Solar Energy 75 (2003) 421–431

www.elsevier.com/locate/solener

Effect of dust deposition on the performance of a solardesalination plant operating in an arid desert area

Ali M. El-Nashar *,1

22 Ahmed Gharbo Street, Apart. #703, Zinzinia, Alexandria, Egypt

Received 6 June 2003; accepted 1 August 2003

Abstract

The performance of a solar desalination plant (whether using thermal or photovoltaic collectors) is influenced by the

ability of the glazing system to transmit solar radiation to the collector absorption surface. This ability is influenced by

such factors as the intensity of solar radiation, the transmittance of the collector glazing, the tilt angle of the absorbing

surface, the operating parameters of the plant, the properties of the materials of construction, etc. This paper discusses

the influence of dust deposition on the evacuated tube collector field on the operating performance of the solar de-

salination plant at Abu Dhabi, UAE. This plant has a collector field area of 1864 m2 of absorber surface and an MED

(multiple effect distillation) unit for seawater desalination with a capacity of 120 m3/day of distilled water. The re-

duction in transmittance due to dust deposition on the amount of heat collected has been measured and its influence on

the distillate production has been estimated using the computer simulation program SOLDES which has been verified

previously as an effective tool for predicting the operating performance of similar plant designs. The frequency of high-

pressure water jet cleaning on the performance of the plant was also investigated. It was found that dust deposition and

its effect on plant performance depend strongly on the season of the year and the frequency of jet cleaning should be

adjusted accordingly.

� 2003 Published by Elsevier Ltd.

1. Introduction

Dust accumulation on the glazing of solar thermal

collectors associated with distillation plants for seawater

desalination is one of the main natural causes for per-

formance degradation. This is particularly so for plants

in operation in remote desert locations subject to sand

storms where the air is laden with fine sand particles.

Dust deposition on flat plate collectors has been

studied by several authors (e.g. Van Goossens and

Kerschaever, 1999; El-Shobokshy and Hussein, 1993;

Sayigh et al., 1985; Hegazy, 2001; Nimmo and Seid,

1979; Garg, 1974). El-Nashar (1994) studied the influ-

* Tel.: +20-3-584-0666; fax: +20-3-582-4822.

E-mail address: elnashar100@hotmail.com (A.M. El-Na-

shar).1 Director, Desalination Laboratory, Research Center, Abu

Dhabi Water and Electricity Authority, Abu Dhabi, UAE.

0038-092X/$ - see front matter � 2003 Published by Elsevier Ltd.

doi:10.1016/j.solener.2003.08.032

ence of dust deposition on the performance of evacuated

tube collectors on a large field of collectors and found

that accumulated dust on this type of collectors can re-

sult in a substantial reduction in collector efficiency. The

airborn particles in the atmosphere affect the amount

and properties of the radiation finally reaching the col-

lectors (see Mastekbayeva and Kumar, 2000; Al-Hassan,

1998). Outdoor measurements on glazing transparency

have been performed by Nahar and Gupta (1990) and

Bonvin (1995). Hegazy (2001) studies dust accumulation

on glass plates with different tilt angles and measured the

transmittance of the plates under different climatic

conditions in Minia, Egypt over a period of one month.

The degradation in solar transmittance during this pe-

riod was found to depend on the tilt angle of the glass

plates with a maximum value when the plate is in a

horizontal position and minimum value when it is ver-

tical. Measurements made by Sayigh et al. (1985) and

Hasan and Sayigh (1992) of dust accumulation on tilted

glass plate located in Kuwait was found to reduce the

Nomenclature

_mmc mass flow rate of collector fluid (water),

kg/s

Ac absorber area of collector block, m2

Cp specific heat at constant pressure for the

collector fluid (water), kcal/kg �CJt solar radiation on tilted surface, kcal/m2 h

M annual distillate production, m3

Mclean annual distillate production with clean col-

lectors, m3

Mdusty annual distillate production with dusty col-

lectors, m3

Qa daily amount of heat collected by collector

block A, kcal/day

Qf daily amount of heat collected by collector

block F, kcal/day

SPCR specific power consumption ratio

T temperature of collector fluid, �C

T1 hourly average temperature of fluid entering

collector block, �CT2 hourly average temperature of fluid leaving

collector block, �CTamb ambient air temperature, �CV voltage of solar sensor, mV

x collector parameter defined as fðT1þT2Þ=2g�Tamb

Jt

Greek symbols

s glass transmittance

g hourly collector block efficiency

aðsÞ function of glass transmittance in Eq. (5)

bðsÞ function of glass transmittance in Eq. (5)

cðsÞ function of glass transmittance in Eq. (5)

gann annual collector block efficiency

gd daily collector block efficiency

gm monthly collector block efficiency

422 A.M. El-Nashar / Solar Energy 75 (2003) 421–431

transmittance of the plate by an amount ranging from

64% to 17% for tilt angles ranging from 0� to 60�, re-spectively after 38 days of exposure to the environment.

Van Goossens and Kerschaever (1999) carried out

wind tunnel experiments to find the effect of wind ve-

locity and airborn dust concentration on the drop of

photovoltaic (PV) cell performance caused by dust de-

position on the cells. It was found that the deposition of

fine dust particles on PV cells significantly affect the

performance of such cells.

The solar desalination plant situated in Abu Dhabi,

UAE was used to study the effect of dust accumulation

on the performance of the plant. This is a demonstration

plant that is located on the site of Umm Al Nar Power

Plant (about 20 km north-east of Abu Dhabi city) and

went online in 1984 and was operated by the Research

Center of Abu Dhabi Water and Electricity Authority

(ADWEA).

2. The solar desalination plant

A simplified schematic of the plant is shown in Fig. 1.

It consists of a field of evacuated tube collectors, ther-

mally stratified heat accumulator and a multiple effect

distillation (MED) unit for seawater desalination. The

collector field consists of 1064 panels of evacuated tube

collectors each having a selective absorber area of 1.75m2

thus making the total absorber area 1862 m2. The col-

lector field is divided into 76 collector arrays each consists

of 14 collector panels connected in series. The field is

divided into six blocks named A, B, C, D, E, and F as

shown in Fig. 2. All arrays in each block are connected in

parallel to the block inlet and outlet headers. The MED

evaporator consists of 18 effects (arranged in a vertical

stack) in which the seawater or its brine is made to boil

under vacuum at different boiling temperatures ranging

from the highest at the first effect (top effect) to the lowest

at the 18th effect (bottom effect). Preheated seawater is

sprayed on the outside surface of the first effect evapo-

rator tube bundle in which hot water from the accumu-

lator flows through the tubes. Part of the seawater is

boiled off and the remaining brine cascades down to the

following effects where it is sprayed on the tube bundles.

Vapor is generated in each effect by using the vapor

produced in the previous effect as a heat source.

The plant was designed to operate in an automatic

fashion where the heat collecting system (consisting of

the collector field, bypass line and heat collecting pump)

is controlled by the solar controller and the MED

evaporator is controlled by the heat accumulator tem-

peratures. The operation of the heat collecting system

depends on the intensity of solar radiation as well as the

temperature of the low temperature tank. The operation

of the evaporator depends on two temperatures mea-

sured using RTDs and located at the medium tempera-

ture and high-temperature tanks. These two RTSs are

connected to setpoint controllers which would allow the

evaporator to initiate it’s startup sequence (when it was

previously shutdown automatically) when the medium

temperature rises above the corresponding setpoint.

When the water temperature in the hot water tank drops

below the corresponding setpoint value, the evaporator

starts it’s shutdown sequence and the evaporator will

continue in this mode until the medium temperature

rises above the corresponding setpoint.

Hot temp. tank

Medium temp. tank

Low temp. tank

Block A

Block B

Block C Block D

Block E

Block F

Heat collecting pump

From evaporator From bypass line

To evaporator

To bypass line

Motorized valves

RTD

Fig. 2. Solar collector field and heat accumulator tanks.

Fig. 1. Schematic diagram of the solar desalination plant.

A.M. El-Nashar / Solar Energy 75 (2003) 421–431 423

424 A.M. El-Nashar / Solar Energy 75 (2003) 421–431

Since the collector field represents the main source of

thermal energy to the evaporator, the condition of the

collectors is expected to affect substantially the perfor-

mance of the whole plant. The purpose of this paper is to

find the effect of dust deposition on the glazing of

commercial evacuated tube collector field on the per-

formance of an operating solar desalination plant that

utilizes this collector field to supply thermally energy to

a seawater distillation plant of the MED type.

3. Site climate

The plant is situated in the Umm Al Nar (UAN) is-

land about 20 km to the north-east of Abu Dhabi city,

(latitude¼ 24.4�, longitude¼ 54.5�), UAE (daily solar

radiation values are shown in Fig. 3). This location is

classified as arid, dry and dusty. The annual precipita-

tion is meager with an annual rainfall averaging about

50 mm. It is noticeable from the weather records that the

highest 24 h rainfall for a typical year is generally about

one-third of the total annual rainfall. Nearly 50% of the

yearly recorded days have visibilities less than 8000 m

due to lifted sand, dust, haze, smoke etc. Sandstorms are

0

1000

2000

3000

4000

5000

6000

7000

daily

sol

ar ra

diat

ion,

kc

al/m

2day

Jan. Feb. Mar. Apr. May. Ju

Fig. 3. Daily solar radiation on a horizontal surfac

Solar sen

Glass tube

Front View

Fig. 4. Measuring the transmitta

typically reported on about seven occasions in a year,

three of them in March.

4. Test equipment and data analysis

Flow measuring and temperature sensing probes

were located at the inlet and outlet of the collector field

in order to be able to estimate the amount of heat col-

lected at any time by the whole field. Similar instruments

are also installed in two individual blocks, namely, A

and F, in order to be able to carry out comparative

studies on the effect of dust accumulation and other

collector operating parameters on the performance of

individual blocks.

4.1. Transmittance measurement

Transmittance measurements were taken at 12:00

mid-day during sunny days using two solar sensors at-

tached near the ends of a support plate (see Fig. 4). The

support plate is inserted inside a sample glass tube such

that one solar sensor is located about the middle of the

glass tube while the other is outside the tube. The sample

n. Jul. Aug. Sep. Oct. Nov. Dec.

e in Abu Dhabi for the reference year (1985).

sors Support plate

Side View

nce of a sample glass tube.

A.M. El-Nashar / Solar Energy 75 (2003) 421–431 425

glass tube has the same material, diameter and wall

thickness as the collector glass tubes. The support plate

is tilted at the same angle as that of the collector ab-

sorber plates. The voltage outputs of the two sensors are

measured simultaneously using two identical millivolt-

meters that has been previously calibrated. The corre-

sponding solar radiation intensity on a tilted surface

inside and outside the tube were estimated and the

transmittance was estimated using the equation:

s ¼ VAVB:Cs

ð1Þ

where VA is the voltage of the inside sensor, VB is that of

the outside sensor and Cs is a calibration factor.

Two sample glass tubes were used, one to simulate

the dust condition of each of collector blocks A and F;

they are referred to as sample tubes A and F. The two

sample tubes are cleaned at exactly the same time their

corresponding collector blocks are cleaned. Thus,

whenever blocks A and F are cleaned, their corre-

sponding sample tubes are also cleaned. This means that

the transmittance of the sample tubes can be assumed to

be identical to the transmittance of the corresponding

collector glass tubes.

4.2. Cleaning of blocks A and F

In order to be able to find the effect of dust accu-

mulation on the performance of a block of collectors, it

was decided to maintain one block (block F) in a rela-

tively clean condition and use it as a reference to com-

pare with another block (block A). Therefore, block F

was cleaned several times per week by the application of

a high-pressure jet of water while block A was kept

uncleaned and subject to dust accumulation for periods

extending from 1 month to one whole year. The amount

of heat collected by both blocks A and F were then

monitored on an hourly basis by the data acquisition

system of the solar plant. Block A was normally cleaned

about once a month by a jet of water. This block was

also kept without cleaning throughout the year in order

to test the effect of long-term dust accumulation on the

heat collected.

5. The simulation program ‘‘SOLDES’’

A computer program named ‘‘SOLDES’’ was pre-

pared to simulate the operating conditions of a solar

desalination plant similar in design to the one in Abu

Dhabi and to help in the design of similar plants. The

program carries out an hour-by-hour heat balances on

each of the major plant components (solar collector

field, heat accumulator and MED evaporator) and cal-

culate the heat input and output from each component

as well as the hourly distillate production from the

evaporator. These calculations are carried out for each

of the 8760 h of the year. The main input data to this

program is:

• site location data: site name, site latitude, site longi-

tude;

• solar radiation data;

• ambient air temperature;

• seawater temperature and salinity;

• collector absorber area (range from 500 to 20,000

m2);

• angles of absorber plate (azimuth and angle with

ground);

• dimensions of absorber plate (width, length and

pitch);

• ratio of capacity of heat accumulator to collector

area (0.05–1.00 m3/m2);

• MED evaporator capacity (100–2000 m3/day);

• number of effects of MED evaporator (13–32);

• frequency of collector cleaning.

The solar radiation data at the plant site is input as

one of four data types:

• hourly solar radiation on a tilted surface having the

same tilt angle as the absorber plate;

• hourly solar radiation on horizontal surface;

• daily solar radiation on a tilted surface having the

same tilt angle as the absorber plate;

• daily solar radiation on horizontal surface.

The ambient air temperature data is input as one of

two data types:

• hourly ambient temperature values;

• daily mean, daily maximum and daily minimum am-

bient temperatures.

Monthly average daily seawater temperatures are

used in the program.

The heat collecting system uses a bypass circuit and a

thermostat to allow the heat collecting water to recir-

culate through the collectors until the water temperature

reaches a preset (set point) value (see Fig. 1). Once the

water temperature rises above the set point value, op-

eration is switched over to the accumulator side and the

hot water from the solar collector field is allowed to

enter the accumulator tank. The operation of the heat

collecting system is controlled by a device called the

solar controller which controls the operation (on–off

control) of the heat collecting pump. The device receives

as input: water temperature at inlet to the collector field

and the instantaneous solar radiation on a titled surface

and sends as output an on-signal (after sunrise) or an

off-signal (before sunset).

426 A.M. El-Nashar / Solar Energy 75 (2003) 421–431

In order to verify the accuracy of this program, the

design specifications, operating parameters and weather

data for the reference year of 1985 (solar radiation, air

temperature and seawater temperature) for the Abu

Dhabi solar plant were submitted as input to the pro-

gram. We compare the program results with the actual

plant output data measured during the month of Janu-

ary. This month was selected because of the wide fluc-

tuations in the daily solar radiation that usually occur

during this month and this would be a good test of the

sensitivity of the program to such fluctuations. The

measured and estimated daily heat collected and deliv-

ered to the heat accumulator is shown in Fig. 5 and the

corresponding daily distillate water production is shown

in Fig. 6. As can be seen from these figures, the agree-

ment between measured and estimated quantities ap-

pears to be quite reasonable which clearly indicate that

0

1

2

3

4

5

6

1 3 5 7 9 11 13 15 17

Day number (Janu

Daily

hea

t to

accu

mul

ator

, 106

kcal

Fig. 5. Daily heat collected for clean collector field f

0

20

40

60

80

100

120

140

1 3 5 7 9 11 13 15

Day numb

Dai

ly w

ater

pro

duct

ion

, m3

Fig. 6. Daily distilled water production for clean collector

the program is able to respond to fluctuations in daily

solar radiation quite well.

6. Results and discussion

6.1. Experimental results

The long-term effect of dust deposition on the

transmittance of a single sample glass tube is shown in

Fig. 7. The data for a whole year is shown in this figure.

The test started with a clean glass tube and the initial

transmittance was measured at 0.98 and no cleaning was

carried out throughout the test period. As can be seen,

the transmittance experienced a gradual drop through-

out the year due to dust accumulation with the final

value reaching 0.80. A sharp drop in transmittance is

19 21 23 25 27 29 31

ary)

measuredsimulation

or the month of January of the reference year.

17 19 21 23 25 27 29 31

er (January)

measuredsimulation

field for the month of January of the reference year.

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400Day number (January 1st = 1)

Tran

smitt

ance

Fig. 7. Long-term effect of dust deposition on the transmittance

of the single glass tube.

0.92

0.94

0.96

0.98

1

1.02

1.04

0.85 0.9 0.95 1Glass transmittance

Qa/

Qf

Fig. 9. Effect of glass transmittance on the daily heat collection

ratio (Qa=Qf ).

A.M. El-Nashar / Solar Energy 75 (2003) 421–431 427

shown during the summer months (particularly June,

July and August) due to dust-laden air from sand storms

that are frequent during this time of the year in Abu

Dhabi. The recovery of part of the transmittance fol-

lowing this sharp drop appears to be due to the natural

phenomena of self-cleaning due to dew point accumu-

lation on the glass tubes during periods of high humidity

as usually happens during the months of July and Au-

gust. In parallel with the single sample tube test referred

to above, Blocks A and F were operated in such a way

that the glass tubes of Block A were to simulate the

operation of the sample glass tube thus it was cleaned

only once at the beginning of the test and was allowed to

accumulate dust throughout the year. Block F was

cleaned at the start of the test and regularly throughout

the test period so as to act as a reference. The daily heat

collected by each block was measured and the ratio of

the collected heat (Qa=Qf ) is shown in Fig. 8.

The ratio (Qa=Qf ) measured during this test is shown

in Fig. 8. This ratio starts at a value of 0.98 when both

blocks are equally clean and subsequently drops to

about 0.65 at the end of the test period. The sudden drop

in the ratio may be attributed to days in which sand-

storms were blowing while sudden jumps usually refer to

days in which precipitation has taken place. These data

are obtained from actual plant measurements.

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350 400Day number (January 1st =1)

Qa/

Qf

Fig. 8. Long-term effect of dust deposition on the heat collec-

tion ratio of blocks A and F (Block F frequently cleaned, Block

A only cleaned at start of test).

The effect of the glass tube transmittance s on

(Qa=Qf ) is shown in Fig. 9. The relationship can be ex-

pressed mathematically by:

Qa

Qf

¼ 0:3467 expð1:1244sÞ ð2Þ

Based on actual plant measurements, the hourly block

efficiency was found to depend on the x-parameter as

well as the transmittance of the glass tube which, in turn,

is affected by the amount of accumulated dust (see Fig.

10). For a clean block, the efficiency was estimated from

the actual plant data using the least square technique as:

g ¼ 0:81� 2:4604x� 1:9174x2 ð3Þ

The x-parameter is defined here as fðT1þT2Þ=2g�Tamb

Jtwhere T1

and T2 are the hourly average inlet and outlet water

temperatures from the collector block, Tamb is the am-

bient air temperature and Jt is the hourly solar radiation

on the absorber plate (tilted surface). The hourly block

efficiency is defined as:

g ¼ _mmcCpðT2 � T1ÞAcJt

ð4Þ

where _mmc is the mass flow rate of collector fluid (water)

through the collector block, Ac is absorber plate area of

00.10.20.30.40.50.60.70.80.9

0 0.05 0.1 0.15 0.2 0.25x-parameter, h.m2.oC/kcal

Hour

ly b

lock

effi

cien

cy

transmittance=0.95transmittance =0.81transmittance =0.7transmittance =0.63

Fig. 10. Effect of glass tube transmittance on the hourly block

efficiency.

428 A.M. El-Nashar / Solar Energy 75 (2003) 421–431

the block and Cp is the specific heat at constant pressure

for water. With dust deposition, the block efficiency can

be expressed as:

gðs; xÞ ¼ aðsÞ þ bðsÞxþ cðsÞx2 ð5Þ

where aðsÞ and bðsÞ and cðsÞ are functions of the

transmittance, s, of the glass tubes. Based on actual

measurements, and using the least square analysis, these

functions were found to take the forms:

aðsÞ ¼ 0:258þ 1:033s

bðsÞ ¼ �9:96þ 0:472s

cðsÞ ¼ 41:10� 19:96s

ð6Þ

It would be interesting to find the effect of dust deposi-

tion on the daily collector block efficiency which is de-

fined here as the ratio of the heat collected by a block

and the amount of solar radiation intercepted by the

collector absorber plates:

gd ¼Psunset

sunrise _mmcCpðT2 � T1ÞPsunset

sunrise AcJt

The monthly and annual collector block efficiency are

defined in a similar way. The difference is in the period

of summation of the amount of heat collected and the

solar radiation intercepted. The period is obviously 1

month for the monthly efficiency and 1 year for the

annual efficiency.

The effect of the reduction of transmittance of the

glass tube due to dust deposition on the collector daily

block efficiency for January is shown in Fig. 11. Actual

solar radiation data for this month is used in the cal-

culation of daily block efficiency. As expected, it can be

seen that the reduction in transmittance of the glass tube

results in a corresponding reduction in the daily block

efficiency. For clean collectors (transmittance¼ 0.98) the

daily block efficiency fluctuates between 0.32 and 0.52

0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Day n

Dai

ly e

ffici

ency

Fig. 11. Daily block efficiency variation with

depending on the daily radiation on the absorber plate;

the higher the solar radiation is, the higher the daily

efficiency and vice versa. For very dusty collectors

(transmittance �0.7) the daily block efficiency fluctuates

between 0.18 and 0.47. For constant transmittance, the

low efficiency values in the figure correspond to cloudy

days with low daily radiation.

The monthly average block efficiency variation with

the transmittance of the glass tube is shown in Fig. 12

for a typical year and the effect of transmittance on the

annual block efficiency is shown Fig. 13. The annual

efficiency can be seen to vary approximately linearly

with the transmittance of the glass tube according to the

following least square straight line:

gann ¼ 0:7013s� 0:1111 ð7Þ

Based on the reference year data for solar radiation and

ambient temperature, monthly average distillate daily

production was obtained from the DESAL program.

Several runs were made with different values of glass

tube transmittance varying from 0.6 to 0.98. The results

of the monthly production ratio (production with dusty

collectors divided by the production with clean collec-

tors) are shown in Fig. 14 and indicate a big influence of

the glass tube transmittance on the production ratio of

the plant.

The effect of dust deposition on the plant annual

distillate ratio (defined as the ratio of annual production

with dusty collectors to the production with clean col-

lectors) is shown in Fig. 15. It can be seen that dust has a

strong influence on the plant production. A plant with

very dusty collectors having a transmittance of, for ex-

ample, 0.7, can have their annual production drop down

to about 60% of their production with clean collectors.

This result is a direct consequence of the reduction in

collector efficiency and therefore the amount of heat

collected due to dust accumulation. A least square fit of

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

umber

transm.=0.98 transm.=0.9transm.=0.8 transm.=0.7

glass tube transmittance for January.

00.10.20.30.40.50.60.7

0.5 0.6 0.7 0.8 0.9 1Glass transmittance

Annu

al c

olle

ctor

effi

cien

cy

Fig. 13. Effect of glass tube transmittance on the annual col-

lector block efficiency.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Month

Mon

thly

col

lect

or b

lock

effi

cien

cy

emittance = 0.98emittance = 0.90emittance = 0.80emittance = 0.70

Fig. 12. Effect of glass tube transmittance on the monthly average collector block efficiency.

00.10.20.30.40.50.60.70.80.9

1

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Month

Mon

thly

pro

duct

ion

ratio

of

dust

y c

olle

ctor

s

transmittance=0.98transmittance=0.9transmittance=0.8transmittance=0.7

Fig. 14. Monthly plant distillate production ratio for the ref-

erence year for different glass tube transmittance (simulation

results).

0

0.2

0.4

0.6

0.8

1

1.2

0.5 0.6 0.7 0.8 0.9 1Glass transmittance

Ann

ual p

rodu

ctio

n ra

tio

Fig. 15. Effect of glass transmittance on the annual production

ratio of Abu Dhabi Plant.

A.M. El-Nashar / Solar Energy 75 (2003) 421–431 429

the data gives the following relation between glass tube

transmittance and the annual production ratio:

Mdusty

Mclean

¼ �0:1084 expð2:3213rÞ ð8Þ

where M is the annual distillate production of the plant.

Dust deposition on the collector tubes also affect the

specific power consumption of the plant (measured as

kWh/m3 of distillate) since, as expected, operating the

plant with dusty collectors would result in a low distil-

late production and result in frequent automatic shut-

downs due insufficient heat accumulator charge (as will

be seen later) (see Fig. 16). Each startup following an

automatic shutdown due to low accumulator charge

takes about 3 h of heating up until the evaporator

reaches its normal operating temperature. During this

period, the evaporator is normally consuming heat and

electrical power without producing any distillate. When

the plant is running with clean collectors, the specific

power consumption has been measured at 5.3 kWh/m3

but when running with very dusty collectors (transmit-

tance �0.6) the specific power consumption increases by

about 56%. The relationship between the specific power

consumption ratio (SPCR) and the glass tube trans-

mittance was obtained by least square as:

1

1.1

1.2

1.3

1.4

1.5

1.6

0.6 0.7 0.8 0.9 1Glass transmittance

Spec

ific

pow

er c

onsu

mpt

ion

ratio

(d

usty

/cle

an)

Fig. 16. Effect of glass transmittance on the specific power

consumption ratio of the plant in Abu Dhabi.

430 A.M. El-Nashar / Solar Energy 75 (2003) 421–431

SPCR ¼ 3:1328 expð�1:1668sÞ ð9ÞBased on the solar radiation data for the reference year,

the annual number of plant automatic start-ups varies

depending on the amount of dust deposited on the glass

tube. Fig. 17 shows the effect of glass transmittance on the

number of start-ups and demonstrates that the number of

start-ups increases exponentially as the collector trans-

mittance drops due to dust deposition. The reason for this

is that with dust accumulation on the collector tubes, the

amount of heat collected cannot match the heat required

by the evaporator thus causing the temperature of the

evaporator heating water (originating from the accumu-

lator top water layers) to drop below the setpoint value

thus causing the evaporator to shut down.

7. Conclusions

The deposition of fine dust particles on the glazing of

solar thermal collectors of desalination plants operating

in sandy areas significantly affects the performance of

such plants. The effect of dust deposition on a number of

Annual number of plant start-ups for different glass transmittances

0

50

100

150

200

250

300

350

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1Glass transmittance

No.

of s

tart

-ups

Fig. 17. Effect of the glass transmittance on the annual number

of start-ups.

major operating parameters of the solar plant in Abu

Dhabi, UAE was investigated using actual plant data as

well as the simulation program SOLDES.

Based on the results of this study, the following

conclusions can be drawn:

• The dust deposition on the collector surface causes a

drop in the transmittance of the glass tube that affects

both the collector efficiency and subsequently the

amount of heat collected.

• Since the plant distillate production depends mainly

on the amount of heat collected, it is therefore very

sensitive to the amount of dust accumulated on the

glass tubes. The larger the amount of dust accumu-

lated, the lower the glass transmittance will be and

the lower the amount of heat collected.

• Higher amount of dust accumulated on the glazing

means lower plant distillate production. Distillate

production can drop by about 40% when the trans-

mittance drops from its clean condition value of

0.98 to a very dusty value of 0.70.

• Dust accumulation results also in a higher plant spe-

cific power consumption due to the higher number of

plant automatic start-ups and shutdowns due to low

accumulator charge. The specific power consumption

increases by about 38% when the glass tube transmit-

tance drops to 0.70 from its clean value.

• As the amount of accumulated dust increases more

frequent evaporator shutdowns occur due to accu-

mulator low charge that is not able to maintain the

heat supply to the evaporator.

• There is, therefore, a big economic incentive in keep-

ing the collectors in a clean condition by conducting

frequent water cleanings to keep the transmittance

at or close to its highest value of 0.98. Since distilled

water is used for washing the collectors, frequent col-

lector cleanings will require large amount of water

that can reduce the net plant water production. There-

fore, it is important to optimize the frequency of

cleaning so as to enhance the economics of the plant.

Acknowledgements

Author is grateful to the support and encouragement

of Darwish M. Al Gubaisi for his keen interest and

encouragement in this project and for his active support

in the research program.

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