Economics of small solar-assisted multiple-effect stack distillation plants

ELSEVIER Desalination 130 (2000) 201-215

DESALINATION

www.elsevier.com/locate/desal

Economics of small solar-assisted multiple-effect stack distillation plants

Ali M. E1-Nashar Research Center, Abu Dhabi Water and Electricity Authority, Abu Dhabi, United Arab Emirates

Tel. +971 (2) 205-1333; Fax +971 (2) 434906; email: [email protected]

Received 22 June 1999; accepted 23 July 2000

Abstract

The objective of this paper is to compare the economics of using solar energy to operate small, multiple-effect seawater distillation systems in remote areas with the conventional method of using fossil fuels. The particular multiple- effect system used is an advanced horizontal-tube, falling-film system called"multiple-effect stack" (MES) in which the pumping energy requirement is relatively low compared with the horizontal in-line system. Three system configurations were investigated: (1) a conventional system using a steam generator to provide steam for the IVIES evaporator and a diesel generator to provide pumping power, (2) a solar-assisted system which uses solar thermal collectors to provide hot water (instead of steam) for the evaporator and a diesel generator for pumping power, and (3) a solar stand-alone system which uses solar thermal collectors for the evaporator heat requirement and a solar PV array to provide electrical energy for pumping. At the present time, solar energy cannot compete favorably with fossil energy, particularly under the present international market prices of crude oil. However, in many remote sunny areas of the world where the real cost of fossil energy can be very high, the use of solar energy can be an attractive alternative. Two important cost parameters affect the relative economics of solar energy vis4t-vis conventional (fossil) energy: the collector cost in dollars per square meter and the cost of diesel oil in dollars per giga Joule. Solar energy becomes more competitive as the local cost of procuring conventional fuel increases and as the collector cost decreases. The water cost from a solar thermal-diesel-MES system (configuration #2) can be seen to approach the water cost from a steam generator-diesel- MES system (configuration #1) when the collector cost drops to $200/m 2 and diesel oil cost at the remote site reaches $50/GL Using a 100% solar system (configuration #3) with solar thermal and solar PV collectors, the economics was seen to improve in favor of the solar system. Even when diesel fuel can be procured at $10/GJ at the remote site, the cost of water from the solar system can be seen to approach that from a conventional plant when thermal collectors costing $200/m 2 are used. The cost of water from the solar system was shown to be always less than that from a conventional system which uses diesel oil procured at the high price of $50/GJ, but always higher than water produced from a conventional system using diesel oil at the low price of $10/GJ.

Keywords: Solar desalination; Seawater distillation; Multiple-effect stack; Economics of solar desalination; Conventional versus solar desalination; Desalination in remote areas

O011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved

PII: S0011-9164-(00)00087 -4

202 A. El-Nashar / Desalination 130 (2000) 201-215

1. Introduction

Many remote areas of the world such as coastal desert areas in the Middle East or some Mediterranean and Caribbean tourist islands are suffering from an acute shortage of drinking water. Drinking water for these locations can be hauled in by tankers or barges or produced by small desalination units using the available saline water. The transportation of water by tankers or barges involves a lot of expense and is fraught with logistical problems which can make fresh water not only very expensive when available but also its supply being very susceptible to frequent interruptions. The use of small conventional desalination units using a fossil fuel such as diesel oil as the energy supply can suffer from the same procurement problems that are encountered with transporting fresh water, namely transportation expenses and supply reliability.

Some of the remote areas are blessed with abundant solar radiation which can be used as an energy source for small desalination units to provide a reliable drinking water supply for the inhabitants of the remote areas. Recently, considerable attention has been given to the use of solar energy as an energy source for desalination because of the high cost of fossil fuel in remote areas, difficulties in obtaining it, interest in reducing air pollution and the lack of electrical power source in remote areas.

Desalination of seawater and brackish water is one of the ways for meeting future fresh water demand. Conventional desalination technology is fairly well established, and some of the processes may be considered quite mature although there is still considerable scope for improvement and innovation. Conventional desalination processes are energy intensive, and one of the major cost items in operating expenses of any conventional desalination plant is the energy cost. Thus, one of the major concerns about using desalination as a means of supplying fresh water to remote communities is the cost of energy.

Apart from energy cost implications, there are environmental concerns with regard to the effects of using conventional energy sources. In recent years it has become clear that environmental pollution caused by the release of green house gases resulting from burning fossil fuels is responsible for ozone depletion and atmospheric warming. The need to control atmospheric emissions of greenhouse and other gases and substances will increasingly need to be based on growing reliance on renewable sources of energy.

In this paper the economics of a solar-assisted and solar stand-alone multiple-effect stack (IVIES) distillation systems for seawater desalination are compared with that of a conventional system consisting of an MES unit operated in association with a steam generator and a diesel generator. The comparison is based on the economic and meteorological environments prevailing in Abu Dhabi, UAE.

2. System configurations

Three system configurations are considered for the current economic study: • a conventional distillation system consisting

of an MES evaporator supplied by steam from a low-pressure steam generator with the pumping power supplied by a diesel generator as shown schematically in Fig, 1 (configuration #1);

• a solar-assisted system consisting of an MES evaporator supplied by thermal energy in the form of hot water from either high-efficiency flat-plate collectors or evacuated tube collectors with pumping power supplied by a diesel generator as shown in Fig. 1 (configuration #2);

• a solar stand-alone system consisting of an MES evaporator supplied by thermal energy from flat plate or evacuated tube collectors, as in configuration 2, with pumping power supplied by a solar PV system as shown in Fig, 1 (configuration #3).

A. El-Nashar I Desalination 130 (2000) 201-215 203

crude oil .I 1

diesel oil

Boiler

Diesel Generator

steam L

"ondensat=

L

r

.-lectricity

MES Evaporator

I [ distillate

brine

seawate

Solar ~ T hermit Collectors

diesel oil .[ Diesel -j Generator

Solar Thermal

Collectors

Solar PV 1 Collectors IX: electricity

Configurat ion (1) ]

(- ~ hot water ! hot water I I--"l i "i n='= I I .

L...H, Y - - Evaporator warm water

electricity

[ ConO ao (,) I T hot water hot water

warm warm water

I Configuration O) ]

distillat

I brine

~anwnt~r

MES Evaporator

distillate

brine blowdown D

seawater

Fig. 1. System configurations.


The system labeled as "configuration #1" needs two types of fossil fuel, namely crude oil for the steam generator and diesel oil for the diesel generator. The thermal energy required by this system is supplied by steam at a pressure ranging from 1-1.5 bar. Several pumps are required to force the different streams to flow through the system and to inject different chemicals into these streams to ensure safe and reliable operation. The major pumps required by the evaporator are the seawater intake, feed water, distillate (product water), brine blowdown, drain, antiscalant dosing (for feedwater stream), NaCIO dosing (for seawater intake and distillate streams), NaHCO3 dosing (for distillate stream) and CaClz dosing (for distillate stream). All these pumps are operated by 3-phase 415 V AC motors, except the chemical dosing pumps which are operated by 3-phase 220V AC motors. Steam ejectors are used to create and maintain the vacuum inside the evaporator.

The system labeled as "configuration #2" needs only one type of fossil fuel, namely diesel oil, for the diesel generator. The thermal energy required by the MES evaporator can be supplied in the form of hot water generated by either fiat- plate or evacuated-tube collectors. Double glazed, well designed, fiat-plate collectors can produce hot water at 80°C with reasonable efficiency while evacuated-tube collectors can easily produce water at more than 95°C with good efficiency albeit higher collector cost. The pumping power for this system is more for the conventional system because at least three additional pumps should be incorporated: mechanical vacuum, heat collecting, and a water heating pump. The mechanical vacuum pump is to replace the steam ejector in the conventional system since no steam is available in the current system. The heat collecting pump is used to circulate the collector fluid (water) through the collector field to enhance solar heat collection. The heating water pump is used to draw hot

water from the heat accumulator and supply it to the first effect of the MES evaporator to initiate seawater boiling in that effect. The capacity of the diesel generator for this system is therefore larger than that of the conventional system.

The system labeled "configuration #3" utilizes solar energy to satisfy both thermal and electrical energy demands of the system. The electrical power requirements of this system is essentially the same as that of"configuration #2". However, the power requirement is supplied by an array of solar PV cells instead of a diesel generator. An inverter is used to change the DC output into AC electricity.

3. Design and sizing considerations

3.1. Thermal energy requirement

The thermal energy requirement of the MES evaporator, Qev, depends essentially on its rated capacity as well as its performance ratio. The performance ratio for MES evaporators depends essentially on the number of effects, N, according to the following relation [1,2]:

PR = -0.809 + 0.932N- 0.0091N 2 (1)

QCv can be obtained from [2]

L (2)

where M d is the rated (design) capacity and L is the latent heat of vaporization.

3.2. Pumping requirements

The electrical power requirements of the MES evaporator depends on the means of achieving the vacuum inside the evaporator. If a vacuum

A. E1-Nashar / Desalination 130 (2000) 201-215 205

pump is used, then its power consumption has to be added to the power consumption of the other pumps to obtain the total pumping power requirement. If vacuum is created with a steam jet ejector, then the total pumping power will only be those needed by the pumps. The power requirements of the vacuum pump and other evaporator pumps [1] are obtained from the following relation, which is based on the manufacturer's data:

a collector is determined by its efficiency defined as the ratio of the amount of heat collected by the collector fluid (water) to the amount of solar radiation intercepted by the absorber plates of the collector. This efficiency, referred to as rh, is essentially dependent on a parameter x defined as

x = (6) i,

Pvae = 1.42 + 0.0395M,~ °3 (3)

Per = 4.36 +O.0965Md-2.2(lO-5)M~ (4)

where the power is in kW and M d is in m3/d. In addition to the pumping power require-

ments given above, we have to add the power required to circulate the water through the solar collectors. Based on data obtained from the Abu Dhabi solar plant, the heat collecting flow rate per meter square of collector absorber area is about 45 kg/hm 2, which is assumed to be main- tained for the current study. Based on a pressure drop per collector panel of about 1 mH20, and assuming a pump efficiency of 70%, the rated power consumption of the heat collecting pump (in kW) can be obtained from the equation

Pc = 2"691 "(10-2)" (0"045Ac~ a3 (5)

where Ac is the collector area in m 2.

3.3. Collector performance

Evacuated tube collectors [1] are assumed to be the collector of choice when collector outlet water temperatures in excess of 80°C are considered. For lower water temperatures, flat- plate collectors can be used. The performance of

where T~ is the collector inlet water temperature, T2 is the outlet water temperature and I is the solar radiation on the absorber area. For evacuated tube collectors of the type used at the solar plant (manufactured by Sanyo), the collector efficiency can be expressed by the following formula:

rl~ = 0.84-2.6x- 1.92x 2 (7)

where x is in units of °C h m 2 kcal- ~. For flat-plate collectors, the collector effi-

ciency is assumed to be

"qc - 0.76 - 4.36x (8)

3.4. Sizing thermal collector area

The optimum thermal collector area required for a particular evaporator rated capacity was defined as the collector area corresponding to the minimum cost of water produced. This optimum area is specific to both weather and economic conditions at the site where the plant is to be constructed. By using the available weather and economic data for Abu Dhabi City, as an example, it was possible to simulate the performance of a solar desalination plant using computer simulation and obtain the optimum collector area for different plant capacities

206 A. EI-Nashar / Desalination 130 (2000) 201-215

1.$.

-~ o.s. 0

2.5.

--.e-- Nrrb-30/o0

- - IF - N/Tt~,24/71

J- N/'rb-18/63

5;0 ,~oo ,~0 2o,o collector atom, m2

2~

Fig. 2. Cost of water produced by a solar MES plant having a rated capacity of 130 m3/d for different collector areas. N is number of effects and T b is maximum brine temperature.

for this location. For example, for an evaporator capacity of 130 m3/d, the relative cost of water for different collector areas is shown in Fig. 2. The minimum water cost can be seen to correspond to a collector area of about 1500m 2 for an evaporator having 30 effects and designed for a maximum brine temperature of 80°C.

3.5. Sizing the P V system

Sizing the elements of the PV system [3] means determining the PV collector area and the amount of energy storage capacity that are needed so that the system can provide the amount of electrical power that is required by the evaporator and the solar thermal collector system. Most methods of sizing the stand-alone system treat the PV array and the battery storage independently. The array size [4] is determined by requiring that the average daily array output matches the average daily load for the worst-case month of the year. A safety factor is introduced to account for degradation in array output due to dirt accumulation, line losses and general uncertainties about "average" outputs and inputs.

The amount of storage capacity is usually fixed by estimating the number of"no-sun days", the number of consecutive days during which overcast conditions prevail, liable to be experienced at the site. Ifa high degree of system

reliability is demanded, the usable daily battery capacity, measured in ampere-hours, is fixed by the maximum daily load, again measured in ampere-hours. The number of no-sun days times the maximum daily load gives the system's required total usable battery capacity.

Sizing the PV array: A PV module, and hence a PV array, is rated according to its maximum power. The maximum power is the power the module will produce at certain reference conditions of (1)module temperature and (2) solar radiation intensity. For example, a 30W module has a maximum power output of 30W at a standard cell temperature of 25°C and solar radiation intensity of 1.0kW/m 2. As a rule of thumb [3], the maximum power decreases by 5% for every 10°C tempera-ture rise. The net power delivered to the load will be less than the power produced due to losses in the inverter, battery and lines. Assuming a 15% loss in power due to temperature rise and 10% loss due to the other factors, we can estimate the amount of energy produced in a typical day (averaging 7.4 full sunshine hours per day) by one module as 7.4 x 0.75 × 30 = 166.5 Wh.

4. Economic ground rules and water cost optimization

4. I. Capital equipment cost

4.1.1. Capital cost of MES evaporator

The cost of an MES evaporator is based on budget offers from different manufacturers [ 1,5] and on cost information available in public literature. The specific capital cost (cost per unit of daily production) was found to depend on the design capacity, number of effects and maximum brine temperature according to the correlation:

Md 1 0 . 7 , 1.16 -2 .254

100~ Md~ 1000

A. EI-Nashar / Desalination 130 (2000) 201-215 207

where ccv is the evaporator specific capital cost, $/m3d -I (1999 price); Md the rated (design) capacity, m3/d; N the number of effects; and T b the design maximum brine temperature, °C.

4.1.2. Capital cost of solar thermal collectors

Solar collectors used for this application should be capable of producing hot water at a temperature ranging between 70°C and 90°C. Evacuated-tube collectors and high-efficiency flat-plate collectors can be used to produce hot water at a temperature in excess of 80°C. The specific cost of the solar collectors is assumed to vary between $200/m 2 (flat-plate collectors) to $1000/m 2 (evacuated-tube collectors). The cost is assumed to include both the solar collector proper as well as the support structure, piping, valves, etc.

4.1.3. Capital cost of PV electricity generating system

The main components of the photovoltaic (PV) system are a PV array, a bank of batteries and an inverter. The cost of the PV array - - including support structure - - is assumed to be $5 per peak watt, which is based on recent quotations for commercial-grade polycrystalline silicon cell material.

Regular 12-V lead acid batteries with a capacity of 96 ampere-hours are considered for this application with an estimated cost of $55 each.

4.1.4. Capital cost of heat accumulator

The heat accumulator is assumed to be a vertical cylindrical tank made of mild steel with a thick layer of fiber glass insulation to reduce heat loss. The tank is designed to operate at atmospheric pressure and is provided with a pressure relief valve as a safety measure. Hot water from the collector field is supplied to the tank at the top via a special water distribution grid which ensures that hot water diffuses slowly

through the surrounding water with causing too much turbulence in order to enhance thermal stratification through the tank.

The capital cost of the heat accumulator as obtained from manufacturer's data is obtained from the following relation:

Cst = 456.6 300 ] 100~ Mst~ 600 (10)

where cst is the specific cost of storage capacity, $/m 3 (1999 price) and Mst is the storage capacity, m 3 .

4.1.5. Capital cost of steam generator

Low-pressure and low-capacity steam generators are required to supply the MES evaporator with the low-temperature thermal energy necessary to drive the unit. The capacity of the steam generator depends on the capacity of the MES unit as well as its performance ratio. A unit with a PR of 13, which produces 200 ma/d at design conditions, requires only 0.6t/h of low- pressure steam.

A fire tube packaged boiler producing steam at 10 bar with an efficiency (LHV) of 86% is considered appropriate. The capital cost of such a boiler, Cb ($), is obtained from Peter and Timmerhaus [6] and is adjusted to the 1999 cost level using the Marshall and Swift Equipment Cost Index. The resulting correlation is:

Csg = 10,824(S)°s7+6973 0.15gS~ 15 (11)

where Csg is the boiler cost (in $) and S is the steam generating capacity, t/h.

4.1.6. Capital cost of diesel generator

The electrical demand of the desalination plant can be supplied by a diesel generator whose


capacity will obviously depend on the capacity of the plant itself. The capital cost of the diesel generator is obtained from the following relation which is based on recent commercial bids:

p ) 0.5495 50,s00 (12)

where Cds is the cost in $ and P is the rated capacity in kW.

4. 2. Operating costs

The main components of the operating costs include: fuel cost, cost of chemicals, cost of spare parts, and salaries for O&M personnel.

4.2.1. Fuel cost

For the all-solar desalination plant option (configuration #3), no fossil fuels are needed. For configuration #2 (solar thermal collectors and diesel generator), only diesel oil is required whereas for configuration #1 (steam generator and diesel generator), crude oil is required for the steam generator and diesel oil is required for the diesel generator. The cost of crude oil in the international market is currently about $2.0/GJ (corresponding to $11 per barrel), while that of diesel oil is about five times as much as crude oil, i.e., $10/GJ.

In remote locations fuel costs will obviously be higher than the costs quoted above due to transportation and logistics.

4.2. 2. Cost o f chemicals

The chemicals used, their dosing rate and their unit cost are shown in Table 1. The unit costs are those prevailing in Abu Dhabi, UAE, during 1999.

4.2.3. Cost o f spare parts

The annual cost of spare parts is assumed to be 2% of the capital cost for the evaporator,

Table 1 Cost of chemicals used in MES desalination plants

Name of chemical Dose Cost Flow (ppm) (S/ton) stream

NaCIO 14 284 Belgard EV 10 1486

antiscalant NaI-ICO3 23 270 NaCIO 3 284 CaCI 2 18 210 Anti-corrosion 5000 2430

chemical (e.g., Nalcool)

Seawater Feedwater

Distillate Distillate Distillate

Table 2 Annual salaries for plant operation and maintenance personnel

O&M personnel Annual salary, $

Plant engineer 44,600 Technician 33,150 Skilled laborer 9,780

steam generator and diesel engine. The cost of spare parts for the thermal and PV collectors is assumed to be negligible compared to the spare parts cost of other equipment. This has been validated by 15 years of operational experience at the Abu Dhabi solar plant.

4.2.4. Salaries for O&M personnel

For plants of configurations # 1 and #2 where a diesel engine and/or steam generator are in operation, the operating staff include one plant engineer, two technicians and one skilled laborer. For configuration #3 plants, which are solar stand-alone systems, only a plant engineer and one skilled laborer are assumed to be needed for operation. The annual salary for each operating staff is shown in Table 2.

A. EI-Nashar / Desalination 130 (2000) 201-215 209

4.3. Optimum cost of water for each plant configuration

The optimum (minimum) water cost for each plant configuration was estimated. The water cost in $/m 3 of fresh water depends on the plant capital cost, O&M costs (which include fuel costs for plants using a steam generator and/or diesel generator) and the annual amount of fresh water produced. The capital cost depends on plant configuration and design parameters such as evaporator capacity, design maximum brine temperature, number of effects, solar collector area, type of solar collector (whether flat-plate or evacuated-tube type), and the capacity of the heat accumulator. The operating cost also depends on plant configuration, plant capacity, number of effects, collector area, heat accumulator capacity and cost of fuel and chemicals used.

The annual amount of fresh water produced by a solar desalination system (configuration #2 and #3) depends on the evaporator design capacity, design maximum brine temperature, number of effects, solar collector area, and heat accumulator capacity. This amount is obtained from the output of the SOLDES program [ 1 ]. The output of the program is correlated with the primary design variables using the least square technique. Thus, for a plant having a design capacity of 500 m3/d, for example, the correlation relating to the annual fresh water production is given below:

Thus, for configuration #2, for example, the optimum cost of water, c,:, was estimated by minimizing the following objective function

+;st~st'Cst)+idg~dg'Cdg)]+Cf~dg'Cf ) (14)

where CRF is the capital recovery factor.

c w = Min w (15)

subject to the following constraints:

2,500 _<_ Ac (m 2) < 12,500

10-<N _<30

60 -< Tb (°C) -< 80

The optimum water cost of a plant of a given capacity is the minimum water cost which can be achieved by the plant when its design parameters are optimized. The optimization process was carried out using the mathematical software Eureka, version I.

M= (-57,304.9+ 49.5475A c - 0.002355A 2)

+ (-190,122.4+16,142.3N-331.81N 2) (13)

where A c is the solar collector area, m 2, ranging between 2500 m 2 and 12,500 m 2, N is the number of effects of evaporators, ranging between 10 and 30, and T b is the design maximum brine temperature, °C, ranging between 60°C and 80°C.

5. Results

The conventional system is that which uses a steam generator to produce steam for the MES evaporator and uses a diesel generator to produce power to run the pumps. This is configuration # 1 (see Fig. 1). The optimum water cost for this system is shown in Fig. 3. In this system the fuel used by the steam generator is assumed to be crude oil with a cost assumed to be equal to one- fifth of that of diesel oil. As can be expected, the

210 A. El-Nashar ~Desalination 130 (2000) 201-215

6 ~

O

• - 4 - - E v a p o r a t o r capaci ty = 130 m31day - - | l - - E v a p o r a t o r capacity = 500 rn31day

JL Evaporator capaci ty = 1000 m31day

| • | w • | " • 1

10 15 20 25 30 35 40 45 50

Oiesel oil cost, $/GJ

Fig. 3. Optimum water cost from conventional MES evaporator using a steam generator and diesel generator (configuration # 1).

12

10

(e j

.,,J"

~ s (J

0

200

. . . . d r " . . . . . A - . . . . ~ _ j

~ ~ . . ° ° o . . - ° . ° ° . - - ° ' " • ~ ' ' ' ' ' ' °

diesel oil cost = 25 $1GJ

~ E v a p o r a t o r capac i ty = 130 m3/day - - • - - Evaporator capac i ty = 500 m31day

- - -A - - Evapo ra to r capac i ty ,, 1000 m3/day

• | " l • w ' ' |

300 400 500 600 700 800

co l lec to r cost, $1m2

Fig. 4. Effect of collector cost on optimum water cost for solar thermal-diesel-MES system (configuration #2).

optimum water cost for this system can be seen to increase steadily as the fuel cost increases.

The optimum (minimum) cost of water produced by the solar thermal-diesel-MES system depends mainly on the collector cost and the cost of diesel oil. The effect of collector cost on the optimum water cost for different evaporator capacities is shown in Fig. 4 for a diesel oil cost of $25/GJ. The cost of water can be seen to increase linearly with the collector cost. At the lowest collector cost of $200/m 2, the cost of water varies between $7-8/m 3, and at the

high collector cost of $800/m 2, the water cost varies between $10-1 ]/m 3.

The effect of collector cost on the optimum collector area for the solar thermal--diesel-MES system is depicted in Fig. 5, assuming a diesel oil cost of $25/GJ. As expected, the optimum collector area increases as the evaporator rated capacity increases because, with higher evaporator capacity, more heat (thus more collector area) has to be collected and supplied to the evaporator.

The effect of diesel oil cost on the optimum

A. El-Nashar / Desalination 130 (2000) 201-215 211

20000 "1

d, /

toooO .o . . . . . . m . . . . . . • . . . . . . . • . . . . . . . . . . . . . • . . . . . . . •

o .-- ;, o ; • i • i

200 300 400 500 600 700 800

Collector cost. $1m2

-----4P----Evaporator capaci ty = 130 m31day - - • - -Evaporator capacity = 500 m31day

- - - J , - Evaporator capacity = 1000 m31day

Fig. 5. Effect of collector cost on optimum collector area for solar thermal-diesel-MES system (configuration #2) (cost of diesel fuel oil = $25/GJ)..

. z

~p

g ,

8, 7. 6, 5 4 3 2 1 0

10 15 20 25

cost of diesel o11, $1GJ

[ ] Evapora to r capaci ty =130 m3/day [ ] Evapora to r capaci ty = 500 m3/day [ ] Evapora to r capaci ty =1000 m3/day I

Fig. 6. Effect of diesel oil cost on optimum water cost for solar thermal-diesel-MES system (configuration #2) (collector cost = $500/m2).

water cost is displayed on Fig. 6 with diesel oil cost varying between $ I 0-25/GJ and assuming a collector cost of $500/m 2. As can be seen, the increase in the cost of diesel oil has a small effect on water cost because this cost item makes a minor contribution to the water cost when compared with capital amortization, which constitutes the major contributor to water cost.

The effect of collector cost on the optimum number of evaporator effects is displayed in Fig. 7 using a diesel oil cost of $25/GJ. It can be seen that the number of effects tends to increase slightly as the collector cost increases because, by increasing the number o f effects, the heat

supplied to the evaporator of a given rated capacity will tend to decrease since the performance ratio increases with the number of effects. The decrease in the heat supply to the evaporator results in a reduction of the collector area required to supply the heat thus the collector cost will tend to decrease. On the other hand, evaporators with larger number of effects have higher capital costs due to the extra construction materials required. Thus, the optimum number of effects has to balance between collector cost and evaporator cost for any given maximum brine temperature in order to achieve a minimum water cost.


25.

20.

5

200 300 400 S00 600 700 800

Collector cost, $1m2

[] Evaporator capacity =130 m31day [] Evaporator capacity = 500 m3/day [] Evaporator capacity =1000 m3/day [

Fig. 7. Effect o f collector cost on optimum number of evaporator effects (diesel oil cost 25 S/G J) (configuration #2).

20000.

E ,16000.

12000,

8000 U

4000, u

0 10 15 20

Diesol oil cost, $/GJ

25

1 Evaporator capacity =130 m3/day m Evaporator capacity = 500 m3/day [] Evaporator capacity =1000 m3/day I

Fig. 8. Effect of diesel oil cost on optimum collector area (collector cost = $500/mZ).

The effect of diesel oil cost on the optimum collector area is shown in Fig. 8, which demonstrates that the cost of diesel oil has a minimal effect on the collector area required because, as mentioned above, this cost component is very small compared with other cost components, particularly capital amortization.

A comparison of the water cost from solar thermal--diesel-MES system and steam gene-

rator--diesel-MES system for different diesel oil cost is shown in Fig. 9 for an evaporator having a rated capacity of 130 m3/d. The comparison is made for two values of collector costs, a low cost of $200/m 2 (typical of fiat-plate collectors) and a medium cost of $500/m 2 (typical of evacuated- tube collectors). It can be seen that the water cost from the conventional plant is lower than the water cost from the solar thermal-diesel-MES plant, but the cost difference between the


Collector cost " 200 $1m2 u" Collector cost = 500 $/m2 --O--Boiler-dleesl-MES

1 2 .

10

s

e ~J

4

boiler-dleseI-MES

• • | | i

0 10 20 30 40 50

Diesel oil cost, $/GJ

Fig. 9. Effect of diesel oil cost on optimum water cost (evaporator capacity = 130 m3/d).

8

6

s

"d

u

4 .

3 .

2 .

1

0

100

0 D ' 0 D 0 D 0

",., Boiler-dieseI.MES(dleesl oi1-10 $/GJ) --U--Boller-dleseI-MES(dlesel o11-50 $1GJ)

• Solar thermaI-PV-MES

, o o soo . o o

Collecor cost, $1m2

Fig. 10. Comparison of cost of water from solar thermal-PV-MES system with boiler--dieseI-MES system.

conventional and solar systems narrows down as collector cost decreases and as the diesel oil cost increases. At a collector cost of $200/m 2 and diesel oil cost of $50/GJ, the water costs from both systems are very close.

A comparison of the water cost from the solar thermal-PV-MES system (configuration #3) and the conventional system (configuration #1) is shown in Fig. 10 where the cost of thermal

collectors is plotted as the abscissa and the water cost as the ordinate and two diesel fuel costs are used in the conventional system, namely, $10/GJ and $50/GJ. Obviously, the water cost from the conventional system is independent of collector cost and is only influenced by the fuel cost. For a particular fuel cost, the cost of water from the conventional system is therefore plotted as a horizontal line in this figure. The cost of water


12

10 t ~

q , , ,

o 4 4 h a v) O O 2

P00

t : ] O C 21 0 C 21 Boller-dlesel-MES(dlesel co$t=10 $1GJ)

--i--Boiler-dlesel-MES(dlesel coat=S0 $1GJ) - - e - - Solar thermal-dlesel-MES(dlesel cost-10 $1GJ) - 4 l - - Solar thermal.dlesel-MES(dleael cost=S0 $/GJ)

an

200 300 400 500 600 700 800

Collector cost. $1m2

Fig. 11. Effect of collector cost on the cost of water-- comparison of solar thermal-dieseI-MES system with a boiler- dieseI-MES system.

from the solar system (configuration #3) is seen to vary between the water cost produced by the two conventional systems with diesel fuel costs of $10/GJ and $50/GJ. At the lowest collector cost ($200/m2), the water cost from the solar system is slightly higher than the water cost from the conventional system operating on the low fuel cost ($10/GJ). At the highest collector cost ($800/m2), the cost of water from the solar system approaches the water cost from the conventional system, which uses the high fuel cost ($50/GJ). One can therefore infer from these results that the solar system has a good potential for remote areas, particularly when low-cost collectors are used.

6. Conclusions

The present study investigated the economics of small-scale, multiple-effect, stack-type distillation plants for seawater desalination using either conventional energy or solar energy sources. The plants are to be considered for the

coastal remote areas which are characterized by very high fuel costs and the unavailability of electricity from the national electrical grid. Three systems configurations were investigated: (1) a conventional system using a steam generator and a diesel generator, (2) a solar-assisted system in which thermal energy required by the evaporator is provided by solar thermal collectors and the pumping power provided by a diesel generator, and (3) a solar system in which thermal energy is supplied by solar thermal collectors and the pumping power by solar PV collectors.

At the present time, solar energy cannot compete favorably with fossil energy, particularly under the present international market prices of crude oil. However, in many remote sunny areas of the world where the real cost of fossil energy can be very high, the use of solar energy can be an attractive alternative.

Two important cost parameters affect the relative economics of solar energy vis-A-vis conventional (fossil) energy: the collector cost in dollars per square meter and the cost of diesel oil in dollars per giga Joule. Solar energy becomes


more competitive as the local cost of procuring conventional fuel increases and as the collector cost decreases. The water cost from a solar thermal--diesel-MES system (configuration #2) can be seen to approach the water cost from a steam generator-diesel-MES system (configuration # 1) when the collector cost drops to $200/m 2 and diesel oil cost at the remote site reaches $50/GJ.

Using a 100% solar system (configuration #3) with solar thermal and solar PV collectors being utilized, the economics was seen to improve in favor of the solar system. Even when diesel fuel can be procured at $10/GJ at the remote site, the cost of water from the solar system can be seen to approach that from a conventional plant when thermal collectors costing $200/m 2 are used. The cost of water from the solar system was shown to be always less than that from a conventional system which uses diesel oil procured at the high price of $50/GJ but always higher than water produced from a conventional system using diesel oil at the low price of $10/GJ.

7 . S y m b o l s

A c - - Collector area, m 2 c~v - - Specific cost of evaporator, $/m3d - cdg - - S p e c i f i c cost of diesel generator,

$/kW

Cst

Csg

M Md

Ms,

N PR

P Tb

Specific cost of heat accumulator, $/m 3

- - S p e c i f i c cost of steam generator, $/td -I Annual distillate production, m3/y

- - R a t e d (design) capacity of evaporator, ma/d

- - Storage capacity of heat accumulator, m 3

- - Number of effects of MES evaporator - - Performance ratio

Pumping power requirement, kW Design maximum brine temperature, o C

R e f e r e n c e s

[1] ENAA & WED, Research and development coop- eration on a solar energy desalination plant, Final Report, 1986.

[2] W.T. Hanbury and T. Hodgkiess, Desalination Technology 95, Porthan, Glasgow, UK, 1995.

[3] Mobil Solar Energy Corporation, Applications of photovoltaics, Report, 1985.

[4] Solarpac Energy Systems, Inc., Photovoltaics, electricity from the sun,.,R..c~port, Ontario, 1985.

[5] G. Fosselard and K. Wangnick, Desalination, 76 (1989) 215.

[6] M. Peters and K.D. Timmerhaus, Plant Design and Economics for Chemical Engineers, 3rd ed., McGraw-Hill International, 1981.

Documents

Economics of small solar-assisted multiple-effect stack distillation plants