Thereconomic Analysis of Seawater Osmosis Reverse Desalination Plant

ELSEVIER Desalination 181 (2005) 43-59

DESAHNATION

www.elsevier.com/locate/desal

Thermoeconomic analysis of a seawater reverse osmosis plant

b Vicente Romero-Ternero a*, Lourdes Garcia-Rodriguez", Carlos Gomez-Camacho aDpto. Fisica Fundamental y Experimental, Electr6nica y Sistemas, Universidad de La Laguna, Avda. Astrofisico Francisco Sdnchez s/n, 38206 La Laguna (Tenerife), Canary Islands, Spain

Tel. +34 (922) 318102; Fax: +34 (922) 318228; email: [email protected]. bDpto. Ingenieria Energdtica y Mecgmica de Fluidos, Universidad de Sevilla, E.S.L,

Camino de los Descubrimientos s/n, 41092 Sevilla, Spain

Received 9 September 2004; accepted 11 February 2005

Abst rac t

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 m3/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. In t roduct ion

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 Garcia-Rodriguez et al. [2-4], Uche et al. [5,6] and E1-Sayed [7,8].

Thermoeconomy is based on the thermodynamic potential exergy, which takes into account

0011-9164/05/$- See front matter 2005 Elsevier B.V. All rights reserved doi: 10.1016/j.desal.2005.02.012

44 V. Romero-Ternero et al. / Desalination 181 (2005) 43-59

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. There- fore, this thermoeconomic analysis can identify where the major chances of improvements are in the production process. In particular, the metho- dology 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 pro- vide an important overview of the performance of the Santa Cruz de Tenerife desalination plant.

2. Thermoeeonomie analysis

The effects ofirreversibility 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.

W30

I REGLII~TION 32 V&LVE

ttI~H PRESSlJRE ~ PUMP

W36

I S~WA~R 1

06

t

~IIJ/qlCIPAL

Fig. 1. Flow chart for the analysis of the Santa Cruz de Tenerife desalination plant.

v. Romero-Ternero et al. / Desalination 181 (2005) 43-59 45

2.1. Thermoeconomics fundamentals

The thermoeconomic analysis is based on the following items: 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.

For any flow from the environment, extemal valuation of the unit exergoeconomic cost is performed. For our system, this involves seawater (free) and external consumption.

For any flow without later usefulness (losses), zero unit exergoeconomic cost is assigned. This involves blowdown.

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 (rip) = fuel cost (1-IF) + fixed costs (Z).

2.2. 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 on- set. It is assumed that building of the desalination plant is done in this basis year. Taxes, amorti- sation 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 (cp) is given by Eq. (1). It is determined by the unit costs (CF) and

exergy rates (ExF) of the fuel components, the rate of discounted fixed costs (X) and the exergy rate of the product (Exp). The rate of discounted fixed costs (JO takes into account discounted fixed costs during the lifetime of the desalination facility (Zn) , availability (A) and discount factor (t3.

ExF + X Ex F + Z. cv=~c v - -=Y~c F - -

F Exp Exp F Exp At rEx e (1)

2.3. Investment costs

According to official figures [11], the total investment cost of the SWRO plant is about 624.6 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

8% Electric

4% Instruments

and contr

15% Piping

and va lves

Others 3% Mechanical

)merit )%

Membranes 20%

. . . . IUTo Civil Work Pret reatment

Fig. 2. Distribution of the total investment cost of the Santa Cruz de Tenerife desalination plant (without distribution).


600C

5000 %

,,,.r 4000

E 3000

2000

1000

1 2 3 4 5 6 7

Equipment

6144

8 9

Fig. 3. Investment cost (1036) 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).

2.4. 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); Building: 7.5% (half of civil works); 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.

/

5000

4000 %

3000 ,,d 0 2000

V. Romero-Ternero et al. / Desalination 181 (2005) 43-59 47

1000

1 2 3 4 5 6 7 8 9

Equipment

Fig. 4. Discounted O&M costs (1031~) 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 E) itemized by equipment for the Santa Cruz de Tenerife desalination plant

1 Seawater pumping

2 Pretreatment

3 High-pressure pump 5 Reverse osmosis skid

6 Pelton turbine 7 Posttreatment 8 Product pumping

Shared costs

Mechanical equipments O&M: 4% of investment [ 12] Intake O&M: 1% of investment

Chemicals: 3 c /m 3 a [19] Cartridge filters replacement: 0.8 c/m 3 [ 19] Mechanical equipment O&M: 4% of investment [ 12] Annual replacement rate: 8% [12,15,16,19], 780/membrane b [19] O&M: 1% of investment Mechanical equipment O&M: 4% of investment [ 12] Chemicals: 0.7 c/m 3 [19] Mechanical equipment O&M: 4% of investment [12] Equipment O&M: 2% of investment Building O&M: 1% of investment Labour: 25,000/worker/year [ 19] x I 1 workers [ 12,19,20] Insurance: 2% of total equipment investment c Auxiliary consumption: 0.5 kWh/m 3 [20] at 6 c/kWh

aPretreatment was designed with a specific cost of 5.5 c/m3; however, the high quality of seawater made a more soft, inexpensive actual pretreatment possible. bin acceptable accordance with the literature [ 17,21,22]. cWithout civil works, distribution and "other" costs.

Calculations establish total annual O&M costs close to 61,557,000 (6.3% of the total investment), 58% due to shared costs. The discounted O&M costs rise approximately to 619,400,000 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. Distri- bution O&M costs were not considered a responsibility of the desalination facility.

48 F. Romero- Ternero et al. / Desalination 181 (2005) 43-59

qla

%

8

u.

~oooo U~,n%~tZent . . . . . . . . . . . . . . . . . . i 8098 8168 [] O&M

8000' I~ ~ I~Fixed costs I~ ~715 I~ 0144

8000 4749

4000 3298

0 l 2 ~ 4 5 6 7 8 9

Equipment

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.

2.5. Fixed costs

Fixed costs involve both investment and O&M. The sum of the discounted total fixed costs rises approximately to 644,000,000 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 t 8%, 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 3.9 0.8 2: Pretreatment 4.2 8.2 3: High pressure pump 4.6 2.3 5: Reverse osmosis skid 8.4 4.1 6: Pelton turbine 1.9 0.9 7: Posttreatment 1.3 1.5 8: Product pumping 0.9 0.5 Total 25.2 18.3

12.4 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.

2.6. Exergoeconomic cost of the flows

Once fixed costs have been calculated, Eq. (1) provides the discounted unit exergoeconomic cost or concise unit cost (c/M J). 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).


t~ ~ 3JO0 ' / i " . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 2.29 2 r31

2,00 ~ . ~ 1.74 . 1.34 162 ~ !+~. I 0962 ~=~ .~ 1.00

ooo 0,00 ~ ~:+'

1 2 3 4 5 6 7 8 9

Equipment

Fig. 6. Rate of discounted fixed costs (cE/s) of the Santa Cruz de Tenerife desalination plant (90% availability, 5% discount rate and 20-year lifetime) itemized by equipment.

25,00

u~ 20.00

U .+ 15.00

10.00

g $ 5.oo

0.00

19.6 19,6

4.25 1

10 21 32 43

FIMass

18.4 [ ] Exergy 16,6

14.o 15.1

+ +

LIIJo 46m 54 65 06 75 87 98 09 10 30 36 80

Flows

Fig. 7. Exergoeconomic cost (c6/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.


~WATER ~ C C ON!~JMPrloN

0 ~:ts I 3~3 e6's L

]L"I'JERIq/LI, HIGH ~.I~SSlJIP~ I~OI)UCT ~ C ttff~ C O~U)Y[FI"ION CONSI/~P'IION

szA~s : ~ STATES ~. 5.04 c~ts 3 77 o61s | 4.,?.5 c~s [ J L4 .~s L __ _: ...................

I

I - 0,727 ~_,,t~ BlbOWDOWN

I~qM.

18A ~/s

Fig. 8. Exergoeconomic cost diagram of the Santa Cruz de Tenerife desalination plant by stages.

0

0

E O e~

X

e-

15.00

10.00

5,00

0.00 t

r 12,1 [ ] Exergy Ill o.7 nl

10 21 32 43 54 65 06 75 87 98 09 10 3(1 36 80

Flows

Fig. 9. Unit exergoeconomic cost (c6/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/M J, the difference with respect to the unit cost of product (9.38 c/M J).

V. Romero-Ternero et aL / Desalination 181 (2005) 43-59 51

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 high- pressure 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/M J) 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/M J); 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 high- pressure 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 domi- nate 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. 1 I. 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).

2.7. 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


EXTERNAL CONSUMPTIONS

SEAWATH~ HIGH PEI~S SUI%E PRODUCT PUMPING PUbIP tKrMPING

S

Oc~/]~J . . . . > 1.67--1 STAGES 14[~'-~',+- 3~+'~"~'~ STAeES 1931+ :'~"~ 7.87 [

/ - 1 ~67 ~/l~lJ BLOWDOWN

IrINAL STAGES

Fig. 10. Exergoeconomic unit cost diagram by stages of the Santa Cruz de Tenerife desalination plant.

FINAL I PRODUCT

1:2.1 e~/M3

100.0 76.7

80,0 69.5 58,5 63,0 ~

60.0

~" 34.4 34.4 o 40.0 26.0 0

20, 0 3 43 7,45 0,0 ' ~ 0 ,0

I0 2[ 32 43 54 65 06 75 87 98 09

Mass f lows

Fig. 11. Cost (c~/m 3) of the mass flows of the Santa Cruz de Tenerife desalination plant.

Table 3 Influence ofexergy 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 0.5 9.8 10.3 0.4 8.5 8.9 2: Pretreatment 0.5 16.5 17.0 0.4 14.4 14.8 3: High pressure pump 3.7 11.8 15.5 3.2 10.2 13.4 4: Regulation valve 1.7 0.0 1.7 1.5 0.0 1.5 5: Reverse osmosis skid 8.6 16.8 25.4 7.4 14.6 22.0 6: Pelton turbine 5.9 8.1 13.9 5.1 7.0 12.1 7: Posttreatment 1.0 8.0 9.0 0.9 6.9 7.8 8: Product pumping 0.4 6.8 7.2 0.3 5.9 6.2 9: Distribution - - - - - - 2.3 11.0 13.3 Total 22.2 77.8 100 21.5 78.5 100

of pumped (98) arid 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.


Reverse osmosis skid leads with about one- quarter of the total increase. About half is attri- buted 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 varia- tions 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. Alter- natively, for all this equipment, a performance reduction of 5% is analysed (i.e., with an efficiency decreasing from 85% to 80%). High- pressure 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 high- pressure 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


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 dis- closes 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.

4.2. 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 de- creases 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 ofexergy (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 kWlgm 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.


5. 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 remain- der 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 3.

4.3. 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 sup- ports 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, Con- clusion 4.2.5 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 4.1.1. 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 4.3.2 and 4.3.3. In


addition, this intensification would be opposed to conclusions 4.1.2, 4.2.6 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 4.3.3. 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 4.2.9 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 4.3.1 and 4.3.2), it is reasonable to choose a high- quality 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 - - rl

r

tr

x

Z z. - -

Subscripts

F p

Availability of the desalination plant Exergoeconomic unit cost or unit cost, ~/kJ Exergy rate, kW Lifetime, y Annual real discount rate

Discount factor (years) = (1 + r) -i i=1

Rate of discounted fixed costs, E/s)

zo A' t

r

Fixed costs, Discounted fixed costs over a lifetime,

Fuel Product

Greek

H - - Rate of discounted exergoeconomic cost or exergoeconomic cost, ~/s

Acknowledgements

This work was financially supported by the Spanish Ministerio de Ciencia y Tecnologia (project SOLARDESAL REN 2000-0176-P4-04) and the Consejeria de Educaci6n, Cultura y De- pones 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 P6rez, for his valuable technical advice and especially for his friendly cooperation.


References

[1] V. Romero-Ternero, Anfilisis termoecon6rnico de la desalaci6n de agua de mar mediante 6smosis inversa con aplicaci6n de energia e61ica, PhD Thesis, Uni- versity ofLa Laguna, Spain, 2003 (in Spanish).

[2] L. Garcia-Rodriguez, A. Palmero-Marrero and C. G6mez-Camacho, Comparison of solar thermal technologies for applications in seawater desalination, Desalination, 142 (2002) 135-142.

[3] L. Garcia-Rodriguez, A. Palmero-Marrero and C. G6mez-Camacho, Thermoeconomic optimization of the SOL-14 plant (Plataforma solar de Almerla, Spain), Desalination, 136 (2001) 219-223.

[4] L. Garcia-Rodriguez and C. Gomez-Camacho, Thermoeconomic analysis of a solar parabolic trough collector distillation plant, Desalination, 122 (1999) 215-224.

[5] J. Uche, L. Serra, L.A. Herrero, A. Valero, J.A. Tur6gano and C. Torres, Software for the analysis of water and energy systems, Desalination, 156 (2003) 367-378.

[6] J. Uche, L. Serra and A. Valero, Thermoeconomic optimization of a dual-purpose power and desalination plant, Desalination, 136 (2001) 147-158.

[7] Y.M. E1-Sayed, Designing desalination systems for higher productivity, Desalination, 134 (2001) 129- 158.

[8] Y.M. El-Sayed, Thermoeconomics of some options of large mechanical vapour-compression units, Desalination, 125 (1999) 251-257.

[9] A. Valero and M.A. Lozano, Curso de termo- economia, Department of Mechanical Engineering, University of Zaragoza, Spain, 1994 (in Spanish).

[10] V. Romero-Ternero, L. Garcia-Rodriguez and C. G6mez-Camacho, Exergetic analysis of a seawater reverse osmosis desalination plant, Desalination, 175 (2005) 197-207.

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[13] A. Valero, J. Uche and L. Serra, La desalaci6n como alternativa al Plan Hidrol6gico National. Centro de Investigaci6n de Recursos y Consumos Energ6ticos (CIRCE), University of Zaragoza, Spain, 2001 (in Spanish).

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Appendix A

Table A. 1 Exergoeconomic unit costs of flows from thermoeconomic balance

Flows Exergoeconomic unit cost

EXlo Wlo + X1 21 Pumped seawater c21 = clO Ex2--~I + cwl 0 Ex21 Ex2-~I

32 Feed (pretreated seawater) C32 = C21 Ex21 X2

+ m Ex32 Ex32

Ex3: e ::3 43 High-pressure feed 43 =c32 E---~43 + ti,30 Ex4--~3 + c~6 Ex43 + Ex4---~3

54 High-pressure feed to skid

65 High-pressure blowdown a

C54 = C43

75 Product

gx43 x,- o

c65 = c54

Ex54 gx65 X5 Cw Ex06 b C75 = C54 EX7"-~5 -C65 EX75 + Ex75 Ex75

Ex65 X6 + W36 Energy recovery c~36 = C65 W36 W36

06 Blowdown a c06 -- 0

Ex75 X 7 87 Posttreated product c87 = c75 + - -

Ex87 Ex87

ExsT + Veso + xs 98 Pumped product c98 = c87 Exg----~s cws 0 Ex98 Exg-~s

Ex9s X9 09 Final product c09 = c98 +

Ex09 Ex09

aBy methodological considerations of Valero-Lozano thermoeconomic analysis. bTerm representing the waste of rejected brine chemical exergy rate, i.e., the waste of its potential use with respect to seawater (cw: unit cost of external consumption). c, exergoeconomic unit cost (E/k J). Ex, exergy rate (kW). X, rate of discounted fixed costs (q/s).

K Romero-Ternero et al./Desalination 181 (2005) 43-59 59

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) andX k is the rate of discounted fixed costs of equipment k (E/s).

D, F ixed costs

c09=cw[1 +. ExD +Ex6]= X Ex09 Ex09 ] Ex09

[ k xo,, xoo] k =CwI+ + +

k=l Ex09 Ex09 k=t Ex09

where c09 is the exergoeconomic unit cost of the final product (q/kJ), cw the unit cost of extemal consumption (t~/kJ), Ex D the rate of exergy destruction of the desalination plant as a whole (kW), EXD, k, rate of exergy destruction in equipment k (kW),Ex06, exergy rate ofblowdown (kW), Ex09, exergy rate of final product (kW),Xthe rate

Ex,, Ex,,, Cw 1+ + '

Ex,,~ Ex ....

X + m

Ex,,~

Exergy destruction

Potential use of rejected brine

chemical exergy rate (It must be cancelled

since this potential use is wasted)

Fig. B 1. Contributions to unit cost of final product (plant as a whole): exergy destruction, fixed costs and potential use of rejected brine chemical exergy rate.

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