product flow. With respect to the plant as a whole, the contribution of global external consumption on the unit cost of the final product is about 30% (23% high pressure pumping + 7% seawater and product pumping, half each); useless exergy of blowdown and fixed costs contribute around 5% and 65%, respectively. The contribution of the annual real discount rate is about 14%, since the unit cost of the final product is reduced to 65.8 c/m 3 for r = 0%. Finally, the influence of environmental parameters on the unit costs is negligible for the Santa Cruz de Tenerife desalination plant, with variations less than 1%. 3. Sensitivity analysis The main thermodynamic and economic parameters of the desalination facility were changed to analyse how they affect the unit cost of the final product. First, performance of mechanical equipment (seawater and product pumps, high-pressure pump and Pelton turbine) is considered. Alternatively, for all this equipment, a performance reduction of 5% is analysed (i.e., with an efficiency decreasing from 85% to 80%). Highpressure pump performance, with a 2.5% increase, presents the most important sensitivity on the unit cost of the final product. For the Pelton turbine a moderate 0.8% increase is achieved, despite that exergy destruction has a higher contribution on the unit cost increase of the final product (see Table 3). Lastly, seawater and product pump performance present a weak influence. The availability of the desalination plant is another operational parameter with a significant potential influence on the unit cost of the final product. The calculations indicate a 3.3% reduction on the unit cost of the final product for an availability increasing from 90% to 95%. Hence, its influence is slightly higher than that of highpressure pump performance, but in a similar order of magnitude. With regard to the main economic parameters, the external consumption unit cost and annual discount rate are considered. For external consumption, a 1 c/kwh drop yields a reduction of about 6%. For the discount rate, the decrease from 5% to 4% provides a 3.3% drop (thus, the same effect as a 5% increase on availability). Consequently, the unit cost of the final product presents a higher sensitivity to economic parameters. Finally, the influence of the most important O&M parameters (chemicals for pretreatment, membrane replacement in reverse osmosis skid) is evaluated. The increase of the chemical costs from 3 c/m 3 to 4 c/m 3 provides a 1.7% reduction of unit cost of the final product. A lower sensitivity is obtained for membrane replacement since only a 1% decrease is achieved when 5% annual membrane replacement cost and the cost of 700 membranes are taken into account. 4. Conclusions 4.1. Fixed costs 1. When distribution is not considered, investment costs and discounted O&M costs including replacement as well along the lifetime of the desalination plant with an annual discount rate of 5% and a lifetime of 20 years, contribute equally to fixed costs. A reduction of the annual discount rate or an increase of the lifetime would increase the contribution due to discounted O&M costs. Additionally, it is important to point out

12 54 V. Romero-Ternero et al. / Desalination 181 (2005) that half of the fixed costs without distribution are shared costs, thus representing a noteworthy contribution. 2. The highest fixed costs are located in the reverse osmosis skid and pretreatment equipment with a contribution slightly greater than one-third of the total fixed costs (about half for each one). The first represents the highest investment cost and the second the highest O&M costs for equipment. This is the first evidence where major improvements on fixed costs may be made, and therefore, any line of work designed to the reduction of fixed costs relating to the pretreatment and membrane cost is a suitable and realistic option. In this way, for example, it would be advisable to fit the pretreatment design to the intake seawater quality to avoid unnecessary over-scale. 3. Core stages contribute only to 40% of the total fixed costs, in contrast to the 80% of the total exergy destruction [10]; therefore, it discloses a significant influence of the no-core stages on the fixed costs. While this happens, it seems reasonable to operate with high-performance mechanical equipment in the core stages, even though it represents an increase of their fixed costs Exergoeconomic analysis 1. High-pressure feed mass flow has the highest exergoeconomic cost, about half of which is due to energy recovery by the Pelton turbine. Consequently, as a universal result, energy recovery always increases the exergoeconomic cost of the high-pressure feed, but conversely, it decreases the exergoeconomic cost of the product in the final stages. However, as a consequence of its high specific exergy, the exergoeconomic unit cost of the high-pressure feed mass flow is relatively low. 2. Mass flows for previous stages present the highest fuel product increases of the exergoeconomic unit cost, with an influence of the fixed costs greater than 90%. Like this, the product from previous stages (feed) exhibits the highest unit cost a significant influence of fixed costs and low gain of exergy (while feed exergy represents only 11% of the core fuel, feed exergoeconomic cost means 51% of the core fuel exergoeconomic cost). In summary, previous stages have a significant influence on the exergoeconomic analysis (supporting conclusion 4.1.2) but is weak on an exergetic one [10] with respect to the final product. 3. Core stages as a whole are characterised by a more balanced contribution between thermodynamic performance and fixed costs (contribution of fixed costs is slightly lesser than 60%) on the fuel product increases of their exergoeconomic unit costs. Similarly, reverse osmosis skid presents the lowest influence of fixed costs (35%) and a significant influence of exergy destruction (53%); the remaining 11% is due to losses (blowdown). Hence, given that pressure drop in membranes has a negligible effect on exergy destruction [10], any thermodynamic improvement on the membrane performance (e.g., development of membranes with similar permeate mass flow rate but operating with a lesser pressure), have an appreciable influence on the unit cost of the final product. Since the theoretical minimum of specific consumption is about 1 kwh/m 3 for the actual recovery factor of a seawater desalination plant (40 45%) and the actual specific consumption range is close to 3 kwh/m 3, it seems reasonable to expect future advancement in this way, even though other factors like mechanical equipment performance are involved in this specific consumption decrease. 4. Final stages without distribution present an influence of fixed costs near to 70% and lower fuel product increases of the exergoeconomic unit cost than previous stages. In view of this result and conclusion 4.2.2, it can be stated that the previous stages are a greater influence on thermoeconomic analysis and particularly on unit exergoeconomic cost of the final product.

13 V. Romero-Ternero et al. / Desalination 181 (2005) There is a strong predominance of fixed costs (79%) on the fuel product increase of the exergoeconomic unit cost of the final product with respect to global fuel (external consumption). Thus, improvements of the thermodynamic performance of the process are clearly limited by fixed costs. In order to reach a reasonable influence of thermodynamic performance, it is necessary to reach a notable reduction of these fixed costs. 6. When influence of equipment on the fuel product increase of the exergoeconomic unit cost of the final product with respect to global fuel is considered, reverse osmosis skid contributes approximately one-fourth of this increase; pretreatment, the high-pressure pump, the Pelton turbine and distribution contribute about a half (in an individual range of 12 15%); and the remainder is due to seawater and product pumping, regulation valve and posttreatment. 7. Core and previous stages contribute 55% and 27% (the latter almost exclusively due to fixed costs), respectively, on the fuel product increase of the exergoeconomic unit cost of the final product when distribution is not considered. 8. Results for the fuel product increase of the exergoeconomic unit cost with respect to global fuel are similar in magnitude when distribution is not taken into account, and thus the main conclusions can be applied to pumped product as well. 9. Energy recovery is economically suitable only when the external consumption unit cost from the grid is higher than 8.6 c/mj (2.4 c/kwh), far enough from the actual 6 c/kwh. 10. Exergoeconomic unit cost of the final product is 12.1 c/mj, 30% due to external consumption 23% high-pressure pump and 7% seawater and product pumping (half each one) 68% to fixed costs and 4% to losses (blowdown). Finally, the cost per cubic meter is 76.7 c/m Sensitivity analysis 1. The exergoeconomic unit cost of the final product presents the highest sensitivity with respect to external unit consumption cost and the annual real discount rate, parameters whose influence is determined by market considerations. 2. The next greatest influences are due to availability, high-pressure pump performance and chemical costs for pretreatment. The latter supports some previous conclusions, but it can be limited by a possible drop in availability. The second one indicates the possible advantages of choosing a high-performance pump even though it were more expensive. 3. From among the parameters analysed, the lower sensitivity corresponds to the Pelton turbine s performance and membrane replacement costs; and thus intensification of the pretreatment does not seem beneficial for reducing only membrane replacement costs. 5. Recommendations First, as a general recommendation, Conclusion indicates an overall reduction of fixed costs without which the influence of equipment performance on the unit cost of final product would be rather limited. The corresponding technological and market tests must be performed both on invest-ment and O&M costs as a result of Conclusion About this general reduction, pretreatment is a principal focus (Conclusion 4.2.7), and it would be suitable to find a higher balance bits role in the process and the fixed costs which its operation generates. With regard to pretreatment, any intensification of chemical treatment that only involves improvements on membrane replacement and not on availability would be unsuitable for the exergoeconomic unit cost of the final product, according to Conclusions and In

14 56 V. Romero-Ternero et al. / Desalination 181 (2005) addition, this intensification would be opposed to conclusions 4.1.2, and 4.2.7, which indicate high fixed costs and greater influence on the exergoeconomic unit cost of the final product for pretreatment. However, on the other hand, the influence of a possible reduction in the pretreatment level on availability must be analysed in detail. In summary, the optimisation of standard pretreatment or the integration of innovative techniques like other membrane processes is entirely justified and clearly supported by reports in the literature. With reference to the Pelton turbine, performance increase is not a priority from the point of view of the product cost according to Conclusion To improve the influence of the Pelton turbine performance on product cost, it is necessary to decrease the fixed costs of the no-core stages (Conclusion 4.1.3) or to increase the influence of equipment performance in agreement with the general recommendations. Conclusion indicates the cost-effective operation of the Pelton turbine, since the unit cost of external consumption (6 c/kwh) is clearly separate from profitable energy recovery value (2.4 c/kwh). The influence of reverse osmosis skid on product cost must be mainly focused on investment cost: membrane technology must point towards a fixed cost reduction (Conclusion 4.1.2) without availability loss (Conclusion 4.3.2) and keep or improve current operational features. According to Conclusion 4.3.3, the influence of membrane replacement on product cost would have a less important order of magnitude. Since fixed costs of the core stages are only 40% of the total (Conclusion 4.1.3) and performance of high-pressure pump is the most influential operational parameter (Conclusions and 4.3.2), it is reasonable to choose a highquality pump like that selected for the Santa Cruz de Tenerife desalination plant, despite its higher cost. However, possible reduction of fixed costs for no-core stages would make a new analysis necessary. 6. Symbols A c Ex n r Availability of the desalination plant Exergoeconomic unit cost or unit cost, /kj Exergy rate, kw Lifetime, y Annual real discount rate t r Discount factor (years) = X Rate of discounted fixed costs, /s) = Z Fixed costs, Z n Discounted fixed costs over a lifetime, Subscripts F P Greek A Fuel Product Rate of discounted exergoeconomic cost or exergoeconomic cost, /s Acknowledgements This work was financially supported by the Spanish Ministerio de Ciencia y Tecnología (project SOLARDESAL REN P4-04) and the Consejería de Educación, Cultura y Deportes of the Autonomous Government of the Canary Islands (project PI2001/012). The authors thank the manager of the Santa Cruz de Tenerife desalination plant, Mr. Jorge Motas Pérez, for his valuable technical advice and especially for his friendly cooperation.

15 V. Romero-Ternero et al. / Desalination 181 (2005) References [1] V. Romero-Ternero, Análisis termoeconómico de la desalación de agua de mar mediante ósmosis inversa con aplicación de energía eólica, PhD Thesis, University of La Laguna, Spain, 2003 (in Spanish). [2] L. García-Rodríguez, A. Palmero-Marrero and C. Gómez-Camacho, Comparison of solar thermal technologies for applications in seawater desalination, Desalination, 142 (2002) [3] L. García-Rodríguez, A. Palmero-Marrero and C. Gómez-Camacho, Thermoeconomic optimization of the SOL-14 plant (Plataforma solar de Almería, Spain), Desalination, 136 (2001) [4] L. García-Rodríguez and C. Gomez-Camacho, Thermoeconomic analysis of a solar parabolic trough collector distillation plant, Desalination, 122 (1999) [5] J. Uche, L. Serra, L.A. Herrero, A. Valero, J.A. Turégano and C. Torres, Software for the analysis of water and energy systems, Desalination, 156 (2003) [6] J. Uche, L. Serra and A. Valero, Thermoeconomic optimization of a dual-purpose power and desalination plant, Desalination, 136 (2001) [7] Y.M. El-Sayed, Designing desalination systems for higher productivity, Desalination, 134 (2001) [8] Y.M. El-Sayed, Thermoeconomics of some options of large mechanical vapour-compression units, Desalination, 125 (1999) [9] A. Valero and M.A. Lozano, Curso de termoeconomía, Department of Mechanical Engineering, University of Zaragoza, Spain, 1994 (in Spanish). [10] V. Romero-Ternero, L. García-Rodríguez and C. Gómez-Camacho, Exergetic analysis of a seawater reverse osmosis desalination plant, Desalination, 175 (2005) [11] Desaladora de Santa Cruz de Tenerife: Manantial de mar, Report of the Spanish Ministry of the Environment, ca (in Spanish). [12] M. Fariñas Iglesias, Ósmosis inversa: fundamentos, tecnología y aplicaciones, McGraw-Hill, Madrid, 1999 (in Spanish). [13] A. Valero, J. Uche and L. Serra, La desalación como alternativa al Plan Hidrológico Nacional. Centro de Investigación de Recursos y Consumos Energéticos (CIRCE), University of Zaragoza, Spain, 2001 (in Spanish). [14] J.A. Ibáñez Mengual, L.M. Berná Amorós and R. Valerdi Pérez, Desalación por Membranas, DM, Murcia, 1997 (text in Spanish). [15] J.C. Ibrahim Perera, Desalación de aguas, Colegio de Ingenieros de Caminos, Canales y Puertos, Madrid, 1999 (in Spanish). [16] J.A. Medina San Juan, Desalación de aguas salobres y de mar: ósmosis inversa, Mundi-Prensa, Madrid, 1999 (in Spanish). [17] A. Hafez and S. El-Manharawy, Economics of seawater RO desalination in the Red Sea region, Egypt. Part I. A case study, Desalination, 135 (2002) [18] Manual of Water of Supply Practices (M46): Reverse Osmosis and Nanofiltration, American Water Works Association, Denver, USA, 1999, p [19] M. Hernández Suárez, Estimación de los costes de explotación de una desaladora de ósmosis inversa de m 3 /día (2001), Canary Islands Water Center, 2004, (in Spanish). [20] J. Motas Pérez, Personal communication, [21] M. Wilf and K. Klinko, Optimization of seawater RO systems design, Desalination, 138 (2001) [22] M. Wilf and M. Schierach, Improved performance and cost reduction of RO seawater systems using UF pre-treatment, Desalination, 135 (2001)

16 58 V. Romero-Ternero et al. / Desalination 181 (2005) Appendix A Table A.1 Exergoeconomic unit costs of flows from thermoeconomic balance Flows Exergoeconomic unit cost 21 Pumped seawater 32 Feed (pretreated seawater) 43 High-pressure feed 54 High-pressure feed to skid 65 High-pressure blowdown a 75 Product b W36 Energy recovery 06 Blowdown a 87 Posttreated product 98 Pumped product 09 Final product a By methodological considerations of Valero-Lozano thermoeconomic analysis. b Term representing the waste of rejected brine chemical exergy rate, i.e., the waste of its potential use with respect to seawater (c W : unit cost of external consumption). c, exergoeconomic unit cost ( /kj). Ex, exergy rate (kw). X, rate of discounted fixed costs ( /s).

17 V. Romero-Ternero et al. / Desalination 181 (2005) Appendix B Unit cost of final product (plant as a whole) From exergy and exergoeconomic balance of the plant as a whole: of discounted fixed costs of desalination plant as a whole ( /s) and X k is the rate of discounted fixed costs of equipment k ( /s). where c 09 is the exergoeconomic unit cost of the final product ( /kj), c W the unit cost of external consumption ( /kj), Ex D the rate of exergy destruction of the desalination plant as a whole (kw), Ex D,k, rate of exergy destruction in equipment k (kw), Ex 06, exergy rate of blowdown (kw), Ex 09, exergy rate of final product (kw), X the rate Fig. B1. Contributions to unit cost of final product (plant as a whole): exergy destruction, fixed costs and potential use of rejected brine chemical exergy rate.