Economic assessment of a two-stage solar organic Rankine cycle for reverse osmosis desalination

Renewable Energy 34 (2009) 1579–1586

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Economic assessment of a two-stage solar organic Rankine cycle for reverse osmosis desalination G. Kosmadakis*, D. Manolakos, S. Kyritsis, G. Papadakis Department of Natural Resources and Agricultural Engineering, Agricultural University of Athens, 75 Iera Odos Street, 11855 Athens, Greece

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 January 2008 Accepted 10 November 2008 Available online 11 December 2008

The current paper presents the economic evaluation of a two-stage Solar Organic Rankine Cycle (SORC) for using the mechanical energy produced during the thermodynamic process to drive a Reverse Osmosis (RO) desalination unit. The developed integrated system is briefly analysed and the specific fresh water cost, as well as the cost of energy is calculated. The economic assessment results are compared with those obtained from a low-temperature SORC-RO and two alternative variants of PhotoVoltaic RO (PV– RO) systems (with and without batteries). It is found that the critical fresh water cost for the system under consideration is 7.48 V/m3 of permeate water and the cost of energy equals to 2.74 V/kWh, when the water cost is slightly higher than the critical one (meaning 8 V/m3). These values are considered satisfactory enough, in comparison to the other autonomous desalination technologies. Additionally, the specific fresh water cost of the developed technology was calculated to be 6.85 V/m3, being very close to the values of the PV–RO systems. The variant of two-stage SORC significantly improves the efficiency and reduces the cost of the already developed prototype system (single-stage low-temperature SORC for RO desalination), because the specific cost is found to be much lower and taking into consideration its reliability, this technology can constitute an alternative desalination method competitive to the PV–RO on the basis of techno-economic feasibility. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Solar organic Rankine cycle Economic assessment RO desalination Specific cost

1. Introduction The current work is focused on the economic assessment of a two-stage SORC, where the mechanical energy produced during the expansion is used to drive the High Pressure Pump (HPP) of the RO desalination unit, which is directly coupled with the Rankine engine. The concept of developing a two-stage SORC for RO desalination demonstrates the evolvement of a single-stage lowtemperature SORC for RO desalination developed within the framework of COOP-CT2003-507997 EC project, in which several technical and economic aspects were explored. From technical point of view, the main conclusions derived from the study and experimental evaluation of the integrated system, clearly prove that the SORC is technically feasible, robust and reliable and has the capacity to be efficiently coupled with the RO unit [1–3]. However, the fresh water cost of the single-stage system is considerably higher than that of a PV–RO of comparable size [4–7]. In view to increase the efficiency of the single-stage SORC with a simultaneous reduction of fresh water cost the two-stage variant was studied within the framework of 05NON-EU-219, partly

* Corresponding author: Tel.: þ30 2105294033; fax: þ30 2105294023. E-mail address: [email protected] (G. Kosmadakis). 0960-1481/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.11.007

financed by the Greek government. The results obtained incorporate, together with significant technical outcomes [8], the radical deduction of fresh water cost. The research concerning the organic Rankine cycle with its various applications is becoming evident, because it is considered to contribute in the exploitation of low-temperature heat, as well as of solar energy. Therefore the efficiency of such systems is kept low, following closely the low Carnot cycle efficiency in such low temperatures, but with the coupling of renewable energy sources, even if the efficiency is in the order of 4%, the annual produced electricity or desalinated water is significant. There are various technologies for desalinating seawater, as described thoroughly in Ref. [9], which cover a variety of systems, using direct or indirect coupling with renewables (i.e. solar collectors, photovoltaic, solar ponds and geothermal energy). There are many examples, which deal not only with the technologies of desalinating seawater, but also with electricity production powered either by geothermal or waste heat or even solar energy, such as those found in Refs. [6,10– 12]. However, the current work is focused on the use of an organic Rankine cycle for RO desalination. The new concept of introducing a second stage in the organic Rankine cycle has been found to increase the efficiency, but most important the annual production of desalinated water [8]. The approach of the authors to the thermal process is quite different than the one followed in Ref. [13], where

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G. Kosmadakis et al. / Renewable Energy 34 (2009) 1579–1586

Nomenclature AEC the annual equivalent cost (V) Bt¼j the benefit of the investment in each year, including the damping value of some subsystems after the end of the life cycle (V) CFt¼0 the present value of the future cash flow (V) CFt¼n a cash flow in n years (V) Ct¼j the cost of the investment in each year, including the installation cost in the beginning of its operation (V) the installation cost of each component (V) ICC the life cycle of each component (years) nC r the interest rate (%)

(V/m3) of the two-stage RO-solar Rankine cycle for RO desalination developed. Also the cost of energy (V/kWh) is calculated for the proposed system. In order to have a more general view of the existing technologies of small-scale systems, which make use of solar energy for RO desalination and are continuously evolving, a comparison of the specific cost of the RO-solar Rankine systems (low-temperature and the two-stage one) with a PV–RO system that uses batteries as energy storage and another one PV–RO without batteries will take place.

2. Economic analysis of the two-stage SORC for RO desalination 2.1. Description of the system

parabolic trough solar collectors have been used and the operating temperatures are much higher than the ones used in the current work, therefore the efficiency of the system in Ref. [13] is expected to be much higher. The effort given here is to exploit solar energy or even low-temperature waste heat for desalinating seawater, which means that the efficiency is expected to be quite low, because of the quite low evaporation/condensation temperature of the two cycles involved. In Ref. [14] an organic Rankine cycle (ORC) for power generation for recovering of low-grade waste heat (below 370  C) has been investigated. The same application has been tested also in Ref. [15], but in that case the evaporation temperature was lower and equal to 120  C. There are also some applications, where the design evaporation temperature is even lower, such as in Ref. [10], where Nguyen et al. developed a small-scale ORC for electricity production with power output of 1.5 kW. The heat supply is low-temperature heat (at around 81  C) and although the thermal efficiency is low and around 4.3%, the whole system concept seems to be promising. The organic Rankine cycle has been also used for RO desalination, such as in Ref. [6], where Mohamed et al. developed an organic Rankine cycle for RO desalination coupled with photovoltaic or wind turbines. They reached a specific mechanical energy consumption equal to 3.3 kWh/m3 using an energy recovery unit and having a quite low specific cost of energy. In the current work only the economic performance of the system will be examined, since the design and the simulation of the system have been explored elsewhere ([8,16,17]). This economic evaluation regards the calculation of the specific fresh water cost

Solar collectors array

The simplified block diagram in Fig. 1, illustrates the basic concept of the system [3]. The process can be briefly described as follows. The heat produced by the solar collectors is transformed into mechanical energy through the High-Temperature Organic Rankine Cycle (HTORC), so called the ‘‘upper stage’’, operating at a temperature range from 130  C to 140  C. The refrigerant R245fa has been selected for this cycle due to its appropriate thermodynamic properties (Critical Point, Tc ¼ 154.05  C, Pc ¼ 36.4 bar) in the defined temperature range [16]. The mechanical energy produced in the upper stage, drives the HPP of the RO unit, representing the only consumption served by this stage itself. During condensation, exchanging of heat to the ‘‘lower stage’’ takes place, thus initiating the Low-Temperature Organic Rankine Cycle (LTORC) process. In other words, the condenser of the upper stage acts as the evaporator of the lower stage. HFC-134a is the refrigerant selected as the heat carrier of the LTORC. The mechanical energy generated through the low-temperature Rankine cycle process drives the remaining system consumptions summing up the energy demand of system pumps, except for that of HPP. The RO system is equipped with an energy recovery system, so that the specific mechanical energy consumption will decrease significantly (approximately 2.5 kWh/m3 [4]) and the production of the desalinated water will increase. Fig. 2 illustrates a detailed layout of the system under investigation, where the two stages are identified and the numbers represent the various components namely: (1) Vacuum tube solar collectors’ array, consisting of 60 collectors (Thermomax model TDS 300) of a total gross area of 240 m2 (2) Circulator, 12 m3/h

Rankine Cycle “Upper Stage”

RO unit

Rankine Cycle “Lower Stage”

System pumps

Thermal energy Mechanical energy

Energy rejected (condensation) Fig. 1. Simplified block diagram of the system.

G. Kosmadakis et al. / Renewable Energy 34 (2009) 1579–1586

High-temperature stage

1581

Low-temperature stage

137 oC

Pre-heater 54 kW

10 s ct or le co l ar So l

Slope 130 oC

Collectors' pump 12 m3/h

88 oC

Pre-heater 32 kW

140 oC

0

kW

Evaporator 48 kW

Expander 8.5 kW

Evaporator 71 kW

Expander 6.9 kW

75.8 oC

37.81 oC

35 oC

Condenser 100 kW

36.11 oC HFC-134a pump 2000 kg/h

78.36 oC

77 oC

Cooling pump 12 m3/h 20 oC

35 oC HFC-245fa pump 2000 kg/h HP pump Membranes

Turbine

Description

Feed pump 10 m3/h

Thermometer Pressure meter Flow meter Conductivity meter

Brine 8 m3/h

Permeate 2 m3/h

Pyranometer

Fig. 2. Layout of the two-stage SORC for RO desalination.

(3) Preheater and evaporator of HFC-134a, 32 and 71 kWth respectively (4) Condenser of HFC-134a, 100 kWth (5) Expander of HFC-134a, 6.9 kW (6) HFC-134a pump, 2000 kg/h (7) RO unit, 2 m3/h fresh water production (8) Water reservoir, 1 m3 (9) RO energy recovery system and HPP (APP type) (10) Expander of HFC-245fa, 8.5 kW (11) HFC-245fa pump, 2000 kg/h

requirements of this building are very few, since its construction is very simple, therefore its cost is also quite low. Also the operational and maintenance cost is each 1% of the cost of the electromechanical equipment [3]. In Fig. 3 the cost significance of every subsystem to the total cost is depicted and in Fig. 4 the comparison with the low-temperature SORC for RO desalination system can be observed [3]. It is

Table 1 Cost of the solar organic Rankine system.

(12) Preheater and evaporator of HFC-245fa, 54 and 48 kWth respectively More details concerning the design and the simulation of the proposed system can be found in Ref. [17]. 2.2. Estimation of investment cost 3

In this section the critical cost of the desalinated water (V/m ), the specific cost of fresh water (V/m3), as well as the cost of energy (V/kWh) for the two-stage solar organic Rankine cycle will be calculated. For this reason the total investment cost and the annual cost will be given next [18]. According to the costs presented in Table 1 the total cost of the solar organic Rankine system using RO desalination is estimated to be 185,712 V. The desalinated water production (incorporating energy recovery) has been found to be equal to 2684.8 m3 during a whole year and the annual mechanical energy production available for desalination equal to 6712 kWh. In Table 2 the annual costs can be seen, which deal with the operation, maintenance, insurance and land required for the solar organic Rankine system. The annual insurance cost is estimated to be equal to 0.65% of the electromechanical equipment (142,712 V) and 0.35% of the building (5000 V). In this building all the electromechanical equipment is stored, together with the control unit of the system, in order to be protected from rain, wind, etc. The

Cost (V)

% Of the total cost

Economic life (years)

Building (10 m2) Thermal energy production system Solar collectors Installation of solar collectors Collectors pump

5000 75,840 50,400 25,000 440

2.69 40.83 27.14 13.46 0.23

20 – 25 – 20

Rankine engine Water/R245fa preheater Water/R245fa evaporator R245fa/R134a preheater R245fa/R134a evaporator R134a/seawater preheater R245fa pump R134a pump Pipes R245fa turbine R134a turbine Labour

44,572 2700 2700 5500 5500 5500 7600 7600 2500 1236 1236 5000

25.36 1.46 1.46 2.96 2.96 2.96 4.09 4.09 1.35 0.67 0.67 2.69

– 20 20 20 20 20 10 10 20 20 20 –

Desalination unit Membranes Other parts (including labour)

44,000 4000 40,000

23.69 2.15 21.54

– 4 15

8000 5800 5000 800

4.31 3.12 2.69 0.43

– – 15 20

185,712

100

20

Civil engineer Other Measuring instruments Water tank Total investment cost

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G. Kosmadakis et al. / Renewable Energy 34 (2009) 1579–1586

Table 2 Annual costs of the solar organic Rankine system.

45% TWO-STAGE SORC SINGLE-STAGE SORC

40%

Operational cost (V) Maintenance cost (V) Insurance (V) Land (0.1 ha) (V)

1427.12 1427.12 945.13 300

35%

Total annual cost

4099.37

20%

30% 25% 15% 10%

concluded that the most important part of the total cost is the thermal energy production system (mainly the evacuated tube solar collectors). If a low or a medium temperature thermal source were exploited (such as industrial waste heat), the total cost would be significantly lower, since no solar collectors are used in that case. The major difference between the two systems is the cost of the Rankine engine and the desalination unit. The first difference is due to the larger number of heat exchangers that are being used and the latter difference exists only in percentage values and not in absolute values, since the total cost of the two systems is not equal.

5% 0% Building (10m2)

Thermal energy production system

X

ðICC ÞðrÞ 1  ð1 þ rÞnC C ¼ 1;N

(1)

The coefficient that correlates a future cash flow with a present value is called the present value coefficient and is given by the following equation:

Civil engineer 4%

Other

n X Bt¼j j ¼ 0 ð1

þ rÞ

j



n X Ct¼j j ¼ 0 ð1

(3)

þ rÞj

The benefit to cost ratio is an index of the ratio of the present value of the benefit cash flows to the present value of the cost cash flows and is given by the following equation:

Pn Bt¼j j ¼ 0 ð1þrÞj Bt¼0 BCR ¼ ¼ P Ct¼j n Ct¼o j

(4)

j ¼ 0 ð1þrÞ

The Pay-Back-Period (PBP) is the years needed for the NPV to reach a zero value and is found by solving Eq. (3) with NPV ¼ 0 V, using the following equation: PBP X Bt¼j j ¼ 0 ð1

þ rÞ

j

¼

PBP X Ct¼j j ¼ 0 ð1

(5)

þ rÞj

The internal rate of return (IRR) is the critical interest rate (where NPV ¼ 0 V) and it should not be lower than the interest rate used, in order for an investment to be economically sustainable. IRR can be calculated from Eq. (6). n X

Bt¼j

j ¼ 0 ð1

þ IRRÞ

j

¼

n X

Ct¼j

j ¼ 0 ð1

þ IRRÞj

(6)

All above equations have been found in Refs. [3,19]. The net present value incorporating the damping value of some subsystems (solar

(2) 50000

Other 3%

Building (10m2) 3%

Desalination unit 24%

Rankine engine 25%

Fig. 3. Cost significance of each subsystem.

Thermal energy production system 41%

ANNUAL CASH FLOW (€)

CFt¼0

CFt¼n ¼ ð1 þ rÞn

Civil engineer

The net present value (NPV), which is actually the present value of the total cash flows during the economic life cycle of an investment, is given next.

NPV ¼ Bt¼0  Ct¼o ¼

AEC ¼

Desalination unit

Fig. 4. Comparison of the two-stage system with the single-stage system.

2.3. Economic analysis of the integrated system For the economic analysis that follows, it is assumed that the installation of the solar organic Rankine cycle for RO desalination is completed in one year. The economic life of the system is 20 years, but not all the components’ life cycle is so long, for example that of membranes. Therefore some parts need to be replaced during the 20-year period and for that reason an extra installation and replacement cost is incurred during the operation of the system. Also after 20 years, which is considered to be the life cycle of the system, the life cycle of some subsystems has not been reached and they have still some damping value, which is taken into consideration in the current study. The price of the fresh water chosen is 8 V/m3, according to Ref. [18], which is a quite high price, but is justified, since the proposed system is a small-scale system and the interest rate (mean annual inflation rate) is equal to 3%. Using all the above data the annual cash flow rate can be calculated, as well as its present values (PV) and the annual equivalent cost of the system, which is found to be equal to 18,401 V. This value has been calculated using the following formula:

Rankine engine

0 0

5

10

15

20

-50000

-100000

-150000

-200000

YEARS Fig. 5. Annual present value of the cash flows during the life cycle of the system.

G. Kosmadakis et al. / Renewable Energy 34 (2009) 1579–1586 Table 4 Results of all cases regarding the NPV.

50000

0 0

NPV (€)

1583

5

10

15

Case

Installation cost (V)

Annual benefit (V)

Annual cost (V)

Interest rate (%)

NPV (V)

NPV change (%)

1 2 3 4 5 6 7 8 9

185,712.00 222,854.40 148,569.60 185,712.00 185,712.00 185,712.00 185,712.00 185,712.00 185,712.00

21,478.00 21,478.00 21,478.00 25,774.08 17,182.72 21,478.00 21,478.00 21,478.00 21,478.00

4099.37 4099.37 4099.37 4099.37 4099.37 4919.24 3279.50 4099.37 4099.37

3 3 3 3 3 3 3 3.6 2.4

20,953.88 30,428.52 72,336.28 84,862.75 42,954.99 8696.22 33,211.53 4923.71 38,458.87

0.0 245.2 245.2 305.0 305.0 58.5 58.5 76.5 83.5

20

-50000

-100000

-150000

-200000

YEARS Fig. 6. NPV during the life cycle of the system.

collectors, desalination unit, measuring instruments) is quite large and equals to 20,953.88 V. The benefit to cost ratio should be larger than unity, in order for the investment to be efficient [20]. In the current case this ratio is 1.06, which is a quite satisfactory value. The Pay-Back-Period can be easily calculated as well. It is found that the investment will gain profit in 20 years from the beginning of its construction and after 19 years of operation. This value is very high and approaches the life cycle of the system. But the main benefit of the system is that it can be installed in isolated areas and especially in islands, where there is lack of fresh water, especially during the summer months. Also since solar energy is used as the heat carrier, no pollutants are produced. The analysis taking place doesn’t take into consideration the CO2 avoidance cost, a fact which would make the proposed system more economically competitive. The Internal Rate of Return (IRR) can be also found easily and is equal to 3.79% (where NPV ¼ 0 V). Also the critical value of the fresh water is calculated and is found to be equal to 7.47 V/m3 (where NPV ¼ 0 V). The cost of every kWh produced, which is equal to the annual equivalent cost divided by the annual mechanical energy produced, has been calculated and it is equal to 2.74 V/kWh. Finally the specific water cost, which is equal to the annual equivalent cost divided by the annual fresh water produced, is found to be 6.85 V/m3, which is a quite low value for a small-scale desalination unit. In Fig. 5 the annual cash flow is illustrated for the economic life cycle of the investment (20 years), including the damping value of some subsystems and in Fig. 6 the Net Present Value (NPV) for the life cycle of the system is presented. From Fig. 6 it can be seen that the sum of the annual cash flows becomes positive at the end of the 19th year of operation (Pay-Back-Period). The sharp gradient of the slope of the two curves during the 11th and 16th year of the system (10th and 15th year of operation respectively) in Figs. 5 and 6 is justified, since at these two time-moments some components are replaced and therefore the cost of the system is increased. More specifically during the 10th year of operation the pumps of the two organic fluids are replaced and during the 15th many parts of the desalination unit (including labour) and some measuring instruments are replaced.

2.4. Sensitivity analysis There is always some uncertainty about the estimation of the basic values and costs of the integrated system. Therefore a sensitivity analysis is important to distinguish, which variables affect the sustainability of the system most. The data of this analysis and the different cases that have been examined in comparison with the standard case are presented in Table 3, whereas in Table 4 the most basic results of the various cases are presented. It is observed that the values that mostly influence the NPV are the change of the installation cost and of the annual benefit and the least significant variation is brought with the change of the annual cost. Moreover in Table 5 all the different criteria, which are important for the evaluation of the investment for all cases examined, are presented. In cases 2 and 5 the NPV takes negative values, which means that the investment under these parameters is not cost effective. The most efficient investment is met under the cases 3 and 4, where the Pay-Back-Period is quite low and the NPV is almost three times higher than the standard case (case 1). In Fig. 7 the variation of NPV for all cases examined is depicted showing the significance of every variable involved (installation cost, interest rate, annual benefit and annual cost). The greatest dependence of NPV is on the installation cost and the annual cost variation. It is also observed that the dependence of NPV from the variation of the parameters in each case considered is linear. In order to have a positive NPV in comparison to the standard case described before, the interest rate should not increase more than 26% (meaning 3.78%), the investment cost should not increase more than 8% (meaning 200,569 V), the annual benefit should not decrease more than 7% (meaning 19,974.91 V) and the annual cost should not increase more than 35% (meaning 5534.15 V). It should be mentioned that any combination of two or more changes in the variables is proportional to each variable, because as it has been discussed before, there is almost a linear dependence between those variables and the NPV. 3. Comparison of the integrated system After the economic assessment of the two-stage SORC for RO desalination unit has been conducted, the conclusion reached is Table 5 Results of all cases regarding investment’s criteria.

Table 3 Cases examined.

Case

NPV (V)

BCR

Pay-Back-Period (years)

IRR (%)

Critical fresh water price (V/m3)

1 2 3 4 5 6 7 8 9

20,953.88 30,428.52 72,336.28 84,862.75 42,954.99 8696.22 33,211.53 4923.71 38,458.87

1.0659 0.9177 1.2711 1.2667 0.8650 1.0263 1.1086 1.0156 1.1195

19 – 11 11 – 20 18 20 18

3.795 1.990 6.230 6.097 1.290 3.332 4.251 3.795 3.795

7.476 8.762 6.190 6.229 9.344 7.782 7.169 7.870 7.090

Case 1 2 3 4 5 6 7 8 9

Standard case 20% Increase of installation cost (IC) 20% Decrease of installation cost (IC) 20% Increase of annual benefit (AB) 20% Decrease of annual benefit (AB) 20% Increase of annual cost (AC) 20% Decrease of annual cost (AC) 20% Increase of interest rate (IR) 20% Decrease of interest rate (IR)

1584

G. Kosmadakis et al. / Renewable Energy 34 (2009) 1579–1586 Table 7 Cost of components of the low-temperature RO-solar Rankine system.

IC change IR change AB change AC change

80 60

NPV (103€)

40 20 0 -20%

-15%

-10%

-5%

0%

5%

10%

15%

20%

-20 -40 -60

VARIATION (%) Fig. 7. Variation of NPV as different variables change (IC: installation cost, IR: interest rate, AB: annual benefit, AC: annual cost).

that it performed quite well. In order to quantify its economic performance, a comparison with other technologies follows. The developed system will be compared with a low-temperature SORC for RO desalination and a PV–RO desalination unit with and without batteries. The low-temperature SORC system is a prototype autonomous system that has been examined experimentally and has been found to operate efficiently [2,3], but with the integration of the solar collectors it needed some concrete steps of optimisation to compete in terms of cost effectiveness with the PV–RO-Batteries system. The latter systems have been also examined thoroughly and their experimental evaluation has been also conducted in Refs. [6,7]. 3.1. Investment costs of the examined alternatives Details for the alternatives of the developed system can be found elsewhere [2,6,18] and in the current work only the comparison of the economic performance will be taken into consideration. Considering the PV–RO system with batteries, according to Ref. [6], the various components, their investment costs, the percentage of each cost to the contribution to the total cost and an estimation of the economic life of every item are listed in Table 6. While Table 7 shows the same values for the lowtemperature SORC for RO desalination [2]. Table 6 Cost of components of the PV–RO system. Total cost (V)

% Of the total cost

Economic life (years)

Building (10 m2) PV system Investment cost (0.8 Wp) Batteries’ replacement cost Operation and maintenance cost

5623 8057.6 6690 1209.6 158

23.84 34.31 28.48 5.15 0.68

20 – 20 4 1

Desalination unit Membrane vessel Feed water pump Feed pump motor High pressure pump Control unit Hydraulics Cabling Feed water reservoir Fresh water reservoir Transportation unit cost Membranes Filters Sensors Chemicals Other

9805 2250 300 1000 3400 500 100 100 150 100 200 1350 80 200 75 23.5

41.75 9.57 1.28 4.26 14.47 2.13 0.43 0.43 0.64 0.43 0.86 5.73 0.34 0.86 0.32 0.10

– 20 20 20 20 20 20 20 20 20 20 4 2 10 1 1

Total investment cost

23,485.6

100

20

Total cost (V)

% Of the total cost

Economic life (years)

Building (10 m2) Thermal energy production system Land rent (400 m2) Solar collectors (22 collectors  1200 V) Installation of solar collectors–hydraulics

5000 37,400 1000 26,400 10,000

6.13 45.78 1.22 32.32 12.24

20 – 1 25 25

Rankine engine

20,000

24.49

20

Desalination unit RO unit (total) Water reservoir Membranes Filters Sensors Chemicals Other

19,230 16,000 100 2700 80 200 150 49.7

23.54 19.60 0.12 3.30 0.10 0.24 0.18 0.06

– 20 20 4 1 1 1 1

Total investment cost

81,679.7

100

20

The investment cost regards a PV–RO system with batteries that under the climatic conditions of Athens is able to produce 500 m3 of fresh water per year [7]. In case that the PV array is directly coupled with the RO unit, the batteries’ cost is neglected, while the fresh water production declines to 273 m3/year approximately. The investment cost for the PV–RO system with batteries is approximately 23,500 V, for the PV–RO without batteries is about 22,200 V, for the low-temperature RO-Rankine system is 82,000 V and for the two-stage RO-Rankine system is approximately 186,000 V, as given before. The investment costs of the different systems can be seen in Table 8. It is observed that the two-stage system considered here has the larger capacity than the rest. Nevertheless all systems examined are considered small-scale systems [21] and therefore the approach in their economic performance is identical. 3.2. Comparison of the examined desalination systems The scope of this section is to compare the different desalination systems, making the same economic assumptions (interest rate, economic life, operating expenses). The interest rate is assumed to be the same and equal to 3% and the economic life of the desalination systems is again 20 years. Under these identical conditions and having estimated the prices for chemicals, membranes, the cost of land and building (different for every technology and varies according to the installation area) and other operating expenses, the specific cost for all systems investigated has been calculated. More specifically PV–RO with batteries specific cost has been estimated to be 4.80 V/m3, while the low-temperature RO-solar Rankine cost is 11 V/m3 and the two-stage RO-solar Rankine specific cost is 6.85 V/m3, which is directly competitive with the PV–RO technology. The specific fresh water cost of the PV–RO without batteries has been estimated to be 7.60 V/m3. All estimated costs consist of both the cost of the energy system and the cost of the desalination system in each case. Table 8 Investment cost of the examined desalination systems.

Energy system cost (V) Desalination system cost (V) Total investment cost (V) Annual fresh water production (m3)

PV–RO system with batteries

PV–RO system without batteries

Low-temperature SORC-RO

Two-stage SORC-RO

8057.6 15,428 23,485.6 500

6848 15,428 22,276 273

57,400 24,279.7 81,679.7 650

141,712 44,000 185,712 2684.8

G. Kosmadakis et al. / Renewable Energy 34 (2009) 1579–1586

1585

Table 9 Total and partial specific cost of the examined desalination systems (V/m3), using solar energy as a heat source. PV–RO with batteries PV–RO without batteries Low-temperature RO-solar Rankine system Two-stage RO-solar Rankine system Specific energy system cost (V/m3) 1.87 Specific desalination system cost (V/m3) 2.93 3 4.80 Specific total cost (V/m )

39% 61% 100%

2.23 5.37 7.60

30% 70% 100%

6.82 4.18 11.00

62% 38% 100%

5.03 1.82 6.85

73% 27% 100%

Table 10 Total and partial specific cost of the examined desalination systems (V/m3), using constant heat source and the grid. PV–RO with batteries PV–RO without batteries Low-temperature RO-solar Rankine system Two-stage RO-solar Rankine system Specific energy system cost (V/m3) 0.20 Specific desalination system cost (V/m3) 1.96 3 2.16 Specific total cost (V/m )

9% 91% 100%

0.20 1.96 2.16

9% 91% 100%

Table 9 shows that in all cases the contribution of the energy and the desalination systems in the total cost is different using solar energy as the heat source. In the case of PV–RO system with batteries the energy part shares 39% of the total system’s cost, while in the case of PV–RO without batteries, the energy system cost is around 30%. For the RO-solar Rankine system the energy part shares a 62% of total cost, due to the innovative but expensive solar Rankine system employed, while for the two-stage RO-solar Rankine system the energy system cost is three times greater than the desalination system cost. The most important conclusion is that the developed system competes in economic terms with the PV–RO without batteries system, because the values of their specific cost are quite near and with further optimisation work, it is expected to perform even better and be able in the future to compete also with the PV– RO with batteries system. This is believed by the authors, because the development of the two-stage SORC for RO desalination has just started and there are many optimisation issues to be resolved. The RO-solar Rankine systems can be supplied by a constant thermal source too, while the PV–RO systems by the electricity grid. This makes their efficiency to rise, since there are not many fluctuations in the heat supply [3]. For the RO-solar Rankine systems the case of thermal wastes or geothermal energy seems to be a direct effective application, with the only constraint the temperature to be in the range of 130–140  C. In Table 10 the specific water cost of the four systems can be seen, when they are supplied by a constant heat source (the RO-solar Rankine systems) and by the grid (the PV–RO systems). It is observed that the specific water cost of the RO-solar Rankine systems is significantly decreased, since the solar collectors are omitted. Also the two PV– RO systems have exactly the same specific water cost, because batteries are not needed. It should be mentioned that the first two

PV-RO-batteries PV-RO-no batteries

RO-Solar-Rankine two-stage RO-Solar Rankine

Specific cost change rate

70%

43% 57% 100%

0.56 0.50 1.06

53% 47% 100%

columns concern substantially the same system that is an RO connected to the grid and are presented for clarity purposes. It deserves to be mentioned that for some expensive components the alternative of replacing them with other of lower cost should be examined. For instance a very expensive component is the diaphragm pistons refrigerant feed pumps. It should be investigated, if any other type of positive displacement pump of lower cost can be integrated into the system. Axial piston pump as those used in the energy recovery system of the RO unit should be tested in priority. Also for the low-temperature RO-solar Rankine system there is a possibility of replacing the expensive evacuated tube collectors with flat plate collectors, which unfortunately have lower efficiency, decreasing the overall efficiency around 45% [3], but they can operate at such temperature levels (77  C). 3.3. Comparison of the sensitivity analysis Fig. 8 shows the differences in the fluctuations of the interest rate. When the interest rate increases from 3% to 10%, the specific desalinated water cost increases to 50% and 59% for the two-stage RO-solar Rankine system and the RO-solar Rankine system respectively. It is observed that the two-stage RO-solar Rankine system is not so vulnerable to the variations of the interest rate. A reason for that is because the annual costs, shown in Table 2, are overestimated and they are considered to be independent of the fluctuations of the interest rate. Therefore in the calculation of the annual equivalent cost of the system a large part is always constant. At the same time the increase for the PV–RO system with batteries is limited to 54%, while for the PV–RO system without batteries is 50%. In other words, the PV–RO systems are stable to uncertainties coming from the fluctuations of interest rates, which also apply for the developed system in the present study. An increase of the interest rate to 6% will cause the specific cost of the developed SORC system to increase from 6.85 V/m3 to 8.23 V/m3, a fact that should be taken into account in the final stage of a techno-economic analysis. 4. Conclusions

60% 50% 40% 30% 20% 10% 0% 1% -10%

0.77 1.04 1.81

2%

3%

4%

5%

6%

7%

8%

9%

10%

-20%

Interest rate Fig. 8. Sensitivity analysis of the four autonomous desalination systems.

The whole concept and the performance of the examined system seem promising and further investigation of the behaviour of the system is needed. Also its experimental evaluation would give more insight into some details of its operation, so that it can be developed and compete in better terms with the PV technology for this kind of applications. An important issue is that all the values of the specific water cost were calculated without considering the CO2 avoidance costs. If this environmental cost was taken into consideration, the specific water cost for every system investigated would be much lower and could compete in better terms with the conventional powered RO desalination units. Among others some major conclusions that can be drawn are listed next.

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 The specific fresh water cost of the developed two-stage ROsolar Rankine system has been estimated to be 6.85 V/m3. If the Rankine engine with the RO system is connected to a steady thermal source, the specific cost is radically reduced to 1.06 V/ m3. This fact makes the system an alternative solution to exploit other thermal sources than solar energy. Especially the case of industrial thermal wastes or geothermal energy seems to be a direct effective application, where the temperature should be in the range of 140  C.  The specific cost of fresh water produced by the two-stage ROsolar Rankine systems is in general higher than that of PV–RO system with batteries, but for the specific application an important increase of its performance has been observed, since the single-stage SORC-RO system has been significantly developed and its further optimisation is in progress, making the two-stage SORC-RO system even more competitive.  For some expensive components the alternative of replacing them with other of lower cost should be examined (e.g. diaphragm pistons refrigerant feed pumps).  Special care should be taken in the estimation of the interest rate together with a realistic estimation of the total cost and the annual cash flow rates for the techno-economic analysis of the integrated system, because the interest rate has a strong influence on the specific cost of the developed system. This stands for all systems under consideration.  The two-stage RO-solar Rankine system is less vulnerable to the variations of the interest rate than the single-stage RO-solar Rankine system and almost the same as the two PV–RO systems.  The low-temperature SORC system has been found to operate efficiently [2,3], but with the integration of the solar collectors it needed some concrete steps of optimisation to compete in terms of cost effectiveness with the PV–RO-Batteries system. The developed technology contributes to this task and can constitute an alternative desalination method competitive to the PV–RO in the basis of techno-economic feasibility.

Acknowledgments This work was conducted within the framework of the projects COOP-CT-2003-507997 (partly financed by European Commission) and 05NON-EU-219 (partly financed by the Greek General Secretary of Research and Technology). References [1] Manolakos D, Papadakis G, Mohamed Essam Sh, Kyritsis S, Bouzianas K. Design of an autonomous low-temperature solar Rankine cycle system for reverse osmosis desalination. Desalination 2005;183:73–80.

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