Cost-effectiveness of solar water production

Desalination, 71 (1989) 165-175 Elsevier Science Publishers B.V., Amsterdam -

165 Printed in The Netherlands

Cost-Effectiveness of Solar Water Production R.K. SURI, A.M.R. AL-MARAFIE, A.A. AL-HOMOUD and G.P. MAHESHWARI Kuwait Institute for Scientific Research, Energy Department, P.O. Box 24885,13109 Safat (Kuwait), Tel. 965-4830776, Cable SCIENCE Kuwait, Telex KISR KT22299 (Received April l&1988)

SUMMARY

In this paper an analysis is given of the technoeconomic viability of solar application technologies for water production. The scope of this study includes photovoltaic and low-grade thermal energy using the solar pond system. The generalized technoeconomic analysis is based on unit cost of water production. This parameter combines capital cost and life cycle of hardware items in addition to energy and power needs. The production of water using a solar pond is on a par with the conventional system cost. Keywords: solar water production, multi-stage flash, reverse osmosis.

INTRODUCTION

In the hot arid region of the Arabian peninsula, fresh water is not available in adequate quantity in its natural form. Multi-stage flash (MSF) and reverse osmosis (RO) are two important desalination processes used at present. Largesize installations, producing millions of gallons of fresh water, are in operation using these techniques. These desalination processes need large amounts of basic thermal and/or electrical energy for fresh water production from seawater or underground brackish water. MSF is the most commonly used process at present. Primary energy requirement of this process is thermal energy in the temperature range of 80-120°C. A smaller amount of electrical energy is also needed to operate miscellaneous pumps and controls. The RO system, unlike the MSF system, needs only mechanical or electrical power. Solar energy can be effectively used to produce the required low-grade thermal energy in a solar pond system whereas direct conversion of solar radiation to electricity is possible using the photovoltaic (PV) system. In this paper is analyzed the technoeconomic viability of solar-based application technologies for water production. The scope of study includes PV and solar pond systems. The generalized technoeconomic analysis is based on unit

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0 1989 Elsevier Science Publishers B.V.

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cost of water production ( KD*/m3). This parameter combines the capital cost and life cycle of hardware items in addition to energy and power needs of the system. METHODOLOGY

An energy analysis of the conventional system was made in order to arrive at the basic power and energy requirements for a unit (1 m3) production of water. The solar-based technologies have been analyzed for Kuwaiti climatological and radiation data to assess their capability to produce water per unit area of solar energy collection system with and without any conventional energy input. The analysis is based on year-round performance in order to arrive at the realistic amount of savings in power and energy. Cost estimates of conventional and solar-based systems have been made realistically along with their life cycle and discount rate to arrive at their annual amortized cost. Also, estimates have been made for the annual operation and maintenance (O&M) costs based on experience and information available from installations in Kuwait and elsewhere. These results in combination with the outcome of technical analysis were used to get the unit cost of water production both by conventional and solar-based technologies, as given below: Unit cost of water production = Costs of amortized MSF plant + annual electricity + annual thermal energy + MSF plant O&M Annual water production based on system effectiveness and plant utilization factor The unit water production cost can be evaluated for the conventional and for the different solar desalination systems for comparison. This single factor which combines the technical and the economic parameters is an important criteria for deciding whether or not a solar-based water production technology is technoeconomically viable. The authors have conducted detailed analyses in an earlier study on the production cost of thermal and electrical energy using solar energy [ 11.These estimates have been used in the present analysis. ENERGY AND COST ANALYSIS OF SOLAR WATER PRODUCTION TECHNOLOGIES

Energy requirements and cost factors have been estimated for the conventional and the solar-energy-based technologies. The systems analyzed are shown in Fig. 1. *lKD=US$3.5.

16’7

Solar

Solar

Fig. 1. Systems analyzed for water production using MSF and RO techniques.

Multi-stage flash desalination system

MSF is the most commonly used desalination system. At present, most of the fresh water in Kuwait is produced by using this technique. Commercial, large-capacity MSF systems, in general, are coupled with thermal power plants. Analyses have been made to estimate the requirements of thermal and electrical energy per unit of water production. These results have been further applied to solar-energy-based water production systems. Energy requirements of MSF systems

Production of fresh water in an MSF system requires a large input of thermal energy and a relatively small amount of electricity. Requirement of thermal

168 TABLE I Comparative technical evaluation of commercial and solar-based MSF systems Parameter

Commercial

Solar-based

Source temperature, ’ C Number of stages Capacity, m3/d Thermal energy input, kJ/kg Electricity input, kJ/kg Equivalent reduction in commercial electricity to provide thermal energy,

120 8 61,000 240 12 37

78 18 125 167 25 0

74

144

kJ/k

Water production, kg/kWh,

TABLE II Technoeconomic results for water production using conventional and solar-powered MSF system” Plant capacity: 1 m3/d. Technoeconomic parameter

Desalination plant cost, KD/m3 db Plant life, y Discount rate, % Plant utilization factor Annual water production, m3 Annual electricity consumption, kWh,/y Annual thermal energy, kWh,/y MSF plant amortized cost, KD/y MSF plant O&M cost, KD/y Type of electrical energy Cost of electrical energy, KD/y Cost of thermal energy, KD/y Total annual expenditure, KD/y Cost of water production KD/m3 Ratio of solar to commercial water production cost

Type Commercial (System 1 of Fig. 1)

Partial solar-based (System 2 of Fig. 1)

Complete solar-based (System 3 of Fig. 1)

410 25 10 0.9 328 4432 -

410 25 10 0.75 274 685 12700 45.1 10.0 Commercial 17.0 68.6 140.7 0.51 1.02

410 25 10 0.75 274 685 12700 45.1 10.0 Solar 98.0 68.6 221.7 0.81 1.62

45.1 10.0 Commercial 109.9 0 165.0 0.50 1.0

“Solar pond costs are based on C, = 75. bl KD=US $3.5.

energy for producing a fixed quantity of fresh water varies significantly with the design of the system. An analysis of basic energy requirements for two MSF plants is shown in Table I. The plants considered for the analysis are a com-

169

mercial plant of large capacity (27,000 m3/d) operating in a cogeneration plant in Kuwait [ 21 and a small-capacity experimental unit (125 m3/d) installed in U.A.E. and using thermal energy from a solar pond system [3]. The small experimental unit has better energy effectiveness since it incorporates more stages for desalination. An electrical energy equivalent of the thermal energy needed in a cogeneration MSF plant system has been estimated and is also shown in Table I. Expressing thermal energy in terms of electrical equivalent is convenient in cost estimations and for comparative evaluations. Cost estimates of water production (MSF systems) Energy cost for water production is estimated by using the quantity and cost of energy. The amortized capital cost of the equipment and annual O&M cost are combined with the energy cost to determine the cost of water production. A desalination capacity of 1 m3/d of water production and annual plant utilization factors of 75 and 90% have been considered for the solar-based and the conventional systems, respectively. Cost estimates for water production have been made for the following cases and the comparative technoeconomic evaluation results are presented in Table II. (1) conventional system (system 1 of Fig. 1): cogeneration electricity and water production plant; (2 ) partial solar system (system 2 of Fig. 1) : thermal energy from solar pond and electricity from commercial source; ( 3 ) full solar system (system 3 of Fig. 1) : thermal energy from solar pond and electricity from organic Rankine power cycle (ORPC ) (or PV) . Reverse osmosis desalination system Technological advances have been made in RO systems during the last decade and the use of RO systems for water production is increasing very rapidly. The RO technology is gaining acceptance at a faster rate, especially for the desalination of brackish water. In an RO system, sea- or brackish water at a high pressure, exceeding the corresponding osmotic pressure, is fed through a semi-permeable membrane. A portion of the fresh water passes through the membrane leaving rest of water with higher salinity. The RO system, unlike the MSF system, needs only mechanical power to raise the pressure of feed(sea- or brackish) water. This mechanical or electrical power is to be provided either as conventional electricity or from solar power electrical systems. Energy requirement of RO systems Salinity of the feedwater and membrane properties are two major factors controlling the energy requirement of an RO process. Higher salinity of feedwater (seawater) requires very high feed pressures to overcome the osmotic pressure, resulting in high energy consumption. Most of the membranes used

170

have a 20-30% recovery (ratio of fresh water to feedwater volume). Energy consumption for seawater desalination has been found to be 12.7 kWh,/m3 for the following operational conditions [ 41: Feed water pressure Water recovery Pumpset efficiency Energy recovery system

= = = =

69 bar 25% 60% none

The Kuwait Institute for Scientific Research (KISR) and the Water Resources Development Centre ( WRDC ) of the Ministry of Electricity and Water in collaboration with GKSS-Forschungszentrum Geesthacht GmbH of F.R.G. have installed three medium-capacity RO pilot plants. The three plants use three different membranes: spiral wound, hollow fine fiber and plate module. TABLE III Performance results of experimental RO system at Doha power plant in Kuwait Capacity of each system: 1000 m3/d. Seawater salinity: 45,000 ppm. Parameter

First stage Type of membrane

Feedwater flow rate, m3/d Feedwater pressure at 25 oC, bar TDS of permeate, mg/l Pump efficiency, % Motor efficiency, % Pump power, kW Second stage Type of membrane Feedwater flow rate, m3/d Feedwater pressure at 25 ’ C, bar TDS of permeate, mg/l Pump efficiency, % Motor efficiency, % Pump power, kW Energy recovery, % Net power demand Water production rating, l/kWh

System Spiral-wound module

Hollow fiber module

Plate module

UOP-PA

DuPont BlO

3922

4536 64

Euro + Schleicher & Schuell + Filmtec 3120 80

56 1080 70 90 413.4 UOP-PA 1177 28 67 70 90 59.4 25 354.6 117.5

900

70 90 546.3 DuPont B9 1270 28 90

70 90 64.0 25 457.7 91.0

2700 70 90 469.7 Hydronautics 1123 35 400 70 90 70.8 25 405.4 102.8

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Each plant has a capacity to produce 1000 m3/day of fresh water from seawater. A comparative evaluation of these membranes is shown in Table III. The average water production capacity of the three membranes is 104 l/kWh,. The corresponding energy requirement of the RO system is 9.6 kWh,/m3. Cost estimates of waterproduction (RO system-s)

Cost estimates for water production have been made for both the conventional and the solar systems. The solar systems are a PV system and an ORPC system coupled to a solar pond. The cost of the RO unit will be the same for all systems irrespective of the source of electricity. An important feature of a PV system is the incorporation of an electricity storage battery bank to facilitate energy availability for a nonstop operation. The PV electricity costing must account for the capital investment on battery storage and the energy loss due to charging and discharging. An energy loss of 14% has been estimated due to the incorporation of the battery bank [ 51. A battery bank is required to store nearly two-third of the electricity produced TABLE IV Technoeconomic results of water production using conventional and solar-powered RO systems Plant capacity: 1 m3/d. Tecbnoeconomic parameter

Type of electrical power system Commercial (System 4 of Fig. 1)

Desalination plant cost including 20% cost for the membrane, KD/m3 da Plant life, y Membrane life, y Discount rate, % Plant utilization factor Annual water production, m3/y Annual electricity consumption, kWhe,/Y Plant amortized cost, KD/y Plant O&M cost, KD/y Unit cost of electricity, fils/kWh Cost of electricity, KD/y Total annual expenditure, KD/y Cost of water production, KD/m3 Ratio of solar to commercial water production cost “1 KD=3.5 US $.

220 25 5 10 0.9 326 3149 41.3 35 25 78.7 155.0 0.37 1.0

Solar pond (System 5 of Fig. 1)

Solar photovoltaic (System 6 of Fig. 1)

220 25 5 10 0.75 274 2630

25 5 10 0.75 274 2630

41.3 30 143 376.1 447.5 1.63 4.4

41.3 30 331 870.7 942.0 3.44 9.7

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during an 8-h period of the day. The effective electrical energy availability thus reduces to 90.6% of the actual production. Thus, both the amortized capital cost and the O&M cost increase. The revised cost estimates are as follows: Annual electricity availability (2.0 x 0.906) Annual amortized plus O&M cost without battery Battery storage requirement Cost of battery bank Battery storage life Annual amortized cost for battery bank Additional O&M for battery bank (5% of capital cost) Total annual expenditure Cost of electricity

= = = = = = = = =

1.812 kWh/W; 0.481 KD/Wp 5.4 Wh/Wp 50 KD/kWh 3 years 0.11 KD/Wp 0.01 KD/Wp 0.601 KD/Wp 0.331 KD/kWh

The electricity cost of the PV system for nonstop operation increases from

Balance of system cost Energy cost

--

1

2 MSF systems

3

4

5

6

RO systems

Fig. 2. Cost of water production by conventional and solar systems using MSF and RO techniques (see Fig. 1 for system definitions). *W, represents peak Watt.

173

241 [l] to 331 fils*/kWb; an increase of 37.5%. The cost estimates for water production based on 1 m3/d of plant capacity have been analyzed and are shown in Table IV. RESULTS AND DISCUSSIONS

Cost analyses of water production using conventional and solar energy have been made for the MSF and the RO desalination techniques. The cost of water production for the different systems of Fig. 1 are shown in Fig. 2 along with a cost partition into energy costs and balance of system costs. Water production using conventional or commercial sources of energy is 26% less expensive with the RO system as compared to the MSF system. The major difference is attributed to the cost of energy since the balance of system cost is nearly equal for both the systems. This is an important factor for the future promotion of RO technology. However, the economics have to be investigated further since

10

20

30

40

Fuel cost, US 0 per barrel

Fig. 3. Cost of water production by conventional and solar systems using MSF technique. *1000 !a=1

KD.

51

174

7

PV cost - 4 KU/w,

2 . .: t R

3

3

100

75

l-

3 B i.i ,m ,::

PI

Conventional (System 4 of Fig. 7)

,

0) LO

I 20

I

I 30

I

L 40

I 5:

Fuel cost, US $ per barrel

Fig. 4.Cost of water production by conventional and solar systems using RO technique.

it is expected that with optimum design, the cost of cogeneration plant will be reduced thereby lowering the cost of electricity production. At this moment none of the solar-based technologies can compete with either of the desalination systems operating with conventional energy sources. A partially solar-powered MSF system (thermal energy from solar pond and using commercial electricity) produces water at the cost of 0.51 KD/m3, which is the lowest cost for all solar-energy-based technologies. This cost is on a par with the cost of water production for the commercial MSF system. Further reduction in the construction cost of the solar pond or increase in its energy extraction efficiency will make water production cheaper. An attempt has been made to predict the conditions of economic viability for water production. The variations considered are an increase in the fuel prices, the construction cost and energy collection efficiency of the solar pond and a reduction in the cost of PV systems. The construction cost and efficiency of solar pond have been combined as the cost efficiency factor (C,). Thus, C, for the present analysis is 75,

175

cost of 15 KD/m2)/(pond efficiency of 0.2). A given by C,= (construction decrease in C,, due to either a reduction in construction cost or an improvement in the pond efficiency, will help to make a solar pond coupled system closer to a conventional system from the cost-effectiveness point of view. C, values of 150, 100, 75 and 50 have been considered for the sensitivity analysis results. In Fig. 3 are shown the costs of water production by conventional and by solar systems using MSF technology. The main parameter is the variation in the fuel price from U.S.$ 10 to 50 per barrel. In Fig. 4 are shown similar results for the system using RO technology.

REFERENCES 1 R.K. Suri, A.M.R. Al-Marafie, A.A. Al-Homoud and G.P. Maheswari, Cost-effectiveness of solar thermal and electrical energy production, Int. J. Ambient Energy, 9 (4) (1988). 2 M.A. Darwish, Critical comparison between consumptions in large capacity reverse osmosis (RO) and multistage (MSF) seawater desalting plants, Desalination, 63 (1987) 143. 3 M. Tleimat, Solar power desalination, Sunworld, 10 (2) (1986) 55-57. 4 G. Belfort (Editor), Synthetic Membrane Process, Academic Press, New York, 1984. 5 H. Al-Busairi, A. Al-Kandari and H. Al-Shami, Photovoltaic powered lighting system for Kuwait English School: Performance evaluation, Kuwait Institute for Scientific Research, Report No. KISR 2320, Kuwait, 1987.